Exploiting quantum interference in dye sensitized solar cells

Scaling of quantum interference from single molecules to molecular cages and their monolayers

The discovery of quantum interference (QI) is widely considered as an important advance in molecular electronics since it provides unique opportunities for achieving single-molecule devices with unprecedented performance. Although some pioneering studies suggested the presence of spin qubit coherence and QI in collective systems such as thin films, it remains unclear whether the QI can be transferred step-by-step from single molecules to different length scales, which hinders the application of QI in fabricating active molecular devices. Here, we found that QI can be transferred from a single molecule to their assemblies. We synthesized and investigated the charge transport through the molecular cages using 1,3-dipyridylbenzene (DPB) as a ligand block with a destructive quantum interference (DQI) effect and 2,5-dipyridylfuran (DPF) as a control building block with a constructive quantum interference (CQI) effect using both single-molecule break junction and large area junction techniques. Combined experiments and calculations revealed that both DQI and CQI had been transferred from the ligand blocks to the molecular cages and the monolayer thin film of the cages. Our work introduced QI effects from a ligand to the molecular cage comprising 732 atoms and even their monolayers, suggesting that the quantum interference could be scaled up within the phase-coherent distance.

Electron Transfer at Quantum Dot–Metal Oxide Interfaces for Solar Energy Conversion

Electron transfer at a donor–acceptor quantum dot–metal oxide interface is a process fundamentally relevant to solar energy conversion architectures as, e.g., sensitized solar cells and solar fuels schemes. As kinetic competition at these technologically relevant interfaces largely determines device performance, this Review surveys several aspects linking electron transfer dynamics and device efficiency; this correlation is done for systems aiming for efficiencies up to and above the ∼33% efficiency limit set by Shockley and Queisser for single gap devices. Furthermore, we critically comment on common pitfalls associated with the interpretation of kinetic data obtained from current methodologies and experimental approaches, and finally, we highlight works that, to our judgment, have contributed to a better understanding of the fundamentals governing electron transfer at quantum dot–metal oxide interfaces.

Substituent Control of σ-Interference Effects in the Transmission of Saturated Molecules

The single-molecule conductance of saturated molecules can potentially be fully suppressed by destructive quantum interference in their σ-system. However, only few molecules with σ-interference have been identified, and the structure-property relationship remains to be elucidated. Here, we explore the role of substituents in modulating the electronic transmission of saturated molecules. In functionalized bicyclo[2.2.2]octanes, the transmission is suppressed by σ-interference when fluorine substituents are applied. For bicyclo[2.2.2]octasilane and -octagermanes, the transmission is suppressed when carbon-based substituents are used, and such molecules are likely to be highly insulating. For the carbon-based substituents, we find a strong correlation between the appropriate Hammett constants and the transmission. The substituent effect enables systematic optimization of the insulating properties of saturated molecular cores.

Chemical transformation of Doubly N-Confused Porphodimethene to variants of (anti)Aromatic Doubly N-Confused Porphyrinoids and σ aromatic Doubly N-Confused Isophlorinoid

Chemical conversion of non-aromatic trans-doubly N-confused porphodimethene to hitherto unknown variants of doubly N-confused porphyrinoids/isophlorinoid have been unravelled by precise interplay between the types of oxidants, the types of meso-aryl...

Coupling of photoactive transition metal complexes to a functional polymer matrix*

Conductive polymers represent a promising alternative to semiconducting oxide electrodes typically used in dye-sensitized cathodes as they more easily allow a tuning of the physicochemical properties. This can then also be very beneficial for using them in light-driven catalysis. In this computational study, we address the coupling of Ru-based photosensitizers to a polymer matrix by combining two different first-principles electronic structure approaches. We use a periodic density functional theory code to properly account for the delocalized nature of the electronic states in the polymer. These ground state investigations are complemented by time-dependent density functional theory simulations to assess the Franck-Condon photophysics of the present photoactive hybrid material based on a molecular model system. Our results are consistent with recent experimental observations and allow to elucidate the light-driven redox chemical processes - eventually leading to charge separation - in the present functional hybrid systems with potential application as photocathode materials.

