Analysis of moisture-induced degradation of thin-film photovoltaic module
Introduction
Solar cell technology has been expected to provide a solution for global energy security and low-carbon society in the near future. Among the many solar cell technologies developed, silicon crystalline solar cell has provided high efficiency and field-proven reliability since 1950s. The solar cell market is now dominated by the silicon technology: crystalline silicon solar cell accounts for more than 90% of the global market [1]. Silicon solar cell has the laboratory-scale conversion efficiency up to 26.1% for mono- and 22.8% for poly-crystalline wafer [2]. Average module efficiencies in the market are 18% for mono- and 17% for poly-crystalline silicon wafer [1]. But growing large crystals of pure silicon is still a very energy-intensive process and requires large amount of material supply. The price of silicon solar cell module depends much on the prices for raw silicon [3].
Thin-film solar cell technologies reduce the amount of material required in creating the active material of solar cell. CuInxGa1-xSe2 (CIGS) is one of the most promising thin-film technologies which could replace the current crystalline silicon solar cell. The first thin-film CIGS solar cell was created with a conversion efficiency of 4.5% in 1976 [4]. The average production efficiencies of CIGS modules are currently between 12% and 15% for commercial modules with leading efficiencies of over 16% [5] and a record efficiency in the lab of 23.4% [2]. Advantages of CIGS compared with silicon solar cell technologies include the high energy yield per material, low temperature coefficient of power loss, low sensitivity to shadowing, and short energy payback time. CIGS has the potential for lowering cost of manufacturing and deploying more economically viable system installations compared to silicon based technologies [6].
Despite the strengths and advantages of CIGS technology, its reliability is not yet competitive. Recent study [7] has shown that thin-film technology has 1.5% per year of degradation rate even though the rate is getting better, while silicon solar cell module has 0.7% per year. Many research papers have explored the degradation mechanism of CIGS and other thin-film techniques [[8], [9], [10], [11], [12], [13], [14], [15]]. Thin-film solar cell technique such as CIGS has been known to be sensitive to moisture that can corrode the module, which is accelerated by exposure to a hot and humid environment [[16], [17], [18], [19], [20], [21]]. To effectively protect the moisture ingression into the module, glass-to-glass structure has been a typical selection for the thin-film technologies.
To enhance the reliability of the thin-film solar cell technologies, it is required to analyze and understand the moisture-induced degradation extensively. Many studies have been focused on the analysis of the degradation of transparent conductive oxide (TCO) layer of the thin-film module structures. However, studies on other layers and quantitative comparison of the layer degradation effect on the power of module are still insufficient.
In this study, the moisture induced degradation of glass-to-glass CIGS module has been comprehensively analyzed. CIGS modules are fabricated and tested under damp-heat conditions with periodical measurement of the electrical characteristics in Section 2. Individual layers are investigated experimentally after reviewing literature and the moisture induced degradation is discussed in Section 3. Using the electrical circuit modeling of the module and extracting the model parameters with a proposed method, the degradations are analyzed in Section 4. Finally, the degradation rates are modelled and the effect of model parameters degradation on the power of module are quantitatively compared in Section 5. The results contribute to the understanding and improving the reliability of glass-to-glass thin-film technologies under hot and humid environments.
Section snippets
Module design
To investigate and analyze the effects of moisture ingression into a CIGS solar cell, mini-sized CIGS modules were fabricated in a monolithic glass-to-glass module by a manufacturer. CIGS absorber layers were sputtered onto 155 × 165 mm2 soda-lime glass substrate with a sputtered molybdenum (Mo) backside contact. The front contact of the solar cells consisted of a Boron-doped zinc oxide (BZO) deposited by a metal-organic chemical vapor deposition (MOCVD) method. Buffer layer between BZO and
Previous studies on CIGS module degradation under damp heat
The reason of the degradation of CIGS module under hot and humid environment has been studied by many previous researches. After exposing the Al-doped zinc oxide (AZO) and Mo layer in a damp heat to discover the failure mechanism of CIGS module, Feist et al. [24] proposed the hydration of the AZO layer may results in increased series resistance, leading to loss of power generation, whereas Mo degradation is considered a non-lifetime-limiting failure. Pern et al. [25] and Dhaka et al. [26] also
Parameter extraction method
The circuit modeling of photovoltaic (PV) module is an essential step in the analysis of the performances and characterization of PV system. It can also help to explore the mechanism and mode of the module degradation. A solar cell can be modelled as an equivalent electric circuit in the so-called diode model. One-diode model which is represented by Eq. (1) has been widely used for the modeling of CIGS solar cell [[29], [30], [31], [32]]:where I and V are the
Linear regression of the degradation rate of the diode model parameter
The linear tendency of shown in Fig. 16(a), in Fig. 19, and and the reciprocal of logarithm of in Fig. 17 implies the constant degradation rate in the metal layers, over layers, and the absorber layer, respectively. With the constant degradation rates, the diode model parameters under a hot and humid condition can be mathematically modelled in the form ofwhere , , , and represent the constant
Conclusion
Previous studies have suggested that the hydration of TCO layer resulting in increased series resistance and leading to loss of power generation is the main cause of the degraded CIGS module performance under damp-heat condition. But this study suggests that the power degradation of CIGS module under the damp-heat environment is more affected by the enhanced recombination of the absorber layer than the degradation of the metal layer or the leakage over layers.
To confirm the argument, the
CRediT authorship contribution statement
Changwoon Han: Conceptualization, Methodology, Software, Validation, Investigation, Visualization, Data curation, Writing - original draft, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was supported by National Research Foundation (NRF, Grant number: 2018R1D1A1A 02086246) and Defense Acquisition Program Administration (DAPA, Grant number: UD180018AD) of Republic of Korea.
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