Increase of charge carrier lifetime in dicyanovinyl-quinquethiophene: fullerene blends upon deposition on heated substrates

The dissociation of excitons and the creation of charges in the active bulk layer in small molecule organic solar cells is significantly influenced by the morphology of the active layer. Here, we influence the active layer morphology of dicyanovinyl-quinquethiophene (donor): fullerene C-60 (acceptor) blend layers by deposition on heated substrates. The signatures of the donor cations and triplet excitons are investigated by photoinduced absorption spectroscopy (PIA) at different measurement temperatures. With increasing measurement temperatures, we observe a decrease in triplet exciton generation rate, accompanied by an increase in cation generation rate. At room temperature we compare the dynamics of donor cations in blend layers deposited at room temperature (T-sub = 30 degrees C) and blend layers deposited on a heated substrate (T-sub = 80 degrees C) by PIA. The cation lifetime (approximate to 10 - 100 mu s) is significantly increased in the heated layer (T-sub = 80 degrees C), whereas the cation generation rate is decreased in the heated layer compared to the unheated layer (T-sub = 30 degrees C). Impedance spectroscopy of heated (T-sub = 80 degrees C) and unheated (T-sub = 30 degrees C) solar cells exhibits a similar increase in carrier lifetime for the heated layer. Furthermore, we determine the lifetimes of free (1-5 mu s) and trapped charges (1 s) by impedance spectroscopy and hence assign the optically detected cation signatures to shallow trap states. (C) 2011 Elsevier B.V. All rights reserved.