Boosting the fill factor and open-circuit voltage of carbon-based perovskite solar cells with a graphene co-doped P3HT/NiO_x hole-transporting bilayer

Low-cost and stable carbon-based perovskite solar cells (C-PSCs) have been regarded as the most promising device architecture for PSC commercialization. However, due to the large interfacial resistance and energy-level mismatch between the carbon-electrode and perovskite, C-PSCs have suffered from a low fill factor (FF) and an open-circuit voltage (V-oc) deficit, limiting their efficiency. Here, a low-temperature, solution-processed P3HT/NiOx organic/inorganic hybrid bilayer, where both elements are doped with graphene (G), is developed as hole-transporting layers (HTLs) for C-PSCs. The optimal G-P3HT/G-NiOx bilayer HTLs not only provide well-matched cascade band alignments that facilitate the charge separation and extraction, and reduce the recombination loss at the perovskite/carbon-electrode interface, but their G co-dopants also build carrier highways within the NiOx and P3HT matrix, accelerating the charge transport and thus reducing FF loss. As a result, a V-oc improvement of 100 mV, an FF of up to 79.2%, and a best power conversion efficiency (PCE) of 19.1% for the C-PSCs are achieved. More importantly, the unencapsulated devices with G-P3HT/G-NiOx bilayer HTLs retain 88% of their initial PCE in ambient air for 2500 h; and 91% upon continuous heating at 85 degrees C for 637 h; and 77% after operation at the maximum power point for over 12000 min. This work provides valuable knowledge for designing bilayer HTLs for highly efficient and stable C-PSCs.