Christopher De Dobbelaere

Perovskite‐Based Hybrid Solar Cells Exceeding 10% Efficiency with High Reproducibility Using a Thin Film Sandwich Approach

Organometal halide perovskites have tremendous potential as light absorbers for photovoltaic applications. In this work we demonstrate hybrid solar cells based on the mixed perovskite CH3 NH3 PbI2 Cl in a thin film sandwich structure, with unprecedented reproducibility and generating efficiencies up to 10.8%. The successfulness of our approach is corroborated by the experimental electronic structure determination of this perovskite.

Aqueous Solutions for Low-Temperature Photoannealing of Functional Oxide Films: Reaching the 400 °C Si-Technology Integration Barrier

Functional oxide films were obtained at low temperature by combination of aqueous precursors and a UV-assisted annealing process (aqueous photochemical solution deposition). For a PbTiO(3) model system, functional ferroelectric perovskite films were prepared at only 400 °C, a temperature compatible with the current Si-technology demands. Intrinsically photosensitive and environmentally friendly aqueous precursors can be prepared for most of the functional multimetal oxides, as additionally demonstrated here for multiferroic BiFeO(3), yielding virtually unlimited possibilities for this low-temperature fabrication technology.

Tuning the dimensions of ZnO nanorod arrays for application in hybrid photovoltaics.

ZnO nanorod arrays are a very eligible option as electron acceptor material in hybrid solar cells, owing to their favorable electrical properties and abundance of available, easy, and low-cost synthesis methods. To become truly effective in this field, a major prerequisite is the ability to tune the nanorod dimensions towards optimal compatibility with electron-donating absorber materials. In this work, a water-based seeding and growth procedure is used to synthesize ZnO nanorods. The nanorod diameter is tuned either by modifying the zinc concentration of the seeding solution or by changing the concentration of the hydrothermal growth solution. The consequences of this morphological tailoring in the performance of hybrid solar cells are investigated, which leads to a new record efficiency of 0.82 % for hydrothermally grown ZnO nanorods of size 300 nm in combination with poly(3-hexylthiophene-2,5-diyl) (P3HT). This improvement is attributed to a combined effect of nanorod diameter and orientation, and possibly to a better alignment of the P3HT backbone resulting in improved charge transport.

Thermal behaviour of arsenic trioxide adsorbed on activated carbon

The thermal stability and desorption of arsenic trioxide (As(2)O(3)) adsorbed on activated carbon (AC) was investigated as this phenomenon is expected to influence the arsenic release during low temperature pyrolysis of chromated copper arsenate (CCA) wood waste. Firstly, a thermogravimetric (TG) experiment with arsenolite, an allotropic form of As(2)O(3), was performed. The sample starts to sublime at temperatures lower than 200 degrees C with a sublimation peak temperature of 271 degrees C. Subsequently, TG experiments with samples of As(2)O(3) adsorbed on AC revealed that only very little (max. 6+/-3 wt%) As(2)O(3) was volatilized at temperatures below 280 degrees C, while still 41.6 (+/-5)wt% of the original arsenic concentration was retained at 440 degrees C and 28.5 (+/-3)wt% at 600 degrees C. The major arsenic volatilization occurred between 300 degrees C and 500 degrees C. The kinetic parameters of desorption, activation energy of desorption (E(d)) and pre-exponential factor (A), were determined by fitting an Arrhenius model to the experimental data, resulting in E(d)=69 kJ/mol, A=1.21 x 10(4)s(-1). It can be concluded that the adsorption of As(2)O(3) on AC can contribute to the thermal stabilisation of As(2)O(3). Consequently, during low temperature pyrolysis of CCA wood arsenic release may be prevented by adsorption of As(2)O(3) on the coal-type product formed during the thermal decomposition of the wood.

Combustion deposition of MoO3 films: from fundamentals to OPV applications

A systematic study of a combustion precursors thermal decomposition pathway is undertaken, in both powders and thin films, to obtain insights in the various parameters influencing the combustion process. The study focuses on MoO3 as a hole transporting layer (HTL) for applications in organic photovoltaics (OPV). Via evolved gas analysis, it was found that fuel volatility occurs prior to the actual combustion reaction of the acetylacetonate based precursor, affecting the optimal oxidizer to fuel ratio. Moreover, close investigation showed that the high rate combustion reaction disappears with increasing surface to volume ratio. Nonetheless, thermal analysis performed on films suggests that with the right heating rate, an oxidative complete decomposition still occurs in films, exemplified by the film composition and specific morphological differences in the resulting layers and through analysis of the evolved components. Finally, the discussed synthesis route allows organic free, crystalline MoO3 films to be obtained affording organic solar cell devices with promising current density–voltage characteristics.

