Optical, photophysical, and electrooptical studies on slot-die polyfluorene-based flexible OLED devices

M. Gioti

doi:10.1364/OME.420697

1. Introduction

Conjugated Polymers (CPs) as well as their blends are attracting worldwide attention due to their potential for use as the active layer in advanced optoelectronic applications and their cost-effective and low thermal processing traits [1,2]. Because most of conjugated polymers preferentially transport one type of charge carrier (electrons or holes), blend of different polymers can be an alternative to create the photoactive material for devices exhibiting improved electronic properties [3–5]. Thus, for the case of Organic Light Emitting Diodes (OLEDs), by combining two different polymers with complementary electrical behavior the emission efficiency can be optimized and enhanced by balancing hole and electron injection [6].

In terms of device fabrication, solution-processable materials possess the advantages of low procession cost attracting thus increasing attention for large scale roll-to-roll (R2R) production. Efficient OLEDs have been developed using as emissive layer (EML) the blend of poly(9,9-dioctylﬂuorene-alt-benzothiadiazole) (F8BT) (acceptor and guest) and poly(9,9-dioctylﬂuorene) (F8) (donor and host) [7–12]. In this blend, an effective Förster resonance energy transfer (FRET) takes place, from F8 (host) to F8BT (guest), resulting in green emission [13]. However, in all these studies the emissive layers were formed via spin coating. Only recently has been reported the fabrication of OLEDs on glass substrates by slot-die coating technique using F8:F8BT-ink formulation. The devices showed uniform light emission, a low turn-on voltage of 3.5 V and a brightness of 357 cd/m² at 10 V [14].

The fabrication of the multilayer OLEDs on flexible substrates in large scale from solution involves the major problem of stack integrity. Since organic semiconductors are soluble in common organic solvents, the fabrication of multilayers is not straightforward, and the solvent used to deposit a subsequent layer may redissolve the layer underneath. Thus, the whole process must be optimized in order to provide efficient, reproducible and reliable devices. Towards to these demands, a fundamental understanding of the connection between optical properties, morphology and carrier transport processes is important to achieve effective control of quantum efficiencies in OLEDs.

For this purpose, non-destructive, rapid, and efficient characterization techniques must be implemented at different stages of the OLED fabrication process. Photoluminescence (PL) and Electroluminescence (EL) Spectroscopies are the most prerequisite and commonly used tools for the characterization and evaluation of OLED materials and devices. In addition, the implementation of Spectroscopic Ellipsometry (SE), as a highly sensitive and non-invasive method to obtain fundamental information about the optical properties of polymer films in the multilayer stacks, reinforces the analytical characterization.

In the present paper, we demonstrate the characterization of flexible OLED devices with F8:F8BT blended films of different ratios (19:1, 1:1, and 1:19) as EMLs, fabricated via slot-die on PET substrates. These specific ratios were selected, in order to study the efficacy of the OLED devices in green color emission due to the blended nature of the photoactive film. These ratios cover a significantly broader range comparing to that commonly presented in the literature and change the role of F8 and F8BT from host to guest and inversely. Visible–far Ultraviolet (Vis-fUV) SE measurements on thin polymeric films in single-layer and multi-layer structures, yield insight into layer thickness values, dielectric function, and absorption coefficient. The determination of the optical properties in combination with the photo- and electro-emission characteristics provide a thorough characterization and evaluation of F8:F8BT photoactive materials and OLED devices.

The ellipsometric data are analyzed according to the modified Tauc–Lorentz (TL) model, in which the energy-depended broadening of the TL oscillators is introduced, providing a precise determination of the optical properties and their dependence on blends’ composition. These findings are well correlated to the PL and EL emission profiles and characteristics. Finally, the evaluation of the functionality and performance of the fabricated flexible OLEDs is presented.

2. Materials and methods

2.1 Materials and ink formulation

For the Hole Transport Layer (HTL) Poly-3,4-ethylene dioxythiophene: poly-styrene sulfonate (PEDOT:PSS) was purchased from Heraeus (Hanau, Germany) with the commercial product name CLEVIOS PVP Al 4083. A solution of PEDOT:PSS AI 4083 mixed with ethanol in the ratio of 3:2 was prepared. For the EMLs Poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO or F8) and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) were obtained from Ossila, UK. The F8 and the F8BT were dissolved in toluene at 19:1, 1:1 and 1:19 ratios giving concentration of 1.5% (w/w). The solvents were purchased from Sigma-Aldrich and used as received without further purification. The materials used for the cathode bilayer, Ca and Ag, were obtained from Kurt J. Lesker.

2.2 OLED fabrication

A mini roller coater by FOM Technologies (slot-die head with 13 mm shim width) was used to coat both the PEDOT:PSS (HTL) and F8:F8BT layers (EMLs) onto a PET/ITO flexible substrate/anode base under ambient conditions (Humidity 30–40% RH). The PEDOT:PSS interlayer was slot-die coated with the coating speed of 0.5 mm min⁻¹, dispensing rate of 45 µL min⁻¹, and substrate temperature of 45 °The layer was thermally annealed at 120 °C for 5 min prior to the deposition of the EML. Thereafter, the F8:F8BT layers were formed with the coating speed of 0.5 m min⁻¹, dispensing rate of 30 µL min⁻¹, and substrate temperature of 45 °C. The EMLs were thermally annealed at 110 °C for 10 min. For the cathode electrode a bilayer of Ca (7 nm) as an Electron Transport Layer (ETL) and an Ag (100 nm) as a cathode, was vacuum deposited using the appropriate shadow masks in order to produce pixels of 1.0 × 1.2 cm². The structure of the fabricated OLED devices is shown in Fig. 1 (a). Control emitting films and OLED devices with single component EMLs of F8 and F8BT films were spin coated (SC) on pre-patterned glass/ITO substrates.

