Evidence for a Neutral Grain-Boundary Barrier in Chalcopyrites

Abstract

Single grain boundaries in CuGaSe 2 have been grown epitaxially. Hall measurements indicate a barrier of 30 -40 meV to majority carrier transport. Nevertheless, local surface potential measurements show the absence of space charge around the grain boundary; i.e., it is neutral. Theoretical calculations [Persson and Zunger, Phys. Rev. Lett. 91, 266401 (2003)] have predicted a neutral barrier for the present 3 grain boundary. Thus, we have experimentally shown the existence of a neutral grain-boundary barrier, however, smaller than theoretically predicted. DOI: 10.1103/PhysRevLett.97.146601 PACS numbers: 72.80.Ey, 61.72.Mm, 71.55.Gs, 72.20.ÿi The electronic structure of Cu chalcopyrites shows numerous peculiarities, including, i.e., their grain-boundary properties. The first model [1,2] for the electronic structure of grain boundaries in chalcopyrites was based on the Seto model [3], where charged defects at grain boundaries cause a barrier for majority carriers (holes in the chalcopyrites discussed here). Barriers and space charge regions have been found in polycrystalline films by Hall measurements and Kelvin probe force microscopy (KPFM) (see, e.g., [1,4 -6] ). Theoretically it has been predicted that grain boundaries in chalcopyrites represent a barrier without charged defects [7,8]. In the Cu chalcopyrites, the top of the valence band is formed from antibonding Cu d states, which lifts it to higher energy compared to the corresponding II-VI compounds [9]. Grain boundaries consisting of f112g tet planes in the tetragonal system [10], corresponding to the f111g cub planes in the cubic system, are proposed to be Cu deficient, which results in a lowering of the valence band maximum. This results in a barrier in the valence band without a space charge at the grain boundary. Recently it has been argued that only a specific structure of a f112g tet grain boundary leads to this barrier and that the experimentally observed structures should not show a neutral barrier [11].Grain boundaries along the f112g tet planes can be described as twins. The coincidence site lattice (CSL) of such grain boundaries is characterized by a value of 3, the lowest value possible. Thus, it is expected that grain boundaries along f112g tet planes show a low defect density. A generating function of the CSL for the cubic system has been derived in Ref. [12]. Since the tetragonal distortion of chalcopyrites is small and since the CSL concerns only the two-dimensional plane of the grain boundary we assume the generating function for the cubic system as an approximation for the chalcopyrite system. Then it becomes clear that a polycrystalline film with f220g tet (i.e., f110g cub ) texture and vertical grain boundaries contains predominantly 3 grain boundaries. In fact, it has been shown that polycrystalline absorber films with f220g tet texture result in higher efficiencies of the corresponding solar cells compared to the usual f112g tet texture [13]. Therefore, 3 grain boundaries are the ones appearing in the most successful solar cells ...

Numerical simulation of carrier collection and recombination at grain boundaries in Cu(In,Ga)Se2 solar cells

“…Note that only the part ⌽ b results from the interface charges Q it whereas the internal valence band offset ⌬E V leads to an additional contribution that is sometimes referred to as neutral barrier. 14,17 This additional barrier prevents holes from the grain interior to reach the location of the GB. As long as the GB is parallel to the direction of carrier transport, we can assume that the band offset does not affect carrier transport in the direction to the collecting junction.…”

Electron Backscatter Diffraction: An Important Tool for Analyses of Structure–Property Relationships in Thin-Film Solar Cells

“…For a CuGaSe 2 bicrystal with a R3 twin boundary, it was found by means of KPFM combined with Hall measurements that this high-symmetrical defect exhibits charge neutrality with a small barrier for holes of about 30 meV. 31 A different situation is found for random grain boundaries. Figure 6 gives an example of combined EBSD and EBIC acquisitions at the identical specimen position.…”

Intergrain variations of the chemical and electronic surface structure of polycrystalline Cu(In,Ga)Se2 thin-film solar cell absorbers

“…4 This correlation has impact on the analysis of the operation of CIGSe-based solar cell devices, as it implies that the grain boundaries-potentially a separate source of energy barriers-can be made to have little or no influence on the lateral charge carrier transport in optimized devices, in agreement with the reported benign/beneficial role that grain boundaries play in polycrystalline chalcopyrite absorbers. 9 The increasing barrier height reported as Ga/(Inþ Ga) decreases, 3 however, cannot likely be explained in terms of a stoichiometry-related energetic barrier, because of both the decreasing dependence of the band gap on composition as x approaches zero and an expected decrease in size of the absolute deviation from the nominal composition.…”

“…Although it has been shown that the grain boundaries can be made to have little or no negative effect on the performance of CIGSe solar cell devices, 9 some predictions suggest that other aspects of polycrystallinity and alloying of the chalcopyrite absorber materials may lead to efficiencies below the Shockley-Queisser limit of ca. 33%: 10 Differences in the electronic and optical properties of the absorbers on the scale of the grain sizes 11 and below 12,13 have revealed numerous laterally resolved variations that may lead to decreased performance.…”