(Invited) Time Resolved EPR Study on Photoinduced Charge-Transfer Trap States in Thiophene-Thiazolothiazole Copolymers Films
Authors: Kobori Yasuhiro , Yamamoto Yuta , Ako Takumi , Nagashima Hiroki , Tachikawa Takashi , Osaka Itaru 
Published: February 2020

Great attentions have been attracted on the organic photovoltaic (OPV) devices as the next generation thin-film solar cells that can be low-cost, flexible and light.[1] Conjugated polymers have extensively been utilized in various organic electronic devices including the solar cells, field-effect transistors (FET)[2] and light-emitting diodes[3] (LED). The spin coating method from mixed solutions consisting of the conjugated polymers as the electron donors (D) and fullerene derivatives as the acceptors (A) has been employed to produce the solid photoactive layer of the OPV cells.[1] In the organic photoactive layers, it is well known that the polymer molecules are self-organized to generate the bulk heterojunction (BHJ)[4] interfaces. Apart from the OPV applications, pristine films of the conjugated polymers are also utilized for the FET and LED applications.[5] The polymers usually form two dimensional lamellae structures as a result of close interchain packing interaction.[4] Such crystalline phases are known to play a key role for the mobilities of the charges and the excitons in the organic devices. Recent studies suggested that non-fluorescent CT characters exist as a competitive channel of the formation of emissive exciton-type states and determine the light emission efficiencies of the LED films.[5]

Despite the significance of the non-fluorescent CT state, no study has experimentally characterized geometry and electronic character of the photoinduced CT state in the pristine polymer films. Consequently, the optoelectric properties of the polymer films have not been fully understood. In our previous study, the geometry and the electronic coupling in the charge-separated states were clarified as the transient radical pairs in regioregular poly(3-hexylthiophene)–fullerene (P3HT–C60) linked dyad molecules.[6] In this study, we have directly observed the photoinduced CT state for a pristine film by thiophene–thiazolothiazole copolymer (PTzBT–BOHD)[7] fabricated by the spin–coating method using time-resolved EPR (TREPR) method to characterize the orbital geometries, the electronic property of the polymer CT state. We show that a long-lived interchain CT states (P+• P–•) are generated as a trap state at disordered region as defects at a cryogenic temperature. We have characterized the interspin separation and the exchange coupling of the interchain CT states in the pristine polymer film of PTzBT–BOHD generated by the 532 nm laser excitation at T = 77 K. It has been revealed that the triplet CT state is generated at the disordered regions of the polymer films as the deep trap site via the triplet exciton. These stable charge−traps would be produced at the polymer defect site possessing the large reorganization energy of ≈ 1 eV. These characteristics of the trapped charges may limit the device performances in the OPV, FET and LED applications. Thus, the TREPR method can provide us with the informative optoelectronic and structural properties of the polymer films for evaluations, designs, and developments of the highly efficient OPV, FET and LED systems.

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