NL2011207C2 - Device and method for testing a photo-voltaic cell. - Google Patents
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J1/00—Photometry, e.g. photographic exposure meter
- G01J1/02—Details
- G01J1/08—Arrangements of light sources specially adapted for photometry standard sources, also using luminescent or radioactive material
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J1/00—Photometry, e.g. photographic exposure meter
- G01J1/02—Details
- G01J1/04—Optical or mechanical part supplementary adjustable parts
- G01J1/0407—Optical elements not provided otherwise, e.g. manifolds, windows, holograms, gratings
- G01J1/0411—Optical elements not provided otherwise, e.g. manifolds, windows, holograms, gratings using focussing or collimating elements, i.e. lenses or mirrors; Aberration correction
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J1/00—Photometry, e.g. photographic exposure meter
- G01J1/02—Details
- G01J1/04—Optical or mechanical part supplementary adjustable parts
- G01J1/0407—Optical elements not provided otherwise, e.g. manifolds, windows, holograms, gratings
- G01J1/0437—Optical elements not provided otherwise, e.g. manifolds, windows, holograms, gratings using masks, aperture plates, spatial light modulators, spatial filters, e.g. reflective filters
Abstract
A system for testing a photo-voltaic cell is provided with a holder for the photo-voltaic cell, with electrodes for receiving a signal from the cell. A light source system provides for light with a spectral composition of terrestrial sunlight. A spatial light modulator with a two dimensional array of controllable mirrors or other pixel modulators is used to form a spatial pattern of this light in an image area that comprises the surface of a photo- voltaic cell. In an embodiment a plurality of spatial modulators is used to modulate different parts of the light from the light source system, which have different light properties. One part may light with the spectral composition of terrestrial sunlight and an other part may overlap with the spectral composition of terrestrial sunlight only in a limited wavelength band for example. Thus a light pattern with different light properties in different areas can be realized without spatial or temporal multiplexing. A control system controls the spectral composition part of light and spatial modulation by the first and second spatial light modulator. Electric signals in response to the control light are captured from the photo-voltaic cell.
Description
Title: Device and method for testing a photo-voltaic cell
Field of the invention
The invention relates to a device for testing a photo-voltaic cell and to a method of testing such a photo-voltaic cell.
Background
An article by W.J. Walecki describes solar cell testing by measuring an electrical output signal from the solar cell while a light projector projects a computer controlled two dimensional light pattern on the solar cell (“Integrated Quantum Efficiency, Reflectance, Topography and Stress Metrology for Solar Cell Manufacturing”, Proc. SPIE 7064, Interferometry XIV: Applications, 70640A, August 11, 2008). The fight patterns may be used to form a scanning spot. Walecki does not identify the type of fight projector used for this measurement.
An article by J. Carstensen et al describe solar cell testing by measuring an electrical output signal while a laser spot is scanned over de surface of the solar cell, This article titled “CELLO: an advanced LBIC measurement technique for solar cell local characterization”, was published in Solar Energy Materials & Solar Cells 76 (2003) 599 - 611. The laser spot provides for a local fighting difference with respect to position independent white background fighting that is applied to the entire surface of the solar cell.
Because a laser source allows only for a limited scope of adjustment of fighting properties of the fighting pattern such a test method provide only for inflexible measurements. Walecki and Carstensen do not disclose the possibility of using conventional beamer technology wherein fight modulated by a computer controlled spatial array of fight modulators like an LCD array could be imaged onto the solar cell. But tests of solar cells using conventional beamer technology would only be of limited value because conventional beamers rely on forms of spatial, temporal and spectral multiplexing that would affect solar cell testing even if they are not distinguishable to the human eye.
Summary
Among others, it is an object to provide for a device for testing photo-voltaic cells that provides for more flexible testing of photo-voltaic cells. A system for testing a photo-voltaic cell according to claim 1 is provided. Herein a controllable spatial light modulator with a two-dimensional array of pixel modulators, such as controllable mirrors or liquid crystal cells, is used to modulate an intensity of light from a light source system in order to project a spatial light pattern onto a photo-voltaic cell. The light source system provides for production of hght with the spectral composition of terrestrial sunlight. In a preferred embodiment this spectral composition is one of a range of selectable spectral compositions that can be supplied by the light source system. Each pixel modulator is configured to switch at least between using hght of the light source system to produce light at a relatively higher and lower intensity, for example by switching a mirror for the pixel between positions wherein light from the hght source system is reflected towards the image area that comprises the photo-voltaic cell or not, respectively, or by switching an LCD cell between a light transmitting state and a non-transmitting state. An array is used wherein all pixel modulators provide light with the same spectral composition to the image area at least in a highest intensity output state. Thus, no time or position multiplexing is needed to provide light with the spectral composition of terrestrial sunhght with a selectable spatial pattern in the image area that comprises the photo-voltaic cell.
