Visualizing Defects in Charge Transport Layers of Halide Perovskite-Based Solar Cells by Fluorescence Quenching Microscopy

Solution-processed perovskite based solar cells (PSCs) have attracted extensive attention by virtue of their simple processing, mechanical flexibility, and scalability, which render a great potential for commercialization. Despite drastic advances in device processing, non-negligible deviations in the performance of PSCs have been consistently observed, hypothetically owing to the presence of defects (<i>i.e</i>., pinholes) within ultrathin charge transport layers (CTLs) and interfacial materials, such as poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) and (9,9-bis(3-(<i>N</i>,<i>N</i>-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene) (PFN). As pinholes exert a significant influence on the quality of interfaces, and ultimately on the performance of PSCs, a high-throughput visualization method will greatly facilitate evaluation of potential candidates for CTLs as well as interfacial engineering for high performance PSCs.<br/>Fluorescence quenching microscopy (FQM) offers high-throughput imaging of various materials at sub-50 nm resolution based on the quenching of emission from neighboring dye molecules. Here, we utilize FQM to analyze size, density, and distribution defects within the commonly used ultrathin (~10 nm) hole transporting layer (HTL), PTAA. When PTAA films were imaged using fluorescein (green dye), high density of bright spots were observed suggesting presence of high density pinholes. In stark contrast to PTAA films, PTAA/PFN films resulted in uniform (featureless) quenching with occasional bright spots associated with pinholes. To correlate these results with the performance of PSCs, we have carried out statistical analysis of standard deviations in open-circuit voltage, short-circuit current density, fill factor, and power conversion efficiency. As expected, PSCs fabricated with PTAA exhibited large deviations in performance, whereas those with PTAA/PFN showed narrow distribution in all figures of merit. We anticipate this straightforward strategy to pave the way for efficient analysis of materials and interfaces not only in the field of halide perovskites, but also in the field of optoelectronic devices.