Highly Efficient Energy Transfer in Light Emissive Poly(9,9

Nov 17, 2017 - Centre for Micro and Nano Devices, COMSATS Institute of Information Technology, Park Road, Islamabad, 44000, Pakistan. ∥ Department o...
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Article Cite This: ACS Photonics XXXX, XXX, XXX−XXX

Highly Efficient Energy Transfer in Light Emissive Poly(9,9dioctylfluorene) and Poly(p‑phenylenevinylene) Blend System Muhammad Umair Hassaan,*,†,‡,§ Yee-Chen Liu,† Kamran ul Hasan,∥ Mohsin Rafique,⊥ Ali K. Yetisen,# Haider Butt,‡ and Richard Henry Friend† †

Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom School of Engineering, University of Birmingham, Birmingham, B15 2TT, United Kingdom § Centre for Micro and Nano Devices, COMSATS Institute of Information Technology, Park Road, Islamabad, 44000, Pakistan ∥ Department of Science and Technology, Campus Norrköping, Linköping University, Bredgatan 34, SE-601 74 Norrköping, Sweden ⊥ Xue Laboratory, Center for Quantum Science and Technology, Beijing Shi, China # Harvard-MIT Division of Health Sciences and Technology, Harvard University and Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ‡

S Supporting Information *

ABSTRACT: A polymer blend system F81−x:SYx based on poly(9,9-dioctylfluorene) (F8) from the family of polyfluorenes (PFO) and a poly(para-phenylenevinylene) (PPV) derivative superyellow (SY) shows highly efficient energy transfer from F8 host to SY guest molecules. This has been realized due to a strong overlap between F8 photoemission and SY photoabsorption spectra and negligibly low self-absorption. The steady-state and timecorrelated spectroscopic measurements show an increased photoluminescence quantum efficiency (PLQE) and lifetime (τ) of SY, with an opposite trend of decreasing PLQE and τ of F8 excitons with increasing SY concentration, suggesting the Förster resonance energy transfer (FRET) to be the main decay pathway in the proposed system. The systematic study of the exciton dynamics shows a complete energy transfer at 10% of SY in the F8 host matrix and a Förster radius of ∼6.3 nm. The polymer blend system exhibits low laser and amplified spontaneous emission thresholds. An ultrahigh efficiency (27 cd·A−1) in F81−x:SYx based light emitting diodes (LED) has been realized due to the intrinsic property of a well-balanced charge transport within the emissive layer. The dual pathway, that is, the efficient energy transfer between the blended molecules via resonance energy transfer, and the charge-traps-assisted balanced transport makes the system promising for achieving highly efficient devices and a potential candidate for lasing applications. KEYWORDS: polyfluorene, poly(para-phenylene-vinylene), polymer blends, Förster resonance energy transfer (FRET), steady state spectroscopy, ultrafast spectroscopy, time-resolved spectroscopy

T

Recently, we reported a blend of poly(9,9-dioctylfluorene) (F8) and one of the poly(para-phenylenevinylene) derivatives superyellow (SY), which showed ultrahigh luminous efficiency in a light-emitting diode (LED) configuration as well as exhibited low thresholds for amplified spontaneous emission and optically pumped lasing.7,18,19 Nevertheless, it is believed that mechanisms responsible for high efficiencies of this blend system in pure optical and electro-optical devices are fundamentally different. In light emitting diodes, it is the predominant stagnancy of the hole charge carriers that brings the emission zone away from the cathode (and related quenching), such that bulk of the active layer is exploited for

here has been a strong interest in developing semiconducting conjugated polymers as gain media for optical amplifiers and lasing applications over the last two decades.1−8 They can be designed to have high photoluminescence (PL) efficiencies, large stimulated emission cross sections, and a wide emission range across the entire visible spectrum.5−9 Polymer systems, such as methyl-substituted ladder-type poly(p-phenylene) (MeLPPP) and poly[2-methoxy-5-(2′-ethylhexyloxy)-pphenylenevinylene] (MEHPPV) exhibit excellent gain properties,10−13 but their low performance in the polymer lightemitting diodes (PLEDs) has limited their potential application in polymer injection lasing.14−16 Hence, it is highly desirable to create a polymer injection laser using a material system that can combine the excellent diode properties with low optically pumped lasing thresholds in a single gain medium.4,15,17 © XXXX American Chemical Society

Received: October 6, 2017 Published: November 17, 2017 A

DOI: 10.1021/acsphotonics.7b01177 ACS Photonics XXXX, XXX, XXX−XXX

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as ASE and lasing (Supporting Information, S1). This report deals mainly with the exciton dynamics of the proposed blend system.

