Study On Carrier Transport Properties Of Hole Tansport Material NPB

Carrier transporting ability is one of many concerns due to its decisive role for the performance of various organic devices, such as organic electroluminescent devices (OLEDs), the organic photovoltaic devices (OPVs) and the organic thin-film field-effect transistor (OFETs), Carrier mobility, as a physical quantity characterizing material carrier charge transport capability, is particularly important. However, a variety of measuring methods currently used for carrier mobility of organic semiconductor material have their own limitations or defects, and can not satisfy the people’s requirement.In this thesis, an approach of mathematical deduction from current-voltage curve based on the space charge-limited-current theory, which is proposed by our research group, is used to study the carrier mobility of a typical semiconductor material in organic electronics field, N,N’-diphenyl-N,N’-bis(1-naphthyl)(1.1’-biphenyl)-4,4’diamine (NPB).First, simple hole only devices with NPB hole transporting layer, using MoO3, CuPc and C60, respectively as buffer layers at anode interface were fabricated. We find that the device using MoO3as a hole buffer layer has the best results and the optimal thickness is approximately0.45-0.5nm.Secondly, from the current-voltage characteristics of the device with MoO3anode buffer layer, the carrier mobility of NPB is determined to be within the range of1.1X10-5to3.5X10-4cm2V-1s-1for the electrical field between600and1000V1/2cm-1/2, which is in very good consistence with other method reported in the literature, indicating that this is a simple and effective way to determine carrier mobility in organic semiconductors.Finally, a simple single layer devices with a structure of ITO/MoO3/NPB/Ca/Al was fabricated, the device emits bright blue under an electric field. The brightness as high as1589cd/m2is achieved. This indicates that electrons are injected into NPB layer through Ca/Al cathode as well as holes through anode. This fact shows the bipolar property of NPB, in other words, NPB can be used to transport both holes and electrons.