Organic light-emitting diodes employing efficientreverse intersystem crossing for triplet-to-singletstate conversionKenichi Goushi, Kou Yoshida, Keigo Sato and Chihaya Adachi*

Light emission from organic light-emitting diodes that make use of fluorescent materials have an internal quantumefficiency that is typically limited to no more than 25% due to the creation of non-radiative triplet excited states. Here, wereport the use of electron-donating and electron-accepting molecules that allow a very high reverse intersystem crossingof 86.5% between non-radiative triplet and radiative singlet excited states and thus a means of achieving enhancedelectroluminescence. Organic light-emitting diodes made using m-MTDATA as the donor material and 3TPYMB as theacceptor material demonstrate that external quantum efficiencies as high as 5.4% can be achieved, and we believe thatthe approach will offer even higher values in the future as a result of careful material selection.

Organic light-emitting diodes (OLEDs) have great potential inthe realization of novel optoelectronic devices (for example,in flat-panel displays and lighting applications) because of

the characteristic features of these organic semiconductor materials,including flexibility over a large area, low-cost fabrication andhigh-performance optical and electrical properties. To enhancethe electroluminescence efficiency of OLEDs, various emissionmaterials based on fluorescence and phosphorescence have beendeveloped1–3. Although OLEDs that use fluorescent materials haveachieved high reliability, their internal electroluminescencequantum efficiency (hint), defined as the number of photonsgenerated per injected carrier, is limited to �25% because of theexciton branching ratio of the singlet excited states under electricalexcitation4,5. In contrast, OLEDs using phosphorescent materialshave achieved hint values of almost 100% (ref. 6). However, theonly phosphorescent materials found practically useful to date areiridium and platinum complexes, which are associated with ratherhigh costs. Thus, both fluorescence- and phosphorescence-basedOLEDs have advantages and disadvantages. With the aim ofachieving higher external electroluminescence quantum efficiencies(hext) of .5%, which is the limit for hext in fluorescence-basedOLEDs, researchers investigating other potential approaches havereported on the use of rare earth metal-free phosphorescentmaterials7,8 such as copper complexes, as well as extra-singletgeneration through triplet–triplet exciton annihilation (TTA)9. Inthe former case, although the copper complexes7 demonstrate arather high hext that is comparable to iridium complex-basedOLEDs, the relatively low reliability (in terms of device stability)and the high driving voltage characteristics (arising from the useof bipolar hosts with a wide energy gap) have proven problematic.In the latter case, meanwhile, the total singlet exciton generationefficiency through TTA is limited to a maximum hint of 62.5%(ref. 9). Accordingly, the two approaches remain unsatisfactoryand other mechanisms must be investigated.

Recently, we proposed a potential mechanism to enable hint¼100% without using phosphorescent materials by introducing anupconversion mechanism from triplet to singlet excited states10.We first demonstrated that the photoluminescence efficiency of

tin(IV) fluoride–porphyrin complexes can be increased from �1 to3% with an increase in temperature, due to activation of thereverse intersystem crossing (ISC) from triplet to singlet excitedstates. However, the activation energy of the reverse ISC (DEST)was rather high at 0.24 eV, so a smaller DEST is required. Here,DEST is proportional to the exchange energy between the highestoccupied molecular orbital (HOMO) and the lowest unoccupiedmolecular orbital (LUMO) in the molecules10. Thus, we expectthat a small DEST can be achieved by separating the HOMO fromthe LUMO because the exchange energy decreases exponentiallywith increasing HOMO–LUMO (electron–hole) separationdistance. Based on this concept, we previously designed amolecule—2-biphenyl-4,6-bis(1,2-phenylindolo[2,3-a] carbazol-11-yl)-1,3,5-triazine (PIC-TRZ)—that has donor–acceptor groupswith steric hindrance incorporated between them to form intra-molecular excited states11. With this molecule, a significant contri-bution of 30% reverse ISC efficiency was realized under bothphotoluminescence and electroluminescence processes.

In the present study, we demonstrate a strategy to realizeradiative-exciton production with higher efficiency than the fluor-escent limit of 25% by using the high reverse ISC efficiency of theintermolecular excited state (that is, exciplex state) betweenelectron-donating and electron-accepting molecules. In principle,the proposed mechanism can realize an internal electrolumines-cence quantum efficiency of 100% once the reverse ISC efficiencyreaches 100%. In fact, we demonstrate a high reverse ISC of 86.5%using an exciplex system in this report.

