Wanpei Hu

Bio: Wanpei Hu is an academic researcher from University of Science and Technology of China. The author has contributed to research in topics: Perovskite (structure) & Passivation. The author has an hindex of 8, co-authored 14 publications receiving 325 citations.

Low-Temperature In Situ Amino Functionalization of TiO2 Nanoparticles Sharpens Electron Management Achieving over 21% Efficient Planar Perovskite Solar Cells.

Abstract: Titanium oxide (TiO2 ) has been commonly used as an electron transport layer (ETL) of regular-structure perovskite solar cells (PSCs), and so far the reported PSC devices with power conversion efficiencies (PCEs) over 21% are mostly based on mesoporous structures containing an indispensable mesoporous TiO2 layer. However, a high temperature annealing (over 450 °C) treatment is mandatory, which is incompatible with low-cost fabrication and flexible devices. Herein, a facile one-step, low-temperature, nonhydrolytic approach to in situ synthesizing amino-functionalized TiO2 nanoparticles (abbreviated as NH2 -TiO2 NPs) is developed by chemical bonding of amino (-NH2 ) groups, via TiN bonds, onto the surface of TiO2 NPs. NH2 -TiO2 NPs are then incorporated as an efficient ETL in n-i-p planar heterojunction (PHJ) PSCs, affording PCE over 21%. Cs0.05 FA0.83 MA0.12 PbI2.55 Br0.45 (abbreviated as CsFAMA) PHJ PSC devices based on NH2 -TiO2 ETL exhibit the best PCE of 21.33%, which is significantly higher than that of the devices based on the pristine TiO2 ETL (19.82%) and is close to the record PCE for devices with similar structures and fabrication procedures. Besides, due to the passivation of the surface trap states of perovskite film, the hysteresis of current-voltage response is significantly suppressed, and the ambient stability of devices is improved upon amino functionalization.

Surface Modification of TiO2 for Perovskite Solar Cells

Abstract: Titanium oxide (TiO2) is commonly used as an electron transport layer (ETL) of regular-structure perovskite solar cells (PSCs); however, it suffers from inherent drawbacks such as low electron mobility and a high density of trap states. Modifying the surface chemistry of TiO2 has proved facile and efficient in enhancing key electron-transport properties, thereby improving device performance. In this review, we summarize recent progress on the surface modification of TiO2 in planar PSCs. The functions of different modifiers in improving device performance are elucidated, revealing the influence of modifier chemical and electronic structure on the properties of TiO2. This offers new opportunities to exploit novel materials for modifying TiO2 toward high-efficiency PSCs.

In Situ Surface Fluorination of TiO 2 Nanocrystals Reinforces Interface Binding of Perovskite Layer for Highly Efficient Solar Cells with Dramatically Enhanced Ultraviolet-Light Stability.

Abstract: Low-temperature solution-processed TiO2 nanocrystals (LT-TiO2) have been extensively applied as electron transport layer (ETL) of perovskite solar cells (PSCs). However, the low electron mobility, high density of electronic trap states, and considerable photocatalytic activity of TiO2 result in undesirable charge recombination at the ETL/perovskite interface and notorious instability of PSCs under ultraviolet (UV) light. Herein, LT-TiO2 nanocrystals are in situ fluorinated via a simple nonhydrolytic method, affording formation of Ti─F bonds, and consequently increase electron mobility, decrease density of electronic trap states, and inhibit photocatalytic activity. Upon applying fluorinated TiO2 nanocrystals (F-TiO2) as ETL, regular-structure planar heterojunction PSC (PHJ-PSC) achieves a champion power conversion efficiency (PCE) of 22.68%, which is among the highest PCEs for PHJ-PSCs based on LT-TiO2 ETLs. Flexible PHJ-PSC devices based on F-TiO2 ETL exhibit the best PCE of 18.26%, which is the highest value for TiO2-based flexible devices. The bonded F atoms on the surface of TiO2 promote the formation of Pb─F bonds and hydrogen bonds between F- and FA/MA organic cations, reinforcing interface binding of perovskite layer with TiO2 ETL. This contributes to effective passivation of the surface trap states of perovskite film, resulting in enhancements of device efficiency and stability especially under UV light.

