Organic photovoltaics (OPVs) have attracted much attention during the last decades due to their potential application for their light weight, low cost, and flexibility [
1]. The simplest form of organic solar cells can be fabricated by sandwiching a layer of an organic electronic material between two metallic conductors, typically a layer of indium-tin-oxide (ITO) with high work function (
WF) (~4.7 eV) and a layer of low
WF metal such as aluminum (Al) (~4.1 eV). However, single layer organic solar cells do not work well, showing low quantum efficiencies (<1%) and low power conversion efficiencies (<0.1%), mainly due to an insufficient light absorption. The diffusion length of excitons in organic electronic materials is typically on the order of 10 nm for most excitons diffusing to the interface of layers and splitting into carriers. Therefore, the layer thickness should be in the same range as the diffusion length [
2]. Now, most of OPV devices with high efficiency took the form of bulk heterojunction, which has an absorption layer consisting of a nanoscale blend of donor and acceptor materials. The large donor-acceptor interfacial area results in a higher likelihood for the short-lived excitons to reach an interface and dissociate [
3]. Bulk heterojunctions have an advantage over layered photoactive structures because they can be made thick enough for effective photon absorption without the difficult processing involved in orienting a layered structure. ITO films are commonly used as an electrode in OPV devices, on which an acidic poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) layer is deposited as the hole selective contact. It is known that PEDOT:PSS layer was detrimental to the ITO substrates due to the instability of PEDOT:PSS/ITO interface. In addition, the PEDOT:PSS is acid and will etch the ITO, making this interface unstable [
4]. And OPV with inverted structure is an effective way to improve the devices’ stability by avoiding the contact between PEDOT:PSS and ITO substrates [
5]. The inverted device architecture, in which ITO collects electrons and a high
WF metal electrode collects holes, has proven to be effective in improving both the power conversion efficiency (PCE) and durability of OPV devices. Recent years, inverted OPV with modified ITO as the transparent cathode has received increasing attention and various kinds of OPV devices with inverted structure have been reported to improve the devices’ efficiency and stability. However, this kind of optoelectronic device requires an electrode with a low
WF to either facilitate the injection or collection of electrons from the lowest unoccupied molecular orbital (LUMO) of a given organic semiconductor [
6]. So it is vital to use appropriate materials or modification to match the energy level in an inverted OPV structure. As the electronic potential of ITO surface has been proved to be very sensitive to the presence of self-assembled molecular layers via tuning the surface dipole, doping and electric field effect, the surface
WF can be changed by applying different methods of modification [
7].