I/V MEASUREMENT OF HIGH CAPACITANCE CELLS WITH VARIOUS METHODS P.-R. Beljean** (corresponding author), A. Lo*, M. Despeisse*, Y. Riesen*, Y. Pelet**, V. Fakhfouri**, N. Wyrsch* and C. Ballif* *Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering IMT, Photovoltaics and Thin Film electronics laboratory, Breguet 2, 2000 Neuchâtel, Switzerland. Phone +41 (0) 32 718 33 23, Fax +41 (0) 32 718 32 01, e-mail :
[email protected] **Pasan SA, Rue Jaquet-Droz 8, 2000 Neuchâtel, Switzerland. Phone +41 32 391 16 00 / Fax +41 32 391 16 99 / e-mail :
[email protected] ABSTRACT: Most of state-of-the-art equipment for I/V characterization and power rating of PV modules implement xenon flashers as the light generating part. These solar simulators are characterized by high-intensity and short duration single or multi-pulses. For more than 30 years this was found very suitable, allowing for a high accuracy in PV modules power rating. However, with the increase of crystalline silicon solar cells efficiency and the strong development of technologies such as hetero-junction solar cells, the pulse duration and consequently the I-V sweeping time become a limitation. This paper presents a review of the impact of the I-V sweeping time duration on the measurement of hetero-junction, monocrystalline and thin film silicon solar modules, and explores some ways to overcome this problem. KEYWORDS: Solar Simulator, Flasher, Long-pulse, Accuracy, HIT, HJT, Transient, Artefact.
1. INTRODUCTION From an electrical point of view, the main characteristic of high-efficiency crystalline silicon solar cells and modules, such as hetero-junction photovoltaic devices, is the high diffusion capacitance presented by the cell under normal operating forward voltages. This capacitance increases for increasing forward voltage, and introduces charge storage effects which prevent rapid changes of voltage, as they then lead to measurement artefacts which depend on the I-V sweep speed and direction [1-5]. For fast sweeping, the charge or discharge of the diffusion capacitance during the measurement will respectively lower or inflate the measured currents for forward (increasing voltage) or reverse (decreasing voltage) directions of the I-V sweep. This high capacitance effect is a direct consequence of the increased performance of the solar cells, with typically increased effective minority carrier lifetime, resulting from improved bulk-material, rear passivation, but also improved front side properties. Any lab or manufacturer has therefore to carefully take this problem into account by applying measurement strategies such as double-sweep, multiple-segments measurements, long pulse measurements or alternative methods [6-10]. In this paper, we present a review of the performance variations of hetero-junction a-Si/c-Si, “standard” diffused mono-crystalline and thin film silicon solar modules with respect to the sweeping time and direction, using a state-of-the-art Pasan flasher and a newly developed LED based flatbed sun simulator [10]. 2. METHOD High efficiency and state-of-the-art crystalline silicon solar modules together with a thin-film silicon module were characterized in this study using two different solar simulators: a Pasan SunSim 3b xenon flasher and an especially designed prototype (1 x 1 m2) flatbed solar simulator developed at IMT [10]. The latter implements a mix of LEDs and halogen lamps, allows for long-pulses and demonstrates an AAA classification according to the IEC standards (details on its characteristics can be found
in [10]). Small modules with tabbing were tested instead of bare cells, so that we do not face the problems of contacting cells that can be a cause of errors in measurements. Thanks to different manufacturers, small modules from two different hetero-junction solar cells manufacturers could be tested, with VOC above 700 mV. In addition standard mono-crystalline silicon modules (with cells VOC around 630 mV) and one micromorph mini-module were tested for comparison. Table I. List of the modules characterized with a Pasan SunSim 3b flasher and an especially designed hybrid LED/halogen flatbed sun simulator [10]. Modules 1-2 3-4 5-6 7
Manufacturer A B C D
Technology Hetero-junction Hetero-junction Mono-crystalline Micromorph
Each of the different modules was measured with the following procedures: - under long illumination pulses (~ 2 seconds of total flash duration), at I-V sweeping speeds of 10 ms, 20 ms, 50 ms, 100 ms, 200 ms, 500 ms and 1 s, in direct and reverse directions, using the flatbed simulator developed at IMT [10]. Special care was taken to minimize modules temperature drifts caused by the large internal heating. - under 10 ms flashes using a Pasan SunSim 3b, with single flash (10 ms) and multiple-flashes to allow for 20 ms, 50 ms, 100 ms and 200 ms I-V sweeping times with a similar number of acquisition points, in direct and reverse directions. - under dark conditions, with different sweeping times of 10 ms, 20 ms, 50 ms, 100 ms, 200 ms, 500 ms and 1 s, in direct and reverse directions, using a Pasan Cell Tester active load and translation to illuminated I-V curve using previously measured ISC under
single-flash and a simple model (this method is further investigated by SUPSI, [7]). 3. EXPERIMENTAL RESULTS The different module performances were analysed as a function of the duration of the I-V sweep, both for a forward sweep direction (increasing voltage, referred to as –D in Figures 1-3) and a reverse sweep direction (decreasing voltage, referred to as –R in Figures 1-3). The incidence of the measurement duration on the module parameters VOC, ISC and MPP (maximum power point) are presented, as determined from the flatbed simulator (referred to as Table in Figures 1-3), the flasher and the dark measurements. For each simulator type and each module parameter, relative data are presented: the parameter value at a given sweep duration is divided by the value averaged over the different sweep durations, to assess parameter variations with the sweep speed.
