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Polycrystalline Cu[In.sub.3][Se.sub.5] thin film photoabsorber deposited by the pulsed laser deposition technique/pulseerival lasersadestamise meetodil valmistatud polukristalliline Cu[In.sub.3][Se.sub.5]-fotoabsorberkile.

INTRODUCTION

Cu[In.sub.3][Se.sub.5] is a promising photoabsorber of n-type conductivity [1,2] for solar cells due to the band gap value of about 1.3 eV [3], which is close to the optimal value of 1.4 eV, and high photoconductivity over a broad wavelength range [2]. On the other hand, this compound is less studied in comparison with CuIn[Se.sub.2] in the pseudo-binary system [Cu.sub.2]Se-[In.sub.2][Se.sub.3] [4]. Moreover, this compound has been mainly investigated in a bulk state and only a few papers describe the preparation of and characterize the thin films fabricated by flash evaporation and laser ablation techniques [1,3,5].

The main purpose of the present work was preparation and investigation of a polycrystalline Cu[In.sub.3][Se.sub.5] thin film photoabsorber and optimization of the deposition technique for its solar cell application. It should be noted that this study was connected with our current aim to prepare a number of n-photoabsorbers for new organic--inorganic hybrid photovoltaic (PV) structures where an inorganic photoabsorber layer of n-type forms an n-p PV junction with an organic layer of p-type (conductive polymers, phthalocyanines, etc.) [6,7]. We demonstrate formation of high-quality polynanocrystalline Cu[In.sub.3][Se.sub.5] photoabsorber layers grown on glass/indium tin oxide (ITO) substrates by using the pulsed laser deposition (PLD) technique. PV properties with both as-deposited and annealed Cu[In.sub.3][Se.sub.5] layers are studied.

EXPERIMENTAL

Polycrystalline bulk Cu[In.sub.3][Se.sub.5] samples for the PLD targets were synthesized from 99.999% pure elements in evacuated quartz ampoules. The ampoules were inserted in a pipe furnace, heated up to 1100'C, kept at this temperature for 5 h, and then very slowly cooled down. The synthesized samples were tested using X-ray diffraction (XRD) analysis carried out on a DRON-3.0 diffractometer equipped with a monochromatic FeK [alpha] source.

The films were deposited by using the PLD technique in accordance with the method developed by us for the CuIn[Se.sub.2] based films [8]. Ablation of the targets was carried out with a XeCl excimer laser. For preventing a decrease of the Se content in the film during the deposition on the heated substrate we used an advanced 3-stage temperature-time regime of deposition: at the first stage the initial Cu[In.sub.3][Se.sub.5] film was deposited at room temperature of the substrate, then deposition was stopped and the deposited glassy film was heated up to temperature [T.sub.1] for crystallization, and finally deposition was continued at a relatively lower temperature of the substrate T2. The value of [T.sub.2] can be lower than of [T.sub.1] because the initial layer of the polycrystalline Cu[In.sub.3][Se.sub.5] is already formed and therefore nucleation is not a limiting stage for the process of crystallization. The appropriate values of [T.sub.1] and [T.sub.2] were chosen in accordance with our previous results [8] ([T.sub.1] = 320[degrees]C, [T.sub.2] =160[degrees]C). Both additional annealing processes--in situ (immediately after the film deposition, without opening the vacuum chamber) and after contact with air--were performed in vacuum at 400 [degrees]C. It should be noted that the ablation rate for Cu[In.sub.3][Se.sub.5] target was much higher in comparison with the CuIn[Se.sub.2] target at the same power of the excimer laser. For the XRD measurement the Cu[In.sub.3][Se.sub.5] films were deposited onto glass substrates; for the investigation of PV properties, the Cu[In.sub.3][Se.sub.5] films were deposited onto glass/ITO substrates. The thicknesses of the Cu[In.sub.3][Se.sub.5] photoabsorber layers on the glass/ITO substrates were determined by using the scanning electron microscopy (SEM) technique. The average thickness of the deposited Cu[In.sub.3][Se.sub.5] films was around 300 nm. The cross-section and surface morphology of the Cu[In.sub.3][Se.sub.5] layers were investigated by the SEM technique, using a commercial high-resolution LEO SUPRA 35 microscope equipped with an energy dispersive spectroscopy (EDS) analyser.

