On the basis of a combination of X-ray photoelectron spectroscopy and synchrotron-based X-ray emission spectroscopy, we present a detailed characterization of the chemical structure of CdS:O thin films that can be employed as a substitute for CdS layers in thin-film solar cells. It is possible to analyze the local chemical environment of the probed elements, in particular sulfur, hence allowing insights into the species-specific composition of the films and their surfaces. A detailed quantification of the observed sulfur environments (i.e., sulfide, sulfate, and an intermediate oxide) as a function of oxygen content is presented, allowing a deliberate optimization of CdS:O thin films for their use as alternative buffer layers in thin-film photovoltaic devices.
The electronic band alignment of the Zn(O,S)/Cu(In,Ga)Se 2 interface in high-efficiency thin-film solar cells was derived using X-ray photoelectron spectroscopy, ultra-violet photoelectron spectroscopy, and inverse photoemission spectroscopy. Similar to the CdS/Cu(In,Ga)Se 2 system, we find an essentially flat (small-spike) conduction band alignment (here: a conduction band offset of (0.09 ± 0.20) eV), allowing for largely unimpeded electron transfer and forming a likely basis for the success of high-efficiency Zn(O,S)-based chalcopyrite devices. Furthermore, we find evidence for multiple bonding environments of Zn and O in the Zn(O,S) film, including ZnO, ZnS, Zn(OH) 2 , and possibly ZnSe.
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.
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...
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