Effect of fluorination of naphthalene diimide–benzothiadiazole copolymers on ambipolar behavior in field-effect transistors

Two naphthalene diimide (NDI)–benzothiadiazole (BT) based conjugated polymers with high molecular weight, P1 and P2, were synthesized by introducing F atoms to modulate the electron-donating ability of the BT moiety. 3-Decyl-pentadecyl branched alkyl side chains were employed and expected to improve the molecular organization and device performance. Both polymers have excellent solubility in common organic solvents. UV-vis-NIR absorption and cyclic voltammetry indicate that the maximum absorption wavelength of P2 is blue-shifted and the HOMO energy level of P2 is decreased in comparison with P1. Two dimensional wide angle X-ray scattering of thin films revealed a similar organization of both polymers. A less balanced transport in field-effect transistors with increased electron mobility of 0.258 cm2 V−1 s−1 and lowered hole transport of 2.4 × 10−3 cm2 V−1 s−1 was found for P2. Polymer devices of P1 exhibited a balanced ambipolar transport, with a hole mobility of 0.073 cm2 V−1 s−1 and electron mobility of 0.086 cm2 V−1 s−1.


Introduction
Ambipolar organic eld-effect transistors (OFETs) have gained increased attention in the past decade due to their potential in complementary logic circuits. [1][2][3][4][5][6] The key design of this kind of polymeric semiconductors is the combination of strong acceptors with suitable donor units in the conjugated backbone, such as naphthalenediimide (NDI), 7-9 diketopyrrolopyrrole (DPP), [10][11][12][13] thiadiazoloquinoxaline (TQ), [14][15][16] and benzobisthiadiazole (BBT). 17,18 The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels are of great importance since they determine the injection and transport of electrons and holes in the active lm. High performance ambipolar transistors of donor (D)-acceptor (A) copolymers have been reported with mobilities above 1 cm 2 V À1 s À1 for electrons and holes. [19][20][21][22] In order to obtain balanced hole and electron charge carrier mobilities, the energy levels of D-A copolymers need to be nely controlled by adjusting donating and accepting groups in the conjugated backbone. However, it might be challenging for some polymers to attain balanced ambipolar transport by only two structural factors.
To circumvent this problem, one approach is to build polymers with two different acceptor units. Therefore, dual-acceptor architectures have been proposed as D-A1-D-A2 copolymers. 23 This dual-acceptor architecture provides a higher freedom in tuning the energy levels due to an enhanced degree of structural combinations. Thereby, oen stronger acceptors as BBT or TQ possess high electron deciency providing well balanced ambipolar characteristics via combination with weaker acceptors as DPP. 17,24 The FET performances of conjugated polymers are also dependent on their side chains. The side chain geometries have crucial inuence on molecular solubility, packing and thin-lm organization, and hence on device performance. 25,26 For example, the branched alkyl chains can provide better solubility and device performance compared to linear alkyl chains in conjugated polymers. 27 The branching positions of alkyl chains were also conrmed to reduce the molecular packing distance and enhance device performance. 28 Therefore, it could be useful strategy to develop well-balanced ambipolar OFETs by using dual-acceptor architecture and tuning branched positions of side chains in conjugated polymers.
In this work, we describe two dual-acceptor based polymers P1 and P2 (Scheme 1) which are composed of either NDI and benzothiadiazole (BT) or NDI and diuorinated BT (FBT). A pair of 3-decyl-pentadecyl alkyl chains were attached to the NDI unit. They are expected to ensure molecular solubility, strengthen molecular self-organization as well as improve charge carrier transport compared to conventional 2-decyl-tetradecyl substituted NDI. 19,29 Both polymers reported here, P1 and P2, were compared to each other and to the literature reports regarding their optical and electrochemical properties, selforganization in thin lms and charge carrier transport in FETs.

