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Voigt M.M.,Center for Plastic Electronics | MacKenzie R.C.I.,Center for Plastic Electronics | MacKenzie R.C.I.,Albert Ludwigs University of Freiburg | King S.P.,Center for Plastic Electronics | And 10 more authors.
Solar Energy Materials and Solar Cells | Year: 2012

We investigate the relationship between processing parameters and the device performance of gravure printed organic solar cells. Ink viscosity, surface energy and surface roughness are studied to optimise the printing process. Surfactants, additives and surface modification by plasma cleaning are utilised to achieve homogeneous printing of multilayer OPV device architectures. It is found that use of high boiling point solvents with high solubilities leads to a desirable surface morphology. The use of a printed metal oxide electrode within the inverted structure leads to devices with lifetimes exceeding those of devices in standard structures without the need for encapsulation. © 2012 Elsevier B.V.


Guilbert A.A.Y.,Center for Plastic Electronics | Reynolds L.X.,Imperial College London | Bruno A.,Imperial College London | Maclachlan A.,Imperial College London | And 7 more authors.
ACS Nano | Year: 2012

The bis and tris adducts of [6,6]phenyl-C 61-butyric acid methyl ester (PCBM) offer lower reduction potentials than PCBM and are therefore expected to offer larger open-circuit voltages and more efficient energy conversion when blended with conjugated polymers in photovoltaic devices in place of PCBM. However, poor photovoltaic device performances are commonly observed when PCBM is replaced with higheradduct fullerenes. In this work, we use transmission electron microscopy (TEM), steady-state and ultrafast time-resolved photoluminescence spectroscopy (PL), and differential scanning calorimetry (DSC) to probe the microstructural properties of blend films of poly(3-hexylthiophene-2,5-diyl) (P3HT) with the bis and tris adducts of PCBM. TEM and PL indicate that, in as-spun blend films, fullerenes become less soluble in P3HT as the number of adducts increases. PL indicates that upon annealing crystallization leads to phase separation in P3HT:PCBM samples only. DSC studies indicate that the interactions between P3HT and the fullerene become weaker with higher-adduct fullerenes and that all systems exhibit eutectic phase behavior with a eutectic composition being shifted to higher molar fullerene content for higher-adduct fullerenes. We propose two different mechanisms of microstructure development for PCBM and higher-adduct fullerenes. P3HT:PCBM blends, phase segregation is the result of crystallization of either one or both components and is facilitated by thermal treatments. In contrast, for blends containing higher adducts, the phase separation is due to a partial demixing of the amorphous phases. We rationalize the lower photocurrent generation by the higher-adduct fullerene blends in terms of film microstructure. © 2012 American Chemical Society.


Soon Y.W.,Center for Plastic Electronics | Clarke T.M.,Center for Plastic Electronics | Zhang W.,Center for Plastic Electronics | Agostinelli T.,Imperial College London | And 5 more authors.
Chemical Science | Year: 2011

In this paper, we compare the photophysics and photovoltaic device performance of two indenofluorene based polymers: poly[2,8-(6,6,12,12-tetraoctylindenofluorene)-co-4′,7′-(2′,1′,3′- benzothiodiazole] (IF8BT) and poly[2,8-(6,6,12,12-tetraoctylindenofluorene)-co-5,5-(40,70-di-2-thienyl-20,10,30-benzothiodiazole] (IF8TBTT) blended with [6,6]-phenyl C61 butyric acid methyl ester (PCBM). Photovoltaic devices made with IF8TBTT exhibit greatly superior photocurrent generation and photovoltaic efficiency compared to those made with IF8BT. The poor device efficiency of IF8BT/ PCBM devices is shown to result from efficient, ultrafast singlet Förster energy transfer from IF8BT to PCBM, with the resultant PCBM singlet exciton lacking sufficient energy to drive charge photogeneration. The higher photocurrent generation observed for IF8TBTT/PCBM devices is shown to result from IF8TBTT's relatively weak, red-shifted photoluminescence characteristics, which switches off the polymer to fullerene singlet energy transfer pathway. As a consequence, IF8TBTT singlet excitons are able to drive charge separation at the polymer/fullerene interface, resulting in efficient photocurrent generation. These results are discussed in terms of the impact of donor/acceptor energy transfer upon photophysics and energetics of charge photogeneration in organic photovoltaic devices. The relevance of these results to the design of polymers for organic photovoltaic applications is also discussed, particularly with regard to explaining why highly luminescent polymers developed for organic light emitting diode applications often give relatively poor performance in organic photovoltaic devices. © The Royal Society of Chemistry 2011.


