SE537301C2 - Device, method and computer program for testing photovoltaic devices - Google Patents

Abstract

16 ABSTRACT The present invention relates to a photocurrent testing device (100) for testingphotovoltaic devices (120) such as solar cells. This device (100) includes a plurality ofpuisating light sources (110) and a signal generator (130) which generates an individualdriving frequency for each light source (110). This causes each light source (110) topuisate at an individual frequency corresponding to its driving frequency. Also included inthe photocurrent testing device (100) is an arnplifier unit (140) which coilects outputcurrent from the photovoitaic device (120) and amplifies the output current at each individual frequency. Further included in the photocurrent testing device (100) are analysing means (150)which analyse the amplitude of the output current at each individual frequency wherelnthe output current at each individual frequency originates from the light source puisatingat the same individual frequency, and where each light source (110) iiiurninates a specificspot (116) on the photovoitaic device. The amplitude of the output current at eachindividual frequency is rnatched with a corresponding specific spot (116) on the photovoitaic device. (Fis 1)

Description

1G A DEVECE, Å METHOÛ, AND Å COMPUTER PROGRAM FÖR TESTENG OF PHOTÛVÛLTÅBCDEVlCES TECHNECAL FlELD The present invention relates to a device, a method, and a computer program for testing of photovoltaic devices.

BACKGRÛUND With dirninishing fossil fuels and increased awareness of the climate change, there is aglobal increase in the demand for renevvahle energy sources. Solar energy is being viewedas one of the most available and reliable renewable energy sources. The use of photovoltaic devices, PVDs, such as solar cells is important in the harvesting of solar energy.There are several different types of solar cells, such as silicon based solar cells, and the recently developed thin organic solar cells, e.g produced by means of printing.

Quality control is an essential step in the production process for photo voltaic devices, toensure that the efficiency of the photo voltaic devices meet the desired requirements.This is important for both large scale and small scale production of photo voltaic devices.Quality control may also be important in the research and development of photovoltaic devices.

Testing of solar cells is at present commonly performed by either illuminating the entiresolar cell at once or in a small area and scanning across the entire surface of the solar cell and measurlng the output current generated by the solar cell.

US8239165 discloses an apparatus for measuring quantum efficiency, QE, of solar cells.The apparatus includes a light source including an array of light ernitting diodes, LEDs,that each emit light corresponding to a differing portion of a test spectrum and each LEBis driven by a sinusoidal power supply that operates at a unique frequency. The lightsource includes an optical coupling focusing the LED light into a test beam targeted on asolar 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 FourierTransform module that processes the digital voltage signals to generate values for eachOperating frequency. The QE measurement module determines the QE values by applyinga Conversion factor to these values. Since all the LEDs can be power-modulatedsirnultaneouslv and the corresponding cell responses to each of the LEDS can be analyzedsimultaneouslv, the QE spectrum measurement time is greatly shortened as compared to conventional methods.

US2013/ÛG2l054 discloses an apparatus used for slmuiating spectrum of solar radiationand testing a photovoltaic device using the simulated spectrum of solar radiation. Theapparatus may include a light-source device configured to reproduce spectrum of solarradiation, the light-source device comprising a radiation plate divided into a plurality ofcells, and each of the cells comprises a piurality of iight-emitting diodes emitting at leasttwo different wavelengths, and a substrate support disposed opposite to the light-sourcedevice. in one example, the plurality of light-emitting diodes emit a wavelength that isselected from the group consisting of colours blue, green, yellow, red, a first and a second colour in infrared having different wavelengths with respect to each other.

Another testing method for photo voltaic devices is that of using a focused laser heam toscan across the surface of the photo voltaic device. The laser heam is moved in at leasttwo directions across the photo voltaic device, iliuminating a small area of the cell at eachmeasuring instance until the total area of the cell has been covered. l-iowever, this testingmethod is slow and thus time consuming since the scanning of the cell is performed hy asingle laser beam and needs to he performed in at least two directions. The techniquefurther has the dravvhack of being expensive and complex since it involves delicate and intricate components to focus and control the laser beam.

