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We designed two solution processable small molecules, 5-((5-(4-(diphenylamino)phenyl)thieno[3,2-b]thiophene-2-yl)methylene)-1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile (1) and 5-((6-(4-(diphenylamino)phenyl)-4-(2-ethylhexyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)methylene)-1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile (2), consisting of a triphenylamine electron donating group and a cyanopyridone electron accepting group linked by two different rigidified π-spacer thiophenes. Ultraviolet–visible absorption spectra revealed that the use of three fused rings (dithienopyrrole) as the conjugated π-spacer resulted in an enhanced intramolecular charge transfer transition and reduction of band gap, when compared with a non-fused bithiophene and two fused thiophenes (thienothiophene) analogues. Power conversion efficiencies (PCEs) of 2.39% and 2.14% were achieved for simple photovoltaic devices based on 1/PC61BM and 2/PC61BM under simulated AM 1.5 illumination (100 mW cm−2), respectively. It has been observed that the degree of conjugation of the central π-bridge was not a key factor for the enhancement of photovoltaic performance.

Here we show that the charge transfer (CT) absorption signal in bulk-heterojunction solar cell blends, measured by photothermal deflection spectroscopy, is directly proportional to the density of molecular donor:acceptor interfaces. Since the optical transitions from the ground state to the interfacial CT state are weakly allowed at photon energies below the optical gap of both the donor and acceptor, we can exploit the use of this sensitive linear absorption spectroscopy for such quantification. Moreover, we determine the absolute molar extinction coefficient of the CT transition for an archetypical polymer:fullerene interface. The latter is ∼100 times lower than the extinction coefficient of the donor chromophore involved, allowing us to experimentally estimate the transition dipole moment as 0.3 D and the electronic coupling between the ground and CT states to be on the order of 30 meV.

The microstructure and photophysics of low-band gap, all-polymer photovoltaic blends are presented. Blends are based on the donor polymer BFS4 (a dithienyl-benzo[1,2-b:4,5-b]dithiophene/5-fluoro-2,1,3-benzothiadiazole co-polymer) paired with the naphthalene diimide-based acceptor polymer P(NDI2OD-T2). Efficiencies of over 4% are demonstrated, with an open circuit voltage of greater than 0.9 V achieved. Transmission electron microscopy reveals a relatively coarse phase-separated morphology, with elongated domains up to 200 nm in width. Near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy and atomic force microscopy (AFM) measurements reveal that the top surface of BFS4:P(NDI2OD-T2) blends is covered with a pure BFS4 capping layer. Depth profiling measurements confirm this vertical phase separation with a surface-directed spinodal decomposition wave observed. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements confirm that BFS4 and P(NDI2OD-T2) are semicrystalline with both polymers retaining their semicrystalline nature when blended. Photoluminescence spectroscopy reveals incomplete photoluminescence quenching with as much as 30% of excitons failing to reach a donor/acceptor interface. Transient absorption spectroscopy measurements also find evidence for rapid geminate recombination.

The results of a study on the effects of encapsulant pre-conditioning (drying) on the durability of “conventional” and “inverted” bulk-heterojunction organic solar cells based on P3HT:PCBM are presented. The architectures of the conventional and inverted devices were ITO/PEDOT:PSS/P3HT:PCBM/Al and ITO/ZnO/P3HT:PCBM/MoO 3 /Ag, respectively. Quantitative analysis of the moisture content, moisture out-gassing, and re-absorbing properties of flexible barrier encapsulant films under ambient conditions was conducted. The effect of moisture out-gassing from the encapsulation materials on device performance was studied and the lifetime of conventional and inverted devices was found to decline significantly for the devices encapsulated using materials without pre-conditioning. This study reveals the essential role of pre-conditioning of materials in the encapsulation of organic photovoltaic devices. Current-voltage characteristics exhibit a clear increase in the series resistance of the devices during storage under ambient conditions, and Spectroscopic Impedance analysis indicates that charge-transport resistance within the devices increases significantly during the degradation process. The results are consistent with the primary degradation mechanism being corrosion at the metal-electrode surface.

Light-emitting electrochemical cells (LECs) consists of a thin film of an ionic organic semiconductor sandwiched between two electrodes. Because of the large density of ions, LECs are often reported to perform independently on the electrodes work function. Here we use metal oxides as charge injection layers and demonstrate that, although electroluminescence is observed independently of the electrodes used, the device performances are strongly dependent on the choice of the interface materials. Relying on metal oxide charge injection layers, such hybrid devices are of interest for real lighting applications and could pave the way for new efficient, stable, low-cost lighting sources.

Direct arylation (DA) is emerging as a highly promising method to construct inexpensive conjugated materials for large-area electronics from simple and environmentally benign building blocks. Here, we show that exclusive α-C–H selectivity is feasible in the DA of π-extended monomers having unsubstituted thiophene or furan units, leading to fully linear materials. Two new naphthalene diimide-based conjugated copolymers—P(FuNDIFuF4) and P(ThNDIThF4), composed of naphthalene diimide (NDI), furan (Fu) or thiophene (Th), and tetrafluorobenzene (F4)—are synthesized. Insight into structure–function relationships is given by density functional theory (DFT) calculations and variety of experimental techniques, whereby the effect of the heteroatom on the optical, structural, and electronic properties is investigated. The use of furan (Fu) allows for enhanced solubilities, a smaller dihedral angle between NDI and Fu as a result of the smaller size of Fu, and a smaller π–π-stacking distance in the solid state. P(FuNDI...

Broad spectral coverage over the solar spectrum is necessary for photovoltaic technologies and is a focus for organic solar cells. We report a series of small-molecule, nonfullerene electron acceptors containing the [(benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile unit as a high electron affinity component. The optoelectronic properties of these molecules were fine-tuned with the objective of attaining strong absorption at longer wavelengths by changing the low-ionization-potential moiety. The electron-accepting function of these materials was investigated with poly(3-n-hexylthiophene) (P3HT) as a standard electron donor. Significant photocurrent generation in the near infrared region, with an external quantum yield reaching as high as 22 % at 700 nm and an onset >800 nm was achieved. The results support efficient hole transfer to P3HT taking place after light absorption by the acceptor molecules. A Channel II-dominated power conversion efficiency of up to 1.5 % was, thus, achieved.

Detailed electrochemical studies have been undertaken on molecular aggregation of the organic semiconductor 7,14-bis((triisopropylsilyl)-ethynyl)dibenzo[b,def]chrysene (TIPS-DBC), which is used as an electron donor material in organic solar cells. Intermolecular association of neutral TIPS-DBC molecules was established by using (1)H NMR spectroscopy as well as by the pronounced dependence of the color of TIPS-DBC solutions on concentration. Diffusion limited current data provided by near steady-state voltammetry also reveal aggregation. Furthermore, variation of concentration produces large changes in shapes of transient DC and Fourier transformed AC (FTAC) voltammograms for oxidation of TIPS-DBC in dichloromethane. Subtle effects of molecular aggregation on the reduction of TIPS-DBC are also revealed by the highly sensitive FTAC voltammetric method. Simulations of FTAC voltammetric data provide estimates of the kinetic and thermodynamic parameters associated with oxidation and reduction of TIPS-DBC. Significantly, aggregation of TIPS-DBC facilitates both one-electron oxidation and reduction by shifting the reversible potentials to less and more positive values, respectively. EPR spectroscopy is used to establish the identity of one-electron oxidized and reduced forms of TIPS-DBC. Implications of molecular aggregation on the HOMO energy level in solution are considered with respect to efficiency of organic photovoltaic devices utilizing TIPS-DBC as an electron donor material.

We report on the use and stability of solution-processed molybdenum oxide (sMoOx) thin films as anode-modifying layers to replace conventionally used poly(3,4-ethyldioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) layers in poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT/PC61BM) bulk heterojunction organic solar cells. Our results show that while as prepared devices using the two anode-modifying layers possess similar performances, the sMoOx devices exhibit a staggering 20-fold stability improvement in its performance half-life compared with PEDOT/PSS devices, ~3400 h versus ~150 h, respectively. A further comparison of the stability between encapsulated and unencapsulated devices demonstrates the necessity for protection from atmospheric moisture and oxygen but again highlights the lucrative nature of sMoOx as a protective anode-modifying layer compared with PEDOT/PSS even under ambient conditions. Copyright © 2014 John Wiley & Sons, Ltd.

The device performance and microstructure of a series of PTB7-based polymers with varied molecular weight and degree of fluorination are investigated. Although the energy level of the highest occupied molecular orbital is found to increase with degree of fluorination, a strong relative molecular weight dependence of device performance dominates any underlying fluorination-dependent trend on overall performance. Microstructural investigation using a combination of X-ray techniques reveals a striking effect of polymer molecular characteristics on film morphology, with the size of PC71BM domains systematically decreasing with increasing polymer molecular weight. Furthermore, the relative purity of the mixed PTB7:PC71BM domain is found to systematically decrease with increasing molecular weight. When domain sizes with and without the use of the solvent additive diiodooctane (DIO) are compared, the effectiveness of DIO in reducing PC71BM domain sizes is also found to be strongly dependent on the molecular weig...

