Recent Progress in Fluorinated Dielectric‐Based Organic Field‐Effect Transistors and Applications

Fluorinated dielectric‐based organic field effect transistors (OFETs) have garnered lots of attention due to some visible superiorities of the incorporation of fluorine functional groups into dielectrics, such as the hydrophobicity, chemical inertness, and low polarizability of the C─F bond for effectively impeding the charge‐trapping process and improving carrier mobility. With the efforts of material design and device optimization, high‐performance multifunctional devices using fluorinated dielectrics have been rapidly developed with the mobility exceeding 8 cm2 V−1 s−1 and the operating voltage lower than −4 V, which provides a promising opportunity for applications in memory devices, wearable electronics, and flexible sensors. On this basis, this review summarizes the recent development of fluorinated dielectric‐based OFETs and OFETs‐based organic optoelectronic devices. In addition, a brief perspective of fluorinated dielectric‐based OFETs and their future challenges is also presented.


Introduction
[19][20][21][22][23] Notably, the fluorinated polymer dielectrics provided smooth surface topographies and low surface energies, which led to a better crystalline morphology in the semiconductor film grown over their surfaces. [24]Pinhole-free dense film morphologies maintain a low leakage current in OFETs and prevent the adsorption of molecules from the air. [25]More importantly, the fluorinated dielectrics can effectively decrease the charge traps density at the semiconductor/dielectric interface and promote the edge-on orientation of semiconductor on the dielectric surface. [26,27][40][41][42][43][44][45] Therefore, it is timely to summarize the recent progress of fluorinated dielectricbased organic optoelectronic devices.In this review, after briefly introducing the structures and the key parameters of OFETs, we will describe the modulating effect of fluorinated dielectrics on electrical characteristics and the gate-bias stress stability in Section 2.Then, the applications using fluorinated dielectrics in memory devices, phototransistors, and flexible organic electronic devices are presented.Last, the current challenges and future opportunities of fluorinated dielectric-based OFETs are prospected. .Reproduced with permission. [49]Copyright 2020, Wiley-VCH.b) Top gate and bottom contact (TGBC).Reproduced with permission. [50]Copyright 2012, Wiley-VCH.

Modulating Effect of Fluorinated Dielectrics on Device Electrical Characteristics and Gate-Bias Stress Stability
It is generally believed that charge transport in OFETs occurs within several monolayers of the semiconductor close to the gate dielectric surface. [46,47]The properties and structures of the interface between the semiconductor and the dielectric affect the carrier transport; and thus, determine the performance of devices. [48][53][54][55][56][57][58] Here, in this review, we pay much attention to the interface between organic semiconductor and dielectric layer, especially the dielectric-based interface engineering.Among many dielectric materials, fluorinated dielectric-based materials (Scheme 1) are promising candidates for organic electronic devices due to their capability of inhibiting the adsorption of water and oxygen molecules in the atmosphere and re-ducing the polarity of dielectrics, [59,60] thereby minimizing trap creation at the semiconductor/dielectric interface and improving the carrier mobility and gate-bias stress stability. [61,62]However, many other factors are known to carrier trapping agents (e.g., the crystallinity of semiconductor layers, interface conditions between atmosphere and semiconductor layers, etc.). [63][67][68] We will clarify their modulation strategies in different functional devices through the influence of different fluorinated dielectric layers on charge transfer, interface modulation, and charge injection in OFETs.

