Easily processed materials with the ability to transport excitons over length scales of more than 100 nanometers are highly desirable for a range of light-harvesting and optoelectronic devices. We ...describe the preparation of organic semiconducting nanofibers comprising a crystalline poly(di-
-hexylfluorene) core and a solvated, segmented corona consisting of polyethylene glycol in the center and polythiophene at the ends. These nanofibers exhibit exciton transfer from the core to the lower-energy polythiophene coronas in the end blocks, which occurs in the direction of the interchain π-π stacking with very long diffusion lengths (>200 nanometers) and a large diffusion coefficient (0.5 square centimeters per second). This is made possible by the uniform exciton energetic landscape created by the well-ordered, crystalline nanofiber core.
Exciton generation, migration, and dissociation are key processes that play a central role in the design and operation of many organic optoelectronic devices. In organic photovoltaic cells, charge ...generation often occurs only at an interface, forcing the exciton to migrate from the point of photogeneration in order to be dissociated into its constituent charge carriers. Consequently, the design and performance of these devices is strongly impacted by the typically short distance over which excitons are able to move. The ability to engineer materials or device architectures with favorable exciton transport depends strongly on improving our understanding of the governing energy transfer mechanisms and rates. To this end, this review highlights recent efforts to better characterize, understand and ultimately engineer exciton transport.
The efficient harvesting of excitons is an essential step in the operation of organic photovoltaic cells. The ability to engineer and tailor exciton transport remains a key challenge for the design of next generation devices. In this review, we examine recent efforts to characterize exciton transport as well as novel material and device-based techniques aimed at extending the distance over which an exciton can migrate.
Photoconversion in planar-heterojunction organic photovoltaic cells (OPVs) is limited by a short exciton diffusion length (L(D)) that restricts migration to the dissociating electron donor/acceptor ...interface. Consequently, bulk heterojunctions are often used to realize high efficiency as these structures reduce the distance an exciton must travel to be dissociated. Here, we present an alternative approach that seeks to directly engineer L(D) by optimizing the intermolecular separation and consequently, the photophysical parameters responsible for excitonic energy transfer. By diluting the electron donor boron subphthalocyanine chloride into a wide-energy-gap host material, we optimize the degree of interaction between donor molecules and observe a ~50% increase in L(D). Using this approach, we construct planar-heterojunction OPVs with a power conversion efficiency of (4.4 ± 0.3)%, > 30% larger than the case of optimized devices containing an undiluted donor layer. The underlying correlation between L(D) and the degree of molecular interaction has wide implications for the design of both OPV active materials and device architectures.
Donor-acceptor organic solar cells often show low open-circuit voltages (V
) relative to their optical energy gap (E
) that limit power conversion efficiencies to ~12%. This energy loss is partly ...attributed to the offset between E
and that of intermolecular charge transfer (CT) states at the donor-acceptor interface. Here we study charge generation occurring in PIPCP:PC
BM, a system with a very low driving energy for initial charge separation (E
-E
~ 50 meV) and a high internal quantum efficiency (η
~ 80%). We track the strength of the electric field generated between the separating electron-hole pair by following the transient electroabsorption optical response, and find that while localised CT states are formed rapidly (<100 fs) after photoexcitation, free charges are not generated until 5 ps after photogeneration. In PIPCP:PC
BM, electronic disorder is low (Urbach energy <27 meV) and we consider that free charge separation is able to outcompete trap-assisted non-radiative recombination of the CT state.
Efficient conjugated polymer optoelectronic devices benefit from concomitantly high luminescence and high charge carrier mobility. This is difficult to achieve, as interchain interactions, which are ...needed to ensure efficient charge transport, tend also to reduce radiative recombination and lead to solid-state quenching effects. Many studies detail strategies for reducing these interactions to increase luminescence, or modifying chain packing motifs to improve percolation charge transport; however achieving these properties together has proved elusive. Here, we show that properly designed amorphous donor-alt-acceptor conjugated polymers can circumvent this problem; combining a tuneable energy gap, fast radiative recombination rates and luminescence quantum efficiencies >15% with high carrier mobilities exceeding 2.4 cm
/Vs. We use photoluminescence from exciton states pinned to close-crossing points to study the interplay between mobility and luminescence. These materials show promise towards realising advanced optoelectronic devices based on conjugated polymers, including electrically-driven polymer lasers.
