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Photocurrent generation in organic bulk heterojunction (BHJ) solar cells is most commonly understood as a process which predominantly involves photoexcitation of the lower ionization potential species (donor) followed by electron transfer to the higher electron affinity material (acceptor) [i.e., photoinduced electron transfer (PET), which we term Channel I]. A mirror process also occurs in which photocurrent is generated through photoexcitation of the acceptor followed by hole transfer to the nonexcited donor or photoinduced hole transfer (PHT), which we term Channel II. The role of Channel II photocurrent generation has often been neglected due to overlap of the individual absorption spectra of the donor and acceptor materials that are commonly used. More recently Channel II charge generation has been explored for several reasons. First, many of the new high-efficiency polymeric donors are used as the minority component in bulk heterojunction blends, and therefore, the acceptor absorption is a significant fraction of the total; second, nonfullerene acceptors have been prepared, which through careful design, allow for spectral separation from the donor material, facilitating fundamental studies on charge generation. In this article, we review the methodologies for investigating the two charge generation channels. We also discuss the factors that affect charge generation via Channel I and II pathways, including energy levels of the materials involved, exciton diffusion, and other considerations. Finally, we take a comprehensive look at the nonfullerene acceptor literature and discuss what information about Channel I and Channel II can be obtained from the experiments conducted and what other experiments could be undertaken to provide further information about the operational efficiencies of Channels I and II.

Triarylamine copolymers are p-type organic semiconducting materials which have been shown to have the crucial advantages of being air-stable, forming amorphous films (critical for device uniformity over large areas) and allowing the creation of high mobility transistors and highly efficient perovskite solar cells. A key area of recent technological progress has been the development of solution-processable metal oxides as charge injection layers in organic semiconducting devices. Here we report on the synthesis of a large ionization potential (IP = 5.65 eV) silafluorene bridged triarylamine copolymer poly(silafluorene-triarylamine) (PSiF-TAA), and compare its time-of-flight (TOF) bulk hole mobility to that of a fluorene bridged triarylamine copolymer poly(fluorene-triarylamine) (PF-TAA), (IP = 5.4 eV) and the homopolymer polytriarylamine (PTAA) (IP = 5.2 eV). Using these mobility values and current–voltage measurements, we then quantify the charge injection efficiency (χ) into these polymers from three ambiently-prepared solution processed hole-injecting contacts MoO3 (aqueous nanoparticle dispersion), V2O5 (sol–gel) and chlorinated indium tin oxide (Cl-ITO) (UV – solvent assisted), and compare them to the more conventional contacts ITO and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Whilst hole injection into PTAA is relatively unaffected by the nature of the contact, injection into PF-TAA and PSiF-TAA is greatly improved by the use of MoO3 and Cl-ITO. Despite its similar mobility and larger ionization potential compared to the homopolymer, the highest injection efficiency is achieved for PF-TAA, indicating the role of chemical design in optimizing charge injection into organic semiconductor devices.

The dielectric properties of three pairs of organic semiconductors that contain increasing numbers of cyclopentadithiophene-co-benzothiadiazole moieties (monomer, dimer and polymer) were studied and compared. The materials in each pair differed in the nature of the ‘solubilizing groups’, which are either alkyl- or glycol-based. At low frequencies (

Post polymerisation fractionation of diketopyrrolopyrrole based conjugated polymers through preparative gel permeation chromatography affords a varying range of molecular weight fractions with narrowed polydispersities. When used as the electron donor material in bulk heterojunction solar cells with both conventional and inverted architecture efficiency enhancements in excess of 50% are observed relative to non-fractionated material with the highest molecular weight fraction demonstrating a power conversion efficiency of 6.3%.

In this review, we describe and discuss recent advances in the performance of polymeric semiconductors in organic field effect transistors (OFETs). Design concepts such as short intermolecular contacts, low conformational disorder, side chain optimization, and noncovalent interactions have all been successfully employed to improve the charge carrier mobility of polymer thin films. The relationship between the molecular design, thin film microstructure, and electrical performance has been exemplified by a range of thiophene, bridged and fused ring, diketopyrrolopyrrole (DPP), and isoindigo containing polymers which are reviewed and discussed.

The low dielectric constants of conventional organic semiconductors leads to poor charge carrier photogeneration in homojunction organic solar cells due to large exciton binding energies. Increasing the dielectric constant can potentially enhance the spontaneous exciton dissociation rate at room temperature in homojunction cells, and decrease the charge carrier recombination in heterojunction solar cells comprising blends of electron donors and acceptors. We report that substituting the ubiquitous alkyl solubilizing groups with short glycol chains can give non-fullerene electron acceptors with a static dielectric constant of up to 9.8.

