We present in-depth analysis of an n/p heterojunction that consists of 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT(CN)(6)) (n-type) and Poly(9-vinylcarbazole) (PVK) (p-type) via X-ray Photoelectron Spectroscopy (XPS) and Ultra-violet Photoelectron Spectroscopy (UPS) measurement. The p-type layer is doped with 2 wt% of 1,1-bis-(4-bis(4-tolyl)-aminophenyl) cyclohexene (TAPC). The energy difference (triangle E) at the hetero-junction, magnitude of band bending (V-b) and the vacuum level shift at the interface is modified when PVK is doped with 2 wt% TAPC. The presence of V-b at the HAT(CN)(6)/PVK (PVK:TAPC) interface makes it easier to reach a triangle E approximate to 0 energy offset in order to facilitate charge generation at the interface. Via a Fowler-Nordheim (FN) tunneling curve, it is found that the electron extraction from PVK to HAT(CN)(6) at the interface could occur via the tunneling process. This finding provides new insights into novel solutions for high efficiency tandem OLEDs. (C) 2017 Elsevier B.V. All rights reserved.