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ABSTRACT Polar organic molecules form spontaneous polarization in vacuum-deposited films by permanent dipole orientations in the films, originating from the molecule’s potential ability to

align itself on the film surface during deposition. This study focuses on developing polar molecules that exhibit spontaneous orientation polarization (SOP) and possess a high surface

potential. In the proposed molecular design, a hexafluoropropane (6F) unit facilitates spontaneous molecular orientation to align the permanent dipoles, and a phthalimide unit induces strong

molecular polarization. Furthermore, the introduction of phthalimides into the molecular backbone raises the glass transition temperature of the molecules, leading to the suppression of

molecular mobility on the film surface during film deposition and an improvement in the dipole orientation. The resulting surface potential slope is approximately 280 mV nm−1 without

substrate temperature control. Furthermore, this work proposes a method using position isomers as a design strategy to tune the SOP polarity. The substitution position of the strong polar

units influences the direction of the total molecular dipoles and affects the SOP polarity of the 6F-based molecules. The proposed molecular designs in this study provide wide tunability of

the SOP intensity and polarity, which contributes to highly efficient organic optoelectronic and energy-harvesting devices. SIMILAR CONTENT BEING VIEWED BY OTHERS SPONTANEOUS ORIENTATION

POLARIZATION DRIVEN BY DESIGNING MOLECULAR ASYMMETRY Article Open access 09 May 2025 SPONTANEOUS FORMATION OF METASTABLE ORIENTATION WITH WELL-ORGANIZED PERMANENT DIPOLE MOMENT IN ORGANIC

GLASSY FILMS Article 30 May 2022 INTERMOLECULAR-FORCE-DRIVEN ANISOTROPY BREAKS THE THERMOELECTRIC TRADE-OFF IN N-TYPE CONJUGATED POLYMERS Article 28 April 2025 INTRODUCTION Precise control

of molecular orientations in organic thin films is a critical requirement to achieve ultimately high-performance organic devices, such as organic light-emitting diodes (OLEDs), organic

photovoltaics, and organic field-effect transistors, because charge mobilities and light-emission performances strongly depend on the molecular orientations1,2,3. Spontaneous orientation

polarization (SOP) is derived from ordered metastable orientation states with aligned permanent dipole moments (PDMs) of polar molecules in solids4,5,6,7. Then, the deposited film of

tris(8-hydroxyquinolinato) aluminium (Alq3) exhibited a positive surface potential (giant surface potential; GSP) of +28 V at a film thickness of 560 nm, indicating that the growth rate of

the GSP to film thickness (GSP slope) was +50 mV nm−1 (Fig. 1a)4. Previous studies have revealed that SOP formation is not a unique event for Alq3 deposition but a universal event for most

amorphous-type polar molecules in their vacuum deposition processes8. On the other hand, there are negative surface charges on the bottom interface of positive GSP films9, that is, the

emission layer (EML)/electron-transport layer (ETL) interface of OLEDs with SOP molecules such as Alq3 as an ETL. The negative surface charges induce hole accumulation at the interface and

cause exciton quenching via exciton-polaron annihilations leading to inferior device performance such as quantum yields and operational stabilities10,11. Furthermore, SOP promotes charge

separation of charge-transfer exciplex excitons between organic donor and acceptor molecules, which would be beneficial for improving the performance of organic photovoltaics and organic

photodetectors12. Additionally, an electret material for vibration power generators is another application of SOP films because a spontaneously formed dipolar film can be used as a

self-assembled electret (SAE)13,14. Thus, SOP can provide a unique perspective on the performance of organic devices, which differs from the widely investigated molecular packings or

orientations that lead to high charge mobilities and efficient light out-coupling. Therefore, to improve and precisely control the performance of these optoelectronic and energy-harvesting

devices, the optimization of the SOP intensity and polarity is critical. However, a design strategy for SOP molecules has not been fully established, because the dipole orientation mechanism

is not entirely clear. The film polarization (_P_) formed by the spontaneous dipole orientations can be expressed as \({P}=p\left\langle \cos \theta \right\rangle n\), where \(p\),

\(\left\langle \cos \theta \right\rangle\), and \(n\) are the PDM of polar molecules, the mean orientation degree of PDMs, and the molecular density of films, respectively. Thus, improvement

of \(\left\langle \cos \theta \right\rangle\), that is, the molecular orientation with the same direction toward the film growth direction, is essential for the enhancement of SOP because

film polarization is cancelled when polar molecules form the inverted direction of the PDM orientation on the film surface by dipole-dipole interactions8,15. The \(\left\langle \cos \theta

\right\rangle\) values of typical SOP molecules, such as Alq3 and 1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBi), were less than 0.1, indicating that molecular PDMs cannot

efficiently contribute to dipolar film formation. Recent studies have proposed that the intrinsic SOP formation mechanism involves anisotropic van der Waals interactions between deposited

molecules and film surfaces16,17. Furthermore, process factors, such as a deposition rate, a deposition step, and a substrate temperature, also affect \(\left\langle \cos \theta

\right\rangle\) values and signs of SOP18,19,20,21. The author’s group recently developed fluoroalkyl-based polar molecules (Fig. 1b) that exhibit high \(\left\langle \cos \theta

\right\rangle\) values of over 0.322. However, the fluoroalkyl-based molecular backbone for SOP formation and the GSP slope value are still limited to realizing high-performance organic

optoelectronic and energy-harvesting devices. In this study, a 4,4ʹ-(hexafluoroisopropylidene)diphthalic imide (6FDI) backbone (Fig. 1c) was used to develop polar molecules with a strong

molecular PDM. The strong molecular polarization of fluoroalkyl-based molecules contributed to the formation of polarized films with a high GSP slope of over 200 mV nm−1 on a substrate

without substrate temperature control (Fig. 1d, Supplementary Table 1). The proposed molecular design helps to improve the SOP and achieve high-performance organic electronic and

energy-harvesting devices. RESULTS AND DISCUSSION Fluoroalkyl units in molecules induce spontaneous orientations in amorphous films owing to their small surface free energy, which the

fluoroalkyl units preferentially face on the film surface side (vacuum-side)22. The small polarizability results in weakend van der Waals interactions between the units and the film surface

during the deposition process, then, the fluoroalkyl units such as the CF3-units escape from the interface between the deposited molecule and the film surface to face the vacuum side.

