edmr of meh-ppv leds
TRANSCRIPT
Physica B 308–310 (2001) 1078–1080
EDMR of MEH-PPV LEDs
G.B. Silvaa, L.F. Santosb, R.M. Fariab, C.F.O. Graeffa,*aDep. de Fisica e Matematica, FFCLRP-USP, Av. Bandeirantes 3900, 14040-901 Ribeir *ao Preto, Brazil
b IFSC-USP, CP 369, 13560-970 S *ao Carlos, Brazil
Abstract
In this work, electrically detected magnetic resonance (EDMR) at X-band is used to study the electronic properties ofpoly(2-metoxy-5-(20-etil-hexiloxy)-1,4-phenylene vinylene) (MEH-PPV) light-emitting diodes (LEDs). The EDMR
signal from MEH-PPV LEDs is found to be composed of two lines, a Lorentzian with peak-to-peak linewidth of 5G,and a Gaussian with peak-to-peak linewidth of 24G. The g-factor of both the components is about 2.002. The EDMRsignal amplitude is typically 10�5, and only observed at forward bias, for V > 10V. The signal is a quenching, and isassigned to the spin-dependent fusion of two like-charged polarons to spinless bipolarons. The Lorentzian component is
attributed to positive polarons fusion, and the Gaussian to negative polarons. The EDMR signal is found to depend onthe process of carrier injection, polaron mobility, temperature and indirectly on bipolarons. r 2001 Elsevier ScienceB.V. All rights reserved.
Keywords: MEH-PPV; LED; Magnetic resonance
1. Introduction
One of the most promising polymers is PPV and its
derivatives, showing good processibility and efficiency inoptoelectronic devices [1]. Among the various techni-ques of investigation, electron spin resonance (ESR) has
been used intensively [2], since it enables one to directly‘‘see’’ the polaron, a paramagnetic charge carrierspecies, which plays a major role in the physics ofconjugated polymers. On the contrary, electrically
detected magnetic resonance (EDMR), which hasprovided insight into various transport and recombina-tion processes in a wide array of semiconductors, has
not been used frequently [3]. In an EDMR experiment,microwave-induced changes in the conductivity aremeasured as the sample is subjected to a swept DC
magnetic field.
2. Experimental details
The poly(2-metoxy-5-(20-etil-hexiloxy)-1,4-phenylene
vinylene) (MEH-PPV) were obtained through standardprocedures [4]. The LED was made using ITO-coatedglass as the positive electrode. Over the ITO, a film of
MEH-PPV was spin coated using chloroform as solvent.Thermally evaporated Al was used as the negativeelectrode. Typical MEH-PPV film thickness was around360 nm. EDMR measurements were done using a
modified, computer interfaced Varian E-4 X-Bandspectrometer in the temperature range of 145–300K.In order to avoid degradation induced by O2 or H2O,
the sample was maintained under a nitrogen flux. Thespin-dependent changes of conductivity were measuredby modulating the static magnetic field (H0) and using
lock-in detection of the resonant current changes.The results discussed are obtaining from the investi-
gation of about 7 diodes. An intrinsic problem found in
the investigation of these devices was the poor reprodu-cibility of the device itself. For example, the active layerthickness varied from sample to sample; these variationswere reflected in the device operation characteristics, in
special the luminescence efficiency, and the I � V curves.
*Corresponding author. Tel.: +55-16-602-3763; fax: +55-
16-633-99-49.
E-mail address: [email protected] (C.F.O. Graeff).
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 8 5 9 - 6
The devices investigated were not encapsulated, andeven though we have avoided exposing the diode to air
and/or light, which are known to be responsible for thedevice degradation [5], the operation lifetime was hardlylonger than 100 h. We have tried to work as close as
possible to the room temperature, in an attempt to makesure that the transport/recombination/injection processthat we observe are those that are important for thecommercial use of these devices.
3. Results and discussion
Fig. 1 shows the typical EDMR spectra from different
MEH-PPV light emitting diodes. The signal could onlybe observed in forward bias. From a phase analysisdependence on modulation frequency, it is found to be a
quenching signal, or in other words, the conductivity ofthe device decreases in resonance. The LEDs spectra inFig. 1 have different emission efficiencies, decreasing
from (a) to (c). Notice, however, that the I � Vcharacteristics of these samples are quite comparable.For the conditions shown, VE18V and TE200K, thecurrent was around 2� 10�4 A. In fact, the LED whose
spectra are shown in Fig. 1(c) had an electrolumines-cence that could not be seen by naked eyes. The spectrain general can be simulated by the composition of two
lines, a Lorentzian and a Gaussian. The g-factor of bothlines varies from sample to sample between 2.001 and2.003. The origin of the variation in g-factor is not well
known; however, the values are closer to those ofcommonly found for positive or negative polarons inconjugated conductive polymers [3]. The Lorentzian line
has a peak-to-peak linewidth of 5.070.5G, while theGaussian line has DHpp ¼ 2471 G. Care was taken sothat the signals were not saturated. In the worst
situation, saturation started at about 20mW. As canbe seen in Fig. 1, the relative intensities of both Gaussian
and Lorentzian lines are dependent on the light emittingefficiency; the bad emitter has the highest Lorentzian/Gaussian line amplitude ratio. In fact, for the sample
just mentioned the Gaussian component is hardlyobservable.In the samples, where the Gaussian component had a
significant amplitude, it was observed that apart from
the in-phase signal, there was also a signal in quad-rature. By the adequate change in phase settings, in fact,the Gaussian component could be isolated. This
phenomenon has been well described by Dersch et al.[6] in a-Si :H. It happens when the EDMR componentsobtained from process have different response times.
