Organic photovoltaic cells (OPVs) are promising solar electric energy conversion systems with impressive recent optimization of active layers. OPV optimization must now be accompanied by the ...development of new charge-selective contacts and interlayers. This Perspective considers the role of interface science in energy harvesting using OPVs, looking back at early photoelectrochemical (photogalvanic) energy conversion platforms, which suffered from a lack of charge carrier selectivity. We then examine recent platforms and the fundamental aspects of selective harvesting of holes and electrons at opposite contacts. For blended heterojunction OPVs, contact/interlayer design is especially critical because charge harvesting competes with recombination at these same contacts. New interlayer materials can modify contacts to both control work function and introduce selectivity and chemical compatibility with nonpolar active layers and add thermodynamic and kinetic selectivity to charge harvesting. We briefly discuss the surface and interface science required for the development of new interlayer materials and take a look ahead at the challenges yet to be faced in their optimization.
In this work, we investigate material design criteria for low-powered/self-powered and efficient organic electrochemical transistors (OECTs) to be operated in the faradaic mode (detection at the gate ...electrode occurs via electron transfer events). To rationalize device design principles, we adopt a Marcus–Gerischer perspective for electrochemical processes at both the gate and channel interfaces. This perspective considers density of states (DOS) for the semiconductor channel, the gate electrode, and the electrolyte. We complement our approach with energy band offsets of relevant electrochemical potentials that can be independently measured from transistor geometry using conventional electrochemical methods as well as an approach to measure electrolyte potential in an operating OECT. By systematically changing the relative redox property offsets between the redox-active electrolyte and semiconducting polymer channel, we demonstrate a first-order design principle that necessary gate voltage is minimized by good DOS overlap of the two redox processes at the gate and channel. Specifically, for p-type turn-on OECTs, the voltage-dependent, electrochemically active semiconductor DOS should overlap with the oxidant form of the electrolyte to minimize the onset voltage for transconductance. A special case where the electrolyte can be used to spontaneously dope the polymer via charge transfer is also considered. Collectively, our results provide material design pathways toward the development of simple, robust, power-saving, and high-throughput OECT biosensors.
We report two strategies toward the synthesis of 3-alkyl-4-fluorothiophenes containing straight (hexyl and octyl) and branched (2-ethylhexyl) alkyl groups. We demonstrate that treatment of the ...dibrominated monomer with 1 equiv of alkyl Grignard reagent leads to the formation of a single regioisomer as a result of the pronounced directing effect of the fluorine group. Polymerization of the resulting species affords highly regioregular poly(3-alkyl-4-fluoro)thiophenes. Comparison of their properties to those of the analogous non-fluorinated polymers shows that backbone fluorination leads to an increase in the polymer ionization potential without a significant change in optical band gap. Fluorination also results in an enhanced tendency to aggregate in solution, which is ascribed to a more co-planar backbone on the basis of Raman and DFT calculations. Average charge carrier mobilities in field-effect transistors are found to increase by up to a factor of 5 for the fluorinated polymers.
Printable electronic devices from organic semiconductors are strongly desired but limited by their low conductivity and stability relative to those of their inorganic counterparts. p-Doping of ...poly(3-hexyl)thiophene (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane increases conductivity through integer charge transfer (ICT) to form mobile carriers in P3HT. An alternate undesired reaction pathway is formation of a partial charge transfer complex (CPX), which results in a localized, traplike state for the hole on P3HT. This effort addresses the stability of the free carrier states, once formed. Herein, we demonstrate that, while the ICT state may be kinetically preferred, the CPX state is thermodynamically more stable. Conversion of the ICT state to the CPX state is monitored here over time using a combination of infrared and photoelectron spectroscopies and supported by a complete loss of film conductivity with an increased CPX state concentration. Both the fraction and the rate of conversion to the CPX state are influenced by polymer molecular weight, dopant concentration, and storage conditions, with ambient storage conditions accelerating the conversion. This work suggests that a renewed focus on dopant–matrix reaction chemistry should be considered in the context of both kinetic and thermodynamic considerations.
Understanding the interaction between organic semiconductors (OSCs) and dopants in thin films is critical for device optimization. The proclivity of a doped OSC to form free charges is predicated on ...the chemical and electronic interactions that occur between dopant and host. To date, doping has been assumed to occur via one of two mechanistic pathways: an integer charge transfer (ICT) between the OSC and dopant or hybridization of the frontier orbitals of both molecules to form a partial charge transfer complex (CPX). Using a combination of spectroscopies, we demonstrate that CPX and ICT states are present simultaneously in F
TCNQ-doped P3HT films and that the nature of the charge transfer interaction is strongly dependent on the local energetic environment. Our results suggest a multiphase model, where the local charge transfer mechanism is defined by the electronic driving force, governed by local microstructure in regioregular and regiorandom P3HT.
