Symposium EP1 : Organic Excitonic Systems and Devices

Emitter orientation fundamentally limits the performance of organic light emitting diodes (OLEDs). If the emissive dopant in an OLED could be efficiently aligned in the thin film the out coupling could be markedly enhanced. To explore the orientation of dopant/host systems in the solid state we synthesized a coumarin based ligand which was cyclometallated onto an iridium core to form heteroleptic molecules. (bppo)₂Ir(acac), (bppo)₂Ir(ppy) and (ppy)₂Ir(bppo) (bppo = benzopyranopyridinone, ppy = 2-phenylpyridinate and acac = acetylacetonate) have large dipole moments ranging between 6.2 D and 8.4 D, with different orientations of both the permanent electrical ground state and transition dipole moments. Resultant orientation of transition dipole moment vectors of the emissive molecules in the films were measured by angular dependent p-polarized photoluminescent emission. (bppo)₂Ir(acac) is found to orient horizontal relative to the substrate, with a net vertical component of the transition dipole moment vector of only 22%, while (ppy)₂Ir(bppo) and (bppo)₂Ir(ppy) are isotropic in the solid films. This result shows that the dipole moment of the complex is not involved in the dopant alignment and supports our hypothesis that the acac group is responsible for the observed orientation in heteroleptic complexes. This leads to new design rules for further improving horizontal orientation. Orientation is unchanged by aggregation even though all species demonstrate concentration quenching and a bathochromic shift in emission at the higher doping levels.

Over the past decade the charge carrier mobility of polymeric semiconductors has increased immensely, now surpassing that found in amorphous silicon¹. Despite this, one of the major challenges facing the field of organic optoelectronics is to develop material systems which possess outstanding charge carrier mobility as well as efficient light emission. This trade-off has so far limited the development of an electrically injected laser diode and is due to a number of fluorescence quenching mechanisms such as the energy gap law, charge transfer state formation and exciton trapping states^2,3. Here we show that it is possible to decouple light emission and electrical performance in the high mobility polymer poly(indacenodithiophene-alt-benzothiadiazole) (IDTBT) through highly efficient resonance energy transfer to a more emissive squaraine based dye molecule which acts to enhance the fluorescence quantum yield of the system by outpacing intrinsic decay mechanisms in the IDTBT. Despite the low fluorescence quantum yield of IDTBT high transfer efficiencies of 0.5 at a dye loading of 0.3 wt. % are achieved by selecting a polymer-squaraine dye combination with an exceptionally large Forster radius of 9.1 nm. In fabricating near infrared organic light-emitting devices we realise almost an order of magnitude enhancement in the external quantum efficiency using the polymer-dye system compared with the neat polymer material without significant detrimental reduction in current density. Moreover, these devices exhibit radiances of up to 5 W/m²str which is comparable to state of the art organic/inorganic NIR LEDs operating at similar wavelengths^4,5. We propose that with further optimisation and material design that this methodology of decoupling charge transport from electroluminescence is a suitable mechanism to developing ultra-bright organic devices.



1. Sirringhaus, H. 25th Anniversary Article: Organic Field-Effect Transistors: The Path Beyond Amorphous Silicon. Adv. Mater. 26, 1319-1335 (2014).

2. Englman, R. & Jortner, J. Energy gap law for radiationless transitions in large molecules. Mol. Phys. 18, 145-164 (1970).

3. Mikhnenko, O. V, Kuik, M., Lin, J., van der Kaap, N., Nguyen, T.-Q. & Blom, P. W. M. Trap- Limited Exciton Diffusion in Organic Semiconductors. Adv. Mater. 26, 1912-1917 (2014).

4. Tan, Z.-K., Moghaddam, R. S., Lai, M. L., Docampo, P., Higler, R., Deschler, F., Price, M., Sadhanala, A., Pazos, L. M., Credgington, D., Hanusch, F., Bein, T., Snaith, H. J. & Friend, R. H. Bright light-emitting diodes based on organometal halide perovskite. Nat. Nanotechnol. 9, 1-6 (2014).

5. Sun, L., Choi, J. J., Stachnik, D., Bartnik, A. C., Hyun, B.-R., Malliaras, G. G., Hanrath, T. & Wise, F. W. Bright infrared quantum-dot light-emitting diodes through inter-dot spacing control. Nat. Nanotechnol. 7, 369-373 (2012).

The degradation of UV and blue OLEDs is monitored via the positive and negative spin 1/2 electroluminescence (EL)- & electrical current-detected magnetic resonance (ELDMR & EDMR). The positive (EL- and current-enhancing) resonance is attributed to reduced quenching of singlet excitons (SEs) by (presumably trapped) polarons and triplet excitons (TEs), whose populations are reduced at resonance. While the SE quenching mechanism is spin-independent, the population of polarons and TEs decreases at resonance due to enhanced spin-dependent quenching of TEs by polarons. This quenching mechanism converts trapped polarons to high-energy mobile polarons, consequently increasing the current through the device. However, as suggested recently, it also raises the possibility that these high-energy polarons, with a total energy of ~5.5 – 7 eV relative to the HOMO level, also cause C-H, C-C, or C-O bond breaking and/or rearrangement.
Concomitant with device degradation, the negative (EL- and current-quenching) spin 1/2 resonance amplitude increases. This resonance is attributed to enhanced spin-dependent formation of positive spinless bipolarons stabilized at negative defect sites. The possibility that these negative defect sites are related to those generated by the high-energy polarons is also discussed.

Thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) had drawn great attention in the past few years. Due to their merits of being free of rare metals and achieving high internal quantum efficiencies up to 100%. However, device operational lifetime was still lag behind phosphorescent OLED. To meet the commercialization requirements, the stability of the devices is critical.
The relation of charge balance and stability in TADF OLEDs was investigated by controlling the charge mobility of host material. An emissive layer of 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) doped with (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN), a green TADF emitter, was selected for this study. The power efficiency, current efficiency and external quantum efficiency were enhanced after doping an electron transporting material into the emissive layer. Under constant current operation starting at a luminescence of 1000 cd/m², the time to reach 90% of initial luminance (LT₉₀) was doubled. Mechanism of lifetime enhancement was also discussed. Cation of host, mCBP, was found to be unstable, which leads to the device degradation. These findings underscore the mechanism of operational lifetime enhancement in the devices. Lifetime enhancement was further enhanced by other methodologies.

The commercially manufactured fluorescent OLEDs have a critical disadvantage in terms of an efficiency since only singlet excitons can be harvested by fluorescent molecules. Recently, thermally activated delayed fluorescence (TADF) and exciplex-emitter based OLEDs have been developed to achieve high external quantum efficiency (EQE) due to its promising nature of exciton harvest from triplet channel. However, these intra- and intermolecular charge transfer complexes have drawbacks such as the broad emission spectra and device stability compared to fluorescent materials. The fluorescent OLEDs with exciplex-forming co-host have been highlighted since it is possible to harvest the host triplet via reverse intersystem crossing (RISC) in exciplex followed by energy transfer to the singlet channel in fluorescent dopant. In this system, increasing the triplet-harvesting is a key factor to approach the ultimately high EQE in fluorescent OLEDs. Here, we report a conventional fluorescent OLED with the highest 24% EQE by increasing triplet harvesting. This research will certainly assist to promote industrial trend toward conventional fluorescent materials with low price and long device lifetime.

Since its first demonstration in 1998, electrophosphorescence has yielded 100% internal quantum efficiency OLEDs (PHOLEDs) across the color gamut. This advance has given birth to a huge global OLED display industry, now estimated to be >$15B, or more than 10% of all displays made. While red and green devices have shown extraordinary operational lifetimes, blue PHOLEDs have had unacceptably short lifetimes. In this talk I discuss the fundamental origins of this deficiency¹, and approaches to extending the lifetime of blue PHOLEDs. Indeed, recent results in our laboratory have already yielded a 10 fold improvement in lifetime, the first such major improvement in almost a decade². The understanding gained from these experiments suggest that much longer lifetimes, even for very deep blue emitting PHOLEDs, is possible. Furthermore, I will discuss recent results in achieving deep blue electrophosphorescence and the relationship between color and molecular structure in this spectral range. Lastly, I will discuss how the lessons learned in improving PHOLED lifetime also extends to organic solar cells.

1. N. C. Giebink, B. W. D’Andrade, M. S. Weaver, P. B. Mackenzie, J. J. Brown, M. E. Thompson and S. R. Forrest, J. Appl. Phys. 103, 044509 (2008).
2. Y. Zhang, J. Lee and S. R. Forrest, Nature Commun. 5, 5008 (2014).

The efficiency of most fluorescent OLEDs is much lower than that of phosphorescent OLEDs because only 25% of the excitons are emissive singlet and other 75% of the excitons are non-emissive triplet. In order to convert the non-emissive triplet excitons to the emissive singlet excitons, two approaches have been reported: triplet-triplet annihilation (TTA)[1] and thermally activated delayed fluorescence (TADF).[2] In addition to these two approaches, reverse intersystem crossing (RISC) at electron-hole pair, charge transfer complex (CT), or hot exciton (Tn ≥ 2 and Sn ≥ 2) state have been proposed.[3,4]
We designed and synthesized a bianthracene compound, in which two anthracene groups were directly linked. X-ray analysis and TD-DFT calculation of the compound showed spatially orthogonal structure and localized frontier orbitals of the two anthracenes. Strong solvent dependence of PL spectra of the compound showed CT characteristics due to the orthogonal bianthracene structure. The OLEDs with the bianthracence compound as a blue fluorescent emitter were fabricated, and the monoanthracene compound MADN was used for comparison. The emitting layers were non-doped neat film. Transient electroluminescence responses of the devices were measured. The device with MADN showed large delayed component derived from TTA, and intensity of the delayed component were strongly dependent on current density and increased as current density increased. This result clearly demonstrated the bimolecular process of the TTA with monoanthracene MADN. On the other hand, the delayed component of the device with the bianthracene was much smaller than that of MADN and independent on current density. This result suggests that the bianthracene was not related with the TTA process even in a neat film, although the internal quantum efficiency was higher than 25%. In order to investigate the contribution of the triplet excitons, magnetic field effect (MFE) of electroluminescence was measured. The EL intensity of MADN decreaced under the high magnetic field, exhibiting negative MFE. In contrast, the EL intensity of the bianthracene device increased when the magnetic field was applied, exhibiting positive MEL. These opposite MFEs of MADN and the bianthracene devices indicate totally different contribution of T1 excitons for the emission in the devices.
[1] D. Y. Kondakov et al., J. Appl. Phys. 106, 124510 (2009).
[2] A. Endo et al., Adv. Mater. 21, 4802 (2009); H. Uoyama et al., Nature 492, 234 (2012).
[3] M. Segal et al., Nature Mater. 6, 374 (2007); S. Difley et al., JACS 130, 3420 (2008); L. Yao et al., Angew. Chem. 126, 2151 (2014); T. Sato et al., J. Mater. Chem. C 3, 870 (2015)
[4] J.-Y. Hu et al., Adv. Funct. Mater. 24, 2064 (2013).

