Geometries and Dynamics of Photoinduced Electron-Hole Pairs in Polyhexylthiophene-Fullerene Systems
Authors: Kobori Yasuhiro , Miura Taku , Tao Ran , Umeyama Tomokazu , Shibata Sho , Tachikawa Takashi , Imahori Hiroshi 
Published: February 2020

For developments of the organic light-energy conversion systems, it is essential to achieve efficient long-range charge-separations (CS). Light-induced electrons and holes generated by the electron-transfer (ET) reactions are thus required to escape from the electrostatic binding at the initial stage. In the natural photosynthetic reaction centers, the efficient distant CS is accomplished with ~100% efficiency by cascading the redox states by the sequential ET processes to a series of cofactors, leading to the light-to-chemical energy conversion. In contrast, the photoinduced CS at the interfaces of organic photovoltaic (OPV) cells generates electron-hole pairs, eventually achieving light-to-electricity conversion after conductions of the electron and the hole through the donor (D) and the acceptor (A) domains, respectively. However, the D/A interfaces in the OPV cells often suffer from the charge-recombination (CR) at the early stage, leading to losses of the input photon energies.

Concerning the mechanism of the efficient photo-career generations in the OPV systems, role of the hot charge-transfer (CT) excitons is currently under intense debate. A recent ultrafast spectroscopic study of a polymer/fullerene (PCPDTBT/PC60BM) OPV films clearly showed that the vibrationally unrelaxed electron-hole pairs (PCPDTBT+/PC60BM-) are generated in < 50 fs before time scales of the internal conversions (IC) of the polymer excited states.[1] On the other hand, Vandewal et al.[2] have demonstrated for the several polymer:PCBM solar cells that the internal quantum efficiency (IQE) is essentially independent of the excitation energy of the lights; even if the low-energy CT band is excited, the IQE is higher than 90 % for some blend films, giving rise to very high OPV performances. This result strongly indicates that the hot CT states are not necessarily required to produce the highly separated photo-careers. To elucidate why and how the photo-carriers escape from the Coulomb binding, it is important to directly observe locations and orientations of the intermediate charges just after the CSs. Time-resolved electron paramagnetic resonance (TREPR) and pulsed EPR methods have been powerful to obtain the electron spin-spin dipolar interaction and the spin-spin exchange coupling in the photoinduced CS states and thus have been utilized to characterize the interspin distances and the electronic couplings of the transient CS states. The TREPR analyses have been useful to obtain the CS state geometries for several systems, since the electron spin polarization (ESP) as the microwave absorption (A) and the emission (E) is sensitive to the molecular conformations.[3] Recent spin polarization analyses have revealed representative geometries of the transient electron-hole pairs in the OPV blends.[4] However, the highly inhomogeneous molecular environments at the bulk-heteojunction (BHJ) interfaces induces the large special distribution in the solid-state OPV materials and would prevent us from understanding the mechanism of the charge conductions.

To unveil fundamental molecular mechanisms of the light-induced charge conductions, it is highly desired to investigate the structure and dynamics of the initial photo-carriers generated in a conjugated polymer-backbone in which the intramolecular orientation and the electronic interaction are well-defined at the initial CS stage. Thus far, no study has characterized the geometry, electronic coupling and charge-conduction dynamics of the photo-carriers in a covalently linked polymer system, although the polymer-fullerene dyads were synthetized and investigated by transient spectroscopies. In this respect, we have synthesized a new series of donor-bridge-acceptor (P3HT-Phm-C60, m = 0, 1, 2, 3) linked polymers composed of regio-regular poly(3-hexylthiophene) (P3HT) as the electron donor, ful-leropyrrolidine (C60) as the acceptor and oligo-p-phenylene bridges as the spacers.[5] We have herein investigated the molecular geometries and the hole-conduction dynamics of the photoinduced intramolecular CS states by using the TREPR in tetrahydrofuran (THF) at room temperature. We clearly show that, without going through the hot CT, localized holes are generated in the conjugated polymer-backbones nearby the phenylene units by quenching triplet excitons and then undergo one-dimensional intramolecular dissocia-tions from the electrons situated at C60 in the dyads.

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