Impedance spectroscopy was applied to investigate the characteristics of dye-sensitized nanostructured TiO 2 solar cells (DSC) with high efficiencies of light to electricity conversion of 11.1% and 10.2%. The different parameters, that is, chemical capacitance, steady-state transport resistance, transient diffusion coefficient, and charge-transfer (recombination) resistance, have been interpreted in a unified and consistent framework, in which an exponential distribution of the localized states in the TiO 2 band gap plays a central role. The temperature variation of the chemical diffusion coefficient dependence on the Fermi-level position has been observed consistently with the standard multiple trapping model of electron transport in disordered semiconductors. A Tafel dependence of the recombination resistance dependence on bias potential has been rationalized in terms of the charge transfer from a distribution of surface states using the Marcus model of electron transfer. The current-potential curve of the solar cells has been independently constructed from the impedance parameters, allowing a separate analysis of the contribution of different resistive processes to the overall conversion efficiency.

Characteristics of high efficiency dye-sensitized solar cells.

This Progress Report highlights recent developments in dye-sensitized solar cells composed of both liquid electrolytes and solid-state hole transport materials. The authors discuss and review the present understanding of and recent developments in the operational processes, such as charge generation, transport, recombination, and charge collection. Also, the merits and challenges of alternative device approaches are discussed, including extremely thin absorber cells, devices containing inorganic p-type hole-transporters and non-TiO 2 mesoporous metal-oxide elctrodes employed in dye-sensitized solar cells.

Advances in Liquid-Electrolyte and Solid-State Dye-Sensitized Solar Cells

Well-crystallized Nb-doped anatase TiO 2 nanoparticles are prepared by a novel synthetic route and successfully used as the photoanode of dye-sensitized solar cells (DSSCs). The homogenous distribution of Nb in the TiO 2 lattice is confirmed by scanning transmission electron microscopy (STEM) elemental mapping and line-scanning analyses. After Nb doping, the conductivity ofthe TiO 2 powder increases, and its flat-band potential (V fb ) has a positive shift. The energy-conversion efficiency of a cell based on 5.0 mol% Nb-doped Ti0 2 is significantly better, by about 18.2%, compared to that of a cell based on undoped TiO 2 . The as-prepared Nb-doped TiO 2 material is proven in detail to be a better photoanode material than pure TiO 2 , and this new synthetic approach using a water-soluble precursor provides a simple and versatile way to prepare excellent photoanode materials.

Improved-Performance Dye-Sensitized Solar Cells Using Nb-Doped TiO2 Electrodes: Efficient Electron Injection and Transfer

We report measurements of thermally induced decay of fiber Bragg gratings patterned by ultraviolet irradiation in germanium‐doped silica fiber. The decay is well characterized by a ‘‘power‐law’’ function of time with a small exponent, which is consistent with the rapid initial decay followed by a substantially decreasing rate of decay. We propose a decay mechanism in which carriers excited during writing are trapped in a broad distribution of trap states, and the rate of thermal depopulation is an activated function of the trap depth. This model is consistent with the observed power‐law behavior. An important consequence of this mechanism is that the decay of the induced index change can be accelerated by increasing temperature. A decelerated‐aging experiment verifies this prediction. This result demonstrates that it is possible to preanneal a device incorporating ultraviolet‐induced refractive‐index changes, wiping out the portion of the index change that would decay over the lifetime of the device, and keeping only the very stable portion of the index change.

Decay of ultraviolet-induced fiber bragg gratings

Luminescence and related investigations of recombination in hydrogenated amorphous silicon prepared by glow discharge and sputtering are described. Emphasis is given to a detailed discussion of the various competing radiative and non-radiative recombination mechanisms. The evidence of the luminescence data is compared to other measurements that involve recombination processes. The experiments demonstrate that the dominant luminescence is a tunnelling transition between band tail electrons and holes. Other luminescence transitions involving defects and impurities, and the role of phonon interactions in the transition are discussed. Experiments in which luminescence is used as a probe of the properties of a-Si:H and related materials are also discussed.

Luminescence and recombination in hydrogenated amorphous silicon

Measurements of transient photoinduced optical absorption in $a$-${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ show that carriers thermalize within a distribution of localized states in the band gap. Photocurrent as well as photoinduced optical absorption transients are explained in a unified way with the multiple-trapping mechanism. Under such circumstances, transient photoconductivity provides a spectroscopy of the density of localized states.

Photocurrent Transient Spectroscopy: Measurement of the Density of Localized States in a -As 2 Se 3

Time-Resolved Optical Absorption and Mobility of Localized Charge Carriers in a-As 2 Se 3

Relaxation of photoinduced optical absorption (PA) and photocurrent (PC) following pulsed excitation is measured as a function of temperature in the range 295 K – 425 K. The decay of PA is well described by bimolecular recombination kinetics with a thermally activated rate coefficient b(T). The magnitude of b(T) yields a lower limit for the mobility of the PA excitation consistent with the hole drift mobility. In addition, first measurements of the time evolution of the PA spectrum are reported. The observed shift of the absorption band towards higher energy with increasing time is interpreted in terms of thermalization within a distribution of localized gap states.

Joseph Orenstein

Papers

Photocurrent Transient Spectroscopy: Measurement of the Density of Localized States in a -As 2 Se 3

Time-Resolved Optical Absorption and Mobility of Localized Charge Carriers in a-As 2 Se 3

Time-resolved measurements of photo-induced optical absorption and photocurrent in a-As2Se3