Optical manipulation of temperature formation of CuInSe2 thin films

Materials Science and Engineering B 134 (2006) 63–67

Optical manipulation of temperature formation of CuInSe2 thin films A. Ashour a,∗ , A.A.S. Akl a , A.A. Ramadan b , N.A. El-Kadry a , K. Abd El-Hady a a

b

Faculty of Science, Physics Department, Minia University, Minia, Egypt Faculty of Science, Physics Department, Helwan University, Helwan, Egypt

Received 22 April 2006; received in revised form 26 June 2006; accepted 23 July 2006

Abstract Polycrystalline thin films of CuInSe2 were grown onto glass substrates using the stacked elemental layer (SEL) technique involving the annealing at different temperatures in air atmosphere for different times. The variation in the structure and optical properties of the CuInSe2 thin films on the annealing temperature and time was investigated using X-ray diffraction (XRD) and optical measurements, respectively. The different structural properties were clearly reflected in X-ray diffraction studies. It was concluded that CuInSe2 phase is dominated at annealing temperature of 300 ◦ C for a time ≥1 h. Ternary phase of CuInSe2 was characterized by highly both transmission and absorption, optimization of the optical constants and band gap. Optical absorption studies indicate a direct band gap range around 1.0 eV. The data on structural and optical properties covering this technique will be presented. © 2006 Elsevier B.V. All rights reserved. Keywords: Stacked elemental layer (SEL); CuInSe2 ; XRD; Optical properties

1. Introduction

2. Experimental details

Copper indium diselenide with a direct band gap of 1.1 eV and high absorption coefficient has been extensively studied for photovoltaic and photoelectrochemical solar cells applications [1–4]. It can be grown with p- as well as n-type doping and, therefore, both homojunction and heterojunction thin film solar cells have already demonstrated terrestrial active area conversion efficiencies of over 18% [5]. Techniques such as flash evaporation, RF and ion beam sputtering, molecular beam epitaxy, spray pyrolysis, electrodeposition, and co-evaporation have all been employed in the production of thin film solar cells [6–9]. Preparation of CuInSe2 (CIS) thin films for photovoltaic application by stacked elemental layers (SEL), and followed by thermal annealing seems more attractive on the economic considerations [10]. In this study, the structural and optical properties of polycrystalline CIS absorbers, which have been grown by SEL technique, were mainly characterized by X-ray diffraction, reflection and transmission measurements. The variation of the parameters on them is discussed.

The preparation of CuInSe2 thin films is described in detail in Ref. [7]. Briefly, the CuInSe2 films were produced by thermal air annealing of stacked elemental layers. Alternate layers of In, Se and Cu were vacuum deposited onto glass substrates by thermal evaporation. A quartz thickness monitor (Edwards Model FTM3) was used to control both film thickness and deposition rate inside the vacuum chamber. The thickness of the deposited CuInSe2 films were measured accurately after deposition by utilizing multiple-beam Fizeau fringes at reflection. The individual layer thicknesses were generally chosen to be in the ratios 1.0:2.2:4.6 to achieve a 1:1:2 stoichiometric ratios for copper, indium and selenium, respectively. The total thickness of each sandwich was approximately 150 nm, as shown in Fig. 1. The annealing in air was performed at 200, 250, 300 and 400 ◦ C for different times (from 1 to 4 h), to study the effect of annealing on the structure and phases. After annealing, the temperature was decreased gradually to room temperature with 35 ◦ C/min. The XRD analysis was carried out for phase identification of the deposits. This was made by JEOL diffractometer (model JSDX-60PA) with Zr-filter Mo K␣ radiation. Continuous scanning was applied with a slow scanning speed and a small time constant. A range of 2θ (from 5◦ to 36◦ ) was scanned, so that the required diffraction peaks for phase. A comparison with JCPDS file cards was done for the establishing the observed peaks.

∗

Corresponding author. Tel.: +2 086 2369149; fax: +2 086 2363011. E-mail address: aashour [email protected] (A. Ashour).

0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.07.028

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3.1. Structural properties of polycrystalline CuInSe2

Fig. 1. Typical stacked elemental layer (SEL) structure of the sandwich.

The absorption coefficient (α) and the band gap energy (Eg) were calculated from the transmission (T) and reflection (R) spectra. The latter were obtained with Shimadzu double beam spectrophotometer, UV-310 PC: UV–vis–NIR scanning spectrophotometer in the wavelength range 400–2400 nm at room temperature. The optical constants were determined by the graphical method.

3. Results and discussion The optimization of the structural and optical properties of polycrystalline CuInSe2 absorber films is an important prerequisite for the successful fabrication of high-ternary phase of CuInSe2 films. The information gained from this study established a basis for the fabrication of high-ternary phase thin films in our laboratory.

