Metal contacts to CuInSe2

Solar Cells, 11 (1984) 301 - 305

301

Short Communication

Metal contacts to CuInSe:

R. J. MATSON, O. JAMJOUM, A. D. BUONAQUISTI*, P. E. RUSSELL t , L. L. KAZMERSKI, P. SHELDON and R. K. AHRENKIEL

Solar Energy Research Institute, Golden, CO 80401 (U.S.A.) (Received November 1, 1983; accepted November 18, 1983)

Copper indium selenide (CuInSe2) is a direct band gap semiconductor that has been used as the active material in some of the highest efficiency single~rystal and thin film polycrystalline heterojunction solar cells reported [1 - 3]. Cells are c o m m o n l y based on an n-(CdZn)S/p-CuInSe2 heterojunction structure. Gold has been identified as a reliable reproducible ohmic contact to the p-type CuInSe2. However, the use of gold in a thin film solar cell structure may severely limit t h e possibility of meeting low cost objectives. A suggested alternative lower cost back-contact material is m o l y b d e n u m which can be sputtered onto an alumina substrate; this substrate can then be used for the solar cell fabrication [4, 5]. Recent investigations, however, have shown that the Mo-p-CuInSe2 contacts can have rectifying properties prior to device thermal processing [6]. In this communication we present the results of several studies of other metal contacts (silver, gold, copper, nickel and m o l y b d e n u m ) to p-type CuInSe:, including electron-beam-induced current (EBIC), celpacitance-voltage (C-V) and interface chemistry analyses. The Mo-p-CuInSe2 contact has been examined using the EBIC technique [6]. Devices are fabricated by sputtering m o l y b d e n u m onto an alumina or glass substrate to form the back contact [3]. P-type CuInSe2 is then thermally vapor deposited onto the m o l y b d e n u m using a three-boat system. An n-type (CdZn)S layer provides the heterojunction window, and ohmic contact is to this layer with an aluminum grid. These devices were fractured and the cross section was examined in a scanning electron microscope b y the EBIC technique. In essentially all the devices examined, an increase in induced current was observed at the Mo-p-CuInSe2 interface. In relatively p o o r devices (i.e. solar cells with a very low efficiency) the effects of the Mo-p-CuInSe 2 interface dominate the EBIC line scans. In very good devices the EBIC line scan is dominated by the heterojunction response, *Present address: University of Florida, Gainesville, FL, U.S.A. tPresent address: Japan Electron Optics Laboratories, Boston, M A , U.S.A. 0379-6787/84/$3.00

© Elsevier Sequoia/Printed in The Netherlands

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while the EBIC signal from the Mo-p-CuInSe2 interface region is relatively small. The strength of the EBIC signal at the Mo-p-CuInSe 2 interface was found to vary considerably with relatively mild heat treatments (i.e. 200 255 °C for less than 1 h) of the devices. Figure 1 illustrates the effect of heat treatment on the position and type of active junction. This micrograph was obtained by heating the device in a scanning electron microscope and performing successive EBIC line scans at the same junction position at different temperatures. The heterojunction response would be considerably larger had the line scan been taken at room temperature after the thermal processing rather than at 225 °C.

Fig. 1. S c a n n i n g e l e c t r o n m i c r o g r a p h o f a C d S / C u I n S e 2 / M o device (edge on). EBIC line scans were p e r f o r m e d at t h e s a m e p o s i t i o n at a series o f d i f f e r e n t t e m p e r a t u r e s . T h e results at t h r e e t e m p e r a t u r e s are sl~own here. T h e h e a t i n g t i m e was generally less t h a n 5 m i n a t a given t e m p e r a t u r e .

When depositing m o l y b d e n u m onto predeposited films of p-type CuInSe2, we found that while the contacts were in general non~)hmic they were also very non-reproducible from run to run. This p r o m p t e d a series of surface analysis studies o f the interface. Thin film 750 A samples of the CuInSe 2 on molybdenum-coated alumina substrates were used for surface analysis studies o f the interface. The CuInSe2-Mo interface was studied by sputter etching the CuInSe2 and simultaneously monitoring the m o l y b d e n u m using secondary ion mass spectrometry. The depth profiling was stopped when m o l y b d e n u m started to appear. The CuInSe2-Mo interface was investigated using X-ray photoelectron spectroscopy (XPS) analysis. The above procedure was repeated for similar samples that had been heated in a sequence of heat treatments for 5 min from r o o m temperature up to 240 °C in an argon atmosphere. Figure 2 shows the Mo 3ds/2, 3/2 and Se 3d lines at the CuInSe2-Mo interface before

303

Se 3d

227,8

Mo 3d,/z3/2 ~

o

Z

Unbaked I I I A -60 -56 -56 -54 -52 Binding

a Unbaked~ I I I I i I -236 -234 -232 -230 -228 -226 Energy (eV)

