Research Article Photoconductive Properties of Brush Plated Copper Indium Gallium Selenide Films

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1 Coatings Volume 213, Article ID 19176, 9 pages Research Article Photoconductive Properties of Brush Plated Copper Indium Gallium Selenide Films N. P. Subiramaniyam, 1 P. Thirunavukkarasu, 2 and K. R. Murali 3 1 Department of Electronics and Communication Systems, A.J.K.College of Arts and Science, Coimbatore 64115, India 2 Department of Electronics, Ramakrishna Mission Vidyalaya College of Arts and Science, Perianaickenpalayam, Coimbatore 6412, India 3 ECMSDivision,CSIR-CECRI,Karaikudi6,India Correspondence should be addressed to K. R. Murali; muraliramkrish@gmail.com Received 26 May 213; Accepted 6 September 213 Academic Editor: Mariana Braic Copyright 213 N. P. Subiramaniyam et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Copper indium gallium selenide (CIGS) films were deposited for the first time by the brush electrodeposition technique. X- ray diffraction studies indicated the formation of single phase chalcopyrite CIGS. Lattice parameters, dislocation density, and strain were calculated. Band gap of the films increased from 1.12 ev to 1.63 ev as the gallium concentration increased. Room temperature transport parameters of the films, namely, resistivity increased from.1 ohm cm to 12 ohm cm, mobility decreased from 125 cm 2 V 1 s 1 to 2.9 cm 2 V 1 s 1, and carrier concentration decreased from cm 3 to cm 3 as the gallium concentration increased. Photosensitivity of the films increased linearly with intensity of illumination and with increase of applied voltage. 1. Introduction In recent years, quaternary chalcopyrite compound CuIn 1 x Ga x Se 2 (CIGS) has been one of the most promising absorber materials for high efficiency thin film solar cells [1]. Thin film solar cells based on coevaporated CIGS absorbers (x close to.3) have reached up to 2.3% conversion efficiencies at the laboratory scale by using a process requiring high vacuum [2]. From lab to large-scale production, one of the main challenges is to find an alternative deposition method using nonvacuum equipment that yields economically viable solar cells and is easily scalable. From this point of view, electrochemical deposition is a simple and nonvacuum technique and has a natural advantage of largearea deposition [3]. Therefore, over the last two decades, there has been considerable work done on the growth of CIGS thin films using electrodeposition technique [4 6]. So far all electrodeposited (ED) CIGS films need a selenization step under a Se-containing atmosphere to recrystallize the films, as in most cases electrodeposition is employed at low temperature. In the present work we report the properties of CIGS films deposited by the brush plating technique for the first time. 2. Experimental Methods Brush plating was carried out using Selectron Power Pack MODEL15A-4V.Layerswerebrushplatedontinoxide coated conducting substrates of about 5 cm 2,whichisthe negative electrode. The stylus, consisting of a carbon rod wrappedincottonwool,servedastheanode.thecottonwool washeldinpositionbyaporoussleeve.asketchofthesystem is shown in Figure 1. Priortoplating,thestyluswaswired tothepowersupplyandthecottonwoolwassoakedinthe electrolyte. The stylus was then brought into contact with the substrate and moved at uniform speed. An electrical current was found passing whenever the stylus was in contact with the substrate. This is associated with the acceleration of the ions in the electrolyte trapped within the cotton wool, which were subsequently reduced at the substrate to form the CIGS film. CIGS films were deposited on tin oxide coated conducting glasssubstratesatatemperatureof8 C. The precursors used

2 2 Coatings Intensity (a.u.) (24/22) (116/312) Intensity (a.u.) (24)/(22) (116)/(312) (112) (112) θ (deg) θ (deg) 8 1 x =.1 x = Intensity (a.u.) (112) (24)/(22) (116)/(312) θ (deg) 8 1 x =.9 Figure 1: X-ray diffraction pattern of CuIn x Ga 1 x Se 2 films of different composition deposited at a substrate temperature of 8 C. Lattice parameter c (Å) Concentration Ga x Figure 2: Variation of lattice parameters a and c with increase of Ga concentration. Lattice parameter a (Å) were 2 mm SeO 2,3mMCuCl 2, and the concentration of the indium chloride and gallium chloride precursors varied, as shown in Table 1, to obtain films of different composition. The deposition current density was 1. ma cm 2.ThepHwas maintained at 1.5 by HCl. Thickness of the films estimated by Mitutoyo surface profilometer varied in the range of μm withanincreaseinsubstratetemperature.thefilms were characterized by Xpertpanalytical X-ray diffraction unit with Cukα radiation. Optical measurements were recorded using a Hitachi UV-Vis-IR spectrophotometer. Composition of the films was estimated by EDS attachment to JOEL SEM. X-ray photoelectron spectroscopic (XPS) studies were made using VGMKII system with Mgkα radiation. Surface morphologyofthefilmswasstudiedbymolecularimagingatomic force microscope. Electrical measurements were made by Hall Van der Pauw Method. The dark and photoconductivity measurements were carried out at room temperature. The measuring system consists of a regulated dc power supply

