Process Engineered Spontaneous Orientation Polarization in Organic Light-Emitting Devices.

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.

Device-Based Probe of Triplet Exciton Diffusion in Singlet Fission Materials.

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 () 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.

Mitigating Damage to Hybrid Perovskites Using Pulsed-Beam TEM.

Using a pulsed-beam transmission electron microscope, we discover a reduction in damage to methylammonium lead iodide (MAPbI) as compared to conventional beams delivered at the same dose rates. For rates as low as 0.001 e·Å·s, we find up to a 17% reduction in damage at a total dose of 10 e·Å.

Sub-turn-on exciton quenching due to molecular orientation and polarization in organic light-emitting devices.

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.

Migration of Charge-Transfer States at Organic Semiconductor Heterojunctions.

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 () have not been elucidated.

Formation of aligned periodic patterns during the crystallization of organic semiconductor thin films.

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.

Lead-free double perovskites CsInCuCl and (CHNH)InCuCl: electronic, optical, and electrical properties.

Searching for alternatives to lead-containing metal halide perovskites, we explored the properties of indium-based inorganic double perovskites CsInMX with M = Cu, Ag, Au and X = Cl, Br, I, and of its organic-inorganic hybrid derivative MAInCuCl (MA = CHNH) using computation within Kohn-Sham density functional theory. Among these compounds, CsInCuCl and MAInCuCl were found to be potentially promising candidates for solar cells. Calculations with different functionals provided the direct band gap of CsInCuCl between 1.

Intrinsic measurements of exciton transport in photovoltaic cells.

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.

Isolating Degradation Mechanisms in Mixed Emissive Layer Organic Light-Emitting Devices.

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.

Impact of Thermal Annealing on Organic Photovoltaic Cells Using Regioisomeric Donor-Acceptor-Acceptor Molecules.

We report a promising set of donor-acceptor-acceptor (D-A-A) electron-donor materials based on coplanar thieno[3,2-b]/[2,3-b]indole, benzo[c][1,2,5]thiadiazole, and dicyanovinylene, which are found to show broadband absorption with high extinction coefficients. The role of the regioisomeric electron-donating thienoindole moiety on the physical and structural properties is examined. Bulk heterojunction (BHJ) organic photovoltaic cells (OPVs) based on the thieno[2,3-b]indole-based electron donor NTU-2, using C as an electron acceptor, show a champion power conversion efficiency of 5.

Probing dark exciton diffusion using photovoltage.

The migration of weakly and non-luminescent (dark) excitons remains an understudied subset of exciton dynamics in molecular thin films. Inaccessible via photoluminescence, these states are often probed using photocurrent methods that require efficient charge collection. Here we probe exciton harvesting in both luminescent and dark materials using a photovoltage-based technique.

Blue-Emitting Arylalkynyl Naphthalene Derivatives via a Hexadehydro-Diels-Alder Cascade Reaction.

We describe here three alkynyl substituted naphthalenes that display promising luminescence characteristics. Each compound is easily and efficiently synthesized in three steps by capitalizing on the hexadehydro-Diels-Alder (HDDA) cycloisomerization reaction in which an intermediate benzyne is captured by tetraphenylcyclopentadienone, a classical trap for benzyne itself. These compounds luminesce in the deep blue when stimulated either optically (i.

Femtosecond to nanosecond excited state dynamics of vapor deposited copper phthalocyanine thin films.

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.

Correlation between the Open-Circuit Voltage and Charge Transfer State Energy in Organic Photovoltaic Cells.

In order to further improve the performance of organic photovoltaic cells (OPVs), it is essential to better understand the factors that limit the open-circuit voltage (VOC). Previous work has sought to correlate the value of VOC in donor-acceptor (D-A) OPVs to the interface energy level offset (EDA). In this work, measurements of electroluminescence are used to extract the charge transfer (CT) state energy for multiple small molecule D-A pairings.

Directing energy transport in organic photovoltaic cells using interfacial exciton gates.

Exciton transport in organic semiconductors is a critical, mediating process in many optoelectronic devices. Often, the diffusive and subdiffusive nature of excitons in these systems can limit device performance, motivating the development of strategies to direct exciton transport. In this work, directed exciton transport is achieved with the incorporation of exciton permeable interfaces.

