Control of Chain Alignment and Crystallization Helps Enhance Charge Conductivities and Thermoelectric Power Factors in Sequentially Doped P3HT:F₄TCNQ Films

Abstract

Thermoelectricity has gained considerable interest in the last decade due to the advent of organic thermoelectric materials. Crystallinity and doping level crucially determine the thermoelectric figure of merit of semi-conducting polymers. Hence, detailed insight into these factors is prerequisite for developing efficient devices. Here we show that the semicrystalline structure of aligned P3HT films shows superior thermoelectric efficiencies as compared to the smectic-like phase because of both a higher in-plane orientation and a higher doping level. Conductivities up to 160 S/cm and power factors of 56 W m-1 K-2 along the rubbing direction are obtained versus a few W m-1 K-2 for non-oriented films. Different intercalation mechanisms of F 4 TCNQ in the layers of alkyl side chains are evidenced by electron diffraction in doped oriented films of the smectic-like and the semi-crystalline phases. We provide compelling evidence that doping of the smectic-like phase promotes ordering of P3HT backbones along the chain direction within individual -stacks whereas for the semi-crystalline phase, dopant intercalation reorganizes the arrangement of successive stacks and perturbs the packing of alkyl side chains. Insight in the orientation of F 4 TCNQanions in the layers of alkyl side chains of P3HT crystals was further retrieved from a detailed polarized UV-vis-NIR spectroscopic analysis. Our results demonstrate that both orientation of the polymer chains and crystallinity enhance the thermoelectric properties as well as the doping level. We anticipate that detailed control of polymer morphology in films further improves the thermoelectric figure of merit of semiconducting polymers.

Influence of dopant size and doping method on the structure and thermoelectric properties of PBTTT films doped with F6TCNNQ and F4TCNQ

“…In the case of P3HT, this absence has been attributed to the fact that the amorphous phase of P3HT is little doped with F 4 TCNQ due to a larger offset between the amorphous polymer's HOMO and the dopant's LUMO as compared to crystalline P3HT. [26] The present results with F 6 TCNNQ suggest a similar effect for C 12 -PBTTT: F 6 TCNNQ cannot dope efficiently the amorphous phase of C 12 -PBTTT.…”

“…The same observation was also obtained for the angular distribution measurements of F 4 TCNQ-doped P3HT films. [15,26] In addition to this, the maximum of the angular distributions of both F 4 TCNQand F 6 TCNNQanions are clearly centered at 90. This implies that the long axis of the dopants is indeed oriented around a plane orthogonal to the average polymer chain direction and not tilted.…”

“…Angle-dependent polarized UV-Vis-NIR spectroscopy was used to further probe the degree of order and the distribution of the dopant anions in the oriented C 12 -PBTTT matrix following the approach by Untilova et al for F 4 TCNQ-doped aligned P3HT. 26 The UV-Vis-NIR spectra were recorded for a variable polarization angle of the incident light with respect to the direction of rubbing in the 0 -90…”

“…Similar trends were obtained in oriented P3HT films for the angular distribution of F 4 TCNQ. [26] This suggests that the F 4 TCNQ dopants are not strictly located in a plane orthogonal to the polymer chains, but are distributed around this average orientation.…”

Insights into the structural complexity of semi-crystalline polymer semiconductors: electron diffraction contributions

“…They can form spontaneously nanostructured bulk heterojunctions via phase separation of donor and acceptor blocks. [77][78][79][80] Preliminary studies show that the alignment methods used for the individual blocks are relevant to orient and crystallize such conjugated copolymers and ultimately study their organization by electron diffraction. In particular, TEM studies demonstrated that the crystallization of the P(NDI2ODT2) block can hamper substantially that of the P3HT block due to a mismatch of lattice parameters of the two blocks at the block junction.…”