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Quantifying Inter-Dopant Exciton Processes in Organic Light Emitting Diodes Carmen Nguyen, Grayson Ingram, and Zheng-Hong Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00526 • Publication Date (Web): 26 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

1

Quantifying Inter-Dopant Exciton Processes in

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Organic Light Emitting Diodes

3

Carmen Nguyen, Grayson Ingram, and Zheng-Hong Lu*

4

Department of Materials Science and Engineering, University of Toronto, 184 College Street,

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Toronto, Ontario M5S 3E4, Canada

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ABSTRACT: Inter-dopant exciton transfer processes play a critical role in engineering emission

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spectra from multiple emitters in white organic light emitting diodes (OLEDs). Developing

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experimental techniques for probing these energy transfer processes are thus vital to gain a better

11

understanding of the device physics and to eventually engineer better white OLEDs. In this

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paper, we present a simple method, based on electroluminescence spectra, to study and quantify

13

the energy transfer mechanisms in a two dopant (triplet-singlet) system. Through a combination

14

of experimental spectra obtained from a collection of donor-acceptor separations and theoretical

15

simulations of spectral variations, we demonstrate that the key physical parameters such as the

16

Dexter van der Waals radii and the Förster radius can be quantified.

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INTRODUCTION

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Over the years, organic light emitting diodes have emerged as a promising candidate for

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general illumination due to its potential to produce high color quality white light with high

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efficiency1–3. However, organic emitters commonly have a narrow emission band, so that a

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combination of emitters is needed in order to achieve a broadband emission for high quality

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white light. Energy transfer mechanisms are thus important in order to distribute excitons to the

25

different color emitters. Additionally, white OLEDs still struggle with maintaining high

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efficiencies at the high brightness levels needed for lighting 4. A better understanding of excitons

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dynamics within complicated multi-dopant devices is needed for the design of high efficiency

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white OLEDs. This work presents a method for quantifying energy transfer in a two-dopant

29

system consisting of a green phosphorescent material and a yellow fluorescent emitter. This is

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known as a sensitized fluorescence system5,6, where the phosphor can transfer triplet excitons

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into the singlet state of the fluorophore, thereby increasing the efficiency of fluorescence.

32

Several studies have investigated energy transfer processes in OLEDs by observing changes in

33

the intensity of electroluminescent (EL) spectra7–11. These studies only present qualitative

34

information on how a factor affects energy transfer efficiency and it is often limited to studying

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only either Dexter or Förster transfer. Additionally, relative energy transfer efficiency are often

36

deduced based on spectral overlap and triplet/singlet energy levels of the donor and acceptor

37

materials

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spectrum of the acceptor is needed to facilitate energy transfer. Another method involving time-

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correlated single photon counting (TCSPC) technique has been used on doped organic films in

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order to measure the energy transfer efficiency based on the transient decay time of the donor

41

emission 12. This method does not distinguish between Förster and Dexter energy transfer. In this

7,12

. A large overlap between the emission spectrum of the donor and the absorption



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work, we present a method that can quantify the amount of Förster and Dexter energy transfer

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occurring between the two dopant in an OLED as well as study the behavior of the two

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competing processes with changing dopant concentrations.

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EXPERIMENTAL AND THEORETICAL METHODS

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Experimental Details. All OLED devices examined were fabricated by thermal evaporation in

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the Kurt J. Lesker Cluster tool, which operates at a base pressure of 10-8 torr. Prior to fabrication,

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pre-patterned indium tin oxide (ITO) substrates are cleaned using Alconox soap, acetone and

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methanol, followed by sonication. The sample is then subjected to UV ozone treatment for 15

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minutes before being loaded into the cluster tool. The devices tested consist of

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ITO/MoO3(1nm)/CBP(35nm)/EML(15nm)/TPBi(65nm)/LiF(1nm)/Al(100nm).

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system is implemented in the emissive layer (EML) with CBP used as the host material and the

54

green triplet phosphor, Ir(ppy)2(acac), and/or the yellow singlet fluorophore, Rubrene, used as

55

the dopant materials. The device structure and energy diagram for the sensitized devices are

56

shown in Figure 1.

