Structure and characteristics of PESt
Semicrystalline polymer is composed of crystalline domains, amorphous domains and domain boundaries. The energetic disorder of localized electronic states in domain boundaries should be much broader than those in completely crystalline or amorphous domains, as the microscopic structures of diverse boundaries are widely dispersed, leading to the multi-dispersed density-of-states (DOS) of electronic sites (Fig. 1a). The tail of the wide DOS profile represents the low energy states, which could maximally stabilize charge carriers, contributing to good charge storage property. Similarly, the energy diversity of dielectric dipoles in crystalline domains, amorphous domains and domain boundaries contributes to a widely-dispersed energy distribution profile. Moreover, the morphological diversity of domain boundaries leads to the fact that some states at the boundary could stabilize the dielectric dipoles, contributing to enhanced dielectric constant and energy storage property. Therefore, solution-processible polycrystalline films with nanoscale crystallites and huge-area boundaries are attracting wide interests for dielectric and electret applications.
Since PE and s-PS are not soluble in any organic solvent at room temperature, copolymerization provides an effective strategy in improving the solubility, while conventional PE-PS block copolymers based on such nonpolar chains still suffer from poor solubility and aggregation in solution. On the contrary, completely random PE-PS copolymers do not crystallize, although they are soluble. From this viewpoint, gradient copolymers with suitable chemical structures, could simultaneously provide good solubility and sufficient crystallinity.
Taking above consideration into account, in this work, we propose a gradient copolymer of ethylene and styrene, PESt, afforded by coordination polymerization, which is quickly dissolvable in o-dichlorobenzene, toluene, chloroform, even in tetrahydrofuran at room temperature (the dissolving could be accelerated at 80 oC). More importantly, once the copolymer is dissolved, the solution is stable at room temperature, without aggregation in solution.
Hence, three PESts with gradually increased styrene content (PESt15: 14.7 mol%, PESt37: 36.7 mol%, PESt59: 59.2 mol%) were synthesized through increasing the initial styrene concentration (Table S1). Fig. S1 and S2 are the 1H and 13C NMR spectra of PESt correspondingly. In the 13C NMR spectrum of PESt, the resonances at 43.8, 40.5 and 29.5 ppm suggest the copolymer contains syndiotactic polystyrene and polyethylene sequences.26 The thermal behavior of PESt was characterized by differential scanning calorimetry (DSC) (Fig. 1b). PESt15 has a melting point at 111 oC which should be derived from polyethylene sequences as obvious 110 and 200 orientation peaks of PE are observed in its X-ray diffraction (XRD) spectrum (Fig. 1c). The Tm value attributed to PE sequences is lower than that of neat PE, suggesting that the PE sequence length is not too long.27 With the styrene content increasing to 36.7 mol%, the length of PE sequences decreases, confirmed by the results that the melting point of PESt37 appears at 104 oC and the diffraction intensities of peaks at 21o and 24o from PE crystalline become weak, while the length of s-PS sequence increases since an endothermic peak appears at 225 oC in the DSC signal and the diffraction peaks at 10o and 18o assigned to s-PS become more obvious. When the styrene content increases to 59.2 mol%, the PE sequence length further decreases, leading to the disappearance of endothermic peak in the DSC curve and the diffraction peaks in the XRD spectrum derive from PE sequence. In contrast, the length of s-PS sequence becomes longer. Thus, the melting point of PESt59 shifts to 233 oC and the diffraction intensities of peaks at 10o and 18o increase. In combination of kinetic study result (Table S2) that with the polymerization prolonging, the styrene content within PESt decreases, the gradient distribution of ethylene and styrene units within PESt is proposed. The thermal decomposition temperature of PESt was determined by thermogravimetric analysis (TGA) (Fig. 1d). The onset temperature at 5% loss (Tonset5%) of PESt15 is 316 oC. With the styrene content increasing from 14.7 mol% to 36.7 mol%, Tonset5% dramatically increases by 47 oC, to 363 oC (copolymer PESt37). Further increasing the styrene content from 36.7 mol% to 59.2 mol%, Tonset5% is only improved from 363 oC to 387 oC. As elevated temperatures over room temperature occur in electrical equipment and electronic devices, the gradually enhanced tolerance to temperature variations from PESt15 to PESt59 contributes to better thermal stability and high temperature operation performance.
The morphology of PESt films were characterized by atomic force microscopy (AFM, Fig. 1e). The film surface of PESt59 is smoother as compared with those of PESt15 and PESt37, showing a height difference within ± 1 nm. Besides, transmission electron microscopy (TEM) image of PESt59 in Fig. 1f shows a high-quality film with staggered arrangement of crystalline and amorphous regions.
