Characterization of thin film CdS-CdTe solar cells

Characterization of thin film CdS-CdTe solar cells

Solar Cells, 31 (1991) 23-38 23 Characterization of thin film CdS-CdTe solar cells Vijay P. Singh, Hagay B r a f m a n and J i t e n d r a M a k w a...

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Solar Cells, 31 (1991) 23-38

23

Characterization of thin film CdS-CdTe solar cells Vijay P. Singh, Hagay B r a f m a n and J i t e n d r a M a k w a n a Electrical Engineering Department, The University of Texas at El Paso, El Paso, 7'X 79968 (U.S.A.)

J o h n C. McClure Metallurgical and Materials Engineering Department, The University of Texas at E1 Paso, El Paso, TX 79968 (U.S.A.)

(Received December 21, 1989; accepted April 25, 1990)

Abstract Current-voltage, junction capacitance and optical characteristics of thin film CdS-CdTe cells on sprayed CdS films were measured. These characteristics have some interesting features such as reversal of the polarity of the a.c. short-circuit current and the a.c. open-circuit voltage when a large forward bias is applied across the cell. The reverse saturation current densityjo increases from 5.9)< 10 -9 A cm -2 in the dark to 18.1 )< 10 -8 A cm -2 under "1 sun" illumination. Diode ideality factors are higher than 2.0 and the slope a of log I vs. V curve is almost temperature independent. The zero-bias depletion layer width is 1.9 p.m. The experimental results are interpreted by a model which proposes a highly compensated layer in CdTe and a high space charge layer in CdTe next to the CdS--CdTe interface. The origin of the high space charge layer is thought to be the ionization of a deep trap level at energy ET below the conduction band edge. For our calculations, we have used ET = 0.45 eV.

1. I n t r o d u c t i o n T h i n film C d S - C d T e h e t e r o j u n c t i o n s o l a r c e l l s a r e a m o n g t h e m o s t promising photovoltaic devices for low-cost large-area terrestrial applications [1, 2 ]. T h e y a r e , h o w e v e r , k n o w n t o e x h i b i t n o n - i d e a l effects s u c h a s " c r o s s over" and voltage shifts b e t w e e n dark and light c u r r e n t - v o l t a g e (I-V) chara c t e r i s t i c s . T h e s e effects l e a d t o a r e d u c e d c e l l efficiency. An u n d e r s t a n d i n g o f j u n c t i o n t r a n s p o r t in t h e s e d e v i c e s is e s s e n t i a l f o r a n a l y z i n g t h e s e e f f e c t s and the loss mechanisms associated with them. Several workers [ 3 - 1 7 ] have studied C d S - C d T e cells p r e p a r e d by different t e c h n i q u e s . A r e v i e w o f C d S - C d T e cell r e s e a r c h h a s b e e n g i v e n b y B u b e e t al. [18]. T h e r e a r e s u b s t a n t i a l d i f f e r e n c e s in t h e c h a r a c t e r i s t i c s a n d m o d e l s p r e s e n t e d by different groups. Some of these differences are p r o b a b l y due to d i f f e r e n c e s in f a b r i c a t i o n p r o c e d u r e s w h i c h c a n r e s u l t in w i d e v a r i a t i o n s in film a n d d e v i c e p r o p e r t i e s . In t h i s p a p e r , we r e p o r t t h e j u n c t i o n t r a n s p o r t c h a r a c t e r i s t i c s o f t h i n film C d S - C d T e c e l l s f a b r i c a t e d a t P h o t o n E n e r g y Inc. b y d e p o s i t i n g C d T e o n t o s p r a y e d CdS films. F u r t h e r d e t a i l s o n p r o c e d u r e s u s e d in f a b r i c a t i n g t h i s t y p e o f cell c a n b e f o u n d in refs. 12 a n d 19. T h e s e

ELsevier Sequoia/Printed in The Netherlands

24 cells exhibit some rather interesting characteristics which have not so far been reported in the literature. Tunneling appears to be an important mechanism in the junction transport in the forward-bias region. Yet the measured depletion layer widths at zero bias are typically in excess of 1 ~m. The effective diode reverse saturation current under illumination is larger Coy three orders of magnitude) than its value in the dark. At present, there does not appear to be any single working model for CdS-CdTe cells which explains all its characteristics. In this paper, we present a preliminary model which describes the junction behavior of Photon Energy Inc. cell in terms of a highly compensated CdTe layer and a high space charge layer next to CdS.

