A series of Cu(In,Ga)Se2 (CIGS) thin film solar cells with differently prepared heterojunctions has been investigated by admittance spectroscopy, capacitance-voltage (CV) profiling, and temperature dependent current-voltage (IVT) measurements. The devices with different CdS buffer layer thicknesses, with an In2S3 buffer or with a Schottky barrier junction, all show the characteristic admittance step at shallow energies between 40 and 160 meV, which has often been referred to as the N1 defect. No correlation between the buffer layer thickness and the capacitance step is found. IVT measurements show that the dielectric relaxation frequency of charge carriers in the CdS layers is smaller than the N1-resonance frequency at low temperatures where the N1 step in admittance is observed. These results strongly contradict the common assignment of the N1 response to a donor defect at or close to the heterointerface. In contrast, an explanation for the N1 response is proposed, which relates the admittance step to a non-Ohmic back-contact acting as a second junction in the device. The model, which is substantiated with numerical device simulations, allows a unified explanation of characteristic admittance, CV, and IVT features commonly observed in CIGS solar cells.

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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 ...

There are still open questions regarding the nature of the positive effect of the presence of Na on the performance of Cu(In,Ga)Se2 based, chalcopyrite thin film solar cells, especially at low processing temperatures. Studying Cu(In,Ga)Se2 thin film devices fabricated from low-temperature coevaporated absorbers on polyimide substrates by admittance and J-V-T measurements, characteristic properties are identified for different amounts of Na present during the growth. A roll-over behavior can be directly correlated with the Na-content. X-ray photoelectron spectroscopy shows the development of a MoSe2 phase at the back contact of the device. Efficiencies of 15.1% with MgF2 antireflection coating are demonstrated.