17.4 / M. Sims

17.4: A Novel Technique to Study OLED Function

Marc Sims, Steven W. Venter and Ian D. Parker DuPont Displays, 6780 Cortona Drive, Santa Barbara, CA 93117, USA

Abstract1 We employ electrically induced absorption (EIA) spectroscopy to probe the internal operation of a series of NPB/Alq3

(1) OLED structures. Our results complement the capacitance-voltage measurements of Brütting et al [3] and support the proposal of a fixed negative charge population at the NPB/Alq3 interface. We show a direct signature for hole injection under reverse bias (as predicted in [3,4]) and demonstrate that the voltage onset for hole injection, Vh (equivalent to transition voltage, Vo in [3,4]) is largely independent of anode material. Conversely, Vh appears to depend sensitively on the cathode material (an exchange of LiF/Al for CsF/Al raises Vh by +0.5V). Finally, we observe a rise in Vh after a prolonged period of current stress and propose that this finding originates in a gradual neutralization of the negative interfacial charge population.

1. Introduction The drive to establish charge carrier balance in organic light emitting diodes (OLEDs) led, in 1987, to the breakthrough development of multilayer architectures [1]. The performance enhancement achievable through this approach meant that a subset of materials could now be designed based almost exclusively on their respective electron or hole mobilities. In addition to facilitating charge balance, the use of multilayers offers the advantage of moving the recombination zone towards the centre of the device, away from electrode-related exciton quenching sites [2]. Moreover, hetero-junctions may be readily set up between layers, leading to charge accumulation and to a subsequent confinement of the recombination zone to the interface. A direct probe of interfacial charges would be useful, particularly as it could enable a study of the influence thereon of factors such as electrode and charge transport materials. In this study, we use electrically induced absorption (EIA) spectroscopy to directly monitor accumulated holes at the interface between the hole transport layer (HTL) and emissive layer (EML) in a common OLED device structure. We examine the influence of anode and

cathode material on the voltage onset for hole injection and on the subsequent density of accumulated holes. We also apply the technique, in conjunction with Stark spectroscopy, to study the effect of prolonged current stress on the accumulated hole density.

2. Experimental The general device structure reported in this work is anode / NPB (60 nm) / Alq3 (60-80 nm) / cathode (150 nm). The cathode structures used were LiF (0.4 nm) / Al and CsF (0.8 nm) / Al. All devices were fabricated on ITO covered substrates that were first patterned in-house, and then O2 plasma-treated immediately prior to any further processing steps. The anodes were either ITO, PEDOT:PSS (Baytron P CH8000)-coated ITO or DuPontTM HIL -coated ITO. Both PEDOT:PSS and DuPontTM HIL-coated ITO substrates were spin-coated from aqueous solution to a resultant film thickness of ~22 nm. PEDOT:PSS substrates were baked in air at 200 °C for 10 min and DuPontTM HIL substrates were baked in air at 140 °C for 7 min. The remaining materials were deposited in a vacuum deposition chamber with a base pressure of 7x10-7 torr. The organic materials (NPB and Alq3 from Sensient) were deposited at a deposition rate of 1.5-2.0 Å/s. The electron injection materials, LiF and CsF (both from Aldrich), were deposited at the lower deposition rates of 0.05-0.10 Å/s and 0.01-0.15 Å/s, respectively. Aluminium was vapor-deposited with a shadow mask at a deposition rate of 5 Å/s; the resulting device area was approximately 5.2 mm2. For the anode comparison experiment, the organic layers and cathodes were deposited in a single vacuum cycle. Devices were then encapsulated using a UV curable epoxy and a glass cover slip. OLED characterization was undertaken with a Keithley 236 source-measure unit coupled to a calibrated silicon photodiode. Electrically induced absorption (EIA) measurements were conducted in trans-reflectance geometry, and a double modulation approach was employed so that ΔT/T data could be reliably gathered without a background electroluminescence (EL) contribution [5]. The above technique was modified to operate in

Figure 1: Schematic of the double modulation set-up (under forward bias) used for the electrically

induced absorption (EIA) experiments described in the main text.