The methods and apparatus disclosed in US8239165 and US2013/Q021054 can he usedfor testing the overall performance of the photo voltaic device, but cannot he used tolocalise defects in a photo voltaic device. Therefore, there is a need in the art for a device, a method, a system and a computer program that enables fast locaiisation of defects. 2G SUMMARY GF iNVENTiON An object of the invention is therefore to provide a device, a method, a system and acomputer program for fast iocaiisation of defects in a photovoitaic device. A further object of the invention is to achieve said iocaiisation in a cost effective manner.

The invention is set forth and characterized in the main ciaims, while the dependent ciaims describe additional aspects of the invention.

The objects of the invention are achieved by a photocurrent testing device for testingphotovoitaic devices such as soiar ceiis. This device inciudes a piuraiity of puisating fightsources and a signai generator which generates an individual driving frequency for eachiight source. This causes each light source to puisate at an individuai frequencycorresponding to its individuai driving frequencyfAlso inciuded in the photocurrenttesting device is an ampiifier unit which coiiects output current from the photovoitaic device and ampiifies the output current at each individual frequency.

Further included in the photocurrent testing device are anaiysing means which analysethe arnpiitude of the output current at each individual frequency wherein the outputcurrent at each individuai frequency originates from the iight source puisating at thesame individuai frequency, and where each Eight source iiluminates a specific spot on thephotovoitaic device. The arnpiitude of the output current at each individuai frequency is matched with a corresponding specific spot on the photovoitaic device. in accordance with the present invention, the photocurrent testing device wiii matchdifferences in the amplitude of the output current of the photovoitaic device to a specificspot on the photovoitaic device, such that a piuraiity of defects can be iocaiised simuitaneously in a fast and cost effective manner. in a further aspect of the invention the iight sources are Eight emitting diodes, LEDs. Lighternitting diodes are inexpensive to buy and operate, and have a long iifetirne. The iightsources are preferahiy adapted to have the same iight emitting spectrum. This aiiovvs for a more efficient caiihrating of the device. in a further aspect of the invention the iight sources are arrangeci in a singie row. in a further aspect of the invention the row of light sources covers the entire width of thephotovoitaic device to he tested, aiiowing the device to scan the entire width of the soiarceii in one run, thereby enabiing a faster and continuous testing of photovoitaic devices arranged as soiar paneis. in a further aspect of the invention the iight sources are arranged in a matrix that iiiurninates the fuii soiar ceii or parts of the soiar ceii. in a further aspect of the invention the iight sources are arranged to move reiative to the photovoitaic device in one direction. in a further aspect of the invention the individuai driving frequencies are eveniydistributed within a predetermined frequency intervai, thereby sirnpiifying caicuiations performed in the anaiyzing means. in a further aspect of the invention the arnpiifier unit is a iock-in anipiifier unit. in a further aspect of the invention the signai generator is cornprised within the iock-in ampiifier unit.

A method for testing a photovoitaic device according to the invention cornprises a firststep of generating an individuai driving frequency for each of a piuraiity of iight sources.in a next step, each of the iight sources is driven with its individuai driving frequency suchthat each iight source puisates at an individuai frequency corresponding to its drivingfrequency. in a next step, each iight source iiiuminates a specific spot on the photovoitaic device. in a next step output current from the photo voitaic device is coiiected and the current ateach individuai frequency is ampiified. in a next step, the ampiitude of the output currentis anaiysed at each individuai frequency, wherein the output current at each individuai frequency originates from the light source puisating at the same individuai frequency. in a 1G next step, the ampiitude of the output current at each individuai frequency is matched with a corresponding specific spot on the photovoitaic device. in an optionai further step a photocurrent map of the photovoitaic device is created, simpiifying the further assessment of the data obtained. in an optionai further step the quaiity of the photovoitaic device is assessed, The objects of the invention are also achieved in a system for testing photovoitaic devicescomprising a photocurrent testing device and a computer program for controiiing a method for testing a photovoitaic device.

BRiEF DESCRiPTEÛN OF DRAWINGSFurther objects, features, and advantages of the present invention wiii appear from thefoiiowing detaiied description, wherein some aspects of the disciosure wiii he described in more detaii with reference to the accornpanying drawings, in which: Figure 1 shows a schematic iayout of the inventive photocurrent testing device, Figure 2 shows a schematic iayout of a part of an embodiment of the photocurrenttesting device, Figure 3 shows a fiow chart iiiustrating the method according to the invention, and Figure 4 shows exampies of photocurrent maps produced with the system accordingto the invention, and Figure 5 shows the principies of quantum efficiency determination for a photovoitaic device.

