Perovskite single crystals (PSCs) are considered the next breakthrough in optoelectronics research due to their free-grain boundary and much lower density of trap states compared to those of their polycrystalline counterparts. However, the inevitable formation of triiodide-based intrinsic defects during high-temperature crystal growth is one of the major challenges impeding the further development of optoelectronic devices based on PSCs. Here, we not only identified the existence of these triiodide ions as hole-trapping centers and their tremendous negative impact on the performance of PSCs, but more importantly, we used a reduction treatment to prevent their formation during crystal growth. The removal of such defect centers resulted in much higher charge carrier mobility and longer carrier lifetime than the untreated counterparts, leading to enhanced photodetection properties. The I3–-free MAPbI3 single crystal (MSC) devices consistently generated a more than 100 times higher photocurrent than that generated by I3–-rich devices under the same light intensity.

In this work, we fabricate photodetectors made of methylammonium lead trihalide perovskite (MLHP) and phosphorus-doped graphitic carbon nitride nanosheets (PCN-S). Using thermal polymerization, PCN-S with a reduced band gap, are synthesized from low-cost precursors, making it feasible to form type-II bulk heterojunctions with perovskites. Owing to the bulk heterojunctions between PCN-S and MLHP, the dark current of the photodetectors significantly decreases from ∼10-9 A for perovskite-only devices to ∼10-11 A for heterojunction devices. As a result, not only does the on/off ratio of the hybrid devices increase from 103 to 105 but also the photodetectivity is enhanced by more than 1 order of magnitude (up to 1013 Jones) and the responsivity reaches a value of 14 A W-1. Moreover, the hybridization of MLHP with PCN-S significantly modifies the hydrophilicity and morphology of the perovskite films, which dramatically increases their stability under ambient conditions. The hybrid photodetectors, described here, present a promising new direction toward stable and efficient optoelectronic applications.

Selective CO2 reduction via photoelectrocatalysis is a creative approach to alleviating the energy crisis and environmental deterioration as well as to producing value-added chemicals. A considerable challenge is the development of electrocatalysts and photoelectrocatalytic systems to selectively and efficiently produce a target product. Here, we report an efficient and selective photocathode for converting CO2 to HCOOH in aqueous solution. A rational compositional screening strategy was first applied to pinpoint In0.4Bi0.6 from ternary In–Bi–Sn alloys as the most HCOOH-selective electrocatalyst composition among the electrocatalysts reported. A photocathode was then fabricated by coating the catalytic, protective, and conductive In0.4Bi0.6 alloy layer on a halide perovskite photovoltaic, effectively utilizing its low melting point eutectic molten state. The generated photovoltage assisted reduction of the overpotential by 680 mV while producing a stable current for nearly exclusive HCOOH production under simulated AM 1.5G irradiation. This work provides a promising approach to achieving efficient and selective solar–fuel conversion.

Twenty-micrometer-thick single-crystal methylammonium lead triiodide (MAPbI3) perovskite (as an absorber layer) grown on a charge-selective contact using a solution space-limited inverse-temperature crystal growth method yields solar cells with power conversion efficiencies reaching 21.09% and fill factors of up to 84.3%. These devices set a new record for perovskite single-crystal solar cells and open an avenue for achieving high fill factors in perovskite solar cells.

Metal-ion doping is one of the most efficient approaches to precisely control the electronic and optical properties of perovskite nanocrystals (NCs). However, the origin of the dramatic contrast in the photoluminescence (PL) behavior of CsPbBr3 NCs incorporating bismuth (Bi3+) and cerium (Ce3+) ion dopants remains unclear. Here, we demonstrate dominant PL quenching/enhancing centers both in the bulk and on the surface of Bi3+/Ce3+-doped CsPbBr3 by calculating the dopant defect formation energies and charge-transition levels using high-level density functional theory (DFT). We show that the Bi3+ dopants introduce deep trap states (antisite BiPb and interstitial Bii) that are responsible for PL quenching. In sharp contrast, the Ce3+ dopants enhance the CsPbBr3 lattice order and enrich the conduction band-edge states through antisite CePb, causing PL enhancement. Our findings not only provide new physical insights into the mechanism of the trivalent metal-ion doping effect but also suggest a new strategy to control the dopant defect states for improving the optical performance of perovskite NCs.

