Optimizing the Thermoelectric Performance of Poly(3-hexylthiophene) through Molecular-Weight Engineering

Enhanced Interface Compatibility by Ionic Dendritic Molecules To Achieve Efficient and Stable Perovskite Solar Cells

Poly(3-hexylthiophene) (P3HT) represents a promising hole transport material for emerging perovskite solar cells (PSCs) due to its appealing merits of high thermal stability and appropriate hydrophobicity. Nonetheless, large energy losses at the P3HT/perovskite interface lead to unsatisfied efficiency and stability of the devices. Herein, two ionic dendritic molecules, 3,3'-(2,7-bis(3,6-bis(bis(4-methoxyphenyl)amino)-9H-carbazol-9-yl)-9H-fluorene-9,9-diyl)bis(N,N,N-trimethylpropan-1-aminium) iodide and 3,3'-(2,7-bis(bis(4-(bis(4-methoxyphenyl)amino)phenyl)amino)-9H-fluorene-9,9-diyl)bis(N,N,N-trimethylpropan-1-aminium) iodide, namely, MPA-Cz-FAI and MPA-PA-FAI, are rationally designed as the interlayer to enhance interfacial compatibility. The dendritic backbone with conjugated structure endows the hole transport layer with high conductivity, derived from the more ordered microstructure with larger crystallization and higher connectivity of domain zones. Besides, a better energy level alignment is established between P3HT and perovskite, which enhances the charge extraction and transport yield. In addition, the peripheral methoxy groups enable effective defect passivation at the interface to suppress nonradiative recombination and the quaternary ammonium iodide serving as side chains enable efficient interfacial hole extraction contributing to enhanced charge collection yield. As a result, the dopant-free P3HT-based PSCs modified with MPA-Cz-PAI deliver a champion efficiency of 19.7%, significantly higher than that of the control devices (15.4%). More encouragingly, the unencapsulated devices demonstrate competitive environmental stability by retaining over 85% of its initial efficiency after 1500 h of storage under humid conditions (70% relative humidity). This work provides an effective molecular design strategy for interface engineering, envisaging a bright prospect for the further development of efficient and stable perovskite solar cells.

Recent Advances in Polyallenes: Preparation, Self-Assembly, and Stimuli-Responsiveness

Polyallenes, as a typical type of reactive polymers, are of great significance and have aroused widespread interest because they contain double bonds that can be post-modified into other functionalities to afford varieties of functional materials. This Minireview firstly highlights the recent advances in the preparation of polyallenes, including preparation of helical polyallenes through directly polymerization of chiral allene monomers or helix-sense-selective polymerization (HSSP) of achiral allene monomers, synthesis of 1,2-regulated polyallenes and 2,3-regulated polyallenes via selective polymerization of allene monomers, polymerization of allene monomers catalyzed by Ni(II)-terminated poly(3-hexylthiophene) (P3HT), and so on. Then, latest progress on the self-assembly and stimuli-responses of polyallene-based diblock, ABA and ABC triblock copolymers is summarized. We hope this Minireview will inspire more interest in developing polyallenes and encourage further advances in functional materials.

Enabling Conducting Polymer Applications: Methods for Achieving High Molecular Weight in Chemical Oxidative Polymerization in Alkyl- and Ether-Substituted Thiophenes

Polythiophenes (PTs) constitute a diverse array of promising materials for conducting polymer applications. However, many of the synthetic methods to produce PTs have been optimized only for the prototypical alkyl-substituted example poly(3-hexylthiophene) (P3HT). Improvement of these methods beyond P3HT is key to enabling the widespread application of PTs. In this work, P3HT and two ether-substituted PTs poly(2-dodecyl-2H,3H-thieno[3,4-b][1,4]dioxine) (PEDOT-C12) and poly(3,4-bis(hexyloxy)thiophene) (PBHOT) are synthesized by the FeCl₃-initiated oxidative method under different conditions. Polymerization was carried out according to a common literature procedure (“reverse addition”) and a modified method (“standard addition”), which differ by the solvent system and the order of addition of reagents to the reaction mixture. Gel-permeation chromatography (GPC) was performed to determine the impact of the different methods on the molecular weights (M_w) and degree of polymerization (X_w) of the polymers relative to polystyrene standards. The standard addition method produced ether-substituted PTs with higher M_w and X_w than those produced using the reverse addition method for sterically unhindered monomers. For P3HT, the highest M_w and X_w were obtained using the reverse addition method. The results show the oxidation potential of the monomer and solution has the greatest impact on the yield and X_w obtained and should be carefully considered when optimizing the reaction conditions for different monomers.

