Said Abdallah

Tuning photocurrent response through size control of CdSe quantum dots sensitized solar cells

The photovoltaic characterization of CdSe quantum dots sensitized solar cells (QDSSCs) by tuning band gap of CdSe quantum dots (QDs) through size control has been investigated. Fluorine doped tin oxide (FTO) substrates were coated with 20 nm in diameter TiO2 nanoparticles (NPs). Pre-synthesized colloidal CdSe quantum dots of different sizes (from 4.0 to 5.4 nm) were deposited on the TiO2-coated substrates using direct adsorption (DA) method. The FTO counter electrodes were coated with platinum, while the electrolyte containing I−/I3− redox species was sandwiched between the two electrodes. The current density-voltage (J-V) characteristic curves of the assembled QDSSCs were measured for different dipping times, and AM 1.5 simulated sunlight. The maximum values of short circuit current density (Jsc) and conversion efficiency (η) are 1.62 mA/cm2 and 0.29 % respectively, corresponding to CdSe QDs of size 4.52 nm (542 nm absorption edge) and of 6 h dipping time. The variation of the CdSe QDs size mainly tunes the alignment of the conduction band minimum of CdSe with respect to that of TiO2 surface. Furthermore, the Jsc increases linearly with increasing intensity of the sun light, which indicates the sensitivity of the assembled cells.

Thermal properties of hydrated cement pastes studied by the photoacoustic technique

Photoacoustic (PA) technique has been applied to measure the effective thermal diffusivity (αeff) of hydrating cement pastes with a varying water to -cement ratio (w/c) and for variable duration (d) of hydration. Four samples with w/c = 0.3, 0.4, 0.5 and o.6 were prepared. The frequency variation of the PA signal for each sample was recorded at the begining (0 d), as well as one week and one month of hydration. The effective thermal effusivity (eeff) was obtained by measuring the variation of the signal with modulation frequency and the corresponding values of the effective thermal conductivity (keff) were calculated. The results for keff show a decrease at higher w/c (0.6), no change for other samples has been observed. The thickness of the duplex film of Ca(OH)2 and C-S-H formed on the surface of the samples of w/c = 0.5 were determined using the effective layer model in the 0 d and after one month of hydration; a remarkable increase was observed in the last case.