Harvesting triplet excitons is an important issue to improve the organic light emitting diode (OLED) efficiency because 75% of electrically generated excitons are directly formed at a triplet state. Therefore, modern OLEDs often contain room-temperature phosphorescent emitters to promote intersystem crossing via spin-orbit coupling to harvest all excitons through emissive phosphorescence. While the emissions occur from a sole energy level of either singlet or triplet excited states, dual emission of both fluorescence and phosphorescence is another possibility to harvest all singlet and triplet excitons for light emission. Further, a significantly slow phosphorescence originated form the pure organic molecule would be useful for afterglow lighting. However long-lived phosphorescence (LLP) cannot be obtained in concentrated environmental conditions due to the presence of noradiative deactivation processes from triplet states. Recently, we developed LLP of over 1 s from a mixture of a hydroxyl-steroid and aromatic emitters. The hydroxyl-steroid provides a rigid and amorphous environment for guest emitters, which significantly minimizes the non-radiative decay of the long-lifetime triplet excitons. Thus, afterglow OLEDs can be realized by introducing an organic LLP emitter layer into an OLED structure.
In this study, we report an OLED having a LLP emitter, demonstrating long transient decay of electroluminescence after turn off. To achieve LLP from an emitter layer, we developed the host molecule, 3-(N-carbazolyl)-androst-2-ene (CzSte), which can minimize the nonradiative decay of guest emitters. Although the CH₂Cl₂ solution of N,N'-Bis(3-methylphenyl)-N,N'-bis(phenyl)-9,9-dimethyl-fluorene (DMFLTPD)-d₃₆ did not exhibit phosphorescence because of the rapid nonradiative decay, the co-deposited films of CzSte and DMFLTPD-d₃₆ exhibited both fluorescence and phosphorescence at room temperature. An OLED consisting of DMFLTPD-d₃₆ : CzSte active layer exhibited both blue and green emissions originating from the fluorescence and LLP (τ_p = 0.39 s). Further, when we applied coronene-d₁₂ as LLP emitter, a long τ_p of 4.31 s was achieved.

Phosphorescent iridium complexes have long been thought to have random orientation when doped in an emitting layer due to their octahedron structures. Recently, however, some heteroleptic iridium complexes have been reported to have preferred emitting dipoles orientation (EDO) along horizontal direction (parallel to substrates). The outcoupling efficiency of the emitted light from the horizontally oriented emitting dipoles in an OLED can reach 45% which is much higher than isotropically oriented transition dipoles.
In this talk, we will present that the preferred EDO of Ir complexes in OLEDs originates from the preferred direction of the triplet transition dipole moments and the strong supramolecular arrangement with host molecules. The EDO is influenced by many factors which can be summarized as follows: (1) Heteroleptic iridium complexes are more likely to have preferred orientation in host materials than homoleptic iridium complexes. (2) There is a linear correlation between the EDO in films and the orientation of the transition dipole moments against the C2 axis of the heteroleptic Ir complexes. (3) The EDO of heteroleptic Ir-complexes varies from horizontal to isotropic, or even to vertical direction depending on host molecules. (4) The preferred molecular orientation of the host molecules does not induce the preferred molecular orientation of the dopant molecules.
Finally high efficiency OLEDs with external quantum efficiencies over 35% will be presented using Ir and Pt based phosphorescent dyes possessing highly oriented emitting dipoles and high photoluminescent quantum yields.

Under electroluminescence operation, about 75% of the excitons formed in organic light-emitting diodes (OLEDs) come to life as triplet states. High quantum yields based on phosphorescent materials have been heavily investigated within the last decades resulting in internal quantum efficiencies (IQE) approaching unity. While phosphorescent OLEDs make use of expensive rare metal complexes to increase triplet harvesting from the non-radiative dark states via efficient spin-orbit coupling to overcome the IQE limit of 25% for fluorescent OLEDs, especially the stability and substantial efficiency roll-off at higher current densities of blue phosphorescent materials remain open issues. The concept of thermally activated delayed fluorescence (TADF) opened new pathways for purely organic material designs to face these fundamental problems. TADF allows via effective reverse intersystem crossing (RISC) for triplet states to be redirected to emissive singlet states, allowing for 100% exciton utilization.
Although there has been an amazing progress in TADF, the overall understanding of the TADF unlocking molecular properties is still in its infancy and the fundamental properties to tackle are still under debate. To increase the IQE of TADF materials, the competition between internal conversion, radiative, and non-radiative rates decides over good and bad emitter molecules. Both from computational and experimental point of view, a great effort has to be made to get access to the relevant photophysical properties of the emitting molecules. In principle, TADF materials are designed to break the conjugation between donor and acceptor units to separate HOMO and LUMO states. As a consequence, the exchange energy is minimized and a small energy split (ΔE_S-T ) between S₁ and T₁ level allows for both increased intersystem crossing and substantial RISC rates.
In this talk, we will present our recent efforts in the understanding of the basic concepts of TADF mechanism. The first part will deal with the rational design of a new TADF material class based on phenyl-carbazoles. Reasonable isomer structures have been constructed theoretically and slight stoichiometric modifications were performed to understand how the molecular structure and its steric hindrance affects the RISC. By this approach, the most promising candidates were identified using TD-DFT schemes and were synthesized accordingly. Photophysical properties and device parameters based on the most promising emitters are discussed. Our discussion will also focus on the interplay of local and charge transfer excited states, which is of key importance to unlock efficient TADF-based electroluminescence. The energetic ordering and the relative transition moments of internal CT states between donor and acceptor subgroups with respect to local excitations of either donor or acceptor depend sensitively on the specific molecular structure, discussed widely as key influencing parameters.

Recently, the emitting dipoles with perfect horizontal orientation parallel to the
substrate result in the theoretical external quantum efficiency (EQE) limit over 45%
without any extra light extraction structures compared to 25~30% for randomly oriented
emitting dipoles. Large effort has been poured to increase the horizontal emitting
dipole ratio (Θ) to get 70~82%. However, it will be difficult to obtain an even higher Θ
due to the amorphous nature of typical emitting layers. In this regard, organic crystals
would be the better emitters because of the orientational and positional ordering.
However, organic crystals have rarely been used for OLEDs due to low stability and
efficiency. In this work, we realized a crystal OLED with unprecedented high EQE of
38.8% using the perfectly oriented Pt based thin film emitting layer possessing PLQY
of 96% and Θ of 93%. We investigated the emitting dipole orientation of the thin films
fabricated using Pt complexes and discussed the structural relationship between X-ray
structural analyses and structures in thin films based on quantum chemical
calculations. The emitting dipole orientation of the crystal emitting layers was largely
affected not only by the crystallinity of the emitting layer but also by the molecular
arrangement in the crystal.

Abrupt interface donor (D) acceptor (A) OLEDs offer a rather unique way of studying pinned exciplex states in the solid state. Here we present time resolved spectroscopy and electroluminescence data from such abrupt interface devices, effectively hole transport layer abutted to the electron transfer layer with no defined, fixed width emitter layer, as a function of the energy offsets between the ground (HOMO) and first excited (LUMO) states of the D and A. We show how these offsets critically control the reverse electron transfer step which dictates exciplex lifetime and final emitting species and emission energy. Further, we observe very strong E-field dependent electroluminescence from which we can determine the electron hole separation across the interface. We also find, counter intuitively that increasing E-field increases the efficiency of radiative exciplex emission. We discuss these findings in terms of the Onsager model. Such abrupt junction devices may be use in optical computing and communications given that simple E-field tuning of emission energy is afforded without quenching of the emitting states.

Metalloporphyrins are an important class of compounds with interesting photophysics and versatile applications, like sensing, two photon absorption, biological and artificial light-harvesting (dye-sensitized solar cells, DSSC′s), organic light emitting diodes, (OLED′s) and photocatalysis^[1]. In particular, the photophysics of Nickel porphyrins is of interest, as they have versatile excited-state properties that are influenced by distortion of the porphyrin macrocycle. In this study, we focus on the excited-state deactivation mechanism of one paradigm Nickel porphyrin, i.e., Nickel-Tetra-Mesityl-Poprhyrin (NiTMP). Its excited states are poorly understood due to the coupling between the molecular orbitals involved in the initial (π, π*) excited state and molecular orbitals dominated by metal 3d electron character. Earlier studies have shown, that within less than 20 ps after the initial excitation, the electron in the π* orbital transfers into the unoccupied d_x2-y2 orbital, while one electron from the d_z2 orbital fills the hole in the initially occupied π orbital^[2]. A detailed experimental analysis of this process was so far not performed due to a lack of sufficient time resolution. But the involvement of a short lived π-d state has been suggested. The quantum mechanical calculations presented here shine light on the initial processes within the first picoseconds after photoexcitation. In particular, we investigate the deactivation mechanism involving conical intersections to unravel the role of the π-d state in the dynamics of NiTMP.
JP and BD want to thank the Nagelschneider Foundation for financial support.

1. Presselt, M., et al., Quantum Chemical Insights into the Dependence of Porphyrin Basicity on the Meso-Aryl Substituents: Thermodynamics, Buckling, Reaction Sites and Molecular Flexibility. Physical Chemistry Chemical Physics, 2015. 17(21): p. 14096-14106.
2. Shelby, M.L., et al., New insight into metalloporphyrin excited state structures and axial ligand binding from X-ray transient absorption spectroscopic studies. Coordination Chemistry Reviews, 2014. 277–278: p. 291-299.

