•ARTICLES• August 2021 Vol.64 No.8: 1410–1416https://doi.org/10.1007/s11426-021-1037-3

Well-balanced ambipolar diketopyrrolopyrrole-based copolymersfor OFETs, inverters and frequency doublers

Jiaxin Yang1,2, Qingqing Liu1,2, Mengxiao Hu1, Shang Ding1, Jinyu Liu1, Yongshuai Wang1,2,Dan Liu1,2, Haikuo Gao1,2, Wenping Hu3 & Huanli Dong1,2*

1Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences,Beijing 100190, China;

2School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China;3Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Science, Tianjin University &

Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China

Received March 10, 2021; accepted May 31, 2021; published online June 8, 2021

Conjugated polymers with well-balanced ambipolar charge transport is essential for organic circuits at low cost and large areawith simplified fabrication techniques. Aiming at this point, herein, a novel asymmetric thiophene/pyridine-flanked diketo-pyrrolopyrrole-based copolymer (PPyTDPP–2FBT) is designed and synthesized. Due to the effect of incorporating F atoms onmolecular energy alignment and conjugation conformation, the PPyTDPP–2FBTcopolymer exhibits typical V-shaped ambipolarfield-effect transfer characteristics with well-balanced hole and electron mobilities of 0.64 and 0.46 cm2 V−1 s−1, respectively.Furthermore, organic digital and analog circuits such as inverters and frequency doublers are successfully constructed based onsolution-processed films of the PPyTDPP–2FBT copolymers which show a typical circuit operating mode with a high gain of133 due to the well-balanced electrical properties. In addition, PPyTDPP–2FBT-based devices also demonstrate good stabilityand batch repeatability, suggesting their great potential applications in organic integrated electronic circuits.

conjugated polymers, balanced ambipolar transport property, organic field effect transistor, inverter, frequency doubler

Citation: Yang J, Liu Q, Hu M, Ding S, Liu J, Wang Y, Liu D, Gao H, Hu W, Dong H. Well-balanced ambipolar diketopyrrolopyrrole-based copolymers forOFETs, inverters and frequency doublers. Sci China Chem, 2021, 64: 1410–1416, https://doi.org/10.1007/s11426-021-1037-3

1 Introduction

Since 1970s, conjugated polymers have drawn considerableattention over the world due to their unique advantages oflow cost and solution processability for potential applica-tions in wearable and flexible electronics [1–4]. Untill now,conjugated polymers with easily tuned optoelectronic prop-erties have shown great applications in various fields, in-cluding organic field-effect transistors (OFETs) [5–7],organic light-emitting diodes (OLEDs) [8–10] and organicphotovoltaic cells (OPVs) [11–15]. Particularly, OFETs as

electrical switches are the most significant component fororganic circuits and other related organic integrated (opto)electronic devices [16–25]. According to the difference ofthe majority charge carriers, OFETs can be classified intounipolar p-channel (hole-only) or n-channel (electron-only)and ambipolar (hole and electron) devices. In general, am-bipolar OFETs with balanced hole and electron mobilities arefavored in simplified fabrication techniques for large-areadevice applications [26,27]. Thus, from this point of view,the development of inherently ambipolar semiconductorswith balanced hole and electron mobilities is highly desirablein organic electronics.Currently, hundreds of high-mobility (>0.1 cm2 V−1 s−1,