XRD studies showed all films to be polycrystalline. The diffractograms of the In/Se/Cu sequence annealed at different temperatures in air for heating time of 1, 2 and 4 h are shown in Fig. 2. Identification of the present phases showed that the binary Cu7 In4 , In2 Se3 as well as the ternary CuInSe2 compounds are present. However, their abundance depends on the annealing temperature and heating time. The obtained results can be discussed according to the ability of the different elements to react together to form binary compounds and then the ternary one due to annealing. A good comparison can also be achieved by comparing the variation of the diffraction patterns of the film annealed at different temperatures but for the same period. Fig. 2 depicts such diffractograms for annealing time of 1, 2 and 4 h. For short time annealing of 1 and 2 h, the diffractograms can be classified into two groups, the first one for 200 and 250 ◦ C and the second for 300 and 400 ◦ C. The first group shows considerable amount of the binary phases. However, the second one exhibits considerable content of the ternary CuInSe2 phase. On the other hand, annealing for 4h shows that the features of second class starts to dominate even from 250 ◦ C. Therefore, optimization of both the annealing temperature and time is of vital importance. At any annealing, the ternary CuInSe2 phase did not form at temperatures below 250 ◦ C. However, growth of the ternary phase is observed at heating temperatures ≥300 ◦ C. It is clear that an increase of the intensity of the (h k l) peaks is achieved after annealing for 1 h. For higher temperature (400 ◦ C), almost no or small variation in intensity is observed. Comparison of the variation of the content of the ternary phase with that of the binary ones is also needed. The growth

Fig. 2. X-ray diffractograms of the annealed films prepared by SEL after annealing times at 1, 2 and 4 h of In/Se/Cu sequence with different temperatures (CI, Cu7 In4 ; IS, In2 Se3 ; CIS, CuInSe2 ).

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Fig. 3. Optical transmission (T) vs. wavelength of CuInSe2 films of different sequences (annealing temperature = 300 ◦ C, for time = 1 h).

of the binary phases at 200 ◦ C is clearly remarkable at any annealing period from 1 h up to 4 h. However, after the decrease in content of the binary phases at 300 ◦ C, another increase is observed at 400 ◦ C. This last increase of the binaries at high temperature seems to be decomposition of the ternary phase. It is clear that, the ternary phase is obtained at annealing temperature equal to 300 ◦ C for any heating time. Therefore, to ensure the presence of the ternary phase only, the annealing temperature must be maintained at a temperature of at least 300 ◦ C and time ≥1 h. The conclusions obtained were found to be in good agreement with published thin films data [10–13]. 3.2. Optical properties of polycrystalline CuInSe2 The different structural properties of CuInSe2 thin films were clearly reflected in the optical studies, indicating high transmission for In/Se/Cu sequence compared to the other sequences. The optical properties and the possible electronic transitions were investigated by measuring the transmission spectra. The dependence of the spectral transmittance curves on wavelength for different sequences is shown in Fig. 3. The flat aspect of the transmission curves without interference fringes emphasis the ternary phase of CuInSe2 and surface uniformity with small crystallite size. The films typically exhibit the semiconductor behaviour and transmittance values of 20–60% at wavelengths above the absorption edge (λg). It was observed that, the sequence with In layer at the top (Se/Cu/In) resulted in poor adhesion of the film with pinholes due to melting and vaporization of In showing dark colour after annealing and very low transmission. When Cu was used as the top layer (Se/In/Cu), the pinholes increased with slight increase of transmission. Also using Cu as the base layer and either In or Se as top layer (Cu/Se/In or Cu/In/Se), showed poor transmission and appear pale dark in colour. Transparent films with good adhesion and surface homogeneity were obtained when In was used as base layer. When using Cu as top layer (In/Se/Cu), resulted relatively higher transmission and sharp optical transition with pale yellow in colour, which can be attributed to the improvement in perfection, stoichiometry and of nearly single-phase formation of the film [10,14]. Also, when the sequence In/Se/Cu is annealed at 300 ◦ C (see Section 3.1), it shows a preferable one for ternary phase formation and most the secondary binary phases are removed. All such evidences recom-

Fig. 4. Optical parameters (A) refractive index (n) and (B) absorption index (k) vs. wavelength for stacked elemental layer CuInSe2 thin film (annealing temperature = 300 ◦ C, for time = 1 h).

mended the sequence In/Se/Cu as the optimum layer sequence. Such conclusions sustain the results and conclusions obtained from the structural investigations using XRD. The refractive index (n) was determined from the reflectance (R) data using [15]: R=

(n − 1)2 (n + 1)2

(1)

As shown in Fig. 4A, the refractive index of the film of sequence In/Se/Cu is greatly influenced by the wavelength (λ). The refractive index decreases as the wavelength increases. This is in a good agreement with semiconductor behaviour [16]. The absorption coefficient, α, and the extinction coefficient (absorption index), k, were obtained from the transmittance, T, and reflectance, R, using the approximate formula [17]: T = (1 − R)2 exp(− αt) 1 − R2 exp(−2αt)

(2)

where t is the thickness and α is related to the extinction coefficient by α=

4πk λ

(3)

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A. Ashour et al. / Materials Science and Engineering B 134 (2006) 63–67

Fig. 6. (αhν)2 plot vs. photon energy (hν) for CuInSe2 thin films.

