Correlating the Local Defect-Level Density with the Macroscopic Composition and Energetics of Chalcopyrite Thin-Film Surfaces

Abstract

The unusual defect chemistry of polycrystalline Cu(In,Ga)Se2 (CIGSe) thin films is a main issue for a profound understanding of recombination losses in chalcopyrite thin-film solar cells. Especially, impurity-driven passivation of electronic levels due to point defects segregating at the surface and at grain boundaries is extensively debated. By combining current imaging tunneling spectroscopy with photoelectron spectroscopy, the local defect-level density and unusual optoelectronic grain-boundary properties of this material are correlated with the macroscopic energy levels and surface composition. Vacuum annealing of different CIGSe materials provides evidence that Na diffusion from the glass substrate does not affect the surface defect passivation or grain-boundary properties of standard Cu-poor materials. Furthermore, we find no major impact on the observed thermally activated dipole compensation or the accompanying change in surface band bending (up to 0.6 eV) due to Na. In contrast, Cu-rich CIGSe shows an opposing surface defect chemistry with only minor heat-induced band bending. Our results lead to a comprehensive picture, where the highly desirable type inversion at the p/n interface in standard chalcopyrite thin-film solar cells is dominated by band bending within the CIGSe absorber rather than the result of Na impurities or an n-type defect phase segregating at the interface. This is in accordance with recent studies suggesting a surface reconstruction as the origin for Cu depletion and band-gap widening at the surface of chalcopyrite thin films.

Improved growth of solution-deposited thin films on polycrystalline Cu(In,Ga)Se2

“…Preferentially, oxygen reacts with Ga, In, and Se ions, rather not with Cu ions [16,17]. Moreover, the formation of Ga 2 O 3 is more likely than that of In 2 O 3 [18,19] also reported on an oxygenation-induced liberation of Cu from the near-surface region of the CIGS absorber, which was not confirmed by Bröker et al [20]. A thermodynamic consideration of CuInSe 2 oxidation is given by Wei et al [21].…”

Time-resolved photoluminescence on double graded Cu(In,Ga)Se2 – Impact of front surface recombination and its temperature dependence

“…Simulations with ΔGGI ¼ 0:1 and μ > 50 cm 2 V À1 s À1 resulted in exponents of a not exceeding the value of 3. However, the band profile for the simulations is quite simplified and changes of the bandgap (and thus potentially the conduction band edge) might also result from a Cu depletion at the surface [26,27] or the formation of alkaline phases [18,28,29]. These effects were not considered in the TCAD model and might induce an additional barrier for the electrons to recombine at the front surface and consequently result in a faster decrease of the PL decay time with increasing temperature for the 'after HCl' configuration, i.e.…”

Compositional and electrical properties of line and planar defects in Cu(In,Ga)Se2 thin films for solar cells – a review

“…4), or by 2 Se V + at the surface [31]. Schmid et al [32], Mönig et al [33,34], as well as Bröker et al [35] provided indications for Cu depletion and In/Ga enrichment, but not for Se depletion or Cu enrichment, at Cu(In,Ga)Se 2 thin-film surfaces.…”