On April 27, 2022, a team of young researchers Liu Yunxi and Wang Yang of the Department of Materials Science of Fudan University published a new study entitled “An all-C–H-activation strategy to rapidly synthesize high-mobility well-balanced ambipolar semiconducting polymers” in the journal Matter.
Through the full C-H activation strategy (2-step C-H activation reaction), the scientific research team quickly synthesized the polymer semiconductor material with controllable molecular weight in only 2 hours, which realized the reduction of the synthesis step and the reduction of time cost, and further improved the yield. Applied to flexible field-effect transistors, the polymers demonstrate excellent balanced bipolar carrier mobility.
Shen Tao, a 2020 doctoral student in the Department of Materials Science of Fudan University, and Wenhao Li, a 2020 master’s student, are the co-first authors of the paper; Young Researcher Wang Yang is the corresponding author of “Lead contact”; and Zhao Yan, a young researcher, and Academician Liu Yunxi, are co-corresponding authors.
Polymer semiconductor materials have attracted great interest from industry and academia for their great potential in areas such as flexible optoelectronic devices, wearable electronics, bionic sensing and nervous systems. In particular, bipolar polymer semiconductor materials that can transmit electrons and holes at the same time are of great significance for the manufacture of flexible organic electronic circuits and organic light-emitting transistors. In recent years, due to the excellent molecular planarity and good electron pulling ability of pyrrolopyrroledione derivatives (DPP), the synthesis and application of DPP-based organic semiconductor materials have received widespread attention. Among them, DPP-based bipolar polymer semiconductors can exhibit hole/electron mobility of more than 3 cm2 V−1 s−1.
However, there are at least three scientific problems in the development of high-performance bipolar semiconductor materials: 1) the imbalance of bipolarity, that is, its p-type mobility is much greater than that of n-type; 2) the difficulty of synthesis, high-mobility polymers are mostly polymerized by Stile or Suzuki coupling polymerization, which require multi-step reactions, especially borate and C-Sn functional group reactions are very challenging and difficult to purify. Among them, especially organotin reagents, and organotin reagents are more toxic, it is difficult to further synthesize a large number of them; 3) the contradiction between high molecular weight and low solution processability. It is generally believed that in order to obtain high performance, it is necessary to make the higher molecular weight of the synthesized polymer. However, high molecular weight often results in lower polymer solubility, resulting in poor processability of the solution. Therefore, when designing and synthesizing high-performance polymer semiconductors, it is necessary to take into account both semiconductor properties and solution processability, that is to say, the control of polymer molecular weight is particularly important.
Recently, a team of young researchers Liu Yunxi and Wang Yang of the Department of Materials Science of Fudan University rapidly synthesized a high-performance DPP-balanced bipolar polymer semiconductor with controllable molecular weight through a full C-H activation strategy (as shown in Figure 1). It only takes two steps from monomer to polymer. The monomer synthesis of the first step and the C-H direct arylation polymerization of the second step can be completed within 1 h, and different molecular weight polymers can be obtained by regulating the time. This is due to the high reactivity of the two new receptors, DTD and DFD. In addition, the molecular energy level and molecular plane are effectively regulated by F substitution, which makes the thin-film transistors have excellent equilibrium bipolar characteristics, and their holes and electron mobility are 3.56 and 3.75 cm2 V−1 s−1, respectively (the ratio of holes to electron mobility is close to 1.0).
Figure 1: Synthetic route of the full C-H activation strategy
As shown in Figure 2, H-Aggregation’s thin films are generally more conducive to carrier transport, and this series of polymers can effectively increase the H-Aggregation ratio by replacing them with F. At the same time, F-atom substitution deepens the HOMO and LUMO energy levels. Deeper LUMO energy levels facilitate the stable transmission of electrons. Thus, F-substituted polymers, such as PDFD-BT, demonstrate higher electron migration. It is worth mentioning that in the polymer replaced by F, there is also F· · · S non-covalent bond interactions result in F-substitution polymers with better molecular planarity and stronger intermolecular interactions, which facilitate carrier transport (as shown in Figure 2H).
Figure 2: Schematic diagram of the molecular skeleton of the polymer physicochemical properties characterization and computational simulation
Using GIWAXS further, the paper also studied the effect of F-atom substitution on the crystallization and accumulation of series polymer films, thereby illustrating the mechanism of performance improvement. As shown in Figure 3(A), all six polymers are faced-on stacked. However, according to the calculation of the π-π accumulation diffraction peak in Figure 3(C), it is known that there is a smaller π-π accumulation distance in the polymer film replaced by F. For example, the π-π stacking distance of PDTD-BT film is 3.69 Å, while the π-π stacking distance of PDFD-BT film replaced by F is reduced to 3.59 Å. Tighter π-π stacking is more conducive to carrier transport. So F-substituted polymers exhibit higher mobility. Combined with the influence of F substitution on the front-line orbital energy level LUMO and the polymer aggregation structure, the F atomic substitution strategy is more conducive to the improvement of electron mobility, resulting in excellent balanced bipolar semiconductor performance. The hole and electron mobility rates are 3.56 and 3.75 cm2 V−1 s−1, respectively, and the ratio of holes to electron mobility is close to 1.0, which is one of the best properties of DPP-based bipolar polymers that have been reported (considering the balance of mobility and bipolar transmission).
Figure 3: GIWAXS for six polymer films. (A) 2D image; (B) in-plane direction; (c) out-of-plane direction
(Source: Science Network)
Related paper information:https://doi.org/10.1016/j.matt.2022.04.008