Thin film Cu(In,Ga)Se2 materials and devices: a versatile material for flat-plate and concentrator photovoltaic applications

Thin-film Cu(In,Ga)Se2(CIGS) is used as the absorber in an all-thin-film solar cell for both conventional 1-sun and concentrator applications. The absorber fabrication process is represented by time-dependent profiles of elemental Cu, In, Ga, and Se fluxes. The Cu/(In+Ga) ratio determines the phase chemistry during growth whereas the Ga/(Ga+In) ratio determines the CIGS band gap profile. All film-growth processes enter the CIGS:CuxSe two-phase field where the CuxSe facilitates large grain growth. Characterization of cells under 1-sun illumination reveals world-record total-area performance of 17.1%. Improvements relative to previous cells are linked to decreased inter-diffusion of In and Ga within the absorber. Cell parameters include an open-circuit voltage (Voc) of 654 mV and short-circuit current (Jsc) of 33.9 mA/cm2. A second cell was fabricated for operation under concentration. The 1-sun, direct-spectrum measurement yielded at 15.1%- efficient cell. Under 5- and 22-sun concentration, the cell improved to 16.5% and 17.2%, respectively. This achievement is significant in that it proves a compatibility of polycrystalline thin-film and concentrator technologies. Further optimization could yield 1-sun performance in excess of 18% and concentrator performance in excess of 20%. A path to this goal is outlined.

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