Mihai Razvan Mitroi

Stress-induced traps in multilayered structures

The trap parameters of defects in Si/CaF2 multilayered structures were determined from the analysis of optical charging spectroscopy measurements. Two kinds of maxima were observed. Some of them were rather broad, corresponding to “normal” traps, while the others, very sharp, were attributed to stress-induced traps. A procedure of optimal linear smoothing the noisy experimental data has been developed and applied. This procedure is based on finding the minimal value of the relative error with respect to the value of the smoothing window. In order to obtain a better accuracy for the description of the trapping-detrapping process, a Gaussian temperature dependence of the capture cross-sections characterizing the stress-induced traps was introduced. Both the normal and the stress-induced traps have been characterized, including some previously considered as only noise features.

Numerical procedure for optimizing dye-sensitized solar cells

We propose a numerical procedure consisting of a simplified physical model and a numerical method with the aim of optimizing the performance parameters of dye-sensitized solar cells (DSSCs). We calculate the real rate of absorbed photons (in the dye spectral range) Greal(x) by introducing a factor β > 1 in order to simplify the light absorption and reflection on TCO electrode. We consider the electrical transport to be purely diffusive and the recombination process only to occur between electrons from the TiO2 conduction band and anions from the electrolyte. The used numerical method permits solving the system of differential equations resulting from the physical model. We apply the proposed numerical procedure on a classical DSSC based on Ruthenium dye in order to validate it. For this, we simulate the J-V characteristics and calculate the main parameters: short-circuit current density jsc, open circuit voltage Voc, fill factor FF, and power conversion efficiency ρ. We analyze the influence of the nature of semiconductor (TiO2) and dye and also the influence of different technological parameters on the performance parameters of DSSCs. The obtained results show that the proposed numerical procedure is suitable for developing a numerical simulation platform for improving the DSSCs performance by choosing the optimal parameters.

Temperature dependence of capture coefficients in trapping phenomena

The temperature dependence of the capture coefficients in trapping phenomena is investigated. It is proved that, besides the dependence induced by the thermal velocity of the carriers, the stress-induced traps at the interfaces of the multi-layered structures present a supplementary temperature dependence. This dependence is found to be of Gaussian type and is in a good agreement with the experimental results.

Optimization of black dye-sensitized solar cells by numerical simulation

The numerical simulation of the physical parameters for dye sensitized solar cells with black dyes was considered based on a new optimizing procedure. The influence of thickness and lifetime on the J−V characteristics parameters was analyzed. In this way, Jsc (short-circuit current density), Voc (open circuit voltage), FF (fill factor), and η (efficiency) were determined. A comparison between the classical ruthenium based solar cells and black dyes ones was possible on the simulation and experimental approach. The obtained optimum values for thickness and lifetime, as well as the analysis of the main parameters of the J−V characteristics of black-dye solar cells, could be used to optimise the manufacturing process. The electron lifetime is in the range of 2–100 ms and has its optimal value of 15 ms. The suitable thickness of TiO2 layer was determined to be in the range of 8–20 μm with the optimal value of 10 μm, where Jsc and η reach their maxima.

DIELECTRIC LOSSES IN ISOTENAX N

The limits of the Maxwell–Wagner model for interfacial losses in composite materials are discussed. The model is improved by taking into account the frequency dependence of the loss resistances. The improved model is applied to the statistical analysis of the experimental data obtained on Isotenax N. The results of the model are in very good agreement with the experiment.

Iodine irradiation induced defects in crystalline silicon

N-type P-doped silicon single crystals with resistivity higher than 8000 Ωcm were irradiated with 127I6+ ions of 28 MeV kinetic energy. The penetration of the ions through the target and the processes of energy loss were simulated using the CTRIM Monte Carlo code, and point defect production was calculated in the frame of our diffusion-reaction model. Trapping phenomena were investigated using the method of thermally stimulated currents without applied bias. The modeling of the current-temperature curves takes into consideration both point defects and stress-type trapping centers, produced by the ions stopped into the crystal.