Thus, the fact that we do observe such an effect is anindication that the Gaussian and Lorentzian lines comefrom different spin dependent transport processes in the
diode. In some diodes it was possible to observe not justan EDMR signal but also the conventional ESR signal.The ESR lineshape in all cases could be well fitted usingthe parameters of the Lorentzian line found in the
EDMR spectra.In Fig. 2, the EDMR signal (Ds=s) is plotted against
the bias voltage as filled symbols, for two different
samples. In the same plot, the signal-to-noise (S/N) ratioas a function of bias voltage is plotted for one of thesamples as open circles. Notice that the two samples
present more or less the same behavior. However, theabsolute value of the EDMR signal is different by afactor of 2–3. We have observed that for the same diode,normally near the end of its operation lifetime, the
EDMR signal could decrease by as much as a factor of10 in exactly the same experimental conditions. For bothdiodes, 10V is just when the diode starts to emit light.
As can be seen for Vo17V, the signal increases, remains
Fig. 1. Typical EDMR signal for different MEH-PPV LEDs.
From (a) to (c) the light emission efficiency decreases.
Fig. 2. EDMR signal amplitude (filled triangles) and signal-to-
noise ratio (open circles) as a function of bias voltage, for
different LEDs.
G.B. Silva et al. / Physica B 308–310 (2001) 1078–1080 1079
constant up to Vo27V, and then drops. The variationsare small, of the order of a factor 10 maximum. This
complex behavior is not understood in detail at present.However one important point to note is that for thesedevices, the luminescence is not homogeneous for the
whole sample, it is in fact concentrated in certain ‘hot’spots. We are assuming that the initial increase of Ds=sis basically derived from an increase in the number orsize of the ‘hot’ spots. As the current injection increases
these hot spots stabilize, and so does the EDMR signalamplitude. For very high injection, a significant localtemperature increase cannot be ruled out, increases in
temperature are followed by a decrease in Ds=s: Notethat the S/N ratio does follow a similar behavior, andgives further support to the simple picture just described.
The consequence of having both a decrease in signalamplitude and S/N ratio, turned the detection at lowervoltages (or current injection) nearly impossible.
The temperature dependence of the EDMR signaldepends on the emitting efficiency of the diode. Fordiodes that have both Gaussian and Lorentzian lines thetemperature dependence for To220K is rather weak,
with a drop when the T reaches room temperature. Thediode with an EDMR signal identified as ‘pure’Lorentzian, has no such change in behavior and can
be simulated by Ds=spT�2:2: This temperature beha-vior is not understood in detail at present.EDMR has already been used for the characterization
of similar diode structures based on PPV, summarized inRef. [3]. However, quite different results have beenfound on ITO/PPV/Ca structures. For example, theEDMR signal amplitude was found to be as strong as
10�3, and temperature independent for 20Ko To296K. The origin of such differences between thepresent and previous studies is not understood.
EDMR only probes paramagnetic states involved inthe conduction process. As a consequence bipolaronswhich have S ¼ 0 for example, are not observed directly
by EDMR (or ESR). The two components observed byEDMR are assigned to polaron-polaron fusion, whichresults in a bipolaron [3]. The Lorentzian line is
observed in ESR as well as in the EDMR and forESR assigned to positive polarons. As mentionedearlier, the diodes with the worst light emission efficiencywere those whose Lorentzian line amplitude was
dominant. One possible explanation for the bad ELefficiency is an unbalanced carrier injection, in our case apoor negative polaron injection. Thus a surplus of
positive carriers exists inside the bad emitting diode,which correlates with a higher Lorentzian line ampli-tude. Thus we attentively assign the Lorentzian line to
p++p+-bp++, while the Gaussian line is attributedto p�+p�-bp��. The differences in linewidth areprobably derived from the differences in mobility. One
could expect that the more mobile p+ has a lineshape
that is motionally narrowed. In this simple explanation,the difference in mobility can also explain the quad-
rature signal. However, at this point more evidence isneeded to further attest this proposition. Note that thefact that the amplitude of the EDMR signal varies from
sample to sample as well as for the same diode, is anindication that non-spin dependent transport paths arepresent in this diode. As discussed elsewhere [7], theeffect of having non-spin dependent transport paths
(bipolarons) parallel to spin dependent ones, is anoverall decrease in Ds=s:
4. Conclusions
EDMR has been used to study the transport/recombination/injection of light emitting diodes based
on MEH-PPV. The EDMR signal was found to becomposed of two lines a g-factor around 2.002. The firstline can be fitted by a Lorentzian with a DHpp=
5.070.5G. The second line is a Gaussian with DHpp=2471G. The relative amplitude of those componentswas found to be dependent on light emission efficiencyof the diode. The Lorentzian is dominant for bad
emitters. It is proposed attentively that the Lorentzianline is related to the fusion of positive polarons, whichcreates a positive bipolaron, while the Gaussian line
comes from the same process however for negativepolarons. It is found that EDMR in ITO/MEH-PPV/AlLEDs can qualitatively provide information about
carrier injection, polaron mobilities as well as indirectlyindicate the presence of bipolarons.
Acknowledgements
This work was supported by FAPESP, CNPq and
CAPES.
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