We have carried out a detailed study into how modifications of the physical, chemical and optical properties of solution-processed, nanocrystalline CdTe layers influence the photovoltaic performance of sintered CdTe/ZnO nanocrystal solar cells. Such solar cells are fabricated through layer-by-layer assembly, which is enabled through an inter layer chemical and thermal treatment cycle. In this manner we are able to fabricate working solar cells with sintered CdTe layers as low as 90 nm, provided that grain size is precisely controlled. We show that the extent of grain growth achieved during the CdTe sintering process is strongly dependent on nanocrystal surface chemistry and chemical environment, with the removal of the organic capping ligands and the introduction of CdCl 2 prior to annealing leading to greatly enhanced growth. Due to the air processing involved and the nanocrystalline nature of the CdTe, the overall performance of these solar cells is shown to be strongly dependent on both annealing temperature and time, with optimal results requiring a balance between crystal growth and degradation due to oxidation. Using this simple bi-layer device structure, optimized treatment conditions result in power conversion efficiencies of up to 7.7% and peak internal quantum efficiencies in excess of 95%.

A facile ligand exchange method for dispersing Cu2ZnSnS4 (CZTS) nanocrystals (NCs) in environmentally benign polar solvents, such as ethanol or n-propanol, at high concentrations (up to 200 mg/mL) is demonstrated. This approach has been applied to CZTS nanocrystals synthesized via scalable, noninjection methods to formulate colloidally stable inks that are suitable for the solution processing of solar cell devices. Unlike other inks currently used to fabricate NC solar cells, the CZTS nanocrystal ink developed here circumvents the need for hydrazine, pyridine, or thiol coordinating solvents. By combining our polar CZTS inks with optimized selenization procedures, substrate CZTSSe solar cells have been successfully fabricated with device efficiencies of 7.7%.

We present the novel 1,4-difunctionalisation of benzocarborane with organometallic groups suitable for cross-coupling and its subsequent insertion for the first time into conjugated polymer backbones. Copolymers with solubilised cyclopentadithiophene and diketopyrrolopyrrole derivatives were prepared by Stille polymerisation in good molecular weight. The physical, material and optoelectronic properties of the resulting polymers were investigated, demonstrating that benzocarborane acts similarly to a stabilised, electron-deficient cis-diene linker. We also report the first polymer field effect transistors incorporating a benzocarborane in the backbone.

Conjugated polymers with fluorine substituents on their backbone have exhibited improved performance over their un-fluorinated analogues by lowering the polymer HOMO level, thereby increasing the open-circuit voltage (VOC). To further investigate how fluorine substituents improve device performance, three polymers with the same donor and acceptor co-monomers, but differing by the number of fluorine atoms on the acceptor unit, were synthesized. Although the HOMO levels of the mono-(P1F) and di-fluorinated (P2F) polymers are essentially the same, an increase in VOC was still observed in the OPV device incorporating P2F. This implies that correlating the VOC to the donor polymer HOMO level is inadequate to fully explain the improvement in VOC. By calculating the charge transfer exciton binding energy from the measured film dielectric constant, it was found that the increase in VOC in going from P1F to P2F matches the decrease in charge transfer exciton binding energy.

A series of six phenyl-flanked benzodipyrrolidone-based copolymers are designed and synthesised: three exhibit backbone in-plane curvature or torsional twisting, while the other three remain planar and linear. While the former may appear less crystalline by X-ray diffraction, they afford a smoother thin-film surface topology and increased electron mobility in top-gate, bottom-contact organic field-effect transistors.

It is well known that even small variations in the solid-state microstructure of polymer:fullerene bulk heterojunctions can drastically change their organic solar cell device performance. We employ pBTTT:PC61BM as a model system and manipulate co-crystal formation of 1 : 1 (by weight) blends with the assistance of fatty acid methyl esters as additives. This allows us to evaluate the role of the intermixed phase in such binary blends through manipulation of their phase morphology—from fully intercalated to partially and predominantly non-intercalated systems—and its effect on the exciton- and carrier- dynamics and the efficiency of charge collection, with relevance for future device design and manufacturing.

A series of donor–acceptor (D–A) conjugated polymers utilizing 4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b′]dithiophene (DTG) as the electron rich unit and three electron withdrawing units of varying strength, namely 2-octyl-2H-benzo[d][1,2,3]triazole (BTz), 5,6-difluorobenzo[c][1,2,5]thiadiazole (DFBT) and [1,2,5]thiadiazolo[3,4-c]pyridine (PT) are reported. It is demonstrated how the choice of the acceptor unit (BTz, DFBT, PT) influences the relative positions of the energy levels, the intramolecular transition energy (ICT), the optical band gap (Egopt), and the structural conformation of the DTG-based co-polymers. Moreover, the photovoltaic performance of poly[(4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b′]dithiophen-2-yl)-([1,2,5]thiadiazolo[3,4-c]pyridine)] (PDTG-PT), poly[(4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b′]dithiophen-2-yl)-(2-octyl-2H-benzo[d][1,2,3]triazole)] (PDTG-BTz), and poly[(4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b′]dithiophen-2-yl)-(5,6-difluorobenzo[c][1,2,5]thiadiazole)] (PDTG-DFBT) is studied in blends with [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM). The highest power conversion efficiency (PCE) is obtained by PDTG-PT (5.2%) in normal architecture. The PCE of PDTG-PT is further improved to 6.6% when the device architecture is modified from normal to inverted. Therefore, PDTG-PT is an ideal candidate for application in tandem solar cells configuration due to its high efficiency at very low band gaps (Egopt = 1.32 eV). Finally, the 6.6% PCE is the highest reported for all the co-polymers containing bridged bithiophenes with 5-member fused rings in the central core and possessing an Egopt below 1.4 eV.

