Effect Of Dopant Structure On Thermoelectric Properties Of Poly(3-hexylthiophene)

Thermoelectric materials can be used to realize the mutual conversion of heat energy and electrical energy by Seebeck effect or Peltier effect,and therefore have been regarded as a sort of green material.Polymer semiconductors have rich and modifiable chemical structures,which makes them attractive as thermoelectric materials with tunable transport properties.Furthermore,they also have the superior advantages compared with traditional inorganic thermoelectric materials,such as low-cost,light-weight,large-scale processibility and capability to fabricate flexible device.Therefore,polymer semiconductors have received increasing attention as potential thermoelectric materials in recent year.Tremendous works have shown that the key factors affecting the performance of polymer thermoelectric materials are the generation and the transport of charge carriers.Chemical doping is an effective method to tune the carrier concentration in polymer.However,the introduction of dopants would probably disturb the packing of polymer chains and therefore block or scatter carrier transport.Therefore,it is very important to understand how the existence of dopants and molecular doping affect the microstructure and the charge transport behavior of polymer.P3HT is a typical polymer semiconductor with advantages of good solubility,high regularity and good stability.In this work,three dopants with different anions group were used to dope two type of P3HT with different molecular weight.The influence of the structure of dopant anions on the doping efficiency,microstructure and thermoelectric properties of P3HT films were systematically investigated.The major results are shown as follows:1.Cyclic voltammetry（CV）measurements reveal that the oxidative ability of the three dopants with different anions follows the sequence as:Fe（OTf）₃≈Fe（Tos）₃>Fe Cl₃.The UV-vis-NIR absorption spectra show that the doping efficiency of Fe（OTf）₃is the highest among the three dopants,which is consistent with the oxidative ability observed in CV.However,the doping efficiency of Fe（Tos）₃ is the lowest,which is likely due to the large size of Tos^-.The steric hindrance from Tos^-makes it hard to insert into the polymer chains and therefore the charge transfer is difficult to occur.It can be concluded that,the generation of carriers requires not only a large driving force for charge transfer between polymer and dopant,but also the good compatibility between dopant and polymer chains.2.The microstructure of pure and doped P3HT films was systematically studied by using grazing incidence X-ray scattering（GIWAXS）.The lamellar distance increases while theπ-πstacking distance decrease after doping for all three dopants.That means the anions of the three dopants tend to intercalate into the space between side chains of the polymer,and the charge transport mainly takes place along theπ-πstacking direction.Compared with Fe Cl₃ and Fe（Tos）₃,although the Fe（OTf）₃ doped P3HT shows the shortestπ-πstacking distance,Fe Cl₃ doped P3HT has a higher mobility than that of Fe（OTf）₃ doped P3HT.It is further found that,the continuous network structures were formed in Fe Cl₃ doped P3HT films,while discrete island-like structures were observed in the Fe（OTf）₃ doped P3HT films.The discontinuous charge transport path is considered in charge of the lower mobility in the Fe（OTf）₃ doped P3HT films.Therefore,it can be concluded that,to realize a high mobility in doped film,an interpenetrating network structure with continuous crystalline regions is desired,which is significantly affected by the size and the shape of dopant anions.3.The P3HT films doped with three kinds of ferric salts were used for fabricating thermoelectric devices.The electrical conductivity and Seebeck coefficient of the obtained films were tested in detail.It was found that the Fe Cl₃ doped P3HT films show the highest electrical conductivity and power factor,and Fe（OTf）₃ doped P3HT has a higher conductivity than that of Fe（Tos）₃ doped P3HT,regardless of the molecular weight of P3HT.This indicates that the higher mobility of Fe Cl₃ doped P3HT plays an important role in its high electrical conductivity and power factor.Furthermore,a thermoelectric module was fabricated by sequentialy connecting five Fe Cl₃ doped L-P3HT films.At the average temperature difference of 23.3 K,the measured maximum output power is about 4.64 nW.