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Solar Energy Materials and Solar Cells 40 (1996) 359-369

Synthesis and characterization of CuInSe 2 thin films from Cu, In and Se stacked layers using a closed graphite box F.O. Adurodija *, M.J. Carter, R. Hill Newcastle Photot,oltaics Applications Centre, Unicersi~" of Northumbria, Newcastle upon Tyne, NE1 8ST, UK

Received l0 March 1995; revised 5 October 1995

Abstract Various techniques have been used to produce CuInSe z, but the problem of producing films with the desired properties for efficient device fabrication over large areas has always persisted. The Stacked Elemental Layer (SEL) technique has been demonstrated as a method for producing films over a large area, but the films normally annealed in vacuum or in Se ambient, mostly exhibited poor morphology with small grain sizes which result in poor devices. A method of synthesizing CuInSe 2 films by annealing or selenization of the Cu, In and Se elemental layers using a closed graphite box was developed. SEM, EDX, XRD, spectrophotometric and Hall measurements were used to characterize all annealed films. Results have shown single phase chalcopyrite films with improved crystal sizes of about 4 ~m. The film composition varied from Cu-rich to In-rich with electrical resistivities of 10 -3 to 10 4 ~ c m , carrier concentrations of 5 × l0 ~s to 10 ~7 cm -3 and mobilities of 0.6 to 7.8 cm 2 V -I s -~. An energy band gap of 0.99 eV and 1.02 eV was obtained for a Cu-rich and near stoichiometric In-rich films respectively. Heterojunction devices using the structure Z n O / C d S / C u I n S e 2 were fabricated with electrical conversion efficiencies of 6.5%. Keywords: Copper indium diselenide; Thin film solar cells; Chalcopyrites; Photovoltaic cells

1. Introduction Ternary chalcopyrite C u l n S e 2 thin film material is very p r o m i s i n g for photovoltaic p o w e r generation because o f its excellent optical and s e m i c o n d u c t i n g properties. Solar

* Corresponding author. 0927-0248/96/$15.00 © 1996 Published by Elsevier Science B.V. SSD10927-0248(95)00160-3

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cells with efficiencies of about 16% have been produced over small areas using sophisticated coevaporation technique [1] and greater than 10% by selenizing elemental precursors of Cu-In-(Se) either with H2Se gas or selenium vapour [2,3]. Although many different techniques have been used successfully to produce CulnSe 2 thin films no process has yet shown the reliability and yield needed for commercial production of large area devices. For a commercial process, the fabrication of CulnSe 2 absorbers over large areas with good reproducible optical and semiconducting properties is essential. The production of CulnSe 2 from the stacked elemental layers is a very promising method since good control of the individual material and film uniformity could be achieved over a large area compared with the coevaporation technique. However, the problem which has hindered development of this technique was the poor quality crystalline structures obtained by vacuum annealing of the films. The potentials of achieving large crystal CulnSe 2 films by selenization of Cu, In and Se in a graphite box have earlier been reported by Yamanaka et al. and Bodegard et al. using different selenization conditions [4,5]. However, to the authors knowledge, no further investigations have been reported on this technique. In this present investigation, the synthesis and further analysis of the properties of improved quality CulnSe 2 films from the stacked elemental layers of Cu, In, and Se deposited on glass or Mo coated substrates using a closed graphite box is discussed subsequent to an earlier report [6]. This synthesis method is simple, economical, does not use toxic gas and could easily be scaled up or integrated into a commercial process. The elemental layer precursors could also be produced by non-vacuum deposition method such as electroplating before synthesizing within the graphite box.

2. Experimental details Cu, In and Se layers were thermally evaporated on glass or Mo coated glass substrates maintained at ambient temperature using I n / C u / I n / S e / I n / C u / I n / S e sequence. The multilayers structure was used in order to enhance inter-diffusion of the elements during annealing. The bottom and top In layer in the sequence was used to (1) enhance film adhesion on the substrate, and (2) prevent possible formation or segregation of CuxSe residue on the surface of the CulnSe 2 films after selenization. The Cu and In layer thickness used for a stoichiometric ratio were 130 nm and 280 nm, respectively. A Se layer thickness of approximately three times the total thickness of (Cu + In) was deposited on all films. The C u / I n ratio was varied by adjusting the thickness of the bottom Cu layer. The Cu, In and Se thicknesses were controlled using three separate quartz crystal monitors. The samples were then placed in a graphite box and covered with a graphite lid. Two cavities were provided at the end within the box to hold some Se shot to ensure the correct Se over pressure to produce good p-type CuInSe 2 films. The sample was arranged face up as shown in Fig. 1. The graphite box was then loaded into an evacuated glass quartz tube in a furnace with a vacuum pressure of 10 -z Torr. All annealing or selenization was carried out at 500°C for 20 minutes. Two types of annealing processes were investigated using the graphite box. The first one involved annealing of the stacked elemental in the box without Se shot. The second annealing

