Role of Bimolecular Exciton Kinetics in Controlling the Efficiency of

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Role of Bimolecular Exciton Kinetics in Controlling the Efficiency of Organic Light Emitting Diodes Amrita Dey, and Dinesh Kabra ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10559 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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ACS Applied Materials & Interfaces

Role of Bimolecular Exciton Kinetics in Controlling the Efficiency of Organic Light Emitting Diodes Amrita Dey1 and Dinesh Kabra1* 1

Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai (India) –

400076 *

Corresponding Authors: [email protected]

Abstract: Here, we have carried out a spectroscopic investigation on the operational organic light emitting diodes (OLEDs) to determine the role of emission layer thickness on the optoelectronic

performance

of

OLEDs

based

on

Poly (9,

9-dioctylfluorene-alt-

benzothiadiazole) (F8BT) copolymer system. Our study shows delayed fluorescence (DF) via triplet-triplet annihilation (TTA) contributes significantly to boost the OLED efficiency through their fractional contribution. Interestingly, we note that DF contribution varies as a function of emissive layer thickness. From the time-resolved electroluminescence (EL) and triplet absorption (TA, under electrical excitation) studies we have seen that the emissive layer thickness controls triplet exciton generation and decay processes. From time-resolved electroluminescence (TREL), we have also shown singlet-triplet annihilation (STA) is the dominant fluorescence quenching mechanism in bulk of emissive layer, whereas, thinner devices have significant exciton quenching at the interface of injection layer/F8BT. The strength of STA differs in thin vs. thick samples, which has been correlated with the spectral & spatial overlap integral of singlet and triplet states. Hence, STA strength and triplet population density are critical parameters for an explanation of high efficiency in unusually thick F8BT OLEDs.

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Keywords: (Polymer light-emitting diodes (PLEDs), Singlet-triplet annihilation, delayed fluorescence, Triplet-Triplet Annihilation (TTA), Photoluminescence Quenching) 1. Introduction: Exciton generation and annihilation processes control the intrinsic efficiency of the organic light emitting diodes (OLEDs).1-3 In case of the conjugated polymerbased light emitting diodes (PLEDs), exciton dynamics is significantly controlled by the film morphology4-7 due to the polymer chain packing. Morphology can be altered by various ways, such as different solvent polarity and polymer concentration, annealing conditions, etc.8-9 In case of fluorescent OLEDs, 25% singlet, and 75% triplet states are formed under electrical injection.10-11 Triplet excitons are in general considered as ‘dark excitons’ since they are not actively participating in the generation of light.11-13 However, triplet excitons can significantly change the device performance via different exciton annihilation processes (such as triplettriplet annihilation (TTA),12, 14 singlet-triplet annihilation (STA))3, 15 due to higher population density along with longer lifetimes compared to singlet-excitons.16 Polymer film morphology (chain packing) can also significantly control the migration process of triplet excitons hence the corresponding energy transfer processes between two triplets and singlet-triplet pairs.5, 1718 19

Among various fluorescent polymeric materials, polyfluorenes are found to be an

attractive class of light emitting polymers due to its high stability, band-gap tuneability to cover entire visible spectra, excellent film formation properties, high photoluminescence efficiency and adequate charge transport properties.20-21

22

PLEDs based on this family of

polymers have shown outstanding performance, where efficiency numbers are beyond the spin-statistical limit.21, 23 Previous studies on Poly (9, 9-dioctylfluorene-alt-benzothiadiazole) (F8BT) have shown that the additional channel of singlet generation via triplet-triplet annihilation (TTA) which leads to enhancing the PLED efficiency beyond spin statistics.2 However, our recent study shows that there is a possibility of occurrence of thermally assisted delayed fluorescence process in this system.10

