Conjugated Polymer Engineering for All-solution Processed Organic Solar Cells

Abstract

Organic solar cells (OSCs) have versatile features such as solution processability, printable device, transparency, and flexibility, thereby OSCs are desired for practical applications such as building-integrated photovoltaics (BIPVs), alternatives for window films, and glass tinting films. For OSCs to be used as those applications, they should accent their advantages, especially transparency and flexibility. Except for electrodes, all layers of OSCs already have the features of semi-transparent and flexible. For this, intensive studies have been focusing on the development of flexible transparent electrodes (FTEs) for various device architectures. Among various candidates of FTEs, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) is one of the most prominent materials due to high transmittance in the visible range, good mechanical properties, solution processability. Since it has relatively low electrical conductivity compared to indium tin oxide (ITO) that is broadly used as transparent conductive electrodes (TCEs), PEDOT:PSS has been hard to develop for the device as electrodes. In this thesis, we have successfully demonstrated various types of PEDOT electrode-based flexible and semitransparent OSCs (ST-OSCs). Chapter Ⅰ describes a basic understanding of the fields of conducting polymers, including the conjugated systems and OSCs. First, the brief science of the doping process in conjugated polymers is introduced. Second, the theoretical backgrounds of OSCs based on bulk-heterojunction (BHJ) are presented. Third, the operating mechanism and structures of OSCs are explained. Chapter Ⅱ presents the development of hole transport layer (HTL) free OSCs based on a work function tunable FTEs. In this chapter, one of the perfluorosulfonic acids (PFSAs), ethanesulfonyl fluoride, 2-[1-[difluoro-[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoro-ethoxy]-1,1,2,2,-tetrafluoro-, with tetrafluoroethylene also known as Nafion is blended with the PEDOT:PSS for tuning the work function of electrodes. The films made with the blending of PEDOT:PSS and Nafion (P:N) are treated with concentrated sulfuric acid for a well-ordered crystalline structure in P:N electrodes. We fabricated HTL free OSCs with P:N films as an anode. We confirmed the electrical and morphological properties of the P:N films with the various ratio of Nafion. Chapter Ⅲ shows a novel method to transfer the PEDOT electrodes through interface engineering for the flexible and ST-OSCs. We analyzed the optical and mechanical characterization of PEDOT electrodes as the top electrode of the devices. The PEDOT electrodes have high transmittance in the visible region (380~780nm), excellent flexibility, and high electrical conductivity so they are suitable for flexible and ST-OSCs. In addition, full devices showed high average visible transmittance (AVT) and good stability in air. Chapter Ⅳ offers a sequential printing method to fabricate the PEDOT top electrodes by using one of the PFSAs, heptadecafluorooctanesulfonic acid, as a solvent in the post-treatment process. With this electrode, we fabricated all-solution processed ST-OSCs. It was confirmed that the heptadecafluorooctanesulfonic acid not only enhances the conductivity of the PEDOT electrode but also has no other effect in the photoactive layer which is underneath the PEDOT electrode. Furthermore, we demonstrated that this device structure has the identical structure as the window films and meets the values of rejection for replacing window films, providing high values of ultraviolet (UV, 300~380nm) rejection, infrared (IR, 780~2500nm) rejection, and total solar energy rejection (TSER, 300~2500nm), respectively, with high power conversion efficiency (PCE). Chapter Ⅴ provides a comprehensive summary of the work and conclusions drawn in this thesis.