The Application of SiNx Layers to Multicrystalline Buried Contact Silicon Solar Cells

The Application of SiN x Layers to Multicrystalline Buried Contact Silicon Solar Cells Shin Hey Won, Sang Wook Park, Jeong Kim and S.H.LeeNational Research Laboratory for Si Photovoltaics, Corporate R&D Center, SAMSUNG SDI Co. Ltd.,P.O.Box111, Suwon, Korea, 440-600AbstractConventional multicrystalline buried contact solar cells (BCSC) need thick SiO2 films which are used as barrier layers for the diffusion and metallization. However, the high temperature process for preparing SiO2 layers causes many difficulties during the production of the multicrystalline BCSC.In this work, silicon nitride (SiN x) films were deposited by plasma enhanced chemical vapor deposition (PECVD) method to replace SiO2 as SiN xin BCSC process. For low thermal budget of SiN x, fabrication process was modified and the results were compared with conventional BCSC.1. IntroductionSilicon nitride (SiN x) films not only act as an antireflection coating layer with a suitable refractive index, but can also improve the performance of photovoltaic devices by defect, surface and bulk passivation. For these reasons, recent studies have shown that the application of PECVD SiN x thin films can be very helpful in improving the efficiency of the multicrystalline silicon solar cells.PECVD SiN x layer is used as barrier layers for the diffusion and metallization of BCSC process. The SiN x layer played an important role as an antireflecting layer and passivating layer. Consequently, we obtained effective lifetime about two times as high as the SiO2 films due to the passivation effect of surface and bulk for solar cell. Therefore we expect that the J sc and the V oc will increase due to the decreasing of the reflectance by the SiN x films and efficiency of the solar cell will be increased.2. Experimental procedureFor the formation of the hydrogen-rich SiN x films, ammonia (NH3), silane (5 % SiH4 in N2) and nitrogen gases were used as source materials. The deposition temperature and the rf power were 300 o C and 60 W, respectively. The refractive index and the extinction coefficient of the SiN x, as measured by spectroscopic ellipsometer, were used as input for the ray-tracing program Sunrays and the thickness of SiN x was optimized. In addition, it is known that the annealing after deposition SiN x films is effective to improve the minority carrier lifetime of mc-Si wafer. In order to increase passivation effect, as-grown SiN x films were post-annealed at 4000C for 30min in forming gas ambient.3. Results and discussion3.1 SiN x layer as a passivation and antireflectionlayerEffective carrier lifetime of multi-Si was measured by microwave photoconductive decay (µW-PCD) to verify the passivation effect before and after annealing.Table 1. Effective carrier lifetimes before and after SiN x deposition.Process AfteremitterAfterSiN xAfterannealing Lifetime 22µs 44µs 65µsTable 2. Comparison of effective carrier lifetimes of the wafer coated with SiO2 and SiN x, respectively on the emitter.Passivation SiO2 layer SiN x layer Carrier lifetime 35µs 65µsWe found that the effective lifetime of mc-Si greatly increased after annealing the introducing SiN x film. It seems that atomic hydrogen in SiN x diffused into the bulk silicon and passivated mc-Si during annealing process and the effective lifetime is higher than that of mc-Si coated with SiO2 films. The surface reflectance of mc-Si with SiN x layer could be remarkably lowered as shown in Fig.1. The refractive index of SiN x film was measured to be 2.01 at 632.6 nm. It is shown that the reflectance of the textured mc-Si wafer with SiN x films is extremely low. It is expected that the SiN x improve the performance of BCSC by increasing V oc and J sc of the cell.3.2 Application of SiNx to BCSC process Multicrystalline –BAYSIX cast silicon wafers were used for fabrication of BCSC cells. The wafers received saw damage etching in isotropic etching solution. The emitter was formed using phosphorous solid source. SiN x films were deposited on theemitter to serve as a diffusion barrier for heavy diffusion and as barrier layer for metallization. The solar cell fabrication process was summarized in Table 3.Figure 1. Reflectances of mc-Si coated with SiN x and SiO2. Texturing of mc-Si wafer was carried out in KOH solution.Table 3. Fabrication process of BCSC cells.1. Saw damage etching2. Phosphorus gettering3. Front junction formation (n+ layer)4. SiNx deposition by PECVD5. Groove formation]6. Groove diffusion (n++ layer)7. Al deposition and sintering8. Metallization (Ni, Cu plating)The output performance of BCSC cell with SiN x is shown Fig. 3. The conversion efficiency of cell was 6.14 % in spite of the improvement of the effective lifetime after the deposition of SiN x films. It is likely that the high temperature processes, heavy diffusion (940 o C) and aluminum sintering (980 o C) degraded the passivation effects of SiN x film. Therefore, we found that the high temperature process should be avoided after depositon of deposition of SiN x film to utilize the advantage of hydrogen passivation effects of SiN x films. Two fabrication processes were tried to use SiN x films effectively. One is to adapt RTP (rapid thermal process) for groove diffusion and aluminum sintering. The other one is to introduce SiN xfilms after all high temperature process. The first one is under study using spin on source and RTA machine in our group. The second one was tried using process sequence similar to that of PESC (Passivated Emitter Solar Cell).By using SiNx films after diffusion and aluminum sintering process, V oc of the cells with PESC structure increased up to 612 mV giving effciency of 15.44 % as shown in Table 3. Therefore it is expected that the efficiency of multi-Si BCSC will be enhanced up to 15 %. Figure 2. The result of the buried contact multicrystalline silicon solar cell fabricated using SiN x films instead of the SiO2 films.Table 3. Output performance of PESC cell fabricated using SiN x film after high temperature process ( AM1.5, 100mW/cm2, 45 cm2 cell area ).ParameterV oc(mV)Jsc(mA/cm2)FF(%)Eff(%) PESC 612 31.8 79.27 15.444. ConclusionThe purpose of this paper is that the fabrication of the buried contact silicon solar cell is applied to the SiN x layer as barrier layers for the diffusion and metallization instead of SiO2 layer and as an antireflection layer. High temperature processes degraded passivation effects of SiN x films. Introduction of SiN x film to after high temperature process gave high V oc resulting in high efficiency of 15.44 %. It is expected that BCSC cell with the efficiency of over 15 will be obtained if the high temperature process is replaced with RTP process or fabrication process is optimized by introducing SiN x after high temperature process.AcknowledgementThis work was supported by the Korean Ministry of Commerce, Industry, and Energy.References[1] Zhizhang Chen, Peyman Sana, CIEEE Transactions on electron devices, V ol. 40, No. 6, Jane (1993)[2] Li Cai, Student Member and Ajeet Rohatgi, IEEE Transactions on electron devices, V ol. 44, No. 1, January(1997)[3] F. Duerinckx, J. Szlufcik, 2nd World conference and exhibition on photovoltaic solar energy conversion 6-10 July(1998)[4] Den Coates, Scott Morrison, 16th European Photovoltaic Solar Energy Conference, 1-5 May(2000)。