DISTINCTION BETWEEN BULK AND INTERFACE STATES IN CUINSE2 CD/ZNO BY SPACE-CHARGE SPECTROSCOPY/

We present a detailed study of admittance spectroscopy and deep level transient spectroscopy on CuInSe2/CdS/ZnO thin film solar cells. The a dmittance spectra reveal an emission from a distribution of hole traps centered at an activation energy of 280 meV and a shallower level wit h a sharp activation energy of similar to 120 meV. After repetitive an nealing of the device in air at 200 degrees C, the activation energy o f the latter level increases continuously from 120 to 240 meV, while t he 280 meV hole traps remain unaffected. Deep level transient spectros copy 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 ascerta ins that the emissions observed in both techniques have the same origi n. The magnitude and continuous shift of the activation energy of the minority carrier emission indicates a distribution of levels in the vi cinity of the CdS/CuInSe2 heterointerface. In the case of interface st ates, the activation energy deduced from admittance spectroscopy corre sponds to the position of the electron quasi-Fermi level at the interf ace, pointing to an inversion of the carrier type at the absorber surf ace. Measurements with an applied de bias indicate that the electron F ermi level is pinned at the interface. (C) 1998 American Institute of Physics.