Formation of Stable Mixed Guanidinium–Methylammonium Phases with Exceptionally Long Carrier Lifetimes for High-Efficiency Lead Iodide-Based Perovskite Photovoltaics

Abstract

Methylammonium (MA)- and formamidinium (FA)-based organic-inorganic lead halide perovskites provide outstanding performance as photovoltaic materials, due to their versatility of fabrication and their power conversion efficiencies reaching over 22%. The proposition of guanidinium (GUA)-doped perovskite materials generated considerable interest due to their potential to increase carrier lifetimes and open-circuit voltages as compared to pure MAPbI. However, simple size considerations based on the Goldschmidt tolerance factor suggest that guanidinium is too big to completely replace methylammonium as an A cation in the APbI perovskite lattice, and its effect was thus ascribed to passivation of surface trap states at grain boundaries. As guanidinium was not thought to incorporate into the MAPbI lattice, interest waned since it appeared unlikely that it could be used to modify the intrinsic perovskite properties. Here, using solid-state NMR, we provide for the first time atomic-level evidence that GUA is directly incorporated into the MAPbI and FAPbI lattices, forming pure GUA MAPbI or GUA FAPbI phases, and that it reorients on the picosecond time scale within the perovskite lattice, which explains its superior charge carrier stabilization capacity. Our findings establish a fundamental link between charge carrier lifetimes observed in photovoltaic perovskites and the A cation structure in ABX-type metal halide perovskites.

Efficient and Stable Inverted Perovskite Solar Cells Incorporating Secondary Amines

“…The photoluminescence (PL) intensity of the optimized DMA doped film is ≈2× higher than that of MA1. [23] This is consistent with the higher formation energy of the perovskite structure obtained in DFT studies. The longer PL lifetime along with the higher steady-state PL of DMA0.11 indicates that the nonradiative recombination is reduced in the DMA-incorporating films, suggesting that the trap state density is reduced.…”

Supramolecular Engineering for Formamidinium‐Based Layered 2D Perovskite Solar Cells: Structural Complexity and Dynamics Revealed by Solid‐State NMR Spectroscopy

“…We have previously shown that bulk mechanochemical perovskites are indistinguishable from those prepared using solution processing only when pure phases are formed. [29] Since there is no clear evidence for the presence of δ-FAPbI 3 in the X-ray diffraction patterns of the solutionprocessed films (Figure 3a,b), we conclude that the ratio of α-FAPbI 3 to δ-FAPbI 3 in the presence of spacer A is affected by the solvent, such that the formation of δ-FAPbI 3 is suppressed. [29] Since there is no clear evidence for the presence of δ-FAPbI 3 in the X-ray diffraction patterns of the solutionprocessed films (Figure 3a,b), we conclude that the ratio of α-FAPbI 3 to δ-FAPbI 3 in the presence of spacer A is affected by the solvent, such that the formation of δ-FAPbI 3 is suppressed.…”

“…We have previously shown that solid-state NMR is well suited to probe the atomic-level microstructure of multicomponent lead halide perovskites, [26][27][28][29] yet this approach has not been systematically employed to analyze the structural features of layered 2D perovskites so far. We have previously shown that solid-state NMR is well suited to probe the atomic-level microstructure of multicomponent lead halide perovskites, [26][27][28][29] yet this approach has not been systematically employed to analyze the structural features of layered 2D perovskites so far.…”

Research Direction toward Scalable, Stable, and High Efficiency Perovskite Solar Cells

“…Photogenerated carries had much longer time in the perovskite film prepared from the 2ME‐based and PbAc 2 ‐containg solution as confirmed by TRPL (Figure d), which was responsible for the higher photovoltaic performance. Addition of guanidinium cation (GA) in the (MAI+PbI 2 +PbAc 2 ) solution improved further PCE over 19% due to further improvement in carrier lifetime …”