Ultrafast exciton dissociation followed by nongeminate charge recombination in PCDTBT:PCBM photovoltaic blends

Aggregation promotes charge separation in fullerene-indacenodithiophene dyad

Fast photoinduced charge separation (CS) and long-lived charge-separated state (CSS) in small-molecules facilitate light-energy conversion, while simultaneous attainment of both remains challenging. Here we accomplish this through aggregation based on fullerene-indacenodithiophene dyads. Transient absorption spectroscopy reveals that, compared to solution, the CS time in aggregates is accelerated from 41.5 ps to 0.4 ps, and the CSS lifetime is prolonged from 311.4 ps to 40 μs, indicating that aggregation concomitantly promotes fast CS and long-lived CSS. Fast CS arises from the hot charge-transfer states dissociation, opening up additional resonant channels to free carriers (FCs); subsequently, charge recombination into intramolecular triplet CSS becomes favorable mediated by spin-uncorrelated FCs. Different from fullerene/indacenodithiophene blends, the unique CS mechanism in dyad aggregates reduces the long-lived CSS dependence on molecular order, resulting in a CSS lifetime 200 times longer than blends. This endows the dyad aggregates to exhibit both photoelectronic switch properties and superior photocatalytic capabilities.

Sub-50 fs Formation of Charge Transfer States Rules the Fate of Photoexcitations in Eumelanin-Like Materials

Eumelanins play a crucial role as photoprotective agents for living organisms, yet the nature of the stationary and transient species involved in the light absorption and deactivation processes remains controversial. Moreover, the critical sub-100 fs time scale, which is key to the characterization of the primary excited species, has remained unexplored. Here, we study the eumelanin analogue polydopamine (PDA) and employ a combination of steady-state and transient optical spectroscopies to reveal the presence of spectrally broad coupled electronic transitions with, at least partial, charge-transfer (CT) character. We monitor the CT state dynamics using tunable sub-20 fs pulses. We find that high photon energy excitation results in accelerated (sub-20 fs) CT formation times while activating pathways, which lead to long-lived (≫1 ns), possibly reactive CT species. On the other hand, visible light excitation results in a slower (≈45 fs) formation of bound CT states, which, however, recombine on the ultrafast sub-2 ps time scale.

Stretchable photothermal membrane of NIR-II charge-transfer cocrystal for wearable solar thermoelectric power generation

Harvesting sunlight into cost-effective electricity presents an enticing prospect for self-powered wearable applications. The photothermal materials with an extensive absorption are fundamental to achieve optical and thermal concentration of the sunlight for efficiency output electricity of wearable solar thermoelectric generators (STEGs). Here, we synthesize an organic charge-transfer (CT) cocrystal with a flat absorption from ultraviolet to second near-infrared region (200 to 1950 nanometers) and a high photothermal conversion efficiency (PCE) of 80.5%, which is introduced into polyurethane toward large-area nanofiber membrane by electrospinning technology. These corresponding membranes demonstrate a high PCE of 73.7% under the strain more than 80%. Sandwiched with carbon nanotube-based thermoelectric fibers, the membranes as stretchable solar absorbers of STEGs could supply a notably increase temperature gradient, processing a maximum output voltage density of 23.4 volts per square meter at 1:00 p.m. under sunlight. This strategy presents an important insight in heat management for wearable STEGs with a desired electricity output.

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.

Mastering morphology of non-fullerene acceptors towards long-term stable organic solar cells

Despite the rapid progress of organic solar cells based on non-fullerene acceptors, simultaneously achieving high power conversion efficiency and long-term stability for commercialization requires sustainable research effort. Here, we demonstrate stable devices by integrating a wide bandgap electron-donating polymer (namely PTzBI-dF) and two acceptors (namely L8BO and Y6) that feature similar structures yet different thermal and morphological properties. The organic solar cell based on PTzBI-dF:L8BO:Y6 could achieve a promising efficiency of 18.26% in the conventional device structure. In the inverted structure, excellent long-term thermal stability over 1400 h under 85 °C continuous heating is obtained. The improved performance can be ascribed to suppressed charge recombination along with appropriate charge transport. We find that the morphological features in terms of crystalline coherence length of fresh and aged films can be gradually regulated by the weight ratio of L8BO:Y6. Additionally, the occurrence of melting point decrease and reduced enthalpy in PTzBI-dF:L8BO:Y6 films could prohibit the amorphous phase to cluster, and consequently overcome the energetic traps accumulation aroused by thermal stress, which is a critical issue in high efficiency non-fullerene acceptors-based devices. This work provides insight into understanding non-fullerene acceptors-based organic solar cells for improved efficiency and stability.

