DC electric and photoelectric measurements of CdTe thin films in Schottky-barrier cells

ARTICLE IN PRESS

Physica B 349 (2004) 296–303

DC electric and photoelectric measurements of CdTe thin films in Schottky-barrier cells S. Darwish Department of Physics, Faculty of Science, Minia University, Mania, Egypt Received 18 February 2003; received in revised form 12 February 2004

Abstract Measurements of the temperature dependence of ohmic and space-charge-limited (SCL) currents on thin films of polycrystalline particles of cadmium telluride in Schottky-junction cells have been carried out in air ambient. These cells showed rectification where p-CdTe material was flanked between an ohmic contact (Au) and a blocking contact (Al). At low voltages, the dark current in the forward direction which corresponds to negative potential at the Al electrode varies exponentially with voltage. At higher voltages, two distinct regions of ohmic and SCL conduction limited by a discrete trapping level are determined. Traps with a density of 3.85
1022 m3 located at 0.58 eV above the valence band edge have been observed. The thickness dependence in the square-law region has been found to confirm the d3 law. Values of conversion efficiency as high as 11.3% and open-circuit voltage of 0.77 V have been evaluated from the photo-measurements of J–V characteristic at input power density of 100 mW cm2. Space-charge concentrations and barrier heights have been estimated from the capacitance–voltage (C–V) measurements both in dark and under constant illumination. The linearity of the C2–V dependence is associated with a homogenous distribution of the impurities inside the space-charge region. r 2004 Elsevier B.V. All rights reserved. Keywords: Electric properties; CdTe thin films; Photoelectric measurements; Schottky-barrier

1. Introduction Among the candidates for thin-film solar cells capable of a significant conversion efficiency of light into electricity, cadmium telluride (CdTe) has shown considerable promise [1]. It has the advantage of a nearly ideal bandgap of 1.45 eV [2] for solar terrestrial photoconversion and a short absorption length when compared to grain sizes typically encountered. This latter property reduces recombination at grain boundaries [3], which is a major problem with other polycrystalline materials. As a result, a large fraction of the

photogenerated carriers are generated within the depletion layer allowing more efficient collection. However, the development of low-cost solar cells depends on the exploitation of thin films. Thus, CdTe films require comprehensive electrical characterization in order to study the defect levels and to obtain a better understanding of physical processes in the CdTe films. In particular, spacecharge-limited (SCL) conduction provides useful information on the trap levels in this material. SCL currents have been observed by several workers, generally dominated by traps either at a signal level [4] or within an uniform [5] or

0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.03.249

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exponential [6] distribution in energy. Ou et al. [7] have observed a hole trap at an energy of 0.54 eV above the valence band edge of concentration 1020 m3 in electrodeposited p-type films. In addition, Ismail and Gould [8] have reported a Fermi-level located in the range 0.747–0.711 eV below the conduction band edge and a total trap concentration of (1.63 to 2.08)
1023 m3 in evaporated CdTe thin films, where a powerlaw current density–voltage characteristics was observed. The object of this investigation is to study the current density–voltage (J–V) characteristics for thin films of p-CdTe in Schottky-junction cells. In addition, a study of the capacitance– voltage (C–V) measurements at room temperature will be presented. From these plots one can better recognize the barrier region, which is formed in the devices. X-ray diffraction analysis is used to obtain information on the structural, preferred orientation and microcrystallite size of CdTe films.

