Organic-silicon heterojunction solar cells on n-type silicon wafers: The BackPEDOT concept

We measure saturation current densities down to J₀=80 fA/cm² for organic-silicon heterojunctions with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as an organic layer. This remarkably low J₀ value corresponds to implied open-circuit voltages around 690 mV, demonstrating the high-efficiency potential of this novel junction type. However, experimentally realized organic-silicon heterojunction solar cells showed relatively moderate efficiencies so far, typically below 12%. We demonstrate in this study that these solar cells were limited by the fact that the organic-silicon junction was localized on the cell front, resulting in a significant parasitic light absorption within the PEDOT:PSS layer. In addition, the rear surface of these front-junction solar cells was either poorly passivated or not passivated at all. In this paper, we overcome these limitations by proposing a back-junction organic-silicon solar cell, the so-called "BackPEDOT" cell. We show that placing PEDOT:PSS on the rear side instead of the front surface avoids parasitic light absorption within the PEDOT:PSS and allows for an improved surface passivation. We fabricate and characterize BackPEDOT solar cells and achieve very high open-circuit voltages of up to 663 mV and short-circuit current densities of up to 39.7 mA/cm². Despite the relatively high series resistances of our first BackPEDOT cells, we achieve an energy conversion efficiency of 17.4%. The measured pseudo efficiency of the best cell of 21.2% suggests that our novel BackPEDOT cell concept is indeed suitable for easy-to-fabricate high-efficiency solar cells after some further optimization to reduce the contact resistance between the PEDOT and the n-type silicon wafer. Based on realistic assumptions we conclude that Back PEDOT cells have an efficiency potential exceeding 22%.