The influence of interfacial charges on the device characteristics of multilayer organic light-emitting diodes (OLEDs) is investigated, and a concept to improve device performance is presented. We studied devices consisting of copper phthalocyanine (CuPc) as hole injection and buffer layer, N, N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) as hole transport layer, and tris(8- hydroxyquinolinato)aluminum (Alq3) as electron transport and emitting layer sandwiched between a high-work-function metal and a semi-transparent calcium electrode. Detailed current-voltage measurements show that the device characteristics in negative bias direction and at low positive bias below the built-in voltage depend strongly on the bias sweep direction, indicating that interfacial charges have a pronounced influence on the device characteristics. Low-frequency capacitance-voltage experiments reveal a voltage-independent capacitance in negative bias direction and a significant increase between 0 and 2 V, evidence of a redistribution of the internal electric field in this device configuration. Time-resolved electroluminescence (EL) measurements proved that also the EL response time at low voltages is governed by the accumulation of charge carriers inside the device rather than by their transport. Optimizing the device structure by grading the organic-organic interfaces results in an enhanced current flow, an improved brightness, and a faster EL response time. Our investigations clearly indicate that the abrupt CuPc-NPB as well as the NPB-Alq3 interface significantly influence the performance of our multilayer OLED.