The optical absorbance of PESt was determined by a UV-Vis light absorption spectrometer (Fig. 1g). The absorption peaks of PESt15, PESt37 and PESt59 are all at 196 nm, showing light absorption signal of PS. The absorbance of the three PESts is smaller than 1%, indicating excellent optical transparency which is applicable in organic transparent optoelectronics.
Dielectric characterization of PESt
As indicated in TEM image, in contrary to the sharp phase boundary in block copolymers, gradient copolymers show broad nano-scale interfacial region,28 which is acknowledged contributing in modulation of dielectric and electrical insulating performance of polymeric materials. Here, the dielectric and charge trapping characteristics of PESt are analyzed by frequency domain dielectric spectrum (FDS), isothermal surface potential decay (ISPD), and thermally stimulated current (TSC) experiments, and compared with PE and s-PS.
For PESt15 with styrene content of 14.7 mol% in Fig. 2a, dielectric constant is 2.5 and is stable in frequency range from 10-1 Hz to 106 Hz at temperatures below 120 oC. At elevated temperatures, dielectric constant rapidly increases at low frequencies, but keeps barely changed at higher frequencies. At the highest temperature of 140 oC, the dielectric constant reaches above 20, indicating broad dielectric relaxations. The dielectric constant of PESt37 is 2.6 in Fig. 2b, showing 0.1 increase as compared with PESt15. At elevated temperatures, the dielectric constant increases to 7.8 at 140 oC, 10-1 Hz while it decreases to 2.3 at higher frequencies. The dielectric constant of PESt59 in Fig. 2c is 3.3 at low frequencies, and shows slight decrease with the increased frequency towards 106 Hz. In frequency range of 10-1~101 Hz, obvious increase in dielectric constant is observed at 120 oC and 140 oC. Comparison in FDS spectra of PESt15, PESt37 and PESt59 at 140 oC demonstrates that the increase in dielectric constant of PESt59 is the smallest at elevated temperatures, indicating the enhanced thermal stability.
In Fig. 2d, the dielectric constant and dielectric loss of PESt15, PESt37 and PESt59 are compared with PE and s-PS. er of PESt15 (er = 2.5) is larger than PE of 2.3, but 0.05 smaller than s-PS of 2.55 (as demonstrated in Fig. S3). As there is only 14.7 mol% styrene in PESt15, the relatively stronger polarization is contributed by both the branched benzene ring and the broad nano-scale interfaces between PE macro-phase and PS macro-phase. The dielectric constant of PESt increases with the enlarged styrene content, the dielectric constants of PESt37 and PESt59 are both extraordinarily larger than s-PS. Especially for PESt59 (er = 3.2), the peculiar large dielectric constant benefits the application in energy and charge storage.
In aspect of dielectric loss, tand of PE is comparatively large in typical insulating polymers, which is 0.012 at 50 Hz, 20 oC in Fig. 2d, otherwise the dielectric losses of PESt15, PESt37, PESt59 and s-PS are all at 10-3 level. Notably, PESt37 (tand = 0.0018) and PESt59 (tand = 0.0025) show smaller dielectric loss than s-PS (tand = 0.0028). Thus, PESt with higher styrene ratio exhibits extraordinarily good dielectric performance as larger dielectric constant and smaller dielectric loss are simultaneously achieved, compared to PE and s-PS.
ISPD is further conducted to investigate charge storage characteristics of PESt, as demonstrated in Fig. 2e. PE experiences the quickest decay, followed by PESt15, s-PS, PESt37, and PESt59 with gradually slowed down decay rate, indicating the formation of deep traps. In aspect of initial surface potential, PESt59 exhibits the highest value of 3015 V. Thus, it could expect larger trap density in gradient copolymers compared with corresponding monomers.
The detailed distributions of trap depth (i.e. trap energy) and trap density are obtained by analyzing the ISPD results with Simmon’s equation,29 the results are demonstrated in Fig. 2f. As the ISPD results are fitted by a double-exponential equation, two peak values are most ordinarily observed in trap depth distributions.30 A shallow trap with activation energy of 0.91 eV and a deep trap with activation energy of 0.985 eV are observed in PE. As for PESt, the trap depth is increasing with increased styrene content. The trap depth of s-PS is 1.02 eV, which is larger than that of PESt15 (0.99 eV), but smaller than that of PESt37 (1.08 eV) and PESt59 (1.09 eV). Therefore, the gradient molecular structure contributes to the formation of deep traps in ethylene-styrene copolymerization system.
