New Organic thermally stable materials for optoelectronics devices - A
  linear spectroscopy study
Microsoft Word - Journal opto-otilia.doc
New Organic thermally stable materials for optoelectronics devices - a linear 
spectroscopy study 
Otilia Sanda Prelipceanu1,3, Marius Prelipceanu1,3, Ovidiu-Gelu Tudose1,3, 
Bernd Grimm2,3, Sigurd Schrader1,3 
1University of Potsdam, Institute of Physics, Condensed Matter Physics, Am Neuen Palais 10, 
D-14469, Germany 
2IDM, Institute of Thin Film Technology and Micro Sensorics, Kantstr.55, 
D-14513 Teltow, Germany 
3 University of Applied Sciences Wildau, Department of Engineering Physics, D- 15745 Wildau, 
Germany 
Contact address: otiliap@rz.uni-potsdam.de 
Published in Abstract Book of ICPAM07 Conference Iasi, Romania 
1. Introduction 
Thermally stable polymers have attracted a lot of interest due to their potential use as the 
active component in electronic, optical and optoelectronic applications, such as light-emitting diodes, 
light emitting electrochemical cells, photodiodes, photovoltaic cells, field effect transistors, 
optocouplers and optically pumped lasers in solution and solid state.  
Polymer-based structures are the focus of intensive investigations as mechanically and 
physically flexible, processible materials for large-area photoemitting and photosensitive devices. 
Their wide practical application is inhibited by present-day limitations in control over luminescent 
spectra, sensitivity and efficiency. We report results of our investigations into the use of thermal 
treatment of poly(p-phenylene vinylene) (PPV) films grown on a variety of substrates (quartz and 
glass). The samples studied had a thickness in the range 50 - 200 nm. Film thickness, morphology 
and structural properties were investigated by a range of techniques in particular: atomic force 
microscope - AFM, DEKTAK method, Ellipsometry and UV-VIS spectroscopy. 
2. Experiment Part 
Thin polymeric films are often used in the microelectronic industry, the development of 
optoelectronic applications. Homogeneous films with thickness varying from 50 – 200 nm are 
commonly prepared by spin coating. I this technique, polymers solution is dropped on the substrate 
surface (our case glass and quartz), which rotates at a given angular velocity during a give period of 
time. The film thickness is controlled by the concentration of the polymer is solution – 5% PPV in 
our experiment -, polymer molecular weight, spinning velocity and solvent evaporation rate. The 
polymers films are annealed at higher temperatures in vacuum and normal atmosphere and after this 
are investigated and results are compared. This work is concerned with the morphology of the thin 
films obtained from spin coating when different annealing method. The interactions between 
substrate, polymer and solvent were qualitatively correlated with the resulting surface morphology of 
spin coated films and treatment applied. We choose quartz and glass as substrates because this is 
transparent and easy for the spectroscopic investigations in transmission mode, and P-PPV dissolved 
in common solvents like toluene and chloroform. Moreover, the determination of the optical 
absorption and transmission, morphology and stability of the films are important for the development 
of electronic applications and waveguides[1].  
Analytical grade toluene and chloroform were used to prepare the solutions at the polymer 
concentration 5 mg mL-1. The P-PPV was dissolved in solvents, where no phase separation takes 
place. The chemical structure for PPV is schematically represented in Figure 1. 
Cl Cl
H2O2 / TeO2
NaOtBu
200oC
III IV
Figure 1. Synthesis of PPV ( after C.J. Brabec, et al.) 
3. Methods and Results  
Spin coating – The PPV films were prepared 
by spin coating on commercial quartz and glass 
substrates. The substrates dimensions of 1 cm x 2 cm 
were previously cleaned in standard manner and dried 
under a stream of N2 [2]. All coatings were performed 
with the spinning velocity of 2000 rpm and the 
spinning time of 60 seconds.  
Ellipsometry – The mean thickness and index 
of refraction (n) of the films were determined by 
means of ellipsometry in a Plasmos SD2000Automatic ellipsometer, Munich, Germany. The samples 
characteristics are shown in Table 1. 
Table 1. Characteristic of PPV films obtained from spin coating. All measurements 
were performed at 24 ± 2 0 C 
Dektak measurements – We measured and compared the morphology, thickness and aspect 
of films before and after treatment. For PPV films before annealing we obtained the thickness of 87 
nm witch is shown in figure 3, and in figure 4 after annealing in vacuum we obtained 45 nm and 
figure 5 and 6 shown aspects layers before and after annealing. [3]. 
Sample Solvent Thickness (nm) 
Reflection index 
P-PPV on quartz 
(before annealing) 
Toluene and 
Chloroform 
87 ± 5 1,3 ± 0.05 
PPV on Quartz 
(annealed in 
vacuum) 
Toluene and 
Chloroform 
45 ± 5 2,590 ± 0.05 
PPV on quartz 
(annealed in 
normal 
atmosphere) 
Toluene and 
Chloroform 
58 ± 5 2,567 ± 0.05 
PPV on quartz 
(after second 
annealing in 
vacuum) 
Toluene and 
Chloroform 
44 ± 5 2,578 ± 0,05 
Figure 2.  Spin coating deposition 
-100 0 100 200 300 400 500 600 700
87 nm
distance µm
-100 0 100 200 300 400 500 600 700
Distance (µm)
Figure 3.PPV film thickness before 
annealing 
Figure 4. PPV film thickness after annealing 
in vacuum 2 h at 200 oC 
Figure 5. PPV film aspects before annealing Figure 6. PPV film aspects before annealing 
Atomic Force Microscopy – Measurements were carried out with an instrument from Park 
Instrument Scientific (Sunnyvale, CA, USA) in non-contact mode in air at room temperature. All 
AFM images represent unfiltered original data and are displayed in color scale in figure 7, 8 and 9 
[4]. 
