Synergistic effect on the efficiency of polymer light ...

JOURNAL OF APPLIED PHYSICS 108, 014503 共2010兲

Synergistic effect on the efficiency of polymer light-emitting diodes upon blending of two green-emitting polymers G. Bernardo,1 Q. Ferreira,1 G. Brotas,1 R. E. Di Paolo,2 A. Charas,1 and J. Morgado1,3 1

Instituto de Telecomunicações, Instituto Superior Técnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal 2 Centro de Química Estrutural, Instituto Superior Técnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal 3 Departamento de Engenharia Química e Biológica, Instituto Superior Técnico, Av. Rovisco Pais, P1049-001 Lisboa, Portugal

共Received 8 February 2010; accepted 26 May 2010; published online 2 July 2010兲 Light-emitting diodes based on blends of the two green-emitting polymers, poly共9,9dioctylfluorene-alt-benzothiadiazole兲, F8BT, and poly共9,9-dioctylfluorene-alt-bithiophene兲, F8T2, show efficiencies that lie in between those of the devices based on the neat polymers 共with a maximum efficiency of approximately 4 cd/A for the devices with magnesium cathodes based on F8BT兲, except for the blend with 5% by weight of F8T2, which is more efficient than the device based on neat F8BT 共a maximum efficiency of approximately 5 cd/A is obtained兲. In view of the lower photoluminescence efficiency of F8T2, we attribute this improvement to the improved hole transport brought about by F8T2, though is surprising that 5% by weight, is enough to significantly improve the charge balance within the emissive layer. A detailed photophysics study was carried out for the neat polymers and their blends and no clear evidence for energy transfer between the components was found. This unanticipated devices performance improvement points to the need of a deeper screening of available conjugated luminescent polymers. © 2010 American Institute of Physics. 关doi:10.1063/1.3456997兴 I. INTRODUCTION

Conjugated polymer blends have been widely explored for light-emitting diodes, LEDs, and other optoelectronics devices.1 In the case of LEDs applications, the use of blends aims to combine materials with complimentary charge transport ability and achieve either efficient energy transfer 共for monocolor devices, in which case we may combine a good transporting polymer and a good emitter兲 or inefficient energy transfer 共for instance to achieve white light emission兲. In addition, a dilution of the emissive component leads, in general, to improved radiative decay efficiency.2 Among such mixtures, those based on poly共9,9-dioctylfluorene兲, PFO, and poly共9,9-dioctylfluorene-alt-benzothiadiazole兲, F8BT, blends3–5 which combine complimentary charge transport abilities 共being the electron mobility higher in F8BT that in PFO while the reverse occurs for hole mobility6兲 and efficient energy transfer 共from PFO to F8BT兲, are a prototypical example. In particular, the blend consisting on 95% PFO and 5% F8BT 共by weight兲 was found to lead to particularly efficient LEDs, with a reported peak luminance efficiency of 14 cd/A.6 More recently, blends of PFO and poly共9,9dioctylfluorene-alt-bithiophene兲, F8T2, where an efficient energy transfer occurs from PFO to F8T2, were explored for LEDs.7 Peak luminance efficiencies up to 2.6 cd/A for a blend consisting of 1% F8T2 and 99% of PFO, by weight, were reported for LEDs with barium cathodes 共work function of 2.7 eV兲. In the same study, the authors also reported a peak luminance efficiency of 3.7 cd/A for similar devices based on the blend of 95% PFO and 5% F8BT. In view of the large number of conjugated luminescent 0021-8979/2010/108共1兲/014503/8/$30.00

polymers available, the search for specific polymer combinations has been guided by the complementary properties of the pure components, as proven for some successful results mentioned above. Here we report on a somewhat unexpected improvement in LEDs performance upon blending F8BT and F8T2. These polymers combine complementary charge transport ability, but they have no tendency, judging from their optical properties, to exhibit an efficient energy transfer process and the photoluminescence efficiency of F8T2 is much lower than that of F8BT. The reported solid-state photoluminescence 共PL兲 efficiency is ⌽PL = 0.65– 0.70 for F8BT 共Ref. 8兲 and much lower, ⌽PL = 0.21, for F8T2.7 Their charge carrier mobilities were previously reported. Time-of-flight studies on F8BT 共Ref. 6兲 showed that electron transport is dispersive, though being the fastest charge carriers, with mobility 共10−3 cm2 V−1 s−1 at 0.5 MV/cm兲 close to that of holes in many polyfluorenes. Hole transport is very poor, which is attributed to strong trapping. The electron mobility value, for F8BT, obtained in field-effect transistors, FETs, is ␮e = 5 ⫻ 10−3 cm2 V−1 s−1, which is comparable to both hole and electron mobilities, ␮h = 5 ⫻ 10−3 cm2 V−1 s−1 and ␮e = 6 ⫻ 10−3 cm2 V−1 s−1, respectively, reported for F8T2-based FETs.9 From the charge mobility point of view, the blending of F8T2 with F8BT could improve hole mobility when comparing with neat F8BT, while little difference is expected in the electron mobility 共assuming that in the blend the charges will drift across the fastest percolation path and neglecting the effect of disorder兲. In summary, the addition of F8T2 to F8BT would improve hole mobility but could have a detrimental effect on the LEDs efficiency due to F8T2 lower PL efficiency, al-