2. Experimental procedures Measurements were per f or m ed on CdS/CdTe devices of 0.3 cm 2 area. The cell structure consisted of glass/tin oxide/cadmium sulfide/cadmium telluride/electrode with light incident from the glass side [12, 19]. Curr en t- v o ltag e characteristics were obtained by an automatic tester driven and monitored with an HP 9845 desk-top computer. " l sun" illumination was simulated with a GE-ENX lamp with appropriate filters. The t em perat ure of the cell was varied in the range from - 1 1 2 to 80 °C by placing it in a liquid-nitrogen-cooled Dewar with a window for illumination. Neutral density filters were used for measurements at various light levels. An HP4271B digital L C R meter was used for capacitance measurements; the frequency was 1 MHz and the signal strength 20 mV. The optical response at a fixed wavelength was measured by exposing Che cell to low intensity (2 mW c m - 2 ) c h o p p e d monochromatic light from a He--Ne laser (Spectra Physics model 157). The c h o p p e r frequency was 20 Hz. The cell could be biased in the forward or reverse region and its a.c. short-circuit response Ms¢ was obtained as a voltage across a 1 ~ resistor in series with the cell. The voltage across the 1 12 load was amplified with a Keithley 103A nanovolt amplifier and monitored on an oscilloscope (HP1220A). The a.c. open-circuit voltage nVo¢ was obtained by replacing the 1 12 load by a 10 000 12 load. The voltage across the 10 000 12 load was measured directly on the oscilloscope; no amplification was necessary.

3. Experimental results The I - V characteristics of cell A were measured in the dark, at seven different temperatures. Log I vs. V plots (after series resistance correction) showed two distinct regions. The slope in the "low voltage" region (V< 0.5 V) was smaller than the slope in the "high voltage" region (V> 0.5 V). The high voltage linear segment of the I - V characteristic was modeled by j =3"0 exp(aV) =Jo exp(qV/AKT). The diode ideality factor A and a were obtained

25

from the slopes of these plots, and the reverse saturation current density Jo was evaluated from the intercept with the V = 0 axis. a, Jo and A are tabulated in Table 1. Similarly a, Jo and A values under "1 sun" illumination were obtained from the plots of log(I+Isc) v s . V. These are tabulated in Table 2. We n o te that a values are relatively insensitive to temperature, both in the dark and unde r illumination. T h e d i o d e ideaUty factor A is larger than 2 in the dark and in excess of 4 in the light. Values o f j o under illumination are three orders of magnitude larger than the Jo values in the dark. In the dark, Jo increases with increasing temperature; no clear trend is obvious in the light. Also, values of A in the "low voltage" region are higher than the "high voltage" region values shown in Table 1. Furthermore, since A and Jo are partially coupled, the effective value o f j o in the "low voltage" region is higher than Jo in the "high voltage" region. When a strong forward bias is applied to the cell with a d.c. supply (at a fixed voltage) and a fixed series resistor, the cell's I - V characteristic drifts toward a higher current a n d / o r a lower voltage. The drift is larger in the dark than u n d er illumination and continues for several minutes. This suggests an effective increase with time in the loss current (or reverse saturation current) at the junction. As is typical for thin film heterojunction solar cells, superposition is not applicable and the dark and light curves cross. TABLE 1 V a l u e s for a, Jo a n d A for v a r i o u s t e m p e r a t u r e s in t h e d a r k f o r cell A T (°C)

a ( V - ')

Jo (A c m -2)

A

46 33 21 7 - 12 - 17 - 38

17.33 17.31 16.93 16.30 15.53 14.65 14.26

18.00 X 8.52 x 5.89 x 4.77X 3.54x 5.66x 2.37 x

2.09 2.19 2.33 2.54 2.86 3.09 3.46

10-9 10 -9 10 -9 10 -9 10 -9 10 -9 10-9

TABLE 2 V a l u e s for a, Jo, A, Voc, Js¢ a n d efficiency at v a r i o u s t e m p e r a t u r e s in t h e light for cell A T (°C)

a (V - l )

Jo (A c m -2)

46 33 21 7 -12 -17 -38

8.81 8.63 7.70 7.40 7.12 7.13 6.93

30.00 x 21.60 X 18.16 X 12.65 X 11.54x 37.00X 38.34X

10 10 10 10 10 10 10

-6 -6 -6 -~ -6 -6 -6

A

Vo¢ (mV)

J~ ( m A c m -2)

Efficiency (%)

