The influence of length of one-dimensional photoanode on the performance of dye-sensitized solar cells

Abstract

The influence of the length of ZnO nanowire arrays (ZnO-NWAs) on the performance of dye-sensitized solar cells (DSSCs) was studied. Three types of ZnO-NWAs were grown using a multi-batch process, continuous flow injection (CFI) process, and NH₃-assisted CFI process. ZnO-NWAs grown by the CFI process are suitable as photoanodes of DSSCs because of their strong optical and structural properties. In addition, the transit time of ZnO-NWAs grown using an NH₃-assisted CFI process was determined to be approximately 0.11 ms. In previous studies, the transit time and electron recombination lifetime were used to determine the collection efficiency (η_CC). For effective improvement of η_CC, a short transit time is needed. However, the transit time is significantly dependent on the photoanode thickness (or length) of ZnO-NWAs. To address this issue, a short (approximately 2.5 μm) ZnO-NWA photoanode was fabricated using the batch process. When analyzing the transport properties, the transit time was found to have decreased from 0.57 ms to 40 μs when the thickness of the ZnO-NWA photoanode was reduced from 10 to 2.5 μm, although they had similar diffusion coefficients (approximately 1.5 × 10⁻³ cm² s⁻¹). The diffusion coefficient better demonstrates the electron transport properties of photoanodes. The diffusion coefficient of ZnO-NWAs fabricated using an NH₃-assisted CFI process was 1.2 × 10⁻² cm² s⁻¹. This is currently the highest value of ZnO-NWA photoanodes. A 25 μm ZnO-NWA was fabricated using the NH₃-assisted CFI process as a photoanode of a DSSC, and the conversion efficiency was 3.92%. Compared with previous studies using long ZnO-NWAs DSSCs, the conversion efficiency of this study was effectively improved. The dye-loading amount increases with the length of ZnO-NWA photoanodes because of the high absorption area. However, the diffusion coefficient must be increased simultaneously to maintain the same collection efficiency.