2.3.1 Introduction
In the introduction to the previous chapter, we have raised the question whether the form-ation of local current paths in composite-based OLEDs could be observed by scanning near-field optical microscopy. In contrast to the fluorescence microscopy described above, such an experiment involves the collection mode of SNOM and has to be performedin situ, i. e. with the OLED in operation. In consequence, there are two essential requirements to be fulfilled. First, the aperture of the SNOM has to be brought into the near-field of the emitter layer of functional OLED. The glass substrate being 1 mm thick makes the detec-tion through the cathode layer the only possible soludetec-tion. Second, the electroluminescence has to be strong enough to supply a signal through the cathode layer which is detectable in an area of about 100×100 nm2 or less. The OLEDs built from a PMMA/TPD blend did not fulfil this requirement. Instead, we have focussed on a different system, in collab-oration with the group of M. Schwoerer, Universit¨at Bayreuth, EPII. The single layer Alq3 devices prepared by this group proved to possess the required intensity for the experiment to work.
2.3.2 Experimental
A single-layer device was prepared by vapour deposition of Alq3 and a 15 nm thin, semi-transparent Al/Ca film on an ITO glass substrate. Measurements of such devices under ambient conditions have to be performed very quickly, as both cathode materials are very sensitive to oxidation and as the cathode layer is very thin. Figure2.10 shows the Alq3 device together with the detection setup. Simultaneous to the recording of the SNOM images optical micrographs can be taken, as the sample is self-luminescent and no illumination is necessary, which would disturb the SNOM measurement.
2.3.3 Results
Figure2.11 shows two optical micrographs of an early and a late stage of degradation, which document the expected instability of the OLED in air. In the early stage we perceive numerous dark spots of different sizes distributed homogeneously over the device.
The spots are all of a round shape and exhibit a protrusion in their centre. At a later stage the dark spots have grown and covered a large part of the device by coalescing.
The centre protrusion has grown and a dark rim has formed around the protrusion. This is in agreement with the results found by other groups [Sat94, Bur94, McE96, Fuj96, Do97, Azi98a, Azi98b, Lie00, Ke01, Kol01, Lim01, Ke02] and is ascribed to oxygen and humidity-induced degradation of the cathode.
After taking the first image, we focussed on one of the defects that was at the limit of visibility and performed collection-mode scans at that position (Figure2.12). The dark
2.3. Near-field Detection of the Electroluminescence of OLEDs 17 spot manifests itself very clearly in the EL signal by an inner part of constant count rate and a rim of increasing EL. Around the defect the EL intensity is nearly constant. A closer look, however, reveals a slight spatial variation which could originate from local current
~
GlassFibre
x-y-z-Piezo Stage
Distance Control Loop
Aperture Uexc
Lock-in
Alq3
ITO Glass
Ca/Al electrode (semi-transparent) APD
Udiode
10x
Figure 2.10: SNOM setup for local EL detection. The near-field condition is achieved by the use of a semi-transparent electrode. Shear-force detection is used to control the distance between tip and sample and the EL is detected by an avalanche photodiode (APD). Bottom view optical micrographs can simultaneously be taken through a 10× or 40× objective.
Beginning formation of dark spots Coalescing of dark spots
Figure 2.11: In situmicrographs of the dark-spot formation. The pictures were taken with a weak darkfield illumination. The main contrast—and the green colour—results from the electroluminescence of the device. Left: Early stage of degradation; many different sizes of dark spots can be found. Right: Late stage of degradation; most of the dark spots have coalesced, restricting the electroluminescent area to very small regions.
18 Chapter 2. Scanning Probe Techniques
Topography Electroluminescence
4µm
0 1 µm 0 5 kHz
APD signal z-Range
0 10 nm 0 100 MHz
EarlyStageFinalStagel
4 µm
Figure 2.12: Topographic and electroluminescence signal of an evolving dark spot. The upper graphs show the very early state of degradation: only a slight elevation is seen in the topographic image whereas the EL shows a clear defect the middle of the image. After 20 min of operation, the electrode is completely torn off and a huge protrusion has formed in the centre of the dark spot.
paths. The topographic image shows a shallow elevation in the middle of the defect which is probably the early stage of electrode delamination, sometimes also referred to as bubble formation [Ke02]. The second image was recorded 20 minutes later after the EL had completely vanished. The topographic image proves that the defect had exploded and torn off the electrode, while in the middle, a µm high protrusion had formed. The remaining EL signal is due to scattered light from different parts of the device.
At this point we want to draw attention to a remarkable fact: in the first scan there is no correlation at all between the EL signal and the topography; the sample is still flat at the border of the defect. We can use this fact to estimate the bare optical resolution of the SNOM without any topographical artefacts involved. For this purpose, we have made a cross section through the EL signal of the defect structure and focussed on the intensity profile at the inner border of the defect. The shape of the profile suggests that the EL intensity increases linearly with the distance from the inner border of the defect. In the
2.3. Near-field Detection of the Electroluminescence of OLEDs 19
Figure 2.13: EL intensity along a cross section through the defect. The shape of the EL at the inner border of the defect is modelled by a linear increase which has to be convoluted with the aperture function to give the detection signal of the APD. The fit leads to an effective optical resolution of 2 w=134 nm.
inner part the intensity is assumed constant:
I(x) =
The transmission of the aperture is modelled by a Gaussian aperture function A(x, y) of width w:
A(x, y) = 2 π w2 e−
2(x2+y2)
w2 (2.2)
The intensity IAPD that is coupled into the glass fibre is then given by the convolution of the emitted intensity and the aperture function. With the above assumptions we are led to an analytically solvable integral that can serve as a fit function for the data:
IAPD(x) =
20 Chapter 2. Scanning Probe Techniques The fit results in an aperture width of 67 nm, corresponding to an optical resolution of 2w=134 nm. Note that this value is a very conservative estimate since the emission intens-ity profile was modelled with a perfectly sharp edge. But already with this conservative value we have overcome the classical diffraction limit ofλ/2.
2.3.4 Conclusion
In summary, scanning near-field optical microscopy is capable of simultaneously detecting the topography and the electroluminescence of OLEDs. The EL intensity was found to be largely homogeneous over areas without dark spots. Very small variations in EL intensity, not resulting from dark spots, have been observed but could not be intensively studied since the experimental time was restricted by the fast degradation in air (≈30 min). The growth of dark spots has been monitoredin situ. In the early stage, the EL vanishes in a circular area, while only slight topographical changes are observed. As a side-effect, this finding allowed us to estimate the SNOM’s resolution to be better than 134 nm. At a later stage, the centre of the dark spot has grown eruptively and the electrode is torn off. We speculate that such micro-explosions are responsible for the self-insulating character of the dark spots. This is the more interesting as recent studies of OLEDs have shown that the degradation in vacuum leads to conducting defects which become insulating upon further operation in inert gas atmosphere (3.10). We are therefore currently planning comparative AFM investigations of devices that degrade in vacuum and in inert gas atmosphere.