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Competition between Exciplex Formation and Photocarrier Generation in Molecular-Scale Donor-Acceptor Heterojunctions Jun’ya Tsutsumi, Toshikazu Yamada, Hiroyuki Matsui, and Tatsuo Hasegawa Flexible Electronics Research Center (FLEC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8562, Japan.

ABSTRACT Donor-acceptor mixed-stack charge-transfer (CT) compounds can be regarded as a model system for charge carrier separation in molecular-scale donor-acceptor heterojunctions. Here we investigated fundamental photocarrier generation characteristics in single crystals of a donoracceptor mixed-stack system, phenothiazine-tetracyanoquinodimethane (PTZ-TCNQ). The laser beam-induced current (LBIC) measurement on the crystals allowed the discrimination between the exciton and the photocarrier diffusion on the basis of the observed spatial decay profiles. We found that the photocarriers are directly generated by higher-lying CT band excitation and exhibit extremely long diffusion length reaching more than 10 m. We discuss the origin of the efficient photocarrier generation in terms of the geminate electron-hole pair formation.

INTRODUCTION Donors and acceptors are the fundamental building blocks for the composition of organic photovoltaic cells (OPCs). The operation of conventional OPCs is initiated by the formation of Frenkel excitons in which the relative motion of electron and hole is confined exclusively either in the donor or acceptor phases [1]. Then the excitons may travel within the respective phases and are separated into free electrons and holes at the donor-acceptor heterojunctions. Because the diffusion length of excitons is quite short both in donor and acceptor phases, it has been demonstrated that the use of bulk heterojunctions, composed of high density donor-acceptor heterojunctions, is useful to realize the efficient exciton dissociation [2,3,4,5]. So far, elementary excitations and charge carrier separations at the donor-acceptor heterojunctions have been studied [6,7,8], but they still remain unclear. We recently investigated photocarrier generations and photovoltaic characteristics in a mixedstack donor-acceptor charge-transfer (CT) compound, dibenzotetrathiafulvalenetetracyanoquinodimethane (DBTTF-TCNQ). The material has molecular-scale order of donors and acceptors [ 9 , 10 , 11 ], which can be regarded as a model system of donor-acceptor heterojunctions. By utilizing laser beam-induced current (LBIC) technique, we found that the photocarriers are directly generated by higher-lying CT band excitation without forming excitons [12]. In addition, we observed an exceptionally long carrier diffusion length reaching as long as 20 μm. Note that the exciton diffusion length of usual organic semiconductors is in a range of a few tens of nanometers [13]. These peculiar features of the donor-acceptor heterojunctions are quite indicative to improve the charge separation efficiency in OPCs. In the present paper, we report the exciton and photocarrier diffusion characteristics of