Ligang Yuan

In this work, the mixed bromide iodide lead perovskites CH3NH3Pb(I1– xBrx)3 (0 ≤ x ≤ 0.67) thin films were prepared by co-evaporation of CH3NH3I, PbI2, and PbBr2. The electronic properties of CH3NH3Pb(I1– xBrx)3 thin films were investigated by X-ray and ultraviolet photoelectron spectroscopy in-situ. The results of core level binding energy show that there is no chemical shift of the C1s, N1s, Br3d5, and I3d5 when the Br composition changes, while there is an approximately linear chemical shift of Pb4f7 to higher binding energy as the Br composition increases. The density functional theory calculation reveals that there is more charge transfer from Pb to Br than I, which results in the chemical shift of Pb4f states. On the other hand, the valence band maximum increases as the Br composition increases, while the work function shows no obvious change, because the conduction band is dominated by Pb 6p orbitals while the valence band is dominated by halide p orbitals. Our work demonstrates the adjustability of the energy level alignment of MAPb(I1– xBrx)3 by the Br composition.

Organometal halide perovskites are highly promising materials for photovoltaic applications, yet their rapid degradation remains a significant challenge. Here, the light-induced structural degradation mechanism of methylammonium lead iodide (MAPbI3) perovskite films and devices is studied in low humidity environment using X-Ray Diffraction, Ultraviolet-Visible (UV-Vis) absorption spectroscopy, Extended X-ray Absorption Fine Structure spectroscopy, Fourier Transform Infrared spectroscopy, and device measurements. Under dry conditions, the perovskite film degrades only in the presence of both light and oxygen, which together induce the formation of halide anions through donation of electrons to the surrounding oxygen. The halide anions generate free radicals that deprotonate the methylammonium cation and form the highly volatile CH3NH2 molecules that escape and leave pure PbI2 behind. The device findings show that changes in the local structure at the TiO2 mesoporous layer occur with light, even in the absence of oxygen, and yet such changes can be prevented by the application of UV blocking layer on the cells. Our results indicate that the stability of mp-TiO2-MAPbI3 photovoltaics can be dramatically improved with effective encapsulation that protects the device from UV light, oxygen, and moisture.

Perovskite single crystals exhibit extraordinary optoelectronic performances due to their advantages such as low trap-state densities, long carrier diffusion, and large absorption coefficient, and thus, photodetectors based on perovskite single crystals have attracted much research interest. Unlike the reported one-component single-crystal perovskite photodetectors, here, we have developed a facile two-step approach to fabricate a core-shell heterojunction based on the CH3NH3PbBr3 single crystal. A photodetector made of the as-prepared perovskite heterojunction renders the feature of self-power attributed to a built-in electric field in the junction and exhibits a wavelength-dependent responsivity with a peak responsivity up to 11.5 mA W⁻¹ under 450 nm irradiation at zero bias, which is one order of magnitude higher than the CH3NH3PbBr3 single crystal and shows a maximum external quantum efficiency of 3.17%, also higher than the reported 0.2% of the CH3NH3PbBr3 single crystal. Our work may lead to more efficient self-powered heterojunction systems based on perovskite single crystals.

We report a reproducible enhancement of the open circuit voltage in Cu2ZnSnS4 solar cells by introduction of a very thin CeO2 interlayer between the Cu2ZnSnS4 absorber and the conventional CdS buffer. CeO2, a non-toxic earth-abundant compound, has a nearly optimal band alignment with Cu2ZnSnS4 and the two materials are lattice-matched within 0.4%. This makes it possible to achieve an epitaxial interface when growing CeO2 by chemical bath deposition at temperatures as low as 50 °C. The open circuit voltage improvement is then attributed to a decrease in the interface recombination rate through formation of a high-quality heterointerface.

Perovskite/crystalline silicon tandem solar cells have the potential to reach efficiencies beyond those of silicon single-junction record devices. However, the high-temperature process of 500 °C needed for state-of-the-art mesoscopic perovskite cells has, so far, been limiting their implementation in monolithic tandem devices. Here, we demonstrate the applicability of zinc tin oxide as a recombination layer and show its electrical and optical stability at temperatures up to 500 °C. To prove the concept, we fabricate monolithic tandem cells with mesoscopic top cell with up to 16% efficiency. We then investigate the effect of zinc tin oxide layer thickness variation, showing a strong influence on the optical interference pattern within the tandem device. Finally, we discuss the perspective of mesoscopic perovskite cells for high-efficiency monolithic tandem solar cells.

A multiple-selenization process for wet-chemically fabricated kesterite-type Cu2ZnSn(S,Se)4 solar cells is reported that significantly improves the overall sample quality of the absorber layer and especially the reproducibility of device characteristics. Conversion efficiencies of up to 7.2% are obtained. With this method, the absorber forms a very compact, hole- and crack-free layer and avoids the formation of multilayer or trilayer structures. Mainly, the series resistance and therefore the short-circuit current can be stabilized, which leads to lower fluctuation of the energy conversion efficiency of the solar cells on the same sample.

A method is proposed that enables the imaging of the photocurrent collected by a solar cell under arbitrary operating conditions. The method uses a series of luminescence images under varying illumination to derive the total photocurrent collection efficiency at a given voltage bias. The resulting total photocurrent collection image directly relates to the difference between the dark and illuminated current-voltage characteristics of the cell. A crystalline silicon solar cell is used to test the method, and the images of the total photocurrent collection efficiency are used to quantify the influence of a crack on the total collected photocurrent of the solar cell.