Research and development efforts on texturization to reduce the optical losses at front surface of silicon solar cell

Renewable and Sustainable Energy Reviews 66 (2016) 380–398

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Research and development efforts on texturization to reduce the optical losses at front surface of silicon solar cell M.F. Abdullah a, M.A. Alghoul b,c,n, Hameed Naser d, Nilofar Asim e, Shideh Ahmadi f, B. Yatim d, K. Sopian e a

Malaysia-Japan International Institute of Technology, Universiti Teknologi Malaysia, Jalan Sultan Yahya Petra, Kuala Lumpur 54100, Malaysia Energy and Building Research Center, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait c Center of Research Excellence in Renewable Energy (CoRe-RE), Research Institute, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia d The School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, Malaysia e Solar Energy Research Institute, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia f NOVITAS, School of Electrical and Electronic Engineering, Nanyang Technological University, 639798, Singapore b

art ic l e i nf o

a b s t r a c t

Article history: Received 26 July 2015 Received in revised form 22 June 2016 Accepted 15 July 2016

Silicon wafer-based solar cell contributes to about 92% of the total production of photovoltaic cells. An average of 30% of the incident light is lost via reflection from the front surface of the silicon solar cell, thus reducing the cell's power conversion efficiency. Texturization is a process of producing the desired unevenness on the surface of solar cell. It is well known as a practical solution to the limitation. Front surface texture reduces cell reflectivity and contributes to more photocurrent generation within active materials. The research and development efforts to reduce the optical losses via texturization are reviewed in this paper. The mechanisms of optical loss reduction, desirable texture feature, methods of texturization, side effects of texturization, and its compatibility with other optical enhancements for crystal silicon cell are elaborated upon. Front surface texture is associated with minimizing optical loss, and negatively affecting carrier and electrical losses. The importance of texturization for crystalline silicon is briefly related with thin film amorphous silicon solar cell to fully encompass this topic. Lesson learned and conclusion is highlighted in the last section. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Silicon solar cell Optical loss reduction Role of texturization Side effect of texturization Power conversion efficiency

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381 Desired textured feature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 2.1. Optimum feature size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 2.2. High coverage, homogeneity and uniformity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 2.3. High surface roughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 Methods of texturization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 3.1. Physical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 3.1.1. Mechanical grooving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384 3.1.2. Reactive ion etching (RIE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384

Abbreviations: mc-Si, Microcrystalline silicon; a-Si, Amorphous silicon; AgNO3, Silver nitride; AR, Anti-reflection; Ar, Argon gas; AZO, Aluminum-doped zinc oxide; CH3COOH, Acetic acid; CH3COONa, Sodium acetate; c-Si, Monocrystalline silicon; Cl2, Chlorine gas; ClF3, Chlorine trifluoride; Cr, Chromium; DIW, De-ionized water; H2O, Water; H2O2, Hydrogen peroxide; H2SO4, Sulfuric acid; HF, Hydrofluoric acid; HNO3, Nitric acid; I-V, Current-voltage; IPA, Isopropyl alcohol; Jsc, Short circuit current; KOH, Potassium hydroxide; mc-Si, Multicrystalline silicon; N2H4, Hydrazine; Na2CO3, Sodium carbonate; Na2S2O8, Sodium persulfate; Na3PO4, Sodium phosphate; NaHCO3, Sodium bicarbonate; NaOCl, Sodium hypochlorite; NaOH, Sodium hydroxide; O2, Oxygen gas; PDMS, Polydimethylsiloxane; PECVD, Plasma-enhanced chemical vapor deposition; PERL, Passivated emitter and localized; R&D, Research and development; RF, Radio frequency; RIE, Reactive ion etching; SEM, Scanning electron microscopy; SF6, Sulfur hexafluoride; Si, Silicon; Si3N4, Silicon nitride; SDE, Saw damage etchings; SnO2, Tin oxide; TBA, Tertiary-butyl alcohol; TCO, Transparent conducting oxide; TMAH, Tetramethyl-ammonium hydroxide; Voc, Open circuit voltage; ZnO, Zinc oxide; ZrO2, Zirconium dioxide n Corresponding author at: Energy and Building Research Center, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait. E-mail address: [email protected] (M.A. Alghoul). http://dx.doi.org/10.1016/j.rser.2016.07.065 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

381

3.1.3. Gas dry etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385 3.1.4. Laser ablation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385 3.2. Chemical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385 3.2.1. Texture etch characteristics by chemicals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385 3.2.2. Texture etching for inverted pyramids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388 3.2.3. Texture etching by acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389 3.3. Pre- and post-treatments of Si . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389 3.4. Alternative processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390 4. Side effects of texturization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390 5. Texture compatibility with other optical enhancement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 6. Thin film Si solar cell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 6.1. Texturization of transparent conducting oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 6.1.1. Sol-gel method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 6.1.2. Magnetron sputtering with acid etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 6.1.3. Magnetron sputtering with inductive coupled plasma etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 6.2. Effects of front and rear surface texture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394 7. Lesson learned and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396

1. Introduction Silicon (Si) wafer-based solar cell contributes to approximately 92% of the total photovoltaic cells production in 2014, while the remaining
9% are contributed by thin films [1]. According to Saga [2], research and development (R&D) efforts on enhancing the efficiency of Si solar cell are focused on reducing the optical loss using several methods as detailed in Table 1. These include studies on the front surface texture, application of anti-reflection (AR) coating on the front surface, attaining high reflectivity and low light absorption on the rear surface, and back contact cell design to avoid shadow on the front surface. The front surface texture is applicable for both conventional crystalline and thin film amorphous Si solar cell. In this study, we cover topics on textured Si solar cell from multiple perspectives based on various research articles. A critical review is performed on the R&D aspects of textured Si solar cell and the associated issues on texturization. There are five main R&D aspects to be highlighted on Si solar cell texturization, which are the optical loss reduction mechanisms, desirable texture features, methods of texturization, side effects of texturization, and texture compatibility with other optical enhancements. Fig. 1 is an important figure illustrating the overall progress in improving the solar cell performance.

Table 1 Key technologies for highly-efficient crystalline Si solar cells [2]. Aspect

Techniques of improvement

Minimizing photon loss

    

Minimizing carrier loss

  

Minimizing electrical loss

   

Textured front surface Anti reflection coating Back-contact cell structure High reflectivity via flat rear surface Rear surface reflector of thin metal with dielectric layer Passivation of (under) front electrode Shallow-doped p-n junction with front surface dielectric passivation layer Locally p þ -doped back surface field and point contact structure Back surface passivation by a dielectric layer Fine front contact gridline Selective emitter of deep and highly doped emitter under the contact n-type or p-type Si substrates with minority carrier diffusion lengths that are longer than the base thickness

Earlier on, Green and Keevers [3] studied the optical properties of intrinsic Si and tabulated the absorption coefficient as a function of wavelength. The data which is later being updated as in Ref. [4] indicates that Si is a poor light absorber especially at longer wavelength approaching near infra-red, supported as in Fig. 2(a). Around 1 cm thick Si wafer is needed to successfully absorb the light energy at wavelength close to 1150 nm. The reflection of Si in the visible wavelength range can be obtained in the manner shown in Fig. 2(b) based on the value of real refractive index and extinction coefficient. More than 30% of the incident light is lost via reflection and this is more pronounced in short wavelength or blue light range. Thus, the optical loss by low absorption in long wavelength and high reflection in short wavelength limit the amount of light that can be converted into electricity in a Si-based solar cell. A technique that addresses both the limitations is the front surface texturization, as mentioned previously, which is our focus of discussion in this article. The mechanism of optical loss reduction in textured Si solar cell can be summarized as in Fig. 3, which are: i. The presence of front surface texture increases the chance for short wavelength absorption by increasing the surface area, which results in carrier generation and collection closer to the junction. ii. Multiply the external reflectance of light onto neighboring inclined surface, promotes neighboring light bouncing instead of outside of the solar cell. iii. Diffraction of the incident light is more than just the zeroth order (happens on planar surface). First and second diffraction orders are promoted into the Si depending on the texture. iv. Lengthen the optical path for long wavelength to more than double of Si thickness to compensate for its low light absorption coefficient. v. Light trapping by total internal reflection of the reflected long wavelength from the rear surface. This occurs when light reaches the internal side of the textured surface with an angle exceeding the critical value. The mechanism of optical loss reduction is initiated once the feature size of the texture reaches its minimum. According to a two dimensional surface grating model and numerical simulation, negligibly very small the texture does not significantly modify the reflectance value. In fact, the reflectance value is similar to that of polished surface if the feature size is not sufficiently big [5] or does not have inappropriate width/height ratio [6]. The reduction of

382

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

Fig. 1. Placement of research in the body of knowledge.

optical loss simultaneously reducing surface reflectance and increasing light absorption coefficient is important due to its association with internal quantum efficiency [7–9], short circuit current density (Jsc), and overall cell efficiency based on detailed modeling [10].

2. Desired textured feature Successful reduction in light reflectance via surface texturization relies on several criteria. The criteria for the desirable textured Si surface can be summarized in the following manner: 2.1. Optimum feature size The optimum feature size ranges from 0.1 to 10 mm. Fukui et al. [11] produced a cone structure on multicrystalline Si using reactive ion etching, which resulted in decreased surface reflectance by increasing cone height/width ratio of the textured Si. They suggested that the optimal cone size is 0.2–0.4 mm high and 0.3– 0.5 mm bottom long. Measurements within this range produce uniform features with lower reflectance value and a higher current density. The short circuit current resulting from this cone size is shown in Fig. 4. On the other hand, too high of aspect ratio is indicative of a deep structure, which can lead to a large current saturation and poor performance. This is in line with the discussed surface grating model, whereby the texture base size between 0.3 and 0.4 mm results in closer ray tracing calculation data, distinguishable with reflectivity plot of polished surface as in Ref. [5] and also within the appropriate height/width ratio [6]. Submicron structures cannot be achieved through low cost chemical etching and mechanical grooving. Medium sized

pyramid of around 5 mm produced through alkali texturization should be optimal for solar cells. Smaller textures around 1 mm have been shown to have increased light reflectance and higher shunt path due to its fragility. It tends to have many crystal defects on the top and having deeper emitters, which lowers the open circuit voltage (Voc) [12]. Uniform and high density 5 mm pyramids are likely the result of optimum alkali texturization parameters as reported in the literatures [13–16]. Pyramid of 3–5 mm high is a good interface for the passivation layer, thus increasing the minority carrier lifetime [17]. 2.2. High coverage, homogeneity and uniformity An optimum feature size alone cannot lower the reflectance of textured Si. Rather, feature density should also be emphasized. Feature density is built from a combination of feature coverage, homogeneity and uniformity. The problem arises from the incapability to texture high density features across Si surface, which depends on the methods of texturization. An obvious limitation found in the mechanical grooving method is the reliance on the shape of the tooling blade [18,19]. Misplaced Si wafer in RIE system also results in non-uniform darkening, which is confined mostly at the center of sample [20]. However, the process should be able to texture random areas effectively. Alkali texturization is proven to be difficult in realizing high density textured structures due to the low wettability between the etchant and Si, if without the presence of alcohol additive or other surfactants [21–23], insufficient texturing duration is allocated [24], or too high etch rate that would collapse out of the formed structures [25,26]. Lower texture density is obtained as the size of every structure increases and the smaller-sized population is replaced by larger sized pyramids [27].

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

383

Fig. 2. (a) Absorption coefficient of flat c-Si wafer at 300 K. (b) Reflectivity of flat c-Si wafer at 300 K. Both graphs are plotted according to the data in Ref. [4].

Fig. 3. Modification of long wavelength travelling path via front surface texture.

