Strong Intermolecular Coupling Research Articles

Organic solar cells and their applications Shah, Razon Oda, F. Hazuan

Enhancing the power conversion efficiency (PCE) is the ma- jor task in the development of solar cells. In 1961, Willi- am Shockley and Hans J. Queisser pointed out that the highest PCE for a single-junction solar cell is limited to 31% due to the thermalization and transmission losses. Singlet fission (SF) is an appealing carrier-multiplication approach to break the Shockley-Queisser limit, since it converts one high- energy singlet exciton into two low-energy triplet excitons. Thus, one absorbed photon generates two electron–hole pairs [1]. SF phenomenon was first observed in anthracene single crystals in 1965. It attracted attention again in 2006. In organic semiconductors, a singlet exciton (S1) forms after the ab- sorption of a photon. If the energy of this singlet exciton is greater than twice of that for triplet exciton (T1), namely E(S1) > 2E(T1), then spin-allowed fission (S1 → 2T 1) could occur. The general kinetic model for SF process is S0 is the ground state, S1 is the excited singlet state, 1(TT) is the intermediate state of a correlated triplet pair, and T1 + T1 are two independent triplet states (Figure 1(a)). The application needs materials with high fission efficiency, suitable bandgap for capturing high-energy photons, appropriate triplet energy and high chemical stability. Owing to the existence of competitive pathways (i.e. S1 to S0 relaxation in picosecond–nanosecond timescale; excimer formation in femtosecond timescale; charge transfer (CT) process in femtosecond timescale, SF needs to occur in pico- second or sub-picosecond timescale. Thus, SF requires strong intermolecular coupling. The intermolecular coupling can be realized via Van der Waals force, weak non-bonded interactions and so on. The coupling distance not only affects SF rate and triplet yield, but also influences the lifetime and diffusion length of triplet excitons [2].

Supramolecular Tuning of Exciton Transport in Pi-Peptide Assemblies

We demonstrate the use of sequence-dependent interactions between peptide-functionalized pi-conjugated pigments to tune exciton transport behavior in their supramolecular assemblies in aqueous solutions. Peptide sequences attached to a perylene diimide (PDI) core were selected based on their ability to foster a wide range of excitonic coupling strengths within organized assemblies, as reflected by experimentally measured steady-state absorption and emission spectra. Photoresponses of organized assemblies of weakly interacting PDIs closely resemble those observed for weakly assembled chromophores. In contrast, organized assemblies that support strong intermolecular coupling exhibit significant excitonic delocalization and transport, as observed through distinct transient signatures of fluence-dependent singlet–singlet annihilation and a short-lived biexcitonic state at high initial exciton densities. A one-dimensional (1D) diffusion model appropriately accounts for exciton–exciton encounters in assemblies, with effective exciton diffusion constants that scale with interchromophore coupling strength, ranging between 0 and 7 sites2/ps (0–1 nm2/ps or LD ranging from 0 to 40 nm). Variations in effective diffusion constant arise from peptide-tuned variations in interchromophore alignment and wavefunction overlap, with mixed Frenkel-CT exciton coupling identified as the key interaction that facilitates site-to-site exciton diffusion within PDI stacks. This work demonstrates the potential to use simple peptide sequence variation as a tool for rational engineering of exciton transport and other excited-state behaviors in supramolecular materials.

Janus dione derivatives: Novel high-mobility hole transport materials for perovskite solar cells

Developing efficient and stable hole-transporting materials (HTMs) is very important for improving the stability and increasing the power conversion efficiency of perovskite solar cells (PSCs). Herein, novel Janus dione based HTMs with X-shaped structure were investigated using quantum chemical calculations. The results showed that introducing appropriate groups on both sides of the core led to a suitable HOMO level, good optical properties, and satisfactory stability. Importantly, due to strong inter-molecular electronic coupling, the calculated hole mobility of JT6, is as high as 7.92 cm2 V−1 s−1, which is 50 times higher than that of Spiro-OMeTAD. Simulating the adsorption properties on perovskite material CH3NH3PbI3 has also shown that these high mobility HTMs are easy to adsorb on the surface of perovskite material and can achieve good charge transfer between surface layers. The present findings highlight the potential of Janus dione core based HTMs for efficient PSCs and provide a meaningful practice for the design of HTMs with controllable HOMO levels.