Reverse set-up (access to frequency domain)

1/(4πωAC)

T ΔT1/(4πωchopper)

Signal out lock-in 1;ref @ ωAC

ΔT (DC)Signal out lock-in 2;

ref @ ωchopper

EL

1/(4πωAC)

ΔELDC term EL ZERO

+ + +

ΔEL

ΔT

1/(4πωchopper)

Reverse set-up (access to frequency domain)

1/(4πωAC)

T ΔT1/(4πωchopper)

Signal out lock-in 1;ref @ ωAC

ΔT (DC)Signal out lock-in 2;

ref @ ωchopper

EL

1/(4πωAC)

ΔELDC term EL ZERO

+ + +

ΔEL

ΔT

1/(4πωchopper)

1 Chemical formulae for the abbreviated compounds are N-N'-diphenyl-N,N'-bis(1-naphtyl)-1,1'-biphenyl-4,4'-diamine (NPB) and 8-tris-hydroxyquinoline aluminium (Alq3).

SID 08 DIGEST • 223ISSN/008-0966X/08/3901-0223-$1.00 © 2008 SID

17.4 / M. Sims

‘reverse mode’, allowing better access to the frequency domain [6]. A schematic for this approach is given in Figure 1. The first lock-in (acting as a low time-constant prefilter) was referenced to the modulation frequency of the applied bias (ωosc = 5056 Hz), and the second to the modulation frequency of the optical chopper (ωprobe = 514 Hz). All the EIA data reported in the present contribution correspond to the in-phase component of the first harmonic of the probe frequency.

3. Results and Discussion In this section, we present the visible region of the EIA spectrum for NPB/Alq3 OLED structures. The main spectral features are first categorized into Stark and interfacial hole absorption bands. Focus is then placed specifically on the region corresponding to hole absorption. We monitor how the hole population at the NPB/Alq3 interface varies with applied voltage for a range of electrode materials. Figure 2 shows EIA spectra for a DuPontTM HIL / NPB / Alq3 / LiF / Al diode structure recorded at +1 V intervals between -5 V and +5 V. A broad absorption band centered at 490 nm is evident at a range of applied voltages. A comparison between the UV-vis absorption spectrum of NPB solutions co-dissolved with an oxidizing salt (FeCl3, spectrum not shown) enabled us to identify the broad feature as the high energy positive polaron (hole) absorption in NPB. The band centered at around 450 nm arises from the Stark response in the Alq3 layer [7]. Confirmation that the device had not degraded significantly over the duration of the experiment was obtained by a final re-measure of the spectrum at -5 V (labeled “-5Vb”). The signal strength (ΔT(X)/T) at 490 nm exhibited no frequency dependence around the 5.056 kHz probe frequency chosen for the experiment, indicating that we are indeed probing in the steady state. We consider that the NPB hole population is predominantly accumulated at the NPB/Alq3 interface due to the energy barrier for holes [ΔHOMO ≡ 5.7 (Alq3) – 5.4 (NPB) = 0.3 eV] and also since the hole mobility of NPB is much larger than in Alq3 [11, 12].

3.1 Influence of Anode Material on the Interfacial Hole Density (IHD)-Voltage Profile Figure 3 shows the interfacial hole density (IHD)-voltage profile for a variety of anode materials. Absorption was monitored at 490 nm and the signal strength can be assumed to be proportional to the hole density following injection from the anode.

For each of the three anode materials, there is a hole injection onset (Vh) at around -0.7 V, well below the built-in voltage (Vbi) of +2 V. This value is in good agreement with capacitance-voltage measurements in which a transition voltage, V0, of -0.5 V was reported [3,4] for a device with similar layer thicknesses. The apparent insensitivity of Vh to the type of anode material is interesting, particularly since there is a considerable variation in the associated work function (as determined from ultraviolet photoemission spectroscopy: DuPontTM HIL, ~5.5 eV [8]; PEDOT:PSS, ~5.1 eV [9]; O2 plasma-treated ITO, 4.35-4.75 eV [10]).The reason for the rise in the interfacial hole density IHD (for Vapp > 0 V) in the order PEDOT:PSS < O2 plasma-treated ITO < DuPontTM HIL is uncertain at this stage. Nevertheless, if we can assume a simple 1-to-1 relationship between the accumulated IHD and the hole current flowing through the NPB layer, then the results would suggest that PEDOT:PSS (Baytron P CH8000) is less effective at delivering holes into the device than O2 plasma-treated ITO. This is somewhat contradictory to what could normally be expected as a result of the higher PEDOT:PSS work

