Organic photovoltaic devices have been created on activated and modified ITO electrodes from electrodeposited poly(3-hexylthiophene) (e-P3HT) donor layers, using pulsed-potential-step (PPS) electrodeposition protocols. PPS electrodeposition uses a series of potential steps of diffusion-controlled e-P3HT deposition, alternated with rest periods where no deposition occurs and the diffusion layer region near the electrode/solution interface refills with thiophene monomer. To create the most photoactive e-P3HT films, a “carpet” layer of polymer was first deposited using dual step chronoamperometry, to create a smooth, pinhole-free film on the ITO electrode. PPS electrodeposition was subsequently used to electrodeposit additional polymer and texture the e-P3HT surface, as revealed by both AFM and SEM. The extent of doping of the polymer film was controlled by the last applied rest potential and monitored by anion incorporation into the e-P3HT film using X-ray photoelectron spectroscopy (XPS). Textured and electrochemically doped e-P3HT films were used as the donor layer in photovoltaic devices, using vacuum deposited C60 as the electron acceptor/electron transport layer: (ITO/e-P3HT/C60/BCP/Al). The performance of these ultrathin OPVs was markedly dependent upon the degree of electrochemical doping of the P3HT layers. The best OPV performance was obtained for e-P3HT films with an average doping level (ratio of oxidized to reduced thiophene units) of approximately 35%, as estimated by XPS. At 100 mW/cm2 white light illumination, optimized devices give a V OC ∼ 0.5 V and a maximum J SC ∼ 3 mA/cm2, with series resistance (R S) below 1 Ω·cm2, shunt resistance (R P) in excess of 160 kΩ·cm2, fill-factors (FF) of approximately 0.65, and an overall power conversion efficiency of approximately 1%. These results demonstrate the promise of electrochemical protocols for the creation of a variety of hybrid energy conversion materials.

We present a comparison of the photovoltaic activity of organic solar cells (OPVs) based on vacuum-deposited and solvent-annealed titanyl phthalocyanine (TiOPc) donor layers with C60 as the electron acceptor, where the TiOPc donor layer exists in three different polymorphic forms: TiOPc included the “as-deposited” form, with a Q-band absorbance spectrum reminiscent of the phase I polymorph, and films subjected to solvent annealing which systematically increased the fraction of either the phase II (α-phase) or the phase III (γ-TiOPc) polymorphs. As-deposited TiOPc/C60 heterojunctions showed open-circuit photopotentials (V OC) of ca. 0.65 V, with estimated AM 1.5G efficiencies of ca. 1.4%. Partial conversion of these thin films to their phase II or phase III polymorphs significantly enhanced the short-circuit photocurrent (J SC), as a result of (i) texturing of the TiOPc layers (ca. 100 nm length scales) and (ii) enhancements in near-IR absorptivity/photoelectrical activity. All TiOPc/C60 heterojunctions, characterized by UV–photoelectron spectroscopy (UPS), showed that estimated E HOMO Pc – E LUMO C60 energy differences, which set the upper limit for V OC, are nearly identical for each TiOPc polymorph. Incident and absorbed photon current efficiency measurements (IPCE, APCE) are consistent with previous studies that showed a majority of the photoactivity in these higher order polymorphs deriving from excitonic states created at λmax ≈ 680 and 844 nm for both the phase II and the phase III polymorphs. The near-IR absorbance features (844 nm) in these Pc polymorphs are believed to have substantial charge-transfer (CT) character, leading to enhanced probabilities for photoinduced electron transfer (PIET). APCE measurements of TiOPc/C60 OPVs, however, show that higher photocurrent yields per absorbed photon arise from the higher energy (680 nm) excitonic state. When C70 is used as the electron acceptor, with its higher electron affinity, APCE was increased throughout the visible and near-IR region, now showing a nearly constant APCE across the entire Q-band spectrum of the TiOPc polymorphs, consistent with increased driving force (E LUMO Pc – E LUMO C60) for PIET involving the lowest energy CT excitonic state. The highest estimated AM 1.5G efficiencies for these textured TiOPc/C60 heterojunctions, with the TiOPc film partially converted to phase II or phase III polymorphs, are 4.5% and 3.5%, respectively.

We use electroabsorption spectroscopy to measure the change in built-in potential (VBI) across the polymer photoactive layer in diodes where indium tin oxide electrodes are systematically modified using dipolar phosphonic acid self-assembled monolayers (SAMs) with various dipole moments. We find that VBI scales linearly with the work function (Φ) of the SAM-modified electrode over a wide range when using a solution-coated poly(p-phenylenevinylene) derivative as the active layer. However, we measure an interfacial parameter of S = eΔVBI/ΔΦ < 1, suggesting that these ITO/SAM/polymer interfaces deviate from the Schottky-Mott limit, in contrast to what has previously been reported for a number of ambient-processed organic-on-electrode systems. Our results suggest that the energetics at these ITO/SAM/polymer interfaces behave more like metal/organic interfaces previously studied in UHV despite being processed from solution.

Poly(3-methylthiophene) (P3MT) was synthesized directly from indium tin oxide (ITO) electrodes modified with a phosphonic acid initiator, using Kumada catalyst transfer polymerization (KCTP). This work represents the first time that polymer thickness has been controlled in a surface initiated KCTP reaction, highlighting the utility of KCTP in achieving controlled polymerizations. Polymer film thicknesses were regulated by the variation of the solution monomer concentration and ranged from 30 to 265 nm. Electrochemical oxidative doping of these films was used to manipulate their near surface composition and effective work function. Doped states of the P3MT film are maintained even after the sample is removed from solution and potential control confirming the robustness of the films. Such materials with controllable thicknesses and electronic properties have the potential to be useful as interlayer materials for organic electronic applications.

Control of charge redistribution across e-P3HT/C60 interfaces is accomplished through the systematic electrochemical oxidation of the thiophene species.