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A significant volume of literature exists that describe minor changes in composition and/or microstructure in perovskite solar cells (PSCs) as the driving force for incremental improvements in device performance. Many authors cite crystallinity as a fundamental driver of performance improvements, yet often do so without quantitatively defining crystallinity or, importantly, addressing the important questions behind their observations. Here I will discuss two recent case studies where processing modifications have been investigated as a means of controllably varying active layer crystallinity and describe the impact on device performance. i) We demonstrate the impact of active layer crystallinity on the accumulated charge and open-circuit voltage (Voc) in solar cells based on methylammonium lead triiodide (CH3NH3PbI3,MAPI). We show that MAPI crystallinity can be systematically tailored by modulating the stoichiometry of the precursor mix, where a small excess of methylammonium iodide (MAI) improves crystallinity increasing device Voc by ~200 mV and, in parallel, that the photoluminescence (PL) yield increases 15x, indicative of a suppression of non-radiative recombination pathways. This is coupled with the development of crystallographic texture (110) in the MAPI. In-situ transient optoelectronic measurements of the charge concentration in PSCs under operation suggest that the concentration of trapped charges (either at interfaces or in the bulk) is some 5x lower at matched Voc. We believe these trap states originate in/near the disordered or amorphous areas between MAPI crystallites, resulting at least in part from orientation mismatch between crystalline domains. ii) Secondly, we identify previously unobserved nanoscale defects residing within individual grains of solution processed MAPI thin films. Using scanning transmission electron microscopy (STEM) we identify the defects to be inherently associated with the established solution-processing methodology and introduce a facile processing modification to eliminate such defect formation. Specifically, defect elimination is achieved by co-annealing the as deposited MAPI layer with the electron transport layer PCBM resulting in devices that significantly outperform devices prepared using the established methodology, achieving PCE increases from 13.6 % to 17.7 %. The use of TEM allows us to correlate the performance enhancements to improved intra-grain crystallinity and show that highly coherent crystallographic orientation results within individual grains when processing is modified. Detailed optoelectronic characterization reveals that the improved intra-grain crystallinity drives an improvement of charge collection, a reduction of surface recombination at the MAPI/PCBM interface and a change in the density of local sub-gap states.
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