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Polar molecules with appreciable permanent dipole moments (PDMs) are widely used as the electron transport layer (ETL) in organic light-emitting devices (OLEDs). When the PDMs spontaneously align, a macroscopic polarization field can be observed, a phenomenon known as spontaneous orientation polarization (SOP). The presence of SOP in the ETL induces considerable surface potential and charge accumulation that is capable of quenching excitons and reducing device efficiency. While prior work has shown that the degree of SOP is sensitive to film processing conditions, this work considers SOP formation by quantitatively treating the vapor-deposited film as a supercooled glass, in analogy to prior work on birefringence in organic thin films. Importantly, the impact of varying thin-film deposition rate and relative temperature is unified into a single framework, providing a useful tool to predict the SOP formation efficiency for a polar material, as well as in blends of polar materials. Finally

Probing triplet transport in singlet fission materials can be challenging due to the presence of multiple diffusing species. We present a device-based method to measure the intrinsic triplet diffusion length (LD) in organic semiconductor thin films exhibiting singlet fission. Triplet states are optically injected into the singlet fission material of interest via energy transfer from an adjacent thin film characterized by strong spin-orbit coupling. Injected triplets migrate through the full thickness of the material before undergoing dissociation at a donor-acceptor interface. By modeling the ratio of injector and acceptor photocurrent as a function of layer thickness, the triplet LD is extracted separate from processes of unknown efficiency including singlet fission and diffusion. In considering three archetypical fission systems, a wide range is found for the triplet LD, ranging from 3.3 ± 0.4 nm for 5,12-bis((triisopropylsilyl)ethynyl)tetracene to 17.1 ± 1.3 nm for pentacene and 32.1 ± 2.6 nm for tetracene.

Heterojunctions underpin the design and performance of virtually all devices based on conventional semiconductors. While metal halide perovskites have received intense attention for applications in...

We characterized the prominent electron transport layer 2,2′,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) via single crystal X-ray diffraction, grazing incidence X-ray diffractio...

Charge-transfer (CT) states formed at organic donor-acceptor (D-A) semiconductor heterojunctions play a critical role in optoelectronic devices. While mobile, their migration has not been extensively characterized. In addition, the factors impacting the CT state diffusion length (LD) have not been elucidated. Here, CT state LD is measured by using photoluminescence quenching for several D-A mixtures, with migration occurring along the bulk heterojunction. All D-A pairings considered yield a similar LD ∼ 5 nm in equal mixtures despite variations in the CT state energy and the constituent molecular structures. The CT state LD varies strongly with mixture composition and is well-correlated to the slowest charge carrier mobility, suggesting a direct method to tune CT state transport. These findings may be applied to elucidate the role of CT state migration in organic photovoltaic and light-emitting devices as well as to broadly explain the transport of interfacial excited states along inorganic and hybrid organic-inorganic heterojunctions.

Fair and meaningful device per- formance comparison among luminescent solar concentrator- photovoltaic (LSC-PV) reports cannot be realized without a gen- eral consensus on reporting stan- dards in LSC-PV research. There- fore, it is imperative to adopt standardized characterization protocols for these emerging types of PV devices that are consistent with other PV devices. This commentary highlights several common limitations in LSC literature and summarizes the best practices moving for- ward to harmonize with standard PV reporting, considering the greater nuances present with LSC-PV. Based on these prac- tices, a checklist of actionable items is provided to help stan- dardize the characterization/re- porting protocols and offer a set of baseline expectations for au- thors, reviewers, and editors. The general consensus combined with the checklist will ultimately guide LSC-PV research towards reliable and meaningful ad- vances.

Much effort has been directed at understanding organic light-emitting device (OLED) efficiency and the role of bimolecular quenching in efficiency roll-off. Quenching is less widely discussed at low-bias, where populations are reduced. Here, we describe lock-in-based photoluminescence measurements on working phosphorescent OLEDs to demonstrate that this assumption is not generally valid, and that significant exciton-polaron quenching is present even prior to turn-on. Exciton-polaron quenching arises with holes accumulated due to spontaneous orientation polarization in the electron transport layer. This low-bias quenching is found to strongly determine maximum achievable efficiency, suggesting a need to refine materials selection and device design rules.

