Performance enhancement of pc-Si solar cells through combination of anti-reflection and light-trapping: Functions of AAO nano-grating

Optics Communications 385 (2017) 205–212

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Performance enhancement of pc-Si solar cells through combination of antireflection and light-trapping: Functions of AAO nano-grating ⁎

Lei Wua, Haiming Zhanga, , Feifei Qinb, Xiaogang Baia, Ziye Jia, Dan Huanga a b

School of Science, Tianjin Polytechnic University, Tianjin 300387, China State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, China

A R T I C L E I N F O

A BS T RAC T

Keywords: Anti-reflection Light-trapping Polycrystalline silicon solar cell AAO nano-grating

Anodic aluminium oxide (AAO) nanogratings are experimentally applied to polycrystalline silicon (pc-Si) solar cells at front surface to improve the light coupling. On the basis of the Fresnel Reflection Principle, the primary reflection loss can be reduced by multi-layer dielectric film with varing refactive index. And this multi-layer film is regarded as anti-reflection coating. An efficient light-trapping structure is significant in absorption enhancement of long wavelength band (around 900–1100 nm) for silicon solar cells. In this paper, we put AAO nanogratings on the front side of pc-Si solar cells to serve as anti-reflecting coating and light-trapping structure. The operation leads to light absorption enhancement eventually. Thanks to AAO nano-grating's structure parameters, the anti-reflecting and light-trapping effects are changeable. This is discussed in three aspects: AAO lattice period, AAO thickness and its pore diameter. Optical interaction between AAO nanograting and Ag electrodes is also discussed. We find an increase of short-circuit current density (1.32 mA/cm2) with SiNx:H/AAO complex coating. The relative power conversion efficiency obtains a growth about 2.2% points. Additionally, AAO nanogratings may facilitate carrier separation. This improves the performance of pc-Si solar cells in electrical aspect.

1. Introduction Minimizing the primary reflection loss at the front surface of solar cells has always been a key challenge for improving the power conversion efficiency [1]. As semiconductor layers' higher refractive index than air, part of sunlight is lost at the air/semiconductor interface due to the Fresnel Reflection Principle [2,3]. Several antireflection methods have been reported to reduce the surface reflection loss, such as deposition of single or multiple layer anti-reflecting (AR) coatings [4,5]. The AR coatings are designed to form a gradually varying refractive index between air and semiconductor so that the surface reflection is lower. AR coatings with several functions are usually used in Si-based solar cells [6,7]. For example, hydrogenated silicon nitride (SiNx:H) coatings are applied in homo-junction crystal silicon cells to passivate the phosphorus-doped emitter surface. Particularly in polycrystalline silicon (pc-Si) solar cells, SiNx:H is the hydrogen source to passivate dangling-bond defects. Thus, SiNx:H are hardly to substitute with other materials. Multi-layer SiNx:H coatings can reduce the reflection loss as much as possible. However, disadvantages limit the application of multi-layer SiNx:H coatings, such as process instability, high cost, and high-temperature firing procedure,

⁎

etc [8,9]. Light-trapping nanostructures are also introduced into crystal or amorphous silicon solar cells in order to improve the light absorption, e.g. moth-eye structures [10], regular or random pyramids [11]. Light-trapping structures can increase the optical path length in the silicon layer so that more photons could be absorbed. A surface layer acts as both AR coating and light trapping structure is preferable in silicon solar cells currently. Furthermore, this surface layer should not exert negative influences on electrical properties [12,13]. We introduce AAO nanograting into traditional industrial pc-Si solar cells to minimize surface reflection, strengthen the light-trapping and facilitate carrier separation within pn-junction. Xingsheng has proved that the rear AAO nanograting offerring better light-trapping effect in thin film amorphous silicon cells [14]. The low price and easy processing made thin film amorphous silicon cells competitive. However, its thickness is not enough leading to insufficient light absorption. The main purpose of our group was to examine the AAO's light trapping effect experimentally, thus commercial pc-Si cell was selected because of its relative cheaper price than monocrystalline Si cell [15], and we hold the view that enhancing the low power conversion efficiency of pc-Si cell is more meaningful. The single layer SiNx:H coating is a necessity while taking the anti-reflection, the

Corresponding author. E-mail address: [email protected] (H. Zhang).

http://dx.doi.org/10.1016/j.optcom.2016.10.039 Received 27 April 2016; Received in revised form 21 September 2016; Accepted 18 October 2016 Available online 08 November 2016 0030-4018/ © 2016 Elsevier B.V. All rights reserved.

