Project

Discipline

Conjugated polymers; Optoelectronic properties; Donor-acceptor copolymers; Organic (bulk heterojunction) solar cells; Ultrafast spectroscopy; Photoconductivity; Photoinduced electron transfer; Photophysics

Lead
Nous vivons dans un monde qui a besoin d'énergie. Le soleil représente une source inépuisable mais qui n'est pas toujours facile à récolter. Le développent de cellules solaires qui permettent de convertir la lumière en électricité, est donc essentiel.
Lay summary
Contenu et objectifs du travail de recherche Les cellules solaires généralement trouvées dans le commerce sont à base de silicone inorganique. Leur production est couteuse en argent et énergie. De nouvelles générations de cellules solaires ont donc été développées. On y compte celles dites plastiques. Elles sont principalement constituées de composés organiques (polymères conjugués, fullerènes) et sont fabriquées selon des procédés peu chers. Elles sont aussi légères et flexibles. L’efficacité des cellules solaires plastiques frôle aujourd'hui les 10 %. Elles deviennent donc commercialement intéressantes, mais doivent encore être améliorées. Pour cela, l'interaction entre les composés photovoltaïques et la lumière, ainsi que la génération de courant électrique, doivent être mieux compris. Dans ce projet, nous utilisons la spectroscopie résolue dans le temps et des mesures de photocourant pour mieux comprendre les cellules solaires plastiques. Il y trois principales parties à cette recherche: 1) Polymères conjugués seuls, en solution ou à l'état solide. A fin de mieux comprendre l'état excité (qui résulte de l'absorption de lumière) et son évolution dans le temps, des séries chimiques de polymères et des composés modèles sont étudiés. 2) Polymères conjugués mélangés à des fullerènes à l'état solide. Il se forme une matière qui permet la conversion de lumière en charges électriques, mais le mécanisme n'est pas bien compris. Nous utilisons la spectroscopie extrêmement rapide (femtoseconde). Des informations complémentaires sont obtenues dans différentes régions spectrales (visible, infrarouge, terahertz). 3) Etude spectroscopique de cellules solaires en marche. Cela permet d'étudier le photocorant macroscopique et l'évolution des charges à échelle femtoseconde, en parallèle. Contexte scientifique et social du projet de recherche  Les résultats de cette recherche vont contribuer au développement ciblé de nouveaux matériaux pour des cellules solaires organiques de plus grande efficacité.

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Last update: 03.01.2013

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Conjugated polymers combine semiconducting electronic behavior with high optical absorption in the visible, plastic-like mechanical properties and easy solution processing. This carbon-based class of materials has therefore emerged over the last few decades as an organic alternative to expensive inorganic semiconductors in optoelectronic applications. Conjugated polymers have been successfully used in light emitting devices, transistors, photo-detectors and organic solar cells. In the solar cells, the solid thin film active layer is typically a blend of the polymer with a fullerene derivative. Spontaneous phase separation leads to the formation of a nanoscale bulk heterojunction (BHJ) material in which there is an interpenetrating polymer:fullerene morphology. Ultrafast photoinduced charge separation (CS) between the polymer donor and the fullerene acceptor is the initial step in the generation of mobile charge carriers and this is favored by the high donor-acceptor interfacial area in the BHJ. The mobile charge carriers can then travel to the electrodes along phase-separated fullerene and polymer networks. The power conversion efficiency in polymer photovoltaic devices has enjoyed a leap form 3% to 8.3% over the last ten years. This rise can largely be attributed to the development of novel donor-acceptor copolymers of an electron-rich unit and an electron-poor unit. The objective of the currently proposed research is not to develop or optimize any applications, but rather to understand why and how solar cells containing conjugated donor-acceptor copolymers work. Indeed, the fundamental Science of the processes underlying the photovoltaic functioning clearly lags behind the rapid advances in device efficiency. The latter are therefore often achieved by trial and error, which involves time-consuming synthesis and laborious device testing/optimization with a large number of materials, only few of which will be successful. Feedback from a fundamental optoelectronic understanding contributes to planned strategy in both the synthesis and solar cell applications, so that higher efficiency can be achieved and this faster. Important questions that need to be addressed to that aim include:• How does the strength of the internal charge transfer character influence the optoelectronic properties of donor-acceptor copolymers?• How can CS in BHJ blends occur in <100 fs when the polymer photoexcitation has to travel by up to 10 nm to reach a fullerene interface?• How does charge recombination (a loss mechanism that reduces efficiency) occur in working solar cells under applied voltage bias?The persisting lack of clarity about the optoelectronic properties and photophysics of polymer solar cell, in spite of ongoing research efforts in the field, arises from various factors. Those include the complexity of conjugated polymers and particularly donor-acceptor copolymers, the limited availability of the non-commercial materials to many photophysically specialized groups, the limited availability of versatile spectroscopic tools to groups who have access to the materials and develop applications, and the difficulty to compare and reproduce results if the experimental conditions (polymer quality, processing, excitation intensity) are not carefully controlled. I therefore suggest a very systematic investigation of the optoelectronic functioning of polymer solar cells, using purely optical spectroscopic techniques as well as more electrical measurements. In order to answer the above and related questions, I plan to lead the investigation from an understanding of the “simplest” systems (the pristine conjugated polymers and even their isolated repeat units), via polymer:fullerene BHJ blends, to working photovoltaic devices. The proposed research relies on the use of a large variety of experimental tools to yield complementary information under well-controlled conditions. Where time-resolved optical measurements are concerned, I will go beyond the most-commonly employed visible transient absorption experiments and will apply very specialized techniques: Time-resolved fluorescence with very high time resolution (<50fs), femtosecond optical-pump terahertz-probe spectroscopy, femtosecond transient absorption with mid-infrared probing, and transient reflectivity (femtosecond and nanosecond resolution) on working solar cells. All apparatuses will be at my disposal at the host institution (École Polytechnique Fédérale de Lausanne, EPFL) in the group of Prof. Jacques Moser, except for the transient infrared setup which will be used in collaboration with the recognized group of Prof. Peter Hamm (University of Zürich), linked to EPFL through the NCCR MUST network. Electrical steady-state photoconductivity measurements, with an apparatus that will be developed if possible by a PhD student, will complement the purely optical methods. Furthermore, the spectroscopic results obtained with working devices can be related to their current-voltage characteristics, since I will have access to the photovoltaic cell characterization equipment in the group of Prof. Michael Grätzel (EPFL, Lausanne). Strength of the project also arises through the often exclusive availability of novel donor-acceptor copolymers through collaborations with internationally leading groups in polymer synthesis: Prof. Fred Wudl (University of California, Santa Barbara) and Prof. Mario Leclerc (Université Laval, Québec City). I will also have access to materials and optimized solar cell devices through collaboration with Nobel Laureate Prof. Alan J. Heeger (University of California, Santa Barbara), pioneer in the Science and applications of conjugated polymers. Moreover, assistance with polymer processing can be obtained from the group of Dr Frank Nüesch (Laboratory for Functional Polymers, Empa, Switzerland), an active collaborator of the Moser group. The majority of investigated compounds (examples are shown in Fig. 11 of the project proposal) have never been studied from a photophysical point of view, especially not with the proposed array of experimental tools. This access to novel materials, combined with a complete set of pertinent measurement techniques and my personal experience with ultrafast spectroscopy and conjugated polymers, promises success an innovation in the proposed research.