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EFFECT OF ZNO:B GROWTH TEMPERATURE DEPOSITED BY MOCVD TECHNIQUE ON FILM PROPERTIES AND THIN FILM SILICON SOLAR CELLS PERFORMANCE J. Sritharathikhun, P. Krudtad, S. Songtrai, A. Moollakorn, A. Limmanee and K. Sriprapha Institute of Solar Energy Technology Development, National Science and Technology Development Agency 111 Thailand Science Park, Paholyothin Rd., Klong Nueng, Klong Luang, Pathumthani 12120 THAILAND Tel. +66 2564 7000 Ext. 2711, Fax. +66 2564 7059, E-mail: jaran@nstda.or.th ABSTRACT: Boron doped Zinc Oxide (ZnO:B) films were deposited at various growth temperatures (120 – 240°C) by the Metal Organic Chemical Vapor Deposition (MOCVD) technique. The gas mixture of Diethyzinc (DEZ) and water (H2O) was used as reactant gas while Diborane (B2H6) was employed as the n-type doping gas. In this paper, the properties of ZnO:B film were investigated by Scanning Electron Microscopy (SEM), Hall’s measurement, step profile and UV/Visible Spectrometer with integrating sphere. It was found that, at the substrate temperature of about 210°C, ZnO:B film showed the carrier concentration, mobility, and resistivity of 4.0x1019 cm-3, 34.4 cm2v-1s-1, and 5.0x10-3 Ωcm, respectively. The total transmittance tended to decrease, while the grain size of the ZnO:B became lager with increasing substrate temperature. The effects of the growth temperatures of ZnO:B coated glass on the performance of thin film amorphous silicon solar cells were carried out with a structure of glass/MOCVD ZnO (2 µm) / p-µc-Si:H (20 nm) / i-a-Si:H (350 nm) / n-µc-Si:H (30 nm) / ZnO (80 nm) / Ag and an effective area of 0.73 cm2. The best solar cell was achieved at MOCVD ZnO:B growth temperature of 180ºC with VOC as high as 0.94 V, JSC of 13.8 mA/cm2, FF of 0.68, and efficiency of 8.7%. Keywords: Transparent conducting oxides, ZnO, thin film solar cell 1 INTRODUCTION Among the transparent conductive oxide (TCO) glasses such as Indium Tin Oxide (ITO), Tin Oxide (SnO2), and Zinc Oxide (ZnO), ZnO film is one of the most promising candidate materials for an application in thin film silicon solar cells. Compared to ITO and SnO2 materials prepared by chemical vapor deposition, the cost of ZnO is the cheapest [1]. Normally, ZnO can be used in three parts of thin film solar cells. ZnO coated glass substrate shows higher stability under relatively aggressive device fabrication processes like high hydrogen dilution plasma [2]. ZnO intermediate reflector can enhance short-circuit current of top cell in tandem cell [3]. And ZnO film can be applied as a back reflector of the solar cells to improve its optical properties [4] and also has advantage of preventing the diffusion of metal atoms at the contact. Recently, Metal Organic Chemical Vapor Deposition (MOCVD) technique has attracted much attention [4,5] since this technique can deposit film in large area, and shows benefits of low cost and easily controllable film properties by changing deposition parameters. In this paper, we have investigated the effect of growth temperature on the structural, electrical, and optical properties of ZnO:B films by using Hall measurement, scanning electron microscopy (SEM), Step Profiler and UV/Visible Spectrometer with integrating sphere. The effect of growth temperature of ZnO:B coated glass on the solar cells performance were also carried out. 2 EXPERIMENTAL DETAILS 2.1 Preparation of ZnO:B with various growth temperature ZnO:B films were prepared by MOCVD technique on soda lime glass. The gas mixture of Diethyzinc (DEZ) and water (H2O) was used as reactant gas while Diborane (B2H6 0.1% in H2 dilution) was employed as the n-type doping gas. The deposition condition was shown in Table I. The [DEZ]/[H2O] flow rate was fixed at 40/80 sccm while B2H6 flow rate was fixed at 15 sccm. The deposition pressure and the thickness of the ZnO:B film were kept at 300 mTorr, and 2.0 µm, respectively. In order to investigated the effect of growth temperature on the ZnO:B film properties, the substrate temperatures were varied from 100-240ºC. The carrier concentrations, resistivity and electron mobility of the ZnO:B films were determined by Hall measurement using the Van der Pauw configuration. Scanning Electron Microscopy (SEM) was used to characterize the surface morphology of the ZnO:B films. The transparency and haze were evaluated from UV/Visible Spectrometer with integrating sphere at wavelength range from 350 to 1100 nm. The film thickness was measured by a step profiler. Table I: Deposition parameters of ZnO:B film DEZ/H2O/B2H6 ratio : 40/80/15 Deposition temperature : 120-240°C Deposition pressure : 0.3 Torr Gap distance : 0.5 cm Film Thickness : 2 µm (* B2H6 0.1% in H2) 2.2 Fabrication of thin film amorphous silicon solar cells on ZnO:B substrate The thin film silicon solar cells with a structure of glass/ MOCVD ZnO:B/ p-μc-SiO:H/ i-a-Si:H/ n-μc-Si:H/ ZnO/ Ag were fabricated in order to investigate the effects of the ZnO:B films growth temperature on the characteristics (performance and quantum efficiency) of solar cells. Thin film silicon solar cell was deposited by very high frequency plasma enhanced chemical vapor deposition (60 MHz VHF-PECVD) using a gas mixture of SiH4, H2 and CO2, whereas PH3 and TMB+B2H6 were employed as dopant gases for n and p type films, respectively. Laser scribing was carried out to divide the active area of the solar cells into 0.73 cm2. Photo current 25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain 3006 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 