This paper describes a newly developed technique for measuring the quantum eficiiency in solar cells in real-time using a unique, electronically controlled, full-spectrum light source

Egaas, B.

Pinegar, S.

Stradins, P.

Young, D. L.

UNT Digital Library

 A national laboratory of the U.S. Department of EnergyOffice of Energy Efficiency & Renewable EnergyNational Renewable Energy Laboratory Innovation for Our Energy Future New Real-Time Quantum Efficiency Measurement System Preprint  D.L. Young, S. Pinegar, and P. Stradins National Renewable Energy Laboratory B. Egaas Colorado School of Mines Presented at the 33rd IEEE Photovoltaic Specialists Conference San Diego, California May 11–16, 2008 Conference Paper NREL/CP-520-42509 May 2008 NREL is operated by Midwest Research Institute ● Battelle     Contract No. DE-AC36-99-GO10337  NOTICE The submitted manuscript has been offered by an employee of the Midwest Research Institute (MRI), a contractor of the US Government under Contract No. DE-AC36-99GO10337. Accordingly, the US Government and MRI retain a nonexclusive royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for US Government purposes. This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.  Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof.  The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Available electronically at http://www.osti.gov/bridge Available for a processing fee to U.S. Department of Energy and its contractors, in paper, from: U.S. Department of Energy Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831-0062 phone:  865.576.8401 fax: 865.576.5728 email:  mailto:reports@adonis.osti.gov Available for sale to the public, in paper, from: U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 phone:  800.553.6847 fax:  703.605.6900 email: orders@ntis.fedworld.gov online ordering:  http://www.ntis.gov/ordering.htm Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste A NEW REAL-TIME QUANTUM EFFICIENCY MEASUREMENT SYSTEM    David L. Young1, Brian Egaas 1, 2, Scott Pinegar1, and Paul Stradins1 1National Renewable Energy Laboratory, Golden, Colorado 80401 (USA) 2Colorado School of Mines, Golden, Colorado 80401 (USA)   ABSTRACT  We have developed a new technique for measuring the quantum efficiency (QE) in solar cells in real-time using a unique, electronically controlled, full-spectrum light source.  Full-spectrum QE graphs can be obtained in less than one second (as opposed to 20 minutes using traditional QE instruments). The high measurement speed is achieved by parallel processing of information from a multitude of spectral channels encoded in modulation frequency bands.  The reduction in time scale makes this QE measurement technique compatible with inline production diagnostics, high-fidelity, spectral-matching cell binning, and thin-film module spatial spectral response uniformity tests.  The instrument is completely solid-state with no moving parts, is robust enough for manufacturing environments, and is significantly less expensive than a traditional QE instrument.  INTRODUCTION  Quantum efficiency (QE) or spectral response measurements are extremely valuable tools for understanding device physics and materials properties of solar cells.  A QE diagram can reveal material band gaps and thicknesses in single and multilayer solar cells, minority carrier diffusion lengths,[1] spectral-dependence of short circuit current and qualitative spatial electronic behavior within cells[1,2]  Traditionally, QE measurements are made with a mechanically driven spectrometer to direct monochromatic, chopped light onto a cell.  Lock-in amplifier techniques measure the cell response (collected electron-hole pairs/photon flux) in a serial manner over the spectrum of interest. To obtain good signal-to-noise in the data and to allow time for mechanical switching of wavelengths within the monochrometer, measurements typically take 20 minutes (after system calibration).   Because of this time burden, QE instruments are typically only found in laboratory settings – never on cell production lines.  This paper discusses the development of a new technique for measuring the quantum efficiency in solar cells that reduces the time for measurement from 20 minutes to less than one second.  This time reduction makes QE measurements compatible with cell and module production-line speeds, bringing a higher level of diagnostics to solar cell and module manufacturing.  We envisage making the new QE system available for in-line cell diagnostics, high fidelity, spectral-matching cell binning, and thin-film module spectral response uniformity tests across the solar cell and module industry. The new real-time quantum efficiency (RTQE) system, developed at NREL, shifts a QE measurement from serial to parallel, thus taking the entire QE spectrum simultaneously in less than one second.  It is similar in nature to the instrument by Bucher and Schonecker[3]; however our approach is less complex, considerably less expensive, completely solid state and significantly faster.  PRINCIPLE OF OPERATION   The high measurement speed is achieved by parallel processing of information from a multitude of spectral channels encoded in modulation frequency bands.  This is achieved by an electronically controlled, full-spectrum light source (ECLS) that allows individual  1on/off frequency control over specified spectral ranges.  The ECLS is an electronically controlled array of light emitting diodes (LED), each with a unique spectral emission, but with a slight overlap in wavelengths such that the ECLS spectrum covers a typical solar cell spectral response (300 nm – 1200 nm).  