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Natural photosystems use protein scaffolds to control intermolecular interactions that enable exciton flow, charge generation, and long-range charge separation. In contrast, there is limited structural control in current organic electronic devices such as OLEDs and solar cells. We report here the DNA-encoded assembly of pi-conjugated perylene diimides (PDIs) with deterministic control over the number of electronically coupled molecules. The PDIs are integrated within DNA chains using phosphoramidite coupling chemistry, allowing selection of the DNA sequence to either side, and specification of intermolecular DNA hybridization. In this way, we have developed a "toolbox" for construction of any stacking sequence of these semiconducting molecules. We have discovered that we need to use a full hierarchy of interactions: DNA guides the semiconductors into specified close proximity, hydrophobic-hydrophilic differentiation drives aggregation of the semiconductor moieties, and local geometry and electrostatic interactions define intermolecular positioning. As a result, the PDIs pack to give substantial intermolecular pi wave function overlap, leading to an evolution of singlet excited states from localized excitons in the PDI monomer to excimers with wave functions delocalized over all five PDIs in the pentamer. This is accompanied by a change in the dominant triplet forming mechanism from localized spin-orbit charge transfer mediated intersystem crossing for the monomer toward a delocalized excimer process for the pentamer. Our modular DNA-based assembly reveals real opportunities for the rapid development of bespoke semiconductor architectures with molecule-by-molecule precision. Title in Web of Science: Deoxyribonucleic Acid Encoded and Size-Defined pi-Stacking of Perylene Diimides

Quantum dot-organic semiconductor hybrid materials are gaining increasing attention as spin mixers for applications ranging from solar harvesting to spin memories. Triplet energy transfer between the inorganic quantum dot (QD) and organic semiconductor is a key step to understand in order to develop these applications. Here we report on the triplet energy transfer from PbS QDs to four energetically and structurally similar tetracene ligands. Even with similar ligands we find that the triplet energy transfer dynamics can vary significantly. For TIPS-tetracene derivatives with carboxylic acid, acetic acid and methanethiol anchoring groups on the short pro-cata side we find that triplet transfer occurs through a stepwise process, mediated via a surface state, whereas for monosubstituted TIPS-tetracene derivative 5-(4-benzoic acid)-12-triisopropylsilylethynyl tetracene (BAT) triplet transfer occurs directly, albeit slower, via a Dexter exchange mechanism. Even though triplet transfer is slower with BAT the overall yield is greater, as determined from upconverted emission using rubrene emitters. This work highlights that the surface-mediated transfer mechanism is plagued with parasitic loss pathways and that materials with direct Dexter-like triplet transfer are preferred for high-efficiency applications.

Hybrid small-molecule/quantum dot films have the potential to reduce thermalization losses in single-junction photovoltaics as photon multiplication devices. Here grazing incidence X-ray scattering, optical microscopy and IR fluorescence microscopy (probing materials at two distinct wavelengths), provide new insight into highly complex morphologies across nm and mu m lengthscales to provide direct links between morphologies and photon multiplication performance. Results show that within the small molecule crystallites three different QD morphologies may be identified; (i) large quantum dot aggregates at the crystallite nucleus, (ii) relatively well-dispersed quantum dots and (iii) as aggregated quantum dots "swept" from the growing crystallite and that regions containing aggregate quantum dot features lead to relatively poor photon multiplication performance. These results establish how combinations of scattering and microscopy may be employed to reveal new insights into the structure and function of small molecule:quantum dot blends.

Triplet energy transfer between inorganic quantum dots (QDs) and organic materials plays a fundamental role in many optoelectronic applications based on these nanocomposites. Attaching organic molecules to the QD as transmitter ligands has been shown to facilitate transfer both to and from QDs. Here we show that the often disregarded thiol anchoring group can achieve quantitative triplet energy transfer yields in a PbS QD system with 6,11-bis[(triisopropylsilyl)ethynyl]tetracene-2-methylthiol (TET-SH) ligands. We demonstrate efficient triplet transfer in a singlet fission-based photon multiplication system with 5,12-bis[(triisopropylsilyl)ethynyl]tetracene generating triplets in solution that transfer to the PbS QDs via the thiol ligand TET-SH. Importantly, we demonstrate the increased thermal stability of the PbS/TET-SH system, compared to the traditional carboxylic acid counterpart, allowing for higher photoluminescence quantum yields.

