Photoconductivity measurements of the electronic structure of organic solar cells

Abstract

Studies of the electronic structure of bulk heterojunction (BHJ) organic solar cells are reported. The photoconductivity spectral response of the solar cells has a weak absorption band extending from the band gap energy down to <1 eV due to charge transfer optical excitation at the interface between the polymer and the fullerene. The low energy absorption indicates band tailing and an absorption model based on the one-electron joint density of states accounts for the data. Transient photoconductivity measurements of the carrier mobility exhibits temperature-dependent carrier dispersion. A multiple trapping model shows that the data is consistent with localized band tail states having a comparable density of states distribution to those observed by optical absorption. First principles calculations of the effects of disorder in the – distance also shows similar band tailing. The evidence for strong or weak phonon coupling is discussed.