Strategies to Enhance Thin Film Silicon Solar Cell Efficiency
S.M. Ghayeni (TU Delft - Electrical Engineering, Mathematics and Computer Science)
Arno Smets – Mentor (TU Delft - Photovoltaic Materials and Devices)
Govind Padmakumar – Mentor (TU Delft - Photovoltaic Materials and Devices)
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Abstract
Thin-film silicon solar cells are an innovative approach to utilizing solar energy. Thanks to their flexibility, lower material usage, and production cost, they have the potential to be used in a range of applications. The conversion efficiencies of thin-film silicon solar cells need to be improved to make them commercially viable. This involves further optimizing the different layers of the solar cells. During this project, different strategies for enhancing thin film silicon solar cells were studied, including the deposition conditions for nc-Si:H layer, the sacrificial texturing used, the use of an additional back reflector layer, and the type of TCO layer used. A more consistent quality of the nc-Si:H samples were obtained by varying the hydrogen flow rate compared to the silane flow rate in the processing chamber. Using a silane flow rate of 2.3 sccm and a hydrogen flow rate of 120 sccm, a silane concentration 2.6 was achieved, resulting in nc-Si:H layer with a crystallinity of 60%, close to the desired amorphous-nanocrystalline silicon transition region. The texturing used in the glass sample can play a significant role in scattering the incident light into the solar cell. Craters of different sizes are formed depending on the material used for the sacrificial layer. Making smaller craters on top of larger craters using modulated surface textures (MST) is also possible. Using intrinsic Zinc Oxide (i-ZnO) sacrificial texturing resulted in the highest spectral utilization in single junction nc-Si: H with a Jsc of 25.2 mA/cm2. Indium-doped tin oxide (ITO) sacrificial texturing created micro-sized textures, resulting in the highest spectral utilization in micromorph samples (Jsc of 24.6 mA/cm2). Using an MST of ITO and i-ZnO also resulted in high spectral utilization with a Jsc of 24.3 mA/cm2 in micromorph cells Having an additional back reflector on top of the metal back contact can further improve photon absorption in solar cells. Using a material with a low refractive index, such as i-ZnO, increased the spectral utilization and improved the short-circuit current (Jsc) by approximately 10%. Depositing the i-ZnO back reflector layer using a higher heater temperature (300°C), improved the spectral utilization of the sample further by 9% The transparent conductive oxide (TCO) layer must be highly transparent and conductive. Usually, ITO is used as a TCO layer, although it has limited absorption in the infrared region and lower conductivity at higher temperatures. A TCO with higher optoelectrical properties was obtained using a bilayer of hydrogenated indium oxide (IOH) and i-ZnO. nc-Si:H samples with the TCO bilayer showed a higher spectral utilization and an improvement in their Jsc by 9-12%. The performance of micromorph samples with TCO bilayer was also higher with Jsc improvement of 6%. The influence of the bilayer thickness on the performance of micromorph samples was also inspected, and it was found that using a thicker bilayer with a thickness of 1100 nm instated of 600nm boosted the performance of the sample further and improved its Jsc by 3%.