We demonstrate the impact of subtle changes in molecular structure on the singlet and triplet exciton diffusion lengths (LD) for derivatives of the archetypical electron-transport material 4,7-diphenyl-1,10-phenanthroline (BPhen). Specifically, this work offers a systematic characterization of singlet and triplet exciton transport in identically prepared thin films, highlighting the differing dependence on molecular photophysics and intermolecular spacing. For luminescent singlet excitons, photoluminescence quenching measurements yield an LD from

Self-organizing patterns with micrometre-scale features are promising for the large-area fabrication of photonic devices and scattering layers in optoelectronics. Pattern formation would ideally occur in the active semiconductor to avoid the need for further processing steps. Here, we report an approach to form periodic patterns in single layers of organic semiconductors by a simple annealing process. When heated, a crystallization front propagates across the film, producing a sinusoidal surface structure with wavelengths comparable to that of near-infrared light. These surface features initially form in the amorphous region within a micrometre of the crystal growth front, probably due to competition between crystal growth and surface mass transport. The pattern wavelength can be tuned from 800 nm to 2,400 nm by varying the film thickness and annealing temperature, and millimetre-scale domain sizes are obtained. This phenomenon could be exploited for the self-assembly of microstructured organic optoelectronic devices. Material depletion and accumulation at the crystallization front of organic semiconductors films induce the formation of large-area regular patterns, with a periodicity relevant to optoelectronic applications in the visible and near-infrared range.

Organic photovoltaic cells are partiuclarly sensitive to exciton harvesting and are thus, a useful platform for the characterization of exciton diffusion. While device photocurrent spectroscopy can be used to extract the exciton diffusion length, this method is frequently limited by unknown interfacial recombination losses. We resolve this limitation and demonstrate a general, device-based photocurrent-ratio measurement to extract the intrinsic diffusion length. Since interfacial losses are not active layer specific, a ratio of the donor- and acceptor-material internal quantum efficiencies cancels this quantity. We further show that this measurement permits extraction of additional device-relevant information regarding exciton relaxation and charge separation processes. The generality of this method is demonstrated by measuring exciton transport for both luminescent and dark materials, as well as for small molecule and polymer active materials and semiconductor quantum dots. Thus, we demonstrate a broadly applicable device-based methodology to probe the intrinsic active material exciton diffusion length., Zhang et al. develop a device-based method to probe intrinsic exciton transport in photovoltaic cells. The broad utility of this method is demonstrated by measuring exciton transport for both luminescent and dark organic semiconductors as well as semiconductor quantum dots.

Spontaneous orientation polarization in OLEDs reduces efficiency but can be eliminated by heating during film deposition., The efficiency of organic light-emitting devices (OLEDs) is often limited by roll-off, where efficiency decreases with increasing bias. In most OLEDs, roll-off primarily occurs due to exciton quenching, which is commonly assumed to be active only above device turn-on. Below turn-on, exciton and charge carrier densities are often presumed to be too small to cause quenching. Using lock-in detection of photoluminescence, we find that this assumption is not generally valid; luminescence can be quenched by >20% at biases below turn-on. We show that this low-bias quenching is due to hole accumulation induced by intrinsic polarization of the electron transport layer (ETL). Further, we demonstrate that selection of nonpolar ETLs or heating during deposition minimizes these losses, leading to efficiency enhancements of >15%. These results reveal design rules to optimize efficiency, clarify how ultrastable glasses improve OLED performance, and demonstrate the importance of quantifying exciton quenching at low bias.