In an embodiment a plurality of distinct spatial light modulators with arrays of pixel modulators are provided, which use different parts of the light from the light source to create overlaid patterns on the image area that comprises the photo-voltaic cell. Preferably, the light source system provides for control of the spectral composition and/or intensity of the part of the light that is spatially modulated by one of the spatial light modulators relative to the part that is modulated by the other spatial light modulator(s). For this purpose, the light source system may comprise a controllable filter to modify the part of the light before and/or after the spatial modulator and/or the light source system may comprise respective light sources (e.g. sets of LEDs) to generate the different parts, so that the light source system controllably affects the lighting properties for at least one of the spatial light modulators as a whole relative to those for the other spatial light modulator(s). In this way no time or position multiplexing is needed to provide light with different lighting properties in different parts of the image area.
By ensuring that at least one of the spatial light modulators can be used to modulate light that substantially has a spectral composition (also termed spectral distribution) of terrestrial sunlight, tests can be performed wherein the electrical effects of localized light with specific properties in the second area can be measured in the presence of simulated sunlight in the first area. Each spatial light modulator may comprise a single array of light modulator elements, or it may comprise a plurality of parallel spatial light modulator arrays, used to project different components of the light overlaid onto the photo-voltaic cell respectively. However, it is preferred that each spatial light modulator comprises a single array, as this avoids local light component variations due to imperfections in the overlay of the light from corresponding pixels in the different arrays in the same modulator.
In an exemplary application, one spatial light modulator may be used to define a first area on a photo-voltaic cell wherein light with first properties is supplied and another spatial light modulator may be used to define a second area wherein light with second, different properties is supplied. The areas may be a spot, such as a circular spot, and an area that surrounds the spot, or a hne and sub-areas on opposite sides along the hne for example. Because distinct spatial hght modulators are used, the light can be supplied with different properties in the different areas without spatial or temporal multiplexing with light with other properties. By ensuring that the a spatial light modulator can be used to modulate hght that substantially has a spectral composition (also termed spectral distribution) of terrestrial sunhght, tests can be performed wherein the electrical effects of localized light with specific properties in the second area can be measured in the presence of simulated sunlight in the first area.
The system comprises a holder system for holding a photo-voltaic cell (possibly as part of an assembly of such cells that is held by the holding system) with electrodes arranged to receive signals dependent on a voltage and/or a current from the photo-voltaic cell, to obtain an electronic response signal in response to the overlaid lighting patterns. A control system provides for control of the spatial light pattern and for measurement of the resulting voltage and/or current from the photo-voltaic cell.
The light source system may provide for control of light properties, e.g. by means of the use of a combination of controllable light source elements with different spectra (e.g. LEDs) to form part of the light or by means of a spectral filter in the light source system. Such a filter may be located in the light path to the photo-voltaic cell in front of or behind the spatial light modulator. In either case, it is considered part of the light source system as it affects the properties of the light as a whole irrespective of position in the spatial modulation.
In a further embodiment temporal modulation may be applied to the properties of the light and electrical measurements may be performed synchronized to the temporal modulation. The properties of the light that is supplied to the photo-voltaic cell via the first spatial light modulator may be similarly controllable. Thus, background light with any properties may be provided, including white light as one possibility. Alternatively, this light may have predetermined fixed properties.
The spatial fight modulators may be used in the creation of a spatial lighting pattern that comprises a probe area wherein the light has different properties compared to light in a background area adjacent the probe area. The spatial light modulator may be used to create probe areas of different shape, for example a probe area in the form of a spot, with a background area surrounding the spot or a line with parts of the background area on opposite sides of the line. Furthermore, the spatial light modulator may be used to step the position of the probe area to different positions on the photo-voltaic cell.
In an embodiment temporal modulation of the light supplied to the photo-voltaic cell via the spatial fight modulator is used and the current and/or voltage is measured synchronized to said temporal modulation. Herein temporal modulation of the light may be achieved by control of the light source, by applying an overall temporal modulation to light emerging from the spatial modulator, or by temporally varying the spatial modulation by the spatial modulator. Synchronized measurement may comprise obtaining respective measured values at times when the lighting properties are set to different values. In this case a difference between these respective different measured values may be computed as part of a differential experiment. Synchronized measurement may also comprise using temporally periodic modulation of properties of the light, such as overall intensity or intensities of one or more spectral components, and detection of a corresponding periodic component of the current and/or voltage. Furthermore, synchronized measurement may comprise measuring current and/or voltage as a function of time delay after a temporal pulse or step in the properties of the light. Thus more detailed measurements of the operation of the photo-voltaic cell and/or response function measurements at non-equilibrium conditions may be obtained. An example is a measurement with a temporal step-bke pulse where, by applying the Photocurrent Decay method, the minority carrier lifetime of the photo-voltaic cell can be derived.