Thermal annealing effect on non-fused ring acceptor based bulk heterojunction investigated by transient absorption spectroscopy

Organic solar cells based on non-fused ring fullerene acceptors have more potential to realize low-cost fabrication and commercialization. However, the power conversion efficiencies (PCEs) are much lower than those of the fused-ring non-fullerene acceptors. Thermal annealing treatment as a performance optimization way is performed on PBDB-T blend film based on fully non-fused ring acceptor, o-4TBC-2F, and the PCE is enhanced from 7.43% to 10.26%. Herein, transient absorption spectroscopy measurement is conducted to unveil the thermal annealing effect in the perspective of ultrafast dynamics. Two types of J-aggregations are present in the o-4TBC-2F film. Thermal annealing treatment induces more o-4TBC-2F molecules in the form of strong intermolecular coupling J-aggregation with long-range order. The aggregation is less heterogeneous, leading to a slow yet efficient hole transfer process, and the generated hole polarons are delocalized and long-lived. The dynamics results well explained the improved performance after thermal annealing treatment. The observations also show that the aggregation in blends can optimize the charge generation, which can be beneficial for designing highly efficient non-fused ring acceptor-based organic solar cells.

Thermal Kinetics of Energetic CL-20/BTF Cocrystal Induced by Strong Intermolecular Coupling

Thermal Kinetics of Energetic CL-20/BTF Cocrystal Induced by Strong Intermolecular Coupling

Red aqueous room-temperature phosphorescence modulated by anion-π and intermolecular electronic coupling interactions.

Aqueous room temperature phosphorescence (aRTP) from purely organic materials has been intriguing but challenging. In this article, we demonstrated that the red aRTP emission of 2Br–NDI, a water-soluble 4,9-dibromonaphthalene diimide derivative as a chloride salt, could be modulated by anion–π and intermolecular electronic coupling interactions in water. Specifically, the rarely reported stabilization of anion–π interactions in water between Cl− and the 2Br–NDI core was experimentally evidenced by an anion–π induced long-lived emission (λAnion–π) of 2Br–NDI, acting as a competitive decay pathway against the intrinsic red aRTP emission (λPhos) of 2Br–NDI. In the initial expectation of enhancing the aRTP of 2Br–NDI by inclusion complexation with macrocyclic cucurbit[n]urils (CB[n]s, n = 7, 8, 10), we surprisingly found that the exclusion complexation between CB[8] and 2Br–NDI unconventionally endowed the complex with the strongest and longest-lived aRTP due to the strong intermolecular electronic coupling between the nπ* orbit on the carbonyl rims of CB[8] and the ππ* orbit on 2Br–NDI in water. It is anticipated that these intriguing findings may inspire and expand the exploration of aqueous anion–π recognition and CB[n]-based aRTP materials.

Effect of Strong Intermolecular Interaction in 2D Inorganic Molecular Crystals.

Strong intermolecular interactions in 2D organic molecular crystals arising from π-π stacking have been widely explored to achieve high thermal stability, high carrier mobility, and novel physical properties, which have already produced phenomenal progress. However, strong intermolecular interactions in 2D inorganic molecular crystals (2DIMCs) have rarely been investigated, severely limiting both the fundamental research in molecular physics and the potential applications of 2DIMCs for optoelectronics. Here, the effect of strong intermolecular interactions induced by unique short intermolecular Se-Se and P-Se contacts in 2D α-P4Se3 nanoflakes is reported. On the basis of theoretical calculations of the charge density distribution and an analysis of the thermal expansion and plastic-crystal transition, the physical picture of strong intermolecular interactions can be elucidated as a higher charge density between adjacent P4Se3 molecules, arising from an orderly and close packing of P4Se3 molecules. More importantly, encouraged by the strong intermolecular coupling, the in-plane mobility of α-P4Se3 nanoflakes is first calculated with a quantum nuclear tunneling model, and a competitive hole mobility of 0.4 cm2 V-1 s-1 is obtained. Our work sheds new light on the intermolecular interactions in 2D inorganic molecular crystals and is highly significant for promoting the development of molecular physics and optoelectronics.

