Semiconductors are the basis of all modern electronics. Chemical doping is crucial in improving the performance of organic semiconductor devices, but traditional doping methods rely on strong oxidants and reducing agents, which have by-products and high costs. Photocatalysts (PCs) are capable of oxidizing or reducing organic compounds based on the presence of weak oxidants or reducing agents, and polychlorinated naphthalene exhibits high efficiency and selectivity in organic synthesis. Therefore, it is important to explore the oxidation or reduction of polychlorinated naphthalene in organic semiconductors and develop new methods for the use of inexpensive and widely available dopanants to improve the compatibility of semiconductor devices.
Here, Prof. Simone Fabiano's research group at Linköping University in Sweden reports a new method in which conductive plastics are immersed in a special salt solution (a photocatalyst) and then irradiated with light for a short period of time. The duration of illumination determines the degree of doping of the material. After that, the solution is recycled for future use, leaving behind a doped conductive plastic in which the only substance consumed is oxygen in the air. This is a versatile method that can be applied to a wide range of organic semiconductors (OSCs) and photocatalysts with conductivity in excess of 3,000 S cm-1. This is an important step towards the future of cheap and sustainable organic semiconductors! The results were published in Nature under the title of "Photocatalytic doping of organic semiconductors", and the first author is Chinese scholar Wenlong Jin. Peking University alumni Assistant Professor Yang Chiyuan and Professor Simone Fabiano are the joint correspondents.
The authors found that neither the dopant nor the PC could extract or contribute electrons from or to the OSC in the ground state (Fig. 1a,b). However, when photoexcitation occurs, PC can oxidize or reduce OSCs and be regenerated by the dopant (Figure 1c-e). The authors selected three different acridine derivatives for photocatalytic doping: Acr-Me+ (10-methylacridine perchlorate), Mes-Acr-Me+ (9-methylmercapto-10-methylacridine tetrafluoroborate), and Mes-Acr-Ph+ (9-methylmercapto-3,6-di-tert-butyl-10-phenyacridine tetrafluoroborate) and compared their activities with those of other air-stable organopolychlorinated biphenyls. Such as perdiimide (PDI-C6C7) and eosin Y (2′,4′,5′,7′-tetrabromofluorescein disodium salt) (Figure 1f).
Figure 1: Photocatalytic doping concept
PBTTT 的光催化 p 掺杂
As shown in the case study of Acr-Me+ oxidized PBTTT in Figure 2, the photocatalytic p-doping method is simple and efficient. Acr-Me+ (0.01 M) was dissolved in a mixture of n-butyl acetate and acetonitrile with salt LiTFSI (0.1 M). The PBTTT film was immersed in Acr-Me:LiTFSI solution and irradiated with 455 nm blue light for 12 minutes to activate the Acr-Me+ light to an excited state. Then, as shown in Figure 2a, the p-doped PBTTT film was removed from the PC solution, washed, and dried. The UV-Visible-NIR absorption spectra of the PBTTT film showed that O2 could not be doped with PBTTT in the absence of Acr-Me+ or in the presence of Acr-Me+ in dark conditions (Figure 2b), but a strong polar absorption elongation was observed in the near-infrared region under blue light irradiation. XPS shows that strong fluorine F(1s) and oxygen-O(1s) XPS core-level signals derived from TFSI- can be observed under light irradiation (Figure 2c). Further analysis of the flat peaks of sulfur S(2p), F(1s), and nitrogen N(1s) XPS nuclear power (Figure 2d) showed that the TFSI- anion content also increased with the irradiation time, resulting in a positive charge of the PBTTT polymer backbone. In addition, ultraviolet photoelectron spectroscopy (UPS) analysis showed that the front-edge site-occupying density of PBTTT was partially detopped and the work function increased significantly after light irradiation (Fig. 2e).
