title: Local band gap measurements by VEELS of thin film solar cells.
abstract: This work presents a systematic study that evaluates the feasibility and reliability of local band gap measurements of Cu(In,Ga)Se2 thin films by valence electron energy-loss spectroscopy (VEELS). The compositional gradients across the Cu(In,Ga)Se2 layer cause variations in the band gap energy, which are experimentally determined using a monochromated scanning transmission electron microscope (STEM). The results reveal the expected band gap variation across the Cu(In,Ga)Se2 layer and therefore confirm the feasibility of local band gap measurements of Cu(In,Ga)Se2 by VEELS. The precision and accuracy of the results are discussed based on the analysis of individual error sources, which leads to the conclusion that the precision of our measurements is most limited by the acquisition reproducibility, if the signal-to-noise ratio of the spectrum is high enough. Furthermore, we simulate the impact of radiation losses on the measured band gap value and propose a thickness-dependent correction. In future work, localized band gap variations will be measured on a more localized length scale to investigate, e.g., the influence of chemical inhomogeneities and dopant accumulations at grain boundaries.
authors.affiliation: 1Empa-Swiss Federal Laboratories for Materials Science and Technology,Laboratory for Thin Films and Photovoltaics,Ueberlandstrasse 129,CH-8600 Duebendorf,Switzerland.
authors.forename: Debora
authors.initials: D
authors.lastname: Keller
authors.affiliation: 1Empa-Swiss Federal Laboratories for Materials Science and Technology,Laboratory for Thin Films and Photovoltaics,Ueberlandstrasse 129,CH-8600 Duebendorf,Switzerland.
authors.forename: Stephan
authors.initials: S
authors.lastname: Buecheler
authors.affiliation: 1Empa-Swiss Federal Laboratories for Materials Science and Technology,Laboratory for Thin Films and Photovoltaics,Ueberlandstrasse 129,CH-8600 Duebendorf,Switzerland.
authors.forename: Patrick
authors.initials: P
authors.lastname: Reinhard
authors.affiliation: 1Empa-Swiss Federal Laboratories for Materials Science and Technology,Laboratory for Thin Films and Photovoltaics,Ueberlandstrasse 129,CH-8600 Duebendorf,Switzerland.
authors.forename: Fabian
authors.initials: F
authors.lastname: Pianezzi
authors.affiliation: 3Institute for Metallic Materials,IFW Dresden,P.O. Box 270116,D-01171 Dresden,Germany.
authors.forename: Darius
authors.initials: D
authors.lastname: Pohl
authors.affiliation: 3Institute for Metallic Materials,IFW Dresden,P.O. Box 270116,D-01171 Dresden,Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Surrey
authors.affiliation: 3Institute for Metallic Materials,IFW Dresden,P.O. Box 270116,D-01171 Dresden,Germany.
authors.forename: Bernd
authors.initials: B
authors.lastname: Rellinghaus
authors.affiliation: 2Empa-Swiss Federal Laboratories for Materials Science and Technology,Electron Microscopy Center,Ueberlandstrasse 129,CH-8600 Duebendorf,Switzerland.
authors.forename: Rolf
authors.initials: R
authors.lastname: Erni
authors.affiliation: 1Empa-Swiss Federal Laboratories for Materials Science and Technology,Laboratory for Thin Films and Photovoltaics,Ueberlandstrasse 129,CH-8600 Duebendorf,Switzerland.
authors.forename: Ayodhya N
authors.initials: AN
authors.lastname: Tiwari
mesh_terms: D003300:Copper; D005708:Gallium; D007204:Indium; D008422:Materials Testing / Q000379:methods*; D012643:Selenium; D012666:Semiconductors; D012993:Solar Energy
chemical_list: D007204:Indium; D003300:Copper; D005708:Gallium; D012643:Selenium