How much gallium do we need for a p-type Cu(In,Ga)Se2?

TL;DR

This study investigates how varying gallium content influences the N-to-P type transition in Cu(In,Ga)Se2, revealing a critical gallium concentration where the semiconductor switches from N-type to P-type, with implications for material doping strategies.

Contribution

It provides a detailed analysis of the gallium concentration threshold for N-to-P transition in Cu(In,Ga)Se2 and proposes a defect formation energy model explaining this transition.

Findings

N-type conductivity persists until 15-19% gallium content.

Carrier concentration drops by about two orders of magnitude near the transition.

A defect formation energy model explains the N-to-P transition based on gallium addition.

Abstract

Doping in the chalcopyrite Cu(In,Ga)Se2 is determined by intrinsic point defects. In the ternary CuInSe2, both N-type and P-type conductivity can be obtained depending on the growth conditions and stoichiometry: N-type is obtained when grown Cu-poor, Se-poor and alkali-free. CuGaSe2, on the other hand, is found to be always a P-type semiconductor that seems to resist all kinds of N-type doping no matter whether it comes from native defects or extrinsic impurities. In this contribution, we study the N-to-P transition in Cu-poor Cu(In,Ga)Se2 single crystals in dependence of the gallium content. Our results show that Cu(In,Ga)Se2 can still be grown as an N-type semiconductor until the gallium content reaches the critical concentration of 15-19%, where the N-to-P transition occurs. Furthermore, trends in the Seebeck coefficient and activation energies extracted from temperature-dependent conductivity measurements, demonstrate that the carrier concentration drops by around two orders of magnitude near the transition concentration. Our proposed model explains the N-to-P transition based on the differences in formation energies of donor and acceptor defects caused by the addition of gallium.