The exciplex is well known as a charge transfer state formedbetween electron-donating and electron-accepting moleculesunder photo- and electrical excitation12–23. Because the exciplexemission occurs as a result of electron transition from the LUMOof an acceptor to the HOMO of a donor, as shown in Fig. 1b, it isreasonable to consider that the HOMO of the donor and theLUMO of the acceptor contribute to the exchange energy, that is,DEST. The electron–hole separation distance corresponds exactlyto the distance between the donor and acceptor molecules,because the HOMO and the LUMO in exciplexes are mainlylocated on the donor and acceptor molecules, respectively.

Center for Organic Photonics and Electronics Research (OPERA), and Center for Future Chemistry, Kyushu University, 744 Motooka, Nishi, Fukuoka819-0395, Japan. *e-mail: adachi@cstf.kyushu-u.ac.jp

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Therefore, the intermolecular excited states should provide a smallerexchange energy compared to that of intramolecular excited states,resulting in the triplet levels being very close to the singlet levels24,25.Although there has been some research on electroluminescencefrom exciplexes, the main focus has been on the formation of aninterface for efficient electron–hole capture with high probabilityto improve the power conversion efficiency in OLEDs12, and forapplication to the tuning of the colour13,18, the white emissions14,19

and control of the carrier recombination zone with the aim of a highhext (ref. 21).

Appropriate selection of the donor and acceptor molecules iscrucial to confirm our concept. We therefore explored a widevariety of molecules that have been used in OLEDs. To find thebest combination, we conducted a transient photoluminescenceexperiment to detect the presence of exciplexes that demonstraterather long decay times compared with that of the promptfluorescence. Supplementary Fig. 1 presents transient decay curvesbased on various combinations of donor and acceptor moleculesin their films. Based on these results, we selected 4,4′,4′′-tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA) as adonor and 2-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-1,3,4-oxadia-zole (t-Bu-PBD) and tris-[3-(3-pyridyl)mesityl]borane (3TPYMB)as acceptors. The exciplex formation between m-MTDATA andt-Bu-PBD in OLEDs has been confirmed in an earlier work, indicat-ing that this combination is a promising candidate for achievinghigh efficiency21.

The photoluminescence spectrum of a 50 mol% m-MTDATA:t-Bu-PBD film is shown in Fig. 1c, with the fluorescencespectra of the neat m-MTDATA and t-Bu-PBD films forcomparison. The photoluminescence peak wavelength of the

m-MTDATA:t-Bu-PBD film is located at �540 nm, which issignificantly redshifted compared to those of the neatm-MTDATA and t-Bu-PBD films. This is due to exciplex formationbetween the m-MTDATA and t-Bu-PBD molecules. To improve thereverse ISC efficiency, the triplet exciplex state must be confinedusing electron-donor and electron-acceptor molecules with highertriplet energy levels. Otherwise, the triplet state of the exciplex iseasily quenched due to energy transfer to the triplet states of thedonor and/or acceptor molecules. To estimate the triplet energyof these molecules, the phosphorescence spectra of these filmswere measured at 10 K. Figure 1c also includes the phosphorescencespectrum of an m-MTDATA film at 10 K, in which thephosphorescence peak was observed at �475 nm, although nophosphorescence was observed for either the t-Bu-PBD films orthe m-MTDATA:t-Bu-PBD films. Although no phosphorescencehas been reported for t-Bu-PBD, phosphorescence spectra forPBD derivatives are available and the peaks are located at�510 nm (ref. 26); the phosphorescence peak of the t-Bu-PBDfilm would therefore be expected to be located at a similar wave-length. The m-MTDATA and t-Bu-PBD films have higher tripletenergy levels than the singlet energy of the exciplex state, whichresults in the confinement of the triplet state of the exciplex in them-MTDATA:t-Bu-PBD host matrix, because the triplet levels ofthe exciplex state are expected to be close to the singlet level.

Figure 2a shows a streak image of a 50 mol% m-MTDATA:t-Bu-PBD film at 300 K, which provides a visual image of thetime-dependent intensities of the prompt and delayed fluorescencecomponents. The intense emissions around t¼ 0 s correspond tothe prompt component, and the long tail emissions correspondto the delayed fluorescence component. The prompt component is

t-Bu-PBD

N CH3

N

CH3

NN t-Bu-PBD

O

N

t-Bu

N

H3C

m-MTDATA

2.0 eV2.4 eV

− LUMO level

m-MTDATA

5.1 eV+

6.1 eV HOMO level

m-MTDATAPhos.

m-MTDATAFluo.

t-Bu-PBDFluo.

