Characterizing the effects of silver alloying in chalcopyrite CIGSCharacterizing the effects of silver alloying in chalc
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1165-M01-07
Characterizing the effects of silver alloying in chalcopyrite CIGS solar cells with junction capacitance methods Peter T. Erslev1, Gregory M. Hanket2, William N. Shafarman2, and J. David Cohen1 University of Oregon Physics Department, Eugene, Oregon 97405 2 Institute of Energy Conversion, University of Delaware, Newark, Delaware, 19716 1
ABSTRACT A variety of junction capacitance-based characterization methods were used to investigate alloys of Ag into Cu(In1-xGax)Se2 photovoltaic solar cells over a broad range of compositions. These alloys show encouraging trends of increasing VOC with increasing Ag content, opening the possibility of wide-gap cells for use in tandem device applications. Drive level capacitance profiling (DLCP) has shown very low free carrier concentrations for all Agalloyed devices, in some cases less than 1014 cm-3, which is roughly an order of magnitude lower than that of CIGS devices. Transient photocapacitance spectroscopy has revealed very steep Urbach edges, with energies between 10 meV and 20 meV, in the Ag-alloyed samples. This is in general lower than the Urbach edges measured for standard CIGS samples and suggests a significantly lower degree of structural disorder. INTRODUCTION Thin film solar cells based on the alloys of CuInSe2 (CIS) hold substantial promise for an economical source of photovoltaic energy. Because the band gap of CIS (~ 1 eV) does not optimally match the solar spectrum, it is normally alloyed with Ga (CIGS) to produce higher band gaps (up to 1.7 eV for CuGaSe2). Unfortunately, the predicted increase in efficiency for band gaps above roughly 1.2 eV (~ 30% Ga) has not yet been realized, primarily due to a deficit in the open circuit voltage [1]. We have examined several sets of samples in which Ag has been alloyed into CIGS devices as (AgxCu1-x)(In1-yGay)Se2 (ACIGS) in hopes that such alloys will produce increases in the band gaps beyond 1.2 eV accompanied by correspondingly higher values of VOC. The endpoint AgInSe2 and AgGaSe2 compounds have band gaps 0.1 eV to 0.2 eV higher than the corresponding Cu compounds, so an increase in the band gap of the alloys might be expected. In addition, because the melting point of the Ag compounds are typically 200 oC lower than that of the Cu compounds, one might expect reduced structural disorder for the Ag alloys and thus better electronic properties [2]. CIGS alloys with Ag have thus far received relatively little attention, partially because it had been believed that single phase chalcopyrite compounds could only be produced over a limited range of compositions [3, 4]. This is in spite of the work by Nakada et. al., who produced a 9.3% efficient single phase chalcopyrite AgInGaSe2 device with a 949 mV VOC [5]. However, by employing our co-evaporation deposition process we have been able to deposit uniform single-phase ACIGS absorbers over a broad range of Ag and Ga concentrations. These films were incorporated into working solar cells, with Mo back contacts, and buffer and window layers of CdS, ZnO and ITO. We then
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