Minimizing non-radiative recombination losses in perovskite solar cells

Abstract: Photovoltaic solar cells based on metal halide perovskites have gained considerable attention over the past decade because of their potentially low production cost, earth-abundant raw materials, ease of fabrication and ever-increasing power conversion efficiencies of up to 25.2%. This type of solar cells offers the promise of generating electricity at a more competitive unit price than traditional fossil fuels by 2035. Nevertheless, the best research cell efficiencies are still below the theoretical limit defined by the Shockley-Queissier theory owing to the presence of non-radiative recombination losses. In this Review, we analyse the predominant pathways that contribute to non-radiative recombination losses in perovskite solar cells, and evaluate their impact on device performance. We then discuss how non-radiative recombination losses can be estimated through reliable characterization techniques, and highlight some notable advances in mitigating these losses, which hint at pathways towards defect-free perovskite solar cells. Finally, we outline directions for future work that will push the efficiency of perovskite solar cells towards the radiative limit.

Hydrothermal deposition of antimony selenosulfide thin films enables solar cells with 10% efficiency

Abstract: Antimony selenosulfide, Sb2(S,Se)3, has attracted attention over the last few years as a light-harvesting material for photovoltaic technology owing to its phase stability, earth abundancy and low toxicity. However, the lack of a suitable material processing approach to obtain Sb2(S,Se)3 films with optimal optoelectronic properties and morphology severely hampers prospects for efficiency improvement. Here we demonstrate a hydrothermal approach to deposit high-quality Sb2(S,Se)3 films. By varying the Se/S ratio and the temperature of the post-deposition annealing, we improve the film morphology, increase the grain size and reduce the number of defects. In particular, we find that increasing the Se/S ratio leads to a favourable orientation of the (Sb4S(e)6)n ribbons (S(e) represents S or Se). By optmizing the hydrothermal deposition parameters and subsequent annealing, we report a Sb2(S,Se)3 cell with a certified 10.0% efficiency. This result highlights the potential of Sb2(S,Se)3 as an emerging photovoltaic material. Antimony chalcogenides are emerging photovoltaic materials, yet difficulties in fabricating high-quality films limit device performance. We show that hydrothermal synthesis affords good morphology and reduced defects in antimony selenosulfide films, enabling solar cells with an efficiency of 10%.

Heterogeneous 2D/3D Tin-Halides Perovskite Solar Cells with Certified Conversion Efficiency Breaking 14.

Abstract: As the most promising lead-free one, tin-halides based perovskite solar cells still suffer from the severe bulk-defect due to the easy oxidation of tin from divalent to tetravalent. Here, a general and effective strategy is delivered to modulate the microstructure of 2D/3D heterogeneous tin-perovskite absorber films by substituting FAI with FPEABr in FASnI3 . The introduction of 2D phase can induce highly oriented growth of 3D FASnI3 and it is revealed in the optimal 2D/3D film that 2D phase embraces 3D grains and locates at the surfaces and grain boundaries. The FPEA+ based 2D tin-perovskite capping layer can offer a reducing atmosphere for vulnerable 3D FASnI3 grains. The unique microstructure effectively suppresses the well-known oxidation from Sn2+ to Sn4+ , as well as decreasing defect density, which leads to a remarkable enhanced device performance from 9.38% to 14.81% in conversion efficiency. The certified conversion efficiency of 14.03% announces a new record and moves a remarkable step from the last one (12.4%). Besides of this breakthrough, this work definitely paves a new way to fabricate high-quality tin-perovskite absorber film by constructing effective 2D/3D microstructures.

Smoothing the energy transfer pathway in quasi-2D perovskite films using methanesulfonate leads to highly efficient light-emitting devices.

Abstract: Quasi-two-dimensional (quasi-2D) Ruddlesden–Popper (RP) perovskites such as BA2Csn–1PbnBr3n+1 (BA = butylammonium, n > 1) are promising emitters, but their electroluminescence performance is limited by a severe non-radiative recombination during the energy transfer process. Here, we make use of methanesulfonate (MeS) that can interact with the spacer BA cations via strong hydrogen bonding interaction to reconstruct the quasi-2D perovskite structure, which increases the energy acceptor-to-donor ratio and enhances the energy transfer in perovskite films, thus improving the light emission efficiency. MeS additives also lower the defect density in RP perovskites, which is due to the elimination of uncoordinated Pb2+ by the electron-rich Lewis base MeS and the weakened adsorbate blocking effect. As a result, green light-emitting diodes fabricated using these quasi-2D RP perovskite films reach current efficiency of 63 cd A−1 and 20.5% external quantum efficiency, which are the best reported performance for devices based on quasi-2D perovskites so far. Owing to large exciton binding energy, quasi-2D perovskite is promising for light-emitting application, yet inhomogeneous phases distribution limits the potential. Here, the authors improve the performance by using MeS additive to regulate the phase distribution and to reduce defect density in the films.