Exciton-Phonon Interactions Govern Charge-Transfer-State Dynamics in CdSe/CdTe Two-Dimensional Colloidal Heterostructures.

CdSe/CdTe core-crown type-II nanoplatelet heterostructures are two-dimensional semiconductors that have attracted interest for use in light-emitting technologies due to their ease of fabrication, outstanding emission yields, and tunable properties. Despite this, the exciton dynamics of these complex materials, and in particular how they are influenced by phonons, is not yet well understood. Here, we use a combination of femtosecond vibrational spectroscopy, temperature-resolved photoluminescence (PL), and temperature-dependent structural measurements to investigate CdSe/CdTe nanoplatelets with a thickness of four monolayers.

Observation of Vibronic-Coupling-Mediated Energy Transfer in Light-Harvesting Nanotubes Stabilized in a Solid-State Matrix.

Ultrafast vibrational spectroscopy is employed to obtain real-time structural information on energy transport in double-walled light-harvesting nanotubes at room temperature, stabilized in a host matrix to mimic the rigid scaffolds of natural light-harvesting systems. We observe evidence of a low-frequency vibrational mode at 315 cm, which transfers excitons from the outer wall of the nanotubes to a crossing point through which energy transfer to the inner wall can occur. This mode is furthermore absent in solution phase.

Order enables efficient electron-hole separation at an organic heterojunction with a small energy loss.

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:PCBM, a system with a very low driving energy for initial charge separation (E -E  ~ 50 meV) and a high internal quantum efficiency (η  ~ 80%).

Elucidation of Excitation Energy Dependent Correlated Triplet Pair Formation Pathways in an Endothermic Singlet Fission System.

Singlet fission is the spin-allowed conversion of a photogenerated singlet exciton into two triplet excitons in organic semiconductors, which could enable single-junction photovoltaic cells to break the Shockley-Queisser limit. The conversion of singlets to free triplets is mediated by an intermediate correlated triplet pair (TT) state, but an understanding of how the formation and dissociation of these states depend on energetics and morphology is lacking. In this study, we probe the dynamics of TT states in a model endothermic fission system, TIPS-Tc nanoparticles, which show a mixture of crystalline and disordered regions.

Vibronically coherent ultrafast triplet-pair formation and subsequent thermally activated dissociation control efficient endothermic singlet fission.

Singlet exciton fission (SF), the conversion of one spin-singlet exciton (S) into two spin-triplet excitons (T), could provide a means to overcome the Shockley-Queisser limit in photovoltaics. SF as measured by the decay of S has been shown to occur efficiently and independently of temperature, even when the energy of S is as much as 200 meV less than that of 2T. Here we study films of triisopropylsilyltetracene using transient optical spectroscopy and show that the triplet pair state (TT), which has been proposed to mediate singlet fission, forms on ultrafast timescales (in 300 fs) and that its formation is mediated by the strong coupling of electronic and vibrational degrees of freedom.

Real-Time Observation of Exciton-Phonon Coupling Dynamics in Self-Assembled Hybrid Perovskite Quantum Wells.

Self-assembled hybrid perovskite quantum wells have attracted attention due to their tunable emission properties, ease of fabrication, and device integration. However, the dynamics of excitons in these materials, especially how they couple to phonons, remains an open question. Here, we investigate two widely used materials, namely, butylammonium lead iodide (CH(CH)NH)PbI and hexylammonium lead iodide (CH(CH)NH)PbI, both of which exhibit broad photoluminescence tails at room temperature.

100 fs photo-isomerization with vibrational coherences but low quantum yield in Anabaena Sensory Rhodopsin.

Anabaena Sensory Rhodopsin (ASR) stands out among the microbial retinal proteins in that, under light-adaptation (LA) conditions, it binds both the 13-cis isomer and the all-trans isomer of the protonated Schiff base of retinal (PSBR). In the dark-adapted (DA) state, more than 95% of the proteins bear all-trans PSBR, and the protein environment adopts a different equilibrium state. We report the excited state and photo-isomerization kinetics of ASR under different LA conditions.