Meiyue Liu

High-Performance Color-Tunable Perovskite Light Emitting Devices through Structural Modulation from Bulk to Layered Film

Adding 2-phenoxyethylamine (POEA) into a CH3 NH3 PbBr3 precursor solution can modulate the organic-inorganic hybrid perovskite structure from bulk to layered, with a photoluminescence and electroluminescence shift from green to blue. Meanwhile, POEA can passivate the CH3 NH3 PbBr3 surface and help to obtain a pure CH3 NH3 PbBr3 phase, leading to an improvement of the external quantum efficiency to nearly 3% in CH3 NH3 PbBr3 LED.

Fabrication of high-performance and low-hysteresis lead halide perovskite solar cells by utilizing a versatile alcohol-soluble bispyridinium salt as an efficient cathode modifier

A novel alcohol-soluble conjugated bispyridinium salt (FPyBr) is developed and used as a cathode modifier to improve the cathode interface of planar heterojunction perovskite solar cells (PHJ PVSCs). The excellent electron-withdrawing ability of bispyridinium rings endows FPyBr with a favorable energy level alignment with phenyl-C60-butyric acid methyl ester (PCBM) and the cathode (e.g., Al), which leads to an ideal ohmic contact and efficient electron transport and collection. The deep-lying highest occupied molecular orbital energy level of FPyBr can also effectively block hole carriers and thus decrease leakage current and hole–electron recombination at the cathode interface. In addition, FPyBr can n-dope PCBM through an anion-induced electron transfer process, which increases the electron mobility of PCBM drastically, thereby diminishing interfacial resistance and promoting electron transport. As a result, by incorporating an FPyBr cathode interlayer with ethanol solvent, high-performance and low-hysteresis PHJ PVSCs with a maximal power conversion efficiency (PCE) of 19.61% can be realized. In contrast, reference devices without any cathode interlayer display a distinctly worse performance, with a PCE of 16.97%. Therefore, this excellent cathode modifier provides a new opportunity to fabricate high performance multilayer PVSCs using low-temperature solution processing without interfacial erosion/mixing.

Effects of organic cations on the defect physics of tin halide perovskites

Tin (Sn) halide perovskite absorbers have attracted much interest because of their nontoxicity as compared to their lead (Pb) halide perovskite counterparts. Recent progress shows that the power conversion efficiency of FASnI3 (FA = HC(NH2)2) solar cells prevails over that of MASnI3 (MA = CH3NH3). In this paper, we show that the organic cations, i.e., FA and MA, play a vital role in the defect properties of Sn halide perovskites. The antibonding coupling between Sn-5s and I-5p is clearly weaker in FASnI3 than in MASnI3 due to the larger ionic size of FA, leading to higher formation energies of Sn vacancies in FASnI3. Subsequently, the conductivity of FASnI3 can be tuned from p-type to intrinsic by varying the growth conditions of the perovskite semiconductor; in contrast, MASnI3 shows unipolar high p-type conductivity independent of the growth conditions. This provides a reasonable explanation for the better performance of FASnI3-based solar cells in experiments with respect to the MASnI3-based solar cells.

Interface Engineering for All‐Inorganic CsPbI2Br Perovskite Solar Cells with Efficiency over 14%

In this work, a SnO2 /ZnO bilayered electron transporting layer (ETL) aimed to achieve low energy loss and large open-circuit voltage (Voc ) for high-efficiency all-inorganic CsPbI2 Br perovskite solar cells (PVSCs) is introduced. The high-quality CsPbI2 Br film with regular crystal grains and full coverage can be realized on the SnO2 /ZnO surface. The higher-lying conduction band minimum of ZnO facilitates desirable cascade energy level alignment between the perovskite and SnO2 /ZnO bilayered ETL with superior electron extraction capability, resulting in a suppressed interfacial trap-assisted recombination with lower charge recombination rate and greater charge extraction efficiency. The as-optimized all-inorganic PVSC delivers a high Voc of 1.23 V and power conversion efficiency (PCE) of 14.6%, which is one of the best efficiencies reported for the Cs-based all-inorganic PVSCs to date. More importantly, decent thermal stability with only 20% PCE loss is demonstrated for the SnO2 /ZnO-based CsPbI2 Br PVSCs after being heated at 85 °C for 300 h. These findings provide important interface design insights that will be crucial to further improve the efficiency of all-inorganic PVSCs in the future.

High performance low-bandgap perovskite solar cells based on a high-quality mixed Sn–Pb perovskite film prepared by vacuum-assisted thermal annealing

Tandem perovskite solar cells are an effective concept to overcome the Shockley–Queisser limit of a single-junction perovskite solar cell. For a high-performance tandem cell, besides a wide-bandgap perovskite top cell, a high-quality low-bandgap perovskite bottom cell with an optimum bandgap of ∼1.2 eV is urgently needed. Moreover, a simple process technique needs to be developed for a high-quality perovskite film with good reproducibility, in order to further simplify the whole tandem-cell fabrication. Accordingly, we develop a simple one-step process (vacuum-assisted thermal annealing) for a high-quality low-bandgap CH3NH3Sn0.5Pb0.5IxCl3−x film, where the absorption edge can exceed 1000 nm. After comparing CH3NH3Sn0.5Pb0.5IxCl3−x films annealed in a vacuum and in a nitrogen environment, we find that vacuum-assisted thermal annealing can result in CH3NH3Sn0.5Pb0.5IxCl3−x films with better film coverage and crystallinity. This process can also accelerate the sublimation of methylammonium chloride and reduce the trap density in the CH3NH3Sn0.5Pb0.5IxCl3−x film. With this process, we successfully fabricated an efficient low-bandgap perovskite solar cell with a power conversion efficiency of more than 12% and good device reproducibility as well as long-term stability.

Recombination Dynamics Study on Nanostructured Perovskite Light-Emitting Devices

The field of organic-inorganic hybrid perovskite light-emitting diodes (PeLEDs) has developed rapidly in recent years. Although the performance of PeLEDs continues to improve through film quality control and device optimization, little research has been dedicated to understanding the recombination dynamics in perovskite thin films. Likewise, little has been done to investigate the effects of recombination dynamics on the overall light-emitting behavior of PeLEDs. Therefore, this study investigates the recombination dynamics of CH3 NH3 PbI3 thin films with differing crystal sizes by measurement of fluence-dependent transient absorption dynamics and time-resolved photoluminescence. The aim is to find out the link between recombination dynamics and device behavior in PeLEDs. It is found that bimolecular and Auger recombination become more efficient as the crystal size decreases and monomolecular recombination rate is affected by the trap density of perovskite. By defining the radiative efficiency Φ(n), which relates to the monomolecular, bimolecular, and Auger recombination, the fundamental recombination properties of CH3 NH3 PbI3 films are discerned in quantitative terms. These findings help us to understand the light emission behavior of PeLEDs. This study takes an important step toward establishing the relationship between film structure, recombination dynamics, and device behavior for PeLEDs, thereby providing useful insights toward the design of better perovskite devices.