System performance loss due to LeTID

Transcription

1 Available online at ScienceDirect Energy Procedia 00 (2017) th International Conference on Silicon Photovoltaics, SiliconPV 2017 System performance loss due to LeTID Friederike Kersten a *, Fabian Fertig a, Kai Petter a, Bernhard Klöter a, Evelyn Herzog a, Matthias B. Strobel a, Johannes Heitmann b, Jörg W. Müller a a Hanwha Q CELLS GmbH, Sonnenallee 17-21, Bitterfeld-Wolfen, Germany b TU Bergakademie Freiberg, Institute of Applied Physics, Leipziger Straße 23, Freiberg, Germany Abstract If not adequately suppressed, Light and elevated Temperature Induced Degradation (LeTID) has been shown to cause severe degradation of multicrystalline (mc-si) silicon solar cells and modules with passivated emitter and rear cell (PERC). Within this work, the system performance of LeTID-sensitive mc-si modules is investigated when operated in temperate and mediterranean climates, and a correlation to predict the observed field performance based on accelerated laboratory testing is presented. Severe degradation induced by LeTID of up to 7% in maximum output power is detected after one thousand hours of laboratory testing, which is shown to correspond to ~ 3 years of field installation time in Cyprus. In contrast, LeTID-sensitive modules installed in Germany show a degradation of 2.5% within the same time period. Due to lower irradiance values and module temperature in Germany the LeTID rate is lower than in Cyprus. A significant dependence of system performance loss and energy yield due to LeTID on installation site is shown. Hence, the loss in system performance due to LeTID affects one-to-one the levelized cost of electricity (LCOE) compared to reference without LeTID. Furthermore, it is shown that LeTID in the field can be suppressed by applying Hanwha Q CELLS Q.ANTUM technology, independently of installation site The Authors. Published by Elsevier Ltd. Peer review by the scientific conference committee of SiliconPV 2017 under responsibility of PSE AG. Keywords: mc-si PERC, light-induced degradation * Corresponding author. Tel.: +49 (0) address: mailto:f.kersten@q-cells.com The Authors. Published by Elsevier Ltd. Peer review by the scientific conference committee of SiliconPV 2017 under responsibility of PSE AG.

2 1. Introduction In 2012, mc-si solar cells with plasma-enhanced chemical vapor deposited (PECVD) aluminum oxide (AlO x) passivated rear side have been reported to potentially degrade more than their Czochralski silicon (Cz-Si) counterparts at elevated temperatures [1]. Furthermore, mc-si aluminum back-surface field (Al-BSF) solar cells have been shown to degrade more at elevated temperatures [2]. Both studies excluded the observed behavior to be due to boron-oxygen (BO) defect formation or iron-boron (FeB) pair dissociation, which has been speculated to be the root cause of the light-induced degradation (LID) observed at lower temperatures in mc-si in previous studies, e.g. [3-8]. Since then, many groups have investigated the degradation of mc-si, confirming cells with dielectrically passivated rear side to be significantly more affected than Al-BSF solar cells [9], showing local inhomogeneities of the defect characteristics [10-12], and more recently, ways to mitigate excessive LeTID [13-20]. As the observed LID is significantly more pronounced at elevated temperatures, Hanwha Q CELLS has introduced the term LeTID for "Light and elevated Temperature Induced Degradation" [14] as denomination for this new degradation mechanism at the time. If not suppressed, LeTID can lead to a loss in relative conversion efficiency of more than 10% [14]. LeTID is relevant under field conditions [14] and, therefore, needs to be suppressed to enable mass production of mc-si solar cells with PERC technology. A methode to suppress LeTID in mass production of dielectrically passivated solar cells and modules has been developed by Hanwha Q CELLS [14] as part of its Q.ANTUM technology [21]. In this work, performance loss measurements for LeTID-sensitive modules in system installations at temperate and mediterranean climates and a correlation between field operation time and accelerated laboratory testing are shown. 2. Observations of LeTID on Module Level in the Laboratory and Field 2.1. Experimental LeTID-sensitive PERC modules were manufactured on standard industrial mc-si substrates. Furthermore, neighbored mc-si substrates were used to process modules with Q.ANTUM technology [21]. The module degradation experiments in the laboratory were performed at a temperature of 75 C in climate chamber, with excess carriers injected by current (CID, current-induced degradation). The current was set to a value to simulate the excess charge carrier density during operation at maximum power point (MPP) and 1 sun illumination, which was previously proposed as standard accelerated test conditions for LeTID [14]. With this method, a high number of modules could be tested in parallel at moderate cost in the laboratory. LeTID-sensitive PERC and LeTID-suppressing Q.ANTUM modules were installed on outdoor test fields in Thalheim, Germany (DE) and Nicosia, Cyprus (CYP). The irradiance (G mod) in module plane and module temperature (T mod) were logged and averaged every 5 min. The energy yield of each system was averaged in 15 min intervals. All night values were eliminated by using a data filter which excluded all values with G mod < 10 W/m 2 due to increased measurement uncertainties. Once every three months, the modules installed in Germany and Cyprus were removed from the system racks and then tested by means of current-voltage measurements under standard test conditions (STC) in the laboratory. After each testing cycle, the modules were reinstalled outdoors Outdoor behavior of LeTID-sensitive modules at temperate and mediterranean climate Fig. 1 shows the annual irradiance distribution measured at the module installation sites in Germany and Cyprus. Each G mod bar shows the sum of irradiated energy for one year in 100 W/m 2 steps. For Thalheim, located in temperate climate compared to mediterranean climate in Cyprus, lower irradiance values with a maximum of 150 kwh/m 2 for the intervals 700 W/m 2 < G mod < 1000 W/m 2 are measured. For G mod values under 400 W/m 2 and, therefore, comparably low-light conditions, the modules operate for ~ 30% of all operation time in Germany. In Nicosia, the highest irradiated energy value of 434 kwh/m 2 was measured in the range of 900 W/m 2 to 1000 W/m 2. As shown in Fig. 2, these relatively high irradiance values around 1000 W/m 2 in Cyprus resulted in comparably high module temperatures. In contrast to the depicted irradiation profile in Fig. 1, the module temperature measured in Nicosia showed a Gaussian distribution curve. The weighted average T mod of 35 C to 40 C is shifted to higher temperatures compared to Thalheim (5 C to 10 C). In Nicosia, elevated temperatures with T mod > 50 C occured for ~ 25% of field operation time per year.