Synergistically Optimized Electrical and Thermal Transport Properties in Copper Phthalocyanine-Based Organic Small Molecule with Nanoscale Phase Separations

A series of copper phthalocyanine (CuPc)-based organic small molecules were prepared through vapor-phase reaction. Nanoscale phase separation was observed with tunable CuPc and copper phthalocyaninato iodide (CuPcI) phase content by changing the iodine ratio. The Seebeck coefficient of the samples was significantly enhanced, which is considered to be attributed to the enhanced surface polarization effect due to the formation of a great number of nanoscale interfaces between the CuPc phase and the CuPcI phase. In addition, these nanointerfaces also gave rise to increased phonon scattering and therefore significantly reduced the lattice thermal conductivity of the small-molecule samples. As a result of the combination of the synergistically optimized electrical and thermal transport properties, the maximum ZT value reaches 3.0 × 10^-2 at room temperature, which is among the highest values for small-molecule charge-transfer complex reported so far. Our results shed light on optimizing the thermoelectric performance of organic small molecules by introducing nanoscale phase separations and tailoring the nanoscale interfaces.

Boosting the Power Factor of Benzodithiophene Based Donor-Acceptor Copolymers/SWCNTs Composites through Doping

In this study, a benzodithiophene (BDT)-based donor (D)–acceptor (A) polymer containing carbazole segment in the side-chain was designed and synthesized and the thermoelectric composites with 50 wt % of single walled carbon nanotubes (SWCNTs) were prepared via ultrasonication method. Strong interfacial interactions existed in both of the composites before and after immersing into the 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) solution as confirmed by UV-Vis-NIR, Raman, XRD and SEM characterizations. After doping the composites by F₄TCNQ, the electrical conductivity of the composites increased from 120.32 S cm⁻¹ to 1044.92 S cm⁻¹ in the room temperature. With increasing the temperature, the electrical conductivities and Seebeck coefficients of the undoped composites both decreased significantly for the composites; the power factor at 475 K was only 6.8 μW m⁻¹ K⁻², which was about nine times smaller than the power factor at room temperature (55.9 μW m⁻¹ K⁻²). In the case of doped composites, although the electrical conductivity was deceased from 1044.9 S cm⁻¹ to 504.17 S cm⁻¹, the Seebeck coefficient increased from 23.76 μV K⁻¹ to 35.69 μW m⁻¹ K⁻², therefore, the power factors of the doped composites were almost no change with heating the composite films.

Enhanced Thermoelectric Performance of Indacenodithiophene-Benzothiadiazole Copolymer Containing Polar Side Chains and Single Wall Carbon Nanotubes Composites

Composite films of indacenodithiophene-bezothiadazole copolymers bearing polar side chains (P1) and single wall carbon nanotubes (SWCNTs) are found to show a competitive thermoelectric performance compared to their analogous polymers with aliphatic side chains (P2). The enhanced power factors could be attributed to the stronger interfacial interactions between the P1/SWCNTs compared to that of P2/SWCNTs containing the same ratio of SWCNTs. A maximum power factor of 161.34 μW m⁻¹ K⁻² was obtained for the composite films of P1/SWCNTs for a filler content of 50 wt%, which is higher than that of P2/SWCNTs (139.06 μW m⁻¹ K⁻², 50 wt%). Our work sheds light on the design of side-chains in efficient conjugated polymers/SWCNTs thermoelectric materials and contributes to the understanding of their thermoelectric properties.

The Molecular Weight Dependence of Thermoelectric Properties of Poly (3-Hexylthiophene)

Organic materials have been found to be promising candidates for low-temperature thermoelectric applications. In particular, poly (3-hexylthiophene) (P3HT) has been attracting great interest due to its desirable intrinsic properties, such as excellent solution processability, chemical and thermal stability, and high field-effect mobility. However, its poor electrical conductivity has limited its application as a thermoelectric material. It is therefore important to improve the electrical conductivity of P3HT layers. In this work, we studied how molecular weight (MW) influences the thermoelectric properties of P3HT films. The films were doped with lithium bis(trifluoromethane sulfonyl) imide salt (LiTFSI) and 4-tert butylpyridine (TBP). Various P3HT layers with different MWs ranging from 21 to 94 kDa were investigated. UV-Vis spectroscopy and atomic force microscopy (AFM) analysis were performed to investigate the morphology and structure features of thin films with different MWs. The electrical conductivity initially increased when the MW increased and then decreased at the highest MW, whereas the Seebeck coefficient had a trend of reducing as the MW grew. The maximum thermoelectric power factor (1.87 μW/mK2) was obtained for MW of 77 kDa at 333 K. At this temperature, the electrical conductivity and Seebeck coefficient of this MW were 65.5 S/m and 169 μV/K, respectively.

Polar Side Chain Effects on the Thermoelectric Properties of Benzo[1,2-b:4,5-b']Dithiophene-Based Conjugated Polymers

The molecular structure of polymers has a great influence on their thermoelectric properties; however, the relationship between the molecular structure of a polymer and its thermoelectric properties remains unclear. In this work, two benzo[1,2-b:4,5-b']dithiophene (BDT)-based conjugated polymers are designed and synthesized, which contain alkyl side chains or polar side chains. The effects of the polymer side chain on the physicochemical properties are systematically investigated, especially the thermoelectric performance of the polymers after doping with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane. It is found that the BDT-based conjugated polymer with polar side chains exhibits good miscibility with the dopants, leading to higher thermoelectric properties than those of the polymer with alkyl side chains. This work can serve as a reference for the future design of high-performance organic thermoelectric polymers.