A New Look at the Electronic Structure of Transparent Conductive Oxides—A Case Study of the Interface between Zinc Magnesium Oxide and Cadmium Telluride

Abstract

and photovoltaic cells. [1][2][3][4] In CdTe-based thin film solar cells, TCOs are employed as front contact layers, which, due to the superstrate configuration of such cells, form the base layer onto which CdTe absorbers are deposited. Thin-film devices based on CdTe have achieved a notable market share as a cost-effective alternative to silicon, with a current world record cell efficiency of 22.1%. [5,6] At a module level, efficiencies have reached 18.2%, which surpasses the best-recorded multi-crystalline silicon module of 17.7%. [5,7] Achieving a desirable band alignment and interface passivation is critical for the performance of high-efficiency thin-film solar cells; here, we report on the electronic structure established by integrating a zinc magnesium oxide layer into a superstrate CdTe device structure. Note that (Zn,Mg)O, or short ZMO, has also shown promise as a buffer material in other chalcogenide solar cells, [8][9][10][11][12][13] while its optimization potential for incorporation as an alternative buffer layer has likely not yet been reached.Furthermore, it is the purpose of this paper to propose an in-depth analysis approach for such TCOs. In particular, we will distinguish between electronic states responsible for the TCO conductivity and the band edge characteristics responsible for the TCO transparency, and thus establish a methodology that is applicable to TCOs in various different application fields. To do this, we have used X-ray (XPS) and UV (UPS) photoelectron spectroscopy, X-ray excited Auger electron spectroscopy (XAES), and inverse photoemission spectroscopy (IPES) to investigate not only the ZMO, but also the interface properties of ZMO and CdTe, with particular emphasis on a proper determination of the band alignment in the conduction band. Results and DiscussionThe Al Kα-excited XPS survey spectrum of the as-received bare substrate surface (Figure 1, bottom) displays all expected ZMO photoemission and Auger lines (of Zn, Mg, and O), which decrease in intensity with increasing CdTe overlayer thickness (2, 5, and 10 nm; Figure 1, upper spectra). The spectra in Figure 1 are magnified by a factor of 3 for binding energies By combining X-ray and ultraviolet photoelectron spectroscopy, X-ray excited Auger electron spectroscopy, and inverse photoemission spectroscopy, a detailed characterization of the chemical and electronic structure at the CdTe/ (Zn,Mg)O interface is presented. In doing so, a novel approach to characterize the electronic structure of transparent conductive oxides, namely, to distinguish between optically and transport-relevant states is presented. Consequently, "optical" and "electronic" band offsets at the CdTe/(Zn,Mg)O interface are derived and it is demonstrated that the "electronic" band offset is suitable for electron transport across the interface (a flat conduction band alignment of 0.02 ± 0.20 eV), while the "optical band offset" is not (a significant 0.48 ± 0.20 eV spike). The new approach can easily be applied to many other transparent conductive oxides and is...

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

“…For a different buffer layer (Zn,Mg)O, it was suggested that such tails are relevant for the charge carrier transport, while the spectral main edges are relevant for the optical properties. [51] Using the main edges, we derive a surface band gap of 3.14 (AE0.14) eV, which is within the region of reported bulk Zn(O,S) band gap values of 2.6-3.4 eV, depending on the S/(S þ O) ratio. [6,52,53] The direct comparison of the band extrema allows a first approximation of the band alignment at the interface, suggesting no pronounced discontinuity in the conduction band and a significant negative valence band offset of %À1.5 eV.…”

“…For a different buffer layer (Zn,Mg)O, it was suggested that such tails are relevant for the charge carrier transport, while the spectral main edges are relevant for the optical properties. [ 51 ] Using the main edges, we derive a surface band gap of 3.14 (±0.14) eV, which is within the region of reported bulk Zn(O,S) band gap values of 2.6–3.4 eV, depending on the S/(S + O) ratio. [ 6,52,53 ]…”

Electronic Structure of Chalcopyrite Surfaces for Photoelectrochemical Hydrogen Production

“…However, these linear extrapolations leave substantial spectral tails undescribed (as shown with the blue linear extrapolation in Figure 6c and blue markings in Figure 6d, leading to a "band gap" of 0.50 (±0.11) eV). While such tails are not uncommon in the description of transparent conductive oxides, 67,68 they are rather unusual in the case of chalcopyrites. We hypothesize that the seemingly narrow "band gap" can be attributed to a much greater number of surface defects compared to its "Cu-poor" counterpart, as we could not find any evidence suggesting the presence of additional binary phases, unique chemical bonding environments, or even any gradual changes in surface composition.…”

In2O3:H-Based Hole-Transport-Layer-Free Tin/Lead Perovskite Solar Cells for Efficient Four-Terminal All-Perovskite Tandem Solar Cells

“…Apart from the quality of a perovskite thin film, which is directly fabricated onto the TCOs in the HTL-free devices, the impact of the TCO substrate on the interface of the TCO/perovskite is also critical for high-performance PSCs. − Here, we investigate three different sets of samples using X-ray photoelectron spectroscopy (XPS) to analyze the chemical and electronic structures of the TCO/perovskite interface: (1) TCO, (2) TCO coated with a thin NBG perovskite (≈3 nm), and (3) TCO coated with a thick NBG perovskite (600 nm, i.e., the nominal thickness) films, with ITO, FTO, or IO:H TCOs. In the XPS survey spectra of these samples (Figure S6), all expected TCO signals are detected and attenuated with increasing perovskite thickness.…”