F,O. Adurodija et aL / Solar Energy Materials and Solar Cells 40 (1996) 359-369

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vacuum wall

sol [

sampl~ graphite box

Fig. 1. The graphite box synthesizingsystem. involved adding Se shot in the box. In addition, films with similar compositions were also subjected to vacuum annealing in order to compare their structural properties with films selenized in the graphite box. The film morphology and the composition were analysed by SEM and EDX (Hitachi 2400 and JOEL JSM 35 equipped with ZAF commercial analysis system), The structural properties were determined by XRD (Siemens Diffractometer D5000). The optical properties were investigated by spectrophotometer (Caryl7D, Varian). The electrical properties were determined by the Van der Pauw, Four Probe l-V, and Hot probe measurements.

3. Results and discussion

Different elemental layer sequences and processing techniques have been investigated by various workers with some degrees of success [7-9]. Others have investigated metallic layers or binary selenides as precursors for CulnSe 2 fabrication [10-12]. However, the problems of non-homogeneity, phase segregation, poor crystallization, etc., still remain to be resolved in most of the processes. This paper presents an alternative potential method for achieving good quality films over large areas from the stacked elemental layers using a closed graphite box compared to vacuum annealing. All films synthesized using the graphite box were dense and exhibited large grain sizes. SEM micrographs revealed grain sizes of the order of 2 txm to 4 p~m measured over areas of 15 cm 2 and film thickness of around 2 I~m. This type of morphology is desirable for boosting the photovoltaic response of the solar cells because large grain sizes indicate reduced losses due to grain boundaries. Fig. 2a-2c shows the surface SEM micrographs of films synthesized in the graphite box compared with vacuum annealed films. Improved adhesion on glass and Mo coated substrates was also exhibited by the films processed in the graphite box over vacuum annealed CulnSe 2 films as shown in cross-sectional SEM micrographs of Fig. 2d and 2e. The morphologies of the films annealed in vacuum were very poor with crystal sizes less than 1 I~m. The poor crystallization observed in these films could be associated with an uncontrolled increase in the vapour pressure of Se within the film at 210°C resulting in a great loss of Se during annealing. An exothermic reaction of Se with binary In-Se resulting in film

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Fig. 2. (a,b,c) Surface SEM micrographs of CulnSe2 films synthesized in: (a) graphite box with Se, (b) graphite box without Se, and (c) in vacuum. (d,e) Cross-sectional SEM micrographs of CulnSe2 synthesized in: (d) the graphite box, and (e) vacuum.

delamination and poor crystallization during vacuum annealing of multiple stacks of Cu, In and Se has been reported [13]. The SEM results of films produced using the graphite box have shown a noticeable improvement over films processed in vacuum even though films with similar composition of the elemental layers were used. The EDX analysis of the CulnSe z films processed without Se shot in the graphite box had compositions with C u / I n of 0.92 to 1.31 and S e / ( C u + In) ratios varying between 0.84 and 0.98. Films synthesized with Se shot in the box during annealing, had C u / I n

