John C. de Mello

Efficient Organic Solar Cells with Solution‐Processed Silver Nanowire Electrodes

Conducting polymers, [ 2–4 ] metal inks, [ 5 ] nanoparticulate metal oxides, [ 6 ] carbon nanotubes, [ 7–9 ] and graphene [ 10–12 ] have been investigated as potential alternatives to brittle indium tin oxide (ITO), but none can yet compete in terms of transparency and sheet resistance. Thin meshes of silver nanowires (AgNWs) [ 13–18 ] have recently emerged as promising electrodes due to their ability to provide transmittances greater than 85% at sheet resistances less than 20 Ω sq − 1 . [ 13 , 14 ] Their application to printed electronics, however, is challenging due to a highly non-uniform topography, which can cause shorting through other layers. This is especially problematic for devices using AgNWs as the lower (substrate-facing) electrode since the mesh presents an extremely rough base layer on which to build the device, leading to signifi cant interelectrode shorting. This in turn leads to low shunt resistances, high dark currents, and poor device effi ciencies. For instance, Peumanns and co-workers reported vacuum-deposited organic solar cells (OSCs) on AgNW-coated glass with low shunt resistances of less than 1 k Ω cm 2 and power conversion effi ciencies (PCEs) less than 0.5%. [ 13 ] They subsequently reported polythiophene/ fullerene OSCs with PCEs of 2.5%, [ 16 , 17 ] using the AgNWs in a top-electrode confi guration where their rough morphology is less detrimental to other layers. However to achieve these effi ciencies they fi rst had to pulse the devices at 10 V to burn-out localized shorts with potentially adverse implications for device lifetimes, suggesting a need for alternative device architectures that better suppress shunt formation. In practice most organic devices utilize a transparent substrate through which light passes, and hence they require a transparent lower electrode. To address this need Zeng et al. embedded AgNWs in a thick fi lm of polyvinyl alcohol, ensuring the exposed nanowires sat fl ush with the top of the fi lm and so provided a planar surface on which to deposit further layers. [ 18 ]

Competition between the Charge Transfer State and the Singlet States of Donor or Acceptor Limiting the Efficiency in Polymer:Fullerene Solar Cells

We study the appearance and energy of the charge transfer (CT) state using measurements of electroluminescence (EL) and photoluminescence (PL) in blend films of high-performance polymers with fullerene acceptors. EL spectroscopy provides a direct probe of the energy of the interfacial states without the need to rely on the LUMO and HOMO energies as estimated in pristine materials. For each polymer, we use different fullerenes with varying LUMO levels as electron acceptors, in order to vary the energy of the CT state relative to the blend with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). As the energy of the CT state emission approaches the absorption onset of the blend component with the smaller optical bandgap, E(opt,min) ≡ min{E(opt,donor); E(opt,acceptor)}, we observe a transition in the EL spectrum from CT emission to singlet emission from the component with the smaller bandgap. The appearance of component singlet emission coincides with reduced photocurrent and fill factor. We conclude that the open circuit voltage V(OC) is limited by the smaller bandgap of the two blend components. From the losses of the studied materials, we derive an empirical limit for the open circuit voltage: V(OC) ≲ E(opt,min)/e - (0.66 ± 0.08)eV.

A Versatile Low Bandgap Polymer for Air‐Stable, High‐Mobility Field‐Effect Transistors and Efficient Polymer Solar Cells

