Discovering new OLED emitters requires many experiments to synthesize candidates and test performance in devices. Large scale computer simulation can greatly speed this search process but the problem remains challenging enough that brute force application of massive computing power is not enough to successfully identify novel structures. We report a successful High Throughput Virtual Screening study that leveraged a range of methods to optimize the search process. The generation of candidate structures was constrained to contain combinatorial explosion. Simulations were tuned to the specific problem and calibrated with experimental results. Experimentalists and theorists actively collaborated such that experimental feedback was regularly utilized to update and shape the computational search. Supervised machine learning methods prioritized candidate structures prior to quantum chemistry simulation to prevent wasting compute on likely poor performers. With this combination of techniques, each multiplying the strength of the search, this effort managed to navigate an area of molecular space and identify hundreds of promising OLED candidate structures. An experimentally validated selection of this set shows emitters with external quantum efficiencies as high as 22%.
The realization of stable, high efficiency blue organic light emitting devices (OLEDs) remains
challenging. The efficiency of blue fluorescent OLEDs is fundamentally limited, and blue
phosphors are typically less stable. We discuss potential solutions to these problems. We show
that device engineering can be used to optimize the color of a relatively stable and efficient bluegreen
phosphor. In addition, we demonstrate the ability to manipulate the fraction of excitons
which form as singlets in fluorescent materials by inserting a mixing layer that affects only the
precursors to excitons.
Organic photovoltaics (PV) are constrained by a tradeoff between exciton diffusion and optical absorption. The short
exciton diffusion length within organic semiconductors demands the use of extremely absorptive materials.
Unfortunately, the excitonic character of most organic materials yields highly structured absorption spectra, with regions
of strong and weak absorption. Here, we describe a device architecture that decouples light absorption and exciton
diffusion in organic PV through the addition of a light absorbing 'antenna' layer external to the conventional charge
generating layers. Radiation absorbed by the antenna is transferred into the thin charge generating layers via surface
plasmon polaritons (SPP) in an interfacial thin silver contact and radiation into waveguide modes. SPPs are a
particularly effective energy transfer mechanism as they propagate in the plane of the PV rather than parallel to the
incident radiation, thereby providing a more efficient means of pumping thin charge generating structures. We exploit
efficient SPP-mediated energy transfer by attaching a resonant cavity antenna to a conventional small-molecular weight
organic PV. We find that the resonant cavity antenna boosts the performance of a phthalocyanine-based PV in the
absorption gap between the phthalocyanine Q and Soret bands. Off resonance the antenna serves as a mirror, but near the
resonant wavelength, the antenna absorption is significantly enhanced, and energy is fed back into the PV cell via SPP-mediated
energy transfer. Thus, the resonant antenna may be employed to supplement the performance of the PV cell at
resonance, with no degradation off-resonance.
We have fabricated saturated red, orange, yellow and green OLEDs, utilizing phosphorescent dopants. Using phosphorescence based emitters we have eliminated the inherent 25% upper limit on emission observed for traditional fluorescence based systems. The quantum efficiencies of these devices are quite good, with measured external efficiencies > 15% and > 40 lum/W (green) in the best devices. The phosphorescent dopants in these devices are heavy metal containing molecules (i.e. Pt, and Ir), prepared as both metalloporphyrins and organometallic complexes. The high level of spin orbit coupling in these metal complexes gives efficient emission from triplet states. In addition to emission from the heavy metal dopant, it is possible to transfer the exciton energy to a fluorescent dye, by Forster energy transfer. The heavy metal dopant in this case acts as a sensitizer, utilizing both singlet and triplet excitons to efficiently pump a fluorescent dye. We discuss the important parameters in designing electrophosphorescent OLEDs as well as their strengths and limitations. Accelerated aging studies, on packaged devices, have shown that phosphorescence based OLEDs can have very long device lifetimes.