Transient Electron Spin Polarization Imaging of Heterogeneous Charge-Separation Geometries at Bulk-Heterojunction Interfaces in Organic Solar Cells

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Despite the importance of elucidating photoinduced charge-generation mechanisms for the development of efficient organic solar cells (OSC), it has been quite difficult to characterize molecular geometries, electronic couplings, and charge mobilities in the initial photoinduced charge-separated (CS) states for the heterogeneous molecular environments in bulk-heterojunction interfaces between electron donor-acceptor domains in the photoactive layers. In this study, we employed a time-resolved electron paramagnetic resonance method to characterize two kinds of electron spin polarizations (ESPs) of the photoinduced CS states as different geometries, exchange couplings, and spin-relaxation times of spin-correlated radical pairs in OSC blend films composed of regioregular poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl-C61-butyric acid methyl ester by applying polarized light excitations (magnetophotoselection) with respect to an external magnetic field direction at a cryogenic temperature. From this, we performed analysis of mapping the ESPs to space directions to obtain image views of the molecular geometries in mobile and trapped CS states. We propose that the heterogeneities in the interfacial charge generations may be correlated with high- and low-frequency phonon modes leading to the mobile and weakly trapped charge pairs, respectively, denoting the great significance of such molecular motions for the efficient photocarriers overcoming the interfacial electrostatic binding potential.

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