Excited-state and charge-carrier dynamics in binary conjugated polymer dots towards efficient photocatalytic hydrogen evolution

Aqueous dispersed conjugated polymer dots (Pdots) have shown promising application in photocatalytic hydrogen evolution. To efficiently extract photogenerated charges from type-II heterojunction Pdots for hydrogen evolution, the mechanistic study of photophysical processes is essential for Pdot optimization. Within this work, we use a PFODTBT donor (D) polymer and an ITIC small molecule acceptor (A) as a donor/acceptor (D/A) model system to study their excited states and charge/energy transfer dynamics via steady-state and time-resolved photoluminescence spectroscopy, respectively. Charge-carrier generation and the recombination dynamics of binary Pdots with different D/A ratios were followed using femtosecond transient absorption spectroscopy. A significant spectral relaxation of photoluminescence was observed for individual D Pdots, implying an energetic disorder by nature. However, this was not seen for charge carriers in binary Pdots, probably due to the ultrafast charge generation process at an early time (<200 fs). The results showed slower charge recombination upon increasing the ratio of ITIC in binary Pdots, which further resulted in an enhanced photocatalytic hydrogen evolution, twice that as compared to individual D Pdots. Although binary Pdots prepared via the nanoprecipitation method exhibit a large interfacial area that allows high charge generation efficiencies, it also provides a high possibility for charge recombination and limits the further utilization of free charges. Therefore, for the future design of type-II heterojunction Pdots, suppressing the charge carrier recombination via increasing the crystallinity and proper phase segregation is necessary for enhanced photocatalytic hydrogen evolution.

Intersystem Crossing in Acceptor-Donor-Acceptor Type Organic Photovoltaic Molecules Promoted by Symmetry Breaking in Polar Environments

The intramolecular electron push-pulling effect has been widely applied to manipulate the excited states in organic photovoltaic (OPV) molecules toward efficient photocurrent generation in working devices with bias fields. However, the effect of field induced polar environments on the excited-state dynamics remains largely unexplored. Here, we investigate the polar environment effect on excited dynamics in acceptor-donor-acceptor type OPV molecules dissolved in solvents with different polarities. By combining ultrafast transient absorption spectroscopy and quantum chemical computation, we observe the stabilization of excited states induced by symmetry breaking in the polar solvent in the molecules exhibiting strong electron push-pulling effects. The stabilized excited states undergo faster intersystem crossing processes with reduced singlet-triplet energy gaps. The findings suggest that the dynamics of charge generation and recombination may be controlled by manipulating the polar environment and electron push-pulling effect to improve the device performance.

The photoprotection mechanism in the black-brown pigment eumelanin

The natural black-brown pigment eumelanin protects humans from high-energy UV photons by absorbing and rapidly dissipating their energy before proteins and DNA are damaged. The extremely weak fluorescence of eumelanin points toward nonradiative relaxation on the timescale of picoseconds or shorter. However, the extreme chemical and physical complexity of eumelanin masks its photoprotection mechanism. We sought to determine the electronic and structural relaxation pathways in eumelanin using three complementary ultrafast optical spectroscopy methods: fluorescence, transient absorption, and stimulated Raman spectroscopies. We show that photoexcitation of chromophores across the UV-visible spectrum rapidly generates a distribution of visible excitation energies via ultrafast internal conversion among neighboring coupled chromophores, and then all these excitations relax on a timescale of ∼4 ps without transferring their energy to other chromophores. Moreover, these picosecond dynamics are shared by the monomeric building block, 5,6-dihydroxyindole-2-carboxylic acid. Through a series of solvent and pH-dependent measurements complemented by quantum chemical modeling, we show that these ultrafast dynamics are consistent with the partial excited-state proton transfer from the catechol hydroxy groups to the solvent. The use of this multispectroscopic approach allows the minimal functional unit in eumelanin and the role of exciton coupling and excited-state proton transfer to be determined, and ultimately reveals the mechanism of photoprotection in eumelanin. This knowledge has potential for use in the design of new soft optical components and organic sunscreens.

Photocatalytic Water-Splitting by Organic Conjugated Polymers: Opportunities and Challenges

The future challenges associated with the shortage of fossil fuels and their current environmental impacts intrigued the researchers to look for alternative ways of generating green energy. Solar-driven water splitting into oxygen and hydrogen is one of those advanced strategies. Researchers have studied various semiconductor materials to achieve potential results. However, it encountered multiple challenges such as high cost, low photostability and efficiency, and required multistep modifications. The conjugated polymers (CPs) have emerged as promising alternatives for conventional inorganic semiconductors. The CPs offer low cost, sufficient light absorption efficiency, excellent photo and chemical stability, and molecular optoelectronic tunable characteristics. Furthermore, organic CPs also present higher flexibility to tune the basic framework of the backbone of the polymers, amendments in the sidechain to incorporate desired functionalities, and much-needed porosity to serve better for photocatalytic applications. This review article summarizes the recent advancements made in visible-light-driven water splitting covering the aspects of synthetic strategies and experimental parameters employed for water splitting reactions with special emphasis on conjugated polymers such as linear CPs, planarized CPs, graphitic carbon nitride (g-C₃ N₄ ), conjugated microporous polymers (CMPs), covalent organic frameworks (COFs), and conjugated polymer-based nanocomposites (CPNCs). The current challenges and future prospects have also been described briefly.

