[0 0 1]-oriented crystalline Potassium-Sodium Niobate thin film fabricated at low temperature for use in piezoelectric energy harvester

In this study, 1.0-μm thick crystalline (K_1-xNa_x)NbO₃ (KNN) films were deposited onto various substrates at 350 °C for use in piezoelectric energy harvesters (PEHs). A Sr₂Nb₃O₁₀ (SN) nanosheet monolayer fabricated on these substrates acted as the seed-layer for the formation of a [0 0 1]-oriented crystalline KNN film at 350 °C. The [0 0 1]-oriented KNN films deposited on a SN/Ni substrate exhibited smaller ε_r (2 7 8) and larger d₃₃ (166 pm/V) values than those of a randomly oriented crystalline KNN film. Because the output power of the PEH is generally proportional to d₃₃²/ε_r, a [0 0 1]-oriented KNN thin film is a good material for use in a PEH. A KNN/SN/Ni PEH shows a large output power density of 20 μW/mm³ (2.9 μW) at 1.0 MΩ based on a soft tapping motion of the finger. The KNN/SN fabricated on the Ni substrate was transferred to a flexible and transparent ITO/polyethylene naphthalate (I-PEN) substrate. The transparent KNN/SN/I-PEN PEH exhibited a promising output power density of 7.02 μW/mm³ with a maximum output voltage of 4.2 V, indicating its applicability as a power source of an electrochromic smart glass window.