Initial Molecular Photocurrent: Nanostructure and Motion of Weakly Bound Charge-Separated State in Organic Photovoltaic Interface

• Published in: Journal of physical chemistry. C 2013-01, Vol.117 (4), p.1589-1599
• Main Authors: Kobori, Yasuhiro, Noji, Ryohei, Tsuganezawa, Shuhei
• Format: Article
• Language: English
• Subjects: Applied sciences; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Electronic transport phenomena in thin films and low-dimensional structures; Electronics; Energy; Exact sciences and technology; Molecular electronics, nanoelectronics; Natural energy; Photoconduction and photovoltaic effects; photodielectric effects; Photovoltaic conversion; Physics; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Solar cells. Photoelectrochemical cells; Solar energy
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp309421s

To elucidate mechanism of the efficient photocarrier generation by the photoactive bulkheterojunction layers of the organic photovoltaic (OPV) devices, we have employed the time-resolved electron paramagnetic resonance method on solid blends composed of [6,6]-C61-butyric acid methyl ester (PCBM) and of poly(3-hexylthiophene-2,5-diyl) (P3HT) with different regioregularities. The photoinduced charge-separated (CS) states have been detected at the boundary regions between the P3HT and PCBM domains at T = 77 K. We have characterized molecular geometries, electronic couplings, and molecular motions of the long-range CS states. From the CS structure, it is indicated that the pentagonal or hexagonal aromatic rings of the buckyball in PCBM directly face the aromatic plane of the π-stacked P3HT surfaces. It has been concluded that the distant CS states are produced via fast hole-delocalization process from the contact charge-transfer (CT) states. Such hole dynamics is explained by a coupling of the hole to librations of chains in the conjugated polymer. It has been concluded that both the enthalpy stabilization and the enhancement of the entropy occur through the orbital delocalization by the electron–phonon coupling, overcoming the initial CT binding to generate the molecular photocurrent.