While those three criteria contribute to lower optical loss, the effect different feature types is also valid in determining the best texture. If they are all comparable, then the type of texture becomes more pronounced. For example, higher current density is generated within the near surface region in regular inverted pyramid compared with regular, random upward pyramid and groove texture, based on the study of spatial profiles of photocurrent generation for shortest distance to the p-n junction in Ref. [29]. It has been proven that the regular inverted pyramid texture produces lower reflectance when compared to random upward pyramids, both before and after application of AR coating [30,31]. Nanotextured upward pyramids result in better internal quantum efficiency when compared to the normal upwards pyramids [32], but the surface roughness is not likely to differ much. Common types of texture featuree are provided in Fig. 5(a) to (h). In terms of reducing optical loss alone that the textured feature could offer, we are convinced that in a descending order it would be: Random conical, micro/nanotexture pyramids, regular inverted pyramid, porous upward pyramids, random upward pyramids, random inverted pyramids, honeycomb, and finally grooves. As many factors are actually present, different researchers may come out with different opinion based on their results. For example, the honeycomb texture might be able to reflect lower light compared to the inverted pyramids as reported in Ref. [33].

3. Methods of texturization

Fig. 4. Influence of RIE-textured cone size on short circuit current as measured in Ref. [11].

Texturization process starts with Si wafer, which is an inorganic material for solar cell purpose. Si wafer can be monocrytalline (c-Si) which has very neat and consistent crystal alignment, or lower cost multicrystalline (mc-Si) which has inconsistent size and arrangement. The most common type of c-Si used as a starting material is grown using the mass scale Czochralski process, being pulled from the melt as a crystal ingot in a standard diameter of either 300, 200 or 150 mm. The Czochralski process is acceptable for electronics manufacturing and solar cell due to minimal defects, higher production and lower cost as opposed to high quality float zone Si. After ingot sawing, physical or chemical texturing process follows.

2.3. High surface roughness 3.1. Physical methods High surface roughness is the product of both aforementioned criteria in which highest surface roughness across textured Si results in the lowest reflectance value [28].

Physical texturization involves portion of Si removal by a dry process. As long as the description fits, any technique can be

384

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

Fig. 5. Several types of textured structures on Si surface. (a) Random conical [34]. (b) Micro/nano pyramids [32]. (c) Regular inverted pyramid [35]. (d) Porous upward pyramids [36]. (e) Random upward pyramids [28]. (f) Random inverted pyramids [37]. (g) Honeycomb [38]. (h) Grooves [19].

regarded as physical texturization, but four are quite common in the laboratory and industry. They are as described below: 3.1.1. Mechanical grooving Mechanical texturing is one of the commonly adapted techniques to texture Si surface. The process was very time-consuming prior to the development of multi-blade tool. Research groups in Refs. [39,40] have developed a low cost tool blade for this purpose. The tool has a metal body with a V-grooved surface coated with an abrasive layer of diamond. Mechanical texturization is realized by printing the structuring wheel with a tool blade at 150–160 mm/s scanning velocity and water as the cooling agent. The scanning velocity depends on the size of the diamond grains, whereby higher cutting speed is possible for larger average grain size (says 5–15 mm). Since the tool is coated with abrasive diamond particles, its wear is significantly reduced. It could reach up to 1 million Si wafers before re-coating is necessary. Therefore, the texturing cost is also reduced. It is important to determine the radius of curvature at the bottom surface of the Si and the angle within the grooves after cutting. The feature of the groove is determined after the cleaning process based on the dimension of the tool blade and post-treatment using chemicals. For example, an original texturization angle of 45° will be lowered to 32° due to the severe degradation through the process of removing the residual damage by quick etching in acid solution, especially at the tips of the groove [18].

Therefore, due consideration is required prior to grooving Si for well shaped and defect free grooves, which is desirable for reducing the optical losses as well as the next stage of cell fabrication [19]. Using this method of mechanical texturing, it is possible to obtain double-sided V-groove where the cell can be designed to have either parallel or perpendicular front/rear grooved surface [41]. 3.1.2. Reactive ion etching (RIE) This process involves dry etching of the Si surface, whereby the surface is attacked by highly reactive ion plasma with Cl2 or SF6/O2 as its reactant gas. The latter is more favorable due to lower toxicity and corrosiveness compared to those of Cl2. Texturization is controllable in term of determining the final feature size of the texture, which can be further enhanced by introducing metal catalyst like Cr and Al in RIE system [42,133]. This practice has resulted in the lowest reflectance for textured Si obtained through Cr-assisted RIE with features of 50–300 nm diameter and 100– 500 nm depth. Despite lowered reflectance, the sample suffered from poorer response in short wavelength. Hence, it should be treated with acid for damage removal, but with a slight increase in final reflectance. Prasad et al. [20] placed the Si wafer on the ground electrode in the RIE setup and used reactive species-dominated plasma etching. The results showed more uniform texture when compared to placing the wafer on the RF-powered electrode. Placing the wafer

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

on the powered electrode causes the sample to be bombarded by heavy ions, resulting in physical sputtering in addition to chemical etching. The situation could be explained by the combination of plasma mode and low RF power density actually helps to increase the controllability of the process and minimize surface damage. Significant reduction in reflectance was obtained using fabricated cell as indicated in Fig. 6(a) and (b), which is followed by improvement in the current generation. RIE etching for mc-Si texturization can be done with or without a mask. Photolithography is required prior to RIE in order to create the mask, that is, to define the holes to be etched. These holes result in a neatly-patterned structure. The masked RIE has been shown to produce better microstructure and have the lowest reflectance for textured wafer compared to those of unmasked [34]. However, the difference between masked RIE, unmasked RIE and acid wet etching is reduced substantially in following AR coating and encapsulation. 3.1.3. Gas dry etching Gas dry etching is carried using gas to attack the Si surface, which is different from the RIE. This process normally etches via isotropic behavior (similar etch rate regardless of crystal orientation) and exhibits high selectivity. However, the anisotropic etching offers the ability to etch at finer resolution and higher aspect ratio when compared to isotropic dry etching, which will be discussed later on. There was a report on the production of honeycomb-texture on both c-Si and mc-Si wafers using Ar and ClF3 in the masked gas etching at room temperature [38]. In that study, the sample was masked with thermally grown oxide, and then placed inside a reaction chamber to be partially etched by ClF3 prior to the introduction of Ar. After evacuation, the sample was etched by buffered acid solution for oxide mask removal. Using this method, similar honeycomb structure shapes and dimensions was observed on both c-Si and mc-Si, thus convincing that the setup was applicable for both. A similar experiment was conducted in Ref. [43] for the unmasked gas etching. It was found that the reflectance for textured c-Si was slightly lower than that of mc-Si after unmasked dry etching with ClF3. 3.1.4. Laser ablation Laser texturization is a method that employs subsequent Si

385

melting and evaporation under pulsed laser-generated radiation. Different features can be textured based on how the laser beam is exposed to the Si surface. For example, parallel groove can be obtained with 1-direction beam movement, or defined patterning with 2-direction movement. In 1-direction laser movement, the grooves are scribed with constant spacing by consecutively scanning the wafer surface in opposite directions [44]. Parameters of concern are maximum output power, speed and pulse repetition frequency of the laser setup. The limitations of this process are melted Si at the bottom and sidewalls of the groove within the heat affected zone and amorphous Si debris [44,45]. These are caused by the laser-generated heat, leading to the deposition of foreign materials during texturization. Thus, laser-textured wafer needs to undergo a material removal process via chemical etching in order to fix its performance parameters accordingly [46]. Table 2 details each physical texturing method for a quick comparison. The described information is helpful to quickly distinguish between several previously-mentioned methods via their advantages and disadvantages. 3.2. Chemical methods 3.2.1. Texture etch characteristics by chemicals Texturization via wet chemical etch is an incomplete chemical wet etching method that is performed to obtain a smooth Si surface for electronic manufacturing and micro-electro-mechanical system. Chemical wet etching can be divided into two, either anisotropic wet etching by alkali or isotropic wet etching by acid. The difference between the two is that alkali etching depends on Si crystal orientation, but not for acid. The basic Si crystal orientations are illustrated in Fig. 7(a). Seidel et al. [47] used potassium hydroxide (KOH) to demonstrate that the slowest etching rate for Si occurs in the (111) direction. Thus, incomplete etching will leave the pyramid structures with (111) sidewall on Si (100) substrate due to the extremely slow etching rate in (111) direction. They also found that the etching rate for alkali is directly proportional to the available hydroxyl ion (OH  ) and free water particles. Here we include a number of known alkali solutions which can texture etch Si wafer according to the desired roughness:

Fig. 6. (a) Reflectance of RIE-textured mc-Si sample in different stages of solar cell fabrication. (b) Comparison of I-V characteristics of mc-Si solar cell with and without RIE texturization [20].

386

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

Table 2 Some remarks on the advantages and disadvantages of individual physical method. Method

Advantage

Disadvantage

Larger feature size compared to other methods (10–100 mm or larger), no randomization in features RIE Capable of producing small structures in the submicron range (less than 1 mm), Sometimes need chemical post-treatment to smoothen sharp cone highly uniform feature, suitable for both c-Si and mc-Si texture (to avoid the side effect) Gas etching Capable of gas-etching Si without plasma, applicable for both c-Si and mc-Si Isotropic etching for c-Si; not as selective as chemical etching (alkali texturization) Laser ablation Laser-accuracy, flexible (depending on the setup), applicable for both c-Si and mc-Si Si has to be treated with chemical after texturization, higher level of sophistication Mechanical

Low cost (can be lowest), high texturization rate, suitable for both c-Si and mc-Si

3.2.1.1. KOH and NaOH. Low concentration and high temperature KOH or sodium hydroxide (NaOH), with the addition of isopropyl alcohol (IPA), is a common or conventional texture etching method for Si. Vazsonyi et al. [21] textured c-Si using NaOH-IPA solution by varying the temperatures, etching durations, and concentrations of NaOH and IPA to determine the best outcome(s). High temperature of around 80 °C, and low concentration of NaOH (1.5–4 wt%) and IPA (3 vol%) were required to maintain an optimum etch rate of 0.60 mm/min. Such rate resulted in uniform and high coverage random pyramid that feature on the Si surface. The addition of the right amount of IPA improves wettability by lowering surface tension [48], liberating water particles in close vicinity to Si surface [25], and promoting pyramid nucleation [21]. Etching using alkali without IPA leads to high etch rate, washing out the pyramid texture which causes non-optimal lowering of the final reflectivity of Si [49]. As indicated in Fig. 7(b), the addition of IPA lowers the overall etching rate in the rest of (hkl), without affecting (100) and (111) directions [50,51]. The hydrogen bubbles produced during the reaction between Si surface and etchant act as a mask on the Si surface. The medium concentration IPA helps to remove the correct amount of bubbles while allowing some to remain as the points of initiation for pyramid texture. The mask can also be built by adding surfactants, native oxides, organic contaminations, or trace metal elements to take part in the texturization process [13]. The mechanisms of nucleation are illustrated in Fig. 8. Tertiary-butyl alcohol (TBA) can be used as a replacement for IPA with comparable results [14]. Another option is to use higher boiling point 1,2-pentanediol, which boils at 206 °C to allow for greater process control as IPA readily evaporates at around 82 °C [52]. Evaporated IPA can be re-condensated by topping the container with ice-cooled reflux condenser [49], or by topping up IPA from time-to-time to maintain the fix amount of etchant. Nevertheless, alcohol fume is regarded as a health hazard. For further economical improvement, adding hydrazine monohydrate (N2H4,

H2O) into NaOH can reduce the use of IPA in batch texturization of c-Si wafer. This practice results in insignificant difference in final reflectance compared to that of conventional NaOH [53]. Alternatively, employing metal grids with suitable openings can eliminate the use of IPA. Such practice is used to appropriately remove and confine hydrogen bubbles at a lower cost [54]. The use of metal grids is compared in Table 3. 3.2.1.2. TMAH. A renowned alternative for conventional KOH/NaOH etching is to use tetramethyl-ammonium hydroxide (TMAH), which is also widely used in microelectronic and MEMS industries due to its compatibility and good etching rate. You et al. [35] conducted texture etching of c-Si using TMAH for solar cell applications. Similarly, for KOH/NaOH texturization, TMAH desired high temperature and low concentration (around 5 wt%) for incomplete etching and leaving pyramids structure on the Si surface with a reflectivity of less than 1%. The pyramids are washed out from the surface using a high concentration alkali from the complete anisotropic etching. The addition of IPA in TMAH plays the exact role as using KOH/ NaOH, which produces more uniform pyramids texture [15,55]. Si can be contaminated with parasitic ions of K þ or Na þ if it is not cleaned up properly after being treated with KOH/NaOH during texturization. This has been proven to reduce the minority carrier lifetime in a complete fabricated solar cell. Although its cost is quite high when compared to KOH and NaOH, it is not regarded as a major problem in using TMAH. The ability of TMAH to texture Si wafer in batched was demonstrated in Ref. [56] where low concentrations (5 wt%) of TMAH without additives took 40 mins to complete the pyramidal formation. In the first 20 mins, a flat surface was prominent, indicating the lack of anisotropic etching. The same solution could be used to texture 10 wafers, with 15% hemispherical reflectance, as shown in Fig. 9(a). This indicates that it has high reproducibility and is promising in terms of chemical and cost-saving processes.

