Unraveling the Role of Non-Fullerene Acceptor with High Dielectric Constant in Organic Solar Cells.

Increasing the relative dielectric constant is a constant pursuit of organic semiconductors, but it often leads to multiple changes in device characteristics, hindering the establishment of a reliable relationship between dielectric constant and photovoltaic performance. Herein, a new non-fullerene acceptor named BTP-OE is reported by replacing the branched alkyl chains on Y6-BO with branched oligoethylene oxide chains. This replacement successfully increases the relative dielectric constant from 3.28 to 4.62. To surprise, BTP-OE offers consistently lower device performance relative to Y6-BO in organic solar cells (16.27% vs 17.44%) due to the losses in open-circuit voltage and fill factor. Further investigations unravel that BTP-OE has resulted in reduced electron mobility, increased trap density, enhanced first order recombination, and enlarged energetic disorder. These results demonstrate the complex relationship between dielectric constant and device performance, which provide valuable implications for the development of organic semiconductors with high dielectric constant for photovoltaic application.

Tracking the Light-Induced Excited-State Dynamics and Structural Configurations of an Extraordinarily Long-Lived Metastable State at Room Temperature.

Time-resolved X-ray (Tr-XAS) and optical transient absorption (OTA) spectroscopy on the pico-microsecond timescale coupled with density functional theory calculations are applied to study the light-induced spin crossover processes of a Fe-based macrocyclic complex in solution. Tr-XAS analysis after light illumination shows the formation of a seven-coordinated high-spin quintet metastable state, which relaxes to a six-coordinated high-spin configuration before decaying to the ground state. Kinetic analysis of the macrocyclic complex reveals an unprecedented long-lived decay lifetime of approximately 42.6 µs. Comparative studies with a non-macrocyclic counterpart illustrate a significantly shortened approximately 568-fold decay lifetime of about 75 ns, and highlight the importance of the ligand arrangement in stabilizing the reactivity of the excited state. Lastly, OTA analysis shows the seven-coordinated high-spin state to be formed within approximately 6.2 ps. These findings provide a complete understanding of the spin crossover reaction and relaxation pathways of the macrocyclic complex, and reveal the importance of a flexible coordination environment for their rational design.

Excited state absorption spectra of dissolved and aggregated distyrylbenzene: A TD-DFT state and vibronic analysis.

A time-dependent density functional theory study is performed to reveal the excited state absorption (ESA) features of distyrylbenzene (DSB), a prototype π-conjugated organic oligomer. Starting with a didactic insight to ESA based on simple molecular orbital and configuration considerations, the performance of various density functional theory functionals is tested to reveal the full vibronic ESA features of DSB at short and long probe delay times.

Elementary Energy Transfer Pathways in Allochromatium vinosum Photosynthetic Membranes.

Allochromatium vinosum (formerly Chromatium vinosum) purple bacteria are known to adapt their light-harvesting strategy during growth according to environmental factors such as temperature and average light intensity. Under low light illumination or low ambient temperature conditions, most of the LH2 complexes in the photosynthetic membranes form a B820 exciton with reduced spectral overlap with LH1. To elucidate the reason for this light and temperature adaptation of the LH2 electronic structure, we performed broadband femtosecond transient absorption spectroscopy as a function of excitation wavelength in A. vinosum membranes. A target analysis of the acquired data yielded individual rate constants for all relevant elementary energy transfer (ET) processes. We found that the ET dynamics in high-light-grown membranes was well described by a homogeneous model, with forward and backward rate constants independent of the pump wavelength. Thus, the overall B800→B850→B890→ Reaction Center ET cascade is well described by simple triexponential kinetics. In the low-light-grown membranes, we found that the elementary backward transfer rate constant from B890 to B820 was strongly reduced compared with the corresponding constant from B890 to B850 in high-light-grown samples. The ET dynamics of low-light-grown membranes was strongly dependent on the pump wavelength, clearly showing that the excitation memory is not lost throughout the exciton lifetime. The observed pump energy dependence of the forward and backward ET rate constants suggests exciton diffusion via B850→ B850 transfer steps, making the overall ET dynamics nonexponential. Our results show that disorder plays a crucial role in our understanding of low-light adaptation in A. vinosum.

Ultra-broadband 2D electronic spectroscopy of carotenoid-bacteriochlorophyll interactions in the LH1 complex of a purple bacterium.

We investigate the excitation energy transfer (EET) pathways in the photosynthetic light harvesting 1 (LH1) complex of purple bacterium Rhodospirillum rubrum with ultra-broadband two-dimensional electronic spectroscopy (2DES). We employ a 2DES apparatus in the partially collinear geometry, using a passive birefringent interferometer to generate the phase-locked pump pulse pair. This scheme easily lends itself to two-color operation, by coupling a sub-10 fs visible pulse with a sub-15-fs near-infrared pulse. This unique pulse combination allows us to simultaneously track with extremely high temporal resolution both the dynamics of the photoexcited carotenoid spirilloxanthin (Spx) in the visible range and the EET between the Spx and the B890 bacterio-chlorophyll (BChl), whose Qx and Qy transitions peak at 585 and 881 nm, respectively, in the near-infrared. Global analysis of the one-color and two-color 2DES maps unravels different relaxation mechanisms in the LH1 complex: (i) the initial events of the internal conversion process within the Spx, (ii) the parallel EET from the first bright state S2 of the Spx towards the Qx state of the B890, and (iii) the internal conversion from Qx to Qy within the B890.

Tracking energy transfer between light harvesting complex 2 and 1 in photosynthetic membranes grown under high and low illumination.

Energy transfer (ET) between B850 and B875 molecules in light harvesting complexes LH2 and LH1/RC (reaction center) complexes has been investigated in membranes of Rhodopseudomonas palustris grown under high- and low-light conditions. In these bacteria, illumination intensity during growth strongly affects the type of LH2 complexes synthesized, their optical spectra, and their amount of energetic disorder. We used a specially built femtosecond spectrometer, combining tunable narrowband pump with broadband white-light probe pulses, together with an analytical method based on derivative spectroscopy for disentangling the congested transient absorption spectra of LH1 and LH2 complexes. This procedure allows real-time tracking of the forward (LH2 â†’ LH1) and backward (LH2←LH1) ET processes and unambiguous determination of the corresponding rate constants. In low-light grown samples, we measured lower ET rates in both directions with respect to high-light ones, which is explained by reduced spectral overlap between B850 and B875 due to partial redistribution of oscillator strength into a higher energetic exciton transition. We find that the low-light adaptation in R. palustris leads to a reduced elementary backward ET rate, in accordance with the low probability of two simultaneous excitations reaching the same LH1/RC complex under weak illumination. Our study suggests that backward ET is not just an inevitable consequence of vectorial ET with small energetic offsets, but is in fact actively managed by photosynthetic bacteria.

Maximizing Performance and Stability of Organic Solar Cells at Low Driving Force for Charge Separation.

Thanks to the development of novel electron acceptor materials, the power conversion efficiencies (PCE) of organic photovoltaic (OPV) devices are now approaching 20%. Further improvement of PCE is complicated by the need for a driving force to split strongly bound excitons into free charges, causing voltage losses. This review discusses recent approaches to finding efficient OPV systems with minimal driving force, combining near unity quantum efficiency (maximum short circuit currents) with optimal energy efficiency (maximum open circuit voltages). The authors discuss apparently contradicting results on the amount of exciton binding in recent literature, and approaches to harmonize the findings. A comprehensive view is then presented on motifs providing a driving force for charge separation, namely hybridization at the donor:acceptor interface and polarization effects in the bulk, of which quadrupole moments (electrostatics) play a leading role. Apart from controlling the energies of the involved states, these motifs also control the dynamics of recombination processes, which are essential to avoid voltage and fill factor losses. Importantly, all motifs are shown to depend on both molecular structure and process conditions. The resulting high dimensional search space advocates for high throughput (HT) workflows. The final part of the review presents recent HT studies finding consolidated structure-property relationships in OPV films and devices from various deposition methods, from research to industrial upscaling.

Bypassing the Single Junction Limit with Advanced Photovoltaic Architectures.

Multijunction devices and photon up- and down-conversion are prominent concepts aimed at increasing photovoltaic efficiencies beyond the single junction limit. Integrating these concepts into advanced architectures may address long-standing issues such as processing complexity, microstructure control, and resilience against spectral changes of the incoming radiation. However, so far, no models have been established to predict the performance of such integrated architectures. Here, a simulation environment based on Bayesian optimization is presented, that can predict and virtually optimize the electrical performance of multi-junction architectures, both vertical and lateral, in combination with up- and down-conversion materials. Microstructure effects on performance are explicitly considered using machine-learned predictive models from high throughput experimentation on simpler architectures. Two architectures that would surpass the single junction limit of photovoltaic energy conversion at reasonable complexity are identified: a vertical "staggered half octave system," where selective absorption allows the use of 6 different bandgaps, and the lateral "overlapping rainbow system" where selective irradiation allows the use of a narrowband energy acceptor with reduced voltage losses, according to the energy gap law. Both architectures would be highly resilient against spectral changes, in contrast with two terminal multi-junction architectures which are limited by Kirchhoff's law.