In an embodiment, the light in the background area is provided by means of spatially uniform light, the light in the probe area being the sum of the spatially uniform light in the probe area and light provided via the spatial light modulator. In another embodiment, the device comprises a further spatial light modulator configured to modulate an intensity of further light. The further spatial light modulator may be used to provide light selectively in the background area without lighting the probe area, or lighting the probe area with reduced intensity compared to the background. Thus it is made possible that the light intensity in the probe area can be made lower as well as higher than in the background area.
In a further embodiment, split off light from the same light source that supplies the spatial hght modulator is supplied to the further spatial light modulator. In an alternative embodiment, a further light source is provided to supply light to the further spatial light modulator. Thus it is made easier to control the light properties in the probe area independent of those in the back ground area, or to control both independent of one another.
In an embodiment the device comprises a sensor configured to sense a property of light reflected from the position for the photo-voltaic cell. The sensor may provide for measurement of reflected light intensity from the photo-voltaic cell as a function of wavelength for example. Localized reflection in a probe area may be measured by using the spatial hght modulator. The spatial light modulator may be used to make the probe area for reflection measurement larger than that for current and/or voltage measurement. Thus more accurate reflection measurements are made possible in a configuration that is also used for current and/or voltage measurement.
Brief description of the drawing
These and other objects and advantageous aspects will become apparent from a description of exemplary embodiments using the following figures.
Figure 1, la show devices for testing photo-voltaic cells
Figure 2a,b show light patterns
Figure 3a,b show light patterns
Figure 4, 5 show devices for testing photo-voltaic cells
Detailed description of exemplary embodiments
Figure 1 schematically shows a device for testing a photo-voltaic cell, comprising a control system 10, a first and second light source 12a,b, a first and second spatial light modulator 14a,b, a beam combiner 16, imaging optics 18 and a holder system 19 for a photo-voltaic cell. First light source 12a is directed to supply light to beam combiner 16 via first spatial light modulator 14a. Second light source 12b is directed to supply light to beam combiner 16 via second spatial light modulator 14b. Imaging optics 18 are configured to image first and second spatial light modulators 14a,b onto a photo-voltaic cell in holder system 19.
First and second light source 12a,b may both be configured to produce white light, the testing device being arranged to supply white light to the photo-voltaic cell from first and second light source 12a,b via each of first and second spatial light modulators 14a,b. White light with a spectral composition substantially equal to that of sunlight may be used for example, standards such as the AW1.5 standard define such spectral compositions.
One or both of first and second light source 12a,b may be controllable light sources that allow for control of the spectral content of the light source, so that a switch from white light to light selectively within a color band is possible for example.
The part of spatial light modulator 14a,b that affects the light may consist of two-dimensional array of identical cells, each cell comprising a controllable mirror for example, or a controllable liquid crystal cell. The cells of first and second spatial light modulator 14a,b provide for at least two modulated intensity levels, a higher and lower intensity such as non-zero and a zero intensity, obtained for example by causing a mirror to reflect light towards the photo-voltaic cell or not respectively. First and second spatial light modulator 14a,b provide for intensity modulation with spectrally uniform position dependence.
The position dependence is spectrally uniform in the sense that neither spatial light modulator 14a,b has interspersed pixels for different colors. Preferably, spatial light modulator 14a,b preserve the spectral distribution of the light from light sources 12a,b in the spatially modulated light. Spectrally uniform position dependence means that, for each spatial light modulator 14a,b, as far as the spatial light modulator influences the spectral distribution of the light at any modulation level at all, at that modulation level it does so in the same way for all spatial positions on the spatial light modulator. In addition, spatial light modulator 14a,b provide for time continuous lighting in the sense all positions are modulated simultaneously in parallel to form the complete light, that is, no time multiplexing of lighting at successive different spatial positions or color components is used.
In the illustrated embodiment, first and second spatial light modulator 14a,b provide for controllable reflection of light from light sources 12a,b dependent on position in the cross-section of the beam, i.e. in a plane transverse to beam axis. The angles between the beam axis and first and second spatial light modulator 14a,b are only shown schematicahy: in practice a different angle may be used, for example an angle that is closer to the perpendicular that the forty five degree angle shown. Although in the ihustrated embodiment uses first and second spatial light modulator 14a,b in reflection, it should be appreciated that the light may instead be supplied via first and second spatial hght modulator 14a,b by transmission through first and second spatial hght modulator 14a,b.