Fluorinated Dielectric Layer in Improving the Electrical Characteristics of Organic Transistors
In the manufacturing process of cutting-edge electronic devices, the electrical characteristics of inorganic semiconductors must be adjusted by doping process. [69]However, doping in OFETs is limited by uncontrolled dopant diffusion and low doping efficiency. [70]Lee et al. proposed using fluorinated functional groups in the polymer dielectric layer as an effective p-type doping strategy to improve the electrical performance of the device without causing structural perturbations. [71]In order to investigate the surface polarization doping effect of fluorinated groups, the device with bottom gate top contact was constructed, as shown in (Figure 2a).The poly(pentafluorostyrene-co-3-azidopropyl-methacrylate-copropargyl-methacrylate) (5F-SAPMA) with fluorinated units Scheme 1.Chemical structures of representative fluorinated polymer as gate dielectrics applied in organic electronic devices.a) DM: dielectric material; b) OS: organic semiconductor; c) μ: mobility (cm 2 V −1 s −1 ); d) V t : threshold voltage (V); e) OV: operating voltage (V); f) DS: device structure, SC: single crystal.
For the semiconducting active channel layer of the OFETs, a diketopyrrolopyrrole (DPP)-based donor-acceptor (D−A)type copolymer (poly-(diketopyrrolopyrrole-alt-thieno[3,2-b] thiophene), PDPP2T-TT (the molecular structure shown in Scheme 2) was selected, which has inherent ambipolar carrier transport characteristics.The I−V characteristics of the OFETs with the PhAPMA and 5F-SAPMA dielectrics were studied.It could be clearly observed that the transfer characteristics of the OFETs based on PhAPMA dielectrics exhibit a typical ambipolar behavior, and when the gate bias changes from positive to negative, the charge transport changes from electron to hole accumulation and vice versa (Figure 2b).For its output characteristics, a significant increase of the current at low gate bias and an appropriate saturation at high gate bias can be observed (Figure 2c).Notably, the OFETs based on the 5F-SAPMA dielectrics show only hole-accumulated transport be-havior under a negative gate bias compared with the device with a fluorine-free PhAPMA dielectric layer (Figure 2d,e).In addition, the hole-accumulated behavior on replacing the PhAPMA dielectrics with the 5F-SAPMA dielectrics in the PDPP2T-TTbased OFETs leads to the hole mobility improving from 0.023 to 0.305 cm 2 V −1 s −1 .It is further clarified that the surface polarization doping effect of the fluorinated functional group in the 5F-SAPMA dielectric-based OFETs is favored for charge transport and improves the electrical characteristics of the device.
bottom layer and pure PMMA as the top layer.When the bilayer sample is annealed, the fluorinated PMMA 51 -ec-FMA 1 will diffuse from the bottom layer to the surface of the top layer.Hence, the diffusion of fluorinated PMMA 51 -ec-FMA 1 to modify the interface between the semiconductor and dielectric layer can be utilized.Figure 3b,d shows the typical transfer curves of the OFETs, which exhibit obvious gate modulation.Moreover, a high on current (I on ) of the OFETs based on the fluorinated PMMA 51 -ec-FMA 1 dielectric was observed as compared to pure PMMA dielec-tric devices.For the P3HT-50K, the average mobility was higher on fluorinated PMMA 51 -ec-FMA 1 dielectrics (3.0 × 10 −3 cm 2 V −1 s −1 ) than on pure PMMA dielectrics (0.9 × 10 −3 cm 2 V −1 s −1 ) (Figure 3c).Similarly, the average mobility of P3HT-83K with fluorinated PMMA 51 -ec-FMA 1 dielectrics (1.9 × 10 −2 cm 2 V −1 s −1 ) was extracted as compared to fluorine-free dielectrics (1.1 × 10 −2 cm 2 V −1 s −1 ) (Figure 3e).In order to explain the excellent electrical performance of fluorinated devices, they calculated the trap density at the interface between semiconductor and dielectric layers.Assuming the density of traps at the interface is independent of the energy, the maximum density of interface traps (N trap ) can be calculated using the following equation: [76] N trap = ( Slog e k B T∕q − 1 where e is the Euler number, k B represents the Boltzmann constant, q represents the elementary charge, and T represents the absolute temperature.Accordingly, compared with pure PMMA (6.65 × 10 12 cm -2 eV -1 ) dielectric devices, the semiconductor/dielectric interfaces of devices with fluorinated PMMA (3.72 × 10 12 cm -2 eV -1 ) dielectrics exhibit lower N trap values, indicating that this type of semiconductor/dielectric interface is more favorable for charge transport.In addition, the morphology of semiconductors plays a critical role in the performance of OFETs. [77,78]To investigate the origin of the better performance of the OFETs based on fluorinated PMMA dielectrics, they also studied the morphology of P3HT on different dielectrics.The AFM topographic images of P3HT films on different dielectric surfaces are presented in Figure 3f-i.For the P3HT-50K, the height profile analysis and partially enlarged AFM images show that the crystalline domains sizes on the fluorinated PMMA dielectric are obviously larger than those on the pure PMMA dielectric (Figure 3f,g).Different from the morphology of P3HT with low molecular weight, the P3HT-83K forms fibrillar crystalline domains which interpenetrate each other to form a network structure.This interconnected crystalline structure formed by high molecular weight P3HT provides a better charge transport performance compared with the low molecular weight P3HT.In addition, the sizes of the P3HT fibrillar crystalline domains on the fluorinated PMMA dielectrics are thicker than those on the surface of pure PMMA (Figure 3h,i).The AFM results indi-cate that the P3HT prefers to form larger size crystalline domains on fluorine-rich dielectrics.As a consequence, these larger size crystalline domains or fibers contribute to the enhanced charge carrier mobility of OFET devices due to the effect of grain boundaries. [79]Clearly, the fluorinated dielectrics can effectively decrease the charge traps density at the semiconductor/dielectric interface and improve the carrier mobility. [80]This work provides a new strategy for device design and interface optimization, which may further improve the performance of OFETs.Moreover, Noh et al. also investigated the large enhancement of carrier transport in solution-processed field-effect transistors by fluorinated dielectric engineering. [81]They fabricated top gate/bottom contact OFETs with P(VDF-TrFE)/PMMA dielectric and selected the semiconductor of DPPT-TT and P(NDI2OD-T2) (Figure 4a).They developed the self-organized surface-directed phase separation technology, mixing PMMA with P(VDF-TrFE), and spincoated the blended solution onto organic semiconductors.During the spin coating process, they successfully achieved gradual vertical phase separation as the surface energy of P(VDF-TrFE) was lower than that of PMMA due to the presence of fluorine (F). [82,83]  Reproduced with permission. [75]Copyright 2021, American Chemical Society.
blended dielectrics, even if it reaches 90 wt%, which is attributed to the same semiconductor/PMMA interface.They explained that the enhanced hole mobility mainly originates from the high value of k and the C─F interface dipoles in P(VDF-TrFE), which superimpose a built-in electric field to the external gate field and improve the accumulation of holes at the semiconductordielectric interface. [50]Simultaneously, with a higher carrier concentration, the additional holes occupy the abundant trap states in disordered organic semiconductors and more holes occupy the mobile states at a smaller gate-field, which leads to a higher overall carrier mobility. [84,85]Meaningfully, they further investigated spontaneous doping at the polymer−polymer interface for high-performance organic transistors. [86]Here, they reported a facile method to modulate the charge transport from ambipolar to unipolar in D─A polymer-based OFETs by applying fluorinated low-k dielectrics, such as poly(perfluoroalkenylvinyl ether) (Cytop) and poly-(tetrafluoroethylene) (Teflon) (Figure 4h).The OFETs with PMMA dielectric show the typical ambipolar charac-teristic.However, this ambipolar charge transport is completely modulated to unipolar p-type by replacing PMMA with Cytop and Teflon, as shown in Figure 4i,j.Notably, the hole mobility shows significantly different tendencies between IDT-BT and DPPT-TT OFETs.In IDT-BT OFETs, the hole mobility strongly depends on the dielectric, and the highest hole mobility obtained with Cytop is up to 1.71 cm 2 V −1 s −1 compared to that of Teflon and PMMA (0.1−0.5 cm 2 V −1 s −1 ).Conversely, the hole mobility in DPPT-TT OFETs is less dependent on the dielectrics and the highest value is obtained with PMMA (Figure 4k).They proposed that this modulation of charge transport results from the rearrangement of C─F bonds at the interface between the fluorinecontaining dielectrics and the conjugated polymer semiconductors by proper thermal annealing. [87]These well-aligned dipole moments lead to an abrupt downshift of the Fermi level of the semiconductor toward the highest occupied molecular orbitals near the dielectric−semiconductor interface, which provides a pdoping effect on the channel transport and results in unipolar Reproduced with permission. [86]Copyright 2019, American Chemical Society.
p-type characteristics in the composed OFETs. [88]This study reveals a new functionality of the fluorinated dielectrics for future organic electronics.Furthermore, the perfluorinated polymer gate dielectric Cytop, known as the most hydrophobic insulating amorphous material, has a low surface energy that can effectively eliminate charge traps [89] and is a good candidate for the gate dielectric of OFETs based on solution-processing. [90][91][92] Hasegawa et al. demonstrated a technology to control the meniscus of solution, [93] which allows the manufacture of single crystal organic semiconductor (OSC) films on the highest hydrophobic amorphous perfluoropolymer Cytop.The schematic diagram and electrical characteristics of the printed Ph-BTNT-Cn singlecrystal TFTs are shown in Figure 5a,b-d.The carrier mobility reached up to 5.5 cm 2 V −1 s −1 in the saturation regime, which is equivalent to that of bottom-gate-top-contact (BGTC)-type TFTs with Ph-BTNT-Cn single-crystal films.
Operating voltage below 2 V, highly sharp switch on/off, small threshold voltage (V th ) or turn-on voltage (V on ) near 0 V, as well as negligible hysteresis are also achieved.Notably, the obtained devices exhibited excellent characteristics, showing a small SS value (63 mV dec −1 ) close to the theoretical limit, which indicated that the excellent interfacial quality is realized in printed OSC singlecrystal films on the Cytop gate dielectric.Notably, the low-voltage  [93] Copyright 2020, American Association for the Advancement of Science.e) A schematic illustration of the PDIF-CN 2 single crystal FETs based on Cytop dielectric and the PDIF-CN 2 film FETs based on SiO 2 , Cytop, and HMDSmodified SiO 2 dielectrics, respectively.f) Transfer characteristics of PDIF-CN 2 based FETs with different degrees of order.g) Temperature dependent mobility of the spin-coated, evaporated, and single crystal FETs devices.(e-g) Reproduced with permission. [96]Copyright 2013, American Institute of Physics.
operation and sharp switching of OFETs are also achieved; although, the Cytop was utilized as a encapsulation layer. [94,95]In addition, in the case of using the same organic semiconductor N,N′-1H,1H-perfluorobutyl dicyanoperylene carboxydiimide (PDIF-CN 2 ), Batlogg and co-workers prepared single crystal, thermally evaporated, and spin-coated thin film transistors (TFTs) using different deposition methods and gate dielectric combinations to study the charge transport characteristics (Figure 5e). [96]mong them, the combination of PDIF-CN 2 single crystal and Cytop as dielectric layer has obtained excellent electrical properties, which are mobility up to 6 cm 2 V −1 s −1 , on/off ratio over 10 8 , and sub-threshold swing of 0.45 V dec −1 (Figure 5f,g), which was consistent with the previous report. [97]They elucidated that Cytop as a gate dielectric layer, greatly reduces the number of trap states; and thus, improves the device performance.
Interestingly, the fluorinated self-assembled monolayer (SAM) can control the holes in ambipolar polymer semiconductors and make them exhibit unipolar p-type transport behavior. [98]akimiya et al. fabricated the PNDTI-BT-based OFETs on a Si/SiO 2 substrate with different SAMs (Figure 6a,b).For the OFETs with octadecyltrichlorosilane (ODTS)-modified Si/SiO 2 substrate, the typical ambipolar transistor characteristics with the hole and electron mobilities of 0.10 and 0.15 cm 2 V −1 s −1 can be observed.On the contrary, unipolar p-channel characteristics are clearly observed in the OFETs with FDTS-SAM-modified sub-strate.The hole mobility extracted from the saturation regime is increased to 0.43 cm 2 V −1 s −1 with the current on/off ratio of 10 5 (Figure 6c).They also fabricated complementary-like inverters on a substrate with two different surface regions modified with FDTS-SAMs and ODTS-SAMs (Figure 6d).The switching characteristics have been greatly improved while maintaining a large voltage gain (Figure 6e).They clarified that SAMs containing fluoroalkyl groups can enhance the hole accumulation in p-channel transistors, which can be explained by the formation of an electronic dipole layer or by modulating the interfacial potential. [99]

