Immobilised nano-particulate photocatalyst sheets under concentrated solar conditions could offer a competitive approach to scaling water-splitting photocatalytic systems for low-emission hydrogen production.

Photosynthesis has been shown to be a highly efficient process for energy transfer in plants and bacteria. Like natural photosynthetic systems, the artificial light harvesting complex (LHC) BODIPY pillar[5]arene exhibits Förster resonance energy transfer (FRET). However, extensive characterisation of the BODIPY pillar[5]arene LHC to determine its suitability as an artificial LHC has yet to occur. In this paper we experimentally and computationally investigate the photophysical properties of the LHC by comparing the light absorption of the BODIPY LHC to individual BODIPY chromophores. Our results show evidence for quantum coherence, with oscillation frequencies of 100 cm-1 and 600 cm-1, which are attributable to vibronic, or exciton-phonon type coupling. Computational analysis suggests strong couplings of the molecular orbitals of the LHC resulting from the stacking of neighbouring BODIPY chromophore units. Interestingly, we find a 40% reduction in the absorbance of light for the BODIPY LHC compared to the individual chromophores which we attribute to electronic interactions between the conjugated π-systems of the BODIPY chromophores and the pillar[5]arene backbone.

Singlet fission (SF) holds the promise to circumvent the photovoltaic efficiency limit to reach a power-conversion efficiency above 34%. SF of TIPS-pentacene (TIPS-Pn) has been investigated but its mechanism is yet to be well elucidated. Recently, we developed a nanoparticle (NP) system, in which doping of TIPS-Pn in a host matrix yields a range of average intermolecular distances, d, to study the dependence of SF in TIPS-Pn on d. At large d values, where the bimolecular SF process should be unfavourable, a relatively high SF quantum yield (ΦSF) is still observed, which implies a deviation from a random distribution of TIPS-Pn throughout the NP. Here, using polarisation-sensitive femtosecond time-resolved spectroscopy and Monte Carlo simulations of exciton migration and SF, we quantify the level of clustering of TIPS-Pn in the host matrix, which is responsible for the higher than expected ΦSF. The experimental data indicate a preservation of polarisation correlation by SF, which is uncommon because energy transfer in amorphous materials tends to result in depolarisation. We show that the preservation of polarisation correlation is due to SF upon exciton migration. Although exciton migration decorrelates polarisation, SF acts to remove decorrelated excitons to give an overall preservation of polarisation correlation.

Two strategies for improving solar energy efficiencies, triplet fusion and singlet fission, rely on the details of triplet-triplet interactions. In triplet fusion, there are several steps, each of which is a possible loss mechanism. In solution, the parameters describing triplet fusion collisions are difficult to inspect. Here we show that these parameters can be determined by examining the magnetic field dependence of triplet fusion upconversion. We show that there is a reduction of the magnetic field effect for perylene triplet fusion as the system moves from the quadratic to linear annihilation regimes with an increase in laser power. Our data are modeled with a small set of parameters that characterize the triplet fusion dynamics. These parameters are cross-validated with molecular dynamics simulations. This approach can be applied to both solution and solid state materials, providing a tool for screening potential annihilators for photon upconversion.

Singlet fission (SF), a process that produces two triplet excitons from one singlet exciton, has attracted recent interest for its potential to circumvent the detailed-balance efficiency limit of single-junction solar cells. For the potential of SF to be fully realized, accurate assignment and quantification of SF is necessary. Intersystem crossing (ISC) is another process of singlet to triplet conversion that is important to distinguish from SF to avoid either over- or under-estimation of SF triplet production. Here, we quantify an upper bound on the rate of ISC in two commonly studied SF chromophores, TIPS-pentacene and TIPS-tetracene, by using transient absorption spectroscopy of solutions of varying concentrations in toluene. We show that SF in solutions of these acenes has previously been misidentified as ISC, and vice versa. By determining a bimolecular SF rate constant in concentrated solutions in which SF dominates over ISC, we distinguish triplet formation due to SF from triplet formation due to ISC, and show that the characteristic time scale of ISC must be longer than 325 ns in TIPS-pentacene, while it must be longer than 118 ns in TIPS- tetracene. We additionally note that no excimer formation is observed in the relatively dilute (up to 8 mM) solutions studied here, indicating that previous excimer formation observed at much higher concentrations may be partially due to aggregate formation. This work highlights that an accurate quantification of ISC is crucial as it leads to accurate determination of SF rate constants and yields.

