Photocurrent Enhancement in Organic Semiconductor/Metal Halide Perovskite Bilayers

Abstract

Abstract Body: Metal halide perovskites (MHPs) have rapidly become a topic of intense interest in both materials physics and device engineering due to their much sought for properties like high charge carrier mobilities, tunable bandgap, and low-cost processability. Various high-performance devices based on MHPs, including photovoltaic cells, light-emitting diodes, transistors and photodetectors have been reported, but a thorough understanding of the mechanism of charge transport and photo-transport in this material is still lacking. Here, we explore the photocurrent properties in bilayers consisting of small molecule organic semiconductors (OSCs) and MHPs. The MHP CH3NH3PbI3-xClx was the primary photoactive layer and electron transport layer, while the OSC hole transport layer was 2,8-Difluoro-6,13-Bis(triisopropylsilylethynyl) anthradithiophene (diF-TIPS-ADT). We found that the responsivity of the bilayer optimized device was 28 times greater than neat perovskite film, reaching values as high as 6.4 A/W. Detectivity also improved by a factor of 9, to 1.4*1011 Jones, in devices with optimum film morphology. We found that the photoresponse can be tuned by altering the microstructure of the organic semiconductor: treatment of the device contacts with (2,3,4,5,6)-Pentafluorothiophenol (PFBT) resulted in grains as large as 155 µm2 (LG) within the OSC, compared to much smaller grains (SG) seen when the OSC was deposited on the bare Au contacts, which resulted in grains in the order of 10 - 15 µm2. Additionally, in the heterostructure devices made on a large grain OSC film, hysteresis was eliminated. X-ray diffraction measurements indicate that the crystal structure of both the OSC and MHP is similar in all samples, with diF-TIPS-ADT preferentially oriented with its (001) plane parallel to the substrate, and MHP adopting its tetragonal phase. We concluded that the observed performance improvement does not originate from differences in the interfacial coupling related to molecular orientation or differences in MHP crystal phase. Instead, we conclude that reducing charge trapping at grain boundaries within the OSC layer extends photocarrier lifetime, leading to an improvement in performance.