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material-dependency and cost-effectiveness into consideration within the cell manufacturing process. However, single layer SiNx:H coating cannot completely reduce the surface reflection loss. Besides, an additional reflection loss occurs inevitably at the air/SiNx:H interface [16]. Moreover, SiNx:H coating is not competent for reducing the loss resulted from sunlight illuminating on the surface metal electrodes (Ag grids) and escaping into air. To address these challenges, AAO nanogratings are transplanted on the surface of commercial pc-Si cells, which can minimize the surface reflection as an AR coating, prolong the light path length in the silicon layer as an light-trapping structure due to its scattering and diffraction effect and thus indirectly improve carrier separation. AAO nanograting, reported firstly by Japanese researcher Masuda, is a kind of photonic crystal due to its periodic array feature [17]. Several analytical investigations have been reported to demonstrate the feasibility of placing AAO nanograting on the top surface of SiNx:H/pc-Si solar cells [18,19]. However, to the best of our knowledge, AAO nanogratings are continually used as membranes in Si-based cell formations [20–22]. Experimental researches about AAO nano-grating as an AR coating are rarely reported. Our group attempt to apply AAO nanogratings as an AR coating and light-trapping structure on the front surface of pc-Si solar cells experimentally for the first time. In this framework, we mainly focus on two approaches, an AAO/ SiNx:H/pc-Si optical model and the preparation of the new cell, to test AAO nanograting's effectiveness on improving pc-Si solar cell's performance. We firstly examine the influence of the equivalent refractive index of AAO nanograting (nAAO) on the reflection loss by treating AAO nanograting as an incident medium film, utilizing the AAO/SiNx:H/pcSi optical model. The anti-reflection result changes with the varying nAAO and an optimum refractive index can be obtained. Then, we demonstrate that the modulation of nAAO can be achieved through tuning of AAO nanograting structure parameters (AAO lattice period, AAO thickness and its pore diameter). Afterwards, we transplant AAO nanograting with the determined preferable nAAO to SiNx:H/pc-Si commercial solar cells. We buy the pc-Si cell samples from manufacturer. The surface texturing process is already made. In other words, samples’ surface are not smooth whether AAO applied or not. Finally, we exhibit the combined construction giving rise to enhancement in short-circuit current density and power conversion efficiency of pc-Si solar cells. We attribute the performace enhancement of commercial pc-Si solar cell to the advanced anti-reflection, improved light-trapping and better carrier separation efficiency.

Fig. 1. An optical model of pc-Si substrate with SiNx:H coating (80 nm) and AAO nanograting (varying refractive indexes, simplified as a dielectric film).

