1. ACS Appl. Mater. Interfaces | Research Article

Approximately 800-nm-Thick Pinhole-Free Perovskite Films via Facile Solvent Retarding Process for Efficient Planar Solar Cells
Zhongcheng Yuan, Yingguo Yang, Zhongwei Wu, Sai Bai, Weidong Xu, Tao Song, Xingyu Gao, Feng Gao, and Baoquan Sun

ACS Appl. Mater. Interfaces, 2016, 8 (50), pp 34446–34454 dol: 10.1021/acsami.6b12637


Device performance of organometal halide perovskite solar cells significantly depends on the quality and thickness of perovskite absorber films. However, conventional deposition methods often generate pinholes within ∼300 nm-thick perovskite films, which are detrimental to the large area device manufacture. Here we demonstrated a simple solvent retarding process to deposit uniform pinhole free perovskite films with thicknesses up to ∼800 nm. Solvent evaporation during the retarding process facilitated the components separation in the mixed halide perovskite precursors, and hence the final films exhibited pinhole free morphology and large grain sizes. In addition, the increased precursor concentration after solvent-retarding process led to thick perovskite films. Based on the uniform and thick perovskite films prepared by this convenient process, a champion device efficiency up to 16.8% was achieved. We believe that this simple deposition procedure for high quality perovskite films around micrometer thickness has a great potential in the application of large area perovskite solar cells and other optoelectronic devices.

2. Applied Physics Letters | Research Article

Inhomogeneous degradation in metal halide perovskites

Rong Yang, Li Zhang, Yu Cao, Yanfeng Miao, You Ke, Yingqiang Wei, Qiang Guo, Ying Wang, Zhaohua Rong, Nana Wang, Renzhi Li, Jianpu Wang, Wei Huang, and   Feng Gao
Appl. Phys. Lett. 111, 073302 (2017); doi: 10.1063/1.4999630

Although the rapid development of organic-inorganic metal halide perovskite solar cells has led to certified power conversion efficiencies of above 20%, their poor stability remains a major challenge, preventing their practical commercialization. In this paper, we investigate the intrinsic origin of the poor stability in perovskite solar cells by using a confocal fluorescence microscope. We find that the degradation of perovskite films starts from grain boundaries and gradually extend to the center of the grains. Firmly based on our findings, we further demonstrate that the device stability can be significantly enhanced by increasing the grain size of perovskite crystals. Our results have important implications to further enhance the stability of optoelectronic devices based on metal halide perovskites.
3. Nature Communication | Research Article

Wei Zou, Renzhi Li, Shuting Zhang, Yunlong Liu, Nana Wang, Yu Cao, Yanfeng Miao, Mengmeng Xu, Qiang Guo, Dawei Di, Li Zhang, Chang Yi, Feng Gao, Richard H. Friend, Jianpu Wang & Wei Huang

Nature Communications, volume 9, Article number: 608 (2018) doi:10.1038/s41467-018-03049-7

Efficiency roll-off is a major issue for most types of light-emitting diodes (LEDs), and its origins remain controversial. Here we present investigations of the efficiency roll-off in perovskite LEDs based on two-dimensional layered perovskites. By simultaneously measuring electroluminescence and photoluminescence on a working device, supported by transient photoluminescence decay measurements, we conclude that the efficiency roll-off in perovskite LEDs is mainly due to luminescence quenching which is likely caused by non-radiative Auger recombination. This detrimental effect can be suppressed by increasing the width of quantum wells, which can be easily realized in the layered perovskites by tuning the ratio of large and small organic cations in the precursor solution. This approach leads to the realization of a perovskite LED with a record external quantum efficiency of 12.7%, and the efficiency remains to be high, at approximately 10%, under a high current density of 500 mA cm−2.

4. Advanced Materials | Communication

Long Electron–Hole Diffusion Length in High‐Quality Lead‐Free Double Perovskite Films

Weihua Ning  Feng Wang  Bo Wu  Jun Lu  Zhibo Yan  Xianjie Liu  Youtian Tao  JunMing Liu Wei Huang  Mats Fahlman  Lars Hultman  Tze Chien Sum*  Feng Gao*

Advanced Materials,

Developing environmentally friendly perovskites has become important in solving the toxicity issue of lead‐based perovskite solar cells. Here, the first double perovskite (Cs2AgBiBr6) solar cells using the planar structure are demonstrated. The prepared Cs2AgBiBr6 films are composed of high‐crystal‐quality grains with diameters equal to the film thickness, thus minimizing the grain boundary length and the carrier recombination. These high‐quality double perovskite films show long electron–hole diffusion lengths greater than 100 nm, enabling the fabrication of planar structure double perovskite solar cells. The resulting solar cells based on planar TiO2 exhibit an average power conversion efficiency over 1%. This work represents an important step forward toward the realization of environmentally friendly solar cells and also has important implications for the applications of double perovskites in other optoelectronic devices.