Correction for ‘Dielectric constant enhancement of non-fullerene acceptors via side-chain modification’ by Jenny E. Donaghey et al., Chem. Commun., 2015, 51, 14115–14118.

The synthesis and characterization of four new donor–acceptor type semiconducting polymers based on the electron-deficient diketopyrrolopyrrole unit for use in photovoltaic devices are reported. The extended 2-decyl-1-tetradecyl alkyl chain (C10C12) is utilized to provide a valuable route toward enhanced solubility. The resultant soluble DPP polymers were synthesized in high polymer molecular weights with a larger range of accessible comonomer units. Bulk heterojunction solar cells containing these polymers show promising device performance with efficiencies around 4%. Organic thin film transistor devices with high ambipolar hole mobilities of >0.1 cm2 V–1 s–1 are also reported.

A new tetradodecyl-substituted DTBTBT donor unit is synthesized by a specific bis-annulation via Suzuki–Miyaura coupling and successfully incorporated into light absorbing electron donor copolymers for OPV and hole and electron transport OFET polymer devices. All copolymers (DTBTBT-co-benzothiadiazole (Bz), DTBTBT-co-thiophene (T) and DTBTBT-co-thienothiophene (TT)) show fully coplanar backbones and strong intermolecular interactions. The DTBTBT-Bz copolymer led to a deep HOMO level (−5.2 eV) and thus a large Voc value of 0.92 V with PC71BM as electron acceptor and a PCE of 3.7% with a Jsc of 6.78 mA/cm2 could be obtained. A hole mobility of 0.1 cm2/(V s) has been observed for the highly coplanar and more crystalline DTBTBT-T copolymer.

Lactam-based semiconducting donor polymer materials often exhibit high performance in the photoactive layer of solution processed organic solar cells. In this review we focus on the structure–property–device performance relationships, offering a set of rational design rules for next generation materials from a chemical structure perspective.

The prospect of using low cost, high throughput material deposition processes to fabricate organic circuitry and solar cells continues to drive research towards improving the performance of the semiconducting materials utilized in these devices. Conjugated aromatic polymers have emerged as a leading candidate semiconductor material class, due to their combination of their amenability to processing and reasonable electrical and optical performance. Challenges remain, however, to further improve the charge carrier mobility of the polymers for transistor applications and the power conversion efficiency for solar cells. This optimization requires a clear understanding of the relationship between molecular structure and both electronic properties and thin film morphology. In this Account, we describe an optimization process for a series of semiconducting polymers based on an electron rich indacenodithiophene aromatic backbone skeleton. We demonstrate the effect of bridging atoms, alkyl chain functionalization, and co-repeating units on the morphology, molecular orbital energy levels, charge carrier mobility, and solar cell efficiencies. This conjugated unit is extremely versatile with a coplanar aromatic ring structure, and the electron density can be manipulated by the choice of bridging group between the rings. The functionality of the bridging group also plays an important role in the polymer solubility, and out of plane aliphatic chains present in both the carbon and silicon bridge promote solubility. This particular polymer conformation, however, typically suppresses long range organization and crystallinity, which had been shown to strongly influence charge transport. In many cases, polymers exhibited both high solubility and excellent charge transport properties, even where there was no observable evidence of polymer crystallinity. The optical bandgap of the polymers can be tuned by the combination of the donating power of the bridging unit and the electron withdrawing nature of co-repeat units, alternating along the polymer backbone. Using strong donors and acceptors, we could shift the absorption into the near infrared.

The synthesis, characterization and testing of four novel pyrroloindacenodithiophene (NIDT) copolymers, in organic field effect transistor (OFET) devices, is reported. Copolymerization of thiophene, thienothiophene, thienopyrrolodione and diketopyrrolopyrrole with NIDT, to form alternating copolymers, is described. Two-dimensional Grazing Incidence Wide Angle X-ray Diffraction (2D GIWAXD) is used to probe the thin film molecular structure and, together with DFT studies, help to explain why the diketopyrrolopyrrole containing polymer outperforms the others in terms of hole transport, exhibiting an average mobility of 0.01 cm2 V−1 s−1.

The synthesis of the novel electron-rich pyrroloindacenodithiophene (NIDT) unit is reported. Stille copolymerization of the distannylated NIDT unit, with the electron-deficient dibrominated benzothiadiazole (BT), difluorobenzothiadiazole (ffBT), thienopyrrolodione (TPD) and 1,1′-bithienopyrrolodione (biTPD) units afforded a series of low band gap semiconducting polymers. Initial testing shows promise for the use of these materials as p-type semiconductors in organic field effect transistors (OFETs) with mobilities as high as 0.07 cm2V−1s−1 being measured. These materials have also been tested as the donor polymer in polymer/fullerene bulk heterojunction organic photovoltaics (OPVs) giving maximum efficiencies of 2.5%.