Multiple-quantum two-dimensional electronic spectroscopy (2DES) can provide direct access to
correlated electronic excited states, revealing their presence and their energetics and dynamics. In
the measurements, coherent light-matter interactions are used to generate multiple-quantum
coherences of biexcitons, triexcitons, etc., in which oscillations of the electronic charge
distribution take place at the multi-exciton frequency. The oscillations are observed by using
additional, time-delayed light-matter interactions to generate single-quantum polarizations that
radiate coherent signals whose amplitudes and phases are modulated as a function of the delay at
the multiple-quantum frequency. The modulation frequency and decay directly reveal the multiexciton
energy and dephasing rate. We developed a method for conducting multiple-quantum
measurements using spatiotemporal femtosecond pulse shaping through which multiple light
beams and multiple pulses can be directed to a sample with full phase coherence among all the
light fields, all controlled by spatial light modulators without the need for interferometric
measurement or feedback [1]. Our initial measurements showed the presence of biexcitons [2] and
triexcitons [3] in GaAs quantum wells and indicated that no higher-order multi-exciton
correlations were present in significant densities. Subsequent measurements in quantum well
microcavities [4] revealed quad-exciton-polaritons, whose high-order correlations were mediated
through the light field so that physical proximity of the electronic excitations was not necessary.
These measurements have been extended into the regime of Bose-Einstein condensation (BEC),
which we explored through separate cw studies as well. Finally, we have observed biexcitons in
Cu2O which are likely responsible for the absence of BEC in that material.
1. “Invited Article: The coherent optical laser beam recombination technique (COLBERT) spectrometer:
Coherent multidimensional spectroscopy made easier,” D.B. Turner, K.W. Stone, K. Gundogdu, and K.A.
Nelson, Rev. Sci. Instrum. 82, 0813301 (2011).
2. “Two-quantum 2D FT electronic spectroscopy of biexcitons in GaAs quantum wells,” K.W. Stone, K.
Gundogdu, D.B. Turner, X. Li, S.T. Cundiff, and K.A. Nelson, Science 324, 1169-1173 (2009).
3. “Coherent measurements of high-order electronic correlations in quantum wells,” D.B. Turner and K.A.
Nelson, Nature 466, 1089-1092 (2010).
4. “Influence of multi-exciton correlations on nonlinear polariton dynamics in semiconductor
microcavities,” P. Wen, G. Christmann, J. J. Baumberg, and K. A. Nelson, New J. Phys. 15, 025005 (2013).

ABSTRACT
Singlet fission has now been observed in a number of molecular systems. However, details regarding the static and dynamical factors governing the process are only beginning to emerge. In this talk, our recent efforts utilizing pump-probe and two-dimensional electronic spectroscopy to investigate the mechanism of singlet fission in nanoparticles of several pentacene derivatives will be described. We find that pentacene derivatives are versatile singlet fission chromophores capable of undergoing singlet fission in the presence of extensive structural disorder. Nanoparticles comprised of weakly coupled chromophores, for example, exhibit non-monoexponential singlet fission kinetics predominantly associated with the migration of energy to sites where singlet fission occurs. In contrast, nanoparticles consisting of more strongly coupled, excitonically shifted chromophores exhibit near monoexponential singlet fission kinetics with rate constants approaching the adiabatic limit. The role of excitonic delocalization in facilitating the highest observed singlet fission rates will be discussed. The nature of vibrational coherences in two-dimensional electronic spectroscopy experiments will be described, and monitoring their time evolution is hypothesized to provide additional insight into the mechanism of singlet fission.

Solution processed organic-inorganic perovskite solar cells are presently the forerunner in the next generation photovoltaic technologies, with power conversion efficiencies approaching 20%. In this talk, I will review the developments in this field and distil the recent findings on the photophysical mechanisms of this remarkable material. I will also highlight some of our latest charge dynamics studies and other investigations on the novel properties of this amazing material.

Hybrid halide perovskites (eg, CH₃NH₃PbI₃) facilitate high power conversion efficiencies in a varierty of solar cell architectures^1,2. The existence and stability of bound electron-hole pairs (excitons) in these materials, and their role in the exceptional photovoltaic performance, remains a controversial issue³. Here we demonstrate, through a combination of femtosecond transient absorption spectroscopy and multiscale modeling as a function of crystal size and temperature, that the electron-hole interaction is sensitive to the microstructure of the material.
We find that by the control of the material processing during fabrication both free carrier and Wannier excitonic regimes are accessible, with strong implications for optoelectronic devices. We elucidate the key role played by the organic cation that creates a dipolar field within the lattice and thus influencing the Coulomb interaction between the electron and hole, eventually dictating the stability of the exciton. We demonstrate that the long range order of the dipole field is disrupted by the polycrystalline disorder that introduces domain walls where dipole twinning breaks down. The resultant variations in electrostatic potential found for smaller crystallites suppresses exciton formation, while larger crystals of the same composition demonstrate an unambiguous excitonic state that is populated in ultrafast timescales.
In addition, within the free carrier regime, we also observe a reduction in the dielectric permittivity of the material after the photo-excitation. We attribute this effect to the saturation of the molecular dipole response induced by the polaronic effects (a result of Frohlich electron-phonon coupling) as the carrier concentration is increased. We show that such a reduction in the dielectric constant results in the quenching of the excitonic screening and thus favouring the formation of a photo-induced meta-stable excitonic transition.
References:
1. Im, J.-H., Jang, I.-H., Pellet, N., Grätzel, M. & Park, N.-G. Growth of CH3NH3PbI3 cuboids with controlled size for high-efficiency perovskite solar cells. Nat. Nanotechnol. (2014). doi:10.1038/nnano.2014.181
2. Ball, J. M., Lee, M. M., Hey, A. & Snaith, H. J. Low-temperature processed meso-superstructured to thin-film perovskite solar cells. Energy Environ. Sci.6, 1739 (2013).
3. D&’Innocenzo, V. et al. Excitons versus free charges in organo-lead tri-halide perovskites. Nat. Commun.5, 3586 (2014).

Solution processing is required for the large-scale manufacture of small-molecule organic semiconductors, and can result in crystalline domains with high charge mobility. However, the interfaces between these domains impede charge transport, degrading device performance. Although understanding these interfaces is essential to improve device performance, their intermolecular and electronic structure is unknown: they are smaller than the diffraction limit, are hidden from surface probe techniques, and cannot be directly resolved using X-ray methods. We have used transient absorption microscopy to inspect a drop-cast thin film of 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene, a material cited for high hole mobility and singlet fission. The crystal and electronic structure of the domains has been characterized, but analogous information for the domain interfaces is unknown. Using a judicious selection of light polarization, we isolate a signal at the interface that is not observed in either of the adjacent bulk domains, exposing the exciton dynamics and inter-molecular structure of this hidden interface. Surprisingly, instead of finding an abrupt grain boundary, we reveal that the interface can be composed of nanoscale crystalline aggregates interleaved by a web of interfaces that compound decreases in charge mobility. The impact of doping and annealing on the interface dynamics will also be reported.

Indium phosphide (InP) quantum dots (QDs) are attractive for hybrid organic/inorganic solar cells due to their excellent electronic properties and widely size-tunable absorption in the visible range.^1-2 The interfacial energy between InP and low-bandgap polymers like P3HT is ideal to form type-II hybrid heterojunctions and promote charge separation.³ In this work we combine femtosecond transient absorption spectroscopy with density functional theory (DFT) to characterize the effects of organic ligands and sizes (2.5 and 4.5 nm) on charge carrier photogeneration and transfer dynamics in a hybrid P3HT/InP QDs system. DFT calculations show that the LUMO energy of the QD-ligand complexes decreases by increasing the size of InP QDs and increases proportionally to the surface coverage of organic ligands. By substituting the original oleylamine stabilizing ligands with a pyridine capping layer, we observe notable quenching of the photoluminescence intensity of InP/P3HT thin films compared to pure P3HT and InP QDs films, an indication of efficient photoinduced charge transfer across the InP/P3HT heterointerface. Photoinduced absorption measurements confirm an instant creation of singlet excitons subsequently dissociate into polarons on an ultrafast time scale (t<1ps) and the yield of polaron formation is significantly enhanced by adding pyridine-InP(4.5 nm) QDs as electron acceptor. Fast electron transfer into the InP QD acceptors is in good agreement with the strong coupling (~15.2 meV) between the lowest unoccupied molecular orbitals of P3HT and InP QDs in the ground state predicted by DFT, demonstrating that our organic/inorganic model accurately captures the key charge transfer dynamics. The yield of polaron formation is found to decrease in magnitude with deceasing QD size from 4.5 nm to 2.5 nm. In 2.5 nm QDs, electron injection is inefficient but reverse energy transfer takes place due to large overlap between absorption of P3HT and QD emission. Thus, both InP QD size and length of the organic ligands can be used as tuneable parameters to optimize performance of hybrid polymer/InP QDs solar cells.
References
1. Xie, R.; Battaglia, D.; Peng, X. J Am Chem Soc, 2007,129, 15432.
2. Dung, M. X.; Mohapatra, P.; Choi, J. K.; Kim, J. H.; Jeong, S.; Jeong, H. D. B Korean Chem Soc, 2012,33, 1491.
3. Selmarten, D.; Jones, M.; Rumbles, G.; Yu, P. R.; Nedeljkovic, J.; Shaheen, S. J Phys Chem B, 2005,109, 15927.

Photo-initiated charge-transfer processes play a central role in natural systems such as human vision and photosynthesis. While researchers have successfully modified these processes to control simple isolated systems, our understanding of photon-to-electronic mechanisms in realistic and complex environments is still in its infancy. In particular, recent experiments have shown that simple descriptions of solvent interactions (either via classical force fields or effective solvent models) are unable to accurately capture the electron dynamics in even relatively simple systems. These ongoing observations open an entirely new field of research in the properties of light-activated processes in complex environments, with the opportunity to deeply understand the real-time electron dynamics between complex interfaces.
To this end, we have developed a new real-time time-dependent density functional tight binding (RT-TDDFTB) approach to calculate the electron dynamics of donor-acceptor complexes in the presence of explicit solvent molecules - all treated at the quantum mechanical level. Our approach significantly differs from previous linear-response TD-DFT methods in that we directly propagate the one-electron density matrix in the presence of a non-perturbative external field. Furthermore, and most importantly, our implementation in the TD-DFT code allows us to calculate the electron dynamics of large solvated systems (~10,000 atoms), whereas conventional approaches are computationally limited to only hundreds of atoms. Using this new capability, we are able to understand and rationalize electron-hole recombination effects as a function of solvent polarity, configuration, and energy transfer. Furthermore, this new capability gives us mechanistic insight into the electron dynamics of new systems in complex environments with the goal of guiding experiments in the exploration of charge-transfer dynamics driven by time-dependent external fields.

Exciton diffusion in molecular aggregates, films or crystals is mainly controlled by the excitonic coupling between the electronic excited states localized in the molecular units. The excitonic coupling can be decomposed into different short-range (exchange, charge-transfer mediated and overlap) and long-range (Coulombic) contributions. The Coulombic term is the only term effective when the molecules are not in contact and this used for example in the Förster theory of excitation energy transfer. However, in organic crystals, which are held together by weak non-covalent intermolecular interactions, there is a large number of low frequency intermolecular vibrations that cause a relatively large displacement of one molecule respect to their neighbors (dynamic disorder). In this context, short intermolecular contacts can be present and the short-range excitonic interactions may be of great relevance to provide a proper description of the modulation of the excitonic coupling owing to the combined thermal motions of the nuclei.
In this contribution, we assess the magnitude and timescale of the fluctuation of the excitonic coupling due to nuclear thermal motions. To do so, we adopt a combined molecular dynamic and quantum chemical approach, and we derive a general diabatization scheme to compute excitonic couplings, in which both short and long-range effects are accounted for. We present the results for the two archetype anthracene and tetracene molecular crystals, which have been widely studied in the context of organic electronics. We show that the inclusion of short-range excitonic interaction is responsible for the large fluctuation of the excitonic coupling introduced by the thermal motions. The large fluctuation of the excitonic coupling in organic crystals is essential for the proper description of exciton transport in molecular materials.

Singlet exciton fission is the process of splitting a single spin zero (singlet) molecular excited state into two spin one (triplet) excited states of different molecules. It is notable because spin conservation disallows the usual competing loss process: thermal relaxation of the high-energy spin zero exciton into a single low-energy spin one exciton. Indeed, the low energy exciton is a dark state, inaccessible by a direct transition from either the high-energy exciton or the ground state. Only the evolution of the high-energy state into two dark excitons is spin-allowed. Consequently, the efficiency of singlet exciton fission can approach unity even in the visible spectrum, harnessing photons of just twice the energy of the child excitons.
In theory, singlet fission can be exploited in silicon solar cells to double the photocurrent from high-energy solar photons, ultimately boosting the efficiency of the silicon cell to 30% or more. The outstanding challenge is how to get the energy from the triplet excitons (energy ~ 1.1eV) from tetracene into silicon (bandgap 1.1eV). We report progress on the coupling of singlet exciton fission to silicon solar cells, including direct excitonic energy transfer from ‘dark&’ triplets in the organic semiconductor tetracene to colloidal PbS nanocrystals, thereby successfully harnessing molecular triplet excitons in the near infrared. Steady-state excitation spectra, supported by transient photoluminescence studies, demonstrate that the transfer efficiency is at least (90±13)%. The mechanism is a Dexter hopping process consisting of the simultaneous exchange of two electrons. Triplet exciton transfer to nanocrystals is expected to be broadly applicable in solar and near infrared light-emitting applications, where effective molecular phosphors are presently lacking. We conclude by demonstrating the usefulness of nanocrystalline sensitization to the reverse process of triplet exciton fusion - a promising approach to upconversion of incoherent light.

