Efficiencies of traditional dye-sensitized solar cells (DSSCs) based on nanoparticle thin films are limited by the competition between electron injection and the loss mechanisms from interfacial charge recombination and back reactions. Without a localized field driving electrons to the current collector, interfacial electrons can recombine with the photoactive dye or back-react with the electrolyte. These undesirable losses limit the photoanode thickness and maximum conversion efficiency. Nanowire arrays are promising architectures that have improved electron transport and charge injection due to an electric field. However, DSSC devices based on nanowire-based photoanodes thus far have not been able to take full advantage of the benefits that nanowires offer to electron transport, short circuit current, and open circuit voltage.
The objective of this proposal is to develop a model that simulates the performance of nanowire-based DSSCs so that field-assisted charge transport effects within these devices can be understood and used more effectively. The proposed work will focus on combining a charge transport model which accounts for the interfacial electric field with experimental studies to validate the transport dynamics and conversion efficiencies calculated for nanowire-based DSSCs. The anodic-alumina-oxide templating approach used to fabricate the semiconductor oxide nanowires takes advantage of the great flexibility in preparing nanowires with different types of materials, aspect ratios, and spacing. This flexibility allows the systematic study of the effect of field-assisted electron transport on DSSC efficiency. The developed model can be used to determine the fabrication parameters (i.e. nanowire diameter, shell thickness, length, and array density) required to maximize efficiency. These results may also provide new insight into the controlling key reactions or processes in charge transport for DSSCs.
Broader Impacts
Beyond photovoltaic applications, the development of nanowire arrays is important to electronic, optoelectronic, environmental, and biomedical applications. The proposed education and outreach activities leverage existing successful programs at the University of Florida (UF). An engineering course sequence in nanotechnology developed through previous a NSF CCLI (Course, Curriculum, and Laboratory Improvement) grant at UF will be enhanced to include solar photovoltaics. Opportunities for undergraduates to participate in the proposed research will be provided through the UF University Scholars Program. Students from under-represented groups will be mentored UF University Minority Mentoring Program through interactions with the PI in the context of the proposed research. Short courses for school teachers on renewable energy and nanomaterials will be offered through the UF Teachers as Scholars Program. High school students will be recruited for summer experiences through the UF Student Science Training Program.
Over the course of this award, there were several major advancements in the understanding of nanowire effects on dye-sensitized solar cell (DSSC) efficiency and performance. Initial work focused on modelling of the charge transport in nanowire arrays, which showed that a significant increase in the efficiency of nanowire-based DSSCs could be achieved by reducing the back reactions within the system. To this end, well-established nanoparticle DSSCs were used to show the effects of back reactions. With the addition of a blocking layer to the nanoparticles to reduce back reactions, the efficiency of the DSSC’s was increased, pointing towards the validity of our model. Based on this result, the validity of our model was further tested by fabricating nanowire-based DSSCs. Several types of semiconductor nanowire arrays were created through chemical vapor deposition, including IZO, ZnO, and ITO. These nanowire arrays were fabricated into DSSCs, which were then tested for efficiency and performance. The actual performance of these nanowire-based DSSCs yielded efficiencies consistent with our model. Along the way, several novel techniques for the handling and transfer of nanowire arrays were established as well as other methods that enhanced our understanding of the behavior of nanowire arrays. We believe that further refinements to nanowire-based DSSCs will yield additional efficiency improvements, which will one day lead to widespread adaptation of DSSCs as a viable alternative energy source.
