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Reports: ND6 49214-ND6: Photoluminescence Upconversion in Solution and Solid-State via Triplet-Triplet Annihilation
The objective of this research project is to explore the potential of the photophysical process triplet-triplet annihilation (TTA; 2T1--> Sn (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.
Initial stated goals in this project were: i) identification of MP-energy acceptor systems and characterization of the efficiency and spectroscopic properties of solution-phase TTA; ii) exploring the influence of solid matrices on TTA, and manipulating matrices to control MP aggregation and hence upconversion efficiency; and iii) construction of a simple upconversion-based DSSC. Significant progress has been made in goals i) and ii), with further development of these areas and exploration of iii) to occur in the second funding cycle. Based on this work, we have four manuscripts accepted for publication, with two additional manuscripts under review or revision (note, two of these publications were based on preliminary studies that took place prior to ACS-PRF funding)1-6. We describe results from these experiments in further detail, along with additional unpublished works and future directions.
In our first published work4, we have performed a detailed mechanistic investigation of upconversion in the model MP system zinc(II) tetraphenylporphyrin (ZnTPP), and mixtures of ZnTPP with other blue emitters (perylene, T1 energy < T1 ZnTPP, and coumarin, T1 energy >T1 ZnTPP). For perylene mixtures, upconversion proceeds via triplet energy transfer from ZnTPP to perylene, followed by perylene-TTA, whereas for coumarin, upconversion occurs via triplet exciplex formation followed by TTA with a second photoexcited ZnTPP molecule. In addition to providing information on rates and mechanisms of upconversion in the model system, this study also definitively showed the importance of aggregation (or co-aggregation with blue emitters) in upconversion via TTA.
As a follow-up to this, we have detected and quantified photon upconversion from ZnTPP in thin polymer films2. It was demonstrated by observing dual S1 and S2 emission of ZnTPP that TTA occurs in solid matrices at MP loadings of ≥ 1 mass%. Additional evidence for ZnTPP aggregation (absorption band broadening and lifetime quenching as a function of dye-loading) was also found. This is the first ever demonstration of fluorescence upconversion in ZnTPP via TTA in solids, and a crucial proof-of-principle demonstration for using upconversion in solid-state devices. Recently, we have also used single-molecule fluorescence spectroscopy to directly probe the aggregation state of sulfonated-ZnTPP in polymer films, and demonstrated both multi-step photobleaching and spectral blue-shifts in S2 emission as direct evidence of MP aggregation1.
We have investigated the use of the ZnTPP upconversion system to measure oxygen permeability of polymer (PVA) films; as the T1 state of the MP is sensitive to oxygen, we can use quenching of the upconverted S2 signal of the MP to probe oxygen diffusion into the polymer7. This is important for DSSC performance, as oxygen-based quenching of the dye-sensitizer will reduce efficiency. It was demonstrated that oxygen permeation coefficients can simply be controlled through adjusting properties of the thin film (solvent capacity, thickness, composition).
More recent experiments have explored MP spectroscopy and photochemistry under conditions which are relevant for DSSC operation. There is significant interest in using room temperature ionic liquids as the electrolyte in DSSCs, because of the low volatility, large electrochemical “window” and high stability against oxidation shown by these compounds. However, there are few reports describing the photophysics of short-lived, highly excited electronic states in ionic liquids, and these are precisely the states used in our ZnTPP upconversion scheme. Hence, we have explored the steady-state and time-resolved spectroscopy of ZnTPP in the highly purified ionic liquid [bmim][PF6] (collaborator R.W. J. Scott, Univ. of Saskatchewan). We found that the ionic nature of the solvent (and associated solvent relaxation times) have no significant effects on the nature of the radiationless decay of the S2 state, which decays quantitatively to S1 at a rate consistent with the weak coupling case of radiationless transition theory5.
In a related study, we have explored the photophysical effect of MP interaction with aqueous iodide ion (and related ionic strength effects), as a simple model of the influence of a typical DSSC electrolyte (often I3- / I-) on excited state dynamics of the MP6. Specifically, the dynamics of water-soluble zinc (II) tetraphenylsulfonatoporphyrin when excited to the S2 state were explored in high ionic strength media, with and without iodide as a potential electron transfer agent.
Current unpublished works include exploring the fullerenes C60 and C70 as highly photostable electron transfer agents in TTA upconversion, the use of functionalized TiO2-based sol-gel materials for controlling aggregation and electron transfer processes of MPs, and use of single-molecule fluorescence spectroscopy for investigating back-electron transfer in model DSSC systems. The second year of the program will explore these areas in greater detail, and efforts to construct simple proof-of-principle devices will proceed, likely in collaboration with other researchers in Canada.
(1) O'Brien, J. A.; Lu, Y.; Hooley, E. N.; Ghiggino, K. P.; Steer, R. P.; Paige, M. F. Canadian Journal of Chemistry 2010 - In Press.
(2) O'Brien, J. A.; Rallabandi, S.; Tripathy, U.; Paige, M. F.; Steer, R. P. Chemical Physics Letters 2009, 475, 220.
(3) Steer, R. P. Journal of Applied Physics 2007, 102, 076102.
(4) Sugunan, S. K.; Tripathy, U.; Brunet, S. M. K.; Paige, M. F.; Steer, R. P. Journal of Physical Chemistry A 2009, 113, 8548.
(5) Szmytkowski, J.; Bond, T.; Paige, M. F.; Scott, R. W. J.; Steer, R. P. Journal of Physical Chemistry A 2010 - Under Revision.
(6) Szmytkowski, J.; Brunet, S. M. K.; Tripathy, U.; O'Brien, J. A.; Paige, M. F.; Steer, R. P. Chemical Physics Letters 2010 - Under Review.
(7) Sugunan, S. K.; Paige, M. F.; Steer, R. P. Canadian Journal of Chemistry 2010, In Press.