Free Charge Carriers in Homo-Sorted π-Stacks of Donor-Acceptor Conjugates

Inspired by the high photoconversion efficiency observed in natural light-harvesting systems, the hierarchical organization of molecular building blocks has gained impetus in the past few decades. Particularly, the molecular arrangement and packing in the active layer of organic solar cells (OSCs) have garnered significant attention due to the decisive role of the nature of donor/acceptor (D/A) heterojunctions in charge carrier generation and ultimately the power conversion efficiency. This review focuses on the recent developments in emergent optoelectronic properties exhibited by self-sorted donor-on-donor/acceptor-on-acceptor arrangement of covalently linked D-A systems, highlighting the ultrafast excited state dynamics of charge transfer and transport. Segregated organization of donors and acceptors promotes the delocalization of photoinduced charges among the stacks, engendering an enhanced charge separation lifetime and percolation pathways with ambipolar conductivity and charge carrier yield. Covalently linking donors and acceptors ensure a sufficient D-A interface and interchromophoric electronic coupling as required for faster charge separation while providing better control over their supramolecular assemblies. The design strategies to attain D-A conjugate assemblies with optimal charge carrier generation efficiency, the scope of their application compared to state-of-the-art OSCs, current challenges, and future opportunities are discussed in the review. An integrated overview of rational design approaches derived from the comprehension of underlying photoinduced processes can pave the way toward superior optoelectronic devices and bring in new possibilities to the avenue of functional supramolecular architectures.

Molecular Structure, Quantum Coherence, and Solvent Effects on the Ultrafast Electron Transport in BODIPY- Derivatives

Photoinduced electron transfer in multichromophore molecular systems is defined by a critical interplay between their core unit configuration (donor, molecular bridge, and acceptor) and their system-solvent coupling; these lead to energy and charge transport processes that are key in the design of molecular antennas for efficient light harvesting and organic photovoltaics. Here, we quantify the ultrafast non-Markovian dissipative dynamics of electron transfer in D-π-A molecular photosystems comprising 1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene (BODIPY), Zn-porphyrin, fulleropyrrolidine, and fulleroisoxazoline. We find that the stabilization energy of the charge transfer states exhibits a significant variation for different polar (methanol, tetrahydrofuran (THF)) and nonpolar (toluene) environments and determine such sensitivity according to the molecular structure and the electron-vibration couplings that arise at room temperature. For the considered donor-acceptor (D-A) dyads, we show that the stronger the molecule-solvent coupling, the larger the electron transfer rates, regardless of the dyads' electronic coherence properties. We find such coupling strengths to be the largest (lowest) for methanol (toluene), with an electron transfer rate difference of 2 orders of magnitude between the polar and nonpolar solvents. For the considered donor-bridge-acceptor (D-B-A) triads, the molecular bridge introduces an intermediate state that allows the realization of Λ or cascaded-type energy mechanisms. We show that the latter configuration, obtained for BDP-ZnP-[PyrC₆₀] in methanol, exhibits the highest transfer rate of all of the computed triads. Remarkably, and in contrast with the dyads, we show that the larger charge transfer rates are obtained for triads that exhibit prolonged electron coherence and population oscillations.

Quantum interference enhances rectification behavior of molecular devices

A theoretical and computational study of the effect of quantum interference on the rectification behavior of unimolecular devices.

Quantum coherence in ultrafast photo-driven charge separation

Coherent interactions are prevalent in photodriven processes, ranging from photosynthetic energy transfer to superexchange-mediated electron transfer, resulting in numerous studies aimed towards identifying and understanding these interactions. A key motivator of this interest is the non-statistical scaling laws that result from coherently traversing multiple pathways due to quantum interference. To that end, we employed ultrafast transient absorption spectroscopy to measure electron transfer in two donor-acceptor molecular systems comprising a p-(9-anthryl)-N,N-dimethylaniline chromophore/electron donor and either one or two equivalent naphthalene-1,8:4,5-bis(dicarboximide) electron acceptors at both ambient and cryogenic temperatures. The two-acceptor compound shows a statistical factor of 2.1 ± 0.2 rate enhancement at room temperature and a non-statistical factor of 2.6 ± 0.2 rate enhancement at cryogenic temperatures, suggesting correlated interactions between the two acceptors with the donor and with the bath modes. Comparing the charge recombination rates indicates that the electron is delocalized over both acceptors at low temperature but localized on a single acceptor at room temperature. These results highlight the importance of shielding the system from bath fluctuations to preserve and ultimately exploit the coherent interactions.

