(576c) Synthesis of Colloidal Cu(In,Ga)Se2 (CIGS) Nanocrystal “Inks” and Their Implementation Into Photovoltaic Devices

Copper indium gallium selenide (CIGS) based solar cells show great promise for widespread use in single junction thin film devices. Large scale production of the CIGS absorber layer, however, is hindered by high cost and poor stoichiometric control associated with the coevaporation of tertiary and quaternary systems in high vacuum. We have devised an inexpensive, solution-based deposition of CIGS under ambient conditions with predetermined stoichiometry. CIGS nanocrystals approximately 10 nm in diameter were synthesized by arrested precipitation and dispersed in organic solvents as ?inks?. We have demonstrated room-temperature deposition of these inks by dip-coating, drop-casting, ink-jet printing, or airbrush spraying to yield device quality films. The ability to deposit CIGS layers under ambient conditions permits alternate device architectures not possible with evaporated CIGS. We have demonstrated functional conventional and inverted device structures employing a variety of back contact materials. An inverted structure removes the need for the CdS buffer layer, making the device ecologically sound. Additionally, we have fabricated functional devices on flexible substrates to demonstrate the ability to integrate this technology into a roll-to-roll production scheme.

Photovoltaic devices fabricated by nanocrystal deposition under ambient conditions have exhibited power conversion efficiencies of 1.5% under AM1.5 illumination. The device performance to date has been limited by the high grain boundary density in the CIGS nanocrystal films that leads to high recombination currents. Films with much larger crystalline CIGS domains and lower grain boundary density can be created by annealing the nanocrystal films at high temperature (500ºC) under a selenium atmosphere. Grain sizes up to 10 μm can be produced and the devices exhibit much reduced surface recombination and higher efficiencies. With the appropriate processing steps, nanocrystal-based PV devices are capable of reaching commercially viable device efficiencies.