3 Annual Sum of Irradiated Energy [kwh/m 2 ] Thalheim (DE) Nicosia (CYP) Irradiance G mod [W/m²] Fig. 1. Annual irradiance distribution of irradiated energy measured in module plane in Thalheim (DE) and Nicosia (CYP). Annual Thalheim (DE) Nicosia (CYP) Fig. 2. Annual module temperature distribution measured in Thalheim (DE) and Nicosia (CYP) Module Temperature T mod [ C] Hence, these climatic conditions in Cyprus are well-suited for the characterization of LeTID in comparison to the temperate climate in Germany with annually ~ 70% of field operation time with T mod < 25 C Comparison of field data and accelerated degradation tests in the laboratory As illustrated in the previous subsection, the real irradiation in module plane and module temperatures over several months and years at two different test fields in Cyprus and Germany were measured. Together with a correlation for the irradiance and temperature dependence of LeTID kinetic, a test-site-specific time constant corresponding to the proposed accelerated laboratory test performed at 75 C is deduced; e.g. 290 h laboratory test correspond to 1 year of field installation time in Cyprus. Fig. 3 shows as filled symbols the relative module power loss due to LeTID for LeTID-sensitive PERC and LeTID-suppressing Q.ANTUM outdoor-tested modules in Cyprus and Germany, referring to the lower x-axis (field operation time t Field). Furthermore, the corresponding values for degradation tests in the laboratory are shown as open squares. The time-dependent relative module power loss under laboratory testing at 75 C with CID in MPP mode refers to the upper x-axis (t Lab). A similar degradation behavior was determined for outdoor and laboratory tests, see Fig. 3. LeTID-sensitive PERC modules showed a significant loss in module power of ~ 7% due to LeTID, both in the field and under accelerated aging in the laboratory. Equivalently installed modules in Germany showed a degradation of ~ 2.5% for the same time period. Due to lower irradiance values and module temperature in Germany the LeTID rate is lower than in Cyprus. Fig. 4 shows the relative loss of specific yield on the left and LCOE on the right y-axis to reference system during 3 years of field installation in Cyprus. The loss in system output power due to LeTID shown in Fig. 3 affects one-to-one the energy yield and the LCOE compared to a reference system without LeTID. It can be seen that the LeTID-suppressing Q.ANTUM modules show a better specific yield and revenue than the reference system. The lower specific yield after approximately 2 years field installation time results from a soiling of the Q.ANTUM system in Cyprus, which was cleaned at the end of October. As shown in Fig. 3-4, applying the optimised defect engineered Q.ANTUM process, LeTID could be fully suppressed in the field. Additional Fig. 5 shows no degradation of Q.ANTUM modules also in the extended laboratory test up to a laboratory time of several thousand hours. With the developed model and test site specific time constant the laboratory degradation time of 4300 h can be calculated to correspond to 15 years of field installation in mediterranean climate.

4 Rel. Module Power Loss due to LeTID [%] Fig. 3. Relative module power loss due to LeTID during laboratory degradation time (upper x-axis) and field operation in Thalheim (DE) and Nicosia (CYP) (lower x-axis). 4. Conclusion Lab Degradation Time t Lab Friederike Kersten et al. / Energy Procedia 00 (2017) Fig. 4. Loss in specific yield and LOCE relative to reference due to LeTID during field operation in Nicosia (CYP). The presented results show for the first time performance loss measurement due to LeTID in system installations in different climates. Long-term degradation tests in a climate chamber with LeTID-sensitive PERC modules resulted in performance loss of up to 7% and the same was observed in mediterranean climate in Cyprus in the first 3 years after installation. Equivalent modules installed in Germany showed a degradation of 2.5% within the same time period. Rel. Module Power Loss due to LeTID [%] LeTID-sens. PERC Lab Q.ANTUM Lab LeTID-sens. PERC Field DE Fig. 5. Time-dependent relative module power degradation due to LeTID. The lower x-axis corresponds to real field operation time in Nicosia (CYP) and the upper x-axis to CID laboratory time in MPP mode at 75 C in climate chamber. Rel. Loss to Reference in Specific Yield [%] Lab Degradation Time t Lab LeTID-sens. PERC Lab Q.ANTUM Lab soiling of system Rel. Loss to Reference in LCOE [%]

5 This demonstrates the high LeTID susceptibility of mc-si PERC modules without LeTID suppression in the field when being installed in areas with high module temperature such as in mediterranean climates. Furthermore, it is shown that the observed degradation behavior in the field can be adequately predicted by using a test site specific time constant, which is easily adaptable to any climate zone. Hanwha Q CELLS Q.ANTUM modules are shown to suppress LeTID, both during outdoor installation in the field and in long-term degradation in a climate chamber. With the determined time constant the laboratory degradation time can be calculated to correspond to 15 years of field installation in mediterranean climate. Beyond that, the benefit of Q.ANTUM technology in energy yield and LCOE were shown in field installation. 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