Wei Luo
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serlw@nus.edu.sg
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Publication Elucidating potential-induced degradation in bifacial PERC silicon photovoltaic modules(WILEY, 2018-10-01) LUO WEI; Hacke, Peter; Terwilliger, Kent; LIANG TIAN SHEN; WANG YAN; SEERAM RAMAKRISHNA; ABERLE,ARMIN GERHARD; KHOO YONG SHENG; Dr Wei Luo; ELECTRICAL AND COMPUTER ENGINEERING; SOLAR ENERGY RESEARCH INST OF S'PORE; MECHANICAL ENGINEERINGCopyright © 2018 John Wiley & Sons, Ltd. This paper elucidates the behavior and underlying mechanism of potential-induced degradation (PID) on the rear side of p-type monocrystalline silicon bifacial passivated emitter and rear cell (PERC) photovoltaic modules. At 50°C, 30% relative humidity, and −1000 V bias to the solar cells with aluminium foil on the rear glass surface, the rear-side performance of bifacial PERC modules at standard testing conditions degraded dramatically after 40 hours with a 40.4%, 36.2%, and 7.2% loss in maximum power (Pmpp), short-circuit current (Isc), and open-circuit voltage (Voc), respectively. The front-side standard testing condition performance, on the other hand, showed less degradation; Pmpp, Isc, and Voc dropped by 12.0%, 5.2%, and 5.3%, respectively. However, negligible degradation was observed when the solar cells were positively biased. Based on I-V characteristics, electroluminescence, external quantum efficiency measurements, and the effective minority-carrier lifetime simulation, the efficiency loss is shown to be caused by the surface polarization effect; positive charges are attracted to the passivation/antireflection stack on the rear surface and reduce its field effect passivation performance. Extended PID testing to 100 hours showed an increase in device performances (relative to 40 hours) due to the formation of an inversion layer along the rear surface. In addition, replacing ethylene-vinyl acetate copolymer with polyolefin elastomer films significantly slows down the progression of PID, whereas a glass/transparent backsheet design effectively protects the rear side of bifacial PERC modules from PID. Furthermore, PID on the rear side of bifacial PERC modules is fully recoverable, and light greatly promotes recovery of the observed PID.Publication Investigation of Potential-Induced Degradation in n-PERT Bifacial Silicon Photovoltaic Modules with a Glass/Glass Structure(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2018-01-01) LUO WEI; KHOO YONG SHENG; JAI PRAKASH; JOHNSON KAI CHI WONG; WANG YAN; ABERLE,ARMIN GERHARD; SEERAM RAMAKRISHNA; Dr Wei Luo; ELECTRICAL AND COMPUTER ENGINEERING; SOLAR ENERGY RESEARCH INST OF S'PORE; MECHANICAL ENGINEERING© 2017 IEEE. Potential-induced degradation (PID) in n-type passivated emitter, rear totally diffused (n-PERT) bifacial crystalline silicon photovoltaic modules with a glass/glass structure is investigated. From front-side measurements, a significant loss in the short-circuit current (Isc ) and a relatively smaller loss in the opencircuit voltage (Voc ) and fill factor (FF) are observed due to PID. A similar degradation behavior is observed from the rear side, except that there is negligible change in Isc . External quantum efficiency and photoluminescence measurements reveal that the losses in Isc and Voc are most likely due to an increase in the front surface recombination. FF loss analysis and two-diode model fitting demonstrate that the FF loss is mainly attributed to an increased recombination in the space charge regions. Moreover, n-PERT bifacial silicon modules also suffer from PID when they are stressed from the rear side. Furthermore, some ethylene-vinyl acetate and polyolefin, which show high PID-resistance to conventional p-type technologies, are found to be not as effective in preventing PID in n-PERT technologies. However, a PID-free n-PERT bifacial module design is possible with the application of the sodium-free glass. Finally, the progression of PID is heavily dependent on the bias voltage and stress temperature.Publication Investigation of the Impact of Illumination on the Polarization-Type Potential-Induced Degradation of Crystalline Silicon Photovoltaic Modules(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2018-09-01) LUO WEI; Hacke, Peter; Hsian, Saw Min; WANG YAN; ABERLE,ARMIN GERHARD; SEERAM RAMAKRISHNA; KHOO YONG SHENG; Dr Wei Luo; ELECTRICAL AND COMPUTER ENGINEERING; SOLAR ENERGY RESEARCH INST OF S'PORE; MECHANICAL ENGINEERING© 2011-2012 IEEE. Accelerated potential-induced degradation (PID) testing of photovoltaic modules is conventionally conducted in the dark and at high temperature and humidity levels