Organic and printed electronics technologies require conductors with a work function that is sufficiently low to facilitate the transport of electrons in and out of various optoelectronic devices. We show that surface modifiers based on polymers containing simple aliphatic amine groups substantially reduce the work function of conductors including metals, transparent conductive metal oxides, conducting polymers, and graphene. The reduction arises from physisorption of the neutral polymer, which turns the modified conductors into efficient electron-selective electrodes in organic optoelectronic devices. These polymer surface modifiers are processed in air from solution, providing an appealing alternative to chemically reactive low-work function metals. Their use can pave the way to simplified manufacturing of low-cost and large-area organic electronic technologies.

The technology-relevant zinc-terminated zinc oxide (0002) polar surface has been studied at the density-functional theory level using both Perdew−Burke− Ernzerhof (PBE) and hybrid Heyd−Scuseria−Ernzerhof (HSE06) functionals. We have considered a number of surface conditions to better understand the impact of surface hydroxylation and intrinsic and extrinsic surface defects, including zinc vacancies, oxygen vacancies, zinc interstitials, and aluminum dopants on the surface electronic properties. Our calculations point to large variations in surface work function and energy band gap as a function of the surface model; these variations can be attributed to changes in surface charge carrier density and to additional surface states induced by the defects. The calculated shifts in O(1s) core-level binding energy of the surface oxygens in different bonding configurations are in good agreement with experimental X-ray photoelectron spectroscopy data and point to the presence of two distinct OH-species on the ZnO surface. Our results also show that the electron-compensation centers induced by zinc vacancies can be stabilized by intrinsic and/or extrinsic n-type doping near the surface; such n-type doping can lead to better performance of organic opto-electronic devices in which zinc oxide is used as an electronselective interlayer.

We report on a systematic study of solvent and polymer matrix effects on the phase segregation behavior of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) blends incorporated into two different amorphous polymer matrices, poly (a-methyl styrene) and poly (triarylamine), and using two solvents, chlorobenzene and tetralin. Optical microscopy, X-ray diffraction analyses, and optical absorption measurements are used to evaluate the film morphology, crystallinity, and optical density, respectively. These analyses are correlated with the extent of vertical segregation of TIPS-pentacene, as observed for the blended films by depth-profile XPS analyses. The microstructure and vertical phase segregation of TIPS-pentacene in blend films are found to be strongly influenced by the choice of solvent. Tetralin, a solvent with a high boiling temperature, was found to be more desirable for achieving distinct phase segregation/crystallization of TIPS-pentacene in blend films and best performance in OFETs with a dual-gate geometry. The electrical properties of top and bottom channels were consistent with the morphological characterization and OFETs processed from tetralin showed higher mobility values than those from chlorobenzene. Further modification of the annealing conditions in the TIPS-pentacene/PTAA/tetralin ternary system led to top-gate OFETs with mobility values up to 2.82 cm 2 /Vs.

We report on the photovoltaic properties of inverted polymer solar cells where the transparent electron-collecting electrode is formed by a ZnO-modified indium−tin oxide (ITO) electrode. The ZnO layers were deposited by atomic layer deposition (ALD) with varying thicknesses from 0.1 to 100 nm. The work function, surface roughness, and morphology of ITO/ZnO were found to be independent of the ZnO thickness. However, the device performance was found to be strongly dependent on a critical ZnO thickness, around 10 nm. Below the critical thickness the device performance was degraded because of the appearance of a “kink” in the current−voltage characteristics. The kink features became less pronounced after ultraviolet (UV) exposure. This was attributed to oxygen desorption, leading to an increased conductivity of the ZnO layer. At and above this critical thickness, the device performance significantly improved and no longer depended strongly on the thickness of the ZnO layer, in agreement with optical simulations. Instead, these optical simulations showed that the thickness of the active layer plays a more important role than the thickness of the ZnO layer in optimizing the photovoltaic properties of inverted solar cells. Inverted polymer solar cells with an increased thickness of the active layer showed a power conversion efficiency (PCE) of 3.06% estimated for AM1.5G, a 100 mW cm−2 illumination.

We report on indium tin oxide (ITO)-free and metal-free semitransparent organic solar cells with a high-conductivity poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (PH1000) as both the bottom and the top electrodes. The PH1000 film showed a conductivity of 680±50 S/cm. A ZnO layer was used as an interlayer to produce an electron-selective electrode. The semitransparent devices with a structure of glass/PH1000/ZnO/poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester/PEDOT:PSS (CPP 105 D)/PH1000 exhibited an average power conversion efficiency of 1.8% estimated for 100 mW/cm2 air mass 1.5 global illumination. This geometry alleviates the need of vacuum deposition of a top electrode.