A set of devices prepared with varying amount of sodium precursor NaF has been characterized by use of junction capacitance techniques. The analysis of the DLTS and admittance spectra shows, that two components contribute to the minority carrier signal-interface states and compensating donors. In Na-free device only a donor level of energy 0.26 eV has been detected. In the other samples both contributions have been found with the values of emission rates from the donor level distorted by electric field-induced tunnel ionization. We conclude, that a fast, tunneling-dominated emission from this stare is a mark of the presence of a defected, p(+) layer adjacent to the heterointerface. Worse photovoltaic performance of the device with not sufficient amount of sodium provided during growth we ascribe to a more pronounced defected layer.