F ormation of Very Low Resistance Contact for Silicon Photovoltaic Cells Baomin Xu, Scott Limb, Alexandra Rodkin, Eric Shrader, and Sean Gamer Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304, USA Abstract - A number of approaches have been developed in order to introduce a nickel-based contact layer between the silver electrode and n+ emitter layer, which can substantially reduce the specific contact resistance. One of them is to use a blanket sputtered nickel film as the contact layer and screen printed silver lines as an etch mask to pattern the underlying nickel film. This approach ensures the use of high quality nickel film as a contact layer to reduce the specific contact resistance, and also avoids the use of standard photolithographic process to reduce the cost. The result shows the specific contact resistance with this approach can be reduced by about two orders of magnitude compared to only using screen printed silver grid lines. The second approach is to use inkjet printed nickel nanoparticle inks instead of the sputtered nickel film to form the contact layer, enabling a very low cost inline process that can be easily implemented into current solar cell production line. The PCID modeling result shows that the absolute efficiency of solar cells can be increased by up to 0.9% with the substantial reduction on contact resistance. Index Terms - silicon, photovoltaic cells, contact resistance, metallization, nickel silicide. I. INTRODUCTION Front side metallization is one of the most critical processes steps for conventional crystalline silicon photovoltaic cells. Currently screen printing silver paste and firing through the silicon nitride antireflection coating layer is the most widely used method for front side contact formation and metallization, but it also produces a poor, very resistive metal-silicon contact [1]. Generally, the specific contact resistance of the fired-through silver lines is in the range of 3 to 10 mohm'cm 2, which is more than four orders of magnitude higher than that used in semiconductor industry (at the order of 10-7 mohm cm 2 ) [2]. To compensate this large specific contact resistance formed by the fired-through silver lines, large contact area and emitter layer with low sheet resistance have to be used, which decreases the cell efficiency. Forming a silicide metal contact layer (e.g., NiTi) has been proved to be a robust approach to achieve the metal-silicon contact with very low specific contact resistance in semiconductor microelectronics industry [3]. The general procedure is to sputter a metal film such as nickel, followed by a high temperature annealing to react with the silicon substrate and form the metal silicide contact. However, the standard photolithographic process used to pattern the sputtered metal film makes it too expensive for solar cell fabri cati on. In this paper we present a number of new approaches to implement a silicide-forming nickel contact layer for silicon solar cells to reduce the specific contact resistance, but without using the complex process steps in microelectronics such as photolithography in order to meet the low cost requirement for solar cell industry. II. CONTACT LAYER FORMATION AND METALLIZATION The first approach is to use a sputtered nickel film to form the contact layer, but in order to avoid the use of complex photolithographic process, we have found the screen printed silver lines can be used as a protection mask to pattern the blanket sputtered nickel film, and the nickel film can be cofired with silver gridlines. The basic procedure is shown in Fig. l. Starting with a solar cell silicon substrate (a) and after drilling the contact holes through the nitride layer using selective laser ablation method (b), a thin nickel film is sputtered on the whole surface (c), followed by screen printing silver gridlines which are aligned with the contact holes (d). Next the uncovered nickel film is etched away using the silver gridlines as a protective mask (e). Finally the silver gridlines and the underlying nickel contact layer are cofired at high temperatures to form the nickel silicide contact and to finish the metallization, as shown in Fig. let)..--sinx (a)! Si r'n+ emiter,- contct (b) openmg F Sputter Ni (c), film (d) r :J S,ren print f - - - Ag lines - - - - _.----Etch Ni film (e) F 9 - - - - Co-firing (t) F 9 Fig. I. Contact layer formation and metallization procedure using sputtered nickel film 003354
The second approach is to use an inkjet printed Ni ink layer instead of the sputtered nickel film. The steps (a) and (b) are the same as that in the sputtered nickel film process, but in step (c) we will inkjet a layer of nickel nanoparticle ink into to the contact holes as a contact layer, followed by screen printing the silver gridlines. Then the samples will be directly heated to high temperatures to form the nickel silicide contact and to finish the metallization. The flowchart for the second approach is shown in Fig. 2. Comparing to Fig. 1. it can be seen that the inkjet printed approach will reduce one step (the etching step) and can be done in an inline process under conventional atmosphere, and hence should be more attractive for solar cell industry. The challenge is that, if the inkjet printed nickel nanoink layer can form the contact as good as the sputtered nickel film..