Energy Harvesting of Organic Semiconductors Based on Push-Pull Type Small Molecules and Carbon Dots

Abstract

Organic semiconductors have received great attention in the past decades due to their flexible structure, low processing cost and unique physical properties. The organic semiconductors have been used for (opto)electronics such as organic photovoltaic devices (OPVs), organic photodetectors (OPDs), organic light-emitting diodes (OLEDs), electrochromic devices (ECDs) and secondary ion battery, etc. In particular, conjugated small organic molecules have attracted considerable attention in relation to energy harvesting because there has been growing interest in an alternative energy harvesting system due to environmental consequences of global warming and the energy crisis. Furthermore, their photophysical and electrical properties can be controlled through the change of the molecular structure. In this thesis, we report on various methods to improve the efficiency of organic photovoltaics such as OPVs and OPDs based on the characteristics of push-pull-structure organic small molecules and nanocomposites. The (carbon-dots)-Mx+ as a new type of the supporting electrolyte is also described for energy harvesting of ECDs.<br/> Part I provides a brief overview of organic semiconductors and organic photovoltaics. Electronic transitions associated with light absorption and emission and mechanisms for the generation and transport of excitons (carriers) are described. It also provides a brief history of development of OPVs and OPDs, push-pull organic small molecules, and device architectures along with working principles of organic photovoltaics. <br/> Part II deals with the development of high performance organic photovoltaics using push-pull small molecules and hybrid materials such as carbon dot@PEDOT:PSS and colloidal quantum dot-silica composites.<br/> The section I describes the effect of the π–linker on electrochemical, thermal and morphological properties of push–pull type D–π–A molecules (3T and DTT) as donor materials of the photoactive layer, and on the performance of OPV devices. 3T with a terthiophene π–linker showed high electrochemical stability with a faster electrode reaction rate. The solution-processed bulk heterojunction (BHJ) OPV devices based on 3T:PC71BM active layer showed higher PCE values than DTT-based devices because of 3Ts superior electrochemical properties. It appears that the elaborate design of the π–linker is required for the development of high performance D–π–A structure-based OPV devices.<br/> The section II presents a study on three push–pull-structure small molecules as OPD donor materials and the performance of OPD devices. It turned out that the π–linker structure of the small molecules affected the photophysical, thermal and morphological properties. H3 with a short π–linker showed weaker intermolecular interactions and lower π–π packing density than other donor molecules. OPD devices based on H3 exhibited high sensitivity and broadband EQE in both planar heterojunction (PHJ) OPDs and BHJ OPDs. Our results suggest that the π–linker structure of push-pull D–π–A molecules could affect the crystallinity of thin films, and the photoactive layer with amorphous-like domains could lead to high performance OPD devices compared to that with polycrystalline domains. These results provide a strategy for effective donor molecule design as well as for reducing the dark current density (Jd) to achieve high performance OPD devices.<br/> The section III introduces a photocrosslinkable oligothiophene as a hole transport layer (HTL) in OPV devices. The oligothiophene (5T-p) consists of diacethylene as a photocrosslinker and quinquethiophene as a π–conjugated backbone. The 5T-p thin films prepared by spin-casting were photopolymerized through irradiation of 254 nm UV hand lamp. Thin film characterization revealed that the intermolecular packing network in thin films significantly influenced the performance of OPV devices.<br/> The section IV describes the self-assembly of poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) organogel films incorporating carbon dots and their application to OPVs as a hole extraction layer (HEL). The carbon dots act as a physical linker among PEDOT chains through electrostatic interactions, resulting in the formation of core-shell nanostructures. The nanostructures were interconnected to each other to form organogels. The carbon dots affected the reorientation of PEDOT chains and the formation of interconnected structure of PEDOT-rich domains in thin films, improving the electrical conductivity. The PEDOT:PSS thin films containing carbon dots were used as the HEL in a typical poly(3-hexythiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) BHJ OPV by solution processing. The power conversion efficiency (PCE) of the OPV devices using carbon dot@PEDOT:PSS thin films was enhanced by up to 26% in comparison to that of the OPV containing pristine PEDOT:PSS as the HEL.<br/> The section V presents the development of gradient colloidal quantum dots (QDs)-silica composite as a luminescent down-shifting (LDS) material to improve light harvesting of the photoactive layer in OPV devices. The QDs were dispersed homogenously in the silica matrix without aggregation and phase separation. The light harvesting of LDS layers was affected by the concentration of QDs and the thickness of the LDS layer. As a result, the PCE of OPV devices were enhanced by 8.9% compared to pristine OPV devices (without an LDS layer) due to increased incident visible light transmittance and ultraviolet light harvesting of the LDS layer.<br/> Part III reports a study on the energy harvesting system of ECDs. Its introduction part briefly describes the characterization of ECD devices and the development of electrochromic materials.<br/> The section I describes a study on a new type supporting electrolyte properties based on carbon dots and the performance of ECDs using carbon dots. The nano-sized carbon dots with high polarizability bear metal counter cations by electrostatic interactions and/or π–cation interactions. The (carbon-dot)-Mx+ (M means metal cations such K+, Na+, and Li+) was characterized by various analytical methods such as thermogravimetric analysis (TGA), Fourier-transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), Raman, and electrochemical impedance spectroscopy (EIS). The (carbon–dot)n-Mn+ electrolytes showed high thermal (Td = 655 oC) and chemical stability, high cation number (n+ > 23), and good ion conductivity. In addition, the (carbon–dot)n-Mn+ is nontoxic and environmentally friendly. The ECDs using (carbon-dot)-Mx+ as an electrolyte showed enhanced electrochromic performance with long cycling life, high coloration efficiency (103.0 cm2C-1), and optical density (0.89). These results suggest that (carbon-dot)-Mx+ exhibits prospective applications in electrochemical cells such as secondary ion battery, supercapacitor and electrochemical transistor based on superior thermal andelectrochemical stability, and ion conductivity.