Perovskite photovoltaics offer a compelling combination of extremely low-cost, ease of processing and high device performance. The optoelectronic properties of the prototypical CH3NH3PbI3 can be further adjusted by introducing other extrinsic ions. Specifically, chlorine incorporation has been shown to affect the morphological development of perovksite films, which results in improved optoelectronic characteristics for high efficiency. However, it requires a deep understanding to the role of extrinsic halide, especially in the absence of unpredictable morphological influence during film growth. Here we report an effective strategy to investigate the role of the extrinsic ion in the context of optoelectronic properties, in which the morphological factors that closely correlate to device performance are mostly decoupled. The chlorine incorporation is found to mainly improve the carrier transport across the heterojunction interfaces, rather than within the perovskite crystals. Further optimization according this protocol leads to solar cells achieving power conversion efficiency of 17.91%.

In this study, we communicate an investigation on efficient CH3NH3PbI3-based solar cells with carbon electrode using mesoporous TiO2 and NiO layers as electron and hole selective contacts. The device possesses an appreciated power conversion efficiency of 14.9% under AM 1.5G illumination. The detailed information can be disclosed with impedance spectroscopy via tuning the interfaces between CH3NH3PbI3 and different charge selective contacts. The results clearly show charge accumulation at the interface of CH3NH3PbI3. The NiO is believed to efficiently accelerate charge extraction to the external circuit. The extracted charge could improve photovoltaic performance by shifting hole Fermi level down, achieving a high device photovoltage. A fast interfacial recombination at the interface of CH3NH3PbI3/electron selective contact layer (mesoporous TiO2), occurring in millisecond domains, is the critical issue for charge carrier recombination loss.