Xuejie Zhu

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.

Stable high efficiency two-dimensional perovskite solar cells via cesium doping

Two-dimensional (2D) organic–inorganic perovskites have recently emerged as one of the most important thin-film solar cell materials owing to their excellent environmental stability. The remaining major pitfall is their relatively poor photovoltaic performance in contrast to 3D perovskites. In this work we demonstrate cesium cation (Cs+) doped 2D (BA)2(MA)3Pb4I13 perovskite solar cells giving a power conversion efficiency (PCE) as high as 13.7%, the highest among the reported 2D devices, with excellent humidity resistance. The enhanced efficiency from 12.3% (without Cs+) to 13.7% (with 5% Cs+) is attributed to perfectly controlled crystal orientation, an increased grain size of the 2D planes, superior surface quality, reduced trap-state density, enhanced charge-carrier mobility and charge-transfer kinetics. Surprisingly, it is found that the Cs+ doping yields superior stability for the 2D perovskite solar cells when subjected to a high humidity environment without encapsulation. The device doped using 5% Cs+ degrades only ca. 10% after 1400 hours of exposure in 30% relative humidity (RH), and exhibits significantly improved stability under heating and high moisture environments. Our results provide an important step toward air-stable and fully printable low dimensional perovskites as a next-generation renewable energy source.

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.

Pt monolayer coating on complex network substrate with high catalytic activity for the hydrogen evolution reaction

A deposition process has been developed to fabricate a complete-monolayer Pt coating on a large-surface-area three-dimensional (3D) Ni foam substrate using a buffer layer (Ag or Au) strategy. The quartz crystal microbalance, current density analysis, cyclic voltammetry integration, and X-ray photoelectron spectroscopy results show that the monolayer deposition process accomplishes full coverage on the substrate and the deposition can be controlled to a single atomic layer thickness. To our knowledge, this is the first report on a complete-monolayer Pt coating on a 3D bulk substrate with complex fine structures; all prior literature reported on submonolayer or incomplete-monolayer coating. A thin underlayer of Ag or Au is found to be necessary to cover a very reactive Ni substrate to ensure complete-monolayer Pt coverage; otherwise, only an incomplete monolayer is formed. Moreover, the Pt monolayer is found to work as well as a thick Pt film for catalytic reactions. This development may pave a way to fabricating a high-activity Pt catalyst with minimal Pt usage.

Graphene oxide – a surprisingly good nucleation seed and adhesion promotion agent for one-step ZnO lithography and optoelectronic applications

Graphene oxide (GO), with its oxygen-containing functional groups (OFGs) attached to its surface and edges, has opened means for graphene to be used in more intriguing applications. We discovered that: ① OFGs on one side of the GO sheet provide excellent nucleation sites to grow a layer of solid ZnO film onto it; ② OFGs on the other side of the GO sheet form chemical bonds with the underlying solid substrate to provide adequate adhesion between the film and the substrate; ③ the combination of these two properties makes it feasible to perform one-step ZnO lithography for microelectronics applications; and ④ it has also made it possible for us to fabricate Ag NP doped ZnO–graphene composite thin films for surface plasmon generation, light trapping and improved solar cell efficiency. XPS analysis confirms that C–O–Si type chemical bonds form when GO was introduced onto the Si wafer surface. As it is expected that GO can also be used to nucleate other metal oxide films on different substrates, it may open a broad arena for new applications.

Effective solvent-additive enhanced crystallization and coverage of absorber layers for high efficiency formamidinium perovskite solar cells

For a high efficiency of planar-type perovskite solar cells, a good crystallization and high surface coverage of the absorber films are required. However, these two key factors are still the main challenges in perovskite film formation to date. Here, 1-chloronaphthalene (CN) is used as a solvent-additive in the precursor HC(NH2)2PbI3 (FAPbI3) solution to control the crystallization and surface coverage of the FAPbI3 films by adjusting its concentration. The CN chlorinated monodentate ligand is likely to temporarily chelate with Pb2+ during the crystal growth, facilitating homogenous nucleation to form relatively high quality FAPbI3 films. Meanwhile, the CN additive with a high boiling point delays the growth rate of the FAPbI3 film, which helps to form homogenous continuous FAPbI3 films with fewer pin-holes. As a result, with the addition of the CN solvent-additive, the efficiency of the FAPbI3 planar-type solar cells is enhanced to 16.53%.

