The optical properties of stoichiometric copper chalcogenide nanocrystals (NCs) are characterized by strong interband transitions in the blue part of the spectral range and a weaker absorption onset up to ~1000 nm, with negligible absorption in the near-infrared (NIR). Oxygen exposure leads to a gradual transformation of stoichiometric copper chalcogenide NCs (namely, Cu(2-x)S and Cu(2-x)Se, x = 0) into their nonstoichiometric counterparts (Cu(2-x)S and Cu(2-x)Se, x > 0), entailing the appearance and evolution of an intense localized surface plasmon (LSP) band in the NIR. We also show that well-defined copper telluride NCs (Cu(2-x)Te, x > 0) display a NIR LSP, in analogy to nonstoichiometric copper sulfide and selenide NCs. The LSP band in copper chalcogenide NCs can be tuned by actively controlling their degree of copper deficiency via oxidation and reduction experiments. We show that this controlled LSP tuning affects the excitonic transitions in the NCs, resulting in photoluminescence (PL) quenching upon oxidation and PL recovery upon subsequent reduction. Time-resolved PL spectroscopy reveals a decrease in exciton lifetime correlated to the PL quenching upon LSP evolution. Finally, we report on the dynamics of LSPs in nonstoichiometric copper chalcogenide NCs. Through pump-probe experiments, we determined the time constants for carrier-phonon scattering involved in LSP cooling. Our results demonstrate that copper chalcogenide NCs offer the unique property of holding excitons and highly tunable LSPs on demand, and hence they are envisaged as a unique platform for the evaluation of exciton/LSP interactions.

Polymeric semiconductors are materials where unique optical and electronic properties often originate from a tailored chemical structure. This allows for synthesizing conjugated macromolecules with ad hoc functionalities for organic electronics. In photovoltaics, donoracceptor co-polymers, with moieties of different electron affinity alternating on the chain, have attracted considerable interest. The low bandgap offers optimal light-harvesting characteristics and has inspired work towards record power conversion efficiencies. Here we show for the first time how the chemical structure of donor and acceptor moieties controls the photogeneration of polaron pairs. We show that co-polymers with strong acceptors show large yields of polaron pair formation up to 24% of the initial photoexcitations as compared with a homopolymer (η = 8%). π-conjugated spacers, separating the donor and acceptor centre of masses, have the beneficial role of increasing the recombination time. The results provide useful input into the understanding of polaron pair photogeneration in low-bandgap co-polymers for photovoltaics.

No abstract

Semiconductor charge transfer (CT) cocrystals are an emerging class of molecular materials which combines the characteristics of the constituent molecules in order to tune physical properties. Cocrystals can exhibit polymorphism, but different stoichiometries of the donor-acceptor (DA) pair can also give different structures. In addition, the structures of the donor and acceptor as pristine compounds can influence the resulting cocrystal forms. We report a structural study on several CT cocrystals obtained by combining the polyaromatic hydrocarbon perylene with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its fluorinated derivatives having increasing electronegativity. This is achieved by varying the amount of fluorine substitution on the aromatic ring, with TCNQ-F2 and TCNQ-F4. We find structures with different stoichiometries. Namely, the system perylene:TCNQ-F0 is found with ratios 1:1 and 3:1, while the systems perylene:TCNQ-Fx (x = 2, 4) are found with ratios 1:1 and 3:2. We discuss the structures on the basis of the polymorphism of perylene as pure compound, and show that by a judicious choice of growth temperature the crystal structure can be in principle designed a priori. We also analyze the structural motifs taking into account the degree of charge transfer between the perylene donor and the TCNQ-Fx acceptors and the optical gap determined from infrared (IR) spectroscopy. This family of materials exhibits tunable optical gaps in the near-IR (NIR), promising applications in organic optoelectronics

