The installed capacity of photovoltaic (PV) technology has increased beyond expectations and is expected to grow even more. One of the most promising PV technologies currently dominating the PV market is the silicon heterojunction (SHJ) solar cells, which hold the highest c-Si single junction efficiency of 27.3% in an interdigitated-back-contacted design. To achieve such high efficiencies, an optimized (i)a-Si:H passivation layer is of great importance. This thesis aims to scale up the lab baseline SHJ technology by optimizing large-area c-Si surface passivation with hydrogenated intrinsic amorphous silicon (i)a-Si:H layers. In view of the development of high-efficiency perovskite/c-Si tandem solar cells, this thesis also covers the development of nanotexture on crystalline silicon (c-Si) wafers used for the bottom cells.
The research for optimization of large-area c-Si passivation with (i)a-Si:H layers first started with obtaining recipes that would deposit uniform (i)a-Si:H layers over M2-sized glasses. The deposition conditions of plasma-enhanced chemical vapor deposition (PECVD) were optimized and various promising recipes that delivered highly uniform (i)a-Si:H layers were obtained. No statistically significant correlation was found between the deposition parameters and the uniformity of the (i)a-Si:H layers.
The passivation quality of the (i)a-Si:H layers was assessed by symmetrically depositing the (i)a-Si:H layers on c-Si wafers and measuring the lifetime. For single-layer passivation strategy, (i)a-Si:H layers prepared with the highly hydrogen-diluted silane (SiH4) resulted in relatively low lifetime values, potentially due to epitaxial growth, while layers deposited with less hydrogen-diluted SiH4 or pure SiH4 resulted in better lifetime values. The best lifetime achieved with single-layer passivation was 8.23 ms. Furthermore, utilizing a bilayer concept that first deposits an ultra-thin (i)a-Si:H layer with pure SiH4 and then stacks on its top a highly hydrogen-diluted (i)a-Si:H layer, can enhance the passivation quality over their single-layer counterparts. With proper selection of the (i)a-Si:H layers that compose the bilayer and the optimum thickness combinations, a lifetime of 16.10 ms was achieved. Moreover, the passivation quality can be further boosted by applying a hydrogen plasma treatment (HPT) of 30 seconds on top of the optimized bilayer. HPT durations longer than 30 seconds resulted in a decrease in lifetime, possibly due to the oversaturation of hydrogen that leads to defect formation. A lifetime as high as 21.53 ms was obtained by combing the bilayer approach and a HPT.
Last but not least, the creation of nanotexture on c-Si wafers was done by anisotropic etching, using KOH, KsSiO3, and surface additive monoTEX 2.6. By varying texturing conditions, namely, temperature and duration, various surface morphologies of c-Si wafers were obtained. According to SEM characterizations, pyramidal features below 1 μm were successfully created mainly thanks to K2SiO3 slowing down the etching rate during the texturing process. The lowest reflectance was obtained by etching the c-Si wafers at 70 °C for 20 minutes. It was also found that K2SiO3 has an extremely slow dissolving rate and the most effective way of dissolving K2SiO3 is by adding KOH and K2SiO3 to DI water at room temperature and heating it to 80 °C while mixing.