Ultra-Fast Quantum Efficiency Solar Cell Test
According to the U.S. Energy Information Administration’s 2010 International Energy Outlook, solar energy is expected to grow globally by 12.7% per year until 2035; more than any other renewable energy source. To meet this demand, the renewable energy industry must develop and manufacture photovoltaic (PV) cells that are less expensive, more reliable, and more efficient in converting solar energy into electricity.
Quantum efficiency (QE) is the standard test which indicates how well a solar cell converts various wavelengths of sunlight into electricity. QE measurements are critical during cell research and manufacturing as it provides an accurate indication of whether a cell is properly designed or operating with an optimal desired response. Currently, QE measurements are relatively expensive and take up to 20 minutes to complete. Using light emitting diodes (LEDs) and mathematical algorithms, NREL scientists have devised a technology that performs QE measurements in a single second. This 2011 R&D 100 award winning technology enables PV designers and manufacturers to quickly and economically modify cell design and monitor the manufacturing process to increase yield of high efficiency PV cells.
In addition, this technology allows wafers to be binned by their QE response. Wafers with similar QE characteristics are identified and kept together. Modules made from cells in the same bin leads to optimal performance. QE binning will not do away with binning based on current-voltage measurements, but it will further improve the wafer-selection process for module production.Description
The technology is based on a full-spectrum array of LEDs instead of traditional halogen bulbs and mechanical monochrometers. All of the LEDs in the array illuminate a solar cell with each LED driven on and off at a unique frequency. The A.C. current generated in the solar cell from the LED array source is Fast Fourier Transformed (FFT) to reveal the power spectrum of the current as a function of frequency from each LED light. A reference cell calibrates the amplitude of the FFT signals to calculate the QE curve.
Using LEDs, a QE measurement is now done in parallel (all frequencies of light at the same time), rather than in series, as is the case for the traditional QE setup. The result is a 1000 x increase in measurement speed, and an ability to envision a reasonable time-scale for a QE area scan of a solar cell module. The intensity of the LED array may be tuned to match different global standards or electronically filtered to match a given experimental need (e.g. QE measurements on multijunction cells).Benefits
- Timely feedback on PV cell design
- Inline QE measurements on every cell
- Improved manufacturing yield through statistical process control
- Reduced QE measurement cost
- QE binning of cells for optimal power output modules
- QE spatial mapping
- PV research and development
- PV manufacturing
|Title and Abstract||
Ultra-fast determination of quantum efficiency of a solar cell
An apparatus for measuring quantum efficiency (QE) of solar cells. The apparatus includes a light source including an array of light emitting diodes (LEDs) that each emit light corresponding to a differing portion of a test spectrum and each LED is driven by a sinusoidal power supply that operates at a unique frequency. The light source includes an optical coupling focusing the LED light into a test beam targeted on a solar cell, and a signal conditioner converts analog current signals generated by the solar cell into digital voltage signals. A QE measurement module determines a QE value corresponding to each of the LEDs based on the digital voltage signals using a Fast Fourier Transform module that processes the digital voltage signals to generate values for each operating frequency. The QE measurement module determines the QE values by applying a conversion factor to these values. Since all the LEDs can be power-modulated simultaneously and the corresponding cell responses to each of the LEDS can be analyzed simultaneously, the QE spectrum measurement time is greatly shortened as compared to conventional methods.
|National Renewable Energy Laboratory||08/07/2012
|Technology ID||Development Stage||Availability||Published||Last Updated|
|NREL 07-31 U.S. Application No. 12/237,452||Licensed||Available||07/08/2011||07/08/2011|