Photosynthesis has been shown to be a highly efficient process for energy transfer in plants and bacteria. It has been proposed that quantum mechanics plays a key role in this energy transfer process. There has been evidence that photosynthetic systems may exhibit quantum coherence. As artificial light-harvesting complexes have been proposed to mimic photosynthesis, it is prudent that artificial photosynthetic materials should also be tested for quantum coherence. To date, such studies have not been reported. In this work, we examine one such system, the BODIPY light harvesting complex (LHC), which has been shown to exhibit classical energy transfer via Förster resonance energy transfer. We compare the photon absorption of the LHC with the BODIPY chromophore by performing UV-visible, transient absorption, broadband pump-probe (BBPP) and two-dimensional electronic (2DES) spectroscopy. The 2DES and BBPP show evidence for quantum coherence, with oscillation frequencies of 100 cm-1 and 600 cm-1, which are attributable to vibronic, or exciton-phonon type coupling. Further computational analysis suggests strong couplings of the molecular orbitals of the LHC resulting from the stacking of neighbouring BODIPY chromophore units may contribute to undesirable hypochromic effects .

TIPS-pentacene is a small-molecule organic semiconductor that is widely used in optoelectronic devices. It has been studied intensely owing to its ability to undergo singlet fission. In this study, we aim to develop further understanding of the coupling between the electronic and nuclear degrees of freedom of TIPS-pentacene (TIPS-Pn). We measured and analyzed the 2D electronic spectra of TIPS-Pn in solutions. Using center line slope (CLS) analysis, we characterized the frequency-fluctuation correlation function of the 0-0 vibronic transition. Strong oscillations in the CLS values were observed for up to 5 ps with a frequency of 264 cm-1, which are attributable to a large vibronic coupling with the TIPS-Pn ring-breathing vibrational mode. In addition, detailed analysis of the CLS values allowed us to retrieve two spectral diffusion lifetimes, which are attributed to the inertial and diffusive dynamics of solvent molecules. Amplitude beating analysis also uncovered couplings with another vibrational mode at 1173 cm-1. The experimental results can be described using the displaced harmonic oscillator model. By comparing the CLS values of the simulated data with the experimental CLS values, we estimated a Huang-Rhys factor of 0.1 for the ring-breathing vibrational mode. The results demonstrated how CLS analysis can be a useful method for characterizing the strength of vibronic coupling.

In two-dimensional electronic spectroscopy (2DES), precise control of the arrival time of ultrashort laser pulses is critical to correlating the molecular states that are accessed in the experiment. In this work, we demonstrate a 2D electronic spectrometer design with an interferometric phase stability of ∼λ/250 at 600 nm. First, we present a new method for controlling pulse delay times based on transmission through pairs of optical flats rotated perpendicular to the beam propagation direction. Second, the calibration methods required to achieve adequate timing precision are also reported. Compared to existing designs using translating wedges, the rotating optical flats can achieve equivalent optical delay with a shorter path length in glass, reducing errors due to spectral dispersion of the broadband laser pulses used in 2DES. Our approach presents a simple, low-cost technique for multidimensional optical spectroscopy that is capable of resolving complex light-induced dynamics.

Materials in which charge delocalization and migration can be tuned are critical for electronic applications.

Potassium (K + ) doping has been recently discovered as an effective route to suppress hysteresis and improve the performance stability of perovskite solar cells. However, the mechanism of these K + doping effects is still under debate, and rationalization of the improved performance in these perovskites is needed. Herein, the photoluminescence (PL) properties and device performance of mixed‐cation mixed‐halide perovskite are dynamically monitored with and without K + doping under bias light illumination via a confocal fluorescence microscope, together with ultrafast transient absorption as well as time‐dependent and time‐resolved PL measurements. It is demonstrated that illumination is essential to trigger the passivation effect of K + by forming KBr‐like compounds, leading to the elimination of interface trapping defects and suppression of mobile ion migration, thus resulting in improved power conversion efficiency and negligible current–voltage hysteresis of solar cells. This work provides novel insight into the hysteresis suppression upon K + doping and highlights the significance of light illumination when using this protocol.