Diradical-Featured Organic Small-Molecule Photothermal Material with High-Spin State in Dimers for Ultra-Broadband Solar Energy Harvesting

Organic materials with radical characteristics are gaining increasing attention, due to their potential implications in highly efficient utilization of solar energy. Manipulating intermolecular interactions is crucial for tuning radical properties, as well as regulating their absorption bands, and thus improving the photothermal conversion efficiency. Herein, a diradical-featured organic small-molecule croconium derivative, CR-DPA-T, is reported for highly efficient utilization of solar energy. Upon aggregation, CR-DPA-T exists in dimer form, stabilized by the strong intermolecular π-π interactions, and exhibits a rarely reported high-spin state. Benefiting from the synergic effects of radical characteristics and strong intermolecular π-π interactions, CR-DPA-T powder absorbs broadly from 300 to 2000 nm. In-depth investigations with transient absorption analysis reveal that the strong intermolecular π-π interactions can promote nonradiative relaxation by accelerating internal conversion and facilitating intermolecular charge transfer (ICT) between dimeric molecules to open up faster internal conversion pathways. Remarkably, CR-DPA-T powder demonstrates a high photothermal efficiency of 79.5% under 808 nm laser irradiation. By employing CR-DPA-T as a solar harvester, a CR-DPA-T-loaded flexible self-healing poly(dimethylsiloxane) (H-PDMS) film, named as H-PDMS/CR-DPA-T self-healing film, is fabricated and employed for solar-thermal applications. These findings provide a feasible guideline for developing highly efficient diradical-featured organic photothermal materials.

Solution-Processed, Inverted AgBiS2 Nanocrystal Solar Cells

AgBiS₂ nanocrystals are a promising nontoxic alternative to PbS, CsPbI₃, and CdS quantum dots for solution-fabricated nanocrystal photovoltaics. In this work, we fabricated the first inverted (p-i-n) structure AgBiS₂ nanocrystal solar cells. We selected spray-coated NiO as the hole-transporting material and used PCBM/BCP as the electron-transporting material. Combining transient photocurrent and photovoltage measurements with femtosecond transient absorption spectroscopy, we investigated the charge collection process on metal oxide/AgBiS₂ interfaces and demonstrated that the NiO/AgBiS₂ NC junction in the p-i-n configuration is more efficient for charge carrier collection. The fabricated p-i-n solar cells exhibited a 4.3% power conversion efficiency (PCE), which was higher than that of conventional n-i-p solar cells fabricated using the same sample. Additionally, inverted devices showed an ultrahigh short-circuit current (J_SC) over 20.7 mA cm^-2 and 0.38 V open-circuit voltage (V_OC), suggesting their potential for further improvements in efficiency and, eventually, for large-scale production.

A Comparison of Charge Carrier Dynamics in Organic and Perovskite Solar Cells

The charge carrier dynamics in organic solar cells and organic-inorganic hybrid metal halide perovskite solar cells, two leading technologies in thin-film photovoltaics, are compared. The similarities and differences in charge generation, charge separation, charge transport, charge collection, and charge recombination in these two technologies are discussed, linking these back to the intrinsic material properties of organic and perovskite semiconductors, and how these factors impact on photovoltaic device performance is elucidated. In particular, the impact of exciton binding energy, charge transfer states, bimolecular recombination, charge carrier transport, sub-bandgap tail states, and surface recombination is evaluated, and the lessons learned from transient optical and optoelectronic measurements are discussed. This perspective thus highlights the key factors limiting device performance and rationalizes similarities and differences in design requirements between organic and perovskite solar cells.

Fabrication of PCDTBT Conductive Network via Phase Separation

Poly[N-9'-hepta-decanyl-2,7-carbazole-alt-5-5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) is a stable semiconducting polymer with high rigidity in its molecular chains, which makes it difficult to organize into an ordered structure and affects the device performance. Here, a PCDTBT network consisting of aggregates and nanofibers in thin films was fabricated through the phase separation of mixed PCDTBT and polyethylene glycol (PEG). Using atomic force microscopy (AFM), the effect of the blending conditions (weight ratio, solution concentration, and molecular weight) and processing conditions (substrate temperature and solvent) on the resulting phase-separated morphologies of the blend films after a selective washing procedure was studied. It was found that the phase-separated structure's transition from an island to a continuous structure occurred when the weight ratio of PCDTBT/PEG changed from 2:8 to 7:3. Increasing the solution concentration from 0.1 to 3.0 wt% led to an increase in both the height of the PCDTBT aggregate and the width of the nanofiber. When the molecular weight of the PEG was increased, the film exhibited a larger PCDTBT aggregate size. Meanwhile, denser nanofibers were found in films prepared using PCDTBT with higher molecular weight. Furthermore, the electrical characteristics of the PCDTBT network were measured using conductive AFM. Our findings suggest that phase separation plays an important role in improving the molecular chain diffusion rate and fabricating the PCDTBT network.

Intrinsic photogeneration of long-lived charges in a donor-orthogonal acceptor conjugated polymer

Efficient charge photogeneration in conjugated polymers typically requires the presence of a second component to act as electron acceptor. Here, we report a novel low band-gap conjugated polymer with a donor/orthogonal acceptor motif: poly-2,6-(4,4-dihexadecyl-4H-cyclopenta [2,1-b:3,4-b']dithiophene)-alt-2,6-spiro [cyclopenta[2,1-b:3,4-b']dithiophene-4,9'-fluorene]-2',7'-dicarbonitrile, referred to as PCPDT-sFCN. The role of the orthogonal acceptor is to spatially isolate the LUMO from the HOMO, allowing for negligible exchange energy between electrons in these orbitals and minimising the energy gap between singlet and triplet charge transfer states. We employ ultrafast and microsecond transient absorption spectroscopy to demonstrate that, even in the absence of a separate electron acceptor, PCPDT-sFCN shows efficient charge photogeneration in both pristine solution and film. This efficient charge generation is a result of an isoenergetic singlet/triplet charge transfer state equilibrium acting as a reservoir for charge carrier formation. Furthermore, clear evidence of enhanced triplet populations, which form in less than 1 ps, is observed. Using group theory, we show that this ultrafast triplet formation is due to highly efficient, quantum mechanically allowed intersystem crossing between the bright, initially photoexcited local singlet state and the triplet charge transfer state. Remarkably, the free charges that form via the charge transfer state are extraordinarily long-lived with millisecond lifetimes, possibly due to the stabilisation imparted by the spatial separation of PCPDT-sFCN's donor and orthogonal acceptor motifs. The efficient generation of long-lived charge carriers in a pristine polymer paves the way for single-material applications such as organic photovoltaics and photodetectors.

