Nematic Liquid Crystalline Hole Transport Materials for OPV

David Jones
University of Melbourne


Abstract

It has been predicted that nematic liquid crystalline organic semiconductors, offering three dimension transport, should outperform smectic or discotic liquid crystalline materials (see Junichi Hanna et al. Thin Solid Films 554, 5863 (2014)). We have recently reported the first high performance example of this new class of materials for use in organic solar cells, that is a molecular nematic liquid crystalline hole transport material which exhibits excellent bulk mobility leading excellent performance, even for thick films (Nature Communications 2015, DOI: 10.1038/ncomms7013). The new molecular electron donor material, BTR, with a benzo[1,2b: 4,5b’] dithiophene (BDT) core and rhodanine peripheral units was developed and used in OPV devices giving PCEs greater than 9% (now max 9.6%, FF from 74-78%). While its pi-conjugated structure is analogous to a high performance compound reported previously (Liu, Y. et al., Sci. Rep. 3, 3356 (2013), Zhou, J. et al., J. Am. Chem. Soc. 135, 84848487 (2013)), the strategic placement of the side chains provided BTR with strong intermolecular interactions, as evidenced by its liquid crystalline behavior. Such interactions were successfully translated into excellent hole transport properties; hole mobilities up to 0.1 cm V-1 s-1 and 1.6 × 10 cm V-1 s-1 were recorded by organic field effect transistor (OFET) and space charge limited current (SCLC) methods, respectively. Therefore, BTR based OPVs with thick active layers (300 to 400 nm) could still afford PCEs of over 8% with high FF of ~70%. This makes BTR a very attractive candidate for roll-to-roll printed OPV modules. An update on the performance of this new class of materials will be given.