Lu-n Li

Fabrication of long TiO2 nanotube arrays in a short time using a hybrid anodic method for highly efficient dye-sensitized solar cells

We report a simple hybrid anodic method, with initial potentiostatic anodization followed by galvanostatic anodization, to grow much longer titania nanotube (TNT) arrays in a much shorter anodization period (t). The length of the TNT arrays (L) depends linearly on t and is controlled by the electric current; the growth rates are 5.3, 10.7 and 20.3 μm h−1 for current densities 3.7, 5.6 and 7.5 mA cm−2, respectively. The produced TNT films of L = 15–57 μm sensitized with N719 dye were fabricated into devices for photovoltaic characterization. The NT-DSSC devices show systematically improved cell performance depending on L, reflecting the excellent intrinsic light-scattering property of the NT-DSSC devices to harvest increased sunlight with long TNT arrays. The great effective surface area inside TNT arrays has been shown to significantly increase the dye loading, which might help to enhance the cell performance of the device with co-sensitizing of different dyes for improved efficiency of light harvesting in the future. The best performance of the NT-DSSC device was achieved at L ∼ 30 μm with a spacer of similar thickness, giving JSC = 14.63 mA cm−2, VOC = 0.741 V, FF = 0.70, and η = 7.6%, which is unprecedented for a back-illumination DSSC.

Detachment and transfer of ordered TiO2 nanotube arrays for front-illuminated dye-sensitized solar cells

We report a method of detachment and transfer of one-dimensional TiO2 nanotube (NT) arrays to fabricate dye-sensitized solar cells (DSSCs) with illumination from the front side. The ordered NT arrays (tube length 20 μm) were detached from the NT–Ti substrate, as annealed, by means of a second anodization at 20 V for 4 h and then transferred, inverted, onto a transparent substrate of conducting oxide (TCO) with an interface of TiO2 nanoparticles of thickness ∼2 μm. Dry etching of the NT surface in a highly dense plasma reactor under BCl3/Cl2 for 90 s opened previously closed ends of the tubes. The inverted (bottom-up) NT–TCO substrate was fabricated into a DSSC device that shows a cell performance (η = 6.24%) significantly improved over those of a front-illuminated counterpart with an upright (face-up) structure (η = 4.84%) and of a conventional back-illuminated device (η = 4.61%). Electrochemical impedance spectra (EIS) of the devices under one-sun irradiation were measured to rationalize the cell performances that are consistent with the corresponding interfacial impedances between the NT arrays and the electrode for collection of electrons.

Characterisation of electron transport and charge recombination using temporally resolved and frequency-domain techniques for dye-sensitised solar cells

We review systematically the kinetic characterisation of electron transport and charge recombination in dye-sensitised solar cells (DSSC) using both temporally resolved and frequency-domain techniques. For the temporally resolved approach, charge extraction (CE), transient-photocurrent decay (TCD) and transient-photovoltage decay (TVD) methods are introduced in detail; for the frequency-domain approach, techniques involving electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) are presented in detail. The TCD and TVD data are obtained under short-circuit and open-circuit conditions, respectively, and the electron diffusion coefficients and electron lifetimes are extracted from fitting the decay curves accordingly; the IMPS/IMVS results are the counterparts of the TCD/TVD results in the frequency domain. Even though the EIS results are readily acquired, an accurate interpretation of the data requires an appropriate model to determine the internal features of the device. This review provides an account of each technique about its operating principle, experimental setup and data analysis. As case studies for each technique, examples are given to rationalise the observed potential shift, decay coefficients of electron transport and of charge recombination in relation to the corresponding photovoltaic performance of the device.

High-Performance Large-Scale Flexible Dye-Sensitized Solar Cells Based on Anodic TiO2 Nanotube Arrays

A simple strategy to fabricate flexible dye-sensitized solar cells involves the use of photoanodes based on TiO2 nanotube (TNT) arrays with rear illumination. The TNT films (tube length ∼35 μm) were produced via anodization, and sensitized with N719 dye for photovoltaic characterization. Pt counter electrodes of two types were used: a conventional FTO/glass substrate for a device of rigid type and an ITO/PEN substrate for a device of flexible type. These DSSC devices were fabricated into either a single-cell structure (active area 3.6×0.5 cm2) or a parallel module containing three single cells (total active area 5.4 cm2). The flexible devices exhibit remarkable performance with efficiencies η=5.40% (single cell) and 4.77% (parallel module) of power conversion, which outperformed their rigid counterparts with η=4.87% (single cell) and 4.50% (parallel model) under standard one-sun irradiation. The flexible device had a greater efficiency of conversion of incident photons to current and a broader spectral range than the rigid device; a thinner electrolyte layer for the flexible device than for the rigid device is a key factor to improve the light-harvesting ability for the TNT-DSSC device with rear illumination. Measurements of electrochemical impedance spectra show excellent catalytic activity and superior diffusion characteristics for the flexible device. This technique thus provides a new option to construct flexible photovoltaic devices with large-scale, light-weight, and cost-effective advantages for imminent applications in consumer electronics.

Morphological control of platinum nanostructures for highly efficient dye-sensitized solar cells

Cyclic electro-deposition (CED) is a cost-effective tool to synthesize nanostructures with a solution process, controllable morphology and high purity. Here we report novel platinum nanostructures fabricated according to CED at room temperature in solution containing H2PtCl6 precursor and NaNO3. Remarkable Pt nanostructures—from nanoclusters, nanosheets, nanograsses to nanoflowers—were produced through morphological control via variation of either period of CED scans or concentration of the precursor. Pt films with uniform nanograss structures have great electro-catalytic performance (electron-transfer resistance = 0.3 Ω) and intrinsic light-scattering (reflectivity ∼50%), perfectly suitable for use as counter-electrodes for dye-sensitized solar cells (DSSCs). The DSSC device made with the Pt-nanograss counter-electrode and N719 dye attained efficiency η = 9.61% of power conversion, which is 12% enhanced from that fabricated according to a conventional thermal decomposition method (η = 8.55%) under similar experimental conditions. When the devices were further optimized with a thick TiO2 film (17 + 5 μm) sensitized by Z907 dye using the CED-Pt counter-electrode, we obtained JSC (mA cm−2) = 19.44, VOC (V) = 0.742, and FF = 0.736, giving an exceptional power conversion efficiency of 10.62%.