Development and Testing of Digital Noise Spectroscopy System: Application to Solar Cells
Open AccessThis thesis reports on two major activities and the results obtained with them.The first is the conception, design, fabrication and testing of a facile double-channeldigital system for obtaining quantitative noise spectra. It consists of four main sub-systems. One is a shielded enclosure containing the device being tested, a probestation for contacting the device and a blocking capacitor to pass frequencies inthe 1 Hz to 100 kHz region. The second major component is a pair of StanfordResearch SR560 Low Noise Amplifiers. Then, another shielded box contains the 200kSample/second, 16 bit analog-to-digital converter 9223 from National Instrumentsin a Compact DAQ chassis. Finally, a laptop computer is employed to store thetwenty million data points obtained in each 100 second run, and to convert suchtime series into spectra.MATLAB was used for the bulk of the computations, and LabVIEW wasemployed for some operations. Fast Fourier Transform algorithms in MATLABwere used to obtain single channel spectra or cross correlated spectra from the dualchannels, each spectrum containing ten million points in the 1 to 105 Hz frequencyrange. Cross correlation removed some of the LNA and ADC noise and improvedthe noise floor of the system by somewhat less than a factor of 10. The singlechannel noise floor of the system is 10-17 V2/Hz, which is competitive with most ofthe systems described in the literature. However, our system is much less expensiveand faster than earlier approaches. It has the potential to be a commercial product.The system development and testing part of the work was highly successful.The second part of the work was to perform exploratory measurements with thenew system on several types of solar cells. The fundamental idea is that both carriertransport through solar cells and noise spectra depend on geometry, composition,and defects in solar cells. Hence, noise spectra offer the possibility of providinginformation on solar cell performance and defects. Silicon, Gallium Arsenide andmore complex compound semiconductor solar cells, mostly with single junctionsbut some with multiple junctions, were measured under various conditions. Theyincluded varying illumination with the cell either not loaded or connected to aresistor load. The triple junction complex compound semiconductor cells had beenirradiated with heavy doses of protons, so noise spectra were obtained as a functionof the proton fluences. Many differences were observed in the dozens of measuredspectra. They included the appearance of some lines of unknown origin, but thoselines do not degrade the utility of the noise spectra. Exhaustive parametric studiesin future work should show that noise spectra are a useful tool for characterizationof solar cells. The possibility of using noise measurements for on-orbit assessmentof solar cell radiation degradation was considered.A creative combination of hardware and software has produced the GWUNoise Spectroscopy System. It is a low cost, user-friendly, rapid, non-destructivedigital capability to acquire and exploit noise spectra. The very low noise floor andbroad frequency range of the system are both important. The new capability has aremarkably wide range of applications in physics, chemistry, biology, brain research,medicine, materials, chemical processing, mechanical and structural engineering,and acoustics.To demonstrate additional applications of the entire new system, noise spectrawere measured from a new MEMS microphone and from batteries with differentchemical systems. Spectra quite different from those of solar cell were obtained.The analysis part of the system was employed to produce spectra from time seriesgotten from electrical probes inserted into a live rat brain. Very unusual spectraresulted. The results indicate that such spectra can be obtained in real-time duringneural probe experiments.
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Mehrabian_gwu_0075M_12099.pdf | 2018-01-17 | Open Access |
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