Long-lived Charge Separation Research Articles

(Invited) Organic/Inorganic Hybrid Interfaces between Perovskite Semiconductors and Semiconducting SWCNTs

Low-dimensional semiconductors provide a rich tapestry of tunable optical and electrical properties for a wide variety of applications. Semiconducting single-walled carbon nanotubes (s-SWCNTs) have shown tremendous potential in applications ranging from digital logic, biological imaging, quantum information processing, photovoltaics, and thermoelectric energy harvesting. Energy harvesting applications rely critically upon the creation of tailored interfaces that enable the movement of energetic species (excitons, electrons, holes) in specified directions. While organic energy harvesting devices often employ interfaces between distinct organic species such as polymers, fullerenes, and carbon nanotubes, hybrid interfaces between organic and inorganic semiconductors have unique properties that are relatively unexplored.In this talk, I will discuss our recent efforts at constructing novel hybrid interfaces between s-SWCNTs and low-dimensional perovskite-based semiconductors. I will highlight time-resolved spectroscopy results that illustrate long-lived photoinduced charge separation across such interfaces, demonstrating utility for photovoltaics and photodetectors. Interestingly, some systems demonstrate additional transient processes that are exceptionally slow and may indicate ion movement within the hybrid systems. Complementary device studies demonstrate exceptional efficiencies for conversion of photons into stable photocurrent, and I will highlight several novel applications in which these interfaces can be employed.

(Invited) Optical Control of the Membrane Potential of Living Cells with Donor-Acceptor-Linked Molecules

In living animal cells, there exists a difference in the electrical charge on the two extreme sides of the plasma membrane, that is, a slight excess of positive and negative ions outside and inside a cell, respectively. Such a charge difference produces a membrane potential (V m) across the plasma membrane. The V m (< 0) is controlled by various ion channels, and, especially, in neuronal cells, it is drastically changed for neurotransmission. In response to some input, the ion channels of neuronal cells get quickly activated such that the abovementioned difference is cancelled (∆V m > 0, depolarization). This electrical signal propagates along the neuronal axons like a wave through coordinated activation and deactivation of the ion channels. Therefore, the control of membrane potential and ion transport across the plasma membrane using a light source is an attractive strategy that can allow targeted, fast control of precisely defined events in the plasma membrane. In this symposium, we have demonstrated a novel general strategy for the control of membrane potential and ion transport by using donor-acceptor-linked molecules (D-A molecules) capable of intramolecular long-lived charge separation, nanocarriers for solubilization, membrane targeting of D-A molecules, and light (Figure 1).For the preparation of the D-A molecule, porphyrin (H2P) derivatives with ferrocene (Fc) and fullerene (C60) were synthesized to create a triadic arrangement (Fc-H2P-C60). We had previously reported that, under light irradiation, Fc-H2P-C60 yields the ferrocenium cation (Fc+)-H2P-C60 radical anion (C60 •-) with a long lifetime of ~0.01 ms [1]. For its solubilization in the physiological media and safer plasma membrane delivery, we focused on a lipoprotein in our body considering its amphiphilic property. High-density lipoprotein (HDL) can be reconstituted with recombinant lipid-binding proteins and phospholipids. We had previously reported that HDL genetically and chemically engineered with cytophilic peptides and fusogenic lipids is capable of high plasma membrane binding without significant cytotoxicity [2]. For this purpose, Fc-H2P-C60 was solubilized with one of our engineered HDL nanoparticles (cpHDL) and added to a cell culture medium of rat pheochromocytoma cells (PC12). The cell membrane delivery of Fc-H2P-C60 was suggested by confocal analysis and freeze-fracture electron microscopy. Subsequent light irradiation of PC12 cells increased the membrane potential (depolarization), which was not observed for the reference molecule (H2P) that was incapable of charge separation [3]. In addition, the inhibition of potassium ion flow across the membrane was found to be responsible for this depolarization (Figure 1). Our following study with 3 D-A molecules, including Fc-H2P-C60, revealed that the degree of the depolarization was positively correlated with the charge separation yield [4]. In this symposium, I will discuss more about the potential of applying our D-A molecules/carriers system in the biomedical fields.Figure 1. Schematic illustration of optical control of membrane potential and cation flow using a D-A molecule (Fc-H2P-C60)[1] Imahori, H. et al., J. Am. Chem. Soc. 2001, 123, 2607-2617.[2] Murakami, T., Biotechncol. J. 2012, 7, 762-767.; Kim, H. et al. Biochim. Biophys. Acta Biomembr. 2019, 1861, 183008.[3] Numata, T. et al., J. Am. Chem. Soc. 2012, 134, 6092-6095.[4] Takano, Y. et al., Chem. Sci. 2016, 7, 3331-3337. Figure 1