24


The different interfacial charge

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injection/transport layers along with the variation of the emissive layer thickness play a crucial role in optimizing the device performances.25-26 Most of the studies in the OLED community are limited to the steady-state operations, without focusing much on the excitonic processes probed via time-resolved electroluminescence (EL) technique on operating devices. Therefore, many important aspects of device physics can’t be unfolded. In our present study, we focus on the strength of STA as a function of emissive polymer layer thickness and therefore DF from triplet density in operational F8BT PLEDs. Here, we have prepared F8BT based PLEDs with three different thicknesses of F8BT emissive layers by varying F8BT wt% in p-xylene. MoO3/Au and ZnO/PEIE are used as hole and electron injecting electrodes, respectively. Devices based on this structure are studied via steady-state current-voltageluminance (J-V-L) characteristics, time-resolved electroluminescence (EL), triplet absorption and PL quenching measurements. Time-resolved EL (TREL) and transient triplet absorption (TA) in-situ on operational devices show that thinner devices with average thickness severely suffer from exciton quenching at the interfaces and, suffer from STA, respectively. These parameters hold back the efficiency of these devices, whereas thicker F8BT film overcomes detrimental factors such as STA and provide higher DF contribution and EL efficiency. It is known that co-existence of singlet alongside long lived triplet excitons is a major obstacle for the injection and optically pumped CW laser.27-28 Thus, by carefully designing molecular structure29 or optimizing the device thickness STA can be minimized in such cases. This paper brings insight into device design rules with the methodological knowledge about exciton dynamics in operational OLEDs. 2. Results: Figure 1a shows the typical device structure used in this study with ZnO/PEIE as the efficient electron injection layer, MoO3/Au as the hole injection contact and F8BT as the emissive layer. The typical chemical structure of F8BT polymer used in this study is shown as inset of Figure 1a. The energy levels corresponding to each segment of the device in the 3 ACS Paragon Plus Environment


unbiased condition has been shown in Figure S1a. Figure S1b shows the current density (J in mA/cm2) vs. applied voltage in F8BT PLEDs with emissive layer thicknesses ~110 nm (device D1), ~310 nm (device D2) and ~560 nm (device D3), respectively. Figure S1c shows the Luminance (L in cd/m2) vs. applied voltage characteristic of F8BT PLEDs with different emissive layer thicknesses. D1 devices give high luminance at lower bias owing to the higher current density through the device, whereas for the thicker devices (D2 and D3) the luminance shows high value at a higher bias voltage. Figure 1b shows the current efficiency (in cd/A) vs. current density for different emissive layer thicknesses of F8BT with outcoupling enhancement using the half-spherical lens. It can be observed that there is almost 5-fold efficiency enhancement with the increase in emissive layer thickness from 110 nm to 560 nm. The current efficiency acquires a steady value concerning injected current density for all the thicknesses after the initial rise. The initial rise (Figure 1b) is much steeper for thicker devices (D3>D2>D1) and indicates towards a better charge carrier balance and lower bimolecular quenching processes. The maximum acquired current efficiency increases with increasing thickness of the emission layer. Figure 1c shows the steady state electroluminescence (EL) spectra of F8BT PLEDs with various emissive layer thicknesses. The EL spectra have peak maximum around ~545 nm and possess narrower emission profile for thicker devices.24 We note the similar trend of emission narrowing with film thickness in the photoluminescence spectral also (see Figure S2). The measured EQEs of the PLEDs are given in Figure S3. To get a better understanding of the thickness dependent performance of the PLEDs, we have studied the time-resolved electroluminescence (TREL) on them. Figure 2a to Figure 2c show the EL intensity after the immediate end of the electrical pulse vs. time for D1, D2, and D3 devices (Figure S4). The origin of this emission has been previously demonstrated as triplet excitons.2, 10 Delayed EL can also arise from polarons recombination after pulse turns4 ACS Paragon Plus Environment

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off30, however, in these PLEDs it is dominated by TTA is confirmed by applying a double (positive + delayed negative pulse in sequence) pulse experiment as reported earlier by king.et al31 and also by us.24 Dynamics of these bimolecular process is dominated by TTA and STA. Polaron and field induced quenching is not observed in these PLEDs is also confirmed by comparing the PL quenching in operatroinal PLED and hole-only devices.10 The DF emission spectra closely match with the pulse ‘ON’ time electroluminescence spectra (Figure S5). The DF emission intensity decreases with increase in current density for all the devices. Inset of Figure 2a shows the DF decays faster with increased current density in D1 devices as compared to D2 (Figure 2b inset) and D3 (Figure 2c inset) devices. We have previously attributed this fluorescence quenching (faster DF decays) to the singlet-triplet annihilation (STA).3, 10 The pulsed current densities are measured using a current probe as described in the experimental portion. For 110 nm device (Figure 2a) the DF intensity decays much faster (IDF J-0.95, where J represents the current density) with current density. It drops from 1 to 0.15 (i.e. 85%) for increasing the current density from 60 mA/cm2 to 375 mA/cm2. However, as the thickness increases we have observed that for 310 nm devices (Figure 2b inset) the DF drops from 1 to 0.4 (IDF J-0.40) i.e. 60% of the initial values for increasing the current density from 40mA/cm2 to 300 mA/cm2. In case of 560 nm (Figure 2c inset) device the drop in DF intensity is even less which falls from 1 to 0.8 (IDF J-0.11) i.e. only 20% of the initial DF intensity for increases in current density from 50 mA/cm2 to 520 mA/cm2.Therefore, thinner devices (D1 and D2) suffer much stronger STA than the thicker devices (D3). In case of the D3 device, DF intensity decays at much slower pace compared to D2 and D1 devices with an increase in similar current density level indicating less bimolecular quenching processes in case of thick devices. The DF quenching at longer time delays also shows the long-lived triplet excitons (having lifetimes in the microsecond time scale) as the cause of fluorescence quenching.10 Our out-coupling calculation (Figure S6) suggests that 5 ACS Paragon Plus Environment


