RESULTS AND DISCUSSION
5.7 OPTICAL PROPERTIES
5.7.1 ABSORBANCE AND TRANSMITTANCE
Chapter-V Results and Discussion Fig. 5.3. (b) X-ray diffraction patterns of annealed Co0.8Cd0.2S thin films.
Chapter-V Results and Discussion nm at room temperature. The absorbance spectra of the as-deposited thin films of
Co1-xCdxS (0≤ x ≤1)having different concentrations are shown in Fig. 5.4(a). These spectra reveal that films grown under the same parametric conditions have low absorbance in the visible and near infrared regions from 500 to 1100 nm. However, absorbance is high in the ultraviolet region. Fig. 5.4(a) shows that absorption increases with the increasing incorporation of Cd in the solution.
Fig. 5.4. (a) Variation of absorbance as a function of wavelength for Co1-xCdxS films for different concentrations. Inset shows the plot for x = 0.0 value.
Fig. 5.4(b). Variation of transmittance as a function of wavelength for Co1-xCdxS films for different concentrations.
Chapter-V Results and Discussion Fig. 5.4(b) shows the optical transmittance spectra for the Co1-xCdxSthin films. All
the films demonstrate more than 70% transmittance at wavelengths longer than 800 nm. Transmittance is very high in the high wave length range i.e. at 850 to 1100 nm range. From 550 to 800 nm range, transmittance increases slightly. Below 500 nm there is a sharp fall in the %T of the films, which is due to the strong absorbance of the films in this region. It has been observed that the over all %T decreases with increasing incorporation of Cd in the solution. This result is quite similar to that of the film deposited by chemical bath deposition technique [19].
5.7.1.1 ABSORPTION COEFFICIENT AND OPTICAL BAND GAP
Absorption coefficient, ( α ) of the film is calculated from the absorbance data using Beer-Lambert’s formula,
2.303A
d α =
where, d is the film thickness and A is the optical absorbance of the film. Fig. 5.4 (00) shows the variation of ‘α’ against photon energy, ( hν ) for different doping concentrations, (x) of as-deposited Co1-xCdxSthin films. From the fig. 5.4( c ) it is seen that the ‘α’ of the as-deposited Co1-xCdxSthin films increases with increasing Cd in the solution. The ‘α’ depends on thickness of the film. From fig. 5.4( c ), it is seen that the value of ‘α’ is very low in the lower energy level and increases slowly in this region. Finally value of ‘α’ tends to become saturated at higher energy level i.e. above 2.5 eV. In the higher energy level ‘α’ is very high which is due to the strong absorbance of the films in this region.
Chapter-V Results and Discussion Fig. 5.4( c ). Variation of Absorption coefficient as a function of photon energy for as-
deposited Co1-xCdxS films.
The energy gap in a semiconductor is responsible for the fundamental optical absorption edge. The fundamental absorption process is one in which a photon is absorbed and an electron is excited from an occupied valance band state to an unoccupied conduction band state. If photon energy is less than the energy gap, such process is impossible and photon energy (hν) will not be absorbed. Such inter band absorption process are possible only if the photon energy is higher than the gap energy. Since ‘α’ is used to describe the reduction in intensity of light in a medium as a function of distance, therefore higher values of ‘α’ is an indication of more reduction in intensity. Fig. 5.4(d-i).shows the plots of (αhν)2 vs. hν for direct transition of as-deposited Co1-xCdxS thin films. The optical band gap ( Eg ) is determined from the plots of (αhν)2 vs. hν for direct transition for Co1-xCdxS thin films. The direct Eg of the films have been obtained from the intercept on the energy axis after extrapolation of the straight line section of (αhν)2 vs. hν curve. Eg of the films varies between 2.20 to 3.10 eV with the increase of Cd in the Co1-xCdxS system, which indicates that the presence of Cd in the films greatly affects the direct Eg. Calculated Eg values are found in good agreement with the Eg of as-deposited Co1-xCdxS films reported by M. Bedir et al [17]. The effects of increasing Eg due to the exchange interactions of the conduction and valence band electrons with the Cd2+
d electrons. The Eg of the compound is altered depending upon the concentration of Cd ions and the 3d levels of transition metal ions are located in the Eg region and d-d
Chapter-V Results and Discussion transition dominates. The nature of this variation in the Eg may be useful to design a
suitable window material for fabrication in solar cells.
Fig. 5.4(d-i). Plots of (αhν) 2 vs. hν curves for as-deposited Co1-xCdxS thin films.
Chapter-V Results and Discussion Table 1.2: Variation of thickness and direct band gap with the doping concentration
value, (x = 0-100 % Cd doping) of Co1-xCdxS thin film.
DopingConcentration values, (x= 0-100%) of Co1-xCdxS film.
Thickness, d (nm) of Co1-xCdxS film.
Band gap, Eg (eV) of Co1-xCdxS film.
0 % 535 2.2
20 % 475 2.4
40 % 420 2.5
60 % 385 2.75
80 % 355 2.8
100 % 287 3.1
Table 1.2 shows the variation of Eg with different molar concentration of the films for direct transitions. From the table 1.2 it is seen that the Eg increases slightly with increasing molar concentrations ( x = 0 to 1 ). Eg obtained for direct transition of the films of different molar concentration and different thickness are also shown in the table 1.2. From the table 1.2 it is seen that in both cases the direct Eg increases slightly with decreasing thickness and that of increasing molar concentration i.e.
increase of Cd incorporation in the solution.