Imaging optics 18 are used to form images of the modulation planes of first and second spatial hght modulator 14a,b onto an image area that comprises the surface of the photo-voltaic cell. The image may coincide with the surface of the photo-voltaic ceh, or it may be larger, extending over an array of such cells for example.
Beam combiner 16 combines the images from the first and second spatial hght modulator 14a,b to form overlaid images in the image area. The beam combiner mat be considered part of imaging optics 18. Beam combiners that allow for projecting overlaid images are known per se, also inversely as beam splitters. Beam combiner 16 may comprise one or more semi-transparent reflective surfaces. Alternatively, the function of the combination of beam combiner 16 and imaging optics 18 may be realized by projecting the beams from spatial modulators 14a,b onto the photo-voltaic cell on holder system 19 from different directions, as shown in figure la, wherein the imaging optics and the combiner are combined in a mirror lens 160 to image spatial hght modulators 14a,b onto the photo-voltaic cell. Alternatively, imaging optics may be used that comprise respective lenses (not shown) to image first and second spatial hght modulators 14a,b onto the photo-voltaic ceh respectively, optionally via one or more mirrors or mirror lenses.
Control system 10, which may comprise a function specific circuit and/or a computer or a combination of computers with a computer program or programs to configure the control system to perform the described functions, has outputs coupled to control inputs of first and second light source 12a,b and first and second spatial light modulator 14a,b. Although control of both first and second light source 12a,b is shown, alternatively only one may be controlled. First and second light source 12a-b, may comprise a light generating element, such as an incandescent lamp, an arc light source, a fluorescent lamp, an electroluminescent lamp a LED or group of LEDs etc, as well as optional spectral filters and beam forming optics.
Holder system 19 comprises a structure to hold the photo-voltaic cell in position and electrodes (not shown) for contacting electrodes on the photo-voltaic cell. Holder system 19 is coupled to control system 10 for obtaining a response signal or signals from the photo-voltaic cell and optionally also for supplying currents and/or voltages to the photo-voltaic cell. Control system 10 may be coupled directly to the electrodes to supply the current and/or voltage as response signals from the photo-voltaic cell to control system 10. Alternatively, holder system 19 may comprise an electronic circuit to convert the current and/or voltage from the photo-voltaic cell into the signal supplied to control system 10, e.g. by amplifications and/or analog to digital conversion. Furthermore holder system 19 may be configured to supply one or more bias voltages via electrodes of holder system 19 to electrodes on the photo-voltaic cell and/or to supply bias currents through such electrodes.
As used herein the holder system is said to hold the photo-voltaic cell, in the sense that it may hold the photo-voltaic cell directly or as part of an assembly held by the holder system. Similarly, response signals dependent on the voltage and/or current may be derived directly from the photo-voltaic cell or through terminals of such an assembly. In an embodiment wherein a solar panel comprising an assembly of solar cells is tested, the photo-voltaic cell may be held by holding the panel and signals dependent on the voltage and/or current may be may be derived through terminals of the panel. In an embodiment measurement and/or bias electrodes of the holder system are mechanically fixed to the part of the holder system that keeps the photo-voltaic cell in place, but alternatively the electrodes may be connected separately.
In an embodiment, first and second spatial light modulator 14a,b each comprise a two-dimensional array of micro mirrors. Such arrays are known for example for beamers and commercially available as a DMD (Digital Mirror Device). A two-dimensional array of 1920x1200 pixels may be used for example. Alternatively, a two dimensional LCD array may be used. As may be noted, such devices provide for true spatial light modulation, wherein the modulated light is provided for a two-dimensional array of spatial locations simultaneously. Furthermore, they provide for spectrally uniform position dependent intensity modulation, in the same way everywhere as a function of position. The spectral distribution does not vary with modulation level, or if varies, it varies in the same way with the modulation level at all spatial positions. Thus true spatial hght modulation of the incoming light is provided, without spatially interspersed elements for modulating different, color components.
In operation, control system 10 controls light properties of the light produced by at least one of first and second light source 12a,b.
Intensity and/or spectral distribution of the light from first and/or second light source 12a,b may be controlled independently of the other for example. Furthermore, control system 10 controls the two dimensional light modulation pattern provided by first and second spatial hght modulator 14a,b, and thereby the light pattern on the photo-voltaic cell in holder system 19. The pattern may comprise a localized probe area that receives light mainly from first light source 12a and a background area with that receives light mainly from second light source 12b, control system controlling the position, shape and/or size of the probe area by controlling first and/or second spatial light modulator 14a,b. Control system 10 receives signals or information from holder system 19 that represent electrical signals generated by the photo-voltaic cell in response to the light pattern.
