Controlling Molecular Doping in Organic Semiconductors

Abstract

The field of organic electronics thrives on the hope of enabling low-cost, solution-processed electronic devices with mechanical, optoelectronic, and chemical properties not available from inorganic semiconductors. A key to the success of these aspirations is the ability to controllably dope organic semiconductors with high spatial resolution. Here, recent progress in molecular doping of organic semiconductors is summarized, with an emphasis on solution-processed p-type doped polymeric semiconductors. Highlighted topics include how solution-processing techniques can control the distribution, diffusion, and density of dopants within the organic semiconductor, and, in turn, affect the electronic properties of the material. Research in these areas has recently intensified, thanks to advances in chemical synthesis, improved understanding of charged states in organic materials, and a focus on relating fabrication techniques to morphology. Significant disorder in these systems, along with complex interactions between doping and film morphology, is often responsible for charge trapping and low doping efficiency. However, the strong coupling between doping, solubility, and morphology can be harnessed to control crystallinity, create doping gradients, and pattern polymers. These breakthroughs suggest a role for molecular doping not only in device function but also in fabrication-applications beyond those directly analogous to inorganic doping.

Influence of crystallinity on the thermoelectric power factor of P3HT vapour-doped with F4TCNQ

“…p-type doping of poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrauoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is a widely used model system (Fig. 1), [7][8][9] where an electron is donated from the highest occupied molecular orbital (HOMO) of P3HT to the lowest unoccupied molecular orbital (LUMO) of F4TCNQ, leading to integer charge transfer.…”

Band Engineering and Majority Carrier Switching in Isostructural Donor–Acceptor Complexes DPTTA‐FXTCNQ Crystals (X = 1, 2, 4)

“…[26,29] Here, the DA complex single crystal is the extreme situation of this doping process. Formation of DA complexes has been wide accepted for explaining the low doping efficiency of organic semiconductors.…”

The Role of Weak Molecular Dopants in Enhancing the Performance of Solution‐Processed Organic Field‐Effect Transistors

“…Similarly, obvious morphology change was also observed in films on the unmodified SiO 2 /Si substrate ( Figure S8, Supporting Information). [13,48] It should be stressed that the insertion of dopant does not disrupt the molecular packing of the host semiconductor. Figure 5a showed that the pristine film had no diffraction peak, suggesting its random molecular orientation and also well illustrating the low mobility of the undoped device.…”

“…Namely, the LUMO of p-dopants is always close to or deeper than the HOMO of host matrix to fulfill the efficient charge transfer. [13,18] The weak charge transfer has been proven to be formed at the interfaces of donor/acceptor mixtures in OPVs and is believed to be benefit for the device performance. F 4 TCNQ can dope poly(3-hexylthiophene) (P3HT) through an integer charge transfer to form ion pairs, [23,24] while doping C 8 BTBT through the partial charge transfer to form the charge-transfer complex.…”

“…For instance, the prototypical p-dopant F 4 TCNQ, with the electron affinities (EAs) of 5.24 eV larger than the ionization energy (IE) of many OSCs, has been widely used to dope various conjugated small molecules and polymers. [12,13,18,25,26] However, the systematic investigation of this weak dopant on the charge transport in OFET is rare. [16] Theoretically, charge transfer only occurs efficiently when the energy levels are correctly matched.…”