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though the final influence of this parameter depends on the F8T2 contribution to the blend emission. In turn, F8T2-based devices are expected to gain in performance upon addition of F8BT, but this performance would be expected to be below that of neat F8BT-based devices. In this study, we compare the performance of LEDs based on the neat polymers and their blends in 5% and 50% by weight. We observe that the addition of F8BT to F8T2 does lead to an improvement of device performance 共maximum luminance and efficiency兲. In addition, LEDs based on the blend consisting on 95% F8BT and 5% F8T2 show even higher efficiencies than the ones based on neat F8BT. The PL, electroluminescence 共EL兲, and picosecond time-resolved fluorescence of the blends indicate that there is no significant energy transfer between the two components nor detectable exciton quenching which could result from excited state charge transfer, which is consistent with the very poorly efficient charge generation in photovoltaic devices based on the 50% blend. The gathered results indicate that, in the blend, the two components behave almost like independent systems. This result points to the need to carry out a thorough screening of the available conjugated luminescent polymers, if LEDs with improved performance are in sight.

II. EXPERIMENTAL

F8BT was obtained from American dye source 共M w = 157 000, polydispersity of 3.0, as provided by ADS兲. F8T2 was synthesized by the Suzuki route 共M w = 38 500, polydispersity of 1.72, as determined by Gel permeation chromatography with respect to polystyrene standards兲.10 Solutions of the neat polymers and their blends were prepared in either chloroform or toluene. Blends composition ranges between either 95% weight of one component 共identified as 95BT5T2 and 5BT95T2兲 and 50% each 共50BT50T2兲. Absorption spectra were recorded with a Cecil 7200 spectrophotometer and PL and excitation spectra were recorded with a SPEX Fluorolog 2121 spectrofluorimeter, for films deposited on spectrosil substrates. LEDs, with an hole-injection layer of poly共3,4-ethylene dioxythiophene兲 doped with polystyrene sulphonic acid, PEDOT:PSS 共or PEDOT, from Bayer, approximately 45 nm thick兲 were prepared under ambient conditions and characterized under vacuum as reported in Ref. 11. Indium-tin oxide 共ITO兲 coated glass substrates were obtained from Technopartner. ITO thickness is 150 nm with a sheet resistance of 13/ 15 ⍀ / cm2. The thickness of the emissive layer is 75–80 nm, as determined with a Dektak 6M profilometer. Magnesium and aluminum were used as cathode materials, deposited at a base pressure of approximately 2 ⫻ 10−5 mbar. In spite of the presence of the PEDOT:PSS layer, we name these devices single-layer LEDs. Doublelayer devices, incorporating a hole-blocking/electrontransporting layer of 2-共4-biphenylyl兲-5-共4-tert-butylphenyl兲1,3,4-oxadiazole 共PBD兲, from Aldrich兴 and with Al cathodes, were also prepared. Thin films of PBD, 18 nm thick, were sublimed on top of the EL layer, prior to the aluminum cathode deposition. LEDs were tested under vacuum, up to the voltages required to achieve maximum luminance or current

values. EL spectra were obtained with a charge coupled device 共CCD兲 Oriel spectrograph. Atomic force microscopy 共AFM兲 measurements were performed with a Molecular Imaging system 共Model 5100兲 operating in noncontact mode using silicon probes with radii lower than 10 nm, at a frequency in the range 200–400 kHz. Picosecond time-resolved fluorescence studies were carried out in films using the time-correlated single-photon counting technique as described in Ref. 12. The pulsed excitation source, working with a repetition rate of 82 MHz, was a Ti:sapphire laser 共Spectra Physics Tsunami兲, with output wavelengths 870 or 880 nm, pumped by a solid-state laser 共Spectra Physics Millennia Xs兲. The output of the Tsunami was frequency-doubled by using a second harmonic generator crystal 共SHG 435 or 440 nm兲. In order to excite the samples, the laser beam 共SHG兲 was attenuated and focused to a spot about 1 mm2 in diameter on the films 共excitation average power of ⬃1 mW兲. Measurements were carried out by exciting the samples with vertically polarized light and the emission was collected at a 90° geometry, setting the emission polarizer 共analyzer兲 at the magic angle 共54.7°兲. The sample plane was adjusted at an angle of 40° with respect to the excitation laser beam. This arrangement prevents reflection of the excitation beam into the analyzer. Then, the fluorescence emission was passed through a monochromator and detected with a microchannel plate photomultiplier 共MCP-PT Hamamatsu R3809u-50兲. The electronics detection system also included a SPC-630 acquisition board 共Becker and Hickl, GmbH兲. Pulse and fluorescence of the sample were collected until 5000 counts had been accumulated at the maximum of the signal. The experimental excitation pulse profile full width at half maximum 共FWHM= 19 ps兲 was measured by using a scattering solution. The fluorescence signals were deconvoluted from the excitation pulse using the George Striker’s program. The time resolution of the system was 3 ps. III. RESULTS A. Photophysical characterization