4.12 4.39 5.12 5.59 6.23 6.34 7.11

701 739 769 803 847 849 904

13.9 13.6 13.3 13.3 13.3 14.5 14.4

5.7 5.9 5.9 6.1 6.3 6.7 6.8

26 C a p a c i t a n c e - v o l t a g e (C-V) characteristics of cell A were m e a s u r e d in the dark and u n d e r t h r e e different intensities o f illumination. 1/C e in the dark is p l o t t e d against bias voltage in Fig. 1. Zero bias c a p a c i t a n c e yields a depletion layer width W o f 1.9 t~m. This value o f W is c o m p a r a b l e with the d e p l e t i o n layer width in devices f a b r i c a t e d b y e l e c t r o d e p o s i t i o n t e c h n i q u e s [20]. W is a l m o s t insensitive to r e v e r s e bias voltage. On the a s s u m p t i o n that the d o p i n g c o n c e n t r a t i o n in CdS is m u c h h i g h e r and the d e p l e t i o n layer is mainly in CdTe, the data in Fig. 1 are suggestive o f a low d o p e d or almost intrinsic layer (i layer) in CdTe close to the h e t e r o j u n c t i o n with CdS. The a s s u m p t i o n o f h i g h e r carrier c o n c e n t r a t i o n in CdS is partly justified b y c a p a c i t a n c e - v o l t a g e (C-V) m e a s u r e m e n t s on m e t a l - C d S S c h o t t k y diodes which yield a d o n o r c o n c e n t r a t i o n in CdS in e x c e s s o f 1017 c m -a. W e note, however, that the carrier c o n c e n t r a t i o n in CdS is likely to be altered during CdTe deposition and s u b s e q u e n t processing. Carrier c o n c e n t r a t i o n in the a l m o s t intrinsic CdTe layer is e s t i m a t e d f r o m the slope of 1/C e v s . V data at bias voltages close to zero (Fig. 1). Its value is NA1 = 3.9 × 1014 cm -3 and c h a n g e s little with illumination (for illumination intensities b e l o w 12 m W c m - e ) ; this is illustrated in Table 3. VO1 Was calculated f r o m the e x t r a p o l a t i o n of 1/C e v s . V c u r v e to the 1 / C e = 0 axis. It s h o u l d be n o t e d that Vdl d e c r e a s e s substantially o n illumination. The carrier c o n c e n t r a t i o n NA2 in the region farther away f r o m the CdS interfaces is e s t i m a t e d f r o m the slope o f 1/C 2 v s . V data at larger negative bias voltages. NA2 values are also s h o w n in Table 3. The excessively large values o f voltage i n t e r c e p t Vd2 o b t a i n e d w h e n the 1/C e v s . V c u r v e s in this regime are e x t r a p o l a t e d to the 1/C 2 = 0 axis should b e noted. Obviously, for this cell, the i n t e r c e p t is not the diffusion voltage. I n t e r p r e t a t i o n o f C - V data in t h e s e cells is quite c o m p l e x ; the N^ values d e d u c e d above should be t a k e n as " e f f e c t i v e " estimates.

t

÷-I"

~, .,.}

-i+ + -t- 4- -I. -I- + - 5.B7

4"+-I-+.1. -1+

+ t-

- 4.87 ÷

I

-2

C2 xlO-B Q~-~Icm4

I 3.87

I

-I

VOLTS (V)

>

Fig. 1. C a p a c i t a n c e a s a f u n c t i o n o f b i a s v o l t a g e in t h e d a r k for cell A; AI is t h e a r e a o f t h e cell, a n d C is its c a p a c i t a n c e .

27 TABLE 3 Values for NA~, Vd~, N~ and Vd2 at different intensities for cell A Intensity (suns)

NAz (cm -3)

Vdl CV)

NA2 (cm -s)

V~ Cv')

Dark 0.023 0.050 0.073 0.100 0.121

3.9 x 1014 4.9 X 1014 4.1 × 10 ~4 4.2 × 1014 4.3 × 10 ~4 2.7 × 1014

1.3 1.13 0.77 0.71 0.65 0.68

5.9 x 1015 3.5 X 10 ~5 2.7 X 10 ~5 2.43 X 10 '5 1.98 X 10 ~5 1.9 × 1015

16.4 11.0 7.67 6.4 4.78 3.96

2.SxlO -B

t

a

b

0 -3.0

[]

BIASVOLTAGEIV]

Fig. 2. Capacitance as a function of bias voltage and frequencies in the dark for cell B: (a) kHz; ( b ) f = 1 0 0 kHz; ( c ) f = l MHz.

f=lO

Figure 2 s h o w s the plots o f c a p a c i t a n c e v s . v o l t a g e at t h r e e different f r e q u e n c i e s for cell B w h i c h w a s f r o m the s a m e b a t c h as cell A. The c a p a c i t a n c e d e c r e a s e s as the f r e q u e n c y is increased. This s u g g e s t s a c o n t r i b u t i o n to the j u n c t i o n s p a c e c h a r g e f r o m relatively slow d e e p level states at or n e a r the CdS--CdTe interface. Figure 3 s h o w s the r e s p o n s e o f the a.c. short-circuit c u r r e n t A/s¢ o f cell C to a m o n o c h r o m a t i c low intensity (2 m W c m -2) c h o p p e d light signal tub o f w a v e l e n g t h 6 3 0 nm. The c h a r a c t e r i s t i c s o f cell C are A/so-- 18.3 m A c m -2, Voc = 776 mV a n d a n efficiency o f 8.5%. A/~ in Fig. 3 is the c u r r e n t leaving the p-CdTe contact. T h u s a positive A/~ in f o r w a r d bias implies t h a t the cell is delivering power. The w a v e f o r m o f AL is s h o w n in Fig. 3(a). Figure 3(b) s h o w s A/s¢ w h e n the cell is at zero bias. Figure 3(c), 3(d), 3(e) and 3(f) s h o w the w a v e f o r m s o f the cell A/~ at f o r w a r d bias v o l t a g e s VF o f 0.69 V, 0 . 8 2 5 V, 0 . 8 9 0 V a n d 1.255 V respectively. T h e s e r a t h e r high values of diode v o l t a g e s are due to the v o l t a g e d r o p a c r o s s the internal r e s i s t a n c e o f the cell w h i c h is quite high in the dark. At zero f o r w a r d bias, A/s~ a n d AL w a v e f o r m s are in phase. The m a g n i t u d e o f A/~¢ r e d u c e s as the f o r w a r d bias is i n c r e a s e d until it is a l m o s t zero at VF = 0 . 8 6 0 V. As VF is i n c r e a s e d further,

28 ,~lsc (a.u.) .5 [

"--/

"--/

(e)

(d)

O, -1.5

i,:sE/

L_A

'5L

L_

(c) 1.5 O,

Co)

}' v ; Ll

1.5 O, ~1.5 - -

(a)

2r5

50

1

75

100

t (msec)

Fig. 3. A.c. short-circuit current A/soas a function of forward-bias voltage using a monochromatic laser source (A= 630 nm) chopped at 20 Hz for cell C: (a) input light AL; (b)-(f) output of cell C at a forward bias of (b) 0.0 V, (c) 0.69 V, (d) 0.825 V, (e) 0.890 V and (f) 1.255 V.

