A system that combines the advantages of fast global electroluminescence (EL) measure-ments and highly detailed spectral EL meas-urements is presented. A Si camera-based EL system is used to measure the intensity of radi-ative recombination of the PV device spatially resolved over its full area. A monochromator-based system is then used to measure local-ised emission spectra at specific points of interest identified, as such as defects and cracks. The first measurement results of a mc-Si and an a-Si PV device show good agreement with reported behaviour of such devices and high-light the potential to distinguish between differ-ent defect types and reveal performance changes that would be missed using camera-based EL only

Karl Bedrich (1257453)

Martin Bliss (1250019)

Ralph Gottschalg (1247661)

Tom Betts (1258395)

Xiaofeng Wu (1254888)

English

Loughborough University Institutional Repository

Draft version, Presented at the PVSAT-10 conference, 23-25 April 2014, Loughborough, UK   201 Spatially and Spectrally Resolved Electroluminescence Measurement System for PV Characterisation  M. Bliss*, X. Wu, K. Bedrich, T.R. Betts, R. Gottschalg Centre for Renewable Energy Systems Technology (CREST), School of Electronic, Electrical and  Systems Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK *Corresponding Author Tel.: +44 1509 635327, Fax: +44 1509 635341, Email: M.Bliss@lboro.ac.uk   Abstract A system that combines the advantages of fast global electroluminescence (EL) measure-ments and highly detailed spectral EL meas-urements is presented. A Si camera-based EL system is used to measure the intensity of radi-ative recombination of the PV device spatially resolved over its full area. A monochromator-based system is then used to measure local-ised emission spectra at specific points of inter-est identified, as such as defects and cracks.  The first measurement results of a mc-Si and an a-Si PV device show good agreement with reported behaviour of such devices and high-light the potential to distinguish between differ-ent defect types and reveal performance changes that would be missed using camera-based EL only.  1 Introduction Electroluminescence (EL) imaging of photo-voltaic (PV) devices has in recent years be-come an invaluable tool for both quality control in mass production and in research & develop-ment in new devices and materials. EL imaging measures the radiative recombination of the device under test (DUT) under forward bias by using a camera. This can highlight problem ar-eas of a DUT even if it is initially performing as it should by showing minute defects as such as cracks, breaks and shunts. Furthermore, EL measurements can be used to elucidate the spatial distribution of minority carrier lifetime and diffusion length and parasitic resistances [1-3]. The main advantages of this measure-ment method are that it takes little time, and is non-destructive. The spectral distribution of the emission is lost during standard EL imaging, but this can pro-vide vital characteristics of the radiative recom-bination mechanisms. Localised, spectrally re-solved EL retains the spectral content of the emitted light from the DUT, but costs measure-ment time. Even though such measurements are much slower, the additional information is useful for determining the quality and material composition of the solar cell junction. It can also be used to probe non-uniformities in the materi-al composition and differentiate between differ-ent defect types that cannot be easily distin-guished using camera-based EL measure-ments.  This paper presents part of work at CREST looking into changes in PV device quality during aging. To gain further insight into the processes involved, a spectrally resolved EL measurement system in combination with a camera-based, global EL imaging system (Figure 1) has been developed.  The aim of the combined meas-urement system is to utilise the speed of global EL imaging to identify areas of interest, as such as possible defects, and to measure those spe-cific areas using spectrally resolved EL to ex-tract maximum useful information from the emission.   Figure 1: Combined EL imaging and spectral EL measurement system In the following, the measurement system configuration and applied measurement meth-ods are described in detail. First measurements of a multi-crystalline silicon (mc-Si) and amor-phous silicon (a-Si) solar cell are presented and compared against reported behaviour. To con-clude, the system optimisation process is dis-cussed.  2 Measurement System 2.1 Global EL Global EL is measured using a 16bit Apogee Alta F-series camera with an 8.3 mega pixel silicon CCD detector. The detector is actively cooled to -10°C to reduce thermal bias currents and to stabilise the spectral response. Depend-ing on the size of the DUT, different optics are used. Normal optics allow for a better ac-ceptance of light and thus a better signal-to-Draft version, Presented at the PVSAT-10 conference, 23-25 April 2014, Loughborough, UK   202 noise ratio, while macro optics are used to fo-cus on much smaller samples with a minimum focused image size of 34x26mm giving a reso-lution of ~10µm. The optics used are without zoom function to maintain good light collection efficiency and short integration times. Instead, the camera is mounted on a guide rail to adjust the distance to the DUT. Filters to reduce ef-fects of unwanted stray light are not used as the enclosure build is light tight, but can be at-tached to aid particular measurements, for ex-ample of diffusion length [2] . To remove the background signal, images are taken with and without sample excitation and subtracted. This also largely reduces the impact of defective bright pixels of the detector, of which the remainders are removed using a 3x3 median filter with threshold applied (i.e. if the pixel value exceeds the median of the sur-rounding pixels by more than the threshold, its value is set to the median). 