Development of Gradient Layers to Improve the Efficiency of Transparent Passivating Contact Solar Cells

One limitation to currently applied passivating contacts like hydrogenated amorphous silicon or polycrystalline silicon is the strong absorption of light in the passivating and contacting layers. To increase the amount of light reaching the absorber material of the solar cell, a more transparent material is needed. Hydrogenated nanocrystalline silicon carbide (nc-SiC:H) is highly transparent, as well as highly conductive and has excellent passivating properties, showing very high generated currents in combination with a good fill factor (FF) and high open-circuit voltage (VOC) when applied in a solar cell. This approach is called transparent passivating contact (TPC). However, the FF is still lower as compared to other state-of-the-art approaches. In this work, a closer look on this reduced FF is taken. It is found that there is a direct trade-off between the passivation and the FF depending on the thickness of the SiC layer. In previous works, the SiC layer consists of two SiC layers, whereas the first deposited SiC layer was grown at a soft deposition condition to not harm the SiO passivation layer and therefore referred as the passivating SiC layer and the subsequent SiC layer was more crystalline and conductive and therefore referred as the conducting SiC layer. We found out that an even thinner passivating SiC layer in combination with a SiC layer with continuous transition of its material properties along the growth direction from more passivation SiC layer-like to conduction SiC layer-like, realized by a slow transition of the temperature of the catalytic filament leads to higher iVOCs in combination with an increased FF, leading to solar cell efficiencies exceeding the previous double-layer nc-SiC:H stack and even the amorphous silicon reference cell