radiative emission, which increases the overall efficiency of the device.18,19 However, a strong overlap between the emission spectrum of F8 and absorption spectrum of SY is presumably related to an efficient energy transfer from F8 to SY molecules, resulting in low thresholds such as in the case of optical pumping.7 The proposed blend was chosen because (i) there is strong overlap between F8 photoluminescence (PL) and SY photoabsorption (PA) spectra, which have great potential to exhibit Förster resonance energy transfer (FRET) in their blended form,7 (ii) the respective absorption and emission spectra of F8 and SY have a negligible overlap−thereby, low self-absorption is expected: FRET shifts the emission spectra toward that of the guest polymer and an efficient energy transfer is expected when there is minimum overlap between the emission and absorption from either polymers (Figure 1), and (iii) both polymers are

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RESULTS AND DISCUSSION Figure 2 shows steady-state PA and PL spectra for F81−x:SYx blend system with increasing SY weight percent concentrations (x = 0.001, 0.01, 0.05, 0.1, 0.5, 0.9 and 1.0) and a reference SY/ chlorobenzene dilute solution. The PA spectra show the expected absorption behavior where increasing SY concentration systematically exhibits its signature along with F8. The PL spectra for F8 shows peaks at ∼423, ∼441, 448, and 468 nm for all concentrations (where resolvable in the spectrum). Since the optical properties of spin-coated polyfluorene films are known to have strong dependence on their morphology, the crystalline (α and α′) and noncrystalline (amorphous, nematic, and β) phases show their distinct features in the spectral analyses.23−26 In the present case, there is some subtle appearance of β phase, generally identified by 0−0 peak at ∼437 nm.27 Similar films deposited using chloroform or toluene solution have been shown to have almost no β phase features and 0−0 peak of emission at ∼423 nm.27 Pure SY exhibits its first electronic 0−0 peak at 547 nm and a second vibronic 0−1 peak at ∼583 nm. 28 Addition of lowconcentration of SY (x = 0.001) in F8 results in the appearance of a pronounced SY peak emission spanning between 500 to 700 nm, although blue emission from F8 still dominates the spectrum. Next, SY takes the lead before long and dominates the PL spectra at only x = 0.01, where F8 emission almost disappears−only 2.4% of the integrated total PL from F8 remains resolvable. This behavior is attributed as a signature of efficient energy transfer via FRET from F8 to SY molecules. Notably, the emission wavelengths for the entire range of 0.01 < x < 1 is considerably away from the host (F8) PA band, providing an ideal condition for such energy transfer.29,30 However, in the case of low SY concentrations, x = 0.001 and 0.01, the overlap between the PA and PL is an absorption loss channel which could lead to an incomplete or inefficient energy transfer. PL spectra of SY for increasing SY concentrations show systematic red-shift across the entire range of samples; however, the position of F8 emission peaks remain unchanged within the range of its detectable signature. The red shift is generally dictated by the interchain relaxation (an additional loss channel) the extent of which varies with increasing concentration of the guest molecules.31−33 The PL spectra of the films having the smallest SY concentrations exhibits considerable resemblance with that obtained for SY/chlorobenzene solution, whereas, it is strongly red-shifted for pure SY films−for example, the first 0−0 vibronic peak exhibits ∼29 nm shift, as observed peaks are at 519, 548 for x = 0.01, 1.0, respectively. There is only the difference of 7 nm between x = 0.01 and SY dilute solution (λpeak = 512 nm). The measured red-(blue-) shift can be regarded as aggregation (dilution) effect for increasing (decreasing) SY concentration, hence, the separation of the SY chains for its larger (smaller) concentrations in F8 matrix decreases (increases) and closely resembles with the analogous situation that of the pure SY film (dilute solution). In the case of closer (farther) proximity where aggregate formation is favored (hindered), the delocalization of electronic wave function among two or more chains in both the ground and excited states is more (less) likely to occur, lowering (maintaining) their corresponding energies.34,35 The interchain

Figure 1. (a) Photoabsorption (PA) and photoluminescence (PA) from thin films of F8 and SY polymers: a strong spectral overlap between F8 emission and SY absorption is the key for efficient Förster resonance energy transfer. The PL spectra were taken using excitation wavelength λexc = 380 nm. The inset shows the layered scheme of the samples used for spectroscopic studies in this work. (b) Illustration of the band diagram and energy transfer mechanism between two polymers. (c) Chemical structures of F8 and SY molecules.

