The Research On The Performance Of Organic Photovoltaics With Interfacial Modification

During the past decades, organic photovoltaics(OPVs) have attracted great attention because of their light weight and potentially low cost from printing on flexible substrates.The main problem on this technology is the unsatisfactory photon-to-current conversion efficiency(PCE) and stability. And the key efforts to improve OPV's performance focuses on understanding the working mechanism, optimizing the morphology of polymer blends through advanced processing methods, and developing high-performance materials. In addition, interface engineering has been a useful approach to facilitate carrier extraction to enhance OPV performance, as the interfacial properties in an OPV system play an important role in determining the efficiency and stability. They are not only responsible for establishing ohmic contact, but also determining different device parameters such as the internal electric field.This paper aims to reveal the roles of interfacial layers by fabricating inverted OPV devices because the inverted OPV device can overcome the interface instability between PEDOT:PSS and low work function metals. Since the surface electronic potential of indium tin oxide(ITO) is very sensitive to the molecular layers, this thesis designs to use polydimethyl diallylammonium chloride(PDDA) to modify the ITO surface through electrostatic adsorption and Molybdenum trioxide(MoO3) to modify the interface between active layer and Aluminium through vacuum evaporation. This thesis also explores the optimum thickness of of MoO3 that is able to facilitate the hole transport and the proper annealing temperature to obtain the best active layer. After different kinds of measurement, it is proved that the work function(WF) of ITO will decrease after the PDDA absorption and the surface properties are improved to achieve a better interlayer with the active layer.Seen from the energy level of this inverted structure, this change can facilitate the electrons injection into and collection from the lowest unoccupied molecular orbital(LUMO) of thegiven organic semiconductor.As a result, the PCE of this inverted device is improved after the interfacial modification, mainly reflecting on the improvement on short current(Isc) and fill factor(FF).As to the stability, we find the interfacial modification on both anode and cathode has a positive effect on the air stability of devices by observing different parameters' degradation during a period of days. To sum up, the properties of OPV devices can be improved by electrode and interfacial modification. And it is promising to develop this type of organic optoelectronic devices by introducing more modification methods.