Photoactive Anthraquinone-Based Host–Guest Assembly for Long-Lived Charge Separation

Photoactive Anthraquinone-Based Host–Guest Assembly for Long-Lived Charge Separation

Conserved residue PsaB-Trp673 is essential for high-efficiency electron transfer between the phylloquinones and the iron-sulfur clusters in Photosystem I.

Despite the high level of symmetry between the PsaA and PsaB polypeptides in Photosystem I, some amino acids pairs are strikingly different, such as PsaA-Gly693 and PsaB-Trp673, which are located near a cluster of 11 water molecules between the A1A and A1B quinones and the FX iron-sulfur cluster. In this work, we changed PsaB-Trp673 to PsaB-Phe673 in Synechocystis sp. PCC 6803. The variant contains ~ 85% of wild-type (WT) levels of Photosystem I but is unable to grow photoautotrophically. Both time-resolved and steady-state optical measurements show that in the PsaB-W673F variant less than 50% of the electrons reach the terminal iron-sulfur clusters FA and FB; the majority of the electrons recombine from A1A- and A1B-. However, in those reaction centers which pass electrons forward the transfer is heterogeneous: a minor population shows electron transfer rates from A1A- and A1B- to FX slightly slower than that of the WT, whereas a major population shows forward electron transfer rates to FX slowed to the ~ 10 µs time range. Competition between relatively similar forward and backward rates of electron transfer from the quinones to the FX cluster account for the relatively low yield of long-lived charge separation in the PsaB-W673F variant. A higher water content and its increased mobility observed in MD simulations in the interquinone cavity of the PsaB-W673F variant shifts the pK of PsaB-Asp575 and allows its deprotonation in situ. The heterogeneity found may be rooted in protonation state of PsaB-Asp575, which controls whether electron transfer can proceed beyond the phylloquinone cofactors.

The synergistic effect of P doping and Ni(II) electron cocatalyst boosting photocatalytic H2-evolution activity of g-C3N4

Doping heteroatoms and coupling cocatalysts are an effective way toward enhancing photocatalytic activity of semiconductor-based photocatalysts. Herein, Ni(II) modified P-doped g-C3N4 (NiPCN) was successfully prepared by an ion impregnation and phosphidation method. The Ni(II) as the electron cocatalyst and P-doping lead to 10 times of H2 evolution activity (1213 μmol h−1 g−1) compared with pristine g-C3N4 photocatalyst (117 μmol h−1 g−1). Detailed experimental results together with theoretical study revealed that the P-doping and Ni(II) as an electron cocatalyst can speed up electron transfer, leading to an enhancement of photocatalytic activity. The ultrafast transient absorption spectroscopy results disclosed that the synergistic effect of Ni(II) as an electron cocatalyst and P doping helps to increase the carrier's lifetime, resulting in the efficient electron transfer and long-lived charge separation. It is believed that this work opens up a possible way for optimizing the separation of photoexcited carriers for photocatalytic H2-evolution. (~1761 ps) is much slower than that (τ2) of the CN (576 ps)

Ultrafast Electron Transfer with Long-Lived Charge Separation and Spin Polarization in WSe2/C60 Heterojunction

