Photoelectrochemical Characteristics of Pulse Plated CuGaSe2 Films

Thin-film copper chalcopyrites have recently received attention as candidate materials for photoelectrochemical water splitting. They exhibit properties that render them particularly attractive for solar energy conversion applications, including: (1) a direct bandgap with steep absorption edge; (2) good carrier transport properties; and (3) band edges that are tunable by composition. Although the photovoltaic properties of p-CuGaSe₂ (CGSe) films have been studied previously by electrochemical means using organic electrolyte or V²⁺

/V³⁺ electrolyte, little work to date has characterized the photoelectrochemical interface of CGSe in aqueous media for possible application in hydrogen evolution devices. CGSe films have been deposited by different techniques like vacuum evaporation, Chemical vapour deposition (CVD), MOCVD etc. In this work, CGSe films were deposited for the first time by the pulse electrodeposition technique.

            CGSe films were deposited at room temperature and at different duty cycles in the range 6 – 50 %. The deposition current density was maintained as – 0.9V (SCE). The precursors used were 10 mM GaCl₃, 5 mM CuCl₂ and 100 mM SeO₂. The pH of the bath was adjusted to 1.5 by HCl. Tin oxide coated glass substrates (5 ohms / sq) were used. A microprocessor controlled pulse plating unit was used. Thickness of the films measured by Mitutoyo surface profilometer was in the range of 0.8 – 1.1 micrometer.

The XRD pattern of CGSe films deposited at different duty cycles, consists of the typical copper chalcopyrite peaks, which can be indexed according to the JCPDS card no. 35-1100. Of the CuGaSe₂ peaks, the (112) and (2 2 0) peaks are most prominent. Preferred orientation along the (2 2 0) direction is observed, as is common  for  films grown by 3-stage process. The peaks are found to increase in intensity and decrease in width with increase of the duty cycle. The crystallite size of the films calculated using Scherrer's formula increased from 58 – 70 nm as the duty cycle increased.

For the films deposited at 6 %, 9 % and 15 % duty cycle, the Cu/Ga ratio was 1.04, 1.035 and 1.03 respectively. As the duty cycle increased, the films became stochiometric. For the films deposited at 33 % duty cycle, Cu/Ga ratio was 1.02. At 50 % duty cycle, the Cu/Ga ratio was equal to unity, 1.00.

Surface morphology of the films studied in an area of  3 μm x 3 μm indicated that the grain size increased from 15 nm to 40 nm as the duty cycle increased from 6 to 50 %. The surface roughness also increased from 1.5 nm to 3.2 nm with increase of duty cycle.

As the duty cycle increases, the band gap decreases due to the increase in grain size with duty cycle. The films deposited at 50 % duty cycle showed an optical band gap of 1.68 eV, which is in good agreement with the reported value.

The PEC cells using these films exhibited low photocurrent and photovoltage. Films deposited at 50 % duty cycle exhibited maximum photo output. In order to increase the photo outpout, the films deposited at 50 % duty cycle were post heated in argon atmosphere at different temperatures in the range of 450 - 550°C for 15 min. The power output characteristics after 80s photoetching indicates a V_oc of 0.70V, J_sc of 12.5 mA cm^-2, ff of 0.71 and h of 10.35 %, for 60 mW cm^-2 illumination.

The results obtained in this investigation points to the possibility of using them in photovoltaic devices.