semiconductor/AAO interface. In this numerical simulation, nAAO is determined by the weighted average of the refractive index of the two matter, air (n=1.0) and aluminium oxide (Al2O3, n=2.0). Thus nAAO can be tuned from 1.0 to 2.0 in the simulation and the weight proportion of air and Al2O3 depends on the practical AAO nanograting structure parameters (AAO lattice period, AAO thickness and its pore diameter). The preferable structure parameters in experimental step are determined according to the optimum nAAO. Furthermore, the situation nAAO=0, means that the Si substate only covered with optimized SiNx:H film, without AAO nanograting. We define the optical property of matter as a matrix [n,k], where n is the refractive index and k is the absorption coefficients at different light wavelengths. In this optical model, wavelength band ranges from 300 to 1100 nm. In FDTD solutions, optical properties of Al2O3 and air could be found directly in material database. The optical property of pc-Si is taken from Palik's optical handbook and imported into FDTD databse [24]. For simplicity, the refractive index of the SiNx:H is defined as a constant 2.0 in the simulation. Reflectivity spectra of this model are available through the frequency-domain field and power monitors in FDTD solutions. Fig. 2(a) exhibits the variation of the reflectivity spectra of the AAO/ SiNx:H/pc-Si stack model when nAAO changes from 1.0 to 2.0 with a step size 0.1. The minimum reflectivity occurring at around 650 nm is found to increase monotonically with the increasing nAAO. Reversely, the reflectivity decreases monotonically with the increasing nAAO at wavelength band less than 450 nm and larger than 900 nm. Since the reflectivity features from 500 to 800 nm in Fig. 2(a) are not distinct, Fig. 2(b) is a reflectivity calculated in logarithmic scale to clearly show specifics, as a supplementary diagram. The optimum nAAO cannot be determined merely according to the anti-reflection effect described above. Because nAAO shows a positive effect in shorter or longer wavelength band (300–500 nm and 900– 1000 nm), but an negative effect in the middle (500–900 nm). Nevertheless, from the perspective of practical application, minimizing reflection loss aims to generate electricity as much as possible over the whole spectra range (300–1100 nm). Therefore, we calculate the generation current density (JG). JG could be written as

2. Influence of the equivalent refractive index of AAO nanogratings 2.1. Analysis based on an optical model In this section, a simplified optical model is exploited to examine the influence of the equivalent refractive index of AAO nanograting (nAAO). Hitoshi Sai has done basic works to discuss the influence of the refractive index of moth-eye structure on Si solar cell [23]. We attempt to discuss the diverse equivalent refractive index of AAO nanogratings by treating AAO nanograting as a dielectric film. A model consists of flat pc-Si substrate coated with common SiNx:H thin film and simplified AAO film on the top, as shown in Fig. 1. At the bottom, one back reflector is added. The thickness of SiNx:H film (n=2) is set as 80 nm because of a minimum reflectivity at wavelength of 650 nm without AAO attached on the surface. The thickness of pc-Si layer is 180 µm referring to the active layer thickness of commercial pc-Si solar cells. One back reflector was set at the bottom to ensure light being sufficiently absorbed in the silicon layer. The material of back reflector is Al. The sunlight illuminates vertically and the optical effect happened at air/ AAO interface is ignored. Finite difference time domain (FDTD) solution is adopted to examine the reflection loss occurred at the

1100 nm

JG =

∫300 nm

eλ (1 − T (λ )−R(λ ))IAM1.5(λ ) dλ hc

(1)

where T(λ) and R(λ) are transmission spectrum and refletance spectrum [25]. In fomula (1), T(λ) and R(λ) are data obtained from the optical simulation above. IAM1.5(λ ) is the standard solar radiation spectrum. As shown in Fig. 3, the curve stands for JG induced by the absorption of the light illuminated into active pc-Si layer in the 206

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Fig. 2. Influence of nAAO on the reflectivity spectra of a flat AAO/SiNx:H/pc-Si stack model. (a) Reflectivity changes with variable nAAO (b) logarithmic reflectivity spectra in logarithmic form to make details of 550–800 nm in (a) more clear.

Fig. 4. Influence of nAAO on Jrefloss of AAO/SiNx:H/pc-Si stack model.

Fig. 3. Influence of nAAO on generation current desity (JG) of AAO/SiNx:H/pc-Si stack model.

2.2. Determination of AAO nanograting structure parameters through the optimized nAAO

situation. The maximum JG could be found when nAAO value is about 1.3 or 1.4. This result indicates that the antireflection reaching a preferable level when nAAO ranges from 1.3 to 1.4. Here, JG appears to be higher in this planar model than in an actual pc-Si cell. Actually, in a real commercial screen-printed cell, some medium materials exist between the silicon layer and the rear metal layer. That means the rear metal is separated with the silicon active layer rather than direct metal contact. Thus, the reflection resulted from the detached rear metal could be very weak in fact. But in the simulation, a direct-contact back metal with the active layer could result in a very strong reflection. Hence, the light path in silicon layer is prolonged, leading to a higher JG in the simulation. To further demonstrate the varing nAAO can exert influence on the suppression of reflection loss occurring on the front surface, Jrefloss (current density loss induced by reflection loss) is defined and calculated similarly as Hitoshi Sai does. The calculation spectrum is also from 300 nm to 1100 nm. As shown in Fig. 4, Jrefloss can be minimized by tuning nAAO within the range 1.3–1.4, and the reduction of Jrefloss can be reached to nearly 0.4 mA/cm2.

