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

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• 作者：Diogenes Placencia ; Weining Wang ; Jeremy Gantz ; Judith L. Jenkins ; Neal R. Armstrong
• 刊名：Journal of Physical Chemistry C
• 出版年：2011
• 出版时间：September 29, 2011
• 年：2011
• 卷：115
• 期：38
• 页码：18873-18884
• 全文大小：1386K
• 年卷期：v.115,no.38(September 29, 2011)
• ISSN：1932-7455

文摘

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 C₆₀ as the electron acceptor, where the TiOPc donor layer exists in three different polymorphic forms: TiOPc included the 鈥渁s-deposited鈥?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 (伪-phase) or the phase III (纬-TiOPc) polymorphs. As-deposited TiOPc/C₆₀ 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/C₆₀ heterojunctions, characterized by UV鈥損hotoelectron spectroscopy (UPS), showed that estimated E_HOMO^Pc 鈥?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 位_max 鈮?680 and 844 nm for both the phase II and the phase III 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/C₆₀ OPVs, however, show that higher photocurrent yields per absorbed photon arise from the higher energy (680 nm) excitonic state. When C₇₀ 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 鈥?E_LUMO^C60) for PIET involving the lowest energy CT excitonic state. The highest estimated AM 1.5G efficiencies for these textured TiOPc/C₆₀ heterojunctions, with the TiOPc film partially converted to phase II or phase III polymorphs, are 4.5% and 3.5%, respectively.