New physical insights of surfaces/ interfaces for bilayer organic solar cells

• Main Authors: JayChang, 張傑
• Other Authors: Wei-Yang Chou
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/91058065079500299977

博士 === 國立成功大學 === 光電科學與工程學系 === 100 === Organic solar cells have been receiving significant attention for research and development studies in the world, not only green energy trend, but also they have advantage of low cost and wide applications. The special characteristic of organic semiconductors: (I) organic molecules can be designed with a great diversity of properties; (II) Low cost due to easy fabrication: low temperature, large area, roll to roll printing, plastic substrates, flexible…; (III) Materials and processing are green. Although, organic solar cells efficiency is still behind non-organic solar cells, but they are with a great potential to be mass production for practical applications. This article focus on the studies of surface and interface physics in the small-molecule organic solar cells (OSCs) system. We analyze the film surface and interface structure compare optical and electrical properties with them. Its purpose is research of the micro-structure and physical mechanism to understand the components of the basic photoelectric mechanism and effective method to improve the efficiency, and to control the technology for the stable devices. All OSCs for testing were fabricated on ITO-covered glass substrates. The ITO substrates were coated with poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) films, which act as hole transport layers. Then, the active layers of the OSCs were constructed by molecular beam depositing a heterojunction of the pentacene and perylene tetracarboxylic derivatives (PTCDI). The investigations in this thesis include four parts. The brief descriptions of four parts are as follows, (I) This study demonstrated oxygen (O2) plasma treatment on ITO surface makes the surface energy of ITO match that of PEDOT:PSS solution, resulting in a uniform distribution of PEDOT:PSS on the ITO substrate, which decreases the bulk resistance and roughness of PEDOT:PSS film. Pentacene films on a smooth PEDOT:PSS surface exhibited better crystallization, which decreases the recombination losses and reduce the number of defects within the device. (II) We use a simple and effective process of ultraviolet (UV)-irradiation performed on thin PEDOT:PSS films to enlarge the conjugate length of PEDOT polymer-chains that underpin the notable changes in bulk film conductivity and the ionization potential for different UV-irradiated PEDOT:PSS conditions. The ionization potential of the PEDOT:PSS film can be tuned by the UV-irradiation to form a gradient band that can simultaneously match the highest occupied molecular orbital of active layer and work function of O2 plasma-treated ITO anode for the OSCs. (III) A standard OSCs embeded with PEDOT:PSS gratings enable carriers to move toward electrodes, resulting in a threefold enhancement of efficiency. Especially, the visualization of mophologies for these heterojunction layers reveals pillar-like grains that are induced by geometric effect of PEDOT:PSS gratings. (IV) A 2% efficient bilayer OSC consisting of pentacene and is fabricated. The morphology of PTCDI-C7 interestingly follows pentacene due to the matched surface energy of these two active layers and the easily deposited PTCDI-C7 monomers on the inclines of the pentacene grains. This condition results in the low trap states in the PTCDI-C7 film and at the pentacene/PTCDI-C7 interface for the enhancement of exciton dissociation and carrier transport. The research of four parts can improve the power conversion efficiency of OSCs; the detailed data are listed in the article.