Figure 1共a兲 shows absorption and PL spectra recorded for the films of neat F8BT and F8T2 and their blends. Figure 1共b兲 shows the corresponding excitation spectra. The absorption spectra of the blends films are well described as a combination of the spectra of the neat films. In particular, the absorption at approximately 320 nm 共high energy absorption band of F8BT兲 is well correlated with F8BT content. PL spectra of the blends are in between those of the neat polymers 共with maximum at 552 nm for F8BT and at 580 nm, with a peak at approximately 558 nm, for F8T2兲, following a trend that correlates with the blend composition. There is a continuous broadening upon increase in F8T2 content, with the spectra tail at the longer wavelengths evidencing a continuous redshift upon increase in F8T2 content, while the shorter wavelength tail 共which is affected by selfabsorption兲 shows little dependence on the composition. From the absorption onset for neat F8BT 共521 nm兲 and F8T2 共515.7 nm兲 films, we estimate the optical energy gap values to be very similar: 2.39 eV for F8BT and 2.41 eV for F8T2.


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FIG. 1. 共Color online兲共a兲 Absorption and normalized PL spectra of films 共65 nm thick兲 of F8BT, F8T2, and their blends on spectrosil. PL spectra were obtained upon excitation at the absorption maximum. 共b兲 Excitation spectra for all films were recorded upon reading the emission at 570 nm.

The excitation spectra resemble the corresponding absorption spectra, except for a higher contribution at shorter wavelength. The excitation spectrum of neat F8BT peaks at about 323 and approximately 444 nm, while the absorption peaks occur at 323 and 466 nm. The excitation spectrum of F8T2 has a maximum at approximately 436 nm, while the absorption peaks at about 454 nm. The blends with 5% content show an excitation spectrum similar to that of the major component, and the 50% blend’s spectrum lies in between the two extreme cases. The fluorescence decay of the films, measured at 550 nm, following excitation at 444 nm, was fitted with sum of exponentials. Table I summarizes the obtained lifetime components and the calculated average lifetime, 具 ␶ 典 = 兺 a i␶ i . i

Figure 2 shows the fluorescence decay for neat F8BT and for the 50% blend.

FIG. 2. 共Color online兲 Fluorescence decays of 共a兲 F8BT and 共b兲 50BT50T2 films, obtained upon excitation at 444 nm and reading at 550 nm, and the fitting according to the parameters shown in Table I. Weighted residuals 共WR兲 and autocorrelation functions 共AC兲 are also represented. This data was obtained using a time/channel relation of 6 ps/channel. 共c兲 Dependence of the average lifetime on the F8T2 content.

Table I and Fig. 2 show that the fluorescence decay is multiexponential and that the average lifetime, being higher for F8BT, decreases in the blends as the F8BT content decreases. As evidenced in Fig. 2共c兲 such decrease in the average lifetime is not linear with the composition. The 50BT50T2 blend was further investigated by measuring the fluorescence decays at the onset 共520 nm兲, center 共570 nm兲, and at the band tail 共620 nm兲 of the PL spectrum,

TABLE I. Fitting parameters 关fluorescence decay times 共␶i兲 and pre-exponential coefficients 共ai兲兴 of the measured decays of the films at 550 nm and the calculated average lifetimes 具␶典. The goodness of the fits is given by the chi-square value 共␹2兲.