well-known for their excellent electro-optical properties, that is, high optical gains and PLED efficiencies.20−22 Therefore, blending the two polymers has been demonstrated to enhance the overall device efficiency and the light output.7,18,19 In this work, we have created a highly efficient F81−x:SYx blend system (x is the SY wt % in the F8) and investigated the exciton dynamics via steady-state, time-correlated single photon counting (TCSPC), and transient absorption (TA) spectroscopic techniques. The spectroscopic measurements showed a reduction (increment) in photoluminescence quantum efficiency and shortening (stretching) of lifetime for F8 (SY) excitons with increasing SY concentration, suggesting that the FRET is the dominant decay pathway in the proposed system. These spectral studies showed significantly reduced optical selfabsorption losses as emission profile moves away from the photoabsorption edge with increasing SY concentration in F8 matrix; a nearly complete energy transfer was measured in F80.9:SY0.1. The Förster radius (R0) calculated from the band overlap between the PL spectra of F8 and PA spectra of SY was from 4.3 to 6.3 nm. The optical output of the blend system strongly depended on the interchain interaction between SY aggregates in F81−x:SYx films. Our findings indicate that the proposed system is promising for photonics applications such B

DOI: 10.1021/acsphotonics.7b01177 ACS Photonics XXXX, XXX, XXX−XXX

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Figure 2. Steady-state spectroscopy: (a) Normalized PA (dashes) and PL (solid line) spectra of the F81−x:SYx blend films with increasing SY concentration and SY dilution in chlorobenzene (lowest panel). The F8 emission is highly suppressed with the inclusion of SY and has almost no signature because of FRET for >5% concentration of SY. (b) PL spectra normalized to SY (0−0) peak, showing the reduction extent of F8 PL upon SY mixing, and hence an efficient energy transfer. (c) PL quantum efficiency of F8 emission (blue), SY emission (red), and SY emission only from SY excitons corrected for FRET (green) with increasing SY concentration. PLQE of pure F8−65% is marked as a blue hollow triangle on the y-axis for reference. (d) CIE chromaticity diagram, shows the spectral shift upon addition of SY in the F8 matrix.

morphology (defects, impurity, concentration), fabrication conditions, and solvent used to fabricate the spin-coated films, consistent with Ariu et al.36 SY offers higher PLQE than other PPV derivatives due to its intramolecular energy transfer between SY copolymer subunits, which lowers the possibility of exciton diffusion to nonemissive defect quenching centers.28,37 Here, we propose that although there is a low possibility of nonemissive quenching between SY copolymer subunits, it is further reduced when SY aggregate formation is hindered if mixed in low concentrations in the F8 matrix. Therefore, it is the degree of SY aggregation in F81−x:SYx films that mainly dictates and improves the efficiency of energy transfer and, hence, overall PLQE of SY. The PLQE of SY/CB dilute solution (∼96%) was measured by comparing the PL emission with Rhodamine 6G in ethanol:38

delocalization of the wave function appears as red-shifted in luminescence spectra−generally, coupled with this is the weakening of the radiative recombination.29,35 Other investigators have reported high photoluminescence quantum efficiency mediated by the intramolecular energy transfer in SY conjugated copolymer.28 Although an optical excitation can cause absorption from different conjugation lengths, emission from the exciton is most likely to occur from the longest segments of SY molecules.28 Therefore, PA spectrum of SY/ chlorobenzene solution is almost similar to that of pure SY film. Concomitant to the PL spectral shift, the shape of the spectra also changes: electronic transition 0−0 peak relative to the first 0−1 vibronic peak is more intense for lower SY concentrations. The suppression of 0−1 peak also supports the idea of formation of lesser SY aggregates for lower SY concentrations. Photoluminescence quantum efficiency (PLQE) was measured to quantify the radiative decay in F81−x:SYx films with increasing SY concentration (Figure 2c). The integrated emission wavelength ranges for F8 and SY are respectively kept between 415 to 500 nm and 500 to 750 nm. The PLQE value for SY (QSY) rises with increasing value of x, that is, QSY ≈ 32%, 62%, and 73% for x = 0.001, 0.01, and 0.05, respectively. However, PLQE for F8 (QF8) emission is highly quenched with increasing SY:QF8 concentration decreases from its maximum value of ∼65% for pure F8 to ∼43% for 0.1% SY (x = 0.001), and reduces to only ∼1.3% for x = 0.01. This increase (decrease) in QSY (QF8) suggests an energy transfer which is maximum for x = 0.1. Increasing SY concentration within 0.001 ≤ x < 0.1 decreases the average effective distance of the excitons located between F8 chain segment and SY molecules, resulting in the increase of QSY. A nonradiative decay channel opens up due to intermolecular interactions as SY aggregation exhibits its signature for x > 0.1, for which QSY again undergoes a decrease to ∼45% and ∼32% with x = 0.5 and 0.9, respectively, consistent with previous studies.33 Pure SY films exhibit QSY ≈ 29%, which is slightly larger than reported literature values (QSY ≈ 17%) based on pure SY films from pxylene solution.28 The difference can be attributed to

