A dense TiO2 electron blocking layer was introduced on a conducting glass in CdS quantum dots sensitized solar cells to investigate electron leakage into the electrolyte. A ferricyanide/ferrocyanide redox couple was employed, as a model electrolyte, possessing a high standard rate constant. The analysis of the I–V characteristics, using a one-diode model, revealed that an increase in this layer, up to 50 nm, significantly suppresses the electron leakage, enhancing the shunt resistance by a factor of 200. An energy conversion efficiency of 1.0% was attained on account of the improved open circuit voltage and fill factor.
We have conducted submicrosecond to millisecond transient absorption studies to elucidate the parameters controlling charge recombination kinetics at an in situ chemical bath-deposited CdS/TiO 2 interface. The CdS/ TiO 2 nanostructures were prepared by depositing CdS in the TiO 2 nanocrystalline films via the SILAR, successive ionic layer adsorption and reaction, technique. Steady-state absorption spectroscopy and XRD measurements of the films indicated that the CdS amount increases as a function of the CdS deposition cycles. In contrast, the CdS crystallinity size remains constant after reaching approximately 4 nm. Comparison of the transient absorption spectra between CdS/TiO 2 and CdS/Al 2 O 3 (Al 2 O 3 is employed as an insulator) suggests that an efficient electron injection occurs from CdS to a TiO 2 conduction band. Charge recombination kinetics for the CdS/TiO 2 appears to be multiexponential, being similar to the transient dynamics observed for dyesensitized TiO 2 films. A detailed analysis of the charge recombination dynamics with nonadiabatic electron transfer theory revealed that the recombination half-lifetime, t 50% , correlates closely with the CdS crystallinity size, resulting in recombination retardation by a factor of 100 with increase in the crystallinity radius from 0.8 to 2.1 nm. This correlation is discussed in relation to the function of CdS quantum dots-sensitized TiO 2 solar cells.
Formation of nanostructured polythiophene/TiO 2 heterojunction films, using photoinduced polymerization of thiophene inside TiO 2 nanopores, was investigated. The resultant film possesses nanohybridization and electronic connection within the TiO 2 nanoporous domain. Photopolymerization proceeded in three stages: (i) photoexcitation of bithiophene covalently attached to the TiO 2 surface, (ii) an electron injection reaction from the surface attached thiophene to the TiO 2 , and (iii) an electron transfer from a thiophene reactant in an electrolyte to the surface-attached bithiophene. Initial rapid photopolymerization and subsequent slow polymer growth were explained by analysis of a series of experiments, e.g., with respect to light irradiation time, applied bias, electrolyte types, thiophene reactant type, and their morphology. Electrochemical measurements for the bithiophene adsorbed on TiO 2 revealed a wide distribution of redox potentials. This was explained by influence of the local electric field on the TiO 2 surface in addition to strong interaction between the surfacebound bithiophene and the TiO 2 . The nanohybrid film was applied to a sensitized-type photoelectrochemical solar cell, substantiating direct application of the nanohybrid film to electronic devices. The solar cell performance was closely associated with the interfacial structure in the nanohybrid film and the photopolymerization degree.
Kinetic studies at Ti0 2 /bithiophene/electrolyte interfaces were conducted, and their parameters to the solar cell functions were related. The solar cell based upon the bithiophene sensitised Ti0 2 films resulted in the maximum IPCE of approximately 25 % at 400 nm. Comparison of emission studies between the bithiophene adsorbed Ti0 2 and Alz0 3 revealed the electron injection from the excited bithiophene into the Ti0 2 with the efficiency of nearly 100 %. The charge recombination between the bithiophene cation and the electron in the Ti0 2 appeared to be fast with a half decay time of 70 J.lS in comparison to the ruthenium dye sensitized Ti0 2 film (~1 ms). The bithiophene regeneration rate with the half time of 20 J.lS was slightly faster, clarifying the inferior photocurrent performance.
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