Although the previous study used 2,2-diphenylhexafluoropropane (6F) as the basic backbone for the proposed CF3-based molecular design, this study used the 6FDI structure to design SOP

molecules with a strong molecular PDM. The 6FDI molecules were synthesized via a one-step chemical reaction of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and amine molecules

with different aromatic groups. The end groups of the 6FDI molecules can be modified by applying a variety of amine molecules to the chemical reaction. In this study, acceptor-type end

groups such as benzothiazole (BTA), triazole (TAZ), and _p_-benzonitrile (_p_BN) were introduced, and 6FDI-2BTA, 6FDI-2TAZ, and 6FDI-2_p_BN were synthesized (Fig. 2a). The calculated PDM

vector of the 6FDA molecule using the density functional theory (DFT) method indicates that the CF3-side is positively polarized in the molecule owing to the strong polarization of phthalic

anhydride. The dipole directions of the 6FDI molecules are almost the same as those of 6FDA, that is, the positively polarized CF3-side in the molecules. The author notes that there are

several stable conformers of these molecules (Fig. 2b), and the effect of the conformers is discussed in a later section. Figure 3a shows the dependence of the surface potentials of the

vacuum-deposited films on the film thickness. The molecules were then deposited on indium tin oxide (ITO)-coated glass substrates. The substrate temperature (_T_s) was room temperature, and

the deposition rate of the molecules was approximately 0.1 nm s−1. Although a 6FDA film showed almost no change in surface potentials (GSP slope: +0.1 mV nm−1), the 6FDI films exhibited

positive GSPs. The GSP slopes of 6FDI-2BTA, 6FDI-2TAZ, and 6FDI-2_p_BN were +66 mV nm−1, +152 mV nm−1, +218 mV nm−1, respectively (Table 1). The positive GSPs indicate that the positively

polarized fluoroalkyl-side of the molecules averagely faced the vacuum-side and formed vacuum-deposited films with ordered molecular PDMs22. Interestingly, 6FDI-2_p_BN exhibited a

significantly high GSP slope compared to 6F-2BN (GSP slope: +69 mV nm−1) which has a molecular structure similar to that of 6FDI-2_p_BN. Furthermore, the GSPs were improved by vacuum

deposition at a higher deposition rate. Figure 3b shows the thickness dependence of the GSP slopes of the developed molecules deposited at a deposition rate of 0.26–0.29 nm s−1. The

developed 6FDI molecules also exhibited a higher GSP slope at a high deposition rate, indicating an improved degree of orientation of the PDMs (Table 1). In particular, the deposited

6FDI-2_p_BN film exhibited a high surface potential of +284 mV nm−1. Thus, the 6FDI-2_p_BN film deposited at a _T_s of RT achieved a GSP slope comparable to that of a film of

bis-4-(N-carbazolyl)phenylphosphine oxide deposited at a _T_s of −70 °C19. Note that the surface charge density (_σ_) of the dipolar films (Table 1) were calculated using the following

equation, _σ_ = (GSP slope) × _ε_r × _ε_0, where _ε_r and _ε_0 are the relative permittivity, the dielectric constant of vacuum. The _ε_r value was assumed to be 3.0 in all organic films23.

Generally, organic molecules possess several stable conformers with different PDM magnitudes and directions. Because the average PDM in a molecule directly affects the magnitude and polarity

of the SOP, conformation control is essential for improving the SOP19,24,25,26. The possible conformations were searched using the force-field theory, and the obtained molecular structures

were optimized using the DFT method. Although 6FDA and 6FDI-2_p_BN had only three stable conformations, the number of conformers of 6FDI-2BTA and 6FDI-2TAZ was 62 and 10, respectively, and

the distribution of the PDM of the conformers and their populations are shown in Fig. 4. Thus, the structural asymmetry of the introduced end-groups was correlated with the number of

conformers, relating to the average PDM magnitude and the direction. The average PDM magnitude, \(\left\langle p\right\rangle\), was calculated as the sum of the production of the PDM

magnitude and the population of each conformer. The \(\left\langle p\right\rangle\) values of 6FDA, 6FDI-2BTA, 6FDI-2TAZ, and 6FDI-2_p_BN were 3.43, 2.26, 3.65, and 5.84 Debye, respectively.

The \(\left\langle p\right\rangle\) of 6FDI-2_p_BN was larger than that of 6F-2BN because of the introduced PI groups, which was one of the reasons for the higher GSP slope of 6FDI-2_p_BN.

