Energy measures of the intra- and intermolecular electronic effects of triisopropylsilylethynyl substitution on pentacene have been obtained from the combination of closely related gas phase and solid phase ultraviolet photoelectron spectroscopy (UPS) measurements along with solution electrochemical measurements. The results show that the shift to lower ionization energy that is expected with this substitution and observed in the gas phase measurements becomes negligible in solution and is even reversed in the solid phase. The principles that emerge from this analysis are supported by electronic structure calculations at the density functional theory level. The relation between the gas phase and solid phase UPS measurements illustrated here provides a general approach to investigating the electronic effects acting on molecules in the condensed phase, which in this case are greater than the direct substituent electronic effects within the molecule. Electronic properties such as lower ionization energies built into the single-molecule building blocks of materials and devices may be reversed in the solid state.

Multilayer donor regions in organic photovoltaics show improved power conversion efficiency when arranged in decreasing exciton energy order from the anode to the acceptor interface. These so-called "energy cascades" drive exciton transfer from the anode to the dissociating interface while reducing exciton quenching and allowing improved overlap with the solar spectrum. Here we investigate the relative importance of exciton transfer and blocking in a donor cascade employing diphenyltetracene (D1), rubrene (D2), and tetraphenyldibenzoperiflanthene (D3) whose optical gaps monotonically decrease from D1 to D3. In this structure, D1 blocks excitons from quenching at the anode, D2 accepts transfer of excitons from D1 and blocks excitons at the interface between D2 and D3, and D3 contributes the most to the photocurrent due to its strong absorption at visible wavelengths, while also determining the open circuit voltage. We observe singlet exciton Förster transfer from D1 to D2 to D3 consistent with cascade operation. The power conversion efficiency of the optimized cascade OPV with a C60 acceptor layer is 7.1 ± 0.4%, which is significantly higher than bilayer devices made with only the individual donors. We develop a quantitative model to identify the dominant exciton processes that govern the photocurrent generation in multilayer organic structures.

We investigate dipolar donor materials mixed with a C 70 acceptor in an organic photovoltaic (OPV) cell. Dipolar donors that have donor-acceptor-acceptor (d-a-a') structure result in high conductivity pathways due to close coupling between neighboring molecules in the mixed films.We analyze the charge transfer properties of the dipolar donor:C 70 mixtures and corresponding neat donors using a combination of time-resolved electroluminescence from intermolecular polaron pair states and conductive tip atomic force microscopy, from which we infer that dimers of the d-a-a' donors tend to form a continuous network of nanocrystalline clusters within the blends. Additional insights are provided by quantum mechanical calculations of hole transfer coupling and hopping rates between donor molecules using nearest neighbor donor packing motifs taken from crystal structural data. The approximation using only nearest neighbor interactions leads to good agreement between donor hole hopping rates and the conductive properties of the donor:C 70 blends. This represents a significant simplification from requiring details of the nano-and mesoscale morphologies of thin films to estimate their electronic 2 characteristics. Using these dipolar donors, we obtain a maximum power conversion efficiency of 9.6 ± 0.5 % under 1 sun, AM1.5G simulated illumination for an OPV comprised of an active layer containing a dipolar donor mixed with C 70 .3

The effects of intermolecular interactions on the electronic properties of bis-triisopropylsilylethynyl-substituted (TIPS) anthracene, tetracene, and pentacene are obtained from comparison of the ionization energies measured by solid-phase ultraviolet photoelectron spectroscopy (UPS) with the ionization energies measured by gas-phase UPS, and with the oxidation potentials measured electrochemically in solution. Additional insight is provided by electronic structure calculations at the density functional theory level. The results show that the solution-phase oxidation potentials correlate linearly with the gas-phase first ionization energies of TIPS oligoacenes, and both energies decrease with the increase in acene core size as expected for the increasing delocalization of the HOMO. However, the solid-phase ionization energies are independent of the acene core size, and thus do not follow the trend indicated by the molecular electronic structures and verified by the gas-phase and solution measurements. The solid-phase electronic properties such as charge injection barriers, ionization energies, and HOMO−LUMO energy gaps are greatly affected by the polarization effects of the surrounding molecules in the solid state, which dominate over the changes in molecular electronic properties caused by the change in acene core size.