A/so r e v e r s e s its p h a s e so t h a t A/s c in t h e d a r k is h i g h e r t h a n it is in the light. In effect, the n e t A/so h a s s w i t c h e d direction. I n c r e a s i n g V~ f u r t h e r also results in a n i n c r e a s e in the m a g n i t u d e of A/so. This is illustrated in Figs. 3(d) a n d 3(e). T h e r.m.s, v a l u e o f A/~¢ as a f u n c t i o n of VF is p l o t t e d in Fig. 4. It s h o u l d b e n o t e d t h a t t h e p h a s e s of A/~ in r e g i o n s I a n d II of Fig. 4 differ b y 180 °. T h e c h a n g e f r o m r e g i o n I to r e g i o n II is n o t always so sharp. S o m e cells s h o w a wide flat r e g i o n o f VF o v e r w h i c h A/~c r e m a i n s at its m i n i m u m v a l u e b e f o r e the c h a n g e in p h a s e ( r e g i o n II) o c c u r s . T h e r e a f t e r , the m a g n i t u d e o f A/~¢ b e g i n s to i n c r e a s e again. All the cells t e s t e d s h o w e d this effect to s o m e degree. T h e r e s u l t s in Figs. 3 a n d 4 are a.c. a n a l o g s of the c r o s s - o v e r effect in thin film h e t e r o j u n c t i o n s o l a r cells. A/s¢ w a v e f o r m s in Fig. 3 w e r e m e a s u r e d as a n a.c. v o l t a g e (at 20 Hz) a c r o s s a 1 ~ r e s i s t o r c o n n e c t e d in series with the cell. T h e 1 ~ l o a d a p p r o x i m a t e d the s h o r t - c i r c u i t c o n d i t i o n f o r t h e cell. Next, t h e 1 12 l o a d

29

~Isc(a.u.) 300.

/

240.

Bo.

I

/

/

/

II

U '

'

'

Li.le

~

'

'

- o J9

....

o:

°

:

~

'o.~

. . . .

i.~

:

VF (volts)

Fig. 4. ~,c as a function of bias voltage for cell C.

was replaced by a 10 000 ~ load which approximated the open-circuit condition for the cell. The voltage waveform 5Vo¢ across the 10 000 l'L load in response to the c h o p p e d light excitation AL was m easured at different values of d.c. bias VF across the cell. The phase reversal in AVocwith increasing VF was not observed in all cells. Only those cells which had a sharp transition between phase I and phase II of A/so (Fig. 4) showed a phase reversal of AVoc. Cell C did not exhibit phase reversal. The r.m.s, value of AVoc as a function of V~ is plotted in Fig. 5. It should be noted that the magnitude of AVoc decreases as the forward bias is increased. Application of reverse bias, on the contrary, increases AVoc by a factor of more than 3, until a m a x imu m at a reverse bias of 3.5 V is reached. This value of reverse bias is close to the breakdown voltage of the diode. Although the data in Figs. 1 - 5 pertain to three different cells, the results are typical of cells fabricated at P h o t o n Energy Inc. The nature of variations depicted in these data is discussed in Section 5 in terms of a model present ed in Section 4 below. The model was developed for understanding and analyzing P h oto n Energy Inc. cells. It may or may not be applicable to CdS-CdTe cells fabricated by other groups. We note, however, that the existence of an intrinsic layer in or next to CdS has been r e p o r t e d by several workers on CdS-based heterojunction solar cells [ 20-23]. It is likely, therefore, that the applicability of model extends to cells fabricated by groups other than P h oto n Energy, Inc.

30

LVoc(mVrms)

I;il I 400.

_41



. . . .

I -3.

.

.

.

.

.

. .

. , :t'.

:

:

:

:

0 [

o. . . . .

1!

'

'

'

I

~

'

.

VF (volts) Fig. 5. A.c. open-c~cmt voltage ~V~ as a function of b ~

voltage using a monochromatic

l a s e r s o u r c e (A = 6 3 0 n m ) c h o p p e d at 2 0 Hz for cell C.

