Mobility of Non-fullerene Acceptors Using a Time of Flight Method

Fullerene acceptors such as PC₇₁BM have shown wide-use and success in organic solar cells partially due to their good electron transport properties. However, the cost of synthesis and poor absorption limit scale-up and device performance. In order to replace these materials, new non-fullerene molecules require an accurate measure of their electron mobility and a better understanding of their charge transport properties. Here we used two acceptors, ITIC and IDTBR, and studied their mobility using a time of flight (TOF) method. This method allowed for the observation of either the hole or electron mobility separately, while also revealing some in-film charge dynamics such as trapping and disorder. With a zero-field electron mobility of 3.5 x 10^-4 cm²/Vs for ITIC in a blend with PTB7-Th, and 6.0 x 10^-5 cm²/Vs for IDTBR in a P3HT blend, these non-fullerene acceptors have slower electron transport compared to fullerenes. However, electron mobility values of this magnitude match the hole mobility of the donor material, minimizing charge build up and recombination. This balance leads to efficient charge extraction and power conversion in solar cells, making these materials a competitive replacement for fullerenes.