Hexagonal selenium with a direct band gap has been developed for optoelectronic applications for more than one century. The major advances in Se solar cells have been made using vacuum or solution-based processing methods. In this work, we demonstrate a new two-stage melt processing (TSMP) method for incorporating Se in printable triple mesoscopic solar cells in the ambient conditions. It is observed that polymerization and depolymerization between several types of selenium chains are simultaneously triggered during the melt processing, from which phase-pure hexagonal selenium is formed in the mesopores of solar cells with high crystallinity. The TSMP method has positive effects on the conduction-band energy level, band gap, and crystal phase of as-deposited Se, as revealed UV electron spectroscopy, UV-vis absorption spectroscopy, and in situ X-ray diffraction. The TSMP-based printable mesoscopic selenium solar cells show a power conversion efficiency of 2%, which is eight times that for devices based on the single-stage melting processing. These findings open up a new research direction of melting processing toward more efficient photovoltaic devices.
Keywords: melt processing; mesoscopic; pure phase; selenium; solar cell.