Figure 1 shows the individual LED spectra for one embodiment of the ECLS which uses 57 LEDs.  A sine-wave generator and an amplifier, both of which are controlled by a computer, power each LED in the ECLS.  This arrangement allows for each LED to operate at a unique frequency and/or intensity.  Light from the LEDs in the ECLS is focused to a common area either by lenses, mirrors or by fiber optics.  A schematic of the ECLS and the RTQE system is shown in Figure 2.    The RTQE system works by focusing light from the ECLS onto a solar cell with each LED in the ECLS switched on and off at a unique, specified drive frequency.  All of the LEDs in the array are driven simultaneously.  The drive frequency of each LED is set above the inverse minority carrier lifetime of the cell and is not a multiple of the other LED drive frequencies.   The response of the cell or the A.C. current vs time signal (Figure 3) is sent through a current-to-voltage preamplifier after which it is recorded by a computer-based analog-to-digital converter (DAQ card).  The digitized signal is then Fourier transformed to determine the power spectrum (Figure 3).  The power spectrum separates out the frequency components of the total signal that exactly match the specified drive  Figure 1. (left axis) Spectra of the individual LEDs in the electronically controlled light source using 57 LEDs. (Right axis) AM1.5 solar spectrum.  Figure 2. Schematic of the real time quantum efficiency instrument.  Figure 3. Schematic of current versus time data from a solar cell illuminated with light from the ECLS. All of the LEDs flash simultaneously at a unique drive frequency onto the solar cell.  The data are transformed into a power spectrum using FFT mathematics.  A power spectrum of the total current vs time signal separates out the current contributions from each LED.  2frequencies of the LEDs in the ECLS.  The amplitude of the power spectrum frequency components are directly related to the current generated in the cell from the light of the corresponding LED.  Calibration of the system is done with a reference cell by scaling the known current to the amplitude of each frequency component in the power spectrum.  A computer program continuously records the data, applies the calibration algorithm, and updates a QE graph.  A QE graph is obtained by assigning the current generated for each drive frequency to the peak wavelength of the associated LED.  The update rate of the QE graph is determined by the capture rate of the analog-to-digital converter and by the integration time of the power spectrum.  Typical graphical update rates are less than 1 second.  Figure 4 shows a QE graph measured by the RTQE system (dots) using a prototype 10-LED ECLS.  The dashed line in fig. 4 is the QE measured by a traditional QE system.  The agreement between the techniques is excellent.  TECHNICAL NOTES  The RTQE system is a very fast and stable technique for measuring the quantum efficiency in solar cells.  To implement the new method, however, several technical considerations should be addressed.   1) Selection of LEDs: An enabling technology for the RTQE system has been the tremendous progress in available LED spectra and intensity.  When choosing LEDs for the ECLS one should consider spectral symmetry and intensity.  Some LEDs appear to the human eye to have a single color but in fact are a combination of two or more colors mixed together.  Only LEDs with a true single color (spectral symmetry) should be used in the ECLS.  If some LEDs are not intense enough to produce good signal to noise, multiple, identical LEDs can be ganged together.   Figure 4. Quantum efficiency data (dots) generated in <1 second using a 10-LED prototype RTQE system.  The dashed line is QE data from a traditional QE system.   2) LED drive signal: A pure sine wave generator should be used to drive the LEDs in the ECLS in order to produce the fewest harmonics during the FFT. 3) Drive frequency: As mentioned above, the drive frequency for each LED should be carefully chosen.  In general, the drive frequencies for each LED should be selected such that there is no overlap in power spectrum peaks.  Practically this means drive frequencies should not be multiples of each other and should be shifted away from large secondary harmonic peaks in the power spectrum due to non-sinusoidal current response in a solar cell.  Drive frequencies should be high enough to allow fast data averaging, but low enough to allow electron hole pairs time to be collected. 4) LED spectral spread: Because light emission from LEDs is not monochromatic,[4] there is a spectral spread in wavelength on the order of 10 nm inherent with each data point in the QE graph.  The wavelength resolution of the QE graph by this technique is determined by the spectral spread of the individual LEDs and by the number of different-spectrum LEDs in the ECLS. The spectral spread of the individual LEDs and the spectral overlap between LEDs can be accounted for in a calibration algorithm that utilizes Singular Value Decomposition (SVD) matrix algebra.  Solid state lasers could also be used instead of LEDs in the ECLS to give sharp spectral bandwidths. 5) Data Acquisition:  Data acquisition rates should be at least twice the highest drive frequency in the ECLS.    ENVISAGE  The RTQE instrument is completely solid-state with no moving parts, is robust enough for manufacturing  3A follow-on to the electronic filtering idea is to match a specific spectrum with the ECLS by varying the intensity of the individual LEDs in the array.  