The Janus structures of transition metal dichalcogenides with an intrinsic dipole have been proposed as efficient photocatalysts for water splitting, and successfully synthesized recently. However, the mechanism for their superior photocatalytic activities are not understood. Here, we systematically investigate the photocatalytic activities of Janus molybdenum dichalcogenides (MoXY, X/Y = O, S, Se, and Te), by studying their band gaps, redox energy levels and electrons and holes separation, by first-principles calculations. The intrinsic dipoles in the Janus structures cause notable band bending to achieve favorable band edge positions relative to water redox potentials, which makes the Janus structures as efficient heterojunction photocatalysts. Electrons and holes are spatially separated on different surfaces of the Janus structure due to the internal electric field, which effectively inhibits the recombination of excitons and ensures photocatalytic activity with high efficiency.

Ji Yujin    Yang Mingye    Lin Haiping    Tingjun Hou    Lu Wang    Youyong Li    李述汤    

Journal of Physical Chemistry C

2018

The rapid advancement of intelligent wearable electronics imposes the emergent requirement for power sources that are deformable, compliant, and stretchable. Power sources with these characteristics are difficult and challenging to achieve. The use of liquid metals as electrodes may provide a viable strategy to produce such power sources. In this work, we propose a liquid-metal-based triboelectric nanogenerator (LM-TENG) by employing Galinstan as the electrode and silicone rubber as the triboelectric and encapsulation layer. The small Young's modulus of the liquid metal ensures the electrode remains continuously conductive under deformations, stretching to a strain as large as ∼300%. The surface oxide layer of Galinstan effectively prevents the liquid Galinstan electrode from further oxidization and permeation into silicone rubber, yielding outstanding device stability. Operating in the single-electrode mode at 3 Hz, the LM-TENG with an area of 6 × 3 cmproduces an open-circuit voltage of 354.5 V, transferred short-circuit charge of 123.2 nC, short-circuit current of 15.6 μA, and average power density of 8.43 mW/m, which represent outstanding performance values for TENGs. Further, the LM-TENG maintains stable performance under various deformations, such as stretching, folding, and twisting. LM-TENGs in different forms, such as bulk-shaped, bracelet-like, and textile-like, are all able to harvest mechanical energy from human walking, arm shaking, or hand patting to sustainably drive wearable electronic devices.

Yang Yanqin    Sun Na    Zhen Wen    Cheng Ping    Zheng Hechuang    Shao Huiyun    Xia Yujian    Chen Chen    Lan Huiwen    Xie Xinkai    Zhou Changjie    Zhong Jun    Xuhui Sun    李述汤    

ACS Nano

2018

As a promising material for optoelectronic devices, cesium lead halide perovskite nanocrystals have attracted wide attention recently. However, the fast anion exchange among CsPbXis a problem for applications (e.g., light emitting diode, LED) that require different colors. In this work, we report the large-scale synthesis of ultrathin cesium lead bromide (CsPbBr) nanoplates (NPs) of tunable edge length and thickness via a simple one-pot colloidal approach. The thickness of NPs can be precisely tuned in a monolayer level by varying the reaction kinetics. The high-quality ultrathin NPs can be prepared in large scale of 0.25 L/batch. The ultrathin CsPbBrNPs emit blue light due to the strong quantum confinement effect, in contrast to the green emission of bulk CsPbBr. For the first time, a blue LED device has been successfully fabricated by using ultrathin CsPbBrNPs as the active layer. The use of CsPbBrperovskite as the emitting layer for blue LED may promote the development of next-generation LEDs and displays.

Yang Di    Zou Yatao    Li Pengli    Liu Qipeng    Wu Linzhong    Hu Huicheng    Xu Yong    Sun Baoquan    张桥    李述汤    

Nano Energy

2018

Solar cells, as promising devices for converting light into electricity, have a dramatically reduced performance on rainy days. Here, an energy harvesting structure that integrates a solar cell and a triboelectric nanogenerator (TENG) device is built to realize power generation from both sunlight and raindrops. A heterojunction silicon (Si) solar cell is integrated with a TENG by a mutual electrode of a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film. Regarding the solar cell, imprinted PEDOT:PSS is used to reduce light reflection, which leads to an enhanced short-circuit current density. A single-electrode-mode water-drop TENG on the solar cell is built by combining imprinted polydimethylsiloxane (PDMS) as a triboelectric material combined with a PEDOT:PSS layer as an electrode. The increasing contact area between the imprinted PDMS and water drops greatly improves the output of the TENG with a peak short-circuit current of ∼33.0 nA and a peak open-circuit voltage of ∼2.14 V, respectively. The hybrid energy harvesting system integrated electrode configuration can combine the advantages of high current level of a solar cell and high voltage of a TENG device, promising an efficient approach to collect energy from the environment in different weather conditions.

