Linwei Yu

Ultrafast Solar‐Blind Ultraviolet Detection by Inorganic Perovskite CsPbX3 Quantum Dots Radial Junction Architecture

Inorganic CsPbX3 (X = Cl, Br, I, or hybrid among them) perovskite quantum dots (IPQDs) are promising building blocks for exploring high performance optoelectronic applications. In this work, the authors report a new hybrid structure that marries CsPbX3 IPQDs to silicon nanowires (SiNWs) radial junction structures to achieve ultrafast and highly sensitive ultraviolet (UV) detection in solar-blind spectrum. A compact and uniform deployment of CsPbX3 IPQDs upon the sidewall of low-reflective 3D radial junctions enables a strong light field excitation and efficient down-conversion of the ultraviolet incidences, which are directly tailored into emission bands optimized for a rapid photodetection in surrounding ultrathin radial p-i-n junctions. A fast solar-blind UV detection has been demonstrated in this hybrid IPQD-NW detectors, with rise/fall response time scales of 0.48/1.03 ms and a high responsivity of 54 mA W-1 @200 nm (or 32 mA W-1 @270 nm), without the need of any external power supply. These results pave the way toward large area manufacturing of high performance Si-based perovskite UV detectors in a scalable and low-cost procedure.

In-plane epitaxial growth of silicon nanowires and junction formation on Si(100) substrates.

Growing self-assembled silicon nanowires (SiNWs) into precise locations represents a critical capability to scale up SiNW-based functionalities. We here report a novel epitaxy growth phenomenon and strategy to fabricate orderly arrays of self-aligned in-plane SiNWs on Si(100) substrates following exactly the underlying crystallographic orientations. We observe also a rich set of distinctive growth dynamics/modes that lead to remarkably different morphologies of epitaxially grown SiNWs/or grains under variant growth balance conditions. High-resolution transmission electron microscopy cross-section analysis confirms a coherent epitaxy (or partial epitaxy) interface between the in-plane SiNWs and the Si(100) substrate, while conductive atomic force microscopy characterization reveals that electrically rectifying p-n junctions are formed between the p-type doped in-plane SiNWs and the n-type c-Si(100) substrate. This in-plane epitaxy growth could provide an effective means to define nanoscale junction and doping profiles, providing a basis for exploring novel nanoelectronics.

Rapid, stable and self-powered perovskite detectors via a fast chemical vapor deposition process

Organometal halide perovskite materials are outstanding candidates not only for solar cells but also for photo-detection. In this work, we develop a well-controlled lower temperature ( 0.1 cm2) without external power supply. Remarkably, the perovskite photovoltaic detectors demonstrate an excellent air-exposure stability for more than two months without particular encapsulation. These excellent performances are attributed to a well-controlled expansive gas–solid reaction and formation of perovskite crystallites that collide and pinch off the pinhole leakage paths at the grain boundaries. More importantly, the accumulated strain at the colliding grain boundaries leads to a selective evaporation of MAI during post-growth annealing, and thus passivate the local defects by the remnant PbI2 layer. These results highlight the potential of LFCVD perovskite materials in developing ultra-fast and self-driven photovoltaic detectors with outstanding stability and scalability.

Mo-O bond doping and related-defect assisted enhancement of photoluminescence in monolayer MoS2

In this work, we report a strong photoluminescence (PL) enhancement of monolayer MoS2 under different treatments. We find that by simple ambient annealing treatment in the range of 200 °C to 400 °C, the PL emission can be greatly enhanced by a factor up to two orders of magnitude. This enhancement can be attributed to two factors: first, the formation of Mo-O bonds during ambient exposure introduces an effective p-doping in the MoS2 layer; second, localized electrons formed around Mo-O bonds related defective sites where the electrons can be effectively localized with higher binding energy resulting in efficient radiative excitons recombination. Time resolved PL decay measurement showed that longer lifetime of the treated sample consistent with the higher quantum efficiency in PL. These results give more insights to understand the luminescence properties of the MoS2.

Dual‐Phase CsPbBr3–CsPb2Br5 Perovskite Thin Films via Vapor Deposition for High‐Performance Rigid and Flexible Photodetectors

Inorganic perovskites with special semiconducting properties and structures have attracted great attention and are regarded as next generation candidates for optoelectronic devices. Herein, using a physical vapor deposition process with a controlled excess of PbBr2 , dual-phase all-inorganic perovskite composite CsPbBr3 -CsPb2 Br5 thin films are prepared as light-harvesting layers and incorporated in a photodetector (PD). The PD has a high responsivity and detectivity of 0.375 A W-1 and 1011 Jones, respectively, and a fast response time (from 10% to 90% of the maximum photocurrent) of ≈280 µs/640 µs. The device also shows an excellent stability in air for more than 65 d without encapsulation. Tetragonal CsPb2 Br5 provides satisfactory passivation to reduce the recombination of the charge carriers, and with its lower free energy, it enhances the stability of the inorganic perovskite devices. Remarkably, the same inorganic perovskite photodetector is also highly flexible and exhibits an exceptional bending performance (>1000 cycles). These results highlight the great potential of dual-phase inorganic perovskite films in the development of optoelectronic devices, especially for flexible device applications.

