TiO2 nanotube arrays (NTA), of 17–37 μm in thickness, detached from anodic oxidized Ti foils were used as photoanodes for dye-sensitized solar cells (DSSCs). Photovoltaic measurements under frontside and backside illumination showed that frontside illumination geometry provided better cell performance than backside illumination did. A cell assembled with 30 μm thick NTA film produced the greatest photocurrent and light conversion efficiency. Despite an advantageous architecture for electron transport, electron trapping remained a limiting factor for both illumination geometries, due to the presence of crystal grains in the NTA walls. Intensity-modulated photocurrent spectroscopy (IMPS) analysis showed that electron transport in the front-illuminated cells comprises both trap-free and trap-limited diffusion modes, whereas electrons in the back-illuminated cells travel only by trap-limited diffusion. The trap-free diffusion mechanism determines front-illuminated cell performance. Electrochemical impedance spectroscopy analysis showed the front-illuminated NTA-based DSSCs have a charge collection efficiency of better than 90%, even at 30 μm NTA film thickness. Large crystal size results in low trap state density in the NTA film, and this effect may result in a more extensive trap-free diffusion zone in the films, which facilitates charge collection.
An electrophoretic deposition (EPD) method, consisting of repetitive short-term depositions with intermediate drying, was developed to prepare nanocrystalline TiO2 films for dye-sensitized solar cells (DSSCs). After calcination, the EPD TiO2 films exhibited a more compact TiO2 network than films derived from the conventional paste-coating (PC) method. X-ray absorption fine structure spectroscopic analysis showed that the EPD films had a higher density of defect states than the PC films because of the higher number of interparticle necking regions created in the EPD films. However, the DSSCs assembled with the EPD films outperformed those with the PC films by 20% in photocurrent and 15% in solar energy conversion efficiency. Intensity-modulated photocurrent spectroscopic analysis showed that the EPD films had a shorter electron transit time than the PC films. Under one-sun illumination on the cells at open-circuit, impedance analysis showed that the EPD films had a constant charge collection efficiency of 95% for thicknesses ranging from 4 to 13 μm, whereas the efficiency of the PC films was not greater than 90% and showed a decreasing trend with increasing film thickness. Concerning the porosity dependence of the electron transport dynamics, the electron diffusivity had much weaker dependence than one would expect from the percolation model with hard spheres. This may result from the fact that interparticle necking causes greater lattice distortion for more compact TiO2 films. The present study demonstrates that an optimized EPD process can construct a nanocrystalline TiO2 architecture with a minimized void fraction to shorten the electron traveling distance and to effectively collect photogenerated charges, even for films with large thicknesses.
Quantitative determination of the amorphous phase content in TiO 2 specimens: 1 In quantitative phase analysis using the Rietveld method, the weight fraction w i of each i th
Next‐generation photovoltaic technologies such as dye‐sensitized solar cells, organic thin‐film photovoltaics and perovskite solar cells are promising to efficiently harvest ambient light energy. However, more and deeper understanding of their photovoltaic characteristics is essential to create new applications under room light illumination. In this study, for the first time, the difference in temperature coefficients and angular dependence of photovoltaic parameters for the large‐area devices are investigated systematically under the compact fluorescent lamp and light‐emitting diode light. These emerging photovoltaic devices, compared with the single crystalline silicon solar cells, not only have higher open‐circuit voltage (up to approximate 1 V) and better power conversion efficiency (in the range of 9.2% ~ 22.6%) but also exhibit less temperature dependent voltage and output power (< −0.6% °C−1), as well as broader angular response (over 75 degrees). The state‐of‐the‐art dye‐sensitized and organic thin‐film devices also show advantageously positive temperature coefficients of current, and the latter even has positive thermal dependence of fill factor. These features suggest the next‐generation photovoltaic devices are more favorable than the conventional crystalline silicon solar cells for real‐life indoor applications.
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