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Pre-metallization processes for c-Si solar cells
Available online at www.sciencedirect.com
ScienceDirect Solar Energy 97 (2013) 388–397 www.elsevier.com/locate/solener
Pre-metallization processes for c-Si solar cells Peraiah Sastry Aakella a,c,⇑, S. Saravanan c, Suhas S. Joshi b, Chetan Singh Solanki a,c a
Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India b Department of Mechanical Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India c National Centre for Photovoltaic Research and Education, Indian Institute of Technology Bombay, Mumbai 400076, India Received 21 May 2013; received in revised form 4 August 2013; accepted 19 August 2013
Communicated by: Associate Editor Takhir M. Razykov
Abstract Crystalline silicon photovoltaic (PV) technology dominates 85% of the PV share due to its ease process and manufacturing flow. Conventional silicon PV technology uses screen printing for the metallization and though most of the PV industries use this technique, it has its own limitations for fabricating solar cells of high efficiency beyond 19%. However this technique is widely used in industries as a major process because of its production feasibility and cost compared to the alternate metallization techniques. In this article, an attempt has been made to study the different printing mechanisms, alternate to screen printing technique. Also, the process and results of alternate printing mechanisms were interpreted and compared with screen printing technique. It is well understood that the PV industries looking for the cost effective advanced metallization technique for fabricating high efficiency solar cells. Ó 2013 Elsevier Ltd. All rights reserved. Keywords: Pre-metallization; Printing; Chemical etching; Laser ablation; Microsintering; Ink-jet printing
1. Introduction For the past few decades crystalline silicon solar cell researchers actively working towards the different technologies to achieve the high efficiency without investing more capital and targeting the cost of $/Wp or lower than that too. The silicon solar cell manufacturing competences is always trying to increase the productivity without compromising the efficiency and quality of the cell. Available literatures reported that this can be achieved by different directions such as minimizing the use of Si, and use of advanced process technologies. One of the ways to minimize the use of Si is to scale down the wafer thickness from 180 to 100 lm and further to ultra-thin wafers of size ⇑ Corresponding author at: National Centre for Photovoltaic Research and Education, Indian Institute of Technology Bombay, Mumbai 400 076, India. Tel.: +91 22 25767895; fax: +91 22 25767890. E-mail addresses: [email protected], [email protected] (P.S. Aakella).
0038-092X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.08.026
50 lm. Researchers reported that the silicon solar cells were fabricated by starting the process with wafer thickness of 50 lm and results the cell of efficiency 21.5% (Dross et al., 2012; Wang et al., 1996). In a typical solar cell fabrication process, the front silver metallization process has potential to improve the efficiency and reduce cost. This is because silver pastes are continuously being evolved and new paste generation with proper composition of silver particles, glass frit and few additives are made available in the market. These new generation pastes in combination with front grid design and screen design delivers better efficiency at lower paste weight, thus reducing the cost as well. Typically a new generation paste offers better aspect ratios (finger height/finger width) if screen design and printing parameters are carefully chosen. By improving the aspect ratio, current is improved as shadow loss is decreased. Improvement of aspect ratio also decreases in series resistance. Besides paste rheology, improvement of the aspect ratio depends upon
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the screen parameters such as mesh count, wire dimensions, emulsion thickness and also on the printing parameters such as squeeze pressure and speed. Apart from this, the appropriate drying and firing with certain ambient, results the proper metallization. This process is referred to as thick film screen printing (Hilali et al., 2004). As mentioned earlier, the silver paste is composed of Ag powder (70–80%) and glass-frit (used for sintering and contact improvement, of 5%) as major components. Many other solvents for suspension, organic resin and additives are also present, which vary with the changes in the rheology of paste. Glass-frit, while making a contact at elevated temperature about 700 °C reacts with Si surface and forms islands. These islands behave as insulators and reduce the charge flow through them, hence the line conductivity. Huge line width with heavily doped underlying silicon and high contact resistance (2 10 7 X cm2) further reduce the blue response. The fill factor is limited to not more than 77% due to poor metal conductivity of the front contact (Resistivity 3 lX cm) and poor aspect-ratio along with high contact resistance (>3 mX cm2) in a screen printed contact (Schubert et al., 2006). Despite these limitations, screen printing of contacts continues in industry as a major technique till date because of its cost effectiveness, simplicity and high productivity. Typical commercial solar cells achieved efficiency in the range of 18–19% and the schematic of the commercial solar cell is as shown in Fig. 1. Also it is well understood that, there is a significant trade-off between the cell performance and cost-effectiveness in the existing c-Si solar cell technology. Record high efficient cells are produced by more complex photo-lithography (PL) technique (Zhao et al., 2001), which is expensive and adds to the cost if employed in commercial production. The screen printed and other laboratory scale high efficiency solar cells have their own limitations in their application in commercial production. Hence the PV researchers explored alternative metallization techniques to achieve the better performance of solar cells at optimum costs and revealed few advanced metallization schemes which involve an additional step of forming the metal seed layer and plating step on it. A seed layer on the silicon surface should be a very thin line and it should be in a good electrical and mechanical contact with silicon surface. Electro plating of seed layer with metals like silver
Fig. 1. Cross section of c-Si solar cell.
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improves the line conductivity and the performance of the solar cell (Glunz, 2007). Similarly they introduced another technique in which the dielectric layer (antireflection coatings) will be opened followed by the plating (Knorz et al., 2009). The selective opening of dielectric layer and the development of metal seed layer are referred to as pre-metallization (PM) techniques. This article reviews the different types of PM techniques and the subsequent processes. Also, this explains the advantages and disadvantages of different PM techniques. Typical silicon solar cell fabrication process with pre-metallization is as shown in Fig. 2. There are two individual steps involved in the PM techniques by which a great control can be achieved on finger dimensions. The curing temperature for the plating contact is lower than the conventional metallization. Hence, it is important to understand these methods along with their limitations and capabilities which will help to improve their performance. Therefore, the objective of this paper is to elucidate various PM techniques followed by their comparative evaluation with their capabilities, parameters and the ways to control them.
2. Classification of Pre-metallization techniques (PM) Fig. 2 shows the solar cell fabrication process flow with pre-metallization technique. It is important to mention here that PM begins once the antireflection (AR) coatings are done on the diffused wafer. In this article the process flow shown in Fig. 2 is common to all the PM techniques unless otherwise specifically mentioned. Route-A: Patterning of dielectric layer. Route-B: Developing metal seed layers. Schematic of the conventional c-Si solar cell and cells fabricated based on the two different PM routes are shown in Fig. 3. Route-A refers the opening of dielectric layers
Fig. 2. Process flow of standard solar cell and pre-metallization preparation.