--sinx (a) Si r'n+ emiter I (b) F (c) F (d),-contct openmg,-inkjeni nanomk :r Sere:" print Ag hnes (e)...,-co-firing Fig. 2. Contact layer formation and metallization procedure using inkjet printed nickel nanoparticle ink In this paper we will mainly focus on forming the nickel silicide contact layer using the procedures given in Figs. 1 and 2, and characterize the specific contact resistance. Based on the contact resistance data we will predict how much cell efficiency improvement can be achieved through PCID modeling. III. RESULTS AND DISCUSSION The silicon wafer is covered by an 80 nm thick PECVD SiNx dielectric layer. Size of the silicon substrate is about 38.Imm (1.5") X 38.Imm (1.5"). To measure the contact resistance using the Transmission Line Method (TLM), first seven lines of contact holes in the nitride layer were drilled using a quadruped Nd:YAG Coherent A VIA laser with 266nm wavelength and 20ns pulse width. Details of the laser drilling method and conditions can be found in an earlier paper [4]. The spacing between the adjacent lines varies from 2.0 mm to 7.0 mm, with the step of l.0 mm. After laser drilling the contact holes, a loo nm-thick nickel film was blanket sputtered, followed by screen printing silver gridlines which were aligned with the lines of contact holes. The silver paste used is Ferro CN33-462. Then the sample was dropped to a FeCI3 solution for a few seconds to etch away the uncovered nickel film, followed a firing at 500 C using a rapid thermal annealer (RTA). After firing the I-V curve between the adjacent AglNi lines (we call it as AglNi line rather than Ag line because there is a Ni layer underneath the Ag line) was measured using the four probe Kelvin method, from which the resistance value was derived, and then the resistance between the adjacent AglNi lines vs. the line spacing was plotted, as shown in Fig. 3. E.c 0.3.. CIl 0.2 J!l.l!l 0.1 0/) 0.4,-----, -- -. - - -+-:- - -.---i-- -.- - --+- - - O +---,---.--.---,--.-- 2 3 4 5 6 7 B Ag/ N i line spacing (mm) Fig. 3. Dependence of measured resistance between adjacent lines on line spacing For our sample the total resistance RT between the AglNi lines is the sum of the contact resistance Rc between the Ni contact layer and the underlying Si plus the silicon substrate resistance RSi between the AglNi lines, and can be expressed as: A. Sputtered nickel film approach In order to simplity the experiment and as a first step to verity the possibility of the new metallization method shown in Fig. 1, we use a polished, heavily As-doped, 0.50 mmthick silicon wafer with the bulk resistivity of about 0.0015 O'cm, hence the whole silicon wafer functionally substitutes for the n+ emitter layer in the solar cells (with 50 OlD sheet resistance and 0.3 to 0.4 11m layer thickness, the emitter layer is equivalent to a bulk resistivity of 0.0015 to 0.002 O cm). Where PSi is the silicon substrate resistivity, d is the line spacing, L is the line length, and t is the substrate thickness. Putting the numbers for each parameter in the equation we found that the silicon substrate resistance RSi is about 0.00770 for the 7mm line spacing. This is about one order of magnitude smaller than the measured resistance RT which is between 0.073 and 0.0870. Hence in our case the measured resistance RT is mainly contributed by the contact resistance 003355
Rc and should be almost not related to the line spacing, which is consistent with the experimental data shown in Fig. 3. Hence the above equation can be simplified to Rr "" 2Rc, from which Rc can be derived. Then considering the contact is through the laser drilled holes the contact area can be calculated, and finally the specific contact resistance can be obtained. Through these calculations we got the specific contact resistance is around 0.04 mohm cm 2. This is about two orders of magnitude smaller than the specific contact resistance formed by the fired-through silver gridlines and the n+ emitter layer, which is generally between 3 to 10 mohm cm 2. In order to confirm all the contacts are through the laser drilled contact holes and the reduction of specific contact resistance is due to the existence of nickel contact layers, three different sample structures were prepared, as shown in Fig. 4. The first one is the normal structure with laser drilled contact holes, sputtered nickel film as the contact layer, and the screen printed silver gridlines as the etching mask and current carrier layer. The second structure has the sputtered nickel film and screen printed silver gridlines, but no laser drilled contact holes. The third structure has the laser drilled contact holes but no sputtered nickel film contact layer. Silver gridlines were directly screen printed on the silicon substrate after the laser hole drilling step. However for the third structure after fired in RTA at 500 C the silver lines did not have any adhesion to the silicon substrate and peeled off right away. This is probably because the normal firing temperature for the Ferro CN33-462 is around SOO C. Nevertheless this does verify the silver gridlines alone cannot form a low resistance contact in the experimental condition we used. iag I I - Fig. 4. Sketch of the three different sample structures Voltage (V) c O. 1 5 - u (b) Voltage (V) Fig. 5. I-V curve between adjacent, 