4. M o d e l The I - V characteristics shown in Tables 1 and 2 exhibit diode ideality factors in excess of 2.0. The diode ideality factor in these cells is, in general, higher than those for the CdTe/CdS cells from the Stanford group [15, 16] where the value of A is typically less than 2.0. Also, a values are almost temperature independent in the dark as well as in the light. These data indicate that tunneling is an important junction transport mechanism in this cell. On the contrary, the zero-bias depletion layer width is as high as 1.9 ~m. The multistep tunneling through a wide depletion layer originally p r o p o s e d by Riben and Feucht [24] for Ge/GaAs heterojunctions would require more than 70 tunneling steps for this cell [25]. A more likely mechanism is tunneling across a narrow high space charge layer near the CdS--CdTe interface, followed by recombination. We pr opos e that such a layer could originate from the ionization of a deep trap level in CdTe. The existence of deep trap levels in this cell is suggested by the dispersion in junction capacitance (Fig. 2) and also by the rather slow drift in the I - V characteristic in forward bias. Furthermore, the C - V characteristics of the device (Fig. 1 and Table 3) indicate the p r es e nc e of a layer at or near the CdS-CdTe junction. This layer may be on the CdS side or the CdTe side of the junction or it may straddle the junction. It is hard to separate the effects of these three possibilities. For simplicity, we first assume the layer to be on the CdTe side and model the cell as a p - i - n device sketched in Fig. 6(a). The space charge distribution

31 Layer NT~/NI T

I

(a)

p-CdTe NA = N3

|-CdTe NA - N2

IC

n-CdS ND = N4

~N4 Charge Density

-d1

-W2 -d 2

-~Nal

il

Layer

<

Co)

S

T(

x,.--~

.iN1

Vs -VT +

Ecp

EFp

Ecn EFn

Evp

Evn

(c) Fig. 6. Proposed structure for the CdS/CdTe cell as a p--i-n device: (a) device configuration; Co) space charge density; (c) energy band diagram in forward bias.

and band diagram under forward bias are sketched in Figs. 6(b) and 6(c). A trap level of density NT cm -3 at an energy ET above the valence band of CdTe is included. This corresponds to the trap level reported by Fortmann et al. [15] at 0.45 eV below the conduction band of CdTe. These bulk traps are assumed to be neutral w h e n empty. It Should be noted that a few of these trap levels are below the Fermi level and are negatively charged. We designate this negatively charged trap region as the T layer. We assume CdS to be much more conductive than CdTe. The junction voltage drop Vbi is therefore entirely on the CdTe side and is shared between the S layer (the space charge layer to the left of T layer) and the T layer. It should be noted that only a minute fraction of the S layer is in the p-CdTe. Contributions to the CdTe space charge come from the CdS-CdTe interface, the T layer, the i layer and the depleted portion of the p-type layer. Let Vs and VT denote the voltage drops across the S layer and the T layer respectively. In equilibrium,

32

Vbi is sufficiently large that the hole population in the valence band of CdTe at the boundary of S and T layers is very small. Current transport across the junction can follow four paths (see Fig. 6(c)). Path 1 is the regular diffusion current. Path 2 is the generat i o n - r e c o m b i n a t i o n current in the depleted i layer. Path 3 is the tunneling between CdS conduction band and empty trap levels followed by hole capture by the trap. Path 4 is recombination via interface states. At zero bias, Vs (Fig. 6(b)) is large so that path 2 is limited by the relatively small hole supply in the i layer. Also, the tunneling current and the interface recombination current are small because hole population at the i - T interface and the CdS-CdTe interface is inadequate. Application of a forward bias, however, results not only in a reduction in the total barrier potential but also in an increase in the width of T layer (Fig. 6(c)). This is caused by a larger num ber of trap levels falling below EF~. The result is that the ratio VT/Vs is now higher than it was in equilibrium. There is a decrease in V~, firstly due to forward bias decreasing the total barrier from Vbi to Vbi--VF and secondly due to VT absorbing a larger proportion of Vb~--VF. This makes a large n u mb er of holes available at the S - T interface. The tunneling current path now b eco mes significant as does the depletion layer recombination current. Interface recombination current will also be enhanced. The effective value of the diode reverse saturation current densityjo thus increases with forward bias. Next, we use the space charge distribution sketched in Fig. 6 to calculate the hole concentration at the S - T interface as a function of forward bias. From Fig. 6(b), we see that the junction voltage in equilibrium is given by

Vbi=EgpkTIln(N,,I q

q [

(Arc)} \N-~a/ + In ~44

(1)

where Nv is the effective density of states for the valence band of CdTe and Nc is the effective density of states for the conduction band of CdS. The electric field distribution in the space charge layer from Fig. 6 is given by the following equations. For - W2 < x < - d2,

qNa %(x + We)

E(x)

and, at x =

- de,

qN3

E ( - d e ) =E2 = For

-d

2
E(x) =E2

(2)

%(We - ct2)

(3)

-dl,

qN2 e,(x + de)

(4)

and, at x = - d l ,

E( - 41) =El =E2

qN2 %(d2 - d l )

(5)

33 For - d l < x < 0 ,

E(x) =E,

qNa

(6)

%(X+dl)

At x = 0 - ,

qNldl

E(0-) =Eol =E~

(7)

%

Also,

(8)

eNE02 -- epEol = Qi

where Ql (C cm -2) is the interface charge and Eo2 is the electric field at x=O +. At zero applied voltage (VF=0),

