Irina
Rörich
ab,
Ann-Kathrin
Schönbein
a,
Deepthi Kamath
Mangalore
a,
Anielen
Halda Ribeiro
a,
Christian
Kasparek
a,
Christian
Bauer
a,
N. Irina
Crăciun
a,
Paul W. M.
Blom
a and
Charusheela
Ramanan
*a
aMax Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany. E-mail: ramanan@mpip-mainz.mpg.de
bDutch Polymer Institute, P.O. Box 902, 5600 AX Eindhoven, The Netherlands
First published on 4th September 2018
We show that the exciton transport and decay processes in two poly(p-phenylene vinylene) (PPV) based semiconducting polymers exhibit distinct temperature dependence based on the energetic disorder of the polymer. Time-resolved photoluminescence (TRPL) spectroscopy and Monte Carlo exciton diffusion simulations are used to disentangle the contributions from radiative and non-radiative decay processes, with the latter including non-radiative decay due to exciton diffusion toward trap sites. In a highly disordered polymer, the exciton lifetime and quantum yield are nearly temperature independent. In the case of a less disordered polymer, the exciton lifetime and quantum yield increase with decreasing temperature, due to both freezing out vibrations and less exciton quenching by slowing down the diffusion toward trap sites. We further demonstrate that the temperature dependence of the electroluminescence of polymer light-emitting diodes comprising these polymers is directly correlated with the photoluminescence behavior.
The device operation of a PLED is determined by three processes: charge carrier (electron and hole) injection from the electrodes, charge transport through the polymer active layer, and charge recombination generating an exciton, which can then decay radiatively.4–6 Charge carrier and exciton transport in organic semiconductors can be regarded as a hopping process between localized states on conjugated chain segments that are inhomogeneously distributed in size and energy within a Gaussian density of states (DOS).1,7–9 The width of the Gaussian DOS (σ) represents the degree of energetic disorder in the system. The three processes that drive PLED operation depend on the DOS: charge injection is mediated by the relative energy level matching to the electrodes, and charge transport and successive exciton diffusion, prior to radiative decay, depends on the energy level distribution. An exciton introduced into an arbitrary energy level of the DOS will subsequently migrate toward the lower-energy tail via dispersive relaxation steps. At quasi-equilibrium, the exciton reaches the energy level of the highest population of states, located at −σ2/kT, where the hopping is determined by thermally-activated transfer.1,9 Thus, the energetic disorder resulting from molecular conformation, structural defects, and the variety of inter- and intrachain interactions due to the irregular morphologies, will govern the exciton diffusion and decay characteristics in PPV films for PLEDs.10,11 Time-resolved photoluminescence (PL) experiments enable characterization of the exciton diffusion behavior in conjugated polymers. An exciton, formed by either photo-excitation or Langevin recombination in a PLED, will decay back to the ground state within a lifetime τf. The PL quantum yield (ϕf) represents the ratio between radiative and non-radiative processes.12 In a thin film with significant chromophore–chromophore interactions, energy-transfer will occur via hopping along inter- and intrachain segments until the exciton decays either radiatively or non-radiatively. Thus, the PL lifetime τf and quantum yield ϕf are governed by the intrinsic radiative (kr) and non-radiative (knr) decay rates, as well as the exciton diffusion with a rate kdiff, which represents the non-radiative decay related to exciton quenching at defect sites (Fig. 1, eqn (1) and (2)).13
(1) |
(2) |
Fig. 1 The photophysical processes of excitons in a conjugated polymer thin film, which determine the exciton lifetime τf and the photoluminescence quantum yield, ϕf. |
Previous work demonstrates that τf and ϕf directly correlate with the energetic disorder of conjugated polymers, where increased disorder slows down the diffusion of the exciton toward non-radiative quenching sites and strongly enhances both τf and ϕf.14 This is consistent with diffusion-limited exciton quenching at defects in organic semiconductors.15 It should be noted that while the electronic DOS relevant for charge transport and exciton DOS are not the same, both represent the energy level distribution in the polymer. Consistent with this and our previous results, Markov et al. observed a three orders of magnitude enhanced charge carrier mobility in PPV derivatives with decreasing energetic disorder.16
Although charge transfer and exciton diffusion processes are well studied in disordered conjugated polymers, a significant open question remains as to how the PL characteristics of a polymer influence the PLED conversion efficiency. For example, the PLED efficiency is charge carrier mobility independent, and is therefore expected to be temperature independent with regards to the active layer material.17,18 However, the PL behavior of a polymer thin film may vary with temperature as the chromophore–chromophore interactions will affect the balance of kr, knr, and kdiff. Since the PLED efficiency is governed by the formation of excitons, it follows that the respective temperature dependencies of the PLED and PL characteristics should have a direct correlation.
