Abstract
Integrated perovskite/organic bulk heterojunction (BHJ) solar cells have the potential to enhance the efficiency of perovskite solar cells by a simple one-step deposition of an organic BHJ blend photoactive layer on top of the perovskite absorber. It is found that inverted structure integrated solar cells show significantly increased short-circuit current (J(sc)) gained from the complementary absorption of the organic BHJ layer compared to the reference perovskite-only devices. However, this increase in J(sc) is not directly reflected as an increase in power conversion efficiency of the devices due to a loss of fill factor. Herein, the origin of this efficiency loss is investigated. It is found that a significant energetic barrier (approximate to 250 meV) exists at the perovskite/organic BHJ interface. This interfacial barrier prevents efficient transport of photogenerated charge carriers (holes) from the BHJ layer to the perovskite layer, leading to charge accumulation at the perovskite/BHJ interface. Such accumulation is found to cause undesirable recombination of charge carriers, lowering surface photovoltage of the photoactive layers and device efficiency via fill factor loss. The results highlight a critical role of the interfacial energetics in such integrated cells and provide useful guidelines for photoactive materials (both perovskite and organic semiconductors) required for high-performance devices.