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etching the dielectrics. In this method, a screen with the required openings is kept in contact with the substrate (solar cell) and then semi-liquid solution (etching paste) will be transferred to the substrate. The cell is then heated to 350 °C around 90 s for chemical etching. Ultrasonic potassium hydroxide (KOH) bath is necessary for removing the chemical residue on the surface of the solar cell. The screen printing of etch paste involves various steps as shown in Fig. 5. Metallization using silver paste with different compositions on the front side is performed, once the dielectric layer is opened in the required grid pattern. With different mesh counts, openings of 85 lm and 50 lm are obtained on both textured and non-textured surfaces respectively (Bahr et al., 2007). Fig. 3. Patterning dielectric layers (Route A) and developing metal seed layer (Route B).
selectively. Since AR is dielectric, it can be opened by either wet chemical etching or selective laser ablation. The wet chemical etching for etching the AR layer can be carried out by different ways and they are screen printing, ink-jet printing, laser grooving and mechanical structure techniques. Route-B discusses developing the metal seed layer which can be done by laser microsintering and printing techniques, namely, pad printing, ink-jet printing and aerosol-jet printing. The classification of PM and its sub-classification are depicted in Fig. 4. The principle and operation of each and every process is explained in detail in their respective sections (see Fig. 4). 2.1. Route-A: Patterning of dielectric layers In this process, AR coating is selectively opened in a required grid pattern. The silicon nitride (Si3N4) is a hard ceramic with inherent properties such as good insulation, good passivation, low thermal expansion coefficient, moderate thermal conductivity and high strength over a broad temperature range, which also serves as an AR coatings for c-Si solar cells (Morosanu, 1980). Based on these properties, silicon nitride is preferred widely as AR coatings for the solar cells. The patterning techniques of dielectric layers has been carried out either by chemical etching or by selective laser ablation method and are described below.
2.1.1.2. Ink-jet printing. Along with screen printing, now-adays ink-jet printing also plays a role in photovoltaic applications. In this process chemical based solution (etch paste) is used on the substrate to open/etch the dielectric layers and this is described elsewhere (Lennon et al., 2008, 2009). Acidic water soluble polymer on the dielectric layer is subjected to ink-jet deposition of a solution which contains fluoride ions. A reaction that takes place between the etch paste and the polymer etches the dielectric layer. Chemical residues formed after the etching process is removed using water. This method has been used for SiO2 and SiNX structure dielectrics. Using this process, 390 nm thick thermally grown SiO2 layer holes are etched. The depth and volume of etched holes depend on the temperature at which inkjet process is carried out. The thickness of etched holes ranges from 270–390 nm. Continuous grooves of 70 lm width are obtained with this method and the minimum width is of 25 lm. The direct ink-jet printing process is schematically shown in Fig. 6.
2.1.1. Chemical etching The dielectric layer, SiNX of a solar cell can be etched chemically by acids such as hydrofluoric acid (HF) and sulfuric acid (H3PO4). The etching depends on the nature of chemical, their viscosity and the method of application. The process is done using different techniques such as screen printing, ink-jet printing and laser engraving.
2.1.1.3. Laser engraving. This process is enabled by covering the solar cells on both the sides with polymer sheet and grooving it using a laser in a required grid pattern. The process of polymer masking and the laser engraving is shown in Fig. 7. Polymer masked diffused wafers are required to groove by laser without affecting its AR coatings. A suitable laser for this kind of application is CO2 laser with 10 lm wavelength. The cells engraved by the laser are kept in 30% HF acid bath for 3 min. Hence etching is happened in the laser grooved areas and the remaining areas are unaffected because the polymer acts as a mask between the acid and AR coatings of solar cell. Further, the cells are processed with Ni-Cu metallization. With this method, 160 lm finger widths of 4 4 cm solar cells were processed and subsequently Ni–Cu plating has been carried out (Chaudhari and Solanki, 2010).
2.1.1.1. Screen printing. Screen printing is the most widely used technique in photovoltaic solar cell process for both front contact metallization and back surface field. Patterning by screen printing technique uses phosphoric acid for
2.1.1.4. Mechanical structure. In this patterning method, etching paste is used for PM through mechanical structure technique. The mechanical structure is usually micromachined in finger-grid pattern, associated with the heating
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Fig. 4. Classification of pre-metallization (PM) techniques.
patterned solar cell obtained with this process is illustrated in Fig. 8. The finger width of 150 lm has been reported using this method (Sastry et al., 2010). As mentioned earlier, along with chemical etching methods, the patterning is demonstrated using selective laser ablation.
Fig. 5. Patterning dielectric layers by screen print etching paste.
elements as shown in Fig. 8(a). The etching paste is applied on mechanical structure and is kept in contact with solar cell. The paste gets transferred to the solar cell in the required grid form. The paste is thermally sensitive and requires heating for activation. Using heating elements on mechanical structure, the paste printed on the solar cells are heated to temperature 390 °C for 90 s. The steps involved in mechanical structure and heating is schematically shown in Fig. 8(b). The heating step etches the AR coating of the solar cell but leaves chemical residues, which are removed with DI water and the ultrasonic bath. The
2.1.2. Selective laser ablation Chemical etching methods use chemicals followed by firing for patterning the AR coatings of the solar cells. These steps are not required in laser ablation method. The solar cell patterning by this method was demonstrated for the first time by Dube and Gonsiorawski (1990). The application of laser ablation process has been widely used and reported by various groups (Engelhart et al., 2006, 2007; Xu et al., 2009; Correia et al., 2007; Payne et al., 2010; Perry et al., 1999; Knorz et al., 2009, 2007 and Lennon et al., 2009). In this method, the laser source should be chosen appropriately in such a way that the AR coatings are grooved by laser source but should not affect the other layers beneath the solar cell. To pattern the solar cells with SiNX as AR coatings, a low refractive index laser source is chosen for grooving and hence the silicon layer underlying the AR coatings is not much affected. Most of the ablation processes use laser of 355 nm wavelength within a nanosecond pulse on- time regime (Grohe et al., 2006). The ultra-short laser pulses interacting with dielectric layer on the solar cell result in a selective and precise ablation. The laser ablation process on non-textured and textured surfaces was done by Knorz et al. (2007, 2009). As compared to planar surfaces, minimal damage is observed on textured surfaces. Using a 355 nm laser source, the dielectric layer is removed selectively and metallization is done separately. The highest fill factor (FF) of 78.6% is achieved using selective laser ablation method on textured surface with efficiency of 19.1% (Knorz et al., 2007). Various groups demonstrated the metallization on the rear side of the solar cell which
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Fig. 6. Direct patterning by ink jet printing (Lennon et al., 2009).
Fig. 7. Processing of polymer masking method to open dielectric layers.
Fig. 8. Processing dielectric layer patterning by mechanical structuring using etching paste.
include the processes like mechanical scribing, point contact solar cells that are not dealt in this paper as the focus of this article is restricted to the front side metallization techniques (Lee, 2009; Emmanuel and Geaucarne, 2006).
various printing techniques which involve penetration of metal through AR coatings and contacting with Si surface. Major processes presented in this section include laser microsintering and printing techniques for developing metal seed layer and is shown in Fig. 3.
2.2. Route-B: Developing metal seed layer The metal seed layer is developed by either printing or laser microsintering technique. The seed layer is then, thickened by plating. The metal seed layer is printed using
2.2.1. Laser microsintering Laser microsintering is a rapid prototyping technique and its applications are in metals/ceramic functional parts with high precision (Regenfuss et al., 2007; Kumar, 2003;
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Kathuria, 1999; Regenfuß et al., 2004; Exner et al., 2003). Application of this technique for solar cell fabrication is reported by Alemn et al. (2006). This method uses a laser source and fine metal powder of micron size. The metal powder is spread on dielectric layer of a solar cell in a finger-grid pattern as shown in Fig. 9(b). Laser source is guided over the metal powder surface by continuous or short pulses which cause melting and adhesion to the Si surface which is beneath AR coatings of solar cell (Fig. 9(c)). Melting causes immediate expansion and creates non equilibrium pressure conditions. This results in condensation and predetermined structure of metal seed layer as shown in Fig. 9(d). Nd-YAG laser with 1064 nm wavelength is chosen as laser source and metals like silver and tungsten are commonly used. The metallization step is processed using the light induced plating. Solar cells processed with laser microsintering are of 15 lm finger widths. With this minimal width, a great control over aspect ratio and reduction in series resistance is achieved which leads to a pseudo fill factor of 78.5%. After light induced plating, laser sintering resulted the cell of efficiency 14.5% (Alemn et al., 2006). 2.2.2. Printing techniques Pad printing, screen printing, ink-jet printing, aerosol-jet printing are the prominent techniques derived from the conventional screen printing technique by which the seed layer can be printed on the solar cells. These methods are extensively used in laboratory scale for the solar cell fabrication. A layer of metal in the form of a powder or solvent is to be deposited by pressurized squeegee wiping on the required pattern and this resulting in print lines with a great control of finger width. The process itself fast and has an added advantage for processing of metallization by plating with suitable metals as compared to the chemical etching methods.