1Exciplex

400 500 6000

Nor

mal

ized

pho

tolu

min

esce

nce

inte

nsity

(a.u

.)

Wavelength (nm)

a

c

b

Figure 1 | Photoluminescence spectra. a, Molecular structures of m-MTDATA and t-Bu-PBD. b, Scheme of exciplex emission formed between m-MTDATA

and t-Bu-PBD with HOMO and LUMO levels, estimated from ultraviolet photoelectron spectroscopy and by subtracting the optical energy gaps from the

HOMO energies, respectively. c, Fluorescence (fluo.) spectra of t-Bu-PBD (black), m-MTDATA (red), and 50 mol% m-MTDATA:t-Bu-PBD (green) films at

300 K, and the phosphorescence (phos.) spectrum of the m-MTDATA film (blue) at 10 K. The excitation wavelength for the films was 337 nm.

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assigned to the exciplex fluorescence, and the delayed component canbe assigned to exciplex fluorescence via the reverse ISC process. Aslight redshift between the prompt and delayed components wasobserved, which can be explained by the fact that the delayed fluor-escence is generated immediately after the reverse ISC process, sothat the nuclear configuration of the singlet excited state formedthrough the reverse ISC is affected by that of the triplet excited state.We propose that the polarization in the host medium at the singletexcited state is different from that at the singlet excited state formedthrough the reverse ISC because the polarization in the hostmedium varies with time27. As a result, the Frank–Condon factor(including polarization in the host medium) of the delayed fluor-escence would be different from that of the prompt fluorescence, sothe photoluminescence spectrum of the delayed component wouldbe slightly different to that of the prompt component. However,photophysical studies of benzophenones and benzil in solutionshave shown that the delayed fluorescence is identical to the promptfluorescence, unlike the exciplex emission28,29. In contrast, a slightredshift between prompt and delayed fluorescence has been observedfor 4-biphenylcarboxylic acid in a solid state30. Both the molecularmotion and the vibration of the emitter and surrounding moleculesare significantly restricted in the solid state compared to that insolution, which explains the difference in the spectral shift betweenthe solution and the solid state.

The photoluminescence decay curves for exciplex emission at 50and 300 K are shown in Fig. 2b. To quantitatively evaluate DEST, theactivation energy of the reverse ISC rate constant (kRISC) wasestimated based on exp(2DEST/kBT), where kB is Boltzmann’sconstant and T the temperature. kRISC can be estimated from theexperimentally observable rate constants and the photolumines-cence quantum efficiencies of the prompt and delayed componentsusing the following equation (Supplementary Fig. 2):

kRISC =kpkd

kISC

Fd

Fp

(1)

where kp and kd are the rate constants of the prompt and delayedfluorescence components, respectively, kISC is the ISC rate constantfrom singlet to triplet states, and Fp and Fd are the photolumines-cence quantum efficiencies of the prompt and delayed components.We show the temperature-dependent photoluminescence efficiencyof the prompt and delayed components of the exciplex formedbetween m-MTDATA and t-Bu-PBD in the SupplementaryInformation. kRISC is estimated using equation (1), and is shown

in Fig. 3 as a function of 1/T (Arrhenius plot) between 200 and300 K, assuming that kISC is independent of temperature. An acti-vation energy of 50 meV was estimated, which indicates that thetriplet energy level of the exciplex is quite close to the singletlevel. It should be noted that below 200 K, the decay curves donot agree with the double exponential decay model, but insteadcorrespond to a multi-exponential decay model. This can beexplained by the widened DEST distribution caused by the differencein the molecular environments at lower temperatures.

