Characterization of the CuGaSe 2 /ZnSe Interface Using Kelvin Probe Force Microscopy
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Characterization of the CuGaSe2/ZnSe Interface Using Kelvin Probe Force Microscopy S. Sadewasser, Th. Glatzel, M. Rusu*, A. Meeder, D. Fuertes Marrón, A. Jäger-Waldau, and M.Ch. Lux-Steiner Hahn-Meitner Institut, Glienicker Str. 100, 14109 Berlin, Germany * State University of Moldova, Department of Physics, 60, a. Mateevici Street, Chisinau, MD2009, Moldova ABSTRACT To improve the efficiency of heterostructure solar cells based on chalcopyrite semiconductors a good understanding of the interface properties is crucial. By Kelvin Probe Force Microscopy it is possible to obtain laterally resolved images of the work function of semiconductor surfaces in addition to the topographical information usually obtained by noncontact atomic force microscopy. We studied the CuGaSe2/ZnSe interface prepared by growth of CuGaSe2 onto the (110) face of freshly cleaved ZnSe single crystals using chemical vapor deposition. We observed different work function values for different crystal facets on single CuGaSe2 grains. From the obtained work function data and surface photovoltage measurements a schematic band diagram for the CuGaSe2/ZnSe heterostructure is proposed.
INTRODUCTION Solar cells with efficiencies of more than 18 % have been realized within the chalcopyrite material system Cu(In,Ga)(S,Se)2 [1]. However, the efficiencies of high band gap chalcopyrites such as CuGaSe2 (CGS) with Eg = 1.68 eV are limited by a too low open circuit voltage (Voc ≤ 0.9 V) [2]. To obtain high efficiencies for the chalcopyrite solar cells, it is necessary to deposit a buffer layer (i.e. CdS or ZnSe) between the p-type absorber and the n-type window layer (usually ZnO). The reason for this buffer layer is still discussed controversially; it could act as a protection against the sputter deposition of the n-ZnO or it might be essential for a good band alignment [3]. Obviously, the band alignment will be influenced by the buffer layer, even if this might not be the dominating effect. Thus, a study of the band alignment at the absorber/buffer interface appears of importance. The most commonly used buffer layer is chemical bath deposited CdS, however, strong research effort is in progress to establish ZnSe as an alternative. The band alignment between CGS and ZnSe has been studied previously by XPS. Bauknecht et al. [4] found the conduction band offset (∆Ec) to be a spike of 0.4 eV when going from CGS to ZnSe, which was evaporated onto CGS deposited by MOVPE onto GaAs(001) substrates. The band alignment in different heterojunctions has previously been studied by contact potential difference (CPD) measurements [5]. In addition to XPS studies, the CPD method also provides information about the band bending by surface photovoltage measurements (SPV), provided that the illumination is sufficient to result in flat band conditions. In a further step, the use of a Kelvin Probe Force Microscope (KPFM) also provides information about lateral changes in the CPD; in addition to the topography a work function (Φ) image with a nanometer resolution (~ 20 nm) is obtain
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