Conjugated polymers; Optoelectronic properties; Donor-acceptor copolymers; Organic (bulk heterojunction) solar cells; Ultrafast spectroscopy; Photoconductivity; Photoinduced electron transfer; Photophysics
Yuen Jonathan, Wang Mingfeng, Fan Jian, Sheberla Dennis, Kemei Moureen, Banerji Natalie, Scarongella Mariateresa, Valouch Sebastian, Pho Toan, Kumar Rajeev, Chesnut Eneida, Bendikov Michael, Wudl Fred (2015), Importance of Unpaired Electrons in Organic Electronics, in Journal of Polymer Science A
, 53, 287.
Cowan Sarah, Banerji Natalie (2015), Spectroscopy of Charge Carrier Dynamics- From Generation to Collection, in Richter Henning, Rand Barry P. (ed.), Pan Stanford Publishing, Singapore, 431.
Paraecattil Arun Aby, Banerji Natalie (2014), Charge Separation Pathways in a Highly Efficient Polymer: Fullerene Solar Cell Material, in Journal of the American Chemical Society
, 136, 1472-1482.
Hayes Sophia, Silva Carlos, Banerji Natalie (ed.) (2014), Physical Chemistry of Interfaces and Nanomaterials XIII
M. Scarongella, A. A. Paraecattil, E. Buchaca-Domingo, J. D. Douglas, S. Beaupré, T. McCarthy-Ward, M. Heeney, J.-E. Moser, M. Leclerc, J. M. J. Fréchet, N. Stingelin, N. Banerji (2014), The Influence of Microstructure on Charge Separation Dynamics in Organic Bulk Heterojunction Materials for Solar Cell Applications, in Journal of Materials Chemistry A
, 2, 6218.
Scarongella Mariateresa, Laktionov Andrey, Rothlisberger Ursula, Banerji Natalie (2013), Charge Transfer Relaxation in Donor-Acceptor Type Conjugated Materials, in Journal of Materials Chemistry C
, 1, 2308-2319.
Physical Chemistry of Interfaces and Nanomaterials XII, (2013), Physical Chemistry of Interfaces and Nanomaterials XII
, SPIE Press, San Diego.
Banerji Natalie (2013), Sub-Picosecond Delocalization in the Excited State of Conjugated Homopolymers and Donor-Acceptor Copolymers, in Journal of Materials Chemistry C
, 1, 3052-3066.
Paraecattil Arun Aby, Beaupre Serge, Leclerc Mario, Moser Jacques-E., Banerji Natalie (2012), Intensity Dependent Femtosecond Dynamics in a PBDTTPD-Based Solar Cell Material, in Journal of Physical Chemistry Letters
, 3, 2952-2958.
Banerji Natalie, Wang Mingfend, Fan Jiang, Chesnut Eneida S., Wudl Fred, Moser Jacques-E. (2012), Sensitization of fullerenes by covalent attachment of a diketopyrrolopyrrole chromophore, in Journal of Materials Chemistry
, 22(26), 13286-13294.
Scarongella Mariateresa, De Jonghe-Risse Jelissa, Buchaca-Domingo Ester, Causa’ Martina, Fei Zhuping, Heeney Martin, Moser Jacques-E., Stingelin Natalie, Banerji Natalie, A Close Look at Charge Generation in Polymer:Fullerene Blends with Microstructure Control, in Journal of the American Chemical Society
, Just Accepted.
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.