The Impact of Disorder on the Energy Level Alignment at Molecular Donor–Acceptor Interfaces

The presence of disorder at this interface broadens the energy distribution of molecular states (HOMO and LUMO) and result in (weak) tailing states. [ 11 ] If these energy level distributions reach or cross the Fermi level, they will lead to additional electron transfer and alter the electrostatic potential across this donor–acceptor interface. Despite the common perception that structural disorder strongly affects the performance of OPVCs in general (e.g., open circuit voltage [ 12 ] and polaron generation, [ 13 ] the specifi c impact of disorder on the electronic structure of prototypical OPVC donor–acceptor interfaces, such as those presented here, has not been addressed before. In this contribution, we argue that disorder, induced by deposition of a second organic layer on top of the fi rst molecular semiconductor, is a common phenomenon at organic heterojunctions. Furthermore, we show that this disorder results in a more complex behavior of the interfacial electronic structure than previously assumed. We specifi cally present two model systems that can be interpreted in this manner and show several cases in literature where the same effects have been observed. The consequences of this disorder manifest themselves in an unexpected kink-like up-and-then downward shift (or vice versa) of the vacuum level (VL) and, more importantly, of the molecular states at the very organic interface on completing the heterostructure, an effect repeatedly observed in literature, but still poorly understood. [ 14 ] We propose that fi lling/ emptying the tails of the broadened frontier energy level distributions induced by disorder plays a major role in inducing this “kink. ” To validate our proposition, we fully characterize the electronic structure of a prototypical donor–acceptor interface, [ 15 ] consisting of zinc phthalocyanine (ZnPc)/C 60 prepared on a molybdenum trioxide (MoO 3 )-covered Au(111) electrode (see Figure 1 a) by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS, respectively). In addition, complementary electrostatic calculations [ 16 ] are carried out, which are in full agreement with the experimental results. The heterostructure was vacuum-deposited at room temperature. The WF increases from 5.3 eV [pristine Au(111)] up to 6.6 eV for a 2 nm thick MoO 3 layer on top (for details see the Supporting Information), which can be seen by in situ UPS measurements of the secondary electron cutoff (SECO) region (Figure 1 b). Upon evaporating 10 nm ZnPc onto the MoO 3 layer, the WF substantially decreases from 6.6 to 4.5 eV (red curve in Figure 1 b) and the ZnPc HOMO is clearly visible near the Fermi level (red curve, zoomed in Figure 1 c). The substantial WF reduction can be attributed to an emptying of ZnPc HOMO derived states. [ 17 ] This can be easily understood, as the ionization energy of ZnPc (4.8 eV, molecules oriented upright in the fi lm) is considerably lower than the WF of the Au/MoO 3 substrate (6.6 eV) and, therefore, occupied molecular states are Organic photovoltaic cells (OPVCs) have attracted considerable interest in the last three decades, because of their advantages over conventional inorganic photovoltaics in terms of easy processability (roll-to-roll), low cost of production, and the possibility of fabricating ultra-thin and fl exible devices. [ 1 ] At the heart of modern OPVCs is a type II heterojunction that splits photogenerated electron–hole pairs and transports the resulting mobile charge carriers to their respective contacts. [ 2 ] This interface, consisting of electron donorand acceptor-type molecular semiconductors, is therefore key to the photocurrent generation in an OPVC. [ 3 ] By optimizing the optical bandgaps of the organic semiconductors and the positions of the electronic levels with respect to each other, power conversion effi ciencies of 10% can be reached, even for single-junction OPVCs. [ 4 ]