Microstructural, optical, and electrical characteristics of Cu-doped CdTe nanocrystalline films for designing absorber layer in solar cell applications

This paper reports the microstructure, optical, and electrical characteristics of
undoped and Cu-doped CdTe nanostructured thin films prepared on glass
substrates by electron beam evaporation technique. The crystallographic study
of X-ray diffraction shows that CdTe and Cu-doped CdTe films crystallize in the
form of a cubic zinc blende structure. Microstructure analysis reveals that as the
Cu-doping level increases, the average crystallite size increases, while the
microstrian decreases due to the improvement of the crystallinity, thereby
reducing defects. XRD and AFM investigations confirmed the nanostructure
characteristic of undoped and Cu-doped films. It was found that the optical
band gap energy increases from 1.485 to 1.683 eV as the Cu concentration
increases from 0 to 10 wt%, which may be related to the Burstein–Moss effect.
The refractive index is calculated from the Swanepoel envelope method and
found to decrease with the increase of the Cu doping due to the decrease in the
polarizability. Similarly, the extinction coefficient decreases with the increase of
Cu in the CdTe matrix. The dc electrical conductivity is found to increase with
increasing Cu doping, which is attributed to the increase in the grain size,
thereby reducing the scattering of the grain boundary. Furthermore, two conduction
mechanisms of the carrier transport in nanostructured undoped and
Cu-doped CdTe films were observed. The low-temperature dependence of the
conductivity of undoped and Cu-doped CdTe nanostructured films is explained
based on Mott’s variable range hopping conduction mechanism model (VRH).
Interestingly, the calculated values of hopping distance R, the hopping energy
W, and the density of states at the Fermi level N(EF) are consistent with Mott’s
VRH. Finally, Hall effect measurements show that all the films have p-type
conduction behavior. Besides, the results show that as the Cu doping level increases, the carrier concentration and the Hall mobility increase due to the
decrease in grain boundary scattering with the increase in grain size. Accordingly,
it can be concluded that by increasing the Cu doping level in the CdTe
film, the conductivity is increased, thereby improving the performance of the
CdTe absorber layer in the solar cell structure.