Additionally, an average orientation degree of PDMs, \(\left\langle \cos \theta \right\rangle\), was calculated based on the following equation, \(P=\left\langle p\right\rangle \left\langle

\cos \theta \right\rangle n\). The calculated \(\left\langle \cos \theta \right\rangle {{\rm{s}}}\) of the 6FDI molecules were approximately 0.2, which were higher than that of the

previously reported 6F-2BN (0.10–0.17), despite the presence of the several 6FDI conformers with the dispersed dipole directions. Therefore, the notably high GSP slopes of the 6FDI molecules

originate not only from the larger \(\left\langle p\right\rangle\), but also from the improved PDM orientations. The \(\left\langle \cos \theta \right\rangle\) of 6FDI-2BTA was relatively

small compared to the other 6FDI molecules because of the presence of conformers with inverted PDM directions owing to the structural asymmetry of the BTA group. The positive and the

negative dipoles of the conformers (Fig. 4a, b) resulted in the cancellation of molecular dipoles in the deposited films, leading to a reduction in \(\left\langle \cos \theta

\right\rangle\). Nevertheless, methyl-substituted 6FDI-2_p_BN (6FDI-2_p_BNMe) exhibited a GSP slope and a \(\left\langle \cos \theta \right\rangle\) value comparable to that of 6FDI-2_p_BN

despite the low structural symmetry of the end-group, leading to an increase in the number of conformers (Supplementary Fig. 1). This is because the methyl-substitution has a minimal impact

on the PDM direction, and there are only a few conformers with a negative dipole reducing the overall \(\left\langle \cos \theta \right\rangle\). Therefore, the management of the PDM

direction (positive or negative) rather than the number of conformers is more important for improving the \(\left\langle \cos \theta \right\rangle\) of 6FDI molecules. The glass transition

temperature (_T_g) of deposited molecules is one of the factors related to the molecular orientations27,28,29. The ratio of _T_s to _T_g (_T_s/_T_g) correlates with the molecular surface

mobility on the deposited surface. Excess molecular mobility interrupts the formation of parallel dipole orientations, leading to the formation of anti-parallel or random orientations with

reduced SOP. A low _T_s/_T_g, limiting the molecular mobility, is beneficial for accelerating the formation of a parallel dipole orientation to improve the SOP. The suppressed PDM

orientation of 6F-2BN originates from its relatively low _T_g (74 °C). The _T_g values measured using differential scanning calorimetry (DSC) of the 6FDI molecules were over 130 °C, which

was significantly higher than that of 6F-2BN (Table 1 and Supplementary Fig. 2). Therefore, the limited mobility of the 6FDI molecules contributes to an improvement in the orientation

degree. Additionally, _T_s during depositions was also controlled using a heater connected to a substrate holder to validate the effect of _T_s/_T_g on the GSP slopes. The _T_s dependence of

the GSP slopes of the deposited 6F-2BN and 6FDI-2_p_BN films is shown in Supplementary Fig. 3. The GSP of the 6FDI-2_p_BN film was maintained at a higher _T_s, however, that of the 6F-2BN

film dramatically decreased with an increase in _T_s, and the GSP became almost zero at a _T_s of 45 °C during the deposition. Thus, the lower _T_g of 6F-2BN resulted in the critical _T_s

dependence of the GSP slopes in these _T_s ranges over room temperature. Therefore, the higher _T_g is one of the reasons for the significant improvement in the orientation degree and the

GSP slope of 6FDI-2_p_BN. Furthermore, it is expected that a decrease in _T_s below room temperature enhances the GSP slopes of these molecules, owing to a low _T_s/_T_g with further limited

molecular mobility, as reported previously19. The author estimated that one of the reasons for the zero polarization of a vacuum-deposited 6FDA film was the low _T_g (61 °C) corresponding

to a high _T_s/_T_g of 0.89 at room temperature, resulting in the randomization of molecular orientations. Hence, cooling _T_s is expected to build a positive GSP of a 6FDA film. Another

factor limiting the GSP slope and the \(\left\langle \cos \theta \right\rangle\) value, except for molecular PDM and low _T_g, is intermolecular dipole-dipole interactions between polar

molecules on the film surface during film formation. The intensity of dipole interactions depends on the PDM of polar molecules, indicating that polar molecules with strong PDM are

considerably influenced by dipole interactions, leading to the formation of anti-parallel dipole pairs to reduce \(\left\langle \cos \theta \right\rangle\) and film polarization. Thus, the

GSP slope of conventional polar molecules is limited, despite the strong molecular PDM (black symbols in Fig. 1d). In contrast, 6FDI-2_p_BN with \(\left\langle p\right\rangle\) of 5.84 Debye

maintained the moderate \(\left\langle \cos \theta \right\rangle\) and the high GSP slopes. This was attributed to the vertical molecular orientation induced by the 6F-units. Thus, the

6F-backbone is useful for designing SOP molecules with strong PDM to achieve extremely high GSP slopes. The SOP polarity design of previously reported 6F molecules was successfully performed

using a wide variety of functional groups with different polarity directions. In this study, the design of position isomers is introduced to control SOP polarity, that is, positive and

negative GSPs. As shown above, the BN group induces strong polarization, in which the nitrogen atom is negatively polarized. Thus, the positional isomers of the BN group are useful for

controlling the direction of the molecular PDM. The positional isomers of 6FDI-2_p_BN, that is, 6FDI-2_o_BN and 6FDI-2_m_BN (Fig. 5a), also possess several conformers with different

molecular PDMs. The calculated results of the PDM of the conformers indicated that the majority of the PDM direction of 6FDI-2_m_BN (Supplementary Fig. 4a) was the same as that of

6FDI-2_p_BN, indicating that the 6F moiety of the molecules was positively polarized, leading to a positive GSP. In contrast, most conformers of the _o_-isomer, 6FDI-2_o_BN (Supplementary