with respect to F8BT thickness, outcoupling is not varying a lot and hence it is being driven by TTA mechanism. To get a better insight of the thickness dependent STA strength, we have optically excited the PLEDs at 490nm laser (which is close to F8BT absorption maxima)23 having a pulse width of ~8 µs (which is digitally modulated using the same function generator source as used for PLEDs).10 The optical pulse is adjusted to fall after 50µs of the electrical excitation (during quasi-steady state of EL intensity) by giving the delay between electrical and optical pulse. Figure S7 shows10 the corresponding experimental set up for the measurement. Figure 3a to Figure 3c show the PL quenching (-PL) during device operation where -PL=(PLfield on +EL)-(PL0+EL), where, PLfield, PL0 is the PL intensity in the presence of electrical Pulse ‘ON’ and ‘OFF’ conditions, respectively (see Figure S4). We have previously established -PL due to singlet-triplet annihilation (STA) mechanism in F8BT PLEDs.10 During the PL quenching experiment, the laser excitation power is kept fixed at 4 mW for all the device thicknesses. It can be observed from the Figure 3d that the modulated PL quenching (-PL/PL0) increases with current density for all the PLEDs suggesting STA prevails in all the devices. However, we can observe that the modulated PL is more in the D2 device (-PL/PL0J0.6, J is the current density) compared to the D1 device (-PL/PL0J0.4). Further, increase in devices thickness to 560nm for D3, lowers the effect of singlet quenching (where, -PL/PL0J0.5) as can also be seen that integrated -PL is lower for D3 devices compared to D2 devices, which indicates less bimolecular quenching process in the thicker devices. Similar behavior has been observed in case of F8BT PLEDs with normal structure (ITO/PEDOT: PSS/TFB/F8BT/Ca/Al),(see Figure S8 in the supporting information). To get a better understanding of the thickness dependent, STA mechanism we have studied triplet absorption on electrically excited PLEDs. Figure 4a to Figure 4c show


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the triplet absorption under electrical excitation for D1, D2, and D3 devices, respectively at different injection current densities. As the current density increases, triplet density increases, which gives enhanced triplet absorption (-R) signal. Triplet absorption signal is more for thicker devices (D3) (Figure 4c). In case of the D1 device, (Figure 4a) the triplet absorption signal at higher current density shows triplet exciton decay during electrical pulse ‘ON time’ which suggests that triplet excitons might suffer from significant interfacial quenching process in thin devices. We note that D3 device triplet population build-up is faster than D2 and in D1 device triplet absorption signal is much weaker than D2 & D3 devices. Figure 4d shows simulated normalized triplet exciton density spatial decay profile as a function of emissive layer thicknesses at t=100 s. It can be noticed the triplet exciton spatial decay length is smaller in case of thin devices (see Figure S9). 3. Discussions: The strength of singlet-triplet annihilation (STA) depends on the successful energy transfer between excited singlet excitons (S1*) to triplet excitons (T1): T1 +S1*= S0+Tn + Phonos

(1)

The energy transfer process is similar to the Forster resonance energy transfer (FRET). 32 The efficiency of non-radiative energy transfer via STA depends on the critical distance RS-T which is defined by the distance at where 50% of the excited singlets get quenched by triplet excitons.33-34 RS-T can be expressed in the following form:32 

 RSTA    6

0

f s ( ) TT ( ) d  4 n 4 ( )

(2a)

Where n is the refractive index of the active polymer system (F8BT in this case). fs is a normalized fluorescent spectrum, TT is the triplet absorption cross-section, and  is the frequency of the incident light.[10] The corresponding effective STA rate can then be expressed as a function of RS-T as,32, 34 7 ACS Paragon Plus Environment


3/2  STA  RSTA  NT

(2b)

