This PDF file contains the front matter associated with SPIE Proceedings Volume 10348, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Conjugated polymers with charge transfer characters in a large extend are responsible for the recent advancement in organic photovoltaic (OPV) applications in bulk heterojunction (BHJ) devices. We have carried out collaborative studies on electronic processes and in-situ morphological development of these low bandgap polymers and small molecules using ultrafast optical spectroscopy and in-situ grazing incident X-ray scattering (GIXS). Conventional organic photovoltaic models, in which donor molecules are treated as anonymous electron sources and charge carrier diffusion channels, are challenged by near-infrared transient absorption results of low bandgap polymers indicating strong correlations between intramolecular donor dynamics in < 100 fs and corresponding device power conversion efficiencies. The other conventional model being challenged is the driving force for exciton splitting in the bulk heterojunction environment which has been described by the LUMO-LUMO energy off-set between conjugated polymer electron donor and fullerene derivative electron acceptor. Our study suggests the intramolecular charge transfer characters must be combined with local and global conformations of conjugated polymer chains to achieve the low band gap. Moreover, the morphology of the BHJ films is also investigated by in-situ GIWAS/GISAX methods including the effects of additives which suggest the interplays of the additives and the polymers in solution. The morphology of the heterojunction films has been correlated directly with the yield of the charge separation on time scales from femtosecond to microsecond. In addition, new photophysical studies are also carried out on a series of metal chelating conjugated polymers showing the capability and potential in photocatalytic hydrogen generation.

The nanoscale organization and corresponding electronic properties of a photoactive donor-acceptor blend layers based on regioregular poly (3-hexylthiophene) (RR-P3HT) donor with fullerene and non-fullerene acceptors on Au(111) substrate has been studied using scanning tunneling microscopy and spectroscopy (STM/STS). Subsequent to annealing treatment, STM topography and dI⁄dV images are observed as a combination of phase-separated donor-rich, acceptor-rich, and mixed donor-acceptor domains. This technique permits to explore simultaneously the quantitative linkage between the nanoscale morphologies and corresponding local electronic properties. We determine the HOMO and LUMO-edges at the individual domains and interfacial band alignments of the donor-acceptor interface. We have observed a noteworthy deeper HOMO energy of RR-P3HT in mixed-region associated primarily with the degree of disorder-induced band gap widening of the polymer and donor:acceptor intermolecular interactions. Similarly, LUMO of the acceptor in the mixed region is also raised due to intermolecular interaction. These energetic difference in the mixed phase is likely to be responsible for the reduced recombination in bulk heterojunction (BHJ). Hence, this characterization provides nanoscale insight to the annealing-induced morphological organization and corresponding local electronic properties account for an impressive increase of the charge generation, transport and corresponding device performance of the BHJ solar cells.

A novel spectroscopy termed single shot transient absorption (SSTA) is presented that can collect a transient absorption spectrum in 6 ms by using laser pulses with tilted wavefronts to spatially encode the delay between pump and probe pulse arrival times at the sample. The transient absorption technique determines the change in sample transmission that results from sample photoexcitation, and tracks this change as a function of the time delay between the arrival of the pump pulse and the probe pulse. Typically, these time delays are generated using a retroreflecting mirror mounted on a motorized translation stage, with a measurement collected at each translation stage position. Because these measurements must be performed in series, data collection requires a significant amount of time. This limits transient absorption to the measurement of systems that are static for the duration of the experiment. SSTA overcomes this restriction by employing pump and probe pulses which are each focused into a line and tilted with respect to each other to spatially encode time delays within the sample. Here, we describe the SSTA technique and instrumentation, demonstrate the principle of this spectroscopy, and present a method for calibrating the spatially encoded time delay by autocorrelation. This instrument will broaden the scop