Accepted in Advanced Materials, 30 March 2018

5. Chem. Commun. | Communication

Room-temperature film formation of metal halide perovskites on n-type metal oxides: the catalysis of ZnO on perovskite crystallization

Zhongcheng Yuan, Sai Bai,* Zhibo Yan, Jun-Ming Liu and Feng Gao*

Chem. Commun., 2018,  dol: 10.1039/C8CC02482K


We investigate the effect of commonly used solution-processed TiOx, SnO2and ZnO interlayers on the perovskite film crystallization process. We find that the ZnO/perovskite interface can efficiently catalyze the perovskite crystallization even without thermal annealing.

Graphical abstract: Room-temperature film formation of metal halide perovskites on n-type metal oxides: the catalysis of ZnO on perovskite crystallization

6. ACS Appl. Mater. Interfaces | Research Article

Oxygen- and Water-Induced Energetics Degradation in Organometal Halide Perovskites

Jianming Yang†, Zhongcheng Yuan‡, Xianjie Liu, Slawomir Braun, Yanqing Li, Jianxin Tang, Feng Gao, Chungang Duan, Mats Fahlman, and Qinye Bao

ACS Appl. Mater. Interfaces201810 (18), pp 16225–16230 DOI: 10.1021/acsami.8b04182


Organometal halide perovskites are under rapid development, and significant focus has been placed on their stability that currently presents a major obstacle for practical application. Energetics plays a vital role in charge injection/extraction and transport properties in devices. Here, we in situ investigate oxygen- and water-induced energetics degradation in organometal halide perovskite films. Oxygen gas induces an upward shift of the vacuum level of the perovskite films because of the formation of an oxygen-induced surface dipole, water vapor causes a significant vacuum-level downshift, and the valence band binding energy referenced to the Fermi level simultaneously increases so as to keep the ionization potential of the perovskite films unchanged. Moreover, the chemical compositions, crystalline structures, surface morphologies, and dynamical properties also are monitored and analyzed in detail. These results are indispensable to understand the degradation mechanisms and to perform the optimizations of stable materials and devices in the future.

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7. J. Phys. Chem. Lett.| Perspective

Organic–Inorganic Hybrid Ruddlesden–Popper Perovskites: An Emerging Paradigm for High-Performance Light-Emitting Diodes

Xiao-Ke Liu and Feng Gao*

J. Phys. Chem. Lett.20189 (9), pp 2251–2258 DOI: 10.1021/acs.jpclett.8b00755


Recently, lead halide perovskite materials have attracted extensive interest, in particular, in the research field of solar cells. These materials are fascinating “soft” materials with semiconducting properties comparable to the best inorganic semiconductors like silicon and gallium arsenide. As one of the most promising perovskite family members, organic–inorganic hybrid Ruddlesden–Popper perovskites (HRPPs) offer rich chemical and structural flexibility for exploring excellent properties for optoelectronic devices, such as solar cells and light-emitting diodes (LEDs). In this Perspective, we present an overview of HRPPs on their structural characteristics, synthesis of pure HRPP compounds and thin films, control of their preferential orientations, and investigations of heterogeneous HRPP thin films. Based on these recent advances, future directions and prospects have been proposed. HRPPs are promising to open up a new paradigm for high-performance LEDs.

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28th May, 2018

8. Advanced Optical Materials | Communication

Optical Energy Losses in Organic–Inorganic Hybrid Perovskite Light‐Emitting Diodes

Xiao‐Bo Shi Yuan Liu Zhongcheng Yuan Xiao‐Ke Liu Yanfeng Miao Jianpu Wang Simone Lenk Sebastian Reineke Feng Gao

Advanced Optical Materials,

Light‐emitting diodes (LEDs) based on organic–inorganic hybrid perovskites, in particular, 3D and quasi‐2D ones, are in the fast development and their external quantum efficiencies (EQEs) have exceeded 10%, making them competitive candidates toward large‐area and low‐cost light‐emitting applications allowing printing techniques. Similar to other LED categories, light out‐coupling efficiency is an important parameter determining the EQE of perovskite LEDs (PeLEDs), which, however, is scarcely studied, limiting further efficiency improvement and understanding of PeLEDs. In this work, for the first time, optical energy losses in PeLEDs are investigated through systematic optical simulations, which reveal that the 3D and quasi‐2D PeLEDs can achieve theoretically maximum EQEs of ≈25% and ≈20%, respectively, in spite of their high refractive indices. These results are consistent with the reported experimental data. This work presents primary understanding of the optical energy losses in PeLEDs and will spur new developments in the aspects of device engineering and light extraction techniques to boost the EQEs of PeLEDs.

Microsoft Word - Table of Contents.docx

Accepted in Advanced Materials, 30 March 2018