Tunable, graded band-gap TiO2 thin film solar cell deposited by HIPIMS on flexible substrate

Among various types of thin-film solar cell (TFSC), the amorphous silicon type exhibits low and unstable conversion efficiency. Compound solar cells, on the other hand, are with a shortage of rare metals and toxic metals usage, not to mention unacceptable high deposition temperature. To overcome these problems and material cost consideration, the titanium-based oxide material known as potential photovoltaic material was chosen to develop a novel TFSC. During film deposition, the oxygen flow rate was adjusted to obtain p-TiO/n-TiO2 as the p-n junction, where p-TiO acts as the absorption layer and n-TiO2 as the window layer, whilst in an attempt in this study is to develop a graded bandgap n-TiO2 by progressive control of N2 flow rate to favor the electron-hole pairs separation and thus increased conversion efficiency. Meanwhile, a high-power impulse magnetron sputtering (HIPIMS) technique was used to conduct film deposition for providing high-density plasma that is beneficial for crystalline growth at low temperature. Ultimately, a flexible photovoltaic device composed of [PI/Ni (V)/ p-TiO/Graded bandgap n-TiO2/IZO/Al] is expected to be achieved. Experimental results show that the p-TiO absorption layer obtained at an oxygen/argon flow rate ratio of 0.28 exhibits a higher degree of crystallinity and enlarged grain size, leading to designated electrical properties. The carrier mobility and electrical resistivity are 2.18 cm2/Vs and 8.45 × 10−4 Ω-cm, respectively. The optical bandgap of the absorption layer p-TiO is about 2.27 eV. On the other hand, the n-TiO2 window layer slightly decreases its bandgap energy from 3.20 eV to 3.18 eV when the nitrogen flow rate increases from 0 to 50 sccm though, the increased external quantum efficiency (EQE) and the associated better conversion efficiency is found (in comparison with the intrinsic n-TiO2 window layer). It is undoubtedly contributed by the graded energy gap structure. Ultimately, the assembled photovoltaic device of [PI/Ni(V)/p-TiO/Graded bandgap n-TiO2/IZO/Al] presents an external quantum efficiency of 45 % under 310 nm monochromatic irradiation and photovoltaic efficiency of 0.2 %.