3 Coatings 3 z (μm) z (μm) x (μm).6.6 y (μm).8 1. x (μ (μm).6.6 y (μm).8 1. z (μm) z (μm) x (μm).6 1. x (μm) y (μm).8.6 y (μm).8 Figure 3: Atomic force micrographs of CuIn x Ga 1 x Se 2 films of different composition deposited at a substrate temperature of 8 C: x =, x =, x =.6 and x =.8. (Aplab) in series with the sample and Keithley electrometer (model 61 C). A tungsten lamp of 2 W was used for illumination. The intensity of the light source was measured with the help of a power meter. The spectral distribution of photocurrent was measured with the help of photophysics monochromator. 3. Results and Discussion Microstructural parameters were estimated by studying the X-ray diffractograms of CuIn 1 x Ga x Se 2 films of different composition. Figure 2 shows the X-ray diffraction patterns of CuIn 1 x Ga x Se 2 films of different composition. All the

4 4 Coatings In3d 5/2 Se3d In3d 3/ Binding energy (ev) Binding energy (ev) 56 Ga3d Cu2p 3/ Binding energy (ev) Binding energy (ev) Figure 4: X-ray photoelectron spectra of CuIn.5 Ga.5 Se 2 thin films deposited at 8 C. Table 1: Concentration of InCl 3 and GaCl 3 precursors for deposition of CuIn 1 x Ga x Se 2 films of different composition. Composition (x) Concentration of InCl 3 (mm) Concentration of GaCl 3 (mm) figures indicate the prominent peaks corresponding to (112), (22)/(24), (312)/(116). These are characteristic of the chalcopyritephase.nootherphaseswereobservedinthexray diffractograms, indicating the formation of single phase material. The peaks shifted from CuInSe 2 side to CuGaSe 2 side as the concentration of Ga increased in the films. The lattice parameters were calculated using the following relation [7]: 1 d 2 = (h 2 +k 2 ) a 2 + I2 c 2, (1) where a and c are the lattice parameters and d isthe lattice spacing. Figure 3 shows the variation of a and c with increase of gallium concentration. This behaviour is similartoanearlierreport[8]. Thegrainsizeofthefilmshasbeencalculatedusing Scherrer s formula: D=.94λ β cos θ, (2) where λ is the wavelength of X-ray used, β the full width half-maximum (FWHM), and θ the Bragg angle. The grain size varied from 3 nm to 7 nm as the gallium concentration

5 Coatings 5 Table 2: Microstructural parameters of CuIn 1 x Ga x Se 2 films of different composition. Composition (x) Thickness (nm) Lattice parameter (Å) a c Grain size (nm) Strain ( 1 4 ) Dislocation density ( 1 15 cm 3 ) Table 3: Composition of CuIn 1 x Ga x Se 2 films deposited at 8 C. Composition (x) Cu (at.%) In (at.%) Ga (at.%) Se (at.%) Cu/(Ga + In) Ga/(Ga + In) increased. The dislocation density δ, defined as the length of dislocation lines per unit volume of the crystal, has been evaluatedusingtheformula[9] δ= 1 D 2. (3) The microstructural parameters are presented in Table 2. From the table it is observed that the dislocation density decreases with the increase of grain size. Information on the particle size and strain for the CuIn 1 x Ga x Se 2 films wasobtainedfromthefullwidthathalf-maximumofthe diffraction peaks. The full width at half-maximum β can be expressed as a linear combination of the contributions from the particle size D and strain ε through the relation [1] β cos θ λ = 1 D + ε sin θ λ. (4) The plot of β cos θ/λ versus sin θ/λ allows us to determine both strain and particles size from slope and intercept of the graph. The estimated values for films deposited at different duty cycles are listed in Table 2. The deviation in the lattice parameter values from the bulk value observed in the present case clearly suggests that the grains in the films are under stress. Such a behaviour can be attributed to the change of nature, deposition conditions, and the concentration of the native imperfections developed in thin films. This results in either elongation or compression of the lattice and the structural parameters. The density of the film is therefore found to change considerably in accordance with the variations observed with the lattice constant values. The stress developed at higher Ga concentrations is likely to be due to the formation of native defects developed from the lattice misfit or dislocations. The defects have a probability to migrate parallel to the substrate surface so that the films will have a tendency to expand and develop an internal tensile stress. Thistypeofchangeininternalstressisalwayspredominant by the observed recrystallization process in polycrystalline films. The stress relaxation is mainly considered as due to dislocation glides formed in the films. The decrease of internal stress may be attributed to a decrease in dislocation density. The reduction in the strain and dislocation density with decrease of Ga concentration may be due to the reduction in concentration of lattice imperfections at lower Ga concentrations. Similar behaviour was reported earlier [11]. Composition of the films was estimated by recording the EDS spectrum of the films deposited from different composition (Table 3). It is observed that films with lower concentrations of gallium were copper rich. As the gallium concentration increased, the films became nearly stoichiometric. Thisisduetothefactthatastheconcentrationofgallium chloride increases, more flux of gallium ions are available fordepositioncomparedtothefluxofindiumions,which results in higher concentration of gallium thus decreasing the Cu/(Ga + In) ratio. Based on the defect chemistry model of ternary compounds [12], compositional deviations of the CuIn 1 x Ga x Se 2 films can be expressed by nonstoichiometry parameter (Δy = [2Se/{Cu +3(Ga + In)}] 1). The parameter Δy is related to the electronic defects. For Δy >, thefilmhasap-type conductivity and it has an ntype conductivityfor Δy <. In this study the value of Δy is greater than zero, and the films exhibit p-type conductivity. Surface morphology of the films (Figure 3)studiedinan area of 3 μm 3 μm indicated that the grain size increased from 4 nm to 1 nm as the In concentration increased.