Recently, solid-state lighting has received considerable attention in both academic and industrial research. [1,2] Of particular interest, for the replacement of the existing light sources, are organic light-emitting diodes (OLEDs) based on phosphorescent molecules. [3][4][5][6] The advantage of using these materials lies in the possibility to internally convert all the spin uncorrelated injected charges into light. Indeed, an internal quantum efficiency of nearly 100% has been achieved in devices based on the green-emitting organometallic complex Ir(ppy) 3 .[7]However, many unresolved issues are the subject of current research in order to implement efficient white light sources and expand the number of applications. In particular, the origin of the efficiency roll-off at high voltages, [8][9][10] the light outcoupling, [11,12] the long-term stability [13,14] and the generation of white light with an all-phosphor device [6,15] are subjects under intense investigation. White light generation is a key issue because of the wide range of applications involving full-color displays and lighting. [1,2] Among the different approaches, solution processed devices based on white light emitting molecules [16] have been demonstrated as well as thermally evaporated red, green and blue (RGB) blends [15] or stacks. To date, white light OLEDs (WOLEDs) with long term operational lifetimes have been obtained mainly with a combination of a blue fluorescent emitter [6] and phosphors for the other colors. Such an elegant approach relies on a well engineered harvesting of singlet and triplet excitons and requires therefore a precise doping of the RGB emitting dyes in the transporting hosts. In contrast, efficient WOLEDs based on blue phosphors can be obtained with all the emitters in one single layer, [17] simplifying the processing. Generally, however, blue phosphors have in the past turned out to be rather unstable. While a physical explanation for blue phosphor based device instability is still lacking, a shorter phosphorescence lifetime, eventually approaching the sub-microsecond time regime, would decrease the residence time of potentially unstable excited states. Moreover, processes detrimental to the efficiency, such as exciton charge-carrier quenching [8] or triplet-triplet annihilation, [9,10] could be strongly reduced with a faster exciton recombination. A shorter phosphorescence lifetime while maintaining high quantum efficiencies requires a large radiative rate. For organometallic complexes this rate is directly proportional to the spin-orbit coupling (SOC) matrix element involving the emitting triplet and the perturbing singlet state and inversely proportional to the degree of mixing between them, i.e., the singlet-triplet splitting (DE ST ). [18][19][20] Photophysical studies of the role of SOC and DE ST in tuning the radiative rate are still sparse, mainly because the large intersystem crossing (ISC) rates ($10 13 s À1 ) of such phosphors, [21] which makes detection (and therefore direct measurement of DE ST ) rather cha...

Molecular doping of conjugated polymers represents an important strategy for improving organic electronic devices. However, the widely reported low efficiency of doping remains a crucial limitation to obtain high performance. Here we investigate how charge transfer between dopant and donor-acceptor copolymers is affected by the spatial arrangement of the dopant molecule with respect to the copolymer repeat unit. We p-dope a donor-acceptor copolymer and probe its charge-sensitive molecular vibrations in films by infrared spectroscopy. We find that, compared with a related homopolymer, a four times higher dopant/ polymer molar ratio is needed to observe signatures of charges. By DFT methods, we simulate the vibrational spectra, moving the dopant along the copolymer backbone and finding that efficient charge transfer occurs only when the dopant is close to the donor moiety. Our results show that the donor-acceptor structure poses an obstacle to efficient doping, with the acceptor moiety being inactive for p-type doping.

We report on the observation of a charge-transfer state forming at the molecular interface between a conjugated polymer and a fullerene based electron acceptor. Electron hole recombination in this state results in a luminescent transition at 840nm, energetically separated from the polymer emission. This transition can be directly photoexcited by tuning the excitation energy below the conjugated polymer bandgap, demonstrating that the charge-transfer state originates from a ground-state interaction. By electric field induced quenching of the photoluminescence, we determine a binding energy of 130meV for excitons in the charge-transfer state.

Single oligo(phenylene-vinylene) molecules constitute model systems of chromophores in disordered conjugated polymers and can elucidate how the actual conformation of an individual chromophore, rather than that of an overall polymer chain, controls its photophysics. Single oligomers and polymer chains display the same range of spectral properties. Even heptamers support π-electron conjugation across ∼80°curvature, as revealed by the polarization anisotropy in excitation and supported by quantum chemical calculations. As the chain becomes more deformed, the spectral linewidth at low temperatures, often interpreted as a sign of aggregation, increases up to 30-fold due to a reduction in photophysical stability of the molecule and an increase in random spectral fluctuations. The conclusions aid the interpretation of results from single-chain Stark spectroscopy in which large static dipoles were only observed in the case of narrow transition lines. These narrow transitions originate from extended chromophores in which the dipoles induced by backbone substituents do not cancel out. Chromophores in conjugated polymers are often thought of as individual linear transition dipoles, the sum of which make up the polymer's optical properties. Our results demonstrate that, at least for phenylene-vinylenes, it is the actual shape of the individual chromophore rather than the overall chromophoric arrangement and form of the polymer chain that dominates the spectroscopic properties.Molecular-level engineering in plastic electronics requires a precise understanding of how a particular physical or chemical structure impacts on the physical properties of the material. Disorder effects on the ensemble level can often mask the subtle interplay between function and structure. Large macromolecules such as conjugated polymers are particularly prone to energetic disorder, which gives rise to substantial spectral broadening and is generally attributed to a "particle in a box"-like picture of varying chromophore lengths. 1 Disorder effects have commonly been investigated in matrix isolated materials, such as polyenes, where subtle interplays between molecular shape and electronic structure have been identified. 2-5 However, matrix isolation on its own is not sufficient to overcome disorder but merely helps to screen intermolecular effects. The intrinsic molecular properties themselves are accessible with single-molecule spectroscopy. Although this technique helps to overcome the ensemble limitations, a single polymer chain can still contain many chromophores. [6][7][8][9][10][11][12] Energy transfer between these chromophores can mask the true photophysics of the individual spectroscopic unit. Although polarization-resolved spectroscopy has yielded detailed insight into the conformation of the polymer chain, 7,8,11,[13][14][15] very little is known about the shape of the indiVidual chromophore. Because a physical bend can potentially interrupt the π-electron conjugation, the chromophore is generally thought to be linearly extended in space...