Improved crystallinity and device performance of CH3NH3PbI3 perovskite solar cells by using a non-stoichiometric PbI2 : CH3NH3I precursor ratio.

Understanding the crystallization process of organic–inorganic halide perovskites is of paramount importance for fabrication of reproducible and efficient perovskite solar cells. We report for the first time on the discovery and interplay of ubiquitous hexagonal polytypes (6H and 4H) during the crystallization process of mixed ion perovskite, namely (FAPbI3) x (MAPbBr3)1–x . These polytypes, the first reported 3D hexagonal lead-halide-based perovskites, orchestrate a perovskite crystallization sequence revealed as 2H (delta phase)-4H-6H-3R­(3C), commonly found among inorganic transition metal oxide perovskites under extreme conditions. We show that the chemical pressure arising from the incorporation of >3% Cs+ cations into the lattice successfully inhibits the formation of these environmentally sensitive polytypes, elucidating the origin of the widely reported improved device stability and reproducibility of Cs+-containing mixed ion perovskites.

Compositional engineering of a mixed cation/mixed halide perovskite in the form of (FAPbI)(MAPbBr) is one of the most effective strategies to obtain record-efficiency perovskite solar cells. However, the perovskite self-organization upon crystallization and the final elemental distribution, which are paramount for device optimization, are still poorly understood. Here we map the nanoscale charge carrier and elemental distribution of mixed perovskite films yielding 20% efficient devices. Combining a novel in-house-developed high-resolution helium ion microscope coupled with a secondary ion mass spectrometer (HIM-SIMS) with Kelvin probe force microscopy (KPFM), we demonstrate that part of the mixed perovskite film intrinsically segregates into iodide-rich perovskite nanodomains on a length scale of up to a few hundred nanometers. Thus, the homogeneity of the film is disrupted, leading to a variation in the optical properties at the micrometer scale. Our results provide unprecedented understanding of the nanoscale perovskite composition.

The small-molecule-based hole-transporting material methoxydiphenylamine-substituted carbazole was synthesized and incorporated into a CH3NH3PbI3 perovskite solar cell, which displayed a power conversion efficiency of 16.91%, the second highest conversion efficiency after that of Spiro-OMeTAD. The investigated hole-transporting material was synthesized in two steps from commercially available and relatively inexpensive starting reagents. Various electro-optical measurements (UV/Vis, IV, thin-film conductivity, hole mobility, DSC, TGA, ionization potential) have been carried out to characterize the new hole-transporting material.

Dopant-free HTM KR321 showed highly ordered characteristic face-on organization leading to increased vertical charge transport and PCE over 19% in PSC with improved stability.