Fei Z.,Imperial College London | Shahid M.,Imperial College London | Yaacobi-Gross N.,Center for Plastic Electronics | Rossbauer S.,Center for Plastic Electronics | And 4 more authors.
Chemical Communications | Year: 2012

We report the first synthesis of a tetrafluorinated 4,7-bis(3,4- difluorothiophen-2-yl)-2,1,3-benzothiadiazole monomer and its polymerisation with dithieno[3,2-b:2′,3′-d]germole by Stille coupling to afford a low band gap polymer with a high ionisation potential. Direct comparison to the non-fluorinated analogue demonstrates that fluorination results in an increase in ionisation potential with no change in optical band gap, and enhanced aggregation over the non-fluorinated polymer. These desirable properties result in a significant enhancement in OPV device performance in blends with PC 71BM. © 2012 The Royal Society of Chemistry.


Fei Z.,Imperial College London | Ashraf R.S.,Imperial College London | Huang Z.,Imperial College London | Smith J.,Center for Plastic Electronics | And 6 more authors.
Chemical Communications | Year: 2012

We report the first synthesis of a fused germaindacenodithiophene monomer and its polymerisation with 2,1,3-benzothiadiazole by Suzuki polycondensation. The resulting polymer, PGeTPTBT, is semicrystalline, despite the presence of four bulky 2-ethylhexyl groups. Blends with P 70CBM afford solar cells with efficiencies of 5.02%. © 2012 The Royal Society of Chemistry.


Zhang X.,U.S. National Institute of Standards and Technology | Richter L.J.,U.S. National Institute of Standards and Technology | Delongchamp D.M.,U.S. National Institute of Standards and Technology | Kline R.J.,U.S. National Institute of Standards and Technology | And 10 more authors.
Journal of the American Chemical Society | Year: 2011

We describe a series of highly soluble diketo pyrrolo-pyrrole (DPP)-bithiophene copolymers exhibiting field effect hole mobilities up to 0.74 cm 2 V -1 s -1, with a common synthetic motif of bulky 2-octyldodecyl side groups on the conjugated backbone. Spectroscopy, diffraction, and microscopy measurements reveal a transition in molecular packing behavior from a preferentially edge-on orientation of the conjugated plane to a preferentially face-on orientation as the attachment density of the side chains increases. Thermal annealing generally reduces both the face-on population and the misoriented edge-on domains. The highest hole mobilities of this series were obtained from edge-on molecular packing and in-plane liquid-crystalline texture, but films with a bimodal orientation distribution and no discernible in-plane texture exhibited surprisingly comparable mobilities. The high hole mobility may therefore arise from the molecular packing feature common to the entire polymer series: backbones that are strictly oriented parallel to the substrate plane and coplanar with other backbones in the same layer. © 2011 American Chemical Society.


Kirchartz T.,Center for Plastic Electronics | Deledalle F.,Imperial College London | Tuladhar P.S.,Imperial College London | Durrant J.R.,Imperial College London | Nelson J.,Center for Plastic Electronics
Journal of Physical Chemistry Letters | Year: 2013

Ideality factors are derived from either the slope of the dark current/voltage curve or the light intensity dependence of the open-circuit voltage in solar cells and are often a valuable method to characterize the type of recombination. In the case of polymer:fullerene solar cells, the ideality factors derived by the two methods usually differ substantially. Here we investigate the reasons for the discrepancies by determining both ideality factors differentially as a function of voltage and by comparing them with simulations. We find that both the dark and light ideality factors are sensitive to bulk recombination mechanisms at the internal donor:acceptor interface, as is often assumed in the literature. While the interpretation of the dark ideality factor is difficult due to resistive effects, determining the light ideality factor differentially indicates that the open-circuit voltage of many polymer:fullerene solar cells is limited by surface recombination, which leads to light ideality factors decreasing below one at high voltage. © 2013 American Chemical Society.


Dalgleish S.,University of Edinburgh | Labram J.G.,Center for Plastic Electronics | Li Z.,University of Cambridge | Wang J.,University of Cambridge | And 4 more authors.
Journal of Materials Chemistry | Year: 2011

The synthesis and full characterisation of a novel indole-substituted nickel dithiolene [Ni(mi-5edt)2] (3) is reported, and compared to its alkyl-substituted analogue [Ni(mi-5hdt)2] (4) that has been previously communicated [Dalgleish et al., Chem. Commun., 2009, 5826] [mi-5edt = 1-(N-methylindol-5-yl)-ethene-1,2-dithiolate; mi-5hdt = 1-(N-methylindol-5-yl)- hex-1-ene-1,2-dithiolate)]. Both complexes are shown to undergo oxidative electropolymerisation, yielding polymer films that retain the redox and optical properties of the monomer. The more soluble analogue 4 is shown to form high quality thin films by spin coating, which have been utilised to fabricate field-effect transistors (FETs) and bulk heterojunction photovoltaic devices (BHJ-PVs). From FET studies, the material shows ambipolar charge transport behaviour, with a maximum carrier mobility of ∼10-6 cm 2 V-1 s-1 for electrons. By using 4 simultaneously as the electron acceptor as well as a NIR sensitiser in BHJ-PVs, the complex is shown to contribute to the photocurrent, extending light harvesting into the NIR region. © 2011 The Royal Society of Chemistry.