Polymer-based organic thin-fi lm transistors (OTFTs) and organic photovoltaics (OPVs) have attracted much interest in recent years due to their solution processability and mechanical fl exibility, which potentially allow them to be manufactured using low-cost, high-throughput processes such as roll-to-roll printing and inkjet printing. [ 1 ] In recent years, the development of novel materials for these applications has contributed to signifi cant improvements in device performance. In the fi eld of OTFTs, the development of polythiophene derivatives such as poly(3,3 ′ ′ ′ -dialkyl-quaterthiophene) (PQT) and poly(2,5bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) has resulted in OTFTs with hole mobilities of between 0.2 and 0.6 cm 2 V − 1 s − 1 . [ 2 , 3 ] The development of low bandgap polymer donors has meanwhile helped to lift solar cell power conversion effi ciencies (PCEs) to above 6%. [ 4–6 ] However, there have been few reports of polymers that perform well in both OTFTs and OPVs. The development of such versatile polymers could be benefi cial for applications which incorporate both types of devices. Although the high hole mobility of polythiophenes is an attractive property for OPV applications, their relatively large bandgap of around 1.9 to 2 eV [ 7 ] means that they are unable to absorb and harvest photons above 650 nm. The incorporation of acceptor moieties into the polythiophene backbone allows the bandgap to be reduced and can thereby improve solar cell performance. A commonly used acceptor moiety in D–A polymers is 2,1,3-benzothiadiazole, which has been copolymerized with donor moieties such as fl uorene, [ 8 ] carbazole, [ 4 , 9 ] and cyclopentadithiophene [ 10 , 11 ] to produce low bandgap polymers and associated solar cells with PCEs up to 6.1%. Copolymers of thiophene and benzothiadiazole have previously been studied in OPVs, with the number of thiophene units in the repeat unit ranging from three to eight. Despite the lowered bandgap and improved absorption properties of the polymers, the PCEs of the resultant devices have been relatively low, ranging from 0.13% to 2.23%. [ 12–15 ]

Reduced Graphene Oxide Electrodes for Large Area Organic Electronics

patterning of molecular electrode materials and carbon nanotubes. The interlayer technique involves the insertion of a layer of photoresist between the substrate and the fi lm to be patterned. The resist layer is exposed through a mask, generating a pattern that can subsequently be developed after deposition of the target material. Immersion in an appropriate developer removes the soluble parts of the resist layer together with the overlying target material, leaving a patterned fi lm of the target material over a likewise patterned fi lm of the resist. The resist and the target material are in effect patterned simultaneously in an expose‐ deposit‐develop step sequence. Furthermore, this technique is compatible with the use of standard solution processing and mechanical transfer methods for deposition of the target fi lms. Since interlayer lithography builds on the existing expertise and equipment of conventional photolithography, it is easy to implement and fully compatible with fast, cost effective sheet-to-sheet processing for large-area electronics.

Microscale synthesis of quantum dots

Microfluidic reactors are emerging as a highly promising technology for quantum dot synthesis due to the unparalleled control they provide over particle properties. In this article, we review recent developments in the microfluidic synthesis of quantum dots, and discuss some of the advantages and challenges of preparing nanocrystalline materials in microscale fluidic channels. The relative merits of continuous-flow and segmented-flow reactors are considered, together with a number of outstanding issues that must be successfully addressed for microfluidics to become a truly viable technology for quantum dot synthesis.

Entanglements in marginal solutions: a means of tuning pre-aggregation of conjugated polymers with positive implications for charge transport

The solution-processing of conjugated polymers, just like commodity polymers, is subject to solvent and molecular weight-dependent solubility, interactions and chain entanglements within the polymer, all of which can influence the crystallization and microstructure development in semi-crystalline polymers and consequently affect charge transport and optoelectronic properties. Disentanglement of polymer chains in marginal solvents was reported to work via ultrasonication, facilitating the formation of photophysically ordered polymer aggregates. In this contribution, we explore how a wide range of technologically relevant solvents and formulations commonly used in organic electronics influence chain entanglement and the aggregation behaviour of P3HT using a combination of rheological and spectrophotometric measurements. The specific viscosity of the solution offers an excellent indication of the degree of entanglements in the solution, which is found to be related to the solubility of P3HT in a given solvent. Moreover, deliberately disentangling the solution in the presence of solvophobic driving forces, leads consistently to formation of photophysically visible aggregates which is indicative of local and perhaps long range order in the solute. We show for a broad range of solvents and molecular weights that disentanglement ultimately leads to significant ordering of the polymer in the solid state and a commensurate increase in charge transport properties. In doing so we demonstrate a remarkable ability to tune the microstructure which has important implications for transport properties. We discuss its potential implications in the context of organic electronics and photovoltaics.

