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 photo-generated 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. We observe the formation of different TT states, with varying yield and different rates of singlet decay, depending on the excitation energy. An emissive TT state is observed to grow in over 1ns when exciting at 480nm, in contrast to excitation at lower energies where this emissive TT state is not observed. This suggests that the pathway for singlet fission in these nanoparticles is strongly influenced by the initial sub-100 fs relaxation of the photoexcited state away from the Frank-Condon point, with multiple possible TT states. On nanosecond timescales, the TT states are converted to free triplets, which suggests that TT states might diffuse into the disordered regions of the nanoparticles where their breakup to free triplets is favored. The free triplets then decay on µs timescales, despite the confined nature of the system. Our results provide important insights into the mechanism of endothermic singlet fission and the design of nanostructures to harness singlet fission.We acknowledge EPSRC and the Winton Programme for the Physics of Sustainability for funding. A.T. thanks the Dr. Manmohan Singh scholarship, St John’s College, University of Cambridge, for funding. M.J.Y.T. acknowledges receipt of a Marie Curie Individual Fellowship

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 OC) relative to their optical energy gap (E g) that limit power conversion efficiencies to ~12%. This energy loss is partly attributed to the offset between E g and that of intermolecular charge transfer (CT) states at the donor-acceptor interface. Here we study charge generation occurring in PIPCP:PC61BM, a system with a very low driving energy for initial charge separation (E g-E CT ~ 50 meV) and a high internal quantum efficiency (η IQE ~ 80%). We track the strength of the electric field generated between the separating electron-hole pair by following the transient electroabsorption optical response, and find that while localised CT states are formed rapidly (<100 fs) after photoexcitation, free charges are not generated until 5 ps after photogeneration. In PIPCP:PC61BM, electronic disorder is low (Urbach energy <27 meV) and we consider that free charge separation is able to outcompete trap-assisted non-radiative recombination of the CT state

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. The full data set allows confirming that the photoisomerization of the 13C isomer occurs within 100 fs and indications of an excited and ground state wavepacket launched by the ultrafast non-adiabatic reaction are reported. Even though this recalls the record isomerization time and the coherent reaction scenario of 11-cis PSBR in rhodopsin, the photoisomerization quantum yield (QY) is much lower, actually the lowest value ever reported for retinal proteins (<15%). Noticeably, in ASR the excited state lifetime (ESL) is at least five times larger and the QY is more than twice as large for AT PSBR as compared to 13C PSBR. We argue that ESL and QY cannot be expected to be correlated at all, but that the latter is decided on, as often anticipated, by the wavepacket pathways leading to the conical intersection seam

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. We show that charge-transfer (CT) excitons across the CdSe/CdTe interface are formed on two distinct time scales: initially from an ultrafast (∼70 fs) electron transfer and then on longer time scales (∼5 ps) from the diffusion of domain excitons to the interface. We find that the CT excitons are influenced by an interfacial phonon mode at ∼120 cm-1, which localizes them to the interface. Using low-temperature PL spectroscopy we reveal that this same phonon mode is the dominant mechanism in broadening the CT PL. On cooling to 4 K, the total PL quantum yield reaches close to unity, with an ∼85% contribution from CT emission and the remainder from an emissive sub-band-gap state. At room temperature, incomplete diffusion of domain excitons to the interface and scattering between CT excitons and phonons limit the PL quantum yield to ∼50%. Our results provide a detailed picture of the nature of exciton-phonon interactions at the interfaces of 2D heterostructures and explain both the broad shape of the CT PL spectrum and the origin of PL quantum yield losses. Furthermore, they suggest that to maximize the PL quantum yield both improved engineering of the interfacial crystal structure and diffusion of domain excitons to the interface, e.g., by altering the relative core/crown size, are required.We acknowledge financial support from the EPSRC [EP/M005143/1] and Winton Program for the Physics of Sustainability. The work of SI is supported by the program ANR JCJC NannoDoSe

