Dissertation Defence: Solution Processing Routes for CuIn(S,Se)2 Photovoltaic Applications

Sunil Suresh, supervised by Dr. Alexander R. Uhl, will defend their dissertation titled “Solution Processing Routes for CuIn(S,Se)2 Photovoltaic Applications” in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Mechanical Engineering.

An abstract for Sunil Suresh’s dissertation is included below.

Examinations are open to all members of the campus community as well as the general public. Registration is not required for in person defences.

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ABSTRACT

Renewable energy sources are necessary to enable persistent economic growth cleanly and sustainably. The sun is one such source irradiating Earth with 120,000 TW of power annually (both land and water), from which humans require only a small fraction of 15 TW globally. Multi-junction can utilize the solar spectrum more efficiently by reducing thermalization losses and can theoretically achieve efficiencies from 42% for a dual junction device under 1 Sun. Distinguished by a direct bandgap, and a large absorption coefficient, CISSe absorber layers are of utmost importance for the growing field of printed solar cells. The widescale adoption of CISSe thin film solar cells relies on device performance, operational stability, and – arguably most importantly – production cost. Solution processing routes for CIS absorbers have generated tremendous interest in the research community as an alternative to state-of-the-art vacuum-based deposition methods addressing challenges of low material usage, throughput, and film homogeneity.

Herein a dimethylformamide (DMF) and thiourea (TU)-based ink deposition route (IDR) was explored to fabricate narrow bandgap (~1.0 eV) CISSe films with Cu-poor, stoichiometric, and Cu-rich) compositions for photovoltaic applications. Characterization of the Cu-rich absorber films using X-ray diffraction and scanning electron microscopy confirmed the partial removal of Cu2-xSe phase from the film surface, while electron backscatter diffraction measurements and depth-dependent energy-dispersive X-ray spectroscopy indicated remnant Cu2-xSe in the absorber layer bulk. Consequently, by carefully tailoring the ink and absorber film composition, active area efficiencies of 13.2% were obtained for devices with air processed, stoichiometric CISSe absorber layers. Next, submicron thick CISSe absorber films were fabricated using the developed DMF-TU based route. Using submicron CISSe absorbers layers lowers the film fabrication costs, however, at the expense of lower power conversion efficiencies (due to rear surface recombination losses). To address the latter, an ultrathin Al2O3 film (~6 nm) with nanosized point openings between the rear contact and the CISSe absorber layer was used to reduce the rear interface recombination losses. Active area efficiencies of 14.2%, and ~12.0% obtained by a 0.75 μm, and 0.55 μm film-based rear passivated device, respectively, which is the highest reported values for solution processed, low bandgap CISSe solar cells with equivalent CISSe film thicknesses.