Liu Yuqiang    Sun Na    Liu Jiawei    Zhen Wen    Xuhui Sun    李述汤     Sun Baoquan   

ACS Nano

2018

The inherent instability of CHNHPbXremains a major technical barrier for the industrial applications of perovskite materials. Recently, the most stable surface structures of CHNHPbXhave been successfully characterized by using density functional theory (DFT) calculations together with the high-resolution scanning tunneling microscopy (STM) results. The two coexisting phases of the perovskite surfaces have been ascribed to the alternate orientation of the methylammonium (MA) cations. Notably, similar surface defect images (a dark depression at the sites of X atoms) have been observed on surfaces produced with various experimental methods. As such, these defects are expected to be intrinsic to the perovskite crystals and may play an important role in the structural decomposition of perovskite materials. Understanding the nature of such defects should provide some useful information toward understanding the instability of perovskite materials. Thus, we investigate the chemical identity of the surface defects systematically with first-principles density functional theory calculations and STM simulations. The calculated STM images of the Br and Br-MA vacancies are both in good agreement with the experimental measurements. In vacuum conditions, the formation energy of Br-MA is 0.43 eV less than the Br vacancy. In the presence of solvation effects, however, the formation energy of a Br vacancy becomes 0.42 eV lower than the Br-MA vacancy. In addition, at the vacancy sites, the adsorption energies of water, oxygen, and acetonitrile molecules are significantly higher than those on the pristine surfaces. This clearly demonstrated that the structural decomposition of perovskites are much easier to start from these vacancy sites than the pristine surfaces. Combining DFT calculations and STM simulations, this work reveals the chemical identities of the intrinsic defects in the CHNHPbXperovskite crystals and their effects on the stability of perovskite materials.

Liu Yunxia    Palot́s Krisztín    Yuan Xiao    Tingjun Hou    Lin Haiping    Youyong Li    李述汤    

ACS Nano

2017

A specially designed n-type semiconductor consisting of Ca-doped ZnO (CZO) nanoparticles is used as the electron transport layer (ETL) in high-performance multicolor perovskite light-emitting diodes (PeLEDs) fabricated using an all-solution process. The band structure of the ZnO is tailored via Ca doping to create a cascade of conduction energy levels from the cathode to the perovskite. This energy band alignment significantly enhances conductivity and carrier mobility in the CZO ETL and enables controlled electron injection, giving rise to sub-bandgap turn-on voltages of 1.65 V for red emission, 1.8 V for yellow, and 2.2 V for green. The devices exhibit significantly improved luminance yields and external quantum efficiencies of, respectively, 19 cd A and 5.8% for red emission, 16 cd A and 4.2% for yellow, and 21 cd A and 6.2% for green. The power efficiencies of these multicolor devices demonstrated in this study, 30 lm W for green light-emitting PeLED, 28 lm W for yellow, and 36 lm W for red are the highest to date reported. In addition, the perovskite layers are fabricated using a two-step hot-casting technique that affords highly continuous (>95% coverage) and pinhole-free thin films. By virtue of the efficiency of the ETL and the uniformity of the perovskite film, high brightnesses of 10 100, 4200, and 16,060 cd m are demonstrated for red, yellow, and green PeLEDs, respectively. The strategy of using a tunable ETL in combination with a solution process pushes perovskite-based materials a step closer to practical application in multicolor light-emitting devices.

Qasim Khan    Baoping Wang    Yupeng Zhang    Li Pengfei    Wang Yusheng    Li Shao-Juan    李述汤     Liangsheng Liao    Weil Lei    Qiaoliang Bao   

Advanced Functional Materials

2017

Compared with polycrystalline films, single-crystalline methylammonium lead halide (MAPbX, X = halogen) perovskite nanowires (NWs) with well-defined structure possess superior optoelectronic properties for optoelectronic applications. However, most of the prepared perovskite NWs exhibit properties below expectations due to poor crystalline quality and rough surfaces. It also remains a challenge to achieve aligned growth of single-crystalline perovskite NWs for integrated device applications. Here, we report a facile fluid-guided antisolvent vapor-assisted crystallization (FGAVC) method for large-scale fabrication of high-quality single-crystalline MAPb(IBr)(x = 0, 0.1, 0.2, 0.3, 0.4) NW arrays. The resultant perovskite NWs showed smooth surfaces due to slow crystallization process and moisture-isolated growth environment. Significantly, photodetectors made from the NW arrays exhibited outstanding performance in respect of ultrahigh responsivity of 12"500 A W, broad linear dynamic rang (LDR) of 150 dB, and robust stability. The responsivity represents the best value ever reported for perovskite-based photodetectors. Moreover, the spectral response of the MAPb(IBr)NW arrays could be sequentially tuned by varying the content of x = 0-0.4. On the basis of this feature, the NW arrays were monolithically integrated to form a unique system for directly measuring light wavelength. Our work would open a new avenue for the fabrication of high-performance, integrated optoelectronic devices from the perovskite NW arrays.