Fig. 7. (a) Si crystallographic planes indicated by (100), (110) and (111). (b) Relative etches rates of Vhkl/V100 in KOH and KOH  IPA solutions [51].

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

387

Fig. 8. The proposed mechanism of texture formation consisting of six stages: (a) Start. (b) Early stage. (c) Formation. (d) Deformation. (e) Reformation. (f) Small texture [13].

Table 3 Economic comparison between KOH with IPA and KOH with metal grid [54]. Stage

KOH with metal grid

KOH with IPA

Pre-treatment

3 mins oxide removal in 8% HF

3 mins oxide removal in 8% HF

Cost: 0.033 USD/wafer

Cost: 0.033 USD/wafer

Texturing

Texturization with metal grid: 20 mins etch in 1 wt% KOH at 90 °C Cost: 0.017 USD/wafer

Conventional texturization: 30 mins in 3 wt% KOH, 5 vol% IPA at 80 °C Cost: 0.066 USD/wafer

Post-treatment 5 mins HCl cleaning at 60 °C þ 1 min 8% HF dipping, then DIW rinsing Cost: 0.055 USD/wafer Total cost: 0.105 USD/wafer

5 mins HCl cleaning at 60 °C þ1 min 8% HF dipping, then DIW rinsing Cost: 0.055 USD/wafer Total cost: 0.154 USD/wafer

3.2.1.3. Na2CO3. Nishimoto and Namba [57] introduced texture etching using sodium carbonate (Na2CO3), which is mostly used in the glass fabricating industry. It is cheaper than the conventional

KOH, NaOH and TMAH, which also eliminates the use of IPA. Na2CO3 requires high temperature and concentration of 23 wt% to sufficiently texture Si at an acceptably low reflectance. Adding NaOH into Na2CO3 degrades the texturization due to the oversupply of OH  . While adding low concentration sodium bicarbonate (NaHCO3) increases the carbonic ion (CO32  ) and decreases OH  in the mixture, resulting in lower reflectance as shown in Fig. 9(b). The carbonic ions and its corresponding compound has similar role with IPA or its compound in KOH/NaOH/TMAH, whereby it acts as an initiator for pyramidal texture, leading to uniform, high density, and high coverage of micro structures. The result in Fig. 9(b) is in agreement with that of few other groups [22,23,58]. Adding NaHCO3 into the solution lowers Si etching rate by increasing the etchant concentration. It also produces more hydrogen bubbles during texturization to act as barriers on the wafer surface, thus lowering the etching rate. A higher etching rate leads to the formation of bigger pyramids, resulting in large non-textured surface area. The HCO3  ions supplied by NaHCO3 plays an important role in moderating the etching reaction. The NaHCO3/Na2CO3 ratio should be 0.16; a higher ratio will leave the excessive amounts of HCO3H  ions and reduce OH  ions, which results in non-optimal etching [23].

Fig. 9. (a) Reflectance of textured c-Si using the same TMAH in subsequent processes [56]. (b) Comparison of the reflectance of textured c-Si using Na2CO3 with and without NaHCO3 [57].

388

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

3.2.1.4. Other solutions. Xi et al. [59] demonstrated Si texturization using low concentration tribasic sodium phosphate (Na3PO4  12H2O) with acceptable results. This solution hydrolyzes in water to provide OH  ions that will attack Si surface. As a matter of fact, the concentration of hydroxyl ion provided exceeds that of Na2CO3. Texturization using this solution favors low concentration (5 wt%) and high temperatures to produce the lowest reflectance. Adding sodium phosphate dibasic (Na2HPO4) does not produce the same effect as adding NaHCO3 into Na2CO3 as the final reflectance of textured Si worsens. IPA is not required as it will react with Na3PO4 and render the solution cloudy. It is suggested that PO43  or its compound from the solution has similar role with IPA in conventional etching such that it promotes the formation of pyramids. Sodium acetate (CH3COONa) at high temperature and low concentration is a cheaper solution for Si texturization [60]. In general, NaOH (without IPA) texture etching results in slightly lower reflectance when compared to CH3COONa. By mixing 2.5 wt% NaOH and 10 wt% CH3COONa, the reduction in reflectance is lower than that obtained in texturization using NaOH. In this case, acetate (CH3COO  ) ensures wettability and expels the hydrogen bubbles from Si surface, which is similar to IPA's roles in conventional texturization. However, IPA and CH3COONa should not be mixed with NaOH simultaneously because they cannot function alongside with one and another. Increasing the concentration of IPA in NaOH-CH3COONa will increase the reflectance of wafer post-processing. Known methods to produce random upward pyramid via alkali texturization are provided in Table 4, indicating their ability to lower the average hemispherical reflection. However, complete c comparison in term of fabricated cell performance is not performed. 3.2.2. Texture etching for inverted pyramids In order to obtain inverted pyramid texture, the Si surface needs to be masked with patterned and thermally grown Si dioxide (SiO2) by photolithography prior to texture etching [30,35].

TMAH is preferred over KOH due to higher undercut rate and (111)/(100) etch rate ratio, which is 2–3 times higher than KOH at similar concentrations to produce an inverted pyramid within an oxide pattern. The (111)/(100) etch rate ratio is determined by dividing the etch depth for (111) plane by (100) plane. The etch depth (111) plane is calculated by multiplying the overhanging oxide depth with 54.74° as illustrated in Fig. 10. TMAH is preferred for the formation of inverted pyramids due to its lower anisotropic factor and lower etch rate of SiO2. Thus, only very thin SiO2 masking is required to allow for the lowering of its oxidation temperature and time. The combination of 5 wt% TMAH and 5 wt% KOH to etch 2.5 mm oxides with masking has been shown to produce extremely smooth (111) sidewall with sharp vertex. While etching by KOH resulted in inverted pyramids with horizontal etching stripes parallel to o1104 on the o111 4 sidewall due to lower (111)/(100) etch rate ratio [30]. The number of insoluble floccules residue observed on the surface of the inverted pyramid in KOH etching is not desirable because it leads to solar cell contamination. The use of low quality porous SiO2 without pattern and deposited by plasma-enhanced chemical vapor deposition (PECVD) as a mask before etching to produce inverted pyramidal textures has also been presented [37]. Due to the excellent etching selectivity between Si and SiO2, a short duration of 5 min for texture etching by TMAH can produce random inverted pyramids. At longer duration, the pyramids collapse to form an upward pyramid. Similarly, a group [62] used Si3N4 as a mask deposited by RF sputtering. Using this method, inverted pyramids can be obtained during the intermediate process of forming upward pyramids. Their final target was obtaining random pyramids texture. In the early stage of etching, few Si3N4 layers were removed, and sparsely distributed inverted pyramids were formed via exposure to the (111) plane. These inverted pyramids act as initial points for forming upward pyramids. Uniform and high quantity inverted pyramids in a moderate masking layer results in highly dense small upward pyramids after certain duration of KOH-IPA etching. In conclusion, using this ‘formation of upward pyramids from

Table 4 Recipes for alkali texturization and their capability to texture c-Si. Ref.

Chemical Optimum Mixture

Temp. Time

Reflectance

Remark

[14]

KOH

0.82 wt% KOH þ 9.7 vol% IPA

80 °C

30 mins

10.7% at λ0.3–1.1 mm

[54]

1 wt% KOH

90 °C

20 mins

15.1% at λ0.4–1.1 mm

[58] [21] NaOH [49] [60]

3 wt% KOH þ 5 vol% IPA 1.5–4 wt% NaOH þ3 vol% IPA 2 wt% NaOH þ 20 vol% IPA 2.5 wt% NaOH þ 10 wt% CH3COONa 5 wt% TMAH 2 wt% TMAH þ 8 vol% IPA 10 wt% TMAH þ 6 vol% IPA

80 °C 80 °C 80 °C 75 °C

40 mins 55 mins 45 mins 35 mins


15% at λ0.4–1.0 mm 12.5% at λ0.4–0.7 mm
12% at λ0.5–1.0 mm 13.5% at λ0.24–0.8 mm

Pre-treatment in HF and KOH SDE. Same parameter with KOH þTBA with reflectance of 11.92% Pre-treatment in 10% HF; using 1 mm separation and 2 mm2 opening metal grid Etch rate of 0.85 mm/min Etch rate of 0.60 mm/min
20 wt% NaOH SDE Pre-treatment in HF

80 °C 80 °C 80 °C


3% at λ0.2–1.2 mm 13% at λ0.35–1.1 mm
10% at λ0.5–1.0 mm

Polished. Etching in 2–9 h did not increase the reflectance 10 s pre-treatment by dipping in 5% HF –


15% at λ0.5–1.1 mm 7% at λ0.4–1.0 mm 11% at λ0.3–1.1 mm 11.5% at λ0.4–1.0 mm 16% at λ0.3–1.2 mm

100 s pre-treatment by dipping in HF Pre-treatment with RCA and then 2% HF for 5 mins Same reflectance at 30 mins with saw damage etch Etch rate of 1.04 μm/min Etch rate of 0.55 mm/min

[35] TMAH [15] [55]

70 °C 90 °C 80 °C 95 °C 90 °C 95 °C

20 mins


15% at λ0.4–1.0 mm

–

90 °C

25 mins

15.4% at λ0.3–1.25 mm

–

[27] [61]

5 wt% TMAH 5 wt% TMAH 20 wt% TMAH þ IPA 20 wt% Na2CO3 þ NaHCO3 12 wt% Na2CO3 þ1.5 wt% NaHCO3 20 wt% Na2CO3 þ 4 wt% NaHCO3 25 wt% Na2CO3 þ 4 wt% NaHCO3 25 wt% Na2CO3 18 wt% Na2CO3

30 mins 30 mins 25– 35 mins 40 mins 10 mins 60 mins 10 mins 40 mins

95 °C 95 °C

10 mins 25 mins

12% at λ0.4–1.1 mm 10.7% at λ0.3–1.1 mm

[59] Na3PO4

5 wt% Na3PO4  12H2O

85 °C

25 mins

12.5% at λ0.25–0.8 mm

Pre-treatment with NaOH SDE and 10% HF Etch rate of 0.70 mm/min; using 1 mm separation and 1 mm2 opening metal grid Pre-treatment in HF

[56] [28] [24] [57] Na2CO3 [58] [22] [23]

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

389

Fig. 10. A cross-sectional schematic to determine the undercut rate of textured Si [30].

inverted pyramids’ technique allows control of upward pyramid size by varying the thickness of masking layer. 3.2.3. Texture etching by acid Due to the disorderly crystal directions in mc-Si, texture etching using alkali fails to match the results of the c-Si sample. The use of acid is more appropriate as it etches isotropically at similar rates regardless of directions, leaving crater-like or oval pits texture on the surface. The mixture of HNO3-CH3COOH-HF or HF-HNO3-H2O is common and suitable for low cost texturization of industrial mc-Si cell [63]. However, it might not be the best option for c-Si compared to conventional alkali texturization due to higher reflectance, except in the case of more complex acid mixture or well patterned photolithography [33,64–66]. Otherwise, anisotropic etching using alkali such as NaOH can be shifted towards isotropic by adding NaOCl as a strong oxidizing agent as demonstrated in ref. [67]. This decreases the etching rate in (110) and (100) directions, while increases it in (111) direction. More positive feedback on the implementation of this solution for industrial mc-Si texturization can be found in Basu et al. [68,69], whereby the authors compared the results with that of acid texturization in terms of cost, smoothness, and environmental friendliness. 3.3. Pre- and post-treatments of Si Si wafers derived from ingot sawing produced slight non-uniformity on the surface prior to physical or chemical texturization (often not emphasized in physical texturization though). If the saw-damaged Si is not removed, the Si wafer toughness can decrease. Comparing Si thinning factor after texturization compared to as-sawed sample, it was estimated that the improvement in toughness could be as high as 150% for alkali etching and 250% for acid etching [70]. This might be due to the damages incurred on Si after the ingot sawing, which severely affect the ultimate material toughness, and limit the mechanical resistance.