4mm separated AglNi lines for the samples (a): with laser drilled contact holes; and (b) without laser drilled contact holes. The I-V curves between adjacent AglNi lines for the samples with laser drilled contact holes and without laser drilled contact holes were measured and the typical results are given in Fig. 4, with the line spacing of 4mm. The very straight I-V curve for the sample with laser drilled holes indicates an excellent ohmic contact between the AglNi electrode and the silicon substrate. The reciprocity of the slope represents the resistance between these two adjacent metal lines. On the other hand, the complicated shape of the I-V curve for the sample without laser drilled contact holes, and the more than 400 times higher resistance (more than 400 times smaller slope) indicates that it cannot form a good ohmic contact with low contact resistance. B. Inkjet printed nickel ink approach The use of inkjet printed nickel nanoparticle inks to replace the sputtered nickel film will enable a very low cost, fully inline process that can be easily implemented into current solar cell production line. The first step to achieve this is to develop the process for printed nickel ink to achieve the comparable contact resistance improvement as that of the sputtered nickel film. In order to more focus on the contact study, in our initial experiments we annealed the nickel ink layer in RTA at high temperatures (700 to 900 C), then screen printed a low temperature (200 C) cured silver paste onto the nickel layer to enable the four probe I-V curve measurement. In order to compare the samples with consistent preparation conditions, we also prepared the samples that were screen printed with the same silver paste directly onto the laser drilled holes (without the nickel ink contact layer), and the samples with a sputtered nickel film layer (annealed at 500 C to form a silicide contact layer) between the substrate and the screen printed low temperature cured silver paste. The result shows that, the formation of a strong adhesion to the silicon substrate and the reaction to form a nickel silicide low resistance contact are very sensitive to the ink printing, drying, and annealing conditions. In one of the processes (denoted as Process # 1), where the sample was quickly dried at 100 C after printing, in most areas the nickel ink layer was peeled off or just "floated" on the silicon substrate, as shown in Fig. 6(a). After annealed in RTA at 700 to 900 C, only in a small portion of the surface area the nickel layer reacted with the silicon substrate as shown in Fig. 6(b). Then we improved the drying condition after printing (denoted as Process #2) and hence the dried nickel layer can have a good adhesion to the silicon substrate, as given in Fig. 7(a). After annealing in most of the surface area the nickel layer reacted with the silicon substrate and stayed on the contact area, as given in Fig. 7(b). The measured contact resistance results under different conditions were presented in Fig. S. The silver paste alone has very high contact resistance value. When the printed nickel ink layer is introduced, the contact resistance is 003356
reduced due to the fonnation of a low resistive nickel silicide contact. For the sample prepared with Process #1, as the nickel silicide only covers some portion of the contact area, the contact resistance is still much higher than that of sputtered nickel film. With improvement on the process and much more nickel silicide coverage on the contact area, that is, the sample prepared with Process #2, the contact resistance can be sharply reduced and start to be close to that of sputtered nickel film (within the same order of magnitude). It should be pointed out that even with the samples prepared using the drying condition as Process #2, the measured contact resistance value is also very sensitive to the annealing conditions. When the samples annealed at 700 to 800 C in RTA, we only got the contact resistance data close to the samples prepared with Process #l. This means even the nickel layer covers most of the contact layer, it does not react with the silicon substrate to form the silicide contact. The low contact resistance can only be obtained when annealed at 850 to 900 C. In semiconductor microelectronics industry, and in our experiment using sputtered nickel film, the low resistive silicide contact can be fonned in the temperature range at about 400 to 500 C. It needs further study to understand the reason why the nickel nanoparticle ink needs much higher temperature to fonn the low resistive silicide contact. No Ni Ni ink Ni ink Sputtered layer process #1 process #2 Ni Ni contactjayer preparation condition Fig. 8. Measured contact resistance values for differently processed Ni contact layer. The experimental phenomena shown in Figs. 6 to 8 can be simply explained using the model presented in Fig. 9. As only partial of the contact area forms the low resistive silicide contact area, we assume this part is ANiSi, while exact form of the silicide compound needs to be further identified. The specific contact resistance of this part is assigned as PNi. Si, which is assumed it has the same specific contact resistance as the sputtered nickel film, and its contact resistance is RNi P. The remaining part, we denoted as AA g, represents the contact area through silver paste, and/or the area which has nickel but does not form the low resistive silicide contact. Its contact resistance is RA g. The total contact A = ANiSi + AA g ' Fig. 6. SEM morphology of nickel ink layer prepared using Process # I condition, bar = 1 f..lm. (a): after printing and drying; (b): after annealing. Fig. 9. Contact resistance structure model Because the contact resistance through the silver contact area is very large, that is, RA g» RNi P, the total contact resistance RT can be expressed: Fig. 7. SEM morphology of nickel ink layer prepared using Process #2 condition, bar = 1 f..lm. (a): after printing and drying; (b): after annealing. 1 RT 1 1 1 RAg + RNi,p RNi,p Or: 003357
