Shengzhong Frank Liu

Two‐Inch‐Sized Perovskite CH3NH3PbX3 (X = Cl, Br, I) Crystals: Growth and Characterization

Two-inch-sized perovskite crystals, CH3 NH3 PbX3 (X=I, Br, Cl), with high crystalline quality are prepared by a solution-grown strategy. The availability of large perovskite crystals is expected to transform its broad applications in photovoltaics, optoelectronics, lasers, photodetectors, LEDs, etc., just as crystalline silicon has done in revolutionizing the modern electronics and photovoltaic industries.

Hysteresis‐Suppressed High‐Efficiency Flexible Perovskite Solar Cells Using Solid‐State Ionic‐Liquids for Effective Electron Transport

An efficiency of flexible perovskite solar cells (Pvs-SCs) of 16.09% is achieved, the highest value reported for flexible Pvs-SCs to date. The outstanding performance is attributed to the superior features of alternative electron-transport materials, such as antireflection, a suitable work function, high electron mobility, and a reduced trap-state density of the perovskite material.

Thinness- and Shape-Controlled Growth for Ultrathin Single-Crystalline Perovskite Wafers for Mass Production of Superior Photoelectronic Devices

Thinness-controlled perovskite wafers are directly prepared using a geometry-regulated dynamic-flow reaction system. It is found that the wafers are a superior material for photodetectors with a photocurrent response ≈350 times higher than that made of microcrystalline thin films. Moreover, the wafers are compatible with mass production of integrated circuits.

Solution-Processed Nb:SnO2 Electron Transport Layer for Efficient Planar Perovskite Solar Cells

Electron transport layer (ETL), facilitating charge carrier separation and electron extraction, is a key component in planar perovskite solar cells (PSCs). We developed an effective ETL using low-temperature solution-processed Nb-doped SnO2 (Nb:SnO2). Compared to the pristine SnO2, the power conversion efficiency of PSCs based on Nb:SnO2 ETL is raised to 17.57% from 15.13%. The splendid performance is attributed to the excellent optical and electronic properties of the Nb:SnO2 material, such as smooth surface, high electron mobility, appropriate electrical conductivity, therefore making a better growth platform for a high quality perovskite absorber layer. Experimental analyses reveal that the Nb:SnO2 ETL significantly enhances the electron extraction and effectively suppresses charge recombination, leading to improved solar cell performance.

Enhancing Efficiency and Stability of Perovskite Solar Cells through Nb-Doping of TiO2 at Low Temperature

The conduction band energy, conductivity, mobility, and electronic trap states of electron transport layer (ETL) are very important to the efficiency and stability of a planar perovskite solar cell (PSC). However, as the most widely used ETL, TiO2 often needs to be prepared under high temperature and has unfavorable electrical properties such as low conductivity and high electronic trap states. Modifications such as elemental doping are effective methods for improving the electrical properties of TiO2 and the performance of PSCs. In this study, Nb-doped TiO2 films are prepared by a facile one-port chemical bath process at low temperature (70 °C) and applied as a high quality ETL for planar PSCs. Compared with pure TiO2, the Nb-doped TiO2 is more efficient for photogenerated electron injection and extraction, showing higher conductivity, higher mobility, and lower trap-state density. A PSC with 1% Nb-doped TiO2 yielded a power conversion efficiency of more than 19%, with about 90% of its initial efficiency remaining after storing for 1200 h in air or annealing at 80 °C for 20 h in a glovebox.

ITIC surface modification to achieve synergistic electron transport layer enhancement for planar-type perovskite solar cells with efficiency exceeding 20%

The electron transport layer (ETL), which also serves as the hole-blocking layer, is a key component in planar perovskite solar cells (PSCs). The commonly used ETL is an anatase-TiO2 (an-TiO2) film due to its excellent optical transmittance, chemical stability and semiconducting characteristics. Nevertheless, its rough surface and plenty of surface defects often lead to a substandard perovskite film and large J–V hysteresis. Herein, a novel low-trap-density ETL is developed by surface modification of the an-TiO2 film using small-molecular ITIC. As a result, the device efficiency has been dramatically increased from 17.12% to 20.08%, entering the league of the highest planar-type perovskite cells. Moreover, the J–V hysteresis has been significantly reduced. Further investigation shows that the ITIC smoothens the TiO2 surface, passivates defects or dangling bands parasitizing the TiO2 surface, and optimizes the device band alignment. In addition, it is demonstrated that the thin ITIC promotes the formation of high quality, uniform perovskite films with better surface coverage and large grain size, implying that there is a synergistic effect between the low-trap-density ITIC and high-mobility TiO2 in improved PSC performance.