The synthesis and characterization four diketopyrrolopyrrole containing conjugated polymers for use in organic photovoltaics is presented. Excellent energy level control is demonstrated through heteroatomic substitution whilst maintaining similar solid state properties as shown by X-ray diffraction and atomic force microscopy. Inverted solar cells were fabricated with the best devices having short circuit currents exceeding 16 mA cm−2 and efficiencies of over 5% irrespective of whether [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) is used. Transient absorption spectroscopy on the bulk heterojunction blends shows efficient charge photo-generation, with the variations in short circuit current correlated to the energetic offset between polymer and fullerene.

A comprehensive study of the effect of intraphase microstructure on organic photovoltaic (OPV) device performance is undertaken. Utilizing a bilayer device architecture, a small molecule donor (TIPS-DBC) is deposited by both spin-coating and by thermal evaporation in vacuum. The devices are then completed by thermal evaporation of C, an exciton blocking layer and the cathode. This bilayer approach enables a direct comparison of device performance for donor layers in which the same material exhibits subtle differences in microstructure. The electrical performance is shown to differ considerably for the two devices. The bulk and interfacial properties of the donor layers are compared by examination with photoelectron spectroscopy in air (PESA), optical absorption spectroscopy, charge extraction of photo-generated charge carriers by linearly increasing voltage (photo-CELIV), time-resolved photoluminescence measurements, X-ray reflectometry (XR), and analysis of dark current behavior. The observed differences in device performance are shown to be influenced by changes to energy levels and charge transport properties resulting from differences in the microstructure of the donor layers. Importantly, this work demonstrates that in addition to the donor/acceptor microstructure, the intraphase microstructure can influence critical parameters and can therefore have a significant impact on OPV performance. The influence of intraphase microstructure on the performance of organic photovoltaics (OPVs) is studied utilizing a bilayer (planar heterojunction) device architecture. Devices in which the donor layer exhibits subtle differences in microstructure are compared, and these differences are shown to affect critical parameters (energy levels, charge transport properties) having a significant impact on performance. Copyright

The effect of direct amine substitution on the photovoltaic performance of donor–acceptor oligothiophenes in dye-sensitized and bulk heterojunction (BHJ) solar cells was investigated. These dyes have a common donor (di-p-tolylamino), π-bridge (terthiophene), and tunable acceptors to provide new materials for evaluation in dye-sensitized solar cells (DSCs) and BHJ solar cells. They were tested with liquid, ionic liquid, and metal-complex based electrolytes for DSCs and PC61BM for solution processable BHJ solar cells. Direct amine substitution of the oligothiophene resulted in a significant spectral red-shift, reduction in band-gaps and improved photovoltaic performance in DSCs but not for BHJ devices.

Bulk heterojunction solar cells were fabricated with flexible webs of aligned multiwalled carbon nanotubes (MWNTs). These webs were drawn from a forest of MWNTs and placed directly onto the device substrate to form the hole collecting electrode. Devices were fabricated on glass substrates with one or two MWNT web layers to study the trade-off between transparency and resistivity on device performance. Devices with two web layers performed better with a fill factor of 0.47 and a device power conversion efficiency of 1.66% due to their higher conductivity. Flexible devices on Mylar substrates were also demonstrated with an efficiency of 1.2% indicating the potential of MWNT webs as a flexible alternative to the more conventional indium tin oxide.

Large-area, flexible organic photovoltaic (OPV) modules are fabricated successfully by gravure printing in air, using an industrial-scale printing proofer of similar performance to commercial roll-to-roll printing processes. Both the hole transport layer, poly-3,4-ethylenedioxy-thiophene:poly(styrene sulfonic-acid) (PEDOT:PSS), and the active layer, poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM), are successively printed on indium tin oxide (ITO) coated polyethylene terephthalate (ITO/PET) substrates with evaporated aluminum (Al) top electrodes. The 45 cm 2 modules, composed of 5 cells connected in series, show power conversion efficiency (PCE) of over 1.0%, in which the short-circuit current ( J sc ) and open-circuit voltage ( V oc ) are as high as 7.14 mA/cm 2 and 2.74 V (0.55 V per cell), respectively. The PCEs could be potentially improved by the further optimization of the layer interface, layer morphology and flexible substrate properties. The results suggest that gravure printing may be a suitable technique for fast commercial processing of large-area, flexible OPVs with high output.

