View of EFFECT OF VOLTAGE ON THE BRIGHTNESS OF ZNS: TM THIN FILM ELECTROLUMINESCENCE DEVICES

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ACCENT JOURNAL OF ECONOMICS ECOLOGY & ENGINEERING Peer Reviewed and Refereed Journal, ISSN NO. 2456-1037

Available Online: www.ajeee.co.in/index.php/AJEEE

Vol. 06, Issue 09,September 2021 IMPACT FACTOR: 7.98 (INTERNATIONAL JOURNAL) 41 EFFECT OF VOLTAGE ON THE BRIGHTNESS OF ZNS: TM THIN FILM

ELECTROLUMINESCENCE DEVICES M. K. Prajapati

Department of Physics, SSSVS Government Post Graduate College, Chunar, Mirzapur Upendra Singh

Department of Physics, Smt. Indira Gandhi Government Post Graduate College, Lalganj, Mirzapur

Har Govind

Department of Physics, H.N.B. Government Post Graduate College, Naini, Prayagraj (U.P.) D.S. Raghuwanshi

Department of Applied Physics, Faculty of Engineering & Technology, Shri Shankaracharya Group of Institutions, Bhilai (C.G.)

Abstract- The aim of this paper is to study of dependence of EL brightness on the applied voltage and dependence of threshold voltage on temperature of ZnS: Mn thin film EL devices The thin film insulator, active layer (ZnS: Tm),and electrode (Al) are deposited by thermal evaporation method by using a vacuum coating unit. The saturation value of EL brightness and the threshold voltage decreases with increasing temperature of EL devices. The rate of decrease of brightness with temperature is faster for the electroluminescent devices in which the active layer contains higher concentration of the activators. The threshold voltage does not change significantly with increasing concentration of the activator in the active layer of thin film electroluminescent devices. The polarization is only very weakly dependent upon temperature. This fact rules out the possibility that the decrease of EL brightness with increasing temperature could be caused mainly by a variation of the number of hot electrons. It may rather be due to a variation of the radiative to non-radiative processes with temperature.

Keywords: Luminescence; Thin film; EL ; Phosphor, Threshold Voltage.

1. INTRODUCTION

The most important aspect of light generation is the response to an electrical stimulus. A variable ac voltage source drives the device and a suitable spectrometer is used to record the brightness data. A critical parameter for device performance is the threshold voltage, Vth. In terms of device physics, this is the voltage at which the electrons are accelerated to sufficiently high energies to excite the luminescent centers and result in light emission. The technology of thin film electroluminescent displays (TFELD) has advanced rapidly since then1980s. Yellow – emitting ZnS:

Mn TFELD are now commercially available in a variety of sizes and resolutions. But the development of full colour displays has been postponed for several years due to the poor luminescence and efficiency of blue emission. The overcoming of this problem needs more detailed investigations on

materials, devices and

electroluminescence mechanisms. Display resistance to environmental stresses such as pressure, temperature and temperature/humidity is very important

to assure EL device reliability. The characteristics of a non-operating thin film EL display are unaffected when subjected to temperature extremes from - 55 to 125⁰C. Typically commercially thin film El display units are specified to operate from 0 to 55 ⁰C, the limitation being due to the electronics rather than the panel. However, thin film El display systems with materialized electronics have been designed to operate over the temperature range of -40 to 85⁰C.

The temperature dependence of EL is interesting not only because it modifies the EL emission but it yields information to understand the nature of the phosphors and the effective trap-depths may be determined. Destriau [1] was probably the first who studied the temperature dependence of ZnS phosphors and observed that the threshold voltage of excitation falls as the temperature is lowered. The effect of temperature on EL output has been studied by a number of workers [2-6].

However, the present study is based on the effect of temperature on brightness of

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Vol. 06, Issue 09,September 2021 IMPACT FACTOR: 7.98 (INTERNATIONAL JOURNAL) 42 ZnS: Tm thin film electroluminescence

devices.

2. EXPERIMENTAL

For present investigation ZnS: Tm phosphors were prepared. Initially the vapors of SnCl2 mixed with air were deposited on heated glass plate. This process was repeated until the desired results were obtained. The sheet resistances of conducting glass plate were nearly 50 Ohm cm. For deposition of insulator active layer and electrode the thin film of insulator MgF2; active layer ZnS: Tm and metal electrode (Al) were deposited by using thin film vacuum coating unit Fig.1. For studying temperature dependence of the brightness, the electroluminescent thin film device was placed on copper disc, attached with one hollow tube containing heating filament. The heating filament was connected to a relay and operated by a variac so that one can maintain the desired temperature. The temperature was measured with the help of an alumel- chromel thermocouple. A varying AC voltage of different frequencies was applied to the cell and the brightness of electroluminescence emission was measured in arbitrary units.

Fig1. Typical Thin Film EL Device Structure.

3. RESULT AND DISCUSSION

Fig. 2 shows the dependence of EL brightness (B) on the applied voltage (U) for ZnS: Tm for different temperatures. It is seen that the EL starts beyond a particular voltage, and then it increases with voltage and tends to attain a saturation value for higher value of the applied voltages. It is evident that the saturation value of the EL device.

Furthermore, the threshold voltage also decreases with increasing value of the temperature.

Fig2: Dependence of EL brightness (B) on the applied voltage Up of ZnS: Mn (1000 ppm) for different temperature

Fig. 3: Dependence of threshold voltage (Vth) on the temperature EL devices

Fig. 3 shows the dependence of EL brightness on the temperature of EL device for different concentrations of activator in the phosphors. It is seen that the EL brightness decreasing with increasing temperature of the sample and decrease of brightness with temperature is faster for the phosphor having higher concentration of the activator. For the study of the effect of temperature on the luminescence efficiency, we consider a luminescent crystal in which the carriers are excited in the conduction band, and suppose the rate of generation of electrons in the conduction band is g. When these carriers recombine with the recombination centre, luminescence is produced. The recombination of electrons woth luminescence centres may be radiative or non-radiative. If α1 and α2 are probability of radiative and non raditaive transitions, respectively, then the rate of change of number of carriers in the conduction band at any time t may be given by

𝑑(∆𝑛)

𝑑𝑡 = 𝑔 − 𝛼₁∆𝑛 − 𝛼₂∆𝑛 (1)

Where ∆n, is the change in the number of electrons in the conduction band at any time t. And resulting equation is

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Vol. 06, Issue 09,September 2021 IMPACT FACTOR: 7.98 (INTERNATIONAL JOURNAL) 43

𝜂 = ¹

1+ ^{𝛼 2}

𝛼 1 exp −_𝐾𝑇^𝐸𝑎

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The above equation shows that the luminescence efficiency should decrease with increasing temperature of the luminescent materials. The saturation value of the EL brightness and the threshold voltage decreases with increasing temperature of the EL devices.

The rate of decreases of brightness with temperature is faster for the electroluminescent devices in which the active layer contains higher concentration of the activators. The threshold voltage does not change significantly with increasing concentration of the activator in the active layer of the thin-film electroluminescent devices. The polarization is only very weakly dependent upon temperature. This fact rules out the possibility that the decrease of EL brightness with increasing temperature could be caused mainly by a variation of the number of hot electrons. It may rather be due to a variation of the ratio of

radiative to non-radiative processes with temperature.

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