CuGaSe2 thin films for photovoltaic applications

CuGaSe2 thin films for photovoltaic applications

Thin Solid Films, 130 (1985) 209-216 ELECTRONICS AND OPTICS 209 CuGaSe z T H I N F I L M S F O R P H O T O V O L T A I C A P P L I C A T I O N S * W...

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Thin Solid Films, 130 (1985) 209-216 ELECTRONICS AND OPTICS

209

CuGaSe z T H I N F I L M S F O R P H O T O V O L T A I C A P P L I C A T I O N S * W. ARNDT, H. DITTRICH AND H. W. SCHOCK Institut fiir Physikalische Elektronik, Universitdt Stuttgart, PfaJfenwaldring 47, D-7000 Stuttgart 80 (F.R.G.)

(Received August 11, 1984; acceptedApril 10, 1985)

Of the I - I I I - V I 2 group chalcopyrites, CulnSe 2 has already proved its suitability for thin film solar cells owing to its excellent optical and transport properties. CuGaSe 2 is expected to exhibit comparable properties from this point of view. With its band gap of 1.7 eV it is a candidate for use in photovoltaic tandem systems. The preparation of CuGaSe 2 thin films by means of the vacuum evaporation of the constituent elements (four-temperature method) is described. The structural, electrical and optical properties of these films were investigated. Secondary electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction examination and measurements of the optical transmission, resistivity and thermoelectric power were used to determine the film properties relative to the preparation parameters and stoichiometry. The growth conditions were optimized for solar cell applications. Heterojunctions were prepared by the in situ evaporation of ZnxCd ~_x S onto the CuGaSe 2 films. The characteristic data of the cells are a short-circuit current of 6 mA and an open-circuit voltage of 620 mV at an illumination at air mass 1.5 on an area of 1 cm 2.

1. INTRODUCTION I - I I I - V I 2 chalcopyrite semiconductors, particularly CulnSe2, are attracting increasing attention because of their favourable optical and electronic properties. Furthermore, their good stability makes them very suitable for photovoltaic and photoelectrochemical applications 1'2. Even though the deposition of thin films of these compounds by vacuum evaporation requires an elaborate arrangement with appropriate crucibles and process control, an economic fabrication of solar cells based on these materials seems to be possible. An ideal combination for a tandem solar cell could be CulnSe2 (Eg = 1.04 eV) and CuGaSe 2 (Eg = 1.7 eV). Very little work on CuGaSe2 thin films has been published3; however, the experimental results for single-crystal devices indicate * Paper presented at the Sixth International Conference on Thin Films, Stockholm, Sweden, August 13-17, 1984. 0040-6090/85/$3.30

© ElsevierSequoia/Printed in The Netherlands

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W. A R N D T , H. D I T T R I C H , H. W . SCHOCK

promising properties 4. The literature data are given in Table I. Important features are the lower electron affinity and lattice constant values and the higher thermal expansion coefficient compared with those of CuInSe 2. TABLE ! PHYSICAL PROPERTIES OF C u G a S e 2 5 7

Lattice constants (~)

Thermal expansion coefficient

Mp

Eg at 300 K

(K)

(eV)

ReJ~active index at850nm

(p type) ( f ~ - l c r n l)

Conductivity

2.90

20to >10

Hole mobility (cm2V-~s

l)

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2.

15.5x10

6

1313

1.68

5

20-50

P R E P A R A T I O N OF FILMS

The CuGaSe2 thin films were prepared by vacuum evaporation from three separate sources in the same equipment as that used previously for the deposition of CuInSe2 films. The graphite crucibles for copper and gallium were contained in a water-cooled box. The crucibles were heated using cylindrical graphite heaters. P t - ( P t - R h ) thermocouples located in a thermowell at the bottom of the crucibles provided exact temperature measurement. A special source consisting of a stainless steel container heated by a wire coil was designed for the evaporation of selenium. The reproducibility of the deposition process was ensured by using a temperature programmer. The individual deposition rates were calibrated using a quartz crystal thickness monitor. A background pressure of 2 x 10-3 Pa was maintained during evaporation. The separation of 7 cm between the single sources together with the sourcesubstrate distance of 3 0 c m causes a composition gradient along the substrate surface. As a high selenium supersaturation (10 to 20 times) is maintained during deposition, geometrical effects do not result in a selenium gradient. The relative deposition rates of copper and gallium change from unity at the centre of the substrate to 0.8 at the edge. Hence the influence of the composition on the properties of the films can be investigated on a single substrate. An additional source for Z n x C d l _ x S provides the possibility of in situ deposition of an n-type layer. Borosilicate glass (Corning 7059) was used as the substrate. It was partially metallized with sputtered molybdenum which could be used for electrodes for electrical measurements and for the back contact of the heterojunction. The thickness of the CuGaSe 2 films was about 3 lam. A deposition rate of 50 nm m i n was achieved at the following temperatures: gallium, 1383 K; copper, 1563 K; selenium, 663 K; substrate, 748 K. 3.