Fig. 1. Schematic representation of the structure of: (a) the OLED devices, and (b) the samples studied by SE and PL in this work.

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2.3 Characterization techniques

The films were characterized in terms of their optical properties using a phase modulated spectroscopic ellipsometer by Horiba/Jobin-Yvon. SE spectra were acquired at an angle of incidence of 70^ο, in the spectral range from 1.5 to 6.5 eV with a step of 20 meV. Photoluminescence (PL) and Electroluminescence (EL) measurements performed using the Hamamatsu absolute PL quantum yield measurement system (C9920-02) and the external quantum efficiency system (C9920-12), which measures brightness and light distribution of the devices.

2.4 SE modeling

SE is a non-destructive and surface sensitive technique useful to measure the dielectric function (ɛ(E)= ɛ₁(E) + iɛ₂(E)) of materials. A standard application of ellipsometry is the determination of the optical constants of a material either in the form of bulk sample or a thin film. In the case of a thin film in a single or multilayer structure the measured quantity by SE is the pseudodielectric function <ɛ(E)>, which also accounts the effect of the films’ thickness. The investigation of multi-layered samples is even more sophisticated, because the layer sequence of the sample must be implemented into the optical model for the simulation-fitting procedures. Thus, by applying the appropriate modeling procedures, even the layers’ thicknesses with sub-nanometer resolution in a sandwich multilayer structure can be calculated, together with the spectral dependence of the dielectric function, the refractive index and the absorption coefficient. Depending on the dispersion equations that are used for the description of the dielectric response of the films, the optical parameters such as the fundamental band gap, the absorption energies (optical gaps) and the strengths and broadenings of the characteristic absorption bands can be derived.

All the experimental spectra were fitted using Delta-Psi software, that comes with the experimental instrumentation, and by applying the Levenberg-Marquardt minimization algorithm. For the data analysis we used the appropriate number of Tauc-Lorentz (TL) oscillators [15,16] to describe the dielectric function ɛ(Ε) of the polymer films. The TL dispersion equation imposes that the imaginary part of the dielectric function to be equal to zero for energies lower than the energy band gap (E_g) of the material, while for energies E > E_g Eq. (1) is applied [16]. The real part of the TL dielectric function is obtained by Kramers-Kronig integration [16,17] (Eq. (3)) and its analytical expression is given in Ref. 16. The characteristic parameters of the TL dispersion model are the energy position of the E_g, the resonance energy E₀, the broadening C and the strength A of the oscillator, which describes the electronic transition. ɛ_∞ accounts the contribution of all the electronic transitions that take place at higher energies above the experimentally measured energy range and are not considered in the theoretically calculated ɛ₂(E).

(1)$${\varepsilon _2}(E) = \frac{{AE_0^{}\Gamma {{(E - {E_g})}^2}}}{{{{({E^2} - E_0^2)}^2} + \Gamma _{}^2{E^2}}} \cdot \frac{1}{E},\quad E > {E_g}$$

(2)$${\varepsilon _2}(E) = 0,\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \;E \le {E_g}$$

(3)$${\varepsilon _1}(E) = {\varepsilon _\infty } + \frac{2}{\pi }P\int_{{E_g}}^\infty {\frac{{\xi {\varepsilon _2}(\xi )}}{{{\xi ^2} - {E^2}}}} d\xi$$

(4)$$\Gamma \equiv \Gamma _j^{\prime}(E )= {\Gamma _j}\textrm{exp} \left( { - \frac{1}{{1 + {\sigma^2}}}{{\left( {\frac{{E - {E_j}}}{{\Gamma {}_j}}} \right)}^2}} \right)$$

Furthermore, in the case of organic materials electron–phonon coupling is of great importance [15]. Thus, for the best description of the optical response of amorphous organic thin films an energy-dependent broadening is employed in the TL oscillator model, which is given by the Eq. (4). In this equation, σ varies from zero to infinity and more specifically for σ=0 a Gaussian line shape is derived whereas, for σ>5 a Lorentzian broadening is established [15,18].

We have to note here that the E_g derived by the analysis isn’t adequately estimated since the TL model does not account the discrete absorption features below the bandgap, which can be associated with defects or polymer structural changes and disorder, contributing however with a relatively low absorbance [19,20]. Thus, for the precise estimation of the band gap the Tauc plot analysis is applied. The Tauc method is based on the assumption that the energy-dependent absorption coefficient α can be expressed for the case of direct transition band gap, by the following equation [21]:

(5)$${({a \cdot E} )^2} = B({E - E_g^{Tauc}} )$$

where, E is the photon’s energy, E_g^Tauc is the band gap energy, and B is a constant. In the resulting plot the distinct linear regime denotes the onset of absorption. Thus, by applying a linear fitting in this spectral region, the energy of the optical band gap of the material is obtained.

