hrough the extensive R&D of organic light-emitting diodes (OLEDs) for more than 30 years, plenty of well-elaborated novel organic optoelectronic materials and device architectures have been extensively developed, resulted in the unique commercial utilization of OLEDs for cutting-edge smartphones, large-area TVs, and further new future display applications by taking advantage of light-weight and flexibility. From the aspect of materials science, the creation of novel light-emitting materials in OLEDs has been the central issue aimed for high electroluminescence quantum efficiency (EQE). Starting from the development of conventional fluorescence materials (1st generation) during 1990-2000th, the room-temperature phosphorescence (2000-) (2nd generation) and thermally activated delayed fluorescence (TADF) (2012-) (3rd generation) continuously pioneered the novel possibilities of organic emitters, resulted in not only high-performance OLEDs but also enriched organic photochemistry. In recent days, there have been a wide variety of studies on TADF-OLEDs because of the unlimited possibilities of TADF molecular design. Further, hyperfluorescence (HP)-OLEDs have been developed since they can realize the compatibility of high efficiency and narrow spectral width, which is ideal for practical display applications. Here we report our recent cutting-edge HP-OLEDs demonstrating high OLED performance by optimizing host, TADF, and terminal emitter (TE) molecules1-3). In particular, we focus on the blue-emission, which is capable of showing narrow FWHM and high EL quantum yield. Blue HP-OLEDs based on two new TEs are fabricated, resulting in high external quantum efficiency (EQE) of over 20%, high color purity, and high brightness. By analyzing the transient PL characteristics of the HP-OLEDs, we found that the presence of efficient FRET between TADF-assistant dopant (TADF-AD) and TE molecules. Further, transient EL analysis confirmed that a smaller EHOMO difference between TADF-AD and TE efficiently helps to decrease hole trapping inside the emitting layer, hence resulting in a lower efficiency rolloff and a longer operational device lifetime. This report provides a designing principle for a TADF and TE in HP-OLEDs with well-matched energy levels, leading to efficient FRET and no significant carrier trapping.

References:
[1] C-Y. Chan et al., Nature Photonics, 15, 203 – 207 (2021).
[2] Y-T. Lee et al., Advanced Electronic Materials, 7, 4, 2001090 (2021).
[3] M. Tanaka et al., ACS Applied Materials & Interfaces, 12, 45, 50668 – 50674 (2020).

Thermally Activated Delayed Fluorescence (TADF) process has appeared as the most popular design strategy towards reaching 100% internal quantum efficiency for Organic Light-Emitting Diodes (OLEDs). TADF consists in promoting upconversion of triplet excited states into emissive singlet ones through Reverse InterSystem Crossing (RISC), a process driven by spin-orbit coupling (SOC) and requiring a small singlet-triplet gap DE_ST. The advancement of the TADF field occurred essentially through materials design, the first strategy, as proposed by C. Adachi and co, consisting in connecting electron donating and accepting units to decrease the DE_ST. However, in doing so, the lower-lying singlet and triplet excited states bear a dominant charge transfer character that translates into a broad emission spectrum.

In this contribution, we will discuss two different topics based on computational considerations:

How doped triangleshaped molecules can lead to (i) concomitant narrow emission, high quantum yield of emission and small DE_ST resulting in a whole new generation of TADF emitters, the multiresonant TADF emitters and to (ii) a new family of compounds with an inverted singlet-triplet gap and potentially, a downwards energy RISC. To do so, we rely on high level quantum chemical calculations and show that an accurate description of electron correlation effects is key to correctly predict the excited states ordering as well as the optical properties of these compounds.

How the interactions in the solid state can turn RISC from a SOCdriven to a hyperfine interaction (HFI)-driven mechanism. Combining time-resolved and transient electron paramagnetic resonance spectroscopies as well as (time-dependent) density functional theory calculations, we demonstrated that HFI-RISC occurs through delocalized charge transfer states in a curcuminoid derivative.

Organic diradicals based on p-conjugated molecules have been proposed to fulfill the thermodynamic requirements for singlet exciton fission (SEF), S₁ ≈ 2 T₁. This is based on the particular stabilization of the first triplet excited state (i.e., T₁) regarding the singlet ground electronic state (i.e., S₀) due to the mitigation of exchange repulsion in the open-shell form. In the case of diradicals made of quinoidal organic cores, the energy distribution of the excited state is controlled by the gaining of aromaticity in the ground electronic state. This provides with a powerful tool to design organic molecules with varying diradical character to be applied as chromophores in SEF. SEF is proposed as a way to increase the charge generation efficiency of organic photovoltaics devices. In this talk, the conceptual basis of the electronic structure of diradicals will be presented together with the implications in the design of molecules for SEF as well as a few examples on organic molecules showing this phenomenon.