Rapid, long-range charge separation in polymer-fullerene organic solar cells (OSCs) enables electrons and holes to move beyond their Coulomb capture radius and overcome geminate recombination. ...Understanding the nature of charge generation and recombination mechanisms in efficient, nonfullerene-acceptor-based OSCs are critical to further improve device performance. Here we report charge dynamics in an OSC using a perylene diimide (PDI) dimer acceptor. We use transient absorption spectroscopy to track the time evolution of electroabsorption caused by the dipolar electric field generated between electron–hole pairs as they separate after ionization at the donor–acceptor interface. We show that charges separate rapidly (<1 ps) and that free charge carriers are generated very efficiently (∼90% quantum yield). However, in the PDI-based OSC, external charge extraction is impaired by faster nongeminate decay to the ground state and to lower-lying triplet states.
The first abiotic foldamer tertiary structures have been recently reported in the form of aromatic helix-turn-helix motifs based on oligo-quinolinecarboxamides held together by intramolecular ...hydrogen bonds. Tertiary folds were predicted by computational modelling of the hydrogen-bonding interfaces between helices and later verified by X-ray crystallography. However, the prognosis of how the conformational preference inherent to each helix influences the tertiary structure warranted further investigation. Several new helix-turn-helix sequences were synthesised in which some hydrogen bonds have been removed. Contrary to expectations, this change did not strongly destabilise the tertiary folds. On closer inspection, a new crystal structure revealed that helices adopt their natural curvature when some hydrogen bonds are missing and undergo some spring torsion upon forming the said hydrogen bonds, thus potentially giving rise to a conformational frustration. This phenomenon sheds light on the aggregation behaviour of the helices when they are not linked by a turn unit.
Fulfilling stabilizing hydrogen bonds in a synthetic helix-turn-helix structure may concomitantly generate conformational frustration.
Electrically injected charge carriers in organic light-emitting devices (OLEDs) undergo recombination events to form singlet and triplet states in a 1:3 ratio, representing a fundamental hurdle for ...achieving high quantum efficiency. Dopants based on thermally activated delayed fluorescence (TADF) have emerged as promising candidates for addressing the spin statistics issue in OLEDs. In these materials, reverse singlet–triplet intersystem crossing (rISC) becomes efficient, thereby activating luminescence pathways for weakly emissive triplet states. However, despite a growing consensus that torsional vibrations facilitate spin–orbit-coupling- (SOC-) driven ISC in these molecules, there is a shortage of experimental evidence. We use transient electron spin resonance and theory to show unambiguously that SOC interactions drive spin conversion and that ISC is a dynamic process gated by conformational fluctuations for benchmark carbazolyl–dicyanobenzene TADF emitters.
Background Although the promotion of social inclusion through sports has received increased attention with other disadvantaged groups, this is not the case for children and adults with intellectual ...disability who experience marked social isolation. The study evaluated the outcomes from one sports programme with particular reference to the processes that were perceived to enhance social inclusion.
Method The Youth Unified Sports programme of Special Olympics combines players with intellectual disabilities (called athletes) and those without intellectual disabilities (called partners) of similar skill level in the same sports teams for training and competition. Alongside the development of sporting skills, the programme offers athletes a platform to socialise with peers and to take part in the life of their community. Unified football and basketball teams from five countries – Germany, Hungary, Poland, Serbia and Ukraine – participated. Individual and group interviews were held with athletes, partners, coaches, parents and community leaders: totalling around 40 informants per country.
Results Qualitative data analysis identified four thematic processes that were perceived by informants across all countries and the two sports to facilitate social inclusion of athletes. These were: (1) the personal development of athletes and partners; (2) the creation of inclusive and equal bonds; (3) the promotion of positive perceptions of athletes; and (4) building alliances within local communities.
Conclusions Unified Sports does provide a vehicle for promoting the social inclusion of people with intellectual disabilities that is theoretically credible in terms of social capital scholarship and which contains lessons for advancing social inclusion in other contexts. Nonetheless, certain limitations are identified that require further consideration to enhance athletes' social inclusion in the wider community.
Donor–acceptor organic solar cells often show high quantum yields for charge collection, but relatively low open-circuit voltages (V OC) limit power conversion efficiencies to around 12%. We report ...here the behavior of a system, PIPCP:PC61BM, that exhibits very low electronic disorder (Urbach energy less than 27 meV), very high carrier mobilities in the blend (field-effect mobility for holes >10–2 cm2 V–1 s–1), and a very low driving energy for initial charge separation (50 meV). These characteristics should give excellent performance, and indeed, the V OC is high relative to the donor energy gap. However, we find the overall performance is limited by recombination, with formation of lower-lying triplet excitons on the donor accounting for 90% of the recombination. We find this is a bimolecular process that happens on time scales as short as 100 ps. Thus, although the absence of disorder and the associated high carrier mobility speeds up charge diffusion and extraction at the electrodes, which we measure as early as 1 ns, this also speeds up the recombination channel, giving overall a modest quantum yield of around 60%. We discuss strategies to remove the triplet exciton recombination channel.
PDF