Cadmium-doped flexible perovskite solar cells with a low-cost and low-temperature-processed CdS electron transport layer

Hybrid perovskite solar cells (PSCs) are promising candidates in exploring high performance flexible photovoltaics, where a low-temperature-processed metal oxide electron transfer layer (ETL) is highly preferable. In this work, we demonstrate perovskite solar cells based on inorganic cadmium-sulfide (CdS) as the electron transfer layer, fabricated using low-temperature chemical bath deposition (CBD) at <85 °C. We show that natural Cd-doping has been achieved in the perovskite fabricated via a physical–chemical vapor deposition (P-CVD), which under properly controlled reaction and post-growth annealing leads to a high power conversion efficiency of 14.68% for the CdS-PSCs on glass substrate. Then, flexible perovskite solar cells with CdS as the ETL are fabricated for the first time and demonstrate a high PCE of 9.9%. These results highlight the exciting potential of a low-temperature-processed and readily scalable CdS-based PSC structure in the development of high performance flexible solar cells.

Enhanced up-conversion luminescence from NaYF4:Yb,Er nanocrystals by Gd3+ ions induced phase transformation and plasmonic Au nanosphere arrays

NaYF4:Yb,Er nanocrystals with different concentrations of Gd3+ ions are prepared via a hydrothermal method. With increasing Gd3+ dopant, up-conversion (UC) emission is gradually enhanced and the strongest UC emission is improved by 50-fold for a sample with 15% Gd3+ doping compared with the one without Gd3+ incorporation. However, further increasing the Gd3+ dopant causes the reduction of UC emission which can be ascribed to the increased surface state recombination. Moreover, the self-assembly technique is used to fabricate Au nanosphere arrays with diameters of 300 nm to enhance the UC luminescence. It is found that the luminescence intensity is increased by 2-fold due to the surface plasmon resonance effect which is confirmed by the absorption spectra.

How tilting and cavity-mode-resonant absorption contribute to light harvesting in 3D radial junction solar cells

Radial junction (RJ) architecture has proven beneficial in boosting light harvesting and fast carrier separation in thin film solar cells. While a comprehensive understanding of the detailed absorption distribution and light incoupling mechanism within such a 3D RJ configuration remains largely unexplored. Taking hydrogenated amorphous Si (a-Si:H) RJ solar cells as an example, we here address in both experimental and theoretical manners the impacts of tilting and spacing configuration on the light absorption and external quantum efficiency (EQE) responses. A nice agreement between the calculated and experimental EQE responses indicates that the light harvesting realized within RJ thin film solar cells is quite robust against geometric variations and shadowing effects. Following the concepts of optical fiber injection, we have been able to single out the contribution arising solely from a resonant-mode-incoupling into the RJ cavities against a sidewall scattering incidence scenario. These results provide insightful viewpoints as well as practical guides in developing a new generation of high performance RJ thin film solar cells.

Enhancing Hybrid Perovskite Detectability in the Deep Ultraviolet Region with Down-Conversion Dual-Phase (CsPbBr3–Cs4PbBr6) Films

Hybrid perovskite photodetectors (PDs) exhibit outstanding performance in the ultraviolet-visible (UV-vis) spectrum but have poor detectability in the deep ultraviolet (DUV) region (200-350 nm). In this work, a novel inorganic-hybrid architecture that incorporates a dual-phase (CsPbBr3-Cs4PbBr6) inorganic perovskite material as a down-conversion window layer and a hybrid perovskite as a light capture layer was prepared to achieve faster, highly sensitive photodetection in the DUV spectrum. A dual-phase inorganic perovskite film coated on the back surface of the photodetector enables strong light absorption and tunes the incident energy into emission bands that are optimized for the perovskite photodetector. The presence of Cs4PbBr6 enhances the capture and down-conversion of the incident DUV light. Due to the down-conversion and transport of the DUV photons, a self-driven perovskite photodetector with this composite structure exhibits a fast response time of 7.8/33.6 μs and a high responsivity of 49.4 mA W-1 at 254 nm without extra power supply.

Deterministic Line-Shape Programming of Silicon Nanowires for Extremely Stretchable Springs and Electronics

Line-shape engineering is a key strategy to endow extra stretchability to 1D silicon nanowires (SiNWs) grown with self-assembly processes. We here demonstrate a deterministic line-shape programming of in-plane SiNWs into extremely stretchable springs or arbitrary 2D patterns with the aid of indium droplets that absorb amorphous Si precursor thin film to produce ultralong c-Si NWs along programmed step edges. A reliable and faithful single run growth of c-SiNWs over turning tracks with different local curvatures has been established, while high resolution transmission electron microscopy analysis reveals a high quality monolike crystallinity in the line-shaped engineered SiNW springs. Excitingly, in situ scanning electron microscopy stretching and current-voltage characterizations also demonstrate a superelastic and robust electric transport carried by the SiNW springs even under large stretching of more than 200%. We suggest that this highly reliable line-shape programming approach holds a strong promise to extend the mature c-Si technology into the development of a new generation of high performance biofriendly and stretchable electronics.