Supplementary material
Efficient Triplet Harvesting by Fluorescent Molecules through
Exciplexes for High Efficiency Organic Light-Emitting Diodes
Young-Seo Park and Jang-Joo Kim*
1. Determination of the reverse intersystem crossing (RISC) rate of exciplex
Transient processes of the singlet and triplet exciplexes can be schematically
represented as in Figure S1. In the presence of the ISC and the RISC between the singlet
and triplet states, the rate constants of the prompt and delayed fluorescence ( k p and
k d ) can be expressed as follows:1
k p  k rS  k nrS  k ISC ,

k ISC
k d  k nrT  1  S
S
 k r  k nr  k ISC
(S1)

k RISC .

(S2)
Here, we assumed that the radiative and non-radiative rate of the singlet state ( k rS
and k nrS ) and the ISC rate ( k ISC ) are much faster than the non-radiative rate of the
Figure S1. Schematic diagram of the transient processes of the singlet and triplet exciplexes.
triplet state ( k nrT ) and the RISC rate ( k RISC ). This assumption is supported by the much
longer decay time of the delay emission in the transient PL data.
The PL quantum efficiencies of the prompt and delayed components (  p and  d )
are given by
k rS
,
p  S
k r  k nrS  k ISC
(S3)

 d   ( ISC  RISC ) k  p ,
(S4)
k 1
where  ISC and  RISC are the quantum efficiencies for ISC and reverse ISC,
respectively. From Equations S1 to S4, following equation can be obtained:1
k RISC 
k p kd  d
k ISC  p
(S5)
Since the PL efficiency of the TCTA:B3PYMPM exciplex is ~100% at 35K, we can
assume k nrS  0 at this temperature. Then, k p ,  p , and k ISC can be expressed as
follows:
k p  k rS  k ISC
(S6)
k rS
p  S
k r  k ISC
(S7)
k ISC  k p (1   p )
(S8)
We can obtain k ISC =1.1×107 s-1 from Equation S8 by substituting k p =1.8×107 s-1
and  p =0.38, which are obtained from the transient PL data in Figure 2a.
The transient PL decay curves of the TCTA:B3PYMPM exciplex shown in Figure 2a
and Figure S2 cannot be fitted with double exponential curves but correspond to a
multi-exponential decay model. Therefore, k d cannot be defined as a specific value,
but rather has a range depending on delay time.
Figure S2 shows the minimum and
maximum values of k d at various temperatures. k d decreases as the delay time
Figure S2. Minimum and maximum values of the decay rate of the delayed fluorescence of the
TCTA:B3PYMPM film at various temperatures. The black lines are experimental transient PL
decay curves. The red and blue lines are the fitting lines with the minimum and maximum
values of the delayed fluorescence decay rate, respectively.
increases. It may due to broad distribution of the singlet and the triplet energy level of
the TCTA:B3PYMPM exciplex. Also, k d decreases as the temperature decreases. It
indicates k nrT is reduced as the temperature decreases.
Assuming that k ISC is independent of temperature because the ISC process is
considered as an adiabatic process, the range of k RISC can be calculated by using
Equation S5. Figure S3 shows the range of k RISC of the TCTA:B3PYMPM exciplex
against temperature. The range of k RISC is 3×106 ~ 3×104 s-1 and k RISC is almost
independent of temperature. It indicates the RISC of the TCTA:B3PYMPM exciplex
occurs without thermal activation.
Figure S3. The range of the RISC rate of the TCTA:B3PYMPM film against the temperature.
The black square and red circle indicate the minimum and maximum values of the RISC rate,
respectively. The sky-blue area indicates the range of the RISC rate.
2. Transient PL decay at different wavelength
The Figure S4 shows the transient PL decays of the TCTA:B3PYMPM film at 450
nm and 550 nm wavelength. At 300 K, the lifetime of the prompt emission of the
exciplex is 61 ns at 450 nm and 82 ns at 550 nm. At 35 K, the lifetime of the prompt
emission of the exciplex is 65 ns at 450 nm and 88 ns at 550 nm. The lifetime at 550 nm
(~80 ns) is slightly longer than at 450 nm (~60 ns) due to the red-shift of the exciplex
emission spectrum. However, the decay rate of the delayed emission at 450 nm is
similar with that at 550 nm. 450 nm is shorter wavelength than prompt emission peak,
and 550 nm is longer wavelength than delayed emission peak. Therefore, it indicates
energy level of the triplet exciplex is broad enough to overlap with that of the singlet
exciplex.
Figure S5. Transient PL decay of the TCTA:B3PYMPM film at 450 nm and 550 nm
wavelength (a) at 300 K and (b) 35 K. Inset: Enlarged image of the main graph.
3. Transient EL measurement of the OLED using TCTA-B3PYMPM exciplex
The Transient EL of the OLED using TCTA-B3PYMPM was measured by an Agilent
8114A pulse generator combined with a monochromator attached photomultiplier tube
(PMT). The excitation pulse width was 100 μs and the baseline voltage was -1 V. The
structure of the devices for the transient EL measurement was as follow: TAPC (30 nm)
/ TCTA (10 nm) / TCTA:B3PYMPM (30 nm) / B3PYMPM (30 nm) / LiF (1 nm) / Al
(100 nm).
The transient EL decays at various current densities shown in Figure S4a. The
transient EL exhibits long delayed emissions up to tens of micro-seconds. One plausible
origin of the delayed emission is the TTA of the triplet exciplex. Since the TTA process
involves second-order kinetics in terms of the concentration of the triplet excited states,
the emission intensity of the delayed fluorescence by the TTA should be proportional to
square of the current density if the formation of the triplet excited states is linearly
proportional to the current density.2 In contrast to the expectation, however, the intensity
of the delayed EL emission of the OLED is linearly proportional to the current density,
as shown in Figure S5b, demonstrating that the TTA of the exciplex is not the origin of
the delayed emission of the OLED in the TCTA-B3PYMPM system. Therefore the
RISC must be the origin of the delayed emission in the OLEDs. It is worth noting that
the exciplex cannot diffuse because the exciplex forming materials have no absorption
in the spectral range of the exciplex emission. Therefore, the collision probability
between the two triplet exciplexes would be extremely low resulting in extremely low
TTA.
Figure S5. (a) Transient EL decay of the OLED at various current densities. (b) Intensity of the
delayed EL emission of the OLED as a function of the current density.
4. Time resolved EL spectra of the OLED using TCTA-B3PYMPM exciplex at two
different temperatures
(a)
(b)
Figure S5. (a) Time resolved EL decay of the OLED at various delay times at room temperature
(a) and 220K (b).
Reference
1. K. Goushi, K. Yoshida, K. Sato, and C. Adachi, Nat. Photonics 6, 253 (2012).
2. C. Ganzorig and M. Fujihira, Appl. Phys. Lett. 81, 3137 (2002).