Page - 182 - in Photovoltaic Materials and Electronic Devices

andconductivityduetotheREDCNPcoatinglayer,withthedominanteffectof the conductivitydueto thehoppingmechanismof theformedoxygenvacancies inside REDCNPs. 3. ExperimentalSection 3.1.NanoparticlesSynthesis Reduced erbium-doped ceria nanoparticles have been synthesized using the chemical precipitation technique which is a relatively simple and inexpensive synthesis process [22]. Cerium (III) chloride (heptahydrate, 99.9%, Sigma-Aldrich Chemicals, St. Louis, MO, USA) of weight 0.485g and erbium (III) chloride (heptahydrate, 99.9%, Sigma-Aldrich Chemicals, St. Louis, MO, USA) (0.015 g) are dissolved in de-ionized (DI) water (40 mL) to obtain a 3% weight ratio of erbium to cerium in the synthesized nanoparticles. This weight ratio is selected after a study by the authors of different weight ratios of erbium-doped ceria nanoparticles, synthesized using the same process, in which it was found that the optimal concentration of erbium in ceria for up-conversion is 3 wt % for the optimum improvement of solar cell efficiency. The solution is stirred constantly at 500 rpm in a water bath, while the temperature of the water bath is raised to 60 ˝C, and ammonia (1.6 mL) is then added to the solution. The solution is kept at 60˝Cfor2hand, then, thesolution isstirredforanother22hat roomtemperature. Then, the wet powder is dried, after being washed using ethanol, on a hot plate for 20 min. The thermal annealing of the dried nanoparticles is performed in a tube furnace (CMFurnace,Model1730-20HT,Bloomfield,NJ,USA)withanatmosphere ofhydrogenandnitrogengases thatare injectedintothefurnaceatflowratesequal to10standardcubic feetperminute(scfm)for twohoursat temperaturesof700˝C. The gases during the annealing assist with the reduction of the cerium ions from the Ce4+ to Ce3+ ionization states and the creation of the oxygen vacancies [23–26], while the thermal energy available during the high temperature anneal promotes the formationof themolecularenergy levelsoferbiuminside theceriahost [15]. 3.2. CharacterizationsofNanoparticles The optical absorption is measured using a dual-beam UV-Vis-NIR spectrometer(UV-3101PCShimadzu,Tokyo, Japan). After theannealingprocedure, a solution of nanoparticles is prepared with a concentration of 0.02 mg of nanoparticles in 10 mL of DI water. The colloidal solution is illuminated with both near-UV and near infra-red (NIR) excitations in an experimental apparatus that was designed to measure the down- and up-conversion process, as described in Figure 8. The fluorescence spectroscopy system consists of two excitation sources. The first one, the near UV excitation, is a Xenon lamp coupled to a 182