Ion migration in Br-doped MAPbI3 and its inhibition mechanisms investigated via quantum dynamics simulations.

MAPb(I1-xBrx)3 is widely used as a window layer in tandem solar cells. Ion migration is one of the most important factors that results in phase separation in MAPb(I1-xBrx)3 and eventually causes a decrease of cell performance. Recent research demonstrates that the doping of Cs+ and the formation of low-dimensional perovskite structures are effective means of inhibiting the migration. To investigate the causes of the migration and its inhibition mechanisms in hybrid halide perovskite materials, large-scale quantum dynamics simulations are conducted on MAPbI3, MAPb(I0.4Br0.6)3 and Cs0.125MA0.875Pb(I0.4Br0.6)3, respectively. By tracking changes in the geometric structures of the perovskite materials before and after doping with Br- and Cs+ in the dynamics processes, the precondition for the ion migration is firstly revealed. The dimension reduction of the perovskite skeleton structures by introducing Cs+ is observed. Furthermore, by combining observations with the variations of the band gap values in all the systems, the inhibition mechanisms of Cs+ doping on ion migration in MAPb(I1-xBrx)3 are revealed.

Dimension-Controlled Growth of Antimony-Based Perovskite-like Halides for Lead-Free and Semitransparent Photovoltaics.

Antimony (Sb) has been identified as a promising candidate for replacing toxic lead (Pb) in perovskite materials because Sb-based perovskite-like halides exhibit not only intrinsic thermodynamic stability but also a unique set of intriguing optoelectronic characteristics. However, Sb-based perovskite-like halides still suffer from poor film morphology and uncontrollable halide constituents, which result from the disorder of the growth process. Herein, we propose a simple strategy to facilitate heterogeneous nucleation and control the dimension transformation by introducing bis(trifluoromethane)sulfonimide lithium (LiTFSI), which produces high-quality two-dimensional MA3Sb2I9-xClx films. As the spacer molecule among Sb-based pyramidal clusters, LiTFSI plays a role in forming a zero-dimensional intermediate phase and retarding crystallization. The slower dimension transformation well stabilizes the band gap of perovskite-like films with a fixed Cl/I ratio (∼7:2) and avoids random "x" values in MA3Sb2I9-xClx films prepared from the conventional method. Based on this method, Sb-based perovskite-like solar cells (PLSCs) achieve the highest recorded power conversion efficiency (PCE) of 3.34% and retain 90% of the initial PCE after being stored under ambient conditions for over 1400 h. More importantly, semitransparent Sb-based PLSCs with PCEs from 2.62 to 3.06% and average visible transparencies from 42 to 23% are successfully obtained, which indicates the great potential of the emerging Pb-free halide semiconductor for broad photovoltaic applications.

Large scale quantum dynamics investigations on the sensing mechanism of H2O, acetone, NO2 and O3 adsorption on the (MA)2Pb(SCN)2I2 surface.

The instability of organometal halide perovskites still remains a key obstacle restricting their practical application in gas sensing research. The first step in gas sensing using a semiconductor material is the recognition of a target gas through gas-solid interaction. In the current work, the adsorption mechanisms of MAPbI3-H2O, (MA)2Pb(SCN)2I2-H2O, (MA)2Pb(SCN)2I2-CH3COCH3, (MA)2Pb(SCN)2I2-NO2 and (MA)2Pb(SCN)2I2-O3 have been investigated by large-scale quantum dynamics simulations. The structural changes of the perovskite skeleton, the adsorption energy, and the charge transfer between the semiconductor material and the gas molecules have been analysed. The suitability and effectiveness of quantum dynamics simulations in adsorption mechanism research are firstly validated by comparing the humidity sensing mechanisms of MAPbI3 and (MA)2Pb(SCN)2I2. Different sensing mechanisms of (MA)2Pb(SCN)2I2 to gases with different oxidising properties have been proposed. These sensing mechanisms hopefully lay a foundation for the development of novel perovskite gas sensing materials with enhanced stability, high sensitivity, and high selectivity.

Moisture-Induced Crystallinity Improvement for Efficient CsPbI3-xBrx Perovskite Solar Cells with Excess Cesium Bromide.

All-inorganic perovskite materials emerge to be potential promising candidates for photoelectric applications. However, their instability to moisture has seriously limited future applications. It is imperative to develop new fabrication methods to make the perovskite films moisture-tolerant. In this work, it is demonstrated that both crystallinity and photoelectronic performance of the CsPbI3-xBrx (x < 1) perovskite were enhanced with the existence of excess CsBr and employing moisture treatment. An intermediate product of CsI1-yBry/CsPbI3-xBrx was obtained by using an extra amount of CsBr, and the structural evolution toward a uniform CsPbI3-zBrz layer (as the Br- incorporation, the mole ratio increases from x to z) was initiated by moisture treatment, forming new (PbX6)4- octahedra with the bonding of CsI1-yBry to Pb2+. Attributed to the homogeneity of the stoichiometry and crystallinity, the photovoltaic performance of the moisture-treated perovskite films was improved, with the open-circuit voltage increased from 0.97 to 1.10 V and the power conversion efficiency from 10.89 to 13.09%.

Theoretical Evaluation of the Influence of Molecular Packing Mode on the Intramolecular Reorganization Energy of Oligothiophene Molecules.

Accurate determination of the relationships among packing mode, molecular structure and charge transfer mobility for oligothiophene analogues has been significantly impeded, due to the lack of crystal structure information. In the current study, molecular dynamics (MD) were used to investigate the packing mode of non-, methyl- and ethyl-substituted poly(3-alkylthiophenes) (P3ATs). Obvious conformational changes were observed when comparing the packed and isolated oligothiophene molecules, indicating the important influence of packing mode on the geometric structures of these materials. Considering the crucial role played by reorganization energy (RE) in the charge transfer process, both quantum mechanics (QM) and quantum mechanics/molecular mechanics (QM/MM) were performed to examine the impact of different conformations on energy. Our simulations revealed that the geometric structures have distinct effects on the RE. Our data suggest that MD could give a reliable packing mode of oligothiophene analogues, and that QM/MM is indispensable for precisely estimating RE.