Using advanced lithography to construct nanostructures for enhancing light harvesting in solar cells

• Main Authors: Shang-Yu Chuang, 莊尚餘
• Other Authors: 陳學禮
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/37837940514439643533

博士 === 臺灣大學 === 材料科學與工程學研究所 === 98 === This thesis aims to study the improvement of light harvesting in various kinds of solarcells. Three parts in photovoltaic devices are needed to be considered for increasing light harvesting: (i) electrode transmission (Chapter 2, 3, 4); (ii) device surface and interface reflection (Chapter 5, 7); (iii) active layer absorption (Chapter 6, 7). In this study, several advanced lithography technologies have been achieved successfully for efficient light harvesting in solar cells: nanoimprint in metal(NIM), reversal imprint of metal(RIM), and colloidal lithography(CL). Two dimension(2D) and three dimension (3D) of periodic structures can be fabricated by NIM, RIM and CL, including nanomesh, corrugated metal films, 3D metal caters, metal pyramids and moth-eye structures. In the nature, metals possess intrinsically excellent conductivity and applicable flexibility. However, metals demonstrate low transmittance that can not satisfy the requirements of being a good transparent electrode. We fabricated the nano-architectures on metal film to reduce the absorption of light in metal and further increase the transmittance. Besides, nano-architectures of metal film have potential on transparent electrode due to the extraordinary transmission, (EOT) which arises from surface plasmons resonance (SPR) on the metal surface. The EOT of nano-architectures of metal were discussed in Chapter 2 to 4.The electric field propagating through corrugated metal films exhibits localized maxima at the tips of the corrugations. Moreover, a strong cavity effect that enhances light transmission was displayed in 3D metal craters. Furthermore, by coupling with a destructive interference antireflection coating, the metal nanomesh structure performed a maximum transmittance of ca. 75% in the visible regime and ca. 60% in near infrared ray regime whereas maintained a low sheet resistance maintains of 8.74 Ω/□, which was about 0.05 to 0.3 times of that for ITO- or single walled carbon nanotubes (SWNT)-based electrodes. To reduce the device interface reflection, nano-architecturing technology of CL and directly imprinting on flexible film were developed to fabricate optimized moth eye antireflection structures. By performing a refractive index gradient between two optical media, we were able to achieve low reflection (~1%) with such graded index of moth eye structures. To know external quantum efficiency (EQE) in organic solar cells well, we discussed optical properties of active layer based on P3HT:PCBM hybrid films in Chapter 6, 7. When a flexible solar cell was bent or illuminated under large incident angle of light, the optical anisotropy of P3HT:PCBM and interface reflection led to EQE loss. It was noticeable that a bent organic solar cell would experience incoming light of various incident angles. We have characterized the optical anisotropy of high- and low-RR P3HT/PCBM hybrid films before and after their thermal annealing. After annealing to 120°C, the degrees of anisotropy of the 90.2%- and 96.7%-RR P3HT:PCBM blends were represented by values of k‖/k⊥ of 1.09 and 1.59, respectively. The results indicated that when the incident TM-mode light propagated into a severely bent solar cell with large incident angles, its oscillating direction of electric field became perpendicular to the main chain of P3HT and resulted in a low light absorption because of the low k⊥ in the active layer. The TE-mode light would be reflected at substrate/air interface at oblique incident angle. Both two factors of optical anisotropy in P3HT:PCBM and interface reflection strongly influenced device efficiency when the device was under bending operation. We used nanoimprint technique to fabricate antireflection structure on PC substrates, and effectively decreased the reflectance of TE-mode light from 40% to below 10% at the incident angle of 70° and successfully reduced the light loss for a bent solar cell.