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

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, V(bi), is of crucial importance. Commonly, V(bi) is assumed to be equal to the voltage V(0,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 V(0,EA) may be interpreted as an effective current density onset voltage, agreeing with the commonly accepted picture, but that for these devices V(bi) is similar to 0.5 V larger than V(0,EA). This is found to be consistent with predictions of V(0,EA) from model calculations of the electric field and light-absorption profiles in the semiconducting layer.