Park et al. [16] compared alkali and acid saw damage etchings (SDE) to investigate their effects after texturization by KOH. In that study, the acid solution was a mixture of HNO3-CH3COOH-HF, while the alkali solution was KOH solution. SDE uses high concentration of KOH to produce lower roughness on the wafer surface. Due to the isotropic etching of highly concentrated alkali against the (100) direction at a high rate of 2 mm/min, square shapes measuring 10 mm wide and 5 mm high are produced on the surface. On the other hand, acid SDE results in crater-like shapes. The defects on the surface following SDE act as initial points of pyramids formation as detailed in Fig. 11. The higher density pyramid texture is obtained via acid when compared to alkali SDE, which is related to increased number of initiation points. Furthermore, highly dense texture results in lower reflectance for textured Si. A simple SDE using highly concentrated alkali or acid solution for short immersion can be used to accelerate the sequence of texturization. This was proven by faster pyramidal structure formation by alkali compared to as-cut and polished wafers [24]. However, the SDE can be omitted if enough time is allowed for texturization [58]. In Fig. 12, a complete duration for texturization by alkali is around 60 mins for 100% feature coverage on Si surface. In addition, similar reflectance is normally obtained from textured sample with or without SDE. The requirement of SDE should only be taken into consideration if other factors such as it would affect the final size, or could give more control during nucleation of pyramids for example by using hot NaOCl pre-treatment as discussed in ref. [71,72]. However, post treatment should not be omitted after texturing as it is necessary for handling some side effects from texturing. Table 5 lists the chemicals for post treatment, which are made up of mostly alkalis and acids. Physically or chemically textured Si is dipped into acid solutions to remove oxides, helps to round the texture [18,44,45] and removes damaged Si prior to junction formation [20]. Marrero et al. [73] stain etched Si using acid to develop porous pyramids texture after producing random pyramids by alkali. The

Fig. 11. KOH texturization for initial surface conditions of Si wafer [16].

390

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

their suitability with fingers and busbar on a completely fabricated cell. This in turn reduced the series resistance and improved the Voc and fill factor. Suitable matching between the plasmonic effect of metal nanoparticles and Si texture potentially increases cell efficiency due to the incident light scattering. However, the design of the texture need to be optimized by considering the combination of Si nanostructure with the correct periodicity, height, filling fraction, and intentioned metal nanoparticles array [76]. 3.4. Alternative processes

Fig. 12. Reflections of textured Si, with and without SDE as a function of texturization time [58].

wafer was etched with a mixture of 69 wt% HNO3 and 48 wt% HF aqueous solutions to form porous Si layer. They found that every stain etch concentration and the duration increased from 3 to 6 s resulted in lower final reflectance. Stain etching in HNO3-HF for a couple of seconds softened and reduced the tetragonal shape of pyramids, leading to lower quality metallic contact during cell fabrication. Effectively, the cell performance is not improved although with slight reduction in reflectance. Nanoscale porous surface increases the front surface area. It can enhance short wavelength absorption while simultaneously increasing the front surface recombination [36], especially for over-stained etched Si. A group as in ref. [32] opted for electroless treatment of c-Si in acidic aqueous solution of Na2S2O8 and AgNO3 for 6 mins, followed by etching in HF-H2O2-H2O for an additional 2 min instead of acid stain etching after alkali texturization. Through this method, the advantages of short wavelength absorption for nanotextured pyramids can be justified as opposed to the normal micro-sized pyramids based on internal quantum efficiency in the blue light range (Fig. 13), as well as increased Jsc. Stain etching using acid following front grid printing on textured Si was investigated [74]. In this method, Ag particles were dissolved from the grid, which then acted as catalysts for porous surface formation. Furthermore, the reduction in contact resistance was attributed to the thinning of the glass layer at the Ag-Si interface. Sardana et al. [75] deposited Ag nanoparticles on the alkali textured c-Si using RF magnetron sputtering as an attempt to utilize the role of metal particles on textured Si surface to improve light scattering and reduce reflectance. Annealing textured Si with Ag nanoparticles for 2 min at 300 °C promotes coalesce of the large coverage of small particles into large particle with small coverage, which is necessary to reduce the parasitic loss caused by Ag. Deposited Ag nanoparticles modified the charge carrier recombination process because of

Another type of texture besides the typical nano-sized structures is one utilizing Si nanowires. The etching is done using mixed HF and AgNO3 solution to fabricate the Si nanowires array, which lowers the front surface reflectance of the cell. Chemical etching can be performed on epitaxial grown Si on c-Si wafer for fabricating lower cost Si nanowires [77]. The use of Si thin film is valid for low cost application, as it can still exhibit valuable cell power conversion efficiency if the film is at an appropriate thickness. A technique was proposed to produce nanowires on the front surface of bare Si wafer, which is by depositing Ag film on the front surface rather than depositing Ag particles from AgNO3 mixture [78]. This can prevent the formation of similar feature on the rear surface, as it is not desirable for cell performance after complete fabrication. The size and distribution of nanowires can be controlled to be more uniform by varying the thickness of the predeposited Ag thin film. Si texturization is not only limited to material removal, but it also includes the addition of material on Si surface. For example, there are reported in uses of optimal at 0.25 mm nano-imprinted ZrO2 and ZnO in Ref. [79], and micro-spherical texture with 3 mm diameter silica particle coated with omni-directional AR coating in Ref. [80] serves similar functions as a damaged Si-surface. Alkali textured c-Si is also reported by a group to be used as a ‘mold’ for depositing polydimethylsiloxane (PDMS) film to produce textured PDMS film. The film was then pasted on glass surface of thin film Si solar cell to act as an incident light scatter [81]. Lowering optical loss provides higher chances for photocurrent generation by the cells. However, increasing current output density does not have a simple proportional relationship with the reduction in reflectance because of the side effects of the process. Forming a junction with the high collection efficiency is an important factor that should be taken into account to avoid further necessity and poor performance [82].

4. Side effects of texturization The most commonly discussed disadvantage of the textured Si solar cell is the front surface recombination. Increased front surface recombination is caused by: i. Increased surface area. The texture produced either via physical or chemical method undoubtedly increases the surface area, which increases the chance for carrier recombination. For c-Si

Table 5 Post-treatment of physically and chemically textured Si. Purpose Remove Remove Remove Remove Rinse

Chemical solution Si residual oxides metal particles organic contaminants

Isotropic etching of KOH, NaOH, or acid mixture HNO3-CH3COOH-HF Diluted HF, HNO3, or mixture of both HCl or mixture of HCl-H2O2 H2SO4-H2O2 De-ionized water (DIW)

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

391

Fig. 14. Minority carrier lifetime measurements of alkali textured Si with AR coating [15].

Fig. 13. Internal quantum efficiency and reflectance of Si solar cells with random pyramids texture and pyramids/ nanotexture [32].

with random pyramidal texture via alkali texturization, its surface area is around 1.73 times greater than that of planar Si [83,84]. The size varies for other textures depending on the size and density. After texturization, surface defect density increases due to greater exposure of (111) than (100) planes. ii. Unsaturated dangling bond. The continuity of Si crystal lattice is broken on textured surface, thereby exposing the unsaturated dangling bond. Exposed bonds can be terminated using the PECVD method by depositing the passivation layer such as a-Si: H on top of the textured surface. The influence of textured surface morphology on the performance of plasma passivation has been simulated previously. During the actual process, hydrogen ions are distributed on Si surface to terminate the dangling bond. This was modeled by considering the electric field of the textured Si microstructure [85]. Flat surfaces indicate a higher degree of uniformity in the hydrogen ions coverage when compared to that of the textured surface. Fortunately, this did not severely affect the final surface passivation output as the dangling bond saturation is only slightly lower for textured surface, and can be ignored with further optimization in plasma process condition. AR coating also serves similar purposes, for example the commonly adapted SiNx or SiO2/SiNx double layers [86], or also a-Si:H/SiNx:H double layers [87]. However, high feature peaks made the passivation layer undesirably higher at the valley [17]. iii. Doping concentration. The capacitance of the space charge region in the p-n junction of a solar cell is a function of the bias voltage, built-in potential and dopant concentration. Then, the area enhancement factor needs to be considered to extract the base-dopant concentration from the capacitance-voltage plots of the textured Si solar cells [88,89]. In addition, the textured surface also leads to uneven p-n junctions [90]. iv. Ion contamination. Texture etching or any post-treatment of Si wafer using KOH/NaOH solution without appropriate cleansing with acid solution will leave K þ and Na þ residues on the surface [21]. As shown in Fig. 14, lower lifetime indicates that the front surface recombination has increased. Edwards et al. [91] obtained the desired reduction in reflectance after texturing c-Si using NaOH. However, the lifetime was as low as 162 μs after depositing the a-Si layer. They confirmed that it is unfavorable due to the remaining Na after the a-Si layer is deposited, hence resulting in lower lifetime. Textured Si before a-Si deposition was polished completely with a mixture of

HNO3-CH3COOH-HF, which improved its lifetime to as high as 1074 ms. However, prolonging the treatment cost results in higher final reflectance. Optimal treatment duration of 5 s was proposed for a balanced tradeoff between optimum lifetime (899 ms) and reflectance. On top of cleaning the ion residue, acid also removes the oxides. The improvement in surface passivation by depositing a-Si buffer layer before the doped layer is distinguishable and stacks its effect with acid post-treatment [92]. This then contributes to a superior electrical performance which increased notably in both Voc and fill factor. Besides the recombination problem, the presence of surface texture also increases the series resistance of completed fabricated cell, mostly due to poor metal contact. A textured surface that is too rough or exceeds the optimal size will cause poorer Si-metal contact. Although the reflectivity is further reduced, it results in unfavorably higher resistance [71]. A study showed that an RIEtextured mc-Si wafer with optimized parameters had lower average reflectance when compared to that of acid-textured solar cell [90]. However in terms of cell performance, the proposed method used in the experiment resulted in slightly lower efficiency poorer fill factor than those obtained with the acid-textured cell as indicated in Fig. 15(a). The investigation in Ref. [90] also revealed that an n-rich layer remained at the bottom valley of the nanostructures with precipitated Ag particles on it, as shown in Fig. 15 (b) and (c). Pores were also present due to the reaction between AgO2 and SiNx. Furthermore, the remaining n-rich layer between Ag-Si with high contact resistance contributed to poor current transmission, while the finely distributed Ag crystallites resulted in low contact resistance because of current transportation near metal-semiconductor contact. Uneven front finger contact by screen-printing and damaged layer on the top/bottom of PECVD-induced texture also occurs in less than optimal textured morphology [13]. As a precaution, the feature size of the texture should not exceed 10 mm. Else, it will interfere with the sequenced cell-processing step [56]. Less sharp pyramidal texture leads to uniform metal coverage on the front grid lines after screen-printing and firing to provide the desired cell efficiency and fill factor [53]. These negative side effects ‘neutralize’ the contribution of reduced optical loss, and negating the very purpose of texturization while increasing the costs of the fabrication stage. Junction breakdown is another problem in chemically textured mc-Si. The breakdown voltage of certain mc-Si solar cell is influenced by two main factors; Si surface morphology and recombination activity. Defect-induced breakdown of mc-Si is because of the presence of etching pit following selective etching of grain boundaries and dislocation [93]. In the area of high

392

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

Fig. 15. (a) I-V characteristics of RIE-textured and acid-textured solar cell. Cross-sectional SEM images of Ag-Si contact in the solar cells: (b) Optimized RIE and (c) acid textured Si [90].

recombination, a lower reverse breakdown voltage is set for deeper etching pit. Fig. 16 provides a comparison between the different levels of texture; damage etches, weak texture and strong texture with the approximate local breakdown voltage for nine different solar cells is plotted against the local etching pit depth. Another type of breakdown in mc-Si related to texturization is the avalanche breakdown, which is another problem after the previously discussed defect-induced breakdown. This problem is more pronounced in alkali textured mc-Si when compared to acid and is mostly because of the isotropic acid-etching. While, selective alkali etches results in a slightly different curvature shape [94].

