The impressive performance of CH₃NH₃PbI₃ perovskite semiconductors in photovoltaics would not be realized without the optimization of highly crystallized perovskite layers. A desirable perovskite thin film with minimal defects is preferred to have a full surface coverage, small surface roughness and a well-defined grain structure. Perovskite crystallinity, grain size and uniform coverage of the perovskite grains is crucial to achieving high performance and has been demonstrated using a 2-step, interdiffusion process. The fast reaction when spinning CH₃NH₃I solution onto PbI₂ film, deposited from a DMF solution, may lead to non-uniform perovskite crystallization as well as epitaxial crystal growth and subsequently increased film roughness. For this purpose, it is often preferred to undertake the secondary step while the PbI₂ film is still ‘wet’. This proves challenging for scale-up of the interdiffusion method, hence we incorporated an intermediate drying step to facilitate batch-to-batch processing. It can be observed from SEM images (Fig.1) that a rough film is formed through uncontrolled nucleation which occurs in the absence of DMF. By adding a small amount of polar and/or non-polar solvent, for example, DMF into CH₃NH₃I solution, the rate of perovskite crystallization will be influenced by the solubilizing capacity of the additive. By controlling the concentration of the compositional solvent, we are able to achieve a very smooth and pin-hole free perovskite film.

We have successfully applied solvent/anti-solvent additives in CH₃NH₃I solution to influence perovskite nucleation and growth. With the optimization of DMF ratio, we achieve a very smooth and pin-hole free perovskite film. With the comprehensive investigation of the perovskite surface and photovoltaic performance, a performance of 11.5% PCE (V_oc to j_sc) was obtained at DMF ratio of 1% with j_sc = 19.2 mA/cm², V_oc = 970 mV, & fill factor (FF) of 0.62 for active cell areas no less than 0.3cm². Hysteresis was observed in our devices, most likely a result of ineffective charge extraction at the interface between TiO₂ and perovskite in this planar structure. Further investigation of the perovskite fabricating process and crystallizing mechanism is an ongoing research activity as applied to our aims of uniform, large-area deposition processes.In this study we investigated the perovskite film formation on compact TiO₂ layers applying a modified interdiffusion method to improve crystallisation and film planarity. We used compositional mixes of solvent/anti-solvent additives in CH₃NH₃I solution to influence both film deposition kinetics and perovskite nucleation and growth.