Hot carrier cooling in halide perovskites is governed by the interplay of carrier-carrier and carrier-phonon interactions, and so the study of their dynamics can reveal important underlying photophysical processes operating in these materials. Here, ultrafast pump-push-probe spectroscopy is used to isolate hot carrier dynamics in lead halide perovskite nanomaterials spanning a range of sizes and shapes. A weak size-dependence is shown by cuboidal CsPbBr3 nanocrystals, while 2D CsPbBr3 nanoplatelets and Ruddlesden-Popper (PEA)2PbI4 display a hot phonon bottleneck that becomes increasingly suppressed with greater excitonic character. This is attributed to an enhanced influence of carrier-carrier scattering in low-dimensional perovskites.

Very few materials are able to absorb visible light without dissipating some of the resulting energy into phonon modes, and these excited modes have the capability to act back on the electronic excitation that is generated. By the same token, very few probes of photophysical processes in materials are able to directly probe the coexistence of both electronic and thermal departures from equilibrium or directly visualize the impact of the spatiotemporal interaction of electronic and thermal excitations. I will nevertheless, describe such a capability that leverages not only the ps time resolution associated with electronic to thermal energy transduction but that also provides direct spatial maps of localized photoinduced electronic and temperature profiles and their coupled evolution. I will how how this approach allows us to investigate thermoelectric effects in few-layer MoS_2 and that it can be more broadly applied to other emerging semiconductors.

We spatiotemporally resolve exciton transport in two-dimensional transition metal dichalcogenides (TMD) in which we incorporate local dielectric inhomogeneities known as nanobubbles. We observe highly efficient exciton funneling in these dielectric inhomogeneities, a process entirely driven by momentum-indirect (dark) excitons whose energies are much more sensitive to dielectric perturbations than bright excitons.

This conference presentation was prepared for the Physical Chemistry of Semiconductor Materials and Interfaces XXI conference at SPIE Optics + Photonics 2022.

We explore the dynamics of strong near field couplings between excitons of a J-aggregated squaraine thin film and surface plasmon polariton excitations using two-dimensional electronic spectra. Pronounced Rabi oscillations of their cross-peaks are resolved for the first time and provide conceptually new insight into the near-field couplings of the system. We show how radiative couplings govern the polariton nonlinearity and provide first access to the two-quantum- excitations of the coupled system.

Exciton-polaritons (EPs) are formed when cavity photons hybridize with electron-hole pairs (excitons) in semiconductors. The part-light part-matter nature of EPs leads to a range of desirable properties associated with light-like energy flow combined with strong matter-like interactions. We demonstrate a momentum-selective ultrafast optical imaging approach that tracks polariton propagation in real space and time in a wide range of emerging semiconductors. Focusing our analysis on 2D halide perovskite microcavities, we leverage our approach to directly resolve polariton-lattice and polariton-polariton interactions, as well as how uncoupled ‘dark’ excitons influence polariton dynamics and transport.The new and generalizable insight on EP propagation and scattering gained in this study testify to the power of our direct imaging approach.

In a combined experimental/numerical study, we show that the slowness of charge carrier thermalization leads to a 0.1-0.2 eV higher open circuit voltage (Voc) in OPV devices than would be expected for instantaneous thermalization. The latter is commonly assumed when analyzing Voc of OPV devices. Second, we demonstrate numerically how this loss channel can be further mitigated in funnel-shaped morphologies of donor- and acceptor-rich domains to direct charge motion. We demonstrate that in optimized funnel morphologies this kinetic, non-equilibrium effect allows to surpass the Shockley-Queisser limit.

This conference presentation was prepared for the Physical Chemistry of Semiconductor Materials and Interfaces XXI conference at SPIE Optics + Photonics 2022.

Helical conjugated polymers are of great interest for their potential as sources of circularly polarized luminescence and as magnetic field sensors. Due to their relatively strong absorption cross sections and high emissivity in the visible wavelength range, these materials permit a detailed investigation of how the transition between helical and random coil forms are driven by polymer structural features such as chain length and chemical defects as well as environmental properties such as solvent and temperature. Bulk methods such as circular dichroism, absorption, and fluorescence as well as single-particle microscopy is used to probe the helix-to-coil phase transition in a model chiral polyfuran and to determine whether the conformations favored in solution are retained in the solid state.

The electronic structure and exciton dynamics of the molecules and polymers that form the active layer in organic electronic devices is governed by the electronic coupling between molecules. Molecules and polymers may become kinetically trapped during spin-coating. Thermal annealing can change these conformations, dramatically changing the exciton dynamics and thus the suitability of the material for electronic devices. In this work, a single-shot transient absorption spectroscopy technique is used that can measure the exciton dynamics of organic semiconducting systems during thermal annealing.

The recently developed combination of vacuum electrospray deposition (ESD) and scanning tunnelling microscopy (STM) is used to analyse pBTTT and its glycated side-chain counterparts, pgBTTT and p(g2T-TT). Our high-resolution images allow us to determine the conformation and assembly of the polymers with sub-monomer resolution. We identify clear differences in the tendency of assembling between the two types of polymers and trace them back to the different nature of their side-chain interactions. The generalisation of these findings from 2D monolayers to 3D thin films is proposed as a means to gain so far inaccessible information on the microstructure of conjugated polymers.