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2.2.2.1. Pad printing. In pad printing, an ink is transferred from print plate to the required surface. Applications of pad printing are in metals, ceramics and automotive. The advantages of pad printing are indirect printing, economic and high throughput production and ability to print on irregular surfaces. Pad printing on solar cell applications is proposed by Bottari et al. (1992). Major components of a pad printing machine are print plate, doctor blade, ink spreader and silicon rubber transfer pad. The etching process of pad printing begins with flooding of ink by reciprocating motion of the printer. Silicon pad gets in contact with the ink which is being imprinted on a solar cell surface by rolling over it. The additional amount of ink is removed using a doctor blade. The printed area is heated to 350–400 °C. At different stages of heating, organics are burnt out and required metal is printed on the substrate. Fast contact firing process results the metal seed layer formation. General sequence of pad printing technique is shown in Fig. 10. Preferably, light induced plating is performed on the seed layer developed by pad printing. Pad printing yields the cell with finger width of 45 lm on 12.5 12.5 cm area cells (Huljic et al., 2002). The paste rheology, viscosity and curvature of pad with hardness are required for pad printing. Rotating pad, known as rotary gravure offset printing and transfer pad (Krebs, 2009) for polymer cell applications are the types of pad printing process. The reported best cell parameters using the pad printing are VOC – 590 mV, JSC – 29.2 mA/cm2, FF – 77.7%, g – 13.4% (Hahne, 2001). 2.2.2.2. Ink-jet printing. Ink-jet printing is a well established printing technology and is used in solar cell fabrication because of the low cost and fine line resolution. Good control over the flow gives very narrow printing on the surface without contacting the surface to be printed. Ink-jet printers use a piezoelectric crystal in nozzle and by applying the
Fig. 9. Laser microsintering process using Nd-YAG laser.
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Fig. 10. Schematic of pad printing sequence (acc-silicones, 2013).
Fig. 11. Schematic of ink jet printing sequence (Teng and Vest, 1988b).
Fig. 12. Schematic of aerosol jet printing.
current to the crystal; it changes its shape or size thereby forcing an ink droplet from the nozzle. All the nozzles include piezoelectric driver, which generates pressure to eject liquid ink. Substrate is kept on programmable XY stage upon which a finger grid is to be printed. Schematic of the ink-jet printing used for solar cell fabrication is shown in Fig. 11 (Teng and Vest, 1988b). The ink-jet printers allow a wider variety of inks such as metal organic deposition (MOD) silver and silver nanoparticulate inks. Different versions of inks for ink-jet metallization are explained in the literature (Vanhest et al., 2009; Shin, 2010; Kaydanova et al., 2003; Teng and Vest, 1987, 1988a; Stockum et al., 2008; Shah et al., 2004; Liu et al., 2009; Curtis et al., 2006 and Gizachew et al., 2011).
It is important to mention here that ink-jet printing is also used for the patterning of dielectric layers (Lennon et al., 2008, 2009). This method is cost-effective for industrial process. Since ink-jet printing as non-contact printing, solar cells from this technique achieves good aspect ratios. Optimized temperature and time period result the fine grid lines. Recent advancements show the potential usage of ink-jet printing in developing a metal seed layer for twostep metallization on the front side of a solar cell. Finger width of 1.81 lm and height of 58.8 lm was achieved with ink-jet printing by using silver nanoparticulate ink (Shin, 2010). Nevertheless, ink-jet printing with metal organic deposition silver inks could not succeed in achieving the better performance of solar cells because of solid content
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Table 1 Comparison of various pre-metallization techniques. Route
Technique
VOC (mV)
JSC (mA/cm2)
FF (%)
g (%)
Height (lm)
Width (lm)
Aspect ratio
Ref.
A
Laser engraving
590
48.9
72.3
13.2
–
160
–
Screen printing Selective laser ablation
605 639
– 38
76 78.6
16 19.10
– –
85–120 –
– –
Chaudhari and Solanki (2010) Bahr et al. (2007) Knorz et al. (2007)
Ink jet printing Laser microsintering (silver) Laser microsintering (tungsten) Pad printing Metal aerosol-jet printing
588 652.7 622 591 624
28.60 37.33 34.12 29.2 36.1
72.66 54.10 73.6 77.7 81
12.22 13.2 14 13.4 18.3
3 – – 1.5–2.5 20
135 40 40 45–140 80
0.02 0.45 0.45 0.01 0.25
Han et al. (2009) Alemn et al. (2006) Alemn et al. (2006) Hahne (2001) Horteis et al. (2007)
B
in the ink and the efficiency of 8 % has been achieved without AR coating (Teng and Vest, 1988b). 2.2.2.3. Aerosol-jet printing. Seed layer metallization by non-contact ink-jet printing is already reported (Teng and Vest, 1988b). Major limitations of ink-jet printing systems are mass production, outlet of nozzle width which is wider than printed line, usage of nano and metal organic inks which are costly (Mette et al., 2006). With some modifications in ink-jet printing, aerosol-jet printing has been developed for solar cell metallization (Mette et al., 2007; Horteis et al., 2007; Glunz, 2007; Drew et al., 2011; Glunz and Horteis, 2008). Working of an aerosol-jet printing is explained by Mette et al. (2007). The printing system (also known as metal aerosol-jet printing) has a deposition head and an atomizer. Metal containing ink becomes aerosol under stream control. Aerosol is transported to the deposition head via heated tube so that preheating of aerosol is possible. The deposition head contains sheath gas, which surrounds the aerosol in suspension and deposits it through tip onto the surface. A shutter is used to interrupt continuous aerosol-jet stream of ink. In the atomizer, gas surrounds the aerosol stream. Through atomizer nozzle, compressed gas enters with high velocities. The ink is drawn off the reservoir by venturi principle and expansion of the compressor gas results in breaking the stream into droplets, which are suspended in the gas flow. Gas droplets impinge against side walls of the reservoir. Due to many collisions, bigger droplets are reduced in size. The smaller droplets remain in the gas and are transported to deposited head. The aerosol is transported to the deposition head which keeps aerosol in suspension without contacting the nozzle with the help of focusing gas. Therefore, great control over printing width on the substrate is achieved. Print head with an atomizer gas is shown in Fig. 12. Good aspect ratio is achieved with this kind of printing. Finger width of 14 lm with nozzle outer diameter of 100 lm has been achieved. The next plating step involves light induced plating. The best cell parameters with aerosol printing are VOC – 624 mV, FF – 88%, g – 18.3% with an average aspect ratio 0.25.