Where NT is the triplet density. Equation (2a) and (2b) indicates STA rate inside PLEDs depends on the strength of overlap integral of electroluminescence (EL) and triplet absorption as well as triplet population density. Triplet exciton generation in OLEDs under electrical excitation occurs majorly due to recombination of electrically injected electrons and holes. There is minute contribution coming from intersystem crossing (ISC) as ISC is not strong in F8BT.11 The possible reason for low DF yield (Figure 2a) or higher triplet quenching (Figure 4a) in thin devices is due to strong exciton quenching at the polymer/electrode interface.25, 35-36 We note that the delayed fluorescence quenching is higher in thin devices (IDFJ-0.95 Figure 2a) compared to thick devices (IDFJ-0.11 Figure 2c). Thus, amount of electrically excited DF contribution is also higher in case of thick devices. Since the electrically excited DF originates from TTA,2, 12 the generation and decay processes of triplet excitons, control DF intensity, effectively.37 Therefore, we need to understand the STA process

10

in F8BT PLEDs as a function of

emissive layer thickness, in the light of singlet emission - triplet absorption overlap vs. triplet population density.32 From PL quenching experiment (Figure 3) it is also clear that less amount of STA occurs in case of a thick device (e.g., STA is low in 560 nm PLED compared to 310 nm PLED), due to less degree of overlap of EL and triplet absorption in thin devices. We have observed that in case of F8BT PLEDs, thin devices possess broader EL spectra compared to thick devices (Figure 1c). The other important factor which controls the singlettriplet overlap integral is the width of the triplet exciton recombination zone, i.e., triplet exciton localization length (in F8BT PLEDs having various emissive layer thicknesses). The more triplet absorption signal in case of a thick device (Figure 4c) points towards that the triplet excitons population density plays a crucial role in enhancing the device efficiency as


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well. To get a clear idea about triplet recombination zone profile we have simulated triplet absorption signal (Figure S6) as a function of emissive layer thicknesses at a current density of ~14010 mA/cm2 ( corresponding to steady EL efficiency concerning J, Figure 1c). The details of the simulation process can be found in supporting information.10 The fitted parameters are tabulated in Table 1 as a function of emissive layer thickness. From Table 1 it can be seen that kT (indicates the monomolecular triplet decay rate, see supporting information) is faster in thin devices compared to thick ones which suggest more nonradiative pathways for triplet excitons in thinner devices. It also appears from our PL quenching (Fig. 3)21 and triplet absorption experiment (Fig. 4) that in case of thinner devices there are more nonradiative paths available both for singlet and triplet excitons. We note that the triplet absorption signal in case of 110 nm thick device is low where triplet excitons decay down during electrical excitation ON time which possibly points towards the fact that the triplet exciton are being quenched at electrodes in thinner devices. It is most probably due to metallic cathode or PEDOT:PSS/F8BT induced nonradiative energy transfer process26 exciton quenching at the polymer/metal (as in the case of thin film under optical excitation). This non-radiative process holds back the device efficiency in case of the thin emissive layer. The kTT (bi-molecular triplet decay rate via TTA) rates are faster in thicker devices (Table 1), as in thick polymer film, compact packing of polymer chain allows easy inter-chain transfer for triplet exciton as well as higher triplet generation rates.37 It avails faster kTT in thick devices which also helps to achieve higher DF yield (since DF intensity, i.e., IDF kTTNT2). Though bimolecular rate constants are material properties and should not have changed with thickness of F8BT layer, however, we note that there is significant difference in delayed EL (Figure 2), triplet density (Figure 4) and FWHM of EL/PL (Figure S2 and S4). This suggests that with thickness of film the packing of the chains favorably assist triplet exciton, as observed earlier in optical and structural studies of F8BT film.4, 24 Besides triplet population



density another critical parameter for STA to occur is singlet emission and triplet absorption overlap. Table 1 shows the width of the triplet recombination zone (1/T) becomes broader in case of thick devices (~60nm) whereas in case of thin devices it is ~25 nm. A narrower recombination zone indicates poor triplet diffusion process. In case of thick device packing of the polymer chains helps in triplet diffusion process ( LT  DT T , where LT, T is the triplet diffusion length and lifetime, respectively)2, which results in larger triplet recombination zone. Hence, there is trade between spatial vs spectral overlap of singlet and triplet states, where poor spectral overlap helps in boosting the DF yield and larger spatial overlap can hamper the DF yield, however, considering the third parameter triplet density suggest overall thicker devices provide the optimal situation for high efficiency OLEDs. This can also explain why the strength of STA is low in case of thin vs. thick devices in F8BT PLEDs. 4. Conclusions: In conclusion, we have studied film thickness dependent STA strength in F8BT PLEDs as a function of triplet excitons density, the spatial & spectral overlap of singlet & triplet excitons. We could correlate a relation between the F8BT emissive layer thickness and device efficiency via triplet dynamics. Different excitonic processes control the fractional contribution of singlet generation yield of the device, which includes different DF lifetimes, DF yields via TTA and STA processes. We demonstrate that the bimolecular quenching process, such as STA, is relatively lower in the thicker devices as observed by, the higher fraction of electrically excited DF. These results are explained by the triplet density at particular injection current denisty J, STA rates, the triplet excitons decay rates and singlet emission vs triplet absorption spectral & spatial overlap integrals as a function of F8BT thickness. This study not only explains the long pending issue for an explanation of high efficiency in unusually thick F8BT PLEDs but also provide thickness optimization rules for future OLEDs device engineering.


