Improving Cu(In,Ga)Se2 solar cell absorbers based on atomic-level modeling

Cu(In, Ga)Se2 (CIGS)-based solar cells are among the most promising candidates to replace crystalline silicon solar cells, thanks to their high efficiencies and low costs. The defect microstructure of the CIGS light absorber layer influences the optical and electronic properties of CIGS solar cells. The recent progress in their efficiency (up to 23.35 %) is mainly due to the incorporation of different alkali metal atoms into the absorber layer. As efficiency increases towards the Shockley-Queisser limit (31 % for a cell with a band gap of 1.3 eV), it becomes more difficult to improve. Thus, knowledge about native point defects and impurities, as well as about the formation of secondary phases in the CIGS absorber layer, is among essential information for optimizing CIGS solar cell performance. In this thesis, the choice of computational methods and their details strongly affects even the qualitative features of the obtained results. Therefore, the effects of the most important computational parameters are studied carefully in the thesis. Native point defects, native point defect complexes, and alkali metal impurities in the CIGS absorber layer are investigated using first-principles calculations within density functional theory (DFT) in order to understand their effect on the electronic structure. Moreover, based on DFT calculations, a mechanism for secondary phase (e.g. alkali indium selenide) formation is suggested. Calculating defects in CIGS is not straightforward. It is impossible to model defects directly in CIGS because In and Ga randomly occupy the same sites. Therefore all the calculations presented in this thesis are performed for CuInSe2 and CuGaSe2. In this thesis, calculations of native point defect formation energies in CuInSe2 provide information about the abundances of acceptors and donors for materials of different Cu concentrations. Moreover, it is shown that light alkali metal atoms prefer to accumulate on the Cu sublattice as impurities, and incorporation of heavy alkali metal atoms contributes mostly by phase separation. The formation of alkali indium/gallium selenide secondary phases during the post-deposition treatment is predicted by considering possible reactions between CuInSe2/CuGaSe2 and different alkali metal compounds by calculating their formation enthalpies. Interfaces between the secondary phases and the CuInSe2 absorber layer are studied in terms of band offsets. Finally, comparisons between the hard X-ray photoelectron spectroscopy (HAXPES) data and the density of states calculations with potassium post-deposition treatment (PDT) as a case study reveal the formation of the KInSe2 phase on the CIGS absorber surface after heavy potassium post-deposition treatment. In summary, the results in this thesis give information about the energetic characteristics of native point defects, native point defect complexes, alkali metal impurities, and alkali metal secondary phases. The results help to analyse already existing experimental observations of the abundances of point defects, migration mechanisms, and the formation of secondary phases.

The fox and the hound

| openaire: EC/H2020/713640/EU//NanoTRAINforGrowthIICu(In,Ga)(S,Se)2 (CIGS) thin film solar cells require appropriate depth and lateral distributions of alkali metal dopants and gallium to attain world record photovoltaic energy conversion. The two requirements are interdependent because sodium is known to hamper In/Ga interdiffusion in polycrystalline films. However, such a fact is challenged by recent findings where sodium appears to enhance In/Ga interdiffusion in monocrystalline films. This contribution reviews closely the two cases to the benefits of grain boundary engineering in CIGS. A computational model reveals why Na induces In accumulation at CIGS grain boundaries, confining Ga to grain interiors. The positive technological implications for wider gap chalcopyrites are stressed.Peer reviewe

The fox and the hound:in-depth and in-grain Na doping and Ga grading in Cu(In,Ga)Se₂ solar cells

Abstract Cu(In,Ga)(S,Se)₂ (CIGS) thin film solar cells require appropriate depth and lateral distributions of alkali metal dopants and gallium to attain world record photovoltaic energy conversion. The two requirements are interdependent because sodium is known to hamper In/Ga interdiffusion in polycrystalline films. However, such a fact is challenged by recent findings where sodium appears to enhance In/Ga interdiffusion in monocrystalline films. This contribution reviews closely the two cases to the benefits of grain boundary engineering in CIGS. A computational model reveals why Na induces In accumulation at CIGS grain boundaries, confining Ga to grain interiors. The positive technological implications for wider gap chalcopyrites are stressed

Alkali Postdeposition Treatment-Induced Changes of the Chemical and Electronic Structure of Cu(In,Ga)Se2 Thin-Film Solar Cell Absorbers

| openaire: EC/H2020/641004/EU//Sharc25The effects of alkali postdeposition treatment (PDT) on the valence band structure of Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorbers are addressed from a first-principles perspective. In detail, experimentally derived hard X-ray photoelectron spectroscopy (HAXPES) data [ Handick, E.; et al. ACS Appl. Mater. Interfaces 2015, 7, 27414-27420 ] of the valence band structure of alkali-free and NaF/KF-PDT CIGSe are directly compared and fit by calculated density of states (DOS) of CuInSe2, its Cu-deficient counterpart CuIn5Se8, and different potentially formed secondary phases, such as KInSe2, InSe, and In2Se3. The DOSs are based on first-principles electronic structure calculations and weighted according to element-, symmetry-, and energy-dependent photoionization cross sections for the comparison to experimental data. The HAXPES spectra were recorded using photon energies ranging from 2 to 8 keV, allowing extraction of information from different sample depths. The analysis of the alkali-free CIGSe valence band structure reveals that it can best be described by a mixture of the DOS of CuInSe2 and CuIn5Se8, resulting in a stoichiometry slightly more Cu-rich than that of CuIn3Se5. The NaF/KF-PDT-induced changes in the HAXPES spectra for different alkali exposures are best reproduced by additional contributions from KInSe2, with some indications that the formation of a pronounced K-In-Se-type surface species might crucially depend on the amount of K available during PDT.Peer reviewe

Heavy Alkali Treatment of Cu(In,Ga)Se 2 Solar Cells: Surface versus Bulk Effects

International audienc