On the critical competition between singlet exciton decay and free charge generation in non-fullerene based organic solar cells with low energetic offsets.

Reducing voltage losses while maintaining high photocurrents is the holy grail of current research on non-fullerene acceptor (NFA) based organic solar cell. Recent focus lies in understanding the various fundamental mechanisms in organic blends with minimal energy offsets - particularly the relationship between ionization energy offset (ΔIE) and free charge generation. Here, we quantitatively probe this relationship in multiple NFA-based blends by mixing Y-series NFAs with PM6 of different molecular weights, covering a broad power conversion efficiency (PCE) range: from 15% down to 1%. Spectroelectrochemistry reveals that a ΔIE of more than 0.3 eV is necessary for efficient photocurrent generation. Bias-dependent time-delayed collection experiments reveal a very pronounced field-dependence of free charge generation for small ΔIE blends, which is mirrored by a strong and simultaneous field-dependence of the quantified photoluminescence from the NFA local singlet exciton (LE). We find that the decay of singlet excitons is the primary competition to free charge generation in low-offset NFA-based organic solar cells, with neither noticeable losses from charge-transfer (CT) decay nor evidence for LE-CT hybridization. In agreement with this conclusion, transient absorption spectroscopy consistently reveals that a smaller ΔIE slows the NFA exciton dissociation into free charges, albeit restorable by an electric field. Our experimental data align with Marcus theory calculations, supported by density functional theory simulations, for zero-field free charge generation and exciton decay efficiencies. We conclude that efficient photocurrent generation generally requires that the CT state is located below the LE, but that this restriction is lifted in systems with a small reorganization energy for charge transfer.

Polymer-acid-metal quasi-ohmic contact for stable perovskite solar cells beyond a 20,000-hour extrapolated lifetime.

The development of a robust quasi-ohmic contact with minimal resistance, good stability and cost-effectiveness is crucial for perovskite solar cells. We introduce a generic approach featuring a Lewis-acid layer sandwiched between dopant-free semicrystalline polymer and metal electrode in perovskite solar cells, resulting in an ideal quasi-ohmic contact even at elevated temperature up to 85 °C. The solubility of Lewis acid in alcohol facilitates nondestructive solution processing on top of polymer, which boosts hole injection from polymer into metal by two orders of magnitude. By integrating the polymer-acid-metal structure into solar cells, devices exhibit remarkable resilience, retaining 96% ± 3%, 96% ± 2% and 75% ± 7% of their initial efficiencies after continuous operation in nitrogen at 35 °C for 2212 h, 55 °C for 1650 h and 85 °C for 937 h, respectively. Leveraging the Arrhenius relation, we project an impressive T80 lifetime of 26,126 h at 30 °C.

Understanding Causalities in Organic Photovoltaics Device Degradation in a Machine-Learning-Driven High-Throughput Platform.

Organic solar cells (OSCs) now approach power conversion efficiencies of 20%. However, in order to enter mass markets, problems in upscaling and operational lifetime have to be solved, both concerning the connection between processing conditions and active layer morphology. Morphological studies supporting the development of structure-process-property relations are time-consuming, complex, and expensive to undergo and for which statistics, needed to assess significance, are difficult to be collected. This work demonstrates that causal relationships between processing conditions, morphology, and stability can be obtained in a high-throughput method by combining low-cost automated experiments with data-driven analysis methods. An automatic spectral modeling feeds parametrized absorption data into a feature selection technique that is combined with Gaussian process regression to quantify deterministic relationships linking morphological features and processing conditions with device functionality. The effect of the active layer thickness and the morphological order is further modeled by drift-diffusion simulations and returns valuable insight into the underlying mechanisms for improving device stability by tuning the microstructure morphology with versatile approaches. Predicting microstructural features as a function of processing parameters is decisive know-how for the large-scale production of OSCs.

