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Straight-stranded anatase TiO2 nanotubes were produced by anodic oxidation on a pure titanium substrate in an aqueous solution containing a 0.45 wt % NaF electrolyte (pH 4.3 fixed). The average length of the TiO2 nanotubes was approximately 3 μm, which had an effect on the level of dye adsorption in the dye-sensitized solar cells. The anodic TiO2 nanotubes were applied as a working electrode in a solid-state dye-sensitized solar cell. An approximately 1 nm ZnO shell was coated on the TiO2 nanotube to improve the open-circuit voltage (V oc) and conversion efficiency of the solar cell, and to retard any back reaction. Although the V oc and short-circuit current (J sc) of the cell were improved, there was a low fill factor as a result of the formation of a thick TiO2 barrier layer in the anodic TiO2/Ti substrate. A parameter on the degradation of fill factor (37%) is related to the formation of a thick TiO2 barrier layer in the anodic TiO2/Ti substrate interface. A hydrogen peroxide treatment was performed in an attempt to narrow the TiO2 barrier layer. This treatment was found to influence not only fill factor (37−49%) but also the conversion efficiency (0.704−0.906%) of the cell by eliminating the remnant after anodic reaction and barrier narrowing through an etching effect. This result was confirmed by X-ray photoelectron spectroscopy (XPS) and photocurrent-voltage measurements. The longer electron lifetime on the ZnO coated TiO2 film was measured by the open-circuit voltage decay. The improvement in the electron lifetime from the thin ZnO coating affects the number of electrons collected on the Ti substrate and the retardation of charge recombination. Therefore, the ZnO coating on the TiO2 nanotube film improves the efficiency of dye-sensitized TiO2 solar cells from the extended V oc from ZnO coating confirmed by the Mott−Schottky plots and the increased J sc through the inhibition of charge recombination confirmed by IPCE measurements.

The photovoltaic performance of dye-sensitized solar cells (DSSCs) employing SnO2 electrodes was investigated while increasing the content of 4-tert-butylpyridine (TBP) in the conventional liquid-type electrolyte. As the added TBP content increased, the open circuit voltage (V oc) and conversion efficiency were highly enhanced while the short circuit current (J sc) was not much affected. With the electrolyte of 2.0 M TBP, the V oc and conversion efficiency were increased by 26 and 33%, respectively, compared with the conventional electrolyte (0.5 M TBP). The electrochemical impedance spectra revealed that the enhancement of V oc resulted from the negative shift of the SnO2 conduction band potential and the increase in resistance of electron recombination by 1 order of magnitude. It is noteworthy that the optimized concentration of TBP for the SnO2 electrode is greatly larger than that for the TiO2 electrode. This may be due to the much faster electron recombination rate and more positive conduction band potential of the SnO2 electrode. The SnO2 electrode modified with TiO2 shell showed only slightly enhanced performance due to the similar effects of shell layer and those of the TBP. In contrast to the SnO2, TiO2 electrodes did not show performance enhancement with the electrolyte of TBP concentration larger than 0.5 M. The impedance spectra of symmetric dummy cells employing Pt counter electrodes indicated that the catalytic effect of Pt was deteriorated, and the resistance of electrolyte diffusion was increased by the higher concentration of TBP. This brings up the need for development of a counter electrode that TBP is not easily adsorbed on, and alternative additives to TBP which are not highly viscous.

We present a new synthetic process of near infrared (NIR)-absorbing copper-indium-selenide (CISe) quantum dots (QDs) and their applications to efficient and completely heavy-metal-free QD-sensitized solar cells (QDSCs). Lewis acid-base reaction of metal iodides and selenocarbamate enabled us to produce chalcopyrite-structured CISe QDs with controlled sizes and compositions. Furthermore, gram-scale production of CISe QDs was achieved with a high reaction yield of ~73%, which is important for the commercialization of low-cost photovoltaic (PV) devices. By changing the size and composition, electronic band alignment of CISe QDs could be finely tuned to optimize the energetics of the effective light absorption and injection of electrons into the TiO2 conduction band (CB). These energy-band-engineered QDs were applied to QDSCs, and the quantum-confinement effect on the PV performances was clearly demonstrated. Our best cell yielded a conversion efficiency of 4.30% under AM1.5G one sun illumination, which is comparable to the performance of the best solar cells based on toxic lead chalcogenide or cadmium chalcogenide QDs.

Photovoltaic performances of TiO2 nanoparticle (NP) electrodes and TiO2 nanotube (NT) electrodes in dye-sensitized solar cells (DSSCs) employing a cobalt bipyridyl redox electrolyte were compared. The TiO2 NP electrodes had pore sizes ranging from 15 to 20 nm while the NT electrodes had a lager pore size of 80 nm. Highly ordered and vertically oriented TiO2 NT electrodes were prepared by a two-step anodization method. In application to DSSCs employing the cobalt redox electrolyte, the 11-μm-thick NP electrode exhibited an efficiency of 1.60% with a J sc of 3.96 mA/cm2. Meanwhile, despite nearly half of the amount of adsorbed dye molecules, the 11-μm-thick NT electrode exhibited a slightly enhanced efficiency of 1.84% with a J sc of 5.86 mA/cm2. In addition, the 35-μm-thick NT electrode showed an efficiency of 2.38% with a J sc of 9.80 mA/cm2. Compared to the 11-μm-thick NP electrode, the 35-μm-thick NT electrode exhibited a 1.5 times higher efficiency with a 2.5 times higher J sc in spite of having a similar amount of adsorbed dye molecules. Photocurrent transient measurements revealed that the mass transport limitation of the cobalt redox electrolyte within the conventional NP electrodes was greatly alleviated within the NT electrodes. In addition, the electrochemical impedance spectra indicated that the interfacial contact between the cobalt redox electrolyte and TiO2 electrode was prominently enhanced in the NT electrodes. Furthermore, the electron lifetime and electron diffusion length were all greatly longer within the NT electrodes. These superior photovoltaic properties may be attributed to the large pore size and vertically oriented structures of the NT electrodes.