Photovoltaics and solid state lighting are examples of technologies that are enablers for a new sustainable energy economy, since these technologies would allow us to use a renewable energy source, and to use energy sources we already have more efficiently. Organic light emitting diodes (OLEDs) with potentially high power efficiency are considered as strong candidate for the next generation of display and illumination devices. Moreover, the use of environmentally benign organic materials in white OLEDs and their potentially low fabrication cost makes them an attractive technological prospect. In this presentation, we will discuss our continuing efforts on the design, synthesis and characterization of novel platinum and palladium complexes for displays and lighting applications. The photo-physics, electrochemistry, electroluminescent properties and operational stability of these novel platinum complexes, including fluorine-free Pt-based deep blue emitters, will be discussed. The rational molecular design enables us to develop cyclometalated Pt complexes with both photon-to-photon (in thin film) and electron-to-photon (in device settings) conversion efficiency close to 100% for OLED applications. We will discuss the development of a wide range of efficient, blue-emitting Pt complexes enabling OLEDs with record purity blue emission of (0.14, 0.08), external quantum efficiency exceeding 25%, or blue OLEDs with improved operational lifetimes.

In contrast to their inorganic counterparts, the majority of molecular materials exhibit an additional degree of freedom due their anisotropic shape. The microscopic orientation of molecules in thin film devices has a strong impact on macroscopic properties such as charge carrier transport and optical properties as well as on the efficiency of optoelectronic devices.
This talk will present different approaches to control the orientation of emitter molecules in organic light-emitting diodes. We find that molecular orientation in vacuum-deposited thin films occurs at the surface of the growing film, however, with significant differences of the driving forces between fluorescent and phosphorescent guest-host systems. For the former type the glass transition temperature of the matrix in relation to the substrate temperature is the key parameter [1]. In the latter case, this parameter is of minor importance; it is rather the intrinsic molecular asymmetry of metal-organic Iridium complexes that lead to non-isotropic phosphor orientation at the boundary between an organic layer and vacuum [2]. We investigate the influence of different processing conditions and discuss consequences for device efficiency of OLEDs.
References:
[1] Control of Molecular Dye Orientation in Organic Luminescent Films by the Glass Transition Temperature of the Host Material, C. Mayr, W. Brütting, Chem. Mater. 27 (2015) 2759-2762; DOI: 10.1021/acs.chemmater.5b00062
[2] Understanding and Predicting the Orientation of Heteroleptic Phosphors in Organic Light-Emitting Materials, M. J. Jurow, C. Mayr, T. D. Schmidt, T. Lampe, P. I. Djurovich, W. Brütting, M. E. Thompson, Nature Materials 15 (2016) 85–91; DOI: 10.1038/NMAT4428

Biluminescence is a property of certain organic molecules, where light is emitted from both their singlet (named fluorescence) and triplet (named phosphorescence) excited states. Although the latter is a quantum mechanically forbidden transition, phosphorescence can be achieved if non-radiative channels are suppressed effectively. For instance, creating a simple host:guest system in which a biluminophore (i.e. materials with biluminescence property) is embedded in an optimum rigid matrix (e.g. polymers, crystals, small molecules), competitive thermal decay is suppressed, allowing emission from the triplet states in addition to the conventional fluorescence at room temperature [1]. Not only the dual state emission is unique to this class of materials, but also the exciton dynamic range spanned by the two spin states is extreme. Here up to nine orders of magnitude lie between nanosecond-lifetime fluorescence and second-lifetime phosphorescence.

In this presentation, we will report on our recent advances in different biluminescent systems. As an archetypicalsystem, we have identified a combination of PMMA [poly(methyl methacrylate)] as host and NPB[N,N’-di(naphtha-1-yl)-N,N’-diphenyl-benzidine] as biluminophore, giving rise to blue fluorescence and green phosphorescence. Due to the sensitivity of the triplet state to oxygen, we have investigated the dependence of the persistent phosphorescence on the oxygen content. These findings are of key importance for the development of novel optical sensing capabilities. Additionally, we will report on the structure-property relationships between the host materials used and the resulting phosphorescence luminescence efficiency.

[1] Reineke, S. and Baldo, M. A. (2014), Room temperature triplet state spectroscopy of organic semiconductors. Sci. Rep. 4: 3797. doi: 10.1038/srep03797

The unique combination of properties (strong absorbance, high fluorescence quantum yield, photostability, n-type semiconductivity) makes perylene bisimides a favored class of dyes for fundamental photophysical studies and various applications. Motivated by the prospects arising from the supramolecular organization of these dyes we have intensively investigated the organization of perylene bisimide dyes by non-covalent forces into desirable nanoscale architectures. In this lecture, I will highlight some of our recent achievements in the preparation of defined perylene bisimide dye assemblies and the functional properties that originate from proper p-p-stacking. In particular, the spectral characteristics and relaxation processes of photoexcited H- and J-aggregates will be discussed as well as the consequences for long range exciton transport. Furthermore I will illustrate our recent achievements in the synthesis of p/n heterojunction model systems composed of oligophenyleneethynylene backbones and appended perylene bisimide dyes.

Organic polaritons are quasiparticles that form in organic microcavities when the exciton-photon interaction rate exceeds that due to dissipation from either the bare exciton or photon component. They obey Bose statistics and inherit a very light effective mass from their photon component. In addition, they interact repulsively through their exciton matter component. At high enough densities, polaritons can condense into their energetic ground state and form a macroscopic coherent state termed a polariton condensate [1]. This state obeys the rich physics of the driven-dissipative Gross-Pitaevskii equation in analogy with certain cold atom systems, but under ambient conditions.

In contrast to many low-temperature systems, however, the coherent light emitted by the condensate can be used to directly probe the phase of the underlying quasiparticles. In this work, we will explicitly show the emergence of long-range spatial coherence in organic polariton condensates, the spontaneous formation of vortices, and the various regimes of stability as a function of the pump shape [2]. Remarkably, the structural disorder present in organic microcavities plays very little role in the observed physics. Our findings are supported by numerical solutions to the Gross-Pitaevskii equation (GPE) under the various experimental pumping conditions. We will also show how repulsive interactions between excitons and polaritons cause a ballistic flow of polaritons away from the pump spot. The polariton propagation length can be orders of magnitude longer than that of bare excitons, which may be interesting for applications that rely on energy transfer in molecular systems. In addition to practical applications as coherent light-sources, the ease with which organic microcavities can be fabricated and characterized makes them ideal systems for studying the rich physics of the GPE.

References
[1] K. D. Daskalakis et al., Nat. Mater. 13, 271-278 (2014)
[2] K. D. Daskalakis et al., Phys. Rev. Lett. 115, 035301 (2015).

Optically-pumped lasers based on solution-processed thin-film gain media have emerged as low-cost, broadly tunable and versatile active photonics components that can fit any substrate and are useful for e.g., chemo- or biosensing or visible spectroscopy. Although single-mode operation has been demonstrated in various resonator architectures with a large variety of gain media (including dye-doped polymers, organic semiconductors, and, more recently, hybrid perovskites) the reported linewidths are always typically on the order of a fraction of a nanometer or broader, i.e. the coherence lengths are no longer than a few millimeters, which does not enable high-resolution spectroscopy or coherent sensing. The linewidth is fundamentally constrained by the short photon cavity lifetime in standard resonators, that are either too short (e.g. Organic VCSELs) or too lossy (e.g. DFB or DBR lasers.)
We demonstrate here an organic thin-film solid-state laser scheme based on a vertical external cavity, wherein a holographic Volume Bragg Grating (VBG) ensures both spectral selection and output coupling in an otherwise very compact (~cm³) design. The combination of an external cavity (closely spaced but very sharp cavity peaks) with a very selective grating reflector (typ. <100 pm FWHM) ensures single mode operation together with a narrow linewidth. The grating was recorded in a photo thermo-refractive-glass by interference of two UV beams. We first used a long (8-mm) high-diffraction efficiency (98%) grating. The gain medium consists of a Rhodamine 640 dye-doped PMMA layer spin-coated on the curved cavity mirror. Under short-pulse (0.4 ns) laser pumping at 532 nm, Fourier-transform-limited laser pulses were obtained, with a FWHM linewidth of 900 MHz (1.25 pm). Using 20-ns-long pump laser pulses, the linewidth could be further reduced to 200 MHz (0.26 pm), corresponding to a coherence length of 1 m, although the spectrum was here 4 times broader than the Fourier limit. This corresponds to an enhancement of 2-3 orders of magnitude compared to the state-of-the-art in terms of spectral purity of organic solid-state lasers.
In order to make the laser tunable, we used interference of spherical wavefronts to make a VBG with a transverse chirp. The grating had a peak reflectivity of 99% for 610 nm, a transverse chirp of 60 pm/mm. By translating the grating and with no impact on the cavity alignment, the laser emission wavelength has been tuned over 3 nm with sub-GHz spectral bandwidth and output peak power in the kW range.
In conclusion, we have shown that the spectral purity of a thin-film organic solution-processed solid-state laser can be enhanced by several orders of magnitude due to the combination of a volume Bragg grating with a vertical external cavity architecture. The concept is potentially transferrable to any type of thin-film laser and can be ultimately made tunable; it also represents a very compact alternative to bulky grating systems in dye lasers.

Planar integrated circuits involving organic or hybrid photonic building blocks such as optical waveguides, interconnects, detectors and light sources, are promising in many applications such as lab-on-a-chip, integrated sensors or portable spectrometers. Organic lasers are ideal integrated light sources in such devices due to their high spectral brightness and their wavelength flexibility across the whole visible spectrum. However, efficient coupling of pump light into a small-footprint (typically a few µm in width) active waveguide is in practice only possible with an external laser source that cannot be easily integrated. The recent development of high-emittance (> W/mm2) Light-Emitting Diodes (LEDs) enables considering them as pump light sources for organic lasers [APL 92, 163306 (2008)] despite their low brightness compared to conventional laser sources. As LEDs are lambertian emitters, it is impossible to enhance their emittance by geometrical concentration in virtue of the brightness conservation law, but this limit can be broken with Luminescent Concentrators (LC), as a result of the Stokes shift between emission and absorption spectra. The concept of pumping an organic laser by LCs was first proposed by Yang et al. [Adv. Mat. 21, 31, p. 3205-3209 (2009)], wherein a LC unit and a separate DFB laser were used, but no lasing under LED pumping was shown. We propose here a simple approach that takes full benefit of the engineering capability offered by ink-jet printing and the availability of photopolymerizable optical materials, enabling easy design of laterally-patterned multicolor optical elements.
We investigate a testbed composed of a 25-µm2 cross section SU8 ridge waveguide doped with a deep-red dye (Rhodamine 700) acting as the gain medium, pumped from both sides by two mm-sized concentrator sheets made of DCM-doped 5-µm thick polymer film, excited by a 1x6 mm2 panel of 450-nm LEDs emitting 60-ns FWHM pulses. We expose the design rules of this new photonic component relying on ray-tracing simulations (using LightTools® software). The importance of index matching between active and concentration units in the weak absorption regime (absorption length >> wavelength) is highlighted; it is also shown that the Stokes shift in the LC part has an optimal value to secure a trade-off between photon energy loss and reabsorption losses. While minimizing passive losses and maximizing absorption is a key priority for low-threshold lasing in the design of the active part, these conditions can be relaxed for the concentrator region wherein low reabsorption losses and high quantum yield become priority. To explore the feasibility of an all-integrated LED pumped laser chip, the test structure was realized by photolithography and subsequent spin-coating of the concentrator sheets, and experiments toward a LED-pumped organic laser using this approach will be shown. We thank the Agence Nationale pour la Recherche (EDELVEIS project) for funding this work.