We next demonstrate the validity of this concept based on thedevice performance in OLEDs. A device structure was prepared,consisting of indium tin oxide (ITO; 110 nm)/m-MTDATA(20 nm)/50 mol% m-MTDATA:t-Bu-PBD (60 nm)/t-Bu-PBD(20 nm)/LiF (0.8 nm)/Al (50 nm). Figure 4a shows the externalelectroluminescence quantum efficiency versus current density(hext–J) characteristics. A maximum hext of 2% was obtainedaround J¼ 1021–100 mA cm22. The total photoluminescencequantum efficiency, including both the prompt and delayedcomponents, was 20% in the 50 mol% m-MTDATA:t-Bu-PBDfilm. Assuming that the singlet-exciton production efficiencyunder electrical excitation is 25% and the light outcouplingefficiency is 20–30% (ref. 31), then hext is limited to 1.0–1.5% in

0

a bT = 300 K

40

20

60

Tim

e (μ

s)

Promptcomponent

Delayedcomponent

80

Delayed com

ponent

Prompt com

ponent

450 500 550 600 650Wavelength (nm)

S1

kISC104

T1

krS knr knr

kRISC103

S0102

101300 K50 KPh

otol

umin

esce

nce

inte

nsity

(a.u

.)

0 50 100 150Time (μs)

S T

Figure 2 | Transient photoluminescence characteristics. a, Streak image and photoluminescence spectra of a 50 mol% m-MTDATA:t-Bu-PBD film at 300 K.

The photoluminescence spectrum was resolved into prompt and delayed components. b, Photoluminescence decay curves of the 50 mol% m-MTDATA:t-Bu-

PBD film at 300 K (black) and 50 K (red). The photoluminescence decay curves show integrated exciplex emission. Inset: energy diagram for the exciplex

formed between m-MTDATA and t-Bu-PBD. krS and knr

S are the radiative and non-radiative rate constants of the singlet state, respectively, kISC and kRISC are

the ISC and reverse ISC rate constants between the singlet and triplet states, and knrT is the non-radiative rate constant of the triplet state.

12.2

11.8

12.0

11.4

11.6

11.0

11.2ln(k

RISC

)

10.6

10.8ΔEST = 50 meV

3.5 4.0 4.5 5.010.4

10−3/T (K−1)

Figure 3 | Activation energy of reverse ISC rate. Arrhenius plot of the

reverse ISC rate from the triplet to the singlet state of the exciplexes formed

between t-Bu-PBD and m-MTDATA, with kISC set to 1× 106 (s21). Error bars

are estimated from the power fluctuation range of the excitation source.

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the OLED based on fluorescent materials with photoluminescencequantum efficiencies of 20%. These results suggest that theradiative-exciton production efficiency of 33–50% is realized dueto the high FRISC of the exciplex states, assuming that the carrierbalance ratio of electrons to holes is equal to 1.

The transient electroluminescence characteristics (Fig. 4b) alsosupport the higher exciton production efficiency by comparingthe photoluminescence and electroluminescence decay curves ofthe 50 mol% m-MTDATA:t-Bu-PBD film and the OLED, respect-ively. In the transient electroluminescence characteristics, thedelayed fluorescence from the reverse ISC process is significantlyenhanced. This corresponds to the proposed mechanism, becausethe triplet-exciton production efficiency (�75%) is higher thanthe singlet-exciton production efficiency (�25%) under electricalexcitation. The quantum efficiency for ISC and reverse ISC (FISCand FRISC) can also be estimated from the obtained transientelectroluminescence characteristics by combining the results ofthe photoluminescence decay curves. Assuming that the singlet-exciton production efficiency is 25%, the ratio between the electro-luminescence efficiencies of the prompt and delayed components(Fp

EL and FdEL) is given by

FELd

FELp

=

∑1

k=00.75×FpFRISC FISCFRISC

( )k+∑1

k=10.25×Fp FISCFRISC

( )k

0.25×Fp

(2)

The experimentally obtained values for Fd/Fp and FdEL/Fp

EL

were 0.639 and 4.882, respectively. From equations (2) and S5 (seeSupplementary Information), FISC and FRISC were estimated tobe 0.449 and 0.865, respectively. The results clarified that a highFRISC can be realized by using exciplexes, which will lead to highradiative-exciton production efficiencies.

The observed electroluminescence spectra are slightly redshiftedcompared to the photoluminescence spectrum. The new peaks inthe electroluminescence spectrum have often been explained bythe formation of electroplexes, which are not found in the photo-luminescence spectra21,32–34. However, the prompt and delayedcomponents of the transient electroluminescence spectra corre-spond to those of the transient photoluminescence spectra.Therefore, the observed redshifted emission is due to enhancementof the delayed fluorescence by electrical excitation.