Two novel indacenodithiophene (IDT) based donor–acceptor conjugated polymers for use in organic field effect transistors and photovoltaic devices are synthesized and characterized. The effect of inclusion of two fluorine atoms on the acceptor portion of the polymer is thoroughly investigated via a range of techniques. The inductively withdrawing and mesomerically donating properties of the fluorine atoms result in a decrease of the highest occupied molecular orbital (HOMO), with little effect on the lowest unoccupied molecular orbital (LUMO) as demonstrated through density functional theory (DFT) analysis. Inclusion of fluorine atoms also leads to a potentially more planar backbone through inter and intrachain interactions. Use of the novel materials in organic field effect transistor (OFET) and organic photovoltaic (OPV) devices leads to high mobilities around 0.1 cm2/(V s) and solar cell efficiencies around 4.5%.

A novel and efficient route to thieno[3,4-c]pyrrole-4,6-dione (TPD) facilitating late-stage alkylation is presented. Four copolymers with alkylated bithiophene (2T) are synthesised, with different TPD N−alkyl side-chain branching points. The effect of steric bulk on solid-state properties is investigated (UV–VIS, differential scanning calorimetry, X-ray diffraction) and C1-branching found to increase crystallinity and solid-state packing in TPD-2T.

We report the synthesis of two novel cyclopenta[2,1-b:3,4-b′]dithiophen-4-one monomers containing solubilizing alkyl groups in the peripheral 3,5 positions. Polymerization with (E)-1,2-bis(tributylstannyl)-ethylene under Stille coupling conditions afforded the first reported examples of soluble poly(cyclopentadithiophen-4-one)vinylenes. The resulting polymers absorb in the near-infrared, with a maximum thin film absorbance around 815 nm and have optical band gaps of 1.25 eV. The polymers exhibit promising ambipolar field effect transistor performance, with average saturated mobilities of 0.5 cm2 V–1 s–1 for holes and 0.12 cm2 V–1 s–1 for electrons. The ambipolar transistors operate in both the hole and electron transport regimes in ambient air. Prolonged exposure to ambient atmosphere leads to a gradual loss of the electron transport behavior, with little change observed in the p-type mobility.

Charge extraction properties of various binary and ternary blends of organic photovoltaic devices covering both polymers and small molecules are studied. Due to their bipolar nature, both slow and fast carrier mobilities are identified from the extraction current transient. The equilibrium carrier concentration is also estimated for each of the blend films. The product of the slow carrier mobility and equilibrium concentration spreading two orders of magnitude can be used to estimate the short circuit current density. A good agreement between the estimated and measured short circuit current density is obtained with the accuracy reliant on the estimation of the slowest carrier mobility. This simplistic approach will be very useful to predict the short circuit current density for devices based on new materials.

Alloying is a versatile tool for engineering the optical and electronic properties of materials. Here, we explore the use of CdTe and CdSe nanocrystals in developing sintered CdSe(x)Te(1-x) alloys as bandgap tunable, light-absorbing layers for solution-processed solar cells. Using a layer-by-layer approach, we incorporate such alloyed materials into single- and graded-composition device architectures. Nanostructured solar cells employing CdSe(x)Te(1-x) layers are found to exhibit a spectral response deeper into the IR region than bulk CdTe devices as a result of optical bowing and achieve power conversion efficiencies as high as 7.1%. The versatility of the layer-by-layer approach is highlighted through the fabrication of compositionally graded solar cells in which the [Se]:[Te] ratio is varied across the device. Each of the individual layers can be clearly resolved through cross-sectional imaging and show limited interdiffusion. Such devices demonstrate the importance of band-alignment in the development of highly efficient, nanostructured solar cells.

Vertically aligned arrays of single walled carbon nanotubes (SWNTs) were chemically attached to an optically transparent electrode of fluorine doped tin oxide (FTO) for use as a working electrode in a photoelectrochemical solar cell. 7,14-Bis[2-[tris(1-methylethyl)silyl]ethynyl]dibenzo[b,def]chrysene (TIPS-DBC) layers were deposited onto the SWNT/FTO electrodes via spin coating. The photovoltaic properties of these new TIPS-DBC/SWNT/FTO electrodes were then investigated. The presence of the additional TIPS-DBC layer was found to increase the open circuit voltage, short circuit current density and power conversion efficiency of the cells compared to those cells made using nanotube only electrodes. In addition, the spin speed at which the TIPS-DBC layer was deposited was found to have an effect on the photovoltaic performance of the cells. These results show much promise for the further development of photovoltaics in which the photoactive elements are carbon based, rather than transition metal based.

