Photovoltaic Study of CZTS Thin Films Grown by Vacuum Evaporation and Chemical Bath Deposition Methods

Transcription

1 Nano Vision, Vol. 5(7-9), , July-September 2015 (An International Research Journal of Nano Science & Technology), ISSN (Print) ISSN (Online) Photovoltaic Study of CZTS Thin Films Grown by Vacuum Evaporation and Chemical Bath Deposition Methods S. Sakthivel and V. Baskaran Thin Film Physics and Nano Science Laboratory, PG and Research Department of Physics, Rajah Serfoji Govt., College (Autonomous), Thanjavur, Tamilnadu, INDIA. Presented in Second National Conference on Thin Film Science and Nano Technology (SECOND-NCTFSANT-2015) March 2-3, 2015, Rajah Serfoji Govt. College, Thanjavur, T.N. (India). ABSTRACT I 2 II IV VI 4 quaternary compounds, such as Cu 2 ZnSnS 4 (CZTS) based solar cells, exhibit optical and electronic properties comparable to CIGS and CdTe materials while consisting entirely of nontoxic constituents and avoid the scarcity issues associated with indium, gallium, cadmium, and tellurium. CZTS has a direct band gap of ev and large absorption coefficient in the order of 10 4 cm 1. Results regarding structural properties, optical properties and photovoltaic characteristics of Cu 2 ZnSnS 4 (CZTS) thin films prepared by sequential deposition of Cu, ZnS and Sn thin films by vacuum evaporation are compared with those obtained with CZTS films grown by sequential deposition of Cu 2 SnS 3 (CTS) and ZnS thin films by CBD Process. X-ray diffraction analysis (XRD) which is mostly used for phase identification cannot clearly distinguish the formation of secondary phases such as Cu 2 SnS 3, since both compounds have similar diffraction patterns; therefore, Raman scattering analysis was used to distinguish these phases. Further, the photovoltaic characteristics short-circuit current density (I sc ), open-circuit voltage (V oc ), a fill factor (FF) and efficiency are studied under AM 1.5(100mW/Cm 2 ) illumination. Keywords: Thin films, CZTS, CBD, Raman, XRD. 1. INTRODUCTION I-III-VI semiconductors have attracted much interest in recent years, as their optical and electrical properties are optimum for photovoltaic and optoelectronic applications. Thin film solar cells based on CIGS heterojunction have been found to exhibit record efficiency 1 of 20.3%. However, alternative materials which are not having expensive or toxic species

2 170 S. Sakthivel, et al., Nano Vision, Vol.5 (7-9), (2015) such as In, Ga, Te or Cd are the objective of many studies. Chalcogenide compounds with stoichiometry Cu 2 (MII)(MIV) (S, Se) 4 (MII =Mn, Fe, Co, Ni, Zn, Cd, Hg and MIV = Si, Ge, Sn) have drawn much attention in recent past, because of availability, non-toxic and have direct band gap, most suited for solar cell applications and other optical devices 2. Cu 2 ZnSnS 4 is an alternative to the traditional p-type semiconductors such as CIS and CIGS. The traditionally obtained structure in literature till now is either Kesterite or Stannite that shows tetragonal crystal symmetry 3. In this context, the Cu 2 ZnSnS 4 is a promising alternative to semiconductor based on Ga and In as a solar absorber material 4.CZTS thin film is one of the most promising photovoltaic materials as an absorber of thin film solar cells because it has a high absorption coefficient above 10 4 cm -1 and a direct bandgap eV 5. Cu 2 ZnSnS 4, which has a similar structure to CIGS, suitable optical bandgap energy and is naturally abundant in the environment, anticipated to be a substitute for CIGS 6. This semiconductor film can be regarded as an alternative to CIS and CIGS materials in which extremely expensive and resource limited Indium is replaced by cheap and abundant Zinc (Zn) and Tin (Sn) 7. Cu 2 ZnSnS 4 thin films have been prepared by different researchers using several techniques such as spray pyrolysis technique 8, dc magnetron sputtering 9, sol-gel sulphurization method 10, pulsed laser deposition 11 and electron beam evaporation 12. So far, the absorber layer of Cu 2 ZnSnS 4 has mainly deposited by thermal evaporation, sputtering or hydrazine slurry-based methods. The problem with using hydrazine as solvent is the toxic and explosive nature of hydrazine. An alternative to hydrazine-based approaches is metal salt solution in organic solvents, where the salt solubility increased and stabilized by adding amine containing complexing agent like monoethonalamine 13. In this work, the structural properties, chemical composition and oxidation states of single phase Cu 2 ZnSnS 4 (CZTS) thin films prepared by reactive thermal evaporation are compared with those of samples grown using a solution based methods like CBD, Spray pyrolysis and SILAR are examined. The structural properties evaluated from XRD and Raman spectroscopy measurements and the elemental composition and oxidation states from XPS analysis. Further, the photovoltaic characteristics short-circuit current density (I sc ), open-circuit voltage (V oc ), a fill factor (FF) and efficiency are studied under AM 1.5 (100mW/Cm 2 ) illumination. 2. EXPERIMENTAL The Cu 2 ZnSnS 4 thin film solar cells were prepared using two different deposition techniques; the first is a vacuum-based thermal evaporation technique that includes sequential evaporation of precursors, followed by annealing at 550 o C and the second one is a solution-based technique, which includes CBD, Spray pyrolysis, and SILAR. The vacuum deposition of CZTS thin films done by sequential thermal evaporation of their precursors at a background pressure around mbar; we fabricated CZTS thin films on Mo coated ITO glass substrates by vapor-phase sulfurization of thermally evaporated precursors. This process consisted of two stages with the sequential evaporation