Fluorinated Dielectric Layer in Improving the Gate-Bias Stress Stability of Organic Transistors
[102] Park and co-workers explained the root cause of the observed gate-bias stress instability in OFETs by studying the properties and electronic structures of the interface between the p-type pentacene layer and a hydrophobic polymer dielectric containing fluorinated functionalized polystyrene. [103]The devices with the same organic semiconductor pentacene based on PMOS, PMS, and PFS dielectrics,  [98] Copyright 2016, Wiley-VCH.
respectively, were constructed (Figure 7a).The smallest ΔV th value after the gate-bias stress of −5.05 V and the negligible hysteresis were observed in the PFS-based OFETs as compared to the PMOS-(−13.14V) and PMS-based (−9.28 V) devices (Figure 7bd).It can be seen that OFETs fabricated with fluorinated polymer dielectrics produced the highest energetic barrier, reduced the carrier density transferred to the dielectric surface, and improved the instability of gate bias-stress.Consequently, OFETs with a fluorinated-polymer dielectric, owing to its high energetic barrier for charge transfer, show excellent gate-bias stress stability under harsh conditions (V G = −80 V for up to 12 h), while preserving the relatively low μ FET of 0.33 cm 2 V −1 s −1 .In addition, fluoropolymer buffer layer in OFETs to improve the stability of the device is also reported. [104]Im et al. proposed a novel method to realize ambient, stable, high-performance, top-gate, patterned WSe 2 FETs on SiO 2 /Si and glass substrates, [105] respectively.The bilayer gate dielectric composed of high-k Al 2 O 3 and low-k organic fluoropolymer (Cytop) layer was utilized for chan-nel charging and as protection buffer, respectively (Figure 7e).As can be seen from the transfer curves, there was little change in the V th of the FETs during the first 7 days; although, a slight increase in I D was observed in the output curves, as well as a slight increase in the mobility (≈30 cm 2 V −1 s −1 ).However, after 10 days of aging in ambient air, a positive shift only 1 V of V th was observed, which resulted in a significant increase in I D and the mobility to ≈36 cm 2 V −1 s −1 (Figure 7f-h).The increase in I D current during aging may be due to the p-doping of oxygen atoms induced by W oxidation, which can diffuse through the layer despite the bilayer passivation.They elucidated that the approach of introducing high-k oxide/low-k organic bilayer dielectrics into OFETs may point the way to future electronic nanodevices and their integration.[108] In Park and co-workers' work, the low operating voltage OFETs of pentaphene and triethylsilylethynyl  and d) PFS dielectric surfaces before, during, and after an applied gate bias stress of V G = −60 V (V D = 0) for 12 h under nitrogen conditions.(a-d) Reproduced with permission. [103]Copyright 2014, Wiley-VCH.e) Device structure of the top-gate FETs with the bilayer Cytop/Al 2 O 3 gate dielectric.f-h) Transfer, output, and mobility behavior of the top-gate 9L WSe 2 FETs on a glass substrate.(e-h) Reproduced with permission. [105]Copyright 2015, Wiley-VCH.
anthradithiophene (TES-ADT) were prepared by using fluorinated PI/Al 2 O 3 bilayer dielectrics (Figure 8a). [109]The transfer curves of the TES-ADT OFETs were based on BPDA−PDA−PDA PI, 6FDA−PDA−PDA PI, and 6FDA−CF3Bz−PDA PI dielectrics, respectively, as a function of the bias stress time (Figure 8b-d).A maintained V G of -3 V (V D = 0 V) was applied to the devices over 3 h; the small shifts in V th from the initial state (ΔV th ) of the OFETs based on 6FDA−CF3Bz−PDA PI after applying a gate bias stress were −0.50 V compared with the BPDA−PDA−PDA PI-based OFETs (−2.11V) and 6FDA−PDA−PDA PI-based OFETs (−2.12 V) (Figure 8e).Obviously, the higher the number of fluorine atoms present in the polyimide skeleton, the smaller the ΔV th , which indicated that the fluorine group effectively prevented the generation of interface traps under gate bias stress.In particular, the TES-ADT OFETs fabricated using highly fluorinated PI (6FDA-CF3Bz-PDA PI) result in higher electrical stability, as well as more negligible hysteresis, even under wet conditions.Similarly, to achieve low-voltage operation and excellent electrical stability in OFETs, Park et al. introduced gPFS treatment into 40 nm thick crosslinked PVP polymer dielectrics (cPVP) (Figure 8f). [110]The bias stability of the OFETs with cPVP and F-cPVP dielectrics was further investigated by measuring the V th value as a function of time at continuous gate bias (Figure 8g-i).The lower ΔV th value (−0.31V) of the F-cPVP-based OFETs was obtained as compared to the devices based on cPVP dielectric (−1.31V), which indicated that the fluorine groups on the surface of F-cPVP inhibit trap generation, as revealed by some previous reports. [23,68]They further used the stretched exponential function to model the experimental ΔV th values: [111] V th − V th,i where V th,i is the initial V th , E A is the activation energy typical of trap creation, k B T 0 is the slope of the activation energy distribution, and A is a constant.E th corresponded to the thermalization energy, defined by k B T 0 ln(t).Here, k B and  are the Boltzmann constant and the frequency of attempted barrier crossing, respectively.The value of ΔV th as a function of the bias-stress time could be clearly fitted to the equation, as shown in Figure 8i.The F-cPVP-based OFETs showed higher E A values of 0.550 eV (0.474 eV in the case of the cPVP-based OFETs) and lower k B T 0 values  (f-i) Reproduced with permission. [110]Copyright 2015, The Royal Society of Chemistry.
of 0.051 eV (0.095 eV), suggesting that the F-cPVP dielectrics reinforced the resistance against trap creation at the semiconductor/dielectric interface and also narrowed the trap energy distribution.Hence, this is the reason why F-cPVP-based OFETs show a higher gate-bias stability compared to cPVP based OFETs.

