Hybrid organo-metal halide perovskites are an exciting new class of solar absorber materials and have exhibited a rapid increase in solar cell efficiencies throughout the past two years to over 17% in both meso-structured and thin-film device architectures. We observe slow transient effects causing hysteresis in the current–voltage characterization of these devices that can lead to an over- or underestimation of the solar cell device efficiency. We find that the current–voltage (IV) measurement scan direction, measurement delay time, and light and voltage bias conditions prior to measurement can all have a significant impact upon the shape of the measured IV light curves and the apparent device efficiency. We observe that hysteresis-free light IV curves can be obtained at both extremely fast and slow voltage scan rates but only in the latter case are quasi-steady-state conditions achieved for a valid power conversion efficiency measurement. Hysteretic effects are also observed in devices utilizing alternative selective contacts but differ in magnitude and time scale, suggesting that the contact interfaces have a big effect on transients in perovskite-absorber devices. The transient processes giving rise to hysteresis are consistent with a polarization response of the perovskite absorber that results in changes in the photocurrent extraction efficiency of the device. The strong dependence of the hysteresis on light and voltage biasing conditions in thin film devices for a period of time prior to the measurement suggests that photo-induced ion migration may additionally play an important role in device hysteresis. Based on these observations, we provide recommendations for correct measurement and reporting of IV curves for perovskite solar cell devices.

Semi-transparent perovskite solar cells with silver nanowires are stacked on CIGS and Si to achieve solid-state polycrystalline tandems with efficiency improvement.

Improved stability and efficiency of two-terminal monolithic perovskite-silicon tandem solar cells will require reductions in recombination losses. By combining a triple-halide perovskite (1.68 electron volt bandgap) with a piperazinium iodide interfacial modification, we improved the band alignment, reduced nonradiative recombination losses, and enhanced charge extraction at the electron-selective contact. Solar cells showed open-circuit voltages of up to 1.28 volts in p-i-n single junctions and 2.00 volts in perovskite-silicon tandem solar cells. The tandem cells achieve certified power conversion efficiencies of up to 32.5%.

Daily temperature variations induce phase transitions and lattice strains in halide perovskites, challenging their stability in solar cells. We stabilized the perovskite black phase and improved solar cell performance using the ordered dipolar structure of β-poly(1,1-difluoroethylene) to control perovskite film crystallization and energy alignment. We demonstrated p-i-n perovskite solar cells with a record power conversion efficiency of 24.6% over 18 square millimeters and 23.1% over 1 square centimeter, which retained 96 and 88% of the efficiency after 1000 hours of 1-sun maximum power point tracking at 25° and 75°C, respectively. Devices under rapid thermal cycling between −60° and +80°C showed no sign of fatigue, demonstrating the impact of the ordered dipolar structure on the operational stability of perovskite solar cells.

Solar cells based on metal-halide perovskite semiconductors inspire high hopes for efficient low cost solar cell technology. This material class exhibits a facile tunability of the band gap making them interesting for multi-junction device technology. We here compare and highlight trends in the band gap tunability and device performance metrics in reported metal halide perovskite alloys of a wide compositional range from low band gap compounds, such as FA_0.75Cs_0.25Sn_0.5Pb_0.5I₃ with an absorption onset of 1.2 eV, to high bandgap compounds, such as CsPbBr₃ with an absorption onset close to 2.4 eV. In between, metal halide perovskites can seemingly be seamlessly tuned by compositional engineering. However, mixed bromide–iodide compounds with band gaps above 1.7 eV often exhibit photo-induced phase segregation inducing domains with lower band gaps that emit photons of low energy. This effect also reduces the photoluminescence quantum yield and hence the maximum open circuit voltage achievable in devices. This highlight summarizes general trends for metal halide perovskites with respect to their absorption onset. Furthermore recent progress as well as possible roadblocks for the band gap tunability of metal halide perovskites are highlighted as this is of particular importance for the development of multifunction solar cell technology.