Lead-free hybrid perovskites have attracted immense interest as environmentally friendly light absorbers. Here, we report on tellurium (Te)-based double perovskites A2TeX6 (A = MA, FA, or BA; X = Br– or I–; MA = CH3NH3; FA = CH(NH2)2; BA = benzylamine) as potentially active materials for optoelectronic devices. These perovskites exhibit a tunable band gap (1.42–2.02 eV), a low trap density (∼1010 cm–3), and a high mobility (∼65 cm2 V–1 s–1). Encouragingly, the MA2TeBr6 single crystal with a band gap of 2.00 eV possesses a long carrier lifetime of ∼6 μs and corresponding carrier diffusion lengths of ∼38 μm, which are ideal characteristics for a material for photodetectors and tandem solar cells. Moreover, A2TeX6 perovskites are relatively robust in ambient conditions, being stable for at least two months without showing any signs of phase change. Our findings bring to the forefront a family of lead-free Te-based perovskites for nontoxic perovskite optoelectronics.

The presence of localized trap states on the surface of CsPbCl3 perovskite nanocrystals (NCs) is one of the greatest challenges precluding the development of optoelectronic applications of these NCs. Passivation of these defect sites provides a promising pathway to remediating their electronic and optical properties, such as photoluminescence quantum yield (PLQY). Herein, we demonstrate a post-synthetic dual-surface treatment using trivalent metal ion salts, i.e., YCl3, as a new passivation approach that enhances the PLQY up to 60 % while preserving the NC size and crystal structure. Such remarkable enhancement of the PLQY along with prolongation of the average PL lifetimes of treated NC samples indicates effective passivation of the surface defects and subsequent suppression of the formation of surface non-radiative recombination centers. As a segue towards optoelectronic applications, we probed the photoelectric performance of the NCs using ultra-flexible devices; we found that YCl3-treated CsPbCl3 NC films exhibit an order of magnitude larger photocurrent compared to their non-treated counterparts. Our experimental and theoretical results provide an insightful understanding of the effective passivating roles of Y3+ and Cl- ions on the surface of CsPbCl3 NCs, as well as offers a new path to synthesize high-quality NCs for UV-light conversion applications.

Cesium lead halide perovskite materials have attracted considerable attention for potential applications in lasers, light emitting diodes and photodetectors. Here, we provide the experimental and theoretical evidence for photon recycling in CsPbBr3 perovskite microwires. Using two-photon excitation, we recorded photoluminescence (PL) lifetimes and emission spectra as a function of the lateral distance between PL excitation and collection positions along the microwire, with separations exceeding 100 µm. At longer separations, the PL spectrum develops a red-shifted emission peak accompanied by an appearance of well-resolved rise times in the PL kinetics. We developed quantitative modeling that accounts for bimolecular recombination and photon recycling within the microwire waveguide and is sufficient to account for the observed decay modifications. It relies on a high radiative efficiency in CsPbBr3 perovskite microwires and provides crucial information about the potential impact of photon recycling and waveguide trapping on optoelectronic properties of cesium lead halide perovskite materials.

Introducing dopants into InGaN NWs is known to significantly improve their device performances through a variety of mechanisms. However, to further optimize device operation under the influence of large specific surfaces, a thorough knowledge of ultrafast dynamical processes at the surface and interface of these NWs is imperative. Here, we describe the development of four-dimensional scanning ultrafast electron microscopy (4D S-UEM) as an extremely surface-sensitive method to directly visualize in space and time the enormous impact of silicon doping on the surface-carrier dynamics of InGaN NWs. Two time regime dynamics are identified for the first time in a 4D S-UEM experiment: an early time behavior (within 200 picoseconds) associated with the deferred evolution of secondary electrons due to the presence of localized trap states that decrease the electron escape rate and a longer timescale behavior (several ns) marked by accelerated charge carrier recombination. The results are further corroborated by conductivity studies carried out in dark and under illumination.

We investigate the intrinsic lead ion (Pb2+) emissions in zero-dimensional (0D) perovskite nanocrystals (NCs) using a combination of experimental and theoretical approaches. The temperature-dependent photoluminescence experiments for both “nonemissive” (highly suppressed green emission) and emissive (bright green emission) Cs4PbBr6 NCs show a splitting of emission spectra into high- and low-energy transitions in the ultraviolet (UV) spectral range. In the nonemissive case, we attribute the high-energy UV emission at approximately 350 nm to the allowed optical transition of 3P1 to 1S0 in Pb2+ ions and the low-energy UV emission at approximately 400 nm to the charge-transfer state involved in the 0D NC host lattice (D-state). In the emissive Cs4PbBr6 NCs, in addition to the broad UV emission, we demonstrate that energy transfer occurs from Pb2+ ions to green luminescent centers. The optical phonon modes in Cs4PbBr6 NCs can be assigned to both Pb–Br stretching and rocking motions from density functional theory calculations. Our results address the origin of the dual broadband Pb2+ ion emissions observed in Cs4PbBr6 NCs and provide insights into the mechanism of ionic exciton–optical phonon interactions in these 0D perovskites.