Quantum dot (QD) solids are an emerging platform for developing a range of optoelectronic devices. Thus, understanding exciton dynamics is essential towards developing and optimizing QD devices. Here, using transient absorption microscopy, we reveal the initial exciton dynamics in QDs with femtosecond timescales. We observe high exciton diffusivity (~10² cm² s¯¹) in lead chalcogenide QDs within the first few hundred femtoseconds after photoexcitation followed by a transition to a slower regime (~10¯¹–1 cm² s¯¹). QD solids with larger interdot distances exhibit higher initial diffusivity and a delayed transition to the slower regime, while higher QD packing density and heterogeneity accelerate this transition. The fast transport regime occurs only in materials with exciton Bohr radii much larger than the QD sizes, suggesting the transport of delocalized excitons in this regime and a transition to slower transport governed by exciton localization. These findings suggest routes to control the optoelectronic properties of QD solids.

Made available in DSpace on 2020-12-12T02:28:45Z (GMT). No. of bitstreams: 0 Previous issue date: 2019-11-01 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) This article appraises the thermal, photo- and photo-oxidative stability of poly(fullerene)-alt-[bismethylbenzene)]s (PFBMBs) prepared by the atom transfer radical addition polymerization (ATRAP) with particular attention paid to their use as additives in organic photovoltaic devices. PFBMBs are of interest due to their well-defined structures based on alternating, main-chain, fullerene-methylene links. This work shows by way of a wide range of characterization techniques and a small library of PFBMBs with varying side chains, however, that PFBMBs are relatively unstable. Given that prior work has shown that other main-chain fullerene polymers, such as poly(pyrrolidinofullerene)s, are inherently stable, we suggest a degradation mechanism specific to the fullerene-methylene links of PFBMBs, which explains their unusual behavior. This work suggests that polymers based on fullerene each have their own specific stabilities and qualities and that PFBMBs might be of more use in purposes other than OPVs where in situ delivery of fullerene is required, for example, in medical applications. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1434–1452. CNRS/Univ Pau & Pays Adour Institut des Science Analytiques et Physico-Chimie pour l'Environnement et les Materiaux Institute for Physical and Theoretical Chemistry Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18 Departamento de Física – FC – UNESP, Av. Luiz Edmundo Carrijo Coube, 14-01 BELECTRIC OPV GmbH (now OPVIUS GmbH), Landgrabenstraße 94 Bavarian Center for Applied Energy Research Solar Factory of the Future, Fürther Str. 250 Departamento de Física – FC – UNESP, Av. Luiz Edmundo Carrijo Coube, 14-01 FAPESP: 2011/02205-3 CAPES: BEX 11216-12-3

Understanding and engineering exciton transport in quantum dot (QD) solids is both of fundamental interest and crucial to their broad applications in devices1-6. Till date, studies of exciton transport in QD solids on pico/nano-second timescales have led to the conclusion that closer packing of QDs enables faster exciton transport, while energetic/structural heterogeneity leads to reduction of exciton diffusivity over time7,8. Here we study PbS QD solids using transient absorption microscopy with 13 femtoseconds time resolution and 10 nm spatial precision. We find exciton diffusivities in the range of ~102 cm2 s-1 within the first few hundred femtoseconds after photoexcitation, followed by the transition to a slower transport regime with diffusivities in the range 10-1 to 1 cm2 s-1. Counterintuitively, the initial diffusivity is higher and the time before the transition to the slower transport phase is longer in QD solids with longer ligand lengths. This suggests a transition from early-time transport of delocalized excitons to later time hopping based transport of localized excitons, where QD packing density and heterogeneity accelerate the localization process. Our results reveal a new regime for exciton transport in QD solids and provide design rules to engineer desired transport properties in these systems on a range of timescales.