We have exhibited that an AAO nanograting with a proper refractive index is of use in the reduction of surface reflection loss. Now, we try to transplant it onto commercial pc-Si cells. But before that, AAO nanograting structure parameters shall be determined according to the former optimized result on account of cost-saving and convenience in experiment preparation. It is presented formerly that the refractive index of air and aluminium oxide are 1.0 and 2.0, and nAAO ranges from 1 to 2.0 distinctly with different weight ratios of air and aluminium in AAO structure [26]. The structure parameters mainly include AAO film thickness, AAO lattice period and duty circle of each hole in an AAO nano-grating. The three structure parameters can maintain a certain weight ratio of air and aluminium. In brief, AAO's structure parameters determine a certain nAAO. We derive a simple analysis to obtain the reasonable AAO structure parameters. On the basis of numerical results previously, the reflection-loss reduction in long wavelength band (about 900–1100 nm) is attributed to the well anti-reflection effect of AAO nano-grating with refractive index ranging

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from 1.3 to 1.4. Generally, it is predicted that the thickness of AAO nanograting would be approximately 100 nm, based on the equation

d =λ /4n

(2)

where d is the thickness of AAO, λ is wavelength and here equals to 550 nm according to the AR film theory. However, it seems that the antireflection effect of AAO nanograting around 550 nm is not desirable. In this work, the thickness of AAO nanograting is set as 200 nm in order to get sufficient reflection reduction in long wavelengths (around 1000 nm). The relation between refractive index and duty circle have been widely reported, hence the duty circle of AAO nanograting is about 0.73 when nAAO equals to 1.3 according to the Maxwell analytical model. AAO nanograting' lattice period is determined as 100 nm when duty circle is 0.73. In the following experiment processes, we adopt the structure parameters exhibited here. 3. Experimental exploration about AAO nano-grating applied to pc-Si substrate and commercial pc-Si cell

Fig. 6. Measured reflectivity spectra of the pc-Si substrate with a bare surface, an SiNx:H coating and the combination of SiNx:H coating and AAO nanograting, respectively.

3.1. Realistic anti-reflection effect of AAO nano-grating on pc-Si substrate

The anti-reflection effectiveness is displayed by the formerly defined AAO nanograting (nAAO=1.3). As shown in Fig. 6, the reflectivity is sharply suppressed over a wide spectral range comparing the bare silicon sample and the sample coated with SiNx:H only. This indicates that SiNx:H film is of great use in antireflection. Furthermore, placing AAO nano-grating on the SiNx:H coating can suppress reflection again both in the shorter (λ < 520 nm) and longer wavelengths (λ > 650 nm), despite the reflectivity from 520 to 600 nm is slightly increased. The experimental reflection result corresponds well with the numerical result previously presented. We draw a conclusion from these results that a preferable AAO nano-grating (appropriate nAAO) with rational parameters (film thickness, lattice period and duty circle of each hole) is effective for reducing reflection loss occurring on the front surface of pc-Si substrate. Moreover, the reduction of reflection loss in longer wavelengths could be a feasible way to promote Si-based solar cells performance.