The strong excitonic effect in monolayer transition-metal dichalcogenides (TMDs) endows them with intriguing optoelectronic properties but also short-lived population and valley polarization. Exciton dissociation by interfacial charge transfer has been shown as an effective approach to prolonging excited-state lifetimes. Herein, by ultrafast spectroscopy and building-block molecule C60, we investigated exciton and valley polarization dynamics in the prototypical WSe2/C60 inorganic-organic hybrid. We show that excitons in WSe2 can be dissociated through ultrafast (∼1 ps) electron transfer to C60, with nanosecond charge separation due to thermally activated electron diffusion in C60 film. Because of suppressed electron-hole exchange interaction after electron transfer, hole in WSe2 exhibits a spin/valley polarization lifetime of ∼60 ps at room temperature, more than 2 orders of magnitude longer than that in WSe2 monolayer. This study suggests exciton dissociation as a general approach to suppress electron-hole interaction and prolong the charge/spin/valley lifetime in TMDs.

Interface engineering Z-scheme Ti-Fe2O3/In2O3 photoanode for highly efficient photoelectrochemical water splitting

Exploiting interface-engineer of In2O3-based photoanode to achieve a higher charge separation efficiency could be regarded as a pivotal but challenging research in water splitting. Herein, the state-of-the-art Ti-Fe2O3/In2O3 photoanodes with different Ti4+ doping concentrations are fabricated for exploring the interface-engineering effect on PEC performance. The optimized 150Ti-Fe2O3/In2O3 photoelectrode with the rapid interfacial hole trapped (∼ 8.96 ps) and long-lived charge separation states could achieve excellent PEC performance by femtosecond time-resolved absorption spectroscopy (fs-TAS). As expected, it shows the highest photocurrent density of 2 mA/cm2 at 1.23 V vs. RHE, which is nearly 7 times higher compared with pure In2O3. Moreover, the Z-scheme mechanism could be fully confirmed by femtosecond time-resolved absorption spectroscopy (fs-TAS) and in-situ double-beam detection strategy (AM 1.5 + 405 nm). This work provides an effective and feasible strategy on designing and regulating high-efficiency composite photoanode with Z-scheme transfer mechanism.

Long-lived charge separation following pump-wavelength-dependent ultrafast charge transfer in graphene/WS2 heterostructures.

Van der Waals heterostructures consisting of graphene and transition metal dichalcogenides have shown great promise for optoelectronic applications. However, an in-depth understanding of the critical processes for device operation, namely, interfacial charge transfer (CT) and recombination, has so far remained elusive. Here, we investigate these processes in graphene-WS2 heterostructures by complementarily probing the ultrafast terahertz photoconductivity in graphene and the transient absorption dynamics in WS2 following photoexcitation. We observe that separated charges in the heterostructure following CT live extremely long: beyond 1 ns, in contrast to ~1 ps charge separation reported in previous studies. This leads to efficient photogating of graphene. Furthermore, for the CT process across graphene-WS2 interfaces, we find that it occurs via photo-thermionic emission for sub-A-exciton excitations and direct hole transfer from WS2 to the valence band of graphene for above-A-exciton excitations. These findings provide insights to further optimize the performance of optoelectronic devices, in particular photodetection.

Anionic Fluorinated Zn-porphyrin Combined with Cationic Endohedral Li-fullerene for Long-Lived Photoinduced Charge Separation with Low Energy Loss.

Here we report an anionic meso-tetrakis(4-carboxymethylthio-2,3,5,6-tetrafluorophenyl) zinc porphyrin (ZnTF4PPTC4-) to form a supramolecular complex with a cationic lithium endohedral [60]fullerene (Li+@C60). The supramolecular ZnTF4PPTC4-/Li+@C60 complex formed by strong electrostatic attraction with a large binding constant generates a long-lived charge-separated (CS) state with low energy loss by photoinduced electron transfer from ZnTF4PPTC4- to Li+@C60. The anionic fluorinated zinc porphyrin with high oxidation potential reduces the energy loss associated with the charge separation and enhances the energy level of the CS state. The energy level of the CS state determined by electrochemical measurements is at 0.94 eV, which is much higher than that of a similar supramolecular complex using an anionic meso-tetrakis(sulfonatophenyl) zinc porphyrin (ZnTPPS4-) at 0.55 eV. Time-resolved transient absorption spectroscopy demonstrates that ZnTF4PPTC4-/Li+@C60 generates a long-lived CS state with a lifetime of 0.29 ms in a binary solvent of acetonitrile and chlorobenzene. The lifetime of the CS state is comparable to that of ZnTPPS4-/Li+@C60 in benzonitrile.

