Overcoming Coulombic Traps: Geometry and Electronic Characterizations of Light-Induced Separated Spins at the Bulk Heterojunction Interface

Abstract

Recent progress is overviewed on experimental elucidations of fundamental molecular functions of the light-energy conversions by the photoactive layers of the organic photovoltalic (OPV) cells by means of the time-resolved electron paramagnetic resonance spectroscopy. Positions and orientations of the unpaired electrons and electronic coupling matrix elements are clarified in photoinduced, primary charge-separated (CS) states. Connections between the molecular geometries and the electronic couplings have been characterized for the initial CS states to elucidate how the structure, orbital delocalization, and molecular libration play roles on exothermic carrier dissociation via a vibrationally relaxed charge-transfer complex with prevention of the energy-wasting charge recombination. Superior functions to biological molecules are presented for the efficient photocurrent generations induced by orbital delocalization and by shallow trap depths at polymer-stacking domains. The above structural and electronic characteristics of the primary electron-hole pairs are essential to evaluations, designs, and developments of the efficient solar cells using organic molecules.

Time-Resolved EPR Study on Photoinduced Charge-Transfer Trap State in Thiophene-Thiazolothiazole Copolymer Film

“…From D = -0.8 mT, the center-to-center separation (r CC ) between the charges is estimated to be 1.5 nm in the present CT state. In the previous study, the photoinduced CS states generated at the BHJ domain interfaces were characterized to be J = +6 T for r CC = 1.5 nm in the RR-P3HT:PCBM blends at T = 77 K [14,15]. This significantly weak J coupling was explained by a weakened electronic coupling (V) between the charges due to highly delocalized distributions in the unpaired orbitals of the mobile holes at the polymer crystalline phase.…”

First Principle Analysis of Charge Dissociation and Charge Recombination Processes in Organic Solar Cells

“…10,51,54,55 There has been much discussion in the literature about the mechanism by which efficient electron-hole dissociation occurs at the donor/acceptor interface, overcoming energy barriers largely exceeding the thermal quantum. 7,11,52,[56][57][58][59][60][61][62] Figure 4, right panel, shows that for all the three donors ln k CT b scales linearly with E b , so that as E b increases dissociation into separated charges becomes rapidly exceedingly slow, no longer competitive with decay to the ground state. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 Table 3: Reorganization energies (eV) of the donor dyes for photo-excitation, and for photoinduced charge separation, charge recombination via triplet, and hole hopping half-reactions.…”

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

“…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.…”