Figure 2a,b illustrates light patterns that first and second spatial light modulator 14a,b may provide under control of control system 10. In this example, first and second spatial light modulator 14a,b are made to provide a pattern with a spot shaped probe area. In this example, first spatial light modulator 14a is made to form a light spot 20 in a dark field to provide light in the probe area. Second spatial fight modulator 14b is made to provide a pattern with a dark spot 22 in a light field to form the background area. Control system 10 makes first and second spatial light modulator 14a,b form a light spot 20 and dark spot 22 of corresponding shape and size at a position that beam combiner 16 and imaging optics 18 image onto a same position on the photo-voltaic cell in holder system 19.
The light field provides for the background area surrounding the spot shaped probe area. Alternatively, a background area may be used that has parts only on opposite sides of the spot along a line through the spot, but preferably the background area includes these sides in a light field that surrounds the spot.
In operation, control system 10 may control first and/or light source 12a-b to make the lighting properties in light spot 20 produced with light from first light source 12a different from those in the field surrounding light spot 20, produced with fight from second light source 12b. The intensities may be made mutually different, or different spectral distributions may be used. During illumination, control system 10 captures signal values at the electrodes of holder system 19 that contact electrodes on the photo-voltaic cell. Thus a cell output voltage and/or current may be captured.
In an embodiment first and/or light source 12a-b each comprise one or more controllable lamps that make it possible to control intensity by varying electrical supply current to the lamps. Alternatively, first and/or light source 12a-b may comprise a controllable attenuator, such as a movable screen that may be moved into and out of the beam path under control of control system 10 to adjust intensity. In an embodiment wherein first, and/or second light source 12a-b comprises a plurality of lamps (e.g. LEDs) with different spectral output, spectral composition can be controlled when control system 10 is configured to vary the intensities of the lamps relative to each other. In another embodiment, first and/or light source 12a-b may comprise an adjustable filter to adjust the spectral composition, e.g. control system 10 may be configured to cause different filter areas in and out of the beam path, or to rotate a grating relative to the beam path.
In embodiment, control system 10 may be configured to perform comparative measurements, wherein results of a first and second experiment are compared. In the first experiment control system 10 controls first and/or second light source 12a-b to provide light of a first spectral composition, and first and second spatial light modulator 14a-b to provide first spatial modulation patterns, and control system 10 obtains a first response signal from a photo-voltaic cell under test from holder system 19.
In the second experiment control system 10 controls first and/or second light source 12a-b to provide light of a second spectral composition, different from the first spectral composition, and control system 10 obtains a second response signal from a photo-voltaic cell under test from holder system 19, using the same first spatial modulation pattern as the first experiment. Control system 10 computes a difference between the first and second response signals, for use to output a test result.
In embodiment, control system 10 may be configured to control a temporal variation of the lighting properties of the light in light spot 20 by supplying time varying control signals to first light source 12a. Instead of using hght source 12a to vary the lighting intensity, control system 10 may control first spatial hght modulator 14a to vary the lighting intensity. The overall intensity may be varied periodically for example, or spectral distributions may be varied periodically. Instead of periodic variation another temporal variation pattern may be used. When temporal variation of the lighting properties is used, control system 10 may be configured to capture a temporal variation signal values at the electrodes of holder system 19. Thus for example control system 10 may be configured to determine an amplitude and/or phase delay of a periodic response to a periodically varying lighting property. As another example, control system 10 may be configured to cause a pulse or stepwise variation of lighting to be applied, control system 10 capturing a temporal response of the signal from the photo-voltaic cell to the pulse or step. Preferably, a time sampling distance is used that is less than a quarter of the life time of minority charge carriers in the photovoltaic cell under test.
In embodiment, control system 10 may be configured to step the position of light spot 20 and dark spot 22 successively to different positions. A two-dimensional line by line stepping pattern may be used for example, realized by successively making different locations on spatial hght modulator 14a, reflect or transmit hght. In synchronism with the steps, control system 10 performs measurements based on signal values at the electrodes of holder system 19. Thus position dependent measurements may be obtained.
As will be appreciated, the device makes it possible to control the actual time-continuous lighting properties as a function of both position and time, rather than merely controlling time or space averaged lighting properties that appear to the human eye as actual lighting properties. First and second spatial hght modulator 14a,b control at least switching between presence and absence of hght with the same lighting properties at adjacent locations on the photo-voltaic ceh without locations with other lighting properties in between. The spectral composition does not vary with modulation, or if varies, it varies with modulation in the same way at ah positions.
Because control system 10 controls light properties of the light produced by first and second light source 12a-b, the patterns of figure 2a-b may result in a lighting pattern of the photo-voltaic cell with continuous light of a first intensity and/or spectral composition at a position corresponding to light spot 20 and a second intensity and/or spectral composition in an area surrounding light spot 20.
