Ni‐doped TiO2/TiO2 homojunction photoanodes for efficient dye‐sensitized solar cells

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SummaryThe 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.