Figure 7. AFM image from PPV films after 
anneling in vacuum at 200 0 C from 2 h 
Figure 8. AFM image from PPV films after 
second anneling in vacuum at 200 0 C from 2 
Figure 9. AFM image from PPV films after 
anneling in mormal atmosphere  at 200 0 C 
from 2 h 
In figure 7 and 8 are shown the image of PPV films after first and second annealing in 
vacuum at 200 0C for 2 hours. We can see not many changes between films, thichness were almost 
the same ( 45 nm respectively 44 nm) [5]. 
Figure 9 shows the surface structure of PPV films after annealing in normal atmophere at 
200 0C for 2 hours and the film structure are diferent, compare with films structure which were 
annealed in vacuum. For all images, films are continuous and smooth with a a root mean square 
(r.m.s) roughness of 2 – 3 nm, from the annealed in vacuum and 5-7 nm from the normal 
athmosphere annealed [6]. We get amorphous films in the both case. The main informations observed 
in Dektak measurements, AFM investigations and ellipsomentry are: the surface roughness of the 
films depend on the speed of heating, slow heat up raises the roughness, quick heat up leads to more 
smoth films. The same situation is meet in case of vacuum annealing and normal atmosphere 
annealing. More over, the thickness of the layers is reduced to about the haltf after annealing in the 
both case. The PPV layers are not orienteded in the both annealing method [6]. 
UV/VIS measurements - were made using the Perkin Elmer – UV/VIS Spectrometer Lambda  
16. Spectra were acquired from 300 to 900 nm for optical excitation.  Figure 9, 10 and 11 shows a set 
of absorption spectra of PPV films obtained from spin coating converted by heating under vacuum 
and normal atmosphere at 200oC for 2 hours [7]. 
Spectra were normalized by dividing absorption spectrum of each individual sample by its 
absorption at the maximum. In this way relative changes within the spectrum and between the spectra 
are easily observed. One can notice differences in the position of the absorption maxima of PPV 
films prepared in different annealing method. The changes in the optical spectra of PPV films 
obtained from the precursors prepared in vacuum conditions and normal atmosphere condition are 
consistent with earlier observations in figure 10, 11 and 12 [8].  
4. Conclusions 
We summarize our findings as follows: (i) Annealing of PPV films causes ordering of polymer 
chains and, as a result, change in the luminescence intensity and spectra. (ii) spectral characteristics 
of the converted PPV-precursor strongly dependent on the preparation condition of the precursor (iii) 
the thickness of layers is reduced to about the half after annealing. (iv) The surface roughness of the 
films depends on the speed of heating: slow heat up raises the roughness; quick heat up leads to more 
smooth films. (v) PPV is thermally stable up to more than 500 0C measured by TGA. (vi) We get 
amorphous films in spin coating deposition. 
5. Acknowledgements 
Financial support of   the   European Commission   under   contract   number: FP6 – 505478-1  
ODEON - Project  and RTN EUROFET – Project is gratefully acknowledged. 
300 400 500 600 700 800 900 1000
after annealing in normal atmosphere
before annealing
after annealing in vacuum
wavelength (nm)
100 200 300 400 500 600 700 800 900 1000
before annealing
after 1st annealing (in vacuum)
after 2nd annealing (in vacuum)
X Axis Title
Figure 10.  UV-VIS spectra of PPV films 
obtained in vacuum conversion and normal 
atmosphere conversion 
Figure 11. UV-VIS spectra of PPV films 
obtained in vacuum conversion made several 
times 
300 400 500 600 700 800 900
before  annealing
after  annealing
wavelength (nm)
Figure 12.  UV-VIS spectra comparation of PPV films after and before vacuum conversion 
6. References 
[1] L. Bakueva, E.H. Sargent, R. Resendes, A. Bartole, I. Manners, J. 
Mater. Sci.: Mater. Electron. 12 (2001) 21. 
[2] M. Pope, C.E. Swenberg, Electronic Processes in Organic Crystals and Polymers, Oxford Science 
Publications, Oxford, 1999. 
[3] L. Bakueva, S. Musikhin, E.H. Sargent, A. Shik, 2001. MRS Fall Meeting, Boston, November 
26–30, 2001 Book of Abstracts. 
[4] D. Moses, A. Dogariu, A.J. Heeger, Synth. Met. 116 (2001) 19. 
[5] B. Hu, F.E. Karaz, Chem. Phys. 227 (1998) 263. 
[6] X.-R. Zeng, T.-M. Ko, J. Polym. Sci. B 35 (1997) 1993. 
[7] C.E. Lee, C.-H. Jin, Synthet. Met. 117 (2001) 27. 
[8] D.F.S. Petri,  J.Braz.Chem.Soc. vol. 13, no 5, 695-699,2002.