Organic radicals have emerged as the basis of more efficient organic light-emitting diodes (OLEDs) with higher performance limits than typical molecular emitters. This is achieved by doublet fluorescence with nanosecond emission in contrast to standard singlet-triplet photophysics found in organic optoelectronics. Here direct charge recombination at radical sites is considered the primary emission process. However, due to the low-energy radical orbitals, severe electron trapping occurs in devices with narrow emission zones at the interface of emitting layers and electron transport layers, causing significant efficiency roll-off, low radiance and poor stability. To improve the optoelectronic performance of organic radical electroluminescence (EL) devices, the device structure is optimised by tuning charge transport layers and utilising an ambipolar host. The concept is tested by transient EL measurements and single-carrier device analysis that show the limiting emission process. We demonstrate highly efficient organic radical light-emitting diodes using tris(2,4,6-trichlorophenyl)methyl (TTM)-based radicals with higher than 15% EQE at 700 nm, including substantially improved efficiency roll-off and maximum radiance over 100,000 mW sr-1 m-2 in an important step to practical organic radical light-emitting diodes.

The light harvesting properties of a solar absorber can be modified by the effect of exciton-photon coupling, as a result of the reconfiguration of the electronic structure of the molecules, that can be achieved when the exciton state of the molecule is in resonance with confined electromagnetic field of an optical cavity. Whenever strong or ultra-strong coupling regime is reached, two new hybrid light-matter states, also known as polaritons, are formed with an energy separation that is proportional to the coupling strength.[1] This form of tailoring of the eigenstate of the system represents a compelling tool in photovoltaics, being conversion efficiency, light harvesting directionality and charge transport direct consequences of such modification.

Recently, subphthalocyanine derivatives have been integrated in a polaritonic organic solar cell acting as an optical resonator in order to modify the absorption onset and tune the optoelectronic properties of the devices.[2] In view of this, a comprehensive analysis on how the different electronic transitions of an absorber can be coupled to the resonant modes of a solar polaritonic device is compelling.

Herein, the light harvesting properties of broad band light harvesting molecules operating under weak, strong and ultra-strong coupling regimes are presented. The effect of directional and spectral response, as well as the effect of polaritons on unproductive absorption due to the presence of metallic films in the structure, are discussed in detail.[3,4] These results allow to determinate the optimum configuration to exploit the potential of solar cells devised as optical resonators.

The great progress in organic photovoltaics (OPV) over the past few years was achieved largely by the development of non-fullerene acceptors (NFAs) with power conversion efficiencies now approaching 20%. To achieve this result and further enhance device performance, loss mechanisms must be identified and minimized. Especially triplet excitons are known as being detrimental to device performance, since they can form energetically trapped states, responsible for non-radiative losses or even device degradation. Using the complementary spin-sensitive methods of photoluminescence detected magnetic resonance (PLDMR), transient electron paramagnetic resonance (trEPR) and transient absorption (TA) corroborated by quantum-chemical calculations, we reveal exciton pathways and identify energetically trapped triplet excitons in OPV blends employing the polymer donors PBDB-T, PM6 and PM7 together with NFAs Y6 and Y7. Thereby, all blends reveal long-lived triplet excitons on the NFA via non-geminate hole back transfer. Further, we detect additional triplet generation on the NFA via spin-orbit coupling induced intersystem crossing in blends with the best performing halogenated polymers PM6 and PM7. In conclusion, we identify triplet formation in all tested solar cell absorber films, which underlines that there is an untapped potential for improved charge collection efficiency and elimination of device degradation caused by triplet excitons.

Singlet exciton fission (SF), and its reverse, triplet–triplet annihilation (TTA), represent two promising ways of manipulating photons in optoelectronic devices via energetic down- or up-conversion, respectively. They are also fascinating physical processes, the study of which offer insights into the fundamental physics of organic semiconductors and photoactive proteins.

In this talk, I will focus on some recent work from my group on understanding the fundamentals of SF and TTA [1] in organic molecular thin films [2,3], photoactive proteins [4] and microcavities [5]. Our recent work shows that SF and TTA both proceed via a real, emissive triplet-pair state in endo- and exothermic singlet fission materials. We find that in microcavities in the strong coupling regime, delayed emission from triplet-triplet annihilation is enhanced compared with bare films and that the spin-statistics governing up-conversion yield can be modified through molecular an intermolecular design [6].

