低成本衬底上多晶硅薄膜电池的探索

Polycrystalline Silicon Thin Film Solar Cells on Low Cost Substrates

在低成本、两种纯度的P++颗粒硅带衬底(SSP)上,采用RT化学气相沉积(CVD)技术制备了多晶硅薄膜电池。无论高纯硅粉还是低纯硅粉制备的SSP衬底杂质含量都很高且表面凹凸不平;采用4μm/min的沉积速率和钝化作用得到了高质量外延多晶硅薄膜。通过扩散工艺制成的多晶硅薄膜太阳电池的转换效率分别为6 25%(高纯硅粉制成的SSP衬底)和4 5%(低纯硅粉制成的SSP衬底)。

The development of polycrystalline silicon thin film on low cost substrates for solar cells is showing an urgent need to reduce cost. In order to achieve high quality, large grains silicon films, high temperature processes have to be used. So the substrates must be high temperature resistant, and fulfill requirements concerning thermal expansion, chemical and mechanical stability. Up to now, silicon based materials are the best choice to satisfy the requirements mentioned above.In this paper, low cost SSP substrates are chosen for our thin film solar cells. SSP substrates are manufactured by melting silicon powder by Fraunhofer institute of solar energy system, Germany. Two kinds SSP substrates, made from high purity silicon powder and from homemade low purity silicon powder, are used in our experiments.The morphology of SSP substrates were measured by using SEM.It can be seen that either made from high purity silicon powder or from homemade low purity powder has very rough surface. The average grain size is about 50～100μm.AES and ICPAES reveal that there are O, B, Fe, Mn and Cr impurity in the low purity SSP substrates and the concentration of Fe and Cr exceed 1017cm-3. High purity SSP substrates have 3%～4% oxygen also.The cell's fabrication processes include: substrate treatments; epitaxy of polycrystalline silicon thin film using rapid thermal chemical vapor deposition (RTCVD); emitter formation; front and rear contacts fabricating; antireflection coating (ARC) deposition.The resistivity characteristics of the epitaxial thin film was 01Ω·cm. Hall mobility is 20～70cm2/V·s. Film thickness is 20～50μm and average grain size are 50～100μm. The largest growth rate is 5μm/min.The best thin film solar cell's conversion efficiency of 636% and 45% (AM15, 25℃, 1cm2) have been achieved on high and low purity substrate. These results show that low cost thin film solar cells have great potentialities.Although antireflect coating (ARC) fabrication improves cells' performance obviously (cell's efficiency increases 14%～22% with SiN), PC1D calculation shows that the best ARC can increase cell's efficiency 40%, that means our cell's ARC is not optimized.Additionally, the cell's efficiencies increases 4%(some results reach 10%～15%) after 30 minutes forming gas annealing in low temperature. That contributes to reduction of contact resistance and partial passivation of grain boundaries and defects.In conclusion, the polycrystalline silicon thin film solar cells on SSP substrates shows a potential way to reduce the cost of solar cells. Further research will be done in our laboratory.