Energy-cascade organic photovoltaic devices incorporating a host-guest architecture.

In planar heterojunction organic photovoltaic devices (OPVs), broad spectral coverage can be realized by incorporating multiple molecular absorbers in an energy-cascade architecture. Here, this approach is combined with a host-guest donor layer architecture previously shown to optimize exciton transport for the fluorescent organic semiconductor boron subphthalocyanine chloride (SubPc) when diluted in an optically transparent host. In order to maximize the absorption efficiency, energy-cascade OPVs that utilize both photoactive host and guest donor materials are examined using the pairing of SubPc and boron subnaphthalocyanine chloride (SubNc), respectively.

Communication: Trapping upconverted energy in neat platinum porphyrin films via an unexpected fusion mechanism.

Direct observation of an unexpected product from excited state fusion of two excited triplet states in platinum octaethylporphyrin is reported. Transient spectroscopy was used to identify the product as a metal centered (d, d) state that decays slowly compared with the rate of fusion. The reaction was demonstrated to be second order with a rate coefficient of k(TTF) = (5.

Connecting molecular structure and exciton diffusion length in rubrene derivatives.

Connecting molecular structure and exciton diffusion length in rubrene derivatives demonstrates how the diffusion length of rubrene can be enhanced through targeted functionalization aiming to enhance self-Förster energy transfer. Functionalization adds steric bulk, forcing the molecules farther apart on average, and leading to increased photoluminescence efficiency. A diffusion length enhancement greater than 50% is realized over unsubstituted rubrene.

Tailored exciton diffusion in organic photovoltaic cells for enhanced power conversion efficiency.

Photoconversion in planar-heterojunction organic photovoltaic cells (OPVs) is limited by a short exciton diffusion length (L(D)) that restricts migration to the dissociating electron donor/acceptor interface. Consequently, bulk heterojunctions are often used to realize high efficiency as these structures reduce the distance an exciton must travel to be dissociated. Here, we present an alternative approach that seeks to directly engineer L(D) by optimizing the intermolecular separation and consequently, the photophysical parameters responsible for excitonic energy transfer.

Long-range, photon-mediated exciton hybridization in an all-organic, one-dimensional photonic crystal.

We demonstrate an all-organic, one-dimensional photonic crystal tuned to the regime of strong exciton-photon coupling. The structure consists of a high index of refraction light-absorbing active layer periodically distributed in a low index of refraction nonabsorbing background medium. The strongly coupled state in this structure is shared between multiple active layers separated by distances longer than the wavelength of light in the structure.

An all-gas-phase approach for the fabrication of silicon nanocrystal light-emitting devices.

We present an all-gas-phase approach for the fabrication of nanocrystal-based light-emitting devices. In a single reactor, silicon nanocrystals are synthesized, surface-functionalized, and deposited onto substrates precoated with a transparent electrode. Devices are completed by evaporation of a top metal electrode.

Diarylindenotetracenes via a selective cross-coupling/C-H functionalization: electron donors for organic photovoltaic cells.

A direct synthesis of new donor materials for organic photovoltaic cells is reported. Diaryindenotetracenes were synthesized utilizing a Kumada-Tamao-Corriu cross-coupling of peri-substituted tetrachlorotetracene with spontaneous indene annulation via C-H activation. Vacuum deposited planar heterojunction organic photovoltaic cells incorporating these molecules as electron donors exhibit power conversion efficiencies exceeding 1.

Nanoporous polyethylene thin films templated by polymeric bicontinuous microemulsions: evolution of morphology on non-neutral substrates.

Polymeric bicontinuous microemulsions (BμE), found in well-designed ternary blends of two homopolymers and a diblock copolymer, have been extensively studied in the bulk, for example, as versatile templates for the synthesis of nanoporous materials. However, there have been few reports regarding BμE-forming blends as films and the potential impact of confinement on the morphology of such blends. We have investigated the morphology of ternary blends of polyethylene (PE), poly(ethylene-alt-propylene) (PEP), and poly(ethylene-b-ethylene-alt-propylene) (PE-PEP) on a variety of substrates.