Interfacial Energetics And Device Performances Of Transition Metal Oxides Based Organic Optoelectronics

Organic optoelectronic devices such as organic light emitting diodes (OLED),organic photovoltaics (OPV) have been focused for decades, due to their low cost,simple manufacture process, and the compatibility with flexible substrate. The organiclight emitting diode (OLED) is expected to be applied into the next generation of flatpanel display and a solid state lighting. However, reducing the driven voltage andincreasing the operating lifetime is a necessity for large-area commercial production. Tosolve this problem, many marvelous methods have been tried, one of which is adding abuffer layer between the organic layer and the electrode. Organic photovoltaics (OPV)are expected to become a truly green and renewable energy resource, making greatcontribution to ease the global energy crisis and environmental pollution. In2013, theGermany Heliatek GmbH company announced on their website that the powerconversion efficiency of their deposited small molecule solar cell device reached12percent, which had fulfilled the commercial level, and would be expected to be up to15percent in2015. Compared with the efficiency and lifetime of conventional solar cell,however, there is a long way to go. The main reason is that the absorption spectrumrange of the organic photosensitive material is very narrow, although which has a strongabsorption capability. The tandem organic solar cell could obtain higher powerconversion efficiency by stacking devices containing photosensitive materials withdifferent absorption spectrum range. Without any doubt, the choice of intermediateconnection layer (ICL) has played a crucial role on device performance. This ICL needsto meet the conditions like high transparency in the range of visible light, intrinsicstability towards oxidation, and good energy-level matching with the sub-devices, etc. The transition metal oxides (TMOs) have a wide range of work functions (fromZrO2~2eV to V2O5~7eV), whose energy levels can be well matched with most oforganic materials, thus effectively reducing the energy barrier and enhancing the hole(electron) injection (export) efficiency. The oxides are widely applied into the OLED,OPV device acting as an efficient buffer layer. In addition, TMOs such as MoO3, WO3are widely used in the Tandem OPV ICL, because of many excellent characteristicssuch as the high transparency in the visible range, good stability, high work function,etc.This paper mainly focuses on TMOs’ application in OLED and OPV. For MoO3and WO3, by photoelectron spectroscopy (XPS and UPS) to study the influences ofoxygen vacancy on the oxides’chemical properties, electronic structures, and the energylevel alignment with organics. The performance of fabricated photovoltaic devicesvalidated the analysis results.In addition, interfacial electronic structures and energy level alignments of TMO(MoO3and WO3) based intermediate layer in Tandem OPV were studied. And theinfluences of ICL structures and materials on devices performance were alsosystematically investigated. The details of research work are as follows：Firstly, the electronic structures of TMOs/organic molecules on different substrateswere systematically studied via XPS and UPS, including (a)ITO/MoOx (5nm,10nm)/NPB interface;(b)ITO/WOx(5nm,10nm)/NPB interface;(c)Si/MoOx interface.Finally setting MoOx5nm as an example, we discussed the impact of oxygen vacancieson the OLED device performance.Secondly, we did some experiments on the tandem solar cell device. Throughinvestigating the ICL’s electronic structure and energy level alignment, includingCsF:Bphen/WO3/CuPc and CsF: Bphen/MoO3/CuPc interface, combining with stackeddevice performance, we studied the carrier generation and injection mechanism, as wellas its influence on the device performance.Finally, we also did some exploration and research on singlet exciton fission’sapplication in OPV.