3. Results and discussions

3.1 Single component films

The dielectric functions of the SC single component films, F8 and F8BT, were derived by fitting the <ɛ(E)> measured by SE. The dispersion equations of 4-TL (F8) and 5-TL (F8BT) oscillators, in combination with the 5-phase model (glass/ITO/PEDOT:PSS/(F8 or F8BT)/air) were employed for the analysis. The real ɛ₁ and the imaginary ɛ₂ parts of the calculated complex ɛ using the best-fit parameters are plotted versus wavelength in Figs. 2 (a) and (b), respectively. By the arrows are denoted the peaks of the oscillators, E_0i^F8 and E_0i^F8BT, that are included within the experimentally measured wavelength (energy) range. For both of F8 and F8BT an additional oscillator at lower wavelengths (higher energies), above the upper experimental energy limit of 6,5 eV, was used. In Table 1 the E_0i and E_g^TL values are presented. In addition, the corresponding calculated Tauc gaps E_g^Tauc are also listed.

Fig. 2. Calculated bulk (a) real ɛ₁, and (b) imaginary ɛ₂ parts of the dielectric function ɛ vs wavelength of single component SC F8 and F8BT films.

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Table 1. SE results for the F8:F8BT blends.

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The band gap energy of a semiconductor describes the energy needed to excite an electron from the valence band to the conduction band. Thus, the accurate determination of the band gap energy is crucial in predicting photophysical and photochemical properties of semiconductors. Particularly, in the case of a photoactive material, its band gap defines the emission characteristic profile. Moreover, the comparison between the E_g^TL and E_g^Tauc values could provide qualitative information for the degree of disorder in the films’ structure.

The absorption coefficients of the F8 and F8BT, calculated by SE results, versus wavelength are presented in Fig. 3 (a). We can distinguish that the edge of the absorption is abrupt in both films. To obtain references of the photo- and electro-emission characteristics of F8 and F8BT as control materials and devices and additionally to verify the expected energy transfer from F8 to F8BT in their blends [5,22], PL measurements carried out on the F8 and F8BT films, as well as EL measurements were performed on the respective OLED devices, fabricated on glass substrates with the following structure: glass/ITO/PEDOT:PSS/(F8 or F8BT)/Ca/Ag. Figure 3 (b) depicts the normalized PL spectrum, measured with an excitation wavelength λ_exc=380 nm, and the normalized EL spectrum measured with an applied bias voltage V_b=10 V, of F8 film and OLED device, and the normalized absorption of the F8BT. Comparing both PL and EL emission spectra of F8 with the absorption spectrum of F8BT, we can distinguish an almost perfect spectral overlap between F8 blue emission, either PL or EL, and F8BT absorption. This allows efficient energy transfer upon photo or electrical excitation, which can be described by FRET theory. FRET is based on the dipole–dipole interaction between energy donor (D) and energy acceptor (A) [23]. In the F8:F8BT, F8 and F8BT are regarded as donor and acceptor, respectively. Highly efficient FRET yields to an emission spectrum that is essentially that of the pure F8BT. Indeed, this has been already reported in the literature for the case of spin-cast blends [5].

Fig. 3. (a) The absorption coefficient vs wavelength of the F8 and F8BT films calculated by SE results. (b) Normalized PL and EL spectra of F8 in comparison to normalized absorption of F8BT.

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3.2 Blended films

In general, blending the emissive material is a good strategy for improving OLEDs performance. However, many issues are addressed concerning the identification of the optimum parameters to achieve it. Phase separation may lead to undesirable non-radiative exciton recombination downgrade the device performance. This work deals with the investigation of the effect of the slot-die coated F8:F8BT blends’ composition on the optical and emission properties of films and devices.

3.2.1 Spectroscopic ellipsometry results

Figures 4 (a), (b) and (c) show the measured SE spectra from the blended F8:F8BT emitting layers formed via slot-die on PET/ITO/PEDOT:PSS. The symbols indicate the experimentally measured spectra and the lines indicate the simulated spectra using the best-fit parameters derived by the applied fitting procedure. The optical properties (bulk dielectric function ɛ) of the PET, ITO and PEDOT:PSS have been determined by the measurement and analysis of PET/ITO and PET/ITO/PEDOT:PSS samples, and they have been used as reference for the performed analysis of the slot-die blended films. The calculated thicknesses d of the F8:F8BT films are presented in Table 1. The E_g^TL, E_g^Tauc and E_0i values are also included in Table 1. We note here that 7-TL oscillators were used for the analysis of the blends with 1:1 and 1:19 ratios, whereas 5-TL oscillators for the 19:1.

Fig. 4. The measured SE spectra of the F8:F8BT films, grown on PET/ITO/PEDOT:PSS, (symbols) and the corresponding fitted ones (lines) with ratios: (a) 19:1 (b) 1:1, and (c) 1:19.

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Figures 5(a) and (b) show the calculated real ɛ₁ and the imaginary part ɛ₂ of the bulk dielectric function of the films, using the best-fit parameters. By the arrows are denoted the peaks of the oscillators that are included within the experimentally measured wavelength (energy) range, and they are listed in Table 1. We found that the characteristic absorptions of the blends can essentially be reproduced by the superposition of the absorptions of the single components with no evidence of additional bands due to ground-state interactions between F8 and F8BT. The assignment of the absorption peaks based on their energy position and in correlation to the peaks obtained by the single components F8 and F8BT (Fig. 2(b)) is presented in Table 1.

Fig. 5. The real (a) and imaginary (b) part of the dielectric function ɛ of the slot-die F8:F8BT blends, calculated using the best-fit parameters derived by SE analysis.