We report the synthesis of a selenophene–diketopyrrolopyrrole monomer and its co-polymerisation with selenophene and thieno[3,2-b]thiophene monomers by Stille coupling. The resulting low band gap polymers exhibit ambipolar charge transport in organic field effect transistors. High and balanced electron and hole mobilities in excess of 0.1 cm2 V−1 s−1 were observed in bottom-gate, bottom-contact devices, suggesting that selenophene inclusion is a promising strategy for the development of ambipolar organic semiconductors.

Organic photovoltaic devices using an electrode of indium tin oxide (ITO) coated with a buffer layer of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) exposed to controlled humidity during fabrication showed a 65–75% decrease in efficiency and displayed S-shaped J–V curves, changes, which are attributed to different levels of indium and tin migration into the PEDOT:PSS film. A distinct shift in the secondary electron cut-off in the UV Photoelectron spectra (UPS) of ITO/PEDOT:PSS samples exposed to controlled humidity indicate an increase of the dipole at the ITO/PEDOT:PSS interface, which could explain the appearance of S-shaped J–V curves. Additionally, the electron density at low binding energies is reduced for the humidity exposed PEDOT:PSS suggesting a second mechanism for decreased device performance. Refereed/Peer-reviewed

The successful design and synthesis of two novel benzo[1,2-b:3,4-b′:5,6-d′]trithiophene-containing polymers is presented. The planar benzotrithiophene core induces strong aggregation effects in alternating co-polymers with both thiophene and thieno[3,2-b]thiophene. Co-monomer choice is found to play a crucial role in determining the backbone conformation, the interchain interactions, and the polymer solubility. Despite molecular disorder introduced by a branched solubilizing alkyl chain and a regiorandom polymerization of the asymmetric benzotrithiophene unit, these co-polymers exhibit hole mobilities as high as 0.24 cm2/(V s).

Here we report the synthesis of novel poly(3-alkyl-2,5-selenylenevinylene)s (PSV) by the polymerization of 2,5-dibromo-3-alkylselenophene and (E)-1,2-bis(tributylstannyl)ethylene under microwave-as...

The preparation of a series of novel polymers comprising pendant electro-active “push–pull” chromophores and their performance in solution-processed organic electronic devices is described. The design of the electro-active pendant chromophores was based on the well-known motif of cyano-substituted poly( p -phenylenevinylene). Optical band-gap engineering within this series of polymers was achieved by varying the conjugation length and the electron donor/acceptor functionalities of the pendant chromophores. The introduction of a cyanoimine group into the electro-active pendant module resulted in a marked narrowing of the optical band-gap compared with the other electro-active pendant chromophores investigated in this work. Bulk heterojunction solar cell devices comprising these polymers were prepared by solution processing blends of each polymer with [6,6]-phenyl-C 61 -butyric acid methyl ester, and their performance was evaluated by measuring power conversion efficiencies. The best-performing solar cell in this series exhibited a power conversion efficiency of 0.29% and a maximum incident photon-to-current conversion efficiency of 22% and was produced using the polymer in which the electro-active chromophore comprised the cyanoimine group.

The photophysics of a homopolymer containing pendant phenoxy-benzothiadiazole-bis(thiophene) (OPhBTDT2) moieties, a block copolymer containing both OPhBTDT2 and triphenylamine-based (TPA) pendant units, and a benzothiadiazole model compound, were investigated using steady-state and time-resolved photo-spectroscopic techniques, and quantum mechanical calculations. Electronic excitation of the OPhBTDT2 chromophores leads to rapid intra-molecular charge re-distribution in the lowest unoccupied molecular orbital resulting in substantially increased electron density on the BTD component. In dilute fluid solution, the fluorescence lifetime of the OPhBTDT2 moieties in the block co-polymer was partially quenched due to photo-oxidation of TPA. The triplet excited-state lifetime of the OPhBTDT2 groups in the block co-polymer in solution was unaffected by the TPA moieties signifying that triplet excited-state OPhBTDT2 groups do not oxidize the TPA moieties. In spin-cast films, the OPhBTDT2 singlet and triplet excitons are shorter-lived than the corresponding excited states of the polymers or the OPhBTDT2 model compound in dilute solution, and the lifetimes are essentially independent of the presence of the TPA groups in the block co-polymer. This quenching of OPhBTDT2 exciton lifetimes in the films suggests efficient non-radiative energy migration to low-energy traps, possibly non-emissive OPhBTDT2 molecular aggregates. The complete quenching of fluorescence from OPhBTDT2 in a 1:1 blend of the OPhBTDT2 homoploymer and the electron acceptor [6,6]-phenyl C 61 butyric acid methyl ester (PC 61 BM) is attributed to efficient photo-induced reduction of PC 61 BM by OPhBTDT2 singlet excitons based on evidence for radical ion formation obtained from nanosecond transient absorbance measurements. The decay kinetics of the absorbance by the resulting charge carriers is consistent with a slow, trap-limited bimolecular recombination mechanism, so the low performance of photovoltaic devices produced using the blend is thought to be limited by extensive phase separation and/or low hole mobility.

