Evaporated organic–MoO3 composite hole transport layers toward stable perovskite solar cells

Abstract

The release and diffusion of corrosive iodine species limits the stable operation of perovskite solar cells (PSCs). In this study, molybdenum trioxide (MoO₃) is employed for efficient oxidation of organic hole transport layers (HTLs), replacing the conventional dopant lithium bis(trifluoromethane)sulfonimide (LiTFSI) which compromises stability. Co-deposition of 2,2′,7,7′-tetra(N,N-di-p-tolyl)amino-9,9-spirobifluorene (spiro-TTB) and MoO₃via thermal evaporation produces a HTL with appropriate ionization energy of −5.06 eV, electrical conductivity of 6.02 × 10⁻⁵ S cm⁻¹, and homogeneous morphology. To investigate the stability of PSCs using the composite HTL, three types of PSCs with different HTLs are fabricated; the 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD) control HTL with conventional dopants, the spiro-TTB:MoO₃ composite HTL, and the double HTL with doped spiro-OMeTAD and spiro-TTB:MoO₃ layers. The most efficient PSC with power conversion efficiency (PCE) of 21.3% is achieved by the double HTL. Since the efficient oxidation of spiro-TTB by MoO₃ and stable morphology under thermal stress mitigate iodine diffusion through the spiro-TTB:MoO₃ HTL, PSCs employing the composite HTLs demonstrate superior thermal stability, retaining 81% of their initial efficiency after 200 h aging at 85 °C.