DETAILED DESCREPTEGN Various aspects of the invention wiii hereinafter be described in coniunction with the appended drawings to iiiustrate, but not to iimit the invention. Like designations denote Eike elements, and variations of the inventive aspects are not restricted to the specificaEEy shown embodlntient, but are appEicabEe on other variations of the invention.

Figure 1 shows a schematic layout of a photocurrent testing device 100 according to thepresent invention. The photocurrent testing device 100 is for testing a photovoitaicdevice 120, such as an organic soiar celE or a silicon wafer solar cell. The photocurrenttesting device 100 comprises a pEuraEity of Eight sources 110. There may be as few as twoEight sources 110, as many as one thousand, any number in between, or rnore than onethousand. The Eight sources could be any kind of Eight sources, such as Eight ernitting diodes, LED, lasers, or gas-discharge Eamps.

The Eight ernitting spectra of the Eight sources 110 should at least partiaEEy overEap withthe absorption spectrum of the photovoltaic device 120 to be testecE. The reason for thisis that at least part of the Eight emitted by the Eight sources 110 has to be absorbed by thephotovoltaic device 120 in order for the photovoltaic device 120 to produce a current,said current being a prerequisite for the photocurrent testing device 100 to function. Foran organic photovoltaic device, a preferred Eight emltting spectrum of the light sources110 should Eie within or at least have significant parts within the waveiength interval 350-1100 nrn, and more specificaliy within 400-700 nm. This is to maxirnise the currentoutput from the photovoltaic device 120 for a given power of the iEEuminating Eight.Preferabiy, aEE the light sources 110 are adapted to have the same light emittingspectrum, since photovoitaic devices 120 norrnaEEy responds differently to differentwavelengths of incorning Eight. With aEi the light sources 110 having the same lighternitting spectrurn, no correction factor for the fight spectrurn has to be taken into äšlCüLlfit.

Each Eight source 110 emits a Eight cone 115 which iiluminates a corresponding spot 116on the photovoltaic device. En an example of the invention, the light sources 110 arearranged such that their respective iliunwinated spots 116 on the photovoitaic device donot significantiy overEap, The Eight intensity in the overiapping part of two spots 116 is in one example of the invention kept one or more orders of magnitude Eower than the Eight intensity in the non-overlapping parts of a spot 116. The term spot 116 refers to a smallarea, typicaily clrcularly shaped. One or more optical elements, such as lenses or filters,may he provided between the light sources 110 and the corresponding spots 116. This isfor controlling or defining the size and shape of the corresponding spot 116, and for controlllng and defining the spectrum of the iiluminating light.

The light sources 110 are driven hy a signal generator 130 which is connected to the lightsources 110 via one or more cahles 135. The signal generator 130 generates for each lightsource 110 an individual driving frequency such that each Eight source 110 pulsates at anindividual frequency corresponding to its individual driving frequency. The signalgenerator is preferahly able to produce many different driving frequencies, since thephotovoltaic testing device 100 may comprise a large number of light sources 110, eachlight source 110 requiring its own individual driving frequency. in one example, a multiplexing unit is provided between the signal generator 130 and the light sources 110.

As the photovoltaic device 120 is iiluminated hy the light sources 110, it will generate aphotocurrent. Every spot 116 will generate a pulsating photocurrent with the individualfrequency equal to the individual frequency of the light source 110 illumlnating thatspecific spot 116. The total photocurrent generated hy the photovoltaic device 120 is thesum of the photocurrents produced by each iiiuminated spot 116. Therefore, the totalphotocurrent cornprises a frequency component for each of the individual driving frequencies of the light sources 110. lf the photovoltaic device 120 is defect in a specific spot 116, the amount of currentgenerated in that spot 116 will be lower than in a non-defect spot. The term defect refersto any deviation from the optimal functionlng of the photovoltaic device 120, such astotal dysfunction, degradation or any other impaired operational Characteristics.Consequently, the contribution to the total photocurrent at the frequency corresponding to the defect spot will be lower than from a non-defect spot.