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363

A

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Fig. 3. (a) XRD spectra of a chalcopyrite Cu-rich and near stoichiometric In-rich CulnSe2 films processed in the graphite box. (b) High resolution XRD scans of the 204/220 and 116/312 peaks of a near stoichiometric In-rich film showing a tetragonal splitting. and S e / ( C u + In) ratios varying from 0.85 to 1.04 and 0.89 to 1.3, respectively. Good reproducibility of the compositional properties were obtained from the deposited elemental materials with compositions varying from Cu-ricb to In-rich. Results of the EDX compositional measurements for films selenized in the graphite box with or without Se shot is shown in Table 1. No significant difference was observed in the compositional properties of these films. Generally, all films annealed in vacuum were deficient in Se. The compositions of C u / I n varied from 0.93 to 1.33 and S e / ( C u + In) ratios from 0.82 to 0.94. XRD studies of the stoichiometric and near stoichiometric In-rich CuInSe 2 films showed only single phase chalcopyrite CulnSe 2, with no extra phases, contamination or oxides present, even though films were synthesized in a low vacuum of 10 -2 Ton'. Cu-rich films exhibited an additional phase identified as Cu2_xSe from the JCPDS files as shown in Fig. 3a. The enhanced crystallinity is evidenced from the narrow diffraction peaks. No significant difference in the diffraction pattern was observed in the films synthesized with or without Se vapour. High resolution XRD scans of a near stoichiometric In-rich film revealed splitting of the 2 0 4 / 2 2 0 and 1 1 6 / 3 1 2 peaks, Fig. 3b. This splitting is a characteristic of good quality material and has been observed by other workers [9].

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Fig. 4. Transmission versus Wavelength spectra for a Cu-rich and near stoichiometric In-rich CulnSe2 films synthesized using the graphite box. The optical transmission spectra of the films with thickness of about 21xm measured at room temperature showed a high transmission with a steep absorption edge at 1250 nm. In-rich films have higher transmission which reduces as the Cu to In ratio increased. The transmission curves show similar pattern as shown in Fig. 4. However, a shift in curve toward higher wavelength end for Cu-rich films was observed. A similar shift of the transmission curve for Cu-rich films was reported by Noufi et al. [14]. They attributed this effect to free carrier absorption and diffusion of light at the longer wavelength end and defects of high disorder due to excess Cu at the shorter wavelength end. For an absorbing material, the absorption coefficient, is given by [ 15,16], a = ( I / t ) I n [ T / ( I - R)2],

(I)

where T is transmittance, R is reflectance, and t is the film thickness. This equation is acceptable in this calculation since the incidence angle of spectrophotometer used in this measurement is less than 10° and is therefore assumed to be normal. The values of o~ were calculated from the measured transmittance and reflectance for a wavelength range 800 nm to 2000 nm using Eq. (I). The absorption coefficient, c(, for a direct transition is related to the band gap of a semiconductor and is given by [17]. (ethv)

= A / ( h v ) [ ( h v - Eg)] '/z,

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where A is a constant, h v is the photon energy, and Eg is the energy gap. A plot of ( a h v ) z against h is shown in Fig. 5. The energy gap, Eg, is estimated from the intercept

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F.O. Adurodija et al. / Solar Energy Materials and Solar Cells 40 (1996) 359-369 1.2X109 ........................................... a-..--~ Cu-rich i

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of the linear portion of the curve on the h axis to be from 0.99 eV and 1.02 eV for a Cu-rich and a near stoichiometric In-rich film. Films exhibited a range of resistivity varying from 10 -3 to 104 cm from the Four Point Probe measurements. All films with C u / I n >_ 1 had low resistivity and sometimes an extra Cu 2-x Se phase, whilst resistivity increased with a decrease in the Cu content in the films. Calculations of the non-molecularity, ~, and valence stoichiometry, e, was carried out. Analysis of the two parameters could provide us with information about the electrical properties of the films for the range of composition considered. The ~ and e relate to the defect and electrical properties of the material [18-20]. Calculations of and e was obtained from the EDX data using equations [18], = ( [ C u l / [ I n ] ) - 1,

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where [Cu], [In] and [Se] are total concentrations of Cu, In and Se atoms respectively in the films. (Error in the EDX was less than 5%.) The atomic composition and the calculated values of g and e are shown in Table 1. Values of ~ > 0 (i.e., C u / I n > 1 or Cu rich films) corresponds to an excess of CU2_xSe. These materials tend towards degenerate state, because so many acceptors are present and states at the top of the valence band are almost completely occupied by holes and the Fermi level lies within the valence band. Cu2_xSe phases were detected in the films with g >> 0. A decrease in the C u / I n (i.e., g < 0, In rich), the material approached non degenerate state. This analysis method was also used by Von Bardeleben and Rincon et al. [19,20] to determine the electrical properties and defect chemistry of their vacuum coevaporated CuInSe 2 films. The value of • determines whether there is an excess or deficiency of Se and it controls the conductivity type of the films. • = 0 indicated intrinsic material and • > 0 or e < 0 is extrinsic and should lead to p- or n-type conductivity for a fixed molecularity. Table 1 showed large variations of from positive to negative values. Large negative