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*Corresponding author (email: dhl522@iccas.ac.cn)

comparable to that of amorphous silicon semiconductors)unipolar p-channel or n-channel conjugated polymers havebeen extensively investigated, especially for the copolymersbased on a diketopyrrolopyrrole (DPP) core as an excellentelectron-deficient building block. In general, DPP-basedcopolymers exhibit efficient charge transport property thanksto good molecular planarity and intense intermolecular π–πinteractions [28,29]. Over the past years, significant ad-vances have been achieved for DPP-based p-channel and n-channel polymer semiconductors by rationally modulatingtheir functional units, conjugated structure and conforma-tion, the ratio of electron-withdrawing and donating motifs inthe polymer chain as well as molecular weight engineering[30–34]. However, the conjugated polymers with in-trinsically balanced ambipolar properties are limited, whichhinders the exploration of organic circuits. Moreover, mostof currently developed conjugated polymers suffer fromserious synthesis problems and difficulty in processing forlarge-area devices.In our previous study, we developed a novel asymmetric

thiophene/pyridine-flanked unit for DPP-based copolymers(PPyTDPP–BT, Scheme 1, top) with the consideration ofaltering the energy levels via the synergistic effect of thio-phene/pyridine in the conjugated unit [35]. As expected, theambipolar charge transport property in OFETs was achievedfor the synthesized PPyTDPP–BT polymers which couldalso be used as donor or acceptor materials in polymer solarcells for high power conversion efficiencies. Such asym-metric flanked unit design strategy was further successfullyapplied for other DPP-based copolymers for optoelectronicdevices [36,37]. From our previous experience, theseasymmetric DPP-based polymers possess features of easysynthesis, and good film-forming characteristic with weakdependence on experimental conditions and the surroundingenvironment, which is very crucial for large-area deviceapplications. However, the balance of ambipolar chargetransport with relatively lower electron mobility should be

further improved if they would be used for organic circuits.With this in mind, herein, we incorporated two fluorineatoms into the thiophene units of PPyTDPP–BT-conjugatedbackbones and synthesized a new copolymer, PPyTDPP–2FBT (Scheme 1, bottom). As designed, the lowest un-occupied molecular orbital (LUMO) energy level of thePPyTDPP–2FBT was decreased due to the strong electron-withdrawing effect of fluorine atoms. Additionally, thePPyTDPP–2FBT exhibited nearly coplanar conjugationowing to the effect of F···S conformation lock, which isbeneficial for efficient charge transport even for improvedelectron carrier transport property [38–40]. OFET devicesfabricated based on the PPyTDPP–2FBT films exhibit well-balanced V-shaped ambipolar field-effect transfer char-acteristics with hole and electron mobilities of 0.64 and0.46 cm2 V−1 s−1, respectively. It is found that the devicescould still work after one year without special package,suggesting its good stability. Due to the well-balanced am-bipolar property of the PPyTDPP–2FBT, we further exploredits application in organic digital and analog circuits such asinverters and frequency doublers, which demonstrated well-defined device operation characters with high performances(a high gain of 133 and a good frequency operating char-acteristic). It should be stated that it is the first demonstrationof single-component ambipolar polymer-based frequencydoublers, which is equally important with logic circuits,especially in the logic gate NOT [41,42]. Additionally, theanalog circuit also shows great potential in radio astronomyand terahertz sensing [43]. The well-balanced ambipolarcharge transport property of the PPyTDPP–2FBT in com-bination of superior film-forming feature and good stabilityand batch repeatability suggests its great potential for organicintegrated circuits after further performance improvements.

2 Results and discussion

2.1 Polymer synthesis

The asymmetric DPP monomer and polymer were obtainedthrough the modification of a reported synthetic route [35].The polymer PPyTDPP–2FBT was synthesized via Stillepolymerization, which was further dried in vacuum. Thedesired polymer was obtained 95% yield; GPC (1,2-di-chlorobenzene, 140 °C):Mn=69 kDa,Mw=141 kDa, polymerdispersity index (PDI)=2.03; anal. calcd. for C63H91F2N3O2S3,C, 71.61; H, 8.68; N, 3.98, found: C, 72.37; H, 9.01; N, 3.63.More detailed synthetic procedures and characterizations ofthe monomers and polymer, as well as computational details,are described in the Supporting Information.

2.2 Photophysical properties and theoretical calculation

The UV-vis absorption spectra of the polymer PPyTDPP–Scheme 1 Molecular design of PPyTDPP–2FBT with fluorine atoms atthe thiophene units for improved ambipolar balance (color online).