Direct correlation of charge transfer absorption with molecular donor:acceptor interfacial area via photothermal deflection spectroscopy

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 influence of polymer purification on the efficiency of poly(3-hexylthiophene):fullerene organic solar cells

We report the influence of different polymer purification procedures on the photovoltaic performance of bulk heterojunction solar cells formed from binary blends of poly(3-hexylthiophene) (P3HT) and fullerenes. Selective Soxhlet extractions and metal scavenging agents were used to remove residual monomer, magnesium salt by-products and catalyst from high-weight P3HT (Mw 121 kg/mol, PDI 1.8, RR 99%) synthesised by the Grignard metathesis (GRIM) polymerization route. Using phenyl-C61-butyric acid methyl ester (PC60BM) as an electron acceptor, we observed an increase in average power conversion efficiency from 2.3 to 4.8% in going from crude to fully purified material. Using indene-C60 bisadduct (IC60BA) in place of PC60BM, we observed a further increase to an average value of 6.6% - high for a bulk heterojunction formed from a binary blend of P3HT and C60 fullerene derivatives.

Amplified fluorescence quenching in high ionic strength media

We report a new cationic poly(phenylene ethynylene) (PPE) derivative that exhibits strong amplified fluorescence quenching in the presence of electron-deficient species, yielding high Stern-Volmer coefficients of 4.7 × 10 M in aqueous solutions. Importantly, with the addition of appropriate non-ionic surfactants, the polymer is found to retain its excellent sensitivity even when transferred to high ionic strength buffered media, which have previously been shown to suppress the amplified quenching effect in other polyelectrolyte systems. The cationic PPE derivative yields Stern-Volmer coefficients as high as 10 M in 25 mM buffer solutions of both tris(hydroxymethyl) aminomethane (Tris) and sodium acetate containing 150 mM sodium chloride, the optimal conditions for many enzymes such as phosphatases. The ability to maintain high Stern-Volmer coefficients in high ionic strength buffered media extends the applicability of ionic conjugated polymers to high sensitivity detection in biological media, and thus greatly enhances their versatility as biological sensors.

Origin of fullerene-induced vitrification of fullerene:donor polymer photovoltaic blends and its impact on solar cell performance

Organic solar cell blends comprised of an electron donating polymer and electron accepting fullerene typically form upon solution casting a thin-film structure made up of a complex mixture of phases. These phases can vary greatly in: composition, order and thermodynamic stability; and they are dramatically influenced by the processing history. Understanding the processes that govern the formation of these phases and their subsequent effect on the efficiency of photo-generating and extracting charge carriers is of utmost importance to enable rational design and processing of these blends. Here we show that the vitrifying effect of three fullerene derivatives ([60]PCBM, bis[60]PCBM, and [60]ICBA) on the prototypical donor polymer (rr-P3HT) can dominate microstructure formation of fullerene/donor polymer blends cast from solution. Using a dynamic crystallization model based on an amalgamation of Flory–Huggins and Lauritzen–Hoffman theory coupled to solvent evaporation we demonstrate that this vitrification, which can result in a large fraction of highly intermixed amorphous solid solution of the fullerene and the polymer, is due to kinetic and thermodynamic reasons. The former is partly determined by the glass transition temperature of the individual components while donor polymer:fullerene miscibility, strongly influenced by the chemical nature of the donor and the fullerene and leading to thermodynamic mixing, dictates the second phenomena. We show that our approximate dynamic crystallization model assists understanding the different solid-state structure formation of rr-P3HT:fullerene blends. Due to the generality of the assumptions used, our model should be widely applicable and assist to capture the influence of the different vitrification mechanisms also of other photovoltaic blends, including the high-efficiency systems based on the strongly aggregating PCE11 (PffBT4T-2OD), which also feature clear signs of vitirfication upon blending with, e.g., [60]PCBM. Hence, our model will provide essential materials design criteria and enable identification of suitable processing guidelines for existing and new high-performing blends from the outset.