Optical transmission spectra of the films were measured with a spectrophotometer SF-8 (USSR) in the spectral range 350-2050 nm.

All the electrochemical PV measurements were performed in a standard three-electrode cell in 0.1 M [H.sub.2][SO.sub.4] background solution using an AUTOLAB PGSTAT 30 potentiostat/galvanostat. White light with an intensity of 100 mW/[cm.sup.2] from a halogen lamp was used for irradiation. Aqueous based graphite suspension (Alfa Aesar) was used for the preparation of contacts on the surface of Cu[In.sub.3][Se.sub.5] for solid-state I-V measurements.

RESULTS AND DISCUSSIONS

In the above-mentioned conditions we obtained wellcrystallized Cu[In.sub.3][Se.sub.5] films, as it was confirmed by the XRD spectroscopy (Fig. 1). The position of the main peak at the interplanar distance of 3.33 [angstrom] coincides with Cu[In.sub.3][Se.sub.5] and CuIn[Se.sub.2] reference data [7]. The XRD patterns of Cu[In.sub.3][Se.sub.5] and CuIn[Se.sub.2] are very similar except for a few additional reflections of low intensity [10].

[FIGURE 1 OMITTED]

Intensive reflection from (112) plane (Fig. 1) means columnar growth of the film crystals with preferred orientation along (112) direction. The columnar structure may be due to the direction of the stream of plasma from the target nonperpendicular to the substrate [11]. It was done in order to increase the thickness uniformity of the films (the appropriate angle was about 45 degrees).

The grain size of film crystals (D) was calculated using Scherrer's equation [12]:

D = 0.94 [lambda] / [beta] cos [theta], (1)

where [lambda] is X-ray wavelength, [beta] is the full width at half maximum in radian, and 6 is the Bragg angle. The grain-size value calculated from (112) reflex for the deposited Cu[In.sub.3][Se.sub.5] thin films was about 20 nm.

Analysis of the composition of the as-deposited polynanocrystalline film by using the EDS technique showed a small excess of In and a small deficiency of Se against stoichiometry (see Table 1).

Figure 2 shows the optical absorption spectra of representative Cu[In.sub.3][Se.sub.5] film annealed in the vacuum chamber. The optical band-gap ([E.sub.opt]) of annealed Cu[In.sub.3][Se.sub.5] films was determined on the basis of Eq. (2), where optical absorption coefficient (a) is related with the energy gap of a semiconductor (for [alpha] > [10.sup.4] [cm.sup.-1]) [13]:

[alpha]hv [congruent to] A(hv - [[E.sub.opt]).sup.n] , (2)

where A is a constant (which equals about [10.sup.5] [CM.sup.-1] [eV.sup.-1] at n = 2), by is energy of the incident photon, n is a parameter that characterizes the process of electronic transition between the valence and conduction bands. For the Cu-In-Se compounds, direct allowed transitions (n =1 / 2) are proposed. Therefore the optical band gap for film can be defined as the intersection of the line approximating the experimental curve in the coordinates [(ahv).sup.2]-hv with abscissa. The determined value of Eopt (1.21 eV) is in a good agreement with the reference data 1.23 eV [1] and 1.26 eV [3] and looks like typical for such photoabsorbers.

[FIGURE 2 OMITTED]

Photosensitivity of the deposited Cu[In.sub.3][Se.sub.5] photo-absorber films was estimated as the difference between the values of 'dark' and 'light' currents in I--V curves. Figure 3 shows that the I--V curve of the glass/ITO/ Cu[In.sub.3][Se.sub.5] structure under chopped white light of 100 mW/[cm.sup.2] intensity has a non-linear character and demonstrates a relatively high photosensitivity of the film with increasing photoconductivity under white light illumination pulses.

In addition, we tried to enhance the morphology and PV parameters of prepared photoabsorber films by additional annealing in vacuum. Figure 4 shows the cross-sectional view of as-deposited and annealed layers of the Cu[In.sub.3][Se.sub.5] photoabsorber.