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The dielectric response of the F8:F8BT films well matched the weighted average. The differences between the films are evident and more pronounced at the higher wavelength (lower energy) range, above 310 nm (up to 4 eV). At this spectral range, the absorption starts, and the first electronic transitions take place. Thus, the fundamental differences between the blue-emitting F8 and the green-emitting F8BT are reflected on the optical response of both single component films and blended films. The values of the band gaps E_g^TL lie within the range defined by the respective values of F8BT (lower limit) and of F8 (upper limit). However, the E_g^Tauc gaps are identically for the F8 and 19:1 blend, and in the same manner for the F8BT, 1:1 and 1:19 blends. The reduction of the strength of the first electronic absorption E₀₁, assigned to the E₀₁^F8BT, follows the reduction of the F8BT content in the blends, whilst its strength is almost negligible for the 19:1. An inversely trend is derived for the E₀₂ which is assigned to the first electronic absorption of the F8 compound. The next three electronic absorptions E₀₃, E₀₄ and E₀₅ are significantly weaker and totally absent in 19:1 blend. Finally, the electronic absorptions E₀₆ and E₀₇ (not shown in graph), attributed to both F8 and F8BT compounds, exhibit similar strength between the blends, independently of their ratio.

Figure 6 illustrates the correlation between the energies of the E_g^TL and E_g^Tauc band gaps and the energies of the 1^st electronic absorption of the single components and the blends. It is easily to distinguish that the F8:F8BT blends of 1:1 and 1:19 ratios exhibit an almost similar optical response with that of F8BT in view of the characteristic energies with no significant energy shift. The effect of the relative content of F8 and F8BT in the blends is revealed only by the strength of the electronic absorption. On the other hand, the 19:1 blend has similar E_g^Tauc and E₀₁ values with that of F8, as this is the dominant phase. However, the presence of the F8BT phase in the blend strongly affects the E_g^TL gap by reducing it. We have already mentioned that the strength of the absorption peak at E₀₁^F8BT in this film is negligible. Thus, the F8BT phase in the 19:1 blend mainly configures its absorption edge.

Fig. 6. The band gaps E_g^TL determined by TL oscillator analysis of SE spectra, E_g^Tauc derived by Tauc plots and the resonance energy E₀₁ of the first electronic transition of the slot-die blended F8:F8BT and SC single component films.

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The absorption coefficients of the blended films, calculated by SE results, versus wavelength are presented in Fig. 7(a). In the same figure, the absorption coefficients of F8 and F8BT compounds have been plotted for completeness. To have a better view of the absorption edge, the absorption coefficient has been also plotted in logarithmic scale in the inset of the figure. The abrupt increase in the absorption of the 19:1 film follows that of the F8 and F8BT. In contrast, a low but non-negligible absorption is derived in the sub-bandgap wavelength region for the 1:1 and 1:19 films. Indeed, from the SE analysis we derived for the first electronic absorption in these films, a σ≈1,3 revealing a Gaussian line shape. This may correspond to disorder-induced and/or intermolecular charge transfer (CT) state absorptions of an exciplex-forming in mixed film [24,25]. On the contrary, the calculated σ values for all the other films studied in this work were remarkably higher, well above 5, revealing their Lorentzian broadening. In addition, it is highlighted that the peak features corresponding to the last two electronic absorption with E₀₆ and E₀₇ energies also exhibit a Gaussian line shape in all blends. These features were attributed to the superposition of the combined contribution of the electronic transitions taking place in F8 and F8BT compounds, since in the single components these are taking place in relatively close energy positions.

Fig. 7. (a) The absorption coefficient vs wavelength of the blended F8:F8BT films in comparison to single components F8 and F8BT films. In the inset figure are the absorption coefficient spectra in a logarithmic scale. (b) The normalized intensities of PL emissions measured from blended F8:F8BT films in comparison to the single component F8 and F8BT films at the excitation wavelength λ^exc=380 nm.

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3.2.2 Photoluminescence results

The PL spectra were recorded after exciting at 380 nm. Figure 7(b) shows the normalized intensities of PL emissions measured from blended F8:F8BT films in comparison to the single component F8 and F8BT films.

The PL spectra of F8 is dominated by 4 distinct peaks at 435, 457, 488 and 520 nm as they were calculated by the deconvolution fitting analysis. For the F8BT the featureless PL emission was deconvoluted by 3 peaks located at 544, 571 and 614 nm. Concerning the PL emissions obtained for the blends, these are clearly dominated by the emission of the F8BT component and only for the blend with the lower F8BT content (19:1) the characteristic emission peaks of F8 appeared, but with a significantly lower intensity. Α gradual blue-shift (denoted by the arrow in the figure) in the short-wavelength edge of the F8BT emission is obtained. This kind of wavelength shift has been attributed to the reduced self-absorption of F8BT since in the spectral range around 500 nm there is a relatively large overlap between emission and absorption [5]. However, in our results there are not observed red-shifted and broadened features, which are often assigned to inter-chain interactions resulting from the formation of excimers. It is evident from Fig. 7 (b) that only a blue-shift of the PL emission band of the blends is established whereas, their broadening is preserved, being identically to that of F8BT.

To get insights into the photoexcitation mechanisms, PL spectra were recorded in variable excitations. Figures 8 (a), (b) and (c) show the sequential recorded PL spectra, by using excitation wavelengths λ^exc from 320 to 420 nm with a step of 10 nm, of the 19:1, 1:1 and 1:19 F8:F8BT blended films, respectively. This wavelength range of excitation was selected because it corresponds to the maximum absorption of F8. As it has been already mentioned, the higher PL intensity was measured for the 19:1 blend being 2,385 (at 400 nm) following by that of 1:1 blend 1,798 (at 400 nm) and the lower that of 1:19 blend 497 (at 420 nm).

