Optimization Of SnO₂ And ZnO Based Photoanodes For Photoelectrochemical Devices

Due to the growing global energy demand and increasing concern for environmental pollution issues, renewable and clean alternative energy sources especially solar energy becomes increasingly hot topic over recent years. Photoelectrochemical cell is a kind of devices that can capture the energy from sunlight and turn it into electric power, such as DSSCs. Conventional photoanodes of DSSCs are fabricated using three-dimensional network film made by mesoporous TiO₂ nanoparticle, which has the large specific surface area enabled a high sensitizer loading capacity. However, the natural drawbacks of high recombination loss and slower electron transport in the conventional TiO₂ nanoparticle photoanode result in a great loss to the photoelectron collection efficiency. In order to improve the performance of the photoelectrochemical cell, it is needed to improve the charge collection efficiency as far as possible under the premise of ensuring the surface area of the photoanode. So, considered the improvement of carrier transport in photoanode, we chose the semiconductor with high carrier mobility（such as ZnO and SnO₂） as the main material, and study the performance of photoelectrode structure from nanosphere to disordered nanotube and then to the ordered nanowire array. Then we optimized the structure of photoanode to prevent the recombination. The concrete research contents were summarized as follows:1. In order to solve the problem of electron transport in the hollow nanosphere photoanode with poor contact, SnO₂ hollow nanospheres（SnO₂ HNSs） have been synthesized by a simple surfactant-free solution route at a low temperature（100°C）. The SnO₂ HNSs enclosed by good crystallinity nanoparticles have an efficient charge transport properties due to the close contact with the neighboring HNSs. The SnO₂ HNS exhibits a high η of 1.51%. With all the advantageous features combined by using the SnO₂ HNS-TiO₂ core-shell structure, namely fast electron transport, slow interfacial electron recombination and effective light absorption, the SnO₂ HNS-TiO₂ photoanode exhibits a high η of 6.54%, increased by 53.9%, 21.5% and 3.3 times as compared with commercial SnO₂ nanoparticle-TiO₂, TiO₂ nanoparticles（P25） and SnO₂ HNS based photoanodes, respectively.2. SnO₂ hierarchical tubular structure（SnO₂-HT） is demonstrated via low-temperature solution route using electrospun carbon nanofiber as templates. Due to the close contact of the nanosheets, the SnO₂-HTs photoanode remains longer tubular structures and has good connection between the neighboring HTs after fabricated by drop-drying method, which may facilitate the electron transport through a longer distance with less diffusive hindrance. The SnO₂-HT exhibits a high η of 1.86%. Then, the SnO₂-HT photoanode was optimized through TiCl₃ treatment to prepare SnO₂@TiO₂-HT core-shell structure. Under this scenario, the assembled SnO₂@TiO₂-HT exhibits a high power conversion efficiency, which is increased by 1.67 times and 13.5% as compared with SnO₂-HT and SnO₂-TiO₂ core-shell structure treated by TiCl₄. These high performances in Sn O₂@TiO₂-HT are mainly benefited from the advantageous features including high electron lifetime and slow interfacial electron recombination.3. Porous ZnO nanowire（NW） arrays with length of 23 μm were synthesized by a one-pot ammonium hydroxide-assisted hydrothermal method. The porous ZnO NW arrays were assembled and a η of 0.71% was attained for porous ZnO NW arrays, much higher than that of ZnO NW with no porous. By TiO₂ modification for different time, the ZnO NW-TiO₂-4 arrays achieve a high η of 3.06% increased by 3.3 times compared with the pure ZnO nanowire array. These results are expected because TiO₂ coating on ZnO NW simultaneously prevents forming Zn2+/dye agglomerate and offers a low degree of charge recombination.4. Compared to FTO glass substrate based solar cell, flexible solar cells have the advantages of light weight, low cost and availability for large-scale production, which has more practical application potential. Therefore, we have carried out a preliminary study on flexible photoanode, which lays the foundation for the research of the flexible device. ZnO NW array DSSCs based on flexible stainless steel mesh（SSM） substrates exhibit a η of 0.66%, much higher than that of the DSSCs based on FTO glass substrates. In order to prevent forming Zn2+/dye agglomerate and further improve the performance of the ZnO solar cell, ZnO NW were optimized by CdS sensitization. The optimization photoanode simultaneously offers excellent visible-light absorption, highly electron transport pathway for charge carrier colloction, and low recombination probability of electron-hole pairs. The Jp of ZnO-CdS-15 can reach 7.08 mA cm?2 at 0 V vs. SCE with the maximum η of 1.88%.