Fig. 4b), possess an inverted PDM direction with 6FDI-2_p_BN, indicating the negatively polarized 6F part in the molecules, leading to a negative GSP. The measured GSPs of the

vacuum-deposited films of the positional isomers are shown in Fig. 5b. As predicted from the calculation results, the GSP polarities of 6FDI-2_m_BN and 6FDI-2_o_BN were positive and

negative, respectively (Fig. 5c). These results clearly indicate that GSP polarity does not originate from native molecular properties, such as molecular mass, elemental ratio, and

functional groups. Although the number of negative SOP molecules is still limited except for the CF3-based molecules, it is essentially required to consider orientation manners and molecular

PDM directions to understand the clear origin of the SOP polarity. The author estimated that one of the reasons for the smaller absolute values of the GSP slopes of 6FDI-2_o_BN and

6FDI-2_m_BN is the wide distribution of the PDM magnitude and the direction, resulting in the smaller effective PDM magnitude of the 6FDI molecules. To reduce the effects of conformers on

the PDM orientations, a molecule with the 6F-backbone and the PI groups, 6F-2PI, was also prepared (Fig. 5a). According to conformation analysis, 6F-2PI has one molecular conformation with a

PDM magnitude of 5.82 Debye (Supplementary Fig. 4c). The GSP polarity of 6F-2PI (Fig. 5b) was negative, and the slope was −210 mV nm−1, corresponding to an orientation degree of −0.17,

which was significantly higher than that of 6FDI-2_o_BN (Supplementary Table 2). This is attributed to the preferable PDM direction along the center of the 6F-backbone, leading to an

efficient dipole alignment toward the substrate in the normal direction. SOP films can be applied as SAEs in vibration-based energy harvesters13,14. Although the stability of GSP is

necessary for electret applications, conventional SOP molecules, such as Alq3, absorb visible light to cancel GSPs. The developed polar molecules have no clear absorption in the visible

light region (Supplementary Fig. 5); thus, the GSPs can potentially be maintained under visible-light irradiation. The thermal stability of the GSPs was discussed in a previous paper22, and

the _T_g of polar molecules is one of the limitations of the electret’s heat-resisting temperature because the GSP vanishes by orientation randomization in the deposited films over _T_g.

Compared to 6F-2BN, the _T_g values of the 6FDI molecules were relatively high (>130 °C), indicating that improved thermal stability can be expected. To verify the operation of the

electret film of 6FDI-2_p_BN, the current generation was demonstrated using a vibration probe of the KP system13. Supplementary Fig. 6 shows the current profile generated via probe vibration

above the deposited 6FDI-2_p_BN film (surface potential of ~20 V), indicating that the deposited film of the developed molecules is applicable for energy harvesters and vibration sensors as

presented in previous studies13,14. The energy levels of the highest occupied molecular orbital (HOMO) were determined to be 7.1–7.9 eV (Supplementary Table 3) using a photoemission yield

spectrometer (Supplementary Fig. 7). The lowest unoccupied molecular orbital (LUMO) levels were estimated using the HOMO levels and the optical gap values (Supplementary Table 3). The

developed molecules possess deep HOMO and LUMO levels compared to typical electron transport molecules, such as Alq3, because of the acceptor-rich 6F-based molecular structures. The

vacuum-deposited films exhibited low-intensity photoluminescence (PL) (Supplementary Fig. 8a–g). The shapes of the PL spectra were relatively broad, and the PL peak wavelength was

approximately 500 nm. These are mainly attributed to charge-transfer (CT)-type emissions because the HOMO and LUMO distributions are spatially separated in the molecules. (Supplementary Fig.

 8h). The carrier transport properties of the developed positive and negative SOP molecules, such as 6FDI-2_p_BN and 6FDI-2_o_BN, were characterized using hole-only and electron-only devices

(HODs and EODs; device structures are shown in Supplementary Fig. 9a, b). The zero-field hole and electron mobilities (_μ_0h and _μ_0e) were estimated using the Child’s law, that is,

\(J=\frac{9}{8}{{\mu }_{0}\varepsilon }_{{{{\rm{r}}}}}{\varepsilon }_{0}\frac{{V}^{2}}{{d}^{3}}\), where _μ_0 and _d_ are the zero-field carrier mobility and thickness of organic films,

respectively30. The calculated _μ_0e and _μ_0h were 1.1–5.7 × 10−9 and 6.8–7.6 × 10−10 cm2 V−1 s−1, respectively (Supplementary Table 4). The author estimates that the low carrier mobilities

are attributed to the broad distribution of the density of states (DOS) owing to dipolar disorder and/or molecular conformations31. Previous studies have revealed that large PDMs in

deposited films induce a broad DOS, lowering the carrier mobilities16,32. Furthermore, as discussed above, the 6FDI-backbone possesses several molecular conformations. Supplementary Figs. 10

and 11 show the computationally calculated HOMO and LUMO energy alignments of the different conformations. Their energy levels are slightly different, which leads to a broadening of the DOS

and a decrease in the charge mobilities. The author examined OLED performance using the developed dipolar films as ETLs. The OLED device structure (Supplementary Fig. 12a) based on a

thermally activated delayed fluorescence emitter, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), was 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN; 10 nm) /

N,N′-di-1-naphthyl-N,N′-diphenylbenzidine (NPD; 30 nm)/3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP; 5 nm)/15 wt% 4CzIPN:mCBP (30 nm)/ETL (50 nm)/LiF (1 nm)/Al (100 nm).

