Bihu Lv

Superior Optical Properties of Perovskite Nanocrystals as Single Photon Emitters.

The power conversion efficiency of photovoltaic devices based on semiconductor perovskites has reached ~20% after just several years of research efforts. With concomitant discoveries of other promising applications in lasers, light-emitting diodes and photodetectors, it is natural to anticipate what further excitements these exotic perovskites could bring about. Here we report on the observation of single photon emission from single CsPbBr3 perovskite nanocrystals (NCs) synthesized from a facile colloidal approach. Compared with traditional metal-chalcogenide NCs, these CsPbBr3 NCs exhibit nearly two orders of magnitude increase in their absorption cross sections at similar emission colors. Moreover, the radiative lifetime of CsPbBr3 NCs is greatly shortened at both room and cryogenic temperatures to favor an extremely fast output of single photons. The above findings have not only added a novel member to the perovskite family for the integration into current optoelectronic architectures, but also paved the way towards quantum-light applications of single perovskite NCs in various quantum information processing schemes.The power conversion efficiency of photovoltaic devices based on semiconductor perovskites has reached ∼20% after just several years of research efforts. With concomitant discoveries of other promising applications in lasers, light-emitting diodes, and photodetectors, it is natural to anticipate what further excitement these exotic perovskites could bring about. Here we report on the observation of single photon emission from single CsPbBr3 perovskite nanocrystals (NCs) synthesized from a facile colloidal approach. Compared with traditional metal-chalcogenide NCs, these CsPbBr3 NCs exhibit nearly 2 orders of magnitude increase in their absorption cross sections at similar emission colors. Moreover, the radiative lifetime of CsPbBr3 NCs is greatly shortened at both room and cryogenic temperatures to favor an extremely fast output of single photons. The above superior optical properties have paved the way toward quantum-light applications of perovskite NCs in various quantum information processing schemes.

Core–shell amorphous cobalt phosphide/cadmium sulfide semiconductor nanorods for exceptional photocatalytic hydrogen production under visible light

Efficient hydrogen (H2) production is considered to be a key pathway for future clean energy supply. Herein we report that a photocatalyst made of core–shell amorphous cobalt phosphide (CoPx) integrated with cadmium sulfide nanorods (CdS NRs) gives exceptional performance of photocatalytic H2 production under visible light. Under optimal conditions, the CoPx/CdS NRs photocatalyst allows an H2 evolution rate of ∼500 μmol h−1 mg−1 based on the photocatalyst (λ > 420 nm) and the apparent quantum yield was ∼35% in aqueous solutions (λ = 450 nm). The turnover numbers (TONs) reached ∼630 000 per mole of cobalt in 70 hours with a TOF of ∼9000 h−1. Such high performance of an artificial photosynthetic H2 production system using a cobalt-based cocatalyst has, to the best of our knowledge, not been reported to date.

Facile Face-Down Annealing Triggered Remarkable Texture Development in CH3NH3PbI3 Films for High-Performance Perovskite Solar Cells

Herein, we demonstrate that the facile face-down annealing route which effectively confines the evaporation of residual solvent molecules in one-step deposited precursor films can controllably enable the formation of (110) textured CH3NH3PbI3 films consisting of high-crystallinity well-ordered micrometer-sized grains that span vertically the entire film thickness. Such microstructural features dramatically decrease nonradiative recombination sites as well as greatly improve the transport property of charge carries in the films compared with that of the nontextured ones obtained by the conventional annealing route. As a consequence, the planar-heterojunction perovskite solar cells with these textured CH3NH3PbI3 films exhibit significantly enhanced power conversion efficiency (PCE) along with small hysteresis and excellent stability. The champion cell yields impressive PCE boosting to 18.64% and a stabilized value of around 17.22%. Particularly, it can maintain 86% of its initial value after storage for 20 days in ambient conditions with relative humidity of 10-20%. Our work suggests a facile and effective route for further boosting the efficiency and stability of low-cost perovskite solar cells.

Dramatically promoted crystallization control of organolead triiodide perovskite film by a homogeneous cap for high efficiency planar-heterojunction solar cells

A homogeneous cap-mediated crystallization concept with face-to-face configuration is proposed here to modulate the crystallization kinetics of organolead triiodide perovskite (OTP) films in a controlled way. The introduced OTP caps, especially those with low surface roughness, play a dramatically positive role in retarding the nucleation rate, promoting the growth, and preventing the composition loss of OTP grains, fully facilitating the formation of pinhole-free OTP films with numerous desirable characteristics, such as greatly enlarged grains, vertically aligned grain boundaries, preferred (110) orientation, significantly improved crystallinity, and proper stoichiometry. As a consequence, planar-heterojunction solar cells incorporating these high-quality films deliver a promising average efficiency of 17.87%, showing a remarkable enhancement of approximately 30% compared with control samples. In particular, large fill factors can be routinely achieved in these high efficiency cells. This excellent performance mainly originates from greatly suppressed non-radiative recombination as well as from the greatly enhanced diffusion and transfer properties of charge carriers in the cells as a result of the improved quality of the OTP films. Our work presents an effective and useful strategy to fabricate high-performance planar heterojunction solar cells based on OTP materials.

