Exploring the theoretical potential of tungsten oxide (WO_x) as a universal electron transport layer (ETL) for various perovskite solar cells through interfacial energy band alignment modulation

Perovskite solar cells (PSCs) play a pivotal role in advancing renewable energy to achieve United Nation's Sustainable Development Goal 7 (SDG 7), which aims to ensure universal access to affordable, sustainable, reliable and modern energy services. Aiming to enhance the performance of PSCs by replacing the typically used electron transport layers (ETLs: TiO₂, SnO₂, ZnO etc.), we theoretically investigated the viability of tungsten oxide (WO_X) as a promising ETL for PSCs. Moreover, the effect of altering the energy levels of WO_X on cell performance has also been analyzed through simulation. Initially, 12 (twelve) PSC structures having the combination of different perovskite (PSK: CsPbBr₃, CsPbI₃, FAPbBr₃, FAPbI₃) absorber layers with different organic hole transport layers (HTLs: Spiro-OMeTAD, P3HT, PEDOT:PSS) and a fixed ETL of WO_X were optimized numerically for comparing their performance. As CsPbBr₃-based PSCs showed the best performance, further simulations were performed by varying some WO_X/CsPbBr₃ interface properties such as interface defect density, conduction band offset (CBO) between WO_X and CsPbBr₃, energy bandgap (E_g) of WO_X etc. Finally, the best-performed PSCs were found for the E_g = 3.5 eV of WO_X and the CBO of - 0.5 eV confirming the conduction band minimum (CBM) of WO_X is lower than that of CsPbBr₃ by 0.5 eV. A properly chosen WO_X layer enhanced the efficiency of CsPbBr₃-based PSCs up to 14.65 %, 14.52 % and 16.09 %, aproaching the Shockley-Queisser (S-Q) limit (16.37% for CsPbBr₃-based solar cell) from the initial values of 11.39 %, 11.27 %, and 12.49 %, respectively. This study ensures WO_X is a promising ETL for which a proper PSC structure having a suitable PSK and an HTL can improve cell performance. Moreover, the importance of modifying energy levels of ETL material in enhancing the performance of PSCs is explored. As a result, this study opens a path for the researchers to develop WO_X having suitable CBM and E_g, so that it can be well-suited with a properly matched PSK material resulting in enhanced cell performance.