Polymer F8BT 95BT5T2 50BT50T2 5BT95T2 F8T2

␶1 共ns兲

a1

␶2 共ns兲

a2

␶3 共ns兲

a3

␹2

具␶典共ns兲

¯ ¯ 0.104 0.041 0.020

¯ ¯ 0.44 0.38 0.52

0.603 0.554 0.375 0.206 0.121

0.18 0.49 0.44 0.47 0.35

1.340 1.318 1.212 0.586 0.398

0.82 0.51 0.12 0.15 0.14

1.09 1.03 0.99 0.99 1.00

1.207 0.944 0.356 0.200 0.109


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TABLE II. Global fitting parameters of the fluorescence decay of 50% blend and neat F8T2 films, collecting the emissions at various wavelengths, with excitation at 435 nm. 50BT50T2 ␭ 共nm兲

␶1 共ns兲

520 570 620

0.01

0.43 ⫺0.26 ⫺0.74

0.06

0.40 0.57 0.51

520 620

0.01

0.59 ⫺0.96

0.03

0.34 0.57

a1

␶2 共ns兲

␶3 共ns兲

a2

a3

␶4 共ns兲

a4

␹2

0.23

0.15 0.37 0.39

1.02

0.02 0.06 0.10

0.94 1.09 1.05

0.11

0.07 0.35

0.54

0.003 0.08

0.99 1.04

F8T2

using lower time/channel relation 共in particular, using 3.0 ps/channel, while in Fig. 2, we have used 6.0 ps/channel兲. The global fit to the three decays 共see Table II兲 shows that there is a short component 共10 ps兲 which appears as a decay time at the band onset, but as a rise time at longer emission wavelength, which points to the existence of energy transfer from high to lower energy sites.13 We note that conformational relaxation has been shown to lead to a similar signature in solution. However, in solid matrices, such relaxation process is frozen.12 To clarify the nature of this energy transfer process, similar studies were also carried out for films of neat F8T2. Global fits to the fluorescence decay at 520 and 620 nm showed the presence of the same short lifetime component 共see Table II兲. This means that the component associated to the energy transfer identified in the blend is most likely related to the energy transfer within the F8T2 domains and not between F8BT and F8T2. This is also consistent with the strong overlap of the absorption and also of the PL spectra of the two blend components, which does not promote a significant energy transfer between them. B. AFM characterization

The device based on neat F8BT reaches a maximum luminance of 11 400 cd/ m2 at 7.5 V, with a light-onset voltage 共corresponding to a minimum luminance of 0.01 cd/ m2兲 of 2.5 V. The peak luminous efficiency is 3.89 cd/A and the peak power efficiency is 2.46 lum/W. The F8T2-based device shows much lower luminance 共maximum value of 155 cd/ m2兲 and efficiency 共0.0075 cd/A and 0.002 lum/W兲 but similar light-onset voltage. The addition of F8BT to F8T2 clearly improves the maximum luminance and efficiency of the LEDs when comparing with neat F8T2. This improvement is particularly impressive when passing from 5% to 50% content of F8BT. Surprisingly, the blend with 95%F8BT shows a maximum luminance of 12 200 cd/ m2 at 8 V and a peak EL efficiency of 4.81 cd/A and 2.85 lum/W, which surpasses that of the LEDs based on neat F8BT. It is worth noting that the current flowing through the devices is higher for the 50BT50T2 blend and lower for the blends with just 5% of the second component. While the higher current for the 50% blend is consistent with a complimentary charge transport ability, the lower current observed for the 5% blends points to an increased effect of disorder and/or charge trapping. The EL spectra show a monotonic variation with composition, indicating that both materials emit, being their con-

AFM studies were carried out to assess the phase separation in these blends. The surface topography of the blends films is similar to that of the films based on the neat components, with similar surface roughness 共Rrms values are 1.36 nm for F8BT, 1.42 nm for 95BT5T2, 1.02 nm for 50BT50T2, 1.20 nm for 5BT95T2, and 1.45 nm for F8T2兲. Furthermore, the phase images fail to discriminate the two components. Figure 3 compares the AFM topography and phase for neat F8BT and 95BT5T2 films. Judging from these results, we conclude that either the two components are compatible, or the solvent 共toluene兲 evaporation is too fast to allow a significant phase separation or the two polymers show similar viscoelastic behavior and cannot be discriminated by the AFM phase imaging. C. LEDs

Figure 4 shows the characteristics of the LEDs based on the neat polymers and their blends, where the electroluminescent layer 共pure polymers and blends兲 was deposited from toluene solutions, and magnesium is the cathode material.

FIG. 3. 共Color online兲 AFM topography and phase images of films of neat F8BT and of the 95BT5T2 blend over a scanned area of 1 ⫻ 1 ␮m2.


