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Band alignments, band gap, core levels, and valence band states in Cu 3 BiS 3 for photovoltaics

DOI: 10.1021/acsami.9b04268 DOI Help

Authors: Thomas J. Whittles (University of Liverpool) , Tim D. Veal (University of Liverpool) , Christopher N. Savory (University College London) , Peter J. Yates (University of Liverpool) , Philip A. E. Murgatroyd (University of Liverpool) , James T. Gibbon (University of Liverpool) , Max Birkett (University of Liverpool) , Richard J. Potter (University of Liverpool) , Jonathan D. Major (University of Liverpool) , Ken Durose (University of Liverpool) , David O. Scanlon (University College London; Diamond Light Source) , Vinod R. Dhanak (University of Liverpool)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Acs Applied Materials & Interfaces

State: Published (Approved)
Published: July 2019

Abstract: The earth-abundant semiconductor Cu3BiS3 (CBS) exhibits promising photovoltaic properties and is often considered analogous to the solar absorbers copper indium gallium diselenide (CIGS) and copper zinc tin sulfide (CZTS) despite few device reports. The extent to which this is justifiable is explored via a thorough X-ray photoemission spectroscopy (XPS) analysis: spanning core levels, ionization potential, work function, surface contamination, cleaning, band alignment, and valence-band density of states. The XPS analysis overcomes and addresses the shortcomings of prior XPS studies of this material. Temperature-dependent absorption spectra determine a 1.2 eV direct band gap at room temperature; the widely reported 1.4–1.5 eV band gap is attributed to weak transitions from the low density of states of the topmost valence band previously being undetected. Density functional theory HSE06 + SOC calculations determine the band structure, optical transitions, and well-fitted absorption and Raman spectra. Valence band XPS spectra and model calculations find the CBS bonding to be superficially similar to CIGS and CZTS, but the Bi3+ cations (and formally occupied Bi 6s orbital) have fundamental impacts: giving a low ionization potential (4.98 eV), suggesting that the CdS window layer favored for CIGS and CZTS gives detrimental band alignment and should be rejected in favor of a better aligned material in order for CBS devices to progress.

Journal Keywords: XPS; photoemission; band gap; ionization potential; Cu3BiS3; density functional theory

Subject Areas: Materials, Chemistry, Energy

Technical Areas: