(Invited) Synthesis, Kinetics, Characterization and Application of Sodium and Potassium (poly)Selenides for Chalcogenide Semiconductor Doping

Abstract

Alkali elements are essential for batteries and solar cells. In batteries, alkali ions are typically intercalated in layered 2D structures, such as transition metal sulfides and selenides, in a reversible fashion 1–4. In solar cells based on Cu(In,Ga)Se2 and Cu2ZnSn(S,Se)4 semiconductors, Na2Sex and K2Sex compounds are sources of extrinsic doping and act as surfactants during or after the growth, ending up as segregations at the grain boundaries, where they are also thought to passivate electronic defects 5–10. Alkali (poly)chalcogenide solutions are also traditionally employed for photoelectrochemical characterization of semiconductors because they offer a wide range of redox states 11,12 and stabilize against photoanodic dissolution 13,14. More recently, they have also been adopted as electrolytes in quantum dot sensitized solar cells 15–17. However, despite their technological relevance, little is known about the single alkali selenide compounds beyond the pioneering works of Zintl and Klemm 18,19 and subsequent structural characterizations 20–24. In this presentation, our recent work on (i) synthesis, (ii) characterization (iii) reactivity to oxygen and (iv) utilization of alkali (poly)selenides is reviewed. The synthesis is performed in liquid ammonia at atmospheric pressure and -55 °C. The progress of the reaction is monitored electrochemically, in order to unveil the kinetics. The decrease of solvated electrons concentration after selenium addition is measured directly by means of cyclic voltammetry at the surface of a Pt microelectrode. The compounds are characterised by X-ray photoelectron, Raman and electronic spectroscopies. Broadening of the Se 3d XPS peaks with the increase of the number of Se atoms in the chain suggests a limited extent of negative charge delocalization among the Se atoms in the solid (poly)selenides. A direct band-gap of 1.83 eV and pseudo-direct band-gap of 2.05 eV are identified for Na2Se and K2Se, respectively. The full width at half maximum of the fluorescence emissions is on the order of 300 meV, and the emission peaks occur at more than 0.5 eV below the respected bandgaps, revealing a pronounced defect-assisted radiative recombination. The extreme sensitivity of the compounds to oxidation is studied and discussed with respect to chalcogenide grain boundary passivation. Additionally, extrinsic doping of epitaxial and polycrystalline CuInSe2 films is investigated by means of a novel gas-phase route 25 that utilizes the synthesised compounds. The resulting films are characterised by photoluminescence spectroscopy, revealing the emergence of a distinct peak at ca. 100 meV higher energy than typical 1 eV peak of Cu-poor CuInSe2, irrespective of the alkali source. Lastly, some of the species likely involved in the alkali gas-phase transport and incorporation in CuInSe2are identified for the first time by Knudsen effusion mass spectrometry.