Highly efficient and stable halide perovskite solar cells
Creators: name: Liu, Xueping
Publisher: name: University of Surrey
Year: 2023

description: The recent success of thin-film photovoltaics (PVs) based on halide perovskite semiconductors is considered to be highly promising. Impressive breakthroughs in power conversion efficiency (PCE) of perovskite solar cells (PSCs) have progressed beyond 25%. However, the qualities of light-absorbing films, electron transport layers (ETLs), and hole transport layers (HTLs) have a large impact on device performance, such as a large number of defects present in semiconductor films, interface energy mismatch, and ion migration. In pursuit of the high performance of PSCs, understanding semiconductor material properties comprehensively and designing rational improvement strategies that correlate the bulk/interface photophysics properties are focused on in this dissertation, which has major implications for academic research and potential applications. <br>
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description: The defects, carrier dynamics, and band energy of perovskite films are the main factors affecting the photogenerated carrier separation and transport and photoelectric performance. The focus herein is on reconstructing the interface of perovskite polycrystalline films by employing novel organic ammonium salts, i.e., neo-pentylammonium (neoPA) halide salts, to minimize non-radiative recombination loss. To this end, the effect of halide choice on quasi-two-dimensional (quasi-2D) perovskite capping layer on a three-dimensional (3D) perovskite light absorber was systematically analysed, including the formation mechanism, crystallography, and photoelectric properties. It was concluded that the iodide-based (neoPA)2(FA)(n-1)PbnI(3n+1) interlayer form, which is independent of the incorporated halide anions. Instead, the halide anions fill the halide vacancies, contributing to the reduction of defect density. Among them, the quasi-2D interlayer induced by chloride-based salts endows perovskite films with superior crystallography, enhanced carrier extraction and transport compared to the iodide and bromide analogues. The quasi-2D/3D-based PSCs boost champion PCE from 20.9% to 23.3% and enhance the shelf life of the device by over 1500 hours. <br>
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description: Furthermore, spin-coated and chemical bath deposition (CBD) processed SnO2 ETL are systematically studied. The films deposited by commercial colloidal SnO2 nanoparticles show insufficient carrier transport at the interface of the ETL and perovskite due to the poor coverage, uneven thickness, and voids at the contact interface. By contrast, the CBD process provides a path towards uniform film coverage and phase purity rutile SnO2 crystals. A growth-controlled deposition strategy for CBD-SnO2 was developed to tune the crystallization rate and grain uniformity, enabling highly dense, controllable film quality, and efficient carrier transport, which is beneficial for Voc and fill factor (FF). This work establishes a highly reproducible, high-performance (22.13% PCE), and stable passivation-free PSC system (over 3192 hours). <br>
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description: Finally, the functionality of ferrocene was proposed to tackle the device instability issue induced by mobile lithium ions (Li+) in the 2,2',7,7'-Tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD). The accumulation of Li+ at the interface and diffusion into the perovskite is significantly reduced, which is attributed to the Li+ tending to combine with an immobile sterically large metallocene to form stable adducts, rendering them immobile. The introduced ferrocene effectively improves the poor stability of Li-doped spiro-OMeTAD, enabling device PCEs retention of up to 74% compared to 24% in the control devices when stored for 1350 hours in elevated 60% relative humidity (RH) conditions. The effective charge carrier separation and transfer at the interface also contributed to increased Jsc from 24.64 mA cm−1 to 25.26 mA cm−1 and PCEs from 21.61% to 23.45% (champion values).<br>