We present a method to measure the exciton diffusion length (LD) of optically dark triplet excitons in organic semiconductor thin films. In order to directly probe only these states, triplets are optically injected into the material of interest via energy transfer from an adjacent phosphorescent thin film. Injected triplets migrate through the full thickness of the material before undergoing energy transfer to a phosphorescent sensitizer. By measuring photoluminescence from the sensitizer as a function of active layer thickness and sensitizer layer concentration, we are able to extract both LD and the transfer rate to the sensitizer. Extraction of the transfer rate is critical, as the assumption of unity quenching can lead to incorrect measurements of LD. We validate the method by measuring the singlet exciton diffusion length in the fluorophores tris-(8-hydroxyquinoline)aluminum (Alq3) and 2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10(2-benzothiazolyl)quinolizine-[9,9a,1gh] coumarin (C545T), and comparing them with values extracted from conventional photoluminescence quenching measurements. The triplet LD is subsequently extracted for a series of archetypical fluorescent organic semiconductors with values falling in the range of 15–30 nm. In addition to probing the diffusion of dark triplets, this method also offers the ability to measure the singlet and triplet LD with only a change in injection layer.

Searching for alternatives to lead-containing metal halide perovskites, we explored the properties of indium-based inorganic double perovskites Cs2InMX6 with M = Cu, Ag, Au and X = Cl, Br, I, and of its organic–inorganic hybrid derivative MA2InCuCl6 (MA = CH3NH3+) using computation within Kohn–Sham density functional theory. Among these compounds, Cs2InCuCl6 and MA2InCuCl6 were found to be potentially promising candidates for solar cells. Calculations with different functionals provided the direct band gap of Cs2InCuCl6 between 1.05 and 1.73 eV. In contrast, MA2InCuCl6 exhibits an indirect band gap between 1.31 and 2.09 eV depending on the choice of exchange–correlation functional. Cs2InCuCl6 exhibits a much higher absorption coefficient than that calculated for c-Si and CdTe, common semiconductors for solar cells. Even MA2InCuCl6 is predicted to have a higher absorption coefficient than c-Si and CdTe across the visible spectrum despite the fact that it is an indirect band gap material. The intrinsic charge carrier mobilities for Cs2InCuCl6 along the L–Γ path are predicted to be comparable to those for MAPbI3. Finally, we carried out calculations of the band edge positions for MA2InCuCl6 and Cs2InCuCl6 to offer guidance for solar cell heterojunction design and optimization. We conclude that Cs2InCuCl6 and MA2InCuCl6 are promising semiconductors for photovoltaic and optoelectronic applications.

We demonstrate carrier-gas assisted vapor deposition (CGAVD) as a promising synthesis technique for high-quality metal halide perovskite thin films. Wide tunability of film microstructure and morphology are accesible with CGAVD via the combination of several independently controllable experimental variables. Here, we examine in detail the material transport mechanisms in CGAVD and develop analytical expressions for deposition rates for the halide perovskite precursors MABr, MAI, SnBr2, and SnI2 as a function of experimentally tunable temperatures, pressures, and flow rates. The method is then applied to systematically control the growth of MASnBr3 thin films via co-deposition across a range of stoichiometries and morphologies. In varying source material temperature, carrier gas flow rate, dilution gas flow rate, substrate temperature, and chamber pressure, corresponding changes are realized in the degree of crystallinity, grain orientation, and average grain size (from ∼0.001 to >0.7 µm2). Thin films of MASnI3 and MASnBr3 deposited using CGAVD show resistivities of 0.6 Ω cm and 7 × 104 Ω cm, respectively, broadly consistent with previous reports.

Low-temperature sequential exposures of N2H4 and BCl3 have been performed on Si0.3Ge0.7(001), Cu, and HOPG surfaces at 350 °C. A novel BN ALD process has been achieved on Si0.3Ge0.7(001) with 60 cycles of BN ALD producing a uniform, pinhole-free thin film with low contamination, as characterized with XPS and AFM. On Cu and Si0.3Ge0.7(001), XPS spectra indicated a near stoichiometric BN film. While AFM imaging indicated the deposition on Cu yielded nanometer-scale etching, conformal deposition was observed on Si0.3Ge0.7(001). The BN ALD also nucleated on inert HOPG via step edges. In situ STM imaging showed that cyclic exposures at 350 °C were able to decorate step edges with features ∼2 nm tall and ∼200 nm wide, indicating the propensity for BN to grow in the planar direction. The N2H4 and BCl3 ALD allows for the deposition of low oxygen, low carbon films, but to avoid etching, the growth should be nucleated by N2H4, since exposure to BCl3 can result in the formation of volatile Cl-containing surface species on many substrates. Therefore, the formation of a stable surface nitride prior to BCl3 exposure is necessary to prevent formation and desorption of volatile species from the substrate.

