Design of Nanostructured Optical Layers for Efficient Light Trapping

DoctorThis thesis explores ways to design of nanostructured optical layers for efficient light trapping in organic solar cells (OSCs). OSCs have much attention for realizing next-generation photovoltaics, because of their full potential as solution printing, lightweight, flexibility and low-cost. However, incommensurate absorption length (~100 nm) and exciton diffusion length (~10 nm) of organic materials have remained a major issue to enhance the power conversion efficiency (PCE) in OSCs. One way to improve light trapping is to employ the various nanostructures, which can change behavior of light in thin optical layers. However, the maximum achievable absorption and optimal nanostructures are still open questions. The work in this thesis focuses on design of novel optical layers, optimization of nanostructures, and theoretical analysis with aim of efficient light trapping in OSCs. In the first part of thesis (chapter 1, 2 and 3), I introduce the basic concepts of OSCs and research trends of light trapping technology for highly efficient OSCs. Fundamental optical modeling tools are presented including characteristic matrix method and rigorous coupled wave analysis. In chapter 4 and 5, I propose the novel structure of polymer/metal/dielectric (PMD) and dielectric/metal/polymer (DMP) as transparent conducting electrodes. Even though the dielectric/metal/dielectric (DMD) multilayer was successfully employed as a transparent electrode in optoelectronic devices, there are some drawbacks such as dewetting of thin metal layer, peak transparency at specific dielectric thickness, and planar structure by physical deposition. In this research, I find that the transmittance insensitive to the thickness could be achieved by empolying the low refractive index of polymer. It brings about excellent transparency without absorption loss even in nanostructured PMDor DMP layers. Also well-ordered nanopatterns can be easily adopted to electrodes becaus of easy structuring and excelleng flexibility of soft materials. In chapter 6, I demonstrate wavelength-scale structures of inverted hexagonal-pyramid on polymer as a haze film for broadband and omnidirectional light harvesting in OSCs. Subwavelength-scale structures have discovered unique antireflection properties in broad solar spectral range. However, the reflectance decreases by only 2 ~ 4%, which could not guarantee effectively enhanced PCE in OSCs. In this research, I have introduced a powerful design for effective light trapping using a wavelength-scale structured films. Through the calculation of devices with different geometry of structures, we identify the roles of wavelength-scale structures in light trapping. In chapter 7, I demonstrate branch shape of indium thin oxide (ITO) nanostructures as an antireflection layers in application to organic solar cells. The three-dimensional ITO nanobranches can be easily fabricated on the front side of glass using a one-step and scalable approach based on electron beam deposition. The optimized structures exhibited excellent broadband antireflection properties in the visible wavelength region