Exciton Regeneration Dynamics in Model Donor−Acceptor Polymer Heterojunctions

Correlation of morphology with photocurrent generation in a polymer blend photovoltaic device

Morphological effects on photovoltaic (PV) properties are studied through scanning photocurrent (PC) and photoluminescence (PL) microscopy of a solution processed, polymer blend PV device composed of PFB [poly(9,9'-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine] and F8BT [poly(9,9'-dioctylfluorene-co-benzothiadiazole]. As PFB and F8BT have unique absorbance bands, it is possible to selectively excite only F8BT (488 nm) or both PFB and F8BT (408 nm). Local voltage-dependent photocurrent (LVPC) measurements from particular regions of interest in the PV show that the diode characteristics between different morphologies are essentially the same, except in regard to the magnitude of PC generated. A local PL spectrum is measured simultaneously with PC generation at each pixel in the image maps. Through integration of the local PL spectrum over particular wavelength ranges, PL image maps are created of PFB-PL (435 to 475 nm), F8BT-PL (530 to 570 nm), exciplex-PL (620 to 685 nm) and total-PL (entire spectrum). These data allow direct correlation of PC generation with local chemical composition variations within the PV device. PL image maps show morphological variations on the order of 0.5 to 1 µm of alternating PFB-rich and F8BT-rich phases. While illuminating only F8BT (488 nm light), the PFB-rich phases produce the most PC, however, while illuminating both polymers but mostly PFB (408 nm light), the F8BT-rich phases produce the most PC. These results show that in the morphology where the light absorbing material is less concentrated, the PC generation is increased. Additionally, the exciplex-PL is found to not be a significant radiative loss mechanism of charge carriers for PC generation.

Phonon-driven ultrafast exciton dissociation at donor-acceptor polymer heterojunctions

A quantum-dynamical analysis of exciton dissociation at polymer heterojunctions is presented, using a hierarchical electron-phonon model parametrized for three electronic states and 28 vibrational modes. Two representative interfacial configurations are considered, both of which exhibit an ultrafast exciton decay. The efficiency of the process depends critically on the presence of intermediate bridge states, and on the dynamical interplay of high- vs low-frequency phonon modes. The ultrafast, highly nonequilibrium dynamics is consistent with time-resolved spectroscopic observations.

Nonadiabatic quantum dynamics based on a hierarchical electron-phonon model: Exciton dissociation in semiconducting polymers

A hierarchical electron-phonon coupling model is applied to describe the ultrafast decay of a photogenerated exciton at a donor-acceptor polymer heterojunction, via a vibronic coupling mechanism by which a charge-localized interfacial state is created. Expanding upon an earlier Communication [H. Tamura et al., J. Chem. Phys. 126, 021103 (2007)], we present a quantum dynamical analysis based on a two-state linear vibronic coupling model, which accounts for a two-band phonon bath including high-frequency C[Double Bond]C stretch modes and low-frequency ring torsional modes. Building upon this model, an analysis in terms of a hierarchical chain of effective modes is carried out, whose construction is detailed in the present paper. Truncation of this chain at the order n (i.e., 3n+3 modes) conserves the Hamiltonian moments (cumulants) up to the (2n+3)rd order. The effective-mode analysis highlights (i) the dominance of the high-frequency modes in the coupling to the electronic subsystem and (ii) the key role of the low-frequency modes in the intramolecular vibrational redistribution process that is essential in mediating the decay to the charge-localized state. Due to this dynamical interplay, the effective-mode hierarchy has to be carried beyond the first order in order to obtain a qualitatively correct picture of the nonadiabatic process. A reduced model of the dynamics, including a Markovian closure of the hierarchy, is presented. Dynamical calculations were carried out using the multiconfiguration time-dependent Hartree method.

Excited state calculations on fluorene-based polymer blends: Effect of stacking orientation and solvation

Polyfluorene-based polymer blends have been utilized in the development of optoelectronic devices. The constituent copolymers are chemically designed to facilitate more efficient electron/hole mobility, thereby enhancing control over exciton formation and dissociation. When appropriate pairs of these are blended together, intermolecular charged-particle localizations are induced, leading to significant intermolecular charge-transfer character and luminescence that exhibit some sensitivity to their interfacial orientation. The authors report on a time-dependent density functional theory quantum chemical investigation of the relevant excited states of the polymer blend poly[9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine]/poly(9,9-dioctylfluorene-co-benzothiadiazole. They show that the calculated excited states generally agree with experimental observations although there is a consistent underestimation of the charge-transfer states. Further, they show sensitivity to lateral shifts in interfacial stacking. Finally, solvation with a low dielectric solvent greatly stabilizes the charge-transfer states.

Exciton dissociation at donor-acceptor polymer heterojunctions: Quantum nonadiabatic dynamics and effective-mode analysis

The quantum-dynamical mechanism of photoinduced subpicosecond exciton dissociation and the concomitant formation of a charge-separated state at a semiconducting polymer heterojunction is elucidated. The analysis is based upon a two-state vibronic coupling Hamiltonian including an explicit 24-mode representation of a phonon bath comprising high-frequency (C==C stretch) and low-frequency (torsional) modes. The initial relaxation behavior is characterized by coherent oscillations, along with the decay through an extended nonadiabatic coupling region. This region is located in the vicinity of a conical intersection hypersurface. A central ingredient of the analysis is a novel effective mode representation, which highlights the role of the low-frequency modes in the nonadiabatic dynamics. Quantum dynamical simulations were carried out using the multiconfiguration time-dependent Hartree method.