Light Harvesting and Charge Recombination in CH₃NH₃PbI₃ Perovskite Solar Cells Studied by Hole Transport Layer Thickness Variation

TLDR

Perovskite solar cell optimization requires detailed understanding of efficiency‑limiting processes. The study investigates how HTL thickness in a mesoscopic TiO2 solar cell affects performance and identifies perovskite film defects as a key optimization target. The authors attribute improved efficiency to enhanced reflection at the smooth HTL/Au interface in the mesoscopic TiO2 architecture. A 200‑nm HTL boosts carrier collection and light harvesting, but a >400‑nm HTL is needed to suppress surface recombination and achieve high open‑circuit voltage; trap‑assisted recombination limits V_oc and fill factor, confirming spiro‑MeOTAD as an effective but not transport‑limited HTL.

Abstract

A tailored optimization of perovskite solar cells requires a detailed understanding of the processes limiting the device efficiency. Here, we study the role of the hole transport layer (HTL) spiro-MeOTAD and its thickness in a mesoscopic TiO2-based solar cell architecture. We find that a sufficiently thick (200 nm) HTL not only increases the charge carrier collection efficiency but also the light harvesting efficiency. This is due to an enhanced reflection of a smooth HTL/Au–electrode interface. The rough CH3NH3PbI3 perovskite surface requires an HTL thickness of >400 nm to avoid surface recombination and guarantee a high open-circuit voltage. Analyses of the electroluminescence efficiency and the diode ideality factor show that the open-circuit voltage becomes completely limited by trap-assisted recombination in the perovskite for a thick HTL. Thus, spiro-MeOTAD is a very good HTL choice from the device physics' point of view. The fill factor analyzed by the Suns-Voc method is not transport limited, but trap-recombination limited as well. Consequently, a further optimization of the device has to focus on defects in the polycrystalline perovskite film.