Characterization of the Electronic Properties of Wide Bandgap CuIn(SeS) 2 Alloys
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Characterization of the Electronic Properties of Wide Bandgap CuIn(SeS)2 Alloys Adam F. Halverson1, Peter T. Erslev1, JinWoo Lee1, J. David Cohen1, and William N. Shafarman2 1 Department of Physics, University of Oregon, Eugene, OR 97403 U.S.A 2 Institute of Energy Conversion, University of Delaware, Newark, DE 19716 U.S.A. ABSTRACT The electronic properties of sulfur containing CIS chalcopyrite alloys have been characterized using junction capacitance methods. Two devices were examined; one containing a CuIn(S,Se)2 alloy with a 1:2 S:Se ratio and a bandgap near 1.15eV, and the other an endpoint CuInS2 alloy with a bandgap slightly above 1.5eV. Drive-level capacitance profiling measurements indicated hole carrier densities of less than 1 x 1015 cm-3 and 1.5 x 1016 cm-3, respectively. Transient photocapacitance (TPC) sub-bandgap spectroscopic measurements revealed sharp bandtails plus a broad defect band within the bandgap of each alloy. The TPC spectra for the CuInS2 sample revealed a couple of unusual features, including a bandtail signal that reversed sign below 250K. This indicated poorer hole collection than electron collection in the low temperature regime. Comparing these results to TPC spectra obtained previously for Cu(InGa)Se2 alloys indicate some similarities but also some striking differences. INTRODUCTION Identifying wide bandgap chalcopyrite materials with good electronic properties is essential for the development of multijunction CIGS based thin film solar cells with efficiencies above 20%. Of particular interest are alloys with gaps greater than 1.5eV. In the current study we have begun to examine one such class of materials in which one replaces some or all of the selenium in Cu(InGa)Se2 materials with sulfur. We have applied junction capacitance methods, in the manner of previous studies on the Cu(InGa)Se2 alloys [1] to investigate how the substitution of sulfur affects the electronic properties of these CIS materials. Our primary measurement methods are the drive-level capacitance (DLCP) profiling method [2] and the transient photocapacitance (TPC) method [3]. These techniques are applied directly to functioning solar cell device samples. Thus, we try to correlate this information about the electronic properties with the performance parameters of the cells themselves. SAMPLES AND SAMPLE TREATMENT All samples studied were fabricated at the Institute of Energy Conversion (IEC) at the University of Delaware. Sample devices were deposited on Mo coated soda lime glass using a physical vapor deposition process [4]. One sample was deposited with a Cu-poor composition with Cu=24.1%, In=26.5%, Se=33.2%, and S=16.3%, and was finished with 30-40nm of CdS using a chemical bath, and then sputtered ZnO/ITO window layers with evaporated Ni/Al grids [4]. A second sample contained an endpoint CuInS2 absorber layer and was deposited with a Curich composition. To form the devices, the CuS surface layer was etched off before finishing the device as above. More details about these samples, including the cell
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