Singlet exciton fission is a process that occurs in certain organic materials whereby one singlet exciton splits into two triplet excitons. In photovoltaic devices these two triplet excitons can each generate an electron, producing quantum yields per photon of >100%. We use ultrafast transient absorption spectroscopy to study the singlet exciton fission dynamics in the thin film of antradithiophene derivatives. By controlling the crystallinity of the film via various solution processing routes and molecular side-chain engineering, we provide direct evidence for the control of the singlet exciton fission rate in the antradiothiophene neat films over a broad range. Singlet exciton fission is observed at the timescale ranged from 10ps to 2ns, and eventually, no singlet exciton fission process is observed in the amorphous neat film. Temperature dependent photoluminescence characterization further reveals peculiar radiative recombination rate observed in the neat film that shows fast fission at room temperature. While singlet exciton fission is dominant at room temperature, the photoluminescence is dominated by the triplet-triplet annihilation and exhibits excimer-like spectra. The radiative recombination rate decreases as the substrate temperature is reduced to 120K. When the substrate temperature is further reduced to 5K, the photoluminescence is dominated by superradiance with the radiative recombination rate enhanced by a factor of 46, with respect to the radiative recombination rate observed at 120K. This work provides the foundation for ways to tailor the multicarrier generation in organic semiconductor by controlling the crystallinity of the organic semiconductor via various solution processing route and molecular side-chain engineering.

In donor-acceptor based bulk-heterojunction solar cells, the splitting of singlet excitons at the donor and acceptor interface is of crucial importance for charge generation and therefore, for photovoltaic performance. The reversed process, in which two initially free charge carriers meet at the interface to form an exciton with singlet or triplet multiplicity is rather beneficial for light emission in OLEDs but considered as one of the loss factors in OPV. The formation of triplet excitons in polymer-fullerene blends was found to depend on the relative energetic position of triplet exciton level of donor polymer and charge transfer (CT) state.[1] In these experiments, the occurrence of triplet excitons and CT states was probed by using spin sensitive detection of the photo- and electroluminescence. A substantial generation of molecular triplet excitons was surprisingly also found in high efficiency donor-acceptor OPV systems based on the low bandgap copolymer PTB7 and in the soluble small molecule p-DTS(FBTTh₂)₂ [2], both blended with PC₇₀BM as acceptor. Note that no triplets could be detected in the reference system P3HT:PC₆₀BM. We ascribe these findings to an electron back transfer from the CT state to the triplet state on the donor material. The triplet formation was found to be strongly influenced not only by the relative energetics of triplet and CT states involved, but also by the blend morphology, which was varied by adding DIO to the investigated blends. In summary, the fundamental understanding of the transformation processes involving the CT states, triplet excitons, as well as free electrons and holes and their dependence on nanoscale morphology and energetics of blends is essential for the optimization of the OPV devices.
[1] M. Liedtke, A. Sperlich, H. Kraus, A. Baumann, C. Deibel, M. J. M. Wirix, J. Loos, C. M. Cardona, and V. Dyakonov, J. Am. Chem. Soc. 133, 9088-9094 (2011).
[2] T. S. van der Poll, J. A. Love, T.-Q. Nguyen, and G. C. Bazan, Advanced Materials 24, 3646 (2012).

Detailed photophysical measurements of intramolecular charge transfer (ICT) states have been made both in solution and solid state. Temperature dependent emission and delayed emission are used to map the energy levels involved in molecule decay, and through detailed kinetic modelling of the thermally activated processes observed, true electron exchange energies and other energy barriers of the systems determined.^1-3 In the solid state it is vital to control the molecular geometry so that all molecules retain a stabalised charge transfer excited state, otherwise large heterogeneities in optical response are observed.⁴
For specific donor acceptor molecular configurations, the CT singlet and triplet states are found to be the lowest lying excited states of ICT molecule with very small electron exchange energies = kT. In these cases the decay kinetics of the molecules become significantly different to normal molecules, and the effect of rapid recycling between CT singlet and triplet states is seen to greatly extend the lifetime of the ‘excited state&’ and yield non-exponential decay. Quantum yields increase markedly, even though the intersystem crossing rate is fast, ? 10⁹ s^-1. Clear evidence will be given to show that TADF reaches 100% efficiency at harvesting triplet states^1,3, and device having > 15% EQE discussed. In ICT molecules with highly controlled structure, we find that there are substantial difference between optical and electroluminescent photophysics resulting in device being far more efficient than is suggested by the molecules PLQY. This will be discussed in a new molecule that has a PLQY of 30% but gives devices having 18% EQE.
¹ V. Jankus, C. J. Chiang, F. Dias, and A. P. Monkman, Adv Mater 25, 1455 (2013).
² F. B. Dias, K. N. Bourdakos, V. Jankus, K. C. Moss, K. T. Kamtekar, V. Bhalla, J. Santos, M. R. Bryce, and A. P. Monkman, Adv Mater 25, 3707 (2013).
³ D. Graves, V. Jankus, F. B. Dias, and A. Monkman, Adv Funct Mater 24, 2343 (2014).
⁴ Vygintas Jankus, Przemyslaw Data, David Graves, Callum McGuinness, Jose Santos, Martin R. Bryce, F. B. Dias, and A. P. Monkman, Advanced Functional Materials DOI 10.1002/adfm.201400948 (2014).

Organic light emitting diodes (OLEDs) have progressed dramatically since their inception and it is now routine to achieve unity internal quantum efficiency and luminescence turn-on voltages on par with the emitted photon energy. Despite this progress, several technical challenges remain for widespread adoption of OLED technology, among which operational lifetime and efficiency roll-off at high brightness are key. Bimolecular exciton annihilation processes impact both of these aspects, however it is notoriously difficult to identify the dominant mode of annihilation in operating OLEDs (exciton-exciton vs. exciton-charge carrier) and then to disentangle its magnitude from competing roll-off processes such as charge imbalance.
Here, we introduce a simple, yet powerful analytical method termed electroluminescence modulation spectroscopy (ELMS) to identify and quantify the impact of different annihilation processes on OLED efficiency roll-off in the presence of changing charge balance. This approach extends admittance spectroscopy into the optical domain by recording the magnitude and phase components of electroluminescence that result from the sinusoidal dither superimposed on the device DC bias. By simultaneously measuring the complex current (J) and light (L) emitted from the OLED at different harmonics of the fundamental dither frequency, we calculate a corresponding complex external quantum efficiency for each harmonic that enables direct calculation of the annihilation rate. The basis of this approach stems from the relationship between recombination current and light output, which becomes nonlinear in the presence of annihilation, thereby mixing the different EQE harmonics in a manner that depends uniquely on the type and magnitude of annihilation.
We derive simple expressions to extract different annihilation rate coefficients and apply this technique to a variety of OLEDs. For example, in devices dominated by triplet-triplet annihilation, the annihilation rate coefficient, K_TT, is obtained directly from the linear slope that results from plotting EQE_dc - EQE_1omega; versus L_DC (2EQE_1omega;-EQE_dc ). In the classic annihilation example involving the phosphorescent emitter platinum octaethylporphyrin (PtOEP), we confirm the impact of triplet-triplet annihilation with a rate coefficient K_TT=6x10¹⁴ cm³/s at intermediate current densities but show that charge imbalance becomes the dominant loss at higher currents (J>30 mA/cm²). We go on to show that, in certain cases it is sufficient to calculate EQE_1omega; directly from the slope of the DC light versus current curve (i.e. via dL_DC/dJ_DC), thus enabling this analysis to be conducted solely from common LIV measurement data. These results represent a significant step forward in OLED analytical capability and should help to more accurately assess the effectiveness of molecular and device structures designed to reduce annihilation and efficiency roll-off.

The development of near infrared (NIR) organic light emitting diodes (OLEDs) is of significant interest for their application in night vision displays, telecommunications and medical imaging. While visible OLEDs have found notable success and are already being commercialized in displays, efficient phosphors in the NIR have been limited.^[1] Moreover, the few NIR LED structures that have been demonstrated contain Pt based phosphors or lower efficiency fluorescent nanocrystals, typically containing toxic lead based compounds.^[2] Here, we report next-generation light emitting diode (LED) emitters with 5 % quantum efficiency based on phosphorescent metal halide nanoclusters that are earth-abundant and inexpensive and entirely unique from fluorescent nanocrystals. LEDs have been fabricated with molybdenum-based nanocluster salts with various cation substitution to demonstrate tunable emission. We will discuss both luminescent and electroluminescent transient dynamics to understand nanocluster photophysics and will show time-dependent density functional theory (TDDFT) calculations performed on the core cluster that give insights about the nature of the emitting state,which involves a strong Pseudo Jahn Teller (PJT) distortion. This demonstration of a new class of entirely inorganic phosphors offers a new direction for developing efficient and long-lifetime phosphors for tunable light emitting diodes across the visible and near-infrared spectrum at low cost.
[1] K. R. Graham, Y. Yang, J. R. Sommer, A. H. Shelton, K. S. Schanze, J. Xue, J. R. Reynolds, Chem. Mater.2011, 23, 5305.
[2] H. Xiang, J. Cheng, X. Ma, X. Zhou, J. J. Chruma, Chem. Soc. Rev.2013, 42, 6128.

Solar energy conversion is a strategy for production of renewable energy with significant potential for long-term worldwide growth. However, electricity production by the photovoltaic effect, or by artificial photosynthesis, is limited by the electronic properties of the light-harvesting materials employed, Conventional solar cells exhibit limited efficiencies in part due to their inability to absorb the entire solar spectrum. Sub-band gap photons are typically lost but could be captured if a material that performs up-conversion (UC), which shifts photon energies higher, is coupled to the device [01]. Recently, molecular chromophores that undergo tripletminus;triplet annihilation (TTA) have shown promise for efficient up-conversion by delayed fluorescence generation at low irradiance, suitable for some types of solar cells. [02] However, the molecular systems that have shown the highest up-conversion efficiency to date, despite in some case the thermodynamic limit for photon UC yield 50% can be reached, are ill suited to broadband light harvesting, reducing their applicability [03] [04]. Here we overcome this limitation by combining an organic TTA system with highly fluorescent CdSe semiconductor nanocrystals. Because of their broadband absorption and spectrally narrow, size-tunable fluorescence, the nanocrystals absorb the radiation lost by the TTA chromophores, returning this energy to the up-converter. The resulting nanocrystal-boosted system shows a doubled light-harvesting ability, which allows a green-to-blue conversion efficiency of sim;12.5% under 0.5 suns of incoherent excitation. [05] This record efficiency at subsolar irradiance demonstrates that TTA-boosting by light-emitting nanocrystals can potentially provide a general route for up-conversion for different photovoltaic and photocatalytic applications.
[01] Ginley, D.; Green, M. A.; Collins, R. MRS Bull. 2008, 33, 355.
[02] Baluschev, S.; Miteva, T.; Yakutkin, V.; Nelles, G.; Yasuda, A.; 469 Wegner, G. Phys. Rev. Lett. 2006, 97, 143903.
[03] Monguzzi, A.; Tubino, R.; Hoseinkhani, S.; Campione, M.; Meinardi, F. Phys. Chem. Chem. Phys.2012, 14, 4322.
[04] Monguzzi, A.; Bianchi, F.; Bianchi, A.; Mauri, M.; Simonutti, R.; Ruffo, R.; Tubino, R.; Meinardi, F. Adv. Energy Mater.2013, 3, 680.
[05] A. Monguzzi, A.; Braga, D.; Gandini, M; Holmberg, V. C,; Kim, D. K.; Sahu, A.; Norris D. J.; Meinardi, F. Nanoletters, 2014, doi: 10.1021/nl503322 (2014)

Organic electronics offer a cheap alternative to conventional inorganic semiconductor based devices while also opening access to new markets as they can be both flexible and lightweight. Organic semiconductors also offer the ability to precisely tailor molecular design to fit particular applications. Organic light emitting diodes (OLEDs) are a particularly exciting area that has already seen successful commercialization, but efficient electroluminescence in OLEDS requires harvesting of both singlet and triplet states. Current OLEDs rely on emitter molecules with heavy metal centers to increase spin orbital coupling to mix the singlet and triplet states to reach high internal quantum efficiency (IQE), but these materials are rare and expensive. Recently, promising new emitters have emerged that rely on thermally activated delayed fluorescence (TADF) to enable high IQE without heavy metal centers.
A direct quantitative understanding of the effect of molecular design on the dynamical processes that TADF materials undergo between exciton formation and emission is needed in order to optimize device performance. TADF materials are known to exhibit prompt fluorescence from the initial population in the singlet state followed by delayed fluorescence from a population that crosses from the triplet to the singlet state at later times. These materials are designed to have low spatial overlap of their HOMO and LUMO orbitals in order to minimize exchange energy, and thereby drive down the triplet-singlet energy splitting to make reverse intersystem crossing possible without heavy metals. The triplet states are thought to act as a reservoir to lower nonradiative losses, and hence IQEs near 100% have been reported for some TADF materials. Nevertheless, little is known about the fast time dynamics for TADF emitters and their effect on device performance.
We have begun to quantify both short and long time dynamics of model TADF emitters through transient absorption and time-resolved photoluminescence studies. The joint utilization of these techniques allows for quantitative mapping of the initial thermalization of the singlet state, the intersystem crossing to the triplet state, and both prompt and delayed fluorescence rates in TADF emitters. Furthermore, by varying the temperature or oxygen levels (a triplet scavenger), clear spectral assignments of the transient data can be achieved. This deeper understanding of the dynamics at all times in TADF materials will provide new information to tailor molecular design for high efficiency and stability. Further studies will analyze the effect of different host materials in solid samples on the dynamics in TADF materials. Variation of the host material&’s rigidity and polarity affects the stability of the charge transfer singlet state and hence the performance of the TADF materials.

The photoactive material of organic solar cells commonly consists of a conjugated polymer blended with a fullerene derivative, yielding a complex network of intermixed and phase-pure domains. The charges that are photo-generated in the blend create an electric field in their vicinity, which can affect neighboring molecules and shift their absorption spectrum (Stark effect). The corresponding electro-absorption signature is a powerful tool to understand charge generation in the organic materials. We have investigated pBTTT:PCBM samples with a variety of well-defined microstructures, exploiting the Stark effect in two complementary ways. First, we have studied the evolution of the electro-absorption signal present in ultrafast transient absorption data (no external bias). This has allowed for the first time to directly visualize the migration of charges from intermixed to phase-pure regions, leading to significant insight to the still poorly understood mechanism by which the neat domains favour spatial separation of charges. Second, we have looked at the field-dependent generation and transport of charges in full solar cell devices with externally applied reverse bias, where the photo-generated charged cause a time-resolved reduction of the electro-absorption induced by the external field.

The promise of organic electronics lies in the synthetic and mechanical flexibility of their constituent materials and in the cost-effectiveness and energy efficiency of their printability. Yet, printing organic films from a solution can lead to complex, heterogeneous physical structure. With the heterogeneity in physical structure comes heterogeneity in the electronic structure, whose weakest local attributes often determine or limit device performance. A cross-cutting theme in my lab is to correlate the nanoscale physical properties in organic semiconducting thin films to their local optical properties in order to inform and improve the non-equilibrium processes used to deposit the materials to make effective devices. I will first describe how we have measured ultrafast exciton dynamics in small-molecule polycrystalline TIPS-pentacene thin films used in transistors by using polarized transient absorption microscopy. In so doing, we not only determine the particulars of excited-state dynamics in the absence of spatial averaging—we also more unusually infer a complex nanoscale structural motif at domain interfaces in the films. Our findings provide an explanation for the surprisingly high observed resistivity of domain interfaces in small-molecule polycrystalline films, and further investigations on annealed films suggest how solution processing could be altered to eliminate it.
I will also describe how we have conceived of and implemented a completely new form of nano-optical imaging, one of whose utilities is to uncover heterogeneity in the optical properties of more disordered conjugated polymer films used in photovoltaics. Our near-field approach is unusual in that it leverages the focus and rapid scanning of a keV electron beam with the non-invasiveness and spectral selectivity of optical fields. I will show how we image nanoscale features in luminescent polymer blends through electron-beam-induced (cathodoluminescence) activation of an oxide thin film scintillator that locally excites the adjacent sample through resonant energy transfer.

We present measurements of the coupled electronic and vibrational dynamics of polaron formation using femtosecond wavepacket techniques. The experiments are carried out on the halide-bridged mixed-valence transition metal linear chain complex [Pt(en)₂][Pt(en)₂Cl₂].(ClO₄)₄ (en = ethylenediamine, C₂H₈N₂), or PtCl(en), a Peierls insulator with strong electron-phonon coupling. Earlier studies in this and similar materials on longer time scales have shown that excitation well above the optical gap energy can result in the formation of charged polarons in addition to the self-trapped excitons (STEs) that form following excitation near the band edge, though the formation mechanism for polarons has not previously been established. In this work, we address the mechanism of polaron formation by impulsively exciting the PtCl(en) complex well above the peak of the intervalence charge transfer band, and probing the response within the subgap photoinduced absorption band. We find that the response is modulated by vibrational wavepacket oscillations that damp as the induced absorbance associated with the nonlinear excitations forms on a time scale of ~ 200 fs. Oscillations are observed at two optical phonon frequencies, ~176 cm^-1 and ~240 cm^-1, both of which are significantly lower than the ground state Raman frequency of 312 cm^-1. The ~176 cm^-1 frequency essentially matches that observed in our previous studies of STE formation in PtCl(en), in which we excited near the low energy onset of the absorption band, a condition that is expected to yield only STEs, and is assigned to the lattice motions that create the lattice distortion that stabilizes the self-trapped state. We assign the new component at ~240 cm^-1 observed under high energy excitation to vibrational motions associated with the self-trapping process that leads to the polaron formation. To our knowledge, this is the first observation of this process. We note that the rapid formation of the self-trapped polaron state, on the time scale of a single vibrational period following photoexcitation, together with the observation of accompanying vibrational coherence strongly suggests that the polarons form directly from the initial photoexcitation, rather than by dissociation of primary excitons. We relate the difference in excited state optical phonon frequencies associated with the formation of polarons vs. STEs to the difference in charge distribution and local structure associated with each type of nonlinear excitation.
This work was supported by the National Science Foundation under grant DMR-1106379. We thank J. A. Brozik (WSU) for preparing the samples used in these studies.

In solar cells that incorporate semiconductor polymers as electron donors and fullerene derivatives as acceptors, a number of reports based on ultrafast optical probes reveal that charges can be generated on timescales significantly faster than ~100 fs in certain solid-state microstructures. Techniques that have been applied in these studies include variants of visible transient absorption and photoluminescence spectroscopy, terahertz spectroscopy, time-resolved infrared spectroscopy, and femtosecond stimulated Raman spectroscopy. These probes allow measurement of population dynamics of relevant photoexcitations (excitons, polarons) but do not reveal directly how these interact to produce photocarriers. Here, we present a non-linear coherent spectroscopy, photocurrent-detected two-dimensional spectroscopy (2DPC), which is an ultrafast optical thechnique belonging to a family of 2D Fourier-domain spectroscopies that allows measurement of correlations between optical transitions induced by short optical pulses. In our implementation, spectral correlations are detected via the time-integrated photocurrent produced in a photovoltaic diode. Four collinear ultrashort laser pulses (10 fs, centered at 600 nm in our experimental setup) excite the semiconductor polymer in the solar cell, with a variable delay that is independently controlled between each pulse in the sequence. Each pulse separately excites a quantum wavepacket with spectral phase and amplitude imparted by that pulse, while the effect of the pulse sequence is to collectively excite multiple quantum coherences. Interferences between the various combinations of the wavepackets determine linear and non-linear contributions to the material optical response. The fourth-order signal terms of the detected photocurrent are read using phase-sensitive detection schemes with reference waveforms corresponding to a modulation of specific phase combinations of the four femtosecond excitation pulses. By scanning the time delay between the pulses 1 and 2, as well as that between pulses 3 and 4 (coherence times), at a fixed delay between pulses 2 and 3 (population waiting time), one measures a two-dimensional coherence decay function that is Fourier transformed to produce a 2D photocurrent correlation excitation spectrum. Measurement of such spectra at different population waiting times provides insight into the role of spectral correlations and state coherence in photocurrent generation in such complex functional materials. We focus on solar cells produced by blends of a common carbazole-thiophene-benzothiadiazole polymer, PCDTBT (the donor polymer), and PCBM (the fullerene acceptor), in which we analyse the dynamics of total photocurrent generation via the time evolution of diagonal and off-diagonal spectral correlations. We address the role of vibronic coherence as well as resonant tunneling in charge separation pathways on ultrashort timescales.

Photovoltaic diodes based on blends of semiconductor polymers and fullerene derivatives now produce power conversion efficiencies exceeding 10% under standard solar illumination. This indicates that photocarriers can be generated efficiently in well-optimized organic het- erostructures. Generally speaking, the dynamics of electron transfer and charge separation in polymer-based photovoltaic systems is dictated by a sequence of elementary steps following photo-excitation: an exciton created within the bulk of the material must diffuse to an region of the material where there is an electronic driving force for charge-separation. Recent evidence, however, suggests that free-polarons produced by dissociation of excitons appear within the first 35-50 fs following photoexcitationlsaquo;suggesting that the process occurs by direct long-ranged tunneling. We will present here our theoretical models and discuss recent experiment experimental evidence that support this notion.

Controlling charge transfer (CT), charge separation (CS), and charge recombination (CR) at the donor-acceptor interface is extremely important to optimize the conversion efficiency in solar cell devices. In general, ultrafast CT and slow CR are desirable for optimal device performance. In this talk, I will present new experimental results on the CT, CS and CR at PbS QDs/PCBM,¹ porphyrin/CdTe QDs,² porphyrin/oligomer³ and porphyrin/graphene⁴ interfaces using femtosecond transient absorption spectroscopy with broad-band capability and 120 fs temporal resolution. For the first two systems, the time-resolved results reveal that the quantum confinement is the key element for efficient electron injection and charge separation processes. For the last two systems, on the other hand, turning the on/off CT has been shown to be possible by controlling the charge density on the nitrogen atom of the porphyrin meso unit and the charge localization on the porphyrin cavity. In these cases, a specific ground-state interaction that brings the donor and acceptor components into close molecular proximity, allowing ultrafast photoinduced CT to occur, giving rise to a CT state that is probed by femtosecond transient absorption spectroscopy. Real-space imaging a donor-acceptor interfaces using four-dimensional electron imaging⁵ will also be presented.
References
1- Alaa O. El-Ballouli, Erkki Alarousu, Marco Bernardi, Shawkat M. Aly, Alec P. Lagrow, Osman M. Bakr and Omar F. Mohammed, J. Am. Chem. Soc.,2014, 136, 6952-6959.
2- Ghada A. Hamdi, Shawkat M. Aly, Anwar Usman, Mohamed S. Eita, Vasily Melnikov, Omar F. Mohammed, Scientific Reports, Submitted.
3- Shawkat M. Aly, Subhadip Goswami, Qana A. Alsulami, Kirk S. Schanze and Omar F. Mohammed, J. Phys. Chem. Lett.,2014, 5, 3386-3390.
4- Shawkat Aly, Manas Parida, Erkki Alarousu, and Omar F. Mohammed, Chem. Commun., 2014, 50, 10452-10455.
5- Omar F. Mohammed, D-S. Yang, Samir K. Pal, and Ahmed. H. Zewail, J. Am. Chem. Soc.,2013, 133, 7708-7711.

The ability to coherently rearrange atoms and chemical bonds at will is among the grand challenges of materials science and chemistry. A primary component of this challenge is to drive a desired reaction without adding excessive residual energy. One common method is to use photons to excite the electronic system, attempting to guide ionic relaxations towards an intended state. While effective in some systems, this approach may incur additional heating or less desirable conformational results. Meanwhile, slower processes without electronic excitations may be thwarted by intramolecular vibrational relaxation (IVR), where anharmonic coupling between vibrational modes leads to irreversible heating of the target.
Motivated by recent advances in the generation and control of strong terahertz (THz) single-cycle pulses, we have investigated the theoretical potential of such pulses to drive chemistry while circumventing both of these problems. This may be possible with single-cycle THz pulses because their spectral content is well separated from electronic excitation frequencies in most molecules, but their timescales may be fast enough to add and remove energy from the ionic system without allowing IVR to take place.
Here we discover that it is, in fact, possible to switch a molecule between several isomers using a strong THz pulse while depositing sufficiently little energy that the molecule stays in the isomerized state. We establish the feasibility of the method we present in this work by addressing the following questions: 1.) How accurately must we know the potential energy surface of the molecule in order to design an effective isomerizing pulse? 2.) How large an activation barrier can be surmounted before the fields become strong enough to ionize the molecule? 3.) How well must the target alignment be prepared in order for the desired trajectory to occur? In this work, we take a first look at these questions by way of density functional theory (DFT) calculations and semi-classical time-dependent DFT (TDDFT)-based Ehrenfest molecular dynamics (MD). TDDFT-Ehrenfest MD offers a way to include a time-varying electric field in the TDDFT Hamiltonian within the adiabatic exchange and correlation (XC) approximations. LiNC is one of the simplest molecules with multiple stable conformations, allowing a relatively thorough study of the DFT potential surface and efficient TDDFT-Ehrenfest simulations. We find that LiNC can be isomerized to either of its metastable conformations in TDDFT-Ehrenfest MD with very low residual heating and ionization rates. This work points the way toward THz coherent control of chemical bonds in materials and biological systems, and provides guidance and limiting factors.