3 S. Sakthivel, et al., Nano Vision, Vol.5 (7-9), (2015) 171 of precursors followed by the vapor-phase sulfurization. We formed the stacked precursors on the substrates by depositing ZnS, Cu and Sn layers with thermal evaporation orderly. A film thickness monitor (FTM) on the evaporation equipment controlled the thickness of each layer. Moreover, the thicknesses of the three layers are shows in Table 1 according to the ratio of the constituents. We sulfurized the precursors in an annealing furnace in the atmosphere of N 2 +H 2 S (5%) at temperature of 550 o C for 2 hours. After sulfurization of the precursors, CZTS thin films formed. The solution based deposition of CZTS thin films done by CBD technique. The bath solution consists of required amount of copper chloride (2M), Zinc chloride (1.2M), tin chloride (1M) and thiourea (8M) were added in a mixture solvent ethanol/water(30:70).clear yellow sol gel was formed after being stirred at room temperature for ten minutes. The substrate dipped in the solution for 24 hours. A thin coating with white Coloration appeared on the substrate. The coated substrate removed at the end of deposition, washed in deionized water, dried in air at C temperature. Finally, the annealing temperature was elevated to C for growing polycrystalline CZTS thin films and they subjected to morphological, optical, and photovoltaic characterizations. To form p-n junction with the p-type CZTS, 50~100 nm n-type CdS thin film deposited on the absorber layer by vacuum evaporation/cbd method. The surface of CZTS thin film is too rough towards entirely covers by CdS thin film, leading to shortage between front contact and back contact. To prevent leakage intrinsic ZnO (i-zno) thin film coated on CdS before 500~1000 nm transparent conducting oxide (TCO) thin film deposited as the front contact layer of the cell. 3. RESULTS AND DISCUSSION 3.1. Structural and Morphological Analysis Single-phase Cu 2 ZnSnS 4 thin films were prepared in this work using both, vacuum and solution based deposition methods. Fig. 1 shows XRD pattern of CZTS thin films prepared by thermal evaporation on molybdenum substrate as well as the diffractogram of a typical CZTS film prepared by CBD method. Fig.1. X-ray diffraction pattern of the CZTS thin film