A

host-guest

57

58 59

Figure 1. Schematic illustration of device architecture and energy diagram for sensitized

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fluorescent OLED devices used in this work 4 ACS Paragon Plus Environment

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For device characterization, the current-voltage-luminance (IVL) characteristics were

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measured in ambient conditions using the HP4140B picoammeter and Minolta LS110

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Luminance meter. The EQE for devices are measured using an integrating sphere and a silicon

64

photodiode. The EL spectrum is measured using an Ocean Optics USB4000 spectrometer.

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Measurements were taken on multiple devices of the same type to ensure reproducible and

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accurate results.

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Theoretical Details. Dexter transfer is a short-range process as significant molecular orbital

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overlap is needed in order to facilitate the exchange of electrons between the two molecules13,14.

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Alternatively, the Förster process occurs through dipole-dipole interactions and it is generally

70

considered a long-range process with interaction distance up to 10nm15. Both mechanisms

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depend on the separation distance, R, between the two interacting molecules, known as the donor

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and the acceptor. The rate constant for Förster and Dexter transfer are given by:

73 74

= = −

(1)

(2)

75

where kR is the decay rate constant of the donor in the absence of energy transfer, R0 is the

76

Förster radius, A is a constant related to the spectral overlap between the donor and acceptor

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molecules and L is the sum of their effective van der Waals radii. The efficiency of either

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process, , can be calculated as the rate constant of that process over the sum of the rate

79

constants for all competing processes. This can be expressed as:

80

= ∑

(3)

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By analyzing the EL spectra from the two-dopant OLEDs and fitting the extracted data to

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Equations 1-3, we will demonstrate in the follow text a method to extract values for R0 and L. 5 ACS Paragon Plus Environment


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Furthermore, this EL spectra data analysis method enables us to study the competition between

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Förster and Dexter transfer mechanisms in the devices.

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The concentration of Rubrene in the EML was varied between 0.2wt% and 16wt.%, with the

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Ir(ppy)2(acac) concentration held at ~7wt.%. Ir(ppy)2(acac) was chosen for its high

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photoluminescence quantum efficiency (ΦPL = 0.94)16 and strong exciton capturing capability in

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CBP17. It is desirable for the higher energy dopant to capture all the excitons first in order to

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allow subsequent energy transfer to lower energy emitters. Generally, excitons will not transfer

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from a lower energy dopant to a higher energy one. The concentration of Rubrene is varied in

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order to change the average distance between Rubrene and Ir(ppy)2(acac) molecules, thus

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affecting the energy transfer efficiency between the two materials.

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Figure 2a displays the normalized EL spectrum for sensitized OLED devices as well as pure

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emission from Ir(ppy)2(acac) and Rubrene, which have a peak emission at ~520nm and ~560nm,

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respectively. The emissive layer composition of each device is summarized Table 1. In the

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sensitized system, there are emission bands from both dopants. The EL spectrum can be resolved

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into its Ir(ppy)2(acac) and Rubrene emission components, as shown in Figure 2b. There is a

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clear trend of decreasing green emission and increasing yellow emission at higher Rubrene

99

concentration. This indicates more efficient energy transfer occurring from the phosphor to the

100

lower energy fluorophore.


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101 102

Figure 2. (a) Normalized EL spectra for select devices tested in this study biased at 7V (EML of

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devices are summarized in table below spectra) (b) Non-normalized EL spectrum of Device B

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resolved into its Rubrene and Ir(ppy)2(acac) emission components, biased at 7V

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Table 1. Emissive layer composition of devices Device

Emissive Layer

A (standard)

7wt.% Ir(ppy)2(acac) in CBP

B (sensitized)

0.2wt.% Rubrene/7wt.% Ir(ppy)2(acac) in CBP

C (sensitized)

0.6wt.% Rubrene/7wt.% Ir(ppy)2(acac) in CBP

D (sensitized)

2wt.% Rubrene/7wt.% Ir(ppy)2(acac) in CBP

E (sensitized)

10wt.% Rubrene/7wt.% Ir(ppy)2(acac) in CBP

F (standard)

2wt.% Rubrene in CBP

G (neat layer)

Neat layer of CBP

106 107

This model assumes that Rubrene has little effect on exciton formation compared to a standard