Figure 2: Electrically induced absorption (EIA) spectra for a DuPontTM HIL / NPB (60 nm) / Alq3 (60 nm) / LiF (0.4 nm) / Al OLED at a range of applied voltages. The broad bandcentered at 490 nm corresponds to the high energy optical transition of the positive polaron (‘hole’) in NPB.

Figure 3: Influence of anode material on the voltage dependence of the 490 nm EIA band for OLEDs with the following structures: anode / NPB (60 nm) / Alq3 (60 nm) / LiF(0.4 nm) / Al; DuPontTM HIL, open circles; O2 plasma-treated ITO, filled diamonds; and PEDOT:PSS (CH 8000), open squares. ΔT(R)/T is proportional to the accumulated hole density at the NPB side of the NPB/Alq3 interface. The inset shows corresponding J-V characteristics for the DuPontTM

HIL- and PEDOT:PSS-based structures. (J-Vs for O2 plasma-treated ITO-based devices are included but are indistinguishable from those of DuPontTM HIL-based devices.)

224 • SID 08 DIGEST

17.4 / M. Sims

function and subsequent improvement in energy level alignment. However, on referral to the J-V characteristics in the inset of Figure 3 it can be seen that there is nearly a 10x current density reduction in PEDOT:PSS based devices compared to DuPontTM HIL and O2 plasma-treated ITO based devices. This finding could be an expected result of the relatively low conductivity of Baytron P CH8000, and therefore may not be inconsistent with the NPB hole density data. Furthermore, the EL efficiency measured at 103 cd/m2 was found to decrease in the order PEDOT:PSS (4.0 cd/A) > O2 plasma-treated ITO (3.7 cd/A) > DuPontTM HIL (3.4 cd/A), which might indicate that reducing the hole delivery could be a favorable approach to improving charge balance. Interestingly, we find that this route towards improved charge balance is highly detrimental to operational lifetime. (The rate of EL degradation for the various anodes (driven at a constant current density of ~ 200 mA/cm2) increased in the order DuPontTM HIL (T70 ~ 80 h) < O2 plasma-treated ITO (T70 ~ 3 h) < PEDOT:PSS (CH 8000) (T70 ~ 0.7 h), where T70 indicates the time at which the total EL intensity had degraded to approximately 70% of its starting value.) Returning to Figure 3, at higher applied voltages (Vbi>Vapp>0 V), the interfacial hole population begins to level off, until at Vbi, when electrons enter the device, the signal increases once again. The latter observation is presumably a result of an accompanying rise in effective field strength in the NPB layer. As the voltage is raised even further (Vapp>Vbi), there is a gradual depletion of the interfacial hole population. These data are consistent with the existence of a sheet of negative charge residing at the NPB/Alq3 interface [3,4]. This charge layer redistributes the electric field, leading to a reduction in Vh. Interestingly, we observe a finite NPB polaron absorption at voltages below Vh in the DuPontTM HIL-based device (i.e. Vapp< -0.7 V). No such absorption exists in devices with O2 plasma-treated ITO or PEDOT:PSS anodes. The associated absorption strength (and therefore the hole density), was generally not found to change down to an applied voltage of -8 V. We attribute the origin of this background absorption to the existence of a residual population of pinned hole states at the HIL/NPB interface. The fact that we have only been able to observe this feature in DuPontTM HIL-based devices is consistent with the higher work function (Φ = 5.5 eV) of this material. Note that DuPontTM HIL is unique in the series in that its chemical potential is greater than the HOMO level of NPB (HOMONPB ~5.4 eV). Similar conditions can facilitate the transfer of charge from the organic layer to the anode, which causes the anode ‘Fermi level’ to pin to transport states of the organic layer [13, 14].

