Exciton annihilation in dye-sensitized nanocrystalline semiconductor films

Abstract

Exciton annihilation in dye-sensitizednanocrystalline semiconductor (Al 2 O 3) films has been studied through laser-induced fluorescence spectroscopy. The relative quantum yield of the fluorescence decreases with increasing excitation light intensity, the indication being that exciton annihilation occurred. The rate constants of the annihilation were estimated for three dyes, N719, D149, and MK2, that are known to be sensitizing dyes for efficient dye-sensitized solar cells. The hopping time between dye molecules and the diffusion length of excitons within their lifetime were also estimated to facilitate discussion of the relevance of exciton annihilation to primary processes in dye-sensitized solar cells.

Interfacial Electron Injection Probed by a Substrate-Specific Excitonic Signature

“…Excitation of the singlet metal-to-ligand charge-transfer ( 1 MLCT, green arrows in Figure 1) state of N719 molecules in a dimethylformamide (DMF) solution leads to population of the 3 MLCT state, which has a lifetime of 20 ns. 44 The deep-UV (shaded area in Figure 1a) transient response of the dye system upon excitation of its 1 MLCT state at 550 nm (2.25 eV) is shown in Figure 2a displaying the resulting color-coded differential absorption (ΔA) map as a function of probe wavelength and time delay between the visible pump and deep-UV probe (at a fluence of 40 μJ/cm 2 ). Figure 2b shows the transients at specific time delays.…”

“…The regions to the blue and the red side of this bleach band are positive. All features (positive or negative) exhibit a long temporal behavior (Figure S2), characteristic of the promptly populated (≤30–40 fs , ) 3 MLCT state . This implies that the positive features in Figure b are due to an excited state absorption of the 3 MLCT state.…”

“…All features (positive or negative) exhibit a long temporal behavior (Figure S2), characteristic of the promptly populated (≤30−40 fs 45,46 ) 3 MLCT state. 44 This implies that the positive features in Figure 2b are due to an excited state absorption of the 3 MLCT state. The weak evolution at early times of all features in Figure 2a is due to vibrational cooling processes.…”

Effect of Adsorbed Water Molecules on Light Harvesting and Electron Injection Processes in Dye-Sensitized Nanocrystalline TiO2 Films

“…To investigate the effect of surface-adsorbed water molecules on electron injection, we first measured the luminescence spectra of N719/n-Al 2 O 3 to determine the effect of surface-adsorbed water on the properties of excited N719. Note that electron injection is suppressed in N719/n-Al 2 O 3 because the edge of the Al 2 O 3 conduction band is higher than the lowest unoccupied molecular orbital (LUMO) level of N719, which is why this system can be used to study the effects of humidity on relaxation of the excited dye molecule. The luminescence from N719 is known to be phosphorescence from the excited triplet MLCT state because intersystem crossing from the initially formed singlet state to the triplet state in N719 is extremely fast (time constant, 100 fs in solution) .…”