108

device consisting of only 7wt.% Ir(ppy)2(acac) in CBP. It has been demonstrated previously that

109

direct exciton formation on the dopant is the likely mechanism for Ir(ppy)2(acac) in CBP18–19. A

110

previous study has also shown that Ir(ppy)2(acac) acts as carrier traps and exciton formation sites

111

in co-doped systems using CBP as the host17. Figure 3a and 3b below shows the turn-on voltages

112

at L = 1 cd/m2 for Devices A-G. The turn-on voltage for Device G, which has a neat layer of 7 ACS Paragon Plus Environment


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CBP as the EML, is 3.8V. A significantly reduction in turn-on voltage is observed for Device F,

114

with 2wt.% Rubrene in CBP, and a further reduction is seen for Device A, with 7wt.%

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Ir(ppy)2(acac) doped into CBP. This indicates that the Ir(ppy)2(acac) atoms play a significant role

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in carrier trapping and exciton formation in a CBP host. All the co-doped devices show the same

117

turn-on voltage of 3V as in Device A, thus suggesting a similar exciton formation mechanism.

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Some excitons may still form on the Rubrene molecules but this fraction of excitons should be

119

small enough that it is negligible. Further studies to justify similar a exciton formation

120

mechanism in the co-doped devices as in the Ir(ppy)2(acac) standard device is provided in the

121

supporting information.

122 123

Figure 3. (a) Luminance vs. voltage and (b) Turn-on voltages at L = 1cd/m2 for OLED devices

124

A-E and G

125

Under these assumptions, the total number of excitons captured in the sensitized system by

126

Ir(ppy)2(acac) can be extracted from the emission spectra of 7wt.% Ir(ppy)2(acac) in CBP as the

127

exciton formation mechanism should be similar in the two systems. Excitons on Ir(ppy)2(acac)

128

will primarily be triplets due to the high intersystem crossing rate in the phosphor20. The ISC rate 8 ACS Paragon Plus Environment

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129

for Ir(ppy)2(acac) has been calculated to be ~1012/s while fluorescence rates for Ir-based

130

phosphors are generally around 107/s and energy transfer rates are on the order of 107-108/s.12,21

131

Due to the many orders of magnitude difference, singlets on the iridium compound will be

132

quickly converted to the triplet state rather than be energy transferred to a Rubrene molecule. In

133

the sensitized system, the triplet excitons can either decay radiative on the Ir(ppy)2(acac) or be

134

energy transferred, through Dexter or Förster mechanism, to a nearby Rubrene molecule. Dexter

135

transfer will result in non-emissive triplets on the fluorophore, as the spin must be conserved in

136

this process, while Förster transfer leads to singlet state excitons that can decay radiatively on the

137

Rubrene. The emission from Rubrene can thus be related to the amount of Förster transfer

138

occurring in the system. Subsequently, the total energy transfer efficiency can be extracted by

139

comparing the Ir(ppy)2(acac) emission to its emission in the 7wt.% Ir(ppy)2(acac) in CBP

140

standard. Any change in its peak height compared to the standard must be due to energy transfer

141

to Rubrene. The total energy transfer is the sum of the Dexter and Förster processes.

142

The non-normalized EL spectrum is examined in order to extract the energy transfer

143

efficiencies. The total optical energy output can be obtained by integrating the spectrum over the

144

wavelength. By dividing this optical energy by the average photon energy emitted (calculated by

145

Equation 4), the number of photons emitted per second is obtained. Each photon corresponds to

146

an emissive exciton. To account for non-radiative processes, this number is divided by the

147

photoluminescence quantum yield (Φ) of the dopant being examined. By carrying out this

148

procedure for the emission spectrum of the 7wt.% Ir(ppy)2(acac) in CBP standard, we obtain the

149

total number of excitons captured by the Ir(ppy)2(acac) molecules, Ntot. As mentioned

150

previously, this can be used as an approximate for the total number of excitons captured by

151

Ir(ppy)2(acac) in the sensitized system. 9 ACS Paragon Plus Environment


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, =

152 153 154

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! "#$%&$

(4)

"#$%&$

where Eph,av is the average photon energy emitted, h is Plank’s constant, c is the speed of light and f(') is the EL spectrum.