In conjugated polymers used in photovoltaics, charges may be produced on ultrafast time scales without requiring exciton diffusion to a donor-acceptor interface. To investigate the role of high-energy, delocalized exciton states in charge generation within polymer domains, we apply a pump-push-probe transient absorption technique to pristine poly(3-hexylthiophene) nanoparticles. The near-infrared push pulse induces exciton dissociation through the S-3 <- S-1 electronic transition, which is predicted to show intra-molecular charge-transfer character. We suggest that the spatial extent of the high-energy exciton, which induces electron-hole separation, is sufficient to overcome the intrinsic Coulombic attraction of the electron-hole pair. We observe that similar to 10% of the pushed excitons undergo dissociation to form free charges. The kinetics of charge recombination indicate that the electron and hole are separated by a distance of similar to 3 nm across the polymer domains.

2D Ruddlesden-Popper type perovskites have attracted enormous attention due to their natural multiquantum-well structure. However, there is still mystery regarding the behavior of photocarriers, especially the exciton fine structure behind the excellent optoelectronic performance. The coexistence of two strikingly different decay components in time-resolved photoluminescence is inconsistent with the high internal quantum yield (QY(IN) = approximate to 0.7) in the conventional model for radiative and nonradiative recombinations (QY(IN) = tau(nr)/(tau(nr) + tau(r)) = 17%). Here it is revealed that there is a special transient energy reservoir outcompeting nonradiative loss in 2D Ruddlesden-Popper type perovskites. Upon optical excitation, the bright excitons rapidly relax into the low-lying energy reservoir before nonradiative recombination occurs. Interestingly, the energy in the reservoir is not lost. The carriers in this energy reservoir can spontaneously transfer back to the bright states and can still effectively contribute to the photovoltaic and photonic properties of the perovskites. This investigation provides a novel insight into the mechanism for the lauded defect tolerance of 2D perovskites by highly efficient energy storage via a transient reservoir.

Singlet fission (SF) is a process with the potential to increase the efficiency of solar cells by reducing losses from thermal relaxation of hot carriers. By generating two triplet excitons from one singlet exciton, the process effectively splits the energy of high-energy photons into two, providing a means to circumvent the Shockley–Queisser limit. Although the applications of SF are promising, questions remain about the mechanistic details and practicalities of implementation in photovoltaic devices that must be resolved to exploit its full potential. In this study, we present a way to investigate the effect of average intermolecular distance on SF by embedding 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) in an amorphous polymer matrix in the form of aqueous nanoparticle dispersions. By controlling the mass ratio of TIPS-Pn to the host polymer, we systematically tune the concentration of TIPS-Pn molecules in a nanoparticle and in turn, the average intermolecular separation, leading to a range of SF quantum yields. We study this system using both steady-state and ultrafast time-resolved spectroscopic techniques and fit the results to a kinetic model to decipher the observed behavior. The quantum yield of SF is shown to decrease with average intermolecular separation, which is explained by diffusion-limited SF and an increase in loss pathways through isolated sites. Additionally, we identify an intermediate species in the SF process and show that a significant proportion of this species decays nonradiatively without dissociating to form separated triplets, revealing a major loss pathway that has important implications for future research and applications.

Although the optoelectronic applications of metal halide perovskites have been intensively investigated in recent years, the fundamental carrier dynamics of zero-dimensional (0D) Cs4PbBr6 perovskites has been relatively underexplored; in particular, the nature of the green fluorescence is highly debated. Nevertheless, the unique photophysical properties are of immense interest for a variety of potential applications. In this work, the green emission of the CsPbBr3-Cs4PbBr6 perovskite composites is studied using temperature dependent photoluminescence (PL). The PL spectra at different temperatures simultaneously contain two sub-peaks (520 nm and 550 nm), which are ascribed to the emissions of the band-edge and the defect trapped exciton of CsPbBr3. This finding will help to understand the controversial photoluminescence currently observed in different 0D Cs4PbBr6 perovskites.

