ND-π-D molecular layer electronically bridges the NiOx hole transport layer and the perovskite layer towards high performance photovoltaics

Abstract Nickel oxide (NiOx) has significant cost and stability advantages over poly[bis (4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) for inverted p-i-n perovskite solar cells (PSCs), but the poor NiOx/perovskite contact stemming from some reactive species at the interface led to suboptimal device performance. To solve this problem, we take a multiple donor molecule approach, using 3,3'-(4,8-bis(hexylthio)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(10-(6-bromohexyl)-10H-phenoxazine) (BDT-POZ) as an example, to modify the NiOx/perovskite interface. The primary goal was to reduce the under-coordinated Ni≥3+ cations via electron transfer from the donor molecules to NiOx, thus mitigating the detrimental reactions between perovskite and NiOx. Equally importantly, the hole extraction at the interface was greatly enhanced after the organic donor modification, since the hydrophobic species atop NiOx not only enabled pinhole-free crystallization of the perovskite but also properly tuned the interfacial energy level alignment. Consequently, the PSCs with NiOx/BDT-POZ HTL achieved a high power conversion efficiency (PCE) up to 20.16%, which compared excellently with that of the non-modified devices (17.83%). This work provides a new strategy to tackle the exacting issues that have so far impeded the development of NiOx based PSCs.