As noted, control system 10 may comprise a computer or a combination of computers. In this case, control system 10 may be programmed with computer readable machine instruction in a computer readable medium such as a (volatile or non-volatile) semi-conductor memory, or a magnetic or optical disc, which when executed by the computer or combination of computers will make the computer or combination of computers control operation of the device as described. In particular, the instructions may be configured to make control system 10 apply control signals to first light modulator 14a and/or second light modulator 14b and/or to first light source 12a and/or second light source 12b. Preferably the instructions may be configured to make control system 10 record electrical signal data based on signals from the photo-voltaic cell under test in holder system 19. The instructions may be configured to make control system 10 capture and record signals synchronized to timing of control of changes of lighting properties and/or stepping of the probe area. The instructions may be configured to make control system 10 control first and second light modulator 14a, b to step the position, shape and/or size of the probe area and the background area in correspondence with each other.
Although an embodiment has been shown wherein a first branch comprising first light source 12a and first light modulator 14a provide hght spot 20 at a location where a second branch comprising second hght source 12b and second hght modulator 14b creates dark spot 22, it should be appreciated that alternatively the second branch may supply spatially uniform hght, without dark spot. In this case, the first branch only adds to the light from the second branch. Creating a dark spot 22 at the location of light spot 20 has the advantage that intensities above and below the intensity of the area surrounding the dark spot can be supplied to the photovoltaic cell, which makes it possible to realize spatial light variations with zero spatial average for example.
In an embodiment the lighting from the second branch has a spatial extent that covers an entire photo-voltaic cell surface. In an embodiment control system 10 may be configured to provide for a partial light field, which is smaller than the entire photo-voltaic cell surface, by causing only part of the locations of second spatial light modulator surrounding light spot 20 to supply light and causing a remainder of the locations further from the position corresponding to hght spot 20 to supply no or less light intensity. In this embodiment, when control system 10 causes the position of the light spot 20 to move, control system 10 is preferably configured to move the partial field along with the hght spot.
This makes it possible on one hand to measure the effect of the light spot with surrounding bias and other hand to reduce the ratio between photo current due light spot 20 and photo current due to the remaining area of the photo-voltaic cell. This may improve signal to noise ratio.
In an embodiment the lighting from the second branch has a spatial extent that covers less than an entire photo-voltaic cell surface. In this case the device may be provided with a motion mechanism configured to move holder system 19 and the lighting system relative to one another, so that a partial hghting field from the second branch can be moved over the photo-voltaic cell.
Although figures 2a,b show an embodiment wherein a light spot 20 is provided by the first branch, it should be appreciated that alternatively control system 10 may be configured to provide for lighting areas of other shape or size.
Figure 3a,b illustrates other light patterns that first and second spatial light modulator 14a,b may provide under control of control system 10. In this example the probe area has the form or a line. In this example, first spatial light modulator 14a provides a pattern with a light hne 30 in a dark field at a position that beam combiner 16 and imaging optics 18 image onto the probe area. Second spatial hght modulator 14b provides a pattern with a dark line 32 in a hght field, at a position that beam combiner 16 and imaging optics 18 image onto the probe area. The light field provides for the background area on mutually opposite sides of the line shaped probe area.
When the photo-voltaic cell contains a linear structure, such as electrode fingers or linear edges between alternating surface field and emitter strips, measurement speed can be increased by using such a light line 30 oriented in parallel to the linear structure. Instead using a time-series of measurements with light spots at different positions along the linear structure a single light line may be used. In an embodiment control system 10 may be configured to step the position of light line 30 and dark line 32 in a direction transverse to the longest direction of the line, e.g. to different distances from a linear structure on the photo-voltaic cell.
Figure 4 shows a device for testing photo-voltaic cells. The device is similar to that of figure 1, except that one or more photo detectors 40, are provided in association with imaging optics 18, directed to holder system 19 so as to capture reflected light from a photo-voltaic cell in holder system 19. Photo detectors 40 may be provided mounted in a ring around a lens opening of imaging optics 18 for example, or at other fixed positions relative to imaging optics 18 and/or the position of the photo-voltaic cell on holder system 19. Photo detectors 40 for different wavelength components may be used for example, for example in combination with filters or a grating (not shown) to select wavelength components for different photo detectors 40. Electrical outputs of one or more photo detectors 40 are coupled to control system 10.
Figure 5 shows a device for testing photo-voltaic cells. The device is similar to that of figure 4, except that a photo-imaging array 50 is provided at an output of beam combiner 16, which is configured to supply light received from photo-voltaic cell in holder system 19 through imaging optics 18 to photo-imaging array 50. A photo-imaging array 50 with interspersed detectors for different wavelength components may be used. In an alternative embodiment, a plurality of arrays for respective different wavelength components may be used.