Hybrid inorganic-organic materials such as quantum dots (QDs) coupled with organic semiconductors have a wide range of optoelectronic applications, taking advantage of the respective materials' strengths. A key area of investigation in such systems is the transfer of triplet exciton states to and from QDs, which has potential applications in the luminescent harvesting of triplet excitons generated by singlet fission, in photocatalysis and photochemical upconversion. While the transfer of energy from QDs to the triplet state of organic semiconductors has been intensely studied in recent years, the mechanism and materials parameters controlling the reverse process, triplet transfer to QDs, have not been well investigated. Here, through a combination of steady-state and time-resolved optical spectroscopy we study the mechanism and energetic dependence of triplet energy transfer from an organic ligand (TIPS-tetracene carboxylic acid) to PbS QDs. Over an energetic range spanning from exothermic (-0.3 eV) to endothermic (+0.1 eV) triplet energy transfer we find that the triplet energy transfer to the QD occurs through a single step process with a clear energy dependence that is consistent with an electron exchange mechanism as described by Marcus-Hush theory. In contrast, the reverse process, energy transfer from the QD to the triplet state of the ligand, does not show any energy dependence in the studied energy range; interestingly, a delayed formation of the triplet state occurs relative to the quantum dots' decay. Based on the energetic dependence of triplet energy transfer we also suggest design criteria for future materials systems where triplet excitons from organic semiconductors are harvested via QDs, for instance in light emitting structures or the harvesting of triplet excitons generated via singlet fission.

Singlet fission, the process of forming two triplet excitons from one singlet exciton, is a characteristic reserved for only a handful of organic molecules due to the atypical energetic requirement for low energy excited triplet states. The predominant strategy for achieving such a trait is by increasing ground state diradical character; however, this greatly reduces ambient stability. Herein, we exploit Baird's rule of excited state aromaticity to manipulate the singlet-triplet energy gap and create novel singlet fission candidates. We achieve this through the inclusion of a [4n] 5-membered heterocycle, whose electronic resonance promotes aromaticity in the triplet state, stabilizing its energy relative to the singlet excited state. Using this theory, we design a family of derivatives of indolonaphthyridine thiophene (INDT) with highly tunable excited state energies. Not only do we access novel singlet fission materials, they also exhibit excellent ambient stability, imparted due to the delocalized nature of the triplet excited state. Spin-coated films retained up to 85% activity after several weeks of exposure to oxygen and light, while analogous films of TIPS-pentacene showed full degradation after 4 days, showcasing the excellent stability of this class of singlet fission scaffold. Extension of our theoretical analysis to almost ten thousand candidates reveals an unprecedented degree of tunability and several thousand potential fission-capable candidates, while clearly demonstrating the relationship between triplet aromaticity and singlet-triplet energy gap, confirming this novel strategy for manipulating the exchange energy in organic materials.

Nanocrystal quantum dots are generally coated with an organic ligand layer. These layers are a necessary consequence of their chemical synthesis, and in addition they play a key role in controlling the optical and electronic properties of the system. Here we describe a method for quantitative measurement of the ligand layer in 3 nm diameter lead sulfide-oleic acid quantum dots. Complementary small-angle X-ray and neutron scattering (SAXS and SANS) studies give a complete and quantitative picture of the nanoparticle structure. We find greater-than-monolayer coverage of oleic acid and a significant proportion of ligand remaining in solution, and we demonstrate reversible thermal cycling of the oleic acid coverage. We outline the effectiveness of simple purification procedures with applications in preparing dots for efficient ligand exchange. Our method is transferrable to a wide range of colloidal nanocrystals and ligand chemistries, providing the quantitative means to enable the rational design of ligand-exchange procedures.

Singlet fission is an exciton multiplication process in organic molecules in which a photogenerated spin-singlet exciton is rapidly and efficiently converted to two spin-triplet excitons. This process offers a mechanism to break the Shockley-Queisser limit by overcoming the thermalization losses inherent to all single-junction photovoltaics. One of the most promising methods to harness the singlet fission process is via the efficient extraction of the dark triplet excitons into quantum dots (QDs) where they can recombine radiatively, thereby converting high-energy photons to pairs of low-energy photons, which can then be captured in traditional inorganic PVs such as Si. Such a singlet fission photon multiplication (SF-PM) process could increase the efficiency of the best Si cells from 26.7% to 32.5%, breaking the Shockley-Queisser limit. However, there has been no demonstration of such a singlet fission photon multiplication (SF-PM) process in a bulk system to date. Here, we demonstrate a solution-based bulk SF-PM system based on the singlet fission material TIPS-Tc combined with PbS QDs. Using a range of steady-state and time-resolved measurements combined with analytical modeling we study the dynamics and mechanism of the triplet harvesting process. We show that the system absorbs >95% of incident photons within the singlet fission material to form singlet excitons, which then undergo efficient singlet fission in the solution phase (135 +/- 5%) before quantitative harvesting of the triplet excitons (95 +/- 5%) via a low concentration of QD acceptors, followed by the emission of IR photons. We find that in order to achieve efficient triplet harvesting it is critical to engineer the surface of the QD with a triplet transfer ligand and that bimolecular decay of triplets is potentially a major loss pathway which can be controlled via tuning the concentration of QD acceptors. We demonstrate that the photon multiplication efficiency is maintained up to solar fluence. Our results establish the solution-based SF-PM system as a simple and highly tunable platform to understand the dynamics of a triplet energy transfer process between organic semiconductors and QDs, one that can provide clear design rules for new materials.

