Perspectives of CdS-Cu2S solar cells at high levels excitations

So/id-Stare

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Hecrronics Vol. 31, No. 10, pp. 1505-1507, Great Britain. All rights reserved

PERSPECTIVES HIGH

0038-IlOlj88 53.00+ 0.00 Copyright 0 1988Pergamon Press plc

1988

OF CdS-Cu,S SOLAR CELLS AT LEVELS EXCITATIONS

D. L. VASILYEVSKY,~L. NINAI and R. VAJTAI Department of Experimental Physics JATE, H-6720, Szeged Dom t. 9, Hungary (Received

5 December

1987; in revised form

19 April 1988)

Abstract-The kinetics of laser-light induced photo e.m.f. is studied using CdSCu,S heterostructures. It is shown that the relaxation curves consist of two parts: a fast one with a duration of 0.5 ms and a slow one with a duration of 10 ms. From an analysis of these results it is concluded that at high intensities the difference between the behaviour of ideal and non-ideal heterostructures is negligible. Therefore it must be possible to use the CdS-Cu,S successfully under concentrated sunlight.

INTRODUCTION In a recent paper we have discussed polycrystalline Cu,S-CdS back-wall photoelements. The samples were prepared using non-vacuum methods. Solutions of CdCl, and SC(NH*)* were pulverized through a capillary onto a substrate, while a high electrical potential (- 5 kV) was kept between the capillary and the substrate[l]. This method is rather economical and makes it possible to produce photoelements on bendable band-type substrates. Our photoelements have a free surface of 20 cm2 with an efficiency of 6-8%, and show a nearly constant sensitivity in a wide range of wavelengths (SOCrlOOO nm). It is well known that the low net efficiency of the cells is associated with the insufficient voltages of non-ideal heterostructures[2]. In spite of the high barrier in Cu,SCdS solar cells (&, - 1 eV), the open circuit voltage ((i,) is not higher than 0.5 V. This is due to a tunnel-recomination mechanism of charge carrier transport, which causes effective shunting of the barrier and decreases U,. The maximum photoelectromotive force (e.m.f.) value has a primary theoretical and practical interest because of its importance in devices’ application. EXPERIMENTAL Samples were irradiated by ruby laser pulses with a wavelength of 1 = 694.3 nm (1.7 eV). This wavelength is matched with the region of highest photosensitivity of our solar cells. a duration of The laser pulses have FWHM = 40 ns. The pulse energy is -0.25 J, and the beam diameter - 8 mm. The intensity of the beam on the samples was varied by a series of calibrated neutral filters. The change of (Cr,) with time for the irradiated heterostructures was recorded by a tPresent address: Department of Experimental (OGU), Odessa State University, U.S.S.R.

Physics

memory scope. In Figs l(aHd) the Um signals are presented at different levels of laser intensity. Apparently the time dependence of U, could be characterized by two types of relaxation; one with a slow time constant (7, - 10ms) and a fast one (7* - 0.5ms). The slow relaxation curve only occurs at higher laser light intensities. Figure 2 presents the dependence of U, amplitudes on E (energy of laser light on the sample) for both slow and fast relaxations, consecutively. It should be noted that the amplitudes in this case are lying in the voltage interval of 0.754.8 V, while at irradiation by sunlight only amount 0.45 V or less.

DISCUSSION

To explain the experimental results first the banddiagram of Cu,SCdS heterostructures is shown (Fig. 3). The light of 1.7 eV quantum energy causes “bandband” electron transitions in Cu,S and transition between the conductivity band and impurity levels in CdS. Photocurrent (or photovoltage) is connected with the transition of minority carriers through the junction. The fast part of relaxation is provided by transitions of photoexcitated carriers into Cu,S. Free holes, however, do not appear in CdS, they remain localised in their centres since the energy of the centres is -0.7eV relative to the valence band. In Ref.[3] the tunnel transport process of holes in Cu,S is described. It is shown there that this process has a quite low probability. It means that no contribution of CdS to the photoresponse should appear as can be seen in Figs l(a) and (b). However with increasing laser pulse energies because of the local temperature rise of the sample the probability of thermal excitation of holes should increase. This process has a higher inertness and therefore corresponds to the slower part of the relaxation. Increasing the energy of the pulses causes an amplitude increase in the slower part (Figs l(c) and (d)). 1505

D L.

