Journal of the Korean Physical Society, Vol. 65, No. 5, September 2014, pp. 739∼744
Copper Conducting Electrode with Nickel as a Seed Layer for Selective Emitter Crystalline Silicon Solar Cells Atteq ur Rehman, Eun Gu Shin and Soo Hong Lee∗ Green Strategic Energy Research Institute, Department of Electronic Engineering, Sejong University, Seoul 143-747, Korea (Received 12 May 2014, in final form 24 June 2014) In this research, we investigated selective emitter formation with a single-step photolithography process having a metallization scheme composed of nickel/copper metal stacks. The nickel seed layers were deposited by applying the electroless deposition process while copper was formed by light induced electro-plating arrangements as the main conducting electrode. The electroless deposition of nickel, along with a sintering process, was employed to create a diffusion barrier between copper and silicon. The nickel metal stack below the copper-conducting electrode also helped in lowering the sheet resistance and improving the contact adhesion. The nickel used as a seed layer was successfully demonstrated in the fabrication of a homogeneous 60 Ω/ emitter and selective emitter cells. Lower series resistances of 0.165 Ω and 0.253 Ω were achieved for the selective emitter and the homogeneous emitter cells, respectively. The best cell efficiency of 18.37% for the selective emitter solar cell was achieved, with average cell efficiencies of 18.17% and 17.3% for the selective emitter and the homogeneous emitter cells, respectively. An approximate efficiency increase of about 0.8% was recorded for the selective emitter solar cells. PACS numbers: 84.60.Jt Keywords: Selective emitter, Nickel/copper, Electrode, Light-induced plating, Sintering, Solar cell DOI: 10.3938/jkps.65.739
I. INTRODUCTION
One promising approach for metallizing the front contact grid is the use of the seed and plate method [4]. The seed and plate method not only may increase the cell efficiency but also could help decrease the production cost by using copper as an alternate to silver. Relatively cheaper copper has a conductivity comparable to that of silver [5]. However, although copper has advantage of lower material cost, it has a major drawback of creating highly-active recombination defects in silicon as it has higher diffusivity in silicon [6,7]. Therefore a diffusion barrier must be built between the copper and the silicon to prevent copper diffusion into silicon. Thus far, nickel has been shown to make a great contribution as a seed layer for copper as it creates an effective barrier and offers lower sheet resistance when sintered at specific temperatures [8–11]. It has already been reported that for various temperature ranges nickel is known to form phases such as Ni2 Si (200 ∼ 300 ◦ C), NiSi (300 ∼ 700 ◦ C) and NiSi2 (700 ∼ 900 ◦ C). [12,13]. The NiSi offers a resistivity as low as about 14 μΩ·cm. [14]. Contact formation using the nickel/copper metallization scheme can be realized by adopting the seed and plate approach, in which nickel is first plated as a seed layer, followed by a copper plating process. Methods such as electroless plating, laser-assisted plating and light-induced plating can be used to form a nickel seed layer. However, the copper-plating process is usually the light-induced plat-
One of the major efficiency-limiting and cost-defining steps in solar-cell manufacturing is contact formation [1]. Silver paste contacts, which are normally fired over SiNx :H antireflection coatings (ARC), are the preferred front-side contacts for silicon solar-cell production in the industry. However, these contacts, which are usually realized by using screen printing technology, pose certain limitations. The major ones are higher contact resistance and lower aspect ratios that result in increased shading losses. Other than the silicon wafer, solar cell metallization is the most important cost element in industrial production. A report from Fraunhofer Institute for Solar Energy-Systems (ISE) suggests that the front- and the rear-side metal accounts for around 40% of the cell’s production costs [2]. The higher cost is due to the use of expensive pastes for contacts, predominantly silver pastes for the front contact grids. According to the standards set out by the international technology roadmap for photovoltaics (ITRPV), the use of a lower portion of silver paste with superior efficiencies is required [3]. Solar-cell modules having lower production costs and higher efficiencies are strongly in demand to reach competitiveness with fossil energy sources. ∗ E-mail:
[email protected]; Fax: +82-2-3408-4157
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Table 1. Reactions occurring at the copper metal (anode), in the electrolyte solution (H2 SO4 /CuSO4 ), and at the substrate (cathode). Reaction taking place at Cu (anode) Electrolyte solution Substrate (cathode)
Reactions Cu + 2e+ → Cu2+ CuSO4 → Cu2+ + SO2− 4 H2 SO4 → 2H+ + SO2− 4 Cu2+ + 2e− → Cu
ing (LIP) process developed at Fraunhofer ISE [15]. The LIP process has the advantage of having the ability of plating the metal contact at higher growth rates even at room temperature [16]. The diffusion of the emitter junction is a very important step in solar-cell manufacture, and it plays a critical role in the conversion efficiency of a solar cell. The types of emitters currently in practice for both laboratory and industry are i. Conventional emitters (homogeneous doping over whole surface area), ii. Selective emitters (heavily- and lightly-doped area underneath the front contacts and between the fingers, respectively) The application of selective doping at the front contact area provides an opportunity to form ohmic contacts and results in a higher blue response [17]. Therefore, selective-emitter solar cells demonstrate a higher open-circuit voltage (VOC ), improved short-circuit current (ISC ) and superior conversion efficiency. In order to investigate the higher efficiency potential offered by selective-emitter solar cells having nickel/copper contacts, we fabricated PESC (passivated emitter solar-cell) structures for both selective and homogeneous emitter cells. A single photolithography step was applied for patterning the front contacts as well as for a heavy diffusion step for selective-emitter solar cells. For the formation of conventional selective emitter, a number of process steps are required (e.g., diffusion mask and front electrode-patterning steps, etching steps). The use of a single photolithography step reduces the number of process sequence steps and helps to make the process easier by avoiding the issues of alignment during the front-electrode’s formation. Moreover, the overall cost of the fabricated cells can be reduced by eliminating the processes of diffusion-mask etching, passivation-layer formation and front-contact patterning. The nickel as a seed layer was successfully implemented in the fabrication of homogeneous, 60-Ω/ emitter and selective emitter cells. The best cell efficiency achieved for the selective-emitter solar cell was 18.37%, with average cell efficiencies of 18.17% and 17.3% for the
Fig. 1. (Color online) Process steps for the selective emitter on the left and the reference cell with a homogeneous emitter on the right.
