Flexible Cu2ZnSn(S,Se)4 solar cells made from nanoparticle inks

Xu, Xinya (2019) Flexible Cu2ZnSn(S,Se)4 solar cells made from nanoparticle inks. Doctoral thesis, Northumbria University.

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Abstract

Cu2ZnSn(S,Se)4 (CZTSSe) is recognised as a promising photovoltaic material for its high theoretical power conversion efficiency, direct band gap and high absorption coefficient. In this work, earth abundant Cu2ZnSnS4 nanoparticle inks were deposited on both rigid and flexible substrates, and subsequently converted to high quality thin film Cu2ZnSn(S,Se)4 photovoltaic absorbers. Integration of these absorbers within a flexible thin film solar cell device structure yields a solar energy conversion efficiency which is comparable to identical devices processed on rigid glass substrates. Importantly, this is only achieved when a thin layer of molybdenum is first applied directly to the foil, as this layer limits the formation of a thick Mo(S,Se)x layer resulting in a substantially reduced series resistance.

Alkali doping is a key factor in the performance of thin film photovoltaic cells with reported benefits including promotion of grain growth, passivation of grain boundaries and increased carrier concentration in chalcopyrite and kesterite based solar cells. Research-grade devices fabricated in substrate configuration often rely on diffusion of Na from a soda lime glass substrate into the photovoltaic absorber layer during high temperature processing. However, for samples on flexible substrates such as foils and plastics, this is not available and requires alternative approaches. In this work, Earth-abundant Cu2ZnSn(S,Se)4 thin film solar cells were fabricated from nanoparticle inks on flexible molybdenum substrates. A simple, low-cost route to incorporating Na in the nanoparticle ink solution is demonstrated and has the advantage of being compatible with large area, high volume manufacturing. The technique is verified to improve the device efficiency relative to a reference flexible device built on molybdenum foil.

The technique results in a 4.4% CZTSSe solar cell on a flexible Mo foil substrate which is comparable with devices built on rigid glass and a significant improvement on flexible devices with non-doping absorbers.

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