International audience; A novel main-chain polyfullerene, poly[fullerene-alt-2,5-bis(octyloxy)terephthalaldehyde] (PPC4), is investigated for its hypothesized superior morphological stability as an electron-accepting material in organic photovoltaics relative to the widely used fullerene phenyl-C61-butyric acid methyl ester (PCBM). When mixed with poly(3-hexylthiophene-2,5-diyl) (P3HT), PPC4 affords low-charge-generation yields because of poor intermixing within the blend. The adoption of a multiacceptor system, by introducing PCBM into the P3HT:polyfullerene blend, was found to lead to a 3-fold enhancement in charge generation, affording power conversion efficiencies very close to that of the prototypical P3HT:PCBM binary control. Upon thermal stressing and in contrast to the P3HT:PCBM binary, photovoltaic devices based on the multiacceptor system demonstrated significantly improved stability, outperforming the control because of suppression of the PCBM migration and aggregation processes responsible for rapid device failure. We rationalize the influence of the fullerene miscibility and its implications on the device performance in terms of a thermodynamic model based on Flory-Huggins solution theory. Finally, the potential universal applicability of this approach for thermal stabilization of organic solar cells is demonstrated, utilizing an alternative low-band-gap polymer-donor system

This work demonstrates for the first time that it is possible to prepare alternating oligomers, containing both dyes and fullerenes in repeating structures, that act as electron acceptors in bulk heterojunction devices. A sterically controlled azomethine ylide cycloaddition polymerization is employed with either C-60 or phenyl-C-61-butyric acid methyl ester (PCBM) and the dye diketopyrrolopyrrole (DPP). The former results in low molecular weights of around 5600 g mol(-1), whereas the latter, PCBM, enables the formation of more soluble chains with higher molecular weights of ca 11 200 g mol(-1). Remarkably, cyclic voltammetry shows that the incorporation of PCBM into the main-chain raises the lowest unoccupied molecular orbital by ca 380 meV due to the in-chain bis-additions. The observation of the complete quenching of DPP fluorescence by the fullerene moiety, combined with computer modelling studies, indicates both electron and energy transfers between intra-chain moieties. Proof-of-concept devices show low efficiencies most likely due to as-yet-unoptimized preparation and structures, but hint at the possibilities of these novel bi-functionalized, in-chain fullerenes due to their high Voc of 0.89 V with an example low-bandgap polymer, KP115, and reasonable charge mobilities of ca 1x10(-4) cm(2) V-1 s(-1),making this new class of materials of strong interest for applications. Furthermore, their good thermal stability to above 300 degrees C and their stabilization of photovoltaic devices against thermal degradation confirm that this new pathway to a wide range of dye/fullerene structures is extremely promising. (C) 2016 Society of Chemical Industry

Hybrid metal sulfide/polymer solar cell active layers are fabricated employing an approach based upon the in-situ thermal decomposition of a single source metal xanthate precursor in a semiconducting polymer film. The nanomorphology of the film, the charge photogeneration yield at the donor-acceptor heterojunction and device performance are shown to be dependent upon the annealing temperature. Photovoltaic devices based upon such layers are shown to exhibit power conversion efficiencies of 2.2% under AM1.5 solar illumination thus demonstrating the potential of such nanocomposite films for photovoltaic device applications.

International audience; Phenyl-C61-butyric acid methyl ester (PCBM) is polymerized simply using a one-pot reaction to yield soluble, high molecular weight polymers. The sterically controlled azomethine ylide cycloaddition polymerization (SACAP) is demonstrated to be highly adaptable and yields polymers with probable Mn ≈ 24600 g mol-1 and Mw ≈ 73800 g mol-1. Products are metal-free and of possible benefit to organic and hybrid photovoltaics and electronics as they form thin films from solution and have raised LUMOs. The promising electronic properties of this new polymer are discussed.