Here, we investigate this relationship by measuring the temperature-dependence of the PL and PLED efficiencies in the PPV derivatives BEH-PPV (poly[2,5-bis(2′-ethylhexyloxy)-1,4-phenylene vinylene]) and SY-PPV (SuperYellow copolymer, Merck) (see Fig. 2 for chemical structures). These materials exhibit different degrees of energetic disorder, as the width of the charge carrier Gaussian DOS (σelectrical) is determined to be 0.092 and 0.140 eV for BEH-PPV and SY-PPV, respectively.16 Furthermore, the exciton lifetime is strongly enhanced with increasing disorder, ranging from 180 ps for BEH-PPV to 1.9 ns for SY-PPV.14 In the new work presented here, time-resolved photoluminescence spectroscopy (TRPL) at varying temperature shows a correlation between the conformational energetic disorder and the balance between radiative (kr) and non-radiative decay processes (knr and kdiff). The thin film of SY-PPV, a highly disordered polymer, exhibits a nearly temperature independent photoluminescence quantum yield, while the more ordered BEH-PPV demonstrates a significant decrease in non-radiative quenching with decreasing temperature (decrease of kdiff and knr). We then fabricated PLEDs with these materials in order to correlate device behavior with the results from our optical experiments. We found that PLEDs based on the disordered SY-PPV show temperature independent current efficiencies, whereas the better ordered BEH-PPV demonstrates enhanced PLED efficiency with lowering temperature, consistent with the temperature dependence of the photoluminescence quantum yield. In addition, we carried out Monte Carlo simulations to model how the exciton diffusion in the semiconductors depends on both the energetic disorder and the concentration of non-radiative quenching-sites. The results are discussed within the context of the diffusion-limited exciton quenching model.
Fig. 2 (a and b) Temperature dependent photoluminescence spectra and (c and d) associated normalized PL decay traces at specified wavelengths for SY-PPV and BEH-PPV. Spectral traces are taken at the maximum from the time-resolved measurements. Fits to the decay traces are from global analysis as described in the text, and are further detailed in Tables 1 and 2. |
Thin films for optical characterization were prepared from 6 mg mL−1 (SY-PPV) and 5.2 mg mL−1 (BEH-PPV) polymer or polymer:PCBM blend solution in chlorobenzene (Sigma-Aldrich, anhydrous 99.8%) on 1 × 2 cm quartz substrates. The substrates were cleaned with acetone and then propanol in an ultrasonic bath and dried for 20 minutes at 140 °C. The films were spin-coated (60 s 1000 rpm; 30 s 4000 rpm) in a nitrogen atmosphere. The polymer layer thicknesses were determined by using a surface profilometer (Bruker, DektakXT), yielding 80 nm for SY-PPV and 160 nm for BEH-PPV. The films were measured the same day they were prepared, in order to limit polymer degradation.
