Flexo-green Polypyrrole – Silver nanocomposite films for thermoelectric power generation Meetu Bharti a,b , Ajay Singh a,⇑ , Soumen Samanta a , A.K. Debnath a , D.K. Aswal a,c , K.P. Muthe a , S.C. Gadkari a a Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India b All India Jat Heroes’ Memorial College, Rohtak 124001, Haryana, India c National Physical Laboratory, New Delhi 110012, India article info Article history: Received 31 December 2016 Received in revised form 15 March 2017 Accepted 6 April 2017 Keywords: Thermoelectrics Conducting polymer Seebeck coefficient Thermal conductivity Figure-of-merit Phonon scattering abstract Conducting polymers offer various advantages over inorganic thermoelectric materials such as eco- friendliness, a reduced manufacturing cost, flexibility, low thermal conductivity and amenability to tun- ing of electrical properties through doping; have recently drawn much attention for conversion of low temperature waste heat (150 °C) into electricity. In this study, we investigated the thermoelectric prop- erties of hybrid films of polypyrrole and silver (PPy-Ag). These films were prepared on biaxially oriented polyethylene terephthalate (BOPET) flexible substrates by eco-friendly one pot photo-polymerization method using aqueous solution of silver nitrate (AgNO 3 ) as photo initiator. Detailed characterization of the samples revealed that morphology of composite films reorganized with the change in AgNO 3 concen- tration during synthesis. Increasing AgNO 3 concentrations resulted in PPy films containing Ag nanopar- ticles, nanoclusters as well as macroclusters. With alteration in concentration and size of Ag particles in PPy matrix, it has been observed that the electrical conductivity of the films increased (1.5–17.3 S cm 1 ), thermal conductivity decreased (0.16–0.002 Wm 1 K 1 ), while Seebeck coefficient moderately reduced from 10.9 lV/K to 5.8 lV/K. Nearly same doping (N + /N  0.35) content, improved conjugation length and incorporation of Ag between the PPy chains resulted in improved charge carrier mobility/electrical conductivity in the PPy–Ag films. It is proposed that the interface of Ag and PPy served as scattering sites for phonons, thus leading to reduction of thermal conductivity. This synergetic combination of high elec- trical conductivity, extremely low thermal conductivity along with moderate Seebeck coefficient in the PPy-Ag films resulted in the highest figure-of-merit of 7.4  10 3 at 335 K among reported PPy based materials. A prototype thermoelectric power generator was fabricated by integrating six numbers of PPy-Ag films. The fabricated device exhibited maximum voltage and power respectively as 6 mV and 30 pW. The present work opens new avenues for the thermoelectric applications of rarely explored flex- ible PPy-Ag films prepared by a simple nature-friendly photo-chemical process at room temperature. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction Thermoelectricity, owing to its various advantages over other energy conversion technologies, is gaining attention in this modern era of industrialization and urbanization that currently is strug- gling hard with the issues like energy economy in terms of waste heat recovery and global warming [1]. Low temperature waste heat generated by human body, geo-thermal sources, electronic and electrical systems can be harnessed using thermoelectric power generators (TEGs) for its conversion into useful electrical energy [1–2]. The efficiency with which this conversion takes place is defined by a dimensionless figure-of-merit ZT  a 2 rT/j, where a is the Seebeck coefficient, r is the electrical conductivity, j is the thermal conductivity and T is the average temperature of operation [3–5]. For a given material, these parameters (a, r and j) are known to maintain a detrimental interdependence which limits further enhancements in ZT. Therefore, for development of thermo- electric materials having superior ZT, it is a pre-requisite that this intrinsic linkage of thermal and electrical transport properties is decoupled [3]. Inorganic semiconductors in particular, are being investigated extensively due to their relatively higher a and r val- ues [4–10]. Recently many efforts are being made to suppress their inherently high j (without comprising the power factor) to achieve further improvement in their ZT. Bi 2 Te 3 based alloys are conven- tional inorganic semiconductor thermoelectric materials that are widely used for power generation applications and thermoelectric http://dx.doi.org/10.1016/j.enconman.2017.04.022 0196-8904/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: asb_barc@yahoo.com (A. Singh). Energy Conversion and Management 144 (2017) 143–152 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman