Quantum Dot Passivation of Halide Perovskite Films with Reduced Defects, Suppressed Phase Segregation, and Enhanced Stability.

Structural defects are ubiquitous for polycrystalline perovskite films, compromising device performance and stability. Herein, a universal method is developed to overcome this issue by incorporating halide perovskite quantum dots (QDs) into perovskite polycrystalline films. CsPbBr3 QDs are deposited on four types of halide perovskite films (CsPbBr3 , CsPbIBr2 , CsPbBrI2 , and MAPbI3 ) and the interactions are triggered by annealing. The ions in the CsPbBr3 QDs are released into the thin films to passivate defects, and concurrently the hydrophobic ligands of QDs self-assemble on the film surfaces and grain boundaries to reduce the defect density and enhance the film stability. For all QD-treated films, PL emission intensity and carrier lifetime are significantly improved, and surface morphology and composition uniformity are also optimized. Furthermore, after the QD treatment, light-induced phase segregation and degradation in mixed-halide perovskite films are suppressed, and the efficiency of mixed-halide CsPbIBr2 solar cells is remarkably improved to over 11% from 8.7%. Overall, this work provides a general approach to achieving high-quality halide perovskite films with suppressed phase segregation, reduced defects, and enhanced stability for optoelectronic applications.

Optimising Non-Patterned MoO3/Ag/MoO3 Anode for High-Performance Semi-Transparent Organic Solar Cells towards Window Applications.

Semi-transparent organic solar cells (ST-OSCs) have attracted significant research attention, as they have strong potential to be applied in automobiles and buildings. For ST-OSCs, the transparent top electrode is an indispensable component, where the dielectric/metal/dielectric (D/M/D) structured electrode displayed a promising future due to its simplicity in the fabrication. In this work, by using the MoO3-/Ag-/MoO3-based D/M/D transparent electrode, we fabricated ST-OSCs based on the PM6:N3 active layer for the first time. In the device fabrication, the D/M/D transparent electrode was optimised by varying the thickness of the outer MoO3 layer. As a result, we found that increasing the thickness of the outer MoO3 layer can increase the average visible transmittance (AVT) but decrease the power conversion efficiency (PCE) of the device. The outer MoO3 layer with a 10 nm thickness was found as the optimum case, where its corresponding device showed the PCE of 9.18% with a high AVT of 28.94%. Moreover, the colour perception of fabricated ST-OSCs was investigated. All semi-transparent devices exhibited a neutral colour perception with a high colour rendering index (CRI) over 90, showing great potential for the window application.

3D characterisation using plasma FIB-SEM: A large-area tomography technique for complex surfaces like black silicon.

This paper demonstrates an improved method to accurately extract the surface morphology of black silicon (BSi). The method is based on an automated Xe+ plasma focused ion beam (PFIB) and scanning electron microscope (SEM) tomography technique. A comprehensive new sample preparation method is described and shown to minimize the PFIB artifacts induced by both the top surface sample-PFIB interaction and the non-uniform material density. An optimized post-image processing procedure is also described that ensures the accuracy of the reconstructed 3D surface model. The application of these new methods is demonstrated by applying them to extract the surface topography of BSi formed by reactive ion etching (RIE) consisting of 2 µm tall needles. An area of 320 µm2 is investigated with a controlled slice thickness of 10 nm. The reconstructed 3D model allows the extraction of critical roughness characteristics, such as height distribution, correlation length, and surface enhancement ratio. Furthermore, it is demonstrated that the particular surface studied contains regions in which under-etching has resulted in overhanging structures, which would not have been identified with other surface topography techniques. Such overhanging structures can be present in a broad range of BSi surfaces, including BSi surfaces formed by RIE and metal catalyst chemical etching (MCCE). Without proper measurement, the un-detected overhangs would result in the underestimation of many critical surface characteristics, such as absolute surface area, electrochemical reactivity and light-trapping.

Progress in Stability of Organic Solar Cells.

The organic solar cell (OSC) is a promising emerging low-cost thin film photovoltaics technology. The power conversion efficiency (PCE) of OSCs has overpassed 16% for single junction and 17% for organic-organic tandem solar cells with the development of low bandgap organic materials synthesis and device processing technology. The main barrier of commercial use of OSCs is the poor stability of devices. Herein, the factors limiting the stability of OSCs are summarized. The limiting stability factors are oxygen, water, irradiation, heating, metastable morphology, diffusion of electrodes and buffer layers materials, and mechanical stress. The recent progress in strategies to increase the stability of OSCs is surveyed, such as material design, device engineering of active layers, employing inverted geometry, optimizing buffer layers, using stable electrodes and encapsulation materials. The International Summit on Organic Photovoltaic Stability guidelines are also discussed. The potential research strategies to achieve the required device stability and efficiency are highlighted, rendering possible pathways to facilitate the viable commercialization of OSCs.

Burn-In Degradation Mechanism Identified for Small Molecular Acceptor-Based High-Efficiency Nonfullerene Organic Solar Cells.

Organic solar cells (OSCs) have again become a hot research topic in recent years. The record power conversion efficiency (PCE) of OSCs has boosted to over 17% in 2020. Apart from the high PCE, the stability of OSCs is also critical for their future applications and commercialization. Recently, many studies have proposed that burn-in degradation can be considered as an ineluctable barrier to long-term stable OSCs. However, there is still lack of studies to explain the detailed mechanism of this burn-in process. In this work, we first investigated the mechanism of the burn-in process in the high-efficiency PM6:N3-based nonfullerene OSCs. The PM6:N3-based device achieved a profound average PCE of 14.10% but also showed a significant performance loss after the burn-in degradation. Following characterizations such as dark J-V, photoluminescence (PL), time-resolved PL, Urbach energy estimation, and electrochemical impedance spectroscopy reveal that the burn-in degradation observed is closely related to the current extraction, energy transfer, nonradiative recombination, and charge transport process in the PM6:N3-based device. At the same time, it has small effects on the exciton dissociation process and energetic disorder in the PM6:N3-based device. Atomic force microscopy, scanning electron microscopy, transmission electron microscopy, and grazing incidence X-ray diffraction measurements gratifyingly found that the morphology of the PM6:N3 active layer is relatively stable during the burn-in degradation. Therefore, these observed degradations are suspected results from the instability of interfaces and electrodes. The atoms in carrier transport layers and electrodes may diffuse to the active layer during the degradation, which changes the energy levels of each layer and causes traps at the interface and in the active layer. Conquering the instability of interfaces and electrodes is proposed as the prior task for PM6:N3-based OSCs to achieve long-term stability. Our study provides insights into the mechanism behind the burn-in degradation of the PM6:N3-based OSCs, which takes the first step to conquer this barrier.

Effects of Hydroiodic Acid Concentration on the Properties of CsPbI3 Perovskite Solar Cells.

Inorganic cesium lead triiodide (CsPbI3) perovskite materials are becoming increasingly attractive for use in perovskite/silicon tandem solar cells, due to their almost ideal band gap energy (E g) of about 1.7 eV. To be useful as photovoltaic absorbers, the CsPbI3 must form the cubic or black phase (α-CsPbI3). To do so at relatively low temperatures, hydroiodic acid (HI) is required as a solution additive. This paper demonstrates CsPbI3 perovskite solar cells with an efficiency of 6.44%, formed using a HI concentration of 36 µL/mL. This value is higher than the previous most commonly used HI additive concentration. Herein, by undertaking a systematic study of the HI concentration, we demonstrate that the structural, morphological, optical, and electrical properties of CsPbI3 solar cells, processed with this HI additive concentration, are superior.