selective emitter and the homogeneous emitter cells, respectively.
II. EXPERIMENTS P-type monocrystalline czochralski (CZ) Si wafers having a resistivity of 1.5 Ω.cm and a thickness of 180 − 220 μ·m were used to prepare the samples. The wafers were initially cleaned with admixed solutions of NH4 OH, H2 O2 and deionized (D.I.) water in order to remove organic contaminants. The surfaces of the wafers were textured using an aqueous alkaline solution of tetramethyl ammonium hydroxide (TMAH) and isopropyl alcohol (IPA), followed by an RCA-2 standard cleaning step to eliminate metallic impurities. The RCA-2 cleaning was performed with an acidic bath containing solutions of HCl, H2 O2 and D.I. water. A shallow doping of about 80 Ω/sq on the wafer surface was done by diffusing the samples in a POCl3 (phosphoryl chloride) tube furnace. The removals of the phospho-silicate glass (PSG) layer after diffusion was done by immersing the wafers in BOE 7 : 1 (buffer oxide etchant). A relatively thick SiO2 layer was grown on the wafer surface by using a tube furnace and was used as a mask layer for heavy diffusion process; It also acts as an antireflection coating (ARC). The front
Copper Conducting Electrode with Nickel as a Seed Layer· · · – Atteq ur Rehman et al.
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Table 2. I-V data for 2 × 2 cm2 cells fabricated using the electroless deposition method to form a nickel seed layer and a copper electrode by using light induced plating. A total of 10 cells were fabricated for the selective emitter and the homogeneous emitter cells. Selective emitter Homogeneous emitter
ISC (mA) 155 ± 2 152 ± 2
VOC (mV) 628 ± 2 604 ± 12
F.F (%) 73.5 ± 0.4 73.4 ± 0.4
Efficiency (%) 18.17 ± 0.2 17.3 ± 0.2
RS (Ω) 0.21 ± 0.04 0.26 ± 0.04
Fig. 3. (Color online) Schematic diagram of the LIP-based copper electroplating process.
Fig. 2. (Color online) Schematic structures of the PESC solar cell with (a) a homogeneous-emitter and (b) a selectiveemitter. Metallization based on Ni/Cu plating techniques was used to form the front contacts for both types.
surface grid was patterned by using a photolithography process, and the unprotected ARC was etched by using a diluted HF solution. A heavy diffusion step of 40 Ω/sq in a POCl3 tube furnace was done for selective doping at the front surface. The PSG and the rear-side oxide were etched in diluted HF solution in subsequent step. The back contact was made by screen printing Al paste on the rear side, and back surface field (BSF) was formed by using a firing step at a peak temperature of 850 ◦ C. The process steps and structures for the fabricated solar cells are shown in Figs. 1. and 2, respectively. The front contacts were formed by using a two-step (seed and plate) process. In the first step, the Ni as a seed layer was deposited by using an electroless deposition process. The immersion plating bath contained solutions of nickel chloride (N iCl2 · 6H2 O) as main the source of Ni, sodium hypophosphite (N aH2 P O2 ·H2 O) as a reduc-
ing agent, and triammonium citrate [(N H4 )3 C6 H5 O7 ] as a buffer or complex agent. The pH of the solution was maintained around 8.5 by adding ammonium hydroxide (N H4 OH), as a uniform Ni layer can be deposited by preserving the bath at higher PH values [18]. The details of the Ni seed layer’s formation by using electroless deposition can be found in our previous article [19]. Prior to the Ni seed layer’s formation, the front surface was dipped in a 2% diluted HF solution for 30 seconds in order to remove any native oxide at the contacts. The plating was performed at a temperature of 82 ◦ C for a duration of 5 minutes. A typical Ni sintering step at 370 ◦ C for 10 minutes was performed in a subsequent step to form Ni silicide. This sintering step resulted in a reduction of the metal silicon contact resistance [8]. Finally, the Ni silicide at the front surface was thickened by using a light-induced plating (LIP) process to electroplate the Cu. The inclusion of a light source with the electroplated bath can help to achieve higher plating rates even at room temperature [16]. The experimental arrangements for Cu plating by using the LIP process is shown in Fig. 3. The main additives of the plating bath are cupric sulfate (CuSO4 · 5H2 O) and sulfuric acid (H2 SO4 ). The samples were immersed in the illuminated electroplating solution at room temperature. A Cu anode connected to the positive electrode was placed into the plating bath, and the sample (solar-cell) was connected to the negative electrode of the battery. Here, the cupric sulfate helps to supply copper ions (Cu2+ ),
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Fig. 4. (Color online) SEM image of a nickel layer plated at the front contact grid formed by a photolithography process, 5 minutes plating duration, and sintered at a temperature of 370 ◦ C for 10 minutes.