2-(9,9′-spirobi[fluoren]-3-yl)-4,6-diphenyl-1,3,5-triazine (SF3-TRZ), 6FDI-2_o_BN, and 6FDI-2_p_BN were used as ETLs with nonpolar, negative SOP, and positive SOP, respectively.

Supplementary Fig. 12b–d shows the _J_–_V_-luminance (_L_), current efficiency (CE) profiles, and electroluminescence (EL) spectra of the OLEDs. The driving voltages of the OLEDs based on

the 6FDI molecules were higher than that of the SF3-TRZ-based OLED. This was attributed to the low electron mobility of the 6FDI molecules (Supplementary Table 4) compared to SF3-TRZ33. Note

that the deep LUMO levels of the 6FDI molecules allow electrons to be directly injected into doped 4CzIPN in the EML (Supplementary Fig. 12e). Because the EL spectra of the OLEDs mainly

attributed to 4CzIPN emission and the delayed EL lifetimes (4.1 μs) approximately correspond to the delayed PL lifetime (3.5 μs) of 4CzIPN doped in mCBP, electrons are directly injected to

4CzIPN molecules and charge recombination occurs in the EML (Supplementary Fig. 13). Although the SF3-TRZ-based OLED exhibited a maximum CE of 83.6 cd A−1 (corresponding to an external EL

quantum efficiency of ~25%)33,34, those of the OLEDs based on 6FDI-2_o_BN and 6FDI-2_p_BN were 30.2 and 1.9 cd A−1. The reason for the low CE was attributed to exciton quenching at the

EML/ETL interface. The low electron mobilities of the 6FDI-based ETL and the high electron injection barrier between the ETLs and the EMLs facilitate hole accumulation and highly

concentrated charge recombination at the interface, resulting in severe exciton-exciton and exciton-charge annihilations35,36,37. Furthermore, the interfacial charge at the EML/ETL interface

also induce charge accumulation. Supplementary Fig. 14a) shows the displacement current measurement (DCM) profiles (ramp rate: 2 kV s−1) of the OLEDs, where the current was measured using a

current amplifier under triangular voltage applications. These results indicate that the ETLs with positive/negative SOPs induce the charge injection and accumulation at the lower voltage

than the EL turn-on (Supplementary Fig. 14b, c). The negative/positive polarization charges at the EML/ETL interface induce hole/electron accumulations to induce exciton-charge annihilations

lowering the EL quantum efficiency11,18,38,39. The author estimated that the reason for the more severe CE drop of the OLED based on the 6FDI-2_p_BN ETL is the positive charge accumulation

at the interface. The accumulated holes induce highly dense cations of 4CzIPN and mCBP, which possess absorption overlap with the 4CzIPN emission36, indicating that 4CzIPN excitons are

efficiently quenched by the cations compared to anions. A previous study investigated the impact of SOP layers on hole injection at the ITO/hole-transport layer interface and revealed that

an introduced negative SOP layer improves the hole current of hole-only devices22. This could be attributed to the adjustment of the work function using a dipole layer that reduces the

carrier injection barrier, which has been well studied using self-assembled monolayers (SAMs)40. Although SAM modifications can form a highly ordered and strong dipolar monolayer on metal

electrodes, they are generally formed by a solution process method and are not applicable to organic/organic interfaces. On the other hand, the SOP layers formed by vacuum deposition are

applicable to organic/organic and organic/metal interfaces. This study examined the impact of SOP interlayers on hole injection at organic/organic interfaces. In the OLED structure shown in

Fig. 6a, the relatively large HOMO level gap (~0.7 eV) between the NPD and the mCBP layers lowers the hole injection at the interface (Fig. 6b). To investigate the impact of the SOP layer at

organic/organic interfaces, 2-nm-thick SOP layers of the developed polar molecules were introduced as interlayers at the interface between the NPD and the mCBP layers (Supplementary Fig. 

15a, b). The reason for the small thickness of the SOP interlayers is the low hole mobility and the deep HOMO levels, reducing hole transport in OLEDs. Therefore, the author expects that the

SOP interlayers act as interfacial dipoles to tune the interfacial energy differences41. Figure 6c shows the _J_–_V_–_L_ characteristics of OLEDs with SOP interlayers. The OLED with the

6FDI-2_p_BN (positive SOP) interlayer exhibited a high driving voltage, whereas the OLED with the 6F-2PI (negative SOP) interlayer exhibited a low driving voltage compared to the OLED

without interlayers. Furthermore, the thickness dependence of the surface potential of the mCBP/SOP-interlayer/NPD/ITO stacks clearly showed that the introduction of 2-nm-thick SOP

interlayers induced a clear energy-level shift between the organic layers (Supplementary Fig. 15a, b). These results clearly indicate that the energy-level shift induced by the negative SOP

interlayer (ca. 0.45 eV) reduces the injection barrier (HOMO level gap) between the NPD and the mCBP layers to improve the hole injection. Note that the author confirmed that the vacuum

level shift at the NPD/mCBP interface was almost negligible (Supplementary Fig. 15c), and the surface potentials on the SOP interlayers were stable under vacuum conditions (Supplementary

Fig. 15d). Additionally, the DCM results (Supplementary Fig. 16a) of these OLEDs indicated that the thin SOP interlayers induced no distinct charge accumulation in the OLEDs observed in the

OLEDs with SOP-ETLs (Supplementary Fig. 14). However, the CEs with the SOP interlayers (Fig. 6d) decreased because of the change in the carrier balance, that is, the hole-rich situation. To

improve the carrier balance of the OLED with the 6F-2PI interlayer, an electron-transport molecule, 4,6-bis(3,5-di(pyridin-4-yl)phenyl)-2-methylpyrimidine (B4PyMPM, Supplementary Fig. 