5. Texture compatibility with other optical enhancement The effect of texturization and AR coating stack improve the current generation and the fill factor of completed fabricated cell, as reported by many groups, regardless of the type of texture [20,30,32,95,96] by using any suitable layer [17,86,87]. A common SiNx layer has a nearly ideal index for AR coating and excellent bulk Si compatibility was selected after the junction formation. A default value of more than 30% reflectance of flat non-textured Si was reduced by
20% in the visible wavelength range after it was fabricated into a solar cell with SiNx. In addition textured Si cell with similar AR coating had further reduction of
10% in visible range, as shown in Fig. 17 for industrial cell. The contribution is not

Fig. 16. Local breakdown voltage versus depth of the etch pits measured at the defect-induced breakdown regions of neighboring solar cells [93].

only in reducing the front surface reflectance as suggested by its name, but also in playing a crucial role in passivating the textured surface to improve carrier lifetime as mentioned previously. Glass encapsulation confines escaped light from the cell and protect the cell from its surroundings. As much as 50% reduction in

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

final reflectance was obtained for encapsulated textured Si when compared to its flat counterpart [18,34]. Low iron float glass is preferred for higher transmittance [97], whereby around 3% of incoming light is absorbed by the glass, while 70% is absorbed by the cell [98]. The rear surface reflector prevents light from being transmitted from the cell. It works well with the front surface texture for weakly-absorbed light trapping. Green [99] presented the Lambertian light trap for weak and strong absorbed lights based on the thickness of the textured Si. The rear surface can either be textured or smooth. Textured rear surface randomizes the reflectance of the rear light to increase the path with its random angle reflectance, while simultaneously increasing the rear surface recombination [100] and parasitic light absorbance by the rear metal electrode. Front metal contact by screen printing which results in higher series resistance can alternatively replace its application with buried contact [101], as the reduction of contact shading and superior blue response do not interfere with the surface texture because of the deep narrow groove cut into the Si for metal plating [19]. Pairing suitable AR coating, metal back reflector and front surface texture can enable the need for thinner Si wafer, which in another perspective preferable in terms of material saving [102].

6. Thin film Si solar cell The light trapping mechanism for thin film a-Si is similar to that of conventional bulk Si cell, except the fact that a direct bandgap a-Si requires less material (100 nm thick is adequate) to absorb fractions of sunlight when compared to an indirect bandgap of bulk c-Si [103,104]. The texture of a-Si is transferred on textured substrate according to the cell configuration based on the deposited layers (p-doped, intrinsic, n-doped): i. p-i-n cell. The substrate for Si deposition is glass with textured transparent conducting oxide (TCO). A typical cell layer consists of glass/TCO/p-i-n/metal. ii. n-i-p cell. The substrate for Si deposition is glass with a textured metal as its back electrode in a complete fabricated cell. A typical cell layer consists of metal-grid/TCO/p-i-n/metal/glass. Both cell configurations is shown in Fig. 18.

Fig. 17. Typical external quantum efficiency and reflectance of c-Si cells with or without alkali texturization with SiNx [58].

393

6.1. Texturization of transparent conducting oxide Before proceeding to TCO texturization, few factors need to be considered for material selection. According to literatures [105,106], these criteria will produce highly efficient thin film cells: i. Low sheet resistance. The sheet resistance should be less than 10 Ω/sq. ii. Low light absorption. The light absorbance by TCO visible range should be less than 5%. iii. Light scattering ability. TCO prefers to have haze values exceeding 5%, or the TCO material needs to be textured for surface modification to increase its light scattering. The optical transparency of TCO is closely related to its electrical conductivity. Thinner TCO results in lower light absorption, but with tradeoff in electrical conductivity, which is indicated by increased sheet resistance. Textured TCOs are available in the market. One is being sold under the name of Asahi-U, which is actually tin dioxide (SnO2) with pre-textured microstructures on the surface. Alternatively, aluminum-doped zinc oxide (ZnO:Al or AZO) is commonly opted for laboratory demonstration due to feasibility in preparation, controllable texturization to obtain the desired microstructures, and also eliminates the limitation of oxidization by exposure to radical hydrogen as faced in SnO2. Common methods to prepare AZO which have been proven to be effective are: 6.1.1. Sol-gel method The sol-gel method is a simple preparation of AZO, followed by spin-coating on the glass substrate and annealing in the furnace [107]. Using this method, the preferred (002) peak of hexagonal ZnO can be obtained, both before and after rapid thermal annealing. Higher temperature annealing not only decreases the resistivity of AZO, but also increases carrier concentration and mobility. If spin coating is conducted on the textured glass surface, it will result in rough-surfaced AZO with the potential to enhance light trapping and photocurrent generations. 6.1.2. Magnetron sputtering with acid etching By using either DC or RF sputtering modes, desirable AZO texture can be obtained by controlling the glass substrate temperature and target alumina concentration (TAC) prior to etching in diluted acid [108,109]. The haze value increases with acid etching duration, where surface morphology changes from smooth to the one resembling lunar landscapes [110]. According to Bergenski et al. [108], there are three types of surface conditions for deposited AZO post wet etching in diluted HF. Type II is said to be the most desirable when compared to I and III due to the uniform coverage of crater-like structure with diameters ranging from 1 to 3 mm and 150–400 nm deep. The recipe to obtain type II AZO surface by magnetron sputtering and acid etching is presented in the schematic distribution shown in Fig. 19. A study also reported a two-step etching process using diluted HCl and using HF to further optimize surface morphology. The effort lead to stronger light scattering capabilities, which is indicated by more than 40% haze value, while maintaining light transmittance [111]. Using similar method for meter-scale industrial production of AZO is beneficial if it lowers the overall manufacturing cost [112]. 6.1.3. Magnetron sputtering with inductive coupled plasma etching Depositing AZO via magnetron sputtering is not limited to acid treatment. It is also doable via sophisticated processes, such as inductively-coupled plasma [113]. This method, which is a part of RIE, circumvents the problems associated with lateral etching and

394

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

Fig. 18. Configuration of typical p-i-n and n-i-p thin film solar cell.

Fig. 19. Regions of three ZnO:Al surface types (post etch) which can be obtained by varying TAC and substrate temperature during magnetron sputtering [108].

temperature-dependent etching rate of acid solution. Surface roughness can be controlled via two important parameters; gas ratio and RF chuck power. Alternative way to deposit Ag as back reflector on the substrate to provide textures in n-i-p cell can be interesting as the roughness of Ag film can be controlled by modifying its deposition temperature. The roughness of Ag film increases in nanoscale from near flat surface of 7 nm to 56.2 nm by elevating the deposition temperature from room temperature to 500 °C, which is actually effected from abrupt grain growth [114]. By increasing the surface roughness of Ag surfaces, the diffuse reflectance of back reflectors can be remarkably enhanced, especially in the visible and nearinfra-red regions without the loss of total reflectance. Furthermore, no further etching is required by the next deposited AZO, which is a good for process simplification. 6.2. Effects of front and rear surface texture Due to the deposited layers in the cell, the roughness of the deposited layer is influenced by the roughness of its predecessor. For the a-Si layer, the roughness of the final surface is a function of a-Si diode thickness. If the feature size of the substrate of the textured surface is comparable to the thickness of the deposited a-Si, then the morphology of the final Si surface will be distinctly

different from the substrate surface [115]. Similarly for AZO deposited on textured glass, the growth of thick AZO layer conforms to the glass substrate, whereby both have comparable surface roughness values [116]. For example, a 25% increase from an original 10 nm p-layer, and a 10% increase from a 300 nm i-layer have been shown in a typical experiment [117]. In terms of the feature size of textured front surface of p-i-n cell, features of TCO-Si interface seems to prefer smaller sizes. However, as learned from the simulation work that was performed in Ref. [118], the increase in height of Si intrinsic layer to improve light absorption is actually in a close relationship with the increase in AR effect by the textured surface. In thin film cell, the rear reflector plays an important role in confining light prior to absorption. The benefits of improved back surface reflector structure include increased reflectivity of back surface reflector, more effective front side light scattering, and more effective rear-side light scattering. Sai and Kondo [119] fabricated an n-i-p type hydrogenated micro-crystalline (mc-Si:H) using Al as the substrate, whereby Al was textured using the anodic oxidation process with the best micro-structural size of around 1 mm for 1 mm-thick mc-Si:H cell. More than 90% of visible light was reflected diffusely for microstructures larger than 0.9 mm due to increased optical path length in a thin layer mc-Si:H. A proper comparison between the effects of rear textured surface in p-i-n (rear texture not intentionally controlled) and n-i-p cell configurations were later established in Ref. [120]. Based on Fig. 20 (a) and (b), the rear surface texture in n-i-p cell resulted in more significant external quantum efficiency improvement in the near infra-red region compared to that of p-i-n cell. This was due to the small critical angle of the total internal reflection on the front side of n-i-p cell. However, rough rear surface in p-i-n cell due to Ag or Al thin film deposition leads to loss in absorption, which can be substantial in the near infra-red region. A cheap solution to this problem is to apply white mat paint. While more sophisticated option includes applying1-dimensional photonic crystal that is designed for high omni-directional reflectivity in wide spectral range [106]. Similar problems are found in n-i-p but are less pronounced. This indicates that the rear texture surface is both beneficial and detrimental. It has been proven to be effective in increasing optical light, however this also resulted in increased absorption loss due to reduced reflectivity (at the rear surface) in both cell configurations [121]. The introduction of an intermediate reflector layer that is in tandem with Si cell in different situation helps to redistribute the mid-range solar spectrum between the top and bottom cells when sandwiched together [122]. The concept of obtaining the texture of the

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

395

Fig. 20. External quantum efficiency and absorbed spectra with/without rear surface texture of: (a) p-i-n cell and (b) n-i-p solar cells [120].

intermediate reflector layer is still similar, which is by depositing on the textured TCO. However, further consideration in its thicknesses is required, as it has been reported that the current yield of fabricated solar cell is reduced as it is thickened. In mc-Si thin film cell, the texture is also related to the Si crystal growth during thermal deposition. Larger Si crystals are grown on rough TCO surfaces as opposed to the smooth surfaces, where it is deposited by electron beam [123]. In other situations,

deterioration in mc-Si (220) preferential growth was observed for higher substrate roughness, which negatively increased carrier recombination rates in solar cell [124,125].

