Space-charge-limited current (SCLC) measurements have been widely used to study the charge carrier mobility and trap density in semiconductors. However, their applicability to metal halide perovskites is not straightforward, due to the mixed ionic and electronic nature of these materials. Here, we discuss the pitfalls of SCLC for perovskite semiconductors, and especially the effect of mobile ions. We show, using drift-diffusion (DD) simulations, that the ions strongly affect the measurement and that the usual analysis and interpretation of SCLC need to be refined. We highlight that the trap density and mobility cannot be directly quantified using classical methods. We discuss the advantages of pulsed SCLC for obtaining reliable data with minimal influence of the ionic motion. We then show that fitting the pulsed SCLC with DD modeling is a reliable method for extracting mobility, trap, and ion densities simultaneously. As a proof of concept, we obtain a trap density of 1.3 × 10 13 cm –3 , an ion density of 1.1 × 10 13 cm –3 , and a mobility of 13 cm 2 V –1 s –1 for a MAPbBr 3 single crystal.

The short exciton diffusion length associated with most classical organic semiconductors used in organic photovoltaics (5-20 nm) imposes severe limits on the maximum size of the donor and acceptor domains within the photoactive layer of the cell. Identifying materials that are able to transport excitons over longer distances can help advancing our understanding and lead to solar cells with higher efficiency. Here, we measure the exciton diffusion length in a wide range of nonfullerene acceptor molecules using two different experimental techniques based on photocurrent and ultrafast spectroscopy measurements. The acceptors exhibit balanced ambipolar charge transport and surprisingly long exciton diffusion lengths in the range of 20 to 47 nm. With the aid of quantum-chemical calculations, we are able to rationalize the exciton dynamics and draw basic chemical design rules, particularly on the importance of the end-group substituent on the crystal packing of nonfullerene acceptors.

Metal halide perovskite semiconductors have sprung to the forefront of research into optoelectronic devices and materials, largely because of their remarkable photovoltaic efficiency records above 25% in single-junction devices and 28% in tandem solar cells, achieved within a decade of research. Despite this rapid progress, ionic conduction within the semiconductor still puzzles the community and can have a significant impact on all metal halide perovskite-based optoelectronic devices because of its influence upon electronic and optoelectronic processes. This phenomenon thus also makes the interpretation of electrical characterization techniques, which probe the fundamental properties of these materials, delicate and complex. For example, space-charge limited current measurements are widely used to probe defect densities and carrier mobilities in perovskites. However, the influence of mobile ions upon these measurements is significant but has yet to be considered. Here we report the effect of mobile ions upon electronic conductivity during space-charge limited current measurements of MAPbBr 3 single crystals and show that conventional interpretations deliver erroneous results. We introduce a pulsed-voltage space-charge limited current procedure to achieve reproducible current−voltage characteristics without hysteresis. From this revised pulsed current−voltage sweep, we elucidate a lower bound trap-density value of 2.8 ± 1.8 × 10 12 cm −3 in MAPbBr 3 single crystals. This work will lead to more accurate characterization of halide perovskite semiconductors and ultimately more effective device optimization.

A fullerene additive adjusts the miscibility between donor and acceptor for morphology optimization and reduces bimolecular recombination, assisting significant improvement of fill factor and efficiency.

The reported power conversion efficiencies (PCEs) of nonfullerene acceptor (NFA) based organic photovoltaics (OPVs) now exceed 14% and 17% for single‐junction and two‐terminal tandem cells, respectively. However, increasing the PCE further requires an improved understanding of the factors limiting the device efficiency. Here, the efficiency limits of single‐junction and two‐terminal tandem NFA‐based OPV cells are examined with the aid of a numerical device simulator that takes into account the optical properties of the active material(s), charge recombination effects, and the hole and electron mobilities in the active layer of the device. The simulations reveal that single‐junction NFA OPVs can potentially reach PCE values in excess of 18% with mobility values readily achievable in existing material systems. Furthermore, it is found that balanced electron and hole mobilities of >10 −3 cm 2 V −1 s −1 in combination with low nongeminate recombination rate constants of 10 −12 cm 3 s −1 could lead to PCE values in excess of 20% and 25% for single‐junction and two‐terminal tandem OPV cells, respectively. This analysis provides the first tangible description of the practical performance targets and useful design rules for single‐junction and tandem OPVs based on NFA materials, emphasizing the need for developing new material systems that combine these desired characteristics.