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2FBT in the thin film and solution are shown in Figure 1a.The copolymer PPyTDPP–2FBT exhibits typical dual-bandabsorption. The lower absorption band (band I: 350–550 nm)can be attributed to the internal charge transfer (ICT) tran-sitions, and the higher absorption band (band II:550–850 nm) is derived from the π–π transition [36,44].Compared to the solution spectrum, the maximum absorp-tion peak (λmax) in the solid state was red-shifted by ap-proximately 7 nm, which demonstrates the effectiveformation of π–π stacking in the PPyTDPP–2FBT, probablydue to its intrinsically good coplanarity. Besides, the opticalbandgap of PPyTDPP–2FBT calculated from the absorptiononset is 1.51 eV, which is slightly less than that ofPPyTDPP–BT. The highest occupied molecular orbital(HOMO) energy level for spin-casted PPyTDPP–2FBT thinfilms determined by ultraviolet photoelectron spectroscopy(UPS) is −5.26 eV (Figure S1). The LUMO energy level is−3.75 eV indirectly obtained utilizing the optical bandgapand HOMO level. The LUMO level of PPyTDPP–2FBT iseffectively lowered due to the introduction of fluorine atoms(−3.60 eV of PPyTDPP–BT), which is consistent with ouroriginal assumption. The narrowed band gap and appropriateenergy levels indicate that the PPyTDPP–2FBT copolymerwould have better balanced hole and electron transportproperty than PPyTDPP–BT under the same symmetric de-vice condition with Au as the electrodes. Meanwhile, somebasic properties of the PPyTDPP–2FBT, including its con-jugated skeletal structure and energy distributions, are un-derstood with the assistance of density functional theory(DFT) calculation. We conducted theoretical optimizationbased on three oligomer fragments of the PPyTDPP–2FBT.Electron cloud distributions of HOMOs and LUMOs of thetwo copolymers were obtained by theoretical calculation(Figure 1b and Figure S2a), where the HOMO and LUMOlevels of PPyTDPP–2FBT are −4.94 and −3.12 eV, respec-tively. We can find that the bandgap obtained by the ex-

periment agrees well with the theoretical calculation result.Therefore, the bandgap of the PPyTDPP–2FBT can be ef-fectively narrowed after the introduction of fluorine atoms.The planarity of a conjugated polymer skeleton makes

quite a difference in the charge transport. By calculating andanalyzing the dihedral angles of two polymers of thePPyTDPP–BT and PPyTDPP–2FBT, we found that the tor-sion angle between the repeated fragments of PPyTDPP–2FBT (1.43°) with the addition of F atoms was much smallerthan that of PPyTDPP–BT (8.44°) (Figure 1c and FigureS2b). The good coplanarity of the PPyTDPP–2FBTmay leadto a better mobility characteristic. The thermogravimetricanalysis (TGA) of the PPyTDPP–2FBT shows its decom-position temperature is around 400 °C (Figure S3). Byanalyzing these experimental and theoretical results, it isclear to see that the effects brought by fluorine atoms includelowering the LUMO energy level and enhancing the copla-narity of the conjugated backbone. As expected, the in-troduction of the F atoms significantly regulates theconjugated structure and energy alignment, making it pos-sible that the PPyTDPP–2FBT may exhibit well-balancedambipolar transporting properties.