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Materials and Methods: F8BT PLEDs are fabricated on cleaned ITO substrate. Before spin coating of F8BT layer, ZnO, from Zinc acetate dihydrate solution with ethanolamine as a stabilizer, is spin coated at 4000 rpm on O2 plasma treated cleaned ITO substrate and annealed at 2000C for 5 minutes in ambient air. PEIE is spin coated on top of ZnO layer with 0.4 wt% ethoxylated polyethylene amine solution in 2-methoxy ethanol, to get a PEIE layer ~10 nm and dried at 1100C. F8BT polymer (from CDT Pvt Ltd, U.K., Mn ~ 81K, PDI ~2.6) with concentration varying from 15mg/ml to 35mg/ml (to achieve thickness from ~140nm-600nm) is spin coated on top of ZnO/PEIE substrates and baked at 1550C for an hour inside the Glovebox under N2 ambient. Finally, an anode layer of 20 nm MoO3 followed by 70 nm Gold capping layer is deposited on F8BT layer using a shadow mask in a thermal evaporator in 310-6 mabar pressure. Current density–Voltage -Luminance (J-V-L) measurement are done outside the GB on encapsulated devices using Keithley 2400 source meter and Keithley 2000 multimeter and a calibrated Si photodiode (from RS component). The transient electroluminescence is measured with the help of avalanche photodiode from Thorlab with rising time of ~40ns. The device is biased by electrical pulses of variable pulse height from 6 to 10V at the 100Hz frequency with 100 µs pulse widths using arbitrary function generator (Arb studio 1104 Lecroy). The transient current is measured with the help of Tektronix current probe (TekCT2). All signals were monitored using a 1 GHz oscilloscope (TektronixDPO4140B). A 490 nm Laser diode (digitally modulated using Function generator) with pulse ~8 µs is used to excite the device optically during operation under electrical injection to monitor the PL quenching. The Optical pulse was adjusted to fall on the device after the transient EL became steady by putting a delay ~50µs between electrical and optical pulse. We have probed triplet population in the operating device (under electrical excitation) by a 780 nm (close to T1 to Tn absorption maxima)38 CW laser (with excitation power ~110 µW). We monitor the change of 11 ACS Paragon Plus Environment


the reflected beam by PIN diode (1 ns rise time) fed to a trans-impedance amplifier (Figure S7). The corresponding signal has been monitored on the oscilloscope. Supporting Information This file contains Figures S1 to S9 and model to understand triplet exciton build-up and decay profile as a function of time. Acknowledgments We thank Cambridge Display Technology (CDT), Ltd., for supplying the F8BT. This work is partially supported by Department of Science and Technology (DST-AISRF), India. We also acknowledge the support of the NCPRE for the device fabrication facility. AD acknowledges the UGC for a research fellowship.

References: (1) Stevens, M. A.; Silva, C.; Russell, D. M.; Friend, R. H. Exciton Dissociation Mechanisms in the Polymeric Semiconductors Poly(9,9-dioctylfluorene) and Poly(9,9-dioctylfluorene-cobenzothiadiazole). Physical Review B 2001, 63 (16), 165213. (2) Wallikewitz, B. H.; Kabra, D.; Gélinas, S.; Friend, R. H. Triplet Dynamics in Fluorescent Polymer Light-Emitting Diodes. Physical Review B 2012, 85 (4), 045209. (3) Zhang, Y.; Forrest, S. R. Triplets Contribute to Both an Increase and Loss in Fluorescent Yield in Organic Light Emitting Diodes. Physical Review Letters 2012, 108 (26), 267404. (4) Donley, C. L.; Zaumseil, J.; Andreasen, J. W.; Nielsen, M. M.; Sirringhaus, H.; Friend, R. H.; Kim, J.-S. Effects of Packing Structure on the Optoelectronic and Charge Transport Properties in Poly (9, 9-di-n-octylfluorene-alt-benzothiadiazole). Journal of the American Chemical Society 2005, 127 (37), 12890-12899. (5) Jankus, V.; Winscom, C.; Monkman, A. P. Dynamics of Triplet Migration in Films of N, N'-diphenyl-N, N'-bis(1-naphthyl)-1, 1'-biphenyl-4, 4''-diamine. J Phys Condens Matter 2010, 22 (18), 185802, DOI: 10.1088/0953-8984/22/18/185802. (6) Thorsmolle, V. K.; Averitt, R. D.; Demsar, J.; Smith, D. L.; Tretiak, S.; Martin, R. L.; Chi, X.; Crone, B. K.; Ramirez, A. P.; Taylor, A. J. Morphology Effectively Controls SingletTriplet Exciton Relaxation and Charge Transport in Organic Semiconductors. Phys Rev Lett 2009, 102 (1), 017401. (7) Kim, J. S.; Lu, L.; Sreearunothai, P.; Seeley, A.; Yim, K. H.; Petrozza, A.; Murphy, C. E.; Beljonne, D.; Cornil, J.; Friend, R. H. Optoelectronic and Charge Transport Properties at Organic-Organic Semiconductor Interfaces: Comparison between Polyfluorene-Based Polymer Blend and Copolymer. J Am Chem Soc 2008, 130 (39), 13120-31