4 172 S. Sakthivel, et al., Nano Vision, Vol.5 (7-9), (2015) As shown in Fig.1, the sample was of good crystalline nature and diffraction peaks of Cu 2 ZnSnS 4 thin films can be conclusively indexed as tetragonal phase with the lattice constants of a=5.426å and c= Å, which were consistent with the values in the standard card (JCPDS # ). Sulfurization of the deposited CZTS thin films possesses a polycrystalline structure. Peaks attributed to (002), (101), (110), (103), (200), (105), (312) and (314) of CZTS appeared, which can be confirmed by the Powder Diffraction File (PDF) Moreover, the intensity of the (002) is the strongest, indicating that the sample corresponds to the kesterite structure, which belongs to the tetragonal system. The XRD patterns of thin film samples indicated that not only the highly oriented and it has polycrystalline nature. The diffraction peaks of (002) and (101), indicates the presence of Copper Tin Sulfide 13. Fig.2. Raman spectra of the CZTS thin film Besides X-ray analysis, Raman spectroscopy measurements studied on thin films of CZTS prepared by both, CBD and thermal evaporation. In Fig. 2 the Raman spectra of a typical CZTS film grown on molybdenum by c thermal evaporation is compared with that of a sample prepared by CBD. In both spectra the dominant peak is located at cm 1, indicating that CZTS with the kesterite/stannite structure is the dominant phase present in samples prepared by thermal evaporation as well as by CBD. This peak arises from corresponds to A1 vibration mode, where only S-anions are involved. The space symmetry changes from kesterite-type I4 to a disordered kesterite- phase I42m symmetry, which is the same for stannite 14. It has also reported 15 that the peak at 331 cm 1 related to the existence of local structural inhomogeneities within the disordered cation sublattice. SEM images, of vacuum thermal evaporated p-type CZTS thin film of thicknesses ranging from 9 KÅ to 17 KÅ shown in Fig 3(a-c). It has been found from Fig.3 that the films are fully covered, homogeneous, well adherent, and free from crystal defects such as pinhole and cracks. The result shows that the film consists of compact structure grains with sub-micron size and low roughness, which is suitable for the absorber of thin film solar cells. As chemical bath deposition, Fig.3 (d-f) shows the surface (SEM) image of annealed CZTS film. Annealed CZTS film composed of particles having size ranging from 250 to 300 nm.

5 S. Sakthivel, et al., Nano Vision, Vol.5 (7-9), (2015) 173 Fig.3. SEM image of the thermal evaporated CZTS thin films (a-c) and CBD CZTS thin films (d-f) Particle size and crystallite size are different. Crystallite size is always smaller than particle size because a particle constitutes many crystallites. Moreover, the efficiency of TFSC increases with increasing crystal size of the absorber layers. From the result of elemental composition there exist four elements, which are zinc (16.83%), copper (23.12%), tin (Sn) (10.95%) and sulphur (49.09%) in atomic percentage Optical properties of the CZTS thin films Optical band gap energy (Eg) of synthesized films was calculated and plotted. Eg was attained from UV-VIS analysis based on the following equation. αhν = k (hν - E g ) m (1) Fig.4&5. Optical spectrum of CZTS thin film

6 174 S. Sakthivel, et al., Nano Vision, Vol.5 (7-9), (2015) Where, α is the light absorption coefficient, h is the Planck constant and ν is the frequency. Extrapolation method in conjunction with the above equation produces Eg, the band gap is estimated to be 1.48 and 1.51 ev by extrapolating the straight line part of the (αhν) 2 versus hν curve to the intercept of the horizontal axis. These values are quite close to the theoretical optimal value for a single-junction solar cell I-V characterization of the CZTS /CdS Solar Cell Fig.6. I-V characteristics of CZTS solar cell Figure.6 shows the photocurrent density-voltage (I-V) characteristics of CZTS heterojunction solar cells. The conversion efficiency (η) of the thermal evaporated CZTS solar cell is 5.6 % with short circuit current density 14.2 ma/cm 2, open circuit voltage 0.63 V and fill factor 63 %. As compared to the conversion efficiency (η) of chemical bath deposited CZTS solar cell is 2.63 % with short circuit current density ma/cm 2, open circuit voltage 0.58 V and fill factor 36 %. 4. CONCLUSION Eco-friendly p-czts/n-cds thin film solar cells were successfully fabricated by sequential vacuum evaporation and chemical bath deposition methods. The sulfurization process that help to increasing high crystalline phase of the deposited film. The deposited film CZTS were annealed at 500 C, to avoid forming binary compounds. A high absorption coefficient as 10 4 cm -1 and an optical band-gap energy of the CZTS samples about 1.48 and 1.51 ev achieved were in this experiments, which is very close to the optimum value for a solar-cell absorber. The photocurrent characteristics of the optimized cell efficiencies of 5.6% and 2.63 % achieved. The experimental results showed that the current-voltage (I-V) characteristic properties of the cell have a strong dependence on the photovoltaic parameters of the thin films.