High efficiency planar-type perovskite solar cells with negligible hysteresis using EDTA-complexed SnO 2

Even though the mesoporous-type perovskite solar cell (PSC) is known for high efficiency, its planar-type counterpart exhibits lower efficiency and hysteretic response. Herein, we report success in suppressing hysteresis and record efficiency for planar-type devices using EDTA-complexed tin oxide (SnO2) electron-transport layer. The Fermi level of EDTA-complexed SnO2 is better matched with the conduction band of perovskite, leading to high open-circuit voltage. Its electron mobility is about three times larger than that of the SnO2. The record power conversion efficiency of planar-type PSCs with EDTA-complexed SnO2 increases to 21.60% (certified at 21.52% by Newport) with negligible hysteresis. Meanwhile, the low-temperature processed EDTA-complexed SnO2 enables 18.28% efficiency for a flexible device. Moreover, the unsealed PSCs with EDTA-complexed SnO2 degrade only by 8% exposed in an ambient atmosphere after 2880 h, and only by 14% after 120 h under irradiation at 100 mW cm−2.The development of high efficiency planar-type perovskite solar cell has been lagging behind the mesoporous-type counterpart. Here Yang et al. modify the oxide based electron transporting layer with organic acid and obtain planar-type cells with high certified efficiency of 21.5% and decent stability.

Record Efficiency Stable Flexible Perovskite Solar Cell Using Effective Additive Assistant Strategy

Even though the power conversion efficiency (PCE) of rigid perovskite solar cells is increased to 22.7%, the PCE of flexible perovskite solar cells (F-PSCs) is still lower. Here, a novel dimethyl sulfide (DS) additive is developed to effectively improve the performance of the F-PSCs. Fourier transform infrared spectroscopy reveals that the DS additive reacts with Pb2+ to form a chelated intermediate, which significantly slows down the crystallization rate, leading to large grain size and good crystallinity for the resultant perovskite film. In fact, the trap density of the perovskite film prepared using the DS additive is reduced by an order of magnitude compared to the one without it, demonstrating that the additive effectively retards transformation kinetics during the thin film formation process. As a result, the PCE of the flexible devices increases to 18.40%, with good mechanical tolerance, the highest reported so far for the F-PSCs. Meanwhile, the environmental stability of the F-PSCs significantly enhances by 1.72 times compared to the device without the additive, likely due to the large grain size that suppresses perovskite degradation at grain boundaries. The present strategy will help guide development of high efficiency F-PSCs for practical applications.

Chelate-Pb Intermediate Engineering for High-Efficiency Perovskite Solar Cells

Crystallization quality and grain size are key factors in fabricating high-performance planar-type perovskite photovoltaics. Herein, we used 1,8-octanedithiol as an effective additive in the [HC(NH2)2]0.95Cs0.05PbI3 (FA0.95Cs0.05PbI3) solution to improve the FA0.95Cs0.05PbI3 film quality via solution processing. 1,8-Octanedithiol would coordinate with lead to form the chelate-Pb compound, leading to smaller Gibbs free energy during the perovskite crystallization process, facilitating formation of high-quality perovskite films with larger grains, smoother surfaces, lower electron trap densities, and longer carrier lifetimes compared to the nonadditive ones. As a result, the champion efficiency for devices with 3% 1,8-octanedithiol-doped FA0.95Cs0.05PbI3 is raised to 19.36% from 18.39% of a device without the additive. The new technique is a promising way to fabricate perovskite photovoltaics with high performance.