We summarize the progress made recently in understanding the electronic structure of chalcopyrites. New insights into the dispersion of valence and conduction band allow conclusions on the effective masses of charge carriers and their orientation dependence, which influences the transport in solar cell absorbers of different orientation. Native point defects are responsible for the doping and thus the band bending in solar cells. Results of optoelectronic defect spectroscopy are reviewed. Native defects are also the source for a number of metastabilities, which strongly affect the efficiency of solar cells. Recent theoretical findings relate these effects to the Se vacancy and the In Cu antisite defect. Experimentally determined activation energies support these models. Absorbers in chalcopyrite solar cells are polycrystalline, which is only possible because of the benign character of the grain boundaries. This can be related to an unusual electronic structure of the GB.

We present a detailed study of admittance spectroscopy and deep level transient spectroscopy on CuInSe2/CdS/ZnO thin film solar cells. The admittance spectra reveal an emission from a distribution of hole traps centered at an activation energy of 280 meV and a shallower level with a sharp activation energy of ∼ 120 meV. After repetitive annealing of the device in air at 200 °C, the activation energy of the latter level increases continuously from 120 to 240 meV, while the 280 meV hole traps remain unaffected. Deep level transient spectroscopy with optical excitation reveals an emission of minority carriers with time constants comparable to those observed for the shallow level in admittance spectroscopy. The shift of the activation energy after annealing also occurs in deep level transient spectroscopy and ascertains that the emissions observed in both techniques have the same origin. The magnitude and continuous shift of the activation energy of the minority carrier emission indicates a distribution of levels in the vicinity of the CdS/CuInSe2 heterointerface. In the case of interface states, the activation energy deduced from admittance spectroscopy corresponds to the position of the electron quasi-Fermi level at the interface, pointing to an inversion of the carrier type at the absorber surface. Measurements with an applied dc bias indicate that the electron Fermi level is pinned at the interface.

Doping distributions in the Cu(In,Ga)Se2 solar cells with various gallium contents are analyzed by the use of capacitance-voltage and drive-level capacitance profiling. The influence of deep traps on the evaluation of the spatial-doping distribution in the bulk of Cu(In,Ga)Se2 absorbers is discussed. An analysis is presented, which shows that traps labeled N1, commonly observed in these devices, are interface states or compensating donors and their input to the capacitance is related only to the width of the depleted n-type insulating layer. We attribute the apparent increase of doping density toward the back electrode to the accumulation of the electrostatic charge in deep bulk acceptors with a concentration at an order of magnitude higher than net shallow doping. The metastable changes of doping distributions induced by light or reverse bias are also investigated and interpreted in terms of the Lany–Zunger model of VSe-VCu divacancies with negative-U property. All conclusions have been tested by numerical modeling. Conductivity of thin films prepared in the same process as absorbers of investigated cells in relaxed and light-soaked states have also been measured. The results provide additional arguments that capacitance methods, if interpreted with care, give credible estimation of doping level in the absorber of Cu(In,Ga)Se2 devices.

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Spectra of hole and electron traps of CuInSe2/CdS/ZnO photovoltaic devices have been investigated using deep-level transient spectroscopy. A decrease of the concentration of shallow electron traps and an increase of the hole trap concentration after an injection of electrons has been observed. The effect is metastable below 200 K. A proposed explanation is based on the idea that both levels belong to the same defect in a different charge state. A resemblance of the phenomena related to that defect and to ‘‘dangling bond’’-type centers in amorphous semiconductors has been indicated. Some consequences of defect conversion for current transport and performance of photovoltaic devices have been discussed.

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