Polarization anisotropy of charge transfer absorption and emission of aligned polymer:fullerene blend films

Abstract

An improved understanding of the electronic structure of interfacial charge transfer (CT) states is of importance due to their crucial role in charge carrier generation and recombination in organic donor-acceptor (DA) solar cells. DA combinations with a small difference between the energy of the CT state (E CT ) and energy of the donor exciton (E D * ) are of special interest since energy losses due to electron transfer are minimized, resulting in an optimized open-circuit voltage. In that case, the CT state can be considered as a resonance mixture, containing character of a fully ionic state (D + A − ) and of the local polymer excited state (D * A). We show that the D * A contribution to the overall CT state wave function can be determined by measurements of the polarization anisotropy of CT absorption and emission of polymer:fullerene blends with aligned polymer chains. We study two donor polymers, P3HT and TQ1, blended with fullerene acceptors with different ionization potentials, allowing variation of the E D * − E CT difference. We find that, upon decreasing E D * − E CT , the local excitonic D * A character of the CT state increases, resulting in a decreased fraction of charge transferred and an increased transition dipole moment. For typical polymer:fullerene systems, this effect is expected to become detrimental for device performance if E D * − E CT < 0.1 eV. This however, depends on the electronic coupling between D * A and D + A − , which we experimentally estimate to be ∼ 6 meV for the TQ1:PCBM system.

Polarization Imaging of Emissive Charge Transfer States in Polymer/Fullerene Blends

“…subsequently free charge carriers. 17,19,21 Moreover, the fluorescence spectrum of blend films contains, in contrast to neat films, an additional red-shifted band, which has been assigned to emission of the interfacial CT state. 21 The spin-casting procedure resulted in a random organization of the TQ1 polymer phase within the film plane, and thus, we denote these films as isotropic.…”

“…Forster radius is proportional to the 6th power root of the oscillator strength (dipole moment squared) of the acceptor multiplied by the fluorescence quantum yield of the donor, where both the donor and acceptor are emissive CT states. By comparing the CT and polymer (TQ1) absorption of blends with low polymer concentration, the average oscillator strength of a CT state has been estimated to be about 1/300 of the oscillator strength of the TQ1 spectroscopic unit, 21 which is consistent with calculations on other polymer:fullerene systems. 26,27 Note that this estimation gives the oscillator strength averaged over all CT states (emissive and nonemissive), while emissive CT states probably have much larger oscillator strengths than those which undergo ultrafast charge separation on subpicosecond time scale.…”

Charge-Transfer State Dynamics Following Hole and Electron Transfer in Organic Photovoltaic Devices

“…25,36 It was shown that electron-transfer process demonstrates a noticeable conservation of the polarisation memory due to the correlation between the excitonic, CT and polaronic transition dipoles of the polymer. [37][38][39][40] In contrast, the polarization of the fullerene excitation has little correlation with the polarization of the polymer's polaronic transition, and thus the anisotropy following hole transfer will be negligibly low.…”

Organic photovoltaic cells with controlled polarization sensitivity

“…6 In addition, alignment of the polymer chains in an OPV cell may provide insight into energy conversion processes. 11 There have been a number of techniques used to uniaxially align conjugated polymer films that have focused on alignment of homogeneous polymer systems. 4,7,[12][13][14] In polymer:fullerene blends, such as poly(3-hexylthiophene): Phenyl-C61-butyric acid methyl ester (P3HT:PCBM), alignment of the P3HT component has recently been demonstrated by (1) mechanically rubbing the film at an elevated temperature (150 C) 6 and (2) through directed crystallization.…”