Fig. 8. The evolution of PL emission of blended F8:F8BT films with the ratio (a) 19:1, (b) 1:1 and (c) 1:19, with the excitation wavelength λ^exc.

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Figure 9 (a) depicts the evolution of the intensity of PL peak versus the applied excitation wavelength for the three blends. It is noticeable that we do not have a monotonic increase of the PL peak intensity with λ^exc. More specifically, the intensities of 19:1 and 1:1 blends exhibit similar dependence on λ^exc and a maximum value is derived for λ^exc=400 nm. For the 1:19 blend the dependence of the maximum PL peak intensity on the λ^exc is significantly weaker. The intensity of 1:1 takes higher values at the lower range of λ^exc up to 350 nm in comparison with that of the 19:1 and above the 350 nm this situation is reversed. Between 300–350 nm the absorption (see Fig. 7 (a)) of 1:1 is higher than that of 19:1 and above the 350 nm the strong absorption of the F8 component dominates the spectrum of the 19:1 blend. For wavelengths larger than 400 nm a remarkable drop in the absorption of the 1:1 and 19:1 blends takes place. Regarding the 1:19 blend the recorded intensity values of the PL peak are comparable with that of 19:1 at low λ^exc up to 330 nm. Nevertheless, the intensity of the PL emission of 1:19 remains low and a characteristic step is observed between 350 and 360 nm and after that a slow monotonic increase is reestablished. In the wavelength range 300–350 nm the absorption coefficient (see Fig. 7 (a)) of 1:19 blend is dominated by the high absorption of the F8BT component following by a drop and starts to increase again from 360 nm and above, due to the combined contribution of the individual absorption of both components. So, it is obvious that the intensity of the peak at 540–550 nm in PL spectra, is well correlated to the characteristics of the absorption coefficient spectra of the blends, calculated by SE results. The direct correlation between electronic absorption and PL emission efficiencies of the films, was derived by considering their absorption features and in addition the numerical data for the absorption coefficient.

Fig. 9. (a) The evolution of the intensity of the maximum PL peak of blended F8:F8BT films with the excitation wavelength λ^exc. (b) The dependence of the parameter ɛ^PL (accounting the ratio I_F8/I_TOTAL) on the excitation wavelength λ^exc.

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As it has been already stated, only in the 19:1 blend the characteristic F8 emission was detected. To quantify the degree of energy transfer between F8 and F8BT in this blend we calculated the relative intensity of the residual F8 emission appeared in PL spectrum within the wavelength range from 400 to 470 nm (I_F8) by integrating the blend emission spectrum and divided it by the total combined F8 and F8BT emission obtained by integrating between 400 and 700 nm (I_TOTAL) [5]. Parameter ɛ^PL accounts this ratio (ɛ^PL=I_F8/I_TOTAL). Figure 9 (b) depicts the results from this analysis. A significant drop in ɛ^PL takes place for the excitation range from 330 to 370 nm, and afterwards its low values ∼0,05–0,07, up to 420 nm, demonstrate the dominance of FRET.

3.2.3 Electroluminescence results

Figure 10 (a) displays the normalized intensities of EL emissions measured from the fabricated OLED devices containing the slot-die F8:F8BT blends and the single component SC F8 and F8BT films, as the emitting layers. Likewise, the PL spectra, the EL spectra of all blended devices are dominated by the F8BT emission with a less pronounced contribution of F8 emission. In the inset of the figure the EL spectra are plotted on a logarithmic scale to better show the weak F8 emission in the blends, which is totally absent in the spectrum of 1:19 blend. For the 19:1 and 1:1 blends the F8 emission appears in the spectra with a relative intensity in relation to the F8BT emission lower by two orders of magnitude. Additionally, the increase of the applied bias voltage did not increase the F8 emission against to that of the F8BT, contrary to what it has been reported previously [22,26,27]. This is verified by the calculation of the corresponding parameter ɛ^EL=I_F8/I_TOTAL. For all the bias voltages we derived ɛ^EL∼5·10⁻³ for both F8:F8BT blends.

Fig. 10. (a) The normalized intensities of EL emissions measured from blended F8:F8BT OLEDs in comparison to the single component F8 and F8BT OLEDs. In the inset figure are the EL spectra in a logarithmic scale. (b) The evolution of the intensity of the maximum EL peak of blended F8:F8BT OLEDs with the applied bias voltage.

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The evolution of EL peak intensity of the F8:F8BT devices with the bias voltage is shown in Fig. 10 (b). A gradual increase is observed in the peak intensity for the 19:1. A delayed and abrupt increase is observed in the peak intensity for the 1:1. Finally, the peak intensity for the 1:19 reaches a saturation with a final value lower by one order of magnitude comparing to the other two devices.

There are several reports on the differences between the EL and PL spectra derived from blends of conjugated polymers containing polyfluorene derivatives [5,28]. In general, PL emission originates from different species either via direct excitation or by energy transfer processes. Commonly, for the excitation are used wavelengths in the UV-A region, which are very close to the band gap of F8 [22]. As a result, the direct photoexcitation of F8 is more pronounced comparing to that of F8BT and the PL emission of F8BT occurs primarily because of energy transfer from the F8.

