Impact of the aliphatic side chain length on photovoltaic properties of fullerenes functionalized with 3-(1-indenyl)propionic acid esters

Abstract

It was shown that the chain length of the aliphatic substituent in the alkyl derivative of 3-(1-indenyl)propionic acid of C₆₀ and C₇₀ fullerenes affects the efficiency of photovoltaic cells with the PTB7-Th:fullerene-derivative bulk heterojunction (BHJ). A series of new C₆₀ and C₇₀ fullerene derivatives containing the n-alkyl ester of 3-(1-indenyl)propionic acid were synthesized using the Diels–Alder reaction. The synthesized fullerene derivatives had aliphatic side chains of 4 to 12 carbon atoms in the ester group. All synthesized compounds were successfully used in the BHJ solar cell, achieving energy conversion efficiencies (PCEs) ranging from 0.8 to 3.9%. We observed an increase in solar cell open-circuit voltage of 30 meV for the 60IPH acceptor material. Long aging measurements of the solar cells with the new fullerene derivatives show long lifetimes for C₆₀ derivatives and shorter for C₇₀. The lifetime significantly depends on the aliphatic chain length. The longest 80% decay period, T80, was about 200 days.