We report the synthesis of a new series of oligomers derived from dibenzo[a,i]pyrene. The compounds were characterized by steady-state and transient absorption spectroscopies. The results indicate that the compounds fluoresce from a formally dark S1 electronic state that is enabled by intensity borrowing from a neighboring bright S2 state. While the monomer exhibits a relatively low photoluminescence quantum yield (PLQY), the dimer exhibits a significantly enhanced PLQY due to a greater S2 intensity borrowing. Moreover, symmetry-breaking charge transfer in the dimer was demonstrated by investigations in solvents of different polarity.

The ability to efficiently up-convert broadband, low-intensity light would be an enabling technology for background-free biomedical imaging, volumetric 3D printing, and sensitizing silicon focal plane arrays to the short-wave infrared. Our approach uses colloidal quantum dots—size-tunable spin-mixing fluorophores—to absorb low-energy photons and sensitize the spin-triplet excitonic states of nearby conjugated molecules. Once there, pairs of these long-lived excitations can combine via triplet fusion (triplet-triplet annihilation) to generate shorter-wavelength fluorescence. I will discuss our ongoing efforts to harness our recent advances in the synthesis of ultra-small PbS quantum dots (d~1.7 nm, hν_peak,abs=2.2eV) to sensitize ‘red-to-blue’ triplet-fusion upconversion in solution. We show that the long (>µs) photoluminescence lifetimes of these particles enable max-efficiency upconversion at modest light intensities (I_th=220 mW/cm2), overcoming a mildly endothermic sensitization scheme that maximizes the anti-Stokes shift (1.04 eV). This architecture facilitates the photo-initiated polymerization of methylmethacrylate using only long-wavelength light (λ: 637 nm); a demonstration of nanocrystal-sensitized upconversion photochemistry. Finally, from the quasi-equilibrium dynamics of triplet energy transfer, we infer that the chemical potential of photoexcited, ultra-small PbS quantum dots is surprisingly high—completing an advantageous suite of properties for upconversion photochemistry, but reinforcing questions regarding the emissive state.

Singlet fission (SF), which allows one singlet state to be converted to 2 triplets, is one of the most perspective phenomena that may facilitate overcoming of the Shockley-Quiser limit in organic and hybrid photovoltaics. Rubrene, mobility champion of organic electronics, is one of the most popular SF materials. Yet, despite its popularity, SF fundamentals in Rubrene remain strongly debated in the literature due to both experimental and computational limitations. In this work we applied sub-10 fs transient absorption spectroscopy (TAS) to fully disentangle SF mechanism in low-defects high-quality Rubrene single crystals. We found that on 0.2 ps – 6 ns timescale, SF may be treated as 2 components process with half of the singlets to be converted into triplets at 10ps. Fascinatingly, at early times (<0.2 ps) we found additional component to be involved, which may be associated with hybrid state facilitating coherent SF. Based on our experimental findings, we have built a complete model of singlet fission in crystalline rubrene, which may help to resolve current debates on SF in the literature.

This conference presentation was prepared for the Physical Chemistry of Semiconductor Materials and Interfaces XXI conference at SPIE Optics + Photonics 2022.

In this contribution, I will discuss our recent work on charge-transport through self-assembled ensembles of the trimeric protein complex, photosystem I (PSI). These remarkable complexes form robust junctions that can be printed and wired into integrated circuits. Current is mediated by long-range tunneling across the complexes, the orientation of which determines their function; in the "up" or "down" position they exhibit temperature-independent current-rectification. Mixing these orientations leads to no rectification. Thus, by controlling the self-assembly process, all-protein logic circuits can be printed onto flexible substrates with yields approaching 100%. These logic circuits operate at speeds of at least 3 kHz.

This conference presentation was prepared for the Physical Chemistry of Semiconductor Materials and Interfaces XXI conference at SPIE Optics + Photonics 2022.

Doping conjugated polymers is an effective way to tune their electronic properties for thin-film electronics applications, including thermoelectrics. Chemical doping of semiconducting polymers involves the introduction of a strong electron acceptor or donor molecule that can undergo charge transfer (CT) with the polymer. The CT reaction creates electrical carriers on the polymer chain (usually positive polarons, a.k.a. holes or polarons) while the dopant molecules remain in the film as counterions. We have shown that the key factor limiting the mobility of the carriers on the polymer is electrostatic attraction between the carriers and the dopant counterions. The electrostatic interaction can be controlled either by the film morphology, or by deliberately designing dopants whose counterions cannot coulombically interact with the carriers they create; polymer crystallinity is important for polaron mobility only to the extent that it helps control the coulomb interaction between the polarons and the dopant counterions. Using such strategies, we are able to improve the carrier mobility and Seebeck coefficient of doped polymer films by more than an order of magnitude. We show that we can use ultrafast spectroscopy to distinguish between highly mobile polarons, coulombically-bound polarons, and bipolarons. We also show that with the correct choice of dopant, we can use the vibrational Stark effect effect to measure the degree of polaron coherence in situ in doped semiconducting polymer films.

This conference presentation was prepared for the Physical Chemistry of Semiconductor Materials and Interfaces XXI conference at SPIE Optics + Photonics 2022.

This conference presentation was prepared for the Physical Chemistry of Semiconductor Materials and Interfaces XXI conference at SPIE Optics + Photonics 2022.