16b)42, with a high electron mobility (1.0 × 10−4 cm2 V−1 s−1) was applied instead of the SF3-TRZ ETL. The CE of the OLED with the 6F-2PI interlayer was successfully improved using a B4PyMPM

ETL, and the maximum CE (101.7 cd A−1) was higher than that of the reference device (Fig. 6d and Supplementary Table 5) because of the well-tuned carrier balance. Therefore, the

introduction of thin SOP layers between the organic/organic layers is beneficial for precisely tuning the carrier balance to simultaneously realize a low driving voltage and a high EL

quantum efficiency. In conclusion, the author developed polar molecules exhibiting a high GSP slope in vacuum-deposited films. The 6FDI backbone provided highly ordered PDM orientations and

strong molecular polarization originating from the 6F and the PI units. The introduction of PI resulted in a strong molecular PDM and an increase in _T_g, leading to improved film

polarization. The presence of conformers directly influenced the effective magnitude of the molecular dipoles to form the film polarization of 6FDI-based molecules. Furthermore, the SOP

polarity can be designed by controlling the dipole direction using the positional isomers of polar molecules. The resulting GSP slope of 6FDI-2_p_BN on an ITO substrate without _T_s control

was approximately 280 mV nm−1, which is 5.6 times higher than that of Alq3. Furthermore, the OLED study revealed that thin SOP interlayers between organic/organic interfaces tune the carrier

injection barrier, leading to improved device performance. The findings of this study provide design strategies for extremely strong SOP for applications in organic electronic and

energy-harvesting devices. METHODS MATERIALS AND GENERAL METHODS All synthesis reagents were purchased from commercial sources (TCI) and used without further purification. All the

synthesized compounds were purified by column chromatography followed by temperature-gradient vacuum sublimation. HAT-CN (>99.7% purity) was purchased from Analysis Atelier Corporation.

NPD (>99.0% purity) and mCBP (>99.0% purity) were purchased from TCI. 4CzIPN (>98.0% purity), SF3-TRZ (>99.0% purity), and B4PyMPM (>99.0% purity) were purchased from Lumtec.

Nuclear magnetic resonance (NMR) spectra were obtained using a JNM-ECX400 NMR spectrometer (JEOL) at ambient temperature. Absorption spectra of organic films on a quartz glass substrate were

measured on an UV-2550 (Shimadzu). A glass transition temperature was determined using differential scanning calorimetry on a DSC7000X (Hitachi). FILM SAMPLE FABRICATION AND EVALUATION

Organic solid-state films of varying thicknesses for surface potential measurements were deposited directly on pre-cleaned, 100-nm-thick, ITO-coated glass substrates using the physical vapor

deposition (PVD) technique. PVD was performed under high vacuum at pressure levels <3 × 10−4 Pa at a monitored deposition rate using an in-house evaporation machine. The surface

potential was measured using the Kelvin probe method under vacuum and dark conditions (UHVKP020, K.P. Technology). Film thickness was estimated using a thickness meter (FR-ES,

ThetaMetrisis). The molecular density was assumed to be the same as that of 6F-2BN to estimate the average orientation degree of the permanent dipole moment in the film. To measure

vibration-based generated current, a probe (stainless steel) with a diameter of 4 mm of a KP measurement system was placed above a deposited organic film on an ITO substrate with a gap of ~1

 mm, and the probe was vibrated with a frequency of 59.2 Hz, then the generated current was collected with an oscilloscope (TBS2104B, Tektronix) using a current/voltage amplifier (SA-604F2,

NF). The HOMO levels were estimated using a photoelectron yield spectrometer (PYS). The PYS measurements of 100-nm-thick films on ITO-coated substrates were performed using AC-3 (RIKEN

KEIKI) and BIP-KV100 (Bunkoukeiki). Because of the deep HOMO levels of the molecules, the experimental results for BIP-KV100 were used to estimate the HOMO levels. DEVICE FABRICATION AND

CHARACTERIZATION OLEDs, HODs, and EODs were fabricated by the vacuum vapour deposition process without exposure to ambient air. After fabrication, devices were immediately encapsulated under

a glass cover using epoxy glue in a nitrogen-filled glovebox (H2O > 0.1 ppm, O2 > 0.1 ppm). All organic layers were deposited at a deposition rate of 0.1 nm s−1. The deposition rate

of LiF and Al layers was 0.01 and 0.5 nm s−1. All device characterizations were performed at room temperature. Current density–luminance–voltage measurements of OLEDs were performed using a

sourcemeter (Keithley 2400) and a luminance meter (LS160, KONICA MINOLTA). Current density-luminance characteristics of HODs and EODs were measured using a sourcemeter (Keithley 2400) For

DCM, repeated-triangular voltage signals were applied to each device using a function generator (AFG1062, Tektronix) and the displacement current amplified using a current amplifier (CA5351,

NF). The applied voltage and amplified current were measured using an oscilloscope (WaveSurfer 4054HD, Teledyne Lecroy). The applied voltage scan rate was 2k V s−1. PL and EL spectra were

collected using a spectrameter (Flame-T, Ocean Photonics). Transient PL and EL profiles were collected using a photomultiplier tube (H10721-01, Hamamatsu Photonics), a current/voltage

amplifier (SA-604F2), and an oscilloscope (WaveSurfer 4054HD). COMPUTATIONAL CALCULATIONS Optimized molecular structures and permanent dipoles of ground-state molecules were calculated using

the B3LYP/6-31 G (d) level with the Gaussian 16 program package. Conformation analysis was performed using force-field theory with CONFLEX 9. SYNTHESIS 6FDI-2BTA:

