Sodium enhances indium-gallium interdiffusion in copper indium gallium diselenide photovoltaic absorbers

Abstract

Copper indium gallium diselenide-based technology provides the most efficient solar energy conversion among all thin-film photovoltaic devices. This is possible due to engineered gallium depth gradients and alkali extrinsic doping. Sodium is well known to impede interdiffusion of indium and gallium in polycrystalline Cu(In,Ga)Se2 films, thus influencing the gallium depth distribution. Here, however, sodium is shown to have the opposite effect in monocrystalline gallium-free CuInSe2 grown on GaAs substrates. Gallium in-diffusion from the substrates is enhanced when sodium is incorporated into the film, leading to Cu(In,Ga)Se2 and Cu(In,Ga)3Se5 phase formation. These results show that sodium does not decrease per se indium and gallium interdiffusion. Instead, it is suggested that sodium promotes indium and gallium intragrain diffusion, while it hinders intergrain diffusion by segregating at grain boundaries. The deeper understanding of dopant-mediated atomic diffusion mechanisms should lead to more effective chemical and electrical passivation strategies, and more efficient solar cells.

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

“…In fact, the accepted paradigm is challenged by newer ndings on monocrystalline CIGS lms. 1 These show that diffusion of Ga from GaAs substrates into epitaxial CIS lms (i.e. free from high-angle grain boundaries) is substantially enhanced by the presence of Na (introduced even from the front surface, Fig.…”

“…22 Here, it should be stressed that Na concentration within the grains is typically one order of magnitude lower than at grain boundaries. 1,19,77 Therefore, it is not surprising to expect different diffusion mechanisms at the two locations. The schematics in Fig.…”

“…Through a parallel approach, a recent study has shown that Na can enhance the interdiffusion of In and Ga in monocrystalline CIGS lms, contrary to the polycrystalline case. 1 Precisely, Na appears to either enhance or impede the diffusion of In and Ga depending on the relative concentration of Na and Cu. 22 This paper reviews Na doping and Ga grading in CIGS with a comparison between the historical and recently discovered interdependences.…”

Role of surface microstructure of Mo back contact on alkali atom diffusion and Ga grading in Cu(In,Ga)Se2 thin film solar cells

“…[12][13][14][15][16][17] The most common approach for the incorporation of alkali atoms in CIGS layer was their natural diffusion from SLG substrate which contained Na and sometimes also K. 18,19 The incorporation of alkali atoms could greatly enhance both the structural and the electrical properties of CIGS absorber materials in comparison with the sodium-free glass. [12][13][14][15][16][17] The most common approach for the incorporation of alkali atoms in CIGS layer was their natural diffusion from SLG substrate which contained Na and sometimes also K. 18,19 The incorporation of alkali atoms could greatly enhance both the structural and the electrical properties of CIGS absorber materials in comparison with the sodium-free glass.…”

“…chalcogenide solar cell materials. [12][13][14][15][16][17] The most common approach for the incorporation of alkali atoms in CIGS layer was their natural diffusion from SLG substrate which contained Na and sometimes also K. 18,19 The incorporation of alkali atoms could greatly enhance both the structural and the electrical properties of CIGS absorber materials in comparison with the sodium-free glass. 20 Tiwari and Schock had determined that alkali elements diffused from SLG through molybdenum (Mo) grain boundaries and subsequently incorporated in the CIGS films.…”