Quantum Interference Effects in Charge Transport through Single-Molecule Junctions: Detection, Manipulation, and Application

Quantum interference effects (QIEs), which offer unique opportunities for the fine-tuning of charge transport through molecular building blocks by constructive or destructive quantum interference, have become an emerging area in single-molecule electronics. Benefiting from the QIEs, charge transport through molecular systems can be controlled through minor structural and environmental variations, which cause various charge transport states to be significantly changed from conductive to insulative states and offer promising applications in future functional single-molecule devices. Although QIEs were predicted by theoreticians more than two decades ago, only since 2011 have the challenges in ultralow conductance detection originating from destructive quantum interference been overcome experimentally. Currently, a series of single-molecule conductance investigations have been carried out experimentally to detect constructive and destructive QIEs in charge transport through various types of molecular junctions by altering molecular patterns and connectivities. Furthermore, the use of QIEs to tune the properties of charge transport through single-molecule junctions using external gating shows vital potential in future molecular electronic devices. The experimental and theoretical investigations of QIEs offer new fundamental understanding of the structural-electronic relationships in molecular devices and materials at the nanoscale. In this Account, we discuss our progress toward the experimental detection, manipulation, and further application of QIEs in charge transport through single-molecule junctions. These experiments were carried out continuously in our previous group at the University of Bern and in our lab at Xiamen University. As a result of the development of mechanically controllable break junction (MCBJ) and scanning tunneling microscope break junction (STM-BJ) techniques, we could detect ultralow charge transport through the cross-conjugated anthraquinone center, which was one of the earliest experimental studies of QIEs. In close cooperation with organic chemists and theoretical physicists, we systematically investigated charge transport through single-molecule junctions originating from QIEs in conjugated centers ranging from simple single benzene to polycyclic aromatic hydrocarbons (PAHs), heteroaromatics, and even complicated metalla-aromatics at room temperature. Then we further investigated the quantitative correlation between molecular structure and quantum interference by altering different molecular patterns and connectivities in homologous series of PAHs and heteroatom systems. Additionally, external chemical and electrochemical gating of single-molecule devices can be used for direct QIE manipulation via not only tuning molecular conjugation but also shifting the electrode Fermi level. Our study further suggested that distinguishable differences in conductance resulting from QIEs offer opportunities to detect photothermal reaction kinetics and to recognize isomers at the single-molecule scale. These investigations demonstrate the universality of QIEs in charge transport through various molecular building blocks. Moreover, effective manipulation of QIEs leads to various novel phenomena and promising applications in molecular electronic devices.

Electron transport behavior of quinoidal heteroacene-based junctions: effective electron-transport pathways and quantum interference

The electron transport behavior through a series of molecular junctions composed of tetracene (TC) and S/O substituted-TC (S/O-TC) has been studied using density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) method. The unique transport behavior has been interpreted using correlated quantum interference and electron transport pathway models. In the TC system, two dominant electron transfer channels exist as demonstrated by a detailed transmission pathway analysis. In the substituted S/O-TC systems, the electron transport behavior is regulated through either constructive or destructive quantum interference due to the existence of additional p-electrons, leading to a significant diversity of current-voltage curves. Compared to the TC molecule in the bias region from 0 to 1.0 V, an α-connected molecular junction exhibits a greater current, whereas a β-connected molecular junction shows a smaller current. The substitution with O and S atoms shows a minor effect on the conductance of the molecular junctions. In order to clarify the role of heteroatoms, a series of artificial models designed by removing specific sulfur and carbon atoms in α-S-TC have been investigated in detail. The results have demonstrated that only the S heteroatom on one side of the molecule contributes to the junction conductivity through constructive quantum interference. It has also been observed that current exchange occurs between the two electron transfer channels.

