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E2.02 - Comparison of Hybrid Tandem Module Technologies Based on c-Si and Wide Band Gap Thin Film Photovoltaics 
April 7, 2015   2:00pm - 2:15pm

By placing a wide band gap semitransparent thin film solar cell in front of a high-efficiency crystalline silicon (c-Si) cell, the practical c-Si single junction conversion efficiency limit of about 26% can be surpassed, which is attractive for further reduction of systems costs provided that the thin film cell is inexpensive and does not have too much parasitic optical absorption or shadow losses. We have investigated by computer simulation (using thin film optics and ray-tracing models) the optical behavior of several hybrid junction stacks and have evaluated their relative potential. As the module concept we adopted the 4-terminal approach, as this avoids constraints regarding band gap values and absorber thicknesses, while in later production also constraints regarding processing temperatures and chemicals used can be avoided in this way. The cost of substrates and encapsulants is eliminated by the concept of depositing the thin film solar cell on the inside of the c-Si module cover glass. We considered three types of thin film PV cells: 1. Enlarged-bandgap oxygenated amorphous silicon (a-SiO:H) cells, 2. Wide band gap chalcopyrite (CuGaSe2) cells, and 3. Perovskite (CH3NH3PbI3) cells. In our model we also included the change in electrical output parameters by the color-filtering effect of the top cells. Our modelling showed a few interesting results. Starting with an interdigitated back contact (IBC) c-Si cell with a conversion efficiency of 20%, and adding the top cell while minimizing the reflection and parasitic absorption losses by choosing proper thicknesses of absorber and contacting layers and implementing light scattering by texturing the glass, we find that +2.0% (absolute) conversion efficiency can be gained using state-of-the-art a?SiO:H (band gap of 2.0 eV) and CuGaSe2 (1.8 eV) top cells and +4.75% can be gained using perovskite cells (1.55 eV). The perovskite cell type appears to be particularly suitable as the overall reflection loss is small due to the low refractive index of the absorber layer. Moreover, in the case of a perovskite top cell, light trapping by texturization of surfaces is not needed which helps avoiding parasitic absorption losses.

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