2.3 Film morphology

To analyze thin-film morphology of the PPyTDPP–2FBT,atomic force microscopy (AFM) images are depicted inFigure 2a. The PPyTDPP–2FBT films showed smooth sur-faces with root-mean-square (RMS) roughness value below1 nm, ensuring close contact between the active layer and thedielectric layer. In order to powerfully reveal the connectionbetween the molecular packing/crystallinity and mobilities,grazing incidence wide-angle X-ray scattering (GIWAXS)techniques were adopted to characterize the polymericPPyTDPP–2FBT films. There are obvious (010) π–π stack-ing peaks in both out-of-plane and in-plane direction, in-dicating a bimodal distribution of face-on and edge-onorientation (Figure 2b). Besides, the copolymer PPyTDPP–2FBTexhibits a more pronounced (010) diffraction pattern inout-of-plane direction than that of in-plane pattern, which isactually not beneficial for carriers in the planar channels tobring about efficient two-dimensional charge transport. ThePPyTDPP–2FBT films show primary peaks locating atd=23.89 Å, 11.38 Å, and 7.93 Å in the out-of-plane direction,respectively, assigned to the diffractions of the lamellarpacking repeat (100), (200) and (300) planes, suggestingrelatively well-ordered and crystalline in the thin film(Figure 2c). Furthermore, in out-of-plane profiles thePPyTDPP–2FBT films demonstrate a pronounced (010) π–πstacking peak at d=3.59 Å. Especially, in-plane (010) π–πpeak was observed in the PPyTDPP-2FBT films, from whichπ–π stacking distance calculated is d=3.59 Å as well (Figure 2d).The crystal coherence length (Lc) is a significant parameter

Figure 1 (a) UV-vis absorption spectra of PPyTDPP–2FBT in solutionand thin films. (b) LUMO and HOMO diagrams of trimer of thePPyTDPP–2FBT repeat units calculated by DFT. (c) Theoretical calcula-tions of dihedral angles and backbone coplanarity for the PPyTDPP–2FBT(color online).

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quantifying the polymer crystalline order range, which iscalculated using the Scherrer equation [45]. The in-plane(010) coherence length for the PPyTDPP–2FBTwas 25.80 Å,clearly indicating the relatively high crystallinity ofPPyTDPP–2FBT polymer films. And other relevant valuesare also calculated and listed in Table S1. Combined with theanalysis of the morphology and structure of the film,PPyTDPP–2FBT films obtained by a simple spin-coatingprocess still show relatively well-ordered and highly crys-talline. In general, the PPyTDPP–2FBT chains in the filmsprepared after thermal annealing adopt a mixed face-on andedge-on orientation model, and the face-on stacking accountsfor a large proportion (Figure 2e). For the most commonplanar OFETs with high mobility, edge-on stacking is actu-ally more important, while face-on stacking is more favoredfor high-performance OPVs. In other words, there is stillspace to efficaciously improve the charge transport ofPPyTDPP–2FBT-based devices by further optimizing mo-lecular structures.

2.4 Charge transport properties

In order to characterize the charge carrier transport of thePPyTDPP–2FBT films, OFETs based on transparent andflexible polyethylene terephthalate (PET) substrates werefabricated with a top-gate bottom-contact (TGBC) deviceconfiguration (Figure 3a, b). The representative transfercurves of PPyTDPP–2FBT thin-film OFETs exhibit well-balanced V-shaped ambipolar field-effect transfer char-acteristics (Figure 3c). The ambipolar OFETs based on the

