Porous, nanostructured sol gel TiO2 (100 nm) has been sensitized with WS2 quantum sheets (approximately 5 nm) with the help of chemical bath deposition. The absorber has been characterized with help of energy dispersive X-ray (EDX), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, and light absorption measurements. The photosensitization was confirmed via electrochemical measurements. The surface of TiO2 has been modified by a thin Al2O3 film, which significantly enhanced the photocurrent density to 0.4-0.7 mA/cm2. Moiré patterns suggest that the S-W-S layers of WS2 are not perfectly aligned in the direction of the c-axis, emphasizing the role of lateral electron transfer, which is also evidenced by surface passivation experiments. With WS2, a new, cheap, environmentally friendly, and stable absorber material for the sensitization of wide band gap nanomaterials has been introduced.
The claim that ruthenium complexes attached to nanostructured TiO 2 can sustain 10 8 electron transfer cycles is in conflict with photocurrent imaging studies, which clearly show photoinduced degradation of solar cell efficiency. HPLC chromatographic studies of the cis-RuL 2 (SCN) 2 with L = 2,2-bipyridyl-4,4-dicarboxylatoacid (bpca) in solution and extracted from longterm operating solar cells were undertaken to identify the spectrum of possible photochemical products. While a dramatic increase of photochemical stability of the ruthenium complex when attached to TiO 2 nanoparticles is confirmed with respect to its photochemical reactivity in solution, photoelectrochemical products of the ruthenium complex can clearly be identified after one to a few months of solar cell operation under simulated solar light. The interpretation is that the ruthenium complex when adsorbed to TiO 2 nanoparticles is exposed to different surface states, which warrant different stability conditions to the oxidized ruthenium intermediate. To cite this article: M.
Nano solar cell materials cannot sustain imprinted thermodynamic potentials yielding electrical fields for a charge separation. They have, therefore, to rely on kinetic mechanisms for current rectification and energy conversion. It is shown that treatment of WS 2 nano-sheet sensitized TiO 2 -material with organic thiols increases the photocurrent efficiency at least three fold. They bind to the WS 2 via the thiol sulfur to produce a charged surface state, which converts into an efficient electron transfer bridge in presence of suitable electron donors. These thiol bridges essentially operate in anodic direction. Thus they increase both the photo induced chemical affinity, which is proportional to the photovoltage generated, and the interfacial reaction rate which is proportional Page 2 of 26 A c c e p t e d M a n u s c r i p t 2 to the photocurrent. The results underline the importance of studying unidirectional electron transfer processes for innovative solar cell applications.
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