without considering the influence of illumination. This study investigates the impact of illumination on the polarization-type PID (PID-p) on two different types of encapsulated (glass/backsheet) crystalline silicon solar cells: 1) n-type bifacial passivated emitter rear totally diffused (the front side is facing glass and PID-stressed); and 2) p-type bifacial passivated emitter and rear cell (the rear side is facing glass and PID-stressed). The samples are stressed under the conditions of -1000 V, 40 °C, and 40% relative humidity and at different irradiance levels (xenon lamps). While the type-A modules show no reduction in PID-p sensitivity under illumination up to 800 W/m2, PID-p in the type-B modules is arrested by the light at an irradiance level as low as 10 W/m2. Furthermore, PID-degraded type-B modules (degradation induced in the dark) exhibit a rapid recovery (full recovery in 20 min) upon exposure to light (40 W/m2). External quantum efficiency measurements on the type-B modules show that ultraviolet from 300 to 400 nm is mainly responsible for the fast recovery.Publication Correction: A review of crystalline silicon bifacial photovoltaic performance characterisation and simulation (Energy & Environmental Science (2019) DOI: 10.1039/c8ee02184h)(Royal Society of Chemistry, 2019) Liang T.S.; Pravettoni M.; Deline C.; Stein J.S.; Kopecek R.; Singh J.P.; Luo W.; Wang Y.; Aberle A.G.; Khoo Y.S.; MATHEMATICS; ELECTRICAL AND COMPUTER ENGINEERING; SOLAR ENERGY RESEARCH INST OF S'POREThere were some typographical errors in the first paragraph of Section 4.6. Modelling software (pp. 26-33). The text should read: "Lo et al.222 combined the capabilities of three simulation software packages: RADIANCE, SMARTS, and PC1D to model and optimise the performance of a bifacial PV module. The function of RADIANCE has been explained in Section 4.1.2.2. Developed by Gueymard,223,224 SMARTS is an open-source tool for simulating the solar irradiance spectrum based on given inputs such as geographical location, time, and local atmospheric parameters. The output from RADIANCE and SMARTS were imported into PC1D,225,226 which executes the electrical and thermal model. The process is summarised in Fig. 26. References 223-226 should read: 223 C. A. Gueymard, SMARTS2: a simple model of the atmospheric radiative transfer of sunshine: algorithms and performance assessment, Report No. FSEC-PF-270-9, 1995. 224 C. A. Gueymard, Sol. Energy, 2001, 71, 325-346. 225 D. A. Clugston and P. A. Basore, Conference Record of the Twenty Sixth IEEE Photovoltaic Specialists Conference - 1997, 1997, pp. 207-210. 226 S. Hargreaves, L. E. Black, D. Yan and A. Cuevas, Energy Proc., 2013, 38, 66-71. The original references 223-230 should be renumbered 227-234 after these 4 new references have been added. The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers. © 2019 The Royal Society of Chemistry.Publication Photovoltaic module failures after 10 years of operation in the tropics(Elsevier BV, 2021-11-01) Luo, W; Clement, CE; Khoo, YS; Wang, Y; Khaing, AM; Reindl, T; Kumar, A; Pravettoni, M; Mui Koon Tan; SOLAR ENERGY RESEARCH INST OF S'POREThis paper presents a case study of photovoltaic (PV) module failures after over 10 years of operation in the tropical climate of Singapore. Three types of modules (two samples from each type) were analysed: multi-crystalline silicon (multi-Si), mono-crystalline silicon (mono-Si), and copper indium selenide (CIS). Visual inspection revealed several problems, including encapsulant discoloration (to different extents), backsheet yellowing, and soiling among others. Different degradation behaviour of the samples was observed from current-voltage (IV) and electroluminescence characterization. The maximum power of the multi-Si samples degraded by more than 9% (on average), likely due to corrosion around the cell edges. The mono-Si modules suffered a catastrophic power reduction (>40%) that could be ascribed to a combination of encapsulant discoloration, potential-induced degradation (PID), and corrosion. While one CIS module was primarily affected by encapsulant discoloration and possibly corrosion, the other sample also exhibited signatures of PID and experienced about 45% power drop. Furthermore, external quantum efficiency measurements of the multi-Si modules identified cell mismatch and changes to the additives in the encapsulant. Overall, PID, corrosion and encapsulant degradation are found to be the most detrimental degradation processes for PV modules in the tropical climate of Singapore.Publication Investigation of polysilicon passivated contact's resilience to potential-induced degradation(ELSEVIER