The previous simulation analysis is verified in this section by applying commercial AAO nanograting (Shanghai Shangmu Technology Co. Ltd, Shanghai, China) onto polycrystalline silicon (pc-Si) substrate coated with SiNx:H film. The experimental sample structure consists of pc-Si substrate, SiNx:H coating, ultraviolet curable polymer coating, AAO nanograting from bottom to top. AAO nanograting here is adhesively adsorbed using ultraviolet-curable polymer coating and processed under 365 nm illumination to make steady contact. Fig. 5 shows the AAO nano-grating used here with thickness 200 nm, period 100 nm and duty circle 0.73, and these structure parameters have been determined in Section 2.2. Thus, The nAAO value of this nano-grating is approximately 1.3, which is within the optimized refractive index range (1.3–1.4) in Section 2.1. The thickness of the pcSi substrate and SiNx:H coating are 180 µm and 80 nm, respectively, corresponding to that in optical model. Aiming to examine the antireflection effectiveness of AAO nano-grating, we also prepare a bare pcSi substrate sample and a pc-Si substrate coating with SiNx:H sample as references. All samples were annealed in a vacuum at 120 °C to evacuate air remaining between each layers. The melting point (660 °C) of Aluminium is higher than 120 °C. Thus it is confirmed that the AAO nanograting can maintain a morphology as before. A spectrometer (Lambda 35, PerkinElmer, USA) was utilized to record reflectivity spectra of these samples at room temperature (25 °C).

3.2. AAO nanograting applied to pc-Si cell and solar cell performance improvements In this section, we transplant AAO nanograting onto commercial pc-Si solar cell with the purpose of improving pc-Si solar cell's performance. The description order is shown in Fig. 7. Firstly, we obtained commercial cell samples from manufacturer (Hengshui city, Hebei province, China). Then we cut them in pieces (1.8*1.8 cm2) and measured the electrical property. At this step, AAO nanograting was not applied. Hence we named the sample as “Original Cell”. The electrical property of this original pc-Si solar cell piece was characterized using the illuminated current density versus voltage characteristics and the external quantum efficiency (EQE). Secondly, AAO nanograting was added on the surface of the sample we used in the first step. The process is presented as following. AAO nanograting used here was obtained from Shanghai Shangmu Technology Co. Ltd, and structure parameters are same as that in Section 3.1. AAO nanograting was gently put into orthophosphoric acid solution to form suspension dispersion solution (orthophosphoric acid is a frequently-used solution for expanding nano-hole's pore diameter within the AAO preparation), and AAO nanograting could float or suspend in the liquid. The temperature of the orthophosphoric acid is 30 °C, acid concentration 6.0 wt%, the processing time is about 40 min in order to remove the barrier layer of AAO membrane [27]. After expanding nanohole's pore diameter of AAO, The pc-Si cell sample above mentioned was immerged into the suspension dispersion solution. Then we extracted the acid liquid through a sucker at the bottom. This process time is short (about 30 s). AAO nanograting remained on

Fig. 5. Scanning electron microscopy image of the AAO nanograting (lattice period 100 nm, thickness 200 nm) used in experiment.

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Fig. 7. The initial commercial pc-Si cell experiences twice electrical property measurements. One is before AAO nanograting applied, the other is after AAO applied.