4,4′-(hexafluoroisopropylidene)diphthalic anhydride (621 mg), 6-aminobenzothiazole (430 mg), and molecular sieve (4A) were added to dehydrated 1,3-dimethyl-2-imidazolidinone (5 mL). The

solution was stirred at room temperature for 20 min and subsequently stirred at 150 °C for 20 h. The resulting solution was washed with water. The precipitate was dried under vacuum, and

purified by chromatography on silica gel (chloroform-ethyl acetate-hexane) to afford 6FDI-2BTA as a white solid in 60% yield. 1H NMR (400 MHz, CDCl3): _δ_ 9.09 (s, 2H), 8.28 (d, _J_ = 8.7 

Hz, 2H), 8.10 (d, _J_ = 8.2 Hz, 2H), 8.07 (d, _J_ = 1.9 Hz, 2H), 7.97 (s, 2H), 7.94 (d, _J_ = 8.2 Hz, 2H), 7.49 (dd, _J_ = 2.0, 8.8 Hz, 2H). 13C NMR (100 MHz, DMSO-d6): _δ_ 166.76, 166.63,

158.48, 153.17, 140.93, 137.94, 136.46, 134.38, 133.60, 133.19, 129.50, 126.49, 125.04, 124.23, 123.73, 122.29. HRMS (ESI) m/z: [M + H]+ calcd. for C33H15F6N4O4S2 709.0433; found 709.0440.

6FDI-2TAZ: 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (621 mg), 1-(4-aminophenyl)-1,2,4-triazole (450 mg), and molecular sieve (4A) were added to 1,3-dimethyl-2-imidazolidinone (5 

mL). The solution was stirred at room temperature for 20 min and subsequently stirred at 150 °C for 20 h. The resulting solution was washed with water. The precipitate was dried under

vacuum, and purified by chromatography on silica gel (chloroform-ethyl acetate-hexane) to afford 6FDI-2TAZ as a white solid in 56% yield. 1H NMR (400 MHz, CDCl3): _δ_ 8.61 (s, 2H), 8.14 (s,

2H), 8.09 (d, _J_ = 7.8 Hz, 2H), 7.93 (d, _J_ = 8.7 Hz, 4H), 7.86 (d, _J_ = 8.7 Hz, 4H), 7.64 (d, _J_ = 9.1 Hz, 4H). 13C NMR (100 MHz, DMSO-d6): _δ_ 166.55, 166.41, 161.54, 144.57, 144.57,

143.19, 137.92, 136.82, 133.58, 133.17, 131.44, 130.95, 129.29, 125.04, 124.25, 122.40, 120.52. HRMS (ESI) m/z: [M + H]+ calcd. for C35H19F6N8O4 729.1428; found 729.1434. 6FDI-2_p_BN:

4,4ʹ-(hexafluoroisopropylidene)diphthalic anhydride (621 mg), 4-aminobenzonitrile (332 mg), and molecular sieve (4 A) were added to 1,3-dimethyl-2-imidazolidinone (5 mL). The solution was

stirred at room temperature for 20 min and subsequently stirred at 150 °C for 20 h. The resulting solution was washed with water. The precipitate was dried under vacuum, and purified by

chromatography on silica gel (chloroform-ethyl acetate-hexane) to afford 6FDI-2_p_BN as a white solid in 80% yield. 1H NMR (400 MHz, CDCl3): _δ_ 8.09 (d, _J_ = 8.2 Hz, 2H), 7.95 (s, 2H),

7.93 (d, _J_ = 8.2 Hz, 2H), 7.83 (d, _J_ = 8.7 Hz, 4H), 7.66 (d, _J_ = 8.7 Hz, 4H). 13C NMR (100 MHz, CDCl3): _δ_ 165.39, 165.20, 139.65, 136.42, 135.43, 133.21, 132.42, 132.12, 126.61,

125.74, 124.66, 118.13, 112.02. HRMS (ESI) m/z: [M+Na]+ calcd. for C33H14F6N4NaO4 667.0811; found 667.0823. 6FDI-2_p_BNMe: 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (622 mg),

4-amino-3-methylbenzonitrile (370 mg), and molecular sieve (4A) were added to 1,3-dimethyl-2-imidazolidinone (5 mL). The solution was stirred at room temperature for 20 min and subsequently

stirred at 150 °C for 20 h. The resulting solution was washed with water. The precipitate was dried under vacuum, and purified by chromatography on silica gel (chloroform-ethyl acetate) to

afford 6FDI-2_p_BNMe as a white solid in 72% yield. 1H NMR (400 MHz, CDCl3): _δ_ 8.09 (_d_, _J_ = 8.3 Hz, 2H), 7.96 (d, _J_ = 8.2 Hz, 2H), 7.93 (s, 2H), 7.70 (s, 2H), 7.66 (dd, _J_ = 1.4,

8.2 Hz, 2H), 7.34 (d, _J_ = 8.2 Hz, 2H), 2.29 (s, 6H). 13C NMR (100 MHz, CDCl3): _δ_ 165.50, 165.24, 139.53, 138.40, 136.28, 135.07, 134.53, 132.75, 132.45, 130.78, 129.75, 125.76, 124.65,

118.05, 113.76, 18.34. HRMS (ESI) m/z: [M+Na]+ calcd. for C35H18F6N4NaO4 695.1124; found 695.1116. 6FDI-2_o_BN: 4,4ʹ-(hexafluoroisopropylidene)diphthalic anhydride (746 mg),