Solvent-dependent activation of intermediate excited states in the energy relaxation pathways of spheroidene.

In carotenoids internal conversion between the allowed (S(2)) and forbidden (S(1)) excited states occurs on a sub-picosecond timescale; the involvement of an intermediate excited state(s) (S(x)) mediating the process is controversial. Here we use high time resolution (sub-20 fs) broadband (1.2-2.5 eV) pump-probe spectroscopy to study the solvent dependence of excited state dynamics of spheroidene, a naturally-occurring carotenoid with ten conjugated double bonds. In the high polarizability solvent, CS(2), we find no evidence of an intermediate state, and the traditional three-level (S(0), S(1), S(2)) model fully accounts for the S(2)→ S(1) process. On the other hand, in the low polarizability solvent, cyclohexane, we find that rapid (~30 fs) relaxation to an intermediate state, S(x), lying between S(1) and S(2) is required to account for the data. We interpret these results as due to a shift of the S(2) energy, which positions the state above or below the energy of S(x) in response to changes in solvent polarizability.

Free-carrier generation in aggregates of single-wall carbon nanotubes by photoexcitation in the ultraviolet regime.

We present evidence for the generation of free carriers in aggregated single-wall carbon nanotubes by photoexcitation in the energetic range of the π→π(*) transition associated with the M saddle point of the graphene lattice. The underlying broad absorption culminating at 4.3 eV can be fit well with a Fano line shape that describes strong coupling of a saddle-point exciton to an underlying free electron-hole pair continuum. Moreover, it is demonstrated that transitions in this energetic region autoionize into the continuum by detecting features unique to the presence of free charges in the transient transmission spectra of the continuum-embedded second sub-band exciton, S(2).

Understanding the Microstructure Formation of Polymer Films by Spontaneous Solution Spreading Coating with a High-Throughput Engineering Platform.

An important step of the great achievement of organic solar cells in power conversion efficiency is the development of low-band gap polymer donors, PBDB-T derivatives, which present interesting aggregation effects dominating the device performance. The aggregation of polymers can be manipulated by a series of variables from a materials design and processing conditions perspective; however, optimization of film quality is a time- and energy-consuming work. Here, we introduce a robot-based high-throughput platform (HTP) that is offering automated film preparation and optical spectroscopy thin-film characterization in combination with an analysis algorithm. PM6 films are prepared by the so-called spontaneous film spreading (SFS) process, where a polymer solution is coated on a water surface. Automated acquisition of UV/Vis and photoluminescence (PL) spectra and automated extraction of morphological features is coupled to Gaussian Process Regression to exploit available experimental evidence for morphology optimization but also for hypothesis formulation and testing with respect to the underlying physical principles. The integrated spectral modeling workflow yields quantitative microstructure information by distinguishing amorphous from ordered phases and assesses the extension of amorphous versus the ordered domains. This research provides an easy to use methodology to analyze the exciton coherence length in conjugated semiconductors and will allow to optimize exciton splitting in thin film organic semiconductor layers as a function of processing.

Adjusting the energy of interfacial states in organic photovoltaics for maximum efficiency.

A critical bottleneck for improving the performance of organic solar cells (OSC) is minimising non-radiative losses in the interfacial charge-transfer (CT) state via the formation of hybrid energetic states. This requires small energetic offsets often detrimental for high external quantum efficiency (EQE). Here, we obtain OSC with both non-radiative voltage losses (0.24 V) and photocurrent losses (EQE > 80%) simultaneously minimised. The interfacial CT states separate into free carriers with ≈40-ps time constant. We combine device and spectroscopic data to model the thermodynamics of charge separation and extraction, revealing that the relatively high performance of the devices arises from an optimal adjustment of the CT state energy, which determines how the available overall driving force is efficiently used to maximize both exciton splitting and charge separation. The model proposed is universal for donor:acceptor (D:A) with low driving forces and predicts which D:A will benefit from a morphology optimization for highly efficient OSC.

Low light adaptation: energy transfer processes in different types of light harvesting complexes from Rhodopseudomonas palustris.

