The frequency-dependent photocurrent response of dye-sensitized TiO2 cells to modulated illumination is analyzed. Analytical expressions are derived that describe generation, collection, and recombination of electrons in a thin layer nanocrystalline solar cell under conditions of steady illumination and with a superimposed small amplitude modulation. The analysis considers illumination from the contact side and from the counter electrode side, and characteristic differences in the intensity-modulated photocurrent response are predicted for the two cases. The attenuation of the ac photocurrent by the RC time constant of the cell is also considered. The theoretical analysis shows that intensity modulated photocurrent spectroscopy (IMPS) can provide new insight into the dynamics of electron transport and collection in the dye-sensitized solar cell. Experimental IMPS data measured for high-efficiency dye-sensitized cells are fitted to the theoretical model using Bode plots in order to derive values of the lifetime (2 × 10-2 s) and diffusion coefficient (5 × 10-5 cm2 s-1) of photoinjected electrons.

Despite the fact that H-terminated, HF-etched Si crystals are the starting point for construction of most contemporary electronic devices, 1 little is known about the chemical reactions of H-terminated Si surfaces under ambient temperature and pressure. 2,3 Functionalization of Si without partial oxidation and/or formation of electrical defects is potentially important in fabricating improved electronic devices 4,5 as well as in measurement of charge transfer rate constants at semiconductor/ liquid contacts. 6 One recently described approach involves the reaction of HF-etched Si(111) with olefins and organic diacyl peroxides, in which formation of a self-assembled (near)monolayer of Si-alkyls was hypothesized. 2 We report here an alternative strategy to functionalize HF-etched Si surfaces involving halogenation and subsequent reaction with alkyl Grignard or alkyl lithium reagents. We report vibrational spectroscopic and temperature programmed desorption data which confirm that the alkyl groups are bonded covalently to the Si surface, and we demonstrate that such overlayer formation can impede the undesirable oxidation of Si in a variety of environments while providing surfaces of high electrical quality.The H-terminated Si surface 7 was first exposed to PCl 5 for 20-60 min at 80-100 °C, in chlorobenzene with benzoyl peroxide as the radical initiator. 8,9 Upon chlorination, the XP survey spectra (Figure 1) showed peaks at 270.2 ( 0.4 binding electron volts, BeV, (Cl 2s) and 199.3 ( 0.4 BeV (Cl 2p), indicating that this procedure yielded Cl on the surface. The high-resolution XP spectrum of the Si 2p peak of this surface displayed, in addition to the substrate Si signal, an additional peak located at 0.98 ( 0.12 BeV higher in binding energy (Figure 2) whose position and intensity was consistent with the formation of a surface Si-Cl bond. 10 Auger electron spectra (AES) also confirmed the presence of Cl on the silicon surface. High resolution electron energy loss spectra (HREELS) of this surface exhibited a characteristic peak at 560 cm -1 that was not present on the H-terminated Si surface, confirming the formation of covalent Si-Cl bonds at the surface. 11 Temperature programmed desorption spectra of the chlorinated surface showed dominant signals at 64 (SiCl), 71 (Cl 2 ), and 135 (SiCl 3 ) amu, peaking at 670 and 850 K, which is characteristic of chlorinated silicon surfaces. 10,12 The 560 cm -1 peak in the HREELS and the Cl peak in the AES disappeared following thermal desorption.Exposure of the chlorinated Si surface to alkyl-Li (RLi: R