2.2 Spectrally resolved EL The system to measure spectral EL emission uses a dual-grating SPEX-270m monochroma-tor with two in-house developed detectors, one Si detector for ultraviolet (UV) to near infrared (NIR) (300-1050nm) and one InGaAs detector for the infrared (IR) (1050–1700nm). The total measurement range is from 0.73eV to 4.1eV. Both detectors are temperature controlled to -20°C to reduce noise, stabilise detector spectral response and to reduce thermally induced self-excitation. The photon-induced detector current is internally amplified by a factor 1 billion (1E+9) and measured using a 16bit NI-DAQ card. To further reduce measurement noise and errors in the digitalisation process, the detectors are low pass filtered at ~3kHz and measured with a 250kHz sampling frequency. The bandwidth of the monochromator has been measured at ap-proximately ±10nm width half maximum. The optical input for spectral EL is currently a direct fibre coupling to the sample with a measure-ment spot size of approximately 3mm diameter, albeit development work is ongoing to change this to a microscope-like input with a spot size of ~100µm. Spectrally resolved EL can be measured us-ing 2 different methods. The first and most commonly used method is the AC mode in which the sample is exited with a forward or reverse current or voltage at a specific rectan-gular pulse frequency (ranging from 10 to 250Hz). The system utilises a digitally pro-grammed lock-in technique to extract the measurement signal at that specific frequency from the background detector white noise and other signal fluctuations due to external influ-ences. The total noise voltage after a 500ms AC measurement interval is ~ 5µVrms (equiva-lent to 5 fArms). The second method applied is the DC mode in which the device under test is energised using a constant excitation. This makes the measurements more prone to exter-nal influences as such as noise, but allows for measuring slow responding devices such as dye-sensitised solar cells.  2.3 System Integration The system is capable of measuring small-area samples, i.e. mini-modules and laboratory samples, of a size of up to 100mm x 100mm. The sample size is currently limited by the pelti-er-based temperature controlled sample plate. The controller can regulate the temperature of the sample from 15°C to 95°C (limited due to the room dew point). As visible in Figure 2, the light-sealed equip-ment enclosure additionally houses an XY-stage on which the DUT with the temperature plate is placed. This is used to move the sam-ple between measurements of global EL and localised spectral EL and to change the meas-urement point during spectral measurements. CameraXY-StageSampleFibre Input Figure 2: Interior of the measurement system showing sample area, EL camera and spectral EL fibre input A Keithley 2045 is used as a 4-wire source unit. An additional switching circuit is used to provide the AC square wave excitation for measuring localised spectral EL emissions. 3 Initial Measurements A single cell amorphous Silicon (a-Si) and a multi-crystalline silicon (mc-Si) mini-module de-vice have been measured to test the functionali-ty and as an initial validation of the measure-ment system. 3.1 Multi-crystalline Silicon The mc-Si device measured is a commercially available mini-module containing 9-cells in se-ries. Its overall size is 60x60mm. Figure 3 shows the global EL image of the device with an injected current of 100% of the short circuit current (ISC) of the device meas-ured at standard test conditions (STC) and at 20% of the ISC value. Note that the emission Draft version, Presented at the PVSAT-10 conference, 23-25 April 2014, Loughborough, UK   203 profile is more uniform at low excitation current. This is due to the stronger influence of series resistance at high current which induces a high operating voltage shift in the area around the busbars. This large series resistance at the same time reduces the current flow to the areas between the busbars and thus the emitted light due to radiative recombination.   Figure 3: EL images of mc-Si single cell; left: ex-citation current at 1xISC; right: current at 0.2xISC; images are normalised, in absolute terms the right image is much darker due to the lower injec-tion current The darker lines within the cells in Figure 3 that are not straight (i.e. not associated with the busbar and fingers) are difficult to distinguish between intrinsic defects (crystal boundaries) and extrinsic defects (breakage, cracks) using a Si CCD camera. However, by measuring the localised EL emission spectra one can distin-guish between them.  0E+05E-71E-62E-62E-63E-60.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4Spoectral EL [arbitrary]Photon energy[eV]Pos 1Pos 2Pos 3 Figure 4: Localised spectral EL measurements of mc-Si sample at different points as indicated As shown in Figure 4, spectral emission has been measured at three points of interest. All curves show the main emission peak at ~1.1eV or 1150nm. This peak is at the indirect bandgap of c-Si and is consistent with what is reported [4]. Furthermore, one can see a second peak at 0.78eV (1600nm). This peak is identical to what has been reported in [4, 5] and originates from the intrinsic deep traps in the grain boundaries of the silicon crystals.  Due to the spectral emission measurement spot being relatively large it was not possible to measure the emis-sion of just the grain defects. Instead the aver-age emission over a large area (3mm diameter) was measured. 3.2 Amorphous Silicon The a-Si sample measured is a large area la-boratory single cell sample with an area of 90x60mm. This cell was chosen as it shows visible discoloration along the cell length. The cell is darker in the middle and less dark on the outer sides, which is a good indicator of non-uniform performance distribution throughout the device.   