However, if FRET is the only process that controls the emission of the F8:F8BT devices, the relative intensities of the F8 and F8BT emissions would be similar in the EL and PL spectra of the blends. But this is not our case. In the 1:1 F8:F8BT blend, F8 emission is absent in PL whilst it appears in EL and moreover is comparable to that of 19:1. Thus, other processes contribute to the diode emission during the EL measurements. EL emission strongly depends on the charge injection from the electrodes, charge transport, exciton generation and finally the recombination process. Furthermore, it has been suggested that the charge injection in polymer blends preferentially favors the exciton formation in a polymer with the lower energy gap [22,29]. The significant reduction or even the elimination of the F8 emission in the EL spectra of the devices investigated in this work suggests that after the injection of electrons and holes into the device, either the excitons are primarily created on F8BT molecules or the recombination preferentially occurs in this polymer phase. Consequently, the smaller F8BT band gap comparing to that of F8 acts as a trap for the charge carriers, and this enhances the recombination of excitons in the F8BT.

The calculated ɛ^EL clearly reveals that the quenching of F8 emission is independent on the bias voltage. Thus, we can assume that the electrons injected from cathode are firstly trapped by F8BT units and combine with holes to form excitons at low voltage. As the bias voltage is increased F8 molecules trap the electrons. Thereupon, F8 transfers a large amount of its exciting energy to surrounding F8BT molecules by inter-chain FRET, resulting in nearly invisible blue emission of F8. In the 19:1 blend with the lower F8BT content the FRET mechanism is more effective possibly due to the higher dispersion of F8BT molecules around chain clusters, whereas in the 1:1 blend the higher amount of F8BT changes the size and shape of phases separating from this blend. Thus, we can speculate that the quenching of F8 emission obtained for 19:1 and 1:1 blends is highly dependent on the microstructure of the polymer blend. Effective FRET from F8 to F8BT takes place in the 19:1, but with a remaining blue emission due to high F8 content, and on the other hand less effective FRET from F8 to F8BT takes place in the 1:1 due to the more extensive phase separation in this blend composition.

3.3 Performance of slot-die OLED devices

The luminance–voltage characteristics of the fabricated OLED devices with the R2R slot-die coated PEDOT:PSS and F8:F8BT layers on PET/ITO are shown in Fig. 11, and the electrophysical parameters are summarized in Table 2. For completeness, the respective luminance–voltage characteristics of the SC F8 and F8BT devices are also included in the figure. The OLED 19:1 has a turn-on voltage of 3.6 V and maximum brightness of 4,025 cd m², demonstrating a fully functioning area of 1.0 × 1.2 cm², R2R compatible coated device. However, the luminous efficiency is lower than that of 1:1 device, which has the highest turn-on voltage. We should remind here that the 1:1 film had significant higher thickness (Table 1) comparing to other two films, and it has been reported that the thickness of emitting layer influences the electrical characteristics of OLEDs such as the turn-on voltage, resistance, power, and emission intensity [30]. 1:19 OLED exhibits inferior operational characteristics. Similar slot-die devices on a patterned glass substrate with 66 nm thick F8:F8BT (19:1) emitting layer and an emissive area of 0.045 cm² exhibited a turn-on voltage of 3.5 V, a maximum brightness of 357 cd m⁻², and a maximum current efficiency (CE) of 1.77 cd A⁻¹ [14].

Fig. 11. Luminance-voltage (L-V) plots of the fabricated slot-die F8:F8BT and SC F8 & F8BT OLED devices.

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Table 2. Summary of device performance for slot-die coating OLEDs based on F8:F8BT blend films.

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It has been reported that in F8:F8BT blends, the small amount up to 5% of F8BT in F8 host (corresponding ratio 19:1) significantly improve the efficiency of EL emission by presumably improving electron-injection and electron transport in the device. However, if the F8BT content exceeds the 5% the injection and transport of electrons becomes more efficient than that of holes, resulting to imbalanced charge fluxes within the devices. As a result, the device efficiency is reduced due to the reduced exciton generation efficiency [5]. The results of this work agree with that of literature referred to spin coated F8:F8BT OLEDs.

Figure 12 (a) displays the CIE chromaticity coordinates obtained for the slot-die F8:F8BT OLEDs as well as for the SC F8 and F8BT OLEDs. The inset presents a photograph of the 19:1 device operated at 10 V, from which the emission homogeneity is demonstrated.

Fig. 12. (a) The CIE color coordinates relative to the EL spectra of the three corresponding OLED devices with slot-die F8:F8BT blended emitting layers and pure F8 and F8BT SC OLED devices. In the inset, the photograph of 19:1 OLED device operating at 10 V. (b) CIE (x, y) stability diagram for increasing operation voltages for the 19:1, 1:1 and 1:19 F8:F8BT OLEDs. Horizontal dotted lines denote the respective (x, y) coordinates of F8BT OLED.