Shoaee S.,Center for Plastic Electronics | Durrant J.R.,Center for Plastic Electronics
Journal of Materials Chemistry C | Year: 2015

Transient absorption spectroscopy is commonly used to probe the yield and kinetics of excited states of materials. We present a transient absorption spectroscopic assay of oxygen diffusion in a series of solution-processed polymer films. The films were partially encapsulated with an epoxy/glass top barrier as a simple model system for organic photovoltaic and light emitting devices with metal top contacts. The results presented herein show that this spectroscopic approach can be a versatile and quantitative in situ assay of local oxygen concentrations in such organic semiconductor films. With our current apparatus, the approach has a time resolution of 5 seconds, thereby enabling direct measurement of oxygen diffusion kinetics into a semiconductor film. The versatility of this approach suggests it could be widely applicable to measurement of oxygen diffusion into organic optoelectronic devices, including for example oxygen diffusion through encapsulation and barrier layers. Employing this approach, we demonstrate significant differences in oxygen diffusion kinetics between different semiconducting polymers. We furthermore demonstrate the impact of an additional getter (ZnO) and light exposure upon the local oxygen concentration, providing new insights into the role of oxygen diffusion kinetics in determining the environmental stability of organic semiconductors. © The Royal Society of Chemistry.


Nielsen C.B.,Center for Plastic Electronics | Holliday S.,Center for Plastic Electronics | Chen H.-Y.,Center for Plastic Electronics | Cryer S.J.,Center for Plastic Electronics | And 2 more authors.
Accounts of Chemical Research | Year: 2015

Conspectus The active layer in a solution processed organic photovoltaic device comprises a light absorbing electron donor semiconductor, typically a polymer, and an electron accepting fullerene acceptor. Although there has been huge effort targeted to optimize the absorbing, energetic, and transport properties of the donor material, fullerenes remain as the exclusive electron acceptor in all high performance devices. Very recently, some new non-fullerene acceptors have been demonstrated to outperform fullerenes in comparative devices. This Account describes this progress, discussing molecular design considerations and the structure-property relationships that are emerging.The motivation to replace fullerene acceptors stems from their synthetic inflexibility, leading to constraints in manipulating frontier energy levels, as well as poor absorption in the solar spectrum range, and an inherent tendency to undergo postfabrication crystallization, resulting in device instability. New acceptors have to address these limitations, providing tunable absorption with high extinction coefficients, thus contributing to device photocurrent. The ability to vary and optimize the lowest unoccupied molecular orbital (LUMO) energy level for a specific donor polymer is also an important requirement, ensuring minimal energy loss on electron transfer and as high an internal voltage as possible. Initially perylene diimide acceptors were evaluated as promising acceptor materials. These electron deficient aromatic molecules can exhibit good electron transport, facilitated by close packed herringbone crystal motifs, and their energy levels can be synthetically tuned. The principal drawback of this class of materials, their tendency to crystallize on too large a length scale for an optimal heterojunction nanostructure, has been shown to be overcome through introduction of conformation twisting through steric effects. This has been primarily achieved by coupling two units together, forming dimers with a large intramolecular twist, which suppresses both nucleation and crystal growth. The generic design concept of rotationally symmetrical aromatic small molecules with extended π orbital delocalization, including polyaromatic hydrocarbons, phthalocyanines, etc., has also provided some excellent small molecule acceptors. In most cases, additional electron withdrawing functionality, such as imide or ester groups, can be incorporated to stabilize the LUMO and improve properties. New calamitic acceptors have been developed, where molecular orbital hybridization of electron rich and poor segments can be judiciously employed to precisely control energy levels. Conformation and intermolecular associations can be controlled by peripheral functionalization leading to optimization of crystallization length scales. In particular, the use of rhodanine end groups, coupled electronically through short bridged aromatic chains, has been a successful strategy, with promising device efficiencies attributed to high lying LUMO energy levels and subsequently large open circuit voltages. © 2015 American Chemical Society.

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