Two-diode organic light amplifiers/converters and peculiarities of photocurrent multiplication

Abstract

The novel tandem two-diode organic amplifier/converter of light based on successively connected photosensitive and light emission cells with spatial disjunction of processes of photocurrent multiplication (PM) and electroluminescence (EL) has been proposed and realized. The terbium complex Tb(Sal) 3(TPPO) 2 (HSal – salicylic acid, TPPO – triphenylphosphine oxide) or aluminum tris-(8-hydroxyquinoline (Alq 3)) have been used as active layers in light emission cell of device as well as the perylene pigment Me-PTC ( N, N″-dimethylperylene-3,4,9,10-bis(decarboximide)) in the photosensitive one. The suggested method of amplification/conversion has some advantages: (1) it avoids the necessity to adjust HOMO- and LUMO levels of photosensitive and electroluminescent layers and thus, extends the range of suitable light emissive materials including ones with high emission characteristics, (2) it also eliminates reabsorption of light by photosensitive part of the system and so, extends spectral band of amplification/conversion. A modernized model of field-activated structural traps has been suggested. Kinetics of PM during light- and voltage switching with time pauses is explained using this model. The peculiarities of PM and its kinetics at different light intensities, temperatures and applied voltages are analyzed too. Optimal values of temperatures, voltages and light excitation intensities providing high PM have been found. The PM gain up to 10 5-fold has been achieved. The conversion of long-wave ( λ = 600 nm) light into narrow emission bands of Tb(Sal) 3(TPPO) 2 ( λ max 545 nm) and wide-band (from 490 nm) emission of Alq 3 has been obtained. Up- and down-conversion of light along with enhancement of emission have been observed in both devices. A computer simulation of the tandem-diode amplifier/converter has been performed and operation conditions providing transition from the up-conversion mode to the enhancement of light mode have been studied. It has been demonstrated that the processes of PM and the bias redistribution between the units of device exert strong influence to one another and only their unification leads to the tandem-diode operation.