Quantifying the Extent of Contact Doping at the Interface between High Work Function Electrical Contacts and Poly(3-hexylthiophene) (P3HT)

We demonstrate new approaches to the characterization of oxidized regioregular poly­(3-hexylthiophene-2,5-diyl) (P3HT) that results from electronic equilibration with device-relevant high work function electrical contacts using high-resolution X-ray (XPS) and ultraviolet (UPS) photoelectron spectroscopy (PES). Careful interpretation of photoemission signals from thiophene sulfur atoms in thin (ca. 20 nm or less) P3HT films provides the ability to uniquely elucidate the products of charge transfer between the polymer and the electrical contact, which is a result of Fermi-level equilibration between the two materials. By comparing high-resolution S 2p core-level spectra to electrochemically oxidized P3HT standards, the extent of the contact doping reaction is quantified, where one in every six thiophene units (ca. 20%) in the first monolayer is oxidized. Finally, angle-resolved XPS of both pure P3HT and its blends with phenyl-C₆₁-butyric acid methyl ester (PCBM) confirms that oxidized P3HT species exist near contacts with work functions greater than ca. 4 eV, providing a means to characterize the interface and “bulk” region of the organic semiconductor in a single film

Surface Modification of Indium–Tin Oxide with Functionalized Perylene Diimides: Characterization of Orientation, Electron-Transfer Kinetics and Electronic Structure

Charge-transfer efficiency at the active layer/transparent conducting oxide (TCO) interface is thought to be a key parameter contributing to the overall efficiency of organic electronic devices such as organic photovoltaics (OPVs). Modification of the TCO surface with a redox-active surface modifier is a possible approach toward enhancing OPV efficiency by providing an efficient charge-transfer pathway between either hole- or electron-harvesting contacts and the organic active layer. Here we report on the modification of indium–tin oxide (ITO) electrodes with two perylene diimides (PDIs), coupled to phosphonic acid (PA) binding groups through a <i>p</i>-phenylene bridge or a biphenyl-4,4′-diyl bridge (PDI–phenyl–PA and PDI–diphenyl–PA, respectively). We used two different deposition techniques: adsorption from solution (SA) and spin coating (SC), to create three types of monolayer films on ITO: SA PDI–phenyl–PA, SA PDI–diphenyl–PA, and SC PDI–phenyl–PA. These thin films, designed to act as “charge-transfer mediators”, were used to study relationships between molecular structure, electron-transfer (ET) kinetics, and electronic structure. Molecular orientation was assessed using polarized attenuated total reflectance (ATR) spectroscopy; the average tilt angle between the PDI molecular axis and the ITO surface normal for both SA films was about 30°, while films deposited using spin-coating were more in-plane, with an average tilt angle of 45°. To our knowledge, these are the first reported measurements of orientation in PDI monolayers on ITO electrodes. Electrochemical and ultraviolet photoemission spectroscopy studies showed that all three PDI–PA films have similar reduction potentials, electron affinities, and ionization energies, indicating that differences in bridge length and molecular orientation did not measurably affect the interfacial electronic structure. ET rate constants ranging from 5 to 50 × 10³ s^–1 were measured using potential-modulated ATR spectroscopy. The kinetic and thermodynamic data, along with a photoelectrochemical comparison of electron injection efficiency, show that PDI–PA films are capable of serving as a charge-transfer mediator between an ITO electrode and an organic active layer, and thus have potential for use as electron-collection contacts in inverted OPV devices

Characterization of ZnO Interlayers for Organic Solar Cells: Correlation of Electrochemical Properties with Thin-Film Morphology and Device Performance

This report focuses on the evaluation of the electrochemical properties of both solution-deposited sol–gel (sg-ZnO) and sputtered (sp-ZnO) zinc oxide thin films, intended for use as electron-collecting interlayers in organic solar cells (OPVs). In the electrochemical studies (voltammetric and impedance studies), we used indium–tin oxide (ITO) over coated with either sg-ZnO or sp-ZnO interlayers, in contact with either plain electrolyte solutions, or solutions with probe redox couples. The electroactive area of exposed ITO under the ZnO interlayer was estimated by characterizing the electrochemical response of just the oxide interlayer and the charge transfer resistance from solutions with the probe redox couples. Compared to bare ITO, the effective electroactive area of ITO under sg-ZnO films was ca. 70%, 10%, and 0.3% for 40, 80, and 120 nm sg-ZnO films. More compact sp-ZnO films required only 30 nm thicknesses to achieve an effective electroactive ITO area of ca. 0.02%. We also examined the electrochemical responses of these same ITO/ZnO heterojunctions overcoated with device thickness pure poly­(3-hexylthiophehe) (P3HT), and donor/acceptor blended active layers (P3HT:PCBM). Voltammetric oxidation/reduction of pure P3HT thin films on ZnO/ITO contacts showed that pinhole pathways exist in ZnO films that permit dark oxidation (ITO hole injection into P3HT). In P3HT:PCBM active layers, however, the electrochemical activity for P3HT oxidation is greatly attenuated, suggesting PCBM enrichment near the ZnO interface, effectively blocking P3HT interaction with the ITO contact. The shunt resistance, obtained from dark current–voltage behavior in full P3HT/PCBM OPVs, was dependent on both (i) the porosity of the sg-ZnO or sp-ZnO films (as revealed by probe molecule electrochemistry) and (ii) the apparent enrichment of PCBM at ZnO/P3HT:PCBM interfaces, both effects conveniently revealed by electrochemical characterization. We anticipate that these approaches will be applicable to a wider array of solution-processed interlayers for “printable” solar cells

Improving the Charge Conductance of Elemental Sulfur via Tandem Inverse Vulcanization and Electropolymerization

The synthesis of polymeric materials using elemental sulfur (S₈) as the chemical feedstock has recently been developed using a process termed inverse vulcanization. The preparation of chemically stable sulfur copolymers was previously prepared by the inverse vulcanization of S₈ and 1,3-diisopropenylbenzene (DIB); however, the development of synthetic methods to introduce new chemical functionality into this novel class of polymers remains an important challenge. In this report the introduction of polythiophene segments into poly­(sulfur-<i>random</i>-1,3-diisopropenylbenzene) is achieved by the inverse vulcanization of S₈ with a styrenic functional 3,4-propylenedioxythiophene (ProDOT-Sty) and DIB, followed by electropolymerization of ProDOT side chains. This methodology demonstrates for the first time a facile approach to introduce new functionality into sulfur and high sulfur content polymers, while specifically enhancing the charge conductivity of these intrinsically highly resistive materials