Exciton fission is a process that occurs in certain organic materials whereby one singlet exciton splits into two independent triplets. In photovoltaic devices these two triplet excitons can each generate an electron, producing quantum yields per photon of >100% and potentially enabling single-junction power efficiencies above 40%. Here, we measure fission dynamics using ultrafast photoinduced absorption and present a first-principles expression that successfully reproduces the fission rate in materials with vastly different structures. Fission is non-adiabatic and Marcus-like in weakly interacting systems, becoming adiabatic and coupling-independent at larger interaction strengths. In neat films, we demonstrate fission yields near unity even when monomers are separated by >5 Å. For efficient solar cells, however, we show that fission must outcompete charge generation from the singlet exciton. This work lays the foundation for tailoring molecular properties like solubility and energy level alignment while maintaining the high fission yield required for photovoltaic applications.

We use transient absorption spectroscopy to demonstrate that the dynamics of singlet exciton fission in tetracene are independent of temperature (10–270 K). Low-intensity, broad-band measurements allow the identification of spectral features while minimizing bimolecular recombination. Hence, by directly observing both species, we find that the time constant for the conversion of singlets to triplet pairs is ~90 ps. However, in contrast to pentacene, where fission is effectively unidirectional, we confirm that the emissive singlet in tetracene is readily regenerated from spin-correlated "geminate" triplets following fission, leading to equilibrium dynamics. Although free triplets are efficiently generated at room temperature, the interplay of superradiance and frustrated triplet diffusion contributes to a nearly 20-fold increase in the steady-state fluorescence as the sample is cooled. Together, these results require that singlets and triplet pairs in tetracene are effectively degenerate in energy, and begin to reconcile the temperature dependence of many macroscopic observables with a fission process which does not require thermal activation.

We present a new fully conjugated diblock copolymer, P3HT-b-PFTBTT, containing donor and acceptor blocks with suitably positioned energy levels for use in a solar cell. This is the first block copolymer to be based on an existing high-performance polymer:polymer blend. We observe phase separation of the blocks and self-assembly behavior. In ternary blends with the respective homopolymers the diblock copolymer introduces lateral nanostructure without restricting P3HT crystallization in the charge transport direction, resulting in standing lamellae. By adding the diblock to the homopolymer blend as a compatibilizer, we prevent phase separation at elevated temperatures and benefit from a dramatic increase in P3HT ordering, allowing us to demonstrate polymer blend photovoltaics where the nanostructure is thermodynamically, rather than kinetically, controlled.

A study of an efficient blue light-emitting diode based on a fluorescent aryl polyfluorene (aryl-F8) homopolymer in an inverted device architecture is presented, with ZnO and MoO 3 as electron-and hole-injecting electrodes, respectively. Charge-carrier balance and color purity in these structures are achieved by incorporating poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)-diphenylamine (TFB) into aryl-F8. TFB is known to be a hole-transporting material but it is found to act as a hole trap on mixing with aryl-F8. Luminance efficiency of ≈6 cd A −1 and external quantum efficiency (EQE) of 3.1% are obtained by adding a small amount (0.5% by weight) of TFB into aryl-F8. Study of charge injection and transport in the single-carrier devices shows that the addition of a small fraction of hole traps is necessary for chargecarrier balance. Optical studies using UV-vis and fluorescence spectroscopic measurements, photoluminescence quantum yield, and fluorescence decay time measurements indicate that TFB does not affect the optical properties of the aryl-F8, which is the emitting material in these devices. Luminance efficiency of up to ≈11 cd A −1 and EQE values of 5.7% are achieved in these structures with the aid of improved out-coupling using index-matched hemispheres.

An energy cascading structure is designed in a polymer photovoltaic device to suppress recombination and improve quantum yields. By the insertion of a thin polymer interlayer with intermediate energy levels, electrons and holes can effectively shuttle away from each other while being spatially separated from recombination. An increase in open-circuit voltage and short-circuit current are observed in modified devices.

We report the electronic properties of the conjugated coupling between a donor polymer and an acceptor segment serving as a model for the coupling in conjugated donor-acceptor block copolymers. These structures allow the study of possible intrachain photoinduced charge separation, in contrast to the interchain separation achieved in conventional donor-acceptor blends. Depending on the nature of the conjugated linkage, we observe varying degrees of modification of the excited states, including the formation of intrachain charge transfer excitons. The polymers comprise a block (typically 18 repeat units) of P3HT, poly(3-hexyl thiophene), coupled to a single unit of F8-TBT (where F8 is dioctylfluorene, and TBT is thiophene-benzothiadiazole-thiophene). When the P3HT chain is linked to the TBT unit, we observe formation of a localized charge transfer state, with red-shifted absorption and emission. Independent of the excitation energy, this state is formed very rapidly (<40 fs) and efficiently. Because there is only a single TBT unit present, there is little scope for long-range charge separation and it is relatively short-lived, <1 ns. In contrast, when the P3HT chain and TBT unit are separated by the wider bandgap F8 unit, there is little indication for modification of either ground or excited electronic states, and longer-lived charge separated states are observed.

Interactions of Ni(0) and thiophene–benzothiadiazole–thiophene (TBT) units during the Kumada catalyst transfer polycondensation (KCTP) of 2-chloromagnesio-5-bromo-3-hexylthiophene (1) are investigated. Monofunctional TBT initiator precursors are used for the external initiation of KCTP, and the mechanism of initiator activation and polymerization is elucidated using NMR and MALDI-ToF MS. We find that the activation of the TBT-bromide initiator precursor using nickel–diethylbipyridine (NiEt2bipy) occurs via a two-step pathway, in which NiEt2bipy coordinates to benzothiadiazole (B) first, followed by the elimination of butane and oxidative addition of liberated Ni(0)bipy into the terminal TBT–Br bond. It is shown that the B unit traps Ni(0) during the KCTP of 1, which results in significant termination, as derived from the degree of bromine-terminated chains. The ability of B units to trap Ni(0) is further illustrated by using a symmetric bifunctional Br-TBT-Br initiator precursor for the KCTP of 1, during which Ni(0) is not able to “ring walk” over the B unit to initiate polymerization at the other end of the chain. These results are important for the design of well-defined and electronically end-functionalized conjugated polymers, but also for understanding termination mechanisms in KCTP in general.

We investigate the properties of long-lived species in F8BT films through time-resolved photoluminescence (PL) measurements at room temperature and 10 K. The kinetics consist of an initial exponential decay (τ = 2.26 ns) followed by a weak power-law decay (I(t) [proportionality] t(-1)) up to at least 1 ms, both of which depend weakly on temperature. From fluence-dependent PL and ultrafast transient absorption (TA) measurements, we confirm that this emission arises from the recombination of geminate charge-pairs generated through singlet-singlet annihilation. This behavior is a consequence of the donor-acceptor nature of this polymer, which enhances singlet-singlet annihilation and facilitates the formation of long-lived geminate-pairs from energetic singlet states.