Polarity switching from p- to n-type in a single thermoelectric donor-acceptor copolymer by p-type doping

The transport mechanism of organic materials is still far away from being well understood and controlled although conducting polymers have been discovered since 1977. It is rare to see conducting polyers possessing high bipolar (p- and n-type) electrical conductivities within a single bulk doped organic polymer without the assistant of gate voltage. Here, we report a novel approach to provide high performance n-type materials by p-type doping. More importantly, the bipolar electrical conductivities of the donor-acceptor conducting polymer are high, resulting high bipolar power factors among the solution-processable ambipolar D-A copolymers. A fully organic p-n junction is created in a planar film, exhibiting a high rectification ratio of 2 x 10 2 at  5 V with a high current density of 3 A/cm 2 . Structural and spectroscopic tests have been performed to provide a fundamental understanding of the polarity switching mechanism. The results open the opportunity of making p- and n-type modules with a single conducting polymer for future modern organic electronics. that is possible to make single material with p- and n-type modules for in the


Introduction
Conducting polymers have the potential advantages of lower manufacturing cost by using traditional printing techniques to produce electronic modules at a large scale, 1 which makes them very attractive for innovative electronic applications, especially for flexible electronic devices. 2,3 Large progress has been achieved in developing functional conducting polymers in many fields such as organic thermoelectrics, 1, 2, 4 organic solar cells, 1 organic field-effect transistors 5 and organic light-emitting diodes, 6 etc since the conducting polymer was discovered for the first time by Heeger, MacDiarmid, and Shirakawa in 1977. 7 However, the transport of holes and electrons in conducting polymers is still far away from being well understood and controlled.
Making p-and n-type modules with a single conducting polymer is still very challenging for organic materials although it can be easily realized for inorganics such as doping silicon with phosphorus and boron to make n-type and p-type semiconductors, respectively. For modern organic electronics, ptype and n-type materials are equally important and desired. Making p-and ntype modules with a single conducting polymer would simplify the fabrication processes of organic devices with large scalability and low cost by combining with traditional printing techniques to pattern a single conducting polymer with p-and n-type dopants. Besides, it may improve the performance of organic devices. For example, Roncali suggested that single material solar cells would be the next frontier for organic photovoltaics since they would have a longer lifetime due to the strong stabilization of the morphology of the interface. 8,9 Toffanin and co-workers reported that single-layer light-emitting transistors might ensure good charge transport together with an efficient light-emission in the solid-state. 10 Wang and Yu et. al. reported that a novel organic Schottky barrier diode created in a single planar polymer film exhibited a remarkable current density of 30 A/cm 2 that is 2-3 orders in magnitude higher superior to that of previously reported organic materials. 6 Because it excluded interfaces that generally exist between p-and n-type modules, resulting in smooth current flow and subsequently improving the performance of the devices. These promising results of single material devices light the enthusiasm in making pand n-type modules with a single conducting polymer.
However, not many conducting polymers can transport both holes and electrons. In general, the carrier polarity of a conducting polymer strongly depends on its molecular structure. Once a conducting polymer is synthesized, it may prefer to transport either electrons or holes. The majority of the conducting polymers exhibit unipolar transport property, which can only transport either holes or electrons. In contrast, ambipolar conducting polymers exhibiting both positive and negative Seebeck coefficient were reported in very limited cases. 4,11,12,13 Especially, it is very challenging to achieve conducting polymers with high bipolar electrical conductivities (possessing high electrical conductivity for both p-and n-type).