The comparison of the normalized PL decay kinetics measured at the S0,0 (550 nm) and S0,1 (590 nm) transitions (Fig. 2c, black and blue traces) demonstrate that the SY-PPV temperature-dependence depends on the emission wavelength. At 550 nm, the 77 K trace decays faster than the 290 K trace, while at 590 nm, the traces are mostly similar, with the 290 K trace decaying faster by a small amount. Fig. 2c shows only the lowest and highest measured temperatures for the S0,0 and S0,1 transition, for clarity, but the observed trend is consistent throughout the entire measured temperature range (Fig. S3, ESI†). The normalized BEH-PPV PL decay traces (Fig. 2d), monitored at 600 and 650 nm, demonstrate that the PL lifetime at 77 K is longer than at 290 K at all wavelengths. This trend is consistent through the measured temperature range (Fig. S4, ESI†).
The TRPL decay dynamics were modeled with a global analysis scheme, which takes into account the decay dynamics across the entire spectral range.19,21 The fits are represented as red lines in Fig. 2c and d and the lifetimes and relative amplitudes are summarized in Tables 1 and 2. For both materials, the data were fit to a sum of a fast and slow exponential contribution. The measured exciton lifetime (τf) is then determined from a weighted average of the two kinetic components, (eqn (3))
(3) |
T (K) | A 1 | τ 1 (ps) | A 2 | τ 2 (ps) | τ f (ps) | ϕ T/ϕ290K |
---|---|---|---|---|---|---|
290 | 0.39 | 360 | 0.61 | 2080 | 1900 | 1.00 |
270 | 0.42 | 370 | 0.58 | 2190 | 1990 | 1.02 |
230 | 0.46 | 410 | 0.54 | 2390 | 2140 | 0.99 |
200 | 0.47 | 420 | 0.53 | 2480 | 2200 | 0.98 |
180 | 0.49 | 440 | 0.51 | 2540 | 2240 | 0.96 |
150 | 0.51 | 450 | 0.49 | 2570 | 2250 | 1.01 |
130 | 0.51 | 460 | 0.49 | 2600 | 2260 | 0.99 |
100 | 0.53 | 480 | 0.47 | 2680 | 2320 | 0.84 |
77 | 0.51 | 490 | 0.49 | 2710 | 2360 | 0.78 |
T (K) | A 1 | τ 1 (ps) | A 2 | τ 2 (ps) | τ f (ps) | ϕ T/ϕ290K |
---|---|---|---|---|---|---|
290 | 0.89 | 160 | 0.11 | 370 | 210 | 1.00 |
270 | 0.91 | 180 | 0.09 | 450 | 240 | 1.32 |
230 | 0.90 | 220 | 0.10 | 550 | 290 | 1.49 |
180 | 0.87 | 270 | 0.13 | 670 | 370 | 1.70 |
150 | 0.83 | 290 | 0.17 | 630 | 390 | 1.84 |
120 | 0.79 | 300 | 0.21 | 600 | 410 | 1.76 |
90 | 0.62 | 190 | 0.38 | 480 | 370 | 1.71 |
77 | 0.56 | 130 | 0.44 | 480 | 390 | 1.76 |
(4) |
The better ordered BEH-PPV exhibits overall higher exciton diffusion coefficients and exciton diffusion lengths than the less well-ordered SY-PPV. With lowering temperature, SY-PPV shows a significant reduction of the exciton diffusion parameters between 298–180 K, with D decreasing from 3.1 × 10−4 cm2 s−1 to 8.0 × 10−5 cm2 s−1 and LD decreasing from 7.7 nm to 4.2 nm. Further cooling from 180 K to 77 K reveals nearly constant exciton diffusion parameters for SY-PPV. In contrast, the exciton diffusion parameters for the better ordered BEH-PPV display less temperature dependence in the higher temperature range (298–120 K), while in the lower temperature range 100–77 K, there is greater dependence on temperature, with D decreasing from 2.5 × 10−3 cm2 s−1 to 6.8 × 10−4 cm2 s−1 and LD decreasing from 9.6 nm to 5.2 nm. Both polymers exhibit an approximately 75% reduction in D at 77 K vs. 290 K.