International audience; The first oligomerisation of phenyl-C61-butyric acid methyl ester (PCBM) using a facile atom transfer radical addition polymerization (ATRAP) and its exploitation for organic photovoltaic devices is described. Oligo\(phenyl-C61-butyric acid methyl ester)-alt-[1,4-bis(bromomethyl)-2,5-bis(octyloxy)benzene]\ (OPCBMMB) shows opto-electronic properties equivalent to those of PCBM but has a higher glass transition temperature. When mixed with various band gap semiconducting polymers, OPCBMMB delivers performances similar to PCBM but with an enhanced stabilization of the bulk heterojunction in photovoltaic devices on plastic substrates under thermal stress, regardless of the degree of crystallinity of the polymer and without changing opto-electronic properties. © 2016 The Royal Society of Chemistry.

International audience; Fullerene is used as a monomer in this simple method to prepare soluble, well-defined polymers. The sterically controlled azomethine ylide cycloaddition polymerization of fullerene (SACAP) yields macromolecules with molecular weights of around 25 000 g mol-1. Importantly, cumbersome comonomers are employed to restrict cross-linking. Extensive characterizations, with the help of modeling studies, indicate that the polymers are regio-irregular with a majority of trans-3 isomers. Of particular interest is the exceptional ease of preparing polymers with zero metal content. © 2016 American Chemical Society.

Development of design rules for hybrid inorganic–organic solar cells through understanding charge generation and recombination dynamics is an important pathway for the improvement of solar cell conversion efficiencies. In this Letter, we study the dynamics of charge generation in CdS:polymer blends by transient absorption spectroscopy. We show that charge generation following excitation of the inorganic component is highly efficient and can occur up to a few nanoseconds after excitation, allowing for diffusion of charges within the inorganic component to an interface. In contrast, charge generation following excitation of the organic component occurs on subpicosecond time scales but suffers from two loss processes, incomplete exciton dissociation and geminate recombination.

Understanding the degradation and failure mechanisms of organic photovoltaic devices is a key requirement for this technology to mature toward a reliable product. Here, an investigation on accelerated temperature and moisture long-term stability testing (>20 000 h) of inverted and glass-encapsulated poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester solar cells is presented. The degradation kinetics is analyzed using the Arrhenius model and the resulting activation energy for the diffusion of water is measured to be ≈43 kJ mol−1. Through comparison of electroluminescence imaging, lock-in thermography, and photoluminescence mapping, the device performance is correlated with the loss of effective cell area and it is shown that the reaction of water at the hole extraction/active layer interface is likely to be the dominant cause for long-term device failure. The diffusion of water through the packaged solar cell is described using classical diffusion theory. Based on an analytical solution of a simple diffusion model, the diffusion coefficient is estimated to be 4 × 10−12 m2 s−1. A shelf life of 100 000 h is anticipated at 65 °C/85% RH using a 9.3 cm wide protective adhesive rim. The findings of this study may inform strategies for predicting lifetimes of organic solar cells and modules based on local in situ tracking of moisture-induced device performance loss using IR imaging.

The influence of morphology on the photophysical properties of blend films containing in situ grown CdS and poly(3-hexylthiophene-2,5-diyl) (P3HT), fabricated utilising a metal xanthate single source precursor, is reported. A combination of transient absorption spectroscopy (TAS), transmission electron microscopy (TEM) and photovoltaic device measurements are employed to study the relationship between the efficiency of charge separation, photocurrent generation and thin film morphology. We identify that a significant proportion of the extractable charge originates from the direct excitation of CdS followed by hole-transfer to the P3HT polymer. The yield of this hole-transfer step from the inorganic CdS to the organic polymer is largely unaffected by the film's nanomorphology, while the dissociation of P3HT excitons into free charges at the CdS:P3HT interface is found to be strongly dependent on this parameter with high yields of charge transfer only being achieved at high CdS loadings. The present study elucidates design rules for the optimization of hybrid inorganic–organic solar energy conversion devices.

We demonstrate that blend films containing poly(3-hexylthiophene-2,5-diyl) and in situ grown CdS display a greater yield of photogenerated charges than a blend containing an equivalent amount of pre-synthesised CdS quantum dots. Moreover, we show that the greater charge yield in the in situ grown films leads to an improvement in device efficiency. The present findings also appear to suggest that charge photogeneration at the CdS/polymer heterojunction is facilitated by the formation of nanoparticle networks as a result of CdS aggregation.

Here a new electron donor is developed to endow a near-IR TADF molecule with good solubility for solution processing and AIE behavior. This is the first AIE TADF material with all PL > 700 nm.