7 ACKNOWLEDGMENT S. Sakthivel, et al., Nano Vision, Vol.5 (7-9), (2015) 175 The authors would like to express their thanks to the University Grants Commission (UGC), New Delhi, India for sanctioning the financial assistance [F. No /2012(SR) Dated: July 2012] to carry out the present research work. REFERENCES 1. K. Wang, O. Gunawan, T. Todorov, B. Shin, S.J. Chey, N.A. Bojarczuk, D. Mitzi, S. Guha,Thermally evaporated Cu2ZnSnS4 solar cells, Appl. Phys. Lett. 97, (3pp) (2010). 2. H. Katagiri, K. Jimbo, S. Yamada, T. Kamimura, W.S. Maw, T. Fukano, T. Ito, T. Motohiro, Enhanced conversion efficiencies of Cu2ZnSnS4-based thin film solar cells by using preferential etching technique, Appl. Phys. Express1, 41201(2pp) (2008). 3. D. A. R. Barkhouse, O. Gunawan, T. Gokmen, T. K. Todorov and D. B. Mitzi, Device characteristics of a 10.1% hydrazine-processed Cu2ZnSn(Se,S)4 solar cell, Progr. Photovolt.: Res. Appl. 20, 6-11 (2012). 4. D. B. Mitzi, O. Gunawan,T.K.Todorov, K. Wang, S. Guha, The path towards a highperformance solution-processed kesterite solar cells, Solar Energy Materials & Solar Cells 95, (2011). 5. A. S. Ionkin, B. M. Fish, W. J. Marshall, and R.H. Senigo, Use of inorganic fluxes to control morphology and purity of crystalline késterite and related quaternary chalcogenides. Solar Energy Materials and Solar Cells 104, (2012). 6. Ingrid Repins, Nirav Vora, Carolyn Beall, Su-Huai Wei.et.al, Kesterites and Chalcopyrites: A Comparison of Close Cousins, Presented at the 2011 Materials Research Society Spring Meeting San Francisco, California, April 25 29, (2011). 7. Bereznev, J. Kois, I. Golovtsov, A. Öpik, E. Mellikov, Thin Solid Films ,425 (2006). 8. M.C. Contreras, K. Ramanathan, J. AbuShama, F. Hasoon, D.L. Young, B. Egaas, R. Noufi, Prog. Photovolt. Res. Appl. 13 (7), 209 (2005). 9. K. Ito, and T. Nakazawa, Electrical and Optical-Properties of Stannite-Type Quaternary Semiconductor Thin-Films, Japanese Journal of Applied Physics, 27(11), (1988). 10. S. W. Shin, S.M. Pawar, C. Y. Park, J. H. Yun, J.Ha Moon, J. H. Kim and J. Y. Lee, Studies on Cu2ZnSnS4 (CZTS) absorber layer using different stacking orders in precursor thinfilms, Solar Energy Materials and Solar Cells, 95(12), (2011). 11. B. Shin, O. Gunawan, Y. Zhu, N. A. Bojarczuk, S. J. Chey, and S. Guha, Thin film solarcell with 8.4% power conversion efficiency using an earth-abundant Cu2ZnSnS4 absorber, Progress in Photovoltaics, Vol. 21, no. 1, pp , (2013). 12. Kasim Uthman Isah, Jibrin Alhaji Yabagi et al., Effect of different copper precursor layer thickness on properties of Cu2ZnSnS4 (CZTS) thin films IOSR Journal of Applied Physics (IOSR-JAP), Volume 2, Issue 6, PP (Jan. - Feb. 2013).

8 176 S. Sakthivel, et al., Nano Vision, Vol.5 (7-9), (2015) 13. S. Schorr and G. Gonzalez Aviles, Phys. Status Solidi A 206, (2009). 14. T. Washio, H. Nozaki, T. Fukano, T. Motohiro, K. Jimbo,and H. Katagiri, J. Appl. Phys. 110, (2011). 15. X. Fontane, V. Izquerdo-Roca, E. Saucedo, S. Schorr, V. O. Yukhymchuk, M. Ya. Valakh, A. Prez-Rodriguez, and J. R. Morante, J. Alloys Compd. 539, 190 (2012).