367

indicated a deficiency of Se and a consequent low acceptor density, but as e increases towards positive values, better p-type conductivity was achieved. For Cu-rich films, low Se content were measured. Cu-rich films are not normally suitable for efficient device fabrication but could be used as starting material for bilayer CulnSe 2 structure with In-rich layer on the top. Results from the hot probe measurements showed similar behaviour with low negative voltages as the p-type conductivity decreased in value. However, some samples revealed p-type for c < 0. These general trends observed in all films are similar to interpretations given by [19,20] for values ~ and ~ in their analysis of vacuum evaporated films. Hall measurements was carried out at room temperature on the samples to determine the hole mobility and carrier concentration. Samples used had composition varying from Cu-rich to In-rich. Hole mobility of 0.6 to 7.8 cm 2 V-T s - l , carrier concentration 5 X 1015 tO 1017 cm -3 were obtained, Very low resistivities and mobilities were obtained for Cu-rich p-type films and these values increased as the Cu content in the films decreased. The use of the graphite box in the synthesis of CuInSe z from the stacked elemental layers was found to play three major roles during the film formation. Firstly, it provided a controlled high vapour pressure (HVP) of Se due to a reduced loss of Se particularly at 210°C where CuInSe z formation starts with an increased heat of formation as a result of an exothermic reaction between 13-1nzSe3 and liquid Se. The HVP of Se within the box was beneficial to CuInSe 2 formation by suppressing the exothermic reaction at this temperature which tended to cause poor crystallization of the films [13,21]. Secondly, it compensated for any excessive loss of Se which could leave the film n-type during the CuInSe 2 formation. Thirdly, it improved the surface uniformity of the films particularly when Se shot are included in the box during annealing. Heterojunction devices were fabricated using the structure Z n O / C d S / C u I n S e 2. Measurements of the J - V characteristics showed a Voc = 400 mV, J,c = 27 m A c m 2 FF = 55%, and efficiencies of 6.5% over an area of 0.25 cm 2. The low efficiency was due to a high series resistance in the cells, A typical J - V characteristic of the solar cells is shown in Fig. 6. It is believed that optimizing all the necessary design and processing parameters required in the graphite box system, films producing more efficient devices over large areas could be produced.

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4. C o n c l u s i o n

The graphite system has produced CulnSe 2 films with single phase chalcopyrite structures and improved crystal sizes suitable for solar cells due to the better control of the Se high vapour pressure during annealing. The films have a homogeneous surface and dense crystallites with sizes of 2 ktm to 4 txm. No segregation or anomalous growth occurred. The problem of poor crystallization and adhesion experienced in vacuum annealed films could be circumvented by annealing or selenizing the films in the graphite box. Optical band gap of 0.99 eV and 1.02 eV were obtained for a Cu-rich and a near stoichiometric In-rich films which agree with the theoretical value for CuInSe 2. The film composition varied from Cu-rich to In-rich with electrical resistivities of 10 -3 to l04 cm, carrier concentrations of 5 X l015 to 1017 c m -3 and mobilities of 0.6 to 7.8 cm 2 V -1 s -1 . Heterojunction devices using the structure Z n O / C d S / C u I n S e 2 were fabricated with efficiencies of 6.5%. The low vacuum (10 -2 Torr) required and the absent of toxic gases during annealing in the graphite box makes this method economical and environmentally acceptable. It has the potential for an in-line integration into a commercial manufacture using any deposition technique to produce the elemental precursors.

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[16] W.K. Chu, J.W. Mayer and M.A. Nicolet, Backscattering Spectroscopy, (Academic Press, New York, 1978). [17] W. Horig, H. Neumann, H. Sobotta, B. Schumann and G. Kuhn, Thin Solid Films 48 (1978) 67. [18] J.A. Groenink and P.H. Janse, Z. Phys. Chem. 110 (1978) 17. [19] C. Rincon, C. Gonzalez and G. Sanchez Perez, Solar Cells, 16, 1986, p. 335. [20] H.J. Von Bardeleben, Solar Cells 16 (1986) 381. [21] F.O. Adurodija, M.J. Carter and R. Hill, Sol. Energy Mater. Sol. Cells 37 (1995) 203.