SCIENCE BV, 2019-06-15) LUO WEI; CHEN NING; KE CANGMING; WANG YAN; ABERLE,ARMIN GERHARD; SEERAM RAMAKRISHNA; SHUBHAM DUTTAGUPTA; KHOO YONG SHENG; Dr Wei Luo; ELECTRICAL AND COMPUTER ENGINEERING; SOLAR ENERGY RESEARCH INST OF S'PORE; MECHANICAL ENGINEERING© 2019 Elsevier B.V. We present clear evidence of excellent resilience to potential-induced degradation (PID) from polysilicon passivated contacts implemented on the rear of n-type solar cells. Under the stress conditions of −1000 V, 50 °C, 30% relative humidity with aluminum foil, no damage was caused to the passivated contact consisting of an ultrathin silicon oxide (SiO x ) film and an n + -doped polysilicon (poly-Si) layer after 168 h. With +1000 V bias and under the same chamber conditions, the SiO x /poly-Si (n + ) passivated contact showed a slight change that translated into about 1% module power loss after 168 h, which is significantly lower than the 5% threshold recommended by IEC 62804-1 PID test standard. Furthermore, the SiO x /poly-Si (n + ) passivated contact, even when encapsulated with ethylene-vinyl acetate copolymer films having a low volume resistivity in the range of 5 × 10 14 Ω⸱cm, exhibited good stability under high-voltage stress. The experimental results were also validated by a generic device simulation, where the SiO x /poly-Si (n + ) stack was shown to be immune to the surface polarization effect. In addition, a promising cell-level solution (i.e., using a stack of aluminium oxide and silicon nitride) to the polarization-type PID for n-type passivated emitter rear totally diffused silicon solar cells was also demonstrated.Publication A comparative life-cycle assessment of photovoltaic electricity generation in Singapore by multicrystalline silicon technologies(ELSEVIER SCIENCE BV, 2018-01-01) Luo, Wei; Khoo, Yong Sheng; Kumar, Abhishek; Low, Jonathan Sze Choong; Li, Yanmin; Tan, Yee Shee; Wang, Yan; Aberle, Armin G; Ramakrishna, Seeram; Dr Wei Luo; ELECTRICAL AND COMPUTER ENGINEERING; SOLAR ENERGY RESEARCH INST OF S'PORE; MECHANICAL ENGINEERING© 2017 This paper presents a comparative life-cycle assessment of photovoltaic (PV) electricity generation in Singapore by various p-type multicrystalline silicon (multi-Si) PV technologies. We consider the entire value chain of PV from the mining of silica sand to the PV system installation. Energy payback time (EPBT) and greenhouse gas (GHG) emissions are used as indicators for evaluating the environmental impacts of PV electricity generation. Three roof-integrated PV systems using different p-type multi-Si PV technologies (cell or module) are investigated: (1) Al-BSF (aluminum back surface field) solar cells with the conventional module structure (i.e. glass/encapsulant/cell/encapsulant/backsheet); (2) PERC (passivated emitter and rear cell) devices with the conventional module structure; and (3) PERC solar cells with the frameless double-glass module structure (i.e. glass/encapsulant/cell/encapsulant/glass). The EPBTs for (1) to (3) are 1.11, 1.08 and 1.01 years, respectively, while their GHG emissions are 30.2, 29.2 and 20.9 g CO2-eq/kWh, respectively. Our study shows that shifting from the conventional Al-BSF cell technology to the state-of-the-art PERC cell technology will reduce the EPBT and GHG emissions for PV electricity generation. It also illustrates that mitigating light-induced degradation is critical for the PERC technology to maintain its environmental advantages over the conventional Al-BSF technology. Finally, our study also demonstrates that long-term PV module reliability has great impacts on the environmental performance of PV technologies. The environmental benefits (in terms of EPBT and GHG emissions) of PV electricity generation can be significantly enhanced by using frameless double-glass PV module design.Publication Membrane-free redox flow cell based on thermally regenerative electrochemical cycle for concurrent electricity storage, cooling and waste heat harnessing of perovskite solar cells(Elsevier BV, 2022-11-15) Zhang, H; Lek, DG; Liu, T; Lin, F; Luo, W; Huang, S; Gao, M; Wang, X; Zhi, Y; Wang, Q; Mui Koon Tan; SOLAR ENERGY RESEARCH INST OF S'PORE; MATERIALS SCIENCE AND ENGINEERINGGlobal climate change requires a significant reduction of greenhouse gas emission through developing sustainable energy technologies such as photovoltaic (PV) devices. However, the inherently variable nature of solar irradiance and consequent electricity generation increase challenges for the system stability of electrical grids. Moreover, the PV devices face significant performance loss under full irradiance conditions with 55–65 °C operating temperature. Herein, we demonstrate