For BEH-PPV, we also compared these results with values derived from a monoexponential fit to the fluorescence decay, which we have reported before for this polymer at room temperature.14 The temperature-dependent exciton diffusion parameters, in this case determined by the Stern–Volmer equation, and exciton lifetimes determined from monoexponential fits of the PL decays are shown in Fig. S5 (ESI†). The results show a similar trend of D and LD in BEH-PPV compared to the results determined from global analysis and Monte Carlo simulations (Fig. 3), with slightly higher values for LD. As D is quite similar for the two fits, the difference in LD is most likely due to the longer fluorescence lifetime from the monoexponential fits, which do not account for the increased energy transfer between polymer chains at lower temperatures.
Temperature dependent EL spectra of SY-PPV and BEH-PPV are shown in Fig. 4c and d. The SY-PPV EL appears qualitatively similar to the PL, while the BEH-PPV EL demonstrates a different spectral line shape than PL spectra in the same temperature range, with the S0,0 peak exhibiting always higher intensity than S0,1. To understand this further, we compared the steady-state PL of the pristine polymer films to that within the PLED stack (Fig. S2, ESI†). Under the same measurement conditions, the BEH-PPV PL spectrum within the PLED stack shows a change in peak ratio relative to the pristine BEH-PPV film. In SY-PPV, there is also a small shift and narrowing of the PL spectrum within the PLED stack. This EL spectral line shape is consistent with an optical microcavity effect in the PLED, wherein the near-field coupling results in a narrowing of the spectral line shape and enhances the spontaneous emission rate.2,32 Our results in Fig. 4 and Fig. S2 (ESI†) show that this is a much stronger effect in the 165 nm BEH-PPV device than in the 80 nm SY-PPV device.
Singlet excited states in disordered polymers are generated at an arbitrary energy-site within the high-energy tail of the Gaussian DOS and simultaneously undergo downhill migration toward lower energy-sites.7,34–36 The down-hill migration, also known as spectral diffusion, ends when the excitons approach the energy level of the most populated excited states, provided that the time needed to approach quasi-equilibrium is shorter than the intrinsic exciton lifetime. At room temperature, spectral diffusion is followed by temperature activated hopping, where balanced downward and thermally activated upward hopping occurs. With decreasing temperature, the equilibrium level shifts deeper into the tail of the Gaussian DOS, causing a red-shift of the PL spectrum.9,25,36 There, the probability to transfer to nearby energy-sites decreases, resulting in a slower exciton transfer. Additionally, thermally activated hopping becomes less pronounced. Thus, the exciton diffusion characteristics (and quantum yield) of a conjugated polymer thin film depend strongly on the disorder parameter σ and the temperature T,37 as well as on the starting singlet excitation energy within the DOS and the coupling strength between the energy sites.38
The excited state transfer rates for the two polymers were calculated in order to quantitatively describe the exciton decay processes (Tables 3, 4 and Fig. S8, ESI†). The lifetime τi correlates with the transfer rate ki with the following relation: τi = 1/ki. The values were determined as follows: the radiative decay rate kr is first calculated by eqn (5),