Modular Assembly of Vibrationally and Electronically Coupled Rhenium Bipyridine Carbonyl Complexes on Silicon.

Hybrid inorganic/organic heterointerfaces are promising systems for next-generation photocatalytic, photovoltaic, and chemical-sensing applications. Their performance relies strongly on the development of robust and reliable surface passivation and functionalization protocols with (sub)molecular control. The structure, stability, and chemistry of the semiconductor surface determine the functionality of the hybrid assembly. Generally, these modification schemes have to be laboriously developed to satisfy the specific chemical demands of the semiconductor surface. The implementation of a chemically independent, yet highly selective, standardized surface functionalization scheme, compatible with nanoelectronic device fabrication, is of utmost technological relevance. Here, we introduce a modular surface assembly (MSA) approach that allows the covalent anchoring of molecular transition-metal complexes with sub-nanometer precision on any solid material by combining atomic layer deposition (ALD) and selectively self-assembled monolayers of phosphonic acids. ALD, as an essential tool in semiconductor device fabrication, is used to grow conformal aluminum oxide activation coatings, down to sub-nanometer thicknesses, on silicon surfaces to enable a selective step-by-step layer assembly of rhenium(I) bipyridine tricarbonyl molecular complexes. The modular surface assembly of molecular complexes generates precisely structured spatial ensembles with strong intermolecular vibrational and electronic coupling, as demonstrated by infrared spectroscopy, photoluminescence, and X-ray photoelectron spectroscopy analysis. The structure of the MSA can be chosen to avoid electronic interactions with the semiconductor substrate to exclusively investigate the electronic interactions between the surface-immobilized molecular complexes.

Efficient Hole Transfer via Delocalized Excited State in Small Molecular Acceptor: A Comparative Study on Photodynamics of PM6:Y6 and PM6:ITIC Organic Photovoltaic Blends

AbstractOrganic solar cells (OSCs) based on small molecular acceptors (SMAs) have made great development with a power conversion efficiency (PCE) over 16% due to the design of novel materials and advances in device preparation technology. This work fabricates two bulk‐heterojunction photovoltaic devices containing the same wide‐bandgap donor PM6, respectively, matched with popular Y6 and ITIC SMAs. The PM6:Y6‐based device achieves a much higher PCE of 15.21% than the PM6:ITIC‐based device of 9.02%. On the basis of comparisons of macroscopic performances in the quasistatic regime, transient absorption spectroscopy (TAS) is further performed to better understand the microscopic dynamics difference in charge separation processes between the two photovoltaic blends. According to the TAS results, the calculated hole transfer efficiency in PM6:Y6 is 71.4%, far greater than the efficiency of 13.1% in PM6:ITIC, demonstrating favorable charge separation at donor/acceptor interfaces via hole transfer channel in PM6:Y6. The favorable hole transfer in PM6:Y6 is accounted for by its better mutual miscibility between the donor and acceptor, and the formation of long‐lived delocalized intramoiety excimer state in the acceptor. These results highlight the important role of proper molecular design strategy with strong intermolecular coupling and beneficial film morphology on facilitating charge generation in OSCs adopting SMAs.

Rational design and crystal structure prediction of ring-fused double-PDI compounds as n-channel organic semiconductors: a DFT study.

In this study, we present an effective molecular design strategy to develop the n-type charge transport characteristics in organic semiconductors, using ring-fused double perylene diimides (DPDIs) as the model compounds. These dimeric-PDIs are formed by joining two separate PDI-units along their bay positions through ring fusion with pyrene, coronene and their N-doped counterparts. The bridging type has a significant steric effect at the annulation positions and controls the molecular geometry, mostly imposing buckling in the structure. The crystal structures of the designed compounds are also theoretically predicted. Thereafter, electronic structure parameters, molecular packing motifs, charge coupling strength and anisotropic mobilities were investigated to understand the charge transport efficiency of these systems. Among all the studied molecules, the 4N-coronene-fused DPDI (DPDI-6) is found to possess a lower LUMO level and a high EA, suggesting air-stable electron injection. Besides, DPDI-6 shows strong intermolecular electron coupling and possesses high electron mobility (μe = 5.31 × 10-2 cm2 V-1 s-1), which is better as compared with the other DPDI-compounds reported here. The DPDIs also possess optical absorption in the UV-visible region, opening up possible applications in organic photovoltaics. Besides, from the non-linear optical (NLO) analysis, DPDI-3 is found to possess the highest first-order hyperpolarizability, which is even better as compared with the reference compound urea, making it a promising candidate for NLO applications.

