Achieving Efficient p‐Type Organic Thermoelectrics by Modulation of Acceptor Unit in Photovoltaic π‐Conjugated Copolymers

Abstract

𝝅-Conjugated donor (D)−acceptor (A) copolymers have been extensivelystudied as organic photovoltaic (OPV) donors yet remain largely unexplored in organic thermoelectrics (OTEs) despite their outstanding mechanical bendability, solution processability and flexible molecular design. Importantly, they feature high Seebeck coefficient (S) that are desirable in room-temperature wearable application scenarios under small temperature gradients. In this work, the authors have systematically investigated a series of D−A semiconducting copolymers possessing various electron-deficient A-units (e.g., BDD, TT, DPP) towards efficient OTEs. Upon p-type ferric chloride (FeCl 3 ) doping, the relationship between the thermoelectric characteristics and the electron-withdrawing ability of A-unit is largely elucidated. It is revealed that a strong D−A nature tends to induce an energetic disorder along the 𝝅-backbone, leading to an enlarged separation of the transport and Fermi levels, and consequently an increase of S. Meanwhile, the highly electron-deficient A-unit would impair electron transfer from D-unit to p-type dopants, thus decreasing the doping efficiency and electrical conductivity (𝝈). Ultimately, the peak power factor (PF) at room-temperature is obtained as high as 105.

Sequential-Twice-Doping Approach toward Synergistic Optimization of Carrier Concentration and Mobility in Thiophene-Based Polymers

“…Great efforts have been given to synthesize alternative donor–acceptor (D–A) polymers because their high μ can contribute to the high electrical conductivity. , Despite lots of D–A polymers showing a high μ in OFET, only a few D–A polymers can be effectively doped by strong dopants . It has been reported that a polymer with a D–A backbone possesses a much lower doping efficiency due to the acceptor moiety being inactive for p-type doping. − Unlike D–A polymers, thiophene polymers, such as poly­(3-hexylthiophene) (P3HT) and poly­(2,5-bis­(3-tetradecylthiophene-2-yl)­thieno­[3,2- b ]­thiophene) (PBTTT-C14), can be easily doped due to the matched energy levels to common dopants, − but their electrical conductivity is limited due to the low intrinsic mobility. In general, the rigidity and coplanarity of conjugated units facilitate effective charge transport by increasing the probability for intermolecular π–π orbital overlap and thus interchain charge transport, leading to a high carrier mobility.…”

“…16,17 Despite lots of D−A polymers showing a high μ in OFET, only a few D−A polymers can be effectively doped by strong dopants. 18 It has been reported that a polymer with a D−A backbone possesses a much lower doping efficiency due to the acceptor moiety being inactive for p-type doping. 18−20 Unlike D−A polymers, thiophene polymers, such as poly(3-hexylthiophene) (P3HT) and poly(2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT-C14), can be easily doped due to the matched energy levels to common dopants, 21−23 but their electrical conductivity is limited due to the low intrinsic mobility.…”

Tailoring the Morphology’s Microevolution for Binary All-Polymer Solar Cells Processed by Aromatic Hydrocarbon Solvent with 16.22% Efficiency

“…To explore more information of the solvent selection's effect in film morphology and the opto-electric property, we carried out the grazing incidence wide-angle X-ray scattering (GIWAXS) experiments. 42−48 49,50 Next, we utilized the space charge limited current (SCLC) method to see whether the charge mobility variation of active layers brought by different solvents follows the previous prediction. The hole and electron mobility (μ h and μ e ) evaluation are plotted in Figure S1, and the calculated results are given in Figure 1c and Table 1.…”

“…These results indicate that the o-XY processed active layer has the fewest IP packings in a single domain but the largest number of OOP packings. Generally, the o-XY processed film should be the most favorable one in achieving efficient electron transport because the PBDB-T family polymers mainly contribute to the IP hole transport. , …”

Remarkably High Conductivity and Power Factor in D–D′‐type Thermoelectric Polymers Based on Indacenodithiophene

“…[3,[8][9][10][11][12][13][14] Owing to the low κ of organic TE materials and the difficulty to measure correctly, the power factor (PF = α 2 σ) is often used to evaluate their TE performance. [9,[15][16][17][18] Recently, extensive studies on TE material design, fundamentals in charge generation/transport, and optimization of device architectures have been performed, successfully demonstrating a record high ZT up to 0.75 for poly(3,4-ethy lenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/ionic liquid (IL) heterostructures. [19] The enhancement of ZT is attributed to only a slight drop in σ and a significant rise in α due to ion accumulation of IL on the polymer surface which can scatter the low energy charge carriers and thus increase the mean energy of the holes at the cold side of the PEDOT:PSS films.To optimize the performance of organic TE generators (TEGs), high σ is the most important prerequisite.…”