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 p-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.

We show for the first time that the frontier orbital energetics (conduction band minimum (CBM) and valence band maximum (VBM)) of device-relevant, methylammonium bromide (MABr)-doped, formamidinium lead trihalide perovskite (FA-PVSK) thin films can be characterized using UV-vis spectroelectrochemistry, which provides an additional and straightforward experimental technique for determining energy band values relative to more traditional methods based on photoelectron spectroscopy. FA-PVSK films are processed via a two-step deposition process, known to provide high efficiency solar cells, on semitransparent indium tin oxide (ITO) and titanium dioxide (TiO) electrodes. Spectroelectrochemical characterization is carried out in a nonsolvent electrolyte, and the onset potential for bleaching of the FA-PVSK absorbance is used to estimate the CBM, which provides values of ca. -4.0 eV versus vacuum on both ITO and TiO electrodes. Since electron injection occurs from the electrode to the perovskite, the CBM is uniquely probed at the buried metal oxide/FA-PVSK interface, which is otherwise difficult to characterize for thick films. UPS characterization of the same FA-PVSK thin films provide complementary near-surface measurements of the VBM and electrode-dependent energetics. In addition to energetics, controlled electrochemical charge injection experiments in the nonsolvent electrolyte reveal decomposition pathways that are related to morphology-dependent heterogeneity in the electrochemical and chemical stability of these films. X-ray photoelectron spectroscopy of these electrochemically treated FA-PVSK films shows changes in the average near-surface stoichiometry, which suggests that lead-rich crystal termination planes are the most likely sites for electron trapping and thus nanometer-scale perovskite decomposition.

Chemisorption of an organic monolayer to tune the surface properties of a transparent conductive oxide (TCO) electrode can improve the performance of organic electronic devices that rely on efficient charge transfer between an organic active layer and a TCO contact. Here a series of perylene diimides (PDIs) was synthesized and used to study relationships between monolayer structure/properties and electron transfer kinetics at PDI-modified indium tin oxide (ITO) electrodes. In these PDI molecules, one of the imide substituents is a benzene ring bearing a phosphonic acid (PA) and the other is a bulky aryl group that is twisted out of the plane of the PDI core. The size of the bulky aryl group and the substitution of the benzene ring bearing the PA were both varied which altered the extent of aggregation when these molecules were absorbed as monolayer films (MLs) on ITO, as revealed by both attenuated total reflectance (ATR) and total internal reflection fluorescence (TIRF) spectra. Polarized ATR measurements indicate that in these MLs, the long axis of the PDI core is tilted at an angle of 33˚-42˚ relative to the surface normal; the tilt angle increased as the degree of bulky substitution increased. Rate constants for electron transfer (k s,opt ) between these redox-active modifiers and ITO were determined by potential-modulated ATR spectroscopy. As the degree of PDI aggregation was reduced, k s,opt declined which is attributed to a reduction in the lateral electron self-exchange rate between adsorbed PDI molecules, as well as the heterogeneous conductivity of the ITO electrode surface. Photoelectrochemical measurements using a dissolved aluminum phthalocyanine as an electron donor showed that ITO modified with any of these PDIs is a more effective electron-collecting electrode than bare ITO.

An electroreflectance method to determine the electron transfer rate constant of a film of redox-active chromophores immobilized on an optically transparent electrode when the surface coverage of the film is very low (<0.1 monolayer) is described herein. The method, potential-modulated total internal reflection fluorescence (PM-TIRF) spectroscopy, is a fluorescence version of potential-modulated attenuated total reflection (PM-ATR) spectroscopy that is applicable when the immobilized chromophores are luminescent. The method was tested using perylene diimide (PDI) molecules functionalized with p-phenylene phosphonic acid (PA) moieties that bind strongly to indium−tin oxide (ITO). Conditions to prepare PDI-phenyl-PA films that exhibit absorbance and fluorescence spectra characteristic of monomeric (i.e., nonaggregated) molecules were identified; the electrochemical surface coverage was approximately 0.03 monolayer. The tilt angle of the long axis of the PDI molecular plane is 58°r elative to the ITO surface normal, 25°greater than the tilt angle of aggregated PDI-phenyl-PA films, which have a surface coverage of approximately one monolayer. The more in-plane orientation of monomeric films is likely due to the absence of cofacial π−π interactions present in aggregated films and possibly a difference in PA-ITO binding modes. The electron transfer rate constant (k s,opt ) of monomeric PDI-phenyl-PA films was determined using PM-TIRF and compared with PM-ATR results obtained for aggregated films. For PDI monomers, k s,opt = 3.8 × 10 3 s −1 , which is about 3.7-fold less than k s,opt for aggregated films. The slower kinetics are attributed to the absence of electron self-exchange between monomeric PDI molecules. Differences in the electroactivity of the binding sites on the ITO electrode surface also may play a role. This is the first demonstration of PM-TIRF for determining electron transfer rate constants at an electrode/organic film interface.

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Electrogenerated chemiluminescence (ECL) was observed from TOPO-capped CdSe@CdSe nanorods (NRs) depositing on a metal electrode in phosphate buffer solution (PBS). Two ECL peaks at-1.05 and-1.33 V in pH 9.2, 0.1 M PBS were found under cyclic voltammetric conditions. Cyclic voltammetry of this solution displayed no distinctive features, on the other hand, light emission was observed during cyclic potential scans. The photoluminescence (PL) spectrum showed an emission maximum at 692 nm. The spectrum of ECL possesses one peak which coincides very well with the PL spectrum of the nanorods film. The mechanism for ECL peaks was proposed. Electron transfer reactions between charged nanorods and molecular redox-active coreactants such as dissolved oxygen and H2O2 or between positively and negatively charged nanorods occurred that led to electron and hole annihilation, producing light.