7. Lesson learned and conclusion Table 6 presents several performance parameters for complete

Table 6 Effects of texturization methods on Si cell performance. Ref.

Method

Texture

Reflectivity

Jsc (mA/cm2) Voc (mV) FF (%) η (%)

Remark

[40] [41] [20]

Mechanical

mc-Si V-groove c-Si V-groove mc-Si random cone/ peaks mc-Si random cones/ hillocks mc-Si random hillocks

n/a n/a
5% at λ0.35–0.8 mm

32.70 38.10 34.00

602.0 651.0 600.0

77.10 77.00 73.00

15.20 19.10 15.10

– Textured front and rear surfaces –

5–10% at λ0.45–0.95 mm (with AR coat)
10% at λ0.5–1.0 mm (with AR coat) 0.7% at λ0.3–1.2 mm (with AR coat)
20% at λ0.4–0.9 mm

34.10

607.0

76.80

15.90

–

34.00

614.0

76.44

39.30 29.60

628.0 550.0

75.80 77.80

16.00 Acid etching performs better; poor contact 18.70 n-type Si PERL cell 12.70 20 mm feature diameter (large)


37.00 17.8% at λ0.3–0.9 mm
4% at λ0.5–1.0 mm (with AR coat) 40.30
20% at λ0.5–1.0 mm 37.49


580.0 662.0 639.0

n/a 74.20 77.60

RIE

[11] [90] [130] [38] Gas etching

c-Si random hillocks c-Si honeycomb or crater-like c-Si random texture c-Si smooth crater mc-Si honeycomb

[63] Acid etching [132] [13] Alkali etching [58] [69] [16]

mc-Si oval pits mc-Si oval pits c-Si random pyramids c-Si random pyramids mc-Si blunt hillocks c-Si random pyramids

15% in visible range 21.41% at λ0.3–0.9 mm
10% at λ0.5–1.0 mm
15% at λ0.4–1.0 mm n/a 5.3% at λ0.3–1.1 mm (with AR coat)

31.74 17.34 35.80 35.00 32.26 36.40

601.4 575.0 629.0 614.0 605.0 524.7

74.00 66.73 79.90 77.50 76.40 67.40

[95] [12] [91]

c-Si random pyramids c-Si random pyramids c-Si random pyramids


3% at λ0.4–1.0 mm (with AR coat) 36.80 33.50
13% at λ0.5–1.0 mm 35.00

622.4 592.0 680.7

79.10 61.80 74.05

c-Si porous pyramids c-Si pyramid with nanotexture c-Si pyramids with Ag particles c-Si pyramid with Ag particles Si nanowires ZrO2 pyramids


7% at λ0.3–1.0 mm 30.40 o 3% at λ0.3–0.9 mm (with AR coat) 35.50

587.4 623.0

48.00 79.30

– Passivated emitter rear contact Optimized screen printing and firing 14.12 – 13.19 – 18.00 – 16.60 – 14.96 – 12.90 No cleaning/acid polishing after texturization 18.10 12.20 Spherical solar cell 17.64 HIT cell with proper cleaning after etching 8.90 Low quality metallic contact 17.50 –


10% at λ0.3–1.0 mm (with AR coat) 5.62% at λ0.3–1.1 mm

25.30

585.0

71.90

10.70

25.62

548.0

45.65

n/a n/a

13.60 40.40

570.0 682.0

71.90 83.00


10% at λ0.5–1.0 mm (with AR coat)
5% at λ0.5–1.0 mm

37.30

618.0

78.10

Ag (front grid) dissolves in stain etch 6.420 Low performance overall instead of low reflectance 5.60 Si/organic hybrid solar cell 22.90 Deposited on c-Si, short λ absorption by ZrO2 18.01 Deposited on c-Si

42.90

696.0

81.00

24.20 PERL solar cell

[43] [45] [131]

[73] [32]

Laser Laser þ acid etching

Alkali þ stain etching Alkali þ electroless oxidation

[74] [75] [77] [79] [80] [126]

Alkali etching þ Ag particles Electroless etching ZrO2 deposition Silica particle deposition

Spherical texture Regular inverted pyramids

n/a 19.80 18.60

396

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

Fig. 21. (a) Buried contact cell as an alternative for screen-printed contact, with texture. (b) Highest homojunction bulk Si cell efficiency, PERL design with inverted pyramids texture [101].

fabricated cell using textured Si. Textured front surface is directly related to reduced reflectance and increased Jsc. However, Voc should not improve significantly; rather it might be negatively affected by texturization which leads to additional resistance in fabricated solar cell. Nevertheless, an accurate comparison for the contribution of optical loss reduction through texturization is not feasible as the performance improvement results from different aspects of the solar cell design. The c-Si cell has reported around 24% increase in efficiency using the design of passivated emitter and localized (PERL) by Wang et al. [126–129]. Although 26 years has passed, cell efficiency still ranged between 17–18% for current industrial usage, including screen printed and buried contact cell with textured front surface as shown in Fig. 21(a). Those PERL design may only be applicable at lab scale, as the cell actually requires multiple photolithography steps and is not easily applied for low-cost industrial production. One of the characteristic of PERL cell design is the inverted pyramids texture on the front surface, as shown in Fig. 21(b). Although the method of production was not disclosed in detail, it would be logical to assume that it would need an appropriate masking prior to alkali etching of c-Si, which relates with more complex processes and a higher production cost. Due to the current advances in R&D, PERL cells seem to adapt itself to random conical/columnar texture by RIE which is straightforward as discussed earlier, especially for implementation on n-type Si. Low reflection textured wafer is commonly referred to ‘black Si’, due to the very low light escaping from the surface. It could be concluded that the conventional bulk Si cell, which is aimed for high powered conversion efficiency, should be opted for proven lower cost alternative and reduced optical losses. Examples include alkali texturization for c-Si, or acid texturization for mc-Si, or RIE. This omits the need for additional oxide masking. In addition, an acceptably low reflectance under 5% in ultra violet to near infra-red range can only be achieved after depositing ARC layer and correct preventive action, such as sufficient cleaning to avoid all detrimental side effects. While for thin film Si cell, texturization is actually more relevant for depositing substrate. Nevertheless, the role of the final texture on Si is also still relevant and closely related with the light entrapment concept.

References [1] Photovoltaics Report. Fraunhofer ISE; 2015. [2] Saga T. Advances in crystalline silicon solar cell technology for industrial mass production. NPG Asia Mater 2010;2(3):96–102. [3] Green MA, Keevers MJ. Optical properties of intrinsic silicon at 300 K. Prog Photo: Res Appl 1995;3:189–92. [4] Green MA. Self-consistent optical parameters of intrinsic silicon at 300 K including temperature coefficients. Sol Energ Mat Sol Cells 2008;92:1305–10.

[5] Llopis F, Tobias I. Influence of texture feature size on the optical performance of silicon solar cells. Prog Photo: Res Appl 2005;13:27–36. [6] Sai H, Kanamori Y, Arafune K, Ohshita Y, Yamaguchi M. Light trapping effect of submicron surface textures in crystalline Si solar cells. Prog Photo: Res Appl 2007;15:415–23. [7] Basore PA. Numerical modeling of textured silicon solar cells using PC-1D. IEEE Trans Electron Devices 1990;37(2):337–43. [8] Basore PA. Extended spectral analysis of internal quantum efficiency. In: IEEE Photovolt Spec Conf; 1993. p. 147–152. [9] Rand JA, Basore PA. Light-trapping silicon solar cells-experimental results and analysis. In: IEEE Photovolt Spec Conf; 1991. p. 192–197. [10] Saha H, Datta SK, Mukhopadhyay K, Banerjee S. Influence of surface texturization on the light trapping and spectral response of silicon solar cells. EEE Trans Electron Devices 1992;39(5):1100–7. [11] Fukui K, Inomata Y, Shirasawa K. Surface texturing using reactive ion etching for multicrystalline silicon solar cells. In: IEEE Photovolt Spec Conf; 1997. p. 47–50. [12] Hayashi S, Minemoto T, Takakura H, Hamakawa Y. Influence of texture feature size on spherical silicon solar cells. Rare Met 2006;25:115–20. [13] Nakamura K, Aoki M, Sumita I, Sato H, Kumagai Y, Kawata Y, Ohshita Y. Texturization control for fabrication of high efficiency mono crystalline Si solar cell. In: IEEE Photovolt Spec Conf; 2013. p. 2239–2242. [14] Park H, Lee JS, Lim HJ, Kim D, Kwon S, Yoon S. The effect of tertiary-butyl alcohol on the texturing of crystalline silicon solar cells. J Korean Phys Soc 2009;55(5):1767–71. [15] Papet P, Nichiporuk O, Kaminski A, Rozier Y, Kraiem J, Lelievre JF, Chaumartin A, Fave A, Lemiti M. Pyramidal texturing of silicon solar cell with TMAH chemical anisotropic etching. Sol Energ Mat Sol Cells 2006;90:2319– 28. [16] Park H, Kwon S, Lee JS, Lim HJ, Yoon S, Kim D. Improvement on surface texturing of single crystalline silicon for solar cells by saw-damage etching using an acidic solution. Sol Energ Mat Sol Cells 2009;93:1773–8. [17] Jeong DY, Kim CS, Song JY, Lee JC, Cho JS, Park SH, Wang JS, Yoon KH, Song J. Effect of texture morphology on the surface passivation and a-Si/c-Si heterojunction solar cells. In: IEEE Photovolt Spec Conf; 2009. p. 642–645. [18] Gee JM, Schubert WK, Tardy HL, Hund TD, Robinson G. The effect of encapsulation on the reflectance of photovoltaic modules using textured multicrystalline-silicon solar cells. In: IEEE Photovolt Spec Conf; 1994. p. 1555–1558(2). [19] Pirozzi L, Arabito G, Artuso F, Barbarossa V, Besi-Vetrella U, Loreti S, Mangiapane P, Salza E. Selective emitters in buried contact silicon solar cells: some low-cost solutions. Sol Energ Mat Sol Cells 2001;65:287–95. [20] Prasad B, Bhattacharya S, Saxena AK, Reddy SR, Bhogra RK. Performance enhancement of mc-Si solar cells due to synergetic effect of plasma texturization and SiNx: H AR coating. Sol Energ Mat Sol Cells 2010;94:1329–32. [21] Vazsonyi E, De Clercq K, Einhaus R, Van Kerschaver E, Said K, Poortmans J, Szlufcik J, Nijs J. Improved anisotropic etching process for industrial texturing of silicon solar cells. Sol Energ Mat Sol Cells 1999;57:179–88. [22] Marrero N, Gonzalez-Diaz B, Guerrero-Lemus R, Borchert D, HernandezRodriguez C. Optimization of sodium carbonate texturization on large-area crystalline silicon solar cells. Sol Energ Mat Sol Cells 2007;91:1943–7. [23] Barrio R, Gonzalez N, Carabe J, Gandia JJ. Texturization of silicon wafers with Na2CO3 and Na2CO3/NaHCO3 solutions for heterojunction solar-cell applications. Mater Sci Semicond Process 2013;16:1–9. [24] Kim H, Park S, Kang B, Kim S, Tark SJ, Kim D, Dahiwale SS. Effect of texturing process involving saw-damage etching on crystalline silicon solar cells. Appl Surf Sci 2013;284:133–7. [25] Zubel I, Kramkowska M. The effect of isopropyl alcohol on etching rate and roughness of (100) Si surface etched in KOH and TMAH solutions. Sens Actuators A-Phys 2001;93:138–47. [26] Volk AK, Glover W, Greulich J, Gutscher S, Wolke W, Zimmer M, Rentsch J, Reinecke H. Optical modelling of the front surface for honeycomb-textured silicon solar cells. In: IEEE Photovolt Spec Conf; 2014. p. 1260–1264.

