Effect of Alkali Metal Atom Doping on the CuInSe2-Based Solar Cell Absorber

M Malitckaya,M J Puska,H.-P Komsa,V Havu

doi:10.1021/acs.jpcc.7b03083

Abstract

The efficiency of Cu(In,Ga)Se2 (CIGS)-based solar cells can be markedly improved by controlled introduction of alkali metal (AM) atoms using post-deposition treatment (PDT) after CIGS growth. Previous studies have indicated that AM atoms may act as impurities or agglomerate into secondary phases. To enable further progress, understanding of atomic level processes responsible for these improvements is required. To this end, we have investigated theoretically the effects of the AM elements Li, Na, K, Rb, and Cs on the properties of the parent material CuInSe2. First, the effects of the AM impurities in CuInSe2 have been investigated in terms of formation energies, charge transition levels, and migration energy barriers. We found that AM atoms preferentially substitute for Cu atoms at the neutral charge state. Under In-poor conditions, AM atoms at the In site also show low formation energies and are acceptors. The migration energy barriers show that the interstitial diffusion mechanism may be relevant only f...

Highlights

The development of the chalcopyrite Cu(In,Ga)Se2 (CIGS)based solar cell devices started fully in the 1970s at Bell Laboratories
On the basis of the calculated reaction enthalpies and migration considerations, we find that mixed phases are more likely in the case of LiInSe2 and NaInSe2, whereas formation of secondary phases is expected for KInSe2, RbInSe2, and CsInSe2
Njμj + q(Ef + Ev) + Ecorr j where Etdoetf,i is the total energy of a supercell with the alkali metal (AM) impurity of type i, Etboutlk is the total energy of the corresponding pristine supercell, μj is the chemical potential of the atom of type j, q is the charge state of the impurity, Ef is the Fermi level measured from the valence band maximum (VBM) Ev, and Ecorr is a correction to artificial electrostatic interactions arising from the supercell approximation

Summary

INTRODUCTION

The development of the chalcopyrite Cu(In,Ga)Se2 (CIGS)based solar cell devices started fully in the 1970s at Bell Laboratories. The electronic effects of Li, Na, and K on the CuInSe2 have already been investigated by first-principles modeling based on the density functional theory (DFT).[15,20,23,24] The calculations show that Li, Na, and K occupy preferably Cu sites as substitutional neutral impurities or positively charged impurity pairs, but they do not reveal any direct way how Li-, Na-, and K-related defects could affect the p-type doping or the efficiency of the CIGS solar cells. Chemical potentials determined for AM atoms under different growth conditions, the ensuing formation energies of AMrelated point defects, and the migration barriers are given and discussed in the Alkali Metal Impurities section. The results are summarized and discussed in the Conclusions section

COMPUTATIONAL DETAILS

RESULTS AND DISCUSSION

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES