Yehao Deng

Bio: Yehao Deng is an academic researcher from University of North Carolina at Chapel Hill. The author has contributed to research in topics: Perovskite (structure) & Grain boundary. The author has an hindex of 38, co-authored 51 publications receiving 7748 citations. Previous affiliations of Yehao Deng include University of Nebraska–Lincoln.

Grain boundary dominated ion migration in polycrystalline organic–inorganic halide perovskite films

Abstract: The efficiency of perovskite solar cells is approaching that of single-crystalline silicon solar cells despite the presence of a large grain boundary (GB) area in the polycrystalline thin films. Here, by using a combination of nanoscopic and macroscopic level measurements, we show that ion migration in polycrystalline perovskites dominates through GBs. Atomic force microscopy measurements reveal much stronger hysteresis both for photocurrent and dark-current at the GBs than on the grain interiors, which can be explained by faster ion migration at the GBs. The dramatically enhanced ion migration results in the redistribution of ions along the GBs after electric poling, in contrast to the intact grain area. The perovskite single-crystal devices without GBs show negligible current hysteresis and no ion-migration signal. The discovery of dominating ion migration through GBs in perovskites can lead to broad applications in many types of devices including photovoltaics, memristors, and ion batteries.

Scaling behavior of moisture-induced grain degradation in polycrystalline hybrid perovskite thin films

Abstract: The stability of perovskite solar cells has shown a huge variation with respect to the film process and film morphology, while the underlining mechanism for the morphology-dependent degradation of the perovskite film has remained elusive. Herein, we report a scaling behavior of moisture-induced grain degradation in polycrystalline CH3NH3PbI3 films. The degradation rates of CH3NH3PbI3 films in moisture were shown to be sensitive to the grain sizes. The duration that was needed for different films to degrade by the same percent showed a linear relationship with the grain size, despite the fact that the films were formed by five different deposition methods. This scaling behavior can be explained by the degradation along the in-plane direction, which is initiated at the grain boundary (GB). The GBs of CH3NH3PbI3 films consist of an amorphous intergranular layer, which allows quick diffusion of moisture into the perovskite films. It was found that thermal annealing induced surface self-passivation plays a critical role in stabilizing the surfaces of thin films and single crystals by reducing the moisture-sensitive methylammonium ions at the surface. The determination of the scaling behavior of grain degradation highlights the importance of stabilizing the GBs to improve the stability of perovskite solar cells.

Scalable fabrication of efficient organolead trihalide perovskite solar cells with doctor-bladed active layers

Abstract: Organolead trihalide perovskites (OTPs) are nature abundant materials with prospects as future low-cost renewable energy sources boosted by the solution process capability of these materials. Here we report the fabrication of efficient OTP devices by a simple, high throughput and low-cost doctor-blade coating process which can be compatible with the roll-to-roll fabrication process for the large scale production of perovskite solar cell panels. The formulation of appropriate precursor inks by removing impurities is shown to be critical in the formation of continuous, pin-hole free and phase-pure perovskite films on large area substrates, which is assisted by a high deposition temperature to guide the nucleation and grain growth process. The domain size reached 80–250 μm in 1.5–2 μm thick bladed films. By controlling the stoichiometry and thickness of the OTP films, highest device efficiencies of 12.8% and 15.1% are achieved in the devices fabricated on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate and cross-linked N4,N4′-bis(4-(6-((3-ethyloxetan-3-yl)methoxy)hexyl)phenyl)–N4,N4′-diphenylbiphenyl-4,4′-diamine covered ITO substrates. Interestingly, the carrier diffusion length in doctor-bladed OTP films is beyond 3.5 μm which is significantly larger than in the spin-coated films, due to the formation of crystalline grains with a very large size by the doctor-blade coating method.

Strained hybrid perovskite thin films and their impact on the intrinsic stability of perovskite solar cells.

Abstract: Organic-inorganic hybrid perovskite (OIHP) solar cells have achieved comparable efficiencies to those of commercial solar cells, although their instability hinders their commercialization. Although encapsulation techniques have been developed to protect OIHP solar cells from external stimuli such as moisture, oxygen, and ultraviolet light, understanding of the origin of the intrinsic instability of perovskite films is needed to improve their stability. We show that the OIHP films fabricated by existing methods are strained and that strain is caused by mismatched thermal expansion of perovskite films and substrates during the thermal annealing process. The polycrystalline films have compressive strain in the out-of-plane direction and in-plane tensile strain. The strain accelerates degradation of perovskite films under illumination, which can be explained by increased ion migration in strained OIHP films. This study points out an avenue to enhance the intrinsic stability of perovskite films and solar cells by reducing residual strain in perovskite films.

Surfactant-controlled ink drying enables high-speed deposition of perovskite films for efficient photovoltaic modules

Abstract: Novel photovoltaic technologies such as perovskites hold the promise of a reduced levelized cost of electricity, but the low-cost potential depends on the ability to scale-up solution-based deposition. So far, complex fluid dynamics have limited the solution deposition of uniform pinhole-free organic–inorganic perovskite thin films over large areas. Here, we show that very small amounts (tens of parts per million) of surfactants (for example, l-α-Phosphatidylcholine) dramatically alter the fluid drying dynamics and increase the adhesion of the perovskite ink to the underlying non-wetting charge transport layer. The additives enable blading of smooth perovskite films at a coating rate of 180 m h–1 with root-mean-square roughness of 14.5 nm over 1 cm. The surfactants also passivate charge traps, resulting in efficiencies over 20% for small-area solar cells. Fast blading in air of perovskite films results in stabilized module efficiencies of 15.3% and 14.6% measured at aperture areas of 33.0 cm2 and 57.2 cm2, respectively. Scaling up perovskite film deposition necessitates controlling the film formation dynamics. Here, Deng et al. use amphoteric choline surfactants to blade-coat well-passivated films, reaching module efficiencies of ~15% for aperture areas up to 57 cm2.

Halide Perovskite Photovoltaics: Background, Status, and Future Prospects

Abstract: The photovoltaics of organic–inorganic lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compounds such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite preparation via simple and inexpensive solution processes demonstrates the immense potential of this thin-film solar cell technology to become a low-cost alternative to the presently commercially available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regardi...

Defect passivation in hybrid perovskite solar cells using quaternary ammonium halide anions and cations

Abstract: The ionic defects at the surfaces and grain boundaries of organic–inorganic halide perovskite films are detrimental to both the efficiency and stability of perovskite solar cells. Here, we show that quaternary ammonium halides can effectively passivate ionic defects in several different types of hybrid perovskite with their negative- and positive-charged components. The efficient defect passivation reduces the charge trap density and elongates the carrier recombination lifetime, which is supported by density-function-theory calculation. The defect passivation reduces the open-circuit-voltage deficit of the p–i–n-structured device to 0.39 V, and boosts the efficiency to a certified value of 20.59 ± 0.45%. Moreover, the defect healing also significantly enhances the stability of films in ambient conditions. Our findings provide an avenue for defect passivation to further improve both the efficiency and stability of solar cells. Losses in solar cells can be caused by material defects in the bulk or at interfaces. Here, Zheng et al. use quaternary ammonium halides to passivate various perovskite absorbers and prepare solar cells with certified efficiency above 20%, suggesting that both anionic and cation defects are affected.