reduction which means absorption improvement in both shorter and longer wavelength range. The EQE peak around 600–800 nm of “AAO applied to the Initial Commercial pc-Si Cell” is a little lower than the “Initial Commercial pc-Si Cell”. We attempt to calculate the JG of “bare pc-Si cell”, “Initial Commercial pc-Si Cell”, and “AAO applied to the Initial Commercial pc-Si Cell” in advance to evaluate the performances of the cell through the analytical method mentioned in Section 2.1, the only difference is that the reflection data in Section 2.1 is obtained by a simulation way but here the reflection spectra is measured from a UV–vis spectrometer (Lamda 35, PekinElmer). The JG results of the three kinds of cells were calculated and exhibited in Table 1. It could be found from chart 1 that the JG increased by about 0.4 mA/cm2 comparing the situations whether AAO nanograting placed on SiNx:H/RC cell or not. This result agrees well with the analysis shown in Fig. 3 in which the JG enhancement is also about 0.4 mA/cm2. Fig. 10 shows the current density-voltage (JV curve) properties of three times measurements. It is seen that a bare pc-Si cell's opencircuit-voltage (Voc) and short-circuit current density are 0.54 V and 25.1 mA/cm2, while a “Initial Commercial pc-Si Cell” can obtain a distinct enhancement in Voc and Isc, respectively. This indicates that a SiNx:H coating which serve as antireflection film and passivation layer is a necessity in the pc-Si cells. On the basis of existence of the SiNx:H coating, the transplant of AAO nanograting to pc-Si cells could bring Voc and Isc an extra increase, although the promotion seems not very considerable. However, when comparing the result of “Initial Commercial pc-Si Cell” and “AAO applied to the Initial Commercial pc-Si Cell”, the effect of AAO nanograting is well highlighted, as is shown in Table 2. As a result, Jsc measured in the designated area (3.24 cm2) is increased from 30.57 to 31.89 mA/cm2 by the application of the AAO nanograting. This is the highest Jsc among the cells we prepared. However, Jsc in Table 2 is low whether AAO applied or not. We attribute this to the low intrinsic Jsc (25.19 mA/cm2). Otherwise, the small-area of cell means that the surface Ag grid electrode area ratio is bigger than that of large area solar cell leading to reduced number the incident photons. Correspondingly, the cell's power conversion efficiency increases from 10.3–11.52% while applying AAO nanograting. Fill Factor is also found to gain a growth relatively. All these improvements in cell behaviors indicate that great application potential of AAO nanogratings in pc-Si solar cell as long as SiNx coatings also exist. We found that the PCE of our samples are below the state-of-the-art comparing with some excellent achievements published already, but we hold the view that applying AAO nano-grating onto photovotaics could be a potential manner to improve device properties and we are pursuing this work to obtain a better result. Comparing the Jsc enhancement before AAO being applied and

the cell surface eventually. The ultimate cell structure is, from the bottom to the top, Al electrode/pc-Si active layer/SiNx:H/Ag grid/AAO. We name the new sample as “AAO applied to the Original Cell”. A potential defect is that hot orthophosphoric acid may harm the SiNx:H film. The orthophosphoric acid is about 160 °C, high concentration and process time is about 40 min in industrial wet etching SiNx:H. But in this experiment, we believe that this could be neglect because of low acid temperature, low acid concentration and short process time. The new cell's electrical properties were characterized again using the illuminated current density versus voltage characteristics and the external quantum efficiency (EQE). The photocurrent is measured with a dual-light solar simulator (Wacom Electric Co. Ltd. Saitama, Japan) under the Air Mass 1.5 Global illumination (AM1.5G 100 mW/ cm2). EQE is analyzed using a monochromator-based spectral response system (Bunkoukeiki Co. Ltd. Japan). All the measurements are performed at room temperature (25 °C). From the twice characterizations of one cell sample before AAO applied and after AAO applied, the effect of AAO nanograting can be displayed by comparing the two successive characterizations. For comarison, we also prepared bare pc-Si cell and pc-Si cell with only SiNx:H coating, respectively. Fig. 8(a) is a cell with only SiNx:H coating before AAO nanograting applied and Fig. 8(b) is the cell after AAO nano-grating applied. The final cell area is around 3.24 cm2 in average. Fig. 8(c) shows the cell structure of the experimental cell applied with AAO nanograting. Reflection could be observed from Fig. 8(b) at oblique incidence. This might be a negative factor for improving light absorption. Actually, the incident light angle indeed exert an influence on the total device properties. We have investigate the AAO's property with incident light angle in paper [28]. We draw a conclusion from this paper that the AAO's property shows a tendency of decline when the incident light angle bigger than 50°. It means, when incident angle bigger than 50°, the reflection may be enhanced. Fig. 8(b) is a picture which taken at an angle larger than 50° to exhibit the existence of AAO on the cell surface. On the other hand, this phenomenon may be contributed to the film interference effect. Just like a pair of glasses, a strong reflectance at a certain wavelength could be observed when looking at the glass surface from a certain angle. Fig. 9 shows the EQE spectra of pc-Si solar cells prepared previously. The cell's surface area including Ag grids is about 3.24 cm2. The SiNx:H film's optical parameters are: thickness 80 nm, refractive index 2.0. Parameters of AAO nanograting used here are as followed. Period and duty circle are 100 nm and 0.73 respectively so that the nAAO of the nano-grating could be equal to 1.3 approximately. It could be found that EQE is enhanced by using the AAO nanograting in the wavelength range from 300 nm to 550 nm as well as from 850 nm to 1100 nm, although the EQE peak around 700 nm is lower than the one without AAO nanograting. The EQE enhancement could be explained by the reflection reduction shown before. The reflection

