Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments.

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The rise in power conversion efficiency of organic photovoltaic (OPV) devices over the last few years has been driven by the emergence of new organic semiconductors and the growing understanding of morphological control at both the molecular and aggregation scales. Non-fullerene OPVs adopting p-type conjugated polymers as the donor and n-type small molecules as the acceptor have exhibited steady progress, outperforming PCBM-based solar cells and reaching efficiencies of over 14 % in 2018. This review starts with a refreshed discussion of charge separation, recombination, and VOC loss in non-fullerene OPVs, followed by a review of work undertaken to develop favorable molecular configurations required for high device performance. We summarize several key approaches that have been employed to tune the nanoscale morphology in non-fullerene photovoltaic blends, comparing them (where appropriate) to their PCBM-based counterparts. In particular, we discuss issues ranging from materials chemistry to solution processing and post-treatments, showing how this can lead to enhanced photovoltaic properties. Particular attention is given to the control of molecular configuration through solution processing, which can have a pronounced impact on the structure of the solid-state photoactive layer. Key challenges, including green solvent processing, stability and lifetime, burn-in, and thickness-dependence in non-fullerene OPVs are briefly discussed.

This paper describes the construction of a phase diagram for the as-cast state in the organic photovoltaic system P3HT:PCBM. Evidence for a transition to a phase-separated state at PCBM concentrations greater than 70 wt % is seen both by DMTA and GIWAXS, and the glass transition temperatures of blends in the single phase state below 70 wt % PCBM are observed to be raised compared to the pure polymer. Pure PCBM is observed to exhibit a thermal transition at 155 °C, an observation unreported to dateoffering insight into crystallites commonly seen in device films. The liquid-crystal phase of P3HT is shown to persist in the presence of up to 41 wt % PCBM. In addition, pure PCBM is shown to be significantly hygroscopic, with important implications for the processing of high-performance devices.

A range of optical probes are used to study the nanoscale‐structure and electronic‐functionality of a photovoltaic‐applicable blend of the carbazole co‐polymer poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and the electronic accepting fullerene derivative (6,6)‐phenyl C70‐butyric acid methyl ester (PC70BM). In particular, it is shown that the glass transition temperature of a PCDTBT:PC70BM blend thin‐film is not sensitive to the relative blend‐ratio or film thickness (at 1:4 blending ratio), but is sensitive to casting solvent and the type of substrate on which it is deposited. It is found that the glass transition temperature of the blend reduces on annealing; an observation consistent with disruption of π–π stacking between PCDTBT molecules. Reduced π–π stacking is correlated with reduced hole‐mobility in thermally annealed films. It is suggested that this explains the failure of such annealing protocols to substantially improve device‐efficiency. The annealing studies demonstrate that the blend only undergoes coarse phase‐separation when annealed at or above 155 °C, suggesting a promising degree of morphological stability of PCDTBT:PC70BM blends.

Pressure-sensitive adhesives (PSAs) adhere instantly and firmly to nearly any surface under the application of light pressure, without covalent bonding or activation.[1] PSAs are increasingly used for demanding applications, such as interconnects in electronic assemblies. [2] The debonding of PSAs occurs by a cavitation process [3,4] followed by cavity expansion to create fibrils [5] that extend in traction. These processes contribute to the energy of adhesion, E a . [6,7] To be an effective PSA, a material must be neither too stiff nor too liquid, and it must dissipate significant energy in deformation. Here, we show that the addition of carbon nanotubes (CNTs) to a PSA makes it stiffer yet more dissipative, which is an ideal, yet unusual, combination. The CNTs impart electrical conductivity to the PSA while increasing E a by 85 % and retaining optical transparency. Because of tightening environmental legislation, [8] there is a trend to manufacture PSAs using waterborne processing, such as colloidal dispersions, [9] to reduce emission of organic solvents. The adhesive properties of waterborne PSAs are inferior to conventional materials, [10,11] creating an urgent need for materials development. A potential remedy is the creation of nanocomposites containing CNTs. Recently, polymer colloid technology, that is, latex, has been exploited to create CNTpolymer composites for structural applications. [12][13][14][15][16][17] CNTs are not naturally water soluble, but several different emulsifiersan anionic surfactant (sodium dodecyl sulfate [12,13,16,17] ), a cationic surfactant, [17] and a water-soluble polymer (gum arabic) [14,15] -have been employed to disperse and stabilize them in water. Surfactants and water-soluble polymers in latex films [18] are known to decrease their water resistance [19] and to degrade adhesive performance.[11] Hence, we avoided dispersions of CNTs prepared with these materials. It has been recognized that the interfaces in polymer/CNT nanocomposites can have an influence on the viscoelasticity. [20][21][22] Soft polymers that are chemically grafted onto CNTs can potentially offer energy dissipation at the interface. To explore this possibility, we prepared nanocomposites using single-walled CNTs (SWNTs) that were functionalized with hydrophilic poly(vinyl alcohol) (PVA) via an esterification reaction [23] to render them dispersible and stable in water and to create a dissipative interface. Transmission electron microscopy [23] has revealed complete coverage of the SWNTs by the polymer and provided evidence for nanotube debundling. The functionalized SWNTs, referred to hereafter as PVA-SWNT, were blended with a poly(butyl acrylate) (P(BuA)) latex dispersion via sonication. A sophisticated means of evaluating adhesive performance is via probe-tack experiments in which a probe is brought into contact with a PSA surface and then removed at a constant velocity. [5,6] Figure 1 shows typical tack curves of P(BuA)/ PVA-SWNT adhesives measured at two debonding speeds of 10 and 100 lm s -1 . The de...

This review highlights the opportunities and challenges in stability of organic solar cells arising from the emergence of non-fullerene acceptors.

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