p-Si heterojunction solar cells

494

Journal of Crystal Growth 61(1983) 494—498 North-Holland Publishing Company

PHOTOVOLTAIC AND ELECTRICAL PROPERTIES OF n-CdS/p-Si HETEROJUNCTION SOLAR CELLS Toshikazu SUDA and Akio KUROYANAGI Department of Electronics, Institute of Vocational Training, 1960 A ihara, Sagamihara, Kanagawa 229, Japan

and Shoichi KURITA Department of Electrical Engineering, Keio University, 3-14-1 I-Iiyoshi, Kohoku, Yokohama 223, Japan

Received 21 October 1982

Heterojunction solar cells of CdS: In/Si have been fabricated by electron-beam evaporation of CdS, and their electrical and photovoltaic properties are studied. The forward I—V characteristics of these cells are in good agreement with the tunneling model. To obtain an optimum condition for cell fabrication several parameters have been varied; such as substrate temperature, amount of In doping in CdS, and annealing temperature in various ambients (H 2, N2 and S vapor). The CdS: In/pt p-Si cell shows no degradation at room temperature in air over 30 months, whereas the CdS: In,Ag/p~p-Si cell shows rapid degradation after 3 months. From the DLTS measurements using a computer controlled digital system five electron traps have been measured in an evaporated CdS: In/p f-Si cell.

1. Introduction

2. Experimental

Recently interest has been shown in photovoltaic properties of CdS/Si heterojunctions using CdS films [1—3]because of their potential application to low cost solar cells. There is still insufficient understanding of the electronic, photovoltaic, and defect properties for these devices. Investigations into photovoltaic and electrical properties of n-CdS on p~p-Siand ptSi were carried out to make their fundamental properties clear. One of the advantages of using heterojunctions is that the surface recombination of minority carriers is reduced since the junction in the narrow-gap semiconductor can be located far from the surface. This advantage, however, can be lost if the interface region between two semiconductors has a high density of traps or defects, which may act as recombination centers. In this paper deep level transient spectroscopy (DLTS) measurements [4—7] were also carried out in order to investigate the effect of these defects.

2.1. Preparation of heterojunctions

0022-0248/83/0000—0000/$03.00

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Cadmium sulfide films doped with In were grown by electron-beam evaporation of CdS (5 nines) [8] onto either chemically etched p~p-Si (p-epitaxial layer with 4—S ~2cm resistivity and 15 ~sm thickness) or p~-Si(0.007 f~cm) substrates with (Ill) orientation. Indium doping [9] into CdS films was performed by sintering CdS powder and 1n2(SO4)3 at 600°Cfor 30 mm in N2 in order to reduce the resistivity of the films. Prior to evaporation the substrates were cleaned by electron bombardment at 300 V for 20 mm, and then the evaporation was accomplished at a pressure of less than 1.0 x IO~Torr. Several fabrication parameters such as substrate temperature, amount of In doping, annealing atmosphere, etc. were varied to obtain the best conversion efficiency. Ohmic contacts were made by evaporating In and Au onto CdS and Si respectively. The In electrode on the

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Photovoltaic and electrical properties of n - CdS/p - Si solar cells

CdS film had a grid structure to allow illumination. An anti-reflective (AR) coating was not deposited.

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~~t~:_.44.

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2.2. DLTS measurement

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A computer controlled digital system was used for measuring DLTS spectra and processing the obtained data. The basic feature is the separation of data acquisition and signal processing into two consecutive steps. The capacitance transients were measured using a Boonton 72B, and the outputs were processed by a specially-designed signal averager which can average the transient data 2—1024 times. The entire transients at each temperature were stored on a flexible disk for the analysis. A micro-computer (Hitachi H68-TR) was used to control the measurement instruments, and a personal computer (Hitachi MB6890) was used to process data.