Vi(O) = - q N i E F ( 0 ) - E v ( 0 ) C cm -2 Eg

(9)

where EF(0) is the Fermi level at VF = 0, Ev(0) is the valence band edge of CdTe at x= 0 and VF= O, Eg is the energy band gap of CdTe and N~ is the total number of interface states (assumed to be uniformly distributed over the band gap of CdTe, at x = 0 ) . Here we have assumed that the interface states are neutral when empty. Furthermore, we have assumed that all states above EF(0) are empty and all below EF(0) are occupied. Application of forward bias splits the Fermi level EF into quasi-Fermi levels for holes ( E ~ ) and for electrons (EF,). The energy differential between EF~ and EFp at the interface is qVF. The interface charge Ql thus becomes less negative and is now given by

Q'(VF) = - qN' ( EF(O) -Ev(O) -qVF

(10)

Combining (8)-(10), we get

Eo2 = --enl(%E°l-qN'EF(O)-E'~g O)-qVF )

(11 )

From Fig. 6 we can see that

E~(0)-Ev(0)--Eg- AEo- 8. +

Eo2W1 2

VF

(12)

where

q

q

\ND/

and AE¢ is the difference between the electron affinities of CdS and CdTe. For 0 < x < W 1 ,

34

qN4 x E ( x ) =Eo2 + - -

(14)

en

The total voltage drop due to s p a c e c h a r g e at the j u n c t i o n is given as

Vsc = q(Vbi - V f - AEc)

(15)

Also, Vs¢ = - 0.5{E2(W2 - d2) + (El +E2)(d2 - d l ) + (E01 + E l ) d 1 + E 0 2 W 1 }

(16)

The condition of global s p a c e c h a r g e neutrality r e q u i r e s that

qNs(W2 - d2) + qN2(d2

- di

) + q N l d l - Qi = qN4W1

(17)

Since x = - - d i is the point at which ET crosses EFt, we can write, f r o m an e x a m i n a t i o n of Fig. 6,

,~,Ec q

~n+

Eo2W1 2

(Eol + E l ) d 1 Ecp-E T =0.45 V 2 q

(18)

Further, f r o m Fig. 6 we see that V1 = potential at x = - dl =

-

0.5{E2(W2 - 42) + (El +E2)(d2 - al)}

The hole c o n c e n t r a t i o n at X = - d l P l = P (at x - d l ) = p p

exp

(

-

(19)

is given b y

kT]

(20)

w h e r e pp is the free hole c o n c e n t r a t i o n in the bulk CdTe which equals the free hole c o n c e n t r a t i o n at x = - W 2 . In the a b s e n c e of interface c h a r g e at the C d S - C d T e interface (Nx = 0), the j u n c t i o n c a p a c i t a n c e can be e x p r e s s e d as [23] / [2e,(Vbi-- VF) c= o/l •

Nsd22

N l d l 2 10.5

(21)

J

F o r assigned values o f W2, Et, NI and carrier c o n c e n t r a t i o n s N1, N2, Na and N4, one can use eqns. ( 1 ) - ( 1 8 ) to evaluate dl, c/2, W1, E l , E2, Eol and Eo2. E q u a t i o n s (19) and (20) are t h e n u s e d for calculating hole c o n c e n t r a t i o n at the S - T interface. Results of s u c h a calculation for W2 = 2.0 ~m, ET = 0.45 eV, N~=-5× 1011 cm -2, N 1 = 5 × 1017 c m -3, N2 = 1 × 10 TM c m -a, N s = 6 × 1015 c m - a and N4 = 1 × 1017 c m - s are s h o w n in Figs. 7 and 8. W e n o t e that in Fig. 8, as VF i n c r e a s e s f r o m zero to 0.44 V, t h e r e is an increase in d2 with the resultant diminishing of s p a c e c h a r g e in the p-CdTe layer; at VF = 0.44 V, d 2 - - W , and only CdTe s p a c e c h a r g e left is in the i-CdTe layer and the T layer. A f u r t h e r increase in VF results in a drastic r e d u c t i o n in depletion layer width d2. At VF= 0.68 V, d2 has d e c r e a s e d all the way d o w n to dl. At this point, all the CdTe s p a c e c h a n g e layer is in the T layer. A f u r t h e r

35 _og of Pcon. (at x= -d I)

28.

21.

14.

7.

O.

0'.

:

~

'

' O,

~

'

' '

: 0.~1

. . . .

O.

i~ . . . .

o.I,

VF (volts)

Fig. 7. Logarithm of

hole concentration at x = - d l

as a function of forward-bias voltage;

computer simulation.

÷

",,~b)

~-.[-

a.

g



~0.

O,

VF ( v o l t s )

Fig. 8. Effect o f f o r w a r d b i a s o n (a) the width of the T layer a n d Co) the width of the i-CdTe layer.

increase in VF will result in a reduction in dl itself. Thus, 8 5 / ~ represents the maximum width of the T layer. Figure 7 illustrates that the hole concentration at the S - T interface increases exponentially with increasing VF.