Microstructure modulation of the CH3NH3PbI3 layer in perovskite solar cells by 2-propanol pre-wetting and annealing in a spray-assisted solution process

As the light absorption layer, a CH3NH3PbI3 perovskite film with a fully and homogeneously covered flat surface is crucially important for perovskite photovoltaic devices. Herein, the sequential method is improved by introducing a spray-assisted solution process (SSP) instead of the commonly used dipping process in CH3NH3PbI3 perovskite film deposition. The morphology analysis for PbI2 films found that the airflow of the spray pre-wetting process provides a driving force for accelerating the dissolution and deep infiltration, which is helpful to form a mesoporous PbI2 scaffold. So the 2-propanol (IPA) pre-wetting has advantages to complete the reaction of PbI2 and CH3NH3I through the diffusion channel in the grain boundary and on the surface of the PbI2 film. Furthermore, the grain size and surface coverage of the film can be controlled effectively by a facile IPA pre-wetting process in the SSP. Meanwhile, the influence of the annealing process on the crystallization, chemical composition, microstructures and carrier transport properties of the perovskite films were investigated. Our results showed that an optimum annealing temperature and time can reduce crystal defects in the film and weaken the hysteresis phenomenon of the perovskite solar cell. Efficient mesoporous structured perovskite solar cells were fabricated with an optimal PCE of 14.2%, because of the improved perovskite film preparation method.

Carrier Multiplication in a Single Semiconductor Nanocrystal.

The UV-excited photoluminescence was measured for single CdSe nanocrystals and an average carrier multiplication efficiency of ~13.1% was obtained when the excitation photon energy was set at ~2.46 times of the nanocrystal energy gap.

Series of ZnSn(OH)6 Polyhedra: Enhanced CO2 Dissociation Activation and Crystal Facet-Based Homojunction Boosting Solar Fuel Synthesis

A series of ZnSn(OH)6 polyhedra are successfully explored with well-controlled area ratio of the exposed {100} and {111} facets. Band alignment of the exposed facet-based homojunction of the elegant polyhedron facilitates spatial separation of photogenerated electrons and holes on {111} and {100} surfaces, respectively. Optimal area ratio of {100} to {111} is the prerequisite for pronounced CO2 photocatalytic performance of high-symmetry cuboctahedra into methane (CH4). The synergistic effect of the excess electron accumulation and simultaneously the enhanced CO2 absorption and low dissociation activation energy on {111} reduction sites promote the yield of CO2 photocatalytic conversion product.

Photon antibunching in a cluster of giant CdSe/CdS nanocrystals

When closely packed into a high-density film, semiconductor nanocrystals (NCs) can interact with each other to yield collective optical behaviours, which are normally difficult to characterize due to the ensemble average effect. Here we synthesized semiconductor NC clusters and performed single-particle spectroscopic measurements to probe the electronic couplings of several giant CdSe/CdS NCs contained in one cluster with nanometer-scale separations. Such a single cluster exhibits multiple emission peaks at the cryogenic temperature with nearly identical photoluminescence decay dynamics, suggesting that the Förster-type energy transfer does not occur among the composing NCs. Surprisingly, strong photon antibunching is still observed from a single cluster, which can be attributed to the Auger annihilation of photo-excited excitons from different NCs. The isolation of several nearby NCs interacting with the above novel mechanism has marked a solid progress towards a full understanding and an efficient control of the operation parameters in NC-based optoelectronic devices.Semiconductor nanocrystals have potential applications in optoelectronic devices including single-photon emission sources. Here, Lv et al. demonstrate that nanocrystals within small clusters can electronically couple, thereby increasing their optical cross-section while the optical emission remains antibunched.

Carrier multiplication in a single semiconductor nanocrystal(Conference Presentation)

When a UV photon is absorbed by a single semiconductor nanocrystal (NC), two or more excitons can be simultaneously generated through the carrier multiplication (CM) process. It is still highly debated whether the CM efficiency is truly enhanced in semiconductor NCs because all the routine CM measurements performed exclusively at the ensemble level are incapable of completely excluding the false CM signals contributed by the charged excitons. Here we place single CdSe NCs above an aluminum film and successfully resolve their UV-excited photoluminescence time trajectories where the true and false CM signals are contained in the blinking “on” and “off” levels, respectively. When the UV photon energy is ~2.46 times of the NC energy gap, an average CM efficiency of ~20.2% can be reliably estimated. The ability to detect UV-excited photoluminescence from a single NC will surely provide a great guidance for the CM applications in various light-to-electric conversion devices.

Carrier multiplication in a single semiconductor nanocrystal

The UV-excited photoluminescence was measured for single CdSe nanocrystals and an average carrier multiplication efficiency of ∼13.1% was obtained when the excitation photon energy was set at ∼2.46 times of the nanocrystal energy gap.