Organic hole transport layers (HTLs) have been known
to be susceptible
to thermal stress, leading to poor long-term stability in perovskite
solar cells (PSCs). We synthesized three 2,5-dialkoxy-substituted,
1,4-bis(2-thienyl)phenylene (TPT)-based conjugated polymers (CPs)
linked with thiophene-based (thiophene (T) and thienothiophene (TT))
comonomers and evaluated them as HTLs in n-i-p PSCs. TPT-T (MB/C6),
which has branched 2-methylbutyl and linear hexyl (MB/C6) side chains,
emerged as a promising HTL candidate, enabling power conversion efficiencies
(PCEs) greater than 15%. In addition, PSCs with this HTL showed an
improvement in long-term stability at elevated temperatures of 65
°C when compared to those with the state-of-art HTL, 2,2′,7,7′-tetrakis(N,N-p-dimethoxyphenylamino)-9,9′-spirobifluorene
(spiro-OMeTAD). This improvement is ascribed to the lack of thermal
transitions within the operational temperature range of PSCs for TPT-T
(MB/C6), which is attributed to the relatively short branched side
chains of this polymer. We propose that the elimination of thermal
transitions below 200 °C leads to HTLs without cracking as-deposited
and after conducting a stress test at 65 °C, which can serve
as a new design guideline for HTL development.