Electronic structure calculations of five crystallography-imitated structures of CsMI3 perovskites with M = Ge, Sn, Pb, Mg, Ca, Sr, and Ba were performed. The formation energy of different perovskite phases, their relative stability, and structural and electronic properties were explored. The sensitivity of the calculations to the choice of the density functional was investigated, and the predictions were compared with experimental results. The outcome of this study is that Mg and Ba perovskites are unlikely to form in the cubic, tetragonal, or orthorhombic phases because they have positive formation energies. Although Ca and Sr perovskites have negative formation energies with respect to the metal-iodide precursors, they exhibit wide band gaps and high hygroscopicity, making these unlikely candidates for applications in photovoltaic devices. Our results suggest that the performance of a local density functional with a nonseparable gradient approximation (NGA) is similar to that of hybrid functionals in t...

Controlling the shape and growth of crystals in molecular organic solids has ramifications impacting diverse fields, but remains challenging to fully exploit. Here, crystal shapes in organic thin films are manipulated from aspect ratios of 1 to over 50, with corresponding growth rates decreased by an order of magnitude simply by mixing a structurally dissimilar minority species into the film. These effects are mapped with composition and temperature in mixtures of two model small-molecular-weight organic compounds, revealing a continuous variation in crystal shape and growth rate. Other combinations of molecules are discussed, showing additive shape selection in multicomponent mixtures and enabling customization of crystal shape.

A thin passivating SiO x control layer has been deposited via self-limiting CVD on the InGaAs(001)-(2x4) surface by first depositing 2 monolayers of silicon with –Cl x termination using Si 2 Cl 6 ,and then subsequently oxidizing the silicon seed layer by employing anhydrous HOOH(g) at a substrate temperature of 350 °C. After HOOH(g)) dosing, XPS spectra show a higher binding energy shoulder peak on Si2p indicative of SiO x bonding, while an unshifted Si 2p component remains, and In 3d, Ga 2p, and As 2p peaks show no higher binding energy components consistent with the prevention of III-V oxidation. Scanning tunneling spectroscopy (STS) measurements show after SiO x deposition on the InGaAs(001)-(2x4) surface, the bandgap broadens towards that of SiO 2 , with the electronic structure free of states in the bandgap leaving the surface ready for subsequent gate oxide ALD. Density functional theory calculations support the experimental STS data following TMA dosing, which shows TMA nucleates directly on the SiO x /InGaAs(001) surface and leaves an electrically passive interface with the bandgap free of defect states and the surface ready for high-K gate oxide nucleation.

Organic light-emitting devices (OLEDs) can exhibit an efficiency roll-off under high current excitation that has been previously modeled using bimolecular quenching. The corresponding transient electroluminescence behavior has not been as effectively treated. Here, we reproduce both the steady-state and transient regimes by introducing into the analysis a dynamic polaron population. This approach permits a rigorous definition of OLED charge balance in terms of dynamics as the exciton formation efficiency.

The amount sublimated of large organic molecules varies with the square of the diameter of the tube in which sublimation occurs. This implies that the velocity profile in the tube is nearly flat, consistent with volume diffusion, but not with laminar flow, Knudsen diffusion, or slip flow. However, molecular simulation calculations show that under the conditions used, the velocity profile is near parabolic when there is no deposition on the tube wall, inconsistent with volume diffusion, but in agreement with laminar flow. These calculations also show that deposition on the wall does result in velocity which is almost constant with radial position. The result is a laminar flow profile which is nearly flat, and hence a total flux proportional to the square of the tube diameter, which is observed both in these simulations and in physical experiments. © 2017 American Institute of Chemical Engineers AIChE J, 2017