Synthesis and characterization
The synthetic route of P1 and P2 is shown in Scheme 1. The synthetic details of monomer M1 is depicted in the ESI. † 30 Monomers M2 and M3 were prepared according to the literature. 31,32 Stille coupling reaction was applied to connect M1 with M2 or M3 to obtain the dual-acceptor copolymers. To avoid intermolecular aggregation and obtain reliable molecular weights, high temperature (135 C) gel permeation chromatography (GPC) was employed using polystyrene as standard and 1,2,4-trichlorobenzene as eluent. The number-average molecular weights (M n ) of 36.8 kg mol À1 for P1 and 44.0 kg mol À1 for P2 were determined with polydispersity indexes (PDI) of 2.5 and 2.8 for P1 and P2, respectively. Their PDI are much smaller than those of polymers with the same conjugated backbones, but different side chains. 19 Due to the long branched side chains, both polymers show excellent solubility in chloroform, toluene and chlorobenzene at room temperature (>10 mg mL À1 ). Additionally, an excellent thermal stability was found up to 455 C with 5% weight loss as shown in Fig. S1 † implying that the uorination does not affect their thermal stability.

Optical and electrochemical properties
UV-vis-NIR absorption spectra of the polymers were recorded in diluted chloroform solution (10 À6 M) as well as in thin lm (Fig. 1). In diluted chloroform solution, the absorption spectra proles of both polymers exhibit three bands, which are similar to other dual-acceptor based polymers. 24 The region of 300-400 nm and 400-550 nm should origin from p-p* transitions of NDI and BT (or FBT) units, respectively. 8,33,34 The vis-NIR region (550-850 nm) is attributed to intramolecular charge transfer (ICT) between the NDI and T-BT (or T-FBT) in the polymer backbone. The uorinated polymer P2 has a signicant blue-shi of l max (39 nm) compared to P1, suggesting that the ICT effect of P2 became weaker aer the introduction of F atoms into BT unit. This behavior can be well explained by DFT calculations which are discussed below (See 2.3). The lms for UV-vis-NIR absorption measurements were prepared by drop-casting onto glass slides from chloroform solution. The polymer thin lms displayed very similar absorption bands compared with those in diluted solutions. The optical bandgaps are 1.50 and 1.62 eV, derived from the absorption onset of the solid lms for P1 and P2, respectively.
The electrochemical properties of both polymers were determined using cyclic voltammetry (CV) from their drop-cast thin lms (Fig. 2). The electron affinities (EAs) and ionization potentials (IPs) of the polymers were calculated from the onset of rst reduction and oxidation potentials. 35 The values of EA are 3.95 and 4.05 eV for P1 and P2, respectively, while the corresponding IP values are 5.40 and 5.67 eV. The electrochemical bandgaps of both polymers were thus deduced to be 1.45 and 1.62 eV for P1 and P2, respectively. The uorinated polymer P2 thus has a larger electrochemical band gap, which is due to weaker donating effect of the T-FBT unit (lowered HOMO).

Quantum chemistry calculations
The density functional theory (DFT) calculations were employed to better understand molecular energy levels of the monomers Scheme 1 Synthetic route for polymers P1 and P2.  and polymers (Fig. 3). 36 The alkyl chains were replaced by methyl substituents during calculation. The LUMO values are 3.41 eV for the monomeric subunits of P1 and P2, respectively, while the corresponding HOMO values are À5.46 and À5.61 eV. Interestingly, the weaker acceptors T-BT and T-FBT exhibit electron-donating nature in dimeric subunits of P1 and P2 (see Fig. S2 †), respectively. Therefore, the LUMO energy levels of both polymers are determined by the strong acceptor NDI part, and their HOMO energy levels are contributed by T-BT and T-FBT moieties, respectively. The T-FBT has lower HOMO energy levels compared to T-BT. The energy level differences between NDI and T-FBT units are smaller than those of NDI and T-BT leading to a weaker ICT in NDI and T-FBT than that of NDI and T-BT. That is the reason why the monomeric subunits of P2 (2.20 eV) have a larger band gap than that of P1 (2.05 eV). The results from calculations are well consistent with the observations from optical absorption and CV conrming that the uorinated polymer P2 has a signicantly blue-shi and a larger bandgap compared to P1.

