Physical Review Applied

Characterizing the energetic disorder of the charge-transfer (CT) state is crucial to understanding organic photovoltaic devices, but is complicated by the discrepancy between temperature-dependent absorption and emission spectra. The authors show that, unlike absorption measurements, electroluminescence measurements do $not$ reveal the full extent of the CT-state distribution at low temperatures. In a highly disordered blend, CT-state filling is incomplete, and injected carriers do not reach equilibrium sites before recombining. CT disorder $can$ be reliably found from temperature-dependent measurements of external quantum efficiency, which can distinguish dynamic from static disorder.

Thermophotovoltaic (TPV) systems hold promise for recycling heat into electricity, but a key challenge is to lower the operating temperature to the range of industrial waste heat, at high efficiency. The authors offer a thermodynamic analysis of the open-circuit voltage of TPV systems operating in the far and near fields, and compare to solar PVs. An analytic model shows that although nonradiative losses increase in the near field, luminescence efficiency improves significantly in this range. This work suggests that an ultrathin near-field TPV system can exhibit ultrahigh open circuit voltage and high efficiency, for emitter temperatures much lower than its far-field counterpart.

Understanding exciton diffusion in organic solar cells is crucial to understanding the recent rise in power-conversion efficiencies afforded by using non-fullerene acceptor molecules (NFAs). This study introduces a technique called pulsed-PLQY for measuring exciton diffusion lengths in organic semiconductors. Compared to existing techniques, pulsed-PLQY is faster, easier, requires less specialized equipment, and is less sensitive to experimental conditions. Using this method, the authors find that modern non-fullerene acceptor semiconductors exhibit longer diffusion lengths than their fullerene forerunners, and that this increase is driven by increases in diffusivity.

Widespread adoption of luminescent solar concentrators (LSCs) would require both high photovoltaic performance and excellent aesthetic quality. In this work, a framework to comprehensively analyze LSC aesthetics is developed. Photoluminescence emitted from the device is incorporated to evaluate the corresponding impact on the key figures of merit for aesthetic quality. In identifying these key aspects, this work can help guide future LSC research along market-adoptable pathways.

New materials are needed to achieve cheap, efficient solar panels, LEDs, and other optoelectronic devices. However, key measurements to reveal the fundamental mechanisms dictating device optimization are typically long and expensive. This study shows that the same level of understanding can be achieved far more rapidly by combining measurements of photoluminescence quantum efficiency and time-resolved photoluminescence. These results plus ultraviolet-visible absorption measurements allow device efficiency to be predicted, providing a tool for the rapid screening and optimization of materials, which is of interest to spectroscopists and device fabricators in both academia and industry.

Developing sustainable and stable visible-light-driven photocatalysts to convert solar energy by splitting water into H${}_2$ and O${}_2$ is a great challenge in applied research. The authors’ calculations show that two-dimensional BCN is a very promising semiconductor for optoelectronic applications. Its excellent stability, sizable band gap, high carrier mobility, and distinct excitonic peaks are promising for photovoltaics, with its valence and conduction bands ideally straddling the oxidation and reduction potentials of water. Its optical absorption coefficient in the visible and near-ultraviolet regions is comparable to that of the perovskites currently employed in solar cells.

Nonreciprocal thermal photonics is an emerging topic in thermal radiation control. Recent advances in using this approach for energy harvesting, thermal management, and even communication have stimulated substantial interest in the subject. The authors review recent developments, challenges, and opportunities in the unidirectional transfer of heat energy via light, to provide a snapshot of the current status of the field, and to advance future research in it.

Compared to traditional solar cells, solar thermophotovoltaic (STPV) systems have the advantage of utilizing the full spectrum of solar radiation, but their thermodynamic efficiency is still far lower than the ultimate limit. This study shows that the efficiency deficit is caused by back emission from the intermediate layer toward the sun, resulting from the reciprocity of the system. The authors therefore propose using an intermediate layer with nonreciprocal radiative properties, which would suppress back emission and funnel more photons toward the cell, for a significant efficiency boost.

Tin halide perovskites present outstanding optoelectronic properties and great application potential, without the toxicity of lead. Here a systematic first-principles investigation reveals that a high-density defect complex, consisting of a tin vacancy plus a hydrogen molecule ($V_{\mathrm{Sn}}$–H${}_2$), is a highly effective center for nonradiative recombination of electrons and holes in this semiconductor. That would explain the experimentally observed significant nonradiative loss in devices based on formamidinium tin triiodide. Therefore, the passivation of this defect complex is expected to improve the performance of tin-based perovskite solar cells and other optoelectronic devices.

Solar cells are made from semiconductors, which inevitably include defects. The defects in Cu(In,Ga)Se${}_2$ (CIGS) are known to be metastable: With excitation they change character, which can have a profound impact on device function. The authors show that these metastable defects reduce the efficiency of the solar cell due to an increased diode factor. It is possible to measure this quantity directly from the bare absorber (without needing to build the entire solar cell), yielding a lower limit for the whole device. Experiments and simulations indicate that metastable defects must be reduced to increase efficiency, and not just in CIGS-based solar cells.

Thermophotovoltaic (TPV) energy conversion is a promising power-generation technology for converting heat to electricity. Recent studies have explored TPV devices featuring nanoscale gaps, which take advantage of near-field effects that enable much larger radiative fluxes and power density. The authors review the physics of near-field thermal radiation, and assess theoretical and experimental advances in predicting and validating near-field enhancements of power output and efficiency in TPV devices. Their discussion of the near-field photonic heat engines presented here will help to guide future engineering solutions in developing practical near-field energy-conversion devices.

As the energy economy becomes increasingly decarbonized, low-cost energy storage grows ever more important. Thermal batteries in combination with thermophotovoltaic (TPV) cells are one major source of storage. The lowest-loss TPV cells utilize an air bridge (AB) with a gold back reflector. In this work, the authors determine a 55.5% thermodynamic efficiency limit for AB-TPVs at an emitter temperature of 1400 K, based on detailed balance. Including losses from nonradiative recombination, finite resistance, and free-carrier absorption, the practical efficiency limit is 48.6%. This work provides a road map for evaluating and limiting losses, leading to even higher AB-TPV efficiencies.