In this study, we suggest a new method for preparing of an ethylene-vinyl acetate copolymer (EVA) film including metal nanoparticles and its application for efficiency enhancement of crystalline silicon solar cells.
Along with the increases of fossil-fuel consumption and greenhouse gas emissions, studies related to the development of clean and renewable energy systems have been attracting considerable interest in recent decades. Among all the renewable energy systems, photovoltaic systems have been attracting widespread attention because they have significant advantages including the avoidance of pollutant emission, silent operation, long lifetime, and stability. However, while photovoltaic systems are promising, their power efficiency is significantly low in comparison with the efficiency of commercial energy sources; this fact is impeding their widespread use. The biggest reason for the low power efficiency of these systems is the limited light absorption property of photovoltaic cells.
To overcome this issue, one promising solution is the deposition of metal nanoparticles on the top surface of photovoltaic cells; this deposition leads to a light scattering effect due to the presence of metal nanoparticles. These approaches can be used to increase the number of light pathway inside the active region, thereby significantly enhancing the light absorption of the photovoltaic cell. In this regard, there have been many studies on method of exploiting metal nanoparticles for light trapping in photovoltaic cells.
Although the previous works successfully demonstrated the efficacy of the light trapping using metal nanoparticles, the thermal deposition method is plagued by low-throughput and the physical casting method is inevitably restricted by unwanted metal nanoparticle aggregation that yields non-homogeneous coverage with metal nanoparticles.
In this study, we suggested a new method that allows a uniform deposition of homogeneously dispersed metal nanoparticles on the top surface of a photovoltaic cell in a simple, cheap, and high-throughput manner. Such homogeneous dispersion of metal nanoparticles allows maximizing the light scattering effect at top surface of a photovoltaic cell, thereby effectively enhancing the efficiency of photovoltaic cell.
In chapter 2, we have developed ethylene-vinyl acetate copolymer resins in the high-pressure autoclave reactor on a mass production scale, highly suitable for crystalline silicon solar cell encapsulating materials. Even though it was polymerized in the autoclave reactor, owing to be designed for having narrower molecular weight distribution than the competitor’s autoclave products, it showed slightly higher mechanical properties sufficient for photovoltaic application. Also the EVA copolymer with high vinyl acetate contents we obtained showed excellent performance on thermal stability at high temperature of $160 \circ C$ and long term reliability at $85 \circ C$ in 85 % in relative humidity for 2,000 hours enough for the very thin (160μm thick) crystalline silicon photovoltaic cells.
In chapter 3, we have introduced silver nanoparticles (diameter ~100 nm) on solar cell encapsulating film to enhance the photovoltaic performance of the c-Si photovoltaic cells. The light scattering effect gradually increased with increase of AgNP content due to light diffraction around the AgNPs. When AgNPs of content 0.02 wt% was used, the power conversion efficiency of c-Si solar cells increased the most, from 16.19 % to 16.72 %. The AgNPs incorporated new type encapsulating film induced the enhancement of short circuit current density and power conversion efficiency, which is presumed to be due to the light scattering effect from the AgNPs, causing an increase in the effective optical path length. Above 0.02 wt%, the scattering effects of the films gradually decreased with AgNP content. We believe that this was attributed to the increased both reflection and interference of incoming light owing to the higher loading of AgNPs. Therefore, the AgNP content was optimized at 0.02 wt%.
In Chapter 4, in order to achieve the homogeneous dispersion of AgNPs, we have developed a new method of introducing a maleic anhydride grafted polypropylene (PPMA) as a compatibilizer onto EVA/AgNPs nanocomposite film. This was accomplished via (i) fabricating AgNP-dispersed EVA thin film from a homogeneous EVA/AgNPs mixture and (ii) subsequently laminating the top surface of a photovoltaic cell. Homogeneous mixing of AgNPs and EVA was achieved through the use of PPMA as a compatibilizer. Owing to a good dispersion of the AgNPs, the resulting EVA/AgNPs/PPMA thin nanocomposite film shows a significantly high light scattering effect; therefore, with the introduction of the EVA/AgNPs/PPMA thin nanocomposite film, containing both 0.02 wt% AgNPs and 5 wt% PPMA, to the photovoltaic cell, the current density was increased from 38.51 mA to 39.13 mA, and the total power conversion efficiency was effectively enhanced from 16.19 % to 17.16 %. These results clearly demonstrate the importance of the homogeneous AgNPs dispersion for enhancing the efficiency of photovoltaic cells. Furthermore, our proposed system is highly suitable for mass production processes because the EVA/AgNPs thin nanocomposite film fabrication and its thermal lamination are carried out in a continuous serial process. Therefore, on the basis of its simplicity and efficacy, our proposed method will provide important progress in the field of photovoltaic systems.