Ali Mirza1, James R. Woodyard1 and David B. Snyder2
1 Wayne State University, Department of Electrical & Computer Engineering, Detroit, MI 48202
2NASA Glenn Research Center, Cleveland, OH 44135
Air mass zero calibration of solar cells has been carried out for several years by NASA Glenn Research Center using a Lear-25 aircraft and Langley plots. The calibration flights are carried out during early fall and late winter when the tropopause is at the lowest altitude. Measurements are made starting at about 50,000 feet and continue down to the tropopause. A joint NASA/Wayne State University program called Suntracker is underway to explore the use of weather balloon and communication technologies to characterize solar cells at elevations up to about 120 kft. The balloon flights are low-cost and can be carried out any time of the year. Results of cell characterization with the Suntracker are reported and compared with the NASA Glenn Research Center aircraft method.
We have carried out an extensive review of mountaintop, aircraft and balloon methods for characterizing solar cells and producing laboratory standards . Our focus in this paper is to compare measurements using aircraft and balloon methods.
The evolution of solar-cell technology for space applications has resulted in “state-of-the-art” cells with four and five junctions in series. Each junction is designed with a spectral response matched to one region of the spectral irradiance of AM0 in order to optimize the efficiency of solar cells. The current in a four-junction solar cell operating at a given voltage is:
whereis the absolute AM0 spectral irradiance of sunlight and is the spectral response of the cell at wavelength and voltage . In the ideal case, the spectral response is independent of the irradiance of the light source.
The evolution of solar-cell technology for space applications has resulted in “state-of-the-art” cells with four and five junctions in series. Each junction is designed with a spectral response matched to one region of the spectral irradiance of AM0 in order to optimize the efficiency of solar cells. The current in a four-junction solar cell operating at a voltage that is given by:
where the variables in Equation 2 are the same as in Equation 1 except andare the lower and upper cut-off wavelength values, above and below which the spectral response is negligible and no longer contributes to cell current. The spectral response in Equation 2 characterizes the overall operation of the four junctions in optical absorption and carrier transport. However, the spectral responses and voltages in Equation 3, and respectively, are subscripted to show that they are different for each of the four junctions. The voltage across the cell is equal to the sum of the voltages across each of the four junctions, namely, . The wavelength ranges on each of the integrals, in the most general case, will overlap since it is not possible to fabricate materials with sharp wavelength cut-offs. Equation 3 shows the series nature of the current in multi-junction solar cells, namely, the current is the same in each of the junctions.
Current reduction due to a change in spectral irradiance in one region of the spectrum must be accomplished through changes in spectral responses of the other three junctions; this is the case because the spectral irradiances in the other three wavelength ranges are assumed to be the same as AM0. The collective interaction of the four junctions will result in redistribution of the cell voltage across the four junctions, which in turn changes the spectral responses of the four junctions and the cell current. It is clear that the voltage dependence of the spectral responses of multi-junction solar cells complicates optimization of cell design. While there are characterization methods that make it possible to use solar simulators in advancing the multi-junction solar cell technology, the series nature of the cells places more demands on the need for standard cells characterized under AM0 conditions. AM0 conditions are available only in space; near AM0 conditions can be achieved at altitudes in excess of 100,000 ft. The demand for greater access to AM0, and the costs associated with AM0 calibration, has generated interest in exploring low-cost methods for AM0 solar cell calibration. The NASA supported Suntracker program is an attempt to meet this challenge.
The aircraft method has been developed over the years by investigators at NASA Glenn Research Center [2-6]. A large body of calibration data has been collected and AM0 standards provided to the PV community. The aircraft has been replaced twice and the method improved. The current aircraft is a Lear 25 that houses the instrumentation and collimator in a pressurized and temperature controlled compartment. Photographs of the Lear 25 aircraft, collimation tube and test cell may be viewed on the NASA Glenn Research Center Web site . Measurements are carried out every nine seconds during a 6E-4 air mass per second rate of descent from 50 kft down to the tropopause. Sources of random error are estimated to be about 0.04 % and agree with measurements. The difference in the average of measurements on a single-junction cell carried out over a twenty-year period and a recent measurement was at the 0.05 % level. Systematic errors were estimated to be at the one percent level. Space shuttle AM0 short-circuit current measurements on two cells were compared with the aircraft method. The aircraft measurements were less than the shuttle measurements by 1.0 and 0.8 % for the two cells; the errors were consistent with the estimated systematic errors.
