Surface trap–mediated nonradiative charge recombination is a major limit to achieving high-efficiency metal-halide perovskite photovoltaics. The ionic character of perovskite lattice has enabled molecular defect passivation approaches through interaction between functional groups and defects. However, a lack of in-depth understanding of how the molecular configuration influences the passivation effectiveness is a challenge to rational molecule design. Here, the chemical environment of a functional group that is activated for defect passivation was systematically investigated with theophylline, caffeine, and theobromine. When N-H and C=O were in an optimal configuration in the molecule, hydrogen-bond formation between N-H and I (iodine) assisted the primary C=O binding with the antisite Pb (lead) defect to maximize surface-defect binding. A stabilized power conversion efficiency of 22.6% of photovoltaic device was demonstrated with theophylline treatment.

Addressing the toxicity issue in lead-based perovskite compounds by seeking other nontoxic candidate elements represents a promising direction to fabricate lead-free perovskite solar cells. Recently, Cs 2 AgBiBr 6 double perovskite achieved by replacing two Pb 2+ with Ag + and Bi 3+ in the crystal lattice has drawn much attention owing to the convenient substitution of its chemical compositions. Herein, the dependence of the optoelectronic properties and corresponding photovoltaic performance of Cs 2 AgBiBr 6 thin films on the deposition methods of vacuum sublimation and solution processing is investigated. Compared to the vacuum sublimation based one, the solution-processed Cs 2 AgBiBr 6 shows inherently higher crystallinity, narrower electronic bandgap, longer photoexcitation lifetime, and higher mobility. The excellent optoelectronic properties are attributed to the accurate composition stoichiometry of Cs 2 AgBiBr 6 films based on solution processing. These merits enable the corresponding perovskite solar cells to deliver a champion power conversion efficiency (PCE) of 2.51%, which is the highest PCE in the Cs 2 AgBiBr 6based double perovskite solar cells to date. The finding in this work provides a clear clue that a precise composition stoichiometry could guarantee the formation of high quality multicomponent perovskite films.

their simple device architecture, easy fabrication procedure, and low temperature processing. Many innovative approaches have been used to improve the performance of planar PSCs, such as PCE in 15.4% by dual-source vapor deposition technique, [ 5 ] 13.9% by a solvent-induced fast crystallization, [ 13 ] 13.1% by alkyl halide additives, [ 14 ] 15.1% by poly(3,4ethylene-dioxythiophene):polystyrenesulfo nate (PEDOT:PSS)-GeO 2 underlayer modifi cation, [ 15 ] 15.7% by cathode and anode dual modifi cations. [ 16 ] Despite the device effi ciency has achieved signifi cant progress, these perovskite solar cells have to be fabricated in very rigorous condition, i.e., in glove box with a humidity less than 1%. [ 17,18 ] To enable PSCs' commercial applications, one large challenge is to resolve the cell stability issue because the perovskite fi lms would be degraded gradually with time in air. [ 12,19 ] To date, a few studies have been focused on the processing circumstance and the stability issue of perovskite solar cells. You et al. [ 20 ] investigated the effect of humid environment on the thermal annealing of perovskite precursor fi lms. Bass et al. [ 21 ] reported that the moisture can prompt the crystallization of lead organohalide perovskites into the expected cubic phase. Yang et al. [ 22 ] demonstrated a fl exible PSC with a PCE of 7.14% which was fabricated under ambient. These studies suggest that the moisture plays an important role in the performance of perovskite solar cells. However, few studies focus on the mechanism how the moisture affects the cell degradation. In addition, controllable growth of the crystalline perovskite fi lms is regarded as the most important factor to obtain high-effi ciency PSCs. In general, the growth of crystalline perovskite fi lms is sensitively dependent on the solution concentration, precursor composition, solvent choice, deposition temperature, and so on. Crystallization controlling by manipulating the perovskite nucleation and growth can improve the fi lm morphology and coverage effectively. [ 22,23 ] Especially, solvent engineering plays very fascinating role in the controllable growth of solution-processed crystalline fi lms. Liang et al. reported that crystallization rate of perovskites could be controlled by incorporating 1,8-diiodooctane (DIO) additives into the precursor solution. [ 24 ] Song et al. demonstrated that perovskite crystallinity could be improved by introducing 1-chloronaphthalene (CN) additive in the perovskite precursor solution, which results in a 30% effi ciency improvement compared with the reference devices. [ 25 ] Most recently, Wu et al. obtained high quality PbI 2 fi lm with good coverage by A key issue for perovskite solar cells is the stability of perovskite materials due to moisture effects under ambient conditions, although their effi ciency is improved constantly. Herein, an improved CH 3 NH 3 PbI 3− x Cl x perovskite quality is demonstrated with good crystallization and stability by using water as an additive during crystal perovskite growt...