Interface Energy Level Offset of Organic Solar Cells Measured by UPS and IPES and Its Correlation with Voc and CT Exciton Energy

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 inability 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/><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 determined 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 the 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 both C60/pentacene and C60/CuPc interfaces so that the factors affecting the voltage losses that led to observed Voc value of the model planar heterojunction organic solar cells can be evaluated.<br/><br/>In addition, we fabricated bilayer model organic solar cells using C60/pentacene and C60/CuPc pairs. The deposition sequence of each layer was alternated resulting in four different types of devices, namely, C60/pentacene, pentacene/C60, C60/CuPc, and CuPc/C60. We confirmed that their exists interfacial energy offset difference between C60/pentacene and pentacene/C60 devices, which is also reflected in Voc values of respective devices. This is due to the difference in molecular orientation for pentacene depending on the deposition sequence in this binary system. On the other hand, we could not observe any dependence of Voc on deposition sequence of C60 and CuPc devices nor could we measure any energy offset at the interface, where the molecular orientation is expected to be independent of deposition sequence. We further investigate the charge transfer (CT) exciton energies for all the devices by using sensitive external quantum efficiency (EQE) measurement. These combined measurements led us to identify the most important parameter in determining the Voc of these model bilayer organic solar cells.<br/><br/>This work was supported by Korea Basic Science Institute (National Research Facilities and Equipment Center) grant funded by the Ministry of Education (2021R1A6C101A437) and National Research Foundation of Korea (NRF) BRL program grant funded by the Korean government (MSIT) (2022R1A4A3030766).