Effect of the Replacement of Some Indium Atoms with Aluminum Atoms on the Behaviour of p-CuInSe2 Thin Films and Schottky Diodes

As environmental and energy resource concerns have increased, greater stress has been placed on the development of renewable energy resources such as photovoltaic electric generators. Chalcopyrite semiconductors have gained prominence as solar cell fabrication materials due to its steady progress in conversion efficiencies. These have emerged as leading absorber semiconducting material for solar power generation purposes.

CulnSe₂ (CIS), a prominent member of the chalcopyrite family, has emerged as a promising material for high efficiency thin film solar cells. Due to its favorable electrical and optical properties, stability and inexpensive means of production that several recent advances have been made to improve its bandgap [E_g]. The bandgap of CIS has been tailored to match the desired portion of the Solar Spectrum by alloying the group III (Ga and Al) or group VI (S and Se) elements. An extensive research work has been carried out on Cu(In,Ga)Se₂ [CIGS] and CuIn(S,Se)₂ [CISS] alloy systems but the efficiency of these devices with E_g >1.3 eV is reduced due to   degradation of the electronic properties of the absorber layer leading to losses in the fill-factor and the open-circuit voltage. The performance of thin film CIS based solar cells has alternatively been improved by the addition of Al in place of Ga to form CuInAlSe₂(CIAS) absorber layers with changed bandgap to match the selective  regions of the solar spectrum. Further, Ga is scarce and expensive material and can preferably be replaced by inexpensive and abundant Al.

            In the present investigation, behaviour of thin film CIAS to that of CIS as an absorber layer portion of the solar cells has been studied. Both the materials possessed tetragonal structure  and found oriented towards  < 112 > set of planes.  The addition of ‘Al’ to CIS films caused reduction in the lattice imperfections’ which resulted in the improved cystallinity of the films. The grain size increased while microstrain and dislocation density decreased in CIAS. The films demonstrated decrease in the cohesive force between film and the substrate material. The optical analysis of the films revealed that the bandgaps and absorption coefficients increased on Al substitution and, thus, CIAS films have been found to be more suitable as an absorber layer in solar cells. The electrical measurements of the films showed that both CIS and CIAS films possessed p-type conductivity. Further, a significant improvement has been observed in the conductivity as well as mobility values in the CIAS films. The phenomenon has been attributed due to the reduction in the grain boundary scattering in the CIAS in comparison to CIS films. The Schottky Diodes of both the types of films were formed using Aluminum metals. The reverse biased leakage currents measured from the I-V/C-V characteristics of CIS were found to lower than that of  CIAS films.  The forward biased parameters such as ideality factors and series resistance decreased while barrier heights increased in the Al/p-CuIn_0.81Al_0.19Se₂ Schottky Diodes. Further, the temperature dependence of the I-V/C-V characteristics of the undertaken CIAS diodes has been explained on the basis of barrier inhomogenities existing over the M-S interface. However, some deviations have been observed between the data generated on the basis of Gaussian Distribution Function for barrier inhomogenities to that obtained from the experimental studies of undertaken Schottky Diodes which indicated the existence of current component due to tunneling alongwith the thermionic emission of the charge carriers  existing over the M-S interface.