Relation between the built-in voltage in organic light-emitting diodes and the zero-field voltage as measured by electroabsorption

Change of title: On the determination of the built-in voltage in organic light-emitting diodes from electro-absorption measurements. For developing understanding of the current density onset voltage and injection barriers in organic light-emitting diodes (OLEDs), a precise determination of the built-in voltage, Vbi, is of crucial importance. Commonly, Vbi is assumed to be equal to the voltage V0,EA at which in an electroabsorption (EA) experiment the reflection of light at the OLED is found to become insensitive to a small voltage modulation. However, this assumption is shown to lead to significant errors for devices with well-injecting contacts. From an analysis of EA experiments for hole-only devices containing a polyfluorene-based copolymer, it is shown that V0,EA may be interpreted as an effective current density onset voltage, agreeing with the commonly accepted picture, but that for these devices Vbi is ~0.5 V larger than V0,EA. This is found to be consistent with predictions of V0,EA from model calculations of the electric field and light-absorption profiles in the semiconducting layer