The preparation of high performance perovskite solar cells

Abstract

Compared to traditional silicon solar cells, the third generation solar cells have many advantages, like low cost, simple fabrication and mechanical flexibility. Organic inorganic perovskites have been one of the most promising light absorbing materials for the third generation solar cells because of their excellent optoelectronic properties, like broad spectral absorption, high carrier mobility and long carrier diffusion length. In a few years, the power conversion efficiency (PCE) of perovskite solar cells (PSC) has increased from 3.8% to more than 25%, which is close to the efficiency of high performance silicon solar cells. In this thesis, 2-dimentional (2D) WS2 flakes with defect-free surfaces are introduced as a template for van der Waals epitaxial growth of mixed perovskite films by solution process. The mixed perovskite films demonstrate a preferable growth along (001) direction on WS2 surfaces. In addition, the WS2/perovskite heterojunction forms a cascade energy alignment for efficient charge extraction and reduced interfacial recombination. This work demonstrates that high-mobility 2D materials can find important applications in PSCs as well as other perovskite-based optoelectronic devices. Then, ammonium hypophosphite is introduced into the FASnI3 perovskite precursor to suppress the oxidation of Sn2+ and assist the growth of perovskite grains, leading to improved perovskite film quality and reduced defect density, and consequently, the device efficiency and open circuit voltage are substantially improved. More importantly, the solar cells exhibit pronounced enhancement of long-term stability. This work provides a facile approach for improving the performance of tin-based perovskite solar cells by introducing ammonium hypophosphite as an antioxidant agent in the precursor solution. Besides, Mixed Sn-Pb perovskites with bandgaps of 1.2 －1.3 eV are ideal candidates for single-junction solar cells according to the Shockley-Queisser limit. However, the efficiency and stability of mixed Sn-Pb perovskite solar cells still lag behind that of its pure Pb counterpart due to the easy oxidation of Sn2+. Here, multiple functional additive 4-hydrazinobenzoic acid (HBA) is introduced to suppress the oxidation of Sn2+. Meantime, incorporation of HBA can regulate the crystallization process of perovskite, leading to improved crystallinity and enlarged grain size. In addition, the HBA-SnF2 complex presented at grain boundaries could passivate trap states and reduce nonradiative recombination. Consequently, the performance of perovskite solar cells increased from 17.16% to 20.14% along with enhanced stability.