Inducting effects of ionic liquid crystal modified-PEDOT:PSS on the performance of bulk heterojunction polymer solar cells†
A type of ionic liquid crystals, 4′-(N,N,N-trimethyl ammonium bromide hexyloxy)-4-cyanobiphenyl (6CNBP-N), was synthesized and explored as a novel additive to modify the PEDOT:PSS anode buffer layer for P3HT:PCBM polymer solar cells (PSCs). Due to the ionic functions of 6CNBP-N, the amount of PEDOT on top surface of PEDOT:PSS was increased with increasing amount of 6CNBP-N, leading to a rougher surface and better conductivity. Most strikingly, when the 6CNBP-N modified PEDOT:PSS anode buffer layer was annealed at 150 °C (in the mesophase temperature region of 6CNBP-N), a more continuous network in the modified PEDOT:PSS film and more ordered microstructure and higher crystallization of P3HT in the overlying P3HT:PCBM active layer were formed, mainly induced by the self-assembling behavior of 6CNBP-N in its mesomorphism, thus favoring excition dissociation, charge transport and collection. Without undergoing the annealing of active layers, the power conversion efficiency (PCE) of 3.17% was obtained. Our results showed that the ionic liquid crystals, which combined the liquid crystal properties and ionic functions, were a promising additive for PEDOT:PSS, which could improve the properties of the PEDOT:PSS buffer layer and the overlying active layer at the same time, thereby enhancing the PCE of PSCs.