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(8) Schwartz, B. J. Conjugated polymers as molecular materials: How Chain Conformation and Film Morphology Influence Energy Transfer and Interchain Interactions. Annual review of physical chemistry 2003, 54 (1), 141-172. (9) Xu, Z.; Tsai, H.; Wang, H.-L.; Cotlet, M. Solvent Polarity Effect on Chain Conformation, Film Morphology, and Optical Properties of a Water-Soluble Conjugated Polymer. The Journal of Physical Chemistry B 2010, 114 (36), 11746-11752. (10) Dey, A.; Rao, A.; Kabra, D. A Complete Quantitative Analysis of Spatio‐Temporal Dynamics of Excitons in Functional Organic Light‐Emitting Diodes. Advanced Optical Materials 2017, 5, 1600678. (11) Köhler, A.; Bässler, H. Triplet States in Organic Semiconductors. Materials Science and Engineering: R: Reports 2009, 66 (4–6), 71-109, DOI: http://dx.doi.org/10.1016/j.mser.2009.09.001. (12) Kondakov, D. Y. Triplet–Triplet Annihilation in Highly Efficient Fluorescent Organic Light-Emitting Diodes: Current State and Future Outlook. Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences 2015, 373 (2044), DOI: 10.1098/rsta.2014.0321. (13) Monkman, A.; Friend, R. H. Organic Semiconductor Spintronics: Utilizing Triplet Excitons in Organic Electronics. Philos Trans A Math Phys Eng Sci 2015, 373 (2044), DOI: 10.1098/rsta.2015.0121. (14) Rothe, C.; King, S. M.; Monkman, A. P. Direct Measurement of the Singlet Generation Yield in Polymer Light-Emitting Diodes. Phys Rev Lett 2006, 97 (7), 076602. (15) Steiner, F.; Vogelsang, J.; Lupton, J. M. Singlet-Triplet Annihilation Limits Exciton Yield in Poly(3-Hexylthiophene). Physical Review Letters 2014, 112 (13), 137402. (16) Monkman, A. Singlet Generation from Triplet Excitons in Fluorescent Organic LightEmitting Diodes. ISRN Materials Science 2013, 2013. (17) Rothe, C.; Monkman, A. P. Triplet Exciton Migration in a Conjugated Polyfluorene. Physical Review B 2003, 68 (7), 075208. (18) Hoffmann, S. T.; Koenen, J.-M.; Scherf, U.; Bauer, I.; Strohriegl, P.; Bässler, H.; Köhler, A. Triplet–Triplet Annihilation in a Series of Poly (p-phenylene) Derivatives. The Journal of Physical Chemistry B 2011, 115 (26), 8417-8423. (19) Lu, L. P.; Finlayson, C. E.; Kabra, D.; Albert‐Seifried, S.; Song, M. H.; Havenith, R. W.; Tu, G.; Huck, W. T.; Friend, R. H. The Influence of Side‐Chain Position on the Optoelectronic Properties of a Red‐Emitting Conjugated Polymer. Macromolecular Chemistry and Physics 2013, 214 (9), 967-974. (20) Gwinner, M. C.; Kabra, D.; Roberts, M.; Brenner, T. J. K.; Wallikewitz, B. H.; McNeill, C. R.; Friend, R. H.; Sirringhaus, H. Highly Efficient Single-Layer Polymer Ambipolar Light-Emitting Field-Effect Transistors. Advanced Materials 2012, 24 (20), 2728-2734, DOI: 10.1002/adma.201104602. (21) Lu, L. P.; Kabra, D.; Friend, R. H. Barium Hydroxide as an Interlayer Between Zinc Oxide and a Luminescent Conjugated Polymer for Light‐Emitting Diodes. Advanced Functional Materials 2012, 22 (19), 4165-4171. (22) Schmidtke, J. P.; Kim, J.-S.; Gierschner, J.; Silva, C.; Friend, R. H. Optical Spectroscopy of a Polyfluorene Copolymer at High Pressure: Intra- and Intermolecular Interactions. Physical Review Letters 2007, 99 (16), 167401. (23) Kabra, D.; Lu, L. P.; Song, M. H.; Snaith, H. J.; Friend, R. H. Efficient Single‐Layer Polymer Light‐Emitting Diodes. Advanced Materials 2010, 22 (29), 3194-3198. (24) Dey, A.; Chandrasekeren, N.; Chakraborty, D.; Johari, P.; McNeill, C. R.; Rao, A.; and Kabra, D. Kinetics of Thermally Activated Triplet Fusion as a Function of Polymer Chain Packing in boosting the Efficiency of Organic Light Emitting Diodes. npj Flexible Electronics, 2018, doi : 10.1038/s41528-018-0042-0. 13 ACS Paragon Plus Environment