We report the synthesis and polymerization of a novel thieno[3,2-b] thiophene-diketopyrrolopyrrole-based monomer. Copolymerization with thiophene afforded a polymer with a maximum hole mobility of 1.95 cm2 V -1 s-1, which is the highest mobility from a polymer-based OFET reported to date. Bulk-heterojunction solar cells comprising this polymer and PC71BM gave a power conversion efficiency of 5.4%. © 2011 American Chemical Society., SCOPUS: ar.j, info:eu-repo/semantics/published

Device function in organic electronics is critically governed by the transport of charge across interfaces of dissimilar materials. Accurate measurements of energy level positions in organic electronic devices are therefore necessary for assessing the viability of new materials and optimizing device performance. In contrast to established methods that are used in solution or vacuum environments, here we combine Kelvin probe measurements performed in ambient environments to obtain work function values with photoelectron spectroscopy in air to obtain ionization potential, so that a complete energy level diagram for organic semiconductors can be determined. We apply this new approach to study commonly used electron donor and acceptor materials in organic photovoltaics (OPV), including poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PCBM), and ZnO, as well as examine new materials. Band alignments across the entire OPV devices are constructed and compared with actual device performance. The ability to determine interfacial electronic properties in the devices enables us to answer the outstanding question: why previous attempts to make OPV devices using 6,13-bis(triisopropylsilylethynyl) (TIPS)-pentacene as the electron donor were not successful.

The synthesis of a series of novel, unsymmetrically substituted benzothiadiazole-containing vinyl monomers and their free radical polymerization with and without the control of a reversible addition−fragmentation chain transfer (RAFT) agent is reported. The resulting polymers with electroactive pendants show tunable absorption and emission spectra depending on their molecular architecture. Using RAFT allows the synthesis of block copolymers using a hole-transporting vinyl-triarylamine as a second monomer. Efficient energy transfer between the two pendants has been detected. Cyclic voltammetry and photoelectron spectroscopy in air measurements have been employed to reveal the location of the HOMO and LUMO of the block copolymers. The block copolymers also influence the morphology of spin-casted films and show rectifying behavior in organic photovoltaic devices.

Treatment of the ligands 3,5-tBu2-2-(OH)C6H2CHNR [R = 2-(CO2H)C6H4 (1a) and 2-(CO2H)C10H6 (1b)] with trimethylborate, B(OMe)3, in toluene yields, after work-up, the yellow crystalline complexes {[3,5-tBu2-2-(O)C6H2CHNR]B(OMe)} [R = 2-(CO2)C6H4 (2a) and 2-(CO2)C10H6 (2b)], respectively. Further treatment of these complexes with trifluoromethanesulfonic (triflic) acid, CF3SO3H, followed by recrystallisation from tetrahydrofuran (thf) afforded the triflate salts [{3,5-tBu2-2-(O)C6H2CHNR}B(thf)][CF3SO3] [R = 2-(CO2)C6H4 (3a) and 2-(CO2)C10H6 (3b)]. An electroluminescent device was constructed using 2a, which produced orange-green light with broad emission spectra (maximum brightness of 5 cd/m2 being observed at 13 V). Compounds 1a and 2b·2MeCN have been characterised by single crystal X-ray structure determinations.

Carbon dioxide (CO2) is arguably the most high-profile molecule of recent times. Although much of its bad press comes from environmental concerns associated with greenhouse gas emissions, there exists the possibility to harness this abundant resource for application to the synthesis and processing of useful materials. This article describes a selection of recent successes in using supercritical carbon dioxide (scCO2) as a solvent for polymerizations. Further, the uses of compressed CO2 as a processing tool in the fabrication of materials for applications such as coatings and biomaterials are discussed. Finally, the application of scCO2 to photolithography is demonstrated, with particular focus on CO2 as a processing solvent for the patterning of organic electronic devices.

The synthesis of easily functionalized and highly soluble fluorene-containing hexabenzocoronenes (FHBC) has been achieved in high yield at a gram scale. Conjugated triarylamine oligomers were coupled to the FHBC cores via Buchwald-Hartwig coupling, and the photophysical properties of resulting dendritic materials were examined by ultrafast laser spectroscopic techniques. Efficient quenching of the triarylamine oligomer fluorescence was observed paving the way for the inclusion of these materials in bulk heterojunction solar cells. In preliminary studies, solar cell devices with external quantum efficiencies above 5% have been fabricated.