Fluorinated Dielectric Layer in Organic Transistor Memory Devices
[114][115] Generally, the fluorinated polymer dielectric layer is a good candidate for designing electronic memory devices due to its high surface hydrophobicity and low surface energy. [116]However, the relationship between molecular structures and charge-trapping properties remains unclear.For this, Lu et al. proposed a series of fluorinated polystyrene isomers by side-chain engineering, namely, ortho-(o-), meta-(m-), and para-fluorinated polystyrene (p-FPS). [117]The charge transport characteristics and memory performance of OFETs depend on the charge trapping ability of polymer dielectrics.The transfer characteristics of OFETs with p-type C 12 -BTBT based on o-FPS, m-FPS, and p-FPS gate dielectrics (Figure 9a), respectively, are shown in Figure 9b-d.As the shift of threshold voltage (ΔV th ) is determined as the memory window, a larger memory window of Reproduced with permission. [117]Copyright 2022, American Chemical Society.
97 V of C 12 -BTBT/p-FPS OFETs is observed as compared to the o-FPS-based OFETs (75 V) and the m-FPS-based OFETs (80 V) (Figure 9b-d).The V th is mainly determined by the state of the interface between the semiconductor and the dielectric polymer, and ΔV th varies greatly with the position of the F atom in the FPS (Figure 9e).Moreover, the long-term memory characteristics of OFETs based on o-FPS, m-FPS, and p-FPS dielectrics are also investigated.Among the three OFETs memories, the p-FPS device shows the best long-term stability at the end of the 10 000 s time interval (Figure 9f).Notably, after testing with 50 cycles of gate voltage scanning, the programming state of the o-FPS-and m-FPS-based devices experiences a significant regression, while the p-FPS-based memory programming state barely changes, indi-cating a strong maintenance capability (Figure 9g-i).They found that the gradually enlarged intramolecular charge separation of o-, m-, and p-FPS enhances molecular electrostatic potential, which promotes polarization and charge trapping performances, resulting in an enlarged dielectric constant, as well as more deep traps toward stable electret. [118]Accordingly, largely improved photon memory performances of p-FPS-based OFETs further suggest the dominating role of dielectric side-chain structures on memory performances.As a result, the deep traps induced by grafted F atoms in p-FPS can securely trap charge under these voltage perturbations; and therefore, help to achieve better repetition stability in keeping the programming/erasure state of the memory device. [119]Reproduced with permission. [123]Copyright 2020, American Chemical Society.

