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Oliver L.A. Monti, University of Arizona
Normal 0 false false false MicrosoftInternetExplorer4 Research has been conducted on interfacial charge transfer kinetics at the single molecule level in model dye sensitized solar cells. We have designed and implemented an ultrahigh vacuum single molecule fluorescence microscope capable of detecting single molecule dynamics under highly controlled conditions and on well-characterized interfaces. Since charge generation kinetics in excitonic solar cells is almost completely controlled by interfacial processes, we emphasize the importance of the defined interface, only available in ultrahigh vacuum conditions. This technical advance has allowed us to utilize single molecule fluorescence as a reporter on interfacial charge transfer kinetics: By monitoring sudden, stochastic changes in fluorescence intensity, the distribution of charge transfer and recombination rates may be observed. As a model system for solar charge generation in excitonic solar cells we chose as an acceptor a single crystalline wide-bandgap semiconductor (GaN, oriented towards (0001)) with a tunable thickness heteroepitaxial insulator layer (Sc2O3, oriented towards (111)) to control the distance between the acceptor and the single molecule donor (PTCDI, a perylene bisimide). We have carefully characterized the PTCDI/Sc2O3/GaN structure by a wide array of surface and solution spectroscopic means and have established that PTCDI is capable of injecting charge into GaN with a driving force of at most 100 meV. Analogs with higher excited state energies are available and will be investigated in the future. Single molecule fluorescence spectroscopy of the PTCDI/Sc2O3/GaN system with 15 Å Sc2O3 at 10-10 mbar shows i) molecular survival times of hundreds of seconds, aiding in obtaining statistically significant measures of fluorescence dynamics; ii) extremely strong intensity fluctuations distributed over many time-scales; and iii) clear indication of multilevel dynamics even in a regime where we believe to observe single molecules only. In order to validate these measurements, we have replaced the acceptor structure with a single crystalline oxide insulator, Al2O3 (0001), which should inhibit electron transfer. Surprisingly, single molecule fluorescence data show unambiguous evidence for excursions to long-lived dark states on sapphire as well, suggestive of charge transfer and trapping on the insulator surface. Moreover, a careful, model-free statistical assessment of our data shows that molecules experience discrete transitions to many more than just two levels while retaining the clear single molecule finger print of one-step photobleaching. The observed dynamics cannot be caused by rotational diffusion. Strikingly, if the dark state excursions are indeed caused by charge transfer to sapphire, the power-law distributed bright periods point towards charge transfer dynamics that does not follow the widely utilized Marcus model for interfacial charge transfer. We believe that such behavior might be much more widely spread in single molecule spectroscopy, but that the absence of adequate statistical methods has thus far prevented the unambiguous identification of complex dynamics at the single molecule level. Our observations, conducted under the most highly-defined conditions, suggest that the frequently observed non-exponential charge transfer dynamics in excitonic solar cells originate in fluctuations at the level of individual molecules and should be addressed at a structural level when designing efficient solar cell architectures based on organic or polymeric materials. We are currently investigating the origin of the fluctuations and non-exponential dynamics; in particular, we have initiated efforts to differentiate between static and dynamic heterogeneity as well as polarization-resolved measurements. The experimental and theoretical tools in place will allow us to tackle the far more complex behavior in the PTCDI/Sc2O3/GaN system. This grant has supported the education of two graduate students (one female), one postdoctoral researcher and two undergraduate students (one now graduate student at Penn State).
Research has been conducted on interfacial charge transfer kinetics at the single molecule level in model dye sensitized solar cells. We have designed and implemented an ultrahigh vacuum single molecule fluorescence microscope capable of detecting single molecule dynamics under highly controlled conditions and on well-characterized interfaces. Since charge generation kinetics in excitonic solar cells is almost completely controlled by interfacial processes, we emphasize the importance of the defined interface, only available in ultrahigh vacuum conditions. This technical advance has allowed us to utilize single molecule fluorescence as a reporter on interfacial charge transfer kinetics: By monitoring sudden, stochastic changes in fluorescence intensity, the distribution of charge transfer and recombination rates may be observed.
As a model system for solar charge generation in excitonic solar cells we chose as an acceptor a single crystalline wide-bandgap semiconductor (GaN, oriented towards (0001)) with a tunable thickness heteroepitaxial insulator layer (Sc2O3, oriented towards (111)) to control the distance between the acceptor and the single molecule donor (PTCDI, a perylene bisimide). We have carefully characterized the PTCDI/Sc2O3/GaN structure by a wide array of surface and solution spectroscopic means and have established that PTCDI is capable of injecting charge into GaN with a driving force of at most 100 meV. Analogs with higher excited state energies are available and will be investigated in the future.
Single molecule fluorescence spectroscopy of the PTCDI/Sc2O3/GaN system with 15 Å Sc2O3 at 10-10 mbar shows i) molecular survival times of hundreds of seconds, aiding in obtaining statistically significant measures of fluorescence dynamics; ii) extremely strong intensity fluctuations distributed over many time-scales; and iii) clear indication of multilevel dynamics even in a regime where we believe to observe single molecules only.
In order to validate these measurements, we have replaced the acceptor structure with a single crystalline oxide insulator, Al2O3 (0001), which should inhibit electron transfer. Surprisingly, single molecule fluorescence data show unambiguous evidence for excursions to long-lived dark states on sapphire as well, suggestive of charge transfer and trapping on the insulator surface. Moreover, a careful, model-free statistical assessment of our data shows that molecules experience discrete transitions to many more than just two levels while retaining the clear single molecule finger print of one-step photobleaching. The observed dynamics cannot be caused by rotational diffusion. Strikingly, if the dark state excursions are indeed caused by charge transfer to sapphire, the power-law distributed bright periods point towards charge transfer dynamics that does not follow the widely utilized Marcus model for interfacial charge transfer.
We believe that such behavior might be much more widely spread in single molecule spectroscopy, but that the absence of adequate statistical methods has thus far prevented the unambiguous identification of complex dynamics at the single molecule level. Our observations, conducted under the most highly-defined conditions, suggest that the frequently observed non-exponential charge transfer dynamics in excitonic solar cells originate in fluctuations at the level of individual molecules and should be addressed at a structural level when designing efficient solar cell architectures based on organic or polymeric materials.
We are currently investigating the origin of the fluctuations and non-exponential dynamics; in particular, we have initiated efforts to differentiate between static and dynamic heterogeneity as well as polarization-resolved measurements. The experimental and theoretical tools in place will allow us to tackle the far more complex behavior in the PTCDI/Sc2O3/GaN system.
This grant has supported the education of two graduate students (one female), one postdoctoral researcher and two undergraduate students (one now graduate student at Penn State).
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