Copper gallium diselenide thin film absorber growth for solar cell device fabrication

A custom-built migration-enhanced epitaxy reactor originally optimized for CuInSe2 (CIS) deposition was modified to grow gallium-containing compound semiconductor thin films, such as CuGaSe2 (CGS) and CuIn1-xGaxSe2 (CIGS). The addition of gallium allows for the manufacturing of solar cell absorber layers with wider band gaps.; Three distinct growth recipes under several growth temperatures and a wide range of metal-composition ratios are used to deposit polycrystalline CGS thin films. The surface morphology of gallium-rich films is typically very uniform, with long needle-like grains when grown by the first recipe, a constant copper-rate process. In contrast, copper-rich films grown by this same recipe or by a modified three-stage process have island structures with very large grains embedded in a matrix region that possesses small grains. The surface morphology becomes more uniform and the grains in the matrix region become larger when a higher growth temperature is used. The third recipe, an emulated three-stage process, does not produce films with an island-matrix structure, and the grains are uniformly large.; The highest conversion efficiency achieved for solar cells based on CGS is 5.3%, delivered by a copper-rich absorber deposited at the highest sustainable growth temperature of 491°C. This device has a large fill factor of 66%, but the open-circuit voltage of 0.48 V is lower than what is expected from a wide band-gap absorber. A set of CIGS solar cells was completely fabricated and characterized in-house. This led to the most efficient device produced from an absorber grown in our reactor, in the form of a 9% CIS solar cell featuring a one-micron film deposited at 491°C.; Finally, a dynamic reactor model was created to describe the deposition environment in our epitaxial reactor. All relevant physical features are incorporated, including the cyclic motion of a rotating platen and the spatial distribution of the flux produced by three metal effusion sources. Reaction occurs under an excess of selenium, and operational variables such as rotational speed and melt height can be simulated. The outputs are predicted film thickness and composition. Further work is proposed to identify the values of adjustable sticking coefficients using experimental data.