Organic photovoltaic cell performance is limited in part by a short exciton diffusion length (LD). While state-of-the-art devices address this challenge using a morphology-optimized bulk heterojunction (BHJ), longer LD would relax domainsize constraints and enable higher efficiency in simple bilayer architectures. One approach to increase LD is to exploit long-lived triplet excitons in fluorescent materials. Though these states do not absorb light, they can be populated using a host-guest triplet-sensitized architecture. Photogenerated host singlets undergo energy transfer to a guest, which rapidly forms triplets that are transferred back to the long-lived host triplet state. Previous efforts have been focused on Pt- and Irbased guests. Here, a host-guest pairing of metal-free phthalocyanine (H2Pc) and copper phthalocyanine (CuPc) is explored, advantageous as the guest also has strong and complementary optical absorption. In optimized devices (20 vol.% CuPc), the short-circuit current is enhanced by 20%. To probe the origin of the enhancement, the exciton LD is measured using a device-based methodology that relies on fitting ratios of donor-to-acceptor internal quantum efficiency as a function of layer thickness. Compared with the neat H2Pc, the LD of the 20 vol.% CuPc doped layer increases from (8.5 ± 0.4) nm to (13.4 ± 1.6 nm), confirming the increased device current comes from enhanced exciton harvesting.

Efficiency roll-off and intrinsic luminance degradation are two of the primary limitations of organic light-emitting devices (OLEDs). While both phenomena have been studied separately in detail, they are rarely considered together. Previous analyses of OLED degradation have largely neglected changes in efficiency roll-off and bimolecular quenching, and the magnitude of these changes and their impact on device lifetime remains unclear. We present experimental and modeling results to quantify the magnitude of these changes, which we find range from ~2% to above 10% in magnitude and increase in importance at high brightness or in devices with significant exciton-exciton annihilation.

Photoconversion in organic photovoltaic cells (OPVs) is limited by carrier recombination that frustrates charge collection at the electrodes. Consequently, identification of the dominant recombination mechanisms is critical to inform device design for improved performance. This analysis is complicated by the need to have a quantitative measure of carrier generation. Here, we demonstrate a photovoltage-based technique to directly investigate the generation of charge carriers in OPVs. This technique allows illuminated current losses to both geminate and non-geminate recombination to be directly quantified as a function of voltage. While broadly applicable, here the technique is demonstrated on OPVs based on the donor–acceptor pairings of 2-((7-(4-N,N-ditolylaminophenylen-1-yl)benzo[c][1,2,5]thiadiazol-4-yl)methylene)malononitrile (DTDCPB)-C60 and copper phthalocyanine (CuPc)-C60. Both structures are limited by geminate recombination at short-circuit and under reverse bias. Under forward bias, the severity of non-geminate recombination depends on both materials selection and device architecture. Consequently, this technique quantitatively casts changes in performance with choice of OPV architecture in terms of the relative roles of geminate and non-geminate recombination.

In Situ gas phase passivation methods can enable new channel materials. Toward this end pure anhydrous HOOH and H2NNH2 membrane gas delivery methods were developed. Implementation led to Si-OH passivation of InGaAs(001) at 350C and Si-N-H passivation of SiGe(110) at 285C. XPS and initial electrical characterization has been carried out. Feasibility for In Situ dry surface preparation and passivation was demonstrated.

While controlled crystallization of organic thin films holds great potential for enhancing the performance of electronic devices, quantitative understanding of the processes involved is limited. Here, we characterize the thin film crystal growth of the organic semiconductor rubrene during annealing using polarized optical microscopy with a heated stage for in situ measurements, followed by atomic force microscopy and X-ray diffraction. During annealing, the film undergoes transitions from predominant growth of a polycrystalline triclinic crystal structure to single crystal orthorhombic, followed by polycrystalline growth of the orthorhombic polymorph. Observation of crystal morphology with time allows determination of the crystal orientation, which is used in conjunction with crystal size measurements to determine the crystallization activation energies for the observed growth phases and crystal planes.