Identification of a bridge-specific intramolecular exciton dissociation pathway in donor-π-acceptor alternating conjugated polymers

Intramolecular exciton dissociation is critical for high efficient mobile charge carrier generations in organic solar cells. Yet despite much attention, the effects of π bridges on exciton dissociation dynamics in donor-π-acceptor (D-π-A) alternating conjugated polymers remain still unclear. Here, using a combination of femtosecond time-resolved transient absorption (TA) spectroscopy and steady-state spectroscopy, we track ultrafast intramolecular exciton relaxation dynamics in three D-π-A alternating conjugated polymers which were synthesized by Qin's group and named HSD-A, HSD-B, HSD-C. It is found that the addition of thiophene unit as π bridges will lead to the red shift of steady-state absorption spectrum. Importantly, we reveal the existence of a new intramolecular exciton dissociation pathway mediated by a bridge-specific charge transfer (CT') state with the TA fingerprint peak at 1200 nm in π-bridged HSD-B and HSD-C. This CT' state results in higher electron capture rates for HSD-B and HSD-C as compared to HSD-A. Depending on the proportion of CT' state and nongeminate recombination are important step for the understanding of high power conversion efficiencies in HSD-B than in HSD-C. We propose that this bridge-specific exciton dissociation pathway plays an important role in ultrafast intramolecular exciton dissociation of organic photovoltaic material D-π-A alternating conjugated polymers.

Optical Gain in Semiconducting Polymer Nano and Mesoparticles

The presence of excited-states and charge-separated species was identified through UV and visible laser pump and visible/near-infrared probe femtosecond transient absorption spectroscopy in spin coated films of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) nanoparticles and mesoparticles. Optical gain in the mesoparticle films is observed after excitation at both 400 and 610 nm. In the mesoparticle film, charge generation after UV excitation appears after around 50 ps, but little is observed after visible pump excitation. In the nanoparticle film, as for a uniform film of the pure polymer, charge formation was efficiently induced by UV excitation pump, while excitation of the low energetic absorption states (at 610 nm) induces in the nanoparticle film a large optical gain region reducing the charge formation efficiency. It is proposed that the different intermolecular interactions and molecular order within the nanoparticles and mesoparticles are responsible for their markedly different photophysical behavior. These results therefore demonstrate the possibility of a hitherto unexplored route to stimulated emission in a conjugated polymer that has relatively undemanding film preparation requirements.

Charge Transfer Excitation and Asymmetric Energy Transfer at the Interface of Pentacene-Perfluoropentacene Heterostacks

High-performance solar cells demand efficient charge-carrier excitation, separation, and extraction. These requirements hold particularly true for molecular photovoltaics, where large exciton binding energies render charge separation challenging at their commonly complex donor-acceptor interface structure. Among others, charge-transfer (CT) states are considered to be important precursors for exciton dissociation and charge separation. However, the general nature of CT excitons and their formation pathways remain unclear. Layered quasiplanar crystalline molecular heterostructures of the prototypical donor-acceptor system pentacene-perfluoropentacene studied at cryogenic temperatures are a paramount model system to gain insights into the underlying physical mechanism. In particular, a detailed experiment-theory analysis on a layered heterojunction featuring perfluoropentacene in its π-stacked polymorph and pentacene in the Siegrist phase indicates that exciton diffusion in unitary films can influence the formation efficiency of CT excitons localized at internal interfaces for these conditions. The correlation of the structural characteristics, that is, the molecular arrangement at the interfaces, with their absorption and photoluminescence excitation spectra is consistent with exciton transfer from pentacene to the CT exciton state only, whereas no transfer of excitons from the perfluoropentacene is detected. Electronic structure calculations of the model systems and investigation of coupling matrix elements between the various electronic states involved suggest hampered exciton diffusion toward the internal interface in the perfluoropentacene films. The asymmetric energy landscape around an idealized internal donor-acceptor interface thus is identified as a reason for asymmetric energy transfer. Thus, long-range effects apparently can influence charge separation in crystalline molecular heterostructures, similar to band gap bowing, which is well established for inorganic pn-junctions.

