Passivation of organometal halide perovskites with polar molecules has been recently demonstrated to improve the photovoltaic device efficiency and stability. However, the mechanism is still elusive. Here, it is found that both polymers with large and small dipole moment of 3.7 D and 0.6 D have negligible defect passivation effect on the MAPbI 3 perovskite films as evidenced by photo thermal deflection spectroscopy. The photovoltaic devices with and without the polymer additives also have comparable power conversion efficiencies around 19%. However, devices with the additives have noticeable improvement in stability under continuous light irradiation. It is found that although the initial mobile ion concentrations are comparable in both devices with and without the additives, the additives can strongly suppress the ion migration during the device operation. This contributes to the significantly enhanced electrical-field stress tolerance of the perovskite solar cells (PVSCs). The PVSCs with polymer additives can operate up to −2 V reverse voltage bias which is much larger than the breakdown voltage of −0.5 V that has been commonly observed. This study provides insight into the role of additives in perovskites and the corresponding device degradation mechanism.

For quantum-dot photodiodes comprising an electron-transporting layer assembled of ZnO nanoparticles, the light emitter/absorber generally exhibits enhanced optoelectronic performance after the device is shelf-aged. To understand the so-called positive aging effect, the optoelectronic properties of ZnO nanoparticles are investigated at the thin film and device level as a function of aging time. It is evidenced that the aging process is driven by a surface-stabilizing mechanism of ZnO nanoparticles, in which the active surface adsorption sites for oxygen are gradually but irreversibly stabilized, i.e.. with surface termination of HO-ZnO, leading to reduced nonradiative recombination and increased built-in potential in the adjacent photoactive layer. This work provides insight into new synthetic routes for minimizing the negative impact caused by the aging process.

Mixed‐halide wide‐bandgap perovskites are key components for the development of high‐efficiency tandem structured devices. However, mixed‐halide perovskites usually suffer from phase‐impurity and high defect density issues, where the causes are still unclear. By using in situ photoluminescence (PL) spectroscopy, it is found that in methylammonium (MA+)‐based mixed‐halide perovskites, MAPb(I0.6Br0.4)3, the halide composition of the spin‐coated perovskite films is preferentially dominated by the bromide ions (Br−). Additional thermal energy is required to initiate the insertion of iodide ions (I−) to achieve the stoichiometric balance. Notably, by incorporating a small amount of formamidinium ions (FA+) in the precursor solution, it can effectively facilitate the I− coordination in the perovskite framework during the spin‐coating and improve the composition homogeneity of the initial small particles. The aggregation of these homogenous small particles is found to be essential to achieve uniform and high‐crystallinity perovskite film with high Br− content. As a result, high‐quality MA0.9FA0.1Pb(I0.6Br0.4)3 perovskite film with a bandgap (Eg) of 1.81 eV is achieved, along with an encouraging power‐conversion‐efficiency of 17.1% and open‐circuit voltage (Voc) of 1.21 V. This work also demonstrates the in situ PL can provide a direct observation of the dynamic of ion coordination during the perovskite crystallization.

A PbI2–(CsI)0.15–(FAI)x intermediate complex associated with preheating enables air-processed, high-efficiency Cs0.15FA0.85PbI3 PSCs for the first time.