3. Comparison of various pre-metallization techniques A comparison of various pre-metallization techniques is presented in Table. 1. The electrical parameters and specific characteristics such as finger width and height of crystalline silicon solar cells fabricated by employing various pre-metallization techniques are listed in Table 1. From table, it has been observed that both Route A and Route B had shown the best performance. However in Route A, selective laser ablation had shown the significant role in electrical performance compared to other techniques. Similarly in Route B, metal aerosol jet printing shown the better electrical performance compared to other techniques. It is important to mention here that most of the PM techniques evaluated are in the laboratory level pilot fabrication and are yet to be commercialized. 4. Conclusions and future directions The article discusses classification of patterning techniques prior to the metallization process of solar cell. Broadly, the classification is made into two groups: patterning of dielectric layers and developing metal seed layers. Screen print metallization of silver is widely used in industries for processing of c-Si solar cells. However in screen print metallization technique, the amount of silver used is gradually reduced and the reason behind this may be the high cost of silver and the researchers looking for an alternate techniques. Advanced metallization processes alternate to the conventional screen printing used for fabricating silicon solar cell are explained in this article. The Ni–Cu metallization discussed in patterning of dielectric layers has a great potential in fabrication level due to the high conductivity nature and the low cost of Cu. Plating of Cu in Ni–Cu metallization is in the pure form and one can expect a good conductivity than a screen printed Ag contact. Similarly light induced plating for metallization plays the significant role in developing metal seed layers and the amount of Silver used for plating is less. Even though the solar cell process is having well developed different metallization and PM techniques, development of
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advanced metallization techniques is still underway. The studies on metallization and PM reveal that the Cu might be the strong candidate which replaces Silver in near future, so as to produce solar cells cost-effectively. References www.acc-silicones.com/applications/tempoprinting.ashx, 2013. Alemn, M., Streek, A., Regenfuß, P., Mette, A., Ebert, R., Exner, H., Glunz, S.W., Willeke, G., 2006. Laser micro-sintering as a new metallization technique for silicon solar cells. In: 21st European Photovoltaic Solar Energy Conference. Bahr, M., Kim, S., Sridharan, S., Khadilkar, C., Shaikh, A., Kohler, I., Reichardt, M., Kumar, M., 2007. A new approach for the front side metallization of industrial type silicon solar cells using a structurization by etching. In: Proc. 22nd EPVSEC, Milan, pp. 2–6. Bottari, F.J., Hanoka, J., Brookline, Sylva, F.W., 1992. Method of applying metallized contacts to a solar cell. Publication Number US5151386 A. Chaudhari, V.A., Solanki, C.S., 2010. A novel two step metallization of Ni/Cu for low concentrator c-Si solar cells. Solar Energy Materials and Solar Cell 94, 2094–2101. Correia, S.A.G.D., Lossen, J., Wald, M., Bahr, M., 2007. Selective laser ablation of dielectric layers. In: 22nd European Photovoltaic Solar Energy Conference, pp. 3–7. Curtis, C.J., Van Hest, M., Miedaner, A., Kaydanova, T., Smith, L., Ginley, D., 2006. Multi-layer inkjet printed contacts for silicon solar cells. In: IEEE 4th World Conference on Photovoltaic Energy Conference, vol. 4, pp. 1392–1394. Drew, K., Hopman, S., Horteis, M., Glunz, S.W., Granek, F., 2011. Combining laser chemical processing and aerosol jet printing: a laboratory scale feasibility study. Progress in Photovoltaics Research and Applications 19, 253–259. Dross, F., Baert, K., Bearda, T., Deckers, J., Depauw, V., Daif, O. El., Gordon, I., Gougam, A., Govaerts, J., Granata, S., Labie, R., Loozen, X., Martini, R., Masolin, A., O’Sullivan, B., Qiu, Y., Vaes, J., Van Gestel, D., Van Hoeymissen, J., Vanleenhove, A., Van Nieuwenhuysen, K., Venkatachalam, S., Meuris, M., Poortmans, J., 2012. Crystalline thin-foil silicon solar cells: where crystalline quality meets thin-film processing. Progress in Photovoltaics: Research and Applications. Dube, C.E., Gonsiorawski R.C., 1990. Improved contact metallization for high efficiency EFG polycrystalline silicon solar cells. In: Photovoltaic Specialists Conference. Conference Record of the Twenty First IEEE, vol. 1, pp. 624–628. Emmanuel, V.K., Geaucarne, 2006. Back-contact solar cells: a review. Progress in Photovoltaics: Research and Application 14, 107–123. Engelhart, P., Harder, N-P., Horstmann, T., Grischke, R., Meyer, R., Brendel, R., 2006. Laser ablation of passivating SiNx layers for locally contacting emitters of high-efficiency solar cells. In: IEEE 4th World Conference on Photovoltaic Energy Conference, vol. 1, pp. 1024–1027. Engelhart, P., Hermann, S., Neubert, T., Plagwitz, H., Grischke, R., Meyer, R., Klug, U., Schoonderbeek, A., Stute, U., Brendel, R., 2007. Laser ablation of SiO2 for locally contacted Si solar cells with ultra-short pulses. Progress in Photovoltaics Research and Applications 15, 521–527. Exner, H., Regenfuss, P., Hartwig, L., Klotzer, S., Ebert, R., 2003. Selective laser microsintering with a novel process. Proceedings of SPIE 5063, 145–151. Gizachew, Y.T., Escoubas, L., Simon, J., Pasquinelli, M., Loiret, J., Leguen, P.Y., Jimeno, J.C., Martin, J., Apraiz, A., Aguerre, J.P., 2011. Towards ink-jet printed fine line front side metallization of crystalline silicon solar cells. Solar Energy Materials and Solar Cells 95, S70–S82. Glunz, S.W., 2007. High-efficiency crystalline silicon solar cells. Advances in OptoElectronics, 1–16. Glunz, S.W., Horteis, M., 2008. Fine line printed silicon solar cells exceeding 20% efficiency. Progress in Photovoltaics: Research and Applications 16, 555–560.
