Pulse Plated CuIn0.5Ga0.5Se2 Films and Their Photoelectrochemical Properties
Authors: Murali Kollegal Ramakrishna 
Published: February 2020

In the recent years, polycrystalline solar cells based on CuInGaSe2 (CIGS) have gained importance. CIGS has shown to be a promising candidate as an absorber material for thin film solar cells. Efficiencies near 19% have been reached on a laboratory scale. Several technique like co-evaporation, selenization of metallic precursors, nano-ink printing and electrodeposition have been developed for the preparation of CIGS absorbers. In this work CIGS thin films have been obtained by the pulse electrodeposition technique and their photoelectrochemical properties are studied.

CuIn0.5Ga0.5Se2 films were pulse electrodeposited on molybdenum substrates at 50 % duty cycle and at room temperature. The precursors used were 20 mM SeO2, 30 mM CuCl2, 50 mM Indium chloride and 50 mM gallium chloride. The deposition potential was maintained at – 0.8V (SCE). The pH was maintained at 1.5 by HCl. A microprocessor controlled pulse plating unit was used. Thickness of the films measured by Mitutoyo surface profilometer was 900 nm.

Microstructural parameters were estimated by studying the x-ray diffractograms of CuIn0.5Ga0.5Se2 films. X-ray diffraction patterns of CuIn0.5Ga0.5Se2 films indicate the prominent peaks corresponding to (112), (220)/(204), (312)/(116). These care characteristic of the chalcopyrite phase. No other phases were observed in the x-ray diffractograms indicating the formation of single phase material.

Composition of the films was estimated by recording the EDS spectrum of the films deposited of different composition. Fig.2 shows the EDS spectrum of CuIn0.5Ga0.5Se2 films. Based on the defect chemistry model of ternary compounds.

The room temperature transport parameters were measured by Hall Van der Pauw technique by providing gold ohmic contact. The magnitude of the room temperature resistivity was 4.85 oohm cm, the value of the room temperature mobility was 26.0 cm2V-1s-1 and the room temperature carrier concentration was 4.95 x 1016 cm-3. The resistivity values are comparable with an earlier report. The Cu/(Ga + In) ratio is greater than unity. The films exhibit p-type conductivity. This is supported by the EDAX measurement.

Photoelectrochemical cells (PEC) cells using these films exhibited low photocurrent and photovoltage. The intensity of the light falling on the films was kept constant at 70 mW cm-2. The as deposited film exhibited low photo output viz., Voc of 0.25 V and Jsc of 80 µA cm-2, hence, in order to increase the photo outpout, the films were post heated in argon atmosphere at different temperatures in the range of 450 - 550°C for 15 min. It is observed that the PEC output parameters, viz., open circuit voltage and short circuit current were found to increase for the electrodes heat-treated upto a temperature of 525°C. Photoelectrodes heat-treated at temperatures greater than this value exhibited lower open circuit voltage and short circuit current due to the reduction in thickness of the films as well as the slight change in stoichiometry. The photovoltaic parameters are Voc of 0.55 V and Jsc of 10.5 mA cm-2, ff of 0.72, efficiency of 5.94 %. A plot of ln Jsc vs Voc yielded a straight line. Extrapolation of the line to the y-axis yields a J0 value of 1.30 x10-7 A cm-2, the ideality factor (n) was calculated from the slope of the straight line and it was found to be 1.95. Photoetching was done by shorting the photoelectrodes and the graphite counter electrode under an illumination of 100 mW cm-2 in 1 : 100 HNO3 for different durations in the range 0 – 100s. Both photocurrent and photovoltage are found to increase up to 70s photoetch, beyond which they begin to decrease. The decrease of the photocurrent and photovoltage after 70s photoetch is attributable to separation of grain boundaries due to prolonged photoetching. The power output characteristics after 70s photoetching indicates a Voc of 0.70V, Jsc of 18.0 mA cm-2, ff of 0.71 and h of 12.78 %, for 70 mW cm-2 illumination.