Near IR sensitization

One of the main attractive features of conjugated polymers is there strong absorption coefficient together with the ability to tune the bandgap. As such, this material class is ideal for photovoltaics. However, since conjugated polymers are dominantly molecular excitonic absorbers, their absorption is characterized by a spectrally narrow absorption of a few 100 nm instead of a flat absorption plateau as seen for inorganic semiconductors.This is especially unsatisfying for solar cell operation, where one wants to absorb the whole spectrum resonant to the bandgap with equal oscillator strength. Various strategies have bee suggested to address this limitation: Synthesis of A-B blockcopolymers, where A and B are different chromophores, was suggested as well as the addition of dyes or small molecular sensitizers. Alternatively, polymers with different bandgaps and absorption can be coupled into tandem cells. Tandem cells allow to collect photons resonant to the bandgap of both polymers with minimal thermalization losses. All these strategies are currently in the exploration phase at different levels of maturity.

In this research proposal we suggest an elegant, alternative strategy to extend the spectral sensitivity of wide bandgap polymers. In detail, we propose to design ternary bulk heterojunction systems, consisting of a wide bandgap polymer, a low bandgap polymer and a fullerene. Each of the components needs to fulfill specific roles:

• The fullerene needs to act as an electron acceptor to both of the polymers as well as an electron transport matrix.
• One polymer needs to act as the p-type transport matrix as well as absorber in one of the two spectral regions, either for the UV/VIS or the NIR.
• The second polymer needs to act as a sensitizer to the first polymer, covering the adjacent spectral regime to the first polymer. This polymer needs to show an efficient charge transfer to the fullerene. Ideally, this polymer shows an efficient hole transfer to the transporting polymer. In that case, the sensitizing polymer does not need to be a good transporter itself.

The design and development of ternary bulk heterojunction (BHJ) composites requires the investigation of three separate aspects and fundamental topics of organic semiconductors and photovoltaics.

1. Synthesis: The project will start with available polymers for the transport matrix as well as for the sensitizers, and then quickly move over to novel polymers, customized for the purpose of near IR sensitizing, i.e. with various bandgap and smaller variations in the HOMO position.
2. Morphology: The morphology assessment and morphology control of ternary BHJ composites will be another challenge is this project. Polymers which are not completely miscible to each other (and that is the common situation) will show demixing in solution as well as in the solid state. . Following Hildebrandt, polymers are miscible to each other if they have comparable intermolecular adhesion forces. Our access to understand and investigate the phase separation mechanisms in ternary systems will be based on an approach similar to Hansen theory. By assessing the solubility parameters for the individual components we investigate the Hansen sphere for a ternary composite.
3. Transport and Charge Transfer: Transport in binary BHJ composites is already a complex process. Here we need to investigate and clarify the transport mechanisms in ternary composites. Specifically, we will explore suitable experimental methods which allow to distinguish the role of the two polymer and their individual contributions to the hole transport. As important as transport is the charge generation process in the ternary blends. Time resolved pump-probe spectroscopy in the ns regime will be used to understand how the two polymer interact with each other. Most importantly, we want to clarify whether the two polymers may show direct charge transfer, energy transfer, or even have no electronic interaction.

Duration:
01.04.2010 - 31.03.2015

Projectleader:
Prof. Dr. U. Scherf