6 6 Coatings Table 4: Transport parameters of CuIn 1 x Ga x Se 2 films of different composition deposited at 8 C. Concentration of Ga (x) Resistivity (ohm cm) Mobility (cm 2 V 1 s 1 ) Carrier density (cm 3 ) Transmission (%) 6 4 (i) (h) (g) (f) (e) Refractive index Figure 5: Transmission spectra of CuIn 1 x Ga x Se 2 films deposited at 8 C: x =.1, x =, x =.3, x =, (e) x =.5, (f) x =.6, (g) x =.7, (h) x =.8, and (i) x =.9. The surface roughness also increased from 5 nm to 3.2 nm with increase of indium concentration. The surface roughness increases due to the increase of grain size. XPS analysis was performed to identify the chemical binding states of the constituents of the CuIn 1 x Ga x Se 2 films deposited at 5% duty cycle. Figure 4 represents the XPS spectra of films of different composition. Cu 2p 3/2 core level spectrum. The observed peak located at ev coincides with the binding energy for Cu 2p 3/2 electrons emitted from CuGaSe 2 compound, and the peak at ev corresponds to the binding energy for Cu 2p 1/2 electrons emitted from Cu element. The Ga 3d core level spectrum was observed at 2 ev. One peak for the Se 3d core level spectrum was observed. The peak at 54 ev corresponds to the electronic state of Se 3d 3/2. The peaks shifted as follows: Cu 2p 3/2 from ev to ev, In 3d 5/2 from ev to ev, In 3d 3/2 from ev to ev, Se 3d from 53.7 ev to 53.8 ev, and Ga 3d from 19.6 ev to 19.2 ev, as In concentration decreases from.9 to Figure 6: Variation of refractive index with wavelength of CuIn 1 x Ga x Se 2 films deposited at 8 C: x =, x =, x =.6, and x =.8. Figure 5 shows the transmission spectra of the CuIn 1 x Ga x Se 2 films deposited at 8 C. The spectra exhibit interference fringes, and the value of the refractive index was estimated by the envelope method [13] as follows: n = [N + (N 2 n s 2 )] 2, N= (n s 2 +1) 2 + 2n s (T max T min ) T max T min, where n s is the refractive index of the substrate and T max and T min are the maximum and minimum transmittances atthesamewavelengthinthefittedenvelopecurveona transmittance spectrum. The value of the refractive index calculated from the above equations was 2.6. The refractive (5)