3.2 Influence of Cathode Material on the Interfacial Hole Density (IHD)-Voltage Profile Next, we have studied the influence of cathode material on hole injection into the NPB layer. This part of the investigation is limited to two cathode types, but forms part of a broader study that addresses a wider range of cathode materials, the results of which will be presented elsewhere. Figure 4(a) shows the IHD-voltage profile for a DuPontTM

HIL/NPB/Alq3/LiF/Al (filled circles) and a DuPontTM HIL/NPB/Alq3/CsF/Al (open circles) OLED structure. Figure 4(b) shows the corresponding J-V characteristics. The Alq3 thickness has been raised to 80 nm in these devices, leading to a decrease in Vh to Vh,LiF ≈ -2 V [3,4]. Overall, the IHD-voltage profiles for both device-types are broadly similar, except for one notable difference. For the case of a CsF/Al, we find that an additional ~0.5 V is required before holes are injected into the NPB layer (Vh,CsF ≈ -1.5

V). Figure 4(c) shows the corresponding NPB hole absorption spectra at -1 V. It can clearly be seen that the accumulated interfacial hole density in the CsF/Al device amounts to approximately 50% of that for the LiF/Al device at -1 V. Moreover, the observation that the Stark response at 450 nm (Alq3) is relatively insensitive to the choice of cathode material allows us to exclude the possibility that the change in population is indeed an artifact related to band overlap.

Figure 4: (a) Influence of cathode material on the voltage dependence of the 490 nm EIA band for OLEDs with the following structures: DuPontTM HIL / NPB (60 nm) / Alq3 (80 nm) / cathode; LiF (0.4 nm) / Al, open circles; and CsF (0.8 nm)/Al, filled circles. J-V characteristics for the same devices are shown in (b) and corresponding EIA spectra (Vapp = -1 V) are given in (c).

SID 08 DIGEST • 225

17.4 / M. Sims

In an attempt to explain these observations, we refer to the fixed negative charge model introduced by Berleb and Brütting [3]. In that seminal work, it was found that the presence of a substantial negative charge density at the NPB/Alq3 interface (6.8×1011

e/cm2) was required in order to explain a set of capacitance-voltage measurements on ITO/NPB/Alq3/Ca OLEDs. An important consequence of the interfacial charge population is that it increases the electric potential at or near the NPB/Alq3 interface, and in-so-doing, re-distributes the internal electric field. The result is that at large reverse bias, there is a lowering of the effective field in the NPB layer and a simultaneous rise in the field in the Alq3 layer. It is the lower field in the NPB layer that leads to the establishment of a flat band well below Vbi, which in turn facilitates a low Vh. The work showed that the field in the NPB layer at large reverse bias depends on the Alq3 thickness and the amount of fixed interfacial negative charge. In our comparison between LiF/Al and CsF/Al cathode-based structures, the layer thicknesses are identical, so the difference in electric field, and hence Vh must be attributed to a difference in amount of fixed interfacial charge. More precisely, it appears that a larger quantity of interfacial charge can be correlated with the evaporation of LiF/Al cathodes compared to CsF/Al cathodes. A rather speculative interpretation for this distinction could be related to differences between diffusion coefficients for LiF and CsF molecules and/or their constituent atoms.

