Limitations of Near Edge X-ray Absorption Fine Structure as a tool for observing conduction bands in chalcopyrite solar cell heterojunctions

Abstract

A non-optimized interface band alignment in a heterojunction-based solar cell can have negative effects on the current and voltage characteristics of the resulting device. To evaluate the use of Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS) as a means to measure the conduction band position, Cu(In,Ga)S 2 chalcopyrite thin film surfaces were investigated as these form the absorber layer in solar cells with the structure ZnO/Buffer/Cu(In,Ga)S 2 /Mo/Glass. The composition dependence of the structure of the conduction bands of CuIn x Ga 1−x S 2 has been revealed for x = 0, 0.67 and 1 with both hard and soft NEXAFS and the resulting changes in conduction band offset at the junction with the buffer layer discussed. A comprehensive study of the positions of the absorption edges of all elements was carried out and the development of the conduction band with Ga content was observed, also with respect to calculated densities of states. Knowledge of these offsets is, therefore, critical to understanding the performance of the resulting solar cell. While the VB offset, ∆E V B , can be determined with established methods, such as combined XPS/UPS [5,6] or Constant Final State Yield Spectroscopy [7], a determination of CB edge positions and offsets, ∆E CB , has proved more difficult. The most common method is simply the assumption that the CB minimum is the energy of the VB plus the band gap. However, the determination of the surface band gap, which is relevant for the band offset, is more involved. Two of the main methods for the direct determination of the CB minimum are inverse photoelectron spectroscopy (IPES) and Near Edge X-ray Absorption Fine Structure (NEXAFS). They have given reliable results in some situations [8,9,10,11], although both have unresolved difficulties and the results must be carefully analyzed. IPES requires high intensity electron irradiation of the sample which often leads to charging of less conductive materials. In the case of NEXAFS these include transition probabilities, spectrum broadening and excitonic or core-hole effects. The latter may cause shifts in the measured position of the absorption edges which do not correspond to the ground state of the material. This is because the position of the absorption edge in NEXAFS represents the energy difference between the initial state (core level) and the final empty state (conduction band) in the material's excited state. The attraction between the core-hole and the excited electron may make the energy difference between the core level and conduction band state appear artificially smaller than it is in the ground state of the material. Also, because the absorption edge represents an energy difference, the energy of the initial state (core level) must be considered to determine whether differences in binding energy could influence the calculated energy of the final conduction band state. Here, while considering only the position of the absorption edge, we assume at first a constant initial state (core level binding) energy, a...

Boron Doped diamond films as electron donors in photovoltaics: An X-ray absorption and hard X-ray photoemission study

“…3, using linear extrapolation towards the onset and assuming that the electronhole interaction is negligible. Compare similar results and extrapolation methods in B€ ar et al, 9 Johnson et al, 10 and B€ ar et al 11 Combining this result with (E F -Cu2p 3/2 ) ¼ 932.22 eV for CIGS/Diamond from step (5) gives (CBM-E F ) ! 0.95 eV.…”

“…It is used to obtain the conduction band minimum of CIGS via linear extrapolation of the L 3 edge, following previous work. [9][10][11] For our CIGS film, the CBM is reached at 933.17 eV. To obtain the position of the CBM relative to the Fermi level (CBM-E F ), we subtract the Cu2p 3/2 core level binding energy obtained by HXPS.…”

“…Here, we focus on CIGS (typically Cu y (In 1Àx Ga x )Se 2 ), whose gap is tunable within the optimal energy range, with x % 0.7 for optimum performance. [3][4][5][6][7][8][9][10][11] A suitable donor has been difficult to find, particularly for dye-sensitized cells. Based on the success with TiO 2 as acceptor, we choose p-doped diamond as a symmetric counterpart.…”

“…The CBM value is extracted from the onset of the Cu2p 3/2 XAS by linear extrapolation, using a previously established method. [9][10][11] Finally, one can take the electron affinity of CIGS from the literature 36 to assign a vacuum level E vac to the CIGS film.…”

Local versus global electronic properties of chalcopyrite alloys: X-ray absorption spectroscopy and ab initio calculations

“…The case of the Cu L 3 -edge is more ambiguous, as discussed in Ref. 3, and more experimental and theoretical research is required to draw reliable conclusions on the behavior at this edge. The theoretical findings thus confirm the experimental observations demonstrating that this unusual behavior of absorption edge positions is indeed a real effect of mixed chalcopyrite alloys.…”