Perovskite light-emitting diodes have recently broken the 20% barrier for external quantum efficiency. These values cannot be explained with classical models for optical outcoupling. Here, we analyse the role of photon recycling (PR) in assisting light extraction from perovskite light-emitting diodes. Spatially-resolved photoluminescence and electroluminescence measurements combined with optical modelling show that repetitive re-absorption and reemission of photons trapped in substrate and waveguide modes significantly enhance light extraction when the radiation efficiency is sufficiently high. In this manner, PR can contribute more than 70% to the overall emission, in agreement with recently-reported high efficiencies. While an outcoupling efficiency of 100% is theoretically possible with PR, parasitic absorption losses due to absorption from the electrodes are shown to limit practical efficiencies in current device architectures. To overcome the present limits, we propose a future configuration with a reduced injection electrode area to drive the efficiency toward 100%.

Metal-halide perovskites have emerged as exceptional semiconductors for optoelectronic applications. Substitution of the monovalent cations has advanced luminescence yields and device efficiencies. Here, we control the cation alloying to enhance optoelectronic performance through alteration of the charge carrier dynamics in mixed-halide perovskites. In contrast to singlehalide perovskites, we find high luminescence yields for photo-excited carrier densities far below solar illumination conditions. Using time-resolved spectroscopy we show that the charge-carrier recombination regime changes from second to first order within the first tens of nanoseconds after excitation. Supported by microscale-mapping of the optical bandgap, electrically-gated transport measurements and first-principles calculations, we demonstrate that spatially-varying energetic disorder in the electronic states causes local charge accumulation, creating p-and n-type photodoped regions, which unearths a strategy for efficient light emission at low charge-injection in solar cells and LEDs. Metal-halide perovskites exhibit outstanding optoelectronic properties, such as low Urbach energies, high carrier mobilities and diffusion lengths, as well as very high photoluminescence quantum efficiencies (PLQEs), 1 which are essential to achieve performance limits in solar cells and light-emitting diodes (LEDs). 2-5 This culminated in reported photovoltaic performances 6,7 exceeding 25 % upon incorporation of a series of monovalent cation mixtures (formamidinium, Cs) and passivating additives (Rb, K) to the methylammonium mixed-halide perovskite prototype MAPb(Br0.17I0.83)3 8,9 , as well as bright LEDs 10-15. Here, we show that local bandgap variations in mixed-halide thin films yield photo-doped regions for efficient photoluminescence, favourable for optoelectronic applications.

We report on individual-InAs nanowire optoelectronic devices which can be tailored to exhibit either negative or positive photoconductivity (NPC or PPC). The NPC photoresponse time and magnitude is found to be highly tunable by varying the nanowire diameter under controlled growth conditions. Using hysteresis characterization, we decouple the observed photoexcitation-induced hot electron trapping from conventional electric field-induced trapping to gain a fundamental insight into the interface trap states responsible for NPC. Furthermore, we demonstrate surface passivation without chemical etching which both enhances the field-effect mobility of the nanowires by approximately an order of magnitude and effectively eliminates the hot carrier trapping found to be responsible for NPC, thus restoring an "intrinsic" positive photoresponse. This opens pathways toward engineering semiconductor nanowires for novel optical-memory and photodetector applications.

A detailed understanding of charge transport is vital to maximize the efficiencies of optoelectronic devices. Using a back-contact architecture, the authors probe transport of electrons and holes separately in polycrystalline hybrid perovskite thin films. Isolating photoexcited charge carriers in separate regions of the device leads to long diffusion ranges of carriers. The authors demonstrate a back-contact perovskite solar cell that operates on majority-carrier diffusion. These results highlight electrode interfaces as limiting aspects of current back-contact architectures, indicating opportunities for improvement.

Films of the semiconducting polymer poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] with 40% fluorinated monomers, denoted PTB7-F40, are spin coated out of different solvents onto PEDOT:PSS films. The influence of the used solvents chlorobenzene, 1,2-dichlorobenzene, and 1,2,4-trichlorobenzene as well as the influence of the additive 1,8-diiodooctane (DIO) is probed with grazing incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS). As seen with GISAXS, without DIO, the films are homogeneous and show roughness correlation with the PEDOT:PSS film surface. With DIO, an inner film structure with a size of 50-75 nm is found and the roughness correlations weaken. In addition, as seen in GIWAXS, the crystalline part of the films is influenced by the used solvent if DIO is added.