The photophysical properties of conjugated materials can be strongly affected by the nature of intermolecular interactions. Toward this aim, supramolecular self-assembly facilitates efficient packing of molecules into ordered architectures, which allows efficient intermolecular coupling. However, the resulting electronic overlap imparts additional sensitivity to the disorder, predominantly because of the "nonlocal" origins of the photophysical properties. Understanding the nature and origin of the disorder in conjugated systems is a prerequisite to exploit the benefits of efficient intermolecular coupling. In this report, we utilize chirality as a marker to sensitively probe the nature of the disorder in thermodynamically assembled helical nanoaggregates of a chiral conjugated polymer poly[(9,9-di-n-octylfluoreny1-2,7-diy1)-alt-(benzothiadiazole)] (PFBT). Surprisingly, we find that one-handed intermolecular coupling in helical PFBT aggregates leads to differences in the decay pathways of left- and right-handed excitations. We attribute the emergence of this sensitivity to the disorder in the excitonic coupling of flexible, nonplanar polymer chain conformations, likely predominant at the edges of the aggregate. Our findings shed insights into the effect of disorder on the photophysical properties, which open up new opportunities for sensitively exploring the links between intermolecular coupling and photophysical properties of conjugated systems.

Singlet fission is a process whereby two triplet excitons can be produced from one photon, potentially increasing the efficiency of photovoltaic devices. Endothermic singlet fission is desired for a maximum energy-conversion efficiency, and such systems have been considered to form an excimer-like state with multiexcitonic character prior to the appearance of triplets. However, the role of the excimer as an intermediate has, until now, been unclear. Here we show, using 5,12-bis((triisopropylsilyl)ethynyl)tetracene in solution as a prototypical example, that, rather than acting as an intermediate, the excimer serves to trap excited states to the detriment of singlet-fission yield. We clearly demonstrate that singlet fission and its conjugate process, triplet-triplet annihilation, occur at a longer intermolecular distance than an excimer intermediate would impute. These results establish that an endothermic singlet-fission material must be designed to avoid excimer formation, thus allowing singlet fission to reach its full potential in enhancing photovoltaic energy conversion.

The carrier dynamics of lead sulphide quantum dot (PbS QD) drop cast films and closely packed ordered Langmuir-Blodgett films are studied with ultra-fast femtosecond transient absorption spectroscopy. The photo-induced carrier temperature is extracted from transient absorption spectra and monitored as a function of time delay. The cooling dynamics of carriers in PbS QDs suggest a reduction of the carrier energy loss rate at longer time delays through the retardation of the longitudinal optical (LO) phonon decay due to partial heating of acoustic phonon modes. A slowed hot carrier cooling time up to 116 ps is observed in the drop cast film. A faster cooling rate was also observed in the highly compact Langmuir-Blodgett film due to the enhanced carrier-LO phonon coupling strength arising from the Coulombic interaction in neighboring QDs, which is verified by temperature dependent steady state PL measurements.

Amorphous carbon nitride (a‐CN) is a less‐explored but promising photocatalyst for hydrogen production. Despite an extended visible light absorption (EVLA) its low quantum efficiency (QE) for water photoreduction is a long standing problem. This implies that EVLA is not proportionally translated into collection of large amounts of photogenerated electrons. Minimizing the mismatch between light‐absorption and charge‐collection remains a scientific challenge. Here a sponge‐like hierarchical structure of a‐CN that addresses this apparent mismatch is reported. Combined experimental and finite difference time domain simulations demonstrate the ability of the a‐CN sponge to induce scattering for total internal light reflection that promotes localized charge carrier generation. Diffused reflectance and transient fluorescence decay studies show good agreement with simulations with a 40% enhanced light‐trapping and an ≈23 times longer electron lifetime in spongy a‐CN compared with that of the bulk material. The result is a new high benchmark for hydrogen production of 203.5 µmol h−1 with a QE of 6.1% at 420 nm in a reaction system of 10 vol% triethanolamine and 1 wt% Pt cocatalyst. The enhanced water photoreduction is a result of amenable photophysical and electrochemical attributes existing within the a‐CN sponge.