PPyTDPP–2FBT exhibit an averaged threshold voltage of−20 V for the p-channel and 31 V for the n-channel. Thehighest hole and electron mobilities of the PPyTDPP–2FBTcalculated from saturation regimes are 0.64 and0.46 cm2 V−1 s−1, respectively. As expected, the PPyTDPP–2FBT-based OFETs exhibited much higher electron chargecarrier mobility than PPyTDPP–BT-based devices due to theelectron-withdrawing effect of incorporated F atoms, thusleading to a well-balanced ambipolar transport property.Moreover, from the corresponding output curves ofPPyTDPP–2FBT-based thin-film OFETs, both p-channeland n-channel transfer behaviors are significantly modulatedby gate voltage with almost perfect symmetry thanks to theintrinsically ambipolar property of the PPyTDPP-2FBT(Figure 3d). The good film-forming feature of thePPyTDPP–2FBT ensures its ability for large-area devicearrays (Figure 3e, f), manifesting that the PPyTDPP–2FBTfilm prepared is uniform and flat. It should be stated thatPPyTDPP–2FBT-based OFETs also demonstrate good sta-bility as the ambipolar property was still obtained after one-year storage under ambient condition without special cap-sulation (Figure S4). The PPyTDPP–2FBT-based ambipolarOFET performances of both as-prepared and after one yearare summarized in Table S2, and the average electron mo-bility of OFETs stored for one year is 1.61×10−2 cm2 V−1 s−1.Additionally, the PPyTDPP–2FBT copolymer exhibits goodbatch stability, which is quite essential for actual deviceapplications (Figure 3g). Yet there is large space to furtherimprove charge transporting properties for the PPyTDPP–2FBT-based OFETs by optimizing molecular packing andcrystallinity in solid states. Although we do not employmeniscus-guided coating techniques to improve the qualityof the PPyTDPP–2FBT films, the edge on arrangement willbe significantly enhanced with nearly an order of magnitudeincrease in mobility according to existing literature reports[46,47].

2.5 Inverters and frequency doublers

Well-balanced ambipolar OFETs are of great significance torealize high-performance organic circuits and simulta-neously reduce the number of units, which is beneficial toefficaciously lower the power consumption and decrease thedevice size [48,49]. Inverters (logic gate NOT) are an im-portant and common application for ambipolar OFETs,which avoids patterning n-channel and p-channel semi-conducting materials into specific areas. Based on the well-balanced ambipolar property of the PPyTDPP–2FBT, wefurther explored its application in organic circuits. The in-verter circuits were constructed by PPyTDPP–2FBT-basedOFETs, which possesses excellent ambipolar charge trans-port property and balanced threshold voltage (Figure 4a).Each inverter circuit is composed of two adjacent OFETs

Figure 2 (a) AFM height images and (b) 2D–GIWAXS images of thePPyTDPP–2FBT films annealed at 150 °C. (c) Out-of-plane and (d) in-plane patterns for 2D–GIWAXS of the PPyTDPP–2FBT films. (e) Sche-matic of PPyTDPP–2FBT stacking in solid state (color online).

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with a common gate as the input voltage (VIN) and a commondrain as the output voltage (VOUT) (Figure S5). In contrast tothe conventional inverters based on unipolar semiconductingmaterials, the inverters consisting of ambipolar transistorscan work in both negative and positive supply biases (VDD).Considering the average threshold voltage of ambipolarOFETs, a VDD of ±60 V was selected to ensure that both then-channel and p-channel OFETs were operated in the sa-turation region. The typical transfer characteristic and noisemargin are shown for an inverter at VDD=60 V with channellength L=150 μm and channel width W=3,000 μm (Figure4b). The noise margin is calculated to be 54.4% by themaximum square fitted within the transfer curve and itsmirror. The gain value, which is defined as the slope of thevoltage transfer characteristics curve, is an important para-meter to evaluate the performance of an inverter. Therefore,gain values and leakage currents for inverters working atpositive or negative supply biases are given in Figure 4c. Foran ideal CMOS-like inverter consisting of equivalent n-channel and p-channel transistors, the ideal inversion voltage(VINS) is located at VDD/2 [50]. For VDD=−60 V, the maximumgain is up to 133 at VINS=−20 V, while under positive biaswith VDD=60 V, the highest gain is up to 127 at VINS=30 V.Both of them are among the highest values reported for in-verters based on single-component DPP-based ambipolarcopolymers (Table S3). High gain value (>100) and idealVINS at positive bias were obtained based on PPyTDPP-2FBT

inverters, owing to the high and well-balanced mobility.However, at VIN=0 or VIN=VDD, there is a certain currentleakage for ambipolar inverters. This is inherently caused bythe V-shaped transfer characteristic of ambipolar OFETs,possibly leading to large static power consumption. Thus,how to reduce power consumption, in terms of realizing low-voltage operation and decreasing leakage currents, is quiteimportant for practical applications in the future.Although there are many researches on organic digital