Grohe, A., Knorz, A., Alemn, M., Harmel, C., Glunz, S.W., Preu, R., Willeke, G.P., 2006. Novel low temperature front side metallization scheme using selective laser. Solar Energy, 2–5. Hahne, P., 2001. Progress in thick-film pad printing technique for solar cells. Solar Energy Materials and Solar Cells 65, 399–407. Hilali, M.M., Rohatgi, A., To, B., 2004. A review and understanding of screen printed contacts and selective-emitter formation. Renewable Energy 1617 (80401-3393). Han-Chang Liu, Chia-Pin Chuang, Yi-Tsun Chen, Chen-Hsun Du, 2009. Ink jet Printing for silicon solar cells. Water air soil polluts: Focus 9, 495–498. Horteis, M., Richter, P.L., Fidorra, F., Glunz, S.W., 2007. Further progress in metal aerosol jet printing for front side metallization of silicon solar cells. Solar Energy 23, 7–10. Huljic, D., Thormann, S., Preu, R., Ludemann, R., Willeke, G., 2002. Pad printed front contacts for c-Si solar cells – a technological and economical evaluation. In: Conference Record of the Twenty-Ninth IEEE Photovoltaic Specialists Conference. IEEE, pp. 126–129. Kathuria, Y.P., 1999. Microstructuring by selective laser sintering of metallic powder. Surface and Coatings Technology 116–119, 643– 647. Kaydanova, T., Miedaner, A., Curtis, C., Perkins, J., Alleman, J., Ginley, D.S., 2003. Ink jet printing approaches to solar cell contacts. In: National Center for Photovoltaics and Solar Program Review Meeting, pp. 919–921. Knorz, A., Grohe, A., Harmel, C., Preu, R., Luther, J., 2007. Progress in selective laser ablation of dielectric layers. In: 22nd European Photovoltaic Solar Energy Conference and Exhibition. Knorz, A., Peters, M., Grohe, A., Harmel, C., Preu, R., 2009. Selective laser ablation of SiNx layers on textured surfaces for low temperature front side metallization. Progress in Photovoltaics: Research and Applications 17, 127–136. Krebs, F.C., 2009. Pad printing as a film forming technique for polymer solar cells. Solar Energy Materials and Solar Cells 93, 484–490. Kumar, S., 2003. Selective laser sintering: a qualitative and objective approach. JOM Journal of the Minerals Metals and Materials Society 55, 43–47. Lee, S.H., 2009. Cost effective progress for high-efficiency solar cells. Solar Energy 83, 1285–1289. Lennon, A.J., Utama, R.Y., Lenio, M.A., Ho-Baillie, A.W., Kuepper, N.B., Wenham, S.R., 2008. Forming openings to semiconductor layers of silicon solar cells by inkjet printing. Solar Energy Materials and Solar Cells 92, 1410–1415. Lennon, A.J., Ho-Baillie, A.W.Y., Wenham, S.R., 2009. Direct patterned etching of silicon dioxide and silicon nitride dielectric layers by inkjet printing. Solar Energy Materials and Solar Cells 93, 1865–1874. Liu, H.C., Chuang, C.P., Chen, Y.T., Du, C.H., 2009. Inkjet Printing for Silicon Solar Cells. Water, Air, & Soil Pollution: Focus 9, 495–498. Mette, A., Richter, P.L., Glunz, S.W., Willeke, G., 2006. Novel metal jet printing technique for the front side metallization of highly gas aerosol ink. In: 21st European Photovoltaic Solar Energy Conference and Exhibition, pp. 1174–1177. Mette, A., Richter, P.L., Horteis, M., Glunz, S.W., 2007. Metal aerosol jet printing for solar cell metallization. Progress in Photovoltaics: Research and Applications 15, 621–627. Morosanu, C., 1980. The preparation, characterization and applications of silicon nitride thin films. Thin Solid Films 65, 171–208. Payne, A.M., Rapolu, K., Davis, P., Chandrasekaran, V., Meier, D., Xu, B., Zesch, J., Littau, K., 2010. Investigation of laser ablation of Silicon Nitride passivation with self doping paste for solar cell contacts. In: Photovoltaic Specialists Conference PVSC. 35th IEEE, pp. 3577–3581. Perry, M.D., Stuart, B.C., Banks, Feit, M.D., Yanovsky, V., Rubenchik, A.M., 1999. Ultrashort-pulse laser machining of dielectric materials. Journal of Applied Physics 85, 6803. Regenfuß, P., Ebert, R., Klo¨tzer, S., Hartwig, L., Exner, H., Brabant, T., Petsch, T., 2004. . In: Proceedings of the 23rd International Congress on Applications of Lasers and Electro-Optics, pp. 413–424.
P.S. Aakella et al. / Solar Energy 97 (2013) 388–397 Regenfuss, P., Streek, A., Hartwig, L., Klo¨tzer, S., Brabant, T., Horn, M., Ebert, R., Exner, H., 2007. Principles of laser microsintering. Rapid Prototyping Journal 13, 204–212. Sastry, A., Chaudhary, V.A., Solanki, C.S., 2010. Novel front metal contact patterning scheme for C-SI solar cells. In: 35th IEEE Photovoltaic Specialists Conference. IEEE, pp. 003491–003493. Schubert, G., Huster, F., Fath, P., 2006. Physical understanding of printed thick-film front contacts of crystalline Si solar cells: review of existing models and recent developments. Solar Energy Materials and Solar Cells 90, 3399–3406. Shah, V.G., Wallace, D.B., Technologies, M., 2004. Low-cost solar cell fabrication by drop-on-demand ink-jet printing. Symposium A Quarterly Journal In Modern Foreign Literatures 37. Shin, D.Y., 2010. Fabrication of an inkjet-printed seed pattern with silver nanoparticulate ink on a textured silicon solar cell wafer. Journal of Micromechanics and Microengineering 20. Stockum, W., Meijer, A., Doll, O., James, M., Plummer, E., Brookes, P., Patterson, K., May, P., Qu, H., Bock, R., Gatz, S., Terheiden, B., Plagwitz, H., 2008. New Inkjet solution for direct printing of local diffusion barriers on solar cells. In: 23rd EU PVSEC. Spain, pp. 1–5. Teng, K., Vest, R., 1987. Liquid ink jet printing with MOD inks for hybrid microcircuits. IEEE Transactions on Components Hybrids and Manufacturing Technology 10, 545–549.
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Teng, K.F., Vest, R.W., 1988a. Application of Ink jet technology on photovoltaic metallization. IEEE Electron Device Letters 56, 591–592. Teng, K.F., Vest, R.W., 1988b. Metallization of solar cells with ink jet printing and silver metallo-organic inks. IEEE Transactions on Components Hybrids and Manufacturing Technology 11, 291– 297. Vanhest, M.F.A.M., Curtis, C.J., Miedaner, A., Pasquarelli, R.M., Kreuder, John., Hersh, P., Ginley, D.S., 2009. Inkjet printed contacts for use in photovoltaics. In: 34th IEEE Photovoltaic Specialists Conference PVSC, pp. 001736–001738. Wang, A., Zhao, J., Wenham, J S.R., Green, M.A., 1996. 21.5% Efficient thin silicon solar cell. Progress in Photovoltaics: Research and Applications 4, 55–58. Xu, B., Littau, K., Zesch, J., Fork, D., Alto, P., 2009. Front side metallization of crystalline silicon solar cells using selectively laser drilled contact openings. In: 34th IEEE Photovoltaic Specialists Conference, pp. 517–522. Zhao, J., Wang, A., Green, M.A., 2001. High-efficiency PERL and PERT silicon solar cells on FZ and MCZ substrates. Solar Energy Materials and Solar Cells 65, 429–435.
ScienceDirect Solar Energy 97 (2013) 388–397 www.elsevier.com/locate/solener
Pre-metallization processes for c-Si solar cells Peraiah Sastry Aakella a,c,⇑, S. Saravanan c, Suhas S. Joshi b, Chetan Singh Solanki a,c a
Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India b Department of Mechanical Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India c National Centre for Photovoltaic Research and Education, Indian Institute of Technology Bombay, Mumbai 400076, India Received 21 May 2013; received in revised form 4 August 2013; accepted 19 August 2013
Communicated by: Associate Editor Takhir M. Razykov
Abstract Crystalline silicon photovoltaic (PV) technology dominates 85% of the PV share due to its ease process and manufacturing flow. Conventional silicon PV technology uses screen printing for the metallization and though most of the PV industries use this technique, it has its own limitations for fabricating solar cells of high efficiency beyond 19%. However this technique is widely used in industries as a major process because of its production feasibility and cost compared to the alternate metallization techniques. In this article, an attempt has been made to study the different printing mechanisms, alternate to screen printing technique. Also, the process and results of alternate printing mechanisms were interpreted and compared with screen printing technique. It is well understood that the PV industries looking for the cost effective advanced metallization technique for fabricating high efficiency solar cells. Ó 2013 Elsevier Ltd. All rights reserved. Keywords: Pre-metallization; Printing; Chemical etching; Laser ablation; Microsintering; Ink-jet printing
1. Introduction For the past few decades crystalline silicon solar cell researchers actively working towards the different technologies to achieve the high efficiency without investing more capital and targeting the cost of $/Wp or lower than that too. The silicon solar cell manufacturing competences is always trying to increase the productivity without compromising the efficiency and quality of the cell. Available literatures reported that this can be achieved by different directions such as minimizing the use of Si, and use of advanced process technologies. One of the ways to minimize the use of Si is to scale down the wafer thickness from 180 to 100 lm and further to ultra-thin wafers of size ⇑ Corresponding author at: National Centre for Photovoltaic Research and Education, Indian Institute of Technology Bombay, Mumbai 400 076, India. Tel.: +91 22 25767895; fax: +91 22 25767890. E-mail addresses: [email protected], [email protected] (P.S. Aakella).
