Abstract
Perovskite solar cells (PSCs) using 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9-spirobifluorene (spiro-OMeTAD) have achieved power conversion efficiencies over 26%, but still suffer from thermal instability. Hole transport layer (HTL) is particularly vulnerable to thermal degradation, mainly due to the chemical nature and ratio of dopants. The typical 6:1 ratio of 4-tert-butylpyridine (tBP) to lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) improves solubility but facilitates de-doping by reducing spiro-OMeTAD•⁺TFSI⁻ radicals. Here, we performed molecular simulations to develop a molecular-level design strategy for heat-stable HTL. Our results revealed that a 1:1 ratio stabilizes Li⁺–tBP coordination, suppressing free tBP and mitigating de-doping. This improves doping stability, raises the glass transition temperature, and enhances thermal durability. These insights provide the molecular-level guideline for designing thermally stable PSCs.