PROPERTIES OF THE FILMS

The composition was determined using an electron microscope with energydispersive X-ray spectroscopy and the results were compared with the data obtained

CuGaSe 2 THIN

FILMS FOR PHOTOVOLTAIC APPLICATIONS

211

from X-ray diffraction measurement, scanning electron micrographs of the surface, and optical transmission and photoluminescence studies. A summary of the X-ray diffraction data shown in Fig. 1 is given in Table II 8 TABLE II SUMMARYOF THE RESULTSOF X-RAY DIFFRACTIONMEASUREMENTS

Vapour composition at vacuum deposition

Cu rich Stoichiometric Ga rich

Composition High Se pressure

Low Se pressure

C u G a S e / + CuxSe CuGaSe2 (sample 13) (CuGaSe2)x+(Ga2Se3)l x

CuGaSe2 + CuxSe + Cu (sample 38) CuGaSe2 + GaSe + Cu (sample 18) (CuGaSe2):,(Ga2Se3)l x + GaSe (sample 43)

A relatively broad region of existence of CuGaSe2 on the gallium-rich side has been reported 5. This is in agreement with our observation of a wide range of singlephase CuGaSe 2 on the substrate despite the composition gradient due to the source separation. Films with a gallium-rich composition also contain GaSe and Ga2Se 3 phases depending on the amount of excess gallium and selenium present. Cu2 _xSe and metallic copper can be found on the copper-rich side. The scanning electron micrographs show tetrahedral growth features for pure CuGaSe2 films (Fig. 2(b)), whereas a platelet-type structure occurs in gallium-rich films (Figs. 2(c) and 2(d)). Coarse CuGaSe2 and CuSe2 grains together with copper whiskers are present in copper-rich films (Fig. 2(a)). The whiskers seem to grow only after the deposition and during the cooling of the substrate. N o pronounced texture is found in any of the films. The optical transmission and photoluminescence (Figs. 3 and 4) show a clear dependence on the composition. A continuous shift in the absorption edge towards higher energies with increasing gallium content is indicated by the experimental results. This may be caused by the superposition of two or three absorption curves due to different phases with different optical properties and a large number of impurity-to-band transitions in GaSe and Ga2Se 3, which can shift the optical absorption curve. An optical gap of 2.32 eV is observed at room temperature in pure Ga2Se 3 films; this value is rather higher than the data given by other workers 9. Normalized optical transmission curves of the samples from Table II and Fig. 1 are plotted in Fig. 3. Owing to the presence of metallic copper in copper-rich films no pronounced absorption edge is observed. The photoluminescence spectra of the stoichiometric and gallium-rich samples are shown in Fig. 4. The sample temperature was 15 K and Ar ÷ laser excitation at 488 nm was applied. The peak at about 1.66 eV is a transition from the free electron to the bound hole (copper or selenium vacancies) 1°. The broad emission band around 1.59eV can be related to donor-acceptor transitions in CuGaSe 2 11. By comparison with X-ray diffraction measurements the peaks at 2.0 eV can be associated with GaSe and those at 1.86 and 1.79 eV can be associated with Ga:Se3. All films show p-type conductivity which decreases at higher gallium contents.

