Elsevier

Synthetic Metals

Volume 106, Issue 2, 15 October 1999, Pages 97-105
Synthetic Metals

Studies on electrical and photoelectrical behaviour of ITO/ArV/In Schottky barrier device

https://doi.org/10.1016/S0379-6779(99)00115-0Get rights and content

Abstract

The electrical and photoelectrical properties of aryl viologen (ArV), chemically known as 1,1′-diphenyl-4,4′-bipyridinium dichloride, in the form of thin film, sandwiched between ITO and In electrode were studied. The current–voltage (JV) characteristics in dark show the rectification effect due to the formation of Schottky barrier at In–ArV interface. The diode quality factor of the device, greater than unity, indicates the recombination of electron-hole in depletion region. Ohmic conduction in low voltage range and space charge limited conduction (SCLC) controlled by an exponential distribution of traps above the valence band edge, for higher voltage region have been observed. Various electrical parameters were calculated from the analysis of JV and capacitance–voltage (CV) characteristics at different temperatures and discussed in details. At higher frequencies, the device exhibit voltage independent capacitance, which is explained in terms of the extremely slow kinetics of space charge and low mobility of charge carriers. The photoaction spectra of the device and absorption spectra of the ArV thin film reveal that the fraction of light, which is absorbed near or within the diffusion length of exciton, is responsible for producing the free charge carriers. The photovoltaic parameters were calculated from the JV characteristics under illumination through ITO and discussed in detail.

Introduction

Many advanced technologies, especially in electronic and optoelectronic devices, require thin layers of new functional materials with special optical and electrical properties. Such properties are determined by a complex combination of many physical factors as well as the chemical nature of the starting material. Organic semiconducting layers are one of such class of prospective thin layer materials. Their low cost production is very simple using recent high vacuum evaporation and spin coating technology and their optical and electrical properties can be modified over a wide range 1, 2, 3, 4, 5. The physical and chemical properties of these material layers depend not only on the molecular structure but also on the structure of the solid thin film [5].

The specific electrical and optical properties of organic semiconductor layer are of special interest, e.g., the absorption of light energy in the thin layer can be converted into electrical, chemical and thermal energy, leading to some interesting applications such as solar cells nonlinear optical devices 6, 7, 8, luminescence devices 9, 10, 11, 12 and energy conversion and storage devices 13, 14, 15, 16, 17, 18. Much research has been carried out on the use of these organic semiconductors as active element for these devices.

Viologens are widely used as electron acceptors for charge transfer mediation in the conversion of photo energy into chemical energy or for solar energy conversion 19, 20. The present communication deals with the investigation of electrical and photoelectrical properties of aryl viologen (ArV) in the form of thin film Schottky device. The electrical and photoelectrical properties of the ArV were measured using the sandwich structure In/ArV/ITO. The dark current–voltage (JV) characteristics of device and comparison of the photoaction spectra of the device with absorption spectra of the thin film reveals that ArV behaves as a p-type organic semiconductor forming the Schottky barrier at In–ArV interface and ohmic contact at ArV–ITO contact. The capacitance–voltage (CV) characteristics of the devices at various frequencies were also discussed. The detail photogeneration processes and photovoltaic effect were also discussed in detail. Various electrical and photoelectrical parameters were also determined from the analysis of JV characteristics in dark and under illumination, and CV characteristics in dark and discussed in detail.

Section snippets

Experimental detail

The synthesis of ArV involves two-step process.

Results and discussion

The JV characteristics of the In/ArV/ITO device in dark show clear rectification behaviour as shown in Fig. 1. On applying positive voltage at the ITO electrode, the device is forward bias. This JV characteristics indicate the formation of a blocking contact at the In–ArV interface and ITO form ohmic contact with ArV layer in In/ArV/ITO device. This behaviour has also been confirmed from the photoaction spectra and CV measurement of the device as discussed in later part of the paper. This

CV characteristics

Fig. 6 shows the variation of capacitance (1/C2) with applied field for device In/ArV/ITO at different frequencies ranging from 40 Hz to 10 kHz. It can be seen that the capacitance is voltage dependent at low frequencies but independent at higher frequencies. In general, the most common cause for the frequency dependent capacitance is the trapping of an electron by surface states. Such electron transfer may be very slow, particularly, for a small barrier. Therefore, when the capacitance is

Photogeneration process and photovoltaic effect

The absorption spectra of the ArV was recorded in DMF with UV–Vis spectrophotometer from 200 to 800 nm (Fig. 8c) showing wide band absorption from 350 to 450 nm. The most predominant electronic transition in the molecular system of ArV is π–π* transition which can be attributed to the delocalisation of π electrons, induced by the extension of conjugation between aryl group and dipyridinium moieties.

The photovoltaic action spectra (variation of quantum yield with) of the device In/ArV/ITO

Conclusions

In this paper, we have described the electrical and photoelectrical properties of In/ArV/ITO device. The JV characteristics of the device in dark show the rectification effect having the diode quality factor greater than unity which indicates the recombination of electron and holes in depletion region. The JV characteristics in dark, comparison of photoaction spectra of the device with the absorption spectra of the ArV, suggest that the ArV behaves as p-type organic semiconductor from the

Acknowledgements

We are grateful to the Department of Science and Technology (DST), Government of India for financial support.

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