Fluorinated Dielectric Layer in Organic Phototransistors
[122] However, the high dark current and low photoresponsivity limit their practical applications.For this, Guo et al. developed a novel vertical organic phototransistor combined with poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) ferroelectric (Figure 10a). [123]The device exhibited typical p-type transistor characteristics in the dark and light, with an average on-off ratio of over 10 5 .At 760 nm light and a power density of 20 μW cm −2 , the I DS current increased remarkably to 40 μA (Figure 10b,c).In addition, it could be seen that the onstate current increased from 7 to 35 μA with the increase of light power intensity, while the threshold voltage V th had a large positive shift (Figure 10d).There are three important merit figures: photoresponsivity (R), photosensitivity (P), and detectivity (D*) to measure their response to light stimulus, which can be calculated as [124][125][126] where I light is the drain current under illumination, P i is the incident light intensity, and S is the area of the illuminated channel. 29] where Δf is the operation bandwidth and i n is the measured noise current.Assuming that the major contribution is the shot noise from dark current, D* is then given by [130][131][132][133] With the increase of light intensity, P increases significantly, reaching the maximum of 5 × 10 7 at 20 μW cm −2 , while R and D* decrease, reaching the corresponding maximum of 5.7 × 10 5 A W −1 and 1.15 × 10 18 Jones at 1 μW cm −2 (Figure 10e-g).The phototransistors are also explored by utilizing them as imaging sensors for recording image information (Figure 10h).The excellent performance of the OPT is due to the ultra-short channel of the active layer and the ferroelectric polarization-induced electrostatic field by the P(VDFTrFE) layer.
To further understand the effect of fluorinated dielectric layers on the performance of organic phototransistors, Ji et al. constructed the OPTs with different gate dielectrics including  [134] Copyright 2022, Wiley-VCH.
fluorine-free PI, ODA-6FDA PI with -2CF 3 , and TFMB-6FDA PI with ─4CF 3 , respectively (Figure 11a). [134]The X-ray diffraction (XRD) characterization was performed to study the crystalline properties of the DNTT films deposited on different dielectrics.It was clearly observed that DNTT films deposited on fluorinated PI dielectric layer exhibited a higher intensity than those on pure PI (Figure 11b) and more of ─CF 3 contributed to the higher crystallinity and higher ordered molecular stacking.Furthermore, they did some investigation about the interfacial issue to identify the reason for the diverse characteristics of different dielectrics.Ultraviolet photoelectron spectroscopy (UPS) was performed to investigate the charge transfer at the semicon-ductor/dielectric interface.The UPS spectrum of DNTT shifted toward low binding energies when the underlying dielectric layer successively changed from PI to ODA-6FDA PI or TFMB-6FDA PI (Figure 11c).More specifically, the highest occupied molecular orbital (HOMO) energy level onset of DNTT changed from 1.14 eV on PI to 0.88 eV on ODA-6FDA PI or 0.62 eV on TFMB-6FDA PI.This indicated a more evident electron transfer from DNTT to ODA-6FDA PI or TFMB-6FDA PI than that to PI, thereby leaving more holes at the DNTT interface when using ODA-6FDA PI or TFMB-6FDA PI as the dielectric layer.In addition, the left holes inside DNTT could be easily driven along the interface under a bias of the drain voltages (V DS ), thereby facilitating the charge transport in DNTT.As a result, the average mobilities of devices based on TFMB-6FDA PI and ODA-6FDA PI were 0.81 and 0.52 cm 2 V −1 s −1 , respectively, which were much higher than those of fluorine-free PI (0.04 cm 2 V −1 s −1 ) devices.It could be observed that the drain current in the on-state was significantly enhanced with increasing light intensity, even when the light intensity was as weak as 0.028 mW cm −2 .Instead of modulating charge carriers without illumination, the best mobility was detected from the TFMB-6FDA PI-based transistors, where the ODA-6FDA PI-based OPT exhibited the best optical figures of merit (Figure 11d-f).The lowest exciton binding energy (E b ) of 31.5 meV was obtained from the DNTT film deposited on ODA-6FDA PI as compared to TFMB-6FDA PI (32.0 meV) and bare PI (32.6 meV).
Therefore, under illumination, the TFMB-6FDA PI dielectrics with more ─CF 3 could inhibit the generation of photogenerated excitons and accelerate the recombination of photogenerated electrons and holes due to the relatively high exciton binding energy and coulomb interaction. [135]On this basis, the synergistic balance effect induced by fluorine functional groups in ODA-6FDA PI-based devices led to the best performance of phototransistors.Moreover, a stable and reproducible dynamic photoresponse behavior could be observed, and the device based on the fluorinated dielectric layer exhibited high photocurrent at the same time interval compared to the device without fluorinated dielectric layer (Figure 11g-i).Simultaneously, compared with fluorine-free PI devices, the fluorinated-based dielectric devices exhibited a relatively fast photoresponse speed with a short rise (0.5 s) and decay (0.6 s) time (Figure 11j-l), which indicates that the fluorinated PI devices could act as promising candidates for a good light-activated witch. [125]Due to the high light sensitivity, the ODA-6FDA PI-based OPT is very suitable for optoelectronic applications such as flexible photosensitive imaging. [136]Clearly, a spatial mapping of the characters can be presented with high accuracy (Figure 11m-o).They elucidated that fluorinated functionalization of PI is an effective way to improve the optoelectronic properties of devices, and fluorinated PI dielectrics show potential applications in organic electronics.

