The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10. 1002/aenm.201700414. used mesoporous TiO 2 ESLs, but the complex synthesis procedure and requirement of sintering at temperatures above 450 °C make the process undesirable for large-scale roll-to-roll manufacturing. [3] Therefore, significant efforts have been made on developing efficient low-temperature planar PVSCs. [23][24][25][26][27] For PVSCs with a regular configuration, the more popular low-temperature processed ESL materials are transition metal oxides such as TiO 2 , [26,28] ZnO, [29,30] SnO 2 , [31,32] and Zn 2 SnO 4 . [23,24] Due to its relatively simple synthesis process, chemical stability, [33][34][35] and high electron mobility, [33,36,37] SnO 2 ESL has shown great promise in making efficient planar PVSCs. For example, Ke et al. first fabricated PVSCs with PCE over 16% using a low-temperature solution processed SnO 2 ESL. [31] Since then, many other groups have also reported efficient PVSCs using low-temperature processed ESLs. [23,33,37] Recently, Gratzel and co-workers reported planar PVSCs with efficiencies approaching 21% using SnO 2 ESLs synthesized by low-temperature chemical bath deposition. [32] Low-temperature SnO 2 ESLs have been deposited by various methods including spin-coating, [31] dual-fuel combustion, [38] chemical-bath deposition, [32] and atomic-layer deposition (ALD). [34,39] ALD produces the most compact thin films compared to the other methods mentioned. However, PVSCs using ALD SnO 2 ESLs often show current-density-voltage (J-V) Adv. Energy

Low‐bandgap mixed tin–lead perovskite solar cells (PSCs) have been attracting increasing interest due to their appropriate bandgaps and promising application to build efficient all‐perovskite tandem cells, an effective way to break the Shockley–Queisser limit of single‐junction cells. Tin fluoride (SnF2) has been widely used as a basis along with various strategies to improve the optoelectronic properties of low‐bandgap SnPb perovskites and efficient cells. However, fully understanding the roles of SnF2 in both films and devices is still lacking and fundamentally desired. Here, the functions of SnF2 in both low‐bandgap (FASnI3)0.6(MAPbI3)0.4 perovskite films and efficient devices are unveiled. SnF2 regulates the growth mode of low‐bandgap SnPb perovskite films, leading to highly oriented topological growth and improved crystallinity. Meanwhile, SnF2 prevents the oxidation of Sn2+ to Sn4+ and reduces Sn vacancies, leading to reduced background hole density and defects, and improved carrier lifetime, thus largely decreasing nonradiative recombination. Additionally, the F− ion preferentially accumulates at hole transport layer/perovskite interface with high SnF2 content, leading to more defects. This work provides in‐depth insights into the roles of SnF2 additives in low‐bandgap SnPb films and devices, assisting in further investigations into multiple additives and approaches to obtain efficient low‐bandgap PSCs.

A detailed investigation of the impact of molecular weight distribution of a photoactive polymer, poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT), on photovoltaic device performance and carrier transport properties is reported. It is found that different batches of as‐received polymers have substantial differences in their molecular weight distribution. As revealed by gel permeation chromatography (GPC), two peaks can generally be observed. One of the peaks corresponds to a high molecular weight component and the other peak corresponds to a low molecular weight component. Photovoltaic devices fabricated with a higher proportion of low molecular weight component have power conversion efficiencies (PCEs) reduced from 5.7% to 2.5%. The corresponding charge carrier mobility at the short‐circuit region is also significantly reduced from 2.7 × 10−5 to 1.6 × 10−8 cm2 V−1 s−1. The carrier transport properties of the polymers at various temperatures are further analyzed by the Gaussian disorder model (GDM). All polymers have similar energetic disorders. However, they appear to have significant differences in carrier hopping distances. This result provides insight into the origin of the molecular weight effect on carrier transport in polymeric semiconducting materials.

Thin-film optoelectronic devices based on polycrystalline organolead-halide perovskites have recently become a topic of intense research. Single crystals of these materials have been grown from solution with electrical properties superior to those of polycrystalline films. In order to enable the development of more complex device architectures based on organolead-halide perovskite single crystals, we developed a process to form epitaxial layers of methylammonium lead iodide (MAPbI) on methylammonium lead bromide (MAPbBr) single crystals. The formation of the MAPbI layer is found to be dominated by the diffusion of halide ions, leading to a shift in the photoluminescence and absorption spectra. X-ray diffraction measurements confirm the single-crystal nature of the MAPbI layer, while carrier transport measurements show that the converted layer retains the high carrier mobility typical of single-crystal perovskite materials. Such heterostructures on perovskite single crystals open possibilities for new types of devices.

The morphological evolution is initiated by L–L or L–S phase separation (left) and further developed by molecular mobility, governed by polymer–solvent interactions which determine the final domain size of the BHJ layer (right).