Influence of intermolecular interactions on the formation of spontaneous orientation polarization in organic semiconducting films

Noguchi, Yutaka; Osada, Kohei; Ninomiya, Kaito; Gunawardana, H. D. C. N.; Koswattage, Kaveenga Rasika; Ishii, Hisao

doi:10.1002/jsid.956

Cited by 22 publications

(50 citation statements)

References 22 publications

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“…We measured the surface potential characteristics of the coevaporated films and found that the molecular orientation of TPBi was enhanced even in the UGH‐2 host, though the apparent enhancement factor was smaller than that in the CBP host. The enhancement of the molecular orientation is attributed to the reduction of the PDM interaction between TPBi molecules, which deteriorates SOP as previously reported 11 . In addition, our results suggest that the SOP of TPBi is robust even in the UGH‐2 host molecules with a weak vdW interaction compared to polyhedral shaped molecules like Ir complexes.…”

Section: Introductionsupporting

confidence: 83%

“…As shown in Figure 2A, all the films exhibited typical GSP behavior; that is, the surface potential induced in the films was proportional to the film thickness 3,5 . The surface potential data of the TPBi neat and TPBi:CBP coevaporated films are adapted from Noguchi et al 10 The GSP slope of the pristine (TPBi neat) film (Δ TP ) was estimated to be 63.1 mV/nm, and that of the coevaporated films with CBP (Δ TC ) and UGH‐2 (Δ TU ) was 67.4 and 44.5 mV/nm, respectively. Δ TC is larger than Δ TP despite the half PDM density, which indicates that the degree of orientation of PDM in TPBi was enhanced in the coevaporated film.…”

Section: Resultsmentioning

confidence: 99%

“…(B) Orientation enhancement factor versus mixed ratio for TPBi:UGH‐2 and TPBi:CBP. The surface potential data of TPBi neat and TPBi:CBP films are adapted from Noguchi et al 10 …”

Section: Resultsmentioning

confidence: 99%

“…We estimated the apparent enhancement factor of the orientation degree of TPBi's PDM (γ) with reference to the degree of orientation in the TPBi neat film (〈cos θ neat 〉), which is defined as follows 10 :

γ ((), x) = \frac{〈 \cos θ (x) 〉}{〈 \cos θ_{neat} 〉} = \frac{normalΔ ((), x) normalε ((), x)}{Δ_{g} ε_{g}} . \frac{1}{x},

where x is the volume ratio of the guest molecule (TPBi) and 〈cos θ ( x )〉, Δ( x ), and ε( x ) are the average degrees of orientation of PDM with respect to the surface normal, GSP slope, and dielectric constant of the mixed film, respectively. The subscript g indicates those for the guest molecule.…”

Section: Resultsmentioning

confidence: 99%

“…Recently, we reported the influence of intermolecular interactions on the formation of SOP 9,11 . We examined the surface potential characteristics of coevaporated films of polar and nonpolar molecules, including 1,3,5‐tris (1‐phenyl‐1Hbenzimidazol‐2‐yl) benzene (TPBi), N,N′‐bis (1‐naphthyl)‐N,N′‐diphenyl‐1,1′‐biphenyl‐4,4′diamine (NPB), 4,4′‐bis (N‐carbazolyl)‐1,1′‐biphenyl (CBP), and 4,4′,4″‐tris (carbazol‐9yl)triphenylamine (TCTA), where TPBi is polar, NPB slightly polar, and CBP and TCTA are nonpolar.…”

Section: Introductionmentioning

confidence: 99%

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Enhancement of the molecular orientation of TPBi in coevaporated films of UGH‐2 host molecules

Gunawardana

Osada

Koswattage

et al. 2021

Surface & Interface Analysis

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In this study, we investigated the spontaneous orientation polarization (SOP) in coevaporated films of polar and nonpolar molecules. SOP has been commonly observed in the evaporated films of organic light‐emitting diode materials, and it influences the device performances. However, the mechanism of the SOP formation is yet to be clarified. Herein, we characterized the molecular orientation of polar molecules of 1,3,5‐tris (1phenyl‐1H‐benzimidazol‐2‐yl) benzene (TPBi) in coevaporated films with nonpolar molecules of 1,4‐bis‐(triphenylsilyl) benzene (UGH‐2) or 4,4′‐bis (N‐carbazolyl)‐1,1′‐biphenyl (CBP). We measured the surface potential characteristics of the coevaporated films. We found that the molecular orientation of TPBi in both UGH‐2 and CBP hosts was enhanced, though the apparent enhancement factor was small in the UGH‐2 host. The enhancement of the molecular orientation is attributed to the reduction of the electrostatic interaction between polar molecules (TPBi), which deteriorates SOP as previously reported. In addition, our results suggest that the SOP of TPBi is robust even in the UGH‐2 host, in contrast to the random orientation of Ir complexes in the UGH‐2 host. Considering a polyhedral shape of Ir complexes, the robust SOP of TPBi in the host molecules with a weak van der Waals interaction, such as UGH‐2, is possibly due to its disk‐like molecular shape.