Fluorinated Dielectric Layer in Flexible Organic Electronic Devices
[139][140][141][142] Significantly, the transition from conventional electronics to flexible electronics necessitates a suitable dielectric material.Fluorinated polyimides (PIs) are considered as promising candidates for next-generation gate insulators because they have unique physicochemical properties, including excellent thermal stability and good adhesion characteristics. [143]Kim et al. designed and synthesized a soluble 6FDA-MDA gate insulator material and successfully demonstrated a flexible OFET using the 6FDA-MDA as a dielectric layer by integrating it with a 3 μm thick Parylene C substrate (Figure 12a,b). [144]The mechanical flexibility of OFETs was demonstrated by measuring the electrical characteristics of manufacturing, bending, and crumpling (Figure 12b).The mobility of 0.10 cm 2 V −1 s −1 and the on/off ratio of 4.23 × 10 6 were observed in the flexible OFETs, which are comparable to the values of OFETs fabricated on glass substrates (Figure 12c).Following this work, Kim and co-workers continued to develop another fluorinated functionalized PI (6FDA-DABC) material and used 6FDA-DABC, DPP-DTT, and Parylene C as the polymer gate dielectric, polymer semiconductor, and flexible substrate to successfully construct room-temperature, printed, low-voltage, flexible OFETs (Figure 12d,e). [145]The OFETs exhibited excellent mechanical flexibility during bending and crumpling tests and showed a negligible change in the device performance compared to those fabricated on glass substrates (Figure 12f).Likewise, Cho et al. also synthesized two solution-processable aromatic fluorinated PIs (6FDA-6FDAM-PI and 6FDA-TFMB-PI) by one-step polymerization method. [146]o test the feasibility of the prepared fluorinated PI films as organic gate insulator layers, transparent flexible OFETs were fabricated using p-type pentacene and n-type (PTCDI-C 8 ) as channel layers, respectively (Figure 12g).Notably, with 6FDA-TFMB-PI, the manufactured pentacene-and PTCDI-C 8 -based OFETs exhibited the average hole and electron mobilities of 1.83 and 0.56 cm 2 •V −1 •s −1 , respectively, which were superior to those of the OFETs with 6FDA-6FDAM-PI.In addition, organic complementary logic devices, including the NOT, NAND, and NOR gates, were demonstrated by connecting the p-type and n-type OFETs based on the 6FDA-TFMB-PI gate insulator (Figure 12h).This work provides great freedom to select gate insulator for the development of flexible electronic applications.
In addition to fluorine-functionalized PIs, Cytop and fluorinated cross-linking agents are also commonly used gate dielectric layers in the preparation of flexible organic electronic devices. [147]ang et al. demonstrated a flexible organic tribotronic transistor (FOTT) without a top gate electrode and used the p-type pentacene and poly(methyl methacrylate)/Cytop composites as the conductive channels and dielectric layers, respectively (Figure 13a). [148]Interestingly, on the basis of FOTT, a flexible magnetic sensor was developed by adding magnetic composites to the triboelectric layer (Figure 13b).When no magnetic field was loaded, the I DS remained unchanged and no significant current modulation was observed.On the contrary, I DS increased significantly, indicating a good potential for magnetic sensing (Figure 13c).Moreover, after 10 000 test cycles, the I DS changed less than 5%, showing its small hysteresis and excellent reproducibility (Figure 13d).In addition, Kim et al. reported a facile, fluorinated, UV-assisted cross-linker series using a fluorophenyl azide (FPA), which reacts with the C─H groups of a conventional polymer. [149]They fabricated flexible printed OFETs based on the UV-treated 4FCOC and untreated 4FPVDF-HFP dielectrics with flexible substrates (Figure 13e).The fluorinated cross-linking agents (FPAs) into dielectric layers for OFETs exhibited excellent dielectric strength and improved chemical durability and surface properties.As mentioned above, all these results promise a bright future of fluorinated polymer dielectric layers in flexible organic electronic devices.(a-c) Reproduced with permission. [144]Copyright 2019, American Chemical Society.d) Schematic diagram of the printed OFETs based on 6FDA-DABC dielectric.e) Optical images of bending and crumpling tests for flexible OFETs.f) Typical transfer curves for OFETs based on DPP-DTT:PS as fabricated, bent, and crumpled.(d-f) Reproduced with permission. [145]Copyright 2020, American Institute of Physics.g) Schematic and photograph of flexible OFETs device fabricated with fluorinated PIs.h) Optical microscopy (OM) image and schematic of complementary inverter (NOT gate) based on 6FDA-TFMB-PI gate insulator.(g,h) Reproduced with permission. [146]Copyright 2020, American Chemical Society.

