Abstract
ABSTRACT First-principles calculations were performed jointly with muon-spin (μSR) spectroscopy experiments in order to examine the electrical activity of hydrogen in mixed-cation chalcopyrite Cu(In,Ga)Se (CIGS) alloys and other related compounds commonly used as absorbers in solar-cell technology. The study targeted the range of Ga concentrations most relevant in typical solar cells. By means of a hybrid-functional approach the charge-transition levels of hydrogen were determined and the evolution of the defect pinning level, E(+/–), was monitored as a function of the Ga content. The use of E(+/–) as a metric of the charge-neutrality level allowed the alignment of band structures, thus providing the band offsets between the CuInSe compound and the CIGS alloys. The μSR measurements in both thin-film and bulk CIGS materials confirmed that the positively charged state is the thermodynamically stable configuration of hydrogen for p-type conditions. The interpretation of the μSR data further addressed the existence of a metastable quasi-atomic neutral configuration that was resolved from the calculations and led to a formation model for muon implantation.
Highlights
Hydrogen is a common impurity in semiconductors and even at trace quantities it can have a strong impact on the macroscopic electronic properties and electrical behavior of the host material [1,2,3,4]
This choice corresponds to hydrogen-rich conditions, whereby the defect supercell is assumed to be in equilibrium with hydrogen gas, H2(g)
Both new and existing experimental results via muon spin spectroscopy on thin-film and bulk CIGS, as well as on the related ternary chalcopyrite semiconductors CuInSe2 and CuInS2, will be discussed in the present work
Summary
Hydrogen is a common impurity in semiconductors and even at trace quantities it can have a strong impact on the macroscopic electronic properties and electrical behavior of the host material [1,2,3,4]. The effects of hydrogen in Cu-based chalcopyrite compounds and related mixed-cation Cu(In1−x,Gax)Se2 (CIGS) alloys employed as absorbers in solar cells have been well documented in a number of studies. The electrical activity of hydrogen in the ternary CIS and CuGaSe2 (CGS) chalcopyrite compounds has been thoroughly studied in the past by first-principles approaches [6, 10, 11] based on density-functional theory [12, 13]. Partial replacement of In by Ga leads to an increase of the band gap of the absorber from the 1.05 eV magnitude of CIS [17] to values in a range from 1.2 to 1.3 eV for the CIGS absorbers with the higher efficiencies [18] These materials usually possess a Ga content, x, defined by the atomic ratio [Ga]/([Ga]+[In]), from 0.20 to 0.30. The amplitude of the diamagnetic fraction was used as a marker for such profiling and a formation model is presented
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