Quantum dots (QDs) are small semi conductive organic spheres whose size (few nanometers) shows quantum phenomena. They possess the unique ability to absorb and emit at different wavelengths depending on their chemical composition and size. Moreover, the full width at half maximum of their emission peak (FWHM) is very narrow (around 30nm). This property enables them, when they are incorporated in light sources, to increase color saturation. Finally, they show a very high quantum emission efficiency. We are interested in using and adapting these properties to generate saturated colors in visualization systems based on LEDs. Our main goal is to ensure the color conversion of blue micro-LEDs by means of QDs. Several technical bottlenecks are related to this approach. One is the minimum thickness necessary to convert all the blue light into red or green. Another is to achieve red, green, and blue structuration on small pixel-sizes, which are typically as small as 10µm for micro display applications. In this talk, we will present conversions from blue to green and red with QDs implemented on array of hybridized LEDs. We already obtained conversion rates of around 15%-20%, which is very promising for the future.

Blending of conjugated polymers is a popular way to get white light from polymer light emitting diodes (PLEDs) and has potential for delivering low cost and large area technology. In this paper, we focus on blends of poly(9,9-dioctyl#64258;uorene-2, 7-diyl)(PFO) and poly (5-methoxy-2-2-ethyl-hexylthio)-p-phenylenevinylene (MEH-PPV) with the objective of understanding the changes in the photophysics and charge transport as a function of MEHPPV concentration. We measure photoluminescence (PL) emission, photoluminescence quantum efficiency (PLQE), PLED characteristics and electroluminescence transient (ELT) as a function of MEHPPV concentration (0 - 15 wt%) in PFO matrix. PL emission exhibits linear increase in the MEHPPV emission with the increase in its concentration, whereas PLQE of the polymer blend decreases. The brightness and EL efficiency of the blend PLEDs, in comparison to performance of pure PFO based PLED, were increased significantly at low concentrations of MEH-PPV and gradually declined at higher concentrations. The best EL performance has been obtained at 2 wt. % of MEH-PPV in pure PFO having white color emission with 1.92 cd A^-1 efficiency and (0.31, 0.32) CIE coordinate at 11 mA.
Correlating information from different optoelectronic techniques, we find that the improvement at the low wt%. MEH-PPV blend OLEDs is due to improved charge balancing in the device. From EL transient measurements, it was found that at relatively low blend concentration of MEH-PPV, less than (2wt.%), the effective hole mobility of PFO:MEH-PPV blend increase with concentration of MEH-PPV. This is derived from the Poole-Frenkel (P-F) plot of mobility(mu;) as a function electric field having a positive slope (β > 0). A decreasing mobility is then observed at higher blend concentration (8-15 wt.%) with negative P-F type dependence as function of electric filed. Gaussian disorder model (GDM) have been used to explain the obtained results which shows that the interplay of both the energetic (σ) and position disorder ( Σ) of dopant molecules in the sample decide the slope of the log mu; versus E^1/2 plot. The decrease in the mobility upon higher concentration of MEH-PPV with negative slope has been attributed to the increase of disorder as well as aggregation of doped polymer in the PFO matrix which is supported by atomic force microscopy (AFM) images.

Photon upconversion (UC), converting low-energy photons to higher-energy photons, is a key methodology to overcome the efficiency limits of sunlight-powered devices including photovoltaic cells and photochemical hydrogen productions [01]. Conventional methods including the second harmonic generation and multi-step excitation of lanthanides require light intensities orders of magnitude higher than the solar irradiance and their low conversion efficiencies detract from their appeal. Meanwhile, a promising strategy developed in the last decade is workable with non-coherent, low-intensity light sources, namely the sensitized triplet-triplet annihilation-based photon upconversion (TTA-UC) in multicomponent donor-acceptor [02] [03]. However, an ideal air-stable, highly efficient, and low-power TTA-UC has not been realized [04]. Here we report a novel self-assembly approach to achieve this, which enabled highly efficient TTA-UC even in the presence of oxygen. A newly developed lipophilic 9,10-diphenylanthracene-based emitter molecule functionalized with multiple hydrogen-bonding moieties a spontaneously co-assemble with a triplet sensitizer in organic media, which reveal efficient triplet sensitization and succeeding triplet energy migration among the pre-organized chromophores. This supramolecular light-harvesting system enjoys the highest record UC quantum yield of 29% optimized at low excitation power in deaerated conditions. Significantly, it also shows the highest in-air UC quantum yield of 18% by virtue of surprising oxygen shielding ability of the self-assembled structures, and this approach is facilely applicable to organogel and solid film systems [05].
[01] Ginley, D.; Green, M. A.; Collins, R. MRS Bull. 2008, 33, 355.
[02] Baluschev, S.; Miteva, T.; Yakutkin, V.; Nelles, G.; Yasuda, A.; 469 Wegner, G. Phys. Rev. Lett. 2006, 97, 143903.
[03] Monguzzi, A.; Tubino, R.; Hoseinkhani, S.; Campione, M.; Meinardi, F. Phys. Chem. Chem. Phys.2012, 14, 4322.
[04] Monguzzi, A.; Bianchi, F.; Bianchi, A.; Mauri, M.; Simonutti, R.; Ruffo, R.; Tubino, R.; Meinardi, F. Adv. Energy Mater.2013, 3, 680.
[05] Ogawa, T.; Yanai, N.; Monguzzi, A.; Kimizuka, N., submitted

Conductive polymer poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) has attracted significant attention as hole transport and electrode layer for flexible organic devices substituting metal electrode. Among the various conductive polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the promising candidates with high conductivity, optical transmittance and work function. Unlike the well investigated electrical properties of PEDOT:PSS, mechanical properties such as adhesion and cohesion failure are less studied even though the reliable mechanical properties are essential to fragile polymer electrode compared to the metal electrode. In addition, the hygroscopic characteristic of PEDOT:PSS induce mechanical instability to moist environment, placing more importance in further investigation of mechanical properties of PEDOT:PSS.
Here, we investigated the interfacial fracture energy and kinetics of decohesion under environmental condition. Also, we added methyl sulfoxide (DMSO) to the PEDOT:PSS aqueous solution to increase the possibility of PEDOT:PSS as an electrode layer. We found that the DMSO addition affects not only the conductivity but also affects the mechanical properties by morphology change depending on the DMSO composition (0, 1, 3, 5 wt.%). Through the results, it was discovered that 3 wt.% DMSO composition displayed optimal condition to maximize the both electrical property and interfacial fracture energy. The addition of DMSO, for conductivity enhancement, defined a particular morphological mechanism, enabling enhancement of mechanical and electrical properties.
Control ID : 2133237

The solubility of semiconducting polymer films can be controlled utilizing a p-type dopant to create an insoluble charge transfer of polymer⁺:dopant^-. In order to control the pattern of the film, we evaporate the dopant through a mask, then dissolve the polymer which has not been doped to leave features where dopant is present. The process of dopant induced solubility control (DISC) is an inexpensive process that allows solution patterning of polymer organics as opposed to evaporation patterning of small molecules. The two-step process avoids mass transfer complications associated with ink-jet patterning techniques, leaving relatively flat features. To measure sharp patterned features of organic thin film polymers we image the edges of our features using atomic force microscopy. The angle between the substrate and the bulk of the polymer will depend on the sharpness. Developer solvents play an important role on the sharpness: it is important that the solvent differentiates between the neat and doped parts of the polymer film without allowing the dopant to diffuse laterally through the film. To choose the optimal developer solvent, we analyze the Flory-Huggins equation and Hildebrand solubility parameters for various solvents compared to our polymer, dopant, and polymer⁺:dopant^-. In addition, we compare these values to how much the film of polymer⁺:dopant^- swells within the solvent. An ideal solvent will leave our features as sharp as possible. Our first priority is that the Gibbs free energy of mixing between the polymer and solvent must be less than zero (ΔG_mix <0). The next priority is that the ΔG_mix >>0 for the solvent and polymer⁺:dopant^- and lastly that the ΔG_mix >0 for the dopant and solvent.

A major barrier to commercialization of white organic light emitting diodes (WOLEDs) as next generation solid state lighting sources is their limited emission brightness, likely requiring large area panels in order to produce a comparable illumination to point sources such as inorganic LEDs. As a result, device structures consisting of 5-10 distinct layers for efficient WOLEDs with multiple emissive species are impractical for widespread adoption. On the other hand, materials which exhibit phosphorescent excimer emission afford the fabrication of simplified WOLEDS with a single emissive material. Select square-planar metal complexes are capable of achieving broad white light through the combination of blue emission from an isolated molecule and broad red emission from phosphorescent excimers formed through 2 or more stacked molecules. Phosphorescent excimers have the benefits of device simplicity, absent energy transfer processes between emissive species, reduced voltage dependence of the emission, and reduced differential aging, making them ideal candidates for white lighting. In my presentation I will report on a series of developments made in our lab on both platinum and palladium complexes for excimer emissive species. The first of these developments is a singly#8209;doped white device with forward direction external quantum efficiency (EQE) over 20% and color rendering index of 80, which is comparable to the best multiple emissive layer devices. Secondly, the development of efficient and stable single-doped white lighting employing tetradentate platinum complexes which exhibit EQE of over 25% for a balanced white emission in a charge confining structure, as well as device operational lifetimes of 500-1000 hours in an electrochemically stable structure. Finally, I will discuss new advances in molecular design utilizing cyclometalating ligands which allow for deep blue emission while remaining electrochemically stable. In particular, I will report on the first efficient palladium complexes for excimer based white devices (to our knowledge) which exhibited both 20% external quantum efficiency and a lifetime to 70% of initial luminance (1000cd/m²) of 1000 hours for the same device. Through the presentation of these various achievements, I will discuss the molecular design principles for efficient and stable emitters, color tuning principles within these molecular designs, the development of efficient and stable device architectures, and the future direction and outlook for this exciting and bourgeoning class of OLEDs.

Solubility of a polymer has been shown to change when a molecular dopant is introduced which is a physical representation of the sign of the Gibbs free energy of mixing (ΔG_mix) with the solvent used. Typically in polymer solvent interactions, ΔG_mix is given by the Flory-Huggins solution theory; however this relation is not able to characterize a polymer⁺:dopant^- complex. Because molecular dopants do not interact with the polymer unless a charge transfer state is formed we proposed a modified model to the Flory-Huggins theory in which the polymer⁺:dopant^- is represented as a third substance in the solution creating a polymer/polymer⁺:dopant^- solvent mix. In order to develop this model we took a temperature and concentration dependent series of UV/Vis and NMR spectra of the polymer⁺:dopant^- complex as it was exposed to various solvents. The spectra allowed us to analyze what conditions lead to the polymer/polymer⁺:dopant^- mixture to dissolve. From this data we were able to model our system in order to predict the ΔG_mix in various conditions. We then took this model and found solvent and temperature conditions where ΔG_mix of the polymer/polymer⁺:dopant^- complex was >> 0 while ΔG_mix of the neat polymer was <0 in order to find conditions in which we can apply dopant induced solubility control (DISC). The difference in ΔG_mix is critical to the process as it allows for solubility to be controlled by the presence of dopant in the system. Finally we tested DISC in a variety of situations we predicted promising in order to confirm the accuracy and applicability of the model.

Organic electronics promise to provide flexible, large-area circuitry that can be fabricated inexpensively from solutions. Due to the mutual solubility of most conjugated organic materials (COMs), solution-based fabrication of multi-layer, micro- to nano-scale features is problematic. Here we demonstrate that the solubility of COMs can be reversibly “switched off" by molecular doping, then recovered with a suitable de-doping solution. Comparing as-cast P3HT films with the same films after doping with F4TCNQ, washing with chlorobenzene, and subsequent de-doping, we observe no changes to film thickness, identical UV-vis-NIR, fluorescence, and HNMR spectra, and unchanged saturation-regime hole mobilities of 0.07 cm²/V-s. Additionally, the film can also be de-doped by illumination at a specific optical transition while in a solvent bath. Functionally, doping with F4TCNQ behaves as an reversible, optically controllable cross-linking reaction for P3HT, a process we call “doping induced solubility control” or DISC.
Using DISC, we are able to stack mutually soluble materials cast from the same solvent, then de-dope the underlying film to form bilayers. We also demonstrate the ability to laterally pattern polymer films by dopant evaporation, achieving micron-scale features, and optically, achieving features as small as 200 nm. After forming these structures, the films can be reverted to their original optical, electrical, chemical state without disrupting the patterned features. This method greatly simplifies solution-based device fabrication, is easily adaptable to current manufacturing workflows, and is potentially generalizable to other classes of materials.

We present a novel platform for multi-stacking organic films by fluorous ligands engineering. By employing fluorinated transition metal complex phosphorescent emitter and its fluorous solvent immiscible with organic molecules, we could fabricate phosphorescent organic light-emitting diodes (PhOLEDs) with emission layer spin casted onto small molecule hole-transporting layer (HTL). Tris[2-phenylpyridinato-C²,N]iridium(III) (Ir(ppy)₃) with highly fluorinated ligands was synthesized and dissolved into hydrofluoroethers, a kind of fluorous solvent, with fluorous host polymer. Then investigation of physical or chemical damage of underlying layer including dissolution, cracking and delamination was carried out by using the cross-sectional scanning electron microscope image of small molecule HTL / highly fluorinated Ir(ppy)₃ doped host polymer layer. The photophysical analysis including UV-Vis absorption and photoluminescence (PL) spectra of host and dopant material and PL decay traces of doped and undoped host film indicated occurrence of energy transfer. From these results, we could fabricate and evaluate green PhOLEDs. Electroluminescence spectra of the devices implied that electron-hole recombination occurs at the emissive layer without charge imbalance which could be caused by destruction of HTL.

