Characteristics of chemical bond and vacancy formation in chalcopyrite‐type CuInSe 2 and related compounds

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.

Section: Resultsmentioning

confidence: 97%

Section: Results and Discussionmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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

Whittles

Veal

Savory

et al. 2019

“…However, the full merit of this practice is questioned when it is clear that the crystal structure of CAS is so different from CIGS, then so should be the DoS as the bonding is what leads to the adopted crystal structure. Clearly, from the CBDoS, Sb 5p states are the main cation contributor in CAS, whereas in CIGS, the bottom of the CB is mainly In/Ga s states [83][84][85] . Consequently, this has been recognised previously and found to be the reason why CAS shows superior absorption to CIGS 86 , but no further discussions have been made 33 .…”

Section: Density Of Statesmentioning

confidence: 99%

Core Levels, Band Alignments, and Valence-Band States in CuSbS₂ for Solar Cell Applications

Whittles

Veal

Savory

et al. 2017

The earth-abundant material CuSbS (CAS) has shown good optical properties as a photovoltaic solar absorber material, but has seen relatively poor solar cell performance. To investigate the reason for this anomaly, the core levels of the constituent elements, surface contaminants, ionization potential, and valence-band spectra are studied by X-ray photoemission spectroscopy. The ionization potential and electron affinity for this material (4.98 and 3.43 eV) are lower than those for other common absorbers, including CuInGaSe (CIGS). Experimentally corroborated density functional theory (DFT) calculations show that the valence band maximum is raised by the lone pair electrons from the antimony cations contributing additional states when compared with indium or gallium cations in CIGS. The resulting conduction band misalignment with CdS is a reason for the poor performance of cells incorporating a CAS/CdS heterojunction, supporting the idea that using a cell design analogous to CIGS is unhelpful. These findings underline the critical importance of considering the electronic structure when selecting cell architectures that optimize open-circuit voltages and cell efficiencies.

“…It was reported that the reductions in CGI and GGI are strongly related to the E A and I P . 61,62 The estimated CGI and GGI by XPS decreased from 0.39 to 0.18 and from 0.33 to 0.07 in XPS analysis, as shown in Figure 3, respectively. The changes in E A and I P for our epi-CIGSe by KF-PDT can be explained qualitatively by the reduction in CGI and GGI.…”

Section: ■ Theoretical Backgroundmentioning

confidence: 89%

Impacts of KF Post-Deposition Treatment on the Band Alignment of Epitaxial Cu(In,Ga)Se₂ Heterojunctions

Nagai

Nishinaga

Tampo

et al. 2022

In this study, we investigated band alignments at CdS/epitaxial CuIn x Ga 1−x Se 2 (epi-CIGSe) and epi-CIGSe/GaAs heterointerfaces for solar cell applications using ultraviolet, inverse, and X-ray photoemission spectroscopy (UPS, IPES, and XPS) techniques. We clarified the impacts of KF postdeposition treatment (KF-PDT) at the CdS/epi-CIGSe front heterointerfaces. We found that KF-PDT changed the conduction band alignment at the CdS/epi-CIGSe heterointerface from a cliff to flat configuration, attributed to an increase in the electron affinity (E A ) and ionization potential (I P ) of the epi-CIGSe surface because of a decrease in Cu and Ga contents. Herein, we discuss the correlation between the impacts of KF-PDT and the solar cell performance. Furthermore, we also investigated the band alignment at the epi-CIGSe/GaAs rear heterointerface. Electron barriers were formed at the epi-CIGSe/GaAs interface, suppressing carrier recombination as the back surface field. Contrarily, a hole accumulation layer is formed by the valence band bending, which is like Ohmic contact.