Kerr Johnson

A transferable model for singlet-fission kinetics

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.

Ternary photovoltaic blends incorporating an all-conjugated donor-acceptor diblock copolymer.

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.

Synthesis and Photophysics of Fully π-Conjugated Heterobis-Functionalized Polymeric Molecular Wires via Suzuki Chain-Growth Polymerization

We present a fast and efficient in situ synthetic approach to obtain fully π-conjugated polymers with degrees of polymerization up to 23 and near quantitative (>95%) heterobis-functionalization. The synthesis relies on the key advantages of controlled Suzuki chain-growth polymerization: control over molecular weight, narrow polydispersity, and ability to define polymer end groups. The first end group is introduced through the initiator metal complex tBu(3)PPd(X)Br, while the second end group is added by quenching of the chain-growth polymerization with the desired boronic esters. In all cases, polymers obtained at 50% conversion showed excellent end group fidelity and high purity following a simple workup procedure, as determined by MALDI-TOF, GPC, and (1)H and 2D NMR. End group functionalization altered the optoelectronic properties of the bridge polymer. Building on a common fluorene backbone, and guided by DFT calculations, we introduced donor and acceptor end groups to create polymeric molecular wires exhibiting charge transfer and energy transfer as characterized by fluorescence, absorption, and transient absorption spectroscopy as well as by fluorescence lifetime measurements.

Suppressing recombination in polymer photovoltaic devices via energy-level cascades.

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.

Control of intrachain charge transfer in model systems for block copolymer photovoltaic materials.

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.

Recombination Dynamics of Charge Pairs in a Push-Pull Polyfluorene-Derivative

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.

Ultrafast endothermic transfer of non-radiative exciplex state to radiative excitons in polyfluorene random copolymer for blue electroluminescence

We report a blue-emitting random copolymer (termed modified Aryl-F8) consisting of three repeat units of polydioctylfluorene (F8), Aryl-polydioctylfluorene (Aryl-F8), and an aromatic amine comonomer unit, poly(bis-N,Ν′-(4-butylphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine) chemically linked to get an improved charge carrier balance without compromising on the photoluminescence (PL) quantum yield with respect to the Aryl-F8 homo-polymer. The measured photoluminescence quantum efficiency (∼70%) of the blue-emitting polymer is comparable to or greater than the individual monomer units. The time resolved PL spectra from the modified Aryl-F8 are similar to those of Arylated-poly(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine) (PFB) even at a time scale of 100–250 ps, indicating an ultrafast energy transfer from the (Aryl-F8 or F8):Arylated-PFB interface to Arylated-PFB, i.e., endothermic transfer of non-radiative exciplex to a radiative molecular exciton. Furthermore, the presence of non-radiative exciplex is confirmed by the photoluminescence decay profile and temperature dependent PL spectra. The luminance efficiency achieved for the modified Aryl-F8 polymer light-emitting diodes is ∼11 cd A−1 with an external quantum efficiency (EQE) of ∼4.5%, whereas it is 0.05 cd/A with an EQE of ∼0.025% for Aryl-F8. Almost two orders of higher efficiency is achieved due to the improved charge carrier balance from the random copolymer without compromising on the photoluminescence yield.We report a blue-emitting random copolymer (termed modified Aryl-F8) consisting of three repeat units of polydioctylfluorene (F8), Aryl-polydioctylfluorene (Aryl-F8), and an aromatic amine comonomer unit, poly(bis-N,Ν′-(4-butylphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine) chemically linked to get an improved charge carrier balance without compromising on the photoluminescence (PL) quantum yield with respect to the Aryl-F8 homo-polymer. The measured photoluminescence quantum efficiency (∼70%) of the blue-emitting polymer is comparable to or greater than the individual monomer units. The time resolved PL spectra from the modified Aryl-F8 are similar to those of Arylated-poly(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine) (PFB) even at a time scale of 100–250 ps, indicating an ultrafast energy transfer from the (Aryl-F8 or F8):Arylated-PFB interface to Arylated-PFB, i.e., endothermic transfer of non-radiative exciplex to a radiative molecular exciton. Furthermore, the ...