kr = ϕf/τf = (ϕT/ϕ290K·ϕabs)/τf | (5) |
T (K) | ϕ f | k f (s−1) | k r (s−1) | k diff (s−1) | k nr (s−1) |
---|---|---|---|---|---|
290 | 0.60 | 5.3 × 108 | 3.2 × 108 | 5.2 × 107 | 1.6 × 108 |
270 | 0.61 | 5.0 × 108 | 3.1 × 108 | 3.6 × 107 | 1.6 × 108 |
230 | 0.59 | 4.7 × 108 | 2.8 × 108 | 2.3 × 107 | 1.7 × 108 |
200 | 0.59 | 4.5 × 108 | 2.7 × 108 | 1.5 × 107 | 1.7 × 108 |
180 | 0.57 | 4.5 × 108 | 2.6 × 108 | 1.3 × 107 | 1.8 × 108 |
150 | 0.60 | 4.5 × 108 | 2.7 × 108 | 1.1 × 107 | 1.7 × 108 |
130 | 0.60 | 4.4 × 108 | 2.6 × 108 | 1.1 × 107 | 1.7 × 108 |
100 | 0.51 | 4.3 × 108 | 2.2 × 108 | 1.1 × 107 | 2.0 × 108 |
77 | 0.47 | 4.2 × 108 | 2.0 × 108 | 1.1 × 107 | 2.1 × 108 |
T (K) | ϕ f | k f (s−1) | k r (s−1) | k diff (s−1) | k nr (s−1) |
---|---|---|---|---|---|
290 | 0.06 | 5.0 × 109 | 3.0 × 108 | 4.3 × 109 | 3.6 × 108 |
270 | 0.08 | 4.7 × 109 | 3.7 × 108 | 4.1 × 109 | 2.1 × 108 |
230 | 0.09 | 3.9 × 109 | 3.5 × 108 | 3.4 × 109 | 1.4 × 108 |
180 | 0.10 | 2.9 × 109 | 2.9 × 108 | 2.5 × 109 | 1.3 × 108 |
150 | 0.11 | 2.7 × 109 | 2.9 × 108 | 2.2 × 109 | 1.4 × 108 |
120 | 0.11 | 2.9 × 109 | 3.0 × 108 | 2.4 × 109 | 2.0 × 108 |
90 | 0.10 | 2.7 × 109 | 2.8 × 108 | 1.8 × 109 | 6.5 × 108 |
77 | 0.11 | 2.7 × 109 | 2.9 × 108 | 1.7 × 109 | 7.5 × 108 |
In order to disentangle knr from kdiff, we used the exciton diffusion model to simulate quenching efficiencies Q (Q = 1 − NR/N0) for a range of diffusion coefficients D. The solid lines in Fig. 5 show the dependence of Q as determined by the ratio of the number of radiatively decaying excitons (NR) and the number of excitons that would decay radiatively in the absence of a quencher (N0). The curves represent Q for a constant intrinsic exciton lifetime τr = 3 ns. The model also assumes a constant number of exciton defects (background quenchers). In SY-PPV an electron trap density of 1 × 1017 cm−3 was determined by charge transport measurements,40 and in BEH-PPV a trap concentration of 6.5 × 1017 cm−3 was determined by using Stern–Volmer analysis, as described previously.14Fig. 5 shows that Q increases with enhanced D, since a high exciton diffusion coefficient represents a fast exciton diffusion toward traps, leading to more efficient exciton quenching, and vice versa. The quenching efficiency is then retrieved from the calculated value of D from TRPL, and kdiff is determined by kf × Q.
Fig. 5 Exciton diffusion coefficient D dependent quenching efficiency Q for SY-PPV and BEH-PPV. The amount of background quenchers equals 1 × 1017 cm−3 and 6.5 × 1017 cm−3 for SY-PPV and BEH-PPV, respectively. The black and red points correspond to the values of D determined from the experimental results (Fig. 3). |
Fig. 5 shows that, at room temperature, almost 90% of the excitons in BEH-PPV are quenched at a defect, whereas for SY-PPV, this value is less than 10%. As seen in Tables 3 and 4, kdiff of BEH-PPV is two orders of magnitude higher than that of SY-PPV. As a result, the non-radiative losses in BEH-PPV are almost fully dominated by diffusion-limited quenching at the defects present in the polymer. Furthermore, both PPV derivatives demonstrate a decrease in kdiff with lowering temperature, in agreement with the diffusion limited exciton quenching model. With the values of kr and kdiff known, the non-radiative transfer rates knr were determined using eqn (1) and (2).
