Studies on solution-processed passivation layers for high performance and long-term stable organic electronic devices

Abstract

Organic electronic devices such as organic field-effect transistors (OFETs) and organic photovoltaic cells (OPVs) have received considerable attention due to their potential applications in low cost, easy-processable, flexible, large-area electronic devices. Despite the rapid progress of electrical performances of OFETs or OPVs, there has been considerable interest in environmental stability of organic electronics because their electrical properties are significantly degraded over time by water and oxygen species present in ambient air. Accordingly, the introduction of a passivation layer is necessary to protect the devices from the detrimental effects of environment. Solution-processing passivation has the advantages of allowing simple and low-cost processing in short times. Since exposure of the devices to solvents is inevitable in solution-processing passivation—and considering that this exposure may affect device performance—it is very important to choose solvents that do not damage the active layer of the OFETs or OPVs. In chapters 2, 3 and 4, we investigated the effects of the solvent on the structure and morphology of various solution-processed semiconductor films, and demonstrated that direct solvent exposure significantly affected the crystallinity, morphology, and device properties. In chapters 2 and 5, we have designed and characterized solution-processed passivation materials having high barrier properties. Finally, we applied them as passivation layers to enhance the lifetimes of OFETs, resulting in long-term stable OFETs operation compared with the unpassivated OFETs. In chapter 2, we investigate the effects of the solvents used in the passivation process on the behavior of pentacene field-effect transistors (FETs) and report on the fabrication of a passivation layer for pentacene FETs via inkjet printing using photocrosslinkable poly(vinyl alcohol), N-methyl-4(4’formylstyryl) pyridinium methosulfate acetal (SbQ-PVA). The passivated pentacene FETs—composed of inkjet-printed SbQ-PVA containing polystyrene/SiO2 and poly(4-vinyl phenol)/SiO2 dual-layer gate dielectrics—retain their electrical properties for much longer periods than the unpassivated devices. Studies of the device performance show that inkjet-printed passivation is better than spin-coated passivation. In chapter 3, the effects of the solvent on the structure and morphology of solution-processed semiconductors containing bulky triisopropylsilylethynyl (TIPS) groups were investigated by using X-ray diffraction (XRD) and atomic force microscopy (AFM) for the solution-processing passivation of organic field-effect transistors (OFETs). Direct exposure of the semiconductor layers to ethanol improved their molecular ordering and crystallinity, thus leading to enhanced electrical characteristics of the OFETs. This improvement of the characteristics of the devices can be attributed to reduction the trap densities of the OFETs after ethanol exposure. In chapter 4, we investigated the effects of direct solvent exposure on the properties of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films and poly(3-hexylthiophene) (P3HT)/PCBM blend films employed as active layers in, respectively, organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). The crystallinity, morphology, and OFET characteristics of the PCBM thin films were significantly influenced by direct exposure to solvent, especially to select alcohols. Control over the nanoscale morphology of the PCBM film, achieved via direct solvent exposure, yielded highly efficient poly(3-hexylthiophene) (P3HT)/PCBM OPVs with a short-circuit current density of 10.2 mA/cm2, an open-circuit voltage of 0.64 V, and a power conversion efficiency of 3.25% under AM 1.5 illumination with a light intensity of 100 mW/cm2. These results indicated that optimal phase separation in the P3HT/PCBM films could be obtained simply by exposing the active layer films for a few seconds to solvent.In chapter 5, in an effort to realize organic field-effect transistors (OFETs) that are stable over long periods of time, we have designed an organic–inorganic hybrid passivation material (TGD622t) prepared via a non-hydrolytic sol–gel process that does not require the use of solvents. Fourier-transform infraredspectroscopy, atomic force microscopy, and UV-visible spectroscopy demonstrated the high density and low porosity of the organic–inorganic hybrid transparent TGD622t film after low-temperature curing (below 100 °C). The dense TGD622t passivation layer, which exhibited a water vapor transmission rate (WVTR) of 0.434 g/m2/day, effectively protected the poly[9,9-dioctylfluorenyl-2,7-diyl]-co-(bithiophene)]-based OFETs from humidity and oxygen in ambient air, resulting in a much more robust OFET performance with long-term stability relative to the operation of unpassivated devices.