Q SY(sol) = Q R

ISY ODR n2 × × 2 IR OD nR

(1)

where Q denotes the PLQE, I is the integrated intensity, and OD is the optical density. The subscripts SY(sol) and R signify the respective values for SY dilute solution and the reference Rhodimine 6G known yield QR (∼0.94).38 To directly compare the PLQEs of SY in the F81−x:SYx films with that of SY/CB dilute solution, incomplete FRET is taken into account.39 The PLQE of SY in the blend films is corrected by the fraction of F8 excitons moving to SY via FRET: Q SY(corrected) =

Q

SY Q F8 − Q F8in F8

1 − x SYx

Q F8

(2)

The corrected PLQE value is ∼96% for x = 0.001, which is almost equivalent to that of calculated for SY/CB dilute solution, indicating that the SY molecules are well dispersed in F8 matrix when mixed in low concentrations as if single molecular chains are spread in a dilute solution. Therefore, suppression of PLQE due to the SY aggregates is absent in these concentrations, consistent with Jakubiak et al.33 Hence, C

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decrease observed in τ, even for the smallest mixing of SY, for example, the lifetimes are ∼228 ps and ∼174 ps for x = 0.001 and 0.01, respectively. F8 excitonic decay becomes shorter than the time resolution of the TCSPC setup (∼100 ps) for all x ≥ 0.05. At this point, PL of F8 also becomes weak to be measured, making F8 exciton lifetime unextractable from the last two curves. Such a strong suppression in τ supports the efficient energy transfer from host to guest molecules.40,41 The increasing concentration of SY has some notable effects: Exponential fit to the SY fluorescence for all blends show shorter lifetimes as compared to pure SY film (∼1841 ps; Figure 3b). Longer lifetime of excited aggregates for high SY concentrations as compared with interachain singlet exciton at lower SY concentration are expected, consistent with Nguyen et al.32 The lifetime for dilute SY/CB solution (∼754 ps) is slightly shorter than that of the lowest SY concentration (867 ps for x = 0.1), where SY molecules are considered to be distributed in F8 matrix in a solution; however, this is subtly longer (∼694 ps) than SY/p-xylene solution, reported by Snedden et al.28 This difference could be because of exciton lifetime dependence on the morphology, solvent and the detection window.36 The rise-time (∼400 ps) measured for x = 0.001 sharply decreases (to 200 ps) as compared to x = 0.01, which is one of the signatures of the increased rate of FRET with increasing SY concentration. With further increase in SY, the rise-time (∼100 ps) curves become almost identical and approach to the resolution of the TCSPC setup. The extent of the radiative decay rate (kr) and the nonradiative decay rate (knr) of the SY excitons within F81−x:SYx blend can be calculated by combining the PLQE with time-resolved photoluminescence data such as QSY = kr(kr + knr)−1, where the lifetime of SY excitons (τSY) is related to the rate constants by τSY = (kr + knr)−1,38 see Supporting Information, S3, for the summary of the values of kr and knr of SY exactions, the corrected QSY and τSY with increasing SY concentrations. Figure 3d shows for major decay channels for F81−x:SYx blend. For x ≥ 0.5, knr ≈ 0.389 × 109 s−1 almost remains unchanged and dominates the kr, which is because of SY-aggregate formation; they are the source major nonradiative decay channel at high SY concentrations. The increasing τSY and decreasing QSY with increasing x suggest that the aggregates between the SY molecules are the H-aggregates (head-to-head), not J-aggregates (head-to-tail), as H-aggregates lead to longer exciton lifetimes and lower the PL quantum efficiency due to the weak radiative coupling to the ground state.42 The strong decrease in kr for pure SY film by a factor of 8 as compared with that of SY/CB solution can also be explained by H-aggregates formation. Differential transmission (ΔT/T) analysis was carried out under selected wavelength range, 500−700 nm, and their temporal evolution was recorded (Figure 4a−f). The TA spectra of F8 shows a weak stimulated emission at a shorter wavelength (∼525 nm) and photoinduced absorption (PIA) of the exciton at a longer wavelength range (525−700 nm), consistent with Stevens et al.43 The PIA signal of F8 decays rapidly with a lifetime of ∼200 ps, which is shorter than τF8 measured by TCSPC, possibly due to exciton−exciton annihilation, though a low excitation fluence of ∼3.5 μJ·cm−2 was used, as suggested in ref 43. Such excitonic annihilation is likely to occur when two molecules in the same aggregate (or closeby) are simultaneously excited, generally causing artifacts such as charged species and faster decay of the excited state.44,45 As two excitations in the S1-state move close enough to each