Organic semiconductors characterized by a small singlet–triplet exciton energy splitting exhibit efficient reverse intersystem crossing and thermally activated delayed fluorescence. Consequently, exciton transport may occur along both the singlet and the triplet excited states, each with unique photophysical behavior and exciton energy transfer mechanisms. Delayed fluorescence systems, therefore, provide a unique test bed for characterizing the role of exciton spin in transport and diffusion. Concentration- and temperature-dependent photophysical characterization combined with measurements of the exciton diffusion length (LD) for 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) elucidate the relative degree and magnitude of transport along the singlet and triplet molecular excited states as well as the role of the local dielectric environment in determining the intersystem balance.

Crystallization under high pressure is an attractive approach to generate novel crystal polymorphs, solvates, and co-crystals of pharmaceuticals and other specialty chemicals. Here, we describe the effect of pressurization on the solubility of two common crystallization standards, paracetamol and piracetam. Simple theoretical models were developed to predict the change in solubility both due to pressurization and due to the temperature increase associated with adiabatic compression of the solution. These models were validated experimentally and provide a basis for experimental design. Interestingly, the decrease in solubility due to pressurization is often balanced by the increase in solubility from the temperature increase due to adiabatic compression of the solution.

The authors produce plasmonic ZnO-TiN nanocomposite films by depositing plasma-synthesized ZnO nanocrystals onto a substrate and then by infilling the nanocrystal network's pores with TiN via remote plasma-enhanced atomic layer deposition (PEALD). This ZnO-TiN nanocomposite exhibits a plasmonic resonance that is blueshifted compared to planar titanium nitride thin films. The authors study the effects of PEALD conditions and the ZnO film thickness on the plasmonic response of these nanocomposites and exploit the optimized film in a device that generates photocurrent at zero bias.

Exciton gates, with an asymmetry in forward and reverse rates of energy transfer, are a means to overcome the diffusive and subdiffusive nature of exciton transport in organic semiconductors. The impact of multiple gating interfaces on exciton transport must be studied, though, to inform application of such gates in devices. Here a molecular site imbalance is engineered to establish gates, and the impact on exciton transport is examined by optically injecting excitons and detecting those that migrate through the structure. This work offers insight into the design of organic optoelectronic devices based on exciton gates, for $e.g.$ planar heterojunction photovoltaic cells.

The arrival of light-emitting diodes based on new materials is posing challenges for the characterization and comparison of devices in a trusted and consistent manner. Here we provide some advice and guidelines that we hope will benefit the community.

Host-guest structures are used in most state-of-the-art organic light-emitting devices, with the host transporting charge and confining excitons on the guest. While the host often plays a critical role in achieving high efficiency and stability, predicting and understanding these effects is a persistent design challenge which slows the discovery of new active materials. Closely related host molecules, which differ only by several functional groups, often show drastically different degradation behavior. Here, we explore this observation for the archetypical carbazole hosts 4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) and 4,4′-bis(carbazole-9-yl)-2,2′-dimethylbiphenyl (CDBP). While devices based on these hosts show similar efficiencies, CDBP-based devices show a tenfold lower lifetime than CBP devices when paired with phosphorescent or fluorescent emitters. Using optically and electrically pumped degradation tests, mass spectrometry, compositional analysis, and low-temperature phosphorescence spectroscopy, the lifetimes of devices containing CDBP are shown to correlate with the formation of intermolecular triplet excimer states. These findings suggest that candidate host molecules should be screened for excimer formation as host excimers may aggravate device degradation and lower device stability.

