Extensive development of new polymer and small molecule donors has helped produce a steady increase in the efficiency of organic photovoltaic (OPV) devices. However, OPV technology would also benefit from the introduction of non-fullerene acceptors. Unfortunately, efforts to replace fullerenes have typically led to significantly reduced efficiencies. A number of possible explanations for reduced efficiencies with non-fullerene acceptors compared to fullerene acceptors have been suggested, including the formation of unfavorable morphologies in non-fullerene systems and/or favorable excitation/carrier delocalization in fullerenes. In addition, enhanced exciton dissociation associated with fundamental characteristics of the fullerene molecular electronic states has also been suggested. We used time-resolved two-photon photoemission (TR-2PPE) to directly compare exciton dissociation at interfaces between zinc phthalocyanine (ZnPc) interfaces and the non-fullerene acceptor, perylene tetracarboxylic dianhydride (PTCDA) versus dissociation measured at the analogous interface with C60, and thus help discriminate between these potential explanations. Exciton dissociation rates are comparable for phthalocyanine interfaces with both acceptors, allowing us to suggest a hierarchy for the importance of various effects producing higher efficiencies with fullerene acceptors.

Exciton dissociation at donor-acceptor (DA) interfaces is critical for the operation of organic photovoltaic (OPV) devices, yet a detailed physical understanding of this process is lacking. This work examines an important aspect of this process, namely the dependence of the exciton dissociation rate on distance from the DA interface. Time-resolved two-photon photoemission (TR-2PPE) measurements were performed on bilayer H2Pc\C-60 heterojunctions fabricated using organic molecular beam epitaxy (MBE) with varying H2Pc thickness. In the measurements, the dynamics of the H2Pc S-1 exciton population created with a 1.55 eV pump pulse were monitored via photoemission with a delayed UV probe pulse. The depth sensitivity of TR-2PPE, due to the short electron attenuation length, provides the means to follow excited state dynamics as a function of H2Pc thickness. Analysis of the S-1 population decay as a function of H2Pc thickness revealed that the electron transfer rate for the first H2Pc layer, adjacent to C-60, is k(CT) = (2.3 +/- 0.4) X 10(12) s(-1). Exciton dissociation is reduced by a factor of at least 10 for the second H2Pc layer and beyond.

Donor–acceptor interfaces are critical for the operation of organic photovoltaic devices. Exciton dynamics at these interfaces play a significant role in determining efficiency and controlling open circuit voltage (VOC), short circuit current (JSC), or fill factor (FF). These fundamental interfacial dynamical processes are dependent on the interfacial electronic and molecular structure. In this report we use time-resolved two-photon photoemission (TR-2PPE) to investigate exciton dissociation, recombination, and relaxation processes occurring at well-characterized prototypical donor–acceptor interfaces of copper phthalocyanine (CuPc) layers on C60. S1 excitons are created by excitation in the CuPc Q-band. The excited S1 population is probed as a function of time via photoemission with a UV probe pulse. TR-2PPE measurements provide a picture of subpicosecond charge separation and recombination processes as a function of distance from the CuPc/C60 interface, starting with a CuPc single layer. Analysis via ra...

The unoccupied electronic structure of C-60 layers on Ag(111) was investigated with two-photon photoemission (2PPE) and scanning tunneling spectroscopy (STS). The focus, in addition to providing unoccupied level assignments for C-60 monolayers, is on identifying signatures of C-60 superatom molecular orbitals (SAMOs), diffuse unoccupied levels weakly bound by the spherical potential of the fullerene shell, in the two complementary techniques. The s-SAMO is identified 3.3 eV above the Fermi level in 2PPE and at a bias of 3.5 eV in STS. Possible contributions from p-SAMO levels are also discussed for both measurement techniques. The results are compared with recent low-temperature scanning tunneling microscopy (LT-STM) work that investigated C-60 SAMOs on Cu(111) and oxygen-covered Cu(110) surfaces and with previous 2PPE measurements for C-60 monolayers on Au(111) and Cu(111).

Charge transport behavior in molecular monolayers based on oligo-(phenylene ethynylene)s (OPEs) shows distinct variations with substitutions on the OPE framework. Explanations of this dependence often center on the impact of the substituted group's electron affinity on charge transfer effects. To examine these effects, we used ultraviolet photoemission to follow changes in the electronic structure of OPE-based thiol-linked self-assembled monolayers induced by charge transfer doping with potassium. The impact of OPE (C(6)H(5)-C C-C(6)H(4)-C C-C(6)H(5)-SH) versus -NO(2)-substituted OPE (NO(2)-OPE, C(6)H(5)-C C-C(6)H(3)NO(2)-C C-C(6)H(5)-SH) on the electronic structure modifications was compared for a range of K additions. Addition of less than 0.5 K/molecule to self-assembled monolayers produced rigid shifts of molecular levels by about 0.5 eV for both monolayers, with larger shifts at lower K addition for NO(2)-OPE. Continued addition of K leads to increased shifts for OPE, while saturation of the effect was observed for NO(2)-OPE. Incorporation of K beyond about 2 K atoms per OPE or NO(2)-OPE leads to an additional structure, broadening, and changes in the relative spacing between molecular levels probably indicating distortion in the monolayer and also of the molecular backbone. The results show clear influence of -NO(2) substitution on K-induced charge transfer effects. The coupled impact of K-induced charge transfer and electrostatic effects from K counterions on molecular level energies is discussed and compared to recent theoretical work.

The ultrafast dynamics at a well-characterized CuPc/C-60 organic photovoltaic heterojunction have been directly measured with time-resolved two-photon photoemission (TR-2PPE). Phthalocyanine/C-60 donor-acceptor interfaces, characterized in detail via scanning tunneling microscopy, provide model systems for studies of critical charge separation and recombination processes that determine device performance. TR-2PPE studies of copper phthalocyanine (CuPc)/C-60 interfaces reveal ultrafast charge separation and provide evidence for recombination via low-lying CuPc triplet levels.

The C60-thin film pentacene interface was investigated using scanning tunneling microscopy, atomic force microscopy, and ultraviolet photoemission spectroscopy. C60 deposition on a multilayer pentacene film (standing) yields an interface dominated by C60 clusters, regardless of the underlying substrate. Three-dimensional cluster growth dominates due to weak interactions with the underlying Pn. C60 cluster size and density on sequential Pn layers suggest an Ehrlich–Schwoebel-type barrier at Pn layer boundaries. Cluster formation reduces the C60 lowest unoccupied molecular orbital–Pn highest occupied molecular orbital (HOMO) separation, while increasing the respective HOMO-HOMO offset. Heterostructure fabrication protocols can alter interface morphology and induce band shifts on the order of 0.3 eV.