Direct Measurements of Donor HOMO – Acceptor LUMO Offset at The Interface of C60/Pentacene Model Organic Solar Cells using Photoemission and Inverse Photoemission Spectroscopy

In organic light-emitting diodes (OLEDs) and heterojunction organic solar cells (OSCs), the energy offset between HOMO and LUMO, also called the transport gap (E_tr) is substantially different from optical gaps due to the relatively large exciton binding energies. However, the difficulty to independently measure the LUMO levels makes it hard to elucidate important things like electron injection barrier at the cathode in OLEDs and the voltage loss mechanisms in Voc of OSCs.<br/>We have recently developed highly sensitive inverse photoemission spectroscopy (IPES) instrument specifically for the LUMO level measurement of organic semiconductors. In combination with the existing ultraviolet photoemission spectroscopy (UPS) we could determine all the relevant transport energy levels (HOMO, LUMO, Fermi level and vacuum level) in any organic semiconductor surface and interface. In addition, combination of the electron source for IPES and the electron energy analyzer for UPS allowed us to utilize reflection electron energy loss spectroscopy (REELS) to determine optical gaps. We first demonstrate that all these measurements can be performed for an identical sample as a function of thickness. We also determine the energy levels at C60/pentacene interface so that the factors affecting the voltage losses that led to observed Voc value of this model planar heterojunction organic solar cells can be evaluated.