Where RNi,S represents the contact resistance with all the contact area fonned as the low resistive silicide contact, that is the case with sputtered nickel film. From the last equation it can be seen that the key point to reduce the contact resistance is to increase the nickel silicide contact area (ANiSi) which can have the specific contact resistance as low as the sputtered nickel film, which we defined as effective nickel contact area. According to last equation even for the Process #2 the effective area is only about 40% of the total contact area. We believe through further improving the ink and processing conditions the effective contact area can be further increased, or the contact resistance can be further reduced, and be more closer to the sputtered nickel film case. C. Potential cell efficiency improvement The potential improvement on cell performance through the significant reduction on specific contact resistance has been simulated by using the PCID software. The increase of cell efficiency comes from three aspects: First, the contact area between the front side electrode and the n+ emitter layer can be reduced. This reduces the front side recombination velocity (FSRV), enabling the increase of cell efficiency. Secondly, as we use laser ablation to make contact holes and nickel film/layer to form the contact, firing-through the nitride layer for the silver paste is no more necessary. Hence we can reduce the resistance of silver grid lines, and thus the reduction of series resistance, by reducing the glass frit content in the silver paste or even using frit-free silver paste. This leads to another improvement on cell efficiency. Thirdly, emitter layer with higher sheet resistance (e.g., 100Q/sq. instead of 50Q/sq.) can be used, which improves the short-wave length response and also cell efficiency. We have simulated the improvement of each of these factors. The results are given in Table 1, where "0" represents the baseline which uses the fire-through silver paste process and the cell efficiency is 17.1 %, and "1" represents the improved situation by using our technology. It can be seen that totally about 0.9% absolute cell efficiency improvement can be reached. IV. CONCLUSION In order to solve the very resistive contact problem caused by the conventional screen printing fire-through silver paste process, we have developed a number of approaches to introduce a nickel-based silicide contact layer between the silver electrode and n+ emitter layer, which can substantially TABLE I EFFICIENCY IMPROVEMENT MODELED BY USING PC 1 D SOFTWARE FSRV R. EmittE!li Eff. f%) In.CI"ease 0 0 0 17.1 Base.line 0 0 17.4 0. 3 0 0 17.2 02 0 17.6 0.5 0 0 17.5 O. 0 17.13 0.7 0 17.7 0.6 18.0 0.9 reduce the specific contact resistance. One of them is to use a blanket sputtered nickel film as the contact layer and screen printed silver lines as an etch mask to pattern the underlying nickel film. This approach ensures the use of high quality nickel film as a contact layer to reduce the specific contact resistance, and also avoids the use of standard photolithographic process to reduce the cost. The result shows the specific contact resistance with this approach can be reduced by about two orders of magnitude compared to only using screen printed silver gridlines. The second approach is to use inkjet printed nickel nanoparticle ink instead of the sputtered nickel film to form the contact layer, enabling a very low cost inline process that can be easily implemented into current solar cell production line. The key point to reduce the contact resistance for the inkjet printed nickel approach is to increase the effective nickel silicide contact area which has the specific contact resistance as low as the sputtered nickel film. We have demonstrated the inkjet printed nickel approach can have the contact resistance close to that of a sputtered nickel film. The PCID modeling result shows that, the substantial reduction of the specific contact resistance can increase the absolute cell efficiency up to 0.9%. REFERENCES [1] D. Neuhaus and A. Munzer, "Industrial silicon wafer solar cells", Adv. OptoElectronics, 2007, Article ID 24521. [2] R. Pierret, Semiconductor Device Fundamentals, Addison Wesley Publishing, Section 14.2, 1996. [3] J. Gambino and E. Colgan, "Silicide and Ohmic Contact", Mat. Chern. Phys., 52,1998, pp. 99-1 46. [4] B. Xu, K. Littau, J. Zesch, and D. Fork, "Front side metallization of crystalline silicon solar cells using selectively laser drilled contact openings", Proceedings of the 34th IEEE Photovoltaic Specialist Conference (2009), pp. 517-522. 003358