Wei Deng    Huang Liming    Xu Xiuzhen    Xiujuan Zhang    Jin Xiangcheng    李述汤     Jie Jian-Sheng   

Nano Letters

2017

Significant effort has been made to develop novel material systems to improve the efficiency of near-infrared organic light-emitting diodes (NIR OLEDs). Of those, fluorescent chromophores are mostly studied because of their advantages in cost and tunability. However, it is still rare for fluorescent NIR emitters to present good color purities in the NIR range and to have high external quantum efficiency (EQE). Here, a wedge-shaped D-π-A-π-D emitter APDC-DTPA with thermally activated delayed fluorescence property and a small single-triplet splitting (ΔE) of 0.14 eV is presented. The non-doped NIR device exhibits excellent performance with a maximum EQE of 2.19% and a peak wavelength of 777 nm. Remarkably, when 10 wt% of APDC-DTPA is doped in 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene host, an extremely high EQE of 10.19% with an emission peak of 693 nm is achieved. All these values represent the best result for NIR OLEDs based on a pure organic fluorescent emitter with similar device structure and color gamut.

Yi Yuan    Hu Yun    Yexin Zhang    Lin Jiu-Dong    Wang Ya-Kun    Zuoquan Jiang    Liangsheng Liao    李述汤    

Advanced Functional Materials

2017

In conventional crystalline silicon (Si) homojunction solar cells, a strategy of doping by transporting phosphorus or boron impurities into Si is commonly used to build Ohmic contacts at rear electrodes. However, this technique involves an energy intensive, high temperature (∼800 °C) process and toxic doping materials. Black phosphorus (BP) is a two-dimensional, narrow bandgap semiconductor with high carrier mobility that exhibits broad light harvesting properties. Here, we place BP:zinc oxide (ZnO) composite films between Si and aluminum (Al) to improve their contact. Once the BP harvests photons with energies below 1.1 eV from the crystalline Si, the ZnO carrier concentration increases dramatically due to charge injection. This photo-induced doping results in a high carrier concentration in the ZnO film, mimicking the modulated doping technique used in semiconductor heterojunctions. We show that photo-induced carriers dramatically increase the conductivities of the BP-modified ZnO films, thus reducing the contact resistance between Si and Al. A photovoltaic power conversion efficiency of 15.2% is achieved in organic-Si heterojunction solar cells that use a ZnO:BP layer. These findings demonstrate an effective way of improving Si/metal contact via a simple, low temperature process. [Figure not available: see fulltext.].

Xia Zhouhui    Li Pengfei    Liu Yu-Qiang    Song Tao    Qiaoliang Bao    李述汤     Sun Baoquan   

Nano Research

2017

Organic-inorganic hybrid solar cells based on n-type crystalline silicon and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) exhibited promising efficiency along with a low-cost fabrication process. In this work, ultrathin flexible silicon substrates, with a thickness as low as tens of micrometers, were employed to fabricate hybrid solar cells to reduce the use of silicon materials. To improve the light-trapping ability, nanostructures were built on the thin silicon substrates by a metal-assisted chemical etching method (MACE). However, nanostructured silicon resulted in a large amount of surface-defect states, causing detrimental charge recombination. Here, the surface was smoothed by solution-processed chemical treatment to reduce the surface/volume ratio of nanostructured silicon. Surface-charge recombination was dramatically suppressed after surface modification with a chemical, associated with improved minority charge-carrier lifetime. As a result, a power conversion efficiency of 9.1% was achieved in the flexible hybrid silicon solar cells, with a substrate thickness as low as ∼14 μm, indicating that interface engineering was essential to improve the hybrid junction quality and photovoltaic characteristics of the hybrid devices.

Jie Zhang    Yang Zhang    Song Tao    Shen Xinlei    Xuegong Yu    李述汤     Sun Baoquan    Baohua Jia   

ACS Applied Materials and Interfaces

2017