(Invited) Time-Resolved EPR Study on Charge Dynamics of Electron-Hole Pairs in Lead Iodide Perovskite Thin Film
Authors: Kobori Yasuhiro , Miyazaki Kosuke , Tachikawa Takashi , Ogomi Yuhei , Hayase Shuzi 
Published: February 2020

For developments of the light-energy conversion systems, it is essential to achieve efficient long-range charge-separations (CS). Light-induced electrons and holes generated by the electron-transfer (ET) reactions are thus required to escape from the electrostatic binding at the initial stage. In this respect, the lead iodide perovskite solar cells have attracted great attentions since they exhibit quite efficient energy conversion performances. This solar cell adopts the hybrid metal iodide perovskites as a photoactive layer with the general formulation of ABX3(CH3NH3PbI3), where A is the methylammonium (CH3NH3
+) cation, B is Pb2+ and M is I-. In the devise structure, this photoactive layer is sandwiched by the compact titanium oxide (TiO2) layer and by the hole-transporting layer to efficiently collect the photocarriers generated by the light irradiations of the perovskite, giving rise to ~20% power conversion efficiencies (PCE).
Recent studies have clearly shown that the carrier mobilities of the electrons and the holes are significantly high in the CH3NH3PbI3 layer, resulting in micrometer ranges of the charge-diffusion lengths.[1] These efficient charge diffusions play a crucial role on the prevention of the unwanted geminate charge-recombination in the electron-hole pairs. However, little is known concerning the electronic interactions of the electron-hole pairs initially generated in the CH3NH3PbI3 layer. At lower temperature (T < 160 K), it is reported that the electron and the hole generated by the light irradiation exhibit a Wannier exciton character with a biding energy of ~30 meV.[2] Thus, low temperature investigations of the electronic characters in the electron-hole pairs are very important to understand the mechanisms of the efficient charge-conductions initiated by the primary electron-hole pairs in the CH3NH3PbI3 layer. Another importance is to understand electronic characters of the trapped-charges generated by the light irradiations of the photoactive layers. It is recently reported that the charge careers are trapped at the surface area of the crystalline CH3NH3PbI3 layer.[3] However, electronic orbital characteristics of the trapped charges are not well known in the devices.
Time-resolved electron paramagnetic resonance (TREPR) and pulsed EPR methods have been powerful to obtain the electron spin-spin interactions including the spin-spin exchange coupling of the photoinduced CS states and thus have been utilized to characterize the interspin distances and the electronic couplings of the transient CS states.[4] Additionally, TREPR can detect the electron spin polarization (ESP) as an effect of the initial photochemical events. This phenomenon is referred to as the chemically induced dynamic electron polarization (CIDEP). According to the radical pair mechanism (RPM) CIDEP, the ESP is generated as a result of the coherent singlet-triplet interconversion in the presence of the exchange coupling (J) of the primary radical-pair states, producing the enhanced microwave absorption and the microwave emission in the separated radicals during the diffusive separations of the individual radicals in the radical-pair.[5] Since the ESP detected as the microwave absorption (A) or the emission (E) is thus sensitive to J, one is able to access the electronic interactions in the primary radical-pairs.[6]
To unveil the above fundamental electronic characteristics for the light-induced charge conductions, we have observed low temperature TREPR spectra for the three layer thin films (Spiro-OMeTAD/CH3NH3PbI3/TiO2 on a glass substrate) composed of the organic hole transport layer (Spiro-OMeTAD), the CH3NH3PbI3 layer and the mesoporous TiO2 fabricated by spin coating method on the mesoporous TiO2 films. From the EPR spectra, we have characterized the spin density distributions of the trapped holes of the CH3NH3PbI3 layer at 110 K from the anisotropic hyperfine structures originating from the iodine atoms (I). Furthermore, from the ESP generated in the trapped hole, we estimated the J coupling in the primary electron-hole pairs which will give us an insight on the electronic coupling of the Wannier excitons.
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