Finally, we have applied the technique to probe the effect of long-term current stress on the internal electric field distribution. A related, but more detailed study that examines the effect of long-term forward bias on the capacitance-voltage characteristics of NPB/Alq3 diode structures can be found elsewhere [15]. Figure 5 shows the IHD-voltage profile for a DuPontTM HIL / NPB / Alq3 / CsF / Al OLED structure before (filled circles) and after (open triangles) stressing for an arbitrarily chosen period of 90 hours at an initial luminance of 6900 cd/m2 (200 mA/cm2). The data show that after this particular degradation protocol, Vh is raised by approximately 2 V, but still lies below the onset for electron injection, Ve (Ve = Vbi ~ 2 V). Clearly a more detailed map of how Vh depends on stress time is desirable, but nonetheless on close inspection, the data appear to be in good agreement with the capacitance-voltage measurements of Kondakov et al [15]. Based on the model of Berleb et al [3], the most obvious interpretation of the present data is that current stress counteracts the influence of the negative interfacial charge, leading, with time, to a reduction in the associated electric potential. Such a deactivation of the fixed negative charge could have an origin in, for instance, its gradual removal from the device and/or its neutralization (e.g. by the local addition of fixed positive charge [15]) during the passing of current. As a test, we measured the Stark response in the Alq3 layer (450 nm [7]) for the same device, over a range of applied voltages. The data are shown in Figure 6.

Unfortunately, given the relatively weak Stark response of Alq3 (and the relative thinness of the film), the signal-to-noise ratio is relatively poor over the voltage range of interest. Nevertheless, repeated measurements made on a number of identical structures proved that the observed differences between fresh and stressed devices were significantly greater than the associated statistical variation. In both devices, the effect of stress is most pronounced below Vh, where overlap with the hole absorption band can be neglected. A clear reduction in the electroabsorption signal strength is observed in the stressed device at 450 nm. This serves

as direct evidence for a stress-induced field reduction in the Alq3 layer. Taken together, the data suggest that long-term forward bias tends to neutralize the interfacial negative charge population.

4. Summary and Conclusions We have developed a new and versatile in-situ optical tool for probing OLED operation, and apply it here to the archetypal NPB/Alq3 device structure. As a complementary technique to capacitance-voltage measurements, EIA spectroscopy provides a

Figure 5: Influence of current stress on the IHD-voltage profile for a DuPontTM HIL / NPB (60 nm) / Alq3 (80 nm) / CsF (0.8 nm) / Al OLED, measured at 490 nm. The device was stressed at a constant current density (200 mA/cm2), for a time t (see main text for details).

Figure 6: Influence of current stress on the voltage dependence of the Alq3 Stark response (450 nm) measured for the same OLED used in Figure 5. (DuPontTM HIL / NPB (60 nm) / Alq3 (80 nm) / CsF (0.8 nm) / Al). At biases below Vh, signal overlap with the hole absorption band can be neglected.

226 • SID 08 DIGEST

17.4 / M. Sims

direct measure of the voltage dependence of the accumulated hole density on the anode side of the NPB/Alq3 interface. This has enabled us to address separately, in detail, the influence of different electrode materials on the interfacial hole density (IHD)-voltage profile, and discuss the results in the context of the interfacial negative charge model. We find that while Vh (equivalent to the transition voltage, V0, in capacitance-voltage measurements [3,4]) is relatively insensitive to our range of anode materials, it is significantly influenced by a relatively subtle change in cathode material (e.g. in this case changing from LiF/Al to CsF/Al). This suggests a link between cathode deposition and the amount of fixed negative charge at the NPB/Alq3 interface. We find the latter to be enhanced in the case of LiF/Al, and speculate that the difference could arise from a higher diffusivity in Alq3. We show preliminary data that sets out to address the effect of prolonged forward bias on the accumulated hole density in the diode. We found Vh to rise significantly after the device was current-stressed to 70% of its initial EL intensity. We attribute this shift in Vh to a partial neutralization of the anomalous negative interfacial charge population. This conjecture would be consistent with a conclusion drawn by Kondakov et al [15], where it was suggested that electrical aging leads to an accumulation of fixed positive charge, although the precise location in that study was undetermined. As an additional comment, we note that a number of the findings reported here are not a peculiarity of the NPB/Alq3 system. Indeed, we have encountered similar phenomena in a number of distinct OLED materials and device structures. Further characterization of the negative charge population and elucidation of its role in device performance would clearly benefit future OLED design.

5. Acknowledgements The authors wish to acknowledge S. Prakash and M. Yasa for help during the early stages of this work and extend their gratitude to P. A. Lane and M.A. Baldo for valuable discussions.