Dye-polyoxometalate coordination polymer as a photo-driven electron pump for photocatalytic radical coupling reactions.

To alleviate diffusion-limited photoinduced electron transfer (PET) in solution, a triphenylamine-derived dye and a Keggin polyoxometalate-type electron relay were coupled into a coordination polymer to photoinduce long-lived charge-separation pairs with enough reductive/oxidative potential to pump multiple electrons unidirectionally from external electron donors to acceptors, thus furnishing photocatalytic radical couplings to afford value-added α-amino C-H arylation products.

Formation of Highly Efficient, Long-Lived Charge Separated States in Star-Shaped Ferrocene-Diketopyrrolopyrrole-Triphenylamine Donor-Acceptor-Donor Conjugates.

The significance of multiple number of donor-acceptor entities on a central electron donor in a star-shaped molecular system in improving light energy harvesting ability is reported. For this, donor-acceptor-donor type conjugates comprised up to three entities ferrocenyl (Fc)-diketopyrrolopyrrole (DPP) onto a central triphenylamine (TPA), (4-6) by the Pd-catalyzed Sonogashira cross-coupling reactions have been newly synthesized and characterized. Donor-acceptor conjugates possessing diketopyrrolopyrrole (1 to 3 entities) onto the central triphenylamine, (1-3) served as reference dyads while monomeric DPP and Fc-DPP served as control compounds. Both DPP and Fc-DPP carrying conjugates exhibited red-shifted absorption compared to their respective control compounds revealing existence of ground state interactions. Furthermore, DPP fluorescence in 4-6 was found to be quantitatively quenched while for 1-3, this property varied between 73-65 % suggesting occurrence moderate amounts of excited state events. The electrochemical investigations exhibited an additional low potential oxidation in the case of Fc-DPP-TPA based derivatives (4-6) owing to the presence of ferrocene unit(s). This was in addition to DPP and TPA redox peaks. Using spectral, electrochemical and computational studies, Gibbs free-energy calculations were performed to visualize excited state charge separation (ΔGCS ) in these donor-acceptor conjugates as a function of different number of Fc-DPP entities. Formation of Fc+ -DPP.- -TPA charge separated states (CSS) in the case of 4-6 was evident. Using spectroelectrochemical studies, spectrum of CSS was deduced. Finally, femtosecond transient absorption spectral studies were performed to gather information on excited state charge separation. Increasing the number of Fc-DPP entities in 4-6 improved charge separation rates. Surprisingly, lifetime of the charge separated state, Fc+ -DPP.- -TPA was found to persist longer with an increase in the number of Fc-DPP entities in 4-6 as compared to Fc-DPP-control and simple DPP derived donor-acceptor conjugates in literature. This unprecedented result has been attributed to subtle changes in ΔGCS and ΔGCR and the associated electron coupling between different entities.

A charge-separated interfacial hole transport semiconductor for efficient and stable perovskite solar cells

