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Molecular Origin of the Asymmetric Photoluminescence Spectra of CsPbBr3 at Low Temperature

Ariadni Boziki, Mohammad Ibrahim Dar, Gwénolé Jean Jacopin, Ursula Röthlisberger

CsPbBr3 has received wide attention due to its superior emission yield and better thermal stability compared to other organic-inorganic lead halide perovskites. In this study, through an interplay of theory and experiments, we investigate the molecular origin of the asymmetric low-temperature photoluminescence spectra of CsPbBr3. We conclude that the origin of this phenomenon lies in a local dipole moment (and the induced Stark effect) due to the preferential localization of Cs+ in either of two off-center positions of the empty space between the surrounding PbBr6 octahedra. With increasing temperature, Cs+ ions are gradually occupying positions closer and closer to the center of the cavities. The gradual loss of ordering in the Cs+ position with increasing temperature is the driving force for the formation of tetragonal-like arrangements within the orthorhombic lattice.

2021

Quantifying Stabilized Phase Purity in Formamidinium-Based Multiple-Cation Hybrid Perovskites

Neha Arora, Thomas Paul Baumeler, Mohammad Ibrahim Dar, Michael Graetzel, Shaik Mohammed Zakeeruddin

A promising approach for the production of highly efficient and stable hybrid perovskite solar cells is employing mixed-ion materials. Remarkable performances have been reached by materials comprising a stabilized mixture of methylammonium (MA(+)) and formamidinium (FA(+)) as a monovalent cation. We compare and quantify the methods of stabilizing FA(-) based perovskites involving the additional blending of the smaller inorganic cations cesium (Cs+) and rubidium (Rb+), which can lead to an improvement in phase purity of black cubic perovskite modification. Even under excess lead iodide conditions, the presence of a separate PbI2 phase as well as hexagonal phases, which are very common for formamidinium-containing perovskites, can be drastically reduced or even completely prevented. In this aspect, adding both Cs+ and Rb+ showed greater effectivity than only adding Cs+, enabling an increase in the percentage of the cubic phase within the material from 45% in the double-cation FA:MA mixture to 97.8% in the quadruple composition. The impact of admixing inorganic cations on the perovskite crystal structure resulted in enlarged homogeneous crystallite sizes and a less pronounced orientational order and indicated also minor modifications of unit cell sizes. Finally, we discuss the impact of the phase purity on charge-carrier recombination dynamics and solar cell performance.

2021

Cyclopentadithiophene-Based Hole-Transporting Material for Highly Stable Perovskite Solar Cells with Stabilized Efficiencies Approaching 21%

Seckin Akin, Neha Arora, Michael Bauer, Mohammad Ibrahim Dar, Michael Graetzel, Gwénolé Jean Jacopin, Yuhang Liu, Ryusuke Uchida, Shaik Mohammed Zakeeruddin

There is an urge to develop new hole-transporting materials (HTMs) for perovskite solar cells (PSCs), which can yield comparable power conversion efficiencies (PCEs) yet mitigate the issue of stability associated with the state-of-the-art HTM Spiro-MeOTAD. Herein, we designed and prepared C-2v-symmetric spiro-configured HTM-1 comprising a central acridine-cyclopentadithiophene core unit flanked with triarylamine moieties. PSCs containing a 40 nm thin HTM-1 layer for hole extraction yielded a stabilized PCE approaching 21% under standard illumination. Owing to its higher hole mobility (mu(h)) at low electric field, an impressive short-circuit current density (J(SC)) of 24.7 mA cm(-2) and a high fill factor (FF) of 0.77 have been achieved. More importantly, HTM-1-based PSCs presented an excellent long-term operational stability under continuous illumination for 400 h and thermal stability at 80 degrees C, which can be ascribed to its high glass transition temperature of 168 degrees C and superior moisture tolerance. Arguably, the confluence of high performance and remarkable stability will lead to the development of technologically interesting new, stable, and efficient PSCs.

AMER CHEMICAL SOC2020