(25) Kumar, A.; Dey, A.; Dhir, A.; Kabra, D. Quantitative Estimation of Exciton Quenching Strength at Interface of Charge Injection Layers and Organic Semiconductor. Organic Electronics 2017, 42, 28-33, DOI: http://dx.doi.org/10.1016/j.orgel.2016.12.004. (26) Markov, D.; Blom, P. Exciton Quenching in Poly (phenylene vinylene) Polymer LightEmitting Diodes. Applied Physics Letters 2005, 87 (23), 233511. (27) Chénais, S.; Forget, S. Recent Advances in Solid‐State Organic Lasers. Polymer International 2012, 61 (3), 390-406. (28) Gärtner, C.; Karnutsch, C.; Lemmer, U.; Pflumm, C. The Influence of Annihilation Processes on the Threshold Current Density of Organic Laser Diodes. Journal of Applied Physics 2007, 101 (2), 023107, DOI: 10.1063/1.2425003. (29) Hajime, N.; Taro, F.; Takuya, H.; Takuji, H.; Chihaya, A. Light Amplification in Molecules Exhibiting Thermally Activated Delayed Fluorescence. Advanced Optical Materials 2017, 5 (12), 1700051, DOI: doi:10.1002/adom.201700051. (30) Gerhard, A.; Bässler, H. Delayed Fuorescence of a Poly (p-phenylenevinylene) Derivative: Triplet–Triplet Annihilation versus Geminate Pair Recombination. The Journal of chemical physics 2002, 117 (15), 7350-7356. (31) King, S. M.; Cass, M.; Pintani, M.; Coward, C.; Dias, F. B.; Monkman, A. P.; Roberts, M. The Contribution of Triplet–Triplet Annihilation to the Lifetime and Efficiency of Fluorescent Polymer Organic Light Emitting Diodes. Journal of Applied Physics 2011, 109 (7), 074502, DOI: doi:http://dx.doi.org/10.1063/1.3561430. (32) Pope, M.; Swenberg, C. E. Electronic processes in organic crystals and polymers, Oxford University Press on Demand: 1999. (33) Yokota, M.; Tanimoto, O. Effects of Diffusion on Energy Transfer by Resonance. Journal of the Physical Society of Japan 1967, 22 (3), 779-784, DOI: 10.1143/JPSJ.22.779. (34) List, E.; Scherf, U.; Müllen, K.; Graupner, W.; Kim, C.-H.; Shinar, J. Direct Evidence for Singlet-Triplet Exciton Annihilation in π-Conjugated Polymers. Physical Review B 2002, 66 (23), 235203. (35) Mikhnenko, O. V.; Cordella, F.; Sieval, A. B.; Hummelen, J. C.; Blom, P. W.; Loi, M. A. Exciton Quenching Close to Polymer− Vacuum Interface of Spin-Coated Films of Poly (p-phenylenevinylene) Derivative. The Journal of Physical Chemistry B 2009, 113 (27), 9104-9109. (36) Markov, D.; Blom, P. Migration-Assisted Energy Transfer at Conjugated Polymer/Metal Interfaces. Physical Review B 2005, 72 (16), 161401. (37) Hoffmann, S. T.; Scheler, E.; Koenen, J.-M.; Forster, M.; Scherf, U.; Strohriegl, P.; Bässler, H.; Köhler, A. Triplet Energy Transfer in Conjugated Polymers. III. An Experimental Assessment Regarding the Influence of Disorder on Polaronic Transport. Physical Review B 2010, 81 (16), 165208. (38) Lee, C.-L.; Yang, X.; Greenham, N. C. Determination of the Triplet Excited-State Absorption Cross Section in a Polyfluorene by Energy Transfer from a Phosphorescent Metal Complex. Physical Review B 2007, 76 (24), 245201.