circuits, especially for inverters, frequency doublers whichhave fundamental analog circuit function are still mostlyreported based on inorganic semiconductors [51,52]. Be-sides, Jiang’s group [53] combined organic-inorganic het-erojunctions to construct high-performance ambipolarOFETs and frequency doublers. Herein, for the first time, wefabricated organic frequency doublers based on the single-component ambipolar polymer PPyTDPP–2FBT. Frequencydoublers, as the name suggests, can double or even multiplethe output signal of the input signal frequency. In the transfercharacteristic curve with the wide-range scanning of the gatevoltage, the ambipolar OFETs are turned on once in the n-channel and p-channel, respectively. This also means thatwell-balanced V-shaped ambipolar transfer characteristicsare of great significance to realize a good frequency doubler.The frequency doubler measurement circuit is constructed byconnecting an ambipolar OFET based on PPyTDPP–2FBT inseries with an off-chip load resistor (Figure 4d). The dynamic

Figure 3 (a) Schematic of PPyTDPP–2FBT-based film OFETs. (b) Photograph of flexible OFET arrays on a PET substrate. Typical (c) transfer and (d)output characteristics of ambipolar OFETs based on the PPyTDPP–2FBT. (e) n-channel and (f) p-channel mobility measured from 7×6 OFETs arrays. (g) n-channel and p-channel mobility statistics measured from the same batch material but fabricated several batches of devices (color online).

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input signal applied to the gate electrode was a sinusoidalsignal VAC. The well-balanced V-shaped transfer character-istics of frequency doubler devices have good repeatabilityafter 5 cycles (Figure 4e). PPyTDPP-2FBT-based frequencydoublers show excellent performance without identifieddistortion (Figure 4f). It is the first demonstration of a single-component ambipolar polymer-based frequency doubler,which hopefully promotes frequency modulation with analogsignal processing in flexible and wearable electronics.

3 Conclusions

In summary, we designed and synthesized a new asymmetricDPP-based copolymer, PPyTDPP–2FBT, with the con-sideration for high-performance well-balanced ambipolarcharge transport property for organic circuits. Well-definedV-shaped ambipolar transporting characteristics forPPyTDPP-2FBT-based OFETs gives balanced hole andelectron carrier mobilities of 0.64 and 0.46 cm2 V−1 s−1, re-spectively. Importantly, a typical ambipolar character couldalso be obtained in the devices stored in the ambient con-dition for one year without special package, suggesting thegood stability of the PPyTDPP–2FBT, which is also im-portant for electron transport. Furthermore, organic digitaland analog circuits including the inverters and frequencydoublers based on PPyTDPP–2FBT-spun-cast films werealso successfully fabricated. The maximum gains of invertersreached 133 and 127 at negative and positive VDD, respec-tively. Moreover, PPyTDPP–2FBT-based frequency dou-blers showed excellent performances without identifieddistortion. Our present research results provide promising

applications of ambipolar conjugated polymers to integratedorganic-electronic devices with simple processing techni-ques at low cost and large area.

Acknowledgements This work is financially supported by the Ministry ofScience and Technology of China (2018YFA0703200, 2017YFA0204503),the National Natural Science Foundation of China (91833306, 61890943,51725304, 22021002), Beijing National Laboratory for Molecular Sciences(BNLMS-CXXM-202012), the Youth Innovation Promotion Association ofthe Chinese Academy of Sciences, and the National Program for Support ofTop-notch Young Professionals.

Conflict of interest The authors declare that they have no conflict ofinterest.

Supporting information The supporting information is available online athttp://chem.scichina.com and http://link.springer.com/journal/11426. Thesupporting materials are published as submitted, without typesetting orediting. The responsibility for scientific accuracy and content remains en-tirely with the authors

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