Conclusions and Outlook
In this review, recent progress of fluorinated dielectric-based OFETs and related organic optoelectronic devices are summarized.[152] Altogether, a variety of applications using fluorinated dielectric materials have been well developed.[155][156] Although various structures of fluorinated dielectrics have been obtained, the relationship between the chemical structures and device performance is still unclear.Generally speaking, for electrical  [148] Copyright 2017, American Chemical Society.e) Flexible printed device configuration and real and CPOM images with cross-linked dielectrics.Reproduced with permission. [149]Copyright 2020, American Chemical Society.
properties, the number of fluorine atoms in the fluorinated polymer dielectrics increased, the surface energy decreased, the fieldeffect mobility improved, and the threshold voltage shifted toward positive values in the OFETs. [109]For optical performance, the synergistic balance effect induced by fluorine atoms is dominant in the fluorinated dielectric layer devices. [134][159] In addition, the development of fluorinated polymer dielectrics still needs to be paid more attention to improve the mobility of the devices.Besides, more functional fluorinated dielectric materials need to be further explored, such as stretchable and self-healing dielectrics, for prospective applications in complex scenarios.In general, the development of fluorinated dielectrics is very promising for the practical application in organic electronics.

Figure 2 .
Figure 2. a) Device structure of the OFETs, chemical structure of the semiconductor (PDPP2T-TT), and terpolymer dielectrics (PhAPMA and 5F-SAPMA).b) Transfer curves of the PDPP2T-TT-based OFETs with the PhAPMA terpolymer dielectric layer.c) Output curves of the OFETs with PhAPMA dielectrics.d) Transfer curves of the OFETs with the 5F-SAPMA dielectric layer.e) Output curves of the OFETs with the 5F-SAPMA dielectrics.Reproduced with permission.[71]Copyright 2021, American Chemical Society.

Scheme 2 .
Scheme 2. Organic semiconductors commonly used in the organic electronic devices with fluorinated polymer dielectrics.
Figure 4b-e shows the transfer characteristics of OFET devices based on two representative ambipolar polymer semiconductors, DPPT-TT and P(NDI2OD-T2), at V d = −30 V and V d = +30 V.It can be observed that all of the blended dielectrics induce a higher output current for both hole and electron transport with respect to the PMMA for DPPT-TT, and at the 9:1 blend ratio, the DPPT-TT OFET exhibits μ FET,h = 2.15 and μ FET,e = 0.21 cm 2 V −1 s −1 , and the P(NDI2OD-T2) exhibits μ FET,h = 0.084 and μ FET,e = 0.33 cm 2 V −1 s −1 .Particularly, the variations in the V on and μ FET according to the change in the blending ratio are shown in Figure 4f,g.Although μ FET,h continuously increases with increasing the weight ratio of P(VDF-TrFE), μ FET,e also increases slightly.However, as the weight ratio increases, V on does not shift for the

Figure 4 .
Figure 4. a) Device configuration of the top gate/bottom contact (TG/BC) FETs.Transfer characteristics of b) p-channel and c) n-channel DPPT-TT OFETs and d) p-channel and e) n-channel P(NDI2OD-T2) OFETs with blended gate dielectrics.The evolution of the turn-on voltage and electron and hole mobilities of f) DPPT-TT and g) P(NDI2OD-T2) OFETs as a function of the concentration of P(VDF-TrFE) in the blended dielectrics.(a-g) Reproduced with permission. [81]Copyright 2015, Wiley-VCH.h) OFET structure with the top-gate and bottom-contact configuration.i) Transfer characteristics of a p-type configuration for IDT-BT OFETs with PMMA, Teflon, and Cytop gate dielectrics.j) Transfer characteristics of the DPPT-TT OFETs based on PMMA, Teflon, and Cytop gate dielectrics, respectively.k) Hole mobility of IDT-BT and DPPT-TT OFETs with PMMA, Teflon, and Cytop gate dielectrics.(h-k)Reproduced with permission.[86]Copyright 2019, American Chemical Society.

Figure 5 .
Figure 5. a) Schematic cross-section of BGBC-type TFTs.b) Transfer characteristics in the saturation regime.c) Output characteristics.d) Transfer characteristics in the switching region.(a-d) Reproduced with permission.[93]Copyright 2020, American Association for the Advancement of Science.e) A schematic illustration of the PDIF-CN 2 single crystal FETs based on Cytop dielectric and the PDIF-CN 2 film FETs based on SiO 2 , Cytop, and HMDSmodified SiO 2 dielectrics, respectively.f) Transfer characteristics of PDIF-CN 2 based FETs with different degrees of order.g) Temperature dependent mobility of the spin-coated, evaporated, and single crystal FETs devices.(e-g) Reproduced with permission.[96]Copyright 2013, American Institute of Physics.

Figure 6 .
Figure 6.a) Chemical structures of PNDTI-BTs.b) Molecules for SAMs.c) Transfer curves of the PNDTI-BT-DP OFETs based on the FDTS-SAM-modified substrate.d) Schematic diagram of a monolithic complementary-like inverter fabricated on Si/SiO 2 substrate with two different surface areas, which are modified with FDTS-and ODTS-SAMs.e) VTC curve, I DD , and voltage gain of complementary-like inverter manufactured on a patterned FDTS/ODTS-SAM-modified substrate.Reproduced with permission.[98]Copyright 2016, Wiley-VCH.

Figure 7 .
Figure 7. a) Schematic diagram of bottom-gate top-contact OFETs.Transfer characteristics of OFETs containing b) PMOS, c) PMS, and d) PFS dielectric surfaces before, during, and after an applied gate bias stress of V G = −60 V (V D = 0) for 12 h under nitrogen conditions.(a-d)Reproduced with permission.[103]Copyright 2014, Wiley-VCH.e) Device structure of the top-gate FETs with the bilayer Cytop/Al 2 O 3 gate dielectric.f-h) Transfer, output, and mobility behavior of the top-gate 9L WSe 2 FETs on a glass substrate.(e-h) Reproduced with permission.[105]Copyright 2015, Wiley-VCH.

