Carrier photogeneration from a higher excited singlet state produced via triplet–triplet annihilation in N-vinylcarbazole–1-vinylnaphthalene copolymer

Abstract

The carrier photogeneration mechanism has been investigated in N-vinylcarbazole(VCz) –1-vinylnaphthalene(VNa) copolymers. The photocurrent as well as the delayed fluorescence of carbazole (Cz) chromophore are enhanced in the copolymer, which contains a few mole percent of VNa units, and the delayed fluorescence of naphthalene (Na) chromophore is observed with selective excitation of Cz in its lowest π–π* transition, i.e., with lower energy than the first excited singlet state of Na. The photocurrent in this copolymer exhibits a quadratic dependence upon the excitation light intensity in a high field. Detailed investigations of the emission spectra and the photocurrent in this VCz–VNa system lead to the ’’intrinsic’’ carrier photogeneration via the triplet state of carbazole moiety as follows: Czhν→1Cz*→3Cz* →3Na*T–Tannihilation→1Na**→1Cz**→carriers. Here, the double asterisk indicates the higher excited state. As evidenced by the existence of this mechanism in the copolymers, also in the VCz homopolymer is suggested the possibility of the intrinsic photocarrier generation from higher singlet via multiphotonic process involving a triplet state.