Excitonic spin plays a crucial role in the design of organic light emitting diodes (OLEDs). The random spin statistics of recombining charge sets a limit of 25% on the fraction of singlet (spin-0) excitons formed by electrical excitation. Without efficient emission from triplet (spin-1) excitons, the same limit applies to the internal quantum efficiency (IQE) of fluorescent OLEDs. Phosphorescent OLEDs, utilising the heavy atom effect to render triplets emissive, and TADF OLEDs, based on thermally-assisted triplet-to-singlet up-conversion, are currently the most promising routes for triplet emission. Here we demonstrate a different approach. The effective exchange energy in a family of linear copper and gold carbene metal amide compounds can be tuned via rotation about the metal-amide bond from positive to negative values. The energetic ordering of spin-states can therefore be inverted, enabling triplet-to-singlet down-conversion. The availability of degenerate states with high oscillator strength allows emission via triplets to occur on sub-microsecond timescales. Using such materials as emissive dopants in solution-processed OLEDs leads to extremely efficient devices with near 100% IQE (external quantum efficiencies >27%), and current efficiency, power efficiency and brightness comparable to or exceeding those of state-of-the-art vacuum-deposited OLEDs and quantum dot LEDs. We describe the experimental and theoretical evidence for rotationally accessed spin-state inversion. Using time-resolved spectroscopy we show how the resulting emission depends strongly in the interplay between rotational energetics, temperature, oscillator strength and the morphology of the emissive layer.

Colloidal quantum dots (QDs) based on lead sulfide (PbS) have acquired scientific interest for infrared optoelectronic devices with potential bandgap tunability and ease of fabrication on arbitrary substrates. In this work, we show how device analysis data feed back into process optimization, towards the realization of high performance QD NIR photodetectors. Using the combination of transient PL, carrier transport and CV measurements we obtain the carrier density, lifetime and diffusion length in the layers. From the measured short diffusion length of the minority carriers, we deduce the need to achieve a wide depletion region to minimize recombination and thus enhance the carrier harvesting. Process optimization lead to a depletion region of more than 150 nm, resulting in high photon to carrier conversion. Furthermore, the complex index of refraction of all layers is characterized using ellipsometry and reflection/transmission, and these values are used as input for a transfer matrix method. Using the first interference peak, we show that a maximum EQE of 25% can be expected from optical modeling, a value that we almost reach experimentally (20%). Combining all of the above, we demonstrate 1450-nm photodetectors with dark current in the range of μA and specific detectivity (D*) of 10^11 Jones.

The fundamental physics in organic-inorganic metal-halide perovskites, is still not sufficiently understood. Applied in photovoltaic devices, organolead halide based solar cells may suffer from hysteresis, that is the difference of the I-V curve during sweeping in two directions. This behaviour significantly influences the large-scale commercial application and seems to have its origin in ionic migration and directly affects stability and device performance. To investigate the mechanisms, we employ a combination of methods that help to identify the origin and effect of ion migration. We use electroabsorption spectroscopy, to explore the built-in potential, track the temperature dependent J-V behaviour and related current relaxation processes and perform X-ray photoemission spectroscopy experiments, showing the redistribution of iodine after applying a constant voltage. In addition, we carry out fluorescence microscopy under an electrical field which allows us to directly track the migration and accumulation of ions. This experiment allows to study the dynamic process of – in this case - iodide ions under an external electrical field and lets us estimate their mobility and diffusion constan. We associate the migration/accumulation of iodide ions with the modulation of interfacial barrier between perovskite and electrodes which gives rise to the shift of the built-in potential. We investigate the influence of PCBM on the migration of ions, which appears to be impeded by the presence of interdiffused PCBM molecules. Furthermore, photoluminescence microscopy gives access to interesting phenomena which are related to the presence of ions, such as blinking or long-term intensity changes in the emission signal.

Polytellurophenes can be synthesized and are stable, processable semiconductors. They have very far red-shifted film optical absorption properties. Their electrical performance matches or exceeds that of thiophene analogue with the same molecular weight.