2-aminobenzonitrile (397 mg), benzoic acid (205 mg), and molecular sieve (4 A) were added to 1,3-dimethyl-2-imidazolidinone (6 mL). The solution was stirred at room temperature for 20 min

and subsequently stirred at 150 °C for 20 h. The resulting solution was washed with water. The precipitate was dried under vacuum, and purified by chromatography on silica gel

(chloroform-ethyl acetate) to afford 6FDI-2_o_BN as a white solid in 75% yield. 1H NMR (400 MHz, CDCl3): _δ_ 8.10 (d, _J_ = 7.8 Hz, 2H), 8.06 (s, 2H), 7.88 (d, _J_ = 8.2 Hz, 4H), 7.79 (t,

_J_ = 7.8 Hz, 2H), 7.62 (t, _J_ = 7.6 Hz, 2H) 7.49 (d, _J_ = 7.8 Hz, 2H). 13C NMR (100 MHz, CDCl3): _δ_ 165.48, 165.29, 139.61, 134.40, 132.42, 132.36, 132.12, 131.81, 130.57, 130.31,

129.69, 125.74, 124.65, 117.84, 113.65. HRMS (ESI) m/z: [M+Na]+ calcd. for C33H14F6N4NaO4 667.0811; found 667.0823. 6FDI-2_m_BN: 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (746 mg),

3-aminobenzonitrile (397 mg), benzoic acid (205 mg), and molecular sieve (4A) were added to 1,3-dimethyl-2-imidazolidinone (6 mL). The solution was stirred at room temperature for 20 min

and subsequently stirred at 150 °C for 20 h. The resulting solution was washed with water. The precipitate was dried under vacuum, and purified by chromatography on silica gel

(chloroform-ethyl acetate) to afford 6FDI-2_m_BN as a white solid in 72% yield. 1H NMR (400 MHz, CDCl3): _δ_ 8.10 (d, _J_ = 7.8 Hz, 2H), 7.95 (s, 2H), 7.93 (d, _J_ = 8.3 Hz, 2H), 7.82 (s,

2H), 7.74 (m, 4H), 7.66 (dd, _J_ = 7.8, 8.2 Hz, 2H). 13C NMR (100 MHz, CDCl3): _δ_ 165.48, 165.29, 153.52, 139.61, 136.40, 132.42, 132.36, 132.12, 131.81, 130.57, 130.31, 129.69, 125.74,

124.65, 117.84, 113.65. HRMS (ESI) m/z: [M+Na]+ calcd. for C33H14F6N4NaO4 667.0811; found 667.0813. 6F-2PI: Phthalic anhydride (550 mg), 4,4′-(hexafluoroisopropylidene)dianiline (561 mg),

benzoic acid (410 mg), and molecular sieve (4A) were added to 1,3-dimethyl-2-imidazolidinone (6 mL). The solution was stirred at room temperature for 20 min and subsequently stirred at 150 

°C for 20 h. The resulting solution was washed with water. The precipitate was dried under vacuum, and purified by chromatography on silica gel (chloroform-ethyl acetate) to afford 6F-2PI as

a white solid in 75% yield. 1H NMR (400 MHz, CDCl3): _δ_ 7.99 (dd, _J_ = 3.2, 5.5 Hz, 4H), 7.82 (dd, _J_ = 3.2, 5.5 Hz, 4H), 7.57 (m, 8H). 13C NMR (100 MHz, CDCl3): _δ_ 167.01, 134.74,

132.73, 132.57, 131.65, 131.11, 125.93, 124.03, 122.69, 64.41. HRMS (ESI) m/z: [M+Na]+ calcd. for C31H16F6N2NaO4 617.0906; found 617.0910. DATA AVAILABILITY The experimental data in this

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Google Scholar  Download references ACKNOWLEDGEMENTS The author thanks Prof. Chihaya Adachi, Prof. Hajime Nakanotani, and Prof. Kenichi Goushi of Kyushu University for the setup of the OLED

fabrication machine. The author appreciates Prof. Nobuhumi Nakamura and Prof. Takahiro Ichikawa of Tokyo University of Agriculture and Technology for their help and discussions. The author

also thanks Prof. Keiichi Noguchi of Tokyo University if Agriculture and Technology for high resolution mass spectroscopy measurements, and Bunkoukeiki Co., Ltd. and RIKEN KEIKI Co., Ltd.

for photoelectron yield spectroscopy measurements. This work was partially supported by JST FOREST Program (JPMJFR223S (M.T.)), JSPS KAKENHI (JP23H05406 and JP23K13716 (M.T.)), Inamori

Foundation (M.T.), Tokuyama Science Foundation (M.T.), Casio Science Promotion Foundation (M.T.), and Advanced Technology Institute Research Grants (M.T.). AUTHOR INFORMATION AUTHORS AND

AFFILIATIONS * Department of Biotechnology and Life Science, Tokyo University of Agriculture and Technology, Koganei, Tokyo, Japan Masaki Tanaka Authors * Masaki Tanaka View author

publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS The project was conceived and designed by M.T. M.T. synthesized the molecules and evaluated their

properties. CORRESPONDING AUTHOR Correspondence to Masaki Tanaka. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing interests. PEER REVIEW PEER REVIEW INFORMATION

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CITE THIS ARTICLE Tanaka, M. Boosting spontaneous orientation polarization of polar molecules based on fluoroalkyl and phthalimide units. _Nat Commun_ 15, 9297 (2024).

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