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Section: Introductionsupporting

confidence: 83%

Section: Resultsmentioning

confidence: 99%

“…(B) Orientation enhancement factor versus mixed ratio for TPBi:UGH‐2 and TPBi:CBP. The surface potential data of TPBi neat and TPBi:CBP films are adapted from Noguchi et al 10 …”

Section: Resultsmentioning

confidence: 99%

γ ((), x) = \frac{〈 \cos θ (x) 〉}{〈 \cos θ_{neat} 〉} = \frac{normalΔ ((), x) normalε ((), x)}{Δ_{g} ε_{g}} . \frac{1}{x},

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Enhancement of the molecular orientation of TPBi in coevaporated films of UGH‐2 host molecules

Gunawardana

Osada

Koswattage

et al. 2021

Surface & Interface Analysis

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Controlling Polarization‐Induced Polaron Accumulation in OLEDs via Device Design

Pakhomenko,

Martin,

et al. 2024

Adv Funct Materials

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Spontaneous alignment of molecular electric dipole moments, known as spontaneous orientation polarization (SOP), is present in many organic electronic materials. The SOP‐induced field changes the internal voltage distribution within the device, altering polaron injection and accumulation. The important role of SOP in organic light‐emitting devices (OLEDs) has recently been emphasized with clear links between dipole alignment and device efficiency and operational lifetime. Prior work has thus sought to engineer SOP via changes in thin film composition or processing conditions. These efforts have generally been directed at the OLED electron transport layer, which can show strong SOP. This work takes a different approach, focusing instead on how changes in device architecture can be applied to tune the magnitude and dynamics of polaron accumulation in response to SOP. In this work, it is demonstrated that the introduction of SOP into the device emissive layer offers control over the spatial location of accumulation. In parallel, insertion of a spacer layer between the hole transport and emissive layers can be used to tune the polaron accumulation rate and density during device operation. Both of these architectural changes permit an increase in OLED efficiency and a pathway around the deleterious impact of SOP‐induced polaron accumulation.

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Controlling Charge Accumulation Properties of Organic Light‐Emitting Diodes using Dipolar Doping of Hole Transport Layers

Noguchi

Hofmann

Brütting

2022

Advanced Optical Materials

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Although the charge accumulation properties at organic hetero interfaces have been often discussed in terms of the energy barrier to charge injection, it is known that the interface charge due to spontaneous orientation polarization (SOP) also plays a significant role. [5,[9][10][11][12] SOP originates from the macroscopic polarization induced by partial alignment of the permanent dipole moments (PDMs) of polar organic semiconducting materials in evaporated films, [9,13,14] and dominates the charge accumulation properties below the turnon voltage of OLEDs. [5,15,16] SOP has been frequently observed in many common OLED materials, particularly emitters and electron transport materials, though it is very small in most hole transport materials. [5,[17][18][19] Recently, Bangsund et al. have demonstrated that the excess charge accumulation due to SOP can reduce the EQE of OLEDs via exciton-polaron quenching, and EQE can be enhanced by eliminating SOP in the device. [6] Thus far, substrate heating during deposition and use of materials with negligible SOP have been examined to eliminate the excess charge accumulation. [6,7] However, substrate heating can influence the properties of underlayers in the device stack, since SOP is often formed in an emission layer (EML) and electron transport layer (ETL) that are commonly deposited on a hole transport layer (HTL). On the other hand, employing negligible SOP materials limits the possible choice of materials and would not be the best for the overall performance of OLEDs. For instance, many phosphorescent and thermally activated delayed fluorescence emitters also exhibit SOP, and ETL materials with negligible SOP are limited. [5,17] As an alternative method, Afolayan et al. have reported very recently that SOP is efficiently eliminated in a co-evaporated film of ETL and medium density of polyethylene. [8] They demonstrated a higher EQE and longer lifetime of a blue OLED by eliminating the SOP of the ETL. Meanwhile, the benefits of SOP in OLEDs are still controversial, e.g., SOP can improve charge injection efficiency and also affect charge blocking property. [20][21][22][23][24] Thus, exploring methods to control the charge accumulation property while keeping SOP is an important issue to optimize the device performance as well as to understand the role of SOP. In that sense, it will be intriguing to use a device stack with identical It is demonstrated that dipolar doping of hole transport layers (HTLs) controls the density and polarity of the accumulated charge at the critical interface between the HTL and the emission layer (EML) in organic lightemitting diodes (OLEDs). Dipolar doping enables spontaneous orientation polarization (SOP) even in nonpolar HTL, and consequently compensates for the negative interface charge originating from the SOP of the adjacent layer. This concept is applied to a phosphorescent OLED, where bis-4(N-carbazolyl) phenylphosphine oxide (BCPO) is employed as a polar dopant for the HTL. The net interface charge is completely compensated at ≈29.5% of d...

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Influence of intermolecular interactions on the formation of spontaneous orientation polarization in organic semiconducting films

Cited by 22 publications

References 22 publications

Enhancement of the molecular orientation of TPBi in coevaporated films of UGH‐2 host molecules

Enhancement of the molecular orientation of TPBi in coevaporated films of UGH‐2 host molecules

Controlling Polarization‐Induced Polaron Accumulation in OLEDs via Device Design

Controlling Charge Accumulation Properties of Organic Light‐Emitting Diodes using Dipolar Doping of Hole Transport Layers

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