Applications of conjugated polymers in photovoltaics and displays drive the need to understand how morphology and aggregation affect emission yields, spectra, and the facility with which charges are generated and migrate through the sample. It is known that solvent-polymer interactions in solution critically affect the properties of thin films formed when these solutions are evaporated onto substrates. Our work demonstrates that the propensity of conjugated oligomers and polymers to form emissive versus non-emissive aggregates in solution and in thin films is likewise governed by the solvent properties. Fluorescence correlation spectroscopy is used as a tool to identify both emissive and non-emissive species in dilute solutions while dynamic light scattering (DLS) is used to measure diffusion properties. In some solvents, such as toluene, conjugated materials form non-emissive aggregates even at picomolar concentrations. Under similar conditions, the same materials exhibit single-emitter properties in more polar solvents such as tetrahydrofuran (THF). These distinctions persist when the molecules are forcibly aggregated by addition of poor solvent and are correlated to variations in chain packing within the aggregate due to differences in their preferred conformation. Transient absorption spectroscopy is used to understand the impact of altering chain packing on the propensity for energy transfer and charge separation in the aggregated state and in films.

Intermolecular charge transfer (CT) states at the interface between electron-donating and electron-accepting (A) materials in organic thin films are characterized by absorption and emission bands within the optical gap of the interfacing materials. In this contribution, we will discuss the fundamental properties of CT states and link them to organic solar cell and light emitting diode performance. Furthermore, we introduce a new device concept, using an optical cavity resonance effect to boost CT absorption at photon energies below the optical gap of both donor and acceptor, enabling sensitive narrow-band, near infrared photo-detection.

The microscopic mechanisms of exciton and charge-transfer-state dissociation in organic semiconductors play a major role for the efficiencies of organic solar cells [1]. One of the most direct experiments for probing the dynamics of these processes is luminescence quenching. Here, we present a comprehensive experimental and simulative study of the field and temperature dependence of the dissociation of singlet excitons in PTB7 and PC71BM, and from charge-transfer states generated across interfaces in PTB7/PC71BM bulk heterojunction solar cells. We deduce the relevant data from time-resolving the near infrared emission of the states from 10K to room temperature and for electric fields ranging from 0 to 2.5 MV/cm. To draw qualitative conclusions from our data, we use an analytical field-assisted hopping model in the presence of disorder [2]. We conclude that singlet excitons can be split by high fields, and that disorder plays a large role in broadening the critical threshold field for which singlet excitons are separated. Charge-transfer (CT) state dissociation can be induced by both field and temperature, and the data imply that a strong reduction of the Coulomb binding potential at the interface facilitates their separation. The observations provided herein of the field dependent separation of CT states as a function of temperature offer a rich dataset against which theoretical models of charge separation can be rigorously tested. References: [1] H. Bässler and A. Köhler, Phys. Chem. Chem. Phys. 17, 28451 (2015) [2] O. Rubel, S. D. Baranovskii, W. Stolz, and F. Gebhard, Phys. Rev. Lett. 100, 196602 (2008).

A series of bilayer films were prepared by vacuum deposition onto Silicon substrates. These films consisted of either Si/SiO₂/donor/C₆₀ or Si/SiO₂/C₆₀/donor, where the organic films were in the 20-40 nm thick range and the donors were 7,7-difluoro-14-phenyl-7H-6l4,7l4-[1,3,2]diazaborinino[4,3-a:6,1-a']diisoindole (bDIP), copper phthalocyanine (CuPC), 3,6,11,14-tetraphenyldiindeno[1,2,3-cd:1',2',3'-lm]perylene (DBP) and 2-(4-(diphenylamino)-2,6- dihydroxyphenyl)-4-(4-(diphenyliminio)-2,6-dihydroxycyclohexa-2,5-dien-1-ylidene)-3-oxocyclobut-1-en-1-olate (DPSQ). The donors chosen here have been reported to give good power efficiencies when incorporated into bilayer photovoltaic cells with a C₆₀ acceptor. These bilayer films were examined by neutron reflectometry to characterize the interface between the donor and C₆₀. In the SiO₂/donor/C₆₀ films, DPSQ, CuPC, and DBP show a discrete interface with C₆₀ while bDIP shows substantial spontaneous mixing at the interface, consistent with a donor/(donor + C₆₀)/C₆₀ structure, where the mixed layer is 14 nm.. In the SiO₂/C₆₀/donor films, all four donors show negligible mixing at the D/A interface consistent with a discrete D/A junction.