Energy transfer processes in photosynthetic light harvesting 2 (LH2) complexes isolated from purple bacterium Rhodopseudomonas palustris grown at different light intensities were studied by ground state and transient absorption spectroscopy. The decomposition of ground state absorption spectra shows contributions from B800 and B850 bacteriochlorophyll (BChl) a rings, the latter component splitting into a low energy and a high energy band in samples grown under low light (LL) conditions. A spectral analysis reveals strong inhomogeneity of the B850 excitons in the LL samples that is well reproduced by an exponential-type distribution. Transient spectra show a bleach of both the low energy and high energy bands, together with the respective blue-shifted exciton-to-biexciton transitions. The different spectral evolutions were analyzed by a global fitting procedure. Energy transfer from B800 to B850 occurs in a mono-exponential process and the rate of this process is only slightly reduced in LL compared to high light samples. In LL samples, spectral relaxation of the B850 exciton follows strongly nonexponential kinetics that can be described by a reduction of the bleach of the high energy excitonic component and a red-shift of the low energetic one. We explain these spectral changes by picosecond exciton relaxation caused by a small coupling parameter of the excitonic splitting of the BChl a molecules to the surrounding bath. The splitting of exciton energy into two excitonic bands in LL complex is most probably caused by heterogenous composition of LH2 apoproteins that gives some of the BChls in the B850 ring B820-like site energies, and causes a disorder in LH2 structure.

Efficient long-range electron transfer processes in polyfluorene-perylene diimide blends.

In bulk heterojunction donor-acceptor (D-A) blends, high photovoltaic yields require charge carrier separation to outcompete geminate recombination. Recently, evidence for long-range electron transfer mechanisms has been presented, avoiding strongly-bound interfacial charge transfer (CT) states. However, due to the lack of specific optical probes at the D-A interface, a detailed quantification of the long-range processes has not been feasible, until now. Here, we present a transient absorption study of long-range processes in a unique phase consisting of perylene diimide (PDI) crystals intercalated with polyfluorene (PFO), as a widely used non-fullerene electron acceptor and donor, respectively. The intercalated PDI : PFO phase possesses specific well-separated spectral features for the excited states at the D-A interface. By use of femtosecond spectroscopy we reveal the excitation dynamics in this blend. PDI excitons undergo a clear symmetry-breaking charge separation in the PDI bulk, which occurs within several hundred femtoseconds, thus outcompeting excimer formation, known to limit charge separation yields when PDI is used as an acceptor. In contrast, PFO excitons are dissociated with very high yields in a one-step long-range process, enabled by large delocalization of the PFO exciton wavefunction. Moreover, both scenarios circumvent the formation of strongly-bound interfacial CT states and enable a targeted interfacial design for bulk heterojunction blends with near unity charge separation yields.

High-time-resolution pump-probe system with broadband detection for the study of time-domain vibrational dynamics.

We present an ultrafast transient absorption spectroscopy system in the visible combining high-sensitivity broadband detection with extreme temporal resolution. The instrument is based on an ultrabroadband sub-10 fs optical parametric amplifier coupled to an optical multichannel analyzer with fast electronics, enabling single-shot detection at 1 kHz repetition rate. For a given pump-probe delay tau, we achieve a differential transmission (DeltaTT) sensitivity of the order of 10(-4) over the lambda(pr)=490-720 nm probe wavelength range by averaging over 1000 shots, allowing the acquisition of complete two-dimensional DeltaTT (lambda(pr),Tau) maps within a few minute measurement time. We present application examples highlighting the capability of this instrument to observe ultrafast dynamical processes, follow impulsively excited vibrational motions with frequency as high as 3000 cm(-1) (11 fs period), and determine the probe wavelength dependence of amplitude and phase of the oscillations.

Lévy Defects in Matrix-Immobilized J Aggregates: Tracing Intra-and Intersegmental Exciton Relaxation.

One-dimensional J aggregates present narrow and intense absorption and emission spectra that are interesting for photonics applications. Matrix immobilization of the aggregates, as required for most device architectures, has recently been shown to induce a non-Gaussian (Lévy type) defect distribution with heavy tails, expected to influence exciton relaxation. Here we perform two-dimensional electronic spectroscopy (2DES) in one-dimensional J aggregates of the cyanine dye TDBC, immobilized in a gel matrix, and we quantitatively model 2DES maps by nonlinear optimization coupled to quantum mechanical calculations of the transient excitonic response. We find that immobilization causes strong non-Gaussian off-diagonal disorder, leading to a segmentation of the chains. Intersegmental exciton transfer is found to proceed on the picosecond time scale, causing a long-lasting excitation memory. These findings can be used to inform the design of optoelectronic devices based on J aggregates as they allow for control of exciton properties by disorder management.