DOI: 10.1002/ente.201500167 Self-poled Efficient Flexible “Ferroelectretic” Nanogenerator: A New Class of Piezoelectric Energy Harvester Sujoy Kumar Ghosh, [a] Tridib Kumar Sinha, [b] Biswajit Mahanty, [a, c] and Dipankar Mandal* [a] Introduction Energy is part and parcel in making life sustainable; energy harvesting from ubiquitous vibrations using nanoscale piezo- electric energy harvesters, known as nanogenerators (NGs), offers a fundamental “battery-free” energy solution in porta- ble and wearable electronics. To date, tremendous attention has been paid to the fabrication of NGs based on several pie- zoelectric materials such as, zinc oxide (ZnO), [1] lead zirco- nate titanate (PZT), [2] lead magnesium niobate–lead titanate (PMN–PT), [3] barium titanate (BaTiO 3 ), [4] zinc stannate (ZnSnO 3 ) , [5] alkaline niobate-based particles (KNLN) with copper (Cu) nanorod filler, [6] and many others. The main ad- vantage of these materials is their superior piezoelectric charge coefficient (d ij ). In contrast, ferroelectric as well as piezoelectric polymer materials-based NGs enjoy their inher- ent advantages, such as adequate flexibility, light weight, ease of large-area processing, low cost, and environmental safety (lead-free). [7] However, the lower piezoelectric charge coefficients limit widespread applications. In this context, polymer electret-based NGs (categories as a new class of NGs, designated as a ferroelectretic NG) with high d ij values are the suitable but challenging choice for next-generation flexible electronics. In convention, charges are created/gener- ated artificially within the polymers by using a high electric poling field to form the electrets, possessing permanent charges. [8, 9] These materials with improved d ij values exhibit square-shaped hysteresis loops, termed as “ferroelectretic”. [9] Recently, several approaches have been undertaken where the high electric poling field was avoided through careful synthesis of the in situ charged materials to serve as electrets, such as a silicon (Si) nanowires electret, [10] zinc oxide (ZnO)/ zinc sulfide (ZnS) core/shell electret, [11] hydroxyapatite thin- film electret [12] etc. However, the preparation of such electret material requires multiple steps, and its inflexibility hindered the fabrication of suitable devices using these materials. The problem can be easily be solved through the generation of a charged porous structure within the ferroelectric polymer material. [13] Thus, a cost-effective poly(vinlidene fluoride-co- hexafluoropropylene) [P(VDF–HFP): a copolymer of Superior ferro- and piezo-electret properties in a self-poled, porous hybrid ferroelectretic polymer nanocomposite film for next-generation device paradigms is introduced by in situ generation of platinum (Pt) nanoparticles (NPs) embedded in poly(vinylidenefluoride-co-hexafluoropropylene) P(VDF– HFP). The cooperative functionality between the self-polar- ized b-phase and the micropores as charge trapping sites is realized by using a simple solvent evaporation method. As a consequence, the resulting film exhibits extraordinary fer- roelectretic behavior, as demonstrated by the superior elec- trical square-shaped hysteresis loop with large remnant po- larization (P r  61.7 mC cm 2 ), piezoelectric charge coefficient (d 33 ~ 686 pC N 1 ), and ultrahigh dielectric properties (e r = 2678, tan d = 0.79 at 1 kHz). A new type of ferroelectretic nanogenerator (FTNG) is fabricated using a flexible hybrid nanocomposite film that effectively converts the applied me- chanical energy into electrical energy upon compressive normal stress (e.g., by actuating with a human finger). The 18 V of the open-circuit output voltage with expected 17.7 mA short-circuit current are generated from the FTNG under 4 MPa of normal stress amplitude. The high piezoelec- tric energy conversion efficiency (h piezo  0.2 %) of the FTNG shows that the hybrid polymer nanocomposite film is well suited for the next generation of piezoelectric-based energy harvesters. The operation of more than 50 blue LEDs, 25 green LEDs, and several capacitors without any subsidiary batteries is demonstrated using the FTNG. [a] S. K. Ghosh, B. Mahanty, Dr. D. Mandal Organic Nano-Piezoelectric Device Laboratory (ONPDL) Department of Physics Jadavpur University Kolkata 700032 (India) Tel.: (+ 91) 33 2414 6666x2880 Fax: (+ 91) 33 2413 8917 E-mail: dipankar@phys.jdvu.ac.in [b] T. K. Sinha Materials Science Centre Indian Institute of Technology Kharagpur, 721302 (India) [c] B. Mahanty Department of Electronics & Communication Engineering Saroj Mohan Institute of Technology (Under Techno India Group) Guptipara, Hooghly 712512 (India) Supporting Information for this article is available on the WWW under http ://dx.doi.org/10.1002/ente.201500167. Energy Technol. 2015, 3, 1190 – 1197 # 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1190