Ultrafast Charge Generation Enhancement in Nanoscale Polymer Solar Cells with DIO Additive

We study the ultrafast photoexcitation dynamics in PBDTTT-C-T (P51, poly(4,8-bis(5-(2-ethylhexyl)-thiophene-2-yl)-benzo[1,2-b:4,5-b']dithiophene-alt-alkylcarbonyl-thieno[3,4-b]thiophene)) film (~100 nm thickness) and PBDTTT-C-T:PC71BM (P51:PC71BM, phenyl-C71-butyric-acid-methyl ester) nanostructured blend (∼100 nm thickness) with/without DIO(1,8-diiodooctane) additives with sub-10 fs transient absorption (TA). It is revealed that hot-exciton dissociation and vibrational relaxation could occur in P51 with a lifetime of ~160 fs and was hardly affected by DIO. However, the introduction of DIO in P51 brings a longer lifetime of polaron pairs, which could make a contribution to photocarrier generation. In P51:PC71BM nanostructured blends, DIO could promote the Charge Transfer (CT) excitons and free charges generation with a ~5% increasement in ~100 fs. Moreover, the dissociation of CT excitons is faster with DIO, showing a ~5% growth within 1 ps. The promotion of CT excitons and free charge generation by DIO additive is closely related with active layer nanomorphology, accounting for Jsc enhancement. These results reveal the effect of DIO on carrier generation and separation, providing an effective route to improve the efficiency of nanoscale polymer solar cells.

The Role of Delocalization and Excess Energy in the Quantum Efficiency of Organic Solar Cells and the Validity of Optical Reciprocity Relations

The photon energy dependence of long-range charge separation is studied for two prototypical polymer:fullerene systems. The internal quantum efficiency (IQE) of PCDTBT:PC₆₁BM is experimentally shown to be independent of the excitation energy. In contrast, for TQ1:PC₇₁BM the IQE is strongly energy-dependent for excitation energies close to charge transfer (CT) electroluminescence peak maximum while it becomes energy-independent at higher excitation energies. Kinetic Monte Carlo simulations reproduce the experimental IQE and reveal that the photon energy-dependence of the IQE is governed by charge delocalization. Efficient long-range separation at excitation energies corresponding to the CT electroluminescence peak maximum or lower requires an initial separation of the hole-electron pair by ∼4-5 nm, whereas delocalization is less important for charge separation at higher photon energies. Our modeling results suggest that a phenomenological reciprocity between CT electroluminescence and external quantum efficiency does not necessarily prove that commonly employed reciprocity relations between these spectra are valid from a fundamental perspective.

Buildup of Triplet-State Population in Operating TQ1:PC71BM Devices Does Not Limit Their Performance

Triplet generation in organic solar cells has been considered a major loss channel. Determining the density of the triplet-state population in an operating device is challenging. Here, we employ transient absorption (TA) spectroscopy on the quinoxaline-thiophene copolymer TQ1 blended with PC₇₁BM, quantify the transient charge and triplet-state densities, and parametrize their generation and recombination dynamics. The charge recombination parameters reproduce the experimentally measured current-voltage characteristics in charge carrier drift-diffusion simulations, and they yield the steady-state charge densities. We demonstrate that triplets are formed by both geminate and nongeminate recombination of charge carriers and decay primarily by triplet-triplet annihilation. Using the charge densities in the rate equations describing triplet-state dynamics, we find that triplet-state densities in devices are in the range of charge carrier densities. Despite this substantial triplet-state buildup, TQ1:PC₇₁BM devices exhibit only moderate geminate recombination and significantly reduced nongeminate charge recombination, with reduction factors between 10^-4 and 10^-3 compared to Langevin recombination.

Geminate recombination in organic photovoltaic blend PCDTBT/PC71BM studied by out-of-phase electron spin echo spectroscopy

The key process in organic solar cell operation is charge separation under light illumination. Due to the low dielectric constant of organic materials, the Coulomb attraction energy within the interfacial charge-transfer state (CTS) is larger than the thermal energy. Understanding the mechanism of charge separation at the organic donor/acceptor interface still remains a challenge and requires knowledge of the CTS temporal evolution. To address this problem, the CTS in the benchmark photovoltaic blend PCDTBT/PC₇₁BM was studied by the out-of-phase Electron Spin Echo (ESE). The protocol for determining the CTS geminate recombination rate for certain electron-hole distances was developed. Simulating the out-of-phase ESE trace for the CTS in the PCDTBT/PC₇₁BM blend allows precise determination of the electron-hole distance distribution function and its evolution with the increase in the delay after the laser flash. Distances of charge separation up to 6 nm were detected upon thermalization at a temperature of 20 K. Assuming the exponential decay of the recombination rate, the attenuation factor β = 0.08 Å^-1 is estimated for the PCDTBT/PC₇₁BM blend. Such a low attenuation factor is probably caused by a high degree of hole delocalization along the PCDTBT chain.

Subtle Molecular Tailoring Induces Significant Morphology Optimization Enabling over 16% Efficiency Organic Solar Cells with Efficient Charge Generation

Manipulating charge generation in a broad spectral region has proved to be crucial for nonfullerene-electron-acceptor-based organic solar cells (OSCs). 16.64% high efficiency binary OSCs are achieved through the use of a novel electron acceptor AQx-2 with quinoxaline-containing fused core and PBDB-TF as donor. The significant increase in photovoltaic performance of AQx-2 based devices is obtained merely by a subtle tailoring in molecular structure of its analogue AQx-1. Combining the detailed morphology and transient absorption spectroscopy analyses, a good structure-morphology-property relationship is established. The stronger π-π interaction results in efficient electron hopping and balanced electron and hole mobilities attributed to good charge transport. Moreover, the reduced phase separation morphology of AQx-2-based bulk heterojunction blend boosts hole transfer and suppresses geminate recombination. Such success in molecule design and precise morphology optimization may lead to next-generation high-performance OSCs.

