wileyonlinelibrary.comenergy, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] as well as to other technical applications, such as batteries, sensors, magnetoelectric devices. [19][20][21] Recently, the successful preparation of piezoelectric nanogenerators (NGs), which can harvest mechanical energy from our surrounding living environments, was demonstrated using piezoelectric PFO materials such as BaTiO 3 (BTO), [2][3][4][5] ZnSnO 3 , [6][7][8] (K, Na) NbO 3 , [ 9,10 ] PMN-PT, [ 11,12 ] and Pb(Zr,Ti) O 3 (PZT). [13][14][15][16][17] The results offer significant potential for various technological applications, such as remote/wireless data transmission, battery charging, and the powering of electronic devices.BTO has been regarded as one of the most technologically promising PFO materials for use in piezoelectric NGs because of its strong piezoelectric properties and the environmental advantages that it offers over lead-based PFO ceramics such as PZT. However, little experimental consensus exists on the critical size at which the size-dependent ferroelectricity of the BTO nanoparticles (BTO NPs ) is suppressed, whereby the critical size of the BTO NPs is generally recognized to be approximately 10-20 nm. [ 22 ] Most studies of BTO-based devices have focused on the use of large BTO NPs (>100 nm), [ 4 ] BTO nanotubes (approximately 5 µm in length or longer) [ 23 ] or vacuumdeposited BTO layers with high-temperature annealing. [ 3 ] Further, although a nanocomposite thin fi lm has been regarded as a very attractive structure for large-scale fl exible piezoelectric NGs due to its facile size scalability, cost-effectiveness, and mechanical robustness, [ 8 ] a high degree of control over the fi lm/ layer dimensions and a uniform internal structure without the requirement of binders or additives to prevent NP segregation or agglomeration are still very challenging for the realization of high-performance nanocomposite thin fi lm-based piezoelectric NGs.With respect to the fabrication of nanocomposite fi lms, the layer-by-layer (LbL) assembly method has demonstrated the highest potential and versatility for the preparation of nanocomposite fi lms with tailored thicknesses, compositions, and functionalities achieved through complementary interactions. [24][25][26][27][28][29][30][31][32][33][34] LbL assemblies can also be effectively applied to various substrates, irrespective of their size or shape. However, the traditional LbL assembly of electrostatically charged inorganic NPs in aqueous solvents relies on electrostatic properties of NP surface, hence the direct application of this approach for Perovskite nanoparticle-based nanocomposite thin fi lms strictly tailored using unconventional layer-by-layer (LbL) assembly in organic media for piezoelectric nanogenerators (NGs) are demonstrated. By employing sub-20-nm BaTiO 3 nanoparticles stabilized by oleic acid ligands (i.e., OA-BTO NPs ) and carboxylic acid (COOH)-functionalized polymers, such as poly(acrylic acid) (PAA), the resulting OA-BTO NP /PAA nanoco...