voltage (I-V) measurement of solar cells was performed under AM1.5 condition, 100 mW/cm2 at a temperature of 25°C. 3 RESULTS AND DISCUSSIONS 3.1 Structural, electrical and optical properties of ZnO:B film Figure 1 showed the morphology of ZnO:B films with growth temperature range from 120 to 240ºC by SEM. It was found that crystalline grain size of the ZnO:B films increased with increasing growth temperature, a dominant orientation were changed from (002) to (110) plane leading to an increase in Haze value (Fig. 2). According to results shown in Fig. 2, we found that the growth temperature significantly affected resistivity, mobility, and carrier concentration of the ZnO:B films. With the increase of growth temperature from 120 to 240ºC, the resistivity of ZnO:B film decreased from 1.5x10-2 to 5.0x10-3 Ω cm due to an increase of crystalline phase of these films as shown in Fig.1 and then increased at the temperature above 210ºC due to a decrease in mobility. Mobility increased gradually with an increase of growth temperature since 120ºC and reached the maximum value of 34 cm2v-1s-1 at the growth temperature of 210ºC, then dropped at the higher temperature. On the other hand, the carrier concentration tended to increase continuously over the observed temperature range. The total and diffuse transmittance of ZnO:B films was shown in Fig. 3. We found that the total transmittance of ZnO:B films was decreased with increasing growth temperature while diffuse transmittance was increased leading to an increase of haze value in Fig. 2. Figure 1 (a-e) SEM picture of ZnO:B films deposited with various growth temperature Figure 2 Resistivity, mobility, aarrier density and haze value of ZnO:B films as a function of ZnO:B film growth temperature 400 500 600 700 800 900 1000 1100 0 10 20 30 40 50 60 70 80 90 100 400 500 600 700 800 900 1000 1100 0 10 20 30 40 50 60 70 80 90 100 Wavelength(nm) Diffuse transmittance Tr an sm itt an ce (% ) Temp 120oC Temp 150oC Temp 180oC Temp 210oC Temp 240oC Total transmittance (a) 120ºC (b) 150ºC (c) 180ºC (d) 210ºC (e) 240ºC Figure 3 Total and diffuse transmittance of ZnO:B films deposited with various growth temperature 3.2 Fabrication of thin film amorphous silicon solar cells using MOCVD ZnO:B substrate First, we fabricated a thin film silicon solar cell by using MOCVD ZnO:B coated glass as a substrate. The structure of the solar cell was glass/MOCVD ZnO:B (2 µm)/p-µc-Si:H (20 nm)/i-a-Si:H (350 nm)/n-µc-Si:H (30 nm)/ZnO (80 nm)/Ag with an effective area of 0.73 cm2. In order to investigate the effect of ZnO:B growth temperature on the solar cell performance, we changed the growth substrate from 120 to 240ºC, and kept the thicknesses of the ZnO:B layers at 2 µm. Figure 4 shows the photovoltaic parameters of the solar cells using ZnO:B substrate with various growth temperature. When the growth temperature increased, Voc, JSC as well as FF increased up to the temperature of 180ºC. Enhancement of the FF was supposed to be due to the reduction of the electrical resistivity of the ZnO:B 25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain 3007 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 films. However, FF and JSC decreased at the temperature above 180ºC. This might be due to deterioration of the thin p layer by textured ZnO:B with large grain size. Figure 5 shows the quantum efficiencies of thin film amorphous silicon solar cells as a function of ZnO:B film growth temperature. The enhancement in the JSC was obtained upto temperature of 180ºC due to an increase of haze value leading to the improvement of light scattering of ZnO:B coated glass. However, the JSC decreased at the ZnO:B growth temperature above 180ºC, where the film showed relatively low total transmittance. Further optimization of deposition conditions and surface texturing of the ZnO:B films are expected to improve solar cell performance. Figure 4 Photovoltaic parameters of thin film amorphous silicon solar cells as a function of ZnO:B film growth temperature. 400 500 600 700 800 0.0 0.2 0.4 0.6 0.8 1.0 Q ua nt um e ffi ci en cy (Q E) Wavelength (nm) 120°C 150°C 180°C 210°C 240°C Figure 5 Quantum efficiencies of thin film amorphous silicon solar cells with various of ZnO:B film growth temperature. 4 CONCLUSIONS In this study, the ZnO:B film was prepared by MOCVD technique. The effects of growth temperature on the structural, electrical and optical properties of ZnO:B films were investigated. With an increasing of growth temperature, crystalline grain size increased. The lowest resistivity of 5.0x10-3 Ωcm was obtained from the growth at 210ºC with carrier concentration and mobility of 4.0x1019 cm-3 and 34.4 cm2v-1s-1, respectively. The best solar cell was achieved at MOCVD ZnO:B growth temperature of 180ºC with VOC as high as 0.94 V, JSC of 13.8 mA/cm2, FF of 0.68, and efficiency of 8.7%. ACKNOWLEDGEMENT This work was supported by Cluster and Program Management Office (CPMO) of NSTDA, Thailand References [1] J.A. Anna Selvan, ZnO for Thin Film Solar Cells, Thesis, (1998) 141. [2] S. Major, K. Satyendra, M. Bhatnagar, and K.L. Chopra, Appl. Phys. Lett. 49 (1986) 394. [3] J. Muller, B. Rech, J. springer, M. Vanecek, Sol. Energy. 77 (2004) 917. [4] X. L. Chen, B. H. Xu, Y. Zhao, C. C. Wei, J. Sun, Y. Wang, X. D. Zhang, X. H. Geng, Thin Solid Films. 515 (2007) 3753. [5] W. W. Wenas, A. Yamada, and K. Takahashi, J. Appl. Phys.70 (11) (1991) 7119. 25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain 3008 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