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FIG. 4. 共a兲 Current and luminance and 共b兲 EL efficiency as a function of the applied voltage for the Mg-based devices prepared from toluene solutions; and 共c兲 the corresponding EL spectra.

tribution to the total emission proportional to the blend composition. In particular, as observed for the PL spectra, the increase in F8T2 leads to a broadening of the EL spectra, due to a redshift of the longer wavelength tail of the spectra, reaching neat F8T2 EL spectrum. LEDs prepared from chloroform solutions show a similar variation in performance with the blend composition. In

particular, the maximum efficiency obtained for neat F8BT is 3.97 cd/A and 2.53 lum/W, which increases to 5.09 cd/A and 3.06 lum/W for the blend with 5% by weight content of F8T2. EL spectra show also a monotonic dependence on the composition, indicating a weighted combination of the EL spectra of the two neat polymers. If aluminum cathodes are used instead of magnesium, we observe a lower current and luminance values, as commonly observed due to Al lower work function and the concomitant increase of electron-injection barrier. Table III compares the results obtained for Al-based devices, prepared from either toluene or chloroform solutions, for the various compositions. As observed for the Mg-based devices, the addition of F8BT leads to significant improvements in efficiency and luminance when comparing with LEDs based on neat F8T2. The improvement, even with respect to neat F8BT, upon blending with 5% content of F8T2, is very significant 共the efficiency nearly doubles兲. For this blend, the peak EL efficiency reaches values of 4.53 cd/A and 2.6 lum/W, which, surprisingly, are very closer to those of the similar devices with Mg cathodes, in spite of the increase in the electron injection barrier 共by 0.5 eV兲. EL spectra are similar to those obtained for the Mg-based devices. The use of chloroform, with lower boiling temperature than toluene, which could affect phase separation, appears not to have a clear effect on device performance. To obtain further insight into the influence of the blends composition, we prepared also double-layer devices with an 18 nm thick hole-blocking/electron-transporting layer of PBD 关2-共4-biphenylyl兲-5-共4-tert-butylphenyl兲-1,3,4oxadiazole, 共ionization potential, Ip= 6.3 eV, and electron affinity EA= 2.6 eV 共Ref. 14兲兴 and with Al cathodes. Figure 5 compares the characteristics of these devices as a function of the EL layer composition. The variation in the EL spectra, as noted for the above devices, is in direct correlation with the blend composition. The insertion of the PBD layer into the LEDs with Al cathodes leads to a similar or higher current. The light-onset voltage, being in the range 2.5–3 V, is similar to that of the single-layer devices 共without PBD兲. The luminance values at 8 V for all devices are higher than those observed for the corresponding single-layer devices. The maximum luminance for the F8BT, 95BT5T2, and 50BT50T2-based devices with PBD, are similar to those of single-layer Mg-based devices mentioned above, but they are attained at higher volt-

TABLE III. Maximum luminance and peak efficiencies 关␩L共cd/ A兲 and ␩P共lum/ W兲兴 of the devices prepared with Al cathodes from F8T2, F8BT, and their blends from either toluene or chloroform solutions. Peak ␩L 共cd/A兲

Lmax 共@8 V兲共cd/ m2兲 EL layer F8BT 95BT5T2 50BT50T2 5BT95T2 F8T2

Peak ␩P 共lum/W兲

Toluene

Chloroform

Toluene

Chloroform

Toluene

Chloroform

1327 1576 155 120 77 共@7 V兲

663 1576 155 112 51 共@7 V兲

2.28 4.53 0.22 0.08 0.08

2.35 4.53 0.22 0.06 0.04

1.41 2.59 0.17 0.03 0.04

1.45 2.60 0.17 0.03 0.02


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FIG. 5. 共a兲 Current and luminance and 共b兲 EL efficiency as a function of the applied voltage for the devices prepared from toluene solutions and having a PBD layer and Al cathodes.

ages. For the other devices, based on neat F8T2 and 5BT95T2, the insertion of the PBD into the Al-based devices leads to the highest luminance values, even above those of the devices with Mg cathodes. As observed for the above discussed device structures, there is a continuous increase in the efficiency upon addition of F8BT to F8T2 and, once more, the device with 95BT5T2/ PBD exhibits the maximum peak luminous efficiency of 3.3 cd/A, compared with the peak value for neat F8BT device of 2.88 cd/A. In terms of power efficiency, we find that the peak value of the 95BT5T2/PBD device, 1.30 lum/W, is, however, slightly lower than the value of 1.33 lum/W for neat F8BT, mainly reflecting the increase in the driving voltage. IV. DISCUSSION

The optical properties data 共absorption, emission, and excitation兲 show that, in the blends, the polymers behave as 共essentially兲 independent systems. The emission spectra indicate that the blends emission can be described as a combination of each component’s emission, with contributions that are in direct correlation with the blends composition. The picosecond-resolved fluorescence decays of the blends films show a multiexponential process, with an average decay time that decreases with F8T2 content increase, from neat F8BT down to neat F8T2, in agreement with the shorter decay time of F8T2. The average decay time of the blends does not vary linearly with the F8T2 content. With time/channel relation of 6 ps/channel no evidence for energy transfer was found. When using 3 ps/channel, a decay component of 10 ps was found and associated to energy transfer.

J. Appl. Phys. 108, 014503 共2010兲

However, as the same component is found in neat F8T2 films, we consider that this component is related to energy transfer within F8T2 domains 共related to energy migration to lower energy sites兲. Therefore, the combination of steady state and time-resolved fluorescence studies provides no evidence for energy transfer between the two components of the blends. This result is consistent with the absence of significant spectral overlap between the absorption of one component and the PL spectrum of the other, this overlap being a required condition for efficient resonant energy transfer from an energy donor to an energy acceptor. The data on Fig. 2共c兲 indicate a fast decrease in the average decay time upon addition of increasing contents of F8T2 to F8BT. Therefore, energy transfer from F8BT to F8T2 is not an obvious explanation for the reduction in the average fluorescence lifetimes of the blends with respect to a linear variation with the blend composition 关see Fig. 2共c兲兴. A likely explanation for fast decrease in the average decay time, determined upon excitation at 444 nm, relates with the relative absorption of the two components at this wavelength. As shown in Fig. 1共a兲, the absorption of F8T2 is nearly twice that of F8BT. If the number of excitons created within F8T2 is nearly twice that of the excitons created in F8BT, then, for a given composition, the contribution of the faster decay time 共F8T2兲 is much higher, leading to a decrease in the average decay time expected if the same number of excitons was created within the two domains and if they would decay independently. AFM studies of the films surface provide no evidence for phase separation between the two blend components; thereby we conclude they are very well intermixed. All the blends device types, even those with a PBD holeblocking/electron transport layer, exhibit an efficiency that is in between those of the similar devices based on the two neat components 共F8TBT and F8T2兲, the same happening for the maximum luminance, with the F8BT-based devices showing the highest luminance and luminous efficiency. Only the devices based on the blend with 95% F8BT and 5% F8T2 are outside this trend, showing higher luminous efficiencies and, in general, higher maximum luminance values. In terms of power efficiencies, the same peak values are found for the single layer devices, while for the PBD-containing devices, the power efficiency is similar to 共or slightly lower than兲 that of neat F8BT-based devices. The EL spectra, similar to the PL, show a weighted contribution of the two components emission, pointing to a contribution that is roughly weighted by the components concentration. In view of the lower PL efficiency of F8T2 and, as far as we can conclude, in the absence of energy transfer, this improved performance of the devices with the 95BT5T2 blend can only be attributed to an improved balance of charge carriers 共which leads to more efficient exciton formation兲 or, possibly, decrease in exciton quenching. Being blends, and as we find no evidence for vertical phase separation, we consider that the exciton quenching by the electrodes is not a factor that could influence the results. The possibility of having excited state charge transfer is not considered relevant. Should this occur, there should be a significant reduction in decay times for the blends, but these


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FIG. 6. Frontiers levels energy of the components used in the LEDs, where “ ⴱ” denotes the reported EA values for the polymers estimated using their optical energy gap and assumed values of the exciton binding energy.

never become lower than that of neat F8T2. In addition, photovoltaics fabricated with the 50% blend 共ITO/ PEDOT:PSS/50BT50T2/LiF/Al兲, showed a very low quantum efficiency 关below 0.01% at absorption maximum 共450 nm兲兴. Therefore, the remaining explanation for the improved performance of the devices based on the 95BT5T2 blend relies on the effect of charge balance. As mentioned above, in terms of charge mobility in the neat polymers, we find that, in terms of electron mobility, this is not much different for the two polymers, but hole mobility in F8T2 is higher. Therefore, in a blend, there would be a better electron/hole mobility balance than in neat F8BT. In addition, the best performance is shown for the devices with just a minor content 共5%兲 of F8T2, which allows for most excitons to decay within F8BT domains, with higher luminescence yield. This appears to be a balanced composition in terms of these two factors determining LEDs performance. As mentioned above, a similar improvement of LEDs performance is obtained when PFO and F8BT are blended, but in this case an efficient energy transfer from PFO to F8BT occurs.3 In a recent work by Bradley and co-workers,7 blends of PFO with 1% F8T2, where an efficient energy transfer also occurs from PFO to F8T2, showed higher peak luminance efficiencies than the pure components. However, there is not a monotonic variation in the efficiency with composition. Our case differs in the absence of energy transfer between the two components. The spectral overlap between the emission spectrum of one component and the absorption of the other is not much different from the spectral overlap between absorption and emission of each pure component, which supports that observation. Furthermore, while in the PFO/F8BT 共Refs. 3 and 15兲 and PFO/F8T2 共Ref. 7兲 blends there is evidence for strong phase separation, we could not find evidence for such phase separation in the F8BT/F8T2 blends. The above observations on the properties of F8BT/F8T2 blends are not easily rationalized in terms of the relevant frontier energy levels diagram, considering the singleelectron levels 共Fig. 6兲. The determination of the energetic position of the frontier levels of conjugated polymers still remains a mater of debate. The simplest technique to assess those energies is

