Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
Publication date: 21st February 2018
Perovskite solar cells have been attracting tremendous attention in recent years, with the perovskite community gradually moving from the classical MAPbI3 to mixed cations and mixed halides perovskites (FA1-x-yMAxCsyPbI3-zBrz) to achieve power conversion efficiency (PCE) >20%. The mixing of FA and MA, as well as the incorporation of Cs cation and Br halide into the perovskite formulation has been linked to increasing efficiency, thermal stability and reduced process sensitivity. However, the underlying reasons for these advantages are still far from clear. In this contribution, we have systematically compared and contrasted the solidification and growth behaviors of FA1-x-yMAxCsyPbI3-zBrz with different compositions through using a multi-probe in situ diagnostics approach consisting in grazing incidence wide angle x-ray scattering (GIWAXS) and UV-Vis absorbance complemented with ex situ characterization of thin film morphology and solar cell devices. We reveal that adding a little amount of Br into mixed cation perovskites benefits the device efficiency and dramatically widens the anti-solvent dripping window, mitigates the crystallization of the δ-FAMACsPbI3, promotes formation of a new Br-rich intermediate phase, and assists the in situ convertion of the ink into the perovskite phase upon anti-solvent dripping. These effects are further enhanced by adding a little amount of Cs into the mixed halide perovskite. We show that maintaining the as-cast film in a disordered sol-gel state prior to conversion to perovskite is crucial for achieving high quality films with uniform halide composition and power conversion efficiency of 20%.