2. Experimental details Thin films of p-CdTe were fabricated by vacuum evaporation at a pressure of 104 Pa on cleaned glass substrate coated with an evaporation film of gold (Au), which formed an ohmic contact with pCdTe material. Deposition rate (R) was 0.5 nm s1 and film thickness in the range from 200 to 700 nm was measured during deposition using a conventional quartz crystal monitor. The substrate temperature (Ts) was maintained constant at 670 K using a thermocouple in direct contact with the substrate. In order to complete the fabrication of the sandwich cell, a metal film of aluminum (Al) was vacuum-vapor-deposited onto p-CdTe film (to produce a so-called Schottky-barrier). The Al electrodes of thickness 50 nm were evaporated from a tungsten spiral with a rate of 0.8 nm s1 at room temperature. Appropriate masks were used to obtain the suitable configuration of the Al/pCdTe/Au sandwich cell. Most of the completed devices had an active area of 8.7
105 m2. The stability in air of the Al electrodes was tested by measuring the surface conductivity with gold

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contacts (back electrode). The film structure was examined by X-ray diffraction using a JEOL diffractometer type 60 PA with Ni filtered Cu-Ka radiation at 35 kV and 15 mA. A slow scanning speed of 2 min1 was maintained to provide a long-time constant. The current through the cell was determined using a Keithly 614 electrometer. The forward bias corresponding to the Al electrode was negative. Electrical measurements were performed over the temperature range 298–368 K. The temperature was measured directly by means of NiCr/Ni–Al thermocouple mounted in close proximity to the sample of interest connected to a Keithley 871 digital thermometer. Optical exposure was carried out with light from a tungsten halogen source, focused onto the front face of the cell through the semitransparent aluminum fingers. A water filter was used to eliminate the large infrared component from the light source. The intensity of the incident power was 100 mW cm2, which was measured using a thermopile connected to a luxmeter (Pasco Scientific, model 9152 B). Capacitance–voltage measurements were performed at a frequency of 1 kHz using a LCR meter (Stander Research System, model SR 720). It should be mentioned that all the measurements in the present work were carried out in air ambient.

3. Results and discussion X-ray diffractograms of the CdTe films of thickness in the range of 200–700 nm are shown in Fig. 1. The main features of the diffraction patterns are the same but only the peak intensity is varied. As the film thickness increases, the diffracted intensity increases due to the growth of the materials incorporated in the diffraction process. However, in all cases the intensities of the (2 2 0) and (3 1 1) peaks were extremely low in comparison with the (1 1 1) one. This indicates a preferential orientation of the microcrystallites with the /1 1 1S direction perpendicular to the substrate. The crystallite size (CS) could be calculated using Scherrer’s formula CS ¼kl=b cos Y;

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where the constant k is a shape factor usually E1, l the wavelength of the X-ray (0.154 nm), Y the Bragg’s angle and b the corrected FWHM. Analysis of the results showed that the films were polycrystalline with a crystallite size ranged between 79 and 125 nm. A typical dark J–V characteristic of Al/p-CdTe Schottky-barrier cells, recorded at 298 K, is shown in Fig. 2. The characteristic was definitely of the diode type, with the forward direction corresponding to the negative potential on the Al electrode. All the measured cells showed a strong rectifying behavior. The rectification factor (forward bias current/reverse bias current) at 1.4 V was ranging from 55 to 65. The diode junction resistance, Rj, is equal to dn/dj, which can be determined from the J–V curves. Typical experimental results of Rj against V are shown in Fig. 3 for an Al/p-CdTe Schottky cell. At sufficiently high forward bias the junction resistance approaches a constant value. This value is the series resistance, Rs,. On the other hand the junction resistance is also constant, at sufficiently high reverse bais, which is equal to the diode shunt resistance, Rsh. The value of Rs varied from 30 to 50 O and was a function of the CdTe film thickness, whereas Rsh had values between 3.4 and 3.9 kO. However, cell series resistance is an important factor in improving cell performance and design. Determination of its magnitude is 10

T=298 K 8

-2

J (mA m )

6

4

2

0

Fig. 1. X-ray diffractograms of CdTe film with different thicknesses (Ts=670 K, R=0.5 nm s1).

-2 1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

-2.0

-2.5

V (V)

Fig. 2. Dark J–V characteristic of Al/p-CdTe Schottky-barrier cell at both forward and reverse bias. The forward bias corresponds to the Al being negative.

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Fig. 3. Junction resistance against applied voltage for an Al/pCdTe Schottky-barrier cell.

10

Fig. 5. Semi-logarithmic plot of forward bias of J–V response at room temperature for an Al/p-CdTe Schottky-barrier cell.

with the thermionic emission function [10].

0

J ¼ Js ½expðqV =mkTÞ  1;

368 K 10

-2

348 K 331 K 10

-3

313 K 298 K

10

ð1Þ

-1

-2

J (Am )

10

299

-4

10

-1

10

0

10

1

V (V)

Fig. 4. Forward J–V plots of Al/p-CdTe Schottky cell at different temperatures.

necessary to continue device improvement, especially for devices exposed to high light intensities. The strong dependence of Rs on the thickness indicates that the bulk resistivity of CaTe is a dominant contributor to Rs [9]. Fig. 4 shows a set of current density–voltage (J–V) curves at different temperatures ranging from 298 to 368 K. Semi-logarithmic plot of the forward current versus applied voltage at room temperature is shown in Fig. 5. This figure shows that the current at low voltages (Vp0.2 V) varies exponentially with voltage. This behavior accorded

where q is the electronic charge, m the diode quality factor, k the Boltzmann’s constant, T the absolute temperature and Js and m may be readily determined from the curve shown in Fig. 4 together with Eq. (1). The mean values of Js and m for three different cells were (3.170.1)
105 Am2 and 1.9170.05, respectively. These devices were nonideal in showing a diode quality factor m>1, which may be attributed to the recombination of electrons and holes in the depletion region and also to the increased effect of the diffusion current on increasing the applied voltage [11,12]. In the low-voltage region, the applied voltage seems to be dropped completely across the Schottky-junction. At higher voltage levels, there were two distinct regions for each characteristic shown in Fig. 4. These experimental curves display an ohmic region (J p V) up to field strength of D2
106 V m1. Above this value a non-ohmic region follows, with J p V2. Such power dependence suggests that J is an SCLC and permits the utilization of SCL theory in the J–V analysis; thus it is possible to recognize the effect of traps on the conduction current, which is dominated by a discrete trapping level [13].

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If the ohmic conductivity is assumed to be due entirely to holes, the expression for the characteristic is [14] V ð2Þ JO ¼ P0 qm ; d where P0 is the concentration of thermally activated holes in the valence band, m the hole mobility and d the thickness of the sample. For the simple case of a single discrete-trapping level, the current density, JSCL, is given by [13] JSCL ¼

9 NV V 2 er e0 m expðEt =kTÞ; 8 Nt d 3

ð3Þ

where er is the permittivity of the p-CdTe, NV is the effective density of states in the valence band and Nt is the density of trapping levels situated at an energy Et above the valence band edge. The quantity (NV/Nt) exp (Et/kT) is normally denoted by y, the trapping factor, which is defined by the free-to-trapped charge ratio. Fig. 4 and Eq. (3) yield a value of y=3.6
109 at room temperature. It is evident from Eq. (3) that the plot of log J vs. 1/T shown in Fig. 6 at different voltages should yield straight lines of slope identical to that of the ohmic conduction. The activation energies for both ohmic and SCL regions were 0.5870.03 eV.The activation energy in the SCL region corresponds to a single dominant hole trapping level situated above the valence band

10

0

V=0.3 V (Ohmic region) V=1.4 V (SCL region) V=2.0 V(SCL region)

-2

J (Am )

10

10

10

10

-1

-2

-3

-4

2.6

2.8

3.0 3

(10 / T ) K

3.2

3.4

-1

Fig. 6. Semi-logarithmic plot of J versus 1/T at different voltages.