J. Appl. Phys. 108, 014503 共2010兲

cyclic voltammetry. In most cases, cyclic voltammetry 共CV兲 provides a value for the ionization potential 共Ip兲 and for the electron affinity 共EA兲. However, it is also common to use the Ip value obtained from CV 共in particular when the reduction peak is not detected兲 and subtract both the optical energy gap 共estimated from the absorption onset兲 and the exciton binding energy, which is commonly taken as a value between 0.3 and 0.5 eV, to obtain EA. In Fig. 6 we compare the single-electron frontier levels energy of the components used in the devices: the work function of PEDOT:PSS 共taken from Ref. 16兲 and of the two cathode metals, and the ionization potential 共Ip兲 and electron affinity 共EA兲 for the two polymers. The lowest EA values were estimated from CV studies, while the higher EA values 共identified in Fig. 6 by ⴱ兲 were estimated from Ip and optical data 共energy gap兲. From CV studies, the estimated values of Ip and EA for F8BT are 5.9 eV and 2.4 eV, respectively,17 and for F8T2 they are 5.5 eV and 2.6 eV, respectively.18 In Ref. 6, the same value of Ip共F8BT兲 = 5.9 eV is reported, while EA= 3.2 eV, which was obtained from Ip minus the optical gap, Eg= 2.4 eV, and an assumed exciton binding energy of 0.3 eV. Reported levels for F8T2 共Ref. 7兲 are Ip= 5.5 eV 共the same value we obtained from CV兲 and EA= 3.1 eV 共which, by considering Eg= 2.4 eV, corresponds to a negligible exciton binding energy兲. If a similar exciton binding energy of 0.3 eV should be considered also for F8T2, than EA should be 2.8 eV instead of 3.1 eV. Calculations for F8BT 共Ref. 19兲 have shown that the single electron levels, highest occupied molecular orbital 共HOMO兲共related to Ip兲 and lowest unoccupied molecular orbital 共LUMO兲共related to EA兲, have different atomic contributions. In particular, it was shown that HOMO is delocalized between both comonomers 共F8 and BT兲, while the LUMO is localized on the BT unit. In addition, the exciton is also mostly localized on the BT unit. Therefore the excitonic levels are essentially localized on BT. Based on these observations, the estimation of EA from Ip using Eg and the exciton binding energy in this case is probably not adequate. In view of the above, we propose to use the energy values, Ip and EA, obtained from CV as the most appropriate approach for the single-electron frontier levels. In terms of the HOMO and LUMO involved in the excitonic levels, being 2.4 eV apart, they should be within the electrochemical energy gap 共Eg, c = Ip− EA兲. In the case of F8T2, they would be symmetrically located with respect to Ip and EA, and the difference between Eg and Eg,c is 0.5 eV. In the case of F8BT, as Ip refers to a molecular orbital with atomic contributions different from those involved in the excitonic levels 共as mentioned above兲, these should not be symmetrically located within the Eg,c, but closer to EA. The position of the excitonic levels within F8BT and F8T2 is such that their separation is similar 共approximately 2.4 eV= Eg兲, thereby explaining the absence of significant energy transfer between the two components. In addition, the mismatch between the levels should not be much higher than 0.3 eV, as we found no evidence for excited state charge transfer. A remark should be made that, as pointed out by Kippelen and Brédas,20 this analysis in terms of the prospects for efficient excited state


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Bernardo et al.

charge transfer by considering this type of diagrams is probably too simplistic, but they constitute a first order approach to assess that possibility. Another remark, arising when correlating the energy diagram shown in Fig. 6 and the observed behavior of the LEDs, is that, judging for the relative Ip and EA values of F8T2 and F8BT, electrons and holes would be mostly trapped at F8T2 sites. This is particularly relevant for hole trapping, as the difference in Ip values is around 0.4 eV, while the difference in EA is minor. Hence, in the absence of energy transfer, we would anticipate that excitons should mostly form and decay within F8T2 domains. This is hard to reconcile with the EL efficiency increase observed for the LEDs based on the 95BT5T2 blend, with respect to LEDs based on pure F8BT. Instead, this performance improvement implies a negligible effect of charge trapping at F8T2 sites. Without disregarding the uncertainties on the determination of Ip and EA values 共though a good agreement exists with respect to reported Ip values, as discussed above兲, the results here presented evidence some uncertainty of anticipating device characteristics and performance if drawn from these diagrams alone. V. CONCLUSIONS

In summary we presented a detailed study of F8BT/ F8T2 blends and the performance of LEDs based on them. In particular we found that the blend with 95% F8BT and 5% F8T2 共by weight兲 leads to particularly efficient devices, either single layer or double layer when a hole-blocking/ electron transporting layer of PBD is used, regardless of the metal cathode used 共Al or Mg兲. We note that the reported PL efficiencies for the neat polymers are very different, being that of F8BT more than three times that of F8T2. We conclude that the higher hole mobility of F8T2 共with respect to F8BT兲 is the likely explanation for the increase in LEDs performance. Still, it is surprising that 5% by weight of F8T2 have a significant effect in terms of hole transport and, therefore, in terms of the hole/electron balance. Based on time-resolved fluorescence studies, the variation in PL and EL spectra with composition and preliminary photovoltaic studies, we have no convincing evidence for the existence of energy transfer between the two components or for excited state charge transfer.