The calculated values show that kr in SY-PPV slowly decreases with decreasing temperature, while in BEH-PPV it barely changes. This is consistent with the predominantly radiative PL observed in BEH-PPV. Considering the non-radiative processes, exciton diffusion towards defects (kdiff) is largely determining the PL quenching in BEH-PPV (Fig. 5). In contrast, in SY-PPV knr is the dominant non-radiative loss process.
The increase of knr with a concomitant slight decrease of kr in SY-PPV is ascribed to increased population of conformational subunits that have higher contribution from non-radiative decay processes, such as may be expected with increased interchain interactions. Also, kdiff remains temperature-independent below 180 K. The decrease in the exciton diffusion rate kdiff with lower temperature leads to a slight increase in the overall (average) exciton lifetime from 1.9 ns to 2.4 ns going from 290 K to 77 K. The wavelength dependent exciton lifetimes are attributed to different sub-populations of emitting species with varying degrees of radiative and non-radiative emission, where at higher energies the radiative species dominates the fluorescence. At lower energies, there are greater contributions due to spectral diffusion caused by energy transfer from shorter to longer conformational subunits as well as from energy transfer to interchain excimer like species, which will have a lower oscillator strength and subsequently longer PL lifetime.25,27 The exciton decay processes in SY-PPV balance out to give a relative quantum yield ϕf, which is fairly temperature independent.
In BEH-PPV, the decrease of knr (300–150 K) results from the decrease in internal vibrational energy redistribution pathways due to the changes in inter- and intrachain packing. This is consistent with the decreased electron-vibrational coupling at lower temperatures, evidenced by the changing peak ratio between S0,0 and S0,1 (Fig. 2d). At lower temperatures (<150 K), knr increases. Previous work, including vibrational spectroscopy, has shown that the electronic-vibrational coupling in BEH-PPV is temperature dependent, with even additional vibronic peaks being resolved at temperatures lower than what we have studied here.26 Such an effect was also observed in MEH-PPV.41 The increase in knr at lower temperatures is therefore attributed to increased energy transfer processes from shorter to longer conformational subunits as well as a possible change in the electron-vibrational coupling. The increased excited state lifetime at lower temperatures in BEH-PPV mainly originates from the decrease in exciton quenching due to slower diffusion toward traps (decrease in kdiff), since kdiff is an order of magnitude larger as compared to knr. The reduction of these non-radiative processes at lower temperatures, dominated by a reduced kdiff, then give rise an increase in the relative quantum yield ϕf as observed in BEH-PPV.
The electronic and optical properties of conductive polymer films may also be influenced by thermal treatment at or around the glass transition temperature, Tg.42,43 For PPV based polymers, Tg values reported in literature are above room temperature.44,45 We therefore do not expect this to influence our results, where the highest temperature measured is 298 K.
In contrast to the exciton diffusion coefficient, the exciton diffusion length LD of BEH-PPV first increases while cooling to 180 K. This increase from 7.7 nm to 9.6 nm is a consequence of the increased exciton lifetime resulting from less competitive non-radiative decay rates with lowering temperature (see eqn (5)). Then, in between 180 K and 120 K the LD of BEH-PPV slightly decreases. The previously noted change in electronic-vibrational coupling could affect the disorder parameter σ and therefore also the temperature dependence of LD. Finally, at a characteristic temperature of 100 K, the exciton diffusion is fully determined by downhill migration and the exciton diffusion length drops off from 8.9 nm to 5.2 nm.
