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The use of dopants is an effective strategy to improve ZnO electron transport layers (ETLs) for application in solution-processed opto-electronic devices. Mg, in particular, has shown significant promise as a dopant and Mg-doped ZnO ETLs have been used to enhance the performance of a number of solution-processed light-emitting diodes and photovoltaics. However, such a use of Mg to dope ZnO ETLs for organic photovoltaics (OPVs) has remained limited, and only investigated in connection with annealing temperatures of 300 °C or so. In this work, with a view to increase sustainability and compatibility with soft and flexible or foldable substrates, we present OPVs incorporating Mg-doped ZnO ETLs fabricated with annealing temperatures of 150 °C. We demonstrate that Mg doping (≈1% at%) in the ZnO ETL reduces leakage currents and recombination losses in our devices, whilst leaving the morphology of the active layer and the work function of the ETL unaffected. A concomitant increase of the short circuit current density, open circuit voltage and fill factor is also observed, thereby leading to a relative enhancement of the power conversion efficiency by ≈18% compared to devices prepared using undoped ZnO.

Electron transport layers (ETLs) have been instrumental in breaking the efficiency boundaries of solution-processed photovoltaics. In particular, bilayer ETLs with an MgO top component have afforded tremendous success in various solution-processed systems, such as perovskite photovoltaics, however, their application in the promising technology of organic photovoltaics is limited. In this work, we fabricate organic photovoltaic devices incorporating a “bilayer” ZnO/MgO ETL instead of a single ZnO ETL, so as to reduce the leakage current and boost the power conversion efficiency. The ZnO/MgO ETL is shown to have a more uniform top surface and a lower work function compared to the single ZnO ETL which is expected to be beneficial to electron extraction. Furthermore, we demonstrate that insertion of the thin (≲ 10 nm) MgO interlayer in devices leads to a reduced leakage current and an increase in the shunt resistance. Application of the MgO interlayer boosts the short circuit current density and fill factor, and enhances the power conversion efficiency by ∼10% (relative increase) thereby demonstrating a facile approach to push the efficiency of organic photovoltaics to higher levels.

Efficient charge photogeneration in conjugated polymers typically requires the presence of a second component to act as electron acceptor. Here, we report a novel low band-gap conjugated polymer with a donor/orthogonal acceptor motif: poly-2,6-(4,4-dihexadecyl-4H-cyclopenta [2,1-b:3,4-b′]dithiophene)-alt-2,6-spiro [cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-fluorene]-2′,7′-dicarbonitrile, referred to as PCPDT-sFCN. The role of the orthogonal acceptor is to spatially isolate the LUMO from the HOMO, allowing for negligible exchange energy between electrons in these orbitals and minimising the energy gap between singlet and triplet charge transfer states. We employ ultrafast and microsecond transient absorption spectroscopy to demonstrate that, even in the absence of a separate electron acceptor, PCPDT-sFCN shows efficient charge photogeneration in both pristine solution and film. This efficient charge generation is a result of an isoenergetic singlet/triplet charge transfer state equilibrium acting as a reservoir for charge carrier formation. Furthermore, clear evidence of enhanced triplet populations, which form in less than 1 ps, is observed. Using group theory, we show that this ultrafast triplet formation is due to highly efficient, quantum mechanically allowed intersystem crossing between the bright, initially photoexcited local singlet state and the triplet charge transfer state. Remarkably, the free charges that form via the charge transfer state are extraordinarily long-lived with millisecond lifetimes, possibly due to the stabilisation imparted by the spatial separation of PCPDT-sFCN's donor and orthogonal acceptor motifs. The efficient generation of long-lived charge carriers in a pristine polymer paves the way for single-material applications such as organic photovoltaics and photodetectors., The spatial separation of PCPDT-sFCN's donor and orthogonal acceptor motifs allows efficient photogeneration of extraordinarily long-lived charge carriers in the pristine polymer, providing an important step towards single-material optoelectronics.

The high power-per-weight ratio displayed by metal-halide perovskite solar cells is a key advantage of these promising devices for applications that require low payload, such as in space and avionics. However, little is known about the effect of the outer space radiation environment on these devices. Here, we report the first in operando study on fast neutron irradiation of perovskite solar cells. We show the remarkable resilience of these devices against one of the most hazardous forms of radiation that can be found at flight altitude and in space. In particular, our results highlight a comparable in operando degradation pattern between light soaked and light + neutron irradiated devices. However, whereas light-induced degradation is fully reversible, fast neutrons lead to permanent effects likely originating from atomic displacement in the active material. We also propose that such irreversible worsening is alleviated by the formation of neutron-induced shallow traps, which act as dopants and contribute to the increase of open circuit voltage and the decrease of leakage current in light + neutron irradiated devices. The high radiation dose that perovskite-based solar cells can potentially withstand renders these devices highly appealing for space and avionic applications.