Recent investigations of the role of ozone on the solar cell AM0 short-circuit current that have improved the aircraft method have been investigated by Snyder and collaborators [8,9]. The ozone correction method called for first correcting the short-circuit current measured at each altitude for ozone absorption, then plotting the data and extrapolating to air mass zero. The approach requires the column ozone at each of the altitudes at which the cell current is measured. The TOMS standard ozone profiles were used to calculate column ozone as function of altitude . Figure 1 shows the TOM ozone profiles used in calculating the column ozone. The ozone density shown in the figure at 156,000 ft includes the ozone above 156,000 ft. The profiles in the figure are for mid latitudes and total column ozone values ranging from 175 to 475 d.u., denoted by 175M-475M in the figure. Mid latitudes are around 45 degrees north, the latitude of the NASA Glenn Research Center flights. The maximum altitude for the flights is about 50,000 ft. Figure 1 shows the fraction of the total column ozone above 55,000 ft decreases with increasing total column ozone. The fraction at 55,000 ft is about 0.87 of the total column ozone when the total column ozone is 175 d.u.; the fraction decreases to about 0.77 at 475 d.u. The fraction is about 0.92 and fairly independent of total column ozone at 40 kft.
The ozone AM0 short-circuit current correction method included converting the aircraft altitudes to atmospheric pressure,. The total column ozone on the day of the flight was obtained and used to select the appropriate TOMS ozone profile in Figure 1. The fraction of the total ozone column above the aircraft during each of the measurements was calculated. The ozone corrected short-circuit current was calculated for each measurementusing:
The ozone corrected short-circuit currents were plotted as a function of pressure on a Langley plot, instead of a function of air mass. The linear plot was extrapolated to zero pressure to determine the AM0 short-circuit current. Data collected over two time intervals, namely a short term and long term, were compared. The short-term data were collected on one cell during twenty flights in one year. The long-term data were collected on the same cell during thirteen flights over a period of eight years. The percent standard deviations are reduced for both the cells as was observed for the Si cell, again confirming the importance of the ozone correction. The differences in the high and low current values are reduced for both cells when the revised method is used. A larger zenith angle results in a larger correction in the AM0 short-circuit currents for both cells. The revised method is more effective in correcting for larger zenith angles as is illustrated by the smaller high-low percentage differences in the AM0 short-circuit currents. However, there is a trend for corrected currents to exhibit a decreasing trend with increasing zenith angle.
A joint NASA Glenn Research Center/Wayne State University program called Suntracker is underway to explore the use of weather balloon and communication technologies to characterize solar cells at elevations up to 120,000 feet [11-13]. The balloon flights are low-cost and can be carried out any time of the year. Details on the program and photographs are available on a Web site .
Seven flights have been attempted with five successful launches. The scientific package was retrieved on the same day for the Suntracker I, III and IV flights. Hardware problems developed during the Suntracker VI and VII flights that resulted in the loss of GPS signals; the package was found within a few days of the launch by individuals and subsequently retrieved. A single-junction silicon solar cell was mounted in the collimator during the flights. The cell voltage data down linked during the Suntracker IV and VI flights have been analyzed using Langley plots to determine the AM0 short-circuit current. The Suntracker IV uncorrected short-circuit current versus altitude is shown in Figure 2. Only the maximum currents were selected for use in the Langley plot. The cell current data illustrate the tracking characteristics during the ascent. For the most part the Suntracker was not locked on the sun during the flight. The video data showed the motors slowed down during the ascent as a result of the low atmospheric temperatures. Motor assemblies using lubricant with lower temperature specifications will be evaluated in future flights. Additionally, the stability of the scientific package and the collimator control algorithm will be investigated in order to improve the performance of the Suntracker system.