Currently, the popular strategy to make ambipolar conducting polymer is to co-polymerize electron-rich groups (donors, D) and electron-deficient groups (acceptors, A). 14,15,16 However, only a small portion of the donor-acceptor copolymers successfully exhibited bipolar transport properties. 17,18,19 Although D-A copolymers have been reported to have very high motilities over 20 cm 2 /(V·s) recently, 19,20 the electrical conductivity for the D-A copolymers are often in the range of 10 -3 -10 -5 S/cm. 21,22 These ambipolar conducting polymers with low electrical conductivities are mainly used in organic field-effect transistors (FETs) or organic solar cells. Moreover, most of these ambipolar conducting polymers exhibit bipolar electrical conductivities in FET devices that require the assistant of a gate voltage. 23 Very rare examples were reported to have bipolar electrical conductivities for bulk-doped conducting polymers without the assistant of the gate voltage. 19,24 To the best of our knowledge, bipolar electrical conductivities over 10 -2 S/cm have never been reported for D-A copolymers up to date.
Here, we report that a solution-processable D-A copolymer poly (2,5-bis(2octyldodecyl)-3,6-di(thiophen-2-yl)diketopyrrolo [3,4-c]pyrrole-1,4-dione-altthieno[3,2-b]thiophen) (DPPTTT, shown in Figure 1) can have high bipolar electrical conductivities after being doped by FeCl3 (DPPTTTFeCl3). It exhibits the highest p-type electrical conductivity of 130.6 S/cm and a high n-type electrical conductivity of 14.2 S/cm superior to that of most of the solutionprocessable D-A copolymers (Table S1 and S2). The high electrical conductivity leads to a high p-type thermoelectric power factor of 23.4 μW/(mK 2 ) as well as a high n-type thermoelectric power factor of 0.66 μW/(mK 2 ), which are all the highest for p-and n-type solution-processable ambipolar D-A copolymers, respectively ( Figure S7). The conversion mechanism was addressed after performing structural and spectroscopic tests. A p-n junction is created in a planar thin film, exhibiting a high rectification ratio of 2 x 10 2 at 5 V for fully printed organic diodes, which further demonstrates the conversion of the p-type D-A copolymer to n-type. The rectification performance meets the requirement for high-frequency radio-frequency-identification (R-ID) tags. 25,26 These results may open the opportunity for developing new organic electronic devices with a single organic material. DPPTTTFeCl3-84wt%, respectively. The larger activation energy should be due to the larger gaps between the agglomerates, leading to lower electrical conductivity of DPPTTTFeCl3-84wt%. Figure 2b shows the Seebeck coefficient of DPPTTTFeCl3 as a function of the FeCl3 concentration. It is interesting to notice that the Seebeck coefficient of DPPTTTFeCl3 switching from p-to n-type. Pristine DPPTTT is a p-type material according to previous works 28,31,32 . The Seebeck coefficient of DPPTTTFeCl3-10wt% is +223 V/K, which indicates that it is still p-type at low FeCl3 concentration. The Seebeck coefficient decreases while more FeCl3 is added, which is due to the increase of carrier concentration at a higher p-type doping level. However, the Seebeck coefficient of DPPTTTFeCl3 becomes negative after the FeCl3 concentration is over 47 wt.%. Further increasing the FeCl3 concentration leads to more negative of the Seebeck coefficient. The negative Seebeck coefficient of DPPTTTFeCl3 reveals that it becomes an n-type material.

Results and discussion
The maximum n-type electrical conductivity and the maximum Seebeck coefficient are 14.2 S/cm and -78.9 V/K for DPPTTTFeCl3-47wt% and DPPTTTFeCl3-84wt%, respectively.