155

For the sensitized fluorescence devices, the EL spectra must be resolved into its Rubrene and

156

Ir(ppy)2(acac) emission components, which can subsequently be used to calculate the number

157

excitons that decayed radiatively on Rubrene, nR, and Ir(ppy)2(acac) molecules, nIr, respectively.

158

Non-radiative processes are factored in using the photoluminescence quantum yield of the

159

emitters at their doping concentrations. From these numbers the Förster energy transfer

160

efficiency and total energy transfer efficiency can be calculated as follows:

161

% )*+,-+ =

162 163 164

% 8*-9: 8 =

./ /1/ 2343

× 100%

.343 ;. @= @A= /

× 100%

B

/

/ / B G CDE; @ @

=

F

/

?

? @> @= @A=

HIJ

× 100%

× 100%

(7a)

(7b)

(8a)


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=

172

/ F / / B G CDE; @ @ F / HIJ

CDE;

K

173

1IJ

= 1+

× 100%

A= =

(8b)

(9)

174 MNOP = +

175

(10)

176

where kD, kF, knr and kr are the rate constants for Dexter transfer, Förster transfer, non-

177

radiative and radiative decay, respectively. By substituting the expressions for the rate constants,

178

as defined in Equations 1 and 2, and dividing through by kr, we arrive at a theoretical model that

179

is a function of separation distance. An approximate separation distance was calculated from the

180

Rubrene weight percentage concentration by assuming molecules are hard-spheres and

181

distributed uniformly throughout a layer. The uniformity of the films is investigated in the

182

supporting information.

183 184

RESULTS AND DISCUSSION

185

Figure 4a displays the energy transfer efficiency as a function of donor-acceptor separation

186

distance. These numbers were calculated from the EL spectrum of devices at a current density of

187

60 mA/cm2. The three transfer efficiency equations are simultaneously fitted to the three sets of

188

experimental data to obtain the unknown parameters L and R0. R0 is the Förster radius and, as

189

mentioned previously, is generally on the order of 50-100Å14. B is related to the sum of the van

190

der Waals radii of the donor and acceptor molecules by L = 2B. L is generally on the order of a

191

few angstroms 14. From the curve fitting, we find R0 = 40.7 ± 2.8Å and L = 6.8 ± 0.06Å, both of

192

which are within theoretical ranges. Repeating these calculations for devices biased at different

193

current densities resulted in similar values for Ro and L, as shown in Figure 4b. At higher current 11 ACS Paragon Plus Environment


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194

densities there is a decline in the Förster radius measured. This is due to high brightness levels at

195

high current densities, which saturates the detector, leading to distorted EL spectra. Using PL

196

measurements and following the method as described in 15, the Förster radius for Ir(ppy)2(acac)-

197

Rubrene was calculated to be 43.4Å, similar to the R0 calculated using the method described in

198

this work.

199 200

Figure 4. (a) Energy transfer efficiency as function of separation distance for sensitized OLED

201

at a current density of 60 mA/cm2 (b) R0 and L values extracted from experimental data for

202

OLED biased at different current densities. Separation distances less than 1nm are shown in

203

dashed lines as they are only theoretical since these distances are on the order of the molecular

204

radii of the emitter molecules.

205

From Figure 4a, it is clear that the energy transfer efficiencies for the Förster and Dexter

206

mechanisms do not follow a simple trend of increasing or decreasing efficiency with separation

207

distance. In the Ir(ppy)2(acac)-Rubrene system, there is an optimal distance range in which

208

Dexter transfer is the major contributor towards energy transfer. At close separation distances, its

209

efficiency begins to drop off while the Förster transfer efficiency increases. A similar trend was 12 ACS Paragon Plus Environment

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210

observed in devices with an emissive layer consisting of Ir(ppy)2(acac) and DCM2. An R0 and B

211

value of 54.8 ± 1.6Å and 4.6 ± 0.1Å, respectively, was measured for this system. The relevant

212

experimental and fitted data in presented in the Supporting Information. It is generally accepted

213

that the Dexter mechanism is dominant at close interatomic distances, however this is not what

214

the fitted data suggests. Whether the Förster or Dexter mechanism is dominant at any given

215

separation distance depends on their relative rate constants. At some close enough distance the

216

Forster mechanism becomes dominant due to its 1/R6 relation. However, in this Ir(ppy)2(acac)-

217

Rubrene system, it is unlikely that the Förster mechanism would actually ever overtake Dexter at

218

small distances since the fitted data shows that this only occurs at separation distances less 1nm.

