Band Alignments, Band Gap, Core Levels, and Valence Band States in Cu₃BiS₃ for Photovoltaics

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.

Wittichenite semiconductor of Cu3BiS3 films for efficient hydrogen evolution from solar driven photoelectrochemical water splitting

“…1b . We observed two major vibrational peaks at 279 cm −1 and 467 cm −1 from the Raman spectrum, which matched well with the reported Raman shifts of Cu 3 BiS 3 films 33 , 38 . The half-widths of the vibrational peaks at ~279 cm −1 and 467 cm −1 decreased with increasing substrate temperature, indicating that the crystallinity increased with increasing substrate temperature.…”

“…The fitted resistance parameters are shown in Table S1 , in which R s is mainly composed of the sheet resistance of the contact and external wire. R 1 is generally influenced by the transport of charge carriers inside the electrode, and R 2 represents the impedance at the electrode/electrolyte interface 33 – 40 . We found that the samples sprayed at 380 °C showed a significantly lower bulk resistance and lower interfacial/bulk recombination ratio of photoexcited carriers due to the compact film structure and appreciate crystalline quality of the Cu 3 BiS 3 film (380 °C), as shown in Fig.…”

“…Clearly, all of the observed diffraction peaks of the Cu 3 BiS 3 film sprayed from the precursor solution with a Cu:Bi = 3:1 ratio corresponded well to the reference (JCPDS NO. 43-1479) 33 – 40 . Nevertheless, we observed obvious secondary phases and impurity phases, such as Cu 2-x S and Cu 3 Bi 3 S 7 (marked with a yellow background in Fig.…”

“…Cu 2 O-based photocathodes have been widely investigated due to their high PEC photovoltage, which has reached as high as 1.2 V RHE , but their low stability is still a major problem 31 , 32 . Fortunately, ternary I − V − VI compounds of Cu 3 BiS 3 are believed to be promising chalcogenide compounds for photovoltaic devices due to their perfect optical band gap ( Eg ≈ 1.4–1.7 eV), high absorption coefficient (>10 5 cm −1 ) compared to those of CuInSe 2 and Cu 2 ZnSnS 4 , and p-type conductivity with a carrier concentration of ~2 × 10 16 cm −3 33 – 35 . Cu 3 BiS 3 , naturally occurring in the “Wittichenite” mineral form, is stable for a broad range of processing temperatures 36 .…”

“…The reported low efficiency and poor PEC performance of Cu 3 BiS 3 -based photoelectrodes are far from their theoretical limit considering their perfect optical band gap value of ~1.7 eV for solar water splitting, and the low working stability itself restricts their further industrial utilization 38 , 39 . The low solar-to -hydrogen conversion efficiency should be caused by the low crystallinity of the material, poor surface/interface conditions and obvious internal defects 33 , 38 . In this work, we presented a world record ABPE of 1.7% for our Cu 3 BiS 3 -based photocathode for solar water splitting, and the photocurrent density ( J 0 ) of ~7 mA/cm 2 , onset potential ( Voc ) of 0.9 V RHE and ABPE of 1.7%.…”