edge. That the activation energies in the ohmic and SCL regions are the same adds further support to the hypothesis of the presence of a single dominant hole trapping level associated with an acceptor level at the same energy separation from the edge of the valence band [15]. It should be mentioned that other trapping states may also be present, but their density is such that they are incapable of dominating the statistics. Values of (mNV/Nt) at different voltages in square-law region, which have been obtained by extrapolating each of the lines in Fig. 6 to 1/T=0, have been found to be approximately identical. In order to estimate values for the hole trap parameters, values of m=8
102 m2 V1 s1, NV=8.65
1023 m3 [16] and er=8.25 [8] are assumed. In addition, according to Eqs. (2) and (3) the temperature-independent-transition voltage, Vt, at which the current converts from ohmic to SCL behavior, is given by   9 q 2 P0 Vt ¼ d : ð4Þ 8 e0 er y This independent behavior implies a sample that is extrinsic [6]. The concentration of thermally activated holes, P0, could be calculated using Eq. (4); its value is found to be D5.84
1012 m3 at room temperature. Thus, an analysis of the temperature dependence of ohmic and SCL currents, which is associated with the extrinsic behavior yields a total trap concentration NtD3.85
1022 m3 located at 0.58 eV above the valence band edge. These values for the CdTe samples are in good agreement with values of Nt=1.08
1022 m3 and Et=0.61 eV calculated by Dharmadhikari [17], and also with values of Nt up to 3
1022 m3 and Et=0.56 eV determined by Basol and Stafsudd [18]. Fig. 7 shows the dependence of current density on sample thickness at room temperature for both ohmic and SCL regions. Each point corresponds to a different specimen. The ohmic dependence (slope D1) indicates the good contacts that had been applied. The slope of D3 in the square-law region verifies that single carrier SCL conduction dominated by a single trap level has occurred. It should be mentioned that the values of y are found to decrease from 7.2 to 1.35
109 as the film

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-1

V=0.35 V V=1.50 V SCL Current Slope=-3

T= 298 K

-2

-2

J (A m )

10

10

10

-3

Ohmic Current Slope=-1

-4

10

-7

d (m)

10

-6

Fig. 7. Thickness dependence of ohmic and SCL current densities for CdTe thin films at applied potentials of 0.35 and 1.5 V for ohmic and SCL currents, respectively.

Fig. 8. J–V characteristic of Al/p-CdTe Schottky-barrier cell under illumination of 100 m W cm2. Area=0.87 cm2.

thickness increases from 200 to 700 nm. This suggests an increase in trap concentration, Nt. The increase of Nt cannot be interpreted in terms of the change of the crystallite size. This is because the size increases and consequently the grain boundary decreases as Nt increases. Therefore, the structural parameters, which can be correlated with Nt is the preferred orientation [16]. A typical J–V curve of Al/p-CdTe Schottkybarrier cell obtained under illumination of 100 mW cm2 is shown in Fig. 8. Analysis of the results provides the characteristic photovoltaic

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parameters, such as the short-circuit current density, Jsc, the open-circuit voltage, Voc, the fill factor, FF, the maximum power output of the cell, Pmax., and the power conversion efficiency, Z. Their values are found to be 21.25 mA cm2, 0.77 V, 0.69, 11.28 mW cm2 and 11.3%, respectively. It should be noted that the quoted Z value refers to an engineering efficiency and that no corrections are made for reflected or transmitted lights. The value of Z is found to be lower than previously reported values (up to 15%) for cells incorporating CdTe Schottky barrier devices [19]. However, a lower value of cell series resistance is an important factor in improving the cell performance and design. In spite of the Al/p-CaTe Schottky barrier devices producing a higher value of open-circuit voltage (0.77 V) than other Schottky devices [19], it remains paramount to improve the photovoltaic properties further before commercial application of a large-area Al/p-CaTe solar cell becomes feasible. It should be mentioned that the J–V characteristics are studied in both vacuum and air ambient. It is found that these curves showed a small rectifying effect in vacuum even though two dissimilar electrodes are used. Therefore, the ambient plays an important role both in the dark electrical behavior and in the photovoltaic effect. Consequently, rectifying behavior and the photovoltaic effect are both found to be particularly significant when the cells are investigated in air ambient [20]. Ponpon and Siffert [21], for example, fabricated Au/ n-Si devices by vacuum evaporation of Au and measured the electrical parameters of the resulting cells, while they were still in vacuum (106 Torr). They found these junctions were not rectifying and showed negligible photovoltaic response before exposure to air. In addition, no correlation was observed between Voc and the work function of Au. On the other hand, when the cells were exposed to air they, then, developed the electrical properties generally seen by other workers [22]. Moreover, Voc seemed to be correlated with the work function of the semitransparent metal electrode. Therefore, it is not surprising that the studies of illuminated J–V curves in air ambient for Al/p-CdTe devices showed a large value of Voc up to 0.77 V even though the junctions

