Doping Studies of Cadmium Telluride, Cadmium Magnesium Telluride, and CdTe/CdMgTe Double Heterostructures Grown Using Molecular Beam Epitaxy
Ogedengbe, Olanrewaju S.
<p>CdTe is one of the leading materials used in thin-film photovoltaic (PV) devices due to some of its basic properties such as its ability to permit both <i>n</i>- and <i>p</i>-type doping, its relatively high absorption coefficient for photons in the visible range, and its direct band gap of 1.514 eV at room temperature, which is near the optimal band gap for solar energy conversion. Despite the near optimal band gap, the highest power conversion efficiency in a CdTe solar cell to date, achieved using polycrystalline CdTe, stands at 21%. This is far less than the Shockley–Queisser limit, which is about 32% for a single-junction cell under AM 1.5 illumination condition. Research efforts have shown that short circuit current (J<sub>sc</sub>) is near its theoretical limit, implying that strategies to improve cell efficiency will have to be contingent on improving open-circuit voltage (V<sub>oc</sub>) and fill factor. Heavy doping has the potential to improve V<sub>oc</sub>. There is also evidence that inclusion of a Cd<sub>1-x</sub>Mg<sub>x</sub>Te barrier in a solar cell structure may improve open circuit voltage, and, ultimately, cell efficiency.</p> <p>Doped and undoped CdTe layers were grown by molecular beam epitaxy (MBE). Secondary ion mass spectrometry (SIMS) characterization was used to measure dopant concentration, while Hall measurement and the capacitance-voltage technique were used for determining carrier concentration. Photoluminescence intensity (PL-I) and time-resolved photoluminescence (TRPL) techniques were used for optical characterization.</p> <p>The incorporation and limits of iodine and arsenic dopants in CdTe were studied. Maximum <i>n</i>-type carrier concentrations of 7.4x10<sup>18</sup> cm<sup>-3</sup> for iodine-doped CdTe and 3x10<sup>17</sup> cm<sup>-3</sup> for iodine-doped Cd<sub>0.65</sub>Mg<sub>0.35</sub>Te were achieved. Studies suggest that electrically active doping with iodine is limited with dopant concentration much above these values. Dopant activation of about 80% was observed in most of the iodine-doped CdTe samples. The estimated activation energy is about 6 meV for CdTe and the value for Cd<sub>0.65</sub>Mg<sub>0.35</sub>Te is about 58 meV. Iodine-doped CdTe samples exhibit long lifetimes with no evidence of photoluminescence degradation with doping as high as 2x10<sup>18</sup> cm<sup>-3</sup> while indium shows substantial non-radiative recombination at carrier concentrations above 5x10<sup>16</sup> cm<sup>-3</sup>. Also, maximum <i>p</i>-type carrier concentration of 2x10<sup>16</sup> cm<sup>-3</sup> for arsenic-doped CdTe was achieved. Dopant activation greater than 20% was observed in most of the arsenic-doped CdTe samples. The process compatibility of iodine and magnesium in CdTe was evaluated for the solar cell device. Iodine was shown to be thermally stable in CdTe at temperatures up to 600°C and magnesium showed a slow diffusion at 500°C. Doped CdTe structures were used to make solar cell device structures, where open circuit voltage up to 880 mV and fill factor up to ~60% were measured.</p>
CdTe, CdMgTe, Doping, Iodine, Arsenic, Double heterostructure, Solar cell, Photovoltaic
Olanrewaju, O. S. (2017). <i>Doping studies of cadmium telluride, cadmium magnesium telluride, and CdTe/CdMgTe double heterostructures grown using molecular beam epitaxy</i> (Unpublished dissertation). Texas State University, San Marcos, Texas.