36 One may thus e xpe c t a "roughly" exponential increase in the tunneling c o mp o n en t of loss current.

5. D i s c u s s i o n We now use the model to analyze Figs. 3 and 4 which measure the effect of ch o p p e d monochromatic low level illumination of amplitude AL on the cell. Incident photons have an energy greater than the band gap of CdTe. The effect of AL is twofold. First, it creates e l e c t r o n - h o l e pairs, mainly in the relatively long S layer; a fraction of these are collected as the a.c. photongenerated current ~PH- Secondly, upon exposure to a.c. illumination, AL effectively forward biases the junction by AVF. This brings about an additional negative charge in T layer with a corresponding decrease in space charge at the bulk edge of the CdTe depletion layer. This redistribution of space charge lowers the barrier to hole flow from p-CdTe to i-CdTe. Consequently, a larger number of holes appear at the S - T boundary and at the CdS-CdTe interface. Hence tunneling and recombination currents are enhanced and the effective loss current A/~oss increases. One could also think of this as an apparent decrease in the diode quality factor, A. The measured a.c. shortcircuit current A/s¢ can therefore be expressed as

Both Iph and I~o~s are, of course, functions of the d.c. forward bias VF. We use Fig. 6(c) to analyze the effect of VF on A/~c. As the d.c. forward bias VF is increased, the angle of intersection between ET and EF~ in Fig. 6(c) b eco mes smaller and smaller. For the same num ber of excited deep levels, there is a larger incremental change in T layer thickness with AL. A/~os~/AL thus increases with forward bias. At the same time, A/ph/AL decreases as VF is increased. This is due to a general decrease in the electric field in the S layer, caused by VF. The r educed field gives a reduced collection efficiency and hence a lower Iph. Accordingly, we can expect the measured A/s~ to decrease (and finally to becom e negative) as VF is increased. This is observed in the experimental results of Figs. 3 and 4. Let us next consider the case where VF is SO large that the entire junction voltage on the CdTe side is supported by the T layer only. In that case, increasing VF reduces the thickness of the T layer and thus makes the tunneling probability more sensitive to AL. A/~o~s/AL therefore still increases with increasing VF. When a reverse bias VR is applied to the cell, the already small hole population at the S - T boundary is further reduced. Tunneling-recombination currents and A/~o~s are thus made even smaller by VR. However, because of an increase in the field in the S layer, A/ph is increased. Accordingly A/s¢ registers an increase with reverse bias. Comparing the responses of A/sc to forward and reverse biases in Fig. 4, it is evident that variations in A/s¢ are much larger in forward bias than in reverse bias. This indicates that A/~o~ is much more sensitive to bias voltage than is 5~/pa.

37 Variations in AVoc with bias voltage seen in Fig. 5 are interpreted in a similar manner. It is interesting to note the large increase in hVoc in reverse bias in Fig. 5. In view of the fact that the increase in A/s¢ with reverse bias is rather small (Fig. 4), we attribute the increase in hVo¢ to a reduction in loss current as the reverse bias is increased; this reduction in A/~o~ follows from our model in the manner described above. In Fig. 5, the decrease in AVoc for reverse voltage in excess of 3.5 V is thought to be due to the beginning of a reverse-bias breakdown. The slow upward drift of I - V characteristics in forward bias is thought to be caused by the increase in loss current with forward bias. Since the deep level states are rather slow, this process is characterized by long time constants.

6. Conclusions I-V, junction capacitance and optical characteristics of thin film CdS-CdTe cells on sprayed CdS films were measured. These characteristics have some interesting features such as the reversal of the polarity of a.c. short-circuit current when a large forward bias is applied across the cell. Many features common to thin film heterojunctions such as cross-over effect, drift in I - V voltage characteristic and increase in loss current upon illumination are observed. The reverse saturation current densityjo increases from 5.9 × 10 -~ A cm -2 in the dark to 18.1 × 10 -6 A cm -2 under "1 sun" illumination. Diode ideality factors are higher than 2.0 and a is almost temperature independent. The zero-bias depletion layer width is 1.9 /~m. The experimental results are interpreted in terms of a model which proposes a highly compensated "intrinsic" layer in CdTe and also a high space charge layer in CdTe (T layer), next to the CdS--CdTe interface. The origin of the high space charge layer is thought to be the ionization of a deep level at an energy ET below the conduction band edge. The value of ET is not yet known. For our calculations, we have used ET = 0.45 eV; this choice is, of course, arbitrary and speculative. However, such a trap level has been reported in CdTe by Fortmann et al. [ 15 ]. Numerical calculations confirm the increase in thickness of the T layer (and the resulting redistribution of the barrier voltage in CdTe) with forward bias. Detailed calculations of various loss currents and junction capacitance variations with VF would be the next step in this analysis. However, the model is successful in explaining the many interesting optoelectronic characteristics of the CdS-CdTe cell fabricated by Photon Energy Inc.

Acknowledgments We are grateful to Photon Energy Inc. for the supply of thin film CdS-CdTe solar cells used in this study. Thanks are due to Lance Lyons

38 for I-V and C-V measurements for the capacitance-frequency

and to the Solar Energy measurements.

