Wide‐Range Refractive Index Control of Organic Semiconductor Films Toward Advanced Optical Design of Organic Optoelectronic Devices

Yokoyama, Daisuke; Nakayama, Keiichi I.; Otani, Toshiya; Kido, Junji

doi:10.1002/adma.201202422

Cited by 37 publications

(33 citation statements)

References 33 publications

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“…Molecular orientation is already recognized as a key factor influencing device performance (1, 2, 4, 5, 10-15, 37, 38) and our results provide a predictive tool for choosing the deposition temperature needed to produce the desired orientation, effectively adding a dimension to device design. The ability to predictably tune refractive indices across a wide range is analogously useful in applications that rely on redirection of light, such as waveguides or antireflective coatings (39). Although the results shown in Fig.…”

Section: Discussionmentioning

confidence: 99%

Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors

Dalal

Walters

Lyubimov

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

202

351

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Physical vapor deposition is commonly used to prepare organic glasses that serve as the active layers in light-emitting diodes, photovoltaics, and other devices. Recent work has shown that orienting the molecules in such organic semiconductors can significantly enhance device performance. We apply a high-throughput characterization scheme to investigate the effect of the substrate temperature (T substrate ) on glasses of three organic molecules used as semiconductors. The optical and material properties are evaluated with spectroscopic ellipsometry. We find that molecular orientation in these glasses is continuously tunable and controlled by T substrate /T g , where T g is the glass transition temperature. All three molecules can produce highly anisotropic glasses; the dependence of molecular orientation upon substrate temperature is remarkably similar and nearly independent of molecular length. All three compounds form "stable glasses" with high density and thermal stability, and have properties similar to stable glasses prepared from model glass formers. Simulations reproduce the experimental trends and explain molecular orientation in the deposited glasses in terms of the surface properties of the equilibrium liquid. By showing that organic semiconductors form stable glasses, these results provide an avenue for systematic performance optimization of active layers in organic electronics.glasses | organic semiconductors | molecular orientation | physical vapor deposition | high-throughput characterization

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Section: Discussionmentioning

confidence: 99%

Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors

Dalal

Walters

Lyubimov

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

202

351

View full text Add to dashboard Cite

show abstract

“…16,17,20 At appropriate substrate temperatures, many of these materials will likely form stable glasses 44 and there are open questions about the transformation mechanism in this geometry. The active layers in OLEDs are thin (30-50 nm) and are in contact on both sides with other solid films.…”

Section: Transformation Mechanism For Thicker Films and Films Withoutmentioning

confidence: 99%

Influence of Substrate Temperature on the Transformation Front Velocities That Determine Thermal Stability of Vapor-Deposited Glasses

Dalal

Ediger

2015

J. Phys. Chem. B

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Stable organic glasses prepared by physical vapor deposition transform into the supercooled liquid via propagating fronts of molecular mobility, a mechanism different from that exhibited by glasses prepared by cooling the liquid. Here we show that spectroscopic ellipsometry can directly observe this front-based mechanism in real time and explore how the velocity of the front depends upon the substrate temperature during deposition. For the model glass former indomethacin, we detect surface-initiated mobility fronts in glasses formed at substrate temperatures between 0.68Tg and 0.94Tg. At each of two annealing temperatures, the substrate temperature during deposition can change the transformation front velocity by a factor of 6, and these changes are imperfectly correlated with the density of the glass. We also observe substrate-initiated fronts at some substrate temperatures. By connecting with theoretical work, we are able to infer the relative mobilities of stable glasses prepared at different substrate temperatures. An understanding of the transformation behavior of vapor-deposited glasses may be relevant for extending the lifetime of organic semiconducting devices.

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“…[ 14 ] For example, Yokoyama et al showed a change of up to 0.58 in refractive index by using a multilayer stack. [ 15 ] In this work, we demonstrated that a multi-alternating 4,4′,4′′-Tris(N-3-methylphenyl-N-phenylamino) triphenylamine (m-MTDATA)/F 16 CuPc stack exhibits ohmic conductivity which is observed neither in the corresponding bilayer nor bulk heterojunction. Figure 1 shows the current density-voltage (J-V) characteristics of two devices with respectively 1 (dashed line) and 30 (solid line) units of "m-MTDATA/F 16 CuPc" bilayers (same total thickness of 300 nm) in the confi guration of ITO/1 or 30 units of bilayer (300 nm)/BPhen (8 nm)/Al.…”

mentioning

confidence: 97%

Multi‐Alternating Organic Semiconducting Films with High Electric Conductivity

Yang

et al. 2014

Adv Funct Materials

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High electric conductivity is observed in multilayer stack of m‐MTDATA/F16CuPc. Impedance data shows that the circuit resistance is significantly dropped by three orders of magnitude from ∼0.2 MΩ to ∼0.4 kΩ when the number of alternating units is increased from one to six, keeping a total thickness of 300 nm. Impedance results show that as the number of alternating units increases, the organic stack shows an increasing capacitance and a decreasing resistance. This result suggests the increasing charges accumulate at the heterojunctions, leading to reduction in overall film resistance. The application of the high conductive units in OLED device results in stability enhancement.

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Wide‐Range Refractive Index Control of Organic Semiconductor Films Toward Advanced Optical Design of Organic Optoelectronic Devices

Cited by 37 publications

References 33 publications

Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors

Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors

Influence of Substrate Temperature on the Transformation Front Velocities That Determine Thermal Stability of Vapor-Deposited Glasses

Multi‐Alternating Organic Semiconducting Films with High Electric Conductivity

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