0038-092X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.08.026
50 lm. Researchers reported that the silicon solar cells were fabricated by starting the process with wafer thickness of 50 lm and results the cell of efficiency 21.5% (Dross et al., 2012; Wang et al., 1996). In a typical solar cell fabrication process, the front silver metallization process has potential to improve the efficiency and reduce cost. This is because silver pastes are continuously being evolved and new paste generation with proper composition of silver particles, glass frit and few additives are made available in the market. These new generation pastes in combination with front grid design and screen design delivers better efficiency at lower paste weight, thus reducing the cost as well. Typically a new generation paste offers better aspect ratios (finger height/finger width) if screen design and printing parameters are carefully chosen. By improving the aspect ratio, current is improved as shadow loss is decreased. Improvement of aspect ratio also decreases in series resistance. Besides paste rheology, improvement of the aspect ratio depends upon
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the screen parameters such as mesh count, wire dimensions, emulsion thickness and also on the printing parameters such as squeeze pressure and speed. Apart from this, the appropriate drying and firing with certain ambient, results the proper metallization. This process is referred to as thick film screen printing (Hilali et al., 2004). As mentioned earlier, the silver paste is composed of Ag powder (70–80%) and glass-frit (used for sintering and contact improvement, of 5%) as major components. Many other solvents for suspension, organic resin and additives are also present, which vary with the changes in the rheology of paste. Glass-frit, while making a contact at elevated temperature about 700 °C reacts with Si surface and forms islands. These islands behave as insulators and reduce the charge flow through them, hence the line conductivity. Huge line width with heavily doped underlying silicon and high contact resistance (2 10 7 X cm2) further reduce the blue response. The fill factor is limited to not more than 77% due to poor metal conductivity of the front contact (Resistivity 3 lX cm) and poor aspect-ratio along with high contact resistance (>3 mX cm2) in a screen printed contact (Schubert et al., 2006). Despite these limitations, screen printing of contacts continues in industry as a major technique till date because of its cost effectiveness, simplicity and high productivity. Typical commercial solar cells achieved efficiency in the range of 18–19% and the schematic of the commercial solar cell is as shown in Fig. 1. Also it is well understood that, there is a significant trade-off between the cell performance and cost-effectiveness in the existing c-Si solar cell technology. Record high efficient cells are produced by more complex photo-lithography (PL) technique (Zhao et al., 2001), which is expensive and adds to the cost if employed in commercial production. The screen printed and other laboratory scale high efficiency solar cells have their own limitations in their application in commercial production. Hence the PV researchers explored alternative metallization techniques to achieve the better performance of solar cells at optimum costs and revealed few advanced metallization schemes which involve an additional step of forming the metal seed layer and plating step on it. A seed layer on the silicon surface should be a very thin line and it should be in a good electrical and mechanical contact with silicon surface. Electro plating of seed layer with metals like silver
Fig. 1. Cross section of c-Si solar cell.
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improves the line conductivity and the performance of the solar cell (Glunz, 2007). Similarly they introduced another technique in which the dielectric layer (antireflection coatings) will be opened followed by the plating (Knorz et al., 2009). The selective opening of dielectric layer and the development of metal seed layer are referred to as pre-metallization (PM) techniques. This article reviews the different types of PM techniques and the subsequent processes. Also, this explains the advantages and disadvantages of different PM techniques. Typical silicon solar cell fabrication process with pre-metallization is as shown in Fig. 2. There are two individual steps involved in the PM techniques by which a great control can be achieved on finger dimensions. The curing temperature for the plating contact is lower than the conventional metallization. Hence, it is important to understand these methods along with their limitations and capabilities which will help to improve their performance. Therefore, the objective of this paper is to elucidate various PM techniques followed by their comparative evaluation with their capabilities, parameters and the ways to control them.
2. Classification of Pre-metallization techniques (PM) Fig. 2 shows the solar cell fabrication process flow with pre-metallization technique. It is important to mention here that PM begins once the antireflection (AR) coatings are done on the diffused wafer. In this article the process flow shown in Fig. 2 is common to all the PM techniques unless otherwise specifically mentioned. Route-A: Patterning of dielectric layer. Route-B: Developing metal seed layers. Schematic of the conventional c-Si solar cell and cells fabricated based on the two different PM routes are shown in Fig. 3. Route-A refers the opening of dielectric layers
Fig. 2. Process flow of standard solar cell and pre-metallization preparation.
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etching the dielectrics. In this method, a screen with the required openings is kept in contact with the substrate (solar cell) and then semi-liquid solution (etching paste) will be transferred to the substrate. The cell is then heated to 350 °C around 90 s for chemical etching. Ultrasonic potassium hydroxide (KOH) bath is necessary for removing the chemical residue on the surface of the solar cell. The screen printing of etch paste involves various steps as shown in Fig. 5. Metallization using silver paste with different compositions on the front side is performed, once the dielectric layer is opened in the required grid pattern. With different mesh counts, openings of 85 lm and 50 lm are obtained on both textured and non-textured surfaces respectively (Bahr et al., 2007). Fig. 3. Patterning dielectric layers (Route A) and developing metal seed layer (Route B).
selectively. Since AR is dielectric, it can be opened by either wet chemical etching or selective laser ablation. The wet chemical etching for etching the AR layer can be carried out by different ways and they are screen printing, ink-jet printing, laser grooving and mechanical structure techniques. Route-B discusses developing the metal seed layer which can be done by laser microsintering and printing techniques, namely, pad printing, ink-jet printing and aerosol-jet printing. The classification of PM and its sub-classification are depicted in Fig. 4. The principle and operation of each and every process is explained in detail in their respective sections (see Fig. 4). 2.1. Route-A: Patterning of dielectric layers In this process, AR coating is selectively opened in a required grid pattern. The silicon nitride (Si3N4) is a hard ceramic with inherent properties such as good insulation, good passivation, low thermal expansion coefficient, moderate thermal conductivity and high strength over a broad temperature range, which also serves as an AR coatings for c-Si solar cells (Morosanu, 1980). Based on these properties, silicon nitride is preferred widely as AR coatings for the solar cells. The patterning techniques of dielectric layers has been carried out either by chemical etching or by selective laser ablation method and are described below.