Fig. 2. XPS studies o f t h e M o - C u I n S e 2 i n t e r f a c e as a f u n c t i o n o f h e a t t r e a t m e n t : spect r u m a, t h i n film CuInSe2 b e f o r e b a k i n g ; s p e c t r u m b, C u I n S e 2 - M o i n t e r f a c e a f t e r a s e q u e n c e o f h e a t t r e a t m e n t s u p t o 1 7 0 °C; s p e c t r u m c, C u I n S e 2 - M o i n t e r f a c e a f t e r a s e q u e n c e o f h e a t t r e a t m e n t s u p t o 2 4 0 °C.

and after a sequence of heat treatments to 240 °C. The position of the 3d peaks in the binding energy is established relative to the Fermi energy of gold. Figure 2, spectrum a, indicates no chemical interaction between the semiconductor and the metal; however, a peak at 232.6 eV is observed which is attributable to MoO3 and this might have been formed during the deposition process of CuInSe2 onto m o l y b d e n u m . This peak did n o t change intensity as the samples underwent heat treatments. As the samples were heated to 170 °C, a peak at 229.0 eV appeared and became stronger in intensity after heating to 240 °C. The position of this peak was f o u n d to correspond to that of MoSe2 [7]. The Se 3d line o f Fig. 2, spectrum c, also provided direct evidence of the existence of MoSe 2 by a peak at 54.5 eV which grew in intensity as a result of heating. This peak demonstrates that the formation of this c o m p o u n d at the CuInSe2-Mo interface is n o t an artifact of the sputtering procedure. The fact that the peaks due to MoSe2 were not observed in the Mo 3d line and Se 3d line before the heat treatment procedure was started confirms that the formation of MoSe2 in the interface is a result of the heat treatment. As the nature of the Mo-p-CuInSe2 contact is n o t completely understood nor reproducible, a series of alternative contact metals were investigated. These studies were c o n d u c t e d b y depositing (magnetron sputtering or thermal resistive deposition) a series of contacts onto predeposited p-type CuInSe~ films. I - V characteristics were then measured across pairs of contact pads, in order to look for ohmic or back-to-back diode characteristics. Any samples showing non-ohmic behavior were then examined using

304 TABLE ] Summary of properties of various metal contacts to p-type CuInSe2 Metal

Configuration

Deposition technique

Behavior

Reproducibility

Au

Substrate a On CuInSe2 b Substrate a On CuInSe2 b

Thermal heating Thermal heating Magnetron sputtering R.f. sputter Magnetron sputtering Magnetron sputtering Electron beam Magnetron sputtering Thermal heating Magnetron sputtering Electron beam

Ohmic Ohmic Non-ohmic Generally ohmic Generally ohmic Ohmic Generally ohmic Ohmic Generally ohmic Generally ohmic Non-ohmic

Very good Very good Good Very poor Poor Very good Poor Good Poor Poor Very poor (degrades with time)

Mo

Ni Al

On CuInSe2b On CuInSe2 b

Ag

On CuInSe2 b

Cu

On CuInSe 2b

a CulnSe2 is deposited onto metal. bMetal is deposited onto CuInSe2.

the EBIC tech n ique b o t h t o de t e r m i ne w h e t h e r the induced current could be measured and to examine t h e j u n c t i o n structure. A summary o f t he results obtained in these studies is given in Table 1. It should be n o t e d that only gold and nickel form a com pl e t e l y reproducible ohmic c o n t a c t in all cases. Silver was f o u n d to p r o d u c e an ohmic c o n t a c t when applied by magnetron sputtering b u t it is well k n o w n that, at elevated temperatures, silver will diffuse into th e CuInSe2 and dope it n t y p e [8]. Aluminum generally provided an ohmic c o n t a c t but it is e x p e c t e d t hat problems with native oxide f o r m a t i o n on aluminum would interfere with its successful use as a back-contact substrate material for CuInSe2 deposition. To predict these p h e n o m e n a better, metal-coated substrates will have to be tested by the deposition o f p - ty pe CuInSe2 films, or heated CuInSe2 films should be used for th e deposition o f candidate metals. In conclusion, these studies have shown that with the except i on of gold, and possibly nickel, metal contacts t o p-type CuInSe: are very complex. The electrical properties o f t he c o n t a c t depend on the sample thermal history and th e m e t h o d o f metal deposition. M o l y b d e n u m appears to be a probable low cost ohmic c o n t a c t material, but f u r t h e r studies are needed to understand and cont r ol fully the electrical properties o f t he Mo-p-CuInSe2 interface. In addition, it would be beneficial t o expand these studies to ot her deposition schemes and, possibly, t o o t h e r metals or t o alloy systems.

The authors acknowledge t h e support o f the U.S. D e p a r t m e n t of Energy under Contract EG-77-C01-4042.

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