[FIGURE 3 OMITTED]

As a result of PLD, well-coherent dense uniform Cu[In.sub.3][Se.sub.5] film was prepared (Fig. 4a). It was found that additional annealing in vacuum (after intermediate contact with air) led to a decrease of the photoconductivity of the deposited Cu[In.sub.3][Se.sub.5] film. At the same time, the composition of the annealed film became closer to the stoichiometric composition and the crystalline structure became more pronounced (Fig. 4b). According to our assumption, oxygen and moisture from air affect dramatically the active 'just-deposited' semi-amorphous Cu[In.sub.3][Se.sub.5] layer. Figure 4b shows that the size of the Cu[In.sub.3][Se.sub.5] crystallites after annealing in vacuum was in the range 50-200 nm.

On the other hand, additional annealing in vacuum in situ (immediately after the film deposition, without opening the vacuum chamber) at 400[degrees]C for 20 min led to increasing photoconductivity of the annealed Cu[In.sub.3][Se.sub.5] layer in comparison with the as-deposited layer (Figs 3, 5), the shape of the I--V curve became typical of a diode junction (Fig. 5). The shape of the I-V curve confirmed the existence of the Schottky junction between n-Cu[In.sub.3][Se.sub.5] and quasi-metallic ITO layers. The SEM micrograph (Fig. 4c) shows the polycrystalline morphology of dense Cu[In.sub.3][Se.sub.5] films prepared in this way. The films include interconnected grains with an average grain size of 50-200 nm. It should be noted that the formation of a more crystalline structure of the Cu[In.sub.3][Se.sub.5] photoabsorber layer resulted in increasing photosensitivity.

The measurements of the photoconductivity of the Cu[In.sub.3][Se.sub.5] film under chopped white light in the background electrolyte confirmed high photosensitivity and n-type of the conductivity of the deposited films.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

The photoelectrochemical characteristics of the prepared Cu[In.sub.3][Se.sub.5] films were determined in 0.1 M sulphuric acid background solution under chopped white light according to the technique described in our previous paper [6]. Figure 6 shows that annealed in situ Cu[In.sub.3][Se.sub.5] film is a photosensitive material and produces a positive photocurrent under white light pulses in the positive range of the applied potential values. In the glass/ITO/Cu[In.sub.3][Se.sub.5] electrode electron-hole pairs can be generated by light absorption. Therefore, photogenerated minority carriers (holes) are driven to the Cu[In.sub.3][Se.sub.5] surface by the electric field, in which they are consumed in photoelectrochemical processes. As it follows from the dependence, the obtained Cu[In.sub.3][Se.sub.5] photoabsorber film has n-type conductivity and could be applied as an n-photoabsorber in complete cell structures; for example, in combination with organic semiconductors of p-type. Thus, by adjusting the deposition temperature and post-deposition annealing in situ, polycrystalline photosensitive Cu[In.sub.3][Se.sub.5] layers can be prepared.

[FIGURE 6 OMITTED]

CONCLUSIONS

A polycrystalline Cu[In.sub.3][Se.sub.5] thin film photoabsorber of ntype was prepared by using the pulsed laser deposition technique. It was found that additional annealing in situ in a vacuum chamber improved the polycrystalline structure and PV properties of deposited Cu[In.sub.3][Se.sub.5] photoabsorber layers. The best crystallinity and photosensitivity of the prepared Cu[In.sub.3][Se.sub.5] layers was reached at the annealing temperature of 400[degrees]C. The Cu[In.sub.3][Se.sub.5] layers prepared in this way can be potentially applied as n-photoabsorber layers for example in heterojunction hybrid organic--inorganic solar cells as well as for the preparation of devices with controlled donor-acceptor interfaces.

ACKNOWLEDGEMENTS

Financial support from INTAS Project Ref. No. 03-51-4561 and from the Estonian Science Foundation (G7595 and G7669) is acknowledged.

Received 29 May 2008, revised 3 September 2008, accepted 11 September 2008

REFERENCES

[1.] Ariswan, G. E. H. M., Abdelali, M., Guastavino, F., and Llinares, C. Structural, optical and electrical properties of the ordered vacancy compound Cu[In.sub.3][Se.sub.5] thin films fabricated by flash evaporation. Solid State Commun., 2002,124,391-396.

[2.] Wang, H. P., Shih, L, and Champness, C. H. Studies on monocrystalline CuIn[Se.sub.2] and Cu[In.sub.3][Se.sub.5]. Thin Solid Films, 2000, 494, 361-362.