7 Coatings 7 (i) (ev cm 1 ) (αh ) (e) Energy (ev) (f) (g) (h) (i) Figure 7: Tauc s plot of CuIn 1 x Ga x Se 2 films deposited at 8 C: x =.1, x =, x =.3, x =, (e) x =.5, (f) x =.6, (g) x =.7, (h) x =.8, and (i) x = Voltage (V) (h) (g) (f) (e) Figure 9: Variation of photocurrent with voltage of CuIn 1 x Ga x Se 2 films of different composition: x =.1, x =, x =.3, x =, (e) x =.5, (f) x =.6, (g) x =.7, (h) x =.8, and (i) x = Intensity (lux) (i) (h) (g) Figure 8: Variation of photocurrent with intensity for CuIn 1 x Ga x Se 2 films of different composition: x =.1, x =, x =.3, x =, (e) x =.5, (f) x =.6, (g) x =.7, (h) x =.8, and (i) x =.9. index decreases with wavelength (Figure 6). The value of the absorption coefficient (α)was calculated using the relation α= 1 d ln {(n 1) (n n s) (n+1)(n n s ) } [(T 2 max/t min ) +1] [(T max /T min ) 2 1], (6) where d is the thickness of the film and the other parameters have the usual meaning as given for (6). The band gap of the films increased from 1.12 ev to 1.63 ev as the gallium concentration increased (from(αh]) 2 versus h] plot) (Figure 7).Thevaluesofthebandgapagreewellwiththe earlier report [14]. (f) (e) The room temperature transport parameters were measured by Hall Van der Pauw technique by providing gold ohmic contact. The influence of composition on the resistivity of the films is shown in Table 4. The magnitude of the room temperature resistivity increased from.1 ohm cm to 12. ohm cm as the gallium concentration increased. The resistivity values are comparable with an earlier report [15]. The variation in resistivity with gallium concentration can be explained in terms of the Cu/(Ga + In) ratio obtained from EDAX measurements. The Cu/(Ga + In) ratio is greater than unity for all compositions. The films exhibit p-type conductivity, as the concentration of gallium increases, the resistivity increases. The variation of room temperature mobility and carrier density with increase of gallium concentration is also shown in Table 4. Theincreaseofresistivitycanalsobe explained in terms of the decrease of carrier density with increase of gallium concentration. Various crystalline imperfections in the film, such as vacancies, dislocations, and grain boundaries, act as trapping or recombination centers of the carriers and play animportantroleinphotoconduction.thesetrapsactas localized positive potential centers for electrons and negative potential centers for holes. Therefore, some localized discrete energy levels are formed in the band gap, in the vicinity of the conduction and valence bands, respectively. Figure 8 shows the variation of photocurrent with light intensity of CuIn 1 x Ga x Se 2 films of different composition. The photocurrent is found to increase with an increase of indium concentration due to increase in film thickness. As the thickness of the film increases, the crystalline nature increases (Table 2), and this helps in the improvement of photocurrent. The increase in photocurrent is attributed to an increase in the majority carrier concentration and/or an increase in impurity centers acting as traps for minority carriers. The variation of photocurrent with applied voltage in CuIn 1 x Ga x Se 2 films of different composition is shown

8 8 Coatings x =.9 x = x =.3 x =.1 Figure 1: Variation of photocurrent with wavelength. in Figure 9. The photocurrent increases with an increase in voltage. Photocurrent spectra of CuIn 1 x Ga x Se 2 films of different composition are shown in Figure1.The photocurrent spectra show a peak near the absorption edge, which was also observed by Rose [16]. The band gap of the films determined from the spectral response coincided with the band gap determined from optical absorption measurements for that composition. The low photocurrent in the short wavelength rangemaybeduetothehighabsorptioncoefficient,and only surface region where defect states give a shorter life time is excited. In the high wavelength region the radiation is only partially absorbed, giving rise to less photocurrent than the peak value. Tails of the spectra extending to long wavelength are attributed to direct excitation of the carriers from the defect levels. Similar results have been reported by several workers for CuInSe 2 films [17, 18]. Photosensitivityistheratiooftheincreaseinconductivity of the material in the presence of light to the conductivity in darknessandisgivenbytherelation Photosensitivity = Δσ σ = I L I d I d, (7) where I L and I d represent the current under illumination and in the dark, respectively. It seems that some transitions