Natural opals are fascinating gemstones with an intriguing play of colors. This specific reflection of individual colors at different angles is a direct result of the microstructure of the material. Opals are composed of monodisperse colloids with diameters in the range of the visible light, which are precisely arranged in a crystalline structure. Such photonic crystals can also be prepared from synthetic monodisperse particles via colloidal self-assembly. These structures show the same reflectance behavior as natural opals, while the photonic bandgap of the self-assembled material can be controlled via the particle diameter and the refractive index of the material.
To make these systems useful, we produce monodisperse particles from semi-conducting polymers to add the quality of an electronic bandgap. We assemble these particles into photonic crystal and control the particle size so that the electronic and the photonic bandgaps coincide. This way, fluorescence from the semiconducting polymers is reflected inside of the photonic crystal structure, allowing resonance of the generated emission. We can deposit these self-assembled active photonic crystals together with a sol-gel matrix to encapsulate the photonic resonator structure. By using ink-jet printing we can produce resonators with any desired shape and structure. When exciting such self-assembled active photonic crystal resonators we can produce laser radiation with thresholds, which are typical for conjugated polymer materials. The fabrication follows a truly additive pathway and represents the easiest way to generate a laser resonator. The use of any nano- and microfabrication techniques can be completely circumvented and chemical etching and backfilling procedures are avoided.

Nonlinear optical properties in organic semiconductors have been attractive for many practical applications such as frequency up-conversion lasers. Although various organic semiconductors have been proposed to date, their up-conversion lasing performances have been severely limited due to the difficulty of simultaneously achieving strong nonlinear optical response and high performance optical gain [1, 2].
Previously, truxene-based star-shaped oligofluorenes having three (T3) and four (T4) fluorene units per arm have been synthesized and these molecules have demonstrated exceptional optical gain performance under single photon absorption pumping [4, 5]. In this work, we report structurally designed star-shaped oligofluorenes exhibiting strong structure-property relationship enabling enhanced nonlinear optical response with favorable optical gain performance [3]. To this end, we introduce and develop T6 molecules, which have six fluorene units per arm. Through systematic investigation of the two-photon absorption (TPA) cross-section in these tailor-made oligomers with varying arm length (i.e., T3, T4 and T6), we reveal a strongly enhanced nonlinear absorption as a function of increasing arm length. The TPA cross-section in T6 molecules is found to be as large as 2200 GM, which is an order of magnitude larger than that of the conventional dyes and other small organic molecules [2]. We further investigate TPA pumped amplified spontaneous emission (ASE) and lasing in these star-shaped molecules with strong nonlinear response. We found a TPA pumped ASE threshold as low as 2.4 mJ/cm² in T6 molecules, which is better than that of the lowest reported threshold in organic semiconductors in general and lower even than colloidal quantum dots having giant TPA cross-sections. Also, using T6 molecules we develop a TPA pumped flexible distributed feedback (DFB) laser. We fabricate the DFB laser from a plastic grating with a period of 266 nm to spectrally match with the optical gain of the T6 molecule in the deep-blue. Above the TPA pumped lasing threshold, which is at ~3.1 mJ/cm², we observe a spectrally and spatially coherent fan-shaped laser beam with a linewidth less than 1.5 nm. To the best of our knowledge, our TPA pumped lasing threshold is the lowest among all organic up-conversion lasers ever reported to date [3].
These achievements suggest that rationally designed star-shaped oligomers are one of the most promising candidates for nonlinearly pumped organic optical gain media. Their frequency up-converted lasers in the deep-blue region make them attractive for many applications such as photodynamic therapy and bio-sensing.

[1] C. Bauer et al. Adv. Mater. 14, 673 (2002).
[2] G. S. He et al. Chem. Rev. 108, 1245 (2008).
[3] B. Guzelturk et al. J. Mater. Chem. C (2015). DOI: 10.1039/C5TC02247A
[4] A. L. Kanibolotsky et al. J. Amer. Chem. Soc. 126, 13695 (2004).
[5] G. Tsiminis et al. Appl. Phys. Lett. 94, 243304 (2009).

Research on exciton-polaritons in microcavities has evolved rapidly since the first demonstration of strong coupling in 1992. Since the experimental realisation of the first polariton laser, the main focus of research lies on exploiting the fundamentally interesting properties of the condensate and on paving the way towards applications.
The latter requires operation of the lasers at room temperature or above, which has been shown to be achievable by using organic semiconductors. These materials support Frenkel excitons which are characterised by small radii and high binding energies leading to reduced interaction and high stability. A remaining major challenge that currently still prevents a break-through in applications lies in the demonstration of an electrically driven organic polariton laser at room temperature.
The work presented here discusses the fabrication and characterisation of metal-clad, optically probed organic microcavities containing different solution-processable organic materials. The analysis was performed by fitting transfer matrix simulations to the experimental reflectance data and comparing the observed polariton modes to predictions from the coupled oscillator model. We found that good agreement of the calculations with the experimental data can only be obtained when taking into account the structural and thus optical anisotropy of the thin films. While in polymers, mainly the absorption strength of the same resonances varies in different orientations, the angle-dependent excitation of a J-aggregate material shows different resonances depending on its direction. Thus we find that it is very important to consider the optical anisotropy when designing organic polariton devices.
Furthermore, these simple structures allow for a focus on the basic strong-coupling characteristics, which enables, by comparison to other material properties, an analysis of the parameters decisive for observing strong coupling. Collating the Rabi splitting with excitonic characteristics in the investigated organic materials ranging from polymers to J-aggregates confirms the absorptivity of the organic materials to be the critical property for large coupling strengths. Other effects such as different linewidths of the excitonic resonances or photoluminescence quantum yields were found to be much less important.

The simple processing and high photoluminescence (PL) efficiency of light-emitting polymers make them good materials for inexpensive photonic devices such as; organic semiconductor lasers (OSLs) high bandwidth colour converters and light emitting optoelectronic devices (e.g. OLEDs). One way to increase the functionality of planar devices is to introduce wavelength scale microstructures. A photonic crystal grating can be used to provide an optical resonator to an organic film gain medium in organic distributed feedback (DFB) lasers, and nanostructured surfaces have been shown to add functionality to OLEDs to increase their external efficiency and give directional output.

A common way to create these structures is to pre-pattern a substrate with a resist and deposit the organic material on top. While very effective there are several limitations imposed by this method; for example the resist layer restricts the grating formed between itself and the polymer to be relatively low-contrast due to the available refractive indices of common resists. A more favourable architecture is to directly pattern the emissive layer itself to take advantage of the higher index contrast at the polymer/air interface. Solvent immersion imprint lithography (SIIL) is a recent solvent based patterning technique reported by Vasdekis et al, has been used to imprint microfluidic channels. Presented here is the use of SIIL to create nanostructured wavelength scale gratings directly in conjugated polymers, creating OSLs and directional emitters comparable to devices made with ultraviolet nanoimprint lithography (UV-NIL).

Conjugated polymer solutions were deposited onto glass slides pre-treated with an adhesion promoter to create light emitting thin films. The films were immersed with acetone, converting them into a surface ‘gel’, and nanoimprinted using a perflouropolyether soft stamp. The soft stamps were cast from silicon master gratings made by e-beam lithography and reactive ion etching. The soft stamps were then pressed into the surface gel using an EVG photomask aligner with custom nanoimprint tooling at a constant uniform pressure. During contact acetone could leave the polymer film through the stamp. After a time of at least 5s the stamp was separated from the polymer film; with an inverted copy of the soft stamp grating imprinted into the surface.

Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images revealed high fidelity gratings made out of conjugated polymers such as poly(9,9-dioctlyfluorenyl-2,7-diyl) (PFO) and poly[2,5-bis(2’,5’-bis(2’’-ethylhexyoxy)phenyl)-p-phenylenevinylene] (BBEHP-PPV). Gratings depths achieved were up to 80 nm in BBEHP-PPV and 25 nm in PFO with lateral feature sizes as low as 70 nm. By engineering grating period and film thickness to achieve 2^nd order distributed feedback at the peak gain of the polymers we achieve lasing in each polymer with similar performance to DFB lasers fabricated with UV-NIL gratings.

Metal nanoparticles are perfectly suited to control and enhance the photoluminescence (PL) properties of organic semiconductors by the strong electrical field components of their localized surface plasmon resonances. We investigated the coupling between silver nanoprism arrays and layers of the archetypical organic semiconductor ZnPc (zinc-phthalocyanine) both prepared by shadow nanosphere lithography. As a measure for the interaction between the excited states of the organic semiconductor and the localized surface plasmons, the spatially resolved PL was measured by confocal microscopy and yielded a strong increase of the ZnPc luminescence on top of the silver nanoprisms. Embedding ZnPc at low concentrations of about 4% by co-evaporation in an Alq₃ matrix further reduces non-radiative decay processes caused by exciton-exciton-annihilation or quenching at metal/organic interfaces. Thus, these periodic ordered Ag/Alq₃:ZnPc/Ag hybrid structures provided large PL factors of up to 50 (700 upon geometrical correction). By means of complementary FDTD simulations we were able to explain the observed ZnPc emission enhancement by the spectral overlap with the localized surface plasmon resonances of the silver nanoarray.

