Direct Graphene Growth, and Flexible Graphene Electrode and Encapsulation for Organic and Hybrid Electronics

Abstract

Graphene is a fascinating material with excellent electrical, optical, mechanical, and chemi¬cal properties. Since the graphene sheet was first reported from Prof. Geim’s group in Manchester University, it aroused massive researches in forming high-quality graphene sheets due to potential applications to various electronic and optoelectronic devices. In this paper, we introduce a variety of graphene-related studies from graphene synthesis to graphene electrodes and graphene barrier films for electronic devices. These studies describe important experimental results that are significantly advancing the field of synthesis of graphene films and their electronic device application. Remarkable progress has been made in the development of methods for syn¬thesizing large-area, high-quality graphene. Recently, the chemical vapor deposition (CVD) method has opened up the possibility of using graphene for electronic devices and other applications. However, CVD requires an additional process to transfer the graphene films onto dielectric substrates that are required in electronic devices. Moreover, the use of explosive hydrocarbon gases as a carbon source, and the limit on film size set by the need to transfer the graphene films after growth, are major obstacles for mass production. Therefore, active research is now in prog¬ress to find a promising way to overcome the obstacles of the CVD method. In chapter 2, we have developed a simple, scalable, transfer-free, ecologically sustainable, value-added method to convert inexpensive coal tar pitch to patterned graphene films directly on device substrates. The method, which does not require an additional transfer process, enables direct growth of graphene films on device substrates in large area. To demonstrate the practical applications of the graphene films, we used the patterned graphene grown on a dielectric substrate directly as electrodes of bottom-contact pentacene field-effect transistors (max. field effect mobility ~0.36 cm2·V-1·s-1), without using any physical transfer process. This use of a chemical waste product as a solid carbon source instead of commonly used explosive hydrocarbon gas sources for graphene synthesis has the dual benefits of converting the waste to a valuable product, and reducing pollution. Organic/inorganic hybrid perovskites (OIPs) are promising light-emitting materials for PeLEDs due to high color purity, low material cost, tunable band gap, and easy fabrication. PeLEDs were based on an indium tin oxide (ITO) anode which is the conventional transparent conductive oxide (TCO) electrode in optoelectronic devices. However, the price of In is increasing due to the limited availability of In sources. In addition, ITO is brittle, so it is not readily applicable in highly flexible PeLEDs. Furthermore, release of metallic In and Sn species from the ITO into overlying layers can cause quenching of excitons which will be more serious in PeLEDs having long LD. Therefore, finding a flexible and chemically inert ITO-free electrode that can realize highly efficient flexible PeLEDs would be a significant advance in flexible perovskite opto-electronic devices. In chapter 3, we achieved highly efficient ITO-free and flexible organic/inorganic hybrid PeLEDs with very bright EL (Lmax >10,000 cd m-2) and high efficiency (CEmax = 18.8 cd A-1) based on graphene anode for the first time. Four-layer graphene (4LG) films with a self-organized gradient buffer hole-injection layer (Buf-HIL; formerly we also called GraHIL in organic light-emitting diodes (OLEDs)) (a composition composed of poly(3,4-ethyleneioxythiophene) : poly(styrene sulfonate) (PEDOT:PSS) and a perfluorinated ionomer (PFI)) and a MAPbBr3 emitter were used to make green-emitting PeLEDs. To overcome the low device efficiency of PeLEDs, we used additive-based nanocrystal pinning, which can modify the MAPbBr3 EML and thus results in high-efficiency PeLEDs. We still suffered from relatively low device efficiency of ITO-PeLEDs (CEmax = 10.9 cd A-1 and EQEmax = 2.83 %) which have the same device structure as Gr-PeLEDs and were fabricated using the same procedure. However, the Gr-PeLEDs showed much higher CEmax = 18.8 cd A-1 and EQEmax = 4.23 % than did ITO-PeLEDs, which implies that the the device using a graphene electrode excludes the adverse effect of ITO electrodes in terms of exciton quenching caused by the In and Sn species migrated into the Buf-HIL. Electronic devices based on organic materials can be large-patterned, formable and designable, so they have great potential for use in large-area flexible electronics, displays, and solid-state lighting. Therefore, OEDs are being continuously investigated for use as next-generation electronics and displays. However, the conventional OEDs are generally non-flexible because of using rigid substrate and encapsulation and thus not suitable for use in flexible displays and lightings. To actualize flexible OEDs, flexible (and transparent especially for organic light-emitting diodes (OLEDs)) encapsulation processes, flexible electrodes, and impermeable flexible substrates should be developed simultaneously, because OEDs are easily degraded by external or internal influences such as moisture and air. Therefore, the feasibility of flexible OEDs could be improved by development of a simple, efficient and inexpensive encapsulation process to fabricate a highly flexible and transparent barrier film with low water vapor transmission rate (WVTR). In chapter 4, we introduce a simple, inexpensive, and large-area flexible transparent lamination encapsulation method that uses graphene films with polydimethylsiloxane (PDMS) buffer on polyethylene terephthalate (PET) substrate. The number of stacked graphene layers (nG) was increased from 2 to 6, and 6-layered graphene-encapsulation shows high impermeability to moisture and air. The graphene-encapsulated polymer light emitting diodes (PLEDs) had stable operating characteristics, and the operational lifetime of encapsulated PLEDs increased as nG increased. Calcium oxidation test data confirmed the improved impermeablity of graphene-encapsulation with increased nG. As a practical application, we demonstrated large-area flexible organic light emitting diodes (FOLEDs) and transparent FOLEDs that were encapsulated by our polymer/graphene encapsulant.