Introducing Solubility Control for Improved Organic P-Type Dopants

Abstract

To overcome the poor solubility of the widely 8 used p-type dopant 2, 3,5,6-tetrafluoro-7,7,8,8-tetracyanoqui-9 nodimethane (F4TCNQ) 31,32 and 3,6-difluoro-2,5,7,7,8,8-75 In this paper, we first demonstrate a straightforward synthetic 88 route to soluble versions of F4TCNQ-type dopants by 89 substituting the cyano groups with either methyl or n-octyl f1 90 esters (Figure 1). A comprehensive study of the electro-91 chemical properties of these F4TCNQ analogues is performed 92 using cyclic voltammetry. With a combination of optical 93 absorption spectroscopy, photoluminescence spectroscopy, and 94 conductivity measurements, we not only demonstrate the p-95 type doping of P3HT using these new dopants, but also show 96 that comparable doping efficiency can be achieved even with 97 slightly reduced electron affinity. These novel molecular 98 dopants establish that the introduction of solubility control is 99 a successful strategy to tailor the properties of organic p-type 100 dopants. 101 ■ EXPERIMENTAL SECTION 102Materials. 1,4-Bis(chloromethyl)-2,3,5,6-tetrafluorobenzene was 103 purchased from Oakwood Products. Sodium hydride (60% dispersion 104 in mineral oil) was purchased from Alfa Aesar. Bromine, 105 perfluorobenzene, potassium carbonate, dimethyl carbonate, malono-106 nitrile, sodium acetate, sodium hydroxide, and phosphorus tribromide 107 were purchased from Sigma-Aldrich. Anhydrous dimethyl sulfoxide 108 (DMSO) and dimethylformamide (DMF) were purchased from Acros 109 Organics. Hydrochloric acid (37% aqueous), p-toluenesulfonic acid 110 monohydrate, ethanol, tetrahydrofuran (THF), dichloromethane 111 (DCM), ethyl acetate, toluene, and hexane were purchased from 112 Fisher Scientific. Acetic acid and sodium cyanide were purchased from 113 Fluka. Octanol was purchased from EM Science. Trifluoroacetic acid 114 (TFA) and acetic anhydride were purchased from EMD. P3HT 115 (Regioregular >98%, M n = 54−75 kDa, HOMO 5 eV and LUMO 3 116 eV) was purchased from Sigma-Aldrich. F4TCNQ (>98%) was 117 purchased from TCI. All chemicals were used as received unless 118 otherwise indicated. All solvents were dried over molecular sieves (3 119 Å) before use. 120Characterization.

The effect of thermal annealing on dopant site choice in conjugated polymers

“…A considerable number of studies have already shown that the molecular dopant F4TCNQ is able to p-type dope P3HT [24,26,30,31]. In this section, this molecular doping process is re-examined from the dopant diffusion perspective.…”

“…It is clear that P3HT and F4TCNQ do not form a stable bilayer structure, but rather the P3HT is substantially doped by the F4TCNQ molecules, as seen by the reduced P3HT absorbance around 2.48 eV and the appearance of the broad peaks centered around 1.5 eV and 0.6 eV which are characteristics of P3HT þ polarons [24,25]. Based on the ground-state/polaron peak ratio, we can determine a doping concentration of~11 wt% [31], which agrees nicely with the amount of F4TCNQ we deposited. This result suggests that P3HT provides a good host for F4TCNQ and that when it is present, the F4TCNQ efficiently dopes the entire P3HT layer at room temperature with an unambiguous change in the optical absorbance.…”

“…However, the F4TCNQ concentration in the P3HT film is not high enough to see these doping features in the absorbance spectrum at any temperature. Based on previous work with P3HT, this means that the doping level is below 0.6 wt% [31].…”

“…S3b. Assuming that all PL quenching is the result of dopants diffusing into the P3HT layer, the doping concentration can be determined by comparison to a calibration curve [31]. The total extrinsic doping concentration in P3HT as a function of annealing temperature is plotted in Fig.…”

“…2 [ 24,25]. Based on the ground-state/polaron peak ratio, we also quantify the doping concentration in the P3HT layer [31] and the results are plotted in Fig. 7b.…”