Fig. 5. (Color online) SEM image of the copper electrode deposited above the nickel seed layer formed with electroless deposition, after light-induced plating.
and the sulfuric acid promotes current induction at lower voltage to increase the conductivity [20]. The Cu plating experiment was performed for a duration of 10 minutes with a 250 mA current. Table 1 summarizes the reactions taking place at each electrode and in the electrolyte solutions during the Cu electroplating process.
III. RESULTS AND DISCUSSION Figure 4 shows a SEM (scanning electron microscope) image of the Ni seed layer formed at the silicon surface by using the electroless plating process. The Ni was sintered at 370 ◦ C for 10 minutes in order to form Ni silicide, which resulted in a lowering of the sheet resistance [21]. The Ni was deposited only on the exposed silicon surface and not on the ARC-covered area as the electroless plating process works only on catalytically-active surfaces. The contact area was fully covered by Ni for a plating duration of 5 minutes, which assured the formation of
Fig. 6. (Color online) Series resistance (RS ) for selective emitter cells against reference (homogeneous emitter) cells.
an effective barrier as the requirement for a Ni barrier is an appropriate thickness and uniformity. The SEM image for Cu, the main conducting electrode electroplated above the Ni seed layer by using the LIP process is shown in Fig. 5. A solar simulator was used to analyze the solar cells, and the recorded current/voltage (I-V) data for both types are presented in Table 2. The peak efficiencies gained from the selective emitter and the homogeneous emitter solar cells were 18.37% and 17.51% respectively. The VOC of the selective emitter cells is about 15 mV higher. The mean efficiency of the selective emitter cells is about 0.8% higher than the reference homogeneous emitter cells. The increases in VOC and ISC are responsible for this enhancement. The fill factor (F.F.) for the selective emitter cells is slightly higher than it is for the reference cells due to the lower series resistance (RS ) estimated from the I-V measurements. Lower RS values of about 0.165 Ω and 0.253 Ω were noted for the selectiveemitter and the homogeneous-emitter (reference) solar cells, respectively. The RS values for the selective emitter cells plotted against the reference cells are shown in Fig. 6. The lower RS values for both types are mainly attributed to the formation of the Ni silicide layer, which has a lower sheet resistance. However, the heavily-doped Si surface beneath the metal contact area further lowers the RS value for selective-emitter solar cells and, hence, increases cell efficiency. The electrical parameters (ISC , VOC , FF and η) for the selective emitter and reference solar cells are plotted against each other in Fig. 7. The light I-V curves for the best cells of both types are shown in Fig. 8.
IV. CONCLUSION The selective emitter formed with single-step patterning using nickel/copper-plated contacts offers a simple
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Fig. 7. (Color online) Electrical parameters (ISC , VOC , FF, and η) for selective emitters cells against reference (homogeneous emitter) cells.
tion and copper electroplating has the potential to be implemented for fabricating Si solar cells with either selective emitters or shallow junctions. The nickel silicide offers a very low resistance after the sintering process, as verified by the low series resistance of the fabricated cells. The low series resistance enabled lightly-doped homogeneous emitter cells with efficiencies exceeding 17.5% to be fabricated. For the cell with selective emitter doping, an efficiency of 18.37% was reached. An approximate efficiency increase of about 0.8% was recorded for selective emitter solar cells. An emphasis on low resistance and adhesive nickelsilicide formation will be given in the future. In this study, the cells metallized with nickel/copper contacts showed good adhesion with silicon; however, quantification in the area of both adhesion strength and contact resistance is required in the future. Fig. 8. (Color online) IV curves of 18.37% efficient selective emitter solar cell and a 17.51% efficient homogeneous emitter cell. For the SE solar cell, JSC: 39.25 mA, VOC: 630 mV, FF: 73.90%, and Rs: 0.165 Ω; homogeneous emitter solar cell, JSC: 38.5 mA, VOC: 616 mV, FF: 73.80%, and RS 0.253 Ω.
and inexpensive method to improve cell efficiency. The formation of the nickel seed layer by electroless deposi-
ACKNOWLEDGMENTS This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Plating (KETEP) who have granted financial support from the Ministry
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of Trade, Industry & Energy, Republic of Korea (No. 20133010011780).
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