[3.] Malar, P. and Kasiviswan athan, S. Characterization of stepwise flash evaporated Cu[In.sub.3][Se.sub.5] films. Sol. Energ. Mat. Sol. C., 2005, 85, 521-533.

[4.] Malar, P. and Kasiviswan athan, S. A comparative study of CuIn[Se.sub.2] and Cu[In.sub.3][Se.sub.5] films using transmission electron microscopy, optical absorption and Ruther ford backscattering spectrometry. Sol. Energ. Mat. Sol. C., 2005, 88, 281-292.

[5.] Bodnar, 1. V., Gremenok, V. F., Nikolaev, Yu. A., Rud, V. Yu., Rud, Yu. V., and Terukov, E. 1. Photosensitivity of thin-film structures based on Cu[In.sub.3][Se.sub.5] and CuIn5Se8 ternary semiconductor compounds. Tech. Phys. Lett., 2007, 33(2), 111-113.

[6.] Bereznev, S., Kois, J., Golovtsov, L, Opik, A., and Mellikov, E. Electrodeposited (Cu-In-Se)/polypyrrole PV structures. Thin Solid Films, 2006, 511-512, 425-429.

[7.] Bereznev, S., Koeppe, R., Konovalov, L, Kois, J., Gunes, S., Opik, A., Mellikov, E., and Sariciftci, N. S. Hybrid solar cells based on CuInS2 and organic buffer-sensitizer layers. Thin Solid Films, 2007, 515, 5759-5762.

[8.] Tveryanovich, A., Borisov, E. N, Vasilieva, E. S., Tolochko, O. V., Vahhi, 1. E., Bereznev, S., and Tveryanovich, Yu. S. CuIn[Se.sub.2] thin films deposited by UV laser ablation. Sol. Energ. Mat. Sol. C., 2006, 90, 3624-3632.

[9.] Wang, H. P., Shih, L, and Champness, C. H. Studies on monocrystalline CuIn[Se.sub.2] and Cu[In.sub.3][Se.sub.5]. Thin Solid Films, 2000, 361-362, 494-497.

[10.] Boehnke, U. C. and Kuhn, G. Phase relations in the ternary system Cu-In-Se. J. Mater. Sci., 1987, 22, 16351641.

[11.] Buzea, C. and Robbie, K. Nano-sculptured thin film thickness variation with incidence angle. J. Optoelectron. Adv. Mater., 2004, 6, 1263-1268.

[12.] Birks, L. S. and Friedman, H. Particle size determination from X-ray line broadening. J. Appl. Phys., 1946, 16, 687-692.

[13.] Mott, N. F. and Davis, E. A. Electron Processes in NonCrystalline Materials. Clarendon Press, Oxford, 1979.

Andrey Tverjanovich (a), Sergei Bereznev (b) *, Evgeny N. Borisov (a), Dongsoo Kim (c), Julia Kois (b), Kristjan Laes (b), Olga Volobujeva (b), Andres Opik (b), Enn Mellikov (b), and Yuri S. Tveryanovich (a)

(a) Saint-Petersburg State University, 198504 Saint-Petersburg, Staryi Petergof, Ul'yanovskaya 5, Russia

(b) Department of Materials Sciences, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia Korea Institute of Machinery and Materials, Sangnam-Dong 66, Kyungnam 641-010, South Korea

* Corresponding author, sergei@staff.ttu.ee
Table 1. Results of EDS analysis of representative as-deposited film
and after thermal annealings. The values in the brackets show the
deviations in the composition of samples vs. Cu[In.sub.3][Se.sub.5]
stoichiometry

                   Content, at.%

                    Annealing in    Annealing in
                    vacuum after   vacuum chamber
                    contact with   of deposition
     As-deposited       air            setup

Cu   11.96 (+0.9)   13.31 (+2.2)    15.47 (+4.4)
In   39.51 (+6.2)   35.41 (+2.1)    34.64 (+1.3)
Se   48.52 (-7.0)   51.28 (-4.2)    49.90 (-5.6)
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Title Annotation:MATERIALS SCIENCE
Author:Tverjanovich, Andrey; Bereznev, Sergei; Borisov, Evgeny N.; Kim, Dongsoo; Kois, Julia; Laes, Kristja
Publication:Proceedings of the Estonian Academy of Sciences
Article Type:Report
Geographic Code:4EXES
Date:Mar 1, 2009
Words:2609
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