The migration of Frenkel excitons, tightly-bound electron-hole pairs, in organic and hybrid organic-inorganic semiconducting films is critical to the function of many next generation optoelectronic devices. While these materials can exhibit a high degree of structural heterogeneity on the nanoscale, traditional measurements of exciton diffusion lengths are performed on bulk samples. Since both the characteristic length scales of structural heterogeneity and the reported bulk diffusion lengths are typically smaller than the optical diffraction limit, we adapt far-field super-resolution fluorescence imaging to determine in-situ exciton diffusivities and to uncover the correlations between the structural and energetic landscapes that the excitons explore.

Thermally Activated Delayed Fluorescence (TADF) has recently gained significant attention in the field of Organic Light Emitting Diodes (OLED). Thanks to efficient reverse T->S intersystem crossing, both singlet and triplet excitons can recombine radiatively through fluorescence from the S₁ state. The design of efficient TADF emitters hinges on minimizing the exchange splitting, as the nearly vanishing singlet-triplet gap enables rapid intersystem crossing even if the magnitude of the spin-orbit coupling is not significant. At the same time, one has to ensure that S₁ is a bright state.

Computational studies have proved to be a useful tool in facilitating the design of molecules with very small singlet-triplet gaps. Quantum chemical methods provide also deep insight into electronic processes occurring in an OLED device; from charge injection, through exciton generation to the final emission of a photon. However, majority of the simulations to date focus on gas phase calculations of isolated molecules or dimers, while in reality, the emission layer is a typical host-guest system, where emitters are dispersed in a matrix. The matrix servers multiple roles like facilitating exciton generation through efficient charge migration, preventing concentration quenching of excitons and fine-tuning of the optical properties of TADF emitters. The careful choice of host and guest systems can significantly improve quantum efficiency of a device.

In this contribution, we present condensed-phase computational studies of TADF OLED emission layers. To generate a morphology of the host-guest system we use classical molecular dynamics (MD) simulations based on the OPLS force field parametrized for a set of TADF host and guest molecules proposed in the literature. Then, we perform electronic structure calculations to study the alignment of HOMO and LUMO levels in the material as well as emission wavelengths of embedded guest molecules. To account for the condensed-phase interactions, we couple quantum mechanical methods with molecular mechanics (MM) within the polarizable QM/MM framework. Our studies allow us to better understand the mechanism of exciton generation in a realistic device setup and the influence of electrostatic interactions with the environment on the energetics of ground and excited states. We hope that this insight will allow for development of new design rules for novel TADF materials.

Organic semiconductors characterized by a small singlet-triplet exciton energy splitting exhibit efficient reverse intersystem crossing and thermally activated delayed fluorescence (TADF). Indeed, much recent attention has been directed at the application of materials that exhibit TADF in efficient organic light-emitting devices. From the perspective of exciton diffusion, these systems are interesting in that transport may occur along both the singlet and triplet excited states, each with unique photophysical behavior and exciton energy transfer mechanisms. Delayed fluorescence systems, therefore, provide a unique test-bed for characterizing the role of exciton spin in transport and diffusion. In this work, concentration and temperature dependent photophysical characterization combined with measurements of the exciton diffusion length (L_D) for 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) elucidate the relative degree and magnitude of transport along the singlet and triplet states as well as the role of the dielectric environment in determining the intersystem balance. Interestingly, transport via both the singlet and triplet states is appreciable in 4CzIPN, with an enhanced role played by the triplet state in dilute films.

Energetic and spatial disorder can profoundly affect exciton transport in soft materials systems. Using colloidal quantum dot (QD) assemblies as a prototypical system, we explore the effects of energetic and spatial disorder on incoherent site-to-site exciton hopping processes. I will present experimental investigations of space-, time-, and energy-resolved exciton transport, including time-resolved optical microscopy, spectrally-resolved transient photoluminescence spectroscopy, and compare these results to kinetic Monte Carlo simulations. In particular, I will emphasize the non-equilibrium nature of transport in inhomogeneously broadened ensembles and the effect it has on our understanding of diffusivity and the diffusion length.

The ability of some animals to navigate using Earth’s magnetic field is truly perplexing. How can tiny fields of one Gauss induce physiologically relevant reactions when Zeeman shifts are over a million times smaller than kT? The secret appears to lie in field-induced modifications to the effect of hyperfine interactions which become relevant because of the exceptionally long spin coherence times of radical pairs. OLEDs provide an unrivaled proving ground to explore the interplay between spin coherence, spin correlations and external fields through spin-dependent transport and luminescence.
Spin-lattice relaxation in OLEDs is virtually independent of temperature and very slow. Spin dephasing over microseconds can be quantified by pulsed magnetic resonance using conventional echo schemes. Slow spin dephasing enables the direct observation of spin-Rabi flopping of both electron and hole species, which, under suitable resonance conditions, couple to each other by spin beating. Such signals are, in principle, sensitive down to the single carrier within the OLED, since the measurement reports on spin permutation symmetry rather than on thermal spin polarization. As the sole parameter determining the resonance condition is the g-factor, compact OLED-based low-frequency resonance circuits can be designed to serve as versatile magnetometers. With novel dual singlet-triplet emitters, singlet-triplet oscillations in the radical-pair can now also be probed directly by a color change in emission.
Recent highlights in exploiting coherent singlet-triplet oscillations in OLEDs include the demonstration of direct control of the hyperfine interaction by NMR [1], quantification of the zero-field splitting of intermolecular excited-state species [2], and the direct manifestation of the elusive ac-Zeeman and spin-Dicke effects [3].

[1] Malissa, Boehme et al., Science 345, 1487 (2014).
[2] van Schooten, Boehme et al., Nature Comm. 6, 6688 (2015).
[3] Waters, Boehme et al., Nature Phys. doi:10.1038/nphys3453 (2015).

The functional unit of an organic light-emitting diode (OLED) device is comprised of an emitter molecule diluted in a host matrix. Such molecular environments present both static and dynamic heterogeneities which are not captured in bulk, ensemble measurements. Through single molecule fluorescence microscopy we probe the spectral and time-resolved emission properties of a thermally-activated delayed fluorescence (TADF) emitter embedded in transparent polymer matrices. We observe two separate emission regimes: one with a short (~6 ns) decay that displays small fluctuations, and a longer (~12 ns) decay with much larger fluctuations. We relate these emission regimes to the intramolecular conformation of the emitter and the charge transfer nature of its excited state, and to the variability in the packing of the host polymer around the emitter. The study of larger clusters of emitter molecules (N~15) allows the extraction of relevant time scales of singlet-triplet equilibration in a local manner. These time scales of ~50 µs are comparable to bulk measures of TADF. Extending these studies to host materials with favorable charge transport properties will enable the tuning of host-guest interactions for improved performance.

Triplet state (or called “dark state”) is a long-lived excited state which is spin forbidden for light absorption and photon emission. Conventionally, it is not easy to spectroscopically measure the dynamics of triplet exciton in organic materials directly. In some organic materials (such as tetracene derivatives), the energy of two triplets is close to that of one singlet (2T₁ ~ S₁), and energy transfer is possible between the singlet excited state and the triplet pair state, known as singlet fission and triplet fusion processes. Hence, by measuring the transient photoluminescence (TrPL), one can obtain the temporal information of singlet and triplet excitons from analyzing prompt and delayed fluorescence. In this study, we found thin (< 20 nm) amorphous rubrene film can effectively confine the triplet exciton “spatially”, which provided a possible way for understanding the diffusion process of triplet excitons. In our experiment, we employed TrPL measurement on amorphous rubrene thin film with different thicknesses (from 5 to 100 nm) fabricated by thermal evaporation. When the incident pulse energy was ultra-low (3.29 nJ/cm²), the exciton dynamics were identical for different rubrene thicknesses. However, with a higher incident pulse energy (43 mJ/cm²), compared to the thicker (100 nm) rubrene film, fluorescence decay of thinner rubrene film deviated from the single exponential decay curve and the intensity increased at the temporal range ~0.2 ns-1 ms, which was even more pronounced when the rubrene film was thinner (5 nm). Such a behavior reflected the enhanced non-germinated recombination (triplet fusion) process due to the triplet confinement in thinner rubrene film under higher excitation pulse energy. A 3D Monte-Carlo simulation was performed to obtain the singlet and triplet populations over time, as well as the fluorescence decays, which showed similar trends with the experimental results.

Organic light emitting diodes (OLEDs) have become a proven technology for display applications, while research into white OLEDs is ongoing. The need for multiple emitters and high current densities in order to meet the strict technical requirements of high color quality and high luminance white light makes the design of white OLEDs more challenging than for their monochrome counterparts. In order to establish rational device design principles, a better understanding of certain fundamental processes will be necessary. To this end, we present the results of several studies of excitons in OLEDs. The life of a typical exciton includes several important processes which contribute to the overall device efficiency, including formation, diffusion, energy transfer and possibly quenching. Each of these processes will be examined in the archetype hole transport layer and host 4'-bis(carbazol-9-yl)biphenyl (CBP). First, analysis of neat CBP using thin sensing layers incorporated into OLEDs will provide insights into singlet exciton dynamics. Moving on to more complicated systems we will then measure the energy transfer efficiency of the competing Förster and Dexter energy transfer mechanisms in multi-doped OLEDs. The implications of these studies for the design of the next generation of OLEDs will be discussed.

The ability to confine and manipulate light below the diffraction limit is a major goal of future multifunctional optoelectronic/plasmonic systems. New near-infrared emitters and device concepts are necessary to combine voltage-controlled electroluminescence and plasmonic structures (e.g. plasmonic waveguides) in a single device. Here, we demonstrate the design and realization of light-emitting field-effect transistors (LEFET) with integrated nanoantennas as tunable sources of localized excitons coupled to surface plasmons. By precise spatial control of the near-infrared emission zone in an LEFET via the applied voltages the near-field coupling between electrically generated excitons and nanoantennas can be turned on or off as visualized by a change of electroluminescence intensity. Narrow bandgap donor-acceptor polymers with broad emission and a large Stokes shift as well as single-chirality semiconducting single-walled carbon nanotube (SWNT) networks with very narrow emission and small Stokes shift are employed as the transport and emission layers. The effect of randomly distributed gold nanorods and large periodic arrays of gold nanodiscs on coupling is investigated for both types of near-infrared emitters.

