Tin oxide (SnO2) is an emerging electron transport layer (ETL) material in halide perovskite solar cells (PSCs). Among current limitations, open‐circuit voltage (VOC) loss is one of the major factors to be addressed for further improvement. Here a bilayer ETL consisting of two SnO2 nanoparticle layers doped with different amounts of ammonium chloride is proposed. As demonstrated by photoelectron spectroscopy and photophysical studies, the main effect of the novel ETL is to modify the energy level alignment at the SnO2/perovskite interface, which leads to decreased carrier recombination, enhanced electron transfer, and reduced voltage loss. Moreover, X‐ray diffraction reveals reduced strain in perovskite layers grown on bilayer ETLs with respect to single‐layer ETLs, further contributing to a decrease of carrier recombination processes. Finally, the bilayer approach enables the more reproducible preparation of smooth and pinhole‐free ETLs as compared to single‐step deposition ETLs. PSCs with the doped bilayer SnO2 ETL demonstrate strongly increased VOC values of up to 1.21 V with a power conversion efficiency of 21.75% while showing negligible hysteresis and enhanced stability. Moreover, the SnO2 bilayer can be processed at low temperature (70 °C), and has therefore a high potential for use in tandem devices or flexible PSCs.

Hybrid perovskites have attracted much attention as promising photovoltaic materials in the past few years. However, the fundamental understanding of their crystallization behavior lags far behind the pace of empirical solar cell efficiency improvement. Methylammonium iodide (MAPbI 3 ) is a widely studied reference compound whose solar cell performance can be improved by chloride addition (e.g., in the form of PbCl 2 ) during the thin-film preparation. Because of the large difference in the ionic radii of both halides, no mixed perovskites MAPbI 3−x Cl x are formed and generally only minute amounts of chlorine can be detected in the final MAPbI 3 thin films. Here, we demonstrate by means of a variety of complementary X-ray diffraction (XRD) techniques that, unexpectedly, the formation mechanism proceeds via an initial MAPbCl 3 layer, which subsequently transforms to MAPbI 3 in an anion exchange reaction during the thermal annealing step, completing the thin-film preparation. The perovskite lattice is highly strained along the process, much more than what is expected from the sole effect of the difference between the thermal expansion coefficients of the perovskite and the substrate. At room temperature, the existence of a double [hh0]/[00l] texture is explained by the ferroelastic character of the cubic/tetragonal transition of MAPbI 3 , which induces the formation of twins. The relative population of these domains is correlated to their strain level. Although strain is known to weaken the stability of the MAPbI 3 phase, our results unambiguously show that it also favors the reproducibility of the thin-film microstructure. When used as active layers in solar cells, the dependence of the cell efficiency and stability on the annealing time is in striking accordance with the formation kinetics of MAPbI 3 , as revealed by the XRD measurements. Therefore, the understanding of the crystallization behavior achieved with the present approach, applicable also to other types of metal halide perovskites, allows for the rational optimization of the device performance and long-term stability.

The impact of the chemical structure and molecular order on the charge transport properties of two donor-acceptor copolymers in their neutral and doped states is investigated. Both polymers comprise 3,7-bis((E)-7-fluoro-1-(2-octyl-dodecyl)-2-oxoindolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b′] difuran-2,6-dione (FBDOPV) as electron-accepting unit, copolymerized with 9,9-dioctyl-fluorene (P(FBDOPV-F)) or with 3-dodecyl-2,2′-bithiophene (P(FBDOPV-2T-C 12 )). These copolymers possess an amorphous and semicrystalline nature, respectively, and exhibit remarkable electron mobilities of 0.065 and 0.25 cm 2 V -1 s -1 in field effect transistors. However, after chemical n-doping with 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl) dimethylamine (N-DMBI), electrical conductivities four orders of magnitude higher can be achieved for P(FBDOPV-2T-C 12 ) (σ = 0.042 S cm −1 ). More charge-transfer complexes are formed between P(FBDOPV-F) and N-DMBI, but the highly localized polaronic states poorly contribute to the charge transport. Doped P(FBDOPV-2T-C 12 ) exhibits a negative Seebeck coefficient of -265 µV K −1 and a thermoelectric power factor (PF) of 0.30 µW m −1 K −2 at 303 K which increases to 0.72 µW m −1 K −2 at 388 K. The in-plane thermal conductivity (κ || = 0.53 W m −1 K −1 ) on the same micrometer-thick solution-processed film is measured, resulting in a figure of merit (ZT) of 5.0 × 10 −4 at 388 K. The results provide important design guidelines to improve the doping efficiency and thermoelectric properties of n-type organic semiconductors.