All-Ambient Processed Binary CsPbBr3–CsPb2Br5 Perovskites with Synergistic Enhancement for High-Efficiency Cs–Pb–Br-Based Solar Cells

All-inorganic CsPbBr3 perovskite solar cells display outstanding stability toward moisture, light soaking, and thermal stressing, demonstrating great potential in tandem solar cells and toward commercialization. Unfortunately, it is still challenging to prepare high-performance CsPbBr3 films at moderate temperatures. Herein, a uniform, compact CsPbBr3 film was fabricated using its quantum dot (QD)-based ink precursor. The film was then treated using thiocyanate ethyl acetate (EA) solution in all-ambient conditions to produce a superior CsPbBr3-CsPb2Br5 composite film with a larger grain size and minimal defects. The achievement was attributed to the surface dissolution and recrystallization of the existing SCN- and EA. More specifically, the SCN- ions were first absorbed on the Pb atoms, leading to the dissolution and stripping of Cs+ and Br- ions from the CsPbBr3 QDs. On the other hand, the EA solution enhances the diffusion dynamics of surface atoms and the surfactant species. It is found that a small amount of CsPb2Br5 in the composite film gives the best surface passivation, while the Br-rich surface decreases Br vacancies (VBr) for a prolonged carrier lifetime. As a result, the fabricated device gives a higher solar cell efficiency of 6.81% with an outstanding long-term stability.

Stable High‐Performance Perovskite Solar Cells via Grain Boundary Passivation

The trap states at grain boundaries (GBs) within polycrystalline perovskite films deteriorate their optoelectronic properties, making GB engineering particularly important for stable high-performance optoelectronic devices. It is demonstrated that trap states within bulk films can be effectively passivated by semiconducting molecules with Lewis acid or base functional groups. The perovskite crystallization kinetics are studied using in situ synchrotron-based grazing-incidence X-ray scattering to explore the film formation mechanism. A model of the passivation mechanism is proposed to understand how the molecules simultaneously passivate the Pb-I antisite defects and vacancies created by under-coordinated Pb atoms. In addition, it also explains how the energy offset between the semiconducting molecules and the perovskite influences trap states and intergrain carrier transport. The superior optoelectronic properties are attained by optimizing the molecular passivation treatments. These benefits are translated into significant enhancements of the power conversion efficiencies to 19.3%, as well as improved environmental and thermal stability of solar cells. The passivated devices without encapsulation degrade only by ≈13% after 40 d of exposure in 50% relative humidity at room temperature, and only ≈10% after 24 h at 80 °C in controlled environment.

Ag Nanoparticle-Sensitized WO3 Hollow Nanosphere for Localized Surface Plasmon Enhanced Gas Sensors

Ag nanoparticle (NP)-sensitized WO3 hollow nanospheres (Ag-WO3-HNSs) are fabricated via a simple sonochemical synthesis route. It is found that the Ag-WO3-HNS shows remarkable performance in gas sensors. Field-emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) images reveal that the Agx-WO3 adopts the HNS structure in which WO3 forms the outer shell framework and the Ag NPs are grown on the inner wall of the WO3 hollow sphere. The size of the Ag NPs can be controlled by adjusting the addition amount of WCl6 during the reaction. The sensor Agx-WO3 exhibits extremely high sensitivity and selectivity toward alcohol vapor. In particular, the Ag(15nm)-WO3 sensor shows significantly lower operating temperature (230 °C), superior detection limits as low as 0.09 ppb, and faster response (7 s). Light illumination was found to boost the sensor performance effectively, especially at 405 and 900 nm, where the light wavelength resonates with the absorption of Ag NPs and the surface oxygen vacancies of WO3, respectively. The improved sensor performance is attributed to the localized surface plasmon resonance (LSPR) effect.

In Situ Synthesis of Few‐Layered g‐C3N4 with Vertically Aligned MoS2 Loading for Boosting Solar‐to‐Hydrogen Generation

In artificial photocatalytic hydrogen evolution, effective incident photon absorption and a high-charge recombination rate are crucial factors influencing the overall efficiency. Herein, a traditional solid-state synthesis is used to obtain, for the first time, novel samples of few-layered g-C3 N4 with vertically aligned MoS2 loading (MoS2 /C3 N4 ). Thiourea and layered MoO3 are chosen as precursors, as they react under nitrogen atmosphere to in situ produce the products. According to the quasi-Fourier transform infrared reflectance and X-ray diffraction measurements, the detailed reaction process is determined to proceed through the confirmed formation pathway. The two precursor units MoS2 and C3 N4 are linked by Mouf8ffN bonds, which act as electronic receivers/conductors and hydrogen-generation sites. Density functional theory is also carried out, which determines that the interface sites act as electron-accumulation regions. According to the photoelectrochemical results, MoS2 /C3 N4 can achieve a current of 0.05 mA cm-2 , which is almost ten times higher than that of bare g-C3 N4 or the MoS2 /C3 N4 -R reference samples. The findings in the present work pave the way to not only synthesize a series of designated samples but also thoroughly understand the solid-state reaction.