Molecular Ink Processed Iron Pyrite Thin Films for Photovoltaics
Thin-film photovoltaics (PV) have the potential to supply our future energy needs, but the dominant commercial thin film technologies rely on rare or toxic elements that may limit their capacity to scale to the terawatt levels of electricity generation needed to impact global energy demand. Iron pyrite (FeS2) is a promising, earth-abundant material that has a suitable band gap of 0.95 eV, a large optical absorption coefficient, and adequate carrier diffusion lengths for use in PV. Unfortunately pyrite solar cells have suffered from low open circuit voltages that hinder overall device performance. Developing high quality, phase pure pyrite thin films may be the key to overcoming these performance drawbacks.
A solution-based process using elemental precursors dissolved in hydrazine has been previously developed at IBM for fabricating solar absorbers such as copper zinc tin selenide/sulfide (CZTS) and copper indium gallium selenide/sulfide (CIGS). Devices using these films have achieved 12.6% and 15.2% efficiencies respectively. In this dissertation, a hydrazine-free molecular ink that uses common organic solvents to dissolve elemental iron and sulfur is described for making pyrite thin films.
To form pyrite thin films, the molecular ink is deposited onto molybdenum-coated glass and then annealed at temperatures between 350-500°C in both hydrogen sulfide and sulfur vapor atmospheres. Phase purity is confirmed using X-ray diffraction and Raman spectroscopy. Film morphology is examined with scanning electron microscopy. Elemental composition is analyzed using secondary ion mass spectrometry and FT-IR spectroscopy. Optical properties are measured using an integrating sphere attachment on a UV-VIS spectrometer. The electrical properties of the films are characterized by temperature-dependent resistivity and qualitative thermopower to verify the carrier type.
The resulting films are measured to be pure phase pyrite that consist of well-connected grains between 50-200 nm in size and are free of pinholes. Optical and electrical properties agree with those commonly cited in literature. Elemental analysis reveals low amounts of oxygen and carbon impurities, compared to other fabrication methods of pyrite used in the same laboratory. The initial results of using these films in p-n heterojunctions will be presented along with on-going strategies being researched to overcome new challenges.