Almost any spectra of choice (AM0, AM1.5, Moon, Mars, Etc) can be quickly and easily simulated.  environments, and is significantly less expensive than a traditional QE instrument.  Except for the highest resolution QE measurements (Δλ ~ 1-2 nm) the RTQE (with LEDs in the ECLS) could replace traditional QE measurement systems in the laboratory saving time and money.  Additionally, the speed of the RTQE instrument allows QE measurements to be made on every cell and module in a manufacturing line during standard contacting for current versus voltage tests.  The ability to measure QE on each cell could allow for finer binning of cells by spectral-matching “identical” short-circuit current cells.  This finer binning of cells would account for daily and yearly AM1.5 solar spectral shifts, which could maximize annual power output from a module.   Additionally, the RTQE system allows new production-line materials diagnostics and device physics feedback not available in current production lines.  NREL has developed prototypes of the system and has secured licensable intellectual property rights.  We are seeking industrial partners to commercialize this technology to the solar industry.  Interested parties should contact the Technology Transfer Department at NREL, (David Christensen) +1 (303) 275-3015.   REFERENCES   1  Basore, P.A., Numerical Modeling of Textured Silicon Solar Cells Using PC-1D. IEEE Transactions on electron devices, 37(2):1990 p337. Spatial spectral-response uniformity mapping at the cell and module (thin-film modules) level is now practical with the RTQE system.  Either serial (single RTQE) scanning or parallel (multiple RTQE units) mapping could be utilized.  2  Hovel, H.J., Semiconductors and semimetals,, Vol. II. Academic Press, New York, 1975. 3  Bucher, K. and Schonecker, A., 1991. Spectral response measurements of mult-junction solar cells with a grating monochromator and a fourier spectrometer. In: A. Luque, G. Sala, W. Palz, G. Dos Santos and P. Helm (Editors), 10th European photovoltaic solar energy conference, Lisbon, Portugal, pp. 107-110. QE measurements of multijunction solar cells can be made with the RTQE system through electronic filtering of the ECLS to light bias different sub-cells within a monolithic tandem cell.  Electronic filtering is achieved by dividing the ECLS LED array into sub-cell-specific spectra so that the sub-cell under test is illuminated with drive-frequency light, while the other sub-cell specific spectra are driven by a constant DC bias.  This electronic filtering allows only the sub-cell under test to be measured (only the drive frequency spectra produce a QE power spectrum), while all the other sub-cells are DC light biased to transport current to the contacts.    4  Sze, S.M. and NG, K.K., Physics of Semiconductor Devices. John Wiley & Sons, INC, Hoboken, 2007 pp 815     4F1147-E(09/2007) REPORT DOCUMENTATION PAGE Form Approved OMB No. 0704-0188 The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION. 1. REPORT DATE (DD-MM-YYYY) May 2008 2. REPORT TYPE Conference Paper 3. DATES COVERED (From - To) 11-16 May 2008 5a. CONTRACT NUMBER DE-AC36-99-GO10337 5b. GRANT NUMBER  4. TITLE AND SUBTITLE New Real-Time Quantum Efficiency Measurement System; Preprint 5c. PROGRAM ELEMENT NUMBER  5d. PROJECT NUMBER NREL/CP-520-42509 5e. TASK NUMBER PVA74101 6. AUTHOR(S) D.L. Young, B. Egaas, S. Pinegar, and P. Stradins 5f. WORK UNIT NUMBER  7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) National Renewable Energy Laboratory 1617 Cole Blvd. Golden, CO 80401-3393 8. PERFORMING ORGANIZATION REPORT NUMBER NREL/CP-520-42509 10. SPONSOR/MONITOR'S ACRONYM(S) NREL 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)  11. SPONSORING/MONITORING AGENCY REPORT NUMBER  12. DISTRIBUTION AVAILABILITY STATEMENT National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161 13. SUPPLEMENTARY NOTES  14. ABSTRACT (Maximum 200 Words) We have developed a new technique for measuring the quantum efficiency (QE) in solar cells in real-time using a unique, electronically controlled, full-spectrum light source.  Full-spectrum QE graphs can be obtained in less than one second (as opposed to 20 minutes using traditional QE instruments). The high measurement speed is achieved by parallel processing of information from a multitude of spectral channels encoded in modulation frequency bands.  The reduction in time scale makes this QE measurement technique compatible with inline production diagnostics, high-fidelity, spectral-matching cell binning, and thin-film module spatial spectral response uniformity tests.  The instrument is completely solid-state with no moving parts, is robust enough for manufacturing environments, and is significantly less expensive than a traditional QE instrument. 15. SUBJECT TERMS PV; quantum efficiency; real-time; solar cells; full-spectrum light source; light emitting diodes 16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON  a. REPORT Unclassified b. ABSTRACT Unclassified c. THIS PAGE Unclassified 17. LIMITATION OF ABSTRACTUL 18. NUMBER OF PAGES  19b. TELEPHONE NUMBER (Include area code)  Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18 

New Real-Time Quantum Efficiency Measurement System: Preprint

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New Real-Time Quantum Efficiency Measurement System: Preprint

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