Controlling interfacial electron-transfer rates is fundamental to maximizing device efficiencies in electrochemical technologies including redox-flow batteries, chemical sensors, bioelectronics, and ...photo-electrochemical devices. Conductive polymer electrodes offer the possibility to control redox properties through synthesis and processing, if critical structure–property relationships governing charge transfer are understood. In this work, we show that the rate and symmetry of electron transfer at conductive polymer electrodes are directly connected to the microstructure and the density of states (DOS) using the model system of poly(3-hexylthiophene) (P3HT) and ferrocene/ferrocenium (Fc/Fc+), as predicted by the Marcus–Gerischer model. Experimentally, crystalline P3HT exhibits a sufficient overlap between the polymer DOS and the DOS of both Fc and Fc+, resulting in a reversible electron transfer. Conversely, the DOS of amorphous electrodeposited P3HT does not overlap with that of Fc+, inhibiting reduction (i.e., kinetic selectivity for oxidation). This proof-of-concept work offers a paradigm to predict and control the kinetics at the polymer/liquid interface for applications from biology to energy.
The influence of protonation reactions between poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and a thiadiazolo3,4‐cpyridine small‐molecule donor are reported; these result in ...poor solar‐cell performance due to a barrier for charge extraction. The use of a NiOx contact eliminates such deleterious chemical interactions and results in substantial improvements in open‐circuit voltage, fill factor, and an increased power conversion efficiency from 2.3% to 5.1%.
Organic semiconductors are increasingly employed in electrochemical devices for energy conversion and storage and chemical sensing. In these systems, the conductivity can be modulated with ...electrochemical doping with substantial variation in electronic charge densities (1016 to 1021 cm–3) stabilized by electromigration of counterions from the electrolyte phase. Herein, we focus on the model system of regioregular poly(3-hexylthiophene) to determine the structural evolution at the onset of conductivity arising from electrochemical doping, specifically targeting elucidation of structural relaxation that precedes volumetric swelling. Using spectroelectrochemical methods, a 20% electrochemical active fraction of the film volume comprised of a nanocrystallite subpopulation serves as a high doping efficiency charge nucleation site with an increase from 1016 to 1020 carriers/cm–3. A small carrier density window is observed where structural reversion of J-to-H aggregates occurs due to electrostatic repulsion of neighboring charges (bipolarons) on the nanocrystallites. After this conformational change, further increase in doping leads to generation of free volume for counterion diffusion in the nanocrystallites along with doping of the amorphous fraction and J-aggregate recovery. This result advances the structural knowledge of conductive polymer electrodes for electrochemical devices beyond what has been reported using X-ray scattering and provides a benchmark for synthetic structural changes to control hybrid electrical–ionic transport, emphasizing the need to control structural conformation relaxations in addition to volumetric swelling.
The characterization and implementation of solution-processed, wide bandgap nickel oxide (NiO x ) hole-selective interlayer materials used in bulk-heterojunction (BHJ) organic photovoltaics (OPVs) ...are discussed. The surface electrical properties and charge selectivity of these thin films are strongly dependent upon the surface chemistry, band edge energies, and midgap state concentrations, as dictated by the ambient conditions and film pretreatments. Surface states were correlated with standards for nickel oxide, hydroxide, and oxyhydroxide components, as determined using monochromatic X-ray photoelectron spectroscopy. Ultraviolet and inverse photoemission spectroscopy measurements show changes in the surface chemistries directly impact the valence band energies. O2-plasma treatment of the as-deposited NiO x films was found to introduce the dipolar surface species nickel oxyhydroxide (NiOOH), rather than the p-dopant Ni2O3, resulting in an increase of the electrical band gap energy for the near-surface region from 3.1 to 3.6 eV via a vacuum level shift. Electron blocking properties of the as-deposited and O2-plasma treated NiO x films are compared using both electron-only and BHJ devices. O2-plasma-treated NiO x interlayers produce electron-only devices with lower leakage current and increased turn on voltages. The differences in behavior of the different pretreated interlayers appears to arise from differences in local density of states that comprise the valence band of the NiO x interlayers and changes to the band gap energy, which influence their hole-selectivity. The presence of NiOOH states in these NiO x films and the resultant chemical reactions at the oxide/organic interfaces in OPVs is predicted to play a significant role in controlling OPV device efficiency and lifetime.
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