Dual-functional electron transport layer design: built-in electric field and defect passivation for efficient perovskite solar cells

• Tri-component ETL: Zn₂SnO₄–ZnO heterojunction with g-C₃N₄ passivation. • Built-in electric field enhances charge separation and directional transport. • g-C₃N₄ significantly reduces trap density and non-radiative recombination. • PCE of 23.5% with >90% retention after 1200 hours under continuous illumination. • Practical, scalable design methodology for stable, high-efficiency PSCs. The electron transport layer (ETL) is a significant aspect of perovskite solar cells (PSCs) that affect their performance in terms of efficiency and lifetime. Our work proposes a new ETL design that integrates three components: a Zn₂SnO₄–ZnO (ZSO-ZO) heterojunction and the graphitic carbon nitride (g-C₃N₄) to create GCN/ZSO-ZO composite ETLs (or GCN/ZSO-ZO composite ETLs). The internal built-in electric field (BIEF) formed at the interface of ZSO-ZO plays an important role in charge separation and the movement of electrons towards their respective electrodes, while g-C₃N₄ accomplishes the task of surface defect passivation at the interface of perovskite and ETL. The result is that perovskite films on GCN/ZSO-ZO substrates have better crystallinity, larger grain sizes, lower trap densities, and less non-radiative recombination. GCN/ZSO-ZO solar cells achieved an outstanding power conversion efficiency (PCE) of 23.5%, which is higher than that of reference cells with ZSO or ZSO-ZO electron transport layers (ETLs) only. Furthermore, the cells demonstrated an excellent long-term thermal and environmental stability with more than 90% of their initial power conversion efficiency retained after testing over 1200 hours. The work in the present study demonstrates a facile and scalable approach to co-optimize IDF and PPF effects, which also tunes passivation for enhanced efficiency and stability in the next-generation PSCs.