TEMPO Radical-Functionalized Supramolecular Coordination Complexes with Controllable Spin-Spin Interactions.

The topic of noncovalent spin-spin interactions is of increasing general interest in supramolecular radical chemistry. In this report, a series of exo- and endo-TEMPO radical-functionalized metallacycles 1-4 and metallacages 5 and 6 were constructed via coordination-driven self-assembly, wherein the number, location, and distance of the spins were precisely controlled. Their intriguing spin-spin interactions were systematically investigated by electron paramagnetic resonance (EPR) and were well interpreted at the molecular level assisted by X-ray crystallography analysis. The results revealed their distinct spin-spin interactions in the solution state, wherein the spin-spin interaction of metallacycle 3 was much stronger than that of the other five assemblies mainly due to its shorter intramolecular spin-spin distance. In the solid state, 1-6 exhibited obvious spin-spin (dipole-dipole) interactions because of the close arrangement of TEMPO units as indicated in their single crystals. Specifically, a large zero-field splitting (ZFS; D = 17.5 mT) was observed in the crystalline form of metallacycle 4, which arose from the strong intermolecular spin-spin coupling. Interestingly, when the counterion of PF6- in 4 was changed to BF4-, the BF4- counterion analog 4a also exhibited a large ZFS, but the ZFS originated from the intramolecular spin-spin interaction due to a small variation in its crystal conformation. Moreover, the reversible on-off switching of the ZFS in 4 and 4a via the crystal-to-amorphous transformation induced by mechanical grinding and solvent vapor stimuli was also successfully realized. The unique and controllable inter- and intramolecular spin-spin interactions in this work reveal new insights for the understanding and manipulation of spin-spin interactions and may open up a new way to develop organic spin materials in the future.

Experimental and theoretical evidence of dihydrogen bonds in lithium amidoborane

In situ high-pressure synchrotron X-ray diffraction, Raman scattering, and complementary first-principles calculations have revealed that structural and spectroscopic properties of lithium amidoborane compound are largely determined by multiple heteropolar dihydrogen bonds. The crystal structure of the compound is stabilized by dimeric complexes, wherein molecular ions bind together by intermolecular dihydrogen bonds of unconventional type. This strong intermolecular coupling determines stable character of the crystal structure in the pressure range up to ~ 30 GPa and is spectroscopically manifested by pronounced changes related to molecular vibrations of the amino group: the splitting of stretching modes, the anomalous behavior of wagging modes as well as Fermi resonance due to vibrational coupling of bending and stretching modes, significantly enhanced above 10 GPa. Unconventional nature of dihydrogen bonds is confirmed by the frequency increase, blueshift, of NH stretching modes with pressure. A role of certain hydrogen mediated interactions in the process of dehydrogenation of ammonia borane and its alkali metal derivatives is speculated. Findings presented here call for reconsideration of hydrogen release mechanism from alkali metal ammonia borane derivatives. The work makes significant contribution towards establishing the general theory of ubiquitous and versatile hydrogen mediated interactions.

Intermolecular electronic coupling of 9-methyl-9H-dibenzo[a,[c] carbazole for strong emission in aggregated state by substituent effect

Bright emission of organic luminogens at aggregated state has attracted increasing attention for their potential applications in opto-electronic devices and bio-/chemo-sensors. In this article, upon the introduction of different substituents (Br, Ph and TPh) to the large conjugated core of 9-methyl-9 H -dibenzo[ a , c ]carbazole (DBC) moiety, the resultant luminogens demonstrated PL quantum yields in solid state ranging from 4.81% to 47.39%. Through the systematic investigation of molecular packing, together with theory calculation, the strong intermolecular electronic coupling in the dimers is proved as the main factor to the bright emission in the solid state. The results afforded a new avenue to investigate the intrinsic relationship among the molecular structures, packing modes and emission properties.