The rate of singlet fission in a dimer is known to be dependent on the coupling between the two chromophores that make up the dimer. A series of ethynyltetraceme dimers have been synthesized with the chromophore units at ortho-, meta- and para- configurations. Broadband femtosecond time-resolved pump-probe spectroscopy and time-correlated single photon counting has been used to observe the singlet state, the multiexciton state and the separated triplet states in these dimers in both solution and neat film phases. In solution, only the para-dimer forms separated triplets while the meta-dimer does not undergo singlet fission at all. In neat films, inter-dimer chromophore interactions lead to singlet fission in both the meta-dimer and the para-dimer. Target analysis using compartmental kinetic models allows for the spectral features of the different excited states in singlet fission to be resolved, along with the rate constants of the excited state processes involved.

A primary source of energy loss within semiconductor photovoltaics is charge carrier thermalization, wherein carriers produced by high-energy photons relax to the semiconductor's band edge, giving up their excess energy as heat. In commercial silicon photovoltaics, this process accounts for nearly 50% of the energy lost by these cells. One strategy to combat this loss is to use a material that undergoes singlet exciton fission (SF) to capture high energy photons. SF, a process that occurs in select organic semiconductors, generates two spin-triplet excitons from a singlet photo-generated spin-singlet excitation. These triplet excitons can each transfer to a silicon layer resulting in a reduction of thermalization losses. However, efforts to couple SF materials with silicon have largely been hindered by inefficient triplet energy transfer across their junction. Currently, the physical basis underlying this result is unclear, and may be tied to poor orbital overlap as well as band edge mismatch. Differentiating between these scenarios and others is central to the design of SF-based photovoltaics. Herein, we use electronic sum frequency generation (ESFG) to probe how the electronic structure of perylenediimides, a family of promising SF materials, is altered at buried interfaces. ESFG measurements of perylenediimide thin films suggest that strain relaxation within crystalline grains can lead to exciton energy shifts of ~150 meV, affecting both their ability to undergo SF near interfaces and transfer triplet excitons to silicon. Our measurements suggest careful control of the arrangement of SF-materials at a semiconductor interface is critical to effecting energy transfer between them.

Perovskite quantum dots (P-QDs) is a new kind of optoelectronic materials in recent years. Compared with organic perovskite QDs (MAPbX3), inorganic perovskite QDs (CsPbX₃) have a better stability. Inorganic P-QDs can be prepared at low temperature. Those novel QDs can be applied in solar cells, light-emitting diodes (LEDs), display, and biolables. Typical synthesis process to prepare CsPbX₃ QDs is used oleic acid (OA) and cesium carbonate (Cs₂CO₃) to form Cs-oleate complex first. Moreover, the oleylamine (OLA) and octadecene (ODE) are used as capping agents. Cs-oleate complex then reacts with PbX₂ to form CsPbX₃ QDs (reacts for 5 s). As we know that the CsPbBr₃ QDs emits green light, and its emission wavelength can be tuned by adding Cland Iions to replace Brion. However, the reaction rate of CsPbX₃ QDs is fast, and it is not easy to control the emission wavelength by particle size. In this study, we use the saturated alkyl amines with difference of carbon chain length such as dodecylamine (DDA), hexadecylamine (HDA), and octadecylamine (ODA) to prepare CsPbBr3 QDs. The result shows that the emission spectra for all samples range from 489 (ODA) to 514 nm (DDA), the full width at half-maximum (FWHM) is between 23 to 28 nm, the surface morphologies of all samples are nearly spherical, and the quantum yields (QYs) are higher up to 130 % (compared with R6G and the excitation wavelength is 450 nm). Based on emission spectra we can find that the emission peaks are fixed even under different excitation wavelength, imply that the particle size distribution of QDs is uniform. Moreover, the emission wavelength blue shifts with increasing carbon chain length of amines. The stability of alkyl amine-capped CsPbBr₃ QDs is good, especially for DDA-capped sample. We also find that a small emission peak around 462 nm can be only observed for DDA-capped sample. Furthermore, this small peak also can be observed even prolong the reaction time to 10 min. The emission wavelengths of CsPbBr₃ QDs can be controlled by carbon chain length of alkyl amines. The small FWHM and high QYs of CsPbBr₃ QDs meaning that it is benefit to enhance the color gamut of display.