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3. Results and discussion

0.2

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0-6 FORWARD

3.1. Electrical and photovoltaic properties of nCdS/p ~p-Si and n-CdS/p k-Si

I

06

•

•

0.8

I

1.0

1 2

VOLTAGE ( V

Fig. 2. Typical dark forward current (I) versus voltage (V) characteristics of an n-CdS:In(0.5 mole%)/p~p-Si cell heat

An analysis of X-ray diffraction for CdS evaporated films grown on Si at temperatures

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400

I

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500 600 700 800 900 1000 11001200 WAVELENGTH (nm)

Fig. I. Typical spectral response of short-circuit current ~ for an n-CdS: In(0.5 mole%)/p~ p-Si cell heat treated in N2 at

350°Cfor 5 mm without AR coating.

above 100°Cshows only the diffraction from (002) and (004) planes. This indicates that the c-axis is along the normal to the plane of polycrystalline CdS films (hexagonal). From capacitance (C) versus voltage (V) measurements, a good linear relation in C 2~ characteristics was obtained for both types of sample (n-CdS/p~p-Sior n-CdS/p~-Si),which indicates an abrupt junction with a built-in potential of 0.90—1.0 V. The carrier concentrations of the CdS films were estimated from C— V measurements and were in the range of 3 X l0’~to 5 X 1016 cm3 depending on the In doping. A typical spectral response of the short-circuit current I~ ts shown in fig. I for an nCdS:In/p~p-Sicell heat treated in N 2 at 350°C for 5 mm. The curve shows good band-pass behavior (window effect) of heterojunctions formed by a CdS window on a Si absorber between 500 and

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Photovoltaic and electrical properties of n CdS/p Si solar cells 30

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450°C I

0

100

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200

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500°C

300

0

TEMPERATURE ( K )

0

Fig. 3. Temperature dependence of open-circuit voltage V . and short-circuit current I for an n-CdS: In/p + p-Si cell heat 2).N2 at 350°C for 5 treated in mW/cm

mm

01

0.2

0.3

0.4

VOLTAGE ( V Fig. 4. Effect of heat treatment in a N

2 atmosphere for 5 mm simulated AM1 illumination mW/cm2). on I—V characteristics for (88 an n-CdS: In/p~ p-Si cell under

under AMI illumination (88

1100 nm corresponding to the approximate bandgaps of the respective semiconductors. The spec-

which is a standard tunneling expression with a 32 V~,$ 0.074 K-1, and jo I 00exp($T) 1.3 2 at room temperature. The cells X l0~ A/cm heat treated in H2 showed the same forward I—V characteristics as those heat treated in N2. Fig. 3 indicates the temperature dependence of open-circuit voltage V~and short-circuit current ‘Sc for an n-CdS: In/p~p-Sicell. The value of extrapolated to 0 K is approximately equal to 1.0 V, the built-in potential [10]. Since the open-circuit voltage is given by =

=

=

=

tral heat response at shorter by treatment in Hwavelengths was improved 2, N2, and S vapor. The structures in fig. 1 can be explained by the interference effect in the CdS film, Typical dark forward current (I) versus voltage (V) characteristics are shown in fig. 2 for an nCdS: In/p~p-Sicell heat treated in N2 at 350°C for 5 mm. The current for this cell can be described by I

=

~

exp($T) exp(aV),

~

(1)

(1/a) ln(ISC/IO),

(2)

Table I Photovoltaic properties of2)evaporated without AR n-CdS: coating In(0.5 mole%)/p~p-Si cells heat treated in various atmospheres under simulated AMI illumination (88 mW/cm Sample I~ FF CdS Cell area Heat treated No. (V) (mA/cm2) (%) thickness (mm2) in (pm) 6l2N

0.29

29.8

0.43

4.2

0.4

25

N

606N 606H 428N 428H 428S

0.30 0.30 0.37 0.35 0.37

25.1 27.4 24.2 25.3 21.7

0.54 0.48 0.52 0.47 0.62

4.6 4.5 5.2 4.8 5.6

1 1 7 7 7

21 20 23 25 6

2 at 350°C N2 at 350°C H2 at 400°C N2 at 350°C H2 at 400°C S at 350°C

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Photovoltaic and electrical properties of n - CdS/p - Si solar cells

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I

I

1

10

20

30

TIME (months)

0.10’

I 50

Fig. 5. Relative power degradation as a function of time in air for (a) an n-CdS:In/p~p-Si cell and (b) an n-CdS:In,Ag(0.2

I 100

mole%)/p~ p-Si cell.