Doping induced solubility control (DISC) is a technique recently discovered by the Moule group (Jacobs, et. Al., Science, in review) which uses molecular dopants to “switch off” the solubility of conductive polymers. In the prototypical system consisting of conductive polymer P3HT and p-type molecular dopant F4TCNQ, doping is shown to be fully reversible, demonstrating complete recovery of the P3HT film&’s optical, electrical, and chemical properties. Intriguingly, illuminating the doped film with 405nm light while immersed in solvent causes the illuminated area to de-dope; if the solvent is a good solvent for P3HT, it dissolves as well. Initial work has demonstrated the ability to pattern sub-micron features using this technique. Here, we study the effects of film morphology, doping level, choice of patterning solvent, and optical power and dose on the resulting feature sharpness, achieving resolution approaching the diffraction limit. Patterning is performed in-situ using a laser scanning confocal microscope, and subsequently characterized with fluorescence microscopy and atomic force microscopy.

Long-lived room temperature phosphorescence (RTP) from metal-free organic material system is very rare due to the very low intersystem crossing rate in organic molecules and long-lived excited triplet state, which can be easily quenched by oxygen and thermal perturbations.
Since the first reports on long-lived emission from pure crystalline organic compounds in 1978, C.S. Bilen¹, there has been limited interest in this area. Recent progress in this area indicates the need for specific conditions to observe such long-lived emission.^2-5 For example, small organic molecules were shielded by larger molecules in order to have their phosphorescence stabilized and protected from oxygen quenching. Other approaches relied on crystallizing the molecules or mixing them in specific host matrix in order to observe the long-lived phosphorescence.
In this work, we will present an intense long-lived RTP originating from organic exciplex in the absence of phosphorescence protector or stabilizer. The experimental observation indicates that such exciplex is relatively resistant to oxygen quenching. Moreover, the ease of forming such materials system by simple grinding of commercially available organic components is an attractive low cost approach.
References:
[1] C. S. Bilen, N. Harrison, D. J. Morantz, Nature1978, 271, 235.
[2] S. Hirata, K. Totani, J. Zhang, T. Yamashita, H. Kaji, S. R. Marder, T. Watanabe, C. Adachi, Adv. Funct. Mater.2013, 23, 3386.
[3] W. Z. Yuan, X. Y. Shen, H. Zhao, J. W. Y. Lam, L. Tang, P. Lu, C. Wang, Y. Liu, Z. Wang,; Q. Zheng, J. Z. Sun, Y. Ma, B. Z. Tang, J. Phys. Chem. C2010, 114, 6090.
[4] O. Bolton, K. Lee, H. J. Kim, K. Y. Lin, J. Kim, Nature chemistry2011, 3, 205.
[5] V. B. Nazarov, T. G. Vershinnikova, M. V. Alfimov, Russ. Chem. Bull.1999, 48, 1998.

2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is an effective p-type dopant for a variety of conjugated polymers. However, being relatively insoluble, F4TCNQ has limited applications. In order to counter this issue, we synthesized four analogs of F4TCNQ and doped various conjugated polymers with each derivative. We demonstrate how the increased solubility affects the morphology of the polymer and consequently affects the optical properties and charge transport of the doped films. The dopant&’s capability correlates to its electron affinity and the F4TCNQ derivatives are limited by an electron affinity of 5.2 eV. We then synthesized dopant derivatives based on benzoquinone structures that have higher electron affinities. We present the effect of these dopants on various conjugated polymers and how the increased electron affinity widens the range of polymers that can be doped.

Carbazole compounds such as 1,3-Bis(N-carbazolyl)benzene (mCP) are good host materials for blue phosphorescent OLEDs, due to their high hole mobility, high triplet energy and small singlet-triplet energy splitting. However, the glass transition temperature (T_g) for mCP is only 60 ^oC, which causes device stability issues. To overcome this problem, three different ter-carbazole host materials (PTC, ETC and BETC) with higher T_g were designed based on mCP. While there is no significant difference in efficiency, devices using ter-carbazole as the host showed a reduction efficiency roll-off at high luminance. Transient photoluminescence and electroluminescence measurements show that the low triplet-triplet annihilation and triplet-polaron quenching rates are the reasons for this reduced efficiency roll-off. The effect from the glass transition temperature of host materials is also illuminated by different driving conditions.

Solid phases of π-conjugated molecules and polymers continue to receive widespread attention as semiconducting materials in field effect transistors, light emitting diodes and solar cells. However, despite the more than five decades of intensive experimental and theoretical research following Kasha's pioneering work on H- and J-aggregates, there are still a great many questions regarding the nature of the photo- excitations in molecular assemblies and how their spectral signatures are related to crystal packing and morphology. The theory of Kasha is based on the long-range Coulombic coupling between chromophores. However, in packing morphologies such as the commonly occurring π-stacking motif, the intimate contact between nearest neighbors allows for charge transfer and the creation of a short-range excitonic coupling mechanism due to wave function overlap. In this talk, it is shown how the simultaneous presence of long-range and short-range intermolecular couplings impact photophysical and transport properties in molecular π-stacks. The effect is similar to that recently described in polymer π-stacks. The analysis is based on a Holstein-style Hamiltonian which includes Coulombic coupling and charge transfer. The interference between short-range and long-range couplings defines four aggregate types: HH, HJ, JH and JJ, based on the sign of the couplings. Each of the four aggregate types possess unique photophysical and tranport properties. For example, HH-aggregates have constructively enhanced exciton mobilities and small radiative decay rates, making them excellent candidates for solar cell absorbers. JJ-aggregates can be superradiant at room temperature and therefore serve as good light emitting materials. The photophysical signatures of the four aggregate types include aggregation-induced changes to the vibronic progressions in the absorption and photoluminescence spectra. The vibronic progression, sourced primarily by the ubiquitous vinyl-stretching mode common to virtually all π-conjugated molecules, therefore serves as a direct probe of the nature of the excitonic coupling, as well as the exciton coherence length and mobility. Specific applications will be made to rylene π-stacks which have been intensively investigated as dye pigments and electron-transporting materials. A new design paradigm for organic electronic materials is presented based on the extreme sensitivity of the short-range coupling to small (sub Angstrom) intermolecular displacements transverse to the stacking axis.

The heterogeneity and anisotropic nature of the solid-state structure of semiconducting polymers prevents a simple description of their electronic and opto-electronic characteristics, which consequently continue to be incompletely understand and debated. In isolated, unfolded chains, the optoelectronic structure of the conjugated backbone can be considered molecular in character with delocalized orbitals. In the complex solid state, H- or J-like coupling of polymer&’s ‘chromophores&’ can occur, which imparts an anisotropic 2D opto-electronic character (with the third dimension being inactive). The fluorescence state with lifetimes of 100s of ps following the much faster absorption is currently thought to be a result of the continuous evolution and relaxation of the same quantum mechanical state. Through spectroscopic studies of the ubiquitous semiconductor poly(3-hexylthiophene) (P3HT), whose H- and J-coupling has been manipulated, we show that even in cases where the absorption spectrum exhibits intrachain excitonic line shapes (J-like), intramonomer repulsive forces and the anisotropic coupling along and transverse to the polymer chains drive formation of interchain excitons, which radiatively recombine to generate H-like emission spectra. Our time resolved studies show that this transformation is radiatively discontinuous and that photogenerated excitons seem to first form an intermediate dark polaron-pair prior to forming an interchain photoluminescent exciton.

The molecular assembly and larger-scale structure significantly influence the
optoelectronic properties of polymer semiconductor blends [1-3]. We will
discuss blends of poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(3hexylselenophene-2,5-diyl)
(P3HS) as a prototype system to study the role of
H- and J- aggregation and how this impacts energy transfer between the two
polymers. We show that systematically changing the phase morphology of
P3HT:P3HS systems by using different polymer molecular weights, we can
manipulate the energy transfer between the two components. We suggest
that our finding is likely broadly applicable to a large variety of
polymer:polymer semiconductor blends.
[1] Felix Peter Vinzenz Koch, Jonathan Rivnay, Sam Foster, Christian Müller, Jonathan M. Downing,
Ester Buchaca-Domingo, Paul Westacott, Liyang Yu, Mingjian Yuang, Mohammed Baklar, Zhuping Fei,
Christine Luscombeg, Martyn A. McLachland, Martin Heeney, Garry Rumbles, Carlos Silva, Alberto
Salleo, Jenny Nelson, Paul Smith Natalie Stingelin. The impact of molecular weight on microstructure
and charge transport in semicrystalline polymer semiconductors-poly(3-hexylthiophene), a model study.
Prog. Polym. Sci., 2013, 38, 1978-1989.
[2] Christoph Hellmann, Francis Paquin, Neil D. Treat, Annalisa Bruno, Luke X. Reynolds, Saif A.
Haque, Paul N. Stavrinou, Carlos Silva, and Natalie Stingelin. Controlling the interaction of light with
polymer semiconductors. Adv. Mater., 2013, 25, 4906-4911.
[3] Francis Paquin, Hajime Yamagata, Nicholas J. Hestand, Maciej Sakowicz, Nicolas Bérubé, Michel
Cocirc;té, Luke X. Reynolds, Saif A. Haque, Natalie Stingelin, Frank C. Spano, and Carlos Silva. Twodimensional
spatial coherence of excitons in semicrystalline polymeric semiconductors: Effect of
molecular weight. Phys. Rev. B, 2013, 88, 155202.

Conjugated P3HT chains are known to exist at least in two distinct conformations, a coiled phase and a better ordered aggregated phase. In an endeavor to correlate these phases with their opto-electronic properties, we investigated the aggregation of P3HT in solution within a temperature range from 300 K to 5 K by emission and absorption spectroscopy.
We show that aggregation is a temperature controlled process, driven by a thermodynamic order-disorder transition. The transition temperature increases with molecular weight of the chains which implies a smearing out of the phase transition in samples with increasing polydispersity, eroding the signature of a first order phase transition.
By detailed spectroscopical investigations on aggregated P3HT, we can give evidence for different polymorphs. At 5 K, we can identify two H-type aggregates with planar polymer backbones yet different degree of order regarding their sidechains. Upon heating, the H#8209;character of the aggregates becomes gradually eroded, until just below the transition temperature the prevailing “aggregate” structure is that of still phase-separated, yet disordered main and sidechains. These conclusions are derived by analyzing the vibrational structure of the spectra and from comparing the solution spectra with those obtained from thin films that were cooled slowly from the melting temperature to room temperature and that had been analysed previously by various X-ray techniques. In addition, site selectively recorded fluorescence spectra show that there is - dependent on temperature - energy transfer from higher energy to lower energy aggregates. This suggests that they must form clusters with dimensions of the exciton diffusion length, i.e. several nanometers in diameter.
(1) Panzer, F.; Bässler, H.; Lohwasser, R.; Thelakkat, M.; Köhler, A.; The Impact of Polydispersity and Molecular Weight on the Order-Disorder Transition in Poly(3-hexylthiophene); The Journal of Physical Chemistry Letters2014.
(2) Panzer, F.; Sommer, M.; Bässler, H.; Thelakkat, M.; Köhler, A.; Spectroscopic Signature of Two Distinct H-Aggregate Species in Poly(3-hexylthiophene); (submitted)

The balance between excited state delocalisation and intermolecular interactions with resonance or charge transfer character plays a key role in photophysical processes such as electron-hole recombination or dissociation. In this talk we shall discuss how excited state localisation and delocalisation controls (i) the recombination/dissociation process at donor-acceptor interfaces in solar cell structures and (ii) the transport of charges and excitations across the bulk.