Figure 8 .
Figure 8. a) Schematic diagram of the top-contact OFETs based on BPDA−PDA−PDA PI, 6FDA−PDA−PDA PI, and 6FDA−CF3Bz−PDA PI, respectively.Transfer characteristics of TES-ADT-based OFETs containing b) BPDA−PDA−PDA PI, c) 6FDA−PDA−PDA PI, and d) 6FDA−CF3Bz−PDA PI before and after an applied gate bias stress of V G = −3 V (V D = 0) for 3 h.e) The threshold voltage shifts (ΔV th ) of OFETs based on TES-ADT are a function of the bias stress time in a N 2 environment.(a-e) Reproduced with permission.[109]Copyright 2014, American Chemical Society.f) Chemical structures and schematic of the top-contact OFETs.Drain current−gate voltage (I D −V G ) transfer characteristics of TES-ADT-based OFETs containing g) F-cPVP and h) cPVP under a gate bias-stress of V G = −3 V over 1 h.i) The threshold voltage shifts of the TES-ADT-based OFETs as a function of the bias stress time under a N 2 environment.(f-i) Reproduced with permission.[110]Copyright 2015, The Royal Society of Chemistry.

Figure 9 .
Figure 9. OFETs with FPS gate dielectrics and the corresponding photon memories.a) Schematic diagram of a bottom-gate top-contact OFETs.Transfer characteristics of OFETs based on b) o-FPS, c) m-FPS, and d) p-FPS dielectric layers, respectively.e) Comparison of V th and ΔV th for OFETs with o-FPS, m-FPS, and p-FPS dielectric layers.f) Photon memory retention characteristics of OFETs with o-FPS, m-FPS, and p-FPS dielectric layers.Repeat stability of photon memories based on g) o-FPS, h) m-FPS, and i) p-FPS dielectric layers repeatedly scanned 50 gate−voltage cycles."C" represents "circles."Reproducedwith permission.[117]Copyright 2022, American Chemical Society.

Figure 10 .
Figure 10.a) Schematic diagram of the novel organic phototransistor (OPT) based on the P(VDF-TrFE) dielectric.b) Typical transfer characteristics of the vertical OPT under different source-drain voltages (I SD ) in the dark and under light illumination.c) Output curves of the OPT in the dark and under 760 nm light illumination with 20 μW cm −2 .d) Transfer characteristics of the device in the dark and under various light intensities at V DS = −40 V. e) The photoresponsivity (R), f) the photosensitivity (P), and g) the detectivity (D*) of the OPT as a function of light intensity.h) Schematic illustration of the experimental setup for the image sensing application of the VOPT array.Reproduced with permission.[123]Copyright 2020, American Chemical Society.

Figure 11 .
Figure 11.a) Schematic diagram of the organic phototransistor (OPT) based on PI, ODA-6FDA PI, and TFMB-6FDA PI dielectrics.b) XRD spectrum of the DNTT film on PI, ODA-6FDA PI, and TFMB-6FDA PI. c) UPS spectra analysis.Transfer characteristics of DNTT OPTs based on d) PI, e) ODA-6FDA PI, and f) TFMB-6FDA PI, respectively, under different light intensities in the air.Photocurrent responses of the DNTT OPTs based on g) PI, h) ODA-6FDA PI, and i) TFMB-6FDA PI, respectively.The enlarged photoswitching curves involving the rise and processes of DNTT OPTs based on j) PI, k) ODA-6FDA PI, and l) TFMB-6FDA PI, respectively.m) Schematic illustration of the experimental setup for the pattern sensing behavior of the flexible OFET array.n) A photograph of devices for the flexible OFET array.o) Current mapping of the 4 × 4 imaging matrix under illumination with "U" type mask (1.5 cm × 1.5 cm) on top.Reproduced with permission.[134]Copyright 2022, Wiley-VCH.

Figure 12 .
Figure 12. a) Typical transfer curves for TIPS-pentacene-based OFETs with various soluble PIs dielectrics.The inset shows the device structure.b) Optical images of bending and crumpling OFETs manufactured on ultra-thin Parylene C substrate for mechanical flexibility test.c) Typical transfer curves of TIPS-pentaphene OFETs manufactured using 6FDA-MDA-soluble PI gate dielectric under bent and crumpled conditions.(a-c)Reproduced with permission.[144]Copyright 2019, American Chemical Society.d) Schematic diagram of the printed OFETs based on 6FDA-DABC dielectric.e) Optical images of bending and crumpling tests for flexible OFETs.f) Typical transfer curves for OFETs based on DPP-DTT:PS as fabricated, bent, and crumpled.(d-f)Reproduced with permission.[145]Copyright 2020, American Institute of Physics.g) Schematic and photograph of flexible OFETs device fabricated with fluorinated PIs.h) Optical microscopy (OM) image and schematic of complementary inverter (NOT gate) based on 6FDA-TFMB-PI gate insulator.(g,h) Reproduced with permission.[146]Copyright 2020, American Chemical Society.

Figure 13 .
Figure 13.a) Schematic diagram of a flexible organic tribotronic transistor (FOTT) without a top gate electrode.b) Schematic illustration of the device structure and the sensing process and the photograph of a flexible sensing device.c) IDS changes of the sensor with a magnetic field on and off.d) Stability test of the sensor.(a-d)Reproduced with permission.[148]Copyright 2017, American Chemical Society.e) Flexible printed device configuration and real and CPOM images with cross-linked dielectrics.Reproduced with permission.[149]Copyright 2020, American Chemical Society.

Table 1 .
Typical examples for the OFETs with fluorinated polymer as gate dielectrics.