I 150

I

I

I

I

200

250

300

350

TEMPERATURE

C K

Fig. 6. Typical DLTS spectrum for an n-CdS: In/p t5i cell heat treated in H

2 at 400°Ctaken at a rate window of 137

a comparison with the tunneling model of eq. (1) shows V,~c to vary linearly with temperature with a slope of (/3/a). The measured slope from fig. 3 of 0.0022 is in good agreement with (/3/a) 0.0023 obtained from the tunneling model. In order to obtain an optimum condition for cell fabrication, several parameters were varied; such as substrate temperature, amount of In doping, and heat treatment in various atmospheres (H2, N2 and 5). The substrate temperature in evaporation of CdS was varied from 100 to 400°C, and the maximum efficiency was obtained at 350°C.The amount of indium in CdS was varied from 0 to 1.0 mole% when evaporated onto a Si substrate at a fixed temperature of 350°C. Approximately 0.5 mole% In in the CdS prior to evaporation gave the best efficiency. The temperature dependence of heat treatment in N2 on photovoltaic parameters was investigated over the temperature range of 300—500°C and is shown in fig. 4 for n-CdS In/p~p-Si. From this result the optimum temperature of 350—400°Cwas obtained for a 5 mm heat treatment. The effect of heat treatment in H2 showed a similar effect as in N2, whereas heat treatment in S vapor increased the resistivity of the CdS film. The photovoltaic parameters of these heterojunctions heatintreated various atmospheres are summarized table in 1. The rather low efficiency compared with those in refs. [2] and [3] is mainly due to the thin p-epitaxial layer (15 ~sm)of the p~p-Sisubstrate. The CdS : In/Si cells showed no significant de—

=

s_I.

gradation in conversion efficiency at room temperature in air with a humidity of 65—75% over the 30 months as shown in fig. 5. In contrast to this property, just as in the case of CdS/Cu2S solar cells [11], CdS: In/Si cells doped with Ag (0.2 mole%) in the form of AgNO3 showed rapid degradation of the efficiency after 3 months in air, due to the deterioration of the fill factor. Here, the CdS was doped with Ag because it enhances recrystallization of CdS films [12]. This degradation may be attributed to Ag ion diffusion like Cu in CdS/Cu2S cells [13,14] rather than an effect caused by moisture [11], since Ag results in deep levels in Il—VI compounds [15]. 3.2. DLTS measurement for n-CdS.’ In/p tSi

An activation energy of trap ~ E~and a capture cross section ~ can be obtained using the equation derived from the principle of detailed balance aKV)NC exp( —~E~/kT), (3) where (v) is the thermal velocity and N. the effective of states the conduction 2 density dependence of in (v)N. was takenband. into The T in the determination of ~ E and a. We account can also obtain the trap concentration using the simplified equation

e~=

N 1

=

2[~C(0)/CO](ND

—

NA),

(4)

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T Soda ci al.

/

Photovoltaic and electrical properties of n - CdS/p - Si solar cells

Table 2 Electron trap parameters obtained from DLTS measurement for an n-CdS: In/p tSi cell

—

Trap

Trap depth

No.