The impact of heteroatom substitution on cross-conjugation and its effect on the photovoltaic performance of DSSCs - a computational investigation of linear vs. cross-conjugated anchoring units

The unusual bonding pattern and proximal heteroatom substitution in π-cross conjugation produced distinct changes in the energy levels and photophysical behaviour of the dyes. To seek an understanding of the origin of these fluctuations, we have carried out a detailed computational investigation on a series of D-π1-π2 (A1)-A2 structured dyes comprised of common donor-spacer (auxiliary acceptor) units but varied the anchoring parts. In this study, we introduced a novel dimethylamino substituted fluorene-based triarylamine donor unit and evaluated its donating strength. Based on the comparison of DFT computed energy levels with experimental results, we have proposed an orbital splitting pattern to explain the energy level and photophysical properties of the linear vs. cross-conjugated dyes with respect to the linking position of the anchoring unit and benzo[1,2,5]thiadiazole (BTD) substitution. The smallest HOMO-LUMO gap of B3 mainly originated from the weak overlap of the directionality mismatch of the orbital interaction imposed by cross-conjugation. The inefficient overlap in B3 can possibly influence the energy levels but failed to enhance the charge transfer transitions upon photoexcitation. On the other hand, β-heteroatom substitution in bridged dyes partially enhanced π-delocalization over the cross conjugation and produced a significant ICT absorption with an optoelectronic response in the NIR region. BTD acceptor substitution increased the HOMO-LUMO gap of the bridged dyes. NBO analysis was performed to corroborate our predictions. DOS-PDOS analysis of the dyes@TiO2 was employed to investigate the electron injection rate of linear vs. bridged dyes. The anchoring pattern and large torsional deviation of the carboxylate anchoring group upon TiO2 adsorption drastically decreased the photovoltaic performance of the bridged dyes. The results obtained from this study provided a detailed understanding of how to surmount the cross-conjugation with the aid of β-heteroatom substitution. These design guidelines would be helpful in developing novel NIR dyes with better hole mobility for various optoelectronic applications. Furthermore, π-delocalization over the cross-conjugation concept opens a new pathway in the field of functional molecular devices to increase the charge conductance over several orders of magnitude with a significant reduction of destructive quantum interference at the molecular junction.

Controlling destructive quantum interference in tunneling junctions comprising self-assembled monolayers via bond topology and functional groups

Quantum interference effects (QI) are of interest in nano-scale devices based on molecular tunneling junctions because they can affect conductance exponentially through minor structural changes. However, their utilization requires the prediction and deterministic control over the position and magnitude of QI features, which remains a significant challenge. In this context, we designed and synthesized three benzodithiophenes based molecular wires; one linearly-conjugated, one cross-conjugated and one cross-conjugated quinone. Using eutectic Ga-In (EGaIn) and CP-AFM, we compared them to a well-known anthraquinone in molecular junctions comprising self-assembled monolayers (SAMs). By combining density functional theory and transition voltage spectroscopy, we show that the presence of an interference feature and its position can be controlled independently by manipulating bond topology and electronegativity. This is the first study to separate these two parameters experimentally, demonstrating that the conductance of a tunneling junction depends on the position and depth of a QI feature, both of which can be controlled synthetically.

Nanoarchitectures in dye-sensitized solar cells: metal oxides, oxide perovskites and carbon-based materials

Dye-sensitized solar cells (DSSCs) have aroused great interest and been regarded as a potential renewable energy resource among the third-generation solar cell technologies to fulfill the 21^st century global energy demand. DSSCs have notable advantages such as low cost, easy fabrication process and being eco-friendly in nature. The progress of DSSCs over the last 20 years has been nearly constant due to some limitations, like poor long-term stability, narrow absorption spectrum, charge carrier transportation and collection losses and poor charge transfer mechanism for regeneration of dye molecules. The main challenge for the scientific community is to improve the performance of DSSCs by using different approaches, like finding new electrode materials with suitable nanoarchitectures, dyes in composition with promising semiconductors and metal quantum dot fluorescent dyes, and cost-effective hole transporting materials (HTMs). This review focuses on DSSC photo-physics, which includes charge separation, effective transportation, collection and recombination processes. Different nanostructured materials, including metal oxides, oxide perovskites and carbon-based composites, have been studied for photoanodes, and counter electrodes, which are crucial to achieve DSSC devices with higher efficiency and better stability.

Protonation tuning of quantum interference in azulene-type single-molecule junctions

The protonation of azulene cores offers significant conductance tuning in single-molecule junctions with quantum interference.