Figure 2: Photocatalytic p-doping of PBTTT
Photocatalytic p-doping mechanism and versatility
In order to understand the charge transfer mechanism during photocatalytic P doping, transient absorption spectroscopy, photoluminescence and absorption spectroscopy experiments were performed. By analyzing the excited state lifetime and fluorescence quenching of Acr-Me+ and PBTTT, it was found that the light-induced electron transfer (PET) process was mainly dominated by the excited state of PC, and the electrons were transferred from the ground state of OSC to the excited state of PC. Using Et3N as a transparent reducing agent, Mes-Acr-Me+ and Mes-Acr-Ph+, which are stable in the presence of Et3N, were selected as representative PCs. Absorption spectra showed that PET with Mes-Acr-Me+ and Et3N formed reduced Mes-Acr-Me- (or Mes-Acr-Ph-), which was subsequently oxidized by O2 or (PhS)2. In situ photoluminescence studies demonstrated the reduction and regeneration of MES-ACR-ME+ and MES-ACR-PH+ (Figure 3A,B). Transient absorption spectra showed that Mes-Acr-Me+ formed an excited state under illumination, which subsequently transformed into reduced Mes-Acr-Me-. Oxidation in air completely regenerates MES-ACR-ME+, ending the photocatalytic oxidation cycle (Figure 3C). Density functional theory (DFT) calculations show that both reduced and excited reduced Acr-Me can be oxidized by O2, and Acr-Me- is preferred.
The researchers also investigated the photocatalytic p-doping versatility of conjugated polymers with different ionization potentials and side chain properties, as well as PCBs with different electron affinities. In the ground state, the electron affinity of the PC is not strong enough for p-doping, but when photoactivated, its excited state can be p-doped. Photocatalytic doping significantly enhances the conductivity of all semiconducting polymers, especially P(g42T-T) with a conductivity of up to 3,000 S cm-1. The increase in conductivity is related to the energy difference between the ionization potential of the OSC and the electron affinity of the excited state of the PC, and the smaller the energy difference, the more significant the increase in conductivity.
Figure 3: Mechanism and generality of the photocatalytic P-doping process
Photocatalytic n-type doping and simultaneous p-type doping and n-type doping
Finally, the authors investigated photocatalytic reduction (n-doping) of OSCs as well as simultaneous photocatalytic p-doping and n-doping (Fig. 4a,b). First, the PC is photoactivated to produce an excited state PC, which oxidizes the weak n dopants. When the n-dopant is oxidized, the PC is converted to a reduced state (Figure 4c) and photoactivated again to obtain an excited reduced state for n-doping of the OSC. In this process, the n-type OSC accepts electrons from the PC, regenerating the ground-state PC. In addition, Et3N is weak in the presence of light or PC to positive-dop BBL (Figure 4d), but in the presence of light and PC, Et3N can significantly positive-doped BBL, producing strong negative pole absorption peaks. After photocatalytic n doping, the conductivity of BBL increased from less than 10-5 S cm-1 to more than 1 S cm-1 (Figure 4e).
By substituting Et3N with P(g42T-T), the authors achieved photocatalytic p-doping and n-doping of P(g42T-T) binding to BBL. Mes-Acr-Me+ acts as a redox shuttle that transfers electrons from P(g42T-T) to BBL, exhibiting unique polarity characteristics. Photocatalytic p-doped P(g42T-T) shows a conductivity of 200 S cm-1, while photocatalytic n-doped BBL shows a conductivity of 0.1 S cm-1 (Figure 4e), with only [EMIM][TFSI] consumed in the process to maintain charge neutrality. Importantly, p-doped or n-doped can only be observed in the presence of Mes-Acr-Me+ (Figure 4F).
brief summary
Here, we report a previously unpublished concept of OSC photocatalytic doping, which provides a simple and efficient solution-based process at room temperature. The doping level can be easily controlled by adjusting the light irradiation dose. In contrast to traditional doping methods that rely on highly reactive dopants consumed during the doping process, photocatalytic doping uses recyclable and air-stable PC, consuming only TSFI-based salts and weak dopants such as O2 (in air). This photocatalytic method is versatile and applicable to a wide range of OSCs, producing p-doped, n-doped, and both p-doped and n-doped highly conductive OSCs. In addition, the method enables redox inert counterions to be inserted directly into the initially undoped OSC film without negatively affecting its microstructure. These results highlight the importance of photocatalytic doping for basic and applied research in organic electronics.
Source: Frontiers of Polymer Science