Said total photocurrent constitutes the output current frorn the photovoltaic device 120.

The output current is coilected hy an electrical cable 145 connecting the photovoltaic device 120 to an amplifier unit 140. The amplifier unit 140 transfers the output currentsignal from the time domain to the frequency domain hy means of a Fourier transform ora Fast Fourier Transform, FFT, and amplifies the output current at each individual drivingfrequency of the light sources 110. The current component at each individual frequency istypically very small since it orlginates from a corresponding, typically very small spot 116on the photovoltaic device. Amplification of these small current components is thusadvantageous in order to more easily distinguish them from white noise. Preferahly, thedriving frequencies generated by the signal generator 130 are also directly input to theamplifying unit 140 to enable amplification at exactly these frequencies. This can forexample he achieved by a lock-in arnplifier. ln one example the amplifying unit 140 is asignal analyser with combined lock-in ampiifier. in one example of the invention, the signal generator 130 and the amplifier unit 140 are comprised within the same unit.

The amplitude of the output current at each individual frequency is analysed hy analysingmeans 150. The analysing means 150 is preferahly constituted hy a computer. The drivingfrequencles, which individual driving frequency corresponds to which light source 110,and the physical position of each spot 116 are preferably predetermined and known tothe analyslng means 150. Since illuminatlng the photovoltaic device 100 with theindividual frequency of the light source results in the generation of a current pulsatlngwith the same individual frequency, the analysing means 150 can match each of theoutput current amplitudes with a specific light source 110, via their common pulsatingfrequency. Since each light source 110 is responsible for illuminating a specific spot 116on the photovoltaic device 120, each output current amplitude component can he paired with the specific spot 116 in which it was generated. ln a preferred example the driving frequencies of the light sources 110 are equallydistributed in a frequency interval. For example, the difference in driving frequenciesbetween two light sources 110 are multiples of 1000 Hz, such that one light source 110 isdriven with 1000 Hz, another with 2000 Hz, yet another with 4000 Hz and so on. lnanother example, the differences in driving frequencies are multiples of 100 Hz. Before performing the Fourier transform or the FFT the output current of the photovoltaic device 120 has to be inputted to the ampiifying unit for at least one full period of the differencesin driving frequencies , i.e., for example for at least 10 ms when the differences in drivingfrequencies is 100 Hz and the lowest driving frequency is at least 100 l-lz and for at least 1ms when the differences in driving frequencies is 1000 Hz and the iowest drivingfrequency is at ieast 1000i~iz. This is to assure that at least one period of the iowestdriving frequency can be integrated in the Fourier transform or the FFT and to assure thatthe differences in frequency are detected. Also measuring the phase from the FFT willenable two light sources driven at the same frequency, as they can be separated by a phase difference.

The analysing means 150 may comprise a computer program which controls the signalgenerator 130, and the amplifier unit 140 during the testing of the photovoltaic device120, as well as the positioning of the light sources 110 relative to the photovoltaic device120. in one example, the anaiysing means 150 also processes the data obtained duringthe testing, for example creates a photocurrent map 400, 450 over the testedphotovoitaic device 120 and/or uses an algorithm in which the current ampiitudes arecompared with a predefined model in order to assess the quality of the tested photovoitaic device 120.

Figure 2 shows a schematic layout of a part of an embodiment of the photocurrenttesting device. Here, the light sources 110 are arranged in a single row in a list 111,wherein the list covers the entire width of the photovoitaic device 120. Hence, a strip 112covering the entire width of the photovoltaic device can be illuminated and thus testedsirnultaneousiy. Letting the list 111 scan the photovoltaic device in a directionperpendicular to said strip 112 enables progressive testing of further strips of thephotovoltaic device. Consequently, the entire or part of the area of the photovoltaicdevice may loe tested in a continuous testing procedure. Preferabiy, said scanning isrealized hy moving the photovoltaic device relative to the list 111 of light sources 110 in adirection 121 perpendicular to the list, Either the list 111 is kept in a fixed position andonly the photovoltaic device 120 moves, or the photovoltaic device 120 is kept in a fixed position and only the list 111 moves, or both the list 111 and the photovoltaic device 120 are arranged to move. in one example, the photovoitaic device 120 is transported on a conveyor helt and the list 111 is fixediy mounted above said conveyor helt.