2.1.1.2. Ink-jet printing. Along with screen printing, now-adays ink-jet printing also plays a role in photovoltaic applications. In this process chemical based solution (etch paste) is used on the substrate to open/etch the dielectric layers and this is described elsewhere (Lennon et al., 2008, 2009). Acidic water soluble polymer on the dielectric layer is subjected to ink-jet deposition of a solution which contains fluoride ions. A reaction that takes place between the etch paste and the polymer etches the dielectric layer. Chemical residues formed after the etching process is removed using water. This method has been used for SiO2 and SiNX structure dielectrics. Using this process, 390 nm thick thermally grown SiO2 layer holes are etched. The depth and volume of etched holes depend on the temperature at which inkjet process is carried out. The thickness of etched holes ranges from 270–390 nm. Continuous grooves of 70 lm width are obtained with this method and the minimum width is of 25 lm. The direct ink-jet printing process is schematically shown in Fig. 6.
2.1.1. Chemical etching The dielectric layer, SiNX of a solar cell can be etched chemically by acids such as hydrofluoric acid (HF) and sulfuric acid (H3PO4). The etching depends on the nature of chemical, their viscosity and the method of application. The process is done using different techniques such as screen printing, ink-jet printing and laser engraving.
2.1.1.3. Laser engraving. This process is enabled by covering the solar cells on both the sides with polymer sheet and grooving it using a laser in a required grid pattern. The process of polymer masking and the laser engraving is shown in Fig. 7. Polymer masked diffused wafers are required to groove by laser without affecting its AR coatings. A suitable laser for this kind of application is CO2 laser with 10 lm wavelength. The cells engraved by the laser are kept in 30% HF acid bath for 3 min. Hence etching is happened in the laser grooved areas and the remaining areas are unaffected because the polymer acts as a mask between the acid and AR coatings of solar cell. Further, the cells are processed with Ni-Cu metallization. With this method, 160 lm finger widths of 4 4 cm solar cells were processed and subsequently Ni–Cu plating has been carried out (Chaudhari and Solanki, 2010).
2.1.1.1. Screen printing. Screen printing is the most widely used technique in photovoltaic solar cell process for both front contact metallization and back surface field. Patterning by screen printing technique uses phosphoric acid for
2.1.1.4. Mechanical structure. In this patterning method, etching paste is used for PM through mechanical structure technique. The mechanical structure is usually micromachined in finger-grid pattern, associated with the heating
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Fig. 4. Classification of pre-metallization (PM) techniques.
patterned solar cell obtained with this process is illustrated in Fig. 8. The finger width of 150 lm has been reported using this method (Sastry et al., 2010). As mentioned earlier, along with chemical etching methods, the patterning is demonstrated using selective laser ablation.
Fig. 5. Patterning dielectric layers by screen print etching paste.
elements as shown in Fig. 8(a). The etching paste is applied on mechanical structure and is kept in contact with solar cell. The paste gets transferred to the solar cell in the required grid form. The paste is thermally sensitive and requires heating for activation. Using heating elements on mechanical structure, the paste printed on the solar cells are heated to temperature 390 °C for 90 s. The steps involved in mechanical structure and heating is schematically shown in Fig. 8(b). The heating step etches the AR coating of the solar cell but leaves chemical residues, which are removed with DI water and the ultrasonic bath. The
2.1.2. Selective laser ablation Chemical etching methods use chemicals followed by firing for patterning the AR coatings of the solar cells. These steps are not required in laser ablation method. The solar cell patterning by this method was demonstrated for the first time by Dube and Gonsiorawski (1990). The application of laser ablation process has been widely used and reported by various groups (Engelhart et al., 2006, 2007; Xu et al., 2009; Correia et al., 2007; Payne et al., 2010; Perry et al., 1999; Knorz et al., 2009, 2007 and Lennon et al., 2009). In this method, the laser source should be chosen appropriately in such a way that the AR coatings are grooved by laser source but should not affect the other layers beneath the solar cell. To pattern the solar cells with SiNX as AR coatings, a low refractive index laser source is chosen for grooving and hence the silicon layer underlying the AR coatings is not much affected. Most of the ablation processes use laser of 355 nm wavelength within a nanosecond pulse on- time regime (Grohe et al., 2006). The ultra-short laser pulses interacting with dielectric layer on the solar cell result in a selective and precise ablation. The laser ablation process on non-textured and textured surfaces was done by Knorz et al. (2007, 2009). As compared to planar surfaces, minimal damage is observed on textured surfaces. Using a 355 nm laser source, the dielectric layer is removed selectively and metallization is done separately. The highest fill factor (FF) of 78.6% is achieved using selective laser ablation method on textured surface with efficiency of 19.1% (Knorz et al., 2007). Various groups demonstrated the metallization on the rear side of the solar cell which
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Fig. 6. Direct patterning by ink jet printing (Lennon et al., 2009).
Fig. 7. Processing of polymer masking method to open dielectric layers.
Fig. 8. Processing dielectric layer patterning by mechanical structuring using etching paste.
include the processes like mechanical scribing, point contact solar cells that are not dealt in this paper as the focus of this article is restricted to the front side metallization techniques (Lee, 2009; Emmanuel and Geaucarne, 2006).
various printing techniques which involve penetration of metal through AR coatings and contacting with Si surface. Major processes presented in this section include laser microsintering and printing techniques for developing metal seed layer and is shown in Fig. 3.
2.2. Route-B: Developing metal seed layer The metal seed layer is developed by either printing or laser microsintering technique. The seed layer is then, thickened by plating. The metal seed layer is printed using
2.2.1. Laser microsintering Laser microsintering is a rapid prototyping technique and its applications are in metals/ceramic functional parts with high precision (Regenfuss et al., 2007; Kumar, 2003;
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Kathuria, 1999; Regenfuß et al., 2004; Exner et al., 2003). Application of this technique for solar cell fabrication is reported by Alemn et al. (2006). This method uses a laser source and fine metal powder of micron size. The metal powder is spread on dielectric layer of a solar cell in a finger-grid pattern as shown in Fig. 9(b). Laser source is guided over the metal powder surface by continuous or short pulses which cause melting and adhesion to the Si surface which is beneath AR coatings of solar cell (Fig. 9(c)). Melting causes immediate expansion and creates non equilibrium pressure conditions. This results in condensation and predetermined structure of metal seed layer as shown in Fig. 9(d). Nd-YAG laser with 1064 nm wavelength is chosen as laser source and metals like silver and tungsten are commonly used. The metallization step is processed using the light induced plating. Solar cells processed with laser microsintering are of 15 lm finger widths. With this minimal width, a great control over aspect ratio and reduction in series resistance is achieved which leads to a pseudo fill factor of 78.5%. After light induced plating, laser sintering resulted the cell of efficiency 14.5% (Alemn et al., 2006). 2.2.2. Printing techniques Pad printing, screen printing, ink-jet printing, aerosol-jet printing are the prominent techniques derived from the conventional screen printing technique by which the seed layer can be printed on the solar cells. These methods are extensively used in laboratory scale for the solar cell fabrication. A layer of metal in the form of a powder or solvent is to be deposited by pressurized squeegee wiping on the required pattern and this resulting in print lines with a great control of finger width. The process itself fast and has an added advantage for processing of metallization by plating with suitable metals as compared to the chemical etching methods.
