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

doi:10.1016/j.synthmet.2006.03.002

Synthetic Metals

Volume 156, Issues 7–8, 5 April 2006, Pages 624-632

https://doi.org/10.1016/j.synthmet.2006.03.002 Get rights and content

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 (Alq₃)) 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 10⁵-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 Alq₃ 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.

Introduction

Recently, Hiramoto et al. demonstrated light converters in which inorganic or organic photoconductive layer were combined with electroluminescent layer [1], [2]. PM (exceeding 10⁴-times) under light irradiation was utilized and studied in their devices. This phenomenon is promising for construction of highly sensitive photosensors and the next generation of optoelectronic systems. The PM was observed in organic films like perilen pigment [3], quinacridone pigment [4] and naphthalene tetracarboxylic anhydride [5].

PM is a phenomenon in which the quantum efficiency of carrier generation exceeds unity. The PM value is calculated as the ratio of the number of carriers flowing through the device to the number of photons absorbed by photosensitive layer.

The authors of the paper [3] have proposed a photoinduced electron tunneling mechanism for the photocurrent multiplication phenomenon observed for perylene pigment having methyl groups (Me-PTC). Fig. 1a illustrates the energy structure of the pigment/metal interface during multiplication. The metal electrode is biased negatively. The following multiplication mechanism was proposed. The applied field is mainly distributed in the highly resistive depletion layer formed between the negatively biased Au and the Me-PTC. Under the light irradiation, the photogenerated electrons and holes move along the potential gradient. The photogenerated holes are then captured by traps near the organic/metal interface. Accumulation of trapped holes builds up a high electric field across the depletion layer, and finally the tunneling injection of electrons occurs from the Au electrode through the shrunken depletion layer, resulting in the large multiplication.

As a candidate of hole trap causing multiplication, the authors have proposed the field-activated structural trap model [6], [7] (Fig. 1b). Organic pigment films having microscopically rough surface and metal do not form uniform contact and there are large number of sites forming blind alleys, which act as hole traps when there is the field toward metal.

Fig. 1c shows the schematic illustration of the Me-PTC/Au interface [8]. The vacuum-evaporated Au film structure shows a congregation of Au nanoparticles of the diameter of about 20-nm and there are spatial gaps between Me-PTC and Au. Non-contact sites and direct contact sites are inevitably formed between the organic surface and Au spheres. The former offer the spatial dead end sites (structural trap), which capture the photogenerated holes under the concentrated electric field toward metal. The latter offer the electron injecting sites.

In light converters based on the PM effect, photosensitive and emissive layers are enclosed in the same structure. In such devices it is necessary to adjust HOMO- and LUMO levels of photosensitive and electroluminescent layers. As a consequence a large number of materials revealing high emitting characteristics and luminescence in blue and ultraviolet region of spectrum are useless because of high energy difference between corresponding levels. To extend the range of organic materials suitable for light up-conversion/amplification an organic light amplifier/converter with spatial separation of PM and emission has been proposed and investigated in this work (Fig. 2a).

The range of amplification/conversion in suggested device has been extended to all EL spectrum of the emissive part of the system due to elimination of light reabsorption by the photosensitive part of the system. So, up- and down-conversion as well as enhancement of an emission light may be obtained.

The layer of perylene pigment (Me-PTC) has been traditionally used in photosensitive part of both devices. As an emissive part of the devices organic diodes based on lanthanide mixed-ligand complex Tb(Sal)₃(TPPO)₂ successfully used in our previous work in multilayer light-emitting diode [10] or well-known compound Alq₃ [11] have been used.

Along with studies of light amplification/conversion the proposed device enables to investigate the voltage redistribution between the photosensitive and electroluminescent parts depending on variation of whole bias voltage applied to the system, as well as of temperature and other factors. It helps to understand the details of internal processes in the suggested device.

Section snippets

Materials and preparation of samples

ITO covered glass plates (“Delta Technologies, Limited”) were used as the substrates. ITO substrates were washed out in ultrasonic bath by alkali (KOH and NaOH) solutions and by purified water.

Alq₃, TPD, PVC, Me-PTC were received from “Aldrich”, PEDOT-PSS – from “Bayer AG” and used without further purification.

Photosensitive part of the investigated device represented the structures consisted of the Me-PTC layer enclosed between ITO layer (anode) and semitransparent gold cathode (Fig. 2b).

The

The photosensitive part of the device

It is important to study the processes taking place in photosensitive part of amplifier/converter and to clarify the factors influencing the PM for optimization of the device performance.

The PM dependence on the applied voltage

The PM dependence on voltage bias applied to the structure displayed noticeable growth of photocurrent starting from 5 V (Fig. 3a). Optimal voltage values corresponding to PM maximum lay in a range around 37 V. Under high bias voltages the potential barrier thickness between the metal electrode and the organic

Conclusions

1.
New tandem two-diode organic amplifier/converter of light with spatial separation of the processes of PM and electroluminescence has been realized and investigated. These devices were based on the emission layers of novel mixed-ligand lanthanide complex (Tb(Sal)₃(TPPO)₂) or well-known compound aluminum tris-(8-hydroxyquinoline) (Alq₃) as well as on the layer of perylene (Me-PTC) pigment as photosensitive material.
2.
The modernized model of field-activated structural traps has been suggested. The

Acknowledgments

This work has been fulfilled under support of grants of Russian Foundation for Basic Research (RFBR) Nos. 04-02-17040, 03-02-16734, 05-03-33090, 03-02-16817 and of Development Program for System of Leading Scientific Schools in Russia, code RI-112/001/039.

Authors are appreciated to P.P. Sverbil for friendly relation and help in adjustment of the working procedure on the interference microscope.

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

Abstract

Introduction

Section snippets

Materials and preparation of samples

The photosensitive part of the device

The PM dependence on the applied voltage

Conclusions

Acknowledgments

Synth. Met.

Synth. Met.

Appl. Phys. Lett.

Appl. Phys. Lett.

Jpn. Appl. Phys. Lett.

Appl. Phys. Lett.