  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.
