Crystal Morphology and Growth in Annealed Rubrene Thin Films

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 <i>in situ</i> 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

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. In a planar heterojunction architecture, excitons generated on the SubPc host rapidly energy transfer to the SubNc guest, where they may migrate toward the dissociating, donor–acceptor interface. Overall, the incorporation of a photoactive host leads to a 13% enhancement in the short-circuit current density and a 20% enhancement in the power conversion efficiency relative to an optimized host–guest OPV combining SubNc with a nonabsorbing host. This work underscores the potential for further design refinements in planar heterojunction OPVs and demonstrates progress toward the effective separation of functionality between constituent OPV materials

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. These interfaces introduce a symmetry-breaking imbalance in exciton energy transfer, leading to directed motion. Despite their obvious utility for enhanced exciton harvesting in organic photovoltaic cells (OPVs), the emergent properties of these interfaces are as yet uncharacterized. Here, directed exciton transport is conclusively demonstrated in both dilute donor and energy-cascade OPVs where judicious optimization of the interface allows exciton transport to the donor–acceptor heterojunction to occur considerably faster than when relying on simple diffusion. Generalized systems incorporating multiple exciton permeable interfaces are also explored, demonstrating the ability to further harness this phenomenon and expeditiously direct exciton motion, overcoming the diffusive limit

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. 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

Effects of Additives on Crystallization in Thin Organic Films

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

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 <i>peri-</i>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 1.5% with open-circuit voltages ranging from 0.7 to 1.1 V when coupled with C₆₀ as an electron acceptor

Intermolecular Interactions Determine Exciton Lifetimes in Neat Films and Solid State Solutions of Metal-Free Phthalocyanine

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 has been commonly reported that excited state lifetimes decrease as the chromophore concentration is increased. The reduced lifetime was correlated to the loss of α-phase ordering as indicated in the UV/vis spectra of the films. Within the context of photovoltaic applications this highlights the importance of both molecular level ordering and chromophore concentration when trying to engineer fundamental material properties such as exciton diffusion length

Effect of Rapid Pressurization on the Solubility of Small Organic Molecules

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

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 <i>peri-</i>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 1.5% with open-circuit voltages ranging from 0.7 to 1.1 V when coupled with C₆₀ as an electron acceptor

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. The devices exhibit electroluminescence centered at a wavelength of λ = 836 nm with a peak external quantum efficiency exceeding 0.02%. This all-gas-phase approach permits controlled deposition of dense, functional nanocrystal films suitable for application in electronic devices