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

[27] Vallejo B, Gonzalez-Manas M, Martinez-Lopez J, Caballero MA. On the texturization of monocrystalline silicon with sodium carbonate solutions. Sol Energy 2007;81:565–9. [28] Pakhuruddin MZ, Ibrahim K, Abdul Aziz A. Effects of different TMAH texturing conditions towards morphology and surface reflectivity of monocrystalline silicon for solar cells applications. Optoelectron Adv Mat 2011;5 (1):16–21. [29] Baker-Finch SC, McIntosh KR. One-dimensional photogeneration profiles in silicon solar cells with pyramidal texture. Prog Photo: Res Appl 2012;20:51– 61. [30] Fan Y, Han P, Liang P, Xing Y, Ye Z, Hu S. Differences in etching characteristics of TMAH and KOH on preparing inverted pyramids for silicon solar cells. Appl Surf Sci 2013;264:761–6. [31] Yang Y, Green MA, Ho-Baillie A, Kampwerth H, Pillai S, Mehrvarz H. Characterization of 2-D reflection pattern from textured front surfaces of silicon solar cells. Sol Energ Mat Sol Cells 2013;115:42–51. [32] Dimitrov DZ, Du CH. Crystalline silicon solar cells with micro/nano texture. Appl Surf Sci 2013;2013(266):1–4. [33] Manea E, Budianu E, Purica M, Cernica I, Babarada F. Technological process for a new silicon solar cell structure with honeycomb textured front surface. Sol Energ Mat Sol Cells 2006;90:2312–8. [34] Macdonald DH, Cuevas A, Kerr MJ, Samundsett C, Ruby D, Winderbaum S, Leo A. Texturing industrial multicrystalline silicon solar cells. Sol Energy 2004;76:277–83. [35] You JS, Kim D, Huh JY, Park HJ, Pak JJ, Kang CS. Experiments on anisotropic etching of Si in TMAH. Sol Energ Mat Sol Cells 2001;66:37–44. [36] Xiao J, Wang L, Li X, Pi X, Yang D. Reflectivity of porous-pyramids structured silicon surface. Appl Surf Sci 2010;257:472–5. [37] Kim J, Inns D, Fogel K, Sadana DK. Surface texturing of single-crystalline silicon solar cells using low density SiO2 films as an anisotropic etch mask. Sol Energ Mat Sol Cells 2010;94:2091–3. [38] Saito Y, Kosuge T. Honeycomb-textured structures on crystalline silicon surfaces for solar cells by spontaneous dry etching with chlorine trifluoride gas. Sol Energ Mat Sol Cells 2007;91:1800–4. [39] Fath P, Borst P, Zechner C, Bucher E, Willeke G, Nayaranan S. Progress in a novel high-throughput mechanical texturization technology for highly efficient multicrystalline silicon solar cells. Sol Energ Mat Sol Cells 1997;48:229– 36. [40] Gerhards C, Marckmann C, Tolle R, Spiegel M, Fath P, Willeke G, Bucher E. Mechanically V-textured low cost multicrystalline silicon solar cells with a novel printing metallization. In: IEEE Photovolt Spec Conf; 1997. p. 43–46. [41] Terheiden B, Fath P. Highly efficient double side mechanically textured novel silicon solar cell concepts. In: World Conference Photovoltaic Energy Conversion; 2003. p. 1443–1446(2). [42] Zaidi SH, Ruby DS, DeZetter K, Gee JM. Enhanced near IR absorption in random, RIE-textured silicon solar cells: The role of surface profiles. In: IEEE Photovolt Spec Conf; 2002. p. 142–145. [43] Kohata H, Saito Y. Masklesstexturization of phosphorus-diffused layers for crystalline Si solar cells by plasmaless dry etching with chlorine trifluoride gas. Sol Energ Mat Sol Cells 2010;94:2124–8. [44] Dobrzanski LA, Drygala A. Surface texturing of multicrystalline silicon solar cells. J Achiev Mater Manuf Eng 2008;31(1):77–82. [45] Zielke D, Sylla D, Neubert T, Brendel R, Schmidt J. Direct laser texturing for high-efficiency silicon solar cells. IEEE J Photo 2012;3(2):656–61. [46] Hegedus S, Das U, Vineis C, Levy-Finklshtein M, Sickler J, Carey J. Laser textured heterojunction solar cells on 45 mm thick Si wafers: Effect of optical configuration and light trapping. In: IEEE Photovolt Spec Conf; 2013. p. 1238– 1241. [47] Saidel H, Csepregi L, Heuberger A, Baumgaretel A. Anisotropic etching of crystalline silicon in alkaline solutions. J Electrochem Soc 1990;137 (11):3612–26. [48] Zubel I, Rola K, Kramkowska M. The effect of isopropyl alcohol concentration on the etching process of Si-substrates in KOH solutions. Sens Actuator A-Phys 2011;171:436–45. [49] Singh PK, Kumar R, Lal M, Singh SN, Das BK. Effectiveness of anisotropic etching of silicon in aqueous alkaline solutions. Sol Energ Mat Sol Cells 2001;70:103–13. [50] Zubel I, Kramkowska M. The effect of alcohol additives on etching characteristics in KOH solutions. Sens Actuators A-Phys 2002;101:255–61. [51] Zubel I, Kramkowska M. Etch rates and morphology of silicon (h k l ) surfaces etched in KOH and KOH saturated with isopropanol solutions. Sens Actuator A-Phys 2004;115:549–56. [52] Zubel I, Granek F, Rola K, Banaszczyka K. Texturization of Si (1 0 0) substrates using tensioactive compounds. Appl Surf Sci 2012;258:9067–72. [53] Gangopadhyay U, Kim K, Kandol A, Yi J, Saha H. Role of hydrazine monohydrate during texturization of large-area crystalline silicon solar cell fabrication. Sol Energ Mat Sol Cells 2006;90:3094–101. [54] Chu AK, Wang JS, Tsai ZY, Lee CK. A simple and cost-effective approach for fabricating pyramids on crystalline silicon wafers. Sol Energ Mat Sol Cells 2009;93:1276–80. [55] Ou W, Zhang Y, Li H, Zhao L, Zhou C, Diao H, Liu M, Lu W, Zhang J, Wang W. Effects of IPA on texturing process for mono-crystalline silicon solar cell in TMAH solution. Mater Sci Forum 2011;685:31–7. [56] Iencinella D, Centurioni E, Rizzoli R, Zignani F. An optimized texturing process for silicon solar cell substrates using TMAH. Sol Energ Mat Sol Cells 2004;87:725–32.

397

[57] Nishimoto Y, Namba K. Investigation of texturization for crystalline silicon solar cells with sodium carbonate solutions. Sol Energ Mat Sol Cells 2000;61:393–402. [58] Sparber W, Schultz O, Biro D, Emanuel G, Preu R, Poddey A, Borchert D. Comparison of texturing methods for monocrystalline silicon solar cells using KOH and Na2CO3. In: World Conference on Photovoltaic Energy Conversion; 2003. p. 1372–1375. [59] Xi Z, Yang D, Que D. Texturization of monocrystalline silicon with tribasicsodium phosphate. Sol Energ Mat Sol Cells 2003;77:255–63. [60] Xi Z, Yang D, Dan W, Jun C, Li X, Que D. Investigation of texturization for monocrystalline silicon solar cells with different kinds of alkaline. Renew Energ 2004;29:2101–7. [61] Li H, Liu W, Liu A, Qiao F, Hu Z, Liu Y. Metal grids-based texturization of monocrystalline silicon wafers for solar cells. Sol Energ Mat Sol Cells 2010;94:942–5. [62] Huang BR, Yang YK, Yang WL. Key technique for texturing a uniform pyramid structure with a layer of silicon nitride on monocrystalline silicon wafer. Appl Surf Sci 2013;266:245–9. [63] Gangopadhyay U, Dhungel SK, Basu PK, Dutta SK, Saha H, Yi J. Comparative study of different approaches of multicrystalline silicon texturing for solar cell fabrication. Sol Energ Mat Sol Cells 2007;91:285–9. [64] Manea E, Budianu E, Purica M, Cristea D, Cernica I, Muller R, Moagar Poladian V. Optimization of front surface texturing processes for high-efficiency silicon solar cell. Sol. Energ Mat Sol Cells 2005;87:423–31. [65] Manea E, Budianu E, Purica M, Podaru C, Popescu A, Parvulescu C, Dinescu A, Coraci A, Cernica I, Babarada F. Front surface texturing processes for silicon solar cells. In: INTERNATIONAL Semiconductor Conference; 2007. p. 191–194. [66] Manea E, Budianu E, Purica M, Podaru C, Cernica I, Coraci A, Babarada F. Silicon solar cells technology using honeycomb textured front surface. In: International Semiconductor Conference; 2005. p. 157–160. [67] Vazsonyi E, Ducso C, Pekker A. Characterization of the anisotropic etching of silicon in two-component alkaline solution. J Micromech Microeng 2007;17:1916–22. [68] Basu PK, Dhasmana H, Varandani D, Mehta BR, Thakur DK. A cost-effective alkaline multicrystalline silicon surface polishing solution with improved smoothness. Sol Energ Mat Sol Cells 2009;93:1743–8. [69] Basu PK, Dhasmana H, Udayakumar N, Thakur DK. A new energy efficient, environment friendly and high productive texturization process of industrial multicrystalline silicon solar cells. Renew Energ 2009;34:2571–6. [70] Stefancich M, Butturi M, Vincenzi D, Martinelli G. Mechanical effects of chemical etchings on monocrystalline silicon for photovoltaic use. Sol Energ Mat Sol Cells 2001;69:371–7. [71] Basu PK, Pujahari RM, Kaur H, Singh D, Varandani D, Mehta BR. Impact of surface roughness on the electrical parameters of industrial high efficiency NaOH–NaOCl textured multicrystalline silicon solar cell. Sol Energy 2010;84:1658–65. [72] Gangopadhyay U, Dhungel SK, Mondal AK, Saha H, Yi J. Novel low-cost approach for removal of surface contamination before texturization of commercial monocrystalline silicon solar cells. Sol Energ Mat Sol Cells 2007;91:1147–51. [73] Marrero N, Guerrero-Lemus R, Gonzalez-Diaz B, Borchert D. Effect of porous silicon stain etched on large area alkaline textured crystalline silicon solar cells. Thin Solid Films 2009;517:2648–50. [74] Chaoui R, Mahmoudia B, Ahmeda YS. Improvement of screen-printed textured monocrystalline silicon solar cell performance by metal-assisted chemical etching. Energy Procedia 2013;36:253–9. [75] Sardana SK, Chava VSN, Thouti E, Chander N, Kumar S, Reddy SR, Komarala VK. Influence of surface plasmon resonances of silver nanoparticles on optical and electrical properties of textured silicon solar cell. Appl Phys Lett 2014;104(73903):1–5. [76] Pudasaini PR, Ayon AA. Modeling the front side plasmonics effect in nanotextured silicon surface for thin film solar cells application. Micro Technol 2013;19(6):871–7. [77] He L, Jiang C, Wang H, Lai D, Tan YH, Tan CS. Rusli. Effects of nanowire texturing on the performance of Si/organic hybrid solar cells fabricated with a 2.2 m thin film Si absorber. Appl Phys Lett 2012;100(103104):1–4. [78] Dou B, Jia R, Li H, Chen C, Ding W, Meng Y, Liu X, Ye T. Rear surface protection and front surface bi-layer passivation for silicon nanostructure-textured solar cells. J Phys D: Appl Phys 2013;46:1–6. [79] Yoshinaga S, Ishikawa Y, Araki S, Uraoka Y. Light trapping effect of nanoimprinted-textured crystalline silicon solar cells. In: IEEE Photovoltaic Specialists Conference; 2013. p. 1310–1313. [80] Chen CH, Juan PC, Liao MH, Tsai JL, Hwang HL. The effect of surface treatment on omni-directional efficiency of the silicon solar cells with micro-spherical texture/ITO stacks. Sol Energ Mat Sol Cells 2011;95:2545–8. [81] Han HV, Chen HC, Tsai YL, Lin CC, Kuo HC, Yu P. Enhance current density and light trapping effect in a-Si thin film solar cells by flexible textured PDMS film. In: IEEE International Nanoelectronics Conference; 2013. p. 150–152. [82] Kwon S, Yi J, Yoon S, Lee JS, Kim D. Effects of textured morphology on the short circuit current of single crystalline silicon solar cells: evaluation of alkaline wet-texture processes. Curr Appl Phys 2009;9:1310–4. [83] McIntosh KR, Johnson LP. Recombination at textured silicon surfaces passivated with silicon dioxide. J Appl Phys 2009;105(124520):1–10. [84] Bachtouli N, Aouida S, HadjLaajimi R, Boujmil MF, Bessais B. Implications of alkaline solutions-induced etching on optical and minority carrier lifetime features of monocrystalline silicon. Appl Surf Sci 2012;258:8889–94.