Efficient triplet pair separation from intramolecular singlet fission in dibenzopentalene derivatives

In intramolecular SF (iSF), the strong coupling nature and confinement of diffusional separation of 1(TT) limits the extraction and harnessing of triplet energy. In order to investigate the possible 1(TT) separation and the role of molecular parameters on it, a series of iSF-capable dibenzopentalene derivatives (DBPs) have been synthesized and their photoinduced dynamics are monitored. iSF takes place in DBPs, accompanied by consecutive 1(TT) separation in polycrystalline film with almost 100% yield. It is suggested the strong intermolecular coupling provided by the closely packing configuration in the film facilitates the disentanglement of correlated 1(TT). Highly efficient triplet pair separation to yield free triplets makes one step forward for utilizing triplet energy from iSF materials for further optoelectronic applications.

Assessing the Role of Intermolecular Interactions in a Perylene-Based Nanowire Using First-Principles Many-Body Perturbation Theory.

We present a first-principles many-body perturbation theory study of the role of intermolecular coupling in the optoelectronic properties of a one-dimensional (1D) π-stacked nanowire composed of perylene-3,4,9,10-tetracarboxylic diimide molecules on a DNA-like backbone. We determine that strong intermolecular electronic coupling results in large bandwidths and low carrier effective masses, suggesting a high-electron mobility material. Additionally, by including the role of finite-temperature phonons on optical absorption via a newly presented approach, we predict that the optical absorption spectrum is significantly altered from that at zero temperature due to allowed indirect transitions, while the exciton delocalization and binding energy, a measure of intermolecular electronic interactions, remains constant. Overall, our studies indicate that strong intermolecular coupling can dominate the optoelectronic properties of π-conjugated 1D systems even at room temperature.

Covalent and Noncovalent Approaches to Rigid Coplanar π-Conjugated Molecules and Macromolecules.

Molecular conformation and rigidity are essential factors in determining the properties of individual molecules, the associated supramolecular assemblies, and bulk materials. This correlation is particularly important for π-conjugated molecular and macromolecular systems. Within such anindividual molecule, a coplanar conformation facilitates the delocalization of not only molecular orbitals but also charges, excitons, and spins, leading to synergistically ensembled properties of the entire conjugated system. A rigid backbone, meanwhile, imposes a high energy cost to disrupt such a favorable conformation, ensuring the robustness and persistence of coplanarity. From a supramolecular and material point of view, coplanarity and rigidity often promote strong intermolecular electronic coupling and reduce the energy barrier for the intermolecular transport of charges, excitons, and phonons, affording advanced materials properties in bulk. In this context, pursuing a rigid and coplanar molecular conformation often represents one of the primary objectives when designing and synthesizing conjugated molecules for electronic and optical applications. Two general bottom-up strategies-covalent annulation and noncovalent conformational control-are often employed to construct rigid coplanar π systems. These strategies have afforded various classes of such molecules and macromolecules, including so-called conjugated ladder polymers, graphene nanoribbons, polyacenes, and conformationally locked organic semiconductors. While pursuing these targets, however, one often confronts challenges associated with precise synthesis and limited solubility of the rigid coplanar systems, which could further impede their large-scale preparation, characterization, processing, and application. To address these issues, we developed and utilized a number of synthetic methods and molecular engineering approaches to construct and to process rigid coplanar conjugated molecules and macromolecules. Structure-property correlations of this unique class of organic materials were established, providing important chemical principles for molecular design and materials applications. In this Account, we first describe our efforts to synthesize rigid coplanar π systems fused by various types of bonds, including kinetically formed covalent bonds, thermodynamically formed covalent bonds, N→B coordinate bonds, and hydrogen bonds, in order of increasing dynamic character. The subsequent section discusses the characteristic properties of selected examples of these rigid coplanar π systems in comparison with control compounds that are not rigid and coplanar, particularly focusing on the optical, electronic, and electrochemical properties. For systems bridged with noncovalent interactions, active manipulation of the dynamic bonds can tune variable properties at the molecular or collective level. Intermolecular interactions, solid-state packing, and processing of several cases are then discussed to lay the foundation for future materials applications of rigid coplanar π conjugated compounds.