Overall, no impact of the measurement duration is observed while the slightly higher variations in the table measurements can be attributed to thermal variations. Similar reproducible VOC performance over the different scanning times is shown for the micromorph thin film silicon tandem module (Figure 1 bottom), with ± 0.1 % variations both for the flasher and for the flatbed simulator. Please note that for this particular module, the multiflash testing has been limited to 50 ms, as no further change is expected and because this test is very timeconsuming. For the hetero-junction modules, as shown as example for the module 3 in Figure 1, systematic small variations of about ± 0.5 to 1 % can be observed between the 10 ms measurement and measurements with a duration > 50 ms, for the flatbed simulator as well as for the flasher and the dark conditions measurements. For sweeping times > 50 ms, similar VOC are again measured (< ± 0.5%).
3.1. Incidence of I-V sweep time on VOC 3.2. Incidence of I-V measurement duration on ISC A similar VOC is measured for the mono-crystalline silicon module for the different sweep times (Figure 1, top): a low variation of ± 0.1 % is measured for the flasher, and ~ ± 0.5 % for the flatbed simulator, at the exception of one measurement at 10 ms sweep time under reverse direction, which can be explained by thermal variation.
Figure 1: VOC variations of the mono-crystalline silicon module 5 (top), of the hetero-junction module 3 (middle), and of the micromorph module 7 (bottom, take care to different Y scale) for 10 ms to 1 s I-V sweep duration as measured using the 3 above-mentioned procedures.
Very similar ISC values (< ± 0.2 %) were measured for all module types, both when using the flatbed simulator and the flasher, as well as in dark conditions. This shows that down to 10 ms for the full I-V measurement, the sweep time and direction have no incidence on ISC rating for the 3 silicon technologies under study.
Figure 2: ISC Variations of the monocrystalline silicon module 5 (top), of the hetero-junction module 1 (middle) and of the micromorph module 7 (bottom) for 10 ms to 1 s I-V sweep duration as measured with the flasher, the table (IMT flatbed [7]) and in dark conditions.
This is consistent with a low cell capacitance governed by the junction capacitance in short circuit current conditions, in comparison to an exponentially increasing high cell capacitance (in particular for the hetero-junction cells) under forward voltage due to the increasing diffusion capacitance [2-5]. For high-efficiency solar cells, precise ISC can thus be determined even from an IV curve measured in 10 ms, and consequently from a single xenon flash. In addition, reproducible ISC measurements are demonstrated for the thin film silicon tandem module for the use of the flatbed simulator, showing a wellcontrolled spectrum (micromorph modules ISC being sensitive to blue and/or red shifts in the spectrum) and intensity of the illumination over the numerous measurements carried out in this study. 3.3. Incidence of I-V measurement duration on MPP Finally, the incidences of the I-V measurement sweep time on MPP are detailed in Figure 3, keeping similar scales for the 4 modules relative MPP data.
10 ms to 1 s I-V sweep duration as measured with the flasher, the table (IMT flatbed [7]) and in dark conditions. The discharge of the cell capacitance and therefore the reverse direction measurement is shown to lead to more pronounced transient effects than the direct measurements. For each hetero-junction, very similar transient effects are measured using the flasher, the flatbed and the corrected dark conditions measurements, showing that pre-illumination, which is occurring during the measurements with the flatbed and not with the flasher or in dark conditions, does not impact the transient behaviour of the measured hetero-junction modules. One remarkable feature is that dark measurements, made with a simple electronic load without illumination, show very similar behaviour, opening the way for composite measurements consisting of ISC determination under single flashlight and I/V curve recording, with no time limitation, under dark conditions.
