A Thermodynamically Favored Crystal Orientation in Mixed Formamidinium/Methylammonium Perovskite for Efficient Solar Cells

Abstract

In the past few years, hybrid organic-inorganic perovskite solar cells (PSCs) have been widely investigated and the power conversion efficiency (PCE) rocket-boosted from 3.8% to 23.7%, [1] originating from the excellent properties of perovskite materials such as appropriate and adjustable bandgap, high absorption coefficient, good charge carrier mobility, long carrier diffusion length, low recombination rate, and high tolerance of chemical defects. [2] Among various compositions, hybrid organic-inorganic PSCs based on mixed-cation formamidinium/methylammonium (FA/MA) perovskites have been attractive not only due to higher efficiency, but also better thermal stability, higher phase stability and more robust long-term stability than the FA cation or MA cation only counterparts. [3] For instance, MA cation only perovskite, i.e., MAPbI 3 , has a very low tetragonal-to-cubic phase transformation temperature of about 60 °C. FA Crystal orientation has a great impact on the properties of perovskite films and the resultant device performance. Up to now, the exquisite control of crystal orientation (the preferred crystallographic planes and the crystal stacking mode with respect to the particular planes) in mixed-cation perovskites has received limited success, and the underlying mechanism that governs device performance is still not clear. Here, a thermodynamically favored crystal orientation in formamidinium/methylammonium (FA/MA) mixed-cation perovskites is finely tuned by composition engineering. Density functional theory calculations reveal that the FA/MA ratio affects the surface energy of the mixed perovskites, leading to the variation of preferential orientation consequently. The preferable growth along the (001) crystal plane, when lying parallel to the substrates, affects their charge transportation and collection properties. Under the optimized condition, the mixed-cation perovskite (FA 1-x MA x PbI 2.87 Br 0.13 (Cl)) solar cells deliver a champion power conversion efficiency over 21%, with a certified efficiency of 20.50 ± 0.50%. The present work not only provides a vital step in understanding the intrinsic properties of mixed-cation perovskites but also lays the foundation for further investigation and application in perovskite optoelectronics.

Crystallization Kinetics in 2D Perovskite Solar Cells

“…Controlling the crystal orientation to achieve rapid carrier transport can effectively avoid or reduce the adverse effects caused by the organic layer. [95][96][97] Further, the distribution of QW also affects the performance of 2D PSCs, and needed to well understanding.…”

Engineering Halide Perovskite Crystals through Precursor Chemistry

“…In addition, the effects of cation species (MA + , FA + , and Cs + ) in precursor solutions on the perovskite crystallization kinetics have been partly illustrated. For example, Xu and co‐workers reported that MA/FA cation ratio would affect the surface energy of the mixed perovskites, leading to the variation of preferential orientation in mixed‐cation perovskites …”

“…c) Crystal structures of three intermediate phases: PbI 2 ‐rich (MA) 2 (DMF) 2 Pb 3 I 8 , stoichiometric (MA) 2 (DMF) 2 Pb 2 I 6 , and MAI‐rich (MA) 3 (DMF)PbI 5 . Reproduced with permission . Copyright 2017, American Chemical Society.…”

Emerging Conductive Atomic Force Microscopy for Metal Halide Perovskite Materials and Solar Cells

“…The facet orientation regulation has been considered as an effective route to elevate the device performance . 2D synchrotron radiation grazing incidence wide‐angle X‐ray scattering has been widely used to detect the crystal orientation . While, in order to clarify the relationship between grain orientation and photoelectric properties of perovskite at the same time, new representations are required.…”