Abstract Perovskite solar cells (PSCs) with high efficiency and high stability are still a challenge to produce although remarkable successes have been achieved since they were first reported in 2009. One strategy to effectively improve both the performance as well as the stability is to introduce an interfacial layer between perovskite and hole transport material. Herein, we report a charge-separated (CS) organic semiconductor as the interfacial layer that forms cascaded energy levels between perovskite and hole transportation material. This CS semiconductor displays high hole and electron mobilities by converting long-lived CS states in solution into permanent polarons (charged carriers) in films. Doping with iodinehydride is able to improve the surface morphology of the CS semiconductor layer. Our devices with an iodinehydride-doped CS semiconductor layer exhibit an efficiency of 17.87%, which is increased by ~25% in comparison with 14.24% of the reference devices that have no interfacial layer. This additional CS semiconductor layer also enhances the unsealed device stability by maintaining 90% of initial PCE, while the reference devices degraded by 35% at a relative humidity of 20–30%, temperature of 25 °C and ambient light for 240 h. This result reveals that the utilization of CS states is an alternative approach to construct high charge transport organic semiconductors. An interfacial semiconductor with proper energy level and a matching hole transport mobility can improve the hole extraction, speed up hole transport and suppress charge recombination of PSCs, and thus may be an effective strategy to improve their efficiency and stability.

Influence of Surface Ligands on Charge-Carrier Trapping and Relaxation in Water-Soluble CdSe@CdS Nanorods

In this study, the impact of the type of ligand at the surface of colloidal CdSe@CdS dot-in-rod nanostructures on the basic exciton relaxation and charge localization processes is closely examined. These systems have been introduced into the field of artificial photosynthesis as potent photosensitizers in assemblies for light driven hydrogen generation. Following photoinduced exciton generation, electrons can be transferred to catalytic reaction centers while holes localize into the CdSe seed, which can prevent charge recombination and lead to the formation of long-lived charge separation in assemblies containing catalytic reaction centers. These processes are in competition with trapping processes of charges at surface defect sites. The density and type of surface defects strongly depend on the type of ligand used. Here we report on a systematic steady-state and time-resolved spectroscopic investigation of the impact of the type of anchoring group (phosphine oxide, thiols, dithiols, amines) and the bulkiness of the ligand (alkyl chains vs. poly(ethylene glycol) (PEG)) to unravel trapping pathways and localization efficiencies. We show that the introduction of the widely used thiol ligands leads to an increase of hole traps at the surface compared to trioctylphosphine oxide (TOPO) capped rods, which prevent hole localization in the CdSe core. On the other hand, steric restrictions, e.g., in dithiolates or with bulky side chains (PEG), decrease the surface coverage, and increase the density of electron trap states, impacting the recombination dynamics at the ns timescale. The amines in poly(ethylene imine) (PEI) on the other hand can saturate and remove surface traps to a wide extent. Implications for catalysis are discussed.

Femtosecond time-resolved spectroscopic observation of long-lived charge separation in bimetallic sulfide/g-C3N4 for boosting photocatalytic H2 evolution

Copper-nickel sulfides could effectively suppress deep trapping states of active charge in carbon nitride. It also improves the efficiency of the shallow trapped electron transfer through CS bond for enhancing visible-light-driven photocatalytic hydrogen production with rates up to 752.8 μmol h−1 g−1, that is 470 times higher than that of pristine g-C3N4 (1.6 μmol h−1 g−1) so far. The kinetic coupling of electron transfer and long-lived charge separation (∼ 4896 ps) are systematically investigated by femtosecond time-resolved absorption spectroscopy (fs-TA). The TA signal of the composite is quenched by hole sacrificial agent, assigning to the effective hole extraction for high photocatalytic activity. Furthermore, a remarkable near-infrared-driven photocatalytic H2 evolution (0.32 μmol h−1 g−1, λ > 800 nm) was achieved due to the hole transfer from copper-nickel sulfide to the trap state of g-C3N4, indicating that the strong interaction between copper-nickel sulfide and g-C3N4 is favorable to charge transfer and long-lived charge separation states.

Generation of Long-Lived Photoinduced Charge Separation in a Supramolecular Toroidal Assembly.