219

This is on the order of the molecular radius of the emitters and the dopants cannot pack closer to

220

one another than the size of the molecules. It is important to note that this is not a universal trend

221

for all materials but depends on the R0 and L values associated with the pair of dopant molecules

222

used.

223 224

We can see in Figure 5 that the Förster and Dexter energy efficiency behaviour as a function of

225

concentration can change drastically with different R0 and L values. Figure 5a shows the effect

226

of changing R0 with L held constant at 6.4Å, while Figure 5b demonstrates the effect of

227

changing L with R0=40.7 bÅ. Each set of R0 and L simulates a different set of donor and

228

acceptor material. Using known values for R0 and L for a set of materials and equations 7-8, it is

229

possible to quickly plot out the Förster and Dexter energy transfer efficiency and determine the

230

optimal dopant concentration that favours the desired transfer mechanism.



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231 232

Figure 5. Förster energy transfer efficiency (dashed line) and Dexter energy transfer efficiency

233

(solid line) with (a) changing R0 and (b) changing L

234 235

CONCLUSIONS

236

In summary, we have demonstrated a simple method for quantifying energy transfer efficiency

237

in a two-dopant system using EL emission spectra. This study allowed Dexter and Förster

238

mechanisms to be studied individually as a function of separation distance in a sensitized

239

fluorescence system. In an Ir(ppy)2(acac)-Rubrene system, a Forster radius of 42.7Å was

240

calculated and we demonstrated a very different Förster and Dexter energy transfer efficiency

241

profile than what is expected when the two mechanisms occur independently. We expect that this

242

work can be used towards designing stable white OLEDs with a single emissive layer for use in

243

general lighting applications.

244 245

ASSOCIATED CONTENT


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246 247


Supporting Information. Film uniformity. Sensitized system. Exciton formation mechanism. Experimental and fitted data for Ir(ppy)2(acac)-DCM2 system. Device lifetime data.

248 249

AUTHOR INFORMATION

250

Corresponding Author

251

*E-mail: [email protected].

252

Author Contributions

253

The manuscript was written through contributions of all authors. All authors have given approval

254

to the final version of the manuscript.

255

Notes

256

The authors declare no competing financial interest.

257 258 259 260

ACKNOWLEDGMENT This research was supported through Canada’s National Science and Engineering Research Council (NSERC) and the Canada Foundation for Innovation.

261 262

REFERENCES

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Figure 1. Schematic illustration of device architecture and energy diagram for sensitized fluorescent OLED devices used in this work 31x18mm (600 x 600 DPI)

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Figure 2. (a) Normalized EL spectra for select devices tested in this study biased at 7V (EML of devices are summarized in table below spectra) (b) Non-normalized EL spectrum of Device B resolved into its Rubrene and Ir(ppy)2(acac) emission components, biased at 7V 343x160mm (150 x 150 DPI)


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Figure 3. (a) Luminance vs. voltage and (b) Turn-on voltages at L = 1cd/m2 for OLED devices A-E and G 340x160mm (150 x 150 DPI)



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Figure 4. (a) Energy transfer efficiency as function of separation distance for sensitized OLED at a current density of 60 mA/cm2 (b) R0 and L values extracted from experimental data for OLED biased at different current densities. Separation distances less than 1nm are shown in dashed lines as they are only theoretical since these distances are on the order of the molecular radii of the emitter molecules. 343x150mm (150 x 150 DPI)


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Figure 5. Förster energy transfer efficiency (dashed line) and Dexter energy transfer efficiency (solid line) with (a) changing R0 and (b) changing L 330x144mm (150 x 150 DPI)



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23x14mm (600 x 600 DPI)


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Quantifying Interdopant Exciton Processes in ... - ACS Publications

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