Quantitative Dedoping of Conductive Polymers

Abstract

Although doping is a cornerstone of the inorganic semiconductor industry, most devices using organic semiconductors (OSCs) make use of intrinsic (undoped) materials. Recent work on OSC doping has focused on the use of dopants to modify a material’s physical properties, such as solubility, in addition to electronic and optical properties. However, if these effects are to be exploited in device manufacturing, a method for dedoping organic semiconductors is required. Here, we outline two chemical strategies for dedoping OSC films. In the first strategy, we use an electron donor (a tertiary amine) to act as competitive donor. This process is based on a thermodynamic equilibrium between ionization of the donor and OSC and results in only partial dedoping. In the second strategy, we use an electron donor that subsequently reacts with the p-type dopant to create a nondoping product molecule. Primary and secondary amines undergo a rapid addition reaction with the dopant molecule 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4TCNQ), with primary amines undergoing a further reaction eliminating HCN. Under optimized conditions, films of semiconducting polymer poly­(3-hexylthiophene) (P3HT) dedoped with 1-propylamine (PA) reach as-cast fluorescence intensities within 5 s of exposure to the amine, eventually reaching 140% of the as-cast values. Field-effect mobilities similarly recover after dedoping. Quantitative fluorescence recovery is possible even in highly fluorescent polymers such as PFB, which are expected to be much more sensitive to residual dopants. Interestingly, treatment of undoped films with PA also yields increased fluorescence intensity and a reduction in conductivity of at least 2 orders of magnitude. These results indicate that the process quantitatively removes not only F4TCNQ but also intrinsic p-type impurities present in as-cast films. The dedoping strategies outlined in this article are generally applicable to other p- and n-type molecular dopants in OSC films.

Controlling Molecular Doping in Organic Semiconductors

“…A similar passivation effect was recently observed by treating P3HT films with 1‐propylamine (PA) . This led to a decrease in conductivity of at least two orders of magnitude and a PL increase of 1.4 relative to untreated films.…”

“…Common environmental impurities in OSCs include water and oxygen . However, even in “impurity‐free” samples (to the extent that such a thing is possible) defects formed by self‐ionization of conjugated bonds into charged defect pairs have been proposed to exist, on the basis of their chemical reactivity with oxidizing or reducing agents . In most OSC samples, positively charged defects are more delocalized than negatively charged defects, causing materials to behave as if they were slightly p‐type doped .…”

“…Diethylamine does, however, increase the PLQE in PBTTT films. Comparison of chemical passivation of defects in P3HT and PBTTT by amines indicates that the chemical nature of defect sites is highly material specific and poorly understood at this point …”

“…Unlike compensation, which never decreases the total charge density in the material, dedoping processes neutralize or remove both free carriers and counterions. This could entail either removal of the neutral dopant molecule or removal of the ionized dopant along with a soluble counterion …”

Performance Improvements in Conjugated Polymer Devices by Removal of Water‐Induced Traps

“…Methanol, on the other hand, which is zeotropic (nonazeotrope) does not lead to improved devices over the reference. It is noteworthy, that this proposed mechanism could also help to explain why azeotrope forming and amine containing solvents have previously been shown to remove traps in, e.g., P3HT films . The clear correlation between device performance and the ability of the solvent to form an azeotrope with water strongly suggests that unbound water molecules trapped in the polymer's microstructure are indeed responsible for device degradation as was argued already in ref.…”