The speed at which the photovoltaic device 120 can be moved relative to the list 111 oflight sources 110 is limited by the integration time as described in connection with theFourier. This is due to that the resolution in the direction of the move of the photovoitaicdevice 120 wiii be limited by how much the photovoitaic device 120 moves under the integration time.

Figure 3 shows a flowchart 300 describing a method for testing a photocurrent device.The method comprises a first step 305 of generating an individual driving frequency foreach of a piurality of light sources 110. The method further comprises an additional step310 of driving each of the light sources 110 with its individual driving frequency, causingeach light source 110 to puisate at an individual frequency corresponding to its drivingfrequency. The next step 315 cornprlses iiluminating for each light source 110 a specificspot on the photovoltalc device 120. The next step 320 comprises coiiecting outputcurrent from the photovoltaic device 120 and amplifying the current at each individualfrequency. The next step 325 comprlses measuring the amplitude of the output current ateach individual frequency, whereln the output current at each individual frequencyoriginates from the ilght source pulsatlng at the same individual frequency. The next step330 comprises matching the measured amplitude of the output current at each individual frequency with a corresponding specific spot 116 on the photovoitaic device 120. ln an optionai further step 340, further spots on the photovoltaic device 120 are to bemeasured, the light sources 110 are moved a predetermined distance relative to thephotovoltalc device 120. Aiternatlveiy the photovoltaic device 120 may be moved relativato the light sources 110. The movement can be executed in a step-wise manner, or continuousiy throughout the testing.

An optional further step 345 comprises creating a photocurrent map 400, 450. This map400, 450 can be a visualizatlon of the measured current amplitudes matched to specific spots 116 on the photovoltaic device, or a data set of such a matchlng. 'li An optional further step 350 comprises assessing the quality of the photovoltaic device.This assessment 350 can be made using the photocurrent map 400, 450 through a visualassessment or other means such as a computer 160 performing calculations on the photocurrent map 400, 450.

Figure 4 shows examples of photocurrent maps produced with the system according tothe present invention. The map in Figure 4a iilustrates a photocurrent map 400 in whichthe photocurrent across a solar cell is iliustrated 410. The photocurrent at different areasof the solar cell is in this example illustrated using gray scale 405, and associatingdifferent values of the photocurrent to different shades of gray. Defects 415 can in thisway be identified as areas of the solar cell that give rise to photocurrents outside apreferred predetermined photocurrent range or value. The map in Figure 4b shows aphotocurrent map in the form of a matrix 450. The matrix 450 comprlses a number ofcells 480 arranged in coiumns 470 and rows 460 wherein each cell 480 corresponds to aspecific spot 116 on the photo voltaic devices. An area of the photo voitaic device 120 isilluminated by a Eight source 110 with a specific frequency and the photocurrent withcorresponding frequency is isolated and amplified and the value of the photocurrent ispresented in the correct cell 480 in the matrix 450. The amount of cells 480 may beincreased or decreased depending on the desired speed of the production of and resolution of the photocurrent map.

The data acquired in the assessment may be retrieved as raw data. The data acquired inthe assessment may be presented on a display. The data acquired in the assessment maybe printed as a raw data, a photocurrent map and/or a matrix. The data may further bepresented as a three dimensional plot. The presentation of data is however not limited tothe above described forms. An operator may assess the presented data. The presented data may further be automatically assessed.

Figure 5 discloses principles of quantum efficiency deterrnination for a photovoltaicdevice. A light source 510, such as a light emitting diode, LED, illuminates a photovoltaic device. The reflectance from the photo voitaic device is measured in a first photo 12 detector 530, When the photovoitaic device is partiy transparent, transmittance of thephotovoitaic device is deterrnined in a second photo detector 540. When the íiiuminationpower from the Eight source 510 is known, externa! quantum efficiency, EQE, can hedeterrnined from the photocurrent for the waveiength of the iiiurninating light. iQE, canbe determined when the refiectance and/or transrnittance of the photovoitaic device has been estabiished.