Efficiencies of artificial photosynthetic and photocatalytic systems depend on their ability to generate long-lived charge-separated (CS) states in photoinduced electron transfer (PET) reactions. PET, in most cases, is followed by an ultrafast back electron transfer, which severely reduces lifetime and quantum yield of CS states. Generation of a long-lived CS state is an important goal in the study of PET reactions. Herein, we report that this goal is achieved using a hierarchically self-assembled anthracene-methyl viologen donor-acceptor system. Anthracene linked to two β-cyclodextrin molecules (CD-AN-CD) and methyl viologen linked to two adamantane units (AD-MV2+-AD) form an inclusion complex in water, which further self-assembled into well-defined toroidal nanostructures. The fluorescence of anthracene is highly quenched in the self-assembled system because of PET from anthracene to methyl viologen. Irradiation of the aqueous toroidal solution led to formation of a long-lived CS state. Rational mechanisms for the formation of the toroidal nanostructures and long-lived photoinduced charge separation are presented in the paper.

Perspectives on Dye Sensitization of Nanocrystalline Mesoporous Thin Films.

Recent advances in our mechanistic understanding of dye-sensitized electron transfer reactions occurring at metal oxide interfaces are described. These advances were enabled by the advent of mesoporous thin films, comprised of anatase TiO2 nanocrystallites, that are amenable to spectroscopic and electrochemical characterization in unprecedented molecular-level detail. The metal-to-ligand charge transfer (MLCT) excited states of Ru polypyridyl compounds serve as the dye sensitizers. Excited-state injection often occurs on ultrafast time scales with yields that can be tuned from unity to near zero through modification of the sensitizer or the electrolyte composition. Transport of the injected electron and the oxidized sensitizer (hole hopping) are both operative in the composite mechanism for charge recombination between the injected electron and the oxidized sensitizer. Sensitizers that contain a pendant electron donor, as well as core/shell SnO2/TiO2 nanostructures, often prolong the lifetime of the injected electron and provide fundamental insights into adiabatic and nonadiabatic electron transfer mechanisms. Regeneration of the oxidized sensitizer by iodide is enhanced through halogen bonding, orbital pathways, and ion pairing. A substantial ∼10 MV cm-1 electric field is created by electron injection into TiO2 nanocrystallites that induces ion migration, reports on the sensitizer dipole orientation, and (in some cases) reorients or flips the sensitizer. Dye-sensitized conductive oxides also promote long-lived charge separation with bias dependent kinetics that provide insights into the reorganization energies associated with electron and proton-coupled electron transfer in the electric double layer.

(Invited) Donor-Acceptor Systems from Triplet Photosensitizers for High Potential, Long-Lived Charge Separation

Generation of high potential, long-lived charge separated states in donor-acceptor systems are invaluable for converting light energy into either electricity or fuels. The majority of the donor-acceptor conjugates designed for this purpose are derived mainly from short-lived singlet sensitizers.1,2 In such conjugates, charge separation from the singlet excited donor or acceptor results in a short-lived charge separated state of singlet spin character. Alternatively, by utilizing high-energy triplet sensitizers one could generate long-lived charge separated states. The radical ion-pairs of triplet spin character slowly charge recombine to the ground state due to spin forbidden process. However, witnessing such a process purely from the triplet-excited state without the interference of the singlet excited state and other competing photochemical events such as excitation energy transfer has been challenging to establish.3-5 Here, we summarize our efforts to synthesize donor-acceptor conjugates bearing triplet sensitizers wherein excited state electron transfer exclusively occurs from the triplet excited state.3-6 To achieve the goal, two types of triplet sensitizers, viz., palladium and platinum porphyrins, and b-iodo functionalized BODIPYs are covalently linked to the well-known electron acceptor, fullerene, C60. Using femtosecond transient absorption studies covering wide temporal and spatial regions, and time-resolved electron paramagnetic studies, photoinduced electron transfer from 3MP* (M = Pd or Pt) to C60 in these dyads, leading to the formation of MP•+-C60 •- charge separated state of triplet spin character is demonstrated. Similarly, the role of triplet sensitizers in promoting charge separation in I2BODIPY-C60 and I2azaBODIPY-C60 dyads6 is also highlighted. 1. B. KC, F. D’Souza, Coord. Chem. Rev. 2016, 322, 104-141.2. D’Souza, and O. Ito ‘Chem. Soc. Rev. 2012, 41, 86-96.3. Obondi, G. N. Lim, F. D’Souza, J. Phys. Chem. C 2015, 119, 176-185.4. Obondi, G. N. Lim, B. Churchill, P. K. Poddutoori, A. van der Est, and F. D’Souza, Nanoscale, 2016, 8, 8333-8344.5. K. Poddutoori,Y. E. Kandrashkin, C. O. Obondi, F. D’Souza and A. van der Est, Phys. Chem. Chem. Phys. 2018, 20, 28223-28231.6. Shao, H. B. Gobeze, V. Bandi, C. Funk, B. Heine, M. J. Duffy, V. Nesterov, P. A. Karr, F. D’Souza, ChemPhotoChem, 2020, in press.