Fig. 5. Absorption coefficient (α) spectra for CuInSe2 thin films vs. photon energy (hν) at different annealing time.

Fig. 4B shows the variation in k as a function of the wavelength, which is found to decrease with increasing wavelength in contrast as the behaviour of refractive index. The absorption coefficient, α, corresponding to the samples of sequence In/Se/Cu and annealed in air at 300 ◦ C for different times (from 45 to 240 min) is drawn in Fig. 5 as a function of photon energy. Curves reveal the absorption by tails of density of states or impurity bands [18]. The films had values for α of the order 104 cm−1 , indicating that the material is of the direct-band-gap type and that transition is allowed [19,20]. Our values of α in the neighbourhood of the energy gap are similar to those obtained by others [10,6]. As it is seen the absorption coefficient is influenced by the time of annealing in air. As shown in Fig. 5, that the film with sequence In/Se/Cu has higher absorption coefficient (α) when it annealed in air at 300 ◦ C for 1 h (60 min). Kazmerski et al. [21] have demonstrated the effect on the absorption characteristics of thin films by annealing. Air annealing increases the absorption coefficients, this influence was not due to an oxide formation, but to an improvement in compositional uniformity [10]. The energy gap is one of the most important parameters in producing solar cells. The optical energy gap of the CuInSe2 thin films was estimated from the optical measurements. The absorption coefficient, α, is related to the energy gap, Eg, according to the equation [22]: α=

A (hν − Eg)n hν

(4)

where Eg is the optical energy gap, A is a constant and n = 1/2 or 2 for an allowed direct, or indirect-transition energy gap, respectively. The last parameter in this study is the band gap energy. It is well known that for direct band gap (αhν)2 versus hν is a straight line with an intercept on the energy axis of Eg as shown in Fig. 6. Since the values of Eg we obtained are close to 1.0 eV. The lower value of Eg is due to the impurity absorption and the higher value of Eg reveals the mixture of several phases [18]. The range of band gap energy obtained in this work is suitable

for absorption of the long wavelength photons in the solar spectra and is in good agreement with previously published data for CIS thin films prepared by other methods [10,6–9]. On the other hand, the optimum value of Eg (∼1.10 eV) is due to the formation of single phase of ternary CIS as shown for the best In/Se/Cu sequence, which annealed at 300 ◦ C for 60 min. This conclusion is confirmed by the X-ray diffraction (see Section 3.1). 4. Conclusions CuInSe2 (CIS) polycrystalline thin films of sufficient quality for solar cell devices have been prepared on glass substrates using a stacked elemental layer (SEL) technique followed by thermal annealing in air at 300 ◦ C for different annealing times (from 45 to 240 min). XRD has shown the mixture of binary and ternary phases of the as-deposited SEL structure. The best film is the sequence In/Se/Cu, which gives a ternary compound content higher than that of the binary compounds. Values of direct band gap close to 1.0 eV are obtained. The great advantage of this study is that, the formation of single phase of ternary CIS can be achieved by the annealing of the best In/Se/Cu sequence at 300 ◦ C for heating times ≥1 h. References [1] A. Shimizu, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 39 (2000) 109. [2] Y. Onuma, K. Takeuchi, S. Ichikawa, M. Harada, H. Tanaka, A. Koizumi, Y. Miyajima, Solar Energy Mater. Solar Cells 69 (2001) 261. [3] R. Trykozko, R. Bacewicz, J. Filipowicz, Solar Cells 16 (1986) 351. [4] J. Szot, U. Prinz, J. Appl. Phys. 66 (1989) 6077. [5] N. Nancheva, P. Docheva, N. Djourelov, M. Balcheva, Mater. Lett. 54 (2002) 169. [6] N. Stratieva, E. Tzvetkova, M. Ganchev, K. Kochev, I. Tomov, Solar Energy Mater. Solar Cells 45 (1997) 87. [7] R. Hill, M.J. Carter, H. Oumous, A. Knowles, Proceeding of the EuropiumNew Energies Congress, vol. 3, 24–28 October, Saarbr¨ucken, FR, Germany, 1988. [8] H. Neumann, B. Perlt, N.A.K. Abdul-Hussein, R.D. Tomlinson, A.E. Hill, Cryst. Res. Technol. 17 (1982) 469. [9] T. Yamaguchi, J. Matsufusa, A. Yoshida, Solar Energy Mater. Solar Cells 27 (1992) 25. [10] N. Kavcar, M.J. Carter, R. Hill, Solar Energy Mater. Solar Cells 27 (1992) 13.

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