Photoinduced Electron Transfer from Conjugated, Semiconducting Polymers to Fullerene: A Molecular Approach to Polymeric Photovoltaic Cells

The recently discovered ultrafast photoinduced electron transfer, with subpicosecond transfer rate, in composites of a conjugated, semiconducting polymer and a molecular acceptor, Buchminsterfullerene C60, opens up a new direction and a new opportunity for photovoltaic research. Since the charge transfer takes place approximately 1000 times faster than the radiative and/or non-radiative decay of photoexcitations on the semiconducting polymer backbone, the quantum efficiency for charge transfer and charge separation is near unity. We are using materials (conjugated polymers) which exhibit an unique combination of properties. The important electronic and optical properties of semiconductors and metals in combination with the attractive mechanical properties and the processing advantages of polymers. The potential advantages of an all-polymer heterojunction solar cell include: low cost (large scale production using existent polymer technology) large area (plastic thin films can be produced at macroscopic dimensions) flexible (mechanical properties of polymers) This discovery provides a rational molecular approach to "plastic" solar cells and phototdetectors. Investigation of this photoinduced electron transfer in conjugated semiconducting polymer/organic electron acceptor composites as well as heterojunctions is the scientific backbone of such an approach. As described in detail below, the photophysics of such supramolecular composites shows quite interesting effects, such as triplet quenching, sensitization of photoconductivity as well as reverse saturable nonlinear absorption etc., whereas each of these photonic effects has scientific importance as well as technological merit. It is the content of this project proposal to focus on this photophysics of conjugated polymers in composite with C60. The primary goal of this project is, to optimize the photophysical parameters for the plastic solar cells; i.e. to find the most efficient photoinduced electron transfer and long lived charge separation from the class of conjugated polymers and fullerenes as well as other acceptors. In special, following problems will be investigated: 1. stability of the composites at ambient conditions, 2. optimum absorption of the solar spectrum, i.e. using low bandgap (Eg <1.7 eV) polymers 3. enhancing the order of the morphology to get better charge mobility

The general scientific goals of this project are summarized as follows: i) To Investigate, analyze and understand the basic photophysical properties of organic, "plastic"solar cells in order to improve their efficiency. ii) To study the stability as well as suitability of new materials for being incorporated into the plastic solar cell devices iii) Investigate methods to analyze and manipulate the morphology of the photoactive films in these plastic solar cells in order to improve the mobility of the photoinduced charge carriers. We have proposed and implemented the following experimental methods to reach that scope: a.) Photoexcited spectroscopy in UV-VI-NIR regime with cryogenic equipment to enable low temperature experiments b.) Light induced electron spin resonance spectroscopy for investigating the photoinduced radicals in organic composites. Cryogenic equipment has been used as standard. c.) Optically detected magnetic resonance as well as photocurrent detected magnetic resonance, to investigate the magneto-photophysics of photoactive composites. d.) Photoinduced FTIR spectroscopy to investigate the charge induced lattice relaxations and IRAV bands. e.) Electrochemically induced in situ FTIR and ESR Spectroscopy to identify the properties of radical ions in organic semiconductors used in plastic solar cells. The project has fulfilled its scientific mission with success. Our understanding of the photophysics of these materials as well as devices has increased enormously and this understanding has lead to organic solar cells with world record power conversion effiencies around 3%, and our scientific success opened up the way to a large publication record, high evaluation of our science internationally and created a critical know-how in our laboratory which is desired and adapted worldwide.