Highly Photoactive Titanyl Phthalocyanine Polymorphs as Textured Donor Layers in Organic Solar Cells

We present a comparison of the photovoltaic activity of organic solar cells (OPVs) based on vacuum-deposited and solvent-annealed titanyl phthalocyanine (TiOPc) donor layers with C-60 as the electron acceptor, where the TiOPc donor layer exists in three different polymorphic forms: TiOPc included the "as-deposited" form, with a Q-band absorbance spectrum reminiscent of the phase I polymorph, and films subjected to solvent annealing which systematically increased the fraction of either the phase II (alpha-phase) or the phase III (gamma-TiOPc) polymorphs. As-deposited TiOPc/C-60 heterojunctions showed open-circuit photopotentials (V-OC) of ca. 0.65 V, with estimated AM 1.5G efficiencies of ca. 1.4%. Partial conversion of these thin films to their phase II or phase III polymorphs significantly enhanced the short-circuit photocurrent (J(SC)), as a result of (i) texturing of the TiOPc layers (ca. 100 nm length scales) and (ii) enhancements in near-IR absorptivity/photoelectrical activity. All TiOPc/C-60 heterojunctions, characterized by UV-photoelectron spectroscopy (UPS), showed that estimated E-HOMO(Pc) - E-LUMO(C60) energy differences, which set. the upper limit for V-OC, are nearly identical for each TiOPc polymorph. Incident and absorbed photon current efficiency measurements (IPCE, APCE) are consistent with previous studies that showed a majority of the photoactivity in these higher order polymorphs deriving from excitonic states created at lambda(max) approximate to 680 and 844 nm for both the phase II and the phase In polymorphs. The near-IR absorbance features (844 nm) in these Pc polymorphs are believed to have substantial charge-transfer (CT) character, leading to enhanced probabilities for photoinduced electron transfer (PIET). APCE measurements of TiOPc/C-60 OPVs, however, show that higher photocurrent yields per absorbed photon arise from the higher energy (680 nm) excitonic state. When C-70 is used as the electron acceptor, with its higher electron affinity, APCE was increased throughout the visible and near-IR. region, now showing a nearly constant APCE across the entire Q-band spectrum of the TiOPc polymorphs, consistent with increased driving force (E-LUMO(Pc) - E-LUMO(C60)) for PIET involving the lowest energy CT excitonic state. The highest estimated AM 1.5G efficiencies for these textured TiOPc/C-60 heterojunctions, with the TiOPc film partially converted to phase II or phase III polymorphs, are 4.5% and 3.5%, respectively.