Impact of the donor polymer on recombination via triplet excitons in a fullerene-free organic solar cell

The greater chemical tunability of non-fullerene acceptors enables fine-tuning of the donor-acceptor energy level offsets, a promising strategy towards increasing the open-circuit voltage in organic solar cells. Unfortunately, this approach could open an additional recombination channel for the charge-transfer (CT) state via a lower-lying donor or acceptor triplet level. In this work we investigate such electron and hole back-transfer mechanisms in fullerene-free solar cells incorporating the novel molecular acceptor 2,4-diCN-Ph-DTTzTz. The transition to the low-driving force regime is studied by comparing blends with well-established donor polymers P3HT and MDMO-PPV, which allows for variation of the energetic offsets at the donor-acceptor interface. Combining various optical spectroscopic techniques, the CT process and subsequent triplet formation are systematically investigated. Although both back-transfer mechanisms are found to be energetically feasible in both blends, markedly different triplet-mediated recombination processes are observed for the two systems. The kinetic suppression of electron back-transfer in the blend with P3HT suggests that energy losses due to triplet formation on the polymer can be avoided, regardless of favorable energetic alignment.

Fluorescent PCDTBT Nanoparticles with Tunable Size for Versatile Bioimaging

Conjugated polymer nanoparticles exhibit very interesting properties for use as bio-imaging agents. In this paper, we report the synthesis of PCDTBT (poly([9-(1'-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophene-diyl)) nanoparticles of varying sizes using the mini-emulsion and emulsion/solvent evaporation approach. The effect of the size of the particles on the optical properties is investigated using UV-Vis absorption and fluorescence emission spectroscopy. It is shown that PCDTBT nanoparticles have a fluorescence emission maximum around 710 nm, within the biological near-infrared "optical window". The photoluminescence quantum yield shows a characteristic trend as a function of size. The particles are not cytotoxic and are taken up successfully by human lung cancer carcinoma A549 cells. Irrespective of the size, all particles show excellent fluorescent brightness for bioimaging. The fidelity of the particles as fluorescent probes to study particle dynamics in situ is shown as a proof of concept by performing raster image correlation spectroscopy. Combined, these results show that PCDTBT is an excellent candidate to serve as a fluorescent probe for near-infrared bio-imaging.

Size-Dependent Photophysical Behavior of Low Bandgap Semiconducting Polymer Particles

The photophysics of water and propan-1-ol suspensions of poly [N-9"-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2',1',3'- benzothiadiazole)] (PCDTBT) nanoparticles and mesoparticles has been studied by ultrafast spectroscopy. High molar mass polymer (HMM > 20 kg/mol) forms nanoparticles with around 50 nm diameter via mini-emulsion post-polymerization, while low molar mass (LMM < 5 kg/mol) polymer prepared by dispersion polymerization results in particles with a diameter of almost one order of magnitude larger (450 ± 50 nm). In this study, the presence of excited-states and charge separated species was identified through UV pump and visible/near-infrared probe femtosecond transient absorption spectroscopy. A different behavior for the HMM nanoparticles has been identified compared to the LMM mesoparticles. The nanoparticles exhibit typical features of an energetically disordered conjugated polymer with a broad density of states, allowing for delayed spectral relaxation of excited states, while the mesoparticles show a J-aggregate-like behavior where interchain interactions are less efficient. Stimulated emission in the red-near infrared region has been found in the mesoparticles which indicates that they present a more energetically ordered system.

Probing charge transfer states at organic and hybrid internal interfaces by photothermal deflection spectroscopy

In organic and hybrid photovoltaic devices, the asymmetry required for charge separation necessitates the use of a donor and an acceptor material, resulting in the formation of internal interfaces in the device active layer. While the core objective of these interfaces is to facilitate charge separation, bound states between electrons and holes may form across them, resulting in a loss mechanism that diminishes the performance of the solar cells. These interfacial transitions appear in organic systems as charge transfer (CT) states and as bound charge pairs (BCP) in hybrid systems. Despite being similar, the latter are far less investigated. Herein, we employ photothermal deflection spectroscopy and pump-push-probe experiments in order to determine the characteristics and dynamics of interfacial states in two model systems: an organic P3HT:PCBM and hybrid P3HT:ZnO photovoltaic layer. By controlling the area of the internal interface, we identify CT states between 1.4 eV and 1.8 eV in the organic bulk-heterojunction (BHJ) and BCP between 1.1 eV and 1.4 eV in the hybrid BHJ. The energetic distribution of these states suggests that they not only contribute to losses in photocurrent, but also significantly limit the possible maximum open circuit voltage obtainable from these devices.

Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends

Despite performance improvements of organic photovoltaics, the mechanism of photoinduced electron–hole separation at organic donor–acceptor interfaces remains poorly understood. Inconclusive experimental and theoretical results have produced contradictory models for electron–hole separation in which the role of interfacial charge-transfer (CT) states is unclear, with one model identifying them as limiting separation and another as readily dissociating. Here, polymer–fullerene blends with contrasting photocurrent properties and enthalpic offsets driving separation were studied. By modifying composition, film structures were varied from consisting of molecularly mixed polymer–fullerene domains to consisting of both molecularly mixed and fullerene domains. Transient absorption spectroscopy revealed that CT state dissociation generating separated electron–hole pairs is only efficient in the high energy offset blend with fullerene domains. In all other blends (with low offset or predominantly molecularly mixed domains), nanosecond geminate electron–hole recombination is observed revealing the importance of spatially localized electron–hole pairs (bound CT states) in the electron–hole dynamics. A two-dimensional lattice exciton model was used to simulate the excited state spectrum of a model system as a function of microstructure and energy offset. The results could reproduce the main features of experimental electroluminescence spectra indicating that electron–hole pairs become less bound and more spatially separated upon increasing energy offset and fullerene domain density. Differences between electroluminescence and photoluminescence spectra could be explained by CT photoluminescence being dominated by more-bound states, reflecting geminate recombination processes, while CT electroluminescence preferentially probes less-bound CT states that escape geminate recombination. These results suggest that apparently contradictory studies on electron–hole separation can be explained by the presence of both bound and unbound CT states in the same film, as a result of a range of interface structures.

Over 12% Efficiency Nonfullerene All-Small-Molecule Organic Solar Cells with Sequentially Evolved Multilength Scale Morphologies

In this paper, two near-infrared absorbing molecules are successfully incorporated into nonfullerene-based small-molecule organic solar cells (NFSM-OSCs) to achieve a very high power conversion efficiency (PCE) of 12.08%. This is achieved by tuning the sequentially evolved crystalline morphology through combined solvent additive and solvent vapor annealing, which mainly work on ZnP-TBO and 6TIC, respectively. It not only helps improve the crystallinity of the ZnP-TBO and 6TIC blend, but also forms multilength scale morphology to enhance charge mobility and charge extraction. Moreover, it simultaneously reduces the nongeminate recombination by effective charge delocalization. The resultant device performance shows remarkably enhanced fill factor and J_sc . These result in a very respectable PCE, which is the highest among all NFSM-OSCs and all small-molecule binary solar cells reported so far.

Energy-Gap Law for Photocurrent Generation in Fullerene-Based Organic Solar Cells: The Case of Low-Donor-Content Blends

The involvement of charge-transfer (CT) states in the photogeneration and recombination of charge carriers has been an important focus of study within the organic photovoltaic community. In this work, we investigate the molecular factors determining the mechanism of photocurrent generation in low-donor-content organic solar cells, where the active layer is composed of vacuum-deposited C₆₀ and small amounts of organic donor molecules. We find a pronounced decline of all photovoltaic parameters with decreasing CT state energy. Using a combination of steady-state photocurrent measurements and time-delayed collection field experiments, we demonstrate that the power conversion efficiency, and more specifically, the fill factor of these devices, is mainly determined by the bias dependence of photocurrent generation. By combining these findings with the results from ultrafast transient absorption spectroscopy, we show that blends with small CT energies perform poorly because of an increased nonradiative CT state decay rate and that this decay obeys an energy-gap law. Our work challenges the common view that a large energy offset at the heterojunction and/or the presence of fullerene clusters guarantee efficient CT dissociation and rather indicates that charge generation benefits from high CT state energies through a slower decay to the ground state.

Ultrafast hole transfer mediated by polaron pairs in all-polymer photovoltaic blends

The charge separation yield at a bulk heterojunction sets the upper efficiency limit of an organic solar cell. Ultrafast charge transfer processes in polymer/fullerene blends have been intensively studied but much less is known about these processes in all-polymer systems. Here, we show that interfacial charge separation can occur through a polaron pair-derived hole transfer process in all-polymer photovoltaic blends, which is a fundamentally different mechanism compared to the exciton-dominated pathway in the polymer/fullerene blends. By utilizing ultrafast optical measurements, we have clearly identified an ultrafast hole transfer process with a lifetime of about 3 ps mediated by photo-excited polaron pairs which has a markedly high quantum efficiency of about 97%. Spectroscopic data show that excitons act as spectators during the efficient hole transfer process. Our findings suggest an alternative route to improve the efficiency of all-polymer solar devices by manipulating polaron pairs.

All-polymer solar cells have shown high efficiencies but the ultrafast charge transfer processes are less known. Here Wang et al. show that polaron pairs play vital role facilitating the hole transfer, which is quite different from the exciton dominated pathway in polymer-fullerene blends.

Long-Lived, Non-Geminate, Radiative Recombination of Photogenerated Charges in a Polymer/Small-Molecule Acceptor Photovoltaic Blend

Minimization of open-circuit-voltage ( V_OC) loss is required to transcend the efficiency limitations on the performance of organic photovoltaics (OPV). We study charge recombination in an OPV blend comprising a polymer donor with a small molecule nonfullerene acceptor that exhibits both high photovoltaic internal quantum efficiency and relatively high external electroluminescence quantum efficiency. Notably, this donor/acceptor blend, consisting of the donor polymer commonly referred to as PCE10 with a pseudoplanar small molecule acceptor (referred to as FIDTT-2PDI) exhibits relatively bright delayed photoluminescence on the microsecond time scale beyond that observed in the neat material. We study the photoluminescence decay kinetics of the blend in detail and conclude that this long-lived photoluminescence arises from radiative nongeminate recombination of charge carriers, which we propose occurs via a donor/acceptor CT state located close in energy to the singlet state of the polymer donor. Additionally, crystallographic and spectroscopic studies point toward low subgap disorder, which could be beneficial for low radiative and nonradiative losses. These results provide an important demonstration of photoluminescence due to nongeminate charge recombination in an efficient OPV blend, a key step in identifying new OPV materials and materials-screening criteria if OPV is to approach the theoretical limits to efficiency.