Since the thermal equilibrium state −σ2/kT shifts with increasing disorder strength σ deeper into the tail states of the Gaussian DOS, the probability to find a nearby energy-site to which a jump is probable decreases noticeably in the case of a more disordered polymer.37 Therefore, in SY-PPV the exciton migration toward quenching defects is much slower at room temperature, leading to an order of magnitude longer exciton lifetime (1.9 ns) compared to BEH-PPV (200 ps).14 At room temperature, the slow exciton diffusion in the disordered SY-PPV is represented by a low exciton diffusion coefficient of 3.1 × 10−4 cm2 s−1 and the two orders of magnitude lower transfer rate kdiff (Tables 3 and 4) compared to BEH-PPV. The decrease in D and LD, while cooling to 180 K (Fig. 3), is a direct consequence of the down-shift of the equilibrium level into the tail of the Gaussian DOS, where the energy difference between occupied and neighboring sites increases, leading to an even lower D (8.0 × 10−5 cm2 s−1) and kdiff. This lower D along with a nearly constant exciton lifetime results in a decrease of LD from 7.7 nm to 4.2 nm in the temperature range of 290–180 K. Below the characteristic temperature (180 K), where thermally activated hopping is switched off, the exciton diffusion becomes fully determined by downhill migration, represented by temperature-independent D (∼6.8 × 10−5 cm2 s−1) and LD (∼3.9 nm).
The better ordered BEH-PPV exhibits PL characteristics that are temperature sensitive and are dominated by the exciton diffusion-limited quenching model. The increase in LD of BEH-PPV with lowering temperature is governed by the increase in τf, while in the case of SY-PPV temperature-dependence of LD is dominated by the temperature-dependence of D. Our findings demonstrate that the temperature at which thermally activated hopping diminishes scales with the energetic disorder. Better ordered PPV derivatives correlate with lower transition temperatures between thermally activated hopping and downhill migration.
However, the PLED efficiency also depends on the decay dynamics of a formed exciton,17,48 which is described by the PL efficiency. The temperature-independence of the CE in the high disordered polymer SY-PPV PLED (Fig. 4a) is explained as follows: after formation of the exciton, it subsequently diffuses along the conjugated chain segments until it reaches a radiative or non-radiative recombination center and decays either radiatively or non-radiatively. The increased population of non-luminescent conformational subunits present in the SY-PPV copolymer compensates for the decrease in exciton diffusion toward non-radiative quenching-sites with lowering temperature. Therefore, the current efficiency, which directly correlates with the PL efficiency ϕf, shows no temperature-dependence in the SY-PPV based PLED.
In contrast, the increased current efficiency in the less disordered BEH-PPV PLED in the temperature-range of 295–215 K (Fig. 4b) is a direct consequence of enhanced ϕf, resulting from less competitive non-radiative diffusion-dominated decay processes with lowering temperature. We note that the temperature-dependence of the current efficiency in BEH-PPV is stronger than that of the relative quantum yield ϕf. This can be attributed to the difference in the relative intensity of peaks in the EL spectra compared to the PL spectra due to interference in the optical microcavity of the EL device. This will influence the device efficiency and the rate of spontaneous emission.32 In the optical microcavity, the BEH-PPV EL spectra still demonstrate decreased electron-vibrational coupling with decreasing temperature. This is evidenced by the increasing ratio between the S0,0 and S0,1 peaks (Fig. 4d), and is consistent with the decrease in knr noted in this temperature range in the PL measurements.
In addition to the different responses to temperature, the SY-PPV PLED exhibits an order of magnitude higher CE than the BEH-PPV PLED. The lower CE in BEH-PPV is a direct result of the faster exciton diffusion (kdiff) towards non-radiative traps, and is consistent with the photoluminescence measurements. As mentioned above, Fig. 5 demonstrates that, at room temperature, the exciton quenching at defects is 90% in BEH-PPV and 10% in SY-PPV. As a result, while the same amount of excitons are formed in the two materials for a given current, the amount of photons generated differs by an order of magnitude due to the difference in the kinetically competitive quenching at non-radiative traps. This results in a higher CE in the more energetically disordered SY-PPV.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8tc01998c |
This journal is © The Royal Society of Chemistry 2018 |