A high reverse ISC efficiency for the exciplex between m-MTDATA and t-Bu-PBD has been demonstrated. Under electricalpulse excitation, the delayed fluorescence by the reverse ISC is

significantly enhanced, because the triplet-exciton productionefficiency (�75%) is higher than the singlet-exciton productionefficiency (�25%). The observed electroluminescence efficiency is,however, still at a low level. To demonstrate superior performance,we therefore explored the choice of material systems consisting ofdonor and acceptor molecules with higher photoluminescenceperformance. As a result, we found a superior exciplex stateformed between m-MTDATA as a donor molecule and 3TPYMBas an acceptor molecule in a 50 mol% m-MTDATA:3TPYMB film(Fig. 5). Although the photoluminescence efficiency still has a lowvalue of 26% (but slightly higher than the 20% of m-MTDATA:t-Bu-PBD), the delayed component of m-MTDATA:3TPYMBin the total photoluminescence efficiency (54.2% of the totalphotoluminescence efficiency) is significantly larger than that ofm-MTDATA:t-Bu-PBD (36.7% of total photoluminescenceefficiency), suggesting a superior electroluminescence performancein an m-MTDATA:3TPYMB-based OLED (SupplementaryFig. 1). The photoluminescence spectrum of a 50 mol% m-MTDATA:3TPYMB film is shown in Fig. 6b, with the fluorescencespectra of neat m-MTDATA and 3TPYMB films shown forcomparison. The photoluminescence peak wavelength of them-MTDATA:3TPYMB film is located at �540 nm, which issignificantly redshifted compared to those of the m-MTDATAand 3TPYMB neat films because of exciplex formation betweenthe m-MTDATA and 3TPYMB molecules. The m-MTDATA and3TPYMB films also have higher triplet energy levels thanthe singlet energy of the exciplex state, resulting in confinement ofthe triplet state of the exciplex in the m-MTDATA/3TPYMB

b

3TPYMB

2.0 eV

3.3 eV− LUMO

levelm-MTDATA

5.1 eV+

6.8 eV HOMO level

a N

N

B

N

3TPYMB

Figure 5 | Formation of exciplex state between m-MTDATA and 3TPYMB.

a, Scheme for exciplex emission between m-MTDATA and 3TPYMB with

HOMO and LUMO levels, estimated from ultraviolet photoelectron

spectroscopy and by subtracting the optical energy gaps from the HOMO

energies, respectively. b, Molecular structure of 3TPYMB.

101

100

10−1

10−4 10−3 10−2 10−1 100 101 10210−2

Exte

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(%)

Current density (mA cm−2)

a b100

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10−2

10−3

Photoluminescence

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(a.u

.)

0 10 20 30 40 50 6010−4

Time (μs)

Figure 4 | Performance characteristics of the OLEDs with m-MTDATA:t-Bu-PBD emitting layer. a, External electroluminescence quantum efficiency

as a function of current density for the ITO/m-MTDATA (20 nm)/50 mol% m-MTDATA:t-Bu-PBD (60 nm)/t-Bu-PBD (20 nm)/LiF (0.8 nm)/Al device.

b, Electroluminescence decay curve (red) for the device under short-pulse electrical excitation with a pulse of 500 ns and a current density of 100 mA cm22,

and photoluminescence decay curve (black) of the 50 mol% m-MTDATA:t-Bu-PBD film at 300 K.

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host matrix (Supplementary Fig. 3). To evaluate the advantageof the improved photoluminescence characteristics in them-MTDATA/3TPYMB exciplexes, we fabricated OLED structuresconsisting of ITO (110 nm)/m-MTDATA (20 nm)/50 mol% m-MTDATA:3TPYMB (60 nm)/3TPYMB (20 nm)/LiF (0.8 nm)/Al(50 nm). The value of hext was found to reach a maximum of5.4% around J¼ 10212100 mA cm22 (Fig. 6a), which is higherthan the limit of hext in fluorescence-based OLEDs. The observedelectroluminescence spectra are slightly redshifted compared tothe photoluminescence spectrum. This is because of enhancementof the delayed fluorescence by the electrical excitation, similar tothat in m-MTDATA:t-Bu-PBD exciplexes as noted above. We notehere that these devices are not optimized, so higher-efficiencyOLEDs would be expected after optimization of the device parameters.