Chain conformation and resulting solid-state microstructure in conjugated polymer films has been shown to have a profound impact on the nature of intra- and interchain dispersion of π electrons. As a consequence, neutral excitations in this class of materials can be considered to be in the Frenkel-exciton limit with respect to interchain interactions, highly localized to a few lattice sites across the lamellar lattice, spanning < 1 nm, and highly influenced by energetic disorder. The limiting case of through bond, intrachain excitonic coupling leads to what can be considered as Wannier-Mott excitons in one-dimensional lattices, In such excitons the electron and hole are bound over several repeat units of the polymer chain. Previous studies demonstrate that in regioregular poly(3-hexylthiophene) (P3HT) films, there is a competition between J-like (intrachain) and H-like (interchain) excitonic coupling (see Paquin et al. ‘&’Two-dimensional spatial coherence of excitons in semicrystalline polymeric semiconductors: Effect of molecular weight&’&’, Phys. Rev. B. 88, 155202 (2013)).
By means of temperature-dependent photoluminescence spectroscopy, it is possible to show that the intrachain and interchain exciton bandwidths corresponding to the emitting aggregate species can be determined entirely from analysis of the PL vibronic progression. In particular, these parameters are accessible from the 0-2/ 0-1 and 0-0/ 0-1 ratios. The results show that the 0-0/0-1 ratio of the PL intensity first increases with temperature (T). The initial increase with T is a result of increasing population of the optically allowed higher energy exciton, as occurs in interchain H aggregates. The peak occurs when kT is approximately equal to the interchain splitting, and the subsequent decrease is the expected behavior for J aggregates.
Here, we examine the interplay between extended exciton states delocalized along the polymer backbones and across polymer chains within the π stack, in P3HT with molecular weight of 125 kg/mol. The relative intensities of the 0-0/ 0-1 vibronic progression at different values of temperature ranging from 10 to 300 K show that the optical emission is dominated by weakly coupled H-aggregates. The obtained free-exciton bandwidth at this molecular weight is about 20 meV. Therefore, it is possible to explore the interplay between the intra- intermolecular interaction by analyzing the temperature dependent PL.
Furthermore, by means of two-dimensional photoluminescence spectroscopy (2D PL), we further investigate the interplay between intra- and intermolecular exitonic coupling in P3HT. 2D PL spectroscopy probes spectral correlations of photoexcitations via cross peaks in the 2D map of the fourth order photoluminescence excitation nonlinear coherent signal. This technique allows us to unravel with intricate detail the nature of intra- and intermolecular electronic coupling.

Optoelectronic properties of organic semiconductors are crucially dependent on the microstructure, and hence may be controlled through manipulating relevant structural features from the molecular to the micrometer-range via processing. Blending with an insulating material thereby opens routes to both tune material properties as well as reduce the amount of expensive active material in the final architecture. We will show here that adding the insulator polyethylene oxide (PEO) to P3HT results in a two-step phase separation process, leading to a substantial change in the absorption and emission spectra of the resulting thin-film structures. Detailed spectroscopic analysis, within the context of the Spano model, indicates a shift from H to J-like aggregation upon blending attributed to greater planarization of the thiophene chain and the enhanced intrachain exciton coherence. This transition to J-aggregation, which enhances the photoluminescent quantum efficiency, also provides insight to a range of fundamental structure/processing/property interrelationships of the complex electronic landscape. As the casting temperature lowers, leading to a reduced evaporation rate, the J-aggregation state becomes more pronounced, suggesting it is thermodynamically favored in the kinetics of thin film formation. The addition of PEO to other functional blends, including the ubiquitous P3HT:PCBM system, will also be presented. Overall the study highlights a new means from which to direct and control energy transfer in semiconductor/insulator systems.

One-dimensional organic nanowires provide a valuable platform for understanding the emergent electronic phenomena in organic semiconductor materials. We have prepared a class of organic nanowires consisting of stacked pi-conjugated building blocks covalently attached to a solubilizing backbone. We have formed self-assembled monolayers from nanowires of various lengths and sequence contexts on gold substrates and characterized their properties with a range of techniques, including x-ray photoelectron spectroscopy (XPS), near-edge x-ray absorption fine structure spectroscopy (NEXAFS), and resonant photoemission spectroscopy (RPES). These studies have elucidated the nanowires&’ electronic structure and charge carrier dynamics, geometric orientation at solid substrates, and interaction with the surrounding environment. Our experiments may offer improved insight into the design of pi-conjugated materials for organic electronic applications.

Vapor deposition and solution processing of materials are two established techniques for fabricating light emitting diodes (OLEDs). High vacuum vapor deposition is used to possibility to obtain devices with complex structures, excellent quality and high purity. However this process is combined with high production costs. A desired goal is the low-cost mass production of devices like flexible large area displays and lighting elements. Thus solution processing of OLED materials could be an alternative. But this method is associated with several disadvantages. Avoiding the partial dissolving of neighbouring layers in the devices is still a great challenge because orthogonal solvent systems are necessary. Therefore cationic small molecule Iridium (III) complexes are interesting because this type of molecules have a good solubility in polar solvents and even in aqueous media. Small molecules tend to crystallise in layers which results in a insufficient device performance. With structure variation of ligands and counter ions, for example through sterical demanding groups, emission wavelength, solubility and degree of crystallinity of cationic Iridium (III) complexes can be influenced.
In this study series of cationic heteroleptic Iridium (III) complexes based on 2-Phenylpyridine, 1,2-Diphenyl-1H-benzimidazole and 1-Phenylpyrazole complexes were synthesized. Especially the variation of counter ions (PF₆^-, SbF₆^-, ClO₄^-, BPh₄^-, BF₄^-, OTf₄^-) was focused to investigate influences on emission wavelength and solubility. Moreover the common matrix materials Triphenylamine and N,Nprime;-Di(1-naphthyl)-N,Nprime;-diphenyl-(1,1prime;-biphenyl)-4,4prime;-diamine were modified for usage as counter ions. The absorption, photoluminescence, cyclic voltammetry and thermal stability of all complexes were investigated. In first experiments emitter layers with acetone as solvent were generated via doctor blading. It was possible to observe homogeneous layers without crystals.

The specific optical absorption of an organic semiconductor is a critical parameter in determining the performance of organic electronic devices. For example in solar cells, higher light harvesting efficiency leads to higher potential efficiency without imposing strict constraints on the electrical transport properties across thick films. We study extinction coefficient as a function of chemical structure for a range of conjugated polymers. In the case of one class of diketopyrolopyrrole copolymers, we find that remarkably high optical absorption can be achieved at relatively low photon energies, for certain conditions on the chemical structure of the polymer. We investigate the origin of the optical absorption in terms of backbone structure and conformation using quantum chemical calculations. We use our results to propose design rules for enhanced light harvesting in conjugated polymers.

When light is emitted by an electronic system, it radiates into the local modes of its optical environment with a characteristic distribution in energy, momentum, and polarization. These distributions can reveal a great deal of information about the electronic structure of the emitter and its local optical environment. For example, energy-momentum spectroscopy can help quantify the multipolar origin of electronic transitions in solid state ions [1] and the orientation of luminescent excitons in layered nano-materials [2].
In this talk, we will present a new experimental method that combines Fourier imaging, optical spectroscopy, and computational optimization to simultaneous acquire the complete, wavelength-dependent, angular emission at two orthogonal polarizations within a single measurement and without the need for bandpass filters or scanning optics [3]. We will present experimental results demonstrating how this method improves optical throughput by orders of magnitude compared to other spectroscopic techniques, while also preserving high spectral resolution. We will also discuss the broader implications of computational optimization for spectroscopic measurements, and investigate the extent to which molecular symmetry may be applied to simplify both the acquisition and analysis of experimental spectra.
[1] T. H. Taminiau, S. Karaveli, N. F. van Hulst, and R. Zia, Nat. Comm. 3, 979 (2012).
[2] J. A. Schuller, S. Karaveli, T. Schiros, K. He, S. Yang, I .Kymissis, J. Shan, and R. Zia, Nature Nanotech. 8, 271-276 (2013).
[3] C. M. Dodosn, J. A. Kurvits, D. Li, and R. Zia, Opt. Lett. 39, 3927 (2014).

Para-distyrylbenzene (DSB), a model compound for the first polymer (PPV) intensely investigated for its electroluminescent properties, has been the focus of numerous experimental and theoretical investigations into the fundamental photophysics of organic semiconductors. The drastic change in optical properties of DSB upon transfer from the dilute solution phase to the bulk crystalline phase has been linked to the herring-bone crystal structure of DSB, which leads to the formation of H-aggregates in which the transition dipole moment of the lowest electronic transition is greatly reduced. While the vibronic-fine structure of DSB seen in low-temperature solution spectra has been fully interpreted on the basis of atomistic simulations combining quantum chemical (QC) calculations and Franck-Condon models of vibronic coupling,¹ the optical spectra of crystal spectra have thus far been simulated using an alternative coarse-grained phenomenological Holstein-type model in which molecules are treated as sites coupled to a single effective intramolecular vibrational mode.² Here we treat single molecules and model DSB-aggregates on an equal footing, with atomistic models combining QC calculations and Franck-Condon and Herzberg Teller models of vibronic coupling. We compare our simulated spectra with coarse-grained Holstein-based simulations and experiments and discuss the theoretical foundations of the two models as well as their advantages and disadvantages when applied to molecular crystals.
(1) Gierschner, J.; Mack, H.-G.; Lu#776;er, L.; Oelkrug, D. J. Chem. Phys.2002, 116, 8596.
(2) Spano, F. C. J. Chem. Phys.2003, 118, 981

Today&’s solid-state luminescent materials are based on inorganic semiconductors, phosphors and quantum dots or consist of synthetic hydrocarbon compounds. Here, we identify a distinctly different class of materials as novel and promising solid-state emitters with unique optical properties - the biologically produced fluorescent proteins. In contrast to many organic dyes, the special structure of fluorescent proteins allows random close packing with an interspacing of 3-4 nm and hence low concentration quenching (7 dB). This enables strong optical amplification (g = 22 cm^-1) in thin protein films and fabrication of efficient solid-state vertical cavity surface emitting micro-lasers with thresholds below 100 pJ and single-frequency operation. We also demonstrate a self-assembly scheme to fabricate protein ring resonator lasers. Moreover, we find that solid-state blends of proteins emitting light of different color support strong Förster resonance energy transfer (FRET). The sensitivity of self-quenching and FRET to the intermolecular distance allows all-optical sensing. Our results demonstrate that the naturally optimized, unique structure of fluorescent proteins can be harnessed in various settings, and provide bio-inspiration for further improvement of synthetic luminescent molecules or nanoparticles.

Organic compounds that exhibit highly efficient, stable blue emission are required to realize inexpensive organic light-emitting diodes for future displays and lighting applications. Here we define the design rules for increasing the electroluminescence efficiency of blue-emitting organic molecules that exhibit thermally activated delayed fluorescence. We show that a large delocalization of the highest occupied molecular orbital and lowest unoccupied molecular orbital in these charge transfer compounds enhances the rate of radiative decay considerably by inducing a large oscillator strength even when there is a small overlap between the two wavefunctions. A compound based on our design principles exhibited a high rate of fluorescence decay and efficient up-conversion of triplet excitons into singlet excited states, leading to both photoluminescence and internal electroluminescence quantum yields of nearly 100%.

Stacked organic light emitting diodes (OLED) have attracted considerable attention in the last decade. Especially the development of efficient large-area OLED luminaires for general lighting requires reliable and easily processable charge generation layers (CGL) with low voltage drop and high optical transparency. OVPD offers the advantage of a precise control of layer morphology, composition and thickness and is a powerful method for the deposition of advanced OLED designs.
In this work, electrical doping of small-molecule organic semiconductors using OVPD is investigated and applied to stacked OLED utilizing hybrid inorganic/organic CGL. The organic p-type dopant NDP-9 of Novaled AG (Dresden, Germany) is used for doping of the hole transport material (HTM) N,N‘-diphenyl-N,N‘-bis(1-naphthylphenyl)-1,1‘-biphenyl-4,4‘-diamine (α-NPD) in an AIXTRON Gen-1 OVPD tool. Hole-only devices of α-NPD with different concentrations of NDP-9 and non-injecting cathode contacts are investigated. The current density-voltage (J-V) characteristics as well as the conductivity measured by impedance spectroscopy are used to analyze the different conduction mechanisms for undoped and doped α-NPD films. Furthermore, the morphology (including a laterally homogeneous distribution of NPD-9 in α-NPD) and optical transmission of thin doped HTM layers are measured by current-sensing atomic force microscopy (CS-AFM) and optical ellipsometry, respectively. A doping concentration of 8 vol.% of NDP-9 in α-NPD is found optimal for hole injection as well as high conductivity. The same dopant concentration was employed in hybrid CGL with the structure: electron transport material ETM001 (PHILIPS GmbH)/ LiF (0.5 nm)/ Al (2 nm)/ α-NPD: 8 vol.% NDP-9. Green phosphorescent OLED units with fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)₃) as emitter doped in an emissive layer (EML), which consists of two cross-faded host materials, were employed for a single-unit reference as well as double- and triple-unit stacked OLED. By simulation, the OLED have been optimized concerning the ETL thicknesses of the individual units for efficient light extraction. Assuming Lambertian emission, external quantum efficiencies (EQE) of 35% and 50% and luminous efficiencies of 45 lm/W and 44 lm/W at 1000 cd/m² are demonstrated for double- and triple-unit green phosphorescent OLED, respectively. The single-unit reference exhibits an EQE of 15% and 37 lm/W at 1000 cd/m². The true luminous efficiency increases merely by 4% from the single-unit OLED to the triple-unit OLED. Due to the cross-faded EML design, a close-to-unity charge carrier balance is maintained even for the stacked devices. Thus, OVPD in combination with a cross-faded two-host EML is shown to be promising for the manufacture of stacked OLED. The next step will be to introduce all-organic CGL employing organic p-type as well as organic n-type dopants.
Support and provision of NDP-9 by NOVALED AG are gratefully acknowledged.

By placing an organic semiconductor material having a relatively narrow electronic transition into an optical microcavity strucutre, it is possible to 'mix' the exction states supported by the semiconductor with the confined photons in the cavity, forming new types of states termed 'cavity polaritons'. Such states are of significant interest, as they have optical properties that are distinctly different from their excitonic or photonic components, and - under suitable excitation conditions - can undergo polariton lasing.
Here, we discuss the formation of polariton states in microcavities containing J-aggregates of two different molecular dyes (cyanine dyes), whose J-band electronic transitons are both coupled to the same cavity photon mode. Under such conditions, three polariton branches are formed, with the "middle" polariton branch composed of an admixture of the cavity photon and the two different exciton states. We show using both photoluminescence excitation spectroscopy and photoluminescence emission measurements (following non-resonant excitation), that such "hybrid" polariton states effectively act as an energy transfer pathway, allowing energy to be transferred between the different exction states [1]. We construct a detailed model that describes exciton scattering into polariton states, and the subsequent decay and energetic relaxation of polaritons. Using our model, we argue that the transfer of middle-branch polaritons to the lower-lying excitonic states is an efficient process, that occurs in time-scale of less than 10 fs.
We then discuss microcavity structures in which a single J-aggregated cyanine dye is placed into a microcavity in which the extended path length of the cavity (3.9 microns) results in the formation of a series of closely spaced (105 meV) cavity photon modes. We show that exctions in the cavity can simultaneously undergo strong coupling with at least four cavity photon-modes, effectively forming a ladder of polariton states [2]. We characterise the polariton population following non-resonant laser excitation, and find a significant polariton population in 3 adjacent polariton branches.
Finally, we discuss the prospects for strong coupling using new types of organic semiconductor materials having high photoluminescence quantum efficiency.
[1] D.M. Coles et al, Nature Materials 13 (2014) 712-719
[2] D.M. Coles et al, Applied Physics Letters 104 (2014) 191108

The coherent coupling between an optical-transition and a confined optical mode, when sufficiently strong, gives rise to a new pair of modes separated in frequency by the vacuum Rabi splitting. Such systems have been widely investigated for electronic-state transitions such as molecular excitons coupled to surface-plasmons and optical microcavities. However, only very recently have vibrational transitions been considered. Here, we bring strong polaritonic-coupling in cavities from the visible into the infrared where a new range of static and dynamic vibrational processes await investigation. We experimentally investigate coherent coupling in several systems.
First, we experimentally and numerically describe coupling between a Fabry-Perot cavity and the carbonyl stretch at an infrared frequency near 1730 cm^#8209;1 in poly-methylmethacrylate (PMMA). As is requisite for the strong coupling regime, the measured vacuum Rabi splitting of 132 cm^#8209;1 is much larger than the full width of either the cavity resonance (34 cm^-1) or the inhomogeneously broadened carbonyl-stretch absorption (24 cm^-1). We have carried out extensive comparison to multiple classical theories and provide evidence that the mixed-state resonances are relatively immune to inhomogeneous vibrational broadening and demonstrate both s- and p-polarized excitation which has implications for orientation and location-specific coupling. We demonstrate the ability to extract splittings by convenient angle tuning of the Fabry-Perot cavity to match the vibrational frequency allowing for rigorous determination of the Rabi splitting in a single sample by observing—as is necessary—the minimum splitting between hybrid modes as one tunes the cavity both red and blue relative to the vibration of interest. Next, we extend this investigation to address the transition from strong to weak coupling regimes through examination of cavities loaded with varying concentrations of the urethane monomer 4,4&’ methylene bis(phenylisocyanate) in PMMA. In this system, the vacuum Rabi splitting at the urethane C=N band (~2260 cm^-1) increases from 0 to ~104 cm^-1 as concentration increases from 0-20 vol%. The measured concentration-dependent splittings are in good agreement to an analytical description that uses no fitting parameters. Finally, we demonstrate that this approach is suitable for investigating microcavities containing liquids by measuring coupling between the C-O stretching band (~1985 cm^-1) of Mo(CO)₆ and a cavity composed of two dielectric mirrors.
Opening the field of polaritonic coupling to vibrational species promises to be a rich arena amenable to a wide variety of infrared-active bonds that can be studied both statically (as here) and dynamically with ultrafast methods. We believe our approach and analysis will initiate studies of coupled-state excitation and dynamics on a wide range of molecular vibrations in both solids and liquids.

Polaritonics has emerged during recent years as a new field of solid-state physics based on the unique quantum properties of mixed light-matter quasiparticles, so called exciton-polaritons. Recent discoveries of Bose-Einstein condensation (BEC) and superfluidity provide opportunities to harness these coherent quantum effects in a new generation of opto-electronic devices. Until now, BECs have been realized either with laser-cooled gases at nano-Kelvin temperatures or with high-quality semiconductor crystals produced by only a few laboratories worldwide. By utilizing the extremely large oscillator strength, exciton binding energy and saturation density of organic semiconductors we demonstrate BEC at room temperature with an amorphous spin-coated polymer film embedded in a Fabry-Pérot microcavity. We observe thermalization of polaritons and, above a critical excitation density, clear evidence of condensation at zero in-plane momentum, manifested by nonlinear behavior, blue-shifted emission and long-range coherence. The key signatures distinguishing the behavior from conventional photon lasing are presented. Since no crystal growth is involved, our approach radically reduces the complexity of experiments to investigate BEC physics and paves the way for a new generation of opto-electronic devices, taking advantage of the processibility and flexibility of polymers.

In the last few years, organic microcavities have shown polariton condensation that can readily be realized at room temperature [1,2,3]. The sudden appearance of off-diagonal long range order in coordinate space has been recognized as a defining feature of polariton condensates. In this report, we use a Michelson interferometer in a mirror-retroreflector configuration to measure the spatial coherence of a polariton condensate using a thermally evaporated film of the oligomer 2,7-bis[9,9-di(4-methylphenyl)-fluoren-2-yl]-9,9-di(4-methylphenyl)fluorene (TDAF). Using an elliptical pump with a Gaussian profile, we find that the intensity patterns of the studied microcavities show non-uniformities related to sample non-uniformity. Despite the non-uniform intensity pattern, clear parallel fringes indicate a flat phase over the entire condensate area.
Upon reaching threshold, we find a sharp increase of the first-order coherence, which extends over the entire pump region. With increasing pump power, we find that the spatial coherence flattens out and that the temporal coherence slowly decreases, similarly to what has been reported in inorganic semiconductors. The measured temporal coherence agrees well with that obtained from the emission linewidth. We measure g⁽¹⁾(r,-r) values of 80% at short distances and of 50% for points separated by nearly 10 mu;m. Moreover, we show that the use of a Gaussian pump spot is necessary for the formation of stable condensates. Measurements with different pump shapes (flat intensity and nearly flat intensity profiles) are shown to lead to highly localized condensate formation that is accompanied by distorted speckle-like interference patterns that are similar to what was observed in Ref. 3.
[1] Kéna-Cohen S, Forrest SR, “Room-temperature polariton lasing in an organic single-crystal microcavity” Nat Phot 4(6), 371-375 (2010).
[2] Daskalakis et al., “Nonlinear interactions in an organic polariton condensate” Nat Mater 13(3), 271-278 (2014).
[3] Plumhof et al., “Room-temperature Bose-Einstein condensation of cavity exciton-polaritons in a polymer” Nat Mater 13(3), 247-252 (2014).

We observe a ten-fold reduction in the lasing threshold of organic microcavities under subpicosecond optical excitation. In contrast to conventional theory of lasing, we find that the lasing threshold depends on the rate at which excitons are created rather than the total energy delivered within the exciton lifetime. The threshold reduction can be described as a microcavity-enhanced super-radiant coupling between the excitons. The interpretation of super-radiance is supported by the temporal relaxation dynamics of the microcavity emission, which follows the super-radiance time rather than the cavity lifetime. This demonstration suggests that room-temperature super-radiant effects could generally lower the threshold in four-level lasing systems of similar relaxation dynamics.

In this report, we present the results of a study of the properties of the organic molecular dye compound, 2,3,6,7-tetracyano-9,10-dimethyl-1,4,5,6,9,10-hexaazaanthracene (DMTCHAA). Previous work with DMTCHAA has demonstrated preparation and fluorescence properties of its vapor deposited thin films [1]. Also, the synthesis, crystal structure, and fluorescence properties have been reported [2,3]. A previous study by one of us has shown the dioctyl derivative of TCHAA possesses the rare property of equal electron and hole mobilities [4]. Here we report the electrochemical and optical properties of DMTCHAA, with particular emphasis on optical waveguiding within its high aspect ratio parallelepiped-shaped microcrystals and sub-microcrystals. The shapes and sizes of DMTCHAA crystals can vary from near spherical to ellipsoidal to parallelepiped depending on crystal growth process conditions. Typically, a mixture of all of these crystal types occurs. However, by following a simple solution process with proper temperature and time profile, an improved yield of high aspect ratio (>6) parallelepiped crystals was obtained. Among the high-aspect ratio parallelepiped DMTCHAA crystals, there is variability not only in the dimensions of length (4 to 10 micrometer), width (0.7 to 1.4 micrometer), and height (0.16 to 0.70 micrometer) but also in the end facet quality. Nevertheless, all of the parallelepiped crystals exhibited active waveguiding. The parallelepiped DMTCHAA crystals were evaluated for their active rectangular dielectric waveguide properties. Excitation (405 nm) of photoluminescence within the DMHAATC crystals leads to tightly confined guided modes. Fluorescence microscopy shows the emission propagates the full length (up to 10 micrometers has been observed) of the DMTCHAA crystals with low loss.
[1] T. Saito, Y. Ueda, K. Harada, K. Fukunishi, Mol. Cryst. Liq. Cryst. 407, 157/[553]-165/[561] (2003).
[2] D. Hou, M. Matsuoka, Dyes and Pigments 22, 57-68 (1993).
[3] J. Jaung, K. Fukunishi, M. Matsuoka, J. Heterocyclic Chem., 34, 653-657 (1997).
[4] Lai, W. W., “Synthesis, structural characterization and mobility measurement of electron accepting pyrazine derivatives” Thesis (Ph.D.)--University of Michigan,; Publication Number: AAI3287675; ISBN: 9780549305606; Source: Dissertation Abstracts International, Volume: 68-11, Section: B, page: 7363.; 195 p. (2006).

Synthetic photonic materials with independent spatial variation in their real and imaginary complex refractive index components have elicited widespread interest as a platform for exploring the physics and applications of parity-time (PT) symmetry. In optics, PT symmetry arises when the refractive index spatial profile and its complex conjugate satisfy ñ(r)=ñ*(-r), a situation that has been predicted to result in a variety of unusual optical phenomena such as unidirectional invisibility and non-reciprocal Bloch oscillations as well as a range of new device applications. PT symmetry has been most widely explored using two-component coupled waveguides and microcavities with balanced gain and loss, however a continuing goal is to realize it in extended periodic systems (e.g. complex gratings and/or lattices) with complex refractive index variation that approximates the form Δñ(x)~Δncos(qx)+iΔκsin(qx).
Here, we introduce a simple route to construct passive PT symmetry in the modal effective index of large area (~cm²) organic thin film waveguides fabricated from S1800 photoresist and the blue pigment copper phthalocyanine (CuPc). Interference lithography is used to write a sinusoidal grating profile into a thin film of the photoresist followed by oblique angle deposition of high extinction coefficient CuPc to lightly coat the 'windward' grating facets. The waveguide is then completed by planarizing it with a top layer of photoresist that is formulated to have a slightly lower refractive index than that of the sinusoidally-patterned underlying photoresist. This arrangement results in a modal effective index with a sinusoidally-varying real part that follows the relative thickness variation between high and low index photoresist, and an imaginary part (loss) that varies in proportion to the local CuPc thickness. Because the CuPc thickness maximizes on the 'windward' grating faces but is negligible on the 'leeward' faces, the loss modulation is shifted by roughly a quarter period from the real index modulation, thereby approximating the targeted PT effective index profile above.
The waveguides are characterized by Kretschmann coupling to the fundamental transverse electric (TE) mode in Littrow, which yields strong asymmetry in the diffracted first order for left versus right incidence that maximizes when Δn=Δκ, marking an exceptional point transition to the broken PT phase that is supported by modeling. These results establish the basis for organic PT waveguide media that can be tuned for operation throughout the visible to near-infrared spectrum and provide a direct pathway to incorporate gain sufficient to achieve active PT symmetric lattices and gratings.