E

ment temperature of 350—400°C,and with an In doping of 0.5 mole% in CdS. The spectral response of 1~ showed good band-pass behavior between 500 and 1100 nm after heat treatment. The

Capture cross

Trap

section 2) (cm

concentration (cm3)

conversion CdS: In/Si efficiency cell showed at room no degradation temperature inin the air over 30 months, whereas the CdS: In,Ag/Si cell

A

5 E1 (eV) 0.25

8.8xlOI2

showed rapid degradation after 3 months. The

B C F D

0.31 037 0.63 0.46

7.2x l0’~ 2.3x l0~~ 6

3.4x 1012 3.8x 103 2.7x 1013

DLTS controlled puter measurements digital were system, performed and five usingelectron a corn-

2.4Xl0~ 4.9X10’5

6.l><

3.6xl0~13

10I2

with L~C(0)being the magnitude of the capacitance transient just after the trap filling pulse (t 0), C0 the capacitance at quiescent reverse bias, and ND NA the compensated carrier density. The DLTS spectrum taken in the majority carrier4-Si mode [4,5] cell heat is shownininH fig. 6 for an n-CdS: In/p treated 2 at 400°C.Note that the DLTS signal is due to the lightly doped CdS layer since the Si substrate is degenerated. The physical quantities obtained from the analysis of DLTS are listed in table 2. Although there are a few reports of DLTS results on single-crystalline CdS [16,17] and on chemical vapor transport CdS [18], no DLTS results have been reported on evaporated CdS films. The electron trap C is especially in good agreement with the level of 0.36 eV in ref. [16] considering its cross section and concentration together.

traps were identified CdS: In/p tSi cell.

in

the

evaporated

References =

—

[1] H. Okimura and R. Kondo, Japan. J. Appl. Phys. 9(1970)

[2] 274. F.M. Livingstone, W.M. Tsang,Dl0 A.J.(1977) Barlow, R.M. Dc La Rue and W. Duncan, J. Phys. 1959. [3] C. Coluzza, M. Garozzo, G. Maletta, D. Margadonna, R. Tomaciello and P. Migliorato, AppI. Phys. Letters 37 (1980) 569. [4] DV. Lang, J. AppI. Phys. 45 (1974) 3023. [5] DV. Lang, J. AppI. Phys. 45 (1974) 3014. [6] G.L. Miller, J.V. Ramirez and D.A.H. Robinson, J. AppI. Phys. 46 (1975) 2638.

[7] H. Lefèvre and M. Schulz, Appl. Phys. 12 (1977) 45. [8] T. Suda and S. Kurita, J. AppI. Phys. 50 (1979) 483.

[9] L.D. Partain, G.J. Sullivan and CE. Birchenall, J. AppI. Phys. Tsai, 50 (1979) [10] M.J. AL. 551. Fahrenbruch and RH. Bube. J. AppI. Phys. 51(1980) 2696.

[II] AG. Stanley, in: Applied Solid State Science, Ed. R. Wolfe (Academic Press, New York, 1975).

4. Conclusions

[12] T. Suda and S. Kurita, Trans. lEE Japan 95A (1975) 341. [13] ER. Hill and B.G. Keramidas, Rev. Physique AppI. I

The n-CdS: In/p~p-Sior n-CdS: In/p ~-Si heterojunctions were prepared by electron-beam evaporation of CdS. They showed abrupt junctions with a built-in potential of 1.0 V. The forward I—V characteristics of the cells were in good agreement with the tunneling model. The maximum efficiency of these cells was obtained at the substrate temperature of 350°C,with a heat-treat-

[14] ER. Hill and B.G. Keramidas, IEEE Trans. Electron

(1966) 189. Devices ED-l4 (1967) 22.

[15] T. Suda, K. Matsuzaki and S. Kurita, J. AppI. Phys. 50 (1979) 3638. [16] C. Grill, G. Bastide, G. Sagnes and M. Rouzeyre, J. AppI. Phys. 50(1979)1375. [17] M. Hussein, G. Lieti, G. Sagnes. G. Bastide and

M

Rouzeyre, J. Appl. Phys. 52 (1981) 261. [18] P. Besomi and B. Wessels, J. AppI. Phys. 51(1980) 4305.