398

M.F. Abdullah et al. / Renewable and Sustainable Energy Reviews 66 (2016) 380–398

[85] Mehrabian S, Xu S, Qaemi AA, Shokri B, Chan C. The effect of microscopic texture on the direct plasma surface passivation of Si solar cells. Phys Plasmas 2013;20(4):1–6. [86] Schmidt J, Kerr M, Cuevas A. Surface passivation of silicon solar cells using plasma-enhanced chemical-vapour-deposited SiN films and thin thermal SiO2/plasma SiN stacks. Semicond Sci Technol 2001;16(3):164–70. [87] Li H, Hallam B, Wenham SR. Comparative study of PECVD deposited a-Si:H/ SiNx:H double passivating layer on CZ crystalline Si substrate. In: IEEE Photovolt Spec Conf; 2011. p. 1481–1485. [88] Hinken D, Milsted A, Bock R, Fischer B, Bothe K, Schutze M, Isenberg J, Schulze A, Wagner M. Determination of the base-dopant concentration of large-area crystalline silicon solar cells. EEE Trans Electron Devices 2010;57 (11):2831–7. [89] Wang X, Moore J, Berdebes D, Lundstrom M. Effects of texturing on the CV analysis of silicon solar cells. Int Semicond Device Res Symp. 2011:1–2. [90] Zhong S, Liu B, Xia Y, Liu J, Liu J, Shen Z, Xu Z, Li C. Influence of the texturing structure on the properties of black silicon solar cell. Sol Energ Mat Sol Cells 2013;108:200–4. [91] Edwards M, Bowden S, Das U, Burrows M. Effect of texturing and surface preparation on lifetime and cell performance in heterojunction silicon solar cells. Sol Energ Mat Sol Cells 2008;92:1373–7. [92] Angermann H, Conrad E, Korte L, Rappich J, Schulze TF, Schmidt M. Passivation of textured substrates for a-Si: H/c-Si hetero-junction solar cells: effect of wet-chemical smoothing and intrinsic a-Si: H interlayer. Mater Sci Eng B 2009;159–160:219–23. [93] Kwapil W, Nievendick J, Zuschlag A, Gundel P, Schubert MC, Warta W. Influence of surface texture on the defect-induced breakdown behavior of multicrystalline silicon solar cells. Prog Photo: Res Appl 2012;21(4):534–43. [94] Bauer J, Lausch D, Blumtritt H, Zakharov N, Breitenstein O. A novel origin of avalanche breakdown in multicrystalline silicon solar cells. In: European Photovoltaic Solar Energy Conference and Exhibition; 2012. p. 857–860. [95] Basu PK, Sarangi D, Boreland MB. Single-component damage-etch process for improved texturization of monocrystalline silicon wafer solar cells. IEEE J. Photo. 2013;3(4):1222–8. [96] Salwa AGN, Cheow SL, Khairy MY, Shahrul AW, Zaharim A, Sopian, K. Effect of plasma enhanced chemical deposition on the textured c-Si solar cells properties. In: WSEAS International Conference on Renewable Energy Sources; 2009. p. 308–311. [97] Deubener J, Helsch G, Moiseev A, Bornhoft H. Glasses for solar energy conversion systems. J Eur Ceram Soc 2009;29:1203–10. [98] Santberge R, van Zolingen RJC. The absorption factor of crystalline silicon PV cells: A numerical and experimental study. Sol Energ Mat Sol Cells 2007;92:432–44. [99] Green MA. Lambertian light trapping in textured solar cells and light-emitting diodes: Analytical solutions. Prog Photo: Res Appl 2002;10:235–41. [100] Ghannam MY, Abouelsaood AA, Alomar AS, Poortmans J. Analysis of thinfilm silicon solar cells with plasma textured front surface and multi-layer porous silicon back reflector. Sol Energ Mat Sol Cells 2010;94:850–6. [101] Green MA, Zhao J, Wang A, Wenham SR. Progress and outlook for high-efficiency crystalline silicon solar cells. Sol Energ Mat Sol Cells 2001;65:9–16. [102] Lu X, Lun S, Zhou T, Zhang M. A low-cost high-efficiency crystalline silicon solar cell based on one-dimensional photonic crystal front surface textures. J Opt 2013;15:1–10. [103] Andreani LC, Bozzola A, Liscidini M. The importance of light trapping in thinfilm solar cells. SPIE Newsroom 2012:1–2. [104] Bozzola A, Liscidini M, Andreani LC. Photonic light-trapping versus Lambertian limits in thin film silicon solar cells with 1D and 2D periodic patterns. J Opt Soc Am 2012;20(2):A224–44. [105] Hegedus S, Buchanan W. Effect of textured tin oxide and zinc oxide substrates on the current generation in amorphous silicon solar cells. In: IEEE Photovoltaic Specialists Conference; 1996. p. 1129–1132. [106] Muller J, Rech B, Springer J, Vanecek M. TCO and light trapping in silicon thin film solar cells. Sol Energy 2004;77:917–30. [107] Sobajima Y, Kato S, Matsuura T, Toyama T, Okamoto H. Study of the lighttrapping effects of textured ZnO: Al/glass structure TCO for improving photocurrent of a-Si: H solar cells. J Mater Sci Mater Electron 2007;18(1):159–62. [108] Berginski M, Hupkes J, Schulte M, Schope G, Stiebig H, Rech B, Wuttig M. The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells. J Appl Phys 2007;101 (074903):1–11. [109] Kang DW, Kuk SH, Ji KS, Ah SH, Han MK. The effect of substrate temperature on optoelectronic characteristics of surface-textured ZnO:Al films for micromorph silicon tandem solar cells. Phys Status Solidi C 2010;7(3–4):925–8.

[110] Wang NF, Tsai YZ, Hsu FH. Effect of surface texture on Al–Y codopedZnO/n-Si heterojunction solar cells. EEE Trans Electron Devices 2013;60(12):4073–8. [111] Yan X, Venkataraj S, Aberle AG. Wet-chemical surface texturing of sputterdeposited ZnO:Al films as front electrode for thin-film silicon solar cells. Int J Photo 2015:1–10. [112] Bu D, Lin Q, Fan J, Liu J, Haug FJ, Bailat J, Lofgren L, Boccard M, Ballif C, Wang Y. A scalable and inexpensive surface-texturization method for advanced transparent front electrodes in microcrystalline and micromorph thin film silicon solar cells. Phys Status Solidi A 2015;212(9):1916–24. [113] Jeong C, Boo S, Kim SH, Chang DR. Investigation on the texture effect of RF magnetron-sputtered ZnO:Al thin films etched by using an ICP etching method for heterojunction Si solar cell applications. J Korean Phys Soc 2008;53(1):431–6. [114] Cho JS, Baek S, Park SH, Park JH, Yoo J, Yoon KH. Effect of nanotextured back reflectors on light trapping in flexible silicon thin-film solar cells. Sol Energ Mat Sol Cells 2012;102:50–7. [115] Jovanov V, Xu X, Shrestha S, Schulte M, Hupkes J, Zeman M, Knipp D. Influence of interface morphologies on amorphous silicon thin film solar cells prepared on randomly textured substrates. Sol Energ Mat Sol Cells 2013;112:182–9. [116] Zhang W, Paetzold UW, Meier M, Gordijn A, Hupkes J, Merdzhanova T. Thinfilm silicon solar cells on dry etched textured glass. Energy Procedia 2014;44:151–9. [117] Jovanov V, Shrestha S, Hupkes J, Ermes M, Bittkau K, Knipp D. Influence of film formation on light trapping properties of randomly textured silicon thin film solar cells. Appl Phys Express 2014;7(8):1–14. [118] Ermes M, Bittkau K, Carius R. Influence of texture modifications in silicon solar cells on absorption in the intrinsic layers. In: Proceedings of the SPIE; 2012. p. 8438(0I):1–10. [119] Sai H, Kondo M. Effect of self-orderly textured back reflectors on light trapping in thin-film microcrystalline silicon solar cells. J Appl Phys 2009;105 (094511):1–7. [120] Sai H, Kondo M. Impact of front and rear-side texturing on light trapping in thin-film silicon solar cells. In: IEEE Photovolt Spec Conf 2010, p. 295–300. [121] Sai H, Jia H, Kondo M. Impact of front and rear texture of thin-film microcrystalline silicon solar cells on their light trapping properties. J Appl Phys 2010;108(044505):1–9. [122] Campa A, Meier M, Boccard M, Mercaldo LV, Ghosh M, Zhang C, Merdzhanova T, Krc J, Haug FJ, Topic M. Micromorph silicon solar cell optical performance: influence of intermediate reflector and front electrode surface texture. Sol Energ Mat Sol Cells 2014;130:401–9. [123] Becker C, Sontheimer T, Steffens S, Scherf S, Rech B. Polycrystalline silicon thin films by high-rate electron beam evaporation for photovoltaic applications-Influence of substrate texture and temperature. Energy Procedia 2011;10:61–5. [124] Matsui T, Tsukiji M, Saika H, Toyama T, Okamoto H. Influence of substrate texture on microstructure and photovoltaic performances of thin film polycrystalline silicon solar cells. J Non-Cryst Solids 2002;299–302:1152–6. [125] Muhida R, Toyama T, Okamoto H. Influence of surface morphology of textured substrate against poly-Si thin film solar cells performance. Mater Sci Forum 2013;737:105–9. [126] Wang A, Zhao J, Green MA. 24% efficient silicon solar cells. Appl Phys Lett 1990;67(6):602–4. [127] Zhao J, Wang A, Green MA. 19.8% efficient “honeycomb” textured multicrystalline and 24.4% monocrystalline silicon solar cells. Appl Phys Lett 1998;73(14):1991–3. [128] Zhao J, Wang A, Campbell P, Green MA. 22.7% efficient perl silicon solar cell module with a textured front surface. In: IEEE Photovoltaic Specialists Conference; 1997. p. 1133–1136. [129] Green MA. The path to 25% silicon solar cell efficiency: History of silicon cell evolution. Prog Photo: Res Appl 2009;17:183–9. [130] Repo P, Benick J, Vahanissi V, Schon J, von Gastrow G, Steinhauser B, Schubert MC, Hermle M, Savin H. N-type black silicon solar cells. Energy Procedia 2013;38:866–71. [131] Morikawa H, Niinobe D, Nishimura K, Matsuno S, Arimoto S. Processes for over 18.5% high-efficiency multi-crystalline silicon solar cell. Curr Appl Phys 2010;10:S210–4. [132] Cheng YT, Ho JJ, Tsai SY, Ye ZZ, Lee W, Hwang DS, Chang SH, Chang CC, Wang KL. Efficiency improved by acid texturization for multi-crystalline silicon solar cells. Sol Energy 2011;85:87–94. [133] Ruby DS, Zaidi SH, Narayanan S, Daminani BM, Rohatgi A. Rie-texturing of multicrystalline silicon solar cells. Sol Energ Mat Sol Cells 2002;74:133–7.