Oleg Shevaleevskiy

Electronic structure study of lightly Nb-doped TiO2 electrode for dye-sensitized solar cells

To improve the conversion efficiency of dye-sensitized solar cells (DSSCs) it is necessary to understand the electronic structure of the TiO2–dye–electrolyte interface in detail. A sturdy junction at the interface can be provided by modifying the electronic structure of the TiO2 electrode with Nb doping. The Nb-doped TiO2 was prepared by a sol–gel method followed by a hydrothermal treatment; the Nb content was varied from 0.5 to 3.0 mol%. The X-ray photoelectron spectroscopy showed that the Fermi level of TiO2 electrode shifted away from the conduction band minimum (CBM) when the Nb content is low (≤1.5 mol%) and shifted toward the CBM when the Nb content is high (≥2.5 mol%). The shift of Fermi level with low Nb doping was due to the passivation of the oxygen vacancies at the TiO2 nanoparticle surface. Intraband states were formed when dopant content was 1.5 and 2.5 mol%. We have found that the photovoltaic parameters of DSSCs based on doped TiO2 sensitized with a cis-[Ru(dcbpyH)2(NCS)2](NBu4)2, N719 dye, are closely related to the electronic structure of the Nb-doped TiO2 electrode. The changes of short circuit current and open circuit voltage of DSSCs were explained in relation to the electronic structure of the TiO2 electrode. The best efficiency of 8.0% was demonstrated by DSSCs with 2.5 mol% Nb-doped TiO2.

Charge transport in hydrogenated boron-doped nanocrystalline silicon-silicon carbide alloys

We have investigated the carrier transport mechanism of mixed-phased hydrogenated boron-doped nanocrystalline silicon–silicon carbide alloy (p-nc-Si-SiC:H) films. From temperature-dependent dark conductivity measurements, we found that the p-nc-Si-SiC:H alloys have two different carrier transport mechanisms: one is the thermally activated hopping between neighboring crystallites near the room-temperature region and the other is the band tail hopping below 150 K.

Nanocrystalline silicon carbide films for solar photovoltaics: The role of dangling-bond defects

Thin films of microcrystalline hydrogenated silicon (µc-Si:H) and nanocrystalline silicon carbide (nc-SiC:H) provide a new class of advanced nanostructured materials for solar photovoltaic (PV) devices. We have worked on the fabrication, characterization, and application of these materials for thin film PV solar cells based on amorphous silicon. Here we present an overview of the preparation and characterization methods for heterogeneous SiC:H-based layers. Hydrogenated nc-SiC:H thin film materials with high crystalline volume fraction were deposited using photo-assisted chemical vapor deposition (photo-CVD) technique. The behavior of spin-containing dangling-bond (DB) defects was performed using electron spin resonance (ESR) and transport measurements as a function of sample crystallinity, doping level, and temperature. The electronic and structural properties of intrinsic and doped µc-Si:H and nc-SiC:H thin films are reviewed with the emphasis of the essential role of DB defects on the photoelectronic transport parameters.

Effects of Bulk Photoconductivity on Photocurrent Action Spectra of Molecular p-n Heterojunction Solar Cells

bInstitute of Biochemical Physics RAS, 119991 Moscow, Russia We have investigated how bulk photoconductivity influences the photovoltaic parameters of zincphthalocyanine-fullerene ZnPc/C60 p-n heterojunction solar cells. The results indicate that the photocurrent action spectrum of a cell depends strongly on the photoconductivity and spectral characteristics of each component material. We therefore propose a model that simulates the action spectra of the short-circuit photocurrent in the molecular organic solar cells, and our model is based on the assumption that

Improvement of electron transport in DSSCs by using Nb-doped TiO 2 electrodes

The performance of dye-sensitized solar cells (DSSCs) is closely related to efficiency of the electron transport within TiO2-dye-electrolyte system. Electron transport can be improved by modification of the electronic structure of TiO2 electrode by doping with niobium (Nb+5). For this purpose, the DSSCs based on undoped and Nb-doped TiO2 layers were fabricated and their PV parameters and electrical properties were studied. The Nb-doped TiO2 was prepared by a sol-gel method followed by a hydrothermal treatment with the Nb content in the range of 0.7 to 3.5 mol%. The transport properties of the TiO2-dye-electrolyte junction were investigated using the electrical impedance spectroscopy. The electron lifetimes, estimated from Bode plots, were found to increase from 8 ms for DSSCs based on undoped TiO2 up to 26 ms for the cells based on TiO2 doped with 2.7 mol% of Nb. We have shown correlation between the parameters of the charge carrier transport and the main DSSC photovoltaic characteristics. The increase in the value of electron lifetime was shown to enhance the value of short circuit current (Jsc). When the Nb doping level was lower than 1.7 mol%, the electrical resistance at the TiO2-electrolyte interface increased leading to rise of the value of the open circuit voltage (Voc). Doping with the 1.7 mol% of Nb led to increase of both Jsc and Voc, and significantly improved the device efficiency.