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The remarkable performance of hybrid perovskite photovoltaics is attributed to their long carrier lifetimes and high photoluminescence (PL) efficiencies. High-quality films are associated with slower PL decays, and it has been claimed that grain boundaries have a negligible impact on performance. We used confocal fluorescence microscopy correlated with scanning electron microscopy to spatially resolve the PL decay dynamics from films of nonstoichiometric organic-inorganic perovskites, CH3NH3PbI3(Cl). The PL intensities and lifetimes varied between different grains in the same film, even for films that exhibited long bulk lifetimes. The grain boundaries were dimmer and exhibited faster nonradiative decay. Energy-dispersive x-ray spectroscopy showed a positive correlation between chlorine concentration and regions of brighter PL, whereas PL imaging revealed that chemical treatment with pyridine could activate previously dark grains.

Organic–inorganic perovskites such as CH 3 NH 3 PbI 3 are promising materials for a variety of optoelectronic applications, with certified power conversion efficiencies in solar cells already exceeding 21%. Nevertheless, state-of-the-art films still contain performance-limiting non-radiative recombination sites and exhibit a range of complex dynamic phenomena under illumination that remain poorly understood. Here we use a unique combination of confocal photoluminescence (PL) microscopy and chemical imaging to correlate the local changes in photophysics with composition in CH 3 NH 3 PbI 3 films under illumination. We demonstrate that the photo-induced ‘brightening’ of the perovskite PL can be attributed to an order-of-magnitude reduction in trap state density. By imaging the same regions with time-of-flight secondary-ion-mass spectrometry, we correlate this photobrightening with a net migration of iodine. Our work provides visual evidence for photo-induced halide migration in triiodide perovskites and reveals the complex interplay between charge carrier populations, electronic traps and mobile halides that collectively impact optoelectronic performance. Visual evidence for photo-induced ionic migration in perovskite films without contacts is lacking. Here, the authors use a unique combination of confocal photoluminescence microscopy and chemical imaging to correlate the local changes in photophysics with composition in CH 3 NH 3 PbI 3 films under illumination.

The interplay of spin, energetics and delocalization of the electronic excitations are shown to create a spin blockade of electron–hole recombination in organic photovoltaic cells, resulting in high quantum efficiencies. Coping with loss in organic optoelectronics State-of-the-art solar cells based on blends of organic semiconductors can show impressive performance characteristics — which is perhaps surprising, given the nature of the interactions that take place between the photogenerated electrons and holes, where recombination of the photogenerated charges might be expected to significantly reduce overall device efficiency. Akshay Rao et al . have looked in detail at this loss mechanism, and identify the key physical factors that can suppress electron–hole recombination in these materials. This mechanistic information should provide valuable guidance for the design of future high-performance organic optoelectronic systems. In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination 1 . In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor–acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors 2 . The nanoscale morphology and high charge densities give a high rate of electron–hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes 3 . Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency 4 . Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer–fullerene systems in which the lowest-energy molecular triplet exciton (T 1 ) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T 1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet ( 1 CT) and spin-triplet ( 3 CT) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of 3 CT states to T 1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of 3 CT states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photoconversion systems, enabling the suppression of electron–hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.

Efficient delocalization of photo-generated excitons is a key to improving the charge-separation efficiencies in state-of-the-art organic photovoltaic (OPV) absorber. While the delocalization in non-fullerene acceptors has been widely studied, we expand the scope by studying the properties of the conjugated polymer donor D18 on both the material and device levels. Combining optical spectroscopy, X-ray diffraction, and simulation, we show that D18 exhibits stronger π–π interactions and interchain packing compared to classic donor polymers, as well as higher external photoluminescence quantum efficiency (~26%). Using picosecond transient absorption spectroscopy and streak camera photoluminescence measurements, we show that the initial D18 excitons form delocalized intermediates, which decay radiatively with high efficiency in neat films. In single-component OPV cells based on D18, these intermediate excitations can be harvested with an internal quantum efficiency >30%, while in blends with acceptor Y6 they provide a pathway to free charge generation that partially bypasses performance-limiting charge-transfer states at the D18:Y6 interface. Our study demonstrates that donor polymers can be further optimized using similar design strategies that have been successful for non-fullerene acceptors, opening the door to even higher OPV efficiencies. Donor exciton delocalization and its impact on photovoltaic performance of organic solar cells remains less explored. Here, the authors found that delocalized excitons are formed in aggregates of the donor polymer D18, and that these delocalized excitons mediate charge generation in solar cells.

Blends of conjugated polymers with fullerenes, polymers, or nanocrystals make promising materials for low-cost photovoltaic applications. Different processing conditions affect the efficiencies of these solar cells by creating a variety of nanostructured morphologies, however, the relationship between film structure and device efficiency is not fully understood. We introduce time-resolved electrostatic force microscopy (EFM) as a means to measure photoexcited charge in polymer films with a resolution of 100 nm and 100 μs. These EFM measurements correlate well with the external quantum efficiencies measured for a series of polymer photodiodes, providing a direct link between local morphology, local optoelectronic properties and device performance. The data show that the domain centres account for the majority of the photoinduced charge collected in polyfluorene blend devices. These results underscore the importance of controlling not only the length scale of phase separation, but also the composition of the domains when optimizing nanostructured solar cells.

