The presence of specific chemical additives in the redox electrolyte results in an efficient increase of the photovoltaic performance of dye-sensitized solar cells (DSCs). The most effective additives are 4-tert-butylpyridine (TBP), N-methylbenzimidazole (NMBI) and guanidinium thiocyanate (GuNCS) that are adsorbed onto the photoelectrode/electrolyte interface, thus shifting the semiconductor's conduction band edge and preventing recombination with triiodides. In a comparative work, we investigated in detail the action of TBP and NMBI additives in ionic liquid-based redox electrolytes with varying iodine concentrations, in order to extract the optimum additive/I 2 ratio for each system. Different optimum additive/I 2 ratios were determined for TBP and NMBI, despite the fact that both generally work in a similar way. Further addition of GuNCS in the optimized electrolytic media causes significant synergistic effects, the action of GuNCS being strongly influenced by the nature of the corresponding co-additive. Under the best operation conditions, power conversion efficiencies as high as 8% were obtained.
Dye-sensitized solar cells (DSCs), are already in the top ten photovoltaic devices attaining the largest overall power-conversion effi ciencies ( > 10% under 1 sun AM1.5G illumination [ 1 ] ), due mainly to their potentially low-cost fabrication, the possibility of transparency and color selectivity, effi cient light absorption by all incident angles and their ability to be integrated into building and automobile applications. [ 2 ] To make the next step towards full commercialization and compete on equal terms with silicon and thin fi lm solar cells, DSCs have to demonstrate enhanced long term stability under thermal and light stress conditions. The most effi cient DSCs utilize a liquid electrolyte consisted of the I − /I 3 − redox couple, dissolved in an organic solvent (e.g., acetonitrile); this kind of volatile solvent (resulting in partial liquid leakage and/or evaporation) is recognized to be the main limiting factor shortening the lifetime of the devices. [ 3 ] To overcome these problems, two alternatives have been proposed: i) the use of solvent-free ionic-liquid-based electrolytes, [ 4 , 5 ] and ii) the application of solidifi ed electrolytes based on organic [6][7][8][9] and inorganic [ 10 ] polymers, nanoparticles, [ 11 , 12 ] and liquid crystals, [ 13 ] attaining maximum effi ciencies on the order of 7-8%. [ 14 ] Following this direction, hybrid composite polymer electrolytes have been prepared by our group in the past: a linear polymer with high viscosity (polyethylene oxide) was used in order to solidify the common liquid electrolytes and a fi ller consisted of a powder of titania nanoparticles was further embodied in the polymer electrolytes to enhance amorphicity. [ 15 ] The obtained photovoltaic effi ciencies (2.5-4.5%) of the DSCs incorporating such solidifi ed polymer electrolytes were (at the time) among the best reported in literature. [16][17][18] These efforts to incorporate metal oxide nanoparticles between the polymer chains has since motivated a lot of research groups to use similar semiconducting nanostructures (TiO 2 , SiO 2 , carbon nanotubes, graphite etc.) to fi ll various polymer electrolytes. [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] Among others, special mention should be made on recently prepared advanced composite polymer (polyethylene glycol and nafi on) electrolytes using hydrothermally prepared titania nanotubes (NTs) as fi llers. [ 34 , 35 ] In this work, taking advantage of the strong experience in the electrochemical preparation of ordered metal oxide nanostructures, [36][37][38] we have prepared novel nanocomposite polymer electrolytes based on potentiostatically grown nanotubular titania fi llers incorporated into polyethylene oxide matrices. The polymeric electrolyte without a fi ller (used as a reference) presented an overall power conversion effi ciency ( η ) of 4.4%, when incorporated in a DSC. When this electrolyte was fi lled with the nanotubular anodic powder, the effi ciency was increased by more than 18% (reaching values up to 5.4%), mai...
Το πειραματικό μέρος της διατριβής εκπονήθηκε στο Εργαστήριο «Φωτοοξειδοαναγωγικής Μετατροπής και Αποθήκευσης της Ηλιακής Ενέργειας» του Ινστιτούτου Φυσικοχημείας του ΕΚΕΦΕ «Δημόκριτος» υπό την επίβλεψη του Διευθυντή Ερευν ών Δρ. Π. Φαλάρα.
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