The recently re-discovered class of organometal-halide perovskites hold great promise for solar cells, LEDs and lasers.[1] Today, their potential has not been fully unlocked partially because of the lack of suitable nano-patterning techniques, which are mandatory to create resonator structures, waveguides etc. with a maximum level of precision directly into perovskite layers. Their chemical and thermal instability prevents the use of established wet-chemical patterning techniques.[2] In contrast to conventional inorganic semiconductors, crystal binding in these perovskites includes significant contributions of van der Waals interactions among the halide atoms and Hydrogen bonding.[3] The formation enthalpy per unit cell is only about 0.1eV in MAPbI3.[4] Here, we take advantage of the “soft-matter properties” of organo-metal halide perovskites and demonstrate that photonic nano-structures can be prepared by direct thermal nano-imprint lithography in MAPbI3 and MAPbBr3 at relatively low temperatures (<150°C). The resulting periodic patterns provide distributed feedback resonators, which afford lasing in MAPbI3 with ultra-low threshold levels on the order of 1 μJ/cm2.[5] Moreover, our results also state the first DFB lasers based on MAPbBr3. We will discuss the applicability of thermal imprinting for perovskite solar cells and LEDs. [1] B. R. Sutherland et al. Nat Photon 2016, 10, 295. [2] D. Lyashenko et al. physica status solidi (a) 2017, 214, 10.1002/pssa.201600302. [3] D. A. Egger et al. Journal of Physical Chemistry Letters 2014, 5, 2728. [4] A. Buin et al. Nano Lett 2014, 14, 6281. [5] N. Pourdavoud et al. Adv Mater 2017, 10.1002/adma.201605003.

In the talk I will focus on the optical properties of lead halide perovskites and their dependence on the amount of excess PbI2. I shall first address that, at low temperature, it is possible to optically and reversibly induce a phase change from the orthorhomic to the tetragonal phase. By exploiting amplified spontaneous emission (ASE) from these phases, fully optical read-write-erase cycles can be conducted. I shall then elaborate on how and why the photoluminescence features are modified in the presence of excess PbI2.

Exciton interactions and dynamics are the most important factors determining the exceptional photophysical properties of semiconductor quantum dots (QDs). In particular, best performances have been obtained for ingeniously engineered core/shell QDs. We have studied two factors entering in the exciton decay dynamics with adverse effects for the luminescence efficiency: exciton trapping at surface and interface traps, and non-radiative Auger recombination in QDs carrying either net charges or multiple excitons. In this work, we present a detailed study into the optical absorption, fluorescence dynamics and quantum yield, as well as ultrafast transient absorption properties of CdSe/CdS, CdSe/Cd_0.5Zn_0.5S, and CdSe/ZnS QDs as a function of shell thickness. It turns out that de-trapping processes play a pivotal role in determining steady state emission properties. By studying the excitation dependent photoluminescence quantum yields (PLQY) in different CdSe/Cd_xZn_1-xS (x = 0, 0.5, 1) QDs, we demonstrate the different role played by hot and cold carrier trapping rates in determining fluorescence quantum yields. Finally, the use of global analysis allows us untangling the complex ultrafast transient absorption signals. Smoothing of interface potential, together with effective surface passivation, appear to be crucial factors in slowing down both Auger-based and exciton trapping recombination processes.

Two-dimensional electronic spectroscopy (2DES) has emerged as an incisive tool for mapping out energy relaxation pathways in complex molecular systems by determining correlation maps between the excitation and emission frequencies. Its enhanced spectral as well as temporal resolution offer new insights into coupling and energy transfer between closely-spaced energy states, which are often hidden in a one-dimensional transient spectrum. However, a major drawback of the current 2DES technique is that the spectral window of detection is directly limited to the laser bandwidth used, which leads to an incomplete visualization of the full energy landscape of the system. As a solution to this limitation, we present an ultrabroadband 2DES apparatus utilizing a 8-fs, 185-nm bandwidth supercontinuum that covers the entire visible region. We demonstrate the utility of our setup by measuring the 2D spectra of laser dyes absorbing at different regions of the laser spectrum, and the major light-harvesting complex of spinach.

Conjugated oligomers and polymers are very attractive for potential future plastic electronic and opto-electronic device applications such as plastic photo detectors and solar cells, thermoelectric devices, field effect transistors, and light emitting diodes. Understanding and optimizing charge transport between an active polymer layer and conductive substrate is critical to the optimization of polymer based electronic and opto-electronic devices. This study focused on the design, synthesis, self-assembly, and electron transfers and transports of a phosphonic acid end-functionalized polyphenylenevinylene (PPV) that was covalently attached and self-assembled onto an Indium Tin Oxide (ITO) substrate. This study demonstrated how atomic force microscopy (AFM) can be an effective characterization technique in conjunction with conventional electron transfer methods, including cyclic voltammetry (CV), towards determining electron transfer rates in polymer and polymer/conductor interface systems. This study found that the electron transfer rates of covalently attached and self-assembled films were much faster than the spin coated films. The knowledge from this study can be very useful for designing potential polymer based electronic and opto-electronic thin film devices.

Silicon (Si) is known to have an indirect bandgap transition, which means it has poor fluorescence properties. However, when engineered into sub-nm sized particles, Si nanoparticles become emissive due to quantum confinement. However, in unmodified Si particles, this effect is limited to generating red or near-infrared emission with low quantum yield. To resolve these limitations, surface-modification methods have successfully generated Si particles that emit in the blue, cyan, and green with quantum yields up to ~90%.1,2 These modifications have also made the Si nanoparticles watersoluble, making them promising in biological applications. To date, the mechanism of emission in these species is still unclear although it has been speculated that charge transfer of Si-O-N could be responsible. To investigate whether emission by these Si nanoparticles proceeds via a charge transfer mechanism, Stark spectroscopy is used. In this method, an external electric field is applied to the Si nanoparticles. Changes in the absorption and/or emission spectra due to the applied field can be taken as strong evidence for a charge transfer mechanism. From the results of Stark spectroscopy, Si nanoparticles are revealed to have ligand to metal charge transfer mechanism along with electric-field quenching, which is useful information for utilization into applications. Addition to the information found, a method of how to tune the emission maxima based on selection of ligands is prosed.

There is a growing demand to improve the operational lifetime of electroluminescent devices utilizing conjugated polymers which are often deposited over metal electrodes. Photo-degradation of the emissive organic layer is one factor that decreases the overall efficiency and longevity of these devices. Therefore, it is important to investigate the underlying photochemistry at metal-polymer interface. Here, the effects of metal films on the emission properties of organic polymers are studied using Total Internal Reflection Fluorescence (TIRF) microscopy and Fluorescence Lifetime Imaging (FLIM). Poly(phenylene vinylene) (MEHPPV) is spun cast over gold films of varying thickness (2 nm to 8 nm). Whereas 8 nm gold films completely quench the MEHPPV fluorescence, thinner gold films (~ 2 nm) cause minimal quenching. However, deposition on the thinner gold films leads to a dramatic increase in photo-stability of MEH-PPV relative to that on glass, even in the presence of molecular oxygen and under continuous laser illumination. Good overlap between the surface plasmon absorbance of gold films and the emission of MEHPPV is required for this effect as it is not observed on metals without a plasmon band in this spectral region.

We report an investigation of the charge carrier dynamics in single layer CVD-grown graphene, by means of ultrafast transient absorption spectroscopy. Our results show a pump-induced negative differential absorption, in a wide spectral range. Decay dynamics are characterized by a biexponential behavior with two characteristic lifetimes on the order of hundreds of femtoseconds and few picoseconds respectively, which can be interpreted as due to phonon assisted recombination mechanisms. The fact that the behavior does not change substantially with the pump energy indicates that Auger processes play a role at the early stages of carrier dynamics. These results provide insights into the photo-physics of two-dimensional graphene, at monolayer level, and pave the way to its applications in optoelectronics and photovoltaics.