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Fig. 8. (a) A pc-Si cell with SiNx:H coating (80 nm), nanmed as “Initial Commercial pc-Si Cell”. The white lines are Ag grids at front surface (b) AAO nanograting placed on the top of cell, named as “AAO applied to the Initial Commercial pc-Si Cell”. The cell sample in (a) and (b) is the same one. (c) Cell structure of the experimental cell applied with AAO nanograting.

Table 1 JG results of “bare pc-Si cell”, “initial commercial pc-Si cell”, and “AAO applied to the initial commercial pc-Si cell”.

Calculated JG (mA/cm2)

Bare pcSi cell

Initial commercial pc-Si cell

AAO applied to the initial commercial pcSi cell

25.30

34.12

34.52

after AAO being applied, we found that the Jsc obtained a growth about 1.32 mA/cm2. This enhancement exceeds the expectations we made in Section 2.1 (about 0.4 mA/cm2). We explain this exceeding Jsc in Section 4. 4. Discussion As the former presentation, we are aware of the increases of cell behaviors when AAO nanograting applied on the top surface of “Initial Commercial pc-Si Cell”. We assume three positive factors of AAO nanograting which result in the promotion of cell performance. The first one, AAO nanogratings which also serve as an AR coating on the front surface of SiNx:H/pc-Si cells. A preferable refractive index of AAO nanograting can induce the refractive index changing gradually at the

Fig. 9. Influences of the AAO nanograting on the external quantum efficiency (EQE). A bare pc-Si cell without any surface anti-reflecting film is nanmed as “Bare pc-Si”. The “initial commercial pc-Si cell” is coated with SiNx:H but not applied with AAO. The “AAO applied to the initial commercial pc-Si cell” is the same commercial sample applied with AAO nanograting. The pc-Si cell indicated in red and blue curve respectively are the same one. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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surface, and this anti-reflecting effect can efficiently suppress Fresnel reflection. As mentioned in Section 2.1, when nAAO varies from 1.3 to 1.4, the reflection loss is minimized which indicate that more sunlight could transmit into the active layer of cell. Secondly, anti-escaping effect can exert positive impact on enhancing the absorptivity of sunlight, as Sai [29] described. In our cells, the anti-escaping effect also exists and plays an important role in absorption enhancement. As shown in Fig. 11(a), the incident light (1) indicates that AAO nanograting can mitigate the shadow loss of the Ag grids on the surface of the cell by preventing the light reflected from Ag grids to air. Conversely, the light reflected by Ag grids can be well trapped within the AAO and active layer, resulting to an additional absorption enhancement. In fact, We attribute the absorption advancement to both anti-reflection effect and anti-escaping effect of AAO nanograting, in other words, the reduction of surface reflection loss is resulted from these two positive effects. The last one, our group have done several works on the scattering effect of AAO nanograting to enhance the light in-coupling in cells. We have verified that AAO nanogratings can facilitate the light-trapping in Si-based cells (not repeat here). Additionally, it is more likely that AAO nanograting can enhance the cell performance indirectly from the perspective of electrical level. Fig. 11(b) showes the AAO's light scattering effect facilitate the carrier separation. The diffusion length for minority carrier (electrons in ptype region) can be decreased resulted from AAO nanograting's light scattering effect. Commonly, the depth for different wavelengths in silicon layer is variable. Longer wavelength photons can go deep into ptype region so that electron-hole pair's birth-position reaches to the bottom of cell. Here, we consider a quantity-equation for minority carriers (electrons in p-type region) in normal incidence.

