www.acsprf.org

The objective of this project has been to explore the potential of the photophysical process triplet-triplet annihilation (TTA; 2T₁--> S_n (n>1) + S₀) for highly-efficient photon upconversion in metalloporphyrin (MP) systems.  As an ultimate goal, the information may lead to improved efficiencies of solar energy utilization in photovoltaic cells (dye-sensitized solar cells; DSSCs) through upconversion of sub-band-gap light in the near infrared that is lost in conventional dye-sensitized and doped semiconductor solar cells.  The proposed scheme increases the fraction of the solar spectrum harvested and minimizes energy losses via heat-producing channels, leading to significant efficiency enhancements.

  Outcomes from this project include publication of ~5 manuscripts in high-quality scientific journals ^1-5, and contributions to the training of ~8 highly-qualified personnel (students, postdocs, technicians).  Funding from the ACS-PRF has helped spawn an industry-academic collaboration between the co-PIs and a Canadian manufacturing company, further financial support from Canadian funding agencies, and played a part in recruiting a world-class faculty member in photovoltaic materials to the University of Saskatchewan (U of S).  In short, return on investment from this research funding has been excellent.

  In terms of a scientific summary, significant inroads have been made on understanding and controlling TTA in metalloporphyrin systems, as well as clarifying mechanisms of energy transfer between MP and “blue emitter” molecules.  Energy transfer in these model systems is important as it provides information on the ability to donate energy into semiconductor films (e.g. electron transfer from a photoexcited dye sensitizer into a semiconductor).  In our initial work (prior to ACS-PRF funding), we demonstrated efficient TTA in solution-based zinc tetraphenylporphyrin (ZnTPP) systems and determined the mechanism of energy transfer between the metalloporphyrin and “blue emitter” molecules (perylene and Coumarin 343); photon upconversion mechanisms differed in both cases, with triplet energy transfer taking place in the first case and triplet exciplex formation in the other ⁶.  We have subsequently demonstrated efficient homomolecular TTA in thin polymer films, a crucial step in preparing solid-state photovoltaic devices⁷. 

  A key insight from this research is that controlling molecular aggregation is important for optimal photon upconversion efficiency.  In solution, this was highlighted by significant differences in upconversion brought about by coordinating and non-coordinating solvents. In the solid state, we have studied this via a single-molecule fluorescence assay for ZnTPP that allows determination of aggregation states of metalloporphyrins¹.  As an extension of this work, we have also demonstrated the potential for measuring the quenching of upconverted fluorescence in ZnTPP for measuring oxygen-diffusion in polymer films ².

  While ZnTPP works well as a model upconversion system, long-term photostability of these dyes is problematic for real-world applications.  Recent work has focused on developing photochemically robust systems capable of achieving photon upconversion.  In particular, we are exploring mechanisms and efficiency of photon upconversion in mixtures of ZnTPP with fullerenes (C60 and C70), because of the high photostability and desirable electron transfer / redox properties of the carbon-based materials.  We have recently reported a spectroscopic study of excited-state quenching of ZnTPP by C60 in which quenching of metalloporphyrin excited states by fullerenes have been demonstrated and details of the mechanism of this effect have been resolved.  These systems have considerable potential for enhancing robustness of dye-sensitized solar cells while enabling efficient electron transfer in upconversion-based devices ³.  Work in this area continues, along with efforts to measure and characterize upconversion on TiO₂ and ZrO₂ semiconductor films. 

  In terms of future research and innovation, funding provided by ACS-PRF for the co-PIs’ photovoltaic research has had a major impact.  In part, based on the strength of the discoveries made here, the applicants have secured two additional Canadian federal grants to continue their photovoltaic research program.  The first of these grants has been used to secure photovoltaic testing equipment to assist with development of functioning devices based on photon upconversion, and the second, in partnership with MW Canada Ltd., is to incorporate upconversion-enhanced photovoltaic devices into engineered materials (textiles and fabrics).  The latter program is highly-interdisciplinary and includes Steer, Paige, as well as collaborators in Canada (U of S, Université de Laval) and in Australia (University of Melbourne, Monash University). 

  An additional unexpected outcome of this work is the recruitment of a new faculty member (CRC Research Chair, Tier II) at the U of S in the area of photovoltaic materials.  Professor Tim Kelly has recently joined the faculty and will be a major contributor to this program through development of novel photovoltaic materials.  Professor Kelly’s recruitment was assisted, in no small part, thanks to this ACS-PRF funding and the resultant success in research and accumulation of world-class photovoltaic research equipment.

  In summary, the timely funding provided by the ACS-PRF program has had a major impact in photovoltaic research, in training and recruiting of highly-qualified personnel, and spawned a larger-scale research and innovation program in this topic.  This, in the view of the co-PIs, has been an excellent return on investment with long-term benefits to the scientific community as a whole.

References:

(1)        O'Brien, J. A.; Lu, Y.; Hooley, E. N.; Ghiggino, K. P.; Steer, R. P.; Paige, M. F. Can. J. Chem. 2011, 89 (2), 122-129.

(2)        Sugunan, S. K.; Paige, M. F.; Steer, R. P. Can. J. Chem. 2011, 89 (2), 195-202.

(3)        Sugunan, S. K.; Robotham, B.; Sloan, R.; Szmytkowski, J.; Ghiggino, K. P.; Paige, M. F.; Steer, R. P. Journal of Physical Chemistry A 2011, 114 (44), 12217-12227.

(4)        Szmytkowski, J.; Bond, T.; Paige, M. F.; Scott, R. W. J.; Steer, R. P. J. Phys. Chem. A 2010, 114 (43), 11471-11476.

(5)        Szmytkowski, J.; Brunet, S. M. K.; Tripathy, U.; O'Brien, J. A.; Paige, M. F.; Steer, R. P. Chem. Phys. Lett. 2011, 501 (4-6), 278-282.

(6)        Sugunan, S. K.; Tripathy, U.; Brunet, S. M. K.; Paige, M. F.; Steer, R. P. Journal of Physical Chemistry A 2009, 113 (30), 8548-8556.

(7)        O'Brien, J. A.; Rallabandi, S.; Tripathy, U.; Paige, M. F.; Steer, R. P. Chemical Physics Letters 2009, 2009 (4), 220-222.