Frequency Resolved Photoluminescence (PL), Delayed Fluorescence and Triplet-Triplet Annihilation in F-Conjugated Polymer
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'Ames Laboratory- USDOE,2Physics andAstronomy Department,and 3InternationalInstitute of TheoreticalandApplied Physics, Iowa State University, Ames, IA 50011 Institute of EnergyProblems of Chemical Physics,Moscow 334, Russia 5Ames Laboratory- USDOE and Chemistry Department,Iowa State University, Ames, IA 50011
ABSTRACT The delayed fluorescence of poly(p-phenylene vinylene) (PPV) and poly(p-phenylene ethynylene) (PPE) derivative solids and frozen solutions at 20 K is described. It provides strong evidence for triplet-triplet annihilation to singlets excitons accounting for up to -3% of the total emission in PPV films and -1.5% in PPE powder. It also yields triplet lifetimes of 70 and 110 Its in PPV films and frozen solutions, and -200 and -500 pis in PPE powder and frozen solutions, respectively. INTRODUCTION Very extensive studies of the photoluminescence (PL) of n-conjugated polymer films and the electroluminescence (EL) of polymer light-emitting devices (LEDs) have been reported during the past fifteen years [1]. Yet several basic issues have remained unanswered. In particular, no quantitative observations of long-lived (microseconds to milliseconds) emission due to delayed fluorescence (DF) have been reported. In contrast, in small t-conjugated molecules such DF due to triplet-triplet (T-7) exciton annihilation to singlet excitons T+T...s*(1) which relax to the lowest (luminescent) 1'Bu state was identified unambiguously and studied in detail [2]. On a broader level, understanding triplet dynamics is important for small organic molecular and polymer LEDs, as they may be the most prolific of states generated by fusion of the positive and negative polaronic carriers. Indeed, the 25% theoretical upper limit of the EL yield of such LEDs is derived from spin statistics, which should yield nonluminescent triplets in
75% of these fusion events. Yet this argument is obviously simplistic, and an established picture of the nature of the coupling between the singlet excitons, triplet excitons, and the intermolecular or interchain polaron pairs may lead to significant modifications of the simple spin-statistical result. The lowest energy level of the interchain polaron pairs is widely believed to be slightly (-0..1 eV) lower than that of the 1 'B,, [3] and consequently decidedly higher than that of the lowest intrachain 13B,, triplet [1,2,4]. Some studies have suggested that under steady-state photoexcitation the population of the triplet (parallel spin) polaron pairs is greater than that of the singlet (antiparallel spin) pairs, which is depleted by nonradiative recombination to the ground state [5]. Other studies, however, have suggested that the triplet pair population, which may recombine to intrachain triplet excitons, is lower than that of the singlet pair population [6]. These latter studies also predicted that in addition to direct nonradiative decay to the ground
state, the intrachain triplets decay by T-T annihilation to singlets (Eq. (1)) resulting in DF.
491 Mat. Res. Soc. Symp. Proc. Vol. 558 ©2000 Materials
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