The maximum p-and n-type electrical conductivities of DPPTTTFeCl3 are compared with those of previously reported solution-processable D-A copolymers as shown in Table S1 and Table S2. Very rare examples were reported to have bipolar electrical conductivities for bulk-doped conducting polymers without the assistant of the gate voltage. 19,24 High electrical conductivities over 2 S/cm for both p-and n-type in a single conducting polymer have never been reported in previous literature. Figure 2c (Table S1). The maximum n-type electrical conductivity of 14.2 S/cm for DPPTTTFeCl3 is one of the best n-type electrical conductivities since only one example was reported previously to have higher n-type electrical conductivities over 14.2 S/cm (Table S2)  Ultra-violet-visible near-infrared spectrophotometer (UV-vis-NIR) was performed to monitor the doping process of DPPTTT and P3HT. Figure 3a shows the UV-vis-NIR spectra of DPPTTTFeCl3 films at different FeCl3 concentration. A major peak appears at 810 nm for pristine DPPTTT film, which is assigned to the -* transition of the DPP unit. 33,34,35 The major peak decreases together with the increase of a new broad peak at the near-infrared region when the FeCl3 concentration increases. The decrease of the major peak should be due to the reduction of the neutral state of DPP units, which are converted into polaron state or bipolaron state 36 that result in the increase of the new peak at the near-infrared region. Figure 3d shows a clear conversion process of P3HT between the neutral state, the polaron state, and the bipolaron state. The neutral state of P3HT can be converted to the polaron state, which may be further converted to bipolaron state 37,38 . At low FeCl3 concentration, the neutral state P3HT was converted into the polaron state P3HT and bipolaron state P3HT, leading to the decrease of the neutral state peak at 532 nm, the increase of the polaron state peak at 832 nm, and the bipolaron state peak in the near-infrared region. At high FeCl3 concentration, the decrease of polaron state peak at 832 nm indicates that the speed of polaron state P3HT converted to the bipolaron state is higher than that of polaron state P3HT generated. The band gaps obtained from the UV-vis-NIR spectra of pristine DPPTTT and P3HT were 1.28 eV and 1.9 eV ( Figure S4), respectively, which are consistent with previously reported works 39,40,41,42,43,44 .
The highest occupied molecular orbitals (HOMOs) of DPPTTT and P3HT were estimated by using the cyclic voltammetry (CV) method with a threeelectrode electrochemical system. Ferrocene was used as the internal reference (see detailed description in the Supporting information). Figure 3b and 3e show that the oxidation peak edges (ox) are 0.97 V and 0.94 V for DPPTTT and P3HT, respectively. Therefore, the HOMOs ( ) are -5.19 eV and -5.16 eV for DPPTTT and P3HT, respectively, according to the following equation 39 : where 1/2 is the average of the oxidation peak potential and the reduction peak of the internal reference, ferrocene (shown in Figure S5). The where E f and E cutoff are respectively high and low kinetic energy cutoff, and ℎ Based on these results, we believe that the polarity switching of DPPTTT films from p-to n-type is due to the crossing of Fermi level (E F ) from above the HOMO (valence band, E v ) to below the HOMO. The hopping model reported by Fritzsche was typically used to describe the charge carrier transport in conducting polymers. 54 For a p-type material, the Seebeck coefficient is described as: where k B , q, and T are the Boltzmann constant, the electron charge, and the absolute temperature. For a neutral conducting polymer, its E F localizes at the middle of LUMO and HOMO. As shown in Figure 3g and  switching from n-to p-type when doped by n-DMBI, poly(pyridinium phenylene) switching from n-to p-type when highly reduced by an organic sodium salt 54 . In contrast, no polarity switching was observed for P3HTFeCl3 under the same conditions. To the best of our knowledge, it is the first time to report that D-A copolymer switches from p-to n-type while doping by a p-type dopant. More importantly, DPPTTTTFeCl3 exhibits high p-type electrical conductivity of 130.6 S/cm as well as a high n-type electrical conductivity of 14.2 S/cm. The high bipolar electrical conductivities result in great p-and n-type power factors of DPPTTTTFeCl3 among the ambipolar D-A copolymers as shown in Figure S7.
These results indicate that it is very possible to make single material organic devices with p-and n-type modules for modern organic electronics in the future as what has been realized in inorganic materials.  Figure S8.
Based on the XPS results, the illustration of the molecular structure of doped DPPTTT is given in Figure 4d. The oxidation of DPPTTT will also lead to the color change of the films. As shown in Figure 4c, the color change from purple to gray was observed while increasing the concentration of FeCl3. For the P3HT film, the color is dark gold, which changes gradually to dark grey as shown in Figure S9. A free-standing DPPTTTFeCl3-84wt.% film was achieved by peeling off the thick film from a glass substrate. Figure 4b shows that the obtained film has good flexibility, which demonstrates the potential application of this material is flexible organic electronics.  Figure S10.
