Hybrid Devices : Morphology Control by Self-Assembly

This thesis focuses on innovative concepts for hybrid photovoltaic devices. In particular, it aims at the improvement of light harvesting and the control of morphology. Hybrid devices combine the advantage of excellent electronic properties of inorganic semiconductors and the convenient processability of organic materials. However, the creation of suitable morphologies which guarantee a high interface area between donor and acceptor as well as continuous charge percolation pathways still remains a challenge. This thesis addresses the key issues utilizing state-of-the-art synthesis and characterization methods for the development of highly functional dyes as well as well-controlled polymers and blockcopolymers. In the first part, BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes were investigated which feature a broad absorption range up to the IR region accompanied by high extinction coefficients. These dyes were tested in solid state dye sensitized solar cells. In addition, triphenylamine based sensitizers were examined which feature beneficial redox potentials and excellent stability but at the expense of the broad absorption. Besides the light harvesting, a second issue concerning solid state dye sensitized solar cells is the lack of control over the morphology of the titania network such as network wall thickness and pore diameter. On basis of polymer brush particles made of a polystyrene core and a polystyrene sulfonate corona, a novel template assisted preparation of a mesoporous TiO2 electrode was established. In contrast to established methods, our template permits the individual control of the TiO2 network density and pore size. With variation of the TiO2 content during the hydrolysis, a fundamental correlation between the TiO2 density, the sensitized interface area and the resulting current generation was discovered. These results contribute to an improved knowledge toward dye sensitized hybrid solar cells. Building up on the knowledge gained from the polymer brush templates, novel amphiphilic semiconducting block copolymers were developed which facilitate the self-assembly of hybrid composites. The semiconductor component is based on well-known triphenylamine derivatives, while the hydrophilic block comprises the polystyrene sulfonate which features unique coordination and catalytic characteristics. The controlled reversible addition−fragmentation chain-transfer (RAFT) process was chosen for polymerization. In order to improve the versatility, the procedure was extended toward a combination of RAFT and “click” chemistry. The resulting amphiphilic semiconductor block copolymers form micelles in polar solvents or water. I utilized the highly charged micelles to attach oppositely charged CdSe nanorods. On the other hand, these block copolymer micelles were assembled on conductive substrates and annealed in saturated DMF vapor leading to microphase separation and a vertical alignment of the microdomains. Altogether, these studies pave the way for a profound basis for the development of morphology controlled hybrid devices. In the final part of this thesis the concept of amphiphilic semiconductor polymers or conjugated polyelectrolyte was developed in order to completely avoid insulating organic blocks. In this regard, a conjugated polythiophene backbone was combined with the sulfonate groups in the side chain. The presented approach significantly improves the control of the polymer synthesis permitting the modulation of the molecular weight, a narrow polymer distribution, a regioregular conformation and well-defined end groups. The highly regioregular conformation induces an unprecedented aggregation phenomenon. Studying the optical properties of the conjugated polyelectrolytes, a distinct correlation between molecular weight and aggregation was observed in aqueous solutions. Further, the electrical properties were studied in detail. A bulk hole transport mobility of 1.3 ± 0.5 x 10^-2 cm^2/Vs was determined which is among the highest values reported so far. In contrast to other conjugated polyelectrolytes impedance spectroscopy revealed a pure electronic transport mechanism without ion reorganization. In conclusion, this thesis comprises innovative concepts to improve light harvesting and gain control over the morphology of hybrid devices. The established synthesis protocols enable the controlled preparation of well-defined functional amphiphilic polymers and block copolymers. The achieved results not only allow new approaches for the optimization of hybrid devices, but also provide the basis for a more comprehensive investigation of physical processes due to the systematic morphology control in such devices.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2013

Author dc:creator

• Brendel, Johannes C.

Contributors dc:contributor

• Thelakkat, Mukundan

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/108/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:108