Ni‐doped TiO 2 / TiO 2 homojunction photoanodes for efficient dye‐sensitized solar cells

The development of novel bilayer photoanodes plays an essential role in dye-sensitized solar cell (DSSC) applications to fabricate efficient devices. Herein, a novel homojunction photoanode consisting of undoped and Ni-doped TiO2 layers with trace amounts of Ni dopant (0.5% at.) was prepared. After a systematic study on photoconversion efficiency that changes with tuning the location of the doped layer, an in-depth understanding was obtained regarding the physical mechanism of the coupling of structural, optical and electrical properties. We demonstrated that the doped layer can be utilized for high-performance DSSCs by employing a homojunction photoanode consisting of an upper-layer of Ni-doped TiO2 and a lower-layer of undoped TiO2 to dramatically increase the photoconversion efficiency. We monitored that the light absorption ability of the dye-loaded photoanodes strongly improved with the bilayer (Ni-doped TiO2/TiO2), leading to a higher short circuit current. From impedance spectroscopy (Mott-Schottky plots), compared to both undoped and the doped single-layer analogues, a greater shift to more negative flat band potential was noticed for the bilayer and, therefore, a higher open-circuit voltage was achieved. From electrochemical impedance spectroscopy analysis, it was found that chemical capacitance decreases, while recombination resistance increases after Ni incorporation in the photoanode. They were associated with a reduction in the number of immobile electrons trapped by defect states. A relatively longer electron lifetime of 10.18 ms and diffusion length of 63.2 μm were obtained for the device assembled with the homojunction (Ni-doped TiO2/TiO2). The PCE (6.08%) of the device assembled with the bilayer was superior to its single-layer analogue (4.13%), owing to its enhanced light harvesting capability, proper band alignment, improved injection ability, fast electron transport and better collection efficiency. Our results shed light on important characteristics of a homojunction photoanode, which can be considered for future studies and applications.