Multiexciton quintet states, ⁵(TT), photogenerated in organic semiconductors using singlet fission (SF) consist of four quantum entangled spins, promising to enable new applications in quantum information science. However, the factors that determine the spin coherence of these states remain underexplored. Here, we engineer the packing of tetracene molecules within single crystals of 5,12-bis(tricyclohexylsilylethynyl)tetracene (TCHS-tetracene) to demonstrate a ⁵(TT) state that exhibits promising spin qubit properties, including a coherence time, T₂, = 3 μs at 10 K, a population lifetime, Tpop, = 130 μs at 5 K, and stability even at room temperature. The single crystal platform also enables global alignment of the spins and, consequently, individual addressability of the spin sublevel transitions. Decoherence mechanisms, including exciton diffusion, electronic dipolar coupling, and nuclear hyperfine interactions, are elucidated, providing design principles for increasing T₂ and the operational temperature of ⁵(TT). By dynamically decoupling ⁵(TT) from the surrounding spin bath, T₂ = 10 μs is achieved. These results demonstrate the viability of harnessing singlet fission to initiate multiple electron spins in a well-defined quantum state for next-generation molecular-based quantum technologies.

Solution processed doped layers are of extreme importance for high throughput production of organic electronic devices via roll-to-roll or ink-jet printing. In this talk, I will discuss tuning the conductivity of conjugated polymers containing Lewis basic sites by Lewis acids. Addition of the Lewis acid effectively p-dopes the hole transport in the parent polymer, leading to increases in the free hole density and conductivity. This methodology is advantageous since the polymer and Lewis acid have excellent solubility in organic solvents, negating the need for co-solvents that uses in molecular dopant such as F₄TCNQ. We use a combination of techniques including electrical measurements, optical absorption, XPS, UPS, IPES, and electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR) and nuclear magnetic resonance (NMR), and gas chromatography techniques in conjunction with density functional theory, to gain insight into the doping mechanism of 4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b′]dithiophene (CPDT) and 2,1,3-benzothiadiazole (BT)-based (PCPDTBT) by Lewis acids, BCF, and to explain why the doping efficiency is much higher for BCF than for F₄TCNQ.

Over the past five years, the solar power conversion efficiencies of organic solar cells (OSC) have greatly improved from 11% to 18%, closing the gap with their inorganic and hybrid counterparts. The major breakthrough behind the rapid efficiency improvement is the development of non-fullerene acceptor molecules, replacing the traditional fullerene molecules as electron-accepting materials. Understanding the photophysical processes underlying these high-performance materials is crucial to OSC research. In this talk, I will present transient optical spectroscopy results on non-fullerene OSC blends with small interfacial energy offsets. By optically probing the time evolution of excited states, we show that free charges are generated via thermal activation of interfacial charge-transfer (CT) states on a hundred picosecond timescale. Reduced charge separation rate is observed at lower temperatures, leading to increasing charge recombination either directly at the donor-acceptor interface or via the emissive singlet exciton state. A kinetic model is used to rationalize the results, showing that although photogenerated charges have to overcome a significant Coulomb potential to generate free carriers, OSC blends can achieve high photocurrent generation yields even at reduced temperatures given that the thermal dissociation rate of charges outcompetes the recombination rate.

Solar cells fabricated from organic semiconductors possess several benefits over traditional inorganic technologies. Organic solar cells are lightweight and flexible, whilst also offering the possibility of semi-transparent modules and superior performance in low-light intensity applications. However, whilst the power conversion efficiencies of organic solar cells have exceeded 18% under sunlight, this is still lower than inorganic technologies, where efficiencies of >20% are commonplace. Of relevance to solar cell operation, organic semiconductors have a reduced ability to screen the Coulomb interaction between two charge carriers of opposite signs compared to their inorganic counterparts. Consequently, organic semiconductors are excitonic materials, where the strong Coulombic interactions between electrons and holes results in a large exciton binding energy (~0.2-0.5 eV) and localised excited states. In these excitonic states, the spins of the paired electrons interact strongly, resulting in the formation of distinct spin-singlet (electron spins anti-parallel) and spin-triplet (electron spins parallel) states. As singlet and triplet excitons possess dramatically different optical and electronic properties, exciton spin becomes a key consideration in the operation of optoelectronic devices fabricated from organic semiconductors, such as solar cells and light emitting diodes.

