HongxiaWang Research Group

We develop an approach that combines molecular dimensionality control and interfacial passivation of perovskite layers using a novel post-device treatment (PDT) with the vapour of ethylene-amine salts of different carbon chain lengths to improve both efficiency and stability of the PSCs. The effect of a series of ligand vapours including ethylenediamine (EDA), diethylenetriamine (DETA) and triethylenetetramine (TETA) was systematically investigated. A thin hydrophobic two-dimensional (2D) perovskite capping layer formed in the device after the 3D perovskite was exposed to the vapour of long chain ethylene-amine molecules, such as DETA and TETA, which protected the underlying bulk 3D perovskite layer from moisture attack. An improved energy level alignment was obtained in the treated devices and that a reduced density of defects was present in the perovskite after treatment with DETA and TETA vapours. Consequently, enhanced efficiency from 17.07% to 18.09% (DETA) and improved moisture stability with PCE retention from 73.8% to 90.0% (TETA) under a relative humidity>65% for 1000 h were achieved by this vapour treatment respectively.
Find out more at Journal of Power Sources.

2D–3D Mixed Organic–Inorganic Perovskite Layers for Solar Cells with Enhanced Efficiency and Stability Induced by n-Propylammonium Iodide Additives

Disheng Yao, Chunmei Zhang, Shengli Zhang, Yang Yang, Aijun Du, Eric Waclawik, Xiaochen Yu, Gregory J. Wilson and Hongxia Wang.

Device instability has become an obstacle for the industrial application of organic–inorganic metal halide perovskite solar cells that has already demonstrated over 23% laboratory power conversion efficiency (PCE). It has been discovered that the sliding of A-site cations in the perovskite compound through and out of the three-dimensional [PbI₆]^4– crystal frame is one of the main reasons that are responsible for decomposition of the perovskite compound. Herein, we report an effective method to enhance the stability of the FA_0.79MA_0.16Cs_0.05PbI_2.5Br_0.5 perovskite film through the incorporation of n-propylammonium iodide (PAI). Both density functional theory calculation and the X-ray diffraction patterns have confirmed the formation of two-dimensional (PA)₂PbI₄ with the Ruddlesden–Popper perovskite as a result of the reaction between PAI and PbI₂ in the perovskite film. X-ray photoelectron spectroscopy reveals less −COOH (carboxyl) groups on the surface of the perovskite film containing (PA)₂PbI₄, which indicates the suppressed penetration of oxygen and moisture into the perovskite material. This is further confirmed by the surface water wettability test of the (PA)₂PbI₄ film that exhibits excellent hydrophobic property with over 110° contact angle. Ultraviolet photoelectron spectroscopy demonstrates the introduction of PAI additives that resulted in the upshift of the conduction band minimum of the perovskite by 160 meV, leading to a more favorable energy alignment with an adjacent electron transporting material. As a consequence, enhanced 17.23% PCE with suppressed hysteresis was obtained with the 5% PAI additive (molar ratio) in perovskite solar cells that retained nearly 50% of the initial efficiency after 2000 h in air without encapsulation under 45% average relative humidity.

Interface Engineering to Eliminate Hysteresis of Carbon-Based Planar Heterojunction Perovskite Solar Cells via CuSCN Incorporation

Yang Yang, Ngoc Duy Pham, Disheng Yao, Lijuan Fan, Minh Tam Hoang, Vincent Tiing Tiong, Zhaoxiang Wang, Huaiyong Zhu and Hongxia Wang.

A carbon electrode with low cost and high stability exhibited competitiveness for its practical application in organic–inorganic hybrid perovskite solar cells (PSCs). Nonetheless, issues such as poor interface contact with an adjacent perovskite layer and obvious hysteresis phenomenon are bottlenecks that need to be overcome to make carbon-based PSCs (C-PSCs) more attractive in practice. Herein, we report an effective method to enhance the interfacial charge transport of C-PSCs by introducing the CuSCN material into the device. Two types of CuSCN-assisted devices were studied in this work. One was based on the deposition of an ultrathin CuSCN layer between the perovskite absorber layer and the carbon cathode (PSK/CuSCN/C), and the other was by infiltrating CuSCN solution into the carbon film (PSK/C-CuSCN) by taking advantage of the macroporous structure of the carbon. We have found that the CuSCN incorporation by both methods can effectively address the hysteretic feature in planar C-PSCs. The origin for the hysteresis evolution was unraveled by the investigation of the energy alignment and the kinetics of interfacial charge transfer and hole trap-state density. The results have shown that both types of CuSCN-containing devices showed improved interfacial charge carrier extraction, suppressed carrier recombination, reduced trap-state density, and enhanced charge transport, leading to negligible hysteresis. Furthermore, the CuSCN-incorporated C-PSCs demonstrated enhanced device stability. The power conversion efficiency remained 98 and 91% of the initial performance (13.6 and 13.4%) for PSK/CuSCN/C and PSK/C-CuSCN, respectively, after being stored under a high humidity (75–85%) environment for 10 days. The devices also demonstrated extraordinary long-term stability with a negligible performance drop after being stored in air (relative humidity: 33–35%) for 90 days.

2 PhD scholarships are available from next year in the area of energy material and device .  Candidates should be solid background in chemistry, physics or material science and should have completed Master degree by research with at least 1 first author publication in decent journal. Research experience in solar cells, in particular perovskite solar cells  or electrochemistry is preferred but not compulsory. For candidate from non-English speaking country, satisfactory IELTS score is required to meet the entry for PhD at QUT.