Alternative Architecture for Solution Processed Thin Film Solar Materials

Robert W Boyne, Purdue University

Abstract

A variety of competing semiconductor materials are used to create p-n solar cells. Currently silicon solar cells dominate more than 90% of the solar market. The major competitors to silicon solar cells are inorganic thin-film solar cells, mainly cadmium telluride and copper indium gallium sulfide. These materials evolved from initial devices based off of cadmium sulfide. All of the non-silicon commercial cells still use cadmium sulfide in their production, which is of concern due to the toxicity of cadmium and limits their application, especially in Europe due to the E.U. Restriction of Hazardous Substances directive. If these devices are to remain market competitive, increased cost efficiency and the removal of cadmium are necessary. ^ First, the cadmium sulfide layers were optimized for nanoparticle CZTS devices, and an in situ measurement apparatus was developed to monitor the deposition thickness during processing. While photons absorbed into cadmium sulfide show low quantum efficiencies, increasing the thickness of the cadmium sulfide buffer layer from 50 to 70nm increased the open circuit voltage and reduced shunting, with only negligible loss in current density, yielding more efficient devices. This agrees with work done on vacuum-based copper indium gallium cells in literature. Thinner films require a partial- electrolyte treatment to achieve comparable efficiencies. ^ Both mesoporous titanium dioxide films were developed as a potential alternative for CdS buffer layers. These layers are comparable to those used in perovskite and dye-sensitized solar cells. Titanium dioxide is a cadmium-free low toxicity alternative to cadmium sulfide as a n-type layer for photovoltaic devices. It has a different conduction band energy, making it a potential candidate for a variety of novel semiconductor materials that may not align with cadmium sulfide. Further, it is resistant to annealing in a selenium atmosphere, allowing solar absorbers to be deposited and annealed in an inverted structure, which, with a proper transparent conductor could create bifacial solar cells. CIGS and CIS devices were prepared on the mesoporous and planar structures, but showed short circuit currents limited to 120µA/cm ². The major limitation of these devices is the poor metallurgical contact between the titanium dioxide and the absorber crystals. ^ Finally, a dissolution mechanism is proposed for the unique action of amine-thiol solvent mixtures. These solvents can be used to create semiconductor films from a variety of metal, chalcogenide and salt precursors. The mechanism relies on the Bronsted acid-base behavior of thiols and amines and the strong complexing potential of thiolates with soft metal ions such as copper, zinc and silver. Evidence suggests that the species in solution consist mainly of metal thiolates and alkylammonium salts. Understanding this mechanism may allow for better control of amine-thiol molecular precursor routes to device fabrication.^

Degree

M.S.Ch.E.

Advisors

Rakesh Agrawal, Purdue University.

Subject Area

Alternative Energy|Chemical engineering|Materials science

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