Organic light emitting field effect transistors (LEFETs) combine the light emitting function of an organic light emitting diode (OLED) with the switching property of a FET in a single device giving access to simpler, cheaper and smaller devices to be used in OLED display technology.[1] One challenge in LEFETs is to develop semiconducting materials which simultaneously exhibit high light emitting efficiency and high charge carrier mobilities.[2] The challenge arises because generally there is a trade-off between the two parameters – in order to achieve high charge carrier mobilities, the chromophores need to pack well in the solid state, but, in many cases this leads to luminescence quenching.

To overcome this, we have developed new square planar Pt(II) complexes as the active materials for LEFETs. The use of these phosphorescent emitters allows harnessing both singlet and triplet excitons generated in the LEFETs, as opposed to simple fluorescent emitters, where triplet excitons are non radiative.[3] The square planar structure of the Pt(II) complexes enables better molecular packing in the solid state and hence has the potential to facilitate high charge carrier mobility. Furthermore, some Pt(II) complexes have been shown to exhibit low aggregation induced luminescence quenching or even aggregation induced emission. This allows for higher chromophore concentrations in the emissive layer and hence increased direct charge injection, bypassing the mobility providing but non emissive host material.

In this presentation we will first discuss the synthesis of new planar Pt(II) complexes using a simple and high yielding one-pot reaction. Their spectroscopic properties along with the effect of aggregation and excimer formation, will then be described. Finally, the fabrication of area emitting (instead of edge emission) LEFETs to give high brightness and good external quantum efficiencies of 855 cd/m² and 0.1%, respectively, will be reported.
This presentation aims to gain insight into the unconventional approach of exploiting the chromophores’ concentration induced electroluminescence in a carefully tailored device architecture.

[1] A. Hepp, H. Heil, W. Weise, M. Ahles, R. Schmechel, H. Von Seggern, Phys. Rev. Lett., 2003, 91,
3157406.
[2] K. Muhieddine, M. Ullah, B. N. Pal, P. L. Burn, E. B. Namdas, Adv. Mater., 2014, 26, 6410-6415.
[3] E. B. Namdas, B. B. Y. Hsu, Z. Liu, S.-C. Lo, P. L. Burn, I. D. W. Samuel, Adv. Mater., 2009, 21, 4957-4961.

Light harvesting antennae rely upon Förster Resonance Energy Transfer (FRET) to funnel the absorbed energy to a reaction center. In such structures, optimizing FRET is critical to efficient energy transport. However, inhomogenities in fluorophore orientation or position lead to reductions in energy transfer efficiency. Specifically, it is well known that dye orientation leads to deviations from the 6^th power dependence of energy transfer efficiency on distance predicted by Förster theory. In particular, isotropic distributions of static dyes have been shown to have a weaker distance dependence, owing to the large number of unfavorable orientations in such an ensemble. Here we report how multiple FRET pathways from the donor to an acceptor compensate for these inhomogenieties.

We examine energy transfer between two identical donors and two identical acceptors, that themselves can undergo homo-FRET, attached to a dual-rail DNA scaffold. We show experimentally that the addition of a second donor yields an unexpected increase in energy transfer efficiency. Additionally, addition of a second acceptor yields a larger increase in efficiency than expected, i.e. beyond a simple doubling of the acceptor absorption. Through monte-carlo simulations of an ensemble of static dyes with an isotropic distribution of orientations, we show that these unexpected increases can be understood as suppression of poor FRET pathways.

Though it is common to assume dynamic dyes that can sample all possible orientations during energy transfer, the linker chemistry holds the dyes static over time scales relevant to energy transfer. In an isotropic distribution of static dyes, there are many more ways to form unfavorable orientations for energy transfer than favorable orientations. However, homo-FRET to a second donor allows access to a second pathway for energy transfer to the acceptor. This suppresses the low efficiency pathway as FRET follows the path of least resistance. We show that, as the number of donors increases, the energy transfer efficiency of a static distribution will approach that of a dynamic distribution, where dyes are free to reorient. These are important considerations when designing light harvesting networks and may aid in the understanding of incoherent hopping transport in other systems.

The control of energy transport in organic materials is of fundamental importance for the development of efficient light-harvesting and charge transport systems. This transport is easily deteriorated by traps in the disordered energy landscape. Throughout the last few years, we have theoretically and computationally proposed and analyzed organic/inorganic nanostructures that support topological edge states of excitons and polaritons (i.e. plexcitons=excitons+plasmons). Backscattering of these chiral quasiparticles is prohibited by symmetry, ensuring that the transport properties of such a system are robust against disorder. To implement our ideas, we explore chemical platforms that recreate exotic phases of matter previously anticipated only in highly controlled model systems in solid state physics. Our proposals are the first blueprints for realizing topological phases of matter in molecular aggregates and suggest novel paradigms for engineering novel excitonic materials.

Excitons in organic and nanostructured materials are fundamental energy carriers that mediate the interaction between light and matter. The manipulation of their lifecycle is at the heart of excitonic optoelectronic devices such as light emitting diodes, lasers, and single photon sources. In this talk I will show how plasmonic nanoantennas can profoundly modify the generation, radiation, and decay rates of excitons. The optical nanoantennas resemble a microwave patch antenna and consist of colloidal silver nanocubes coupled to a metallic film, separated by a controlled sub-10 nm spacer layer embedded with emitters. The large electric field enhancements in this unique plasmonic platform allows for unprecedented control over spontaneous emission. Molecules coupled to the nanoantenna experience an enhancement of their spontaneous emission rate by a factor of ~1,000, while maintaining high quantum efficiency and directionality of emission (Nat. Photon. 8, 835 (2014)). By coupling colloidal quantum dots to the nanoantenna, we demonstrate ultrafast spontaneous emission with a bandwidth of ~100 GHz (Nat. Commun. 6, 7788 (2015)). We also use this versatile plasmonic platform to manipulate two-dimensional materials and achieve enhanced single-photon generation.

Organic excitonic devices, such as organic solar cells (OSCs) and light-emitting diodes (OLEDs) rely on the ability of Frenkel excitons either to dissociate at the interface between p- and n-type materials to produce separated charges or to recombine within the recombination layer to produce photon emission. The efficiency of the aforementioned devices directly depends on the exciton diffusion within the active layer. Various techniques to measure exciton diffusion length and diffusion coefficient in organics have been proposed and utilized to date [1] with their pros and cons. Here we combine the advantages of a few of them into a novel approach based on ultrafast photoinduced absorption (PIA) spectroscopy, to follow the exciton diffusion process in real time. The material of interest (absorber) with known thickness is covered by an exciton quencher, the hole or electron accepting layer. After spectrally-selective photoexcitation of the absorber by the ultrashort visible pulse, the photogenerated excitons diffuse to the interface with the quencher where they dissociate into pairs of charges. The concentration of charges (holes) in the quencher is measured via photoinduced polaron absorption. The resulting transient directly yields the exciton diffusion coefficient; therefore, only a single sample of well-defined thickness is required.
We applied the new method for real-time tracking of singlet exciton diffusion to vacuum-deposited layers of the well-known OSC acceptor, C₇₀ fullerene. In our experiments, vacuum-deposited C₇₀ layers of different thicknesses from 6 nm to 196 nm (absorber) are sandwiched between 10 nm thick layers of tris[4-(5-phenylthiophen-2-yl)phenyl]amine (TPTPA) which acts as a quencher and hole acceptor. We obtained the exciton diffusion coefficient of D~3.5*10^-3 cm²/s which is a factor of 15 higher than in the prototypical soluble OSC acceptor material [70]PCBM [2]. The high diffusion rate is explained by extremely small energetic disorder of ~5 meV, as demonstrated by temperature-dependent measurements. Experimental results are well-described with a simple model of 1D diffusion and supported by extensive Monte-Carlo simulations of the exciton 3D random walk in the C₇₀ cubic grid. We foresee the proposed noninvasive time-of-flight technique as a powerful tool for further development of organic opto-electronic components, such as simple layered solar cells [3], thin-film light-emitting transistors, and electrically pumped lasers.

[1] S. M. Menke and R. J. Holmes, Energy Environ. Sci., 7(2014).
[2] Hedley et al., Nat Commun, 4 (2013)
[3] Cheyns et al., Appl. Phys. Lett., 104 (2014)

Exciton multiplication mechanisms offer a way of boosting photovoltaic efficiencies beyond theoretical single cell limits[i]^,[ii]. Singlet exciton fission, the process in organic semiconductors whereby two triplet excitons are generated per absorbed photon, offers a way of improving existing inorganic technologies via organic-inorganic single-junction architectures[iii]^,[iv]. In these bilayer devices energy is harvested from the organic material via triplet excitons, and a good understanding of triplet exciton diffusion is critical. Recent studies have shown that triplet diffusion in singlet fission chromophores may not follow expected Dexter transfer mechanisms and that transport can be significantly enhanced if the triplet excitons can couple to the singlet exciton manifold via triplet-triplet annihilation (TTA)[v]. Clearly, a good understanding of the mechanism of fission in such systems, such as the nature of intermediate states, is advantageous for device design.

Here, we investigate an efficient singlet fission system, TIPS-tetracene, where fission occurs via a stabilized triplet-pair intermediate state in disordered thin films. This result follows our previous work where we identified the same state in concentrated solutions[vi]. In both the film and solution we show this state dissociates into free triplet excitons that do not couple back to the initial singlet, with possible implications for their diffusion. In contrast, we show that fission also occurs in crystals of TIPS-tetracene, but without an identifiable low-energy, emissive intermediate state. We present results that suggest a triplet –pair state still plays a role in crystalline TIPS-tetracene and may never dissociate into isolated triplet excitons. We consider these findings indicate that bound triplet-pair states could live for tens of nanoseconds in certain systems, and in some cases much longer. This result could have important implications for device design where separation of triplet pairs is necessary for high quantum efficiencies.


[i] Hanna MC, Nozik AJ (2006) Solar conversion efficiency of photovoltaic and photoelectrolysis cells with carrier multiplication absorbers. J Appl Phys 100(7):074510.

[ii] Smith M-B, Michl J (2010) Singlet fission. Chem Rev 110(11):6891–6936.

[iii] Thompson NJ, Wilson MWB, Congreve DN, Brown PR, Scherer JM, Bischof TS, Wu M, Geva N, Welborn M, Voorhis TV, Bulovic V, Bawendi MG and Baldo MA (2014) Energy harvesting of non-emissive triplet excitons in tetracene by emissive PbS nanocrystals. Nat Mat 13,1039-1043

[iv] Ehrler B, Walker BJ, Bohm ML, Wilson MWB, Vaynzof Y, Friend RH and Greenham NC (2012) Nat Comm, 3, 1019.

