Structural and Transport Properties of Hot Wall Deposited $CuInSe_2$Thin Films and the Fabrication of $CuInSe_2$ Based Solar Cells

Abstract

Films of $CuInSe_2$ were deposited onto glass substrates by a hot wall deposition method under thermodynamic equilibrium condition using bulk $CuInSe_2$ as a source material. All the deposited CuInSe2 films were found to be polycrystalline in nature exhibiting the chalcopyrite structure with the crystallite orientation along (101), (112), (103), (211), (220), (312) and (400) directions. The crystallites were found to have a preferred orientation along the (112) direction. Microstructural parameters of the films, such as grain size, dislocation density, tetragonal distortion and strain have been determined. The grain sizes in the films have been found in the range of 89 to 244 nm. Hall effect studies have been carried out on the hot wall deposited $CuInSe_2$ thin films. Hall voltage has been measured as a function of temperature in the range 80 to 460 K. The obtained positive Hall voltage indicates that the prepared $CuInSe_2$ films are of p-type nature with holes as majority charge carriers. The Hall co-efficient is found to decrease with increase in film thickness. The carrier concentration in $CuInSe_2$ films is found to be of the order of $10^{17} cm^{-3}$. The carrier concentration is observed to increase with increase in film thickness. The Hall mobility is found to increase very slowly in the low temperature region from 80 to about 215 K, and then rapidly with increase in temperature above 215 K. The mobility is found to increase with increase in film thickness. The dependence of $T^{3/2}$ law for Hall mobility indicates that scattering on ionized impurities is predominant in the temperature region above 215 K in hot wall deposited $CuInSe_2$ thin films. The grain boundary potential (Eb) of $CuInSe_2$ thin films of different thickenesses has been evaluated. The barrier height is observed to decrease with increase in carrier concentration indicating the presence of partially depleted grains in $CuInSe_2$ films. $CuInSe_2$-based solar cells with CdS as buffer layer were fabricated. The fabricated solar cells were illuminated using 100 $mW/cm^2$ white light under AM1 conditions. The current and voltage were measured using an optical power meter and an electrometer, respectively. The solar cell parameters, open circuit voltage (V_{oc}), short circuit current (Isc), series resistance ($R_s$), shunt resistance ($R_{sh}$), power maximum ($P_{max}$) and fill factor (FF) were determined