FRET is completed for x ≥ 0.1, as there is no resolvable F8 emission above this concentration. The Förster radius (R0) can be calculated by the overlap between PL and PA spectra of F8 and SY, respectively: R 0 = 0.211(κ 2n−4Q DJ(λ))1/6

(3)

where κ (2/3) is the orientation parameter, n (1.8) is the refractive index of the material, QD (0.65) is the PLQE of the donor in the absence of the acceptor, and J(λ) (2.56 × 1015 M−1cm−1 nm4) is dimensionless overlap integral calculated as39 2

∞

J (λ ) =

∫0

∞

∫0 FD(λ)εA (λ)λ 4 dλ ∞

∫0 FD(λ)dλ

(4)

where FD(λ) is the normalized spectral distribution of the host and εA(λ) is the molar extinction coefficient of the acceptor (Supporting Information, S2). The value for Förster radius was calculated to be ∼43 Å. Considering that the exciton may delocalize between 5 and 10 repeat units along the SY polymer chain, molar extinction coefficient of SY may be taken as 5− 10× larger, which would result in a slightly extended R0. To understand FRET dynamics, the lifetime changes of F8 and SY excitons were studied using time-correlated single photon counting (TCSPC). The decay dynamics are monitored at 450 and 550 nm corresponding to F8 and SY PL emissions, respectively. Figure 3a shows the time-resolved photoluminescence intensity curves of F8 emission from F81−x:SYx blend films with increasing value of x. The decay curves at low SY concentrations fit well with single exponential decays. For pure F8, the lifetime (τ) of excitons is ∼413 ps. There is a sharp

Figure 3. Time-correlated spectroscopic measurements of F81−x:SYx blend films. (a) Time-resolved photoluminescence from F8 PL regions measured at 450 nm for F81−x:SYx blend films with increasing SY weight concentration: for low concentrations (0%, 0.1%, and 1.0%) the decay curves are fitted with single exponential decays (black curves). Decay becomes faster with the inclusion of SY and becomes shorter than the time resolution of the equipment. (b) Time-resolved photoluminescence from SY PL regions measured at 550 nm for increasing SY weight concentration of F81−x:SYx films and dilute SY solution in chlorobenzene. (c) The rates of radiative (kr), nonradiative (knr), and combined (kr + knr) decay channels for SY excitons in F81−x:SYx films with increasing SY concentration. The values for SY dilute solution are also marked on the y-axis. (d) The illustration of the general decay pathways. D

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Figure 5. TA kinetics of F81−x:SYx blend films with increasing SY weight concentration. All films were excited at λexc = 400 nm and with excitation fluence of ∼3.5 μJ·cm−1.

is more dominant, consistent with the TA spectra as discussed above. The combination of efficient FRET and charge transport properties of F81−x:SYx renders this blend-system attractive for electro-optical devices. The FRET process transfers the exciton (as a neutral excitation) from the higher gap F8 to the SY. For this, the positions of the HOMO and LUMO do not matter as such - it is the overlap of emission from the F8 and absorption by the SY is critical. However, in LED operation, these devices are efficient mainly because it is possible to balance the electron and hole currents.18 Since in both polymers, F8 and SY, charge transport is hole-dominant, that is, hole currents are larger than electron currents. As a result, in their pure forms, the excitonic recombination takes place close to the cathode where cathode quenching is inevitable. In the situation where low fractions of SY in the F8 is used as the active layer, electrons transport does not change significantly, however, holes are trapped at the SY sites and their transport experience the severe trap-assisted stagnancy; holes now have to switch from one SY site to the next to reach the cathode. In this way, the longer stay of holes at trap sites increases the probability of excitonic recombination at SY molecules only (Supporting Information, S4). Apart from LEDs, such high-performance polymer systems possessing excellent optical gain properties are attractive candidates for many other applications including lasers,7 waveguides47 and other advanced devices such as integrated gyroscopes.48 We believe that F81−x:SYx is an interesting optical system offering enhanced optical performance and can find its application in many devices.

Figure 4. Transient absorption of F81−x:SYx. (a−f) Transient absorption (TA) spectra of F81−x:SYx blend films with increasing SY weight content.

other, their excitation energy can be used to create a higher excited Sn-state at one molecule, and the other molecule returns to the S0 ground state: S1 + S2 → Sn + S0. As the Sn-state can be relaxed by fast internal conversion (usually