Immobilizing tBP via tailor-made π-conjugated hole transport materials for efficient and stable perovskite solar cells

Abstract

In n-i-p structured perovskite solar cells (PSCs), 4-tert-butylpyridine (tBP) is commonly used as an additive to control the morphology and mitigate Li-ion accumulation in the organic hole transport layer (HTL). However, the high volatility and corrosive nature of tBP significantly compromise the long-term stability and overall performance of PSCs. To address these challenges, we designed a novel π-conjugated hole transport material (HTM), termed X87, incorporating spiro[fluorene-9,9′-flavonoids] (SFX) as the core unit and anthracene as the substituent group. The unique molecular structure of X87 promotes robust π–π interactions with tBP, attributed to the planar π-conjugated system of the anthracene substituent. NMR and FTIR spectroscopy confirmed the strong π–π interactions between X87 and tBP, highlighting the effectiveness of the molecular design in stabilizing tBP. Beyond stabilizing tBP, X87 exhibited several desirable properties, including high thermal stability, excellent hole mobility, superior film-forming ability, and well-aligned energy levels. As a result, PSCs incorporating X87 achieved an impressive power conversion efficiency (PCE) of 24.07%, along with remarkable long-term stability. Under continuous one-sun illumination at 60 °C in an N₂ atmosphere, unencapsulated X87-based PSCs retained 91% of their initial efficiency after 1000 hours of operation. This study underscores the potential of rational molecular design in developing advanced organic HTMs. The demonstrated properties of X87 provide valuable insights into improving PSC stability and efficiency, offering promising applications in optoelectronic devices.