Perovskite photovoltaics offer a compelling combination of extremely low-cost, ease of processing and high device performance. The optoelectronic properties of the prototypical CH3NH3PbI3 can be further adjusted by introducing other extrinsic ions. Specifically, chlorine incorporation has been shown to affect the morphological development of perovksite films, which results in improved optoelectronic characteristics for high efficiency. However, it requires a deep understanding to the role of extrinsic halide, especially in the absence of unpredictable morphological influence during film growth. Here we report an effective strategy to investigate the role of the extrinsic ion in the context of optoelectronic properties, in which the morphological factors that closely correlate to device performance are mostly decoupled. The chlorine incorporation is found to mainly improve the carrier transport across the heterojunction interfaces, rather than within the perovskite crystals. Further optimization according this protocol leads to solar cells achieving power conversion efficiency of 17.91%.

Perovskite solar cells have received considerable attention in recent years as a promising material capable of developing high performance photovoltaic devices at a low cost. Its high absorption coefficient, tunable band gap, low temperature processing and abundant elemental constituents provide numerous advantages over most thin film absorber materials. In this feature article, we discuss the current status of CH3NH3PbX3 (X = I, Br, Cl) based photovoltaic devices and provide a comprehensive review of CH3NH3PbX3 device structures, film properties, fabrication methods, and photovoltaic performance. We emphasize the importance of perovskite film formation and properties in achieving highly efficient photovoltaic devices. The flexibility and simplicity of perovskite fabrication methods allow use of mesoporous and planar device architectures. A variety of processing techniques are currently employed to form the highest quality CH3NH3PbX3 films that include precursor modifications, thermal annealing and post-deposition treatments. Here we outline and discuss the resulting material qualities and device performances. Suggestions regarding needed improvements and future research directions are provided based on the current field of available literature.

Based on rotational dynamics measurements carried out with isomorphic co-crystals formed by halogen-bonding (XB) between tritylacetylene halides (TrX) and diazabicyclo[2.2.2]octane (dabco), we were able to distinguish the sources of the enthalpic and entropic components in the rotational free energy barrier. We describe the formation of the 1:1 co-crystals (TrX···N(R)N) obtained from 1 equiv of dabco and 1 equiv of either TrI or TrBr, respectively, to give 4a and 4b instead of the potential 2:1 complexes. The co-crystals were prepared by solvent evaporation and mechanochemical synthesis. No co-crystal with TrCl was obtained, reflecting the weaker nature of the TrCl···NR interaction. Single-crystal X-ray diffraction confirmed structures that resemble a spinning top on a tripod and revealed that the two XB co-crystals are isomorphous, with slightly different C-X···NR (X = I, Br) distances and packing interactions. Quadrupolar-echo H NMR experiments withH-labeled samples showed that fast rotation of dabco in these co-crystals follows a six-fold potential energy surface with three lowest energy minima. Variable-temperature H NMR spin-lattice relaxation (VTH T) data revealed rotational dynamics with indistinguishable pre-exponential factors and small but distinguishable activation energies. The activation energy of 4b (E = 0.71 kcal mol) is the lowest reported in the field of amphidynamic crystals. Using the Eyring equation, we established that their activation entropy for rotation is small but negative (ΔS = -3.0 cal mol K), while there is almost a 2-fold difference in activation enthalpies, with 4a having a higher barrier (ΔH = 0.95 kcal mol) than 4b (ΔH = 0.54 kcal mol). Analysis of the rotator cavity in the two co-crystals revealed subtle differences in steric interactions that account for their different activation energies.

A facile and quantitative approach has been demonstrated to improve the TiO2 ETL in perovskite solar cells, with enhanced carrier extraction.