I506

Fig.

I. Relaxation

VASILYEVSKY

et

3.3 x IO ~’ J (a),

of open-circuit voltage (l’,,). L = 694.3nm, rim,, = 40 ns. energies 7 x IO ‘J (h). 2 x IO ‘J (c), 2 x IO 2 J (d).

It should be noted that U,, vs E curves has two regions in every experiment. Non-ideahty coefficients (q) of the CFI characteristics were in the 2.7.-5.1 interval which is characterized by major tunnelmechanism recombination transport in heterojunctions[4]. This process shunts the barrier thereby and therefore decreases the value of U,,,. At

high intensities of however--if excitation, 4 = &, - elJoc is near to 0.24.3 eV-the contribution of thermal mechanism becomes higher and more pronounced. At these intensities the value of q is between l.OlL1.3 which supports the reality of epitaxiai heterostructure transport phenomena[4]. It means that at low intensities the difference between

lo-'

Fig. 2. Amplitide

al

-E

10“

I

I

loo

lo2

[JI

loo I

SUN

L

10’

of fast (1 12,3) and slow (l’, 2’) part of C’,, relaxations YS laser pulse energy for different samples of CuzS CdS. Numbers at curves show the value of q.

Perspectives of CdS-Cu,S solar cells

1507

CdS

Fig. 3. Energy

band-diagram

ideal and non-ideal heterostructure is big, at high intensities, however, the non-ideal heterostructure works as an ideal one. The change in the curves U, (E) appears at a flux interval of 1.25 x 105-IO6 W/cm*. In order to compare these values with solar illumination levels it is necessary to take into consideration that the collection of electrons in Cu,S takes places at a time as short as 7, - 0.5 ms (fast part of the relaxation). Thus, in contrast to the case of short laser pulses at continuous excitation the carriers are collected in the Cu,S within TV. This means that a laser pulse of 0.25 J energy is identical to a power density I = lo3 W/cm*. Taking into account that AM 1 is equivalent to 0.1 W/cm* it can be calculated that the laser pulse is equivalent to IO4 suns. From the breaking of curves in Fig. 2, lo’-lO*sun could be obtained. It should be noted that the form of U, v/s E curves (slow part of recombination) are not linear which shows the influence of extra thermal effects. CONCLUSION

On the basis of our investigations we conclude that non-ideal heterostructure solar cells of the types

of Cu,S-CdS

heterojunction.

Cu, S-CdS, Cu, S-ZnCdS , CuInSe,-ZnCdS can be optimally used for energy conversion of concentrated sunlight. In this case their advantages (large surface, low prices) can be kept and their disadvantages be eliminated. A sufficient rise of photovoltage makes it possible to reach a 1.5-2.0 times increase in efficiency. Of course, the new system requires new types of effective current-collecting contacts, etc. Acknowledgemenu-The authors would like to thank Professors Dr 1. Hevesi from Department of Experimental Physics JATE and V. V. Serdyuk from the Department of Experimental Physics OGU for their stimulating and informative discussions.

REFERENCES

S. A. Korepanov. Yu. N. Karakis, D. L. Vasilyevsky and V. V. Serdyuk, 2nd Conf. on Reneoables EnergySources, Yerevan, p. 135 (1985). K. Chopra and S. Das, Thin Film Sun Cells. Mir, Moscow (1968). In Russian. M. S. Vinogradov, V. A. Borschak and D. L. Vasilyevsky, Sou. Phys. Semicond. 20, 10 (1986). A. G. Mimes and D. L. Feucht, Heterojuncrions and Metal-Semiconductor Junctions. Academic Press, London (1972).