Widely Dispersed Intermolecular Valence Bands of Epitaxially Grown Perfluoropentacene on Pentacene Single Crystals.

Strong intermolecular electronic coupling and well-ordered molecular arrangements enable efficient transport of both charge carriers and excitons in semiconducting π-conjugated molecular solids. Thus, molecular heteroepitaxy to form crystallized donor-acceptor molecular interfaces potentially leads to a novel strategy for creating efficient organic optoelectronic devices via the concomitance of these two requirements. In the present study, the crystallographic and electronic structures of a heteroepitaxial molecular interface, perfluoropentacene (PFP, C22F14) grown on pentacene single crystals (Pn-SCs, C22H14), were determined by means of grazing-incidence X-ray diffraction (GIXD) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS), respectively. GIXD revealed that PFP uniquely aligned its primary axis along the [11̅0] axis of crystalline pentacene to form well-crystallized overlayers. Valence band dispersion (at least 0.49 eV wide) was successfully resolved by ARUPS. This indicated a significant transfer integral between the frontier molecular orbitals of the nearest-neighbor PFP molecules.

Exploring through-bond and through-space magnetic communication in 1,3,2-dithiazolyl radical complexes.

Reaction of the methyl-benzodithiazolyl radical (MBDTA) with M(hfac)2 complexes (M = Mn, Co, Zn) affords the complexes M(hfac)2(MBDTA)2. Strong antiferromagnetic exchange interactions are observed between M(ii) ions and the two S = 1/2 radicals (M = Mn, Co), whereas weak antiferromagnetic interactions are observed between radicals when using the diamagnetic Zn(ii) ion. Strong intermolecular exchange coupling is also evident in Mn(hfac)2(MBDTA)2 and attributed to π*-π* contacts between MBDTA radicals which are absent for the Co and Zn derivatives.

Conjugated Block Copolymers as Model Systems to Examine Mechanisms of Charge Generation in Donor–Acceptor Materials

AbstractFully conjugated donor–acceptor block copolymers are established as model systems to elucidate fundamental mechanisms of photocurrent generation in organic photovoltaics. Using analysis of steady‐state photoluminescence quenching, exciton dissociation to a charge transfer state within individual block copolymer chains is quantified. By making a small adjustment to the conjugated backbone, the electronic properties are altered enough to disrupt charge transfer almost entirely. Strong intermolecular coupling of the electron donor is introduced by synthesizing block copolymer nanoparticles. Transient absorption spectroscopy is used to monitor charge generation in block copolymer isolated chains and nanoparticles. While efficient charge transfer is observed in isolated chains, there is no indication of complete charge separation. In the nanoparticles, long‐lived polarons are observed as early as ≈15 ns. Thus, aggregation of electron donors can facilitate efficient charge generation.

Photophysical Tuning of Organic Ionic Crystals from Ultralong Afterglow to Highly Efficient Phosphorescence by Variation of Halides.

Manipulation of photophysical properties of pure organic materials via simple alteration is attractive but extremely challenging because of the lack of valid design strategies for achieving ultralong afterglow or efficient room-temperature phosphorescence. Herein, we report a first photophysical manipulation of organic ionic crystals from ultralong afterglow to highly efficient phosphorescence by variation of halides in the crystals. Crystal structural analysis reveals ultralong organic afterglow of tetraphenylphosphonium chloride is promoted by strong intermolecular electronic coupling in the crystal, and theoretical analysis demonstrates that the tremendous boost of the phosphorescence of tetraphenylphosphonium iodide is caused by the coupling effects of significant heavy atom effect from iodine atoms and a small energy difference between the first singlet and triplet states. This work contributes to regulating long-lived emissive behaviors of pure organic ionic crystals in a controlled way and will promote the development of optical switches controlled by external stimuli.