Self-organization
The lm microstructure of spin-coated P1 and P2 was investigated by tapping-mode atomic force microscopy (AFM) (Fig. 4) to nd a correlation between supramolecular organization and lm morphology. The 50 nm thick lms were annealed at 120 C to remove residual solvents. The lm of P1 shows a brous microstructure with relatively large void areas and distinct grain boundaries between crystalline domains (Fig. S4a †). A surface area (SA) of 17.5 mm 2 is determined for this lm. This value describes the total area that the grains occupy in the lm.
Annealing at 200 C slightly improves the microstructure leading to an interconnection between domains and increases SA to 20.2 mm 2 . The root-mean-square roughness (R ms ) of the surface remains at the same level of 3.0 nm (Fig. 4a). Further annealing of P1 at 300 C (Fig. S4b †) increases the roughness up to R ms ¼ 4.0 nm, whereby the size of domains stays almost unchanged (19.6 mm 2 ) in comparison to annealing at 200 C. Polymer P2 exhibited a less textured lm topography with signicantly reduced grain boundaries with respect to P1. The lm of P2 annealed at 120 C (Fig. S4c †) contains grains built from smaller bers as expressed by a lower SA value of 15.1 mm 2 and a lower roughness of R ms ¼ 1.5 nm compared to P1. Additional annealing at 200 C (Fig. 4b) and 300 C (Fig. S4d †) of P2 improves the interconnection between separated domains and slightly increases the surface area to 17.7 mm 2 , while the roughness persists on the same level. In summary of the AFM studies, only slight differences in surface morphology between P1 and P2 were found.
Grazing incidence wide-angle X-ray scattering (GIWAXS) of the spin-coated lms was performed in order to investigate the effects of the F-containing units on the organization of the dualacceptor polymers. Aer annealing at 120 C, P1 reveals an outof-plane 100 reection at q z ¼ 0.24Å À1 and q xy ¼ 0Å À1 that corresponds to an interlayer distance of 2.61 nm of polymer chains indicating that P1 is preferentially arranged edge-on on the substrate (Fig. S3a †). An additional in-plane reection at q z ¼ 0Å À1 and q xy ¼ 0.347Å À1 is related to the monomeric unit length of the polymer backbone of 1.81 nm. This value is in agreement with model calculations. The increase in annealing temperature to 200 C does not affect the position of the reections, but slightly improves the long-range organization as conrmed by the appearance of higher order interlayer reections (Fig. 4c). On the other hand, the higher temperature of 300 C reduced the crystallinity again (Fig. S3b †). It has to be emphasized that P1 did not show any reections corresponding to the p-stacking indicating poor intralayer molecular order. The GIWAXS pattern of P2 annealed at 120 C exhibited similar structures as P1 (Fig. S3c †). An interlayer distance of 2.69 nm was found for P2, whereby two distinct surface orientations of the backbone with face-and edge-on are evident from the inplane and out-of-plane positions of the corresponding reections. Annealing at 200 C and specially at 300 C improved (Fig. S3d †) the organization of the polymer as conrmed by the appearance of higher order reections. The in-plane reection at q xy ¼ 0.347Å À1 and q z ¼ 0Å À1 is related to the d-spacing of  1.81 nm which corresponds to the theoretical calculated monomer length. Summarizing, GIWAXS results indicated that both polymers have a similar organization in thin lms.