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In F8:F8BT blends, an amount of excitons from the higher-band-gap (F8) polymer converts to the one of lower band gap (F8BT). Part of excitons deactivate by radioactive transition to emit blue emission, while the remaining excitons deactivate by FRET to excite F8BT and emit green emission from F8BT. This leads to dual emission [22,26,27]. The fractions of this distinct emissions, which are related to the blends’ composition, define the final emitting color of the device. Moreover, a color shift from green to blue has been observed by increasing the bias voltage [22]. In our results, all the blend devices emit in the green color range close enough to the F8BT emission. The evolution of (x, y) coordinates with the applied voltage (above the V_on of each device) is shown in Fig. 12 (b). The horizontal dotted lines denote the respective (x, y) coordinates of the F8BT for comparison. The 19:1 exhibit good color stability in the whole voltage range. On the other hand, deviations in the chromaticity coordinates of the 1:19 and 1:1 devices’ emission noticed at low and high voltages, respectively. These results can be explained based on the distribution of F8 and F8BT phases in the blends. In the 19:1 the molecularly homogeneous dispersion of guest F8BT polymer without aggregation in the host F8 polymer, leads to less disorder microstructure, which also results from the sharp increase of the absorption coefficient (see Fig. 7 (a)), in contrast to the 1:1 and 1:19 in which absorption was observed below the fundamental gap revealing a possible disorder. Disordering can justify the high V_on values, especially for the case of 1:1, whereas can contribute to the lower luminance values measured for the 1:1 and 1:19 OLEDs.

The performances of the fabricated blended OLED devices using slot-die process for the development of PEDOT:PSS and F8:F8BT emitting layers are very promising. Thus, this work shows that large-area flexible OLEDs can be fabricated using slot-die coating techniques. The thorough investigation of the parameters of slot-die process to optimize R2R coating of OLED layers is a viable pathway towards large-scale manufacturing high-performance OLEDs [31].

4. Conclusions

In summary, we have herein studied the optical and emitting properties of polymeric F8:F8BT blends of 19:1, 1:1 and 1:19 ratios, produced by slot-die coating technique. Spin coated F8 and F8BT single component layers were used as control films and devices. The films were examined using spectroscopic ellipsometry, and the thickness, the dielectric function and the absorption coefficient were obtained. Precise determination of the band gaps and the absorption bands was accomplished by employing the dispersion equation of multiple modified Tauc-Lorentz oscillators with energy-dependent broadening in the fitting – modelling of the SE spectra. Depending on the average content of the F8 and F8BT in the blended films, different number of electronic transitions were identified and attributed to the respective transitions of the F8 and F8BT compounds. Furthermore, the characteristics of the absorption edge, identifying by the band gap, as well as the low absorption derived for energies below the band gap, reveal the disordering of the films. The SE results enabled the interpretation of the PL and EL emission data of the blended F8:F8BT films and the fabricated OLED devices. We have demonstrated the fabrication of flexible OLED devices, in which the PEDOT:PSS (HIL) and F8:F8BT (EML) layers formed using the R2R slot-die coating technique in ambient conditions. The green light emitting devices confirmed to be fully functional with 1.0 × 1.2 cm² active area, uniform light emission, good color stability with CIE coordinates of (0.37, 0.61), low-voltage operation (3.6 V) and luminance above 4,000 cd m⁻². Thereby, we believe that the optimization of the slot-die processing parameters, which will be realized with the aid of the analytical and comparative SE, PL and EL characterization, and the implementation of the appropriate encapsulation processes of the devices will reinforce the large-scale fabrication of slot-die flexible OLEDs with superior efficiency and functionalities.

Funding

European Regional Development Fund of the European Union and Greek national funds through the Operational Program Competitiveness, Entrepreneurship, and Innovation, under the call RESEARCH – CREATE – INNOVATE (project code: T1EDK-01039).

Acknowledgments

Author is thankful to Mr K. Stavrou for his support with the experiments and to Prof. S. Logothetidis for the support and for the fruitful discussions.

Disclosures

The author declares no conflicts of interest.

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	d (nm)	$E_{g}^{T L}$ $(e V)$	$E_{g}^{T a u c}$ $(e V)$	$E_{01}^{F 8 B T}$ $(e V)$	$E_{01}^{F 8}$ (eV)	$E_{02}^{F 8 B T}$ $(e V)$	$E_{03}^{F 8 B T}$ (eV)	$E_{02}^{F 8}$ (eV)	$E_{03}^{F 8}$ $E_{04}^{F 8 B T}$ (eV)	$E_{04}^{F 8}$ $E_{05}^{F 8 B T}$ (eV)
F8	32,5 ±1,1	2,85 ±0,01	3,05		3,11 ±0,01			5,21 ±0,01	5,44 ±0,14	8,23 ±3,71
F8BT	46,9 ±1,0	2,27 ±0,01	2,51	2,59 ±0,01		3,59 ±0,01	3,97 ±0,02		5,77 ±0,05	7,99 ±0,10
				$\begin{matrix} E_{01}^{F8BT} \\ ↕ \\ E_{01} \end{matrix}$	$\begin{matrix} E_{01}^{F8} \\ ↕ \\ E_{02} \end{matrix}$	$\begin{matrix} E_{02}^{F8BT} \\ ↕ \\ E_{03} \end{matrix}$	$\begin{matrix} E_{03}^{F8BT} \\ ↕ \\ E_{04} \end{matrix}$	$\begin{matrix} E_{02}^{F8} \\ ↕ \\ E_{05} \end{matrix}$	$\begin{matrix} E_{03}^{F8} \\ E_{05} \end{matrix}$ & $\begin{matrix} E_{04}^{F8BT} \\ ↕ \\ E_{06} \end{matrix}$	$\begin{matrix} E_{04}^{F8} \\ E_{05} \end{matrix}$ & $\begin{matrix} E_{05}^{F8BT} \\ ↕ \\ E_{07} \end{matrix}$
F8:F8BT	d (nm)	$E_{g}^{T L}$ $(e V)$	$E_{g}^{T a u c}$ $(e V)$	$E_{01}$ $(e V)$	$E_{02}$ (eV)	$E_{03}$ $(e V)$	$E_{04}$ (eV)	$E_{05}$ (eV)	$E_{06}$ (eV)	$E_{07}$ (eV)
19:1	31,0 ±0,3	2,44 ±0,06	3,02	2,49 ±0,01	3,14 ±0,01	-	-	5,02 ±0,20	5,63 ±0,03	7,00 ±0,23
1:1	53,0 ±0,4	2,23 ±0,03	2,50	2,59 ±0,01	3,12 ±0,01	3,66 ±0,03	3,89 ±0,07	5,09 ±0,30	5,58 ±0,01	7,63 ±1,01
1:19	35,2 ±0,5	2,28 ±0,01	2,49	2,56 ±0,01	3,13 ±0,02	3,66 ±0,01	3,90 ±0,03	4,60 ±0,15	5,62 ±0,03	7,39 ±0,49

