Role of Surface Band Gap Widening in Cu(In, Ga)(Se, S) 2 Thin-Films for the Photovoltaic Performance of ZnO/CdS/Cu(In, G
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Role of surface band gap widening in Cu(In,Ga)(Se,S)2 thin-films for the photovoltaic performance of ZnO/CdS/Cu(In,Ga)(Se,S)2 heterojunction solar cells
U. Rau and M. Turcu Institute of Physical Electronics, University of Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany ABSTRACT Numerical simulations are used to investigate the role of the Cu-poor surface defect layer on Cu(In,Ga)Se2 thin-films for the photovoltaic performance of ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells. We model the surface layer either as a material which is n-type doped, or as a material which is type-inverted due to Fermi-level pinning by donor-like defects at the interface with CdS. We further assume a band gap widening of this layer with respect to the Cu(In,Ga)Se2 bulk. This feature turns out to represent the key quality of the Cu(In,Ga)Se2 surface as it prevents recombination at the absorber/CdS buffer interface. Whether the type inversion results from n-type doping or from Fermilevel pinning is only of minor importance as long as the surface layer does not imply a too large number of excess defects in its bulk or at its interface with the normal absorber. With increasing number of those defects an n-type layer proofs to be less sensitive to material deterioration when compared to the type-inversion by Fermi-level pinning. For wide gap chalcopyrite solar cells the internal valence band offset between the surface layer and the chalcopyrite appears equally vital for the device efficiency. However, the unfavorable band-offsets of the ZnO/CdS/Cu(In,Ga)Se2 heterojunction limit the device efficiency because of the deterioration of the fill factor. INTRODUCTION The surface properties of Cu(In,Ga)Se2 (CIGS) polycrystalline thin films are especially important as this surface becomes the active interface of the completed ZnO/CdS/CIGS heterojunction solar cells. Despite the importance of this part of the solar cell, its electronic properties are not satisfactorily understood. The free surfaces of Cu-poor grown CIGS polycrystalline thin films exhibit two prominent features: (i) The Fermi level lies above the valence band energy EV by about 1.1 eV for CuInSe2 films [1]. This energy is larger than the band gap energy Egb of the bulk absorber material. This finding was taken as an indication for a widening of the absorber band gap towards the surface of the film. A recent direct measurement of the surface band gap EgS of polycrystalline CuInSe2 films by Morkel et al. [2] proofed that EgS is about 1.4 eV, i.e., more than 0.3 eV larger than Egb ≈ 1.04 eV. (ii) The surface composition of Cu-poor Cu(In,Ga)Se2 films corresponds to a (Ga+In)/(Cu+Ga+In) ratio of approximately 0.75 for a range of bulk compositions of 0.5 < (Ga+In)/(Cu+Ga+In) < 0.75. Both observations (i) and (ii) have led to the assumption that a phase segregation of Cu(In,Ga)3Se5, the so-called Ordered Defect Compound (ODC), occurs at the surface of the films [1]. The question whether the type inversion of the Cu(In,Ga)Se2 surface of as grown films as well as in completed h
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