Crystal Orientation, Strain, and Microstrain of Perovskite Films in a Complex Compositional Parameter Space

The chemical and functional stability issues of halide perovskite films integrated in solar cells are presently catalyzing large interest in the scientific community. Strategies are explored in which A-site doping and mixture of halides and incorporation of large organic cations (LOCs) and/or natural polymers are considered, often concomitantly. In this context, multiple parameters interfere with the crystallization process, and understanding the interplay of microscopic properties with the device stability and efficiency remains a challenging task. Herein, we screened the complex compositional parameter space of the FAxMA1–xPbI3 film series modified by LOCs [in the form of benzylammonium iodide (BzAI), 1,5-n-pentyldiammonium iodide (PDAI2), or 1,4-xylylendiammonium iodide (XDAI2)] and cornstarch incorporation. By combining GIWAXS, XRPD, and imaging analyses, we explore the influence of major constituents on the crystal structure, microstructure, and orientation and on the morphological film properties. By increasing the starch amount, the crystal structure and lattice strain of films are rather unaffected, whereas microstrain systematically rises, which explains the parallel optical band gap widening. By incorporating BzA and PDA cations, the films adopt the [100] orientation in comparison to the [110] of the LOC-free film (cubic notation), but increasing the starch amount induces a systematic crystal randomization in all formulations. These findings suggest a minor influence of texture and microstrain on the excellent photovoltaic performances of MAPbI3-based devices. In contrast, the surface compactness and optimal matching of film thickness and crystal domain size, all influenced by the starch additive, may play a favorable role. XDA incorporates into the 3D perovskite structure and greatly increases the air stability, providing a promising platform toward stable and flexible starch-based devices. This work demonstrates the importance of quantifying and correlating the micro- and macroscopic properties to deepen our understanding of the fundamental mechanisms of perovskite solar cells.