• Energy Research

Fast diffusion of charge carriers is crucial for efficient charge collection in perovskite solar cells. While lateral transient photoluminescence microscopies have been popularly used to characterize charge diffusion in perovskites, there exists a discrepancy between low diffusion coefficients measured and near-unity charge collection efficiencies achieved in practical solar cells. Here, we reveal hidden microscopic dynamics in halide perovskites through four-dimensional (directions x, y and z and time t) tracking of charge carriers by characterizing out-of-plane diffusion of charge carriers. By combining this approach with confocal microscopy, we discover a strong local heterogeneity of vertical charge diffusivities in a three-dimensional perovskite film, arising from the difference between intragrain and intergrain diffusion. We visualize that most charge carriers are efficiently transported through the direct intragrain pathways or via indirect detours through nearby areas with fast diffusion. The observed anisotropy and heterogeneity of charge carrier diffusion in perovskites rationalize their high performance as shown in real devices. Our work also foresees that further control of polycrystal growth will enable solar cells with micrometres-thick perovskites to achieve both long optical path length and efficient charge collection simultaneously.

AbstractSolar fuels offer a promising approach to provide sustainable fuels by harnessing sunlight1,2. Following a decade of advancement, Cu2O photocathodes are capable of delivering a performance comparable to that of photoelectrodes with established photovoltaic materials3–5. However, considerable bulk charge carrier recombination that is poorly understood still limits further advances in performance6. Here we demonstrate performance of Cu2O photocathodes beyond the state-of-the-art by exploiting a new conceptual understanding of carrier recombination and transport in single-crystal Cu2O thin films. Using ambient liquid-phase epitaxy, we present a new method to grow single-crystal Cu2O samples with three crystal orientations. Broadband femtosecond transient reflection spectroscopy measurements were used to quantify anisotropic optoelectronic properties, through which the carrier mobility along the [111] direction was found to be an order of magnitude higher than those along other orientations. Driven by these findings, we developed a polycrystalline Cu2O photocathode with an extraordinarily pure (111) orientation and (111) terminating facets using a simple and low-cost method, which delivers 7 mA cm−2 current density (more than 70% improvement compared to that of state-of-the-art electrodeposited devices) at 0.5 V versus a reversible hydrogen electrode under air mass 1.5 G illumination, and stable operation over at least 120 h.

Abstract Photolithography is used as an alternative method to overcome the challenge of making anode and cathode contacts on a Si nanocrystal solar cell deposited on non-conductive substrates instead of reactive ion etching (RIE). The advantages of this method include better control of isolation mesa fabrication and the avoidance of device exposure to highly energetic particles which may cause unpredictable damage. The photovoltaic device fabricated shows an open-circuit voltage (VOC) and a short-circuit current density (JSC) of 270 mV and 0.124 mA/cm2 respectively at room temperature under one-sun illumination. Current–voltage measurements were performed at temperatures (T) from 77 K to 300 K. A model that includes recombination-generation current in the depletion region is considered to explain the observed current behaviour of the device. An ideality factor very close to 2 was calculated based on Suns-VOC measurement, which indicates that the device is limited by recombination in the depletion region. A discrepancy was observed between the peaks (1.47 eV) in the photoluminescence spectrum and maximum VOC (0.81 V) extrapolated from the VOC–T relation at 0 K. This discrepancy has been attributed to the temperature dependence of the carrier lifetime in the depletion region characterized by an activation energy later defined in this article.