Dopant-free hole transport materials with a long alkyl chain for stable perovskite solar cells

Hole transport materials are indispensable to high efficiency perovskite solar cells. Two new hole transporting materials (HTMs), named 4,4^′-(9-nonyl-9H-carbazole-3,6-diyl)bis (N,N-bis(4-methoxyphenyl)aniline) (CZTPA-1) and 4,4^′-(9-methyl-9H-carbazole-3,6-diyl)bis (N,N-bis(4-methoxyphenyl)aniline)(CZTPA-2), were developed by different alkyl substitution methods. The two compounds, containing a carbazole core and triphenylamine (TPA) groups with different lengths of the alkyl chain, were designed and synthesized through a two-step synthesis approach. The power conversion efficiency (PCE) was found to be affected by the length of the alkyl chain, reaching 7% for CZTPA-1 and 11% for CZTPA-2. Furthermore, the CZTPA-2 still maintained 89.7% of its original performance after 400 h. The proposed results demonstrate the effect of carbon chain substituents on the efficiency of perovskite solar cells (PSCs).