This Small Grant for Exploratory Research (SGER) project is aimed at developing a new design and synthesis methodology based on initiated chemical vapor deposition (iCVD) to make semiconducting, conjugated polymers for use in solar cells with enhanced efficiency and performance.
Intellectual merit: In bulk heterojunction polymer-based solar cells, inefficiencies are attributed to the high band gap of polymers leading to a mismatch with the solar spectrum, and generally poor charge generation and transport in the polymer due to structural and morphological defects. In hybrid systems using inorganic, nanocrystalline titania, with or without dye sensitization, solar cell efficiency has been hampered by the poor filling of the mesoporous titania with the polymer. Some of these issues stem from using liquid-based processing methods to form the conjugated polymer thin films in the solar cell stack. Since unmodified conjugated polymers are typically intractable and insoluble, addition of solubilizing side chains on these polymers induce solubility and therefore processability. However, addition of side groups may result in undesirable changes in crystallinity, morphology and stability and lead to degradation of the polymer. Further, polymer properties are often sensitive to the choice of solvent and the conditions for solvent removal.
iCVD relies on the direct polymerization of a solid polymer thin film on a surface using monomers and thermally activated initiators/catalysts in the vapor form. By circumventing the liquid-phase, there is no requirement for solvents, and therefore iCVD gains significant processing freedom, not only in making unmodified conjugated polymers tractable, but also opening up the possibility of designing novel materials which would otherwise have been unattainable in the liquid phase due to solvent constraints or incompatibilities. Further, without the issues of macroscale phase separation and demixing from adverse solvent interactions, iCVD as a solventless processing technique is expected to enable intimate, nanoscale mixing of dissimilar phases between the conjugated polymer and its counter-junction.
The specific research objectives are (1) to demonstrate the synthesis of conjugated polymers using iCVD for a selected list of monomers and co-monomers, which will have viable properties as solar cell materials; and (2) to demonstrate the tight contact between the donor and acceptor phases by integrating iCVD and processing techniques like thermal evaporation and spin coating to produce the heterojunctions, similar to the paradigm that has been successfully applied in microelectronics fabrication through the utilization of CVD and other processing techniques.
Broader impact: This program is motivated by the need to discover alternative energy solutions to reduce our dependency on fossil fuel, protect our environment from its damaging effects, and enable a more sustainable Earth. With solar energy remaining a largely untapped resource, this program is a first step that aims to fuel the technological development of this alternative energy. Beyond this, well-designed conjugated polymers from this work could find applications in flexible electronics, organic light-emitting diode (OLED) devices, and biomedicine. This program will integrate a strong educational component through mentoring high school and undergraduate students from Drexel?s many outreach programs; by growing relationships with area high schools to provide hands-on opportunities to test the long term stability of solar cells as a way to inform students of responsible technology. This program aims to actively recruit from minority and underrepresented groups. Research results will be disseminated and are expected to germinate multidisciplinary research because of the applicability and versatility of the iCVD approach.