9 Coatings 9 Photosensitivity 1 5 Intensity (lux) (f) (e) Figure 11: Variation of photosensitivity with intensity of illumination of CuIn 1 x Ga x Se 2 films of different composition: x =.1, x =.3, x =.5, x =.7, (e) x =.8, and (f) x =.9. that create additional free carriers effectively increase the free life time increasing the photosensitivity of the material. Figure 11 shows a plot of photosensitivity versus light intensity of thin films of different compositions. Thinner films exhibit moderate photosensitivity, whereas thicker films are found to exhibit higher photosensitivity. Crystallographical imperfections acting as trapping centers will enhance the photosensitivity, whereas the recombination centers decrease the photosensitivity. 4. Conclusions The brush plating technique has been successfully employed forthedepositionofcuin 1 x Ga x Se 2 films of different compositions. Films with crystallite size in the range of 3 7 nm weredeposited.thebandgapofthefilmscouldbevariedin the range of 1.12 ev to 1.63 ev as the gallium concentration increased. The resistivity of the films was in the range of.1 ohm cm to 12 ohm cm. Linear photocurrent-voltage characteristics were observed in all cases. The photocurrent increased linearly with intensity of illumination. References [1] D. Abou-Ras, D. Rudmann, G. Kostorz, S. Spiering, M. Powalla, and A. N. Tiwari, Microstructural and chemical studies of interfaces between Cu(In,Ga) Se 2 and In 2 S 3 layers, Applied Physics,vol.97,no.8,ArticleID8498,25. [2] P. Jackson, D. Hariskos, E. Lotter et al., New world record efficiency for Cu(In,Ga)Se 2 thin-film solar cells beyond 2%, Progress in Photovoltaics,vol.19,no.7,pp ,211. [3] D. Lincot, Electrodeposition of semiconductors, Thin Solid Films,vol.487,no.1-2,pp.4 48,25. [4] I. M. Dharmadasa, N. B. Chaure, G. J. Tolan, and A. P. Samantilleke, Development of p +,p,i,n,andn + -type CuInGa Se 2 layers for applications in graded bandgap multilayer thinfilm solar cells, the Electrochemical Society, vol.154, no. 6, pp. H466 H471, 27. [5] T. Delsol, M. C. Simmonds, and I. M. Dharmadasa, Chemical etching of Cu(In, Ga)Se 2 layers for fabrication of electronic devices, Solar Energy Materials and Solar Cells, vol.77,no.4, pp , 23. [6] M. E. Calixto, K. D. Dobson, B. E. McCandless, and R. W. Birkmire, Controlling growth chemistry and morphology of single-bath electrodeposited Cu(In,Ga)Se 2 thin films for photovoltaic application, the Electrochemical Society, vol. 153, no. 6, pp. G521 G528, 26. [7] D.-Y. Lee, S. Park, and J. Kim, Structural analysis of CIGS film prepared by chemical spray deposition, Current Applied Physics, vol. 11, no. 1, pp. S88 S92, 211. [8] C. A. Rincon, E. Hernandez, M. T. Alanso et al., Optical transitions near the band edge in bulk CuInxGa1 xse 2 from ellipsometric measurements, Materials Chemistry and Physics, vol.7,no.3,pp.3 34,21. [9]H.Mustafa,D.Hunter,A.K.Pradhan,U.N.Roy,Y.Cui, and A. Burger, Synthesis and characterization of AgInSe 2 for application in thin film solar cells, Thin Solid Films, vol.515, no. 17, pp , 27. [1] M. C. Santhosh Kumar and B. Pradeep, Formation and properties of AgInSe 2 thin films by co-evaporation, Vacuum, vol.72,no.4,pp ,24. [11] M. Venkatachalam, M. D. Kannan, S. Jayakumar, R. Balasundaraprabhu, and N. Muthukumarasamy, Effect of annealing on the structural properties of electron beam deposited CIGS thin films, Thin Solid Films,vol.516,no.2,pp ,28. [12] J. A. Groenik and P. H. Janse, A generalized approach to the defect chemistry of ternary compounds, Zeitschrift für Physikalische Chemie,vol.11,no.1,pp.17 28,1978. [13] H. Y. Joo and H. J. Kim, Spectrophotometric analysis of aluminum nitride thin films, Vacuum Science & Technology A,vol.17,no.3,pp ,1999. [14] C.J.Huang,T.H.Meen,M.Y.Lai,andW.R.Chen, Formation of CuInSe 2 thin films on flexible substrates by electrodeposition (ED) technique, Solar Energy Materials and Solar Cells, vol. 82, no. 4, pp , 24. [15]K.T.R.ReddyandR.B.V.Chalapathy, Preparationand properties of sprayed CuGa.5 In.5 Se 2 thin films, Solar Energy Materials and Solar Cells, vol. 5, no. 1 4, pp , [16] A. Rose, Space-charge-limited currents in solids, Physical Review,vol.97,no.6,pp ,1955. [17] R. Pal, K. K. Chattopadhya, S. Chandhuri, and A. K. Pal, Photoconductivity in CuInSe 2 films, Solar Energy Materials and Solar Cells,vol.33,no.2,pp ,1994. [18] D. Fischer, T. Dylla, N. Meyer, M. E. Beck, A. Jäger-Waldau, and M. C. Lux-Steiner, CVD of CuGaSe 2 for thin film solar cells employing two binary sources, Thin Solid Films, vol. 387, no. 1-2, pp , 21.

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