Hydrogen states in mixed-cation CuIn_(1−x)Ga_xSe₂ chalcopyrite alloys: a combined study by first-principles density-functional calculations and muon-spin spectroscopy

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${}_{1-x}$,Ga${}_x$)Se${}_2$ (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${}_2$ 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.

KEYWORDS:

• Chalcopyrite solar cells
• ab initio calculations
• hydrogen
• muon spectroscopy

Acknowledgments

The computer resources of the Department of Physics of the University of Coimbra were used, including the Navigator cluster at the Laboratory for Advanced Computing. ISIS muon beam time allocation from the Science and Technology Facilities Council, through experiment RB1920408 [60], and the support of the ISIS muon team are gratefully acknowledged.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported with funds from FEDER (Programa Operacional Factores de Competitividade COMPETE) and from FCT-Fundação para a Ciência e Tecnologia (Portugal) under projects UID/FIS/04564/2016 and PTDC/FIS-MAC/29696/2017. The support by the European Regional Development Fund, Project TK141 is also gratefully acknowledged.