Electro-chemical development of CuInGaSe2-based photovoltaic solar cells.

The aim of this work was to make low cost, high efficiency, graded bandgap, thin film CuInGaSe2 solar cells by electrodeposition, using novel device designs proposed by Dharmadasa et al. These new designs were first experimentally tested using well researched GaAs and AlxGa(1-x)As materials grown using MOCVD, these ideas were then transferred to electrodeposited CuInGaSe2.New designs of graded bandgap solar cells based on p-type window materials, using the well researched GaAs and AlxGa(1-x)As alloy system, have been experimentally tested. The size of the cell was gradually scaled up from 0.5 mm diameter (0.002 cm2) to 3x3 mm2 (0.090 cm2) and to 5x5 mm2 (0.250 cm2), these were then assessed using I-V and QE techniques. The devices showed Voc in the range of 1070-1175 mV, exceeding reported values, FF in the range 0.80-0.87, and Jsc in the range 11-12 mA cm-2. The reason for the low current density was believed to be due to the GaAs capping layer used in the device, which acted as a filter. To confirm this, a second set of devices was fabricated, replacing the GaAs cap with GaAlP, this increased the Jsc to ~14 mA cm-2, Voc and FF remained the same.New PV device structures based on CuInGaSe2 starting from the front contact, instead of the conventional Mo back contact, have been grown by electrodeposition from aqueous solutions using a single bath. In order to investigate the effect of bath concentrations on the film properties, 3 different bath concentrations were used. PEC was used to determine the electrical conduction of these layers, and it was found that it was possible to grow p+, p, i, n, n+ layers by changing the deposition voltage. XRF was used to determine the stoichiometry of the corresponding layers, and XRD to investigate the bulk structure. The morphological properties were studied using AFM and SEM. A four-layer n-n-i-p solar cell structure was initially fabricated and I-V measurements were carried out to assess the devices. The devices were PV active with parameters Voc~235 mV, Jsc~22 mA cm-2, FF~0.38 and n~2.0%.Due to problems annealing CdS at high temperature and the difficulty of incorporating gallium into the layer, CuInSe2 cells with Mo as the substrate were deposited. To understand the mechanisms of film growth, detailed cyclic voltammetry was carried out, leading to the construction of a Pourbaix diagram for the Cu-In-Se system. Depositing the films at -0.476 V for 20 minutes, followed by 50 minutes at -0,576 mV gave the best quality films, with p-type electrical conduction. XRF and XRD were used to determine stoichiometry and structural properties respectively. A method to anneal the CuInSe2 layers without the use of H2Se was devised, and a detailed study using SEM to determine the effects of annealing time and temperature was carried out. Annealing the films at 550°C for 30 minutes gave the best results. I-V measurements were carried out using an electrolyte contact, the devices were photo active, (Voc~866 mV, Jsc~0.9 mA cm-2, FF~0.40).