Interface Control In Organic Light Emitting Diodes And Solar Cells

Organic light emitting diodes and solar cells are the novel photoelectric conversion devices based on organic semiconductor materials, which possess a wide variety of advantages, such as light weight, mechanical flexibility, low cost and so on. Many interfaces (inorganic/organic, organic/organic, organic/metal) exist in these organic-based electronics. It has been proved that device performance and lifetime depend critically on the properties of the interfaces. Research works of this article are mainly focused on cathode interface control of organic-based electronics, and donor/acceptor interface control of the active layer. On cathode interface control, a kind of anionic conjugated polyelectrolyte PFEOSO3Na was synthesized and successfully applied to organic light emitting diodes and solar cells, which could effectively reduce interfacial energy barrier between the organic layer and cathode, leading to better electron injection or extract and significantly improving the device efficiency. On donor/acceptor interface control, two amphiphilic block polymer P3HT-PEO and fullerene derivatives PCB-C8oc were introduced to the active layer as interfacial compatilizer, which could located at the interfaces between the donors and the acceptors due to the intermolecular interaction, reducing the interfacial tension of two phases. As a consequence, much more stable morphology of the active layer was obtained. The concrete research contents are as follows:1) several cathode modified materials(PFEO, PFOCSO3Na, PFEOSO3Na) were used in polymer light-emitting diode to investigate how the backbone structure and side chains influence the wetting property and electron injection characteristic, we demonstrated a novel anionic conjugated polyelectrolyte, in which combining with polyfluorene backbone, sulfonate sodium and PEO derivative in side chains (PFEOSO3Na), to modify the cathode in PLEDs. The obtained hydrophilic material showed super wetting property with the hydrophobic light emitting layer due to the polyfluorene backbone. Compared with the anionic CPE without PEO derivatives, the PEO derivative side chains in exhibited key role to the ion migration in the cathode interface layer, which is crucial to the device performances. The PLEDs performances highly depend on the corresponding thickness of PFEOSO3Na due to its insulated properties.2) Given good device performance with PFEOSO3Na in the PLED devices, we also used it as cathode interface modification for organic solar cells and systematically studied device efficiency based on P3HT.PCBM, PCDTBT:PC71BM, PCDTTT-C:PC71BM. We also studied work function’s changes of the conductive substrates (ITO,AZO) and metal (Au) with PFEOSO3Na. Furthermore, High efficiency inverted solar cells with ITO/PFEOSO3Na as cathode, PBDTTT-C:PC71BM as the active layer were received. In addition, PFEOSO3Na was also employed in flat structure perovskite battery to modified PCBM and Al cathode interface, resulting in much higher efficiency.3) A diblock amphiphilic polymer P3HT-PEO was synthesized with accurate molecular structure and narrow molecular weight distribution (PDI=1.21). By introducing P3HT-PEO to the P3HT:PCBM system, the supramolecular interaction between the P3HT-PEO and donor/acceptor(D/A) molecule in the blends could make P3HT-PEO located at P3HT/PCBM domains’ interface, which effectively reduce the interfacial tension and suppress acceptor PCBM from severe aggregations under prolonged annealing time, resulting in much more stable morphology of the bulk-heteroj unction. In addition, when P3HT-PEO was introducing to the P3HT:ICBA system, the PEO chains spontaneously migrated to the surface of the active layer during the solvent annealing, which could form an interfical dipole moment with its negative pole pointing towards the Al, leading to high power conversion efficiency of the device with only Al cathode.4) A new type fullerene derivative (PCB-C8oc) was synthesized by modifying PCBM molecules, which possess a similar optical band gap and energy level. When PCB-C8oc was introduced to the P3HT:PCBM blends, the strong intermolecular interactions between C8 and C6could induce the P3HT self assemble to crystallization, which could improved efficiency of the unannealing devices. Due to similar structure, it can be confined to interfaces between donor and acceptor, leading to reduction of the interfacial tension and thin film thermal stability. So the PCB-C8oc possess the double function of additive and compatilizer. In addition, we also added the PCB-C8oc to the PTB7:PCBM blends, the efficiency and thermal stability of the device were both improved.