Reports: G6
46516-G6 Interfacial Electron Transfer One Molecule at a Time
Research in LabMonti has been conducted on interfacial charge transfer kinetics at the single molecule level in order to model fundamental processes in dye sensitized solar cells. We have recently developed a novel single molecule confocal scanning optical microscope (CSOM) and sample preparation system capable of operating under ultrahigh vacuum (UHV) 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. We used this development to investigate single molecule fluorescence intermittency of N,N′-dibutylperylene-3,4,9,10-dicarboxyimide (C4-PTCDI) physisorbed on Al2O3 (0001). Remarkably, even though deposited onto a bare, crystalline insulator surface, the molecules exhibited prolonged periods of high ("on") as well as periods of no ("off") fluorescence. The "off" periods were attributed to molecules undergoing charge transfer to long-lived dark states: UHV SMS is capable of observing interfacial charge transfer one molecule at a time and under highly controlled conditions. Mechanisms other than charge transfer (triplet excursions, spectral diffusion) could be excluded based on the known photophysics of PTCDI and the short triplet lifetime. The probability densities for both "on" and "off" periods ton and toff showed power-law behavior over three decades, with low power-law coefficients for both "on" and "off" distributions of m ≈ 1.2.
A detailed analysis of these distributions using an unbiased change point detection algorithm suggested activated interfacial charge transfer kinetics to and from a distribution of states in the sapphire substrate.The likely chemical nature of states involved in interfacial charge transfer was identified, facilitated by a complete understanding of band and energy level alignment for the C4-PTCDI/Al2O3 (0001) system. On the basis of the observed activated interfacial charge transfer kinetics and extensive theoretical work by others, we proposed an interfacial charge transfer mechanism capable of quantitatively explaining not only the power-law distribution of "off" durations – a number of scenarios have been discussed in the literature – but also of "on" times by activated charge recombination of specific polaron pairs in Al2O3. The sequence is as follows: Formation of an electron polaron (PA, Al interstitials and F-centers), followed by generation of a hole polaron (PD, O and Al vacancies); this polaron pair prevents further charge transfer from the molecule, thus explaining the extended "on" periods observed; recombination occurs by polaron recombination from a distribution of energies, thus giving rise to power-law distributed "on" times. Activated neutralization from a distribution of energies explains the power-law distributed "off" periods. Therefore, SMS on highly-defined surfaces under UHV conditions provides an unprecedented opportunity to obtain quantitative mechanistic insight into interfacial charge transfer processes even in the event of power-law statistics. We emphasize that even at single crystalline surfaces, there exist surface and defect states that may participate in interfacial charge transfer. Until recently, their role in interfacial charge transfer has however been difficult to assess. Strikingly, the power-law distributed bright and dark periods point towards charge transfer dynamics that do not follow the widely utilized Marcus model for interfacial charge transfer.
We are currently extending these efforts to a system including a wide-bandgap semiconductor: GaN, oriented towards (0001) serves as the single crystalline wide-bandgap semiconductor, with a tunable thickness heteroepitaxial insulator layer (Sc2O3, oriented towards (111)) to control the distance between the acceptor and the single molecule donor (PTCDI). 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 injection 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.
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.
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).
