Identifying barriers to charge-carriers in the bulk and surface regions of Cu2ZnSnS4 nanocrystal films by x-ray absorption fine structures (XAFSs)

Abstract

Solar cell performance is most affected by the quality of the light absorber layer. For thin-film devices, this becomes a two-fold problem of maintaining a low-cost design with well-ordered nanocrystal (NC) structure. The use of CuZnSnS (CZTS) NCs as the light absorber films forms an ideal low-cost design, but the quaternary structure makes it difficult to maintain a well-ordered layer without the use of high-temperature treatments. There is little understanding of how CZTS NC structures affect the photoconversion efficiency, the charge-carriers, and therefore the performance of the device manufactured from it. To examine these relationships, the measured photoresponse from the photo-generation of charge-carrier electron-hole pairs was compared against the crystal structure, as short-range and long-range crystal orders for the films. The photoresponse simplifies the electronic properties into three basic steps that can be associated with changes in energy levels within the band structure. These changes result in the formation of barriers to charge-carrier flow. The extent of these barriers was determined using synchrotron-based X-ray absorbance fine structure to probe the individual metal centers in the film, and comparing these to molecular simulations of the ideal extended x-ray absorbance fine structure scattering. This allowed for the quantification of bond lengths, and thus an interpretation of the distortions in the crystal lattice. The various characteristics of the photoresponse were then correlated to the crystallographic order and used to gain physical insight into barriers to charge-carriers in the bulk and surface regions of CZTS films.

Favorable Bonding and Band Structures of Cu2ZnSnS4 and CdS Films and Their Photovoltaic Interfaces

“…Antisite formation within the crystal structure again accounts for these changes. Taken together with the observations on the Cu spectrum, the presence of antisite markers in both the Cu and Zn spectra is indicative of a possible Cu-on-Zn single acceptor, or a Zn-on-Cu single donor site, as discussed previously on CZTS nanocrystal films. − Single-acceptor Cu vacancies and double-acceptor Zn-vacancies are also possible explanations. These defects are theoretically calculated disorder formations arising during CZTS fabrication, and they have been shown in sputtered CZTS films. , The deeper acceptor sites contribute favorably to charge-carrier physics at the barrier, while shallow acceptors and donor sites are less favorable and often affect performance adversely. , Cu vacancies would therefore contribute favorably to performance, as would Cu-on-Zn antisites.…”

Short- and Long-Range Cation Disorder in (AgxCu1–x)2ZnSnSe4 Kesterites

“…As shown in Figure a–e, the absence of significant changes in the absorption edges ruled out the presence of significant amounts of secondary phases, in good agreement with the XPS results. The Cu K-edge spectra in Figure a display a strong Cu + component in the Cu centers of the materials due to the absence of a pre-edge peak; i.e., dipole-forbidden and quadrupole-allowed 1s → 3d transitions are unavailable. , These features reveal that copper remained in the Cu + state in all samples, and no Cu 2+ species were present . Similarly, the lack of pre-edge features in the Zn spectra suggests a well-conserved symmetry, with a d 10 structure maintaining the Zn 1s → 4p transition (Figure b). , On the other hand, the Sn K-edge spectra exhibit fewer features, owing to the increased broadening of the absorption peaks, which is a common effect for heavier elements (Figure c) .…”

“…The Cu K-edge spectra in Figure 5a display a strong Cu + component in the Cu centers of the materials due to the absence of a pre-edge peak; i.e., dipole-forbidden and quadrupole-allowed 1s → 3d transitions are unavailable. 56,57 These features reveal that copper remained in the Cu + state in all samples, and no Cu 2+ species were present. 58 Similarly, the lack of pre-edge features in the Zn spectra suggests a wellconserved symmetry, with a d10 structure maintaining the Zn 1s → 4p transition (Figure 5b).…”

Local Structure Studies of As-Made Cu2ZnSnS4 Nanoparticles

“…Nanoparticle films have been studied by Turnbull et al 37 using several techniques including EXAFS. Their sample 1 has a similar composition to our sample #2, but none of the others are comparable to our sample #1 which has a high Sn and low Cu content.…”