Improvement of Quantum Efficiency of Amorphous Silicon Thin Film Solar Cells by Using Nanoporous PMMA Antireflection Coa
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Improvement of Quantum Efficiency of Amorphous Silicon Thin Film Solar Cells by Using Nanoporous PMMA Antireflection Coating Liang Fang, Jong San Im, Sang Il Park and Koseng Su Lim Division of Electrical Engineering, School of Electrical Engineering and Computer Science, KAIST, 335 Gwahangno, Yuseong-gu, Daejeon 305-701, Republic of Korea ABSTRACT The enhancement of optical transmittance at the air/glass interface of amorphous silicon thin film solar cells was shown by application of a nanoporous polymethyl methacrylate (PMMA) antireflection (AR) coating. The PMMA coating was prepared by spin coating of PMMA solution in chloroform in the presence of a small amount of nonane. Because of the difference of the vapor pressure of chloroform and nonane, phase separated structure formed after complete evaporation of both of them during spin coating process. The Corning 1737 glass with the AR coating has high transmittance near 95% from 450-1100nm wavelengths. The amorphous silicon solar cells with the nanoporous PMMA AR coating realize an improvement in quantum efficiency (QE) up to 4% in 450-650nm spectral regions. Introduction Recently, antireflection coatings have been widely employed in optical and optoelectronic fields, and most importantly, for solar cells application. Antireflection (AR) coatings play a vital role in a wide variety of optical technologies by enhancing light transmittance at interface. For glass and common plastics, refractive index (n) is within the range of 1.45-1.7. As a result, reflection forms 4% to more than 6.5% of normally incident light from air-medium interface [1]. Until now antireflection coatings for solar cells application have been concentrated on inorganic material. These thin dielectric films with a low refractive index can enhance transmission via the destructive interference of the reflected light at the air/film and the film/glass interfaces. With the current trend of technology moving towards optically transparent polymer media and coatings, nanoporous polymer AR coatings have attracted wide attention. Because polymer materials have relatively simple and economical fabrication process compared to inorganic materials, they also can be easily coated on a large area and flexible substrate. In the case of a single layer AR coating, two criteria must be satisfied: a film thickness must be a quarter of a reference wavelength in the optical medium, and the refractive index n f of the film material must be satisfied by the equation n 0 /n f =n f /n s , here n 0 , and ns are the refractive indices of air, and
substrate, respectively. Film thickness can be easily met. However, the refractive index of the film poses a problem, because the ns of Corning 1737 glass is around 1.52. Based on above equation, the ideal n f of an AR film should be 1.23. But the smallest n f of available dielectric material is 1.38 of MgF2. Therefore, any dense film can’t satisfy the ideal value. Instead of a homogeneous layer, a nanoporous film can be used. When the pore size is much smaller than th
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