Study of the transient EL slow rise in single layer OLEDs

Study of the transient electroluminescence (EL) behavior at turn-on upon the application of a single voltage pulse is presented. Based on the continuity equations which describe the carrier dynamics, and on the recombination current which describe the EL output, we present a theoretical model to explain the slow rise of the EL associated to the electron packet motion in the bulk. The model enables us to determine, more than the mobility of majority carriers, the equilibrium density of free holes in the bulk. The dependence of free hole density (pe) of the applied voltage shows that the unipolar current flow for holes in the bulk follows as expected, an SCLC regime characterized by a linear dependence of pe with applied voltage. Our analysis allows us also to estimate the trap density (for holes) in the bulk of about 2 × 1015 cm−3. Both experimental and theoretical results are compared to each other and to other works, and were found to be consistent with the proposed model. The device under investigation is the conventional ITO/MEH-PPV/Al OLED.