Study of Thin Film Semiconductor Materials for The Fabrication of Solar Cells

Abstract

The CdTe, CdS, Al doped ZnO (AZO), Sn02 (TO), Al doped Sn02 (ATO), In doped Sn02 (ITO) thin film are successfully deposited by thermal evaporation method. The films are deposited on different types of substrates with different thickness. In the CdTe/CdSIFTO glass and CdTe/CdS/AZOI FTO glass structures, modified double layer thermal evaporation technique is developed for the preparation ofCdTe and CdS thin films. Aluminum (AI) doped Zinc Oxide (AZO) thin films were deposited on bare glass and fluorine doped tin oxide (FTO) coated glass substrates and the structural, optical, electrical and surface properties were investigated. The thickness of the films was determined by means of in situ Quartz Crystal (FTM5) thin film thickness monitor during the deposition of the materials. X-ray diffraction (XRD) technique has been used to identify the different phases present in the deposited films and it also confirmed the crystalline nature of the films. The surface morphology of the films are characterized using scanning electron microscopy (SEM) and to ascertain that homogeneous and uniform particles were grown onto FTO coated glass substrates. The transmittance, reflectance, absorption co-efficient, optical band gap, refractive index, extinction coefficient and optical conductivity were analyzed using double beam UV -VIS-NIR spectrophotometer and the band gaps were changed from 2.95 to 3.18 eV due to thickness and substrate effects. The electrical conductivity, mobility, carrier concentration were analyzed by Hall-effect measurement system and the properties are changed with temperature for different thickness and substrates. The conductivity was drastically increased from the order of 10- 3 to 10 2 (O-cm) for the thin film deposited on glass and FTO coated glass substrates respectively. The electrical, optical, structural and surface characteristics showed that the AZO thin films deposited on FTO coated glass substrate can be used for photovoltaic application. The Sn02 (TO) thin films of thickness 150, 300 and 600 nm showed that the films are highly transparent (about 80%) in the visible, near infrared and infrared region of light. Interference fringes are indication of the uniformity of the films. The values of absorption coefficient of the films are estimated in the range of 10 cm -I and the band gap energy values are obtained as 2.7 eV, 2.74 eV and 2.86 eV respectively. The IV 4 _10 5 refractive index (1.64 - 2.48) and the extinction coefficient (0.02406 to 0.24642) of the films are highly varied with thickness. The X-ray diffraction of the Sn02 (TO) thin films, Al doped Sn02 (TO) thin film and In doped Sn02 (TO) thin film showed that the films are crystalline fonnation with hexagonal structure. The conductivity values of Sn02 (TO), ITO and FTO thin films are measured as 11.1 (n cm)"), 716.1 (n cm)") and 6360 (n cm)") respectively. The temperature dependence of conductivity reveals that the films are homogeneous compound fonnation. The mobility and carrier concentration properties show the stability nature. The high transmittance (greater than 80 %), low reflectance, high absorption coefficient (> 10 are observed for different thickness of CdS thin films deposited on glass substrates. The films deposited on FTO glass and AZOIFTO glass revealed that the films are polycrystalline cubic structure. It is observed that the surface images of the CdS thin films are changed due to different substrates. The film deposited by modified double layer thennal evaporation on CdS/AZOIFTO glass substrate showed high conductivity, mobility and carrier concentration. The CdS thin films can be used as a window layer in solar cell applications. The XRD analysis of CdTe thin films showed that the films are polycrystalline with cubic structure. The lower FWHM showed higher grain size of the CdTe/CdS/AZOIFTO film results better compound fonnation. The optical transmission spectra of 900 nm CdTe films on glass substrate shows a high transmission of about 82% in visible region with a sharp fall near the fundamental absorption at 850 nm. Films were subjected to scanning electron microscopy (SEM) to study the growth features of the annealed films. The SEM image showed homogeneous and unifonn surface structure of the films. High conductivity of the CdTe thin films were observed at room and for the variation of higher temperature. The conductivity, mobility and carrier concentration of CdTe thin films of structure CdTe/CdSI AZOIFTO showed better stability. The high optical absorption co-efficient about 10 5 1 em and ~ 1.5 e V direct band gap of the films indicate that the films are useful as an absorber for photovoltaic application in the solar cell. The research work is done to optimize deposition conditions, deposition technique, thickness and substrates of the CdTe, CdS, AZO, Sn02 and doped Sn02 thin film materials. The achievement of the modified double layer thennal evaporation method might be helpful to fabricate solar cells with the thin film semiconductor materials.