Figure 3 is a Langley plot of the data for a single-junction silicon solar cell from the Suntracker IV and VI flights. The data have been corrected for the earth-sun distance, ozone and cell temperature, and fit with straight lines. The extrapolated AM0 short-circuit currents are 144.32 and 144.38 mA for the Suntracker IV and VI measurements, respectively. The average AM0 short-circuit current is 144.35 mA 0.02 %. The resolution of the eight-bit ADC in the Suntracker data acquisition system is 0.2 %, showing that the agreement between the two flights is better than the uncertainty in the measurements and probably reflects the statistics of the curve fitting etc. The AM0 short-circuit current of the single-junction silicon solar cell flown on the Suntracker flights was determined using the aircraft method at NASA Glenn Research Center. The AM0 short-circuit current was 144.88 mA and within 0.36 % of the Suntracker average value. The results agree to within the statistics of the two methods, namely about 0.2 % for the Suntracker measurements and 0.6 % for the aircraft method.
The cell temperature versus altitude during the Suntracker VI flight is shown in Figure 4. Also shown are the radiosonde data reported by the National Weather Service (NWS) on the day of the flight. The effect of solar heating on the cell current is apparent. While the solar cell temperature increased from –10 to about 0 oC as the balloon ascended from 80,000 to 96,000 ft, the atmospheric temperature remained at about –45 oC. The dependence of the cell and NWS temperatures in this altitude range suggests that the cell temperature may be higher at higher altitudes. If this is the case, it will be possible to operate cells closer to 25 oC at higher altitudes, and to determine the temperature coefficient of the short-circuit current as the package ascends.
It is instructive to determine the atmospheric optical absorption coefficients for the two Langley plots. The slopes of the two straight lines in Figure 3 were analyzed to determine the absorption coefficients; the coefficients are 0.265 and 0.293 per air mass for the Suntracker IV and VI data, respectively. The average value of the atmospheric optical absorption coefficient is 0.280 per air mass5 %. An analysis of the Langley plot produced with aircraft data gives absorption coefficients of 0.125 per air mass. The Suntracker value is somewhat larger than the 0.20 per air mass determined from the earlier aircraft measurements  while the absorption coefficients determined with the current aircraft data is considerably less. The reasons for these differences are not understood and will be the subject of future investigations.
The voltage dependence of the spectral responses of multi-junction solar cells complicates optimization of cell design. The series nature of multi-junction solar cells places more demands on the need for standard cells characterized under AM0 conditions. The AM0 short-circuit current of a single-junction silicon solar cell was determined using data collected during two Suntracker flights. The agreement in the two measurements was 0.02 %. The agreement in the AM0 short-circuit current of the cell measured with the Suntracker balloon method and NASA Glenn Research Center aircraft method was 0.36 %, which is within the uncertainty of the two methods. There is a need to understand the role of ozone and atmospheric optical absorption on the calibration of solar cells in the stratosphere.
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8. David B. Snyder, David A. Scheiman, Philip P. Jenkins, William J. Rieke and Kurt S. Blankenship, “Ozone Correction for AM0 Calibrated Solar Cells for the Aircraft Method,” Proceedings of the 29th Photovoltaic Specialists Conference, page 832, 2002.
9. David B. Snyder, Philip P. Jenkins and David A. Scheiman, “Historical Precision of an Ozone Correction Procedure for AM0 Solar Cell Calibration,” Proceedings of the Space Photovoltaic Research and Technology Conference, 2003, In Press.
10. Richard D. McPeters, P. K. Bahartia, Arlin J. Krueger, Jay R. Herman, Charles G. Wellemeyer, Colin J. Seftor, Glen Jaross, William Byerly, Steven L. Taylor, Tom Swissler and Richard P. Cebula, “Earth Probe Total Ozone Mapping Spectrometer (TOMS) Data Products User’s Guide,” NASA Technical Publication 1998-206895, page 53, 1998.
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12.Glenroy A. Bowe, Qianghua Wang and James R. Woodyard, ”Investigations to Characterize Multi-junction Solar Cells in the Stratosphere Using Low-Cost Balloon and Communication Technologies,” Twenty-Eight IEEE Photovoltaic Specialists Conference Proceedings, 2000, page 1328.
13.Ali Mirza, David Sant, James R. Woodyard, Richard R. Johnston and William J. Brown, “Report on Project to Characterize Multi-junction Solar Cells in the Stratosphere Using Low-Cost Balloon and Communication Technologies,” Seventeen Space Photovoltaic Research and Technology Conference, 2001, page 137.