In operation control system 10 may use photo detectors 40 and/or photo-imaging array 50 to determine a reflection coefficient R in response to the lighting applied to a photo-voltaic cell under test, preferably as a function of wavelength. From this reflection coefficient, control system 10 may compute normalized signals N normalized for reflection from E electrical signals obtained using signals from holder system 19 according to N=E/(1-R). In an example, E may be the electrical quantum efficiency computed from the electric signals.
Control system 10 may use a calibration coefficient that relates detected reflected intensity I to the reflection coefficient R. Control system 10 may be configured to perform a calibration measurement of reflection from a reference reflector with a known reflection coefficient to determine the calibration coefficient.
Control system 10 may be configured to determine reflection from a selected location on the photo-voltaic cell under test by causing light to be applied selectively only to that location, the photo-voltaic cell kept dark outside the location. Alternatively control system 10 may be configured to determine reflection from the selected location from a difference RI1-RI2 between detected reflected intensities Ril, RI2 obtained with lighting patterns that differ only at that location, normalized by the intensity difference between the lighting patterns. In an embodiment control system 10 is configured to enlarge a size (diameter or width) of the light spot 20 or light line 30 used for reflection measurements relative to the size used for the electrical measurement of an electrical voltage or current from a corresponding location on the photo-voltaic cell. This provides for improved signal to noise ratio. In an embodiment control system 10 may be configured to switch off the background light during the reflection measurement. This provides for improved signal to noise ratio of the reflection measurement.
Although embodiments have been shown wherein two hght sources 12a,b, a beam combiner 16 and a programmable spatial hght modulator system consisting of two spatial hght modulators 14a,b are used to form a pattern onto a surface of the photo-voltaic cell, it should be appreciated that other configurations are possible. For example, more than two hght sources 12a,b, and/or more than two spatial hght modulators 14a,b may be used to form a single lighting pattern on a photo-voltaic cell under test. In another embodiment, hght from one hght source may be split into parts and the parts may be fed to the photo-voltaic cell under test in parallel via different spatial hght modulators, optionally using different spectral filters and/or attenuators for the different split parts, or a spectral filter and/or attenuator for one of the split parts. This one hght source may be the only hght source used to generate the lighting pattern. However, use of a plurality of hght sources may provide for more flexible and more rapidly variable lighting, for example when the lamps of the hght sources are electrically controlled.
Although embodiments with two or more spatial hght modulators have been described, it should be appreciated that instead only one spatial hght modulator may be used. In this case a lighting pattern with a shadow area may be realized. This may be useful for a number of tests. However, it is not possible to realize non-multiplex patterns wherein different parts of the image area have different spectral composition, and/or subtly different intensity. Use of two or more spatial hght modulators to form overlaid images in the image area makes it possible to perform more extensive tests without multiplex patterns. Preferably this is done in combination with a light source system that allows for control of the part of the light that is used to form an image in the image area by means of modulation by one of the spatial hght modulators independent of the parts of the light modulated by spatial light modulators.
In another embodiment the programmable spatial light modulator system may comprise a spatial light modulator only for one of the split parts, or only for one of a plurality of light sources, light from the another part or other light source be passed to the photo-voltaic cell without spatial modulation. However, this makes it impossible to provide for a controllable “hole” in correspondence with the light spot, line or other pattern produced using the single spatial light modulator.
Although embodiments have been described wherein a single photo-voltaic cell is tested, it should be understood that alternatively a solar panel comprising an array of such photo-voltaic cell may be tested. In this case the device may be used to simulate the effect of shadows for example.
In this embodiment the control system may be configured to cause the light patterns to define a light area with relatively higher intensity simulating a sunlit part and a relatively darker shadow area, preferably both with adjustable light properties.