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exhibit thermionic emission that dominated the conduction. Fig. 9a shows the dark C2–V characteristic of the Al/p-CdTe Schottky-barrier cell measured at 298 K. The forward direction corresponds to the Al electrode being negative. It can be seen from this figure that C2 increases linearly with the applied voltage. The capacitance is examined with aid of the used Schottky relation [23]:
2 Vb  Va  kT=q C 2 ¼ ; ð5Þ qer e0 NA2 where N is space-charge density, Vb the diffusion potential, Va the applied voltage and A the effective area of the device. From Eq. (5) it is clear that the plot of C2 vs. Va should give a straight line whose intercept on the voltage axis given by Vb+(kT/q) and which has a slope 2/qere0NA2. The barrier height of the Schottky diode, fb, is related to Vb by the following Eq. (6):   kT NV 1 þ ln fb ¼ Vb þ  Df; ð6Þ q N where Df is a relatively small term arising due to the image force lowering of the barrier height, and can be neglected. The results shown in Fig. 9a are interpreted in terms of the Schottky-barrier analysis described above. The derived values of N, Vb and fb are found to be 4.33
1023 m3,

T=298 K

0.3

(a)

-2

14

(C x10 ) F

-2

0.4

0.2

(b) 0.1

0.0 -0.8

-0.6

-0.4

-0.2

Forward Direction

0.0

0.2

0.4

Reverse Direction

V (V) 2

Fig. 9. C –V characteristics of Al/p-CdTe devices at 1 kHz (a) in the dark, and (b) under illumination of 100 mW cm2. Forward bias corresponds to Al being negative.

0.63 eV and 0.673 eV, respectively. The capacitance of the device (C0) at zero bias is measured and found to be D180 nF, which corresponds to a thickness, o, of the depletion region (o=e er A/C0) of 36 nm. The effect of light on C2–V plot is presented in Fig. 9b at input power density of 100 mW cm2. The derived values of N, Vb, C0, o, fb are also found to be 8.7
1023 m3, 0.66 eV, 246 nF, 26 nm and 0.69 eV, respectively. Comparing these results with those obtained from the dark C2–V characteristic shown in Fig. 9a, the capacitance of the device is found to increase, o decreased and N increased at an input power density of 100 mW cm2. However, the presence of a large value of N caused a shrinkage of o, which may severely limit the performance of the Schottkybarrier photovoltaic cells [24].

4. Summary and conclusions Current density–voltage measurements on thin films of p-CdTe particles in Schottky-junction cells have been studied. At low voltages, the current varies exponentially with voltage, whereas at higher voltages two separate regions of ohmic and SCL currents are presented. The latter process has been observed with a single dominant trap level. X-ray studies of CdTe polycrystalline films prepared by thermal evaporation show a /1 1 1S fiber texture and a crystallite size in the range 79– 125 nm.The increase in the trapping concentration is correlated with the ascending degree of preferred orientation of the highest atomic density plane. The Al/p-CdTe Schottky cells exhibit a photovoltaic response showing a conversion efficiency as high as 11.3% and an open-circuit voltage of 0.77 V under illumination by light with a power density of 100 mW cm2. Information concerning the barrier width, built-in potential and spacecharge density can be successfully obtained by analyzing the capacitance–voltage characteristics. The main conclusion obtained from C–V measurements is that the increase in space-charge density due to illumination is a major concern of the possible limiting factor for the performance of the Al/p-CdTe photovoltaic Schottky cells.

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