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2.2.2.1. Pad printing. In pad printing, an ink is transferred from print plate to the required surface. Applications of pad printing are in metals, ceramics and automotive. The advantages of pad printing are indirect printing, economic and high throughput production and ability to print on irregular surfaces. Pad printing on solar cell applications is proposed by Bottari et al. (1992). Major components of a pad printing machine are print plate, doctor blade, ink spreader and silicon rubber transfer pad. The etching process of pad printing begins with flooding of ink by reciprocating motion of the printer. Silicon pad gets in contact with the ink which is being imprinted on a solar cell surface by rolling over it. The additional amount of ink is removed using a doctor blade. The printed area is heated to 350–400 °C. At different stages of heating, organics are burnt out and required metal is printed on the substrate. Fast contact firing process results the metal seed layer formation. General sequence of pad printing technique is shown in Fig. 10. Preferably, light induced plating is performed on the seed layer developed by pad printing. Pad printing yields the cell with finger width of 45 lm on 12.5 12.5 cm area cells (Huljic et al., 2002). The paste rheology, viscosity and curvature of pad with hardness are required for pad printing. Rotating pad, known as rotary gravure offset printing and transfer pad (Krebs, 2009) for polymer cell applications are the types of pad printing process. The reported best cell parameters using the pad printing are VOC – 590 mV, JSC – 29.2 mA/cm2, FF – 77.7%, g – 13.4% (Hahne, 2001). 2.2.2.2. Ink-jet printing. Ink-jet printing is a well established printing technology and is used in solar cell fabrication because of the low cost and fine line resolution. Good control over the flow gives very narrow printing on the surface without contacting the surface to be printed. Ink-jet printers use a piezoelectric crystal in nozzle and by applying the
Fig. 9. Laser microsintering process using Nd-YAG laser.
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Fig. 10. Schematic of pad printing sequence (acc-silicones, 2013).
Fig. 11. Schematic of ink jet printing sequence (Teng and Vest, 1988b).
Fig. 12. Schematic of aerosol jet printing.
current to the crystal; it changes its shape or size thereby forcing an ink droplet from the nozzle. All the nozzles include piezoelectric driver, which generates pressure to eject liquid ink. Substrate is kept on programmable XY stage upon which a finger grid is to be printed. Schematic of the ink-jet printing used for solar cell fabrication is shown in Fig. 11 (Teng and Vest, 1988b). The ink-jet printers allow a wider variety of inks such as metal organic deposition (MOD) silver and silver nanoparticulate inks. Different versions of inks for ink-jet metallization are explained in the literature (Vanhest et al., 2009; Shin, 2010; Kaydanova et al., 2003; Teng and Vest, 1987, 1988a; Stockum et al., 2008; Shah et al., 2004; Liu et al., 2009; Curtis et al., 2006 and Gizachew et al., 2011).
It is important to mention here that ink-jet printing is also used for the patterning of dielectric layers (Lennon et al., 2008, 2009). This method is cost-effective for industrial process. Since ink-jet printing as non-contact printing, solar cells from this technique achieves good aspect ratios. Optimized temperature and time period result the fine grid lines. Recent advancements show the potential usage of ink-jet printing in developing a metal seed layer for twostep metallization on the front side of a solar cell. Finger width of 1.81 lm and height of 58.8 lm was achieved with ink-jet printing by using silver nanoparticulate ink (Shin, 2010). Nevertheless, ink-jet printing with metal organic deposition silver inks could not succeed in achieving the better performance of solar cells because of solid content
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395
Table 1 Comparison of various pre-metallization techniques. Route
Technique
VOC (mV)
JSC (mA/cm2)
FF (%)
g (%)
Height (lm)
Width (lm)
Aspect ratio
Ref.
A
Laser engraving
590
48.9
72.3
13.2
–
160
–
Screen printing Selective laser ablation
605 639
– 38
76 78.6
16 19.10
– –
85–120 –
– –
Chaudhari and Solanki (2010) Bahr et al. (2007) Knorz et al. (2007)
Ink jet printing Laser microsintering (silver) Laser microsintering (tungsten) Pad printing Metal aerosol-jet printing
588 652.7 622 591 624
28.60 37.33 34.12 29.2 36.1
72.66 54.10 73.6 77.7 81
12.22 13.2 14 13.4 18.3
3 – – 1.5–2.5 20
135 40 40 45–140 80
0.02 0.45 0.45 0.01 0.25
Han et al. (2009) Alemn et al. (2006) Alemn et al. (2006) Hahne (2001) Horteis et al. (2007)
B
in the ink and the efficiency of 8 % has been achieved without AR coating (Teng and Vest, 1988b). 2.2.2.3. Aerosol-jet printing. Seed layer metallization by non-contact ink-jet printing is already reported (Teng and Vest, 1988b). Major limitations of ink-jet printing systems are mass production, outlet of nozzle width which is wider than printed line, usage of nano and metal organic inks which are costly (Mette et al., 2006). With some modifications in ink-jet printing, aerosol-jet printing has been developed for solar cell metallization (Mette et al., 2007; Horteis et al., 2007; Glunz, 2007; Drew et al., 2011; Glunz and Horteis, 2008). Working of an aerosol-jet printing is explained by Mette et al. (2007). The printing system (also known as metal aerosol-jet printing) has a deposition head and an atomizer. Metal containing ink becomes aerosol under stream control. Aerosol is transported to the deposition head via heated tube so that preheating of aerosol is possible. The deposition head contains sheath gas, which surrounds the aerosol in suspension and deposits it through tip onto the surface. A shutter is used to interrupt continuous aerosol-jet stream of ink. In the atomizer, gas surrounds the aerosol stream. Through atomizer nozzle, compressed gas enters with high velocities. The ink is drawn off the reservoir by venturi principle and expansion of the compressor gas results in breaking the stream into droplets, which are suspended in the gas flow. Gas droplets impinge against side walls of the reservoir. Due to many collisions, bigger droplets are reduced in size. The smaller droplets remain in the gas and are transported to deposited head. The aerosol is transported to the deposition head which keeps aerosol in suspension without contacting the nozzle with the help of focusing gas. Therefore, great control over printing width on the substrate is achieved. Print head with an atomizer gas is shown in Fig. 12. Good aspect ratio is achieved with this kind of printing. Finger width of 14 lm with nozzle outer diameter of 100 lm has been achieved. The next plating step involves light induced plating. The best cell parameters with aerosol printing are VOC – 624 mV, FF – 88%, g – 18.3% with an average aspect ratio 0.25.
3. Comparison of various pre-metallization techniques A comparison of various pre-metallization techniques is presented in Table. 1. The electrical parameters and specific characteristics such as finger width and height of crystalline silicon solar cells fabricated by employing various pre-metallization techniques are listed in Table 1. From table, it has been observed that both Route A and Route B had shown the best performance. However in Route A, selective laser ablation had shown the significant role in electrical performance compared to other techniques. Similarly in Route B, metal aerosol jet printing shown the better electrical performance compared to other techniques. It is important to mention here that most of the PM techniques evaluated are in the laboratory level pilot fabrication and are yet to be commercialized. 4. Conclusions and future directions The article discusses classification of patterning techniques prior to the metallization process of solar cell. Broadly, the classification is made into two groups: patterning of dielectric layers and developing metal seed layers. Screen print metallization of silver is widely used in industries for processing of c-Si solar cells. However in screen print metallization technique, the amount of silver used is gradually reduced and the reason behind this may be the high cost of silver and the researchers looking for an alternate techniques. Advanced metallization processes alternate to the conventional screen printing used for fabricating silicon solar cell are explained in this article. The Ni–Cu metallization discussed in patterning of dielectric layers has a great potential in fabrication level due to the high conductivity nature and the low cost of Cu. Plating of Cu in Ni–Cu metallization is in the pure form and one can expect a good conductivity than a screen printed Ag contact. Similarly light induced plating for metallization plays the significant role in developing metal seed layers and the amount of Silver used for plating is less. Even though the solar cell process is having well developed different metallization and PM techniques, development of
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