Influence of Nitrogen Doping on Device Operation for TiO2-Based Solid-State Dye-Sensitized Solar Cells: Photo-Physics from Materials to Devices

Chemical composition of the powders. The carbon and nitrogen contents were measured using two elemental analysis instruments (Carbon/Sulfur Analyzer EMIA-320V Series, Horiba, and Oxygen/Nitrogen/Hydrogen Analyzer EMA-800 series, respectively). Optical absorption of the dye-sensitized electrodes. The optical properties of the dye-sensitized electrodes were measured in transmission mode by a UV-visible spectrometer SAFAS DES 200. IPCE spectra of ssDSSC devices. The incident photon to charge carrier efficiency spectra were measured in static regime using a monochromated 75 W Xenon lamp (Newport) and a calibrated picoamperemeter (Keithley 485). A certified silicon photodiode of known spectral response was used for the calibration of the incident photon flux. Photoluminescence spectra of the un-doped and N-doped nanopowders. Steady-state photoluminescence spectra were probed by a FLS980 spectrometer (Edinburgh Instruments). The excitation was performed using a Xenon lamp and a monochromator and the detection was made between 200 and 870 nm by a R928P Hamamatsu photomultiplier (cooled, dark count <50 cps). The powders were placed in a quartz sample holder and monochromator slits were adjusted to lead to a bandwidth of around 1.5 nm. All the spectra are normalized with regard to the emission pic at 3.15 eV, which is attributed to the free exciton emission of anatase TiO2. TRMC experiments. The TRMC technique is based on the measurement of the time evolution of the microwave power ∆∆(() reflected by a sample, following an excitation pulse delivered by a laser [1,2]. For small perturbations, the relative variation of the reflected microwave power ∆∆(() ⁄ can be correlated with the variation of conductivity ∆σ() of the probed material as: ∆∆(() = ∆σ() = ∑ ∆∆ () • μ (1) where, ∆∆ () is the number of excess charge carriers of species at time , and μ is the corresponding charge mobility. The sensitivity factor is independent of time, but depends on different factors such as the microwave frequency or the dielectric constant of the materials studied. Considering that the mobility of trapped species is usually several orders of magnitude lower than that of mobile charge carriers, and considering the n-type behavior of TiO2 materials, ∆∆ () reduces to mobile electrons in the conduction band of the metal oxide [3]. Photoconductivity measurements. In a constant-voltage photo-response measurement, the change in conductivity ∆σ due to generation of electron-hole pairs by a constant light intensity will be observed as a current jump, ∆∆, which will depend on the density of photo-generated charge …