Organic-inorganic perovskites are creating a great deal of excitement in the photovoltaic community, with power conversion efficiencies already matching those of established thin film technologies. It has recently been shown that, at high charge densities, the dominant recombination pathways are radiative bimolecular processes involving free electrons and holes, with photoluminescence quantum efficiencies approaching unity^1,2. The dominance of radiative pathways is particularly important, not only for pushing the open-circuit voltages in photovoltaic devices towards thermodynamic limits, but also for fabricating highly efficient light emission devices based on these materials.
Several groups have recently demonstrated amplified stimulated emission (ASE) from thin films of perovskites, as well as optically-pumped lasing when incorporating the perovskites into suitable cavity structures^1,3-6. The components of the perovskite structure can be easily tuned, enabling bandgap and hence emission color tunability.
Cholesteric liquid crystals (CLCs) possess a 1D photonic band-gap (PBG) which suppresses emission in the PBG and enhances it at the band-edges, thus enabling “mirrorless” lasers⁷. In this work, we demonstrate optically-pumped ASE from a thin film of perovskite which is sandwiched within a cavity comprised of a CLC reflector (reflection optimized at the perovskite emission peak) and a metal back-reflector. The threshold for ASE is significantly reduced in the device stack containing the CLC layer. We study the pump-dependent temporal evolution of the ASE, and also discuss routes towards single-mode lasing. The device structure is particularly promising due to its potential as a wavelength-tuneable single-mode laser for use in a variety of applications.
(1) Deschler, F.; Price, M.; Pathak, S.; Klintberg, L. E.; Jarausch, D.-D.; Higler, R.; Hüttner, S.; Leijtens, T.; Stranks, S. D.; Snaith, H. J.; Atatüre, M.; Phillips, R. T.; Friend, R. H. The Journal of Physical Chemistry Letters2014, 5, 1421.
(2) Stranks, S. D.; Burlakov, V. M.; Leijtens, T.; Ball, J. M.; Goriely, A.; Snaith, H. J. Physical Review Applied2014, 2.
(3) Xing, G.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X.; Sabba, D.; Gratzel, M.; Mhaisalkar, S.; Sum, T. C. Nat Mater2014, 13, 476.
(4) Zhang, Q.; Ha, S. T.; Liu, X.; Sum, T. C.; Xiong, Q. Nano Letters2014.
(5) Sutherland, B. R.; Hoogland, S.; Adachi, M. M.; Wong, C. T. O.; Sargent, E. H. ACS Nano2014.
(6) Dhanker, R.; Brigeman, A. N.; Larsen, A. V.; Stewart, R. J.; Asbury, J. B.; Giebink, N. C. Applied Physics Letters2014, 105.
(7) Coles, H.; Morris, S. Nat Photon2010, 4, 676.

Organic optoelectronic materials attracted particular attention for the development of low-cost multifunctional devices, such as photo-transistors and optical memories. In these devices, light is used as a remote control to modulate electrical properties. In particular, conductivity can be tuned by incorporating photochromic molecules, which are able to undergo a reversible light-induced interconversion between two (or more) isomers possessing markedly different physical and chemical properties, such as different ionization potentials. [1]
In this lecture I will present different recent approaches we have undertaken in order to develop high performing, solution processable, optically switchable thin-film transistors either by decorating planar [2] or non-planar [3] interfaces between Au (nano)structures and the organic/polymeric semiconductor, or via blending or organic[4]/polymeric[5] semiconductors with ad-hoc photochromic molecules. The latter method, which relies on the generation of phototunable and bistable energy levels, was also used to fabricate memories on flexible substrates.
Our findings are of interest for the development of high-performing optically gated electronic devices, towards printable logic circuits.
[1] E. Orgiu, P. Samorigrave;, "Organic Electronics marries Photochromism: Generation of Multifunctional Interfaces, Materials and Devices", Adv. Mater.2014, 26, 1827-1845.
[2] N. Crivillers, E. Orgiu, F. Reinders, M. Mayor, P. Samorigrave;^*, "Optical modulation of the charge injection in an organic field-effect transistor based on photochromic SAM functionalized electrodes" Adv. Mater.2011, 23, 1447-1452..
[3] C. Raimondo, N. Crivillers, F. Reinders, F. Sander, M. Mayor, P. Samorigrave;, "Enhanced current photo-switching in a field-effect transistor based on photoresponsive gold nanoparticles blended with poly(3-hexylthiophene)", Proc. Natl. Acad. Sci. U.S.A.2012, 109, 12375-12380.
[4] M. El Gemayel, K. Börjesson, M. Herder, D.T. Duong, J.A. Hutchison, C. Ruzié, G. Schweicher, A. Salleo, Y. Geerts, S. Hecht, E. Orgiu, P. Samorigrave;, "Optically switchable transistors by simple incorporation of photochromic systems into small molecule semiconducting matrices" 2014 submitted.
[5] E. Orgiu, N. Crivillers, M. Herder, L. Grubert, M. Pätzel, J. Frisch, E. Pavlica, G. Bratina, N. Koch, S. Hecht, and P. Samorigrave;, "Optically switchable transistor via energy level phototuning in a bi-component organic semiconductor", Nat. Chem.2012, 4, 675-679.

Shape memory polymers that can be optically actuated have received increased attention because of their scientific and technological significance. One application for these optically active materials is ultra-lightweight optics in telescopes and spacecraft, as their shape can be remotely controlled using a laser beam or sunlight. We have synthesized a new photo-responsive shape-memory polymer and seek to understand the energy conversion mechanism and build a phenomenological model describing its actuation. Of specific interest is how micro-scale structure helps control optomechanical properties. These polymers consist of polyhedral oligomeric silsesquioxane (POSS) core functionalized with eight identical polymer arms. Polylactides (PLAs) of length 20 were grafted to the octahyroxlated POSS core via ring opening polymerization of D,L lactide to give a star-branched macromer POSS-(PLA₂₀)₈. The reactive hydroxyl end groups were then crosslinked with azobenzene units to form a POSS-Azo photo-responsive polymer. The structure of the crosslinker, core, and polymer were characterized by IR and NMR. The photo-responsive behavior was evaluated by polarized light irradiation. Our results indicate the formation of organic-inorganic hybrid materials, in this case POSS and azobenzene, and demonstrate excellent thermal stability, and reversible cis-trans photo-isomerization.

We present our recent progress on the fabrication of 3D nanostructures using Direct Laser Writing (DLW) and a novel process which involves initiator-free multiphoton polymerization of hybrid organic-inorganic materials. We report for the first time on the redox multiphoton polymerization of an organicminus;inorganic composite material, in which one of the components, a vanadium metallo-organic complex, initiates the polymerization. The composite employs multiphoton absorption to self-generate radicals by photoinduced reduction of the metal species from vanadium (V) to vanadium (IV). We exploit this material for the fabrication of fully 3D structures by multiphoton polymerization with 200 nm resolution, employing a femtosecond laser operating at 800 nm, in the absence of a photoinitiator. Nonlinear absorption measurements indicate that the use of an 800 nm laser initiates the photopolymerization due to three-photon absorption of the vanadium alkoxide. The laser power required to induce this three-photon polymerization is comparable to what is required for inducing two-photon polymerization in materials using standard two-photon absorbers, most likely due to the high content of vanadium in the final composite (up to 50% mole).

By dispersing certain photosensitive molecular species in liquid crystalline materials, a transition between phases with different order, or between an ordered and a disordered phase, can be achieved isothermally by exposure to light of appropriate wavelength. These transitions are typically associated with molecules that undergo photoisomerization or other photoinduced interconversion process between two differently shaped conformers. Upon exposure, the photosensitive molecule becomes either more or less compatible with the host liquid crystal fluid. Azobenzene derivatives have been widely used to build photoresponsive materials in which light disrupts the ordered phase. Recently, isothermal order-increasing transitions have been demonstrated with a new class of photoresponsive materials, namely liquid crystals doped with naphthopyran derivatives. In these systems, the excitation of the isolated molecules (the dopants are typically present at low weight fractions in the liquid crystal medium) and the local switch in molecular conformation of the dopants can lead to the macroscopic change in the properties of the host system. These can also be envisioned as systems in which the energy of the excitation photons is converted into molecular motions of the environment surrounding the excited molecules, as needed to counteract the randomization of orientations that would be energetically favored in the absence of light. We explore how the operating range and changes in order parameter of liquid crystal devices containing naphthopyrans depend on the molecular structure of the dopants and exposure conditions.
An improved understanding of the photophysical processes taking place at the molecular level in these material systems could lead to an enhancement in the performance of multifunctional liquid crystal devices triggered by light.

To overcome the poor solubility of widely accepted p-type dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), a series of organic p-type dopants with modified structures have been synthesized. The lowest unoccupied molecular orbital (LUMO) and the optical bandgaps are estimated based on cyclic voltammetry (CV) and UV-vis measurements. The results show that a range of soluble versions of F4TCNQ with a range of LUMO values are successfully developed. From UV-vis, photoluminescence and in-plane conductivity data, we also demonstrate new series of dopants successfully p-type dope poly(3-hexylthiophene-2,5-diyl) (P3HT), and the effectiveness of p-type dopants has been found to be significantly relative to the geometry of dopant molecules. This study will potentially be beneficial to the dopant molecules design with better solubility as well as superior doping performance for solution processed fabrication.

Sub-bandgap electroluminescence, a phenomenon in which the electroluminescence turn-on voltage is lower than the bandgap voltage of the emitter, has been observed in several fluorescent OLEDs. Based on the results of our transient electroluminescence and transient photoluminescence measurements, we conclude that the sub-bandgap luminescence in Rubrene/C₆₀ OLED is due to singlet emission via triplet-triplet annihilation (TTA). Transient electroluminescence and photoluminescence measurements confirm that singlet excitons are generated through the triplet states and the formation of the triplet states is a direct result of energy transfer through the charge transfer exciplex at the Rubrene/C₆₀ interface. The existence of the charge transfer exciplex is determined by our electro-absorption spectroscopy data. This study could shed light on the design of OLEDs with an extremely low driving voltage and thus a high power efficiency.

We studied the influence of external magnetic and electrical fields during the deposition of organic thin films. Because of the internal dipoles of the organic molecules one can expect a high impact of the external field strengths onto the orientation of the molecules and therefore onto the morphology and properties of the deposited films. Thus, a possibility to improve the efficiencies of organic devices is evaluated.
Organic thin films, which are used for OLEDs and OPV, commonly are of amorphous structure. With ordered structures one can achieve advantages in terms of efficiency and charge carrier transport, which is why several approaches have been discussed to deposit such films. To keep the advantages of an OMBD process - the fast deposition at room temperature onto various substrates - the use of crystals of organic molecules is left out of consideration. Instead, self assembled layers and molecules with high intrinsic dipole momentum have been characterized by several scientific groups on a wide range of substrates. When vapor depositing such molecules, the question arises if the orientation on regularly used substrates, e.g. ITO coated glass substrates, is controllable via external magnetic and electrical fields during deposition.
In this study we investigated commonly used OLED materials regarding their ordering properties during the film growth process. The molecules have been selected to have a wide range of dipole strengths within themselves. A vacuum chamber has been specially modified to allow for a controlled electrical or magnetic field during the deposition. Additionally the substrate temperature has been varied in order to analyze the effect of different mobilities on top of the substrate.
The resulting thin films were then measured regarding their horizontal and vertical charge carrier mobility compared to reference films without the influence of an external field. ITO coated glass substrates were used, patterned to either measure the vertical transport or, with an interdigital structure, to measure the horizontal transport. The optical properties in terms of photoluminescence efficiency and photoluminescence decay time, which are affected by the ordering of the molecules, were determined in comparison.

Amorphous organic semiconductor films used for OLEDs are roughly categorized into two groups by the difference in fabrication processes: (a) vacuum-processed films and (b) solution-processed films. In addition, (b) solution-processed films can be further separated into two groups by molecular size: (b-1) solution-processed small-molecule films and (b-2) solution-processed polymer films. Although most of commercialized OLEDs are currently based on (a) vacuum-processed films, (b) solution-processed films have recently drawn increasing attention because of their low-cost simple fabrication. However, differences between (a) and (b-1), and also between (b-1) and (b-2), have not been systematically discussed from the viewpoint of higher-order structures of molecules.
In particular, their difference in molecular orientation is very important, because it directly affects light extraction in OLEDs. We now know that vacuum-deposited small molecules can be horizontally oriented even in amorphous films [1] and have known for a long time that spin-coated polymers tends to be oriented substantially [2]. Thus, the comparison of molecular orientation is very interesting. In addition, their difference in density are also important, because it is related to thermal stabilities of the films and devices.
In this presentation, we will show and discuss the important differences in physical properties of vacuum-deposited and spin-coated OLED films of more than eight materials. Using some spectroscopic techniques such as ex-situ/in-situ ellipsometry, we systematically analyzed the difference in molecular orientation, density, and transition temperature of the films. We found that the order of the degree of horizontal orientation of small molecules is (a)>(b-1) for commonly used OLED materials irrespective of the experimental conditions of spin coating. It was also found to be difficult to make small molecules in spin-coated films significantly oriented in horizontal directions. We also confirmed the order of orientation of (b-2)>(b-1) using a small molecule and polymer with a similar unit structure. The order of density was (a)>(b). The higher density of (a) is due to the extraordinary kinetic stability of vacuum-deposited organic glasses [3], and we found that it also contributes to the higher thermal stability of (a) compared to (b). It should also be noted that the molecular orientation and glass transition temperature of (b-1) were identical to those of the “deteriorated” vacuum-deposited films that experienced transition through thermal annealing.
Our results demonstrate the fundamental differences between vacuum- and solution-processed films. We should consider their differences carefully when discussing the optical/electrical properties and thermal stability of the films and devices in detail.
[1] D. Yokoyama, J. Mater. Chem. 2011, 21, 19187. [2] J.-S. Kim et al., J. Appl. Phys. 2000, 88, 1073. [3] S. S. Dalal et al., J. Phys. Chem. Lett. 2012, 3, 1229.