Two dye sensitized solar cells (DSC) can be joined to form a tandem cell with two separate absorption ranges for the two different absorber materials. This can enhance the solar conversion efficiency and in particular the photovoltage of the DSC. Water splitting appears as a realistic long term target. The DSC tandem can be realized as n-n junction employing known dye molecules with optimal absorption spectra. Dye molecules with elongated shapes can be realized by covalently attaching a conducting bridge group terminated by an anchor group to a desired chromophore. Due to the long conducting bridge group separating the hole state of the dye from the surface of the semiconductor recombination is slowed down. The ordered molecular structure can be self-assembled on the recently introduced rod or cylinder shaped oxide electrodes but will not slow down recombination in the nm-cavities of the conventional TiO2 Graetzel electrode.
Ultrafast heterogeneous electron transfer (HET) from the excited singlet state of the organic chromophore perylene
into the inorganic semiconductor rutile TiO2 was investigated with femtosecond time-resolved two-photon
photoemission (2PPE). With 2PPE one can address adsorbates at coverages far below a monolayer on single
crystal surfaces. With the same chromophore perylene fixed with different anchor and bridge groups at the surface
of rutile TiO2(110) the corresponding 2PPE transients revealed the relevant parameters that characterize
the contributing processes. Instantaneous optical injection on one hand and slow injection over a long distance
on the other hand were realized. Direct optical charge transfer was realized with the chromophore catechol that
is known to form a charge transfer complex with Ti atoms on the surface of TiO2. The slow injection cases
were realized by inserting rigid molecular bridges. Comparison of the different 2PPE signals with corresponding
transient absorption (TA) signals for the identical systems revealed the physical processes and time scales that
control the 2PPE transients. On the surface of the single crystals only one long time constant was measured via
2PPE also in the case of a long rigid bridge/anchor group in contrast to a broad distribution of time constants
observed for the same molecules anchored in the nm-size cavities of an anatase TiO2 film measured via TA.
The broad distribution of time constants in the latter measurements can be attributed to different microscopic
environments giving rise to different distances between the chromophore and the nearest TiO2 wall.
Hot electron injection from the excited electronic singlet state of perylene chromophores into the (110) surface of rutile TiO2 single crystals was measured with femtosecond two-photon photoemission (2PPE) for different anchor/bridge groups attached to the perylene chromophore. Femtosecond 2PPE probes the time and energy dependence of the population of firstly the excited state of the chromophore and secondly of the hot electrons injected into the surface layer of the semiconductor. Measuring both these contributions gives a complete picture of the ultrafast photo-induced injection process and bridges the gap to conventional measurements of the rise time of the corresponding photocurrent. Studying the system in ultra-high-vacuum (UHV) makes all the tools of surface science available. Impurities on the surface were studied with XPS, the alignment of the occupied and unoccupied electronic levels at the interface with UPS and with 2PPE, respectively. The orientation of the elongated chromophores with respect to the crystal surface was deduced from angle and polarization dependent 2PPE signals making use of the known orientation of the dipole moment for the optical transition, the energy distribution of the injected hot electrons was determined with 2PPE from the energy distribution of the photoemitted electrons, and finally the escape of the injected electrons from the surface to bulk states of the semiconductor was obtained from femtosecond 2PPE transients.
The reconstruction of P-rich InP(100) requires at least a (2x4) surface unit cell to stay semiconducting and uncharged (electron counting rule). Recently it has been shown that the much smaller (2x2) unit cell obtained from MOCVD (metalorganic vapor deposition) growth contains P-H bonds. Orientation and polarization dependent Fourier Transform Infrared Spectroscopy (FTIR) of the P-H bonds in the Attenuated Total Reflection (ATR) mode have confirmed the specific form of the (2x2) surface unit cell (T. Letzig et al., Phys. Rev. B 71 (2005) 033308) earlier proposed by W.G. Schmidt and coworkers (W.G. Schmidt et al., Phys. Rev. Lett. 90 (2003) 126101). Surface unit cells with a higher concentration of P-H bonds also obey the electron counting rule. A c(2x2) LEED image and two matching FTIR peaks were observed when the (2x2) reconstructed surface was exposed to atomic hydrogen. The corresponding c(2x2)-2P-3H surface unit cell can be shown to form a stable surface phase (T. Letzig et al., Phys. Rev. B, submitted). The complete transformation of the (2x2) surface to this new phase is not observed since the surface deteriorates when exposed to a higher dose of atomic hydrogen.
Hot electron injection from the aromatic chromophore perylene into TiO2 was measured with transient absorption signals for different rigid anchor-cum-spacer groups revealing 15 fs as the shortest and 4 ps as the longest injection time. The energetic position of the donor orbital of the chromophore with respect to the conduction band edge was determined at about 0.8 eV employing ultraviolet photoelectron spectroscopy (UPS)and simple absorption
spectroscopy.It is not clear in the case of rutile or anatase TiO2 whether unoccupied surface states are involved
in the electron injection process as acceptor states. Since the surface reconstruction of TiO2 is difficult to control
electron scattering between a well-defined surface state and isoenergetic unoccupied bulk states was studied with InP(100). Electron scattering was time-resolved employing two-color two-photon-photoemission (2PPE). Scattering from isoenergetic bulk states to the empty C1 surface state was found to occur with a 35 fs time
constant, and the reverse process showed a time constant in the range of 200 fs. The latter was controlled by energy relaxation in bulk states, i.e. via the emission of longitudinal optical phonons in InP. In general, the injection of a hot electron from a molecular donor into electronic states of a semiconductor as to be distinguished
from consecutive electron scattering processes between surface states and bulk states. Distinguishing between the different processes may become difficult, however,if the electronic interaction becomes large for a small chromophore directly attached to the semiconductor.
In this paper the operation principles of a new folded ultra-thin-layer solar cell are being discussed. The cell was introduced recently by Graetzel and coworkers. The relationship between photocurrent and luminescence in this cell is derived for the simplest kinetic scheme appropriate for this type of solar cell. It involves three stages, i.e. four levels. In our simpel model these stages are connected by rate constants. We show picosecond time-resolved measurements of the luminescence decay curve and of the luminescence spectrum of the triplet state of the adsorbed
trinuclear-Ruthenium dye molecules. Picosecond time-resolution of these signals is essential for distinguishing between relevant and irrelevant luminescence signals emitted from the cell. Moderately fast electron injection from the triplet state of this dye with 172 ps time constant yields very efficient conversion of absorbed photons
to injected electrons. The time-response of the photocurrent is determined by the filling and emptying of traps in the depletion layer. We discuss the potential of this cell for the photovoltaic solar energy conversion.
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