OFET properties
To nd the relation between chemical polymer structure (BT and F-containing BT units) and charge carrier transport, P1 and P2 were employed as semiconducting thin lms in organic eldeffect transistors (OFET). The transistor characterization was performed aer thermal annealing at 120 C (to ensure evaporation of residual solvent), 200 C and 300 C. Highly doped silicon wafers were used as gate electrode, while the 300 nm thick thermally grown silicon oxide was surface modied by octadecyltrichlorosilane (OTS) to be exploited as the dielectric layer. Both polymers exhibit an ambipolar eld-effect in a bottom-gate, top-contact conguration with gold electrodes. The best device parameters with the highest charge carrier mobility and lowest threshold voltages were observed for thin lms thermally annealed at 200 C (Table 1). Fig. 5 shows representative transfer and output characteristics for P1 (Fig. 5a and c) and P2 (Fig. 5b and d) aer annealing at 200 C. Polymer P1 exhibited a balanced ambipolar charge transport with mobilities of 0.073 cm 2 V À1 s À1 and 0.086 cm 2 V À1 s À1 for holes and electrons, respectively. These values are more balanced compared to previous work that hole and electron mobilities are 0.1 cm 2 V À1 s À1 and 0.05 cm 2 V À1 s À1 , respectively, in the same device conguration, 29 and less than 10 À3 cm 2 V À1 s À1 and 0.19 cm 2 V À1 s À1 , respectively, in bottomgate bottom-contact device conguration. 19 The ambipolar transport of P2 provided less balanced transport with an electron mobility of 0.258 cm 2 V À1 s À1 and with a low hole transport of only 2.4 Â 10 À3 cm 2 V À1 s À1 . However, the same conjugated backbone of P2 was previously reported that the polymer only exhibits an electron molibility of 0.15 cm 2 V À1 s À1 in bottomgate bottom-contact device conguration. 19 The decline in threshold voltage for electrons of P1 and P2 from 35 V to 6 V upon annealing at 200 C is related to the enhancement of the polymer organization and lm morphology described in the AFM and GIWAXS parts. Interestingly, aer annealing at 200 C, we have found for P1 a balanced ambipolar mobility with 10 À2 cm 2 V À1 s À1 for both types of carriers, while the electron mobility for P1 is one and for P2 two orders of magnitude higher in comparison to literature values for devices of the same structure. 19,29 The electron mobility is higher in P2 in comparison to P1 what is attributed to the additional F atoms at the BT unit. The lower hole transport in P2 is assigned to the two strong acceptors lowering the HOMO of the polymer.
In top-gate bottom-contact device architectures, P1 and P2 backbones substituted by 2-decyl-tetradecyl side chains showed superior electron mobilities of 3.1 cm 2 V À1 s À1 . 19 The reason for this improved electron transport is the polymer dielectric (PMMA) reducing the interfacial trapping in comparison to the inorganic SiO 2 gate isolator. 37 The difference in performance between P1 and P2 in our work can be explained as following. The metal/semiconductor interface is described as a Mott-Schottky barrier determined by the difference between the work function of the metal electrodes and the semiconductor HOMO or LUMO levels. When the work function is close to the HOMO or LUMO level of the semiconductor an ohmic contact is expected. Otherwise, the potential barrier signicantly reduces the injection of the charges resulting in a change from a wellbalanced to an imbalanced or even unipolar operation mode of the transistor. Optimization of the electrode work function towards the polymer energy levels is expected to favor a balanced electron and hole transport with mobilities above 1 cm 2 V À1 s À1 . 38 As it was mentioned, the introduction of uorine does not alter the LUMO level resulting in typical ohmic behavior. Aer improvement of the lm microstructure by annealing, the threshold voltage for electron transport is reduced for both polymers down to 0 V. In literature, the lack of hole transport was mainly attributed to the mismatch of the HOMO level and Au work function. 19 However, in our case the difference between work function of the Au electrodes (À5.1 eV) and IP of P1 (5.40 eV) is smaller than to the metal work function and IP of P2 (5.67 eV). In contrast, the difference between work function of the Au electrodes and EA of P1 (3.95 eV) is larger  This journal is © The Royal Society of Chemistry 2018 than to the metal work function and EA of P2 (4.05 eV) suggesting a more facile electron injection into P2. However, at the same time the hole injection would become more difficult into P2 in comparison to P1. This is the reason for the higher electron, but lower hole mobility of P2 than of P1.

Conclusions
In summary, we have presented two copolymers based on NDI and BT with a pair of 3-decyl-pentadecyl alkyl chains attached to the NDI unit. The introduction of uorine atoms in the BT moiety plays a signicant role on the optoelectronic and electrochemical properties. Compared to P1 without F atoms, the l max of P2 is blue-shied, and the IP energy level of P2 is decreased more than their EA energy level. The introduction of 3-decyl-pentadecyl alkyl chains into NDI unit has an important inuence on the device performance. Compared to the same conjugated backbone polymers, the FET device of P1 exhibits a well-balanced ambipolar charge carrier transport with hole and electron mobilities of 0.073 cm 2 V À1 s À1 and 0.086 cm 2 V À1 s À1 , respectively. However, aer introduction of F atoms into the BT unit, P2 shows a less balanced transport with an electron mobility of 0.258 cm 2 V À1 s À1 and with a poor hole transport of 2.4 Â 10 À3 c cm 2 V À1 s À1 . The understanding the inuence of uorination and alkylation of NDI-BT copolymers is benecial to further design dual-acceptor polymers towards well-balanced high-performance ambipolar transistors.

Conflicts of interest
There are no conicts to declare.