When a target photo-voltaic cell in an array of such cells is tested, series connected cells may be used in the array. This means that if one wants to analyze the current response of the target cell, this current may be limited by any cell that is connected in series. It is desirable to prevent such current limiting. This may be realized for example by keeping non-target cells in the dark. In an alternative embodiment, the spatial light modulators are controlled to project background light on all non-target cells and probe light on the target cell. The projected probe light may cover an area that coincides with the entire target cell. Alternatively the projected probe light may cover or part of the target cell only, if measurement of spatial performance of the target cell is required. The probe light of the target cell can be white hght or a desired spectrum yielding physical information of this cell. A method of testing an assembly of solar cells is described in an article titled “Hot spot tests for crystalline silicon modules” by John Wohlgemuth et al. This may short circuiting the assembly and shadowing each cell in turn, in order to select the cell which gives the biggest decrease in short-circuit current when shadowed. In an embodiment the shadowed area of the cell is decreased and current is measured for a series of different, shadowing areas are in order to determine worst case conditions. Conventionally, selective shadowing comprises placing a light obstructing screen over the cell. In an embodiment of the present invention, shadowing in this test is replaced by projection an image onto the cells in the assembly using a spatial light modulator, the image comprising a darker part at the location of a single cell and a brighter part at the locations of the remaining cells in the assembly. In a series of steps, different cells may successively be singled out in this way. In another embodiment, different groups of cells may singled out instead of the single cell. In a further embodiment, light may be projected onto the single cell, or single group of cells, using a second spatial light modulator to define the pattern that hghts only this single cell or group of cells using light with different light properties compared to those used by a first spatial light modulator for the remaining cells.
Although a system with two spatial light modulators may be used for this purpose, it should be appreciated that a system with a single spatial light modulator may suffice to replace shadowing during testing of an assembly of photo-voltaic cells.
Adaptation of the pattern of projected probe light to the cell is useful for example when monolithically manufactured thin-film photovoltaic modules are tested, like organic photo-voltaic, thin-film Silicon, CI(G)S and CdTe. It is known to create such modules by deposition of thin layers that together constitute the pn-junction, the cells in the module being created after the deposition process by isolating cells and by realizing interconnection of the p-layer to the n-layer of the adjacent cell and vice versa. By using a device with spatial light modulators that provide for adaptable light patterns, and controlling the spatial hght modulators to fit the cells created in the module tests of individual cells in monohthically manufactured thin-film photo-voltaic modules can easily be performed.
Claims (20)
Priority Applications (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| NL2011207A NL2011207C2 (en) | 2013-07-23 | 2013-07-23 | Device and method for testing a photo-voltaic cell. |
| TW103125141A TW201510495A (en) | 2013-07-23 | 2014-07-22 | Device and method for testing a photo-voltaic cell |
| PCT/NL2014/050503 WO2015012689A1 (en) | 2013-07-23 | 2014-07-23 | Device and method for testing a photo-voltaic cell |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| NL2011207A NL2011207C2 (en) | 2013-07-23 | 2013-07-23 | Device and method for testing a photo-voltaic cell. |
| NL2011207 | 2013-07-23 |
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| Publication Number | Publication Date |
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| NL2011207C2 true NL2011207C2 (en) | 2015-01-26 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| NL2011207A NL2011207C2 (en) | 2013-07-23 | 2013-07-23 | Device and method for testing a photo-voltaic cell. |
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|---|---|
| NL (1) | NL2011207C2 (en) |
| TW (1) | TW201510495A (en) |
| WO (1) | WO2015012689A1 (en) |
Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3185834A (en) * | 1962-04-25 | 1965-05-25 | Optical Coating Laboratory Inc | Light source |
| US6623122B1 (en) * | 1999-09-30 | 2003-09-23 | Semiconductor Energy Laboratory Co., Ltd. | Light source optical system and projector having first and second lamps of different spectral distributions |
| US20110227598A1 (en) * | 2009-04-20 | 2011-09-22 | Seung Nam Park | Apparatus and method for inspecting homogeneity of solar cell quantum efficiency using imaging device |
| DE102011052046A1 (en) * | 2011-07-21 | 2013-01-24 | Wavelabs Solar Metrology Systems Gmbh | Device for generating a homogeneously illuminated surface and corresponding method |
-
2013
- 2013-07-23 NL NL2011207A patent/NL2011207C2/en not_active IP Right Cessation
-
2014
- 2014-07-22 TW TW103125141A patent/TW201510495A/en unknown
- 2014-07-23 WO PCT/NL2014/050503 patent/WO2015012689A1/en active Application Filing
Patent Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3185834A (en) * | 1962-04-25 | 1965-05-25 | Optical Coating Laboratory Inc | Light source |
| US6623122B1 (en) * | 1999-09-30 | 2003-09-23 | Semiconductor Energy Laboratory Co., Ltd. | Light source optical system and projector having first and second lamps of different spectral distributions |
| US20110227598A1 (en) * | 2009-04-20 | 2011-09-22 | Seung Nam Park | Apparatus and method for inspecting homogeneity of solar cell quantum efficiency using imaging device |
| DE102011052046A1 (en) * | 2011-07-21 | 2013-01-24 | Wavelabs Solar Metrology Systems Gmbh | Device for generating a homogeneously illuminated surface and corresponding method |
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| Publication number | Publication date |
|---|---|
| TW201510495A (en) | 2015-03-16 |
| WO2015012689A1 (en) | 2015-01-29 |
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