PhD thesis defense of Wim Dexters
Apr 15, 2015 - May 01, 2015
Wim Dexters would like to invite you to the public defence of his doctoral thesis entitled: "The structural and electronic properties of lead phthalocyanine thin films on single crystal diamonds."
Promotor is Prof. dr. Ken Haenen, copromotor is Prof. dr. Jan D'Haen.
This work investigates the use of diamond as a part of a solar cell. As the earth's population is rising, the demand for energy is also rising. With the growth of countries like China, there is a need for new solutions. The goal is to see if diamond can helps with these issues.
For several years, diamond has been getting more attention as a possible candidate in solar cells. The main focus has been on the use of diamond in combination with organic dyes, other molecules and polymers. The interface between the diamond substrate and the organic layer will determine the charge transfer between the layers. Therefore it is important to understand the influence of the diamond surface on the deposited layer.
In the case of diamond, the properties of the surface can be altered. It can be terminated with a variety of atoms. In this work, hydrogen and oxygen termination will be used. This changes the surface from hydrophobic in the hydrogen terminated case to hydrophilic in the oxygen terminated case. The main question to be answered is what the influence of the termination of the diamond surface is on the growth, structure and opto-electronic properties of deposited organic layers.
The deposited molecule for this study is lead phthalocyanine. The symmetrical structure of the molecule renders it an excellent candidate to deposit on diamond. Although the phthalocyanines have been around for 100 years, the scientific interest resurfaced the last few years for gas sensors and other applications. PbPc stands aside from the other phthalocyanines as it is not a planar molecule but has a more umbrella-like shape, also called a shuttlecock shape. Due to this structural difference, the molecule has a different crystal structure from the planar phthalocyanines and additionally it has a dipole moment. All of these factors will influence the growth of the layers on any surface. At this point in time, little is known about the interaction of PbPc with the surface of diamond.
In order to deposit the PbPc layers, an organic molecular beam deposition system (OMBD) was set up. The machine consists of two parts, the load lock and a deposition chamber. The load lock is there to maintain a good vacuum in the main chamber and to store the substrate before and after deposition. The deposition chamber has a base pressure of 2 10-9 mbar after baking for 24 hours at 200°C. The substrates are mounted on a holder that can be heated. After annealing of the substrate, the PbPc in heated in a homemade effusion cell. Deposition rates are recorded with a quartz crystal oscillator.
The PbPc growth is first investigated on silicon. Using kinetic roughening theory, AFM images of the layers are analyzed. The island growth is shown to be similar to the growth of CoPc.
Lead phthalocyanine thin films of 5 and 50 nm have been deposited on hydrogen and oxygen terminated (100) type IIa single crystal diamond. Atomic force microscopy and X-ray diffraction (XRD) studies showed that PbPc grown on hydrogen terminated single crystal diamond (SCD:H) forms layers with a high degree of crystallinity, dominated by the monoclinic (320) orientation parallel to the diamond surface. The oxygen terminated diamond leads to a randomly oriented PbPc film. Absorption and photocurrent measurements indicated the presence of both polymorphs of PbPc, however the ratio differed depending on the termination of the SCD. Finally, polarized Raman spectroscopy was used to determine the orientation of the molecules in the thin film. The results confirmed the random orientation on the O-terminated diamond. On SCD:H, the PbPc molecules are lying down, in accordance with the XRD results.
In order to analyze the electrical properties and the work function of the PbPc, 50 nm thick films were deposited on diamond and gold. Peak force TUNA images of PbPc films on O- and H-terminated diamond, performed under an N2 environment, showed a preference to the boundary conductivity. This was expected as previous investigations indicated the need for defects to assist in the conduction. Local J-V curves on the boundaries and islands give an insight into the type of conductivity. The H-terminated diamond led to an Ohmic/SCLC type conductivity. This type has previously been found in Au/PbPc/Au structures. The O-termination showed Poole-Frenkel/Schottky emission type behavior with β-values between the theoretical values for the two emission types. Kelvin probe force microscopy measurements of diamond and PbPc were performed under N2 environment with illumination of a solar simulator. The H-termination remains remarkably stable under the illumination in contrast to previous publications. The O-termination is unstable even without the illuminations which is attributed to the surface states of the diamond. The PbPc film has a work function close to the diamond and Au, which is expected because of the Ohmic/SCLC conductivity. Under illumination the work function changes as a function of time. This behavior was previously seen in pulsed photocurrent measurements on ZnPc. The cause was attributed to the trapping of the photogenerated charge carriers. The half-life of the growth and decay are dependent on the substrate and thus on the structure of the film.