Tailoring device-scale properties in organic electronics: morphological, optical and electrode-interface related approaches

摘要

In era of dwindling fossil fuel supplies, increasing energy demand and high rates of carbon emission, investment in the clean and renewable-energy market is now the goal of many governments. This global prospect pushes the countries to consolidate new policies and rules to increase the production of cost-effective resources and grow the deployment in renewable energy. Sunlight, wind, waves and geothermal heat are the natural energy resources that strongly contribute to our global energy consumption. Organic materials - restricted to those that have conjugated structure and exhibit semiconducting properties - have gained intense interest in research and academia, leading to efficient and commercially applicable devices. Organic photovoltaic (OPV), organic field-effect transistors (OFETs) and organic light-emitting diodes (OLEDs) are the most prominent devices in the field of organic electronics. These devices are promising due to potentially low cost, mechanical flexibility, lightweight as well as high and ease of processability from solution (such as, spin-coating, drop casting, roll painting, and ink jet printing). Significant improvements have been achieved - especially for OLEDs, which have now been commercialized for cellular telephone applications as well as high-definition television screens. For OPVs, however, inferior performance and short lifetimes hinder their successful commercialization. The work presented in this dissertation focuses on three different performance-related issues and strategies for OPVs; electrode-interface engineering, morphology tuning, and optical absorption enhancement. The work on morphology tuning is also extended and applied to OLEDs and OFETs.In chapter one, a schematic showing the organization of this dissertation is presented. Inthe same chapter, a general introduction on organic materials and thin-films is also discussed. Furthermore, the architecture and basic operation of OPV, OFET and OLED devices are considered.Chapter two discusses the possibility of fabricating new OPV devices on previously used Indium-Tin-Oxide (ITO) substrates, which went through prior device processing with popular acidic interfacial layer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). We show that, contrary to the concerns in the literature, only the top few nanometers of ITO are etched by PEDOT:PSS in typical device processing and storage thereafter. Conductivity losses are offset by transmission gains leading to an increased power conversion efficiencies (PCE) for PTB7-based (PTB7: poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]) OPVs on used ITO substrates compared to devices on fresh ITO.In chapter three, we introduce a generic morphology tuning technique with anisotropic applicability -exposure to static electric-field (E-field) gradient during the solidification of solution-processed polymeric thin-films. This technique improves the connectivity between polymer chains; by changing the E-field direction, radiative pathways in polymeric thin-films can be altered, charge transport in- and out-of-plane can be improved, and phase-separation in polymer-fullerene blends can be coarsened in the bulk and perpendicular to the substrate. In exemplary cases, we improved the hole mobility in OFETs, power conversion effciency in OPVs, and electroluminescence efficiency in OLEDs.In the last part of this dissertation, we studied the effect of using microlens array (MLA)to increase the light absorption inside the active-layer of OPVs. Our MLA approach does not hinder the fabrication of the OPV because MLA lies on the non-conductive side of the ITO glass. In chapter four, we initially investigated the effect of using MLA with 2000 nm feature size. We found that thick (P3HT:PCBM) and thin (PCDTBT:PCBM) OPVs exhibit an improved short-circuit current. This enhancement stems from the increased light path coupled with the constructive interference patterns inside the OPV photoactive layer. In chapter five, we used MLAs with smaller feature sizes. In addition to feature size of 2 micrometer, 1.5 micrometer, 1 micrometer and 0.6 micrometer MLAs were also investigated. The experimental and simulations results show agreement on an increased light absorption inside the photoactive layer; improved current-density and PCE were realized for PTB7:PCBM and PCDTBT:PCBM OPVs (w.r.t. control) using 1 micrometer and 1.5 micrometer feature size MLAs, respectively.