A UV-absorber bismuth(III)-N-methyldiethanolamine complex as a low-temperature precursor for bismuth-based oxide thin films

Novel synthetic methods in solution that reduce the formation temperature of bismuth-based electronic oxides are essential for their successful integration with substrates of low thermal stability within micro- and flexible-electronic devices. This has become crucial for these oxides, since they appear as promising low-toxic functional materials alternative to other electronic oxides containing heavy metals. However, this is a challenge, since the crystallization of bismuth oxides occurs at high temperatures. To overcome these problems, we synthesize here a UV-absorber charge transfer metal complex in solution between the Bi(III) ion and an alkanolamine, N-methyldiethanolamine (Bi(III)–mdea). We take advantage of the photoreactivity of this complex to prepare bismuth-based oxide thin films at low temperature, which cannot be achieved by traditional thermal processing methods. Room temperature stable oxide thin films of the high-temperature δ-Bi2O3 phase are prepared from these solutions by UV-irradiation and annealing at 350 °C. The efficiency of this synthetic strategy is additionally proven for the low temperature preparation of thin films of much more complex bismuth based functional oxides: the multiferroic bismuth ferrite, BiFeO3, and the relaxor-ferroelectric perovskite of bismuth, sodium and barium titanate, (Bi0.5Na0.5)0.945Ba0.055TiO3.

SnO2 thin films from an aqueous citrato peroxo Sn(IV) precursor

In this work, tin(II) oxalate was studied as a novel chloride-free starting material for the preparation of a stable Sn-containing precursor solution. This precursor was applied for the chemical solution deposition (CSD) of transparent conducting coatings of SnO2 on Si/SiO2 substrates. An influence of synthesis parameters, such as pH, complexing agent to metal ion ratio on the stability of the citrato peroxo Sn(IV) precursor has been investigated in this study. Insights into the precursor chemistry and its thermal decomposition based on TG-DSC analysis are also presented. The obtained SnO2 films were characterized by high temperature X-ray diffraction (HT-XRD) and scanning electron microscopy (SEM) to evaluate phase purity and film thickness, respectively.

Thermal decomposition synthesis of Al-doped ZnO nanoparticles: an in-depth study

Al-doped ZnO nanoparticles are synthesized by means of a heating up solution based thermal decomposition method. The synthesis involves a reaction of zinc acetylacetonate hydrate, aluminium acetylacetonate and 1,2-hexadecanediol in the presence of oleic acid and oleyl amine. A proposed reaction mechanism from reagents to monomers is corroborated by analysis of the evolving gases using headspace GC-MS analysis. The Al-doped ZnO nanoparticles synthesized are dynamically stabilized by adsorbed oleate ions, after deprotonation of oleic acid by oleyl amine, as was found by NOESY proton NMR and complementary FTIR spectroscopy. Precession electron diffraction shows a simultaneous increase in lattice parameters with Al concentration. This, together with HAADF-STEM and EDX maps, indicates the incorporation of Al into the ZnO nanoparticles. By the combination of complementary characterization methods during all stages of the synthesis, it is concluded that Al is incorporated into the ZnO wurtzite lattice as a dopant.

Steering the Properties of MoOx Hole Transporting Layers in OPVs and OLEDs: Interface Morphology vs. Electronic Structure

The identification, fine-tuning, and process optimization of appropriate hole transporting layers (HTLs) for organic solar cells is indispensable for the production of efficient and sustainable functional devices. In this study, the optimization of a solution-processed molybdenum oxide (MoOx) layer fabricated from a combustion precursor is carried out via the introduction of zirconium and tin additives. The evaluation of the output characteristics of both organic photovoltaic (OPV) and organic light emitting diode (OLED) devices demonstrates the beneficial influence upon the addition of the Zr and Sn ions compared to the generic MoOx precursor. A dopant effect in which the heteroatoms and the molybdenum oxide form a chemical identity with fundamentally different structural properties could not be observed, as the additives do not affect the molybdenum oxide composition or electronic band structure. An improved surface roughness due to a reduced crystallinity was found to be a key parameter leading to the superior performance of the devices employing modified HTLs.