In this talk, I will present our recent work on spin management in organic solar cells[1]. We find that in benchmark organic solar cells fabricated using non-fullerene electron acceptors, up to 90% of the charge carrier recombination proceeds via low energy triplet exciton states. As the radiative decay of triplet excitons back to the spin-singlet ground state is spin-forbidden, recombination via dark triplet excitons constitutes a significant voltage loss pathway in organic solar cells. Through the identification of systems where recombination via triplet excitons is suppressed, we demonstrate a novel tactic to turn off this loss pathway that involves the strong electronic coupling between the electron donor and acceptor components in the organic solar cell. Consequently, we propose molecular design rules to engineer-out recombination via triplet excitons in future systems. Therefore, our findings provide a framework to further reduce the performance gap between organic and inorganic solar cell technologies.

Organic thin films that emit and absorb circularly polarised (CP) light have been demonstrated with the promise of achieving important technological advances; from efficient, high-performance displays, to 3D imaging and all-organic spintronic devices. As a result, interest in the identification of materials that can emit CP light has surged in recent years. Unfortunately, the selectivity in absorption or emission (so-called dissymmetry or g-factor) of left-handed versus right-handed CP light is low for many molecular systems. Conjugated polymer thin films can give rise to very large dissymmetry (|g_abs| >1).The precise origins of the large chiroptical effects in such films had, until recently, remained elusive. We have investigated the emergence of such phenomena in achiral polymers blended with chiral small-molecule additives and intrinsically chiral-sidechain polymers using a combination of spectroscopic methods and structural probes. We show that – under conditions relevant for device fabrication – the large chiroptical effects are caused by magneto-electric coupling (natural optical activity), not structural chirality as previously assumed, and may occur because of local order in a cylinder blue phase-type organisation and increased delocalisation of their chromophore/excitonic coupling. We show that optimised chiral polymer systems can be used as the active layers of CP Organic Photodetectors (OPDs) and CP Organic Light Emitting Diodes (OLEDs) with exceptional device performance. For example, CP OLEDs based on the widely studied π-conjugated polymer F8BT can achieve current efficiencies of 16.4 cd/A, power efficiency of 16.6 lm/W, a maximum luminance of over 28,500 cd/m², and a high electroluminescence dissymmetry of 0.57, whilst CP OPDs achieve photocurrent dissymmetries of 0.85, low dark currents (10 pA) and ultrafast response times. Our disruptive mechanistic insight into chiral polymer thin films will offer new approaches towards the development of chiroptical materials and devices.

Single-component organic solar cells (SCOSCs) have witnessed great improvement during the last few years with the champion efficiency jumping from the previous 2-3% to currently 6-11% for the representative material classes. However, the photophysics in many of these materials has not been sufficiently investigated, lacking essential information regarding charge carrier dynamics as a function of microstructure, which is highly demanded for a better understanding and potential guidance to further improvements.

 In this work, for the first time, the charge carrier dynamics on different time scales of a representative double-cable polymer, which achieves efficiencies of over 6% as an active layer in SCOSCs, has been investigated across 7 orders of magnitude in time scale, from fs-ps TAS and ps-ns TRPL for probing charge generation to ns-µs TAS for charge recombination. Specific emphasis is placed on understanding the evolution of charge generation, transport as well as charge recombination behind the gradually improved photovoltaic performance upon thermal annealing treatment of the representative double-cable polymer PBDBPBI-Cl. By increasing the thermal annealing temperature, geminate recombination is reduced accompanied by more efficient charge dissociation and suppressed bimolecular recombination. Annealing the photoactive layer at 230 ^oC results in the highest photovoltaic performance correlating well with the findings from transient studies. This work successfully presents a complete picture of the charge carrier dynamics in SCOSCs.

Spin doublet radical organic semiconductors can show near unity luminescence yield from their lowest energy excited state and are attractive as the emissive component in organic light-emitting diodes (OLEDs). [1]

Here I will present our recent measurements of direct, rapid, spin-allowed energy transfer from triplet excitons generated within a closed-shell organic host to a doublet chromophore. We use a carbene-metal-amide (CMA-CF₃) as a model host, since following photoexcitation it undergoes extremely fast intersystem crossing to set up a population of triplet excitons within a few picoseconds. We track the subsequent energy transfer to the TTM-3PCz radical using transient absorption and temperature-dependent transient photoluminescence spectroscopies. These show that direct triplet-to-doublet energy transfer is the dominant channel that accounts for over 90% of all radical emission. OLEDs based on the CMA-CF₃:TTM-3PCz blend show improved device characteristics compared to TTM-3PCz radical OLEDs without triplet-enhanced energy transfer.