Electronic supplementary information (ESI) available: Synthetic details and materials characterization. See DOI: 10.1039/c4tc00342j A family of phthalimide–thiophene copolymers with linear and branched alkyl chains attached to the imide nitrogen have been synthesized. Their optical and electronic properties were investigated along with their applications in OFETs and LEFETs. The phthalimide–thiophene copolymer having a C₁₆ straight alkyl chain on the phthalimide yielded the highest mobilities and PLQE with mobilities of 1 × 10̄̄̄̄⁻³ cm² V⁻¹ s⁻¹ for holes and 1 x 10⁻² cm² V⁻¹-s⁻¹ for electrons with a PLQE of ~28% in the solid state. Since these polymers are ambipolar and emissive, they have proven to be useful for applications as a host material for NIR PLEDs. In this study a 1% loading of NIR emitting DAD segments based on bisthienyl(thiadiazoloquinoxaline) or bisthienyl(benzotriazolothiadiazole) were incorporated into the phthalimide–thiophene polymerization. Using the branched CH(C₈H₁₇)₂ alkyl chain on the host phthalimide–thiophene copolymer combined with the bisthienyl(benzotriazolothiadiazole) emitter resulted in the most efficient (emission maximum ≥ 850 nm) single layer NIR-emitting PLED to date with an EQE of 0.27% emitting at 885 nm. Refereed/Peer-reviewed

We present the synthesis of a novel diphenylanthracene (DPA) based semiconducting polymer. The polymer is solubilised by alkoxy groups attached directly to a DPA monomer, meaning the choice of co-monomer is not limited to exclusively highly solubilising moieties. Interestingly, the polymer shows a red-shifted elecroluminescence maximum (510 nm) when compared to its photoluminescence maximum (450 nm) which we attribute to excimer formation. The novel polymer was utilised as a host for a covalently-linked platinum(ii) complexed porphyrin dopant. Emission from these polymers was observed in the NIR and again showed almost a 100 nm red shift from photoluminescence to electroluminescence. This work demonstrates that utilising highly aggregating host materials is an effective tool for inducing red-shifted emission in OLEDs.

The development of semiconducting polymers with high solid-state luminescence efficiencies has enabled the fabrication of efficient polymer light-emitting diodes. Luminescence is often quenched in well-ordered molecular solids, as a result of inter-molecular dipolar coupling, and the general observation of efficient luminescence in semiconducting polymers is unexpected. We report here the synthesis and characterisation of a series of model 'glassy' poly(arylenevinylene)s where we control the cis to trans ratio about the vinylene linkage and also the phenylene linkage geometry. Photoluminescence efficiency is enhanced for more disordered materials, with highest values for 50 : 50 cis : trans ratios, when it exceeds 50%. We also find that the free volume associated with these glassy disordered polymers allows conformational relaxation of the excitonic state, via ring rotation at the vinylene linkage, causing a large Stokes' shift of the emission. We propose that high luminescence efficiency in these glassy polymers is due to emission from the more disordered regions, and that two effects due to disorder are simultaneously required: firstly that these regions are luminescent (prevention of aggregation by disorder), and, secondly, that the electronic excited state (exciton) can lower its energy below its value in more ordered regions by means of the ring-rotational coupling.

We present picosecond time-resolved studies of thin films of two polyfluorene blends, combining a relatively high energy-gap poly(9,9-dioctylfluorene), PFO, and a polyfluorene derivative with a smaller gap. These studies show both efficient energy transfer from the donor to the acceptor, as well as spatial confinement of the excitons in the acceptor polymers. The optoelectronic characteristics of light-emitting diodes fabricated with such blends reflect the contribution of these two processes. The electroluminescence is almost exclusively from the acceptor polyfluorenes and we observe a significant improvement of the electroluminescence efficiency of the light-emitting diodes fabricated with such blends in comparison to those based on neat films of the polyfluorenes.

We report on the luminescence of a tetraphenylporphyrin, TPP-d, blended into poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene], MEH-PPV. We find significant energy transfer from MEH-PPV to the porphyrin, in spite of the low absorption of the porphyrin at the emission wavelength of MEH-PPV, reflected in a Forster transfer radius (2.5 nm) smaller than for materials with larger spectral overlap. The overall photoluminescence, PL, efficiency decreases monotonically with increasing porphyrin content, whereas the porphyrin contribution to the total efficiency, referred as an “apparent” PL efficiency, exhibits a maximum at 1.4% porphyrin content (by weight). We attribute this non-monotonic behaviour to the interplay of the exciton transfer probability and PL quenching, both of which increase with concentration. We also observed the energy transfer under electrical excitation, but noticed that, at low concentrations, the porphyrin contribution to the electroluminescence is higher than that observed in PL. This indicates significant emission from excitons formed directly at the porphyrin sites, which are likely to act as charge trapping sites. We also compare the luminescence properties of the blends with those of copolymers based on the same host–guest pair.

We report the preparation of high-quality, solvent-free [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) large single crystals (size up to 0.5 mm) by slow drying of a chlorobenzene solution at room temperature. The monoclinic structure containing four PCBM molecules per unit cell was successfully solved (R factor = 0.0512) via micro-focused X-ray diffraction and employed as a reliable experimental model for further molecular dynamics simulations. We find that the first peak of the simulated fullerene-fullerene radial distribution function is centred at 10.05 A, giving a nearest neighbour coordination number of 7.0. The work reported herein provides the structural basis for a fundamental understanding of charge transport in this important functional material that is particularly relevant to organic solar cells.

2,3-Dialkoxy-substituted poly(1,4-phenylene vinylene) (PPV) homo- and co-polymers have been prepared by the Gilch dehydrohalogenation polycondensation of the corresponding bishalomethyl-substituted benzene monomers, and double layer light emitting devices fabricated with these materials exhibited high electroluminescence efficiencies with low turn-on voltages.

Highly luminescent inclusion complexes consisting of poly(para-phenylene) (PPP) or poly(4,4'-diphenylene-vinylene) (PDV) in the helical cavity of amylose have been synthesised, structurally characterised by nuclear Overhauser spectroscopy and used to fabricate electroluminescent light-emitting diodes.

A one-pot polycondensation of 4,4′-(hexafluoroisopropylidene)bis(benzoic acid)1 with terephthalic dihydrazide 2 and/or hydrazine sulfate 3 affords aromatic oxadiazole polymers 4, 5 and 6, respectively, the optical and electrochemical properties of which have been investigated for applications as charge transporting layers in electroluminescent devices.