However, no n-type Seebeck coefficient was obtained. The reason should be due to that no enough 4 − was generated inside of the DPPTTT film since not many FeCl3 can penetrate inside the DPPTTT films.
To identify the dominant carriers in the n-type DPPTTTFeCl3-84wt.%, the frequency-dependent (1-10 6 Hz) AC impedance was performed to distinguish electronic and ionic current. 4,6 The n-type sample DPPTTTFeCl3-84wt.% exhibited a deviation from a very high frequency ( ~ 10 6 Hz, Figure 5c), which is even higher than the deviation frequency of pristine DPPTTT (10 3 Hz). Typically, the deviation frequency for ionic conduction is about 0.3 Hz. The results suggest that electronic transport is dominant in the n-type material other than the ionic effect. 6 Figure 5d shows a p-n junction made of p-type DPPTTTFeCl3-21wt.% and ntype DPPTTTFeCl3-84wt.%. The sample was prepared by drop-casting p-type DPPTTTFeCl3-21wt.% on half of the glass substrate first, and then drop-casting ntype DPPTTTFeCl3-84wt.% on the other half of the glass substrate after the p-type film was dry (See Figure S11 in the supporting information). The current density was measured at room temperature as a function of the bias voltage in the range of -5 V-5 V with a scan rate of 0.5 V/s for DPPTTTFeCl3-84wt.%. The rectification was observed when a current was passed from the p-type side to the n-type side as shown in the inserted image and Figure 5d(1) (reverse bias).
The forward bias (Figure 5d(2)) current density of the organic diode is 3.0 A/cm 2 at 5 V. A high rectification ratio (current density at 5 V divided by that of -5 V) was obtained to be ~2 x 10 2 , which meets the requirement for high-frequency R-ID tags of rectification ratio > 100 at about 5 V. 25,26 The current-voltage relations of single p-type DPPTTTFeCl3-21wt.% and single n-type DPPTTTFeCl3-84wt.% were measured for comparison. No rectification was obtained as shown in Figure S12.
The responses of a transient and steady-state current of the organic diode were measured while alternating input voltage of 5 V. The reverse bias current was quickly recovered in less than 3 s to reach a steady-state by switching the bias polarity. The relaxation behavior should be due to the capacitive effects that may be derived from the high resistance of the films and dissociated ions by moisture. The quick response further indicates that this organic diode is electronic dominant since a typical ion-based organic diode is hard to maintain a steady current and requires a much longer time to reach a stationary current. 59,60 Kelvin probe techniques 6 were performed to identify the work function difference between the p-type DPPTTTFeCl3-21wt.% side and the n-type DPPTTTFeCl3-84wt.% side. Figure 5f shows the work function differences between the samples (WFsample) and the probe tip (WFtip), which is also called contact potential difference (CPD): WFsample = WFtip -CPD. Assuming the work function of the used tip is 5.1 eV, the work functions for the p-type DPPTTTFeCl3-21wt.% side and the n-type DPPTTTFeCl3-84wt.% are 5.14 eV and 5.39 eV, respectively.
The results match well with those values obtained by UPS as shown in Figure   3c, which further demonstrate the p-to n-type switching of DPPTTT by FeCl3 doping.

Conclusion
We have demonstrated that solution-processable D-A copolymer can exhibit both high p-and n-type electrical conductivities over 10 S/cm during the p-type doping process. The polarity switching from p-to n-type is suggested to These values are also among the top values of nowadays state-of-the-art solution-processable D-A copolymers including both unipolar and ambipolar D-A copolymers. A fully organic p-n junction was created with the p-and n-type material exhibiting a high rectification ratio of over 2 x 10 2 at 5 V with a high current density of 3 A/cm 2 , which meet the requirement for high-frequency R-ID tags. 25,26 The results open the opportunity of making p-and n-type modules with a single conducting polymer for future modern organic electronics.