Processus de conversion d'énergie ultra-rapide dans des protéines photosensibles et nanostructures organiques à visée photovoltaïque

Femtosecond transient spectroscopies are used to investigate photonic energy conversion inorganic systems. These techniques allow to observe the ground and excited states of themolecules at the timescale of the photoreactions. It is used to understand the inter- andintramolecular energy and charge transfers leading to the desired photochemical process.The natural photoswiching retinal protein Anabaena sensory Rhodopsin is studied to understand the key parameters ruling the isomerisation quantum yield. We could determine the isomerisation quantum yield of both stable forms and their dynamics in the very same experimental conditions.Charge generation is investigated in small molecule bulk heterojunction active layers for organic solar cells made of PCBM and a BODIPY dye-derivative donor. The influence of the active layer morphology on charge generation is studied. The charge generation is limited by charge recombination but also by exciton diffusion to the donor-acceptor interface. The active layer morphology has to be improved to achieve more efficient organic solar cells with these materials.Les techniques de spectroscopie femtoseconde permettent d’étudier les processus de conversion d’énergie dans les système organiques. Elles permettent d’étudier les populations photo-générées et leur évolution à l’échelle de ces photoréactions. Elles permettent de comprendre les transferts d’énergie et de charge intra- et inter-moléculaires à l’origine du fonctionnement de ces systèmes.La protéine de rétinal Anabaena sensroy Rhodopsin est un photocommutateur naturel, qui est étudié afin de comprendre les paramètres à l’origine de l’efficacité quantique d’isomérisation. Nous avons pu déterminer cette efficacité quantique pour les deux formes stables du rétinal ainsi que leur dynamique d’isomérisation dans les mêmes conditions expérimentales.La génération de charge dans des couches actives pour le photovoltaique organique est étudiée dans un système composé d’un mélange de PCBM et d’un donneur organique dérivé du colorant BODIPY. L’influence de la nanostructuration de la couche active sur la génération de charge est étudiée. La génération de charge est limitée dans ce système par la recombinaison des charges générées et par la diffusion des excition aux interfaces donneur-accepteur. Ces observations indiquent que l’amélioration de la nanostructuration de la couche active peut permettre d’augmenter les rendements de photo-génération de charge

Steady state emission of the fluorescent intermediate of Anabaena Sensory Rhodopsin as a function of light adaptation conditions

Steady-state fluorescence measurements of the first excited state of the anabaena sensory rhodopsin (ASR), and Bacteriorhodopsin are reported for different light stabilization conditions, including the dark-adapted state. We determine the fluorescence spectra of both all-trans (AT), and 13-cis (13C) protonated Schiff base of retinal, and compare the effect of the proteins. Referenced against the fluorescence quantum yield of AT-bR (2.5 x 10(-4)) we find for AT-ASR, 13C-ASR, and 13C-bR the values of 3.3 x 10(-4), 0.8 x 10(-4), and 1.7 x 10(-4), respectively. Using reported excited state lifetimes, the radiative rates are deduced, and their differences discussed on the basis of a configuration-dependent oscillator strength

Ultrafast Photoisomerization in Anabaena Sensory Rhodopsin

International Conference on Ultrafast Phenomen

Supporting data for "Elucidation of Excitation Energy Dependent Correlated Triplet Pair Formation Pathways in an Endothermic Singlet Fission System"

Data supporting the experimental claims of this publication helps inestablishing formation pathways of intermediates formed during the process of singlet fission. Data includes that from steady state and transient absorption, photoluminescence measurements, structural information from electron microscopy etc

Research Data Supporting Order Enables Efficient Electron-hole Separation at an Organic Heterojunction with a Small Energy Loss

Pump Push Probe Transient Absorption Images for PIPCP and PIPCP:PCBM Films. Images were acquired as described in the associated manuscript. Images were acquired as a function of Pump Probe delay time at a variety of Pump Push delays, Push energies, Push fluences, and Pump fluences. Also included are the Pump Probe and Push Probe images that are acquired simultaneousl