212

W. ARNDT, H. DITTRICH, H. W. SCHOCK

Selenium defects also cause a decrease in the conductivity, probably as a result of compensation by deep donors as has been observed in single crystals ~2. 4. CuGaSez-ZnxCd~ x S HETEROJUNCTIONS CuGaSez-ZnCdS heterojunctions were fabricated by the vacuum deposition of a Zn~Cd~ ~S film about 3 I.tm thick from a coaxial source ~. Additional n-type doping was provided by the co-evaporation of gallium or indium. The zinc content was limited to x = 0.5 because at higher concentrations the resistivity of the films

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THIN FILMS FOR PHOTOVOLTA1C APPLICATIONS

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increases rapidly. An electron affinity mismatch of about 0.4 eV between CuGaSe z and Zno.sCdo.sS has to be taken into account. The optical absorption coefficient of C u G a S e / p l o t t e d in Fig. 5 gives a value of about 1 lam for the minimum thickness of the active layer. The quantum efficiency curve shown in Fig. 6 indicates that the electron diffusion length is of the same order of magnitude. The blue response is limited by the band gap of Zn~Cd~ _xS. In this case x ~ 0.35.

214

W, ARNDT, H. DITTRICH, H. W. SCHOCK

Fig. 2. Scanning electron micrographs of the same films as shown in Fig. 1.

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X[nm] Fig. 3. Optical transmission spectra of films with various compositions defined in Table lI. Fig. 4. Photoluminescence spectra at 15 K for the films shown in Figs. 1-3.

CuGaSe2 THIN FILMS FOR PHOTOVOLTAIC APPLICATIONS

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Fig. 5. Optical absorption coefficient ofa CuGaSe 2 thin film. Fig. 6. Quantum efficiency ofa CuGaSe2-Zno.3Cd0.vS thin film solar cell.

A short-circuit current of 6 mA and an open-circuit voltage of 620 mV on an area of 1 cm 2 were obtained in initial experiments under illumination at air mass 1.5. The current corresponds to an external quantum efficiency of about 50~o; however, the open-circuit voltage is still too low with respect to the band gap of 1.7 eV. 5. CONCLUSIONS

The experiment showed that CuGaSe2 thin films with suitable properties for photovoltaic applications can be obtained by vacuum deposition. The single-phase region of CuGaSe2 is much broader than that of CuInSe2. Further investigations of the phase composition of thin films of the C u - G a - S e system are required. Optimization of the n-type collector and window layer is necessary in order to increase the open-circuit voltage of the devices. ACKNOWLEDGMENTS

The authors are indebted to B. Dimmler who performed much of the experimental work. This work was supported by the Bundesministerium fiir Forschung und Technologie under Contract 03-E-8019-A. REFERENCES 1 K. Zweibel, A. Hermann and R. Mitchell, Sol. Cells, 12 (1984) 257-261. 2 D. Cahen, Y. W. Chen, P. J. Ireland, R. Noufi, J. A. Turner and K. Bachmann, Proc. 17th IEEE Photovoltaic Specialists' Conf., Orlando, FL, 1984, IEEE, New York, 1984, p. 786. 3 W. H6rig, H. Neumann, B. Schumann and G. Kiihn, Phys. Status Solidi B, 85 (1978) K57. 4 N. Romeo, G. Sberveglieri, L. Tarricone and C. Paorici, Appl. Phys. Lett., 30 (1977) 108. 5 J.L. Shay and J. H. Wernick, Ternary Chalcopyrite Semiconductors, Pergamon, Oxford, 1975, pp. 4, 22, 118, 191.

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A . U . Mal'sagov, Soy. Phys.-Semicond., 4 (1971) 12. L. Mandel, R. D. Tomlison and M. J. Hampshire, J. Appl. Crystallogr., 10 (1977) 130. Powder Diffraction File, Joint Committee on Powder Diffraction Standards, Center for Diffraction Data, Swarthmore, PA 19081, 1960, Cards 4-0809, 4-0836, 5-0724; 1975, Card 19-401 ; 1976, Card 20-437; 1980, Card 29-628; 1981, Card 31-455. W . H . Strehlow and E. L. Cook, J. Phys. Chem. Ref Data, 2 (1973) 163-185. M . P . Vecchi, J. Ramos and W. Giriat, Solid-State Electron., 21 (1978) 1609-1612.

A. Poure, J.P, LeyrisandJ. P. Aicardi, J. Phys. C, 14(1981)521-530. W. Giriat and J. Stankiewicz, Proe. 14th IEEE Photovoltaic Specialists' Conj,, San Diego, 1980, IEEE, New York, 1980, p. 647. W. H, Bloss, J. Kimmerle, F. Pfisterer and H. W. Schock, Proc. l 7th IEEE Photovoltaic Specialists' Cot¢[i, Orlando, FL, 1984, 1EEE, New York, 1984, p. 715.