Presentation on theme: "Structural Properties of Electron Beam Deposited CIGS Thin Films Author 1, Author 2, Author 3, Author 4 a Department of Electronics, Erode Arts College,"— Presentation transcript:
Structural Properties of Electron Beam Deposited CIGS Thin Films Author 1, Author 2, Author 3, Author 4 a Department of Electronics, Erode Arts College, Erode, India b Thin Film Center, PSG College of Technology, Coimbatore, India c Department of Physics, Coimbatore Institute of Technology, Coimbatore, India Thin layers of CuIn x Ga 1-x Se 2 (CIGS) are considered to be the most suitable materials for high efficiency solar cells because of their high absorption coefficient (>10 5 cm -1 ), direct band gap and stability against photo-degradation. Of the thin film photovoltaic devices that have been developed to date, those fabricated with CIGS absorbers have shown the highest efficiency of 19.52%. In this paper, we report the influence of annealing temperature on the structure of the CIGS thin films prepared by EB-gun evaporation technique. EXPERIMENTAL DETAILS STRUCTURAL PROPERTIES CONCLUSION ACKNOWLEDGEMENT : The authors thank the encouragement and support from the Principal and the Management of PSG College of Technology, Coimbatore, India INTRODUCTION 1. Bulk compound of CuIn x Ga 1-x Se 2 with three different compositions x = 0.75, 0.80 and 0.85 have been prepared by direct reaction of high purity elemental copper, indium, gallium and selenium. 2. Stoichiometric amounts of the individual elements copper, indium, gallium and selenium according to the required composition are taken in a quartz ampoule in which a pressure of 10 -5 Torr is achieved and then sealed. 3. The quartz ampoule with the material is placed in a furnace and heated at a rate of 100 o C/hr in steps up to 950 o C and maintained at this temperature for about five hours and then allowed to cool slowly to room temperature. During the process of heating and cooling the quartz ampoule has been rotated continuously to ensure homogeneity in the molten mixture. 4. Thin films of CuIn x Ga 1-x Se 2 of thickness in the range 275 – 325 nm have been deposited on to well-cleaned glass and silicon substrates by electron beam evaporation method using the prepared bulk alloy in a vacuum of 10 -6 Torr The shift of the diffraction peak to higher angle and the decrease of lattice parameter with increase of gallium content is in accordance to Vegards law in the studied compositions. These results indicate that CIGS solid solution has been successfully obtained without any phase separation into CIS or CGS or any other secondary phases in the EB-gun evaporation technique Substrate x Lattice parameter Grain size (Å) Dislocation density (lines/ m 2 ) x 10 14 Strain x 10 -4 BulkThin film a(Å)c(Å)a(Å)c(Å) Glass0.75 0.80 0.85 5.72 5.73 5.75 11.63 11.66 11.69 5.72 5.74 5.76 11.57 11.59 11.67 530 585 682 3.56 2.92 2.14 8.26 6.31 5.91 Silicon0.75 0.80 0.85 -- 5.72 5.74 5.75 11.59 11.57 11.62 568 607 940 3.07 2.71 1.12 7.44 6.13 3.87 X-ray diffraction pattern of bulk CuIn x Ga 1-x Se 2 X-ray diffractogram of CuIn x Ga 1-x Se 2 films deposited on glass substrates annealed at 400 °C X-ray diffraction pattern of CuIn x Ga 1-x Se 2 films annealed at 200 °C Micro structural parameters of CuIn x Ga 1-x Se 2 bulk and thin films Composition x Bulk(at%)Thin film(at%) CuInGaSeCuInGaSe 0.85 0.80 0.75 23.89 26.02 26.13 18.21 19.66 21.86 6.85 4.76 3.28 51.05 49.56 48.73 25.20 25.56 25.10 18.56 19.32 21.36 6.38 4.62 3.12 49.86 50.50 50.42 Composition of the prepared CuIn x Ga 1-x Se 2 bulk and thin films Scale: 1×1 µm 40.0 nm 20.0 nm 0.0 nm abc AFM images of the CuIn x Ga 1-x Se 2 (x = 0.85) films on silicon substrate as-deposited (b) annealed at 200 ºC (c) annealed at 400 ºC The deposition of homogeneous CIGS thin films with tunable lattice parameters and band gap values is a prerequisite for the fabrication of high efficiency solar cells. The CIGS films annealed at 400 °C have been found to be crystalline in nature exhibiting a linear shift in the lattice parameter values of the chalcopyrite lattice with increase in gallium content as indicated by Vegard’s law. The films deposited on silicon substrates have been found to be of better crystalline quality than those deposited on glass substrates. The crystalline nature has been observed to improve with increase in annealing temperature. The AFM studies confirm that the roughness of the CIGS films decreases with annealing temperature. The photoluminescence studies showed an increase in band gap with increasing gallium concentration confirming the homogeneous incorporation of gallium in the prepared thin films. Photoluminescence spectra of CuIn x Ga 1-x Se 2 films EMRS SPRING MEETING STRASBOURG, FRANCE May 28 – June 1, 2007 X-ray diffraction pattern of CuIn x Ga 1-x Se 2 films deposited on silicon substrates and annealed at 400 °C.