Soft X-ray Spectroscopy of a Complex Heterojunction in High-Efficiency Thin-Film Photovoltaics: Intermixing and Zn Speciation at the Zn(O,S)/Cu(In,Ga)Se₂ Interface

Abstract

The chemical structure of the Zn(O,S)/Cu(In,Ga)Se interface in high-efficiency photovoltaic devices is investigated using X-ray photoelectron and Auger electron spectroscopy, as well as soft X-ray emission spectroscopy. We find that the Ga/(Ga+In) ratio at the absorber surface does not change with the formation of the Zn(O,S)/Cu(In,Ga)Se interface. Furthermore, we find evidence for Zn in multiple bonding environments, including ZnS, ZnO, Zn(OH), and ZnSe. We also observe dehydrogenation of the Zn(O,S) buffer layer after Ar ion treatment. Similar to high-efficiency CdS/Cu(In,Ga)Se devices, intermixing occurs at the interface, with diffusion of Se into the buffer, and the formation of S-In and/or S-Ga bonds at or close to the interface.

Improving performance by Na doping of a buffer layer—chemical and electronic structure of the InxSy:Na/CuIn(S,Se)2 thin‐film solar cell interface

“…One of the most challenging tasks for the chalcopyrite photovoltaics industry is the replacement of the “state‐of‐the‐art” CdS buffer layer with a Cd‐free, more transparent alternative that is compatible with an in‐line production process. Promising candidates like Zn(O,S), (Zn,Mg)O, and In(OH,S) buffer layers, as well as the possibility of fully avoiding the buffer layer, have been investigated. However, for small solar cells, the record efficiencies for alternative buffer layers are lower, typically by about 1% to 2% .…”

Chemical and Electronic Structure at the Interface between a Sputter‐Deposited Zn(O,S) Buffer and a Cu(In,Ga)(S,Se)2 Solar Cell Absorber

“…Comparing the results of the here‐presented sputter‐deposited Zn(O,S)/CIGSSe with the CBD‐Zn(O,S)/CIGSSe interface, [ 9,10 ] we find some similarities and distinct differences. Both buffer layers consist of various bonding environments, e.g., sulfates and hydroxides, in addition to the expected mixture of Zn–O and Zn–S bonds.…”

“…Similar effects are also reported for prolonged X‐ray exposure and elevated temperatures. [ 9,13,44 ] In summary, thus, the analysis of the buffer layer suggests that a multitude of different bonding environments are presents, i.e., Zn–O, Zn–OH, Zn–S, and S–O bonds.…”

“…Previous studies on CBD Zn(O,S) buffer layers with Cu(In,Ga)Se 2 (CIGSe) absorbers from the National Renewable Energy Laboratory (NREL) revealed a flat conduction band alignment at the buffer/absorber interface, an intermixing of Se into the buffer, as well as Zn in various bonding environments, e.g., ZnS, ZnO, ZnSe, and Zn(OH) 2 . [ 9–11 ] For other Zn(O,S)/CIGSe samples, a high defect concentration, inducing Fermi level pinning, was reported. [ 12 ] Studies of CuInS 2 (CIS) absorbers show the formation of a Zn(O,S)/ZnS bilayer at the CBD‐Zn(O,S)/CIS interface [ 13 ] and a small spike in the conduction band alignment.…”

Electronic Structure of Chalcopyrite Surfaces for Photoelectrochemical Hydrogen Production

“…In the case of chalcopyrite absorbers, such additional band bending is expected to be rather small, based on the experimentally observed additional band bending in the absorber induced by interface formation with various buffer layers in photovoltaic devices [including CdS and Zn­(O,S)]. In all of these cases, interface-induced band bending changes in the absorber are typically on the order of 0.1 eV or less (see, e.g., refs , , and ), and hence this aspect will not be further considered here.…”