Figure 5: normalized EL images of a-Si single cell; left: excitation current at 1xISC; right: current at 0.2xISC; the bright side of the cell indicates the positive contact As one would expect form such a large thin-film single cell, it suffers from a significant se-ries resistance loss over the width of the cell between the contacts. Thus, at 1x ISC current injection (left of Figure 5) the signal is very high at the positive side but drops of sharply to a very low single at the negative contact. At 0.2x ISC the signal drop is still visible, but not as strong. Apart from the large series resistance, the cell has a small defect in the low middle part, which is more noticeable in the low excita-tion current image (right of Figure 5). However, since the global EL intensity distribution is uni-form along the length of the cell (top to bottom of Figure 5) changes due to discoloration are not noticeable. 0E+05E-81E-72E-72E-73E-70.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5Spectral EL [arbitrary]Photon energy [eV]Left Centre Right Figure 6: Spectral EL measurements of a-Si sample at different points as indicated 23 1Draft version, Presented at the PVSAT-10 conference, 23-25 April 2014, Loughborough, UK   204 As detailed in Figure 6, the spectral emission of the cell has been measured at points along the positive contact side of the cell. It is noticed that the main spectral emission peak shifts be-tween the measurements on the left to the right of the cell from ~1.05eV to 0.9eV (1170nm - 1370nm). A second peak is visible on the right side emission spectrum at 1.2eV (1040nm). The emission spectrum is in relative terms al-most constant across the width (direction posi-tive to negative contact) of the cell, and only changes in absolute signal strength (results not shown). The EL emission peak of a-Si is not at its effec-tive band gap which would be at ~1.55eV (no measurable signals were detected during this work). Instead the dominant recombination and thus the emission peak is at much lower photon energy. In principle the shape of the emission is comparable with reported behaviour of hydro-genated a-Si:H [6, 7]. However, the emission is dependent on many factors as such as temper-ature, i-layer thickness and material composi-tion (i.e. hydrogen content). At the point of writ-ing the exact cause of the shift in the emission spectrum has not been determined.  To note is that a second sample of the same type, showing no discoloration, was measured and exhibited no change in emission spectrum. This was similar to the right side emission curve in Figure 6. 4 System Development At the time of writing the system, even though functional, was to some extent still in the opti-misation phase. One of the next steps in the project is to change the optical input at the DUT from a direct fibre coupling to a microscope op-tic. An approximate spot size of 100µm (de-pendent on wavelength) will ensure that details as such as defects and grain boundaries can be measured accurately. To be able to keep a good signal to noise ratio, the optical efficiency of the monochromator and fibre input will have to be improved at the same time. The localised spectral response measure-ments had an initial calibration applied. Howev-er, to improve measurement accuracy light spectra as well as linearity will have to be cali-brated fully. Nevertheless, the high sensitivity of the detectors makes this a challenging task. Calibration will also have to be applied to the EL imaging camera in the form of uniformity and linearity. This should also correct for the signals of most defective pixels. Finally, to gain an insight into the measure-ment uncertainty, the measurements need to be validated with, and compared against other EL imaging and spectrally resolved EL measure-ment systems. 5 Summary The set-up of a combined Si-CCD camera-based EL imaging and monochromator-based spectral EL emissions measurement system has been described. Furthermore, details of the next steps in the optimisation of the system are detailed, with the most important improvements being in the optical set-up of the spectrally re-solved EL system. Initial measurements of a mc-Si and a-Si de-vice show a comparable agreement with re-ported behaviour. This is an important first evaluation step of measurement quality and accuracy and means the system can be used for characterising radiative recombination. Measurement results also highlight the ad-vantage of using a combination of a fast EL im-aging technique with detailed EL emission spectrum measurements. Investigating defects and other points of interest is possible with much greater detail and one can further distin-guish between different types of defects and gain more information on the material composi-tion and uniformity, details that otherwise might have been missed. Acknowledgements This work has been supported by a joint UK-India initiative in solar energy through a joint project ‘Stability and Performance of Photovol-taics (STAPP)’ funded by Research Councils UK (RCUK) Energy Programme in UK (contract no: EP/H040331/1) and by Department of Sci-ence and Technology (DST) in India. References [1] T. Fuyuki and A. Kitiyanan, "  Applied Phys-ics A, vol. 96, pp. 189-196, 07/01, 2009.  [2] P. Würfel, et.al., J.  Appl.  Phys. 101(12), 2007. [3] B. Li, et.al., Prog Photovoltaics Res Appl, vol. 20, pp. 936-944, 2012.  [4] T. Fuyuki, et.al., in Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE, 2010, pp. 001380-001382. [5] K. Bothe, et.al., Journal of Applied Physics, vol. 106, pp. 104510-104510-8, 2009.  [6] K. Wang, et.al., J.  Non Cryst.  Solids, vol. 164–166, Part 1, pp. 595-598, 12/2, 1993.  [7] D. Han, et.al., J.  Appl.  Phys. 80(4), pp. 2475-2482. 1996.  

Spatially and spectrally resolved electroluminescence measurement system for PV characterisation

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Spatially and spectrally resolved electroluminescence measurement system for PV characterisation

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