(Invited) Transient Electron Spin Polarization Imaging of Photoinduced Interfacial Charge Separation Geometries in Organic Photovoltaic Cell
Authors: Kobori Yasuhiro , Ako Takumi , Tachikawa Takashi , Marumoto Kazuhiro 
Published: February 2020

The organic photovoltaic (OPV) devises have recently attracted much attention as the next generation thin-film solar cells that can be low-cost, flexible and light compared to the conventional silicon solar cells. The spin coating method from mixed solutions consisting of conjugate polymers as the electron donors (D) and fullerene derivatives as the acceptors (A) are employed to produce the solid photoactive layer of the OPV cells. Recent progresses in the organic materials and in device structures have enabled the remarkably efficient OPV cells as high as 12 % in the light energy conversion efficiency, which is however still lower than that in the silicon solar cells. In the organic photoactive layers, it is well known that the molecules tend to be self-organized to form the phase segregation by the polymers and by the fullerene derivatives to generate the bulk heterojunction (BHJ) by the domain interfaces. The BHJ thin-films composed of poly(3-alkylthiophene) (P3AT) and [6,6]-C61-butyric acid methyl ester (PC60BM) have been regarded as a standard and representative material of the photoactive layer in the OPV devices.

Extensive studies have been performed to investigate the physical and photochemical properties of the OPV-related materials. The ultrafast photoinduced charge-injection dynamics has been clarified by the light-excitations of poly(3-hexylthiophene-2,5-diyl) (P3HT) domains in the P3HT:PCBM blend films. Recent studies have demonstrated that the photoinduced, contact charge-transfer (CT) states are initially generated at the D/A domain-interfaces and play significant roles on the photocurrent generations in the P3AT:PCBM blends. Thus, the electrons and holes need to escape from the Coulomb attraction which energy is supposed to be several hundreds of meV in the CT state. To elucidate why and how the photo-carriers escape from the CT binding, it is important to directly observe not only the separation distances between the electron and the hole but also the relative molecular positions and orientations of the charged species just after the interfacial charge conductions. As for such separated electron-hole pairs, the electronic coupling matrix elements (V
CR) are key interaction to understand how the energy-wasting charge-recombination (CR) processes are inhibited. Connection between the geometry and the V
CRin the CS state will also lead to unveiling 1) the initial photoconduction pathways in the molecular orbitals and 2) the electronic conduction characteristics relating to the trap-depth at the D/A domain interface.[1] The molecular motion in the organic semiconductors is another key to the efficient dissociation. The molecular libration in the blend should enhance the entropy of the dissociated CS state as compared to that of the bound CT state. [2]

The time-resolved electron paramagnetic resonance (TREPR) spectroscopy is a powerful tool to detect unpaired spins and to investigate their electronic structures, geometries and dynamics.[1,2] In our previous studies, the geometries and the V
CRvalues were reported for the interfacial CS states in the BHJ blend films at cryogenic temperature.[1] These distant CS states separated by c.a. 2 nm were hypothesized to be shallow traps that are weakly bound by the electrostatic attraction between delocalized charges. However, the properties of the electronic states and of the charge motions are unknown in those separated charges.

In the present study, we have developed a three-dimensional mapping tool of the “electron spin polarization imaging method” obtained by quantum mechanical analysis of the TREPR signals to clarify precise charge geometries in the distant CS states.[3] We herein show directional charge-separation dynamics and motions at the domain interfaces in an organic solar cell composed of ITO(150 nm)/PEDOT:PSS(~40 nm)/P3HT:PCBM (~160 nm)/LiF(0.6 nm)/Al(100 nm) by using the spin polarization imaging map.

[1] Kobori, Y.; Miura, J. Phys. Chem. Lett.
2015,
6, 113-123.

[2] Miura, T.; Tao, R.; Shibata, S.; Umeyama, T.; Tachikawa, T.; Imahori, H.; Kobori, Y. J. Am. Chem. Soc.
2016,
138, 5879-5885.

[3] Hasegawa, M.; Nagashima, H.; Minobe, R.; Tachikawa, T.; Mino, H.; Kobori, Y. J. Phys. Chem. Lett.
2017,
8, 1179-1184.