[v] Wan Y, Guo Z, Zhu T, Yan S, Johnson J and Huang L (2015) Nat Chem 7, 785-792.

[vi] Stern HL, Musser AJ, Gelinas S, Parkinson P, Herz LM, Bruzek MJ, Anthony J, Friend RH and Walker BJ (2015) PNAS, 112, 7656-7661.

Photochromic molecules provide an intriguing and relatively untapped alternative to traditional materials utilized in organic memory devices. We have recently reported on a new prototype of a nonvolatile light-emitting organic memory (LE-OMEM) that integrates a layer of crosslinkable dithienylethene photochromes (XDTE) into a solution-processed, multilayer OLED. The XDTE molecules undergo a change in both their UV-visible absorption and energy level position due to a photo- and/or electrically-induced ring-opening/-closing reaction. Exploiting the difference in HOMO and LUMO energies of both isomers and the subsequent change in hole-injection barrier we use this XDTE layer as an electrical switch within our OLED layer stack. Optimized devices have displayed ON/OFF ratios in both current and electroluminescence of greater than 10⁴. We investigate both optical and electrical programming of the OMEM devices and show that precise control of the ratio of both isomers in the active layer enables access to a multitude of intermediate states demonstrating the potential of these devices for future multilevel memory applications. We also discuss the difference in the molecular-scale mechanisms that are responsible for the optically- and electrically-induced switching effect in these devices by in-situ monitoring of the fraction of closed molecules as a function of the external stimulus.

Due to its superior properties including ultrafast response, thermal and chemical stability, and flexible processing, phthalocyanine (Pc) molecules with the conjugated π-electron system have been widely investigated for use in various optoelectronic devices. Pcs are two-dimensional aromatic molecules with an inner ring, and various kinds of metals can be coordinated to the center of rings. Thus, the chemical and electronic properties of metallophthalocyanines (MPcs) can be tuned through the variation of the metal center and the molecular arrangement. MPcs are of particular interest for photovoltaic and photoconductivity applications due to their high absorption coefficient in a wide spectral range of solar radiation and high energy conversion efficiency.
Attributed to its low-dimensional molecular structure, intermolecular charge transfer in Pcs is possible and it is well known that the energy relaxation of the exciton is critically dependent of the molecular arrangement in molecular assemblies. The exciton relaxation of monomeric form of Pc molecule in solution is different from that in solid film with ordered stacking of molecular columns. In this work, by means of ultrafast time-resolved pump-probe spectroscopy, we investigated the dependence of exciton relaxation on the size and the orientation of molecular arrangement in the nanostructured zinc phthalocyanine (ZnPc) and zinc hexadecafluoro phthalocyanine (F₁₆ZnPc) films. Transient transmittance responses measured at different probe wavelengths and polarizations enabled us to understand the various optical absorption and relaxation processes. The ground state bleaching near the Q-band edge followed by multi-exponential recovery was commonly observed for ZnPc and F₁₆ZnPc nanorods. The diffusive migration of singlet excitons before annihilation dominates the exciton dynamics. The singlet exciton lifetime shows a strong dependence on the length of nanorods as well as the polarization of the probe beam. Longer exciton lifetime along the axis of nanorod implies the diffusive migration of excitons through the molecular columns stacked parallel to the substrate. In lateral direction to the axis of nanorod, the diffusion of excitons is limited by randomly oriented molecular columns. These results suggest the evidence of anisotropic growth of Pc molecules in nanostructures and identify the anisotropic singlet exciton lifetimes in Pc organic materials. In addition, X-ray diffraction measurement of our Pc films suggests that Pc columns are consisted of micro-size domains and their orientations are random along in-plane direction. Finally, this work provides a full understanding of dynamics of excitons in nanostructured organic materials and the reduced singlet exciton diffusion within nanostructures offer their potentials in small-molecule photovoltaic devices.

In some organic materials such as tetracene derivatives, energy of triplet exciton is about half of singlet exciton. The singlet exciton resides on one molecule may split into two triplet excitons by sharing its energy between the triplet pair that reside on two adjacent molecules. This spin allowed process is called singlet fission which can be utilized to improve efficiency of solar energy harvesting. The reverse process where two triplets fuse into a singlet is called triplet fusion. It’s promising for triplet harvesting in organic light-emitting diode, and for wavelength conversion in (bio-) sensing. In this study, we investigated the fission and fusion process in rubrene (a tetracene derivative) thin film by transient photoluminescence (TrPL) measurement. Amorphous rubrene thin film was deposited on the glass substrate by thermal evaporation with the thickness of 100 nm. In TrPL measurement, prompt and delayed fluorescence were monitored at 5 ns and 2 ms window, respectively, with varying excitation pulse energy and temperature. To investigate the activation energy of fission and fusion dynamics in amorphous rubrene thin film, moderate excitation pulse energy (0.047 mJ/cm²) was applied to avoid singlet-singlet annihilation while keeping satisfactory signal-to-noise level under different temperatures (77 to 300 K). As the temperatures rise, both fission and fusion rates increases following the Arrhenius law with activation energy of 29.1 and 12.9 meV, respectively. These results showed non-adiabatic energy transfer characteristics in amorphous rubrene thin film due to the small electronic coupling between the singlet excited state and the triplet pair state.

Inkjet-printing (IJP) is today a widely-used technique in organic electronics, enabling large area manufacturing with high throughputs and low costs. IJP renders simple the lateral patterning of different active materials with a good resolution. Although it has been used for realizing organic LEDs, transistors or solar cells, it remains largely unexplored for organic lasers [App. Phys. Exp. 2012, 5, 072101]. Solid-state organic lasers offer a huge potential for building potentially disposable and broadly tunable coherent sources in the visible spectrum. They enable wavelength coverage over the whole visible spectrum, with no limitations on the number of materials that can be deposited laterally side by side, in contrast with inorganic active materials. An additive printing technique is especially attractive in the context of organic lasers at it enables printing “pixels” with different materials emitting at different wavelengths, tuning being obtained by a simple translation of the laser pump spot. In addition, it is a solution to the photodegradation issue as the laser pump sot can be easily scanned over a large pixel when the laser output power drops below a defined level. In this paper, we present an IJP organic laser based on a vertical external cavity, that enables high-energy (>30 µJ) and a high conversion efficiency (>30 %) in a diffraction-limited beam, that is, in which the emission is funneled into the fundamental gaussian mode of a stable optical cavity. Laser pixels are printed onto a transparent slide forming a disposable capsule which is then set between two dielectric mirrors to form the resonant cavity. While “coarse” tuning is realized by sending the pump spot on the desired pixel with a selected dye, a 2-µm free-standing rotating polymer film acts as a fine tuning Fabry-Perot etalon to achieve quasi-monochromatic (~1 nm wide) emission. The laser based on 2 dyes (Pyrromethene 597 and Rhodamine 640) is here tunable between 570 and 670 nm.
The laser ink has been formulated in order to obtain thick films (> 10 µm to obtain full absorption of the 532-nm pump light with low dye concentration to avoid aggregation) that have the optical quality and transparency required for laser applications. For this purpose we used a commercial UV-curable dielectric ink, EMD6415 from Sun Chemical, known for its dielectric properties but which had never been used in the context of printed photonics. A small (<1% vol.) fraction of dye is added after dissolution in ethanol. After UV curing at 365 nm, 20-µm thick non-porous films are obtained for a drop spacing of 20 µm, in the form of square mm-sized pixels. The films show good planarity without any scalloped effect, a low roughness (<2 nm RMS), and high transparency (>95%). The laser performance is shown to be comparable to the reference device made from a spin-coated PMMA uniform layer, establishing the interest of IJP in the realization of organic lasers.

Here, we introduce a novel ray-optical configuration, named compound parabolic trapper (CPT), using 1D compound parabolic concentrator arrays as a light trapping structure. CPT has been realized by plastic molding process and the PCE of a PTB7-Th-based polymer photovoltaic cell (PV) was improved from 9.38 % to 10.0 %. Further enhancement in optical path length is expected when the CPT is integrated with V-groove textured surface (VCPT). As a result, the PCE of the polymer PV was further improved to 10.4 % by attaching VCPT film on the device, with a broadband enhancement of light absorption. Meanwhile, surface plasmon resonance (SPR) in the active layer can lead to the strong absorption enhancement possibly higher than Yablonovitch limit at specific wavelength region. By locating the resonant wavelength at the band-edge region of active materials, low absorption can be largely compensated. We propose a multi-scale approach to achieve perfect light trapping. VCPT was combined with metal nanogratings with a period of 417 nm, and the PCE of the polymer PV was improved from 9.38 % to 10.8 %. Especially, the visible light absorption of the polymer PV was increased from 77.3 % to 94.1 %, validating the design principle to achieve the black PV system.

The effects of the thiophene donor length on film morphology and device performance were investigated for two thiophene-isoindigo copolymers. The polymer backbones are composed of the isoindigo acceptor unit along with different numbers of electron-donating thiophene units: one thiophene unit in P(T1-iI) and three thiophene units in P(T3-iI). Despite the similar chemical structures, nearly identical band-gaps and photo-physical properties, the device characteristics upon mixing with PC₇₁BM were found to be very different.
Atomic force microscopy images revealed very different domain sizes for the two polymer-fullerene blends and suggested better polymer-fullerene intermixing for P(T3-iI):PC₇₁BM. Transient photovoltage measurements revealed similar charge carrier lifetimes suggesting that bimolecular recombination is not a limiting factor in device performance. Sub-bandgap external quantum efficiency spectra along with transient photoluminescence data confirmed that charge transfer (CT) states in P(T3-iI):PC₇₁BM devices were more effective, and therefore charge carrier generation was more efficient leading to higher short circuit current (J_SC). The energetic position of the CT manifolds were investigated using charge modulated electro-absorption spectroscopy. It was found that the CT manifold for P(T3-iI):PC₇₁BM devices was redshifted compared to that of P(T1-iI):PC₇₁BM devices. The CT cutoff redshift was correlated to an increase in dielectric constant for P(T3-iI):PC₇₁BM and a difference in the effective bandgap measured using photoemission spectroscopy. Further, the CT cutoff redshift was shown to be responsible for the lower V_OC observed for P(T3-iI):PC₇₁BM devices.
We propose that the high blend dielectric constant and improved exciton dissociation observed for P(T3-iI):PC₇₁BM are due to a more favorable polymer-fullerene interaction originating from more intimate mixing due to the larger donor moiety that allows fullerene to come closer to the isoindigo acceptor moiety.