Enhancement of the Power-Conversion Efficiency of Organic Solar Cells via Unveiling an Appropriate Rational Design Strategy in Indacenodithiophene- alt-quinoxaline π-Conjugated Polymers

We report on the photovoltaic parameters, photophysical properties, optoelectronic properties, self-assembly, and morphology variations in a series of high-performance donor-acceptor (D-A) π-conjugated polymers based on indacenodithiophene and quinoxaline moieties as a function of the number-average molecular weight ([Formula: see text]), the nature of aryl substituents, and the enlargement of the polymer backbone. One of the most important outcome is that from the three optimization approaches followed to tune the chemical structure toward enhanced photovoltaic performance in bulk heterojunction solar cell devices with the fullerene derivative [6,6]-phenyl-C₇₁-butyric acid methyl ester as the electron acceptor, the choice of the aryl substituent is the most efficient rational design strategy. Incorporation of thienyl rings as substituents versus phenyl rings accelerates the electron-hole extraction process to the respective electrode, despite the slightly lower recombination lifetime and, thus, improves the electrical performance of the device. Single-junction solar cells based on ThIDT-TQxT feature a maximum power-conversion efficiency of 7.26%. This study provides significant insights toward understanding of the structure-properties-performance relationship for D-A π-conjugated polymers in solid state, which provide helpful inputs for the design of next-generation polymeric semiconductors for organic solar cells with enhanced performance.

Hot kinetic model as a guide to improve organic photovoltaic materials

The model yields that the most promising ways to increase the OSC performance are decreasing the reorganization energy, increasing the dielectric permittivity and enhancing the charge delocalization.

Ultrafast Long-Range Charge Separation in Nonfullerene Organic Solar Cells

Rapid, long-range charge separation in polymer-fullerene organic solar cells (OSCs) enables electrons and holes to move beyond their Coulomb capture radius and overcome geminate recombination. Understanding the nature of charge generation and recombination mechanisms in efficient, nonfullerene-acceptor-based OSCs are critical to further improve device performance. Here we report charge dynamics in an OSC using a perylene diimide (PDI) dimer acceptor. We use transient absorption spectroscopy to track the time evolution of electroabsorption caused by the dipolar electric field generated between electron-hole pairs as they separate after ionization at the donor-acceptor interface. We show that charges separate rapidly (<1 ps) and that free charge carriers are generated very efficiently (∼90% quantum yield). However, in the PDI-based OSC, external charge extraction is impaired by faster nongeminate decay to the ground state and to lower-lying triplet states.

Visualizing excitations at buried heterojunctions in organic semiconductor blends

Interfaces play a crucial role in semiconductor devices, but in many device architectures they are nanostructured, disordered and buried away from the surface of the sample. Conventional optical, X-ray and photoelectron probes often fail to provide interface-specific information in such systems. Here we develop an all-optical time-resolved method to probe the local energetic landscape and electronic dynamics at such interfaces, based on the Stark effect caused by electron-hole pairs photo-generated across the interface. Using this method, we found that the electronically active sites at the polymer/fullerene interfaces in model bulk-heterojunction blends fall within the low-energy tail of the absorption spectrum. This suggests that these sites are highly ordered compared with the bulk of the polymer film, leading to large wavefunction delocalization and low site energies. We also detected a 100 fs migration of holes from higher- to lower-energy sites, consistent with these charges moving ballistically into more ordered polymer regions. This ultrafast charge motion may be key to separating electron-hole pairs into free charges against the Coulomb interaction.

Significantly improved photovoltaic performance in polymer bulk heterojunction solar cells with graphene oxide /PEDOT:PSS double decked hole transport layer

This work demonstrates the high performance graphene oxide (GO)/PEDOT:PSS doubled decked hole transport layer (HTL) in the PCDTBT:PC₇₁BM based bulk heterojunction organic photovoltaic device. The devices were tested on merits of their power conversion efficiency (PCE), reproducibility, stability and further compared with the devices with individual GO or PEDOT:PSS HTLs. Solar cells employing GO/PEDOT:PSS HTL yielded a PCE of 4.28% as compared to either of individual GO or PEDOT:PSS HTLs where they demonstrated PCEs of 2.77 and 3.57%, respectively. In case of single GO HTL, an inhomogeneous coating of ITO caused the poor performance whereas PEDOT:PSS is known to be hygroscopic and acidic which upon direct contact with ITO reduced the device performance. The improvement in the photovoltaic performance is mainly ascribed to the increased charge carriers mobility, short circuit current, open circuit voltage, fill factor, and decreased series resistance. The well matched work function of GO and PEDOT:PSS is likely to facilitate the charge transportation and an overall reduction in the series resistance. Moreover, GO could effectively block the electrons due to its large band-gap of ~3.6 eV, leading to an increased shunt resistance. In addition, we also observed the improvement in the reproducibility and stability.