Solid State Solvation in Amorphous Organic Thin Films

Madigan, Conor; Bulović, Vladimir

doi:10.1103/physrevlett.91.247403

Cited by 115 publications

(124 citation statements)

References 10 publications

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“…Consistent with previous observations, we observe a redshift of E CT with increasing C 60 fraction [10][11][12][13][14] , but find that modelling based on the accompanying change in dielectric constant via the solid-state solvation effect (SSSE) 15 can only explain the data at high (450%) C 60 loading. We find that E CT at low fullerene concentration is higher than expected from the change in dielectric constant and attribute this discrepancy to increased localization of the electron component of the CT state because of a reduction in average C 60 crystallite size below B4 nm.…”

supporting

confidence: 79%

“…The energy shift can be partially explained via the SSSE, which successfully describes the emission red-shift of other polar excited states with increasing dielectric constant in organic thin films 15 . Here, the energy shift DE em ¼ À AL static À BxL optical accounts for screening by the induced local electric field and involves contributions from both the static and optical frequency dielectric constants according to the local field factors L ¼ 2(e À 1)/(2e þ 1) and constants A and B that depend on the ground and excited state dipole moments involved in the optical transition.…”

Section: Resultsmentioning

confidence: 99%

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Delocalization and dielectric screening of charge transfer states in organic photovoltaic cells

et al. 2014

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Charge transfer (CT) states at a donor-acceptor heterojunction have a key role in the charge photogeneration process of organic solar cells, however, the mechanism by which these states dissociate efficiently into free carriers remains unclear. Here we explore the nature of these states in small molecule-fullerene bulk heterojunction photovoltaics with varying fullerene fraction and find that the CT energy scales with dielectric constant at high fullerene loading but that there is a threshold C 60 crystallite size of B4 nm below which the spatial extent of these states is reduced. Electroabsorption measurements indicate an increase in CT polarizability when C 60 crystallite size exceeds this threshold, and that this change is correlated with increased charge separation yield supported by CT photoluminescence transients. These results support a model of charge separation via delocalized CT states independent of excess heterojunction offset driving energy and indicate that local fullerene crystallinity is critical to the charge separation process.

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supporting

confidence: 79%

Section: Resultsmentioning

confidence: 99%

Delocalization and dielectric screening of charge transfer states in organic photovoltaic cells

et al. 2014

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“…This failure likely refl ects physical aspects not captured in the basic EMA model such as differences in molecular packing/fi lm density that vary with composition as well as the solid-state solvation effect, where DCM absorption red-shifts with increasing background refractive index. [ 18 ] This latter complication is clearly evident in Figure 2 c, where the extinction coeffi cient of 44% DCM in Tefl on is substantially lower than for an equivalent concentration in Alq 3 because DCM absorption in the former is blue-shifted by approximately 80 nm.…”

Section: Doi: 101002/adom201400021mentioning

confidence: 91%

Passive PT Symmetry in Organic Composite Films via Complex Refractive Index Modulation

Yan

Giebink

2014

Advanced Optical Materials

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“…Instead, we conclude that luminescence is due entirely to intramolecular TADF and attribute the observed color tunability to the solid-state solvation effect that is commonly observed in polar OLED dyes. 17 The energy levels of TPA-QNX(CN)2 and TPA-PRZ(CN)2 were estimated using cyclic voltammetry ( Figure S8 We obtained external quantum efficiencies (EQE) in OLEDs based on TPA-QNX(CN)2 with values up to 9.4% (Figure 5c, red line). This exceeds the highest EQE values for OLED devices based on simple fluorescence materials of around 5%.…”

Section: 13mentioning

confidence: 99%

Thermally Activated Delayed Fluorescence Materials Based on Homoconjugation Effect of Donor–Acceptor Triptycenes

Kawasumi¹,

Wu²,

Zhu³

et al. 2015

J. Am. Chem. Soc.

360

216

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Donor-acceptor triptycences, TPA-QNX(CN)2 and TPA-PRZ(CN)2 were synthesized and their emissive properties were studied. They exhibited a blue-green fluorescence with emission lifetimes on the order of a microsecond in cyclohexane at room temperature. The long lifetime emission is quenched by O 2 and is attributed to thermally activated delayed florescence (TADF). Unimolecular TADF is made possible by the separation and weak coupling due to homo-conjugation of the HOMO and LUMO on different arms of the three-dimensional donoracceptor triptycene. Organic light emitting devices (OLEDs) were fabricated using TPA-QNX(CN)2 and TPA-PRZ(CN)2 as emitters which displayed electroluminescence with efficiencies as high as 9.4% EQE.Since the first report by Tang and Van Slyke in 1987 1 , multi-layered organic light emitting diodes (OLEDs) have attracted interest for utilization in high efficiency illumination and flexible displays.2 OLEDs using fluorescent materials 3 have low internal quantum efficiencies (IQEs) of ≈ 25%, 4 due in part to the inherent limitation of electrical excitation, which generates singlets and triplets in a 1:3 ratio. 5High quantum yield OLEDs with Ir or Pt phosphorescent materials have been intensely investigated for the last several decades 5,6 and now achieve 100% IQE. 5aAlthough phosphorescent materials have defined the present state of OLED technology, there are significant issues including cost, stability of blue emitters, and strong triplet-triplet annihilation at high current density.7 As a result of recent efficiency increases, thermally activated delayed fluorescence (TADF) has become a viable alternative for harvesting both singlet and triplet state in OLEDs. 8,9 TADF is based on reversible intersystem crossing from thermally equilibrated triplet and singlet excited states, and competitive luminescence from the singlet states. If non-radiative pathways are negligible then TADF can achieve 100% electroluminescence IQE. 9f An advantage of TADF materials is that they can be purely organic materials and do not Typical TADF designs employ an electron donor and electron acceptor, which are connected directly but have a twisted geometry ( Figure 1a) to minimize the HOMO-LUMO overlap. 9,13An alternative approach is a through-space interaction wherein electronic systems are in communication by homo-conjugation 14 but are sufficiently separated to create a small singlet-triplet ∆E ST (Figure 1b). The design we report herein places the donor and acceptor on the different fins of a triptycene scaffold. These structures display homoconjugation and many triptycene derivatives display intrinsically high thermal stability, which is critical to OLED manufacturing and opperation. We designed the donor-acceptor triptycences, TPA-QNX(CN)2 and TPA-PRZ(CN)2, as novel TADF materials ( Figure 1c). The triphenylamine functions as the donor and dicyanoquinoxaline or dicyanopyrazine as the acceptor. Our designs were guided by time-dependent density functional theory (TD-DFT) calculations, which prov...

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Solid State Solvation in Amorphous Organic Thin Films

Cited by 115 publications

References 10 publications

Delocalization and dielectric screening of charge transfer states in organic photovoltaic cells

Delocalization and dielectric screening of charge transfer states in organic photovoltaic cells

Passive PT Symmetry in Organic Composite Films via Complex Refractive Index Modulation

Thermally Activated Delayed Fluorescence Materials Based on Homoconjugation Effect of Donor–Acceptor Triptycenes

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