Fabrication and Characterization of Cu(In,Ga,Al)Se2 Thin Films for Solar Cells
sputtering, phelevlile
Cu(In,Ga,Al)Se2 (CIGAS) thin films were studied as an alternative absorber layer material to Cu(InxGa1-x)Se2. Polycrystalline Cu(In,Ga,Al)Se2 thin films with different Al composition (0 to 12 at.% and 0 to 7.3 at. %) have been deposited by pulsed laser deposition and RF/DC magnetron sputtering on Si(100) and soda-lime glass substrates. the effect of substrate temperature (from 25 to 500 °C) on the physical properties of both CIGAS and CIGS prepared by sputtering has been studied extensively. Several solar cells with three different absorber thicknesses were fabricated using a soda lime glass/Mo/CIGS/CdS/i- ZnO/ZnO:Al/Al stack structure and show efficiency of 7 %. Electron probe micro-analysis and secondary ion mass spectroscopy were used to determine the composition of the films and the distribution of Al across the film thickness, respectively. Phase evolution and structure of the films was studied by X-ray and SAED image of HRTEM. The FESEM and AFM has been used to examine the morphology and topography of the thin films. EPMA results has indicated that Al content increase as the width of the Al strips on the CIGS target increases for films prepared by PLD and linearly increases with time of Sputtering for films sputtered by RF/DC magnetron sputtering. The ratio of Cu/(In + Ga) varied slightly from 1.03 to 0.98 and the ratio of Ga/(In+Ga) maintained almost constantof ≈ 0.3 which matches with the optimum composition for the highest efficient CIGS solar cell. Furthermore, EMPA indicated that all the compounds are Cu-poor which is permanently used in solar cell fabrication. Secondary ion mass spectroscopy(SIMS) results show that the concentration of In and Ga in as-deposited films is reduced for higher Al concentrations, suggesting some Al atoms are substituted for In as well as Ga. After annealing, the Al distribution across film thickness became almost uniform.X-ray diffraction studies indicated that CIGAS are single phase with chalcopyrite structure and that the (112) peak clearly shifts to higher 2θ values with increasing Al content which in turn result in the reduction of d-values. Moreover the Structure of CIGAS of Constant Al Content (5.3 %) has slightly affected by Change of growth temperature and Show no change after annealing. This behavior was observed also in Cu(In, Ga)Se2.The selected area diffraction (SAD) pattern acquired From HRTEM indicate that CIGAS thinfilm has a polycrystalline nature with a d-spacing matching this acquired from XRD measurements and well agree with ICCD card. AFM analysis showed that CIGAS films exhibited well-defined and uniformly distributed grains with decreasing surface roughness as Al content increased. Field Emission Scanning electronic microscopy (FE-SEM) plane images revealed dense and well-defined grains, as well as sharp CIGAS/Si(100) interfaces for all films. Crosssectional FESEM images revealed columnar grains of the films and confirmed that all films have 200 nm thickness. The grain size increases over the increase of temperature and further increasing after annealing Indicating an advanced stage of recrystallization.The bandgap of CIGAS films was determined from the optical transmittance and reflectance spectra and was found to increase as Al content increased to reach closely from the theoretical optimum value. The band gap increase from 1.22 to 1.47eV as Al content increase from 0 % at. to 12 % at. for samples prepared by pulsed Laser Deposition and increase from 1.20 to 1.32 eV as Al content increase from 0 % at. to 7.4 % at. for thin films prepare by RF/DC sputtering. The Current density-voltage measurements show that the efficiency of 7 % is achieved at the area of 0.5 cm2 with open circuit voltage (Voc) of 440 mV, short current density (Jsc) of 27.5 mA/cm2 and the fill factor (FF) of 0.57 %. The efficiency increase from 3.4 to 7% as thickness of the absorber layer increase from 0.8 to 1.5 μm.