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Figure Captions: Figure 1. (a) Schematic of the device structure of F8BT PLED and the chemical structure of F8BT. (b) Current efficiency (cd/A) vs. J and (c) steady-state electroluminescence spectrum of three devices with different emissive layer thicknesses in F8BT PLEDs. Figure 2. (a) Normalized delayed electroluminescence intensity for (a) 110 nm (b) 310 nm (c) 560 nm thick F8BT PLEDs at different injected pulse current densities. The inset shows the integrated DF intensity decay vs. current density in log-log scale as a function of the emissive layer thickness. Figure 3. Fluorescence quenching during device operation at different current density levels for (a) 110 nm (b) 310 nm and (c) 560 nm thick F8BT film based PLEDs. (d) Integrated (PL) vs. current density for 110 nm to 560 nm F8BT films based PLEDs. Figure 4. Transient absorption signal for triplet population density during 100µs electrical pump pulse of different pulse height to vary the injected current density for (a) 110 nm (b) 310 nm (c) 560 nm thick active layer respectively at triplet probe beam (780 nm). (d) Normalized triplet excitons spatial profile for J~14010 mA/cm2 at t= 100 s with recombination zone position at ITO/ZnO/PEIE/F8BT interface.



Figure 1.

(a) V

Au /MoO3 F8BT ZnO/PEIE ITO Glass

(b)

(c) 25

1.0

20

0.8

Normalized EL

Current Efficiency (cd/A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 21

15 10 110nm 310nm 560nm

5 00

50

100 150 J (mA/cm2)

0.6 0.4 0.2 0.0 500

200

110nm 310nm 560nm


550 600 650 Wavelength(nm)

700

Page 17 of 21

Figure 2.

ELPulse OFF Intensity

(a)

100

60 mA/cm2

375 mA/cm2

10-1 1 0.8 0.6 0.4

IDF (Norm.)

10-2

(b)

10

0.2

Slope~-0.95 100 J (mA/cm2)

Time (s)

10-5

0

D2~310nm

40 mA/cm2 70 mA/cm2 115 mA/cm2

10

170 mA/cm2

-1

300 mA/cm2

1

10-2

IDF (Norm.)


150 mA/cm2 280 mA/cm2

10-6

0.8

slope~0.4

0.6

0.4 10

100 J (mA/cm2)

10-6 10

Time (s)

10-5

0

D3~560nm

10-1

50 mA/cm2 130 mA/cm2 250 mA/cm2 350 mA/cm2 520 mA/cm2

1 IDF (Norm.)

(c)

90 mA/cm2

D1~110nm

10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.9

Slope~-0.11 0.8

10-2

10

100 J (mA/cm2)

10-6

10-5 Time (s)



Figure 3.

0.00

D2~310 nm

-PL (arb.unit)

-0.01 -0.02 D1~110 nm

-0.03

60 mA/cm2 280 mA/cm2 375 mA/cm2 545 mA/cm2

-0.04 -0.05 40

(c)

0.00 -0.02

0.00

(b)

45

50 55 60 Time (s)

65

-0.04 40 mA/cm2 115 mA/cm2 170 mA/cm2 300 mA/cm2 485 mA/cm2

-0.08

-0.12 40

70

(d)

45

50 55 60 Time (s)

65

70

0.16

D3~560 nm

0.12

-PL/PL0

-PL (arb.unit)

(a)

PL (arb.unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 21

-0.04 50 mA/cm2 130 mA/cm2 250 mA/cm2 350 mA/cm2 520 mA/cm2

-0.06 -0.08 40

45

50 55 60 Time (s)

65

~J0.6 ~J0.4

0.08

~J0.5 110 nm 310 nm 560 nm

0.04

70

0


200 400 J (mA/cm2)

600

Page 19 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4.



Page 20 of 21

Table 1. Thickness-dependent triplet exciton decay constants, the width of the recombination zone at current density J~14010 mA/cm2

Thickness (nm)

kT (s-1)

kTT (cm3s-1)

1/T (nm)

110

1.4105

6.010-12

25

310

1.1105

5.010-12

50

560

2.8104

2.010-12

60


Page 21 of 21

Au MoO3 d (nm) F8BT ZnO/PEIE ITO Glass

Singlet-Triplet Annihilation (STA)

TOC:

Triplet-Triplet Annihilation (TTA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EQE 1/STA

EQE TTA

d (nm)


Role of Bimolecular Exciton Kinetics in Controlling the Efficiency of

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