Degradation in organic light-emitting devices (OLEDs) is generally driven by reactions involving excitons and polarons. Accordingly, a common design strategy to improve OLED lifetime is to reduce the density of these species by engineering an emissive layer architecture to achieve a broad exciton recombination zone. Here, the effect of exciton density on device degradation is analyzed in a mixed host emissive layer (M-EML) architecture which exhibits a broad recombination zone. To gain further insight into the dominant degradation mechanism, losses in the exciton formation efficiency and photoluminescence (PL) efficiency are decoupled by tracking the emissive layer PL during device degradation. By varying the starting luminance and M-EML thickness, the rate of PL degradation is found to depend strongly on recombination zone width and hence exciton density. In contrast, losses in the exciton formation depend only weakly on the recombination zone, and thus may originate outside of the emissive layer. These results suggest that the lifetime enhancement observed in the M-EML architectures reflects a reduction in the rate of PL degradation. Moreover, the varying roles of excitons and polarons in degrading the PL and exciton formation efficiencies suggest that kinetically distinct pathways drive OLED degradation and that a single degradation mechanism cannot be assumed when attempting to model the device lifetime. This work highlights the potential to extract fundamental insight into OLED degradation by tracking the emissive layer PL during lifetime testing, while also enabling diagnostic tests on the root causes of device instability.

Purification of large organic molecules in a tubular sublimator occurs by a combination of laminar flow, Knudsen diffusion, and volume diffusion. For laminar flow, the amount purified per area per driving force varies with pd2, where p is pressure and d is tube diameter. For Knudsen diffusion, it varies with d and is not a function of pressure. For volume diffusion, it is constant, consistent with experiment. This volume diffusion mechanism may offer an alternative explanation to slip flow for dilute gas transport of both organic semiconductors and common low molecular weight gases. © 2015 American Institute of Chemical Engineers AIChE J, 62: 861–867, 2016

Vapor deposited thin films of copper phthalocyanine (CuPc) were investigated using transient absorption spectroscopy. Exciton-exciton annihilation dominated the kinetics at high exciton densities. When annihilation was minimized, the observed lifetime was measured to be 8.6 ± 0.6 ns, which is over an order of magnitude longer than previous reports. In comparison with metal free phthalocyanine (H2Pc), the data show evidence that the presence of copper induces an ultrafast relaxation process taking place on the ca. 500 fs timescale. By comparison to recent time-resolved photoemission studies, this is assigned as ultrafast intersystem crossing. As the intersystem crossing occurs ca. 10(4) times faster than lifetime decay, it is likely that triplets are the dominant excitons in vapor deposited CuPc films. The exciton lifetime of CuPc thin films is ca. 35 times longer than H2Pc thin films, while the diffusion lengths reported in the literature are typically quite similar for the two materials. These findings suggest that despite appearing to be similar materials at first glance, CuPc and H2Pc may transport energy in dramatically different ways. This has important implications on the design and mechanistic understanding of devices where phthalocyanines are used as an excitonic material.

Temperature dependent measurements of the exciton diffusion length (LD) are performed for three archetypical small-molecule, organic semiconductors: aluminum tris-(8-hydroxyquinoline) (Alq3), dicyanovinyl-terthiophene (DCV3T), and boron subphthalocyanine chloride (SubPc). The experimental results are well-reproduced with stochastic simulations for LD by accounting for the presence of energetic disorder and thermal activation within both the inhomogeneously broadened density of states and the rate of intermolecular Forster energy transfer, respectively. In turn, activated and non-activated transport regimes can be distinguished, and exciton energy transfer within these materials can be deconvoluted from energetic disorder—providing insight regarding the fundamental parameters limiting LD.

Thin films of vapor-deposited metal-free phthalocyanine (H2Pc) were studied using ultrafast transient absorption spectroscopy in the visible region. Following photoexcitation, an excited state absorption feature located near 532 nm was observed which served as a probe of the excited state. For exciton densities larger than 5 × 1018 excitons/cm3 the time-dependent measurements of the excited state absorption included the presence of nonexponential decay kinetics attributed to exciton–exciton annihilation. At exciton densities less than 5 × 1018 excitons/cm3 annihilation was negligible, and the decay kinetics appeared single exponential within the signal-to-noise. The fitted time constant, 239 ± 24 ps, was attributed to the lifetime decay of the singlet excitons. When the H2Pc was diluted into a wide energy gap host via vapor deposition, the observed lifetime was significantly reduced, reaching 87 ± 9 ps for a concentration of 25% H2Pc. The decreased exciton lifetime with dilution was remarkable since it ha...