Organic bulk-heterojunction solar cells are one of the most promising next generation solar cell technologies on the brink of commercialization. Printing and coating techniques with eco-compatible solvents used in roll-to-roll production for all functional layers enable competitive low-cost fabrication perspectives with short energy payback times. Semi-transparent solar cells on mechanically flexible substrates open up avenues to new markets for truly transparent photovoltaics in building integration and automotive applications, formerly only insufficiently addressed by inorganic solar cells. Additional value is generated by pushing solar cells to the consumer and lifestyle industry promoted by the inherent colorful and artistic design options.
In this work, we investigate truly all-solution processed organic solar cells and monolithically interconnected sub modules on mechanically flexible substrates employing doctor blading for the deposition of all functional layers, mimicking the process conditions of large-area deposition, e.g., by slot-die coating. All layers were deposited from eco-compatible solvents as required by industry standards. Using the highly efficient ternary polymer:fullerene absorber blend PffBT4T-2OD:PC₆₁BM:PC₇₁BM and employing two transparent hybrid electrodes comprising conductive polymers (PEDOT:PSS), silver nanowires and printed microscopic metal grids, enables the fabrication of semi-transparent solar cells with very homogenous appearance. Solar sub modules yield power conversion efficiencies of 6% and peak transparency in the visible of more than 13%.

[Ref.: F. Nickel, Sol. Energy Mater. Sol. Cells, 2014, 130, 317]
[Ref.: C. Sprau, Energy Environ. Sci., 2015, 8, 2744]

Steady development of more efficient materials and advanced cell architectures over the past decades have made OPV more competitive with other thin film technologies - record performances surpassed 10%, and the first products have been launched in the market. However, their comparably low stability positions them on the market merely as an exotic niche product.
We are reporting on the possibilities of long-term stability improvement by ternary blending the active layers with small amounts of stabilizing compounds of different classes of antioxidants, radical scavengers and light stabilizers. We present the results of the lifetime investigations using a variety of compounds investigated in bulk-heterojunction, and characterized under ISOS-3 degradation conditions. Different microscopic and spectroscopic methods were applied to trace chemical degradation over time, and the observed differences in the stabilization of tested additives are discussed in terms of energetic trap states formation within the HOMO/LUMO gap of the photoactive material, morphological changes, and chemical structure.

Organic light emitting diodes (OLEDs) have become a proven technology for display applications, while research into white OLEDs is ongoing. The need for multiple emitters and high current densities in order to meet the strict technical requirements of high color quality and high luminance white light makes the design of white OLEDs more challenging than for their monochrome counterparts. In order to establish rational device design principles, a better understanding of certain fundamental processes, such as the formation, diffusion, and quenching of excitons in a working device will be necessary. This talk will discuss recent experiments designed to study the formation of excitons and their subsequent diffusions in organic light emitting diodes (OLEDs). The focus will be on singlet excitons in the archetype hole transport layer and host 4'-bis(carbazol-9-yl)biphenyl (CBP). The emission from ultrathin exciton sensing layers is analyzed to determine both the singlet exciton diffusion length and the mechanism through which the excitons form. The implications for models of efficiency roll off are discussed in light of the improved understanding of exciton dynamics.

In recent work (Jacobs, et. Al., ACS Nano, 2015, 9 (2), pp 1905–1912), it was demonstrated that films of semiconducting polymer P3HT doped with strong electron acceptor F4TCNQ are completely insoluble in a range of organic solvents, but regain their solubility upon illumination with 405 nm light. Previous work had indicated the reaction to be highly sensitive to illumination wavelength, suggesting an optical transition resulting in charge transfer back to the neutral states of the polymer and dopant molecule. Using this mechanism, it was demonstrated that P3HT films could be patterned with sub-micron resolution, but the underlying mechanism by which solubility is reintroduced remains unclear.

Here, we demonstrate a “one-shot” method for determining the functional form and rate constant of the rate law corresponding to the optically induced reaction. A P3HT:F4TCNQ film is illuminated under solvent through a small pinhole, resulting in a diffraction pattern being projected onto the film. This diffraction pattern contains a range of intensities at different positions on the film, resulting in a spatially modulated dissolution rate; the resulting film is then imaged by atomic force microscopy. The diffraction pattern corresponding to the features observed on the film can then be fit numerically using a model-free method, and reduced to a plot of optical intensity vs dissolution rate. This method eliminates errors due to temporal variance in optical power, and allows for a single experiment to produce a large number of data points. By repeated tests at varying pinhole film distances, we are able to extract the full rate law of the optically-induced reaction.

Using this information, we were able to optimize optical intensity and dwell time for in-situ nanoscale patterning in a laser scanning confocal microscope. We demonstrate the ability to pattern and image sub-300 nm features, including 1-D and 2-D gratings and arbitrary curves. These structures have a wide range of potential applications in optical trapping structures for photovoltaics or outcoupling structures in OLEDs, as well as cavities for lasers or ultra-small OTFTs.

Non-emissive triplet excitons can undergo triplet-triplet annihilation (TTA) to produce significant singlet excitons in fluorescent polymer light emitting diodes (PLEDs)[1] is being used to qualitatively explain recent high efficiency results, which are beyond spin-statistics, but failed to explain quantitatively observed high external quantum efficiency (EQE) vs current density (J) in many PLEDs.[2] Our recent studies not only explain singlet exciton generation yield in operational PLED, which successfully modeled the EQE vs J, but also indicates spin-dependent exciton formation, which provides one more booster other than TTA to EQE in Poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) PLED. Our two independent studies on PLEDs and light emitting field transistors [3] offers an clear insight of spin dependent exciton formation in this conjugated polymer, where optical out-coupling is also modeled using anisotropic optical constants of polymeric material.

Keywords: Electroluminescence, Spin-statistics, Exciton-dynamics

Reference:
“Barium Hydroxide as an Interlayer Between Zinc Oxide and Luminescent Conjugated Polymer for Light Emitting Diodes” L. P. Lu, Dinesh Kabra and R. H. Friend Adv. Func. Mater. Vol. 22, p-4165 (2012)
Highly Efficient Single-Layer Polymer Ambipolar Light-Emitting Field-Effect Transistors” M. Gwinner, Dinesh Kabra, M. Roberts, T. J. K. Berner, B. H. Wallikewitz, R. H. Friend and H. Sirringhaus, Advanced Materials Vol. 24, pp- 2728 (2012)
“Triplet Dynamics in a Fluorescent Polymer Light-Emitting Diodes”, B. H. Wallikewitz, Dinesh Kabra, S. Gelinas and R. H. Friend for Phys. Rev. B Vol. 85, pp-45209 (2012)

In recent years, thermally activated delayed fluorescence (TADF) materials have received a considerable attention as a new concept for overcoming limitation of fluorescence light emission. Adachi et al. group reported non-metal organic fluorescence materials which have minimized the energy gap between the singlet and triplet excited states (ΔE_ST). The formed small ΔE_ST allowed energy up-conversion from T₁ to S₁ (RISC, reverse intersystem crossing) for lighting emission through fluorescence decay channels. This process lead to theoretically 100% IQE which is close to that achieved in phosphorescence emitter based OLEDs.
In this report, we present a new blue TADF emitter by introducing heteroatom into the donor moiety, which could increase non-radiative process and shorten the exciton lifetime of TADF materials. In addition, this heteroatom could increase TADF process by having higher triplet energy. 4,5-di(5H-pyrido[3,2-b]indol-5-yl)phthalonitrile (2PIPN) were synthesized by modifying 4,5-bis-9-carbazol-9-yl-phthalonitrile (2CzPN) with pyridoindole electron donor moiety. Material characteristics and device performances of newly synthesized TADF emitter are evaluated by comparison with reported 2CzPN blue TADF emitter. 2PIPN shows high triplet energy (2.70 eV) and small ΔE_ST (0.38 eV) which are expected to be blue TADF emitter. Additionally, delayed fluorescence exciton lifetime was measured by using time-resolved spectroscopy. The exciton lifetime of 2PIPN (τ_d : 180 µs) could achieve shorten as compared to 2CzPN (τ_d : 270 µs) by incorporating heteroatom into the donor moiety due to increasing non-radiative process. We also evaluated photoluminescence quantum yields (PLQYs). Even though exciton lifetime was decreased due to increasing non-radiative process, 2PIPN film shows high PLQY (92%). Based on these photophysical properties and PL characteristics, we fabricated blue OLED devices using 2CzPN and 2PIPN blue TADF emitters. The fabricated device structure is as follows : ITO (50 nm) / HATCN (7 nm) / TAPC (55 nm) / DCDPA (10 nm) / mCP : 20% emitter (20 nm) / TmPyPB (35 nm) / LiF (1.5 nm) / Al (100 nm). The J-V characteristics are almost same due to similar molecular structures. However, as we expected in PLQY, external quantum efficiency (EQE) of 2PIPN based device is quite higher than that of 2CzPN. The device made with 2PIPN shows high maximum EQE with 23% by extended non-radiative rate through the pyridoindole moiety. And 2PIPN device emits blue color with CIE 1931 color coordinates of (0.19, 0.35).

A comprehensive study on omnidirectional and broadband absorption enhancement in organic solar cells (OSCs) with 2-D photonic structures for excitation of surface plasmon polaritons, waveguide modes, localized surface plasmons and their mutual coupling was carried out. Performance of the nano-structured OSCs and the corresponding control planar cells, fabricated with the blend of PTB7:PC₇₀BM was analyzed. We invested the absorption enhancement, angular dependent cell performance and interfacial exciton dissociation processes occurred at organic/electrode interfaces and their impact on the performance of nano-structured OSCs.

With nanostructured active layer, the device performance is increased for 12% in power conversion efficiency. Both the Jsc and FF are improved. Our results reveal that there is broadband light absorption enhancement in the active layer. Specially, we find some interesting relationship between the cell performance and the angle of the incident light on the cells. A special testing system was set up to measure the incident photo to current efficiency (IPCE) with different incident angle. The Jsc integrated from the IPCE results indicates that for smaller incident angle (