In summary, we have demonstrated a pronounced enhancementof electroluminescence efficiency under electrical excitation byusing the reverse ISC process as a means to enhance the radiative-exciton production efficiency. Using m-MTDATA:3TPYMB, wedemonstrated hext greater than 5%, even with the rather lowphotoluminescence efficiency of 26%, showing that efficient ISCsignificantly contributes to increased hext. To further improve theelectroluminescence efficiencies in OLEDs based on delayedfluorescence from exciplexes, an appropriate choice of materialsystems consisting of donor and acceptor molecules will definitelyenhance device performance. Compatibility of the shallowHOMO and deep LUMO levels in the donor and acceptormolecules, as well as high photoluminescence efficiency, arekey factors for enhancement of OLED device performance. Thetransition dipole moment of the exciplexes is expected to beproportional to that of the donor and acceptor materials35,36,so highly luminescent donor and acceptor molecules shouldbe selected.

MethodsOrganic films for optical measurements were fabricated by thermal evaporationunder high vacuum (�7 × 1024 Pa) onto clean quartz and silicon substrates. Thephotoluminescence spectra of these films were recorded using a spectrofluorometer(Horiba Jobin Yvon, FluoroMax-4), and the photoluminescence quantumefficiencies were measured using an absolute photoluminescence quantum yieldmeasurement system (Hamamatsu, C9920-02). The transient photoluminescencecharacteristics were measured under vacuum using a streak camera (Hamamatsu,C4334). A nitrogen-gas laser with a wavelength of 337 nm and a pulse width of�500 ps (Lasertechnik Berlin, MNL200) was used as an excitation source.Low-temperature measurements were conducted using a cryostat (Iwatani IndustrialGases, CRT-006-2000) with application of an InGa alloy as an adhesive to ensuregood thermal conductivity between the silicon substrate and the sample holder.

The OLED devices were fabricated by thermal evaporation under high vacuum(�7 × 1024 Pa) onto clean ITO-coated glass substrates. Current density/voltage/luminance (J–V–L) characteristics were obtained using a semiconductor parameter

analyser (Agilent, E5273A) with an optical power meter (Newport, 1930C). Theelectroluminescence spectra of the OLEDs were obtained using a multichannelspectrometer (Ocean Optics, SD2000). For measurement of the transientelectroluminescence characteristics, short-pulse excitation with a pulse width of500 ns and a peak current density of 100 mA cm22 was used in combination witha streak camera.

Received 9 May 2011; accepted 25 January 2012;published online 11 March 2012

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101

a b

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10−3 10−2 10−1 100 101 10210−1

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cy (%

)

Current density (mA cm–2)

1

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3TPYMBFluo.

m-MTDATAFluo.

300 400 500 600 7000

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ized

inte

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(a.u

.)

Wavelength (nm)

Figure 6 | Performance characteristics of the OLEDs with m-MTDATA:3TPYMB emitting layer. a, External electroluminescence quantum efficiency as

a function of current density for the ITO/m-MTDATA (20 nm)/50 mol% m-MTDATA:3TPYMB (60 nm)/3TPYMB (20 nm)/LiF (0.8 nm)/Al device.

b, Fluorescence spectra of 3TPYMB (blue), m-MTDATA (green) and 50 mol% m-MTDATA:3TPYMB (red) films at 300 K, and the electroluminescence

spectrum of an ITO/m-MTDATA/50 mol% m-MTDATA:3TPYMB/3TPYMB/LiF/Al device at a current density of 10 mA cm22 (black).

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AcknowledgementsThe authors thank K. Tokumaru for fruitful discussions. This work was supported inpart by the Funding Program for World-Leading Innovative R&D on Science andTechnology (FIRST) and the Konica Minolta Science and Technology Foundation. Theauthors thank the Global Centers of Excellence (COE) programme ‘Science for FutureMolecular Systems’ of the Ministry of Education, Culture, Sports, Science and Technologyof Japan (MEXT).

Author contributionsK.G. and K.Y. designed the experiments, carried out the measurements of thephotoluminescence and electroluminescence characteristics and discussed the experimentaldata with C.A. K.S. provided experimental support and suggestions. K.G. and C.A. wrotethe manuscript.

Additional informationThe authors declare no competing financial interests. Supplementary informationaccompanies this paper at www.nature.com/naturephotonics. Reprints and permissioninformation is available online at http://www.nature.com/reprints. Correspondence andrequests for materials should be addressed to C.A.

ARTICLES NATURE PHOTONICS DOI: 10.1038/NPHOTON.2012.31

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