Compositional heterogeneity in wide-bandgap (1.8 − 2.1 eV) mixed-halide perovskites is a key bottleneck in the processing of high-quality solution-processed thin films and prevents their application in efficient multijunction solar cells. Notably, mixed-cation (formamidinium-methylammonium) wide-bandgap perovskite films are prone to form micrometer-scale wrinkles which can interfere with the smooth surfaces ideal for multijunction devices. Here, we study the formation dynamics of wrinkled mixed-halide perovskite films and its impact on the local composition and optoelectronic properties. We use in situ X-ray scattering during perovskite film formation to show that crystallization of bromide-rich perovskites precedes that of mixed-halide phases in wrinkled films cast using an antisolvent-based process. Using nanoscopic X­-ray fluorescence and hyperspectral photoluminescence imaging, we also demonstrate the formation of iodide- and bromide-rich phases in the wrinkled domains. This intrinsic spatial halide segregation results in an increased local bandgap variation and Urbach energy. Morphological disorder and compositional heterogeneity also aggravate the formation of sub-bandgap electronic defects, reducing photostability and accelerating light-induced segregation of iodide and bromide ions in thin films and solar cells. Surface wrinkling reduces the performance of mixed-halide perovskite solar cells. Here, the authors identify that sequential nucleation of bromide-rich and iodide-rich domains results in compositional heterogeneity and subsequent wrinkling.

Few-photon optical nonlinearity in planar solid-state systems is challenging yet crucial for quantum and classical optical information processing. Polaritonic nonlinear metasurfaces have emerged as a promising candidate to push the photon number down – but have often been hindered by challenges like the poor photon-trapping efficiency and lack of modal overlap. Here, we address these issues in a self-hybridized perovskite metasurface through critical coupling engineering, and report strong polaritonic nonlinear absorption at an ultra-low incident power density of only 519 W/cm 2 , with an estimated photon number of 6.12 per cavity lifetime. Taking advantage of a quasi-bound-state-in-the-continuum design with asymmetry-controlled quality-( Q )-factor, we systematically examine the Q -dependent device nonlinearity and determine the optimal cavity critical coupling condition. With the optimized device, we demonstrate at 6 Kelvin a tunable nonlinear response from reverse saturable absorption to saturable absorption at varying pump powers, with a maximal effective nonlinear absorption coefficient up to 29.4 ± 5.8 cm/W at 560 nm wavelength. In addition, the cavity-exciton detuning dependent device response is analyzed and well explained by a phase-space-filling model, elucidating the underlying physics and the origin of giant nonlinearity. Our study paves the way towards practical flat nonlinear optical devices with large functional areas and massive parallel operation capabilities. Achieving nonlinear optical response of free-space planar solid devices in the few-photon regime will provide several technological advances. Here, the authors demonstrate a self-hybridised perovskite metasurface with strong nonlinear absorption at record low incident powers, by means of cavity critical coupling engineering

A family of semiconductors known as perovskites has great promise for use in optoelectronic devices. A much-needed strategy for adjusting the density of charge carriers in these materials unleashes their potential. An approach for changing the density of charge carriers in perovskites.

This paper describes a facile method of preparing cubic Au nanoframes with open structures via the galvanic replacement reaction between Ag nanocubes and AuCl 2 − . A mechanistic study of the reaction revealed that the formation of Au nanoframes relies on the diffusion of both Au and Ag atoms. The effect of the edge length and ridge thickness of the nanoframes on the localized surface plasmon resonance peak was explored by a combination of discrete dipole approximation calculations and single nanoparticle spectroscopy. With their hollow and open structures, the Au nanoframes represent a novel class of substrates for applications including surface plasmonics and surface-enhanced Raman scattering.

Reducing non-radiative recombination in semiconducting materials is a prerequisite for achieving the highest performance in light-emitting and photovoltaic applications. Here, we characterize both external and internal photoluminescence quantum efficiency and quasi-Fermi-level splitting of surface-treated hybrid perovskite (CH3NH3PbI3) thin films. With respect to the material bandgap, these passivated films exhibit the highest quasi-Fermi-level splitting measured to date, reaching 97.1 ± 0.7% of the radiative limit, approaching that of the highest performing GaAs solar cells. We confirm these values with independent measurements of internal photoluminescence quantum efficiency of 91.9 ± 2.7% under 1 Sun illumination intensity, setting a new benchmark for these materials. These results suggest hybrid perovskite solar cells are inherently capable of further increases in power conversion efficiency if surface passivation can be combined with optimized charge carrier selective interfaces.