ZnO hollow sphere prepared by flame spray pyrolysis serves as an anti-reflection layer that improves the performance of dye-sensitized solar cells

Abstract

In this study we investigated the use of sub-micrometer ZnO hollow spheres as anti-reflection layers (ARLs) in dye-sensitized solar cells (DSSCs). The ZnO hollow sphere was synthesized by flame spray pyrolysis (FSP) followed by post-heating treatment. The ZnO hollow sphere was analyzed via x-ray diffraction (XRD), which revealed a hexagonal-wurtzite structure. A scanning electron microscope (SEM) image showed a uniform and spherical hollow particle with a narrow size distribution of the diameter: dav ~900 nm. In this research, the DSSCs were fabricated via manual screen printing with TiO2 nanoparticles (20 nm) as transparent layers and ZnO hollow spheres as anti-reflection layers. The performance of the DSSCs was investigated using a Keithley 2440 source meter under an AM 1.5 G solar simulator with a power intensity of 100 mW/cm2. The addition of the ZnO anti-reflecting layers resulted in a four-fold improvement in the efficiency of these DSSCs. In this study, we also noted that the photocurrent density was dependent on the size of the crystals in the ZnO ARLs. The photocurrent was further improved when crystal size was increased via heat treatment. As a result, the DSSCs using ZnO hollow spheres annealed at 900 °C and coated onto TiO2 film achieved a PCE of 2.61% (Voc of 0.75 V, Jsc of 10.03 mA cm−2, ad FF of 0.35), whereas DSSCs using TiO2 with a non-annealed ZnO ARL achieved a PCE of only 2.18% (Voc of 0.78 V, Jsc of 6.78 mA cm−2, and FF of 0.34), and the DSSCs with only TiO2 showed a paltry PCE of 0.64%.