Investigation of electrodeposited copper indium selenium(2) films for photovoltaic cells

Uniform polycrysalline p-type CuInSe{dollar}sb2{dollar} films, thicknesses ranging from 1 to 4 {dollar}mu{dollar}m, were deposited by a stable electrodeposition process developed in our laboratory. A (112) preferred orientation was found for the films. Vacuum annealing experiment results showed an improvement in the crystalline quality of the films after the annealing. The metal ratio of the films was not altered significantly by the annealing process. Al/CuInSe{dollar}sb2{dollar} Schottky junctions fabricated on p-type CuInSe{dollar}sb2{dollar} films showed that the current transport mechanism in the intermediate voltage region was governed by a recombination component. A dispersion effect of the capacitance-voltage curves with frequency, observed on the Schottky junctions suggested the presence of either enhanced interface states or deep levels in the depletion region. Solar cells of CdS/CuInSe{dollar}sb2{dollar} were fabricated by vacuum evaporating low resistivity n-type CdS layers on the electrodeposited p-type CuInSe{dollar}sb2{dollar} films. The highest AM1 active area conversion efficiency, for the 0.9 cm{dollar}sp2{dollar} cells, was 5.2% after an air heat treatment at 200{dollar}spcirc{dollar}C. The efficiency value was increased to 5.6% in small area devices fabricated using a similar procedure. The air heat treatment was found to reduce the apparent carrier concentration in the CuInSe{dollar}sb2{dollar} films. Current-voltage (I-V) characteristics of the cells showed a definite relationship between the short circuit current and the indium to copper ratio in the CuInSe{dollar}sb2{dollar} films. Temperature dependent I-V curves revealed a dominant recombination current component for the heterojunction cells. The value of barrier height for a cell was estimated to be 0.64 eV. It was found that the differential capacitance values depended on measurement frequencies, resulting in frequency-dependent voltage intercepts. The C{dollar}sp{lcub}-2{rcub}{dollar}-V curves were also observed to shift to more negative voltages under illumination. Interface states were believed to be responsible for all these effects. The density of the charged interface states under AM1 illumination condition for a cell with efficiency of 5.2% was estimated to be 5 {dollar}times{dollar} 10{dollar}sp{lcub}12{rcub}{dollar} cm{dollar}sp{lcub}-2{rcub}{dollar}. Admittance spectroscopy results showed that there was a relation between the composition of the CuInSe{dollar}sb2{dollar} films and the activation energy of the deep levels. For the films with an indium/copper ratio of 1.1, the activation energy was about 50 meV. The trap states were much deeper (160 meV) for the films with an In/Cu ratio of 1.25.