Department Interface Design

Examples from current research

Investigating the Alkali Postdeposition Treatment of Cu(In,Ga)Se₂ Thin-Film Solar Cell Absorbers

We have investigated the effects of alkali fluoride postdeposition treatments (PDT) on the chemical and electronic surface and near-surface structure of Cu(In,Ga)Se₂ (CIGSe) absorbers using a variety of x-ray based spectroscopies. Laboratory-based x-ray photoelectron spectroscopy (XPS) and synchrotron-based hard x-ray photoelectron spectroscopy (HAXPES) have been used to determine the surface and near-surface composition of the absorbers, which significantly changes depending on the performed postdeposition treatments focusing on the influence of NaF and NaF/KF-PDT when compared to alkali-free (i.e. untreated) CIGSe absorbers. The alkali-free and NaF-PDT absorbers show similar chemical properties, having a Cu and Ga poor surface region compared to the nominal bulk and the same chemical environment for indium and selenium. For the NaF/KF-PDT samples a K-In-Se compound is present on top of a Cu-In-Ga-Se compound, with a nanopatterned surface. Synchrotron-based x-ray emission spectroscopy (XES) and x-ray absorption spectroscopy (XAS) have been used to also probe the chemical environment of selenium and potassium. Based on the comparison of the spectra of the untreated (alkali-free) and the NaF-PDT CIGSe absorber, we find no significant change in the Se environment. However, the spectrum of the NaF/KF-PDT CIGSe absorber indicates the presence of a different Se species. Both, the Se and K related spectra of the NaF/KF-PDT CIGSe sample, are in good agreement with the spectra of a KIn_xSe_y-like compound.

HAXPES was also used to depth-dependently study the valence band together with ultraviolet photoelectron spectroscopy (UPS). Inverse photoemission was employed to derive the conduction band minimum (CBM). For the NaF/KF-PDT CIGSe absorber, we find a significant shift of the valence band maximum (VBM) and CBM away from the Fermi level compared to the alkali-free and NaF-PDT CIGSe, resulting in a significantly increased surface band gap of 2.52 (+0.14/-0.51) eV. This is in agreement with a Cu- and Ga-devoid surface region and the formation of a K-In-Se surface compound Furthermore, we find that for all measurements and all samples the VBM moves away from the Fermi level with increasing surface sensitivity, indicating chemically and/or electronically modified CIGSe surfaces.

Schematics of the main findings for the chemical structure for NaF/KF-PDT CIGSe absorbers showing a K-In-Se/CIGSe bilayer structure.

Schematic of the energy band positions for NaF-PDT and NaF/KF-PDT CIGSe thin-film solar cell absorbers with respect to the Fermi level E_F, determined using direct and inverse photoemission measurements having different inelastic mean free path. The gray boxes indicate the experimental uncertainty.

For more details see:

E. Handick, P. Reinhard, J.-H. Alsmeier, L. Köhler, F. Pianezzi, S. Krause, M. Gorgoi, E. Ikenaga, N. Koch, R. G. Wilks, S. Buecheler, A. N. Tiwari, and M. Bär, Potassium Postdeposition Treatment-Induced Band Gap Widening at Cu(In,Ga)Se₂ Surfaces – Reason for Performance Leap?, ACS Appl. Mater. Interfaces 7, 27414 (2015)

E. Handick, P. Reinhard, R. G. Wilks, F. Pianezzi, T. Kunze, D. Kreikemeyer-Lorenzo, L. Weinhardt, M. Blum, W. Yang, M. Gorgoi, E. Ikenaga, D. Gerlach, S. Ueda, Y. Yamashita, T. Chikyow, C. Heske, S. Buecheler, A. N. Tiwari, and M. Bär, Formation of a K—In—Se Surface Species by NaF/KF Postdeposition Treatment of Cu(In,Ga)Se₂ Thin-Film Solar Cell Absorbers, ACS Appl. Mater. Interfaces 9, 3581 (2017)

Direct Observation of an Inhomogeneous Chlorine Distribution in CH₃NH₃PbI_3-xCl_x Layers: Surface Depletion and Interface Enrichment

We have used hard X-ray photoelectron spectroscopy (HAXPES) at different photon energies and fluorescence yield X-ray absorption spectroscopy (FY-XAS) to non-destructively investigate CH₃NH₃PbI_3-xCl_x perovskite thin films on compact TiO₂. This combination of spectroscopic techniques allows the variation of information depth from the perovskite layer surface to the top-most part of the underlying compact TiO₂ layer. We have taken advantage of this to understand the distribution of chlorine throughout the perovskite/TiO₂ layer stack. No Cl is detected using HAXPES, indicating surface depletion of Cl and allowing us to place an upper limit on the amount of Cl in the perovskite layer: x < 0.07 and x < 0.40 to depths of ~10 nm and ~26 nm, respectively, beneath the perovskite film surface. Our FY-XAS results, however, demonstrate that there is a higher average concentration of Cl throughout the perovskite layer than at the surface (x > 0.40) consistent with both enhanced concentrations of Cl deep beneath the perovskite film surface and near the CH₃NH₃PbI_3-xCl_x perovskite/TiO₂ interface. The consequences of this distribution of Cl in the CH₃NH₃PbI_3-xCl_x perovskite layer on device performance are discussed.

Cl 2p spectra taken with a photon energy of 2 keV of a 3 : 1 mole ratio, CH₃NH₃I : PbCl₂, solution in DMF, drop-cast onto a compact TiO₂ layer and dried in UHV; at room temperature before heating (bottom), after heating to 140 °C (top).

The Cl 2p region as a function of temperature taken with a photon energy of 2 keV. Yellow indicates higher intensities in the plot. The Cl 2p region is indicated at the top of the plot.

Cl 2p and I 4s spectra taken with a photon energies of 2 keV (black) and 6 keV (red) of a 60 nm thick CH₃NH₃PbI_3-xCl_x film on compact TiO₂. Note the absence of any detectable Cl in both spectra.

Cl K edge FY-XAS spectra of a 60 nm thick CH3NH3PbI3-xClx layer on compact TiO2 (blue) and the compact TiO2 layer without a CH3NH3PbI3-xClx layer on top (green). Note the distinctively different shapes of the two spectra.

 

For more details see:

D.E. Starr, G. Sadoughi, E. Handick, R.G. Wilks, J.H. Alsmeier, L. Köhler, M. Gorgoi, H.J. Snaith, and M. Bär, Direct observation of an inhomogeneous chlorine distribution in CH₃NH₃PbI_3−xCl_x layers: surface depletion and interface enrichment,  Energy Environ. Sci. 8, 1609 (2015)