The protonation of azulene derivatives with quantum interference effects is studied by the conductance measurements of single-molecule junctions. Three azulene derivatives with different connectivities are synthesized and reacted with trifluoroacetic acid to form the protonated states. It is found that the protonated azulene molecular junctions produce more than one order of magnitude higher conductance than the neutral states, while the molecules with destructive interference show more significant changes. These experimental observations are supported by our recently-developed parameter free theory of connectivity, which suggests that the largest conductance change occurs when destructive interference near the Fermi energy in the neutral state is alleviated by protonation.

A naphthalene-fused dimer of an anti-aromatic expanded isophlorin

We report the first synthesis of a covalent expanded isophlorin dimer from two 24-π doubly S-confused sapphyrin-like pentathiaisophlorins. It exhibits marginal peripheral aromaticity rather than strong global diatropicity or paratropicity and weak intermacrocycle electronic communication. Quantum chemical methods discern that cross-conjugation is responsible for these unusual electronic features.

Does the position of the electron-donating nitrogen atom in the ring system influence the efficiency of a dye-sensitized solar cell? A computational study

We have reported a number of new metal-free organic dyes (2-6) that have cyclic asymmetric benzotripyrrole derivatives as donor groups with peripheral nitrogen atoms in the ring, fluorine and thiophene groups as π-spacers, and a cyanoacrylic acid acceptor group. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were employed to examine the influence of the position of the donor nitrogen atom and π-conjugation on solar cell performance. The calculated electron-injection driving force (ΔG inject), electron-regeneration driving force (ΔG regen), light-harvesting efficiency (LHE), dipole moment (μ normal), and number of electrons transferred (∆q) indicate that dyes 3, 4, and 6 have significantly higher efficiencies than reference dye 1, which exhibits high efficiency. We also extended our comparison to some other reported dyes, 7-9, which have a donor nitrogen atom in the middle of the ring system. The computed results suggest that dye 6 possesses a higher incident photon to current conversion efficiency (IPCE) than reported dyes 7-9. Thus, the use of donor groups with peripheral nitrogen atoms appears to lead to more efficient dyes than those in which the nitrogen atom is present in the middle of the donor ring system. Graphical Abstract The locations of the nitrogen atoms in the donor groups in the designed dye molecules have an important influence on DSSC efficiency.

Charge Recombination Suppressed by Destructive Quantum Interference in Heterojunction Materials

We show that charge recombination in ordered heterojunctions depends sensitively on the degree of coherent delocalization of charges at the donor-acceptor interface. Depending on the relative sign of the electron and hole transfer integrals, such delocalization can dramatically suppress recombination through destructive quantum interference. This could explain why measured recombination rates are significantly lower than predictions based on Langevin theory for a variety of organic bulk heterojunctions. Moreover, it opens up a design strategy for photovoltaic devices with enhanced efficiencies through coherently suppressed charge recombination.

Exponential Attenuation of Through-Bond Transmission in a Polyene: Theory and Potential Realizations

An exponential falloff with separation of electron transfer and transport through molecular wires is observed and has attracted theoretical attention. In this study, the attenuation of transmission in linear and cyclic polyenes is related to bond alternation. The explicit form of the zeroth Green's function in a Hückel model for bond-alternated polyenes leads to an analytical expression of the conductance decay factor β. The β values calculated from our model (β(CN) values, per repeat unit of double and single bond) range from 0.28 to 0.37, based on carotenoid crystal structures. These theoretical β values are slightly smaller than experimental values. The difference can be assigned to the effect of anchoring groups, which are not included in our model. A local transmission analysis for cyclic polyenes, and for [14]annulene in particular, shows that bond alternation affects dramatically not only the falloff behavior but also the choice of a transmission pathway by electrons. Transmission follows a well-demarcated system of π bonds, even when there is a shorter-distance path with roughly the same kind of "electronic matter" intervening.

Can silicon substituted metal-free organic dyes achieve better efficiency compared to silicon free organic dyes? A computational study

DFT and TD-DFT calculations performed using metal free organic dyes containing silicon substituted silole units and/or donor systems exhibit significantly improved optical properties compared to their corresponding silicon free dyes.