We consider that the observed improvements of the EL efficiency could not be anticipated, in view of the properties of the neat components. These results point to the need to carry out a careful screening of polymer blends when aiming at improving LEDs performance. ACKNOWLEDGMENTS

We thank A. L. Maçanita for helpful discussions and FCT-Portugal 共under the Contract Nos. PTDC/QUI/65474/ 2006 and PTDC/FIS/72831/2006兲 for financial support. G.B. thanks FCT for a postdoctoral research grant 共SFRH/BPD/ 38169/2007兲. C. R. McNeill and N. C. Greenham, Adv. Mater. 共Weinheim, Ger.兲 21, 3840 共2009兲. A. Charas, J. Morgado, J. M. G. Martinho, A. Fedorov, L. Alcácer, and F. Cacialli, J. Mater. Chem. 12, 3523 共2002兲. 3 J. Morgado, E. Moons, R. H. Friend, and F. Cacialli, Adv. Mater. 共Weinheim, Ger.兲 13, 810 共2001兲. 4 C. I. Wilkinson, D. G. Lidzey, L. C. Palilis, R. B. Fletcher, S. J. Martin, X. H. Wang, and D. D. C. Bradley, Appl. Phys. Lett. 79, 171 共2001兲. 5 J. Morgado, R. H. Friend, and F. Cacialli, Appl. Phys. Lett. 80, 2436 共2002兲. 6 A. J. Campbell, D. D. C. Bradley, and H. Antoniadis, Appl. Phys. Lett. 79, 2133 共2001兲. 7 P. A. Levermore, R. Jin, X. Wang, J. C. De Mello, and D. D. C. Bradley, Adv. Funct. Mater. 19, 950 共2009兲. 8 J.-S. Kim, R. H. Friend, I. Grizzi, and J. H. Burroughes, Appl. Phys. Lett. 87, 023506 共2005兲. 9 L.-L. Chua, J. Zaumseil, J.-F. Chang, E. C.-W. Ou, P. K.-H. Ho, H. Sirringhaus, and R. H. Friend, Nature 共London兲 434, 194 共2005兲. 10 A. Charas, L. Alcácer, A. Pimentel, J. P. Conde, and J. Morgado, Chem. Phys. Lett. 455, 189 共2008兲. 11 J. Morgado, A. Charas, J. A. Fernandes, I. S. Gonçalves, L. D. Carlos, and L. Alcácer, J. Phys. D: Appl. Phys. 39, 3582 共2006兲. 12 R. E. Di Paolo, H. D. Burrows, J. Morgado, and A. L. Maçanita, ChemPhysChem 10, 448 共2009兲. 13 F. B. Dias, J. Morgado, A. L. Maçanita, F. P. da Costa, H. D. Burrows, and A. P. Monkman, Macromolecules 39, 5854 共2006兲. 14 J. Kalinowski, M. Cocchi, P. Di Marco, W. Stampor, G. Giro, and V. Fattori, J. Phys. D: Appl. Phys. 33, 2379 共2000兲. 15 J. Morgado, E. Moons, R. H. Friend, and F. Cacialli, Synth. Met. 124, 63 共2001兲. 16 T. M. Brown, J. S. Kim, R. H. Friend, F. Cacialli, R. Daik, and W. J. Feast, Appl. Phys. Lett. 75, 1679 共1999兲. 17 A. Charas, H. Alves, J. M. G. Martinho, L. Alcácer, O. Fenwick, F. Cacialli, and J. Morgado, Synth. Met. 158, 643 共2008兲. 18 G. Bernardo, A. Charas, L. Alcácer, and J. Morgado, J. Appl. Phys. 103, 084510 共2008兲. 19 J. Cornil, I. Gueli, A. Dkhissi, J. C. Sanch-Garcia, E. Hennebicq, J. P. Calbert, V. Lemaur, D. Beljonne, and J.-L. Brédas, J. Chem. Phys. 118, 6615 共2003兲. 20 B. Kippelen and J.-L. Brédas, Energy Environ. Sci. 2, 251 共2009兲. 1

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