(Invited) Organic/Inorganic Hybrid Interfaces between Perovskite Semiconductors and Semiconducting Swcnts for Energy Harvesting and Conversion

Low-dimensional semiconductors provide a rich tapestry of tunable optical and electrical properties for a wide variety of applications. Semiconducting single-walled carbon nanotubes (s-SWCNTs) have shown tremendous potential in applications ranging from digital logic, biological imaging, quantum information processing, photovoltaics, and thermoelectric energy harvesting. Energy harvesting applications rely critically upon the creation of tailored interfaces that enable the movement of energetic species (excitons, electrons, holes) in specified directions. While organic energy harvesting devices often employ interfaces between distinct organic species such as polymers, fullerenes, and carbon nanotubes, hybrid interfaces between organic and inorganic semiconductors have unique properties that are relatively unexplored.In this talk, I will discuss our recent efforts at constructing novel hybrid interfaces between s-SWCNTs and low-dimensional perovskite-based semiconductors. I will highlight time-resolved spectroscopy results that illustrate long-lived photoinduced charge separation across such interfaces, demonstrating utility for photovoltaics and photodetectors. Interestingly, some systems demonstrate additional transient processes that are exceptionally slow and may indicate ion movement within the hybrid systems. Complementary device studies demonstrate exceptional efficiencies for conversion of photons into stable photocurrent, and I will highlight several novel applications in which these interfaces can be employed.

Long-lived charge separation in two-dimensional ligand-perovskite heterostructures.

Rational design of heterojunctions using nanostructured materials is a useful strategy for achieving efficient interfacial charge separation in photovoltaics. Heterojunctions can be constructed between the organic ligands and the inorganic layers in two-dimensional perovskites, taking advantage of their highly programmable structures. Here, we investigate charge transfer and recombination at the interface between the thiophene-based semiconducting ligands and the lead halide inorganic sublattices using time-resolved photoluminescence and transient reflection spectroscopy in single two-dimensional perovskite crystals. These measurements demonstrate the charge transfer time around 10 ps and long-lived charge-separated state over the nanosecond time scale in two-dimensional ligand-perovskite heterostructures. The efficient charge transfer processes coupled with slow charge recombination suggest the potential for improving exciton dissociation and charge transport in two-dimensional perovskite solar cells.

Vibronic coherence evolution in multidimensional ultrafast photochemical processes

The complex choreography of electronic, vibrational, and vibronic couplings used by photoexcited molecules to transfer energy efficiently is remarkable, but an unambiguous description of the temporally evolving vibronic states governing these processes has proven experimentally elusive. We use multidimensional electronic-vibrational spectroscopy to identify specific time-dependent excited state vibronic couplings involving multiple electronic states, high-frequency vibrations, and low-frequency vibrations which participate in ultrafast intersystem crossing and subsequent relaxation of a photoexcited transition metal complex. We discover an excited state vibronic mechanism driving long-lived charge separation consisting of an initial electronically-localized vibrational wavepacket which triggers delocalization onto two charge transfer states after propagating for ~600 femtoseconds. Electronic delocalization consequently occurs through nonadiabatic internal conversion driven by a 50 cm−1 coupling resulting in vibronic coherence transfer lasting for ~1 picosecond. This study showcases the power of multidimensional electronic-vibrational spectroscopy to elucidate complex, non-equilibrium energy and charge transfer mechanisms involving multiple molecular coordinates.