Multi-functional MXene quantum dots enhance the quality of perovskite polycrystalline films and charge transport for solar cells.

Recently, two-dimensional (2D) transition metal carbides/nitrides (MXenes) find applications in perovskite solar cells (PSCs), due to their high conductivity, tunable electronic structures, and rich surface chemistry, etc. However, the integration of 2D MXenes into PSCs is limited by their large lateral sizes and relatively-small surface volume ratios, and the roles of MXenes in PSCs are still ambiguous. In this paper, zero-dimensional (0D) MXene quantum dots (MQDs) with an average size of 2.7 nm are obtained through clipping step by step combining a chemical etching and a hydrothermal reaction, which display rich terminals (i.e., -F, -OH, -O) and unique optical properties. The 0D MQDs incorporated into SnO2 electron transport layers (ETLs) of PSCs exhibit multifunction: 1) increasing the electrical conductivity of SnO2, 2) promoting better alignments of energy band positions at the perovskite/ETL interface, 3) improving the film quality of atop polycrystalline perovskite. Particularly, the MQDs not only tightly bond with the Sn atom for decreasing the defects of SnO2, but also interact with the Pb2+ of perovskite. As a result, the defect density of PSCs is significantly decreased from 5.21 × 1021 to 6.4 × 1020 cm-3, leading to enhanced charge transport and reduced nonradiative recombination. Furthermore, the power conversion efficiency (PCE) of PSCs is substantially improved from 17.44% to 21.63% using the MQDs-SnO2 hybrid ETL compared with the SnO2 ETL. Besides, the stability of the MQDs-SnO2-based PSC is greatly enhanced, with only ～4% degradation of the initial PCE after storage in ambient condition (25 °C, RH: 30-40%) for 1128 h, as compared to that of the reference device with a rapid degradation of ～60% of initial PCE after 460 h. And MQDs-SnO2-based PSC also presents higher thermal stability than SnO2-based device with continuous heating for 248 h at 85 °C. The unique MQDs exhibited in this work might also find other exciting applications such as light-emitting diodes, photodetectors, and fluorescent probes.

Two-Dimensional Transitional Metal Disulfides as Charge Transport Layers in Organic-Inorganic Perovskite Solar Cells.

In the past decade, organic-inorganic perovskite solar cells (PSCs) have received significant attentions due to their high efficiencies and low costs. However, the commercialization of PSCs is stilled hindered by several issues such as device performance (especially for large-area cells) and stability. Recently, two-dimensional (2D) transition metal disulfides (TMDs) show great potentials in solving aforementioned problems due to their unique morphological structure and electrical properties. Herein, we summarize the advancements in the recent applications of various TMDs materials as charge transport layers in PSCs. Although some progress have been made, there are considerable issues to be tackled in this field. The challenges and development directions of these 2D TMDs materials for PSCs are also clarified. Lastly, the most recent advancements about TMDs materials in some other electronic (or optoelectronic) fields are also summarized and discussed.

Interdiffusion Stomatal Movement in Efficient Multiple-Cation-Based Perovskite Solar Cells.

The composition and crystallization process are essential for high-quality perovskite films. Cesium (Cs) and methylammonium chlorine (MACl) were found to affect the crystallization kinetics of perovskite, and the performance and stability of corresponding devices were greatly improved. We adopted an ion exchange method to remove MACl vapor and add Cs to form a multiple-cation-based perovskite film. With the increase of annealing time, Cl- from cesium chloride (CsCl) and MA from methylammonium bromide (MABr) formed gradually MACl vapor, and the porosity of surface morphology improved accordingly. The highly crystallized and compact CsyMAx - yFA1 - xPbI3 - xBrx perovskite film with different compositions was eventually obtained. The effects of the amount of MABr on the property of perovskite films and on the performance of the corresponding perovskite solar cells (PerSCs) were systematically studied. The PerSCs derived from 12 mg of MABr exhibit the best photovoltaic performance with a power conversion efficiency of 21.57% under 1 sun illumination.