ABSTRACT
The study of defects in the space charge region of actual thin film solar cells based on the chalcopyrite CuInSe2 and related materials gives insight to bulk and interface properties. The combination of admittance spectroscopy and deep level transient spectroscopy consistently identifies states at the heterointerface. The narrow spectrum of the corresponding activation energies indicates pinning of the Fermi-level close to the CuInSe2 conduction band with lateral fluctuations below 20meV. The measurements further reveal a distribution of bulk hole traps peaked 270–320meV above the valence band and the main acceptor level in Cu-rich grown CuInSe2. Electron injection into the space charge region persistently increases the space charge capacitance, which is most likely due to a lattice relaxation involving interstitial Cu, acting as compensating donors in the dark equilibrium condition. The implications of DLTS and capacitance voltage measurements lead to a new model for the Cu-poor surface layer that ascribes an additional driving force for its formation to a positive surface charge inducing Cu migration.