According to an aspect of the invention, the photovoitaic testing device, as iiiustrated inFigure 1, inciudes first and/or second photo cietectors for cietermining refiectance and/ortransniittance of the photoeiectric device. The anaiysing means are in accordance withthis aspect of the invention, arranged to perform the anaiysis to determine EQE as weii as iQE. in the drawings and specification, there have been disciosed exernpiary aspects of theinvention. Hovvever, many Variations and modifications can he made to these aspectswithout suhstantiaiiy departing from the principies of the present invention. Thus, thedisciosure shouid be regarded as iiiustrative rather than restrictive, and not as beingiimited to the particular aspects discussed above. Accorciingiv, although specific terms areempioyed, they are used in a generic and descriptive sense oniy and not for purposes of iimitation.

Claims (15)

13 CLAiMS

1. A photocurrent testing device (109) for testing photovoltaic devices (120) such as solarcelEs, cornprising:a piurailty of puisating Eight sources (116), - a signal generator (lštl) which generates for each Eight source (110) anindividual driving frequency, such that each Eight source (110) puisates at anindividual frequency corresponding to its driving frequency, - an arnpiifier unit (140) which coliects output current from the photovoitaicdevice (120) and ampiifies the output current, - analysing means (150) for analysing the amplitude of the output current ateach individual frequency, Whereih the output current at each individualfrequency originates from the Eight source puisating at the same individualfrequency, characterised in that each light source (110) iilumlnates a specific spot (116) on the photovoltaic device, such that the amplitude of the output current at each individual frequency is matched with a corresponding specific spot (116) on the photovoltaic device.

2. The photocurrent testing device according to claim 1, wherein the light sources (110) are light ernitting diodes (LED).

3. The photocurrent testing device according to any of the precedlng ciaims, vvherein the Eight sources (119) are adapted to have the sarne light emitting spectrum.

4. The photocurrent testing device according to any of the preceding clairns, wherein the light sources (110) are arranged in a single row.

5. The photocurrent testing device according to clairn 4, wherein the row of light sources covers the entire width of the photovoltaic device to be tested. 14

6. The photocurrent testing device according to any of the preceding ciairns, vvherein thelight sources (110) are arranged in a matrix that iilurninates the full solar ceEi or parts of the soiar celi.

7. The photocurrent testing device according to any of the preceding ciairns, wherein theiight sources (119) are arranged to move relative to the photovoitaic device (120) in one direction.

8. The photocurrent testing device according to any of the preceding ciaims, vvherein theindividuai driving frequencies are eveniy distributed within a predetermined frequency intervai.

9. The photocurrent testing device (100) according to any of the preceding ciairns, wherein the arnpiifier unit (140) is a Eock-in ampiifier unit.

10. The photocurrent testing device according to any of the preceding ciairns, furtherincluding first and/or second photo detectors (530, 540) for determining the refiectanceand/or transrnittance of the photovoltaic device, and wherein the anaiysing means (150)is arranged to receive input on the refiectance and/or transmittance of the photovoitaicdevice and to determine external quantum efficiency, EQE, and! or interna! quantum efficiency, iQE, based on this input.

11. A method for testing a photovoitaic device, comprising the steps of: - generating an individual driving frequency for each of a piuraEity of Eightsources, - driving each of the iight sources with its individual driving frequency such thateach Eight source puisates at an individuaE frequency corresponding to itsdriving frequency, - iiiuminating the photovoitaic device such that each Eight source iiiuminates a specific spot on the photovoitaic device, coiiecting output current frorn the photovoitaic device and ampiifying thecurrent,ahah/sing the ampiitude of the output current at each individuai frequency,wherein the output current at each individuai frequency originates from theEight source puisating at the same individuai frequency, and ~« matching the amplitude of the output current at each individuai frequency with a corresponding specific spot on the photovoitaic device.

12. The method according to ciairn 11, further comprising the step of creating a photocurrent map of the photovoitaic device.

13. The method according to ciaim 11 or 12, further comprising the step of assessing the quaiity of the photovoitaic device.

14. The method according to any of the preceding ciaims, further comprising the step ofdetermining externai quantum efficiency and/or interna! quantum efficiency of the photovoitaic device.

15. Computer program for executing the steps of the method according to ciaims 11-14.