Conjugated polyelectrolytes comprise an electronically delocalized backbone with pendant groups bearing ionic functionalities. Important new developments regarding their use to improve charge injection from metallic electrodes into organic semiconductors, a key requirement in emissive devices, have recently appeared. This article provides an overview of recent studies concerning the basic properties of conjugated polyelectrolytes as a function of molecular structure, and of optoelectronic devices with conjugated polyelectrolytes as essential functional components. Processes where more insightful mechanistic understanding is needed and areas of opportunity are discussed.

Polymer bulk heterojunction (BHJ) solar cells have generated notable scientific interest due to the potential for lowering device fabrication costs by taking advantage of solution-deposition methods. [1][2][3][4] One of the more critical factors in determining the power conversion efficiencies (PCEs) of these devices is the morphology of the BHJ interpenetrating network comprising the donor (D, a conjugated polymer) and acceptor (A, typically a fullerene derivative) components. [5][6][7][8][9][10][11] For fabrication to be simple, the self-assembly processes that control the final D/A organization at the nanoscale need to operate during the timescale of film formation. The spatial dimensions and connectivity of the D/A domains are dictated by the optoelectronic properties of the individual materials and the general requirements for operation of the device. For example, domain sizes are constrained by the need for excitons to reach D/A interfaces, where the charge-generation processes take place. There also needs to be continuous pathways for charge carriers to reach the electrodes and internal order within the individual phases for optimizing charge-carrier mobilities.A number of options, including solvent and thermal annealing, and the chemical structure of the materials have been identified experimentally as significant factors affecting the nanoscale morphology.[6] These choices affect the complex relationships between parameters that govern phase separation, such as the Flory-Huggins interaction, the tendency of the components to aggregate, as well as the kinetic constraints. [6][7][8] One particularly effective way to control the phase separation in BHJ devices precisely at the time of thin-film deposition and thereby improve performance, is by processing with solvent additives. [12][13][14] This method circumvents the need for postdeposition optimization via various annealing protocols. [15,16] Criteria that have emerged for selecting a potentially useful additives include: 1) it needs to have a higher boiling point than the parent solvent, and 2) it must be a poor solvent for the conjugated polymer and a better solvent for the fullerene component. [17][18][19] Such a combination of properties leads to aggregation of the polymer in solution, which translates to an increase in the sizes of the BHJ domains in the film. [12][13][14][17][18][19][20][21][22][23] Here we show a substantial improvement in the performance of a polymer BHJ solar cell by introducing an additive that is a good solvent for both components and the use of which ultimately leads to better mixing of the individual phases. This work increases the scope of function for additives to control the nanoscale organization of BHJ active layer films, to improve the efficiency of devices, and to open new lines of inquiry regarding the mechanism of film formation in multicomponent organic semiconducting films.In this work we introduced a novel donor polymer, namely poly [(4,4-didodecyldithieno[3,2-b, as shown in Scheme 1. This low-bandgap polymer was ...

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We report on the design, synthesis and characterization of light harvesting small molecules for use in solution-processed small molecule bulk heterojunction (SM-BHJ) solar cell devices. These molecular materials are based upon an acceptor/donor/acceptor (A/D/A) core with donor endcapping units. Utilization of a dithieno(3,2-b;2 0 ,3 0 -d)silole (DTS) donor and pyridal[2,1,3]thiadiazole (PT) acceptor leads to strong charge transfer characteristics, resulting in broad optical absorption spectra extending well beyond 700 nm. SM-BHJ solar cell devices fabricated with the specific example 5,5 0 -bis{7-(4-(5hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine}-3,3 0 -di-2-ethylhexylsilylene-2,2 0bithiophene (6) as the donor and [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) as the acceptor component showed short circuit currents above À10 mA cm À2 and power conversion efficiencies (PCEs) over 3%. Thermal processing is a critical factor in obtaining favorable active layer morphologies and high PCE values. A combination of UV-visible spectroscopy, conductive and photoconductive atomic force microscopies, dynamic secondary mass ion spectrometry (DSIMS), and grazing incident wide angle X-ray scattering (GIWAXS) experiments were carried out to characterize how thermal treatment influences the active layer structure and organization.

We show that polymer light-emitting diodes with high workfunction cathodes and conjugated polyelectrolyte injection/transport layers exhibit excellent efficiencies despite large electroninjection barriers. Correlation of device response times with structure provides evidence that the electron-injection mechanism involves redistribution of the ions within the polyelectrolyte electron-transport layer and hole accumulation at the interface between the emissive and electron-transport layers. Both processes lead to screening of the internal electric field and a lowering of the electron-injection barrier. The hole and electron currents are therefore diffusion currents rather than drift currents. The response time and the device performance are influenced by the type of counterion used.conjugated polyelectrolytes ͉ ion motion ͉ polymer light-emitting diodes ͉ electron transporting layer ͉ charge injection L ight-emitting diodes and thin-film transistors fabricated with semiconducting (conjugated) polymers are examples of an emerging technology with potential impact in low-cost displays and solid-state lighting (1). Balanced charge injection (holes into the -band from the anode and electrons into the *-band from the cathode) is a basic requirement of high-efficiency polymer light-emitting diodes (PLEDs) (2). In the absence of interfacial effects, the barrier for electron injection is determined by the difference between the energy of the bottom of the *-band (lowest occupied molecular orbit, LUMO) of the polymer and the Fermi energy of the metal used as the cathode; similarly, the barrier for hole injection is determined by the difference between the energy of the top of the -band (highest occupied molecular orbit, HOMO) and the Fermi energy of the anode. Because the charge injection is described (in first approximation) by a combination of Fowler-Nordheim tunneling and thermionic emission mechanisms, these barriers limit the device performance (3). Large and unequal barriers reduce power and optical output efficiencies by increasing the turn-on voltages and creating unbalanced injection of charge carriers. Electron injection continues to be an important problem because low workfunction metals such as Ca or Ba, with Fermi energies that match the *-bands of organic semiconductors, are unstable and decrease device operational lifetimes.Inserting injection/transport layers (TLs) between the emissive layer (EL) and the electrodes can improve charge injection into organic LEDs via different mechanisms. For example, a favorable dipole can be introduced that shifts the vacuum level at the electrode/TL interface (4). Charge-carrier blocking and accumulation at the EL/TL interface can also lead to improved injection by redistributing the field toward the TL and thereby reducing the charge tunneling distance (5-8). Efficient electron injection from stable metals into PLEDs incorporating a conjugated polyelectrolyte (CPE) electron TL (ETL) was recently demonstrated. CPE materials are characterized by a -delocalized backbone with p...