Long Zhou

Intermediate Phase Intermolecular Exchange Triggered Defect Elimination in CH3NH3PbI3 toward Room-Temperature Fabrication of Efficient Perovskite Solar Cells

The solvent-engineered one-step spin-coating method has been widely used to produce full-coverage CH3NH3PbI3 films for perovskite solar cells by forming an intermediate phase. However, the resultant CH3NH3PbI3 films usually contain numerous structural and compositional defects mainly resulting from the fast crystallization of the intermediate phase as well as the escape of CH3NH3I species induced by the inevitably thermal annealing recipe. Herein, a facile room-temperature intermolecular exchange route is proposed to enable conversion of the intermediate phase into uniform and ultra-flat CH3NH3PbI3 films. It can effectively inhibit the formation of structural and compositional defects in the resultant films, and even repair their inherent defects. As a result, the efficiency of perovskite solar cells can be boosted to 19.45% with a stabilized value of 18.55%, which is much higher than that from the ones fabricated by thermal annealing. This study suggests a facile and low-cost route to room-temperature fabrication of highly efficient perovskite solar cells including flexible ones.

Investigation on the structural, morphological, electronic and photovoltaic properties of a perovskite thin film by introducing lithium halide

The performance of perovskite solar cells (PSCs) including device efficiency and stability is mainly dependent on the perovskite film properties which are critically related to the organic cations used. Herein, we studied the role that the inorganic lithium (Li) cation played in perovskite thin films and its influence on crystal growth, film properties, and device performance. We found that within the threshold limit of a 1.0% molar ratio, the Li dopant had a positive effect on the film formation and properties. However, after replacing more MA+ with Li+, the device performance was degraded significantly with reduced short-circuit current density (Jsc) and fill factor (FF) values. With a doping ratio of 10 mol%, the film morphology, crystallinity, photophysical, and electronic properties totally changed due to the unstable nature of the Li doped, distorted 3-D perovskite structure. The Li doping mechanism was discussed, and it was thought to contain two different doping mechanisms. One is interstitial doping at the much lower doping ratio, and the other is substitutional doping for the MA cation at the higher doping ratio.

Efficient Semitransparent Perovskite Solar Cells Using a Transparent Silver Electrode and Four-Terminal Perovskite/Silicon Tandem Device Exploration

Four-terminal tandem solar cells employing a perovskite top cell and crystalline silicon (Si) bottom cell offer a simpler pathway to surpass the efficiency limit of market-leading single-junction silicon solar cells. To obtain cost-effective top cells, it is crucial to develop transparent conductive electrodes with low parasitic absorption and manufacturing cost. The commonly used indium tin oxide (ITO) shows some drawbacks, like the increasing prices and high-energy magnetron sputtering process. Transparent metal electrodes are promising candidates owing to the simple evaporation process, facile process conditions, and high conductivity, and the cheaper silver (Ag) electrode with lower parasitic absorption than gold may be the better choice. In this work, efficient semitransparent perovskite solar cells (PSCs) were firstly developed by adopting the composite cathode of an ultrathin Ag electrode at its percolation threshold thickness (11u2009nm), a molybdenum oxide optical coupling layer, and a bathocuproine interfacial layer. The resulting power conversion efficiency (PCE) is 13.38% when the PSC is illuminated from the ITO side and the PCE is 8.34% from the Ag side, and no obvious current hysteresis can be observed. Furthermore, by stacking an industrial Si bottom cell (PCEu2009=u200914.2%) to build a four-terminal architecture, the overall PCEs of 17.03% (ITO side) and 11.60% (Ag side) can be obtained, which are 27% and 39% higher, respectively, than those of the perovskite top cell. Also, the PCE of the tandem cell has exceeded that of the reference Si solar cell by about 20%. This work provides an outlook to fabricate high-performance solar cells via the cost-effective pathway.

Investigation of Fe2+-incorporating organic–inorganic hybrid perovskites from first principles and experiments

The development of high efficiency perovskite solar cells (PSCs) has been proved to depend on the stability and optical properties of perovskite materials. A lot of efforts have been applied to improving these properties. Among them, the alternative mixed-metal perovskite composition has been considered as a new solution for photovoltaic device applications to satisfy the demand for exploring efficient photovoltaic performance. Here, we have systematically performed first-principles calculations using density-functional theory (DFT) to study the structural, electronic, magnetic and optical properties of the perovskite CH3NH3(Pb:Fe)I3, and investigated the effect of iron (Fe) metal ion doping on the properties of the perovskites and solar cell performance. The calculated results reveal that the perovskite CH3NH3(Pb:Fe)I3 exhibits half-metallic behavior due to the impurity bands induced by the Fe dopant crossing the Fermi level. Consequently, it is found that the absorption intensities of CH3NH3(Pb:Fe)I3 are slightly higher than those of CH3NH3PbI3 in the near-infrared light region. It is unexpected that the perovskite CH3NH3(Pb:Fe)I3 exhibits a large magnetic moment of 4 μB and its magnetic coupling belongs to the antiferromagnetic (AFM) configuration. Meanwhile, we found that the Fe incorporation can distort the structure due to its small ionic size, which significantly changes the device performance. Our findings provide a reference for exploring the properties of perovskite materials.

High-Performance Simple-Structured Planar Heterojunction Perovskite Solar Cells Achieved by Precursor Optimization

Planar perovskite solar cells (PSCs) with perovskite and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) heterojunction have attracted much interest because they can be fabricated by a low-temperature process and exhibit high power conversion efficiency (PCE). Various compositional engineering and interface engineering approaches have been applied to produce high-performance PSC devices. In this study, we found that high-quality lead halide crystal precursor has a significant effect on the perovskite film quality and it could enhance perovskite film light absorption, reduce crystal defects, and suppress charge recombination, resulting in enhanced device performance (from 8.9 to 15.0% for CH3NH3PbI3 and from 14.2 to 18.0% for MA0.7FA0.3PbI3–xClx). Moreover, electron and hole interlayer free devices were achieved by using high-quality PbI2 crystals and the devices exhibited a high PCE of 13.6 and 10.4% for glass and flexible substrates, respectively. This is important for simplifying the perovskite solar cell fabrication process without complex interface engineering involved, especially for printed PSCs.

Performance of field-effect transistors based on Nb(x)W(1-x)S2 monolayers.

The Schottky barrier has been detected in many field-effect transistors (FETs) based on transition metal dichalcogenide (TMD) semiconductors and has seriously affected the electronic properties of the devices. In order to decrease the Schottky barrier in WS2 FETs, novel Nb doping in WS2 monolayers has been performed and p-FETs based on Nb-doped WS2 (Nb(x)W(1-x)S2) monolayers as the active channel have been fabricated for the first time. The monolayer Nb0.15W0.85S2 p-FET has a drain current of 330 μA μm(-1), an impressive I(ON)/I(OFF) of 10(7), and a high effective hole mobility of ∼146 cm(2) V(-1) s(-1). The novel Nb doping in monolayer WS2 has eliminated the ambipolar behavior and reduced the Schottky barrier in WS2 FETs. The reduction of the Schottky barrier is ascribed to the hybridization between W 5d, Nb 4d and S 3p states near the EF and to the enhancement of the metallization of the contact between the Pd metal and monolayer Nb(x)W(1-x)S2 after Nb doping.