Understanding the Effects of Molecular Dopant on n‐Type Organic Thermoelectric Properties

Abstract

Molecular doping is a powerful method to fine-tune the thermoelectric properties of organic semiconductors, in particular to impart the requisite electrical conductivity. The incorporation of molecular dopants can, however, perturb the microstructure of semicrystalline organic semiconductors, which complicates the development of a detailed understanding of structure-property relationships. To better understand how the doping pathway and the resulting dopant counterion influence the thermoelectric performance and transport properties, a new dimer dopant, (N-DMBI) 2 , is developed. Subsequently, FBDPPV is then n-doped with dimer dopants (N-DMBI) 2 , (RuCp*mes) 2 , and the hydride-donor dopant N-DMBI-H. By comparing the UV-vis-NIR absorption spectra and morphological characteristics of the doped polymers, it is found that not only the doping mechanism, but also the shape of the counterion strongly influence the thermoelectric properties and transport characteristics. (N-DMBI) 2 , which is a direct electron-donating dopant with a comparatively small, relatively planar counterion, gives the best power factor among the three systems studied here. Additionally, temperature-dependent conductivity and Seebeck coefficient measurements differ between the three dopants with (N-DMBI) 2 yielding the best thermoelectric properties.

Synergistically Improved Molecular Doping and Carrier Mobility by Copolymerization of Donor–Acceptor and Donor–Donor Building Blocks for Thermoelectric Application

“…As shown in Figure 6a, σ of all doped films increases with increasing temperature, indicating that a thermally activated charge transport is expected. [42][43][44] The transport activation energy (E a ) extracted from σ = σ 0 exp(−E a /k B T) (Figure 6b) shows that E a would be around 60 meV when x < 0.3, then it decreases to be around 50 meV when 0.3 ≤ x ≤ 0.5. For Pg 3 2T-TT, E a drops to 18 meV.…”

Surface Functionalization of Black Phosphorus with a Highly Reducing Organoruthenium Complex: Interface Properties and Enhanced Photoresponsivity of Photodetectors

“…The variation of electron concentration is very small; the value at V g =40 V increased from 2.0×10 12 cm −2 for pristine BP to 2.8×10 12 cm −2 when molecular thickness was 0.4 nm and then dropped to 2.6×10 12 cm −2 at 0.8 nm. However, considering two electrons donated per dimer and the volume of the Ru cation being 350 Å 3 , a nominal molecular thickness of 0.2 nm in principle corresponds to a cation density of 6×10 13 cm −2 and electron injected at the level of 1×10 14 cm −2 . The electron concentration derived from the transport behavior is thus surprisingly low.…”

“…The φ of the electron analyzer was measured to be 4.30±0.05 eV. [RuCp*(mes)] 2 molecules (synthesized as previously described) were thermally evaporated from a Knudsen cell onto the substrate. The nominal thickness of deposited molecules was monitored by a quartz crystal microbalance exactly located in front of the sample stage and further calibrated by the attenuation of substrate core level peak intensity.…”

n‐Doping of a Low‐Electron‐Affinity Polymer Used as an Electron‐Transport Layer in Organic Light‐Emitting Diodes

“…F8BT (purchased from Sigma Aldrich) was dissolved in chlorobenzene and stirred at 80 °C for 1 h. The concentration of the F8BT solution was 6.67 mg mL −1 for photoelectron spectroscopy, 15 mg mL −1 for absorption and conductivity measurements, and 10 mg mL −1 for OLEDs. [RuCp*Mes] 2 (synthesis described elsewhere [ 18 ] ) was dissolved in toluene and stirred at 80 °C for 1 h. Doped F8BT:[RuCp*Mes] 2 films with the desired dopant concentration were spin‐coated from solutions made by mixing appropriate amounts of the F8BT and dopant solutions. The mixed solutions were then stirred at 80 °C for 30 min, and spin‐coated onto the substrates at 2000 rpm for 45 s. The films were then immediately annealed at 100 °C for 10 min.…”

“…[ 15 ] The focus of the present work is to demonstrate the use of solution processing for incorporating [RuCp*Mes] 2 into, and efficiently n‐doping, a low EA polymer. n‐Doping with the ruthenium‐based dimer has been previously demonstrated in solution‐processed polymers; however, it has only been used with hosts that have significantly larger EA (>3.5–4 eV), including P(BTP‐DPP), [ 16 ] P(NDI2OD‐T2), [ 17 ] and FBDPPV, [ 18 ] for which the initial electron transfer from the dimer is considerably less endergonic than that to POPy 2 , leading to facile n‐doping. Here, we employ the polymer poly[(9,9‐dioctylfluorene‐2,7‐diyl)‐ alt ‐(benzo[2,1,3]thiadiazol‐4,7‐diyl)] (F8BT) (Figure 1b), with a low EA equal to 2.8 eV and which is therefore much more challenging to n‐dope.…”