The interfaces between absorber and transport layers are shown to be critical for perovskite device performance. However, quantitative characterization of interface recombination has so far proven to be highly challenging in working perovskite solar cells. Here, methylammonium lead halide (CH3NH3PbI3) perovskite solar cells are studied based on a range of different hole‐transport layers, namely, an inorganic hole‐transport layer CuOx, an organic hole‐transport layer poly(triarylamine) (PTAA), and a bilayer of CuOx/PTAA. The cells are completed by a [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM)/bathocuproine/Ag electron contact. Energy levels are characterized using photoelectron spectroscopy and recombination dynamics by combining steady‐state photoluminescence and transient photoluminescence with numerical simulations. While the PTAA‐based devices hardly show any interface recombination losses and open‐circuit voltages >1.2 V, substantial losses are observed for the samples with a direct CuOx/perovskite interface. These losses are assigned to a combination of energetic misalignment at the CuOx/perovskite interface coupled with increased interface recombination velocities at the perovskite/PCBM interface.

Organic–inorganic hybrid lead halide perovskites have gained significant attention as light-harvesting materials in thin-film photovoltaics due to their exceptional optoelectronic properties and simple fabrication process. The power conversion efficiency of perovskite solar cells (PSCs) has surged beyond 25% in a short time span. Their transition to commercial market is a “work in progress” due to limited long-term operational stability and the persisting environmental concern due to the presence of lead. Comprehensive investigations on the interplay of material composition and interfacial effects on the device performance of PSCs based on methylammonium lead iodide have shown the crucial role of an A-site cation in incipient deterioration of the material through external stimuli (moisture, light, oxygen, or heat). Consequently, a partial or complete replacement of A-site cations by up to four isoelectronic substituents has resulted in many new perovskite compositions. The correlations between the chemical composition and the optoelectronic properties are, however, not always easy to determine. A-site cation management is governed by stability and charge neutrality of the lattice, and the choices include Cs+-cations and organic cations such as CH3NH3+ or CH(NH2)2+ and combinations thereof. Since the size of the cations is an important structural parameter, an adequate compositional engineering of the A-site could effectively optimize the stability by reducing non-radiative defect sites and enhancing carrier lifetimes. This Perspective reflects on the experimental strategies for A-site cation management and their direct impact on the stability and device performance. It also highlights the opportunities and challenges for further research and industrial commercialization of PSCs.

An absorber layer that does not fully cover the substrate is a common issue for thin-film solar cells such as perovskites. However, models that describe the impact of pinholes on solar cell performance are scarce. Here, we demonstrate that certain combinations of contact layers suppress the negative impact of pinholes better than others. The absence of the absorber at a pinhole gives way to a direct electrical contact between the two semiconducting electron and hole transport layers. The key to understand how pinholes impact the solar cell performance is the resulting nonlinear diodelike behavior of the current across the interface between these two layers (commonly referred to as a shunt current). Based on experimentally obtained data that mimic the current−voltage characteristics across these interfaces, we develop a simple model to predict pinhole-induced solar cell performance deterioration. We investigate typical contact layer combinations such as TiO 2 /spiro-OMeTAD, poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate)/phenyl-C 61 -butyric acid methyl ester, and TiO 2 /poly(3-hexylthiophene). Our results directly apply to perovskite and other emerging inorganic thin-film solar cells, and the methodology is transferable to CIGS and CdTe. We find substantial differences between five commonly applied contact layer combinations and conclude that it is not sufficient to optimize the contact layers of any real-world thin-film solar cell only with regard to the applied absorber. Instead, in the context of laboratory and industrial fabrication, the tolerance against pinholes (i.e., the mitigation of shunt losses via existing pinholes) needs to be considered as an additional, important objective.

Electron transporting layers facilitating electron extraction and suppressing hole recombination at the cathode are crucial components in any thin-film solar cell geometry, including that of metal–halide perovskite solar cells. Amorphous tantalum oxide (Ta2O5) deposited by spin coating was explored as an electron transport material for perovskite solar cells, achieving power conversion efficiency (PCE) up to ~14%. Ultraviolet photoelectron spectroscopy (UPS) measurements revealed that the extraction of photogenerated electrons is facilitated due to proper alignment of bandgap energies. Steady-state photoluminescence spectroscopy (PL) verified efficient charge transport from perovskite absorber film to thin Ta2O5 layer. Our findings suggest that tantalum oxide as an n-type semiconductor with a calculated carrier density of ~7 × 1018/cm3 in amorphous Ta2O5 films, is a potentially competitive candidate for an electron transport material in perovskite solar cells.