a novel solar energy conversion and storage (SECS) system by integrating a perovskite PV device with a low-cost membrane-free Zn/Mn-based redox flow battery (RFB) which has a quite negative temperature coefficient. The proof-of-concept SECS system shows great potential for future large-scale deployment of sustainable energy by three concurrent processes: (1) Liquid electrolyte of RFB provides an effective cooling process for the PV device, mitigating its performance loss at high temperature. (2) Thermally regenerative electrochemical cycle (TREC) allows RFB to generate additional electricity from the low-grade heat gathered from the solar cell at a high absolute thermoelectric efficiency, which adds to the overall system efficiency. (3) RFB timely stores the electricity produced by the solar cell, buffering the fluctuation of solar power, stabilizing the electrical grids, and reducing the energy curtailment.Publication Space-Time Patterns, Change, and Propagation of COVID-19 Risk Relative to the Intervention Scenarios in Bangladesh(MDPI, 2020-02-01) Masrur, Arif; Yu, Manzhu; Luo, Wei; Dewan, Ashraf; Dr Wei Luo; SOLAR ENERGY RESEARCH INST OF S'POREThe novel coronavirus (COVID-19) pandemic continues to be a significant public health threat worldwide, particularly in densely populated countries such as Bangladesh with inadequate health care facilities. While early detection and isolation were identified as important non-pharmaceutical intervention (NPI) measures for containing the disease spread, this may not have been pragmatically implementable in developing countries due to social and economic reasons (i.e., poor education, less public awareness, massive unemployment). Hence, to elucidate COVID-19 transmission dynamics with respect to the NPI status-e.g., social distancing-this study conducted spatio-temporal analysis using the prospective scanning statistic at district and sub-district levels in Bangladesh and its capital, Dhaka city, respectively. Dhaka megacity has remained the highest-risk "active" cluster since early April. Lately, the central and south eastern regions in Bangladesh have been exhibiting a high risk of COVID-19 transmission. The detected space-time progression of COVID-19 infection suggests that Bangladesh has experienced a community-level transmission at the early phase (i.e., March, 2020), primarily introduced by Bangladeshi citizens returning from coronavirus epicenters in Europe and the Middle East. Potential linkages exist between the violation of NPIs and the emergence of new higher-risk clusters over the post-incubation periods around Bangladesh. Novel insights into the COVID-19 transmission dynamics derived in this study on Bangladesh provide important policy guidelines for early preparations and pragmatic NPI measures to effectively deal with infectious diseases in resource-scarce countries worldwide.Publication In-Situ Characterization of Potential-Induced Degradation in Crystalline Silicon Photovoltaic Modules Through Dark I-V Measurements(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2017-01-01) LUO WEI; Hacke, Peter; JAI PRAKASH; CHAI JING; WANG YAN; SEERAM RAMAKRISHNA; ABERLE,ARMIN GERHARD; KHOO YONG SHENG; Dr Wei Luo; ELECTRICAL AND COMPUTER ENGINEERING; SOLAR ENERGY RESEARCH INST OF S'PORE; MECHANICAL ENGINEERING© 2011-2012 IEEE. A temperature correction methodology for in-situ dark I-V (DIV) characterization of conventional p-Type crystalline silicon photovoltaic (PV) modules undergoing potential-induced degradation (PID) is proposed. We observe that the DIV-derived module power temperature coefficient (γdark ) varies as a function of the extent of PID. To investigate the relationship between γdark and DIV-derived module power (Pdark (Ts), measured in situ and at the stress temperature) two parameters are defined: change in the DIV-derived module temperature coefficient (δγdark ) and DIV-derived module power degradation at the PID stress temperature (δPdark (Ts)). It is determined that there is a linear relationship betweenδγdark andδPdark (Ts). Based on this finding, we can easily determine the module γdark at various stages of PID by monitoring Pdark (Ts) in situ. We then further develop a mathematical model to translate Pdark (Ts) to that at 25 °C (Pdark (25 °C)), which is correlated with the module power measured at the standard testing conditions (PSTC ). Our experiments demonstrate that, for various degrees of PID, the temperature correction methodology offers a relative accuracy of ±3% for predicting PSTC . Furthermore, it reduces the root-mean-square error (RMSE) by around 70%, compared with the PSTC estimation without the temperature correction.