Material Properties of a-SiGe:H Solar Cells as a Function of Growth Rate
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Material Properties of a-SiGe:H Solar Cells as a Function of Growth Rate Peter G. Hugger1, Jinwoo Lee1, J. David Cohen1, Guozhen Yue2, Xixiang Xu2, Baojie Yan2, Jeff Yang2 and Subhendu Guha2 1 University of Oregon, Department of Physics. Eugene, Oregon, United States 2 United Solar Ovonic LLC. Troy, Michigan, United States ABSTRACT We have examined a series of a-SiGe:H alloy devices deposited using both RF and VHF glow discharge in two configurations: SS/n+/i (a-SiGe:H)/p+/ITO nip devices and SS/n+/i (aSiGe:H)/Pd Schottky contact devices, over a range of deposition rates. We employed drive-level capacitance profiling (DLCP), modulated photocurrent (MPC), and transient junction photocurrent (TPI) measurement methods to characterize the electronic properties in these materials. The DLCP profiles indicated quite low defect densities (mid 1015 cm-3 to low 1016 cm-3 depending on the Ge alloy fraction) for the low rate RF (~1Å/s) deposited a-SiGe:H materials. In contrast to the RF process, the VHF deposited a-SiGe:H materials did not exhibit nearly as rapid an increase of defect density with the deposition rate, remaining well below 1017 cm-3. up to rates as high as 10Å/s. Simple examination of the TPI spectra on theses devices allowed us to determine valence band-tail widths.. Modulated photocurrent (MPC) obtained for several of these a-SiGe:H devices allowed us to deduce the conduction band-tail widths. In general, the aSiGe:H materials exhibiting narrower valence band-tail widths and lower defect densities correlated with the best device performance. INTRODUCTION Hydrogenated amorphous silicon germanium alloys (a-Si1-xGex:H) have been critical materials in the development of multi-junction thin film photovoltaics. However, despite this success and the easily tunable nature of its optical bandgap to energies from 1.7 eV to 1.3 eV, a substantial challenge remains in the deposition of high quality a-SiGe:H films at rates above ~3 Å/s using PECVD glow discharge techniques. Typically, as deposition rates increase cell performance parameters and carrier mobility-lifetimes drop while, simultaneously, deep defect densities increase. To maintain high-quality photovoltaic material and at the same time increase deposition rates, United Solar has developed a modified VHF glow discharge deposition technique that has been shown to allow deposition rates 3 to 5 times greater than typical RF methods while still maintaining low densities of deep defects [1,2]. Our previous studies for series of a-SiGe:H alloys with similar bandgaps indicate that the Urbach energies determined from transient photocapacitance (TPC) and transient photocurrent spectra (TPI) correlate very well with the measured defect densities determined by our drive-level capacitance profiling (DLCP) in the manner predicted by certain models of defect creation. [3,4] In light of these previous results, here we evaluate the application of a complementary technique, the “modulated photocurrent” (MPC) method. This technique has been shown capable of providing g
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