Thiol-Anchored TIPS-Tetracene Ligands with Quantitative Triplet Energy Transfer to PbS Quantum Dots and Improved Thermal Stability.

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

Engineering Molecular Ligand Shells on Quantum Dots for Quantitative Harvesting of Triplet Excitons Generated by Singlet Fission.

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.ER

Singlet exciton fission in a modified acene with improved stability and high photoluminescence yield

Abstract: We report a fully efficient singlet exciton fission material with high ambient chemical stability. 10,21-Bis(triisopropylsilylethynyl)tetrabenzo[a,c,l,n]pentacene (TTBP) combines an acene core with triphenylene wings that protect the formal pentacene from chemical degradation. The electronic energy levels position singlet exciton fission to be endothermic, similar to tetracene despite the triphenylenes. TTBP exhibits rapid early time singlet fission with quantitative yield of triplet pairs within 100 ps followed by thermally activated separation to free triplet excitons over 65 ns. TTBP exhibits high photoluminescence quantum efficiency, close to 100% when dilute and 20% for solid films, arising from triplet-triplet annihilation. In using such a system for exciton multiplication in a solar cell, maximum thermodynamic performance requires radiative decay of the triplet population, observed here as emission from the singlet formed by recombination of triplet pairs. Combining chemical stabilisation with efficient endothermic fission provides a promising avenue towards singlet fission materials for use in photovoltaics

Deoxyribonucleic Acid Encoded and Size-Defined π-Stacking of Perylene Diimides.

Funder: University of CambridgeNatural 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 π-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 π 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.ERC Horizon 2020 (grant agreement No 670405 and No 803326) EPSRC Tier-2 capital grant EP/P020259/1. Winton Advanced Research Programme for the Physics of Sustainability. Simons Foundation (Grant 601946). Swedish research council, Vetenskapsrådet 2018-0023

Ligand-Directed Self-Assembly of Organic-Semiconductor/Quantum-Dot Blend Films Enables Efficient Triplet Exciton-Photon Conversion

Blends comprising organic semiconductors and inorganic quantum dots (QDs) are relevant for many optoelectronic applications and devices. However, the individual components in organic-QD blends have a strong tendency to aggregate and phase-separate during film processing, compromising both their structural and electronic properties. Here, we demonstrate a QD surface engineering approach using electronically active, highly soluble semiconductor ligands that are matched to the organic semiconductor host material to achieve well-dispersed inorganic–organic blend films, as characterized by X-ray and neutron scattering, and electron microscopies. This approach preserves the electronic properties of the organic and QD phases and also creates an optimized interface between them. We exemplify this in two emerging applications, singlet-fission-based photon multiplication (SF-PM) and triplet–triplet annihilation-based photon upconversion (TTA-UC). Steady-state and time-resolved optical spectroscopy shows that triplet excitons can be transferred with near unity efficiently across the organic–inorganic interface, while the organic films maintain efficient SF (190% yield) in the organic phase. By changing the relative energy between organic and inorganic components, yellow upconverted emission is observed upon 790 nm NIR excitation. Overall, we provide a highly versatile approach to overcome longstanding challenges in the blending of organic semiconductors with QDs that have relevance for many optical and optoelectronic applications

Optoelectronic and spectroscopic characterisation of polymer-cadmium sulfide nanocomposite solar cells

Reducing the band gap of organic materials to accommodate absorption in the near IR has been a long standing problem of fully organic systems. In contrast, the size control of nanoparticles has enabled the tuning of the electronic band gap such that near IR absorption is achievable by a variety of inorganic materials. Furthermore, electron and hole mobilities in single crystal inorganic materials have been reported consistently well in excess of those in organic materials. However, these advantages have not translated into superior performances in devices based on solution-processed nanocrystals, which rely on good charge generation properties and an effective transport network in the photoactive layer to extract a current. To achieve an effective dispersion of nanoparticles throughout an organic matrix, traditionally, capping ligands have been employed on the surfaces of the nanoparticles. These have been shown to hinder both interfacial charge transfer, between the organic material and nanoparticle, and charge transport between nanoparticles. As a result much effort has been directed at facilitating good mixing between nanoparticles and organic materials without the use of ligands. One method that has shown promise utilises a single-source xanthate organometallic precursor, which is soluble in common organic solvents and thus easily dispersed throughout a polymer thin film. The application of heat results in the decomposition of this compound yielding volatile organic side products and a percolating metal sulfide nanostructure. The research presented in this thesis focuses on the design and function of solar cells employing photoactive layers of the semiconducting polymer, P3HT, and nanostructured CdS fabricated employing the in-situ strategy described above. Firstly, investigation of the role of the architecture of the solar cell is undertaken. The materials involved in the electron extraction process are scrutinised, followed by those responsible for hole extraction. The introduction of new materials and layers as well as their method of fabrication are considered in order to ascertain the optimum combination to support the effective operation of the active layer. The focus is then shifted on to the optimisation of the photoactive layer. Study into the effects of changing composition and processing conditions on the behaviour of devices is conducted and the potential routes for further improvement of the CdS/P3HT system are established. Next, the relationship between morphology and the photophysical properties of CdS/P3HT blends is subjected to a comprehensive spectroscopic investigation. Altering the morphology of the blends is achieved through a change in the composition ratio of the samples and confirmed by TEM. Transient and steady state absorption techniques as well as photoluminescence are used to monitor the behaviour of excited states and identify the drawbacks of the CdS/P3HT combination. Finally, the environmental stability of typical CdS/P3HT devices is examined. Devices and films are subjected to inert and ambient conditions both in the light and dark and the change in performance and charge separation yield (as determined by transient absorption measurements) is monitored over time. Furthermore a comparison is made between the stability of CdS/P3HT systems with their fully organic P3HT/PCBM counterparts. These are subjected to oxygen and water separately in order to shed light on the mechanism of degradation in both systems.Open Acces

Engineering Molecular Ligand Shells on Quantum Dots for Quantitative Harvesting of Triplet Excitons Generated by Singlet Fission.

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.ER

Research data underlying: Direct vs Delayed Triplet Energy Transfer from Organic Semiconductors to Quantum Dots and Implications for Luminescent Harvesting of Triplet Excitons

Files with data underlying the figures in the manuscript and SI are attached. Data for Absorption in file Absorption.csv, with OA dots/ Ligand Exchanged dots and ligand spectra according to column labels. ns-Transient absorption data in *.cor files with time in ns accross columns in first row, first column in wavelength /nm and DT/T signal accross wavelength and time in column and rows. Extracted ns-TA data used for fitting in files TA-data-*.csv including fitted curves. Includes column labels. ps-TA, in *.wtf files. with time in fs accross columns in first row, first column in wavelength /nm and DT/T signal accross wavelength and time in column and rows. Sample numbering for TA (ns and ps) is as folowing: 1 = 0.9 eV PbS With TET-CA ligand 2 = 1.0 eV PbS With TET-CA ligand 3 = 1.08 eV PbS With TET-CA ligand 4 = 1.18 eV PbS With TET-CA ligand 5 & 6 = 1.3 eV PbS With TET-CA ligand 14 = 0.9 eV PbS With OA ligand 15 = 1.0 eV PbS With OA ligand 16 = 1.08 eV PbS With OA ligand 17 = 1.18 eV PbS With OA ligand 19 = 1.3 eV PbS With OA ligand Excitation wavelength included in filename as "_XXX_nm_" Measured PLQE values and rate constants are listed in PLQE and k values.csv according the column labels