Fig. 10. The current density versus voltage curve of “bare pc-Si cell”, “initial commercial pc-Si cell”, and “AAO applied to the initial commercial pc-Si cell”. Table 2 Solar cell behaviors of “bare pc-Si cell”, “initial commercial pc-Si cell” and “AAO applied to the initial commercial pc-Si cell”. The measured area is designate using a shadow mask with an value of 3.24 cm2.

Bare pc-Si cell Initial commercial pc-Si cell AAO applied to the initial commercial pc-Si cell

Voc (V)

Jsc (mA/ cm2)

Fill factor (%)

PCE (%)

0.549 0.566 0.572

25.19 30.57 31.89

61.55 63.75 63.22

8.51 10.3 11.52

d ⎛ dN ⎞ N ⎜ −D ⎟ − =0 dz ⎝ dz ⎠ τ

(3)

where z is the position in the z axis, N is the number of minority carriers, D is diffusion coefficients of minority carriers, τis the lifetime of minority carriers. It describes the quantity distribution of minority carrier. We assume that N0 is the nember of electrons at the generation position of electron-hole pair, and in this case we set z equals to zero (N(z)|z=0=N0). Then, a function N(z) could be obtained and written as N (z )=N0 exp (−z /L ), where L=(Dτ)1/2. L is the diffusion distance of electrons. When AAO nanogrting was applied onto the cell's surface, the light-scattering effect changes the generation position of electronhole pairs. As Fig. 10(b) shows, the scattering angle is θ. We introduce s as the depth where photon absorbed. It could be obtained that s(θ )=s0 cosθ , where s0 is the absorption depth at normal incidence(θ=0°). At last, we can calculate the number of minority carriers going across the pn-junction. It can be written as +∞

Num (θ )=

∫s (θ )

1 1⎞ N (z ) dz=N0 (Dτ ) 2 exp (−s0 cosθ /(Dτ ) 2 ⎟⎟ ⎠

(4)

where Num is the number of separated electron-hole pairs. Num is monotonous to θ. To utilize the improvement in light in-coupling via AAO nanograting as much as possible, the illustrated sunlight shall be absorbed in cells before it overflow from the bottom side and thickening the active layer is available. Normally, the thickness of pcSi (180 µm) is sufficient for the light to be absorbed, however, the carrier generate position is relatively very far away from the space charge region where electron-hole pairs separated. In other words, the distance between space charge region and carrier generation position is long for an electron or a hole to diffuse. Here, the scattering effect of AAO nano-grating can incline the light path in the active layer of cell. That means the position where carrier born at is close to the p-n junction, and this shorter distance makes the carrier separation more likely to happen.

Fig. 11. (a) A cross-sectional schematic diagram of the anti-escaping effect and lightscattering effect of AAO nano-grating (b) AAO nanograting's light scattering effect facilitate the carrier separation.

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5. Conclusion [9]

In summary, we explore the improvement of light-incoupling for pc-Si solar cell when AAO nano-grating included. AAO/SiNx:H double AR can minimize surface average reflectivity of pc-Si solar cells over a broad spectral range. nAAO could be modified with parameters of AAO nano-gratings. An AAO nanograting with rational parameters applied to SiNx:H/pc-Si solar cell could play a significant role in improving solar cell performance. The effectiveness of AAO nanogratings in our solar cells for improving cells performance is owing to these three positive factors. Firstly, gradual variation in refractive index between air and semiconductor layer induced by AAO/SiNx:H double AR layer, which minimized the Fresnel reflection as much as possible. Furthermore, AAO nanograting on the top of Ag grids resulted in anti-escaping effect which prevent the sunlight reflected from Ag grid into air, leading to an additional gain in Jsc. Last one, the scattering effect of AAO indirectly facilitate the carrier separation in deep position of pc-Si layer, ascribed to the shortening of diffusion distance of electrons. As a result of these three positive effects, the Jsc and PCE of a pc-Si solar cell in a designated area have increased to 31.89 mA/ cm2 and 11.52%, respectively. The moderate fabrication method makes AAO nanograting suitable for applications in pc-Si solar cells, even other Si-based solar cells.

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Acknowledgement

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The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 61274064). Special thanks to all editors and reviewers for their earnest comments and suggestions.

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