Our design overcomes triplet-imposed performance limits for optoelectronics by activating spin-allowed triplet-doublet transfer on picosecond–nanosecond timescales, with light emission obtained orders of magnitude faster than derived from conventional triplet(-singlet) management technologies.

This method allows photophysical studies to reflect the mechanisms "Behind the Device" by mimicking spin statistics present under electrical charge injection, which may be a powerful tool for the wider organic electronics community.

Excitonic transport in organic semiconductors underpins charge generation in all manner of organic electronic devices[1]. In this presentation I will discuss the role that exciton diffusion length plays in organic photovoltaic devices and introduce a steady-state technique to measure the diffusion length in organic semiconductors through exciton-exciton annihilation (EEA), named pulsed-PLQY. Previously, accurate measurements of exciton diffusion length through EEA required highly specialized time-resolved equipment[2]. I will show that by measuring in steady-state the required equipment specialization, cost, and time can be reduced, while the accuracy is improved.

Specifically, a Kinetic Monte-Carlo hopping model is used to simulate the exciton dynamics in an organic semiconductor. These dynamics are analysed using traditional and steady-state EEA techniques. It is found that pulsed-PLQY is less sensitive to the choice of initial density and has increasing confidence with increasing densities used. These simulations are validated by preforming both steady-state and traditional EEA experiments on organic semiconductors, including technologically relevant non-fullerene acceptors (NFAs). Overall, it is found that NFAs show an increase in diffusion length, driven primarily by an increases in diffusivity, when compared to the benchmark fullerene acceptor [3].

Conjugated polymers and molecular semiconductors are emerging as a viable semiconductor technology in industries such as displays, electronics, renewable energy, sensing and healthcare.  A key enabling factor has been significant scientific progress in improving the charge transport properties and carrier mobilities of these materials, which has been made possible by better understanding of the molecular structure-property relationships and the underpinning charge transport physics. Here we aim to present a coherent review of how we understand the unique charge transport physics in these van-der Waals bonded semiconductors, that can be understood within the framework of transient localisation which is intermediate between classical hopping and band transport. Specific questions of interest include a discussion of the coupling between charge and structural dynamics, how the transport physics of conjugated polymers and small molecule semiconductors are related and how the incorporation of a high concentration of counterions in doped films, as used for example in thermoelectric devices, affects the electronic structure and charge transport properties and how these processes can be studied by spectroscopic tools.   

The function of essentially all present and future quantum devices, from quantum computers over quantum sensors to photocatalytic systems or solar cells, relies on the motion of charges and spins on ultrafast time and exceedingly short length scales. Usually these dynamics are governed by such a complex interplay between electronic and nuclear motion that their understanding is quite limited and we rely on particle-like transport models for describing the dynamics and function of those systems.

In my talk, I will introduce and discuss several systems in which this classical, particle-like transport regime breaks down and the wave-like coherent transport of energy and charge becomes dominant, even in disordered nanostructures and at room temperature. I will try to demonstrate this for different transport processes and in quite diverse systems, ranging from artificial light-harvesting molecules to technologically-relevant materials such as halide perovskite crystals. Specifically, I will discuss coherent intramolecular charge transport processes in donor/acceptor molecules (1) and organic thin films (2), and coherent intermolecular charge transfer in P3HT/PCBM solar cell layers (3). More recently, we have found strong evidence for the role of intermolecular conical intersections (CoIns) in the energy transport in aggregated donor/acceptor thin films (4). Using an advanced spectroscopic method, two-dimensional electronic spectroscopy (2DES), we have followed the coherent wavepacket motion across such a CoIn (4). Finally, I will discuss what 2DES can tell us about electron-phonon couplings in halide perovskites (5). Our studies of charge transport in organic materials give unexpected evidence for long-lived vibronic quantum coherence and outline strategies for molding the flow of charge in nanostructures by tailoring and controlling their coherent coupling to vibrational modes of the materials. These advances became possible by probing the optical properties of nanostructures with a time resolution of few femtoseconds only, faster than any of the functionally relevant vibrational modes of the material, and by comparing the experimental results to advanced theoretical modelling..