Evaporated Organic-MoO₃ Composite Hole Transport Layers Toward Stable Conventional Perovskite Solar Cells

The release of iodine species and subsequent diffusion in perovskite solar cells (PSCs) is problematic due to their volatile and corrosive nature. Since hole transport layers (HTLs) have been shown to be a channel for iodine diffusion and metal anode corrosion in conventional PSCs, developing an HTL that inhibits iodine diffusion is necessary. Efficient oxidation (i.e., doping) of HTLs is crucial in that it improves the hole transport of organic HTLs with low electrical conductivity and mitigates the redox reaction between organic HTLs and iodine. In this study, molybdenum trioxide (MoO₃) is employed for efficient oxidation, replacing the conventional dopant lithium bis(trifluoromethane)sulfonimide (LiTFSI) having stability issues including hygroscopicity, Li⁺ diffusion to perovskite layer, and aggregation and pinhole formation in HTLs. Co-deposition of 2,2',7,7'-tetra(N,N-di-p-tolyl)amino-9,9-spirobifluorene (spiro-TTB) and MoO₃ via thermal evaporation leads to an 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 double HTL with doped spiro-OMeTAD and spiro-TTB:MoO₃ layers, and the spiro-TTB:MoO₃ composite 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, the PSC employing the composite HTL outperforms the control device employing the doped spiro-OMeTAD in the thermal stability test performed at 85 °C. While the control device reaches 80% of the initial efficiency after 1h of aging, the device with the composite HTL survives for 200 h. The PSC with the double HTL exhibits moderate stability by surviving for 45 h.