Time-Resolved EPR Study of Electron–Hole Dissociations Influenced by Alkyl Side Chains at the Photovoltaic Polyalkylthiophene:PCBM Interface

Abstract

Nanosecond time-resolved electron paramagnetic resonance (TREPR) spectroscopy has been utilized at T = 77 K to characterize alkyl side-chain effects on geometries and on the electronic couplings (VCR) of transient charge-separated (CS) states in the photoactive layers fabricated by the spin-coating of mixed solutions of regioregular polyalkylthiophenes (RR-P3AT) and [6,6]-C61-butyric acid methyl ester (PCBM). By increasing the alkyl side-chain number from 6 to 12 in P3AT, a highly distant and long-lived CS state has been obtained. This result is explained by a coupling of the hole dissociation to the polymer librations by the side-chains. From an exponential decay of VCR with respect to the CS distance, the attenuation factor (βe) has been determined to be βe = 0.2 Å(-1). Such a long-range tunneling feature is explained by the generations of the shallowly trapped, delocalized electron-hole pairs by the dissociation of the hole toward π-stacking directions at the organic photovoltaic interface.

Morphology Effect on Geometry of Photoinduced Charge-Separated State in P3HT:PCBM Blend Films as Studied by Time-Resolved EPR Spectroscopy

“…To clarify the electron-hole dissociation mechanism and the electronic property of the photoinduced CS states, the nanostructures and the 2J were obtained by the analyses of the TREPR data (fitted curves in Fig. 3) on the basis of the stochastic-Liouville equation (SLE) by which the time-dependent quantum mechanical phenomena in the density matrix () of the singlet-triplet system are solved in the rotating frame on the basis spin-functions of ‫|‬T + › = ‫|‬›, ‫|‬T 0 › = ‫|‬ ›/√2, ‫|‬T -› = ‫|‬› for the triplet CS state, and of ‫|‬S› = ‫|‬ ›/√2 for the singlet CS state [6,8,9]. The effects of the spin relaxations (the spin lattice relaxation time of T 1 , the transverse relaxation times T 2 * and Fig.…”

“…the relaxation time of T 23 by the S-T 0 dephasing) [10] and the CR kinetics were taken into account in the SLE [6,8]. Theoretical details how one can compute and evaluate the time-evolutions of the transverse magnetizations have been reported in our previous studies [5,6,11].…”

“…Theoretical details how one can compute and evaluate the time-evolutions of the transverse magnetizations have been reported in our previous studies [5,6,11]. Figure 4 shows the geometries of the interfacial CS states to reproduce the TREPR spectra in Fig.…”

“…Blend films composed of the conjugated polymer of regioregular poly(3-hexylthiophene) (RR-P3HT) and the fullerene derivative of [6,6]-phenyl-C 61 -butyric acid ester (PC 60 BM) have attracted attention as a model system for the study of photoactive bulkheterojunction layer in the organic thin-film solar cells. However, only a few studies have directly characterized the geometries of photoinduced charge-separated (CS) state at the domain interface of the blend films [5,6]. Consequently, initial photoelectric conversion mechanisms have not been fully understood.…”

Theoretical study on the cooperative exciton dissociation process based on dimensional and hot charge-transfer state effects in an organic photocell

“…To explain this mystery, various researchers have been actively discussing the exciton dissociation process and have proposed several explanatory mechanisms such as the contribution of entropy (dimensional effect), the hot charge-transfer (CT) state, doping or defects, and structural heterogeneity. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] Among these, a strong candidate is the hot CT mechanism. 1, [22][23][24][25][26][27][28] In our previous study, we proposed a theoretical method to consider the hot CT state and examined the exciton dissociation process at the donor-acceptor interface.…”