Controlling the microstructure of hybrid halide perovskite thin films is essential for optimizing their performance in optoelectronic devices. It is well established that the strain state of the perovskite layer affects its stability. Likewise, the orientation of the perovskite lattice is a determining parameter as these materials have shown pronounced anisotropies in their physical and mechanical properties. In this work, the authors focus on the understanding of the mechanisms that govern the strain and texture observed in MAPbI3 thin films deposited on various oxide substrates. A thorough study of the evolution of the strain of the perovskite layer upon cooling down to room temperature from the crystallization temperature (100 °C) shows an essentially relaxed behavior of the perovskite layers. This result contradicts the commonly accepted hypothesis according to which MAPbI3 layers synthesized above ambient temperature are strained due to the large mismatch in the thermal expansion coefficients of the perovskite and its substrate. The texture in MAPbI3 layers is investigated by means of synchrotron full‐field diffraction X‐ray microscopy. This technique allows the direct observation of the [hh0] and [00l]‐oriented domains at the origin of the observed textures, demonstrating both their twin and ferroelastic nature. The stability of the different domain orientations is investigated by DFT calculations, illustrating the determining role of the chemical environment at the film‐substrate interface. PbI2‐ terminated surfaces are found to favor the [hh0] orientations while for MAI‐terminated ones, both [hh0] and [00l] domains are equally stabilized. The different results constitute an important step of clarification and understanding from the perspective of controlling the microstructure of perovskite layers.

Nickel oxide (NiO x ) is an emerging hole transport layer (HTL) material in halide perovskite solar cells (PSCs), combining high hole mobility, transparency, and stability. Current limitations of the device performance are mainly related to the inefficient hole extraction caused by contact problems between NiO x and the perovskite layer. Based on its expected strong interaction with both the NiO x surface and the perovskite layer, we selected 4-dimethylaminopyridine (DMAP) as a molecular passivation agent for the HTL. Photoelectron spectroscopy and photophysical studies demonstrate that DMAP passivation creates a more favorable band alignment at the NiO x /perovskite interface. This leads to decreased carrier recombination near the interface and enhanced hole transfer. In addition, X-ray diffraction reveals reduced strain, improved crystalline quality, and a redistribution of excess PbI2 in perovskite layers grown on DMAP-passivated NiO x . As a consequence, PSCs with the DMAP-modified HTL exhibit a strongly increased fill factor and power conversion efficiency with values close to 80 and 18%, respectively. Moreover, they show negligible hysteresis and enhanced environmental stability compared to devices with untreated HTLs.

In article number 2005671, Jiajiu Ye, Yueli Liu, Peter Reiss, and co‐workers use a bilayer of tin oxide nanoparticles, doped with different amounts of ammonium chloride, as the electron transport layer in lead halide perovskite solar cells. It enables improved electron transfer from the solar light harvesting perovskite and gives access to high open circuit voltage, power conversion efficiency and enhanced stability.

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Tin (IV) oxide is a highly promising electron transport layer (ETL) for lead halide perovskite solar cells due to its high conductivity, transparency, wide band gap, and the possibility of low-temperature processing. Nonetheless, charge carrier recombination processes at the SnO2/perovskite interface diminish the device performance. Here, we demonstrate that SnO2 doping with guanidine hydrochloride (G-SnO2) leads to efficient surface passivation and a larger band offset between the ETL and the perovskite layer, resulting in reduced voltage losses and faster electron transfer. Moreover, G-SnO2 facilitates the growth of highly crystalline perovskite layers. Consequently, a power conversion efficiency of up to 23.48% and a high open-circuit voltage of 1.18 V are obtained in solar cells incorporating the G-SnO2 ETL. These devices also exhibited negligible hysteresis and maintained more than 96% of their initial power conversion efficiency after 1,250 h exposure to the air without encapsulation.