6. References [1] C.W. Tang, S.A. Van Slyke, “Organic Electroluminescent

Diodes,” Applied Physics Letters, Vol. 51 (12) pp. 913-915, 1987.

[2] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.-L. Bredas, M. Logdlund, W.R. Salaneck, “Electroluminescence in conjugated polymers,” Nature, Vol. 397 (6715) pp. 121-128, 1999.

[3] S. Berleb, W. Brütting, G. Paasch, “Interfacial charges and electric field distribution in organic hetero-layer light-emitting devices” Org. Electron., Vol. 1, 41-47, 2000.

[4] W. Brütting, S. Berleb, A.G. Mückl, “Device physics of organic light-emitting diodes based on molecular materials,” Org. Electron., Vol. 2 (1) pp. 1-36, 2001.

[5] M.P. Pires, P.L. Souza, J. P. V d. Weid, “New improved technique to measure photoreflectance,” Braz. J. Phys., Vol. 26, pp. 252-255, 1996.

[6] P.J. Brewer, J. Huang, P.A. Lane, A.J. de Mello, D.D.C. Bradley, J.C. de Mello, “Influence of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) in polymer LEDs” Phys. Rev. B, Vol. 74, pp. 115202/1-115202/6, 2006.

[7] F. Rohlfing, T. Yamada, and T. Tsutsui, J. “Electroabsorption spectroscopy on tris-(8-hydroxy-quinoline) aluminum-based light emitting diodes” J. Appl. Phys., Vol. 86, pp. 4978-4984, 1999.

[8] C.-H. Hsu, Y. Shen, E.M. Smith, D.D. LeCloux, H. Skulason, S. Kim, I.D. Parker, “A high work function hole-injection material for enhanced OLED performance,” SID Symposium Digest, Vol. 37, pp. 49-51, 2006.

[9] N. Koch, A. Kahn, J. Ghijsen, J. –J. Pireaux, J. Schwartz, R.L. Johnson, A. Elschner, “Conjugated organic molecules on metal versus polymer electrodes. Demonstration of a key energy level alignment mechanism,” App. Phys. Lett., Vol. 82, pp. 70-72, 2003.

[10] J. S. Kim, M. Granström, R. H. Friend, N. Johansson, W. R. Salaneck, R. Daik, W. J. Feast, and F. Cacialli, “Indium-tin oxide treatments for single- and double-layer polymeric light-emitting diodes: The relation between the anode physical, chemical, and morphological properties and the device performance,” J. Appl. Phys., Vol. 84, pp. 6859-6870, 1998.

[11] S. Naka, H. Okada, H. Onnagawa, J. Kido, T. Tsutsui, “Time-of-flight measurement of hole mobility in aluminum (III) complexes,” Jpn. J. App. Phys., Vol. 38, pp. L1252-L1254, 1999.

[12] S.-W. Liu, J.-K. Wang, “Charge mobility of mixed organic semiconductors: a NPB-AlQ3 study,” Proc. SPIE 6333, pp. 63331R/1-63331R/8, 2006.

[13] C. Tengstedt, W. Osikowicz, W.R. Salaneck, I. D. Parker, C.-H. Hsu, M. Fahlman, “Fermi-level pinning at conjugated polymer interfaces,” Appl. Phys. Lett., Vol. 88(5), pp. 053502/1-053502/3, 2006.

[14] P.A. Lane, P.J. Brewer, J. Huang, D.D.C. Bradley, J.C. de Mello, “Elimination of hole injection barriers by conducting polymer anodes in polyfluorene light-emitting diodes,” Phys. Rev. B, Vol. 74(12), pp. 125320/1-125320/7, 2006.

[15] D. Y. Kondakov, J. R. Sandifer, C. W. Tang, and R. H. Young, “Nonradiative recombination centers and electrical aging of organic light-emitting diodes: Direct connection between accumulation of trapped charge and luminance loss,” J. Appl. Phys. , Vol. 93, pp. 1108-1119, 2003.

SID 08 DIGEST • 227