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

Abstract

Despite the importance of elucidating photoinduced chargegeneration 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 spincorrelated radical pairs in OSC blend films composed of regioregular poly(3hexylthiophene-2,5-diyl) and [6,6]-phenyl-C 61 -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.

Electron spin polarization generated by transport of singlet and quintet multiexcitons to spin-correlated triplet pairs during singlet fissions

“…1, when the triplet excitons are separated from each other to result in a negligible magnitude of exchange coupling (J z 0), spin correlations in the triplet-triplet pair may occur to induce diagonalized nine spin-states as the quantum superpositions of the singlet-triplet-quintet character through the spin-spin dipolar couplings of individual triplet species. This is quite similar to the spin-correlated radical pair (SCRP) model 33,34,[36][37][38][39][40][41][42] in the photoinduced long-range chargeseparation system generated by singlet or triplet precursors, causing the superpositions of the singlet-triplet characters when the J-coupling is small in the radical pairs. We herein demonstrate that the transportations of the SF-induced 5 (TT) and 1 (TT) characters can be distinguished on the T + T dissociation using the ESP of the correlated T-T state by applying TREPR spectroscopy to thin lms of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn).…”

“…This mechanism is also relevant to our recent low-temperature TREPR study for chargeseparation geometries (distance z 3 nm) in the bulkheterojunction interfaces of organic solar cells; it was concluded that the high-frequency in-plane stretching reorganization within aromatic polymer backbones promoted 1D charge-dissociation at the D:A domain interfaces. 41 The present SCTP spin polarization mechanism may thus pave new avenues to understand the nature of ultimate de-coupling and dissociation of two excitons for energy conversion. Although the coherent superposition of spin eigenstates was not directly detected, the present SCTP may be applicable to scalable quantum information processing via one-photon excitation because the sixteen possible EPR transitions from the nine sublevels ( Fig.…”

“…S1 of the ESI. † 41 This quantity of powder-like samples ensured an optical density of ca. 1 by the 532 nm laser and powder-pattern EPR analyses for the fabricated thin lms, because the laser irradiation spot area was ca.…”

“…The X-band TREPR measurements were carried out using a Bruker EMX Plus system without using eld modulation, as reported previously. 41 The microwave power was 1.0 mW. Light excitations were performed by the second harmonic (532 nm) of a Nd:YAG laser (Continuum, Minilite II, fwhm $5 ns with 0.5 mJ mm À2 ).…”

“…The slower rise in the E/A/E/A polarizations than those of the A/E/A/E is also explained by the above slow spin-relaxation characteristics resulting in large phase memory time (T 2 ). 41 For the delay times larger than 0.4 ms, the TREPR spectra were reproduced (the red lines in Fig. 3a and b) by using combinations of the green spectrum component in Fig.…”