• Energy Research

Doping SnO2 with Mg can lead to non-linear effects on the conduction band edge potentials, first-principles calculations allow dissecting the origin of such behavior.

We show that a simple ethanol (EtOH) refluxing treatment at mild temperature (120 °C) allows producing blue-colored and reduced titanium dioxide (TiO2-x) exhibiting improved visible-light (VIS) photocatalytic properties. The treatment causes an increase in the density of Ti(III) species and the appearance of two optical absorption features: a broad absorption band--responsible for the blue coloration--extending from the green region (~2.3 eV) up to the near-infrared and a subgap absorption tail close to the band gap energy. The experimental results combined with a computation of the density of states via hybrid Hartree-Fock density functional support the hypothesis that the EtOH reflux treatment leads to formation of surface and subsurface oxygen (O) vacancies. We also show that the excitation-resolved photoluminescence technique allows a high-contrast detection of a subgap optical excitation band peaked at about 430 nm (~2.9 eV), associated with anatase photoluminescence, whose intensity increases after the EtOH reflux treatment. This result gives a very direct support to the debated hypothesis identifying O vacancy states as the energy levels involved in the radiative transition of anatase TiO2. Improved photocatalytic degradation by the processed TiO2 under VIS illumination is demonstrated, and the possible mechanism involved in the formation of surface O vacancies is discussed. The method outlines a very simple, low-cost, and fast procedure to target the formation of O vacancies in the TiO2 surface region.

Abstract The effects of discharge parameters on the properties of hydrogenated silicon–carbon films deposited by PECVD system have been investigated. Hydrogenated microcrystalline silicon–carbon films have been grown at low RF power. The increase in RF power from 5 to 25 W leads to a decrease of the crystallinity degree and an enhancement of the carbon content from 0.025 to 0.10. The optical energy gap can be tuned in the 1.86–1.96 eV range and the dark conductivity decreases by about three orders of magnitude. In the investigated pressure range (1.7–2.9 Torr) microcrystalline samples show approximately the same optical and electrical properties. Microcrystalline films with high dark conductivity present an n-type character and the micro-mono crystalline silicon heterostuctures fabricated with these materials reveal a rectifying behaviour of the junctions.

In this work, the apparatus and the method for a simultaneous measurement of the optical losses and short-circuit current in a solar cell, at variable incidence angle of the light, is presented. The method has been applied to an n + /p c-Si cell with a polished surface. The investigation has been performed over an angular range from 81 to 801, using a linearly polarized laser beam, either normally or parallelly polarized with respect to the incidence plane. The experimental curves of reflectance seem to be in a good agreement with the theoretical ones derived from the Fresnel equations. Since the measurements are performed inside an integrating sphere, a procedure has been developed to derive, from the total current I tot ; the calculated direct one, IdirCal excluding contributions from the incoming light backdiffused to the cell under investigation; the results are compared with real direct-current measurement. Since with the proposed method both reflectance and current are measured for the same surface region and under identical illumination conditions, the results have been combined to get the internal spectral response of the n + /p c-Si polished surface solar cell. r 2002 Published by Elsevier Science B.V.