An Attempt to Synthesize Mn-doped Pyrite Thin Films

Abstract

Solar energy is a good choice to meet energy and environmental challenges. Researchers have been searching for inexpensive solar absorber materials which can be used in photovoltaic devices. Pyrite FeS2 has long been recognized as a potential candidate for its high theoretical efficiency, low cost, earth-abundance and non-toxicity. However, the performance of pyrite based solar cells has been limited for some unknown reasons. The poorly understood doping mechanisms have impeded the progress in FeS2-based solar cell research. Identifying the unknown unintentional dopants, however, remains a big challenge, as does controlled n- and p-type doping. In this work, we explore doping p-type pyrite films via intentional introduction of Mn. We focus on producing p-type films because unintentionally doped films were recently shown to be n-type, likely due to S vacancies. Development of a p-type dopant would therefore enable p-n junctions, a key step in creating a FeS2-based solar absorber. Specifically, we synthesized Mn-doped pyrite thin films via ex situ sulfidation of Fe1−xMnx films. First, a metallic Fe1−xMnx thin film was deposited via D. C. magnetron sputtering. Subsequently, this metallic film was sulfidized in a sulfur atmosphere at 600 ◦C. We studied the chemical and structural properties of the metallic Fe1−xMnx thin films before and after sulfidation. It was found that the Fe1−xMnx thin films form a metastable bcc phase, which partly transforms into an fcc phase when the films are annealed at (or above) 400 ◦C. We studied the chemical, structural and electronic transport properties for 10 such films after sulfidation. The Mn-doped pyrite thin films contained nanoscale inhomogeneities which led to hopping transport. None of the thin films were p-type. At high Mn concentrations, we detected the formation of MnS in pyrite FeS2 films. Though we did not demonstrably succeed in synthesizing phase pure Mn-doped p-type pyrite thin films, the following findings provide insight into how to proceed in the future: we propose to sufidize the Fe1−xMnx films in two steps. First, sulfidize the film at a temperature between 200 ◦C and 400 ◦C. Subsequently, further anneal this film in sulfur atmosphere at 600 ◦C after all metastable bcc phases convert.