Understanding the impact of impurities in solar absorbers is critical to engineering high-performance in devices, particularly over extended periods of time. Here, we use hybrid functional calculations to explore the role of hydrogen interstitial (Hi) defects in the electronic properties of a number of attractive solar absorbers within the chalcopyrite and kesterite families to identify how this common impurity may influence device performance. Our results identify that Hi can inhibit the highly p-type conditions desirable for several higher-band gap absorbers and that H incorporation could detrimentally affect the open-circuit voltage (Voc) and limit device efficiencies. Additionally, we find that Hi can drive the Fermi level away from the valence band edge enough to lead to n-type conductivity in a number of chalcopyrite and kesterite absorbers, particularly those containing Ag rather than Cu. We find that these effects can lead to interfacial Fermi-level pinning that can qualitatively explain the observed performance in high-Ga content CIGSe solar cells that exhibit saturation in the Voc with increasing band gap. Our results suggest that compositional grading rather than bulk alloying, such as by creating In-rich surfaces, may be a better strategy to favorably engineering improved thin-film photovoltaics with larger-band gap absorbers.

To enable an understanding and optimization of the optoelectronic behavior of CdTe-ZnO nanocomposites, the morphological and chemical properties of annealed CdTe/ZnO interface structures were studied. For that purpose, CdTe layers of varying thickness (4–24 nm) were sputter-deposited on 100 nm-thick ZnO films on surface-oxidized Si(100) substrates. The morphological and chemical effects of annealing at 525 °C were investigated using X-ray Photoelectron Spectroscopy (XPS), X-ray-excited Auger electron spectroscopy, energy dispersive X-ray spectroscopy, scanning electron microscopy, and atomic force microscopy. We find a decrease of the Cd and Te surface concentration after annealing, parallel to an increase in Zn and O signals. While the as-deposited film surfaces show small grains (100 nm diameter) of CdTe on the ZnO surface, annealing induces a significant growth of these grains and separation into islands (with diameters as large as 1 μm). The compositional change at the surface is more pronounced for Cd than for Te, as evidenced using component peak fitting of the Cd and Te 3d XPS peaks. The modified Auger parameters of Cd and Te are also calculated to further elucidate the local chemical environment before and after annealing. Together, these results suggest the formation of tellurium and cadmium oxide species at the CdTe/ZnO interface upon annealing, which can create a barrier for charge carrier transport, and might allow for a deliberate modification of interface properties with suitably chosen thermal treatment parameters.

We have investigated the surface and near-surface bulk chemical properties of differently stressed (Au/Cu/)CdTe/CdS thin-film solar cells. The employed Au/Cu contact design created unique samples in which both the back contact surface/interface and the (back) surface of the CdTe absorber were exposed. X-ray photoelectron spectroscopy and soft x-ray emission spectroscopy were employed to determine the chemical composition of the surface and near-surface bulk of both the exposed CdTe surface and the Au/Cu back contact surface of each sample. Sulfur is observed to have migrated (from the CdS) into the CdTe bulk, forming Au2S and Cu2-xS compounds under the back contact. Chlorine is found at the surface and in the near-surface bulk of both the back contact and the exposed CdTe absorber. Under the back contact, Cl is observed to form Cu-Cl bonds only in the samples that were heat stressed under AM 1.5 illumination.

We present a study of low-energy ion surface cleaning treatments and their impact on the surface electronic structure of an air-exposed CdTe thin film treated with CdCl 2 . In order to determine the electronic structure using surface-sensitive photoemission, surfaces need to be free of contaminants. This is achieved by subsequent low-energy ion treatment steps, carefully monitoring the chemical and electronic surface structure. We present data on the valence band maximum (VBM), and core-level binding energies, that suggest that neither preferential sputtering occurs nor metallic states are formed using our cleaning procedure. For a clean CdTe surface, the VBM is determined to be (0.8 ± 0.1) eV below the Fermi energy.

We have investigated the electronic and chemical surface properties of a Cu(In1-xGax)Se2 (CIGSe) thin-film solar cell absorber and a CdS/CIGSe interface sample taken from Nanosolar's manufacturing line. Using soft x-ray and UV photoelectron spectroscopy, inverse photoemission, and soft x-ray emission spectroscopy employing high-brilliance synchrotron radiation, we have determined the chemical composition of the surface and near-surface bulk, as well as some of the relevant electronic structure parameters (e.g., the surface band gap of the absorber). We find that the (previously air-exposed) surfaces show a surprisingly low degree of carbon-containing surface adsorbates, the presence of sodium and selenium oxide species on the surface of both samples, a significant S/Se intermixing at the CdS/CIGSe interface, and, as is common for high-efficiency CIGSe absorbers after surface cleaning, an electronic surface band gap (1.45 ± 0.15 eV) that is noticeably larger than the optical bulk band gap.