Figure 4: Relative variations between MPP measured in forward and in reverse directions in hetero-junction modules, expressed in %, for different I-V sweep durations. Hetero-junction modules 1 and 3, which are from different manufacturers, have similar cells VOC and similar transient behaviour with a similar minimum sweep time of about 120 ms. Hetero-junction module 4 implements cells with a higher VOC (+ 20 mV), and exhibit stronger transient effects, with a minimum sweep time of about 300 ms. From Figure 3, it can be seen that the variations with the sweep time is quite similar for the hetero-junction modules 1 and 3, which are respectively from two different manufacturers, A and B. This is clearly confirmed by direct comparisons in Figure 4 of the relative difference between the MPP measured in the forward and in the reverse directions, referred to as ΔMPP in the following. Interestingly, the perfect match between the two modules transient behaviour correlates with similar VOC measured for the cells in these two modules coming from different manufacturers. In order to provide an accurate measurement, one can consider that ΔMPP must be below ± 0.5 %. This condition is met for heterojunction modules 1 and 3, which have similar cell VOCs, for similar sweeping times above 120 ms, even though they were developed by different manufacturers.
Figure 3: MPP variations of the monocrystalline silicon module 5 (top), of the hetero-junction modules 1 and 3 (middle) and of the micromorph module 7 (bottom) for
Similar analysis of the relative MPP difference between forward and reverse direction ΔMPP was conducted on the hetero-junction module 4, which implements cells with a higher VOC than the cells in modules 1 and 3 (typically ~ 20 mV more per cell). Results compared in Figure 4
clearly demonstrate more pronounced transient effects on this module than on modules 1 and 3, with a minimum sweep time of about 300 ms to allow for ΔMPP < ± 0.5 %. This can be explained by higher effective minority carrier lifetime in the cells of module 4, yielding an increased VOC, and an enhanced diffusion capacitance, which in turn results into more pronounced transient artefacts during the measurement. These final results show that by further enhancing the cells performance, the measurement artefacts get even more pronounced, and imply the use of further enhanced minimum sweeping time, clearly showing that the transient behaviours in high efficiency module power rating will get more and more pronounced in the future. This problem also arises for very high efficiency back-contact cells, will also start to arise for diffused solar cells with enhanced VOC, following the introduction of selective emitters and backside passivation. 4. DISCUSSION & CONCLUSIONS The characterizations of mono-crystalline silicon module and thin film silicon tandem module have shown that a similar MPP is measured in direct and reverse directions and for sweeping durations down to 10 ms. Long sweeping times (> 120 ms for modules 1 and 3 and > 300 ms for module 4) were then demonstrated to be needed for an accurate measurement of the hetero-junction modules. Long-pulse is the method of choice, but can be very sensitive to thermal errors, due to the heating of the module in test. However, quite stable VOC measured in this study even for modules on the flatbed simulator, showing that a good thermal control can be achieved with these small area modules. Multi-flash can be conducted and was shown to give accurate results for laboratory use. However it is difficult to implement in production lines. Some strategies may be used with sweeping limited to regions of interest like MPP and VOC to reduce the required number of flashes. Dark I/V measurement plus translation shows interesting potential to overcome dynamic effects as this can be measured without the need of a light source. However, as some of the cells parameters are influenced by illumination, determination of translation parameters may be tricky (see [7]). Other methods like those proposed by Sinton [4] have not been evaluated in this paper. They also seem to be sensitive to some adjustment parameters. 5. ACKNOWLEDGEMENTS The authors thank Vincent Trachsel, Valentin Chapuis and Laure-Emmanuelle Perret-Aebi for the encapsulation and preparation of the mono-crystalline and thin film silicon modules, Corinne Droz for careful relecture of this paper, and the Swiss commission for technology and innovation (KTI) for funding under project no. 10880.2. 6. REFERENCES [1] D.L. King, J. M. Gee and B.R. Hanson. Measurement precautions for high-resistivity silicon solar cell, in Proc. of the 20th IEEE PVSC, pp. 555-559, (1988). [2] G. Friesen and H. A. Ossenbrick. Capacitance effects in high-efficiency cells, Solar Energy Materials and Solar Cells 48: 77 – 83, (1997). [3] W. M. Keogh, A. W. Blakers, and A. Cuevas. Constant voltage I-V curve flash tester for solar cells,
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