Research

Institute

References 1 S. P. Albright, B. Ackerman and J. F. Jordan, Efficient CdTe/CdS solar cells and modules by spray processing, IEEE Trans. Electron Devices, 37 (1990) 434--437. 2 K. Zwiebel, H. S. Ullal and R. L. Mitchell, Proc. 20th Photovoltaic Specialists' Conf., IEEE, New York, 1988, pp. 1 4 6 9 - 1 4 7 6 . 3 K. Y. Yamaguchi, H. Matsumoto, N. Nakayama and S. Ikegami, Jpn. J. Appl. Phys., 15 (1976) 1575. 4 Y. Y. Ma, A. L. F a h r e n b r u c h and R. H. Bube, AppL Phys. Lett., 30 (1977) 423. 5 B. M. Basol, E. S. Tseng, R. L. Rod, S. Ou and O. M. Stafsudd, Ultra-thin electrodeposited CdS/CdTe heterojunction with 8% efficiency, Proc. 16th Photovoltaic Specialists' Conf., San Diego, CA, 1982, IEEE, New York, 1982, pp. 8 0 5 - 8 0 8 . 6 H. Uda, H. Matsumoto, Y. Komatsu, A. Nakano and S. Ikegami, AU screen printed CdS/ CdTe solar cell, Proc. 16th Photovoltaic Specialists' Conf., San Diego, CA, 1982, IEEE, New York, 1982, pp. 8 0 1 - 8 0 4 . 7 N. Nakayama, H. Matsumoto, A. Nakano, S. Ikegami, H. Uda and T, Yamashita, Jpn. J. Appl. Phys., 19 (1980) 703. 8 F. G. Courreges, A. L. F a h r e n b r u c h and R. H. Bube, J. Appl. Phys., 51 (1980) 2175. 9 J. A. Aranovich, D. Golmayo, A. L. F a h r e n b r u c h and R. H. Bube, J. Appl. Phys., 51 (1980) 4260. 10 K.W. Mitchell, A. L. F a h r e n b r u c h and R. H. Bube, Evaluation of the CdS/CdTe heterojunction solar cell, J. Appl. Phys., 48 (1977) 4 3 6 5 - 4 3 7 1 . 11 Y. S. Tyan and E. A. Perez-Albuerne, Efficient thin-film CdS/CdTe solar cells, Proc. 16th Photovoltaic Specialists' Conf., San Diego, CA, 1982, IEEE, New York, 1982, pp. 794--800. 12 S. P. Albright, V. P. Singh and J. F. Jordan, Junction characteristics of CdS/CdTe solar cells, Sol. Cells, 24 (1988) 4 3 - 5 6 . 13 J. Werthen, A. L. F a h r e n b r u c h and R. H. Bube, J. Appl. Phys., 54 (1983) 2750. 14 T. C. Anthony, A. L. Fahrenbruch, M. G. Peters and R. H. Bube, Electrical properties of CdTe films and junctions, J. Appl. Phys., 57 (1985) 4 0 0 - 4 1 0 . 15 C. M. Fortmann, A. L. F a h r e n b r u c h and R. H. Bube, Relative carrier densities and trap effects on the properties of CdS/CdTe, J. App£ Phys., 61 (5) (1987) 2 0 3 8 - 2 0 4 5 . 16 M. G. Peters, A. L. F a h r e n b r u c h and R. H. Bube, Properties of CdS/ZnCdTe heterojunctions, J. Appl. Phys., 64 (6) (1988) 3106--3111. 17 V. P. Singh, R. H. Kenny, J. C. McClure, S. P. Albright, B. Ackerman and J. F. Jordan, Proc. 19th Photovoltaic Specialists' Care., IEEE, New York, 1987, pp. 2 1 6 - 2 2 1 . 18 R. H. Bube, A. L. Fahrenbruch, R. Sinclair, T. C. Anthony, C. Fortmann, W. Huber, C. T. Lee, T. Thorpe and Y. Yamashita, Cadmium telluride films and solar cells, IEEE Trans. Electron Devices, 31 (1984) 5 2 8 - 5 3 8 . 19 J. F. J o r d a n and S. P. Albright, Large area CdS/CdTe photovoltaic cells, SoL Cells, 23 (1988) 1 0 7 - 1 1 3 . 20 H. S. Ullal, Electronic structure of electrodeposited thin film CdTe solar cells, SERI/TP211-3361 Rep., May 1988 (Solar Energy Research Institute). 21 P. V. Meyers, Sol. Cells, 23 (1988) 5 9 - 6 7 . 22 L. R. Shiozawa, G. A. Sullivan and F. Augustine, Co~. Record 7th Photovoltaic Specialists' Conf., Pasadena, CA, 1968, IEEE, New York, pp. 3 9 - 4 6 . 23 R. B. Hall and V. P. Singh, Analysis of capacitance--voltage m e a s u r e m e n t s on heat treated Cu2S/CdS heterojunctions, J. Appl. Phys., 50 (10) (1979) 6406--6412. 24 A. R. Riben and D. L. Feucht, n-Ge/p-GaAs Heterojunctions, Solid.StateElectron., 9 (1966) 1055. 25 Lance D. Lyons, Electronic characterization of thin film CdS/CdTe photovoltaic cells, M.S. Thesis, The University of Texas at E1 Paso, December 1988.