Study on transport properties of bulk hetero-junction OPVs and simple structure OLEDs

Abstract

Recently organic devices are widely investigated in the world. In this study weare investigating the transport physical parameters like mobility, trap energy levels anddensity of states of OLEDs and OPVs by several methods to contribute the under-standing physics. To extract the mobility of the Alq3 and P3HT:PCBM-71, we usethe photo-CELIV(charge extraction in linearly increasing voltage) and compare withresults of other groups. Mobility of thin film device (~0.1 um) could be measuredby photo-CELIV while ToF method needs thick devices ( > 1 um) because of pene-tration depth. For the trap energy level, we could use the several methods like trap-limited SCLC(space charge limited current), PICTS (photo-induced current transientspectroscopy) and turn-off dynamics of transient-electroluminescence(EL). We extractthe trap energy level of P3HT:PCBM-71 as 70 meV by trap-limited SCLC and PICTS.For phosphorescent OLEDs triplet trap energy levels are extracted by temperature de-pendence of dispersive parameter. The triplet trap energy levels represent the effectiveenergy gap between host and dopant triplet levels. By impedance spectroscopy, junc-tion parameters and density of state near HOMO level are investigated. We also couldextract the attempt-to-escape frequency from temperature dependence of capacitance-frequency characteristics. Extracted attempt-to-escape frequency of P3HT:PCBM arearound 108 109(s??1). Large boundary leads to small attempt-to-escape frequency.The domain sizes of P3HT in bulk heterojunction are determined by analogy with theattempt-to-escape frequency. We also fabricated the multi-layer OLEDs with new con-ductive polymers to improve the performance and stability. These HIL material arenon acidic (pH 6-7) thus we avoid the damage on ITO. Eciency, driving voltage andstability are improved with new HIL1.3N because of its high work function, preventingthe spike of ITO by cladding and its neutral property.