F8:F8BT EML	Substrate	V_on ^a [V]	L_max ^b [cd m⁻²]	LE_max ^c [cd A⁻¹]	CIE x, y ^d
19:1	PET/ITO	3.6	4,025	0.42	0.37, 0.61
1:1	PET/ITO	7.5	1,446	0.58	0.40, 0.58
1:19	PET/ITO	5.7	301	0.20	0.35, 0.60

	d (nm)	$E_{g}^{T L}$ $(e V)$	$E_{g}^{T a u c}$ $(e V)$	$E_{01}^{F 8 B T}$ $(e V)$	$E_{01}^{F 8}$ (eV)	$E_{02}^{F 8 B T}$ $(e V)$	$E_{03}^{F 8 B T}$ (eV)	$E_{02}^{F 8}$ (eV)	$E_{03}^{F 8}$ $E_{04}^{F 8 B T}$ (eV)	$E_{04}^{F 8}$ $E_{05}^{F 8 B T}$ (eV)
F8	32,5 ±1,1	2,85 ±0,01	3,05		3,11 ±0,01			5,21 ±0,01	5,44 ±0,14	8,23 ±3,71
F8BT	46,9 ±1,0	2,27 ±0,01	2,51	2,59 ±0,01		3,59 ±0,01	3,97 ±0,02		5,77 ±0,05	7,99 ±0,10
				$\begin{matrix} E_{01}^{F8BT} \\ ↕ \\ E_{01} \end{matrix}$	$\begin{matrix} E_{01}^{F8} \\ ↕ \\ E_{02} \end{matrix}$	$\begin{matrix} E_{02}^{F8BT} \\ ↕ \\ E_{03} \end{matrix}$	$\begin{matrix} E_{03}^{F8BT} \\ ↕ \\ E_{04} \end{matrix}$	$\begin{matrix} E_{02}^{F8} \\ ↕ \\ E_{05} \end{matrix}$	$\begin{matrix} E_{03}^{F8} \\ E_{05} \end{matrix}$ & $\begin{matrix} E_{04}^{F8BT} \\ ↕ \\ E_{06} \end{matrix}$	$\begin{matrix} E_{04}^{F8} \\ E_{05} \end{matrix}$ & $\begin{matrix} E_{05}^{F8BT} \\ ↕ \\ E_{07} \end{matrix}$
F8:F8BT	d (nm)	$E_{g}^{T L}$ $(e V)$	$E_{g}^{T a u c}$ $(e V)$	$E_{01}$ $(e V)$	$E_{02}$ (eV)	$E_{03}$ $(e V)$	$E_{04}$ (eV)	$E_{05}$ (eV)	$E_{06}$ (eV)	$E_{07}$ (eV)
19:1	31,0 ±0,3	2,44 ±0,06	3,02	2,49 ±0,01	3,14 ±0,01	-	-	5,02 ±0,20	5,63 ±0,03	7,00 ±0,23
1:1	53,0 ±0,4	2,23 ±0,03	2,50	2,59 ±0,01	3,12 ±0,01	3,66 ±0,03	3,89 ±0,07	5,09 ±0,30	5,58 ±0,01	7,63 ±1,01
1:19	35,2 ±0,5	2,28 ±0,01	2,49	2,56 ±0,01	3,13 ±0,02	3,66 ±0,01	3,90 ±0,03	4,60 ±0,15	5,62 ±0,03	7,39 ±0,49

F8:F8BT EML	Substrate	V_on ^a [V]	L_max ^b [cd m⁻²]	LE_max ^c [cd A⁻¹]	CIE x, y ^d
19:1	PET/ITO	3.6	4,025	0.42	0.37, 0.61
1:1	PET/ITO	7.5	1,446	0.58	0.40, 0.58
1:19	PET/ITO	5.7	301	0.20	0.35, 0.60

Optical, photophysical, and electrooptical studies on slot-die polyfluorene-based flexible OLED devices

Abstract

1. Introduction

2. Materials and methods

2.1 Materials and ink formulation

2.2 OLED fabrication

2.3 Characterization techniques

2.4 SE modeling

3. Results and discussions

3.1 Single component films

3.2 Blended films

3.2.1 Spectroscopic ellipsometry results

3.2.2 Photoluminescence results

3.2.3 Electroluminescence results

3.3 Performance of slot-die OLED devices

4. Conclusions

Funding

Acknowledgments

Disclosures

References

Cited By

Figures (12)

Tables (2)

Equations (5)

Optical Materials Express