L. Weinhardt

Nondestructive depth-resolved spectroscopic investigation of the heavily intermixed In2S3/Cu(In,Ga)Se2 interface

The chemical structure of the interface between a nominal In2S3 buffer and a Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorber was investigated by soft x-ray photoelectron and emission spectroscopy. We find a heavily intermixed, complex interface structure, in which Cu diffuses into (and Na through) the buffer layer, while the CIGSe absorber surface/interface region is partially sulfurized. Based on our spectroscopic analysis, a comprehensive picture of the chemical interface structure is proposed.

Sulfur gradient-driven Se diffusion at the CdS/CuIn(S,Se)2 solar cell interface

The diffusion behavior of Se at the CdS/Cu(In,Ga)(S,Se)2 thin film solar cell interface was investigated by x-ray photoelectron spectroscopy and x-ray excited Auger electron spectroscopy. Buffer/absorber structures with S/Se ratios between zero and three at the initial Cu(In,Ga)(S,Se)2 surface were analyzed. Samples from a high-efficiency laboratory process (NREL) as well as from an industrial large-area process (AVANCIS) were investigated. We find selenium diffusion into the CdS buffer layer, the magnitude of which strongly depends on the S content at the absorber surface. The associated modification of the heterojunction partners has significant impact on the electronic structure at the interface.

Setup for in situ investigation of gases and gas/solid interfaces by soft x-ray emission and absorption spectroscopy

We present a novel gas cell designed to study the electronic structure of gases and gas/solid interfaces using soft x-ray emission and absorption spectroscopies. In this cell, the sample gas is separated from the vacuum of the analysis chamber by a thin window membrane, allowing in situ measurements under atmospheric pressure. The temperature of the gas can be regulated from room temperature up to approximately 600u2009°C. To avoid beam damage, a constant mass flow can be maintained to continuously refresh the gaseous sample. Furthermore, the gas cell provides space for solid-state samples, allowing to study the gas/solid interface for surface catalytic reactions at elevated temperatures. To demonstrate the capabilities of the cell, we have investigated a TiO2 sample behind a mixture of N2 and He gas at atmospheric pressure.

A closer look at initial CdS growth on high-efficiency Cu(In, Ga)Se 2 absorbers using surface-sensitive methods

In this contribution we present further evidence that the post-deposition treatment with alkali elements (PDT) on Cu(In,Ga)Se2 (CIGS) films can positively influence the initial growth of chemical-bath-deposited CdS. We investigate the surface of CIGS films with and without PDT during initial growth of CdS by various surface-sensitive methods — x-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), and high-resolution scanning electron microscopy — in order to gather information about the growth rate and coverage of CdS on the two surface types. We find that the CdS film deposited for 180 seconds on PDT-CIGS is mostly closed, while samples without PDT show very inhomogeneous coverage. Furthermore, for growth on PDT-CIGS, the effective CdS film thickness is determined to be higher by both XPS and SE. Finally, we present a discussion of the information content of the methods employed here.

X-ray Emission Spectroscopy of Proteinogenic Amino Acids at All Relevant Absorption Edges

Nonresonant N K, O K, C K, and S L2,3 X-ray emission spectra of the 20 most common proteinogenic amino acids in their solid zwitterionic form are reported. They represent a comprehensive database that can serve as a reliable basis for the X-ray absorption spectroscopy (XES) studies of peptides and proteins. At the most important N and O K edges, clear similarities and differences between the spectra of certain amino acids are observed and associated with the specific chemical structure of these molecules and their functional groups. Analysis of these spectra allows the generation of spectral fingerprints of the protonated amino group, the deprotonated carboxylic group, and, using a building block approach, the specific nitrogen- and oxygen-containing functional groups in the side chains of the amino acids. Some of these fingerprints are compared to the spectra of reference compounds with the respective functional groups; they exhibit reasonable similarity, underlining the validity of the spectral fingerprint approach. The C K and S L2,3 XES spectra are found to be specific for each amino acid, in accordance with the different local environments of the involved C and S atoms, respectively.

Electronic and chemical properties of non-vacuum deposited chalcopyrite solar cells

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 Nanosolars 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.

Cu 2 ZnSnS 4 thin-film solar cell absorber composition revealed by energy-dispersive and soft x-ray emission spectroscopy

Different Cu2ZnSnS4 (CTZS) thin-film solar cell absorbers have been investigated by bulk-sensitive energy-dispersive spectroscopy (EDS) and surface-near bulk-sensitive soft x-ray emission spectroscopy (XES). While we find a good agreement between the computed Zn/Sn composition ratio based on the EDS and XES data, the XES determined Cu/(Zn+Sn) composition ratio significantly deviates from that based on EDS measurements for some samples. While the first can be explained by a homogenous Zn/Sn composition throughout the CTZS samples, the latter is interpreted as a variation of the Cu depth profile in the respective thin-film solar cell absorbers.

Spectroscopic analysis of the chemical structure at the CdS/Cu(In,Ga)Se 2 interface in high-efficiency solar cell devices

High-efficiency Cu(In1-xGax)Se2 (CIGSe)-based solar cells utilize a CdS buffer layer between the window and the chalcopyrite absorber. Soft x-ray spectroscopies were employed to investigate the chemical properties of the CdS/CIGSe interface and its dependence on the details of the chemical bath deposition (CBD) of CdS. We have investigated the CdS/CIGSe interface after various CdS CBD times (0, 4, and 12.5 minutes). We find evidence for the presence of Cd(OH)2 at the CdS/CIGSe surface for the thickest CdS sample.