We report a spray deposition technique as a screening tool for solution processed solar cells. A dual-feed spray nozzle is introduced to deposit donor and acceptor materials separately and to form blended films on substrates in situ. Using a differential pump system with a motorised spray nozzle, the effect of film thickness, solution flow rates and the blend ratio of donor and acceptor materials on device performance can be found in a single experiment. Using this method, polymer solar cells based on poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C61 butyric acid methyl ester (PC61BM) are fabricated with numerous combinations of thicknesses and blend ratios. Results obtained from this technique show that the optimum ratio of materials is consistent with previously reported values confirming this technique is a very useful and effective screening method. This high throughput screening method is also used in a single-feed configuration. In the single-feed mode, methylammonium iodide solution is deposited on lead iodide films to create a photoactive layer of perovskite solar cells. Devices featuring a perovskite layer fabricated by this spray process demonstrated a power conversion efficiencies of up to 7.9%.

Significant progress has been realized in the design and synthesis of light emitting polymers that emit over the entire visible spectrum. However, up to seventy-five percent of charge recombination events can lead to triplet states that decay non-radiatively. Following the pioneering work in the field of small molecule organic light emitting devices, it has been found that solution processible iridium polymer complexes can be used to harness the wasted triplet energy. In this paper, new results with respect to the electrophosphorescence of solution processible tethered iridium polymer derivatives are presented. Furthermore, our approaches to the design of new high triplet energy conjugated polymer hosts are also reported.

We report the first soluble poly(3-dodecyl tellurophenylenevinylene) and compare its properties to the analogous thiophene and selenophene containing polymers., We report the first soluble poly(3-dodecyl tellurophenylene-vinylene) polymer (P3TeV) by Stille copolymerization and compare its properties to the analogous thiophene and selenophene containing polymers. The optical band gap of the polymers is shown to systematically decrease as the size of the heteroatom is increased, mainly as a result of a stabilization of the LUMO energy, resulting in a small band gap of 1.4 eV for P3TeV. Field effect transistors measurements in variety of architectures demonstrate that the selenophene polymer exhibits the highest mobility, highlighting that increasing the size of the heteroatom is not always beneficial for charge transport.

Three new nitrogen-donor ligands with ferrocenyl pendants, namely ferrocenylmethylbis(2-pyridylethyl)amine (L1), ferrocenylmethylbis(2-pyridylmethyl)amine (L2), and 1-ferrocenylmethyl-4,7-diisopropyl-1,4,7-triazacyclononane (L3), have been synthesised. Copper(II) complexes of these ligands have been made and their physicochemical and redox properties characterised. Crystal structures of [Cu(L2)X2·0.5Et2O (X = Cl, Br), [Cu(L2)(OTf)2(CH3OH)], [Cu(L3)Cl2], and [{(L3)Cu}2(μ-OH)2](OTf)2·0.5Et2O are reported. Oxidation of each complex with ceric ion affords the corresponding FeIII ∼ CuII species. Variations in ligand design, the choice of co-ligand (chloride, bromide, trifluoromethanesulfonate (OTf) or solvent) and the choice of solvent allowed, in the case of acetonitrile solutions of [Cu(L1)(H2O)2](OTf)2, the close matching of the FeIII–FeII and CuII–CuI electrochemical couples. [Cu(L1)(H2O)2](OTf)2 reacts with dioxygen in acetonitrile, but is stable in other solvents such as tetrahydrofuran. Electronic spectra of the complex in acetonitrile exhibit a prominent ferrocenium ion band. These results are interpreted in terms of an intramolecular electron transfer equilibrium between FeII ∼ CuII and FeIII ∼ CuI tautomers for the complex that leads to it reacting with dioxygen; their possible biological relevance is discussed.

Fully printed perovskite solar cells are demonstrated with slot-die coating, a scalable printing method. A sequential slot-die coating process is developed to produce efficient perovskite solar cells and to be used in a large-scale roll-to-roll printing process. All layers excluding the electrodes are printed and devices demonstrate up to 11.96% power conversion efficiency. It is also demonstrated that the new process can be used in roll-to-roll production.

A high molecular weight donor-acceptor conjugated polymer is synthesized using the Suzuki polycondensation method. Using this polymer, a single-junction bulk-heterojunction solar cell is fabricated giving a power conversion efficiency of 9.4% using a fullerene-modified ZnO interlayer at the cathode contact.

The planar benzotrithiophene unit (BTT) was incorporated into four different donor polymers, and by systematically changing the nature and positioning of the solubilizing alkyl side chains along the conjugated backbone, the polymers’ frontier energy levels and optoelectronic properties were controlled. Reducing the steric hindrance along the polymer backbone lead to strong interchain aggregation and highly ordered thin films, achieving hole mobilities of 0.04 cm2/(V s) in organic thin film transistors. In an attempt to increase the polymer’s processability and reduce chain aggregation, steric hindrance between alkyl side chains was exploited. As a result of the increased solubility, the film forming properties of the polymer could be improved, but at the cost of reduced hole mobilities in OFET devices, due to the lack of long-range order in the polymer films.