In Fig. 2g, TSC of PE shows Tpeak at 56.5 oC, followed by PESt15 of 88.5 oC, PS of 119.5 oC, PESt37 of 128.0 oC, and PESt59 of 138.5 oC in ascending order. As the peak temperature is proportional to trap depth, TSC result shows perfect consistency with ISPD. Theoretically, TSC is expressed as,31
where ITSC is the TSC in A; T is the test temperature in K; p is the polarization intensity in C/m2; t is the relaxation time in s; ET is the activation energy (trap depth) in eV; k is the Boltzmann constant; b is the temperature ramping rate in K/s.
The trap depth and trap density are thus calculated based on a Matlab fitting, and the results are shown in Fig. 2h. Based on TSC calculation, PE owns the smallest trap depth of 0.8 eV, and PESt59 exhibits the largest trap depth of 1.6 eV. The trap depths of PESt15, PESt37 and PS are between those of PE and PESt59, which are PESt15 of 0.9 eV, PS of 1.2 eV and PESt37 of 1.3 eV, respectively. The comparison in trap depth obtained from ISPD and TSC perfectly matches, indicating a more stable dielectric electret with increased trap depth in gradient copolymers, both on surface and in bulk of the film.
More importantly, the gradient molecular structure contributes in building up deeper traps with activation energy far beyond corresponding amorphous dielectric polymers. The related mechanism is physically similar with that of nano-filler reinforced dielectric, as the broad nano-scale interface exhibits strong charge immobilization characteristics which acts as trapping centers to capture free charges. With the increased styrene content towards 50% in PESt, the interfacial region is maximally broadened, which increases the deep trap content, leading to increase in average trap depth as demonstrated in Fig. 2f.
In aspect of trap amount, as TSC expresses more information of bulk traps than ISPD, and is obtained with the pre-treatment of long-term high temperature polarization, it could expect a comprehensive trap fulfillment and detrapping process. Thus, the integral area of TSC curve represents intrinsic bulk trap density in dielectric polymers. In Fig. 2h, it is observed that the trap density of PESt is around 7´10-9 A·oC, which is larger than simply PE (4.8´10-9 A·oC) or s-PS (5.6´10-9 A·oC). Thus, the extended interfacial region generates 20~40% more deep traps by contributions of gradient structure.
In mechanism, the conjugated p bond in benzene ring forms strong charge withdrawing ability compared with unsaturated C-C bond on the backbone of polyethylene and polystyrene, which physically absorbs free charges. As styrene content is gradually increased from PESt15 to PESt59, the charge trapping ability is enlarged by contributions of increased benzene density. However, with the solitary contribution of benzene, the charge trap density of PESt could never reach that of polystyrene. Thus, it should be expected that the large amount of deep traps is generated by the broad nano-scale interfaces, formed by the gradient structure.
The enlarged deep trap density modulates charge transfer and accumulation characteristics within the polymer film, leading to enhanced dielectric and electret performance. With the characteristic of solution processability, PESt is further applied as energy storage film in power capacitors, dielectric layer in OFET memories and stable electret in porous filter materials.
Application in power energy storage
In order to investigate the energy storage performance of PESt, room temperature DC breakdown test was first conducted, the results are demonstrated in Weibull distributions in Fig. 3a. PE exhibits the smallest breakdown strength of 327.2 kV/mm, while that of s-PS is 377.1 kV/mm. The breakdown strengths of PESt15, PESt37 and PESt59 are 360.8 kV/mm, 441.7 kV/mm and 521.8 kV/mm, respectively, which are increasing with increased styrene ratio. To be noticed that PESt37 and PESt59 exhibit extraordinary breakdown strengths, showing 22.4% and 44.6% increase as compared with s-PS.
Another key parameter in characterizing electrical insulating performance of a dielectric is volume resistivity, as shown in Fig. 3b. PE exhibits the smallest volume resistivity of 2.71´1015 W·cm, while PESt59 exhibits the highest resistivity of 3.86´1016 W·cm. The result of volume resistivity shows strong correlation with breakdown strength in Fig. 3a, indicating contributions of nano-scale interfacial traps in determining electrical insulating performance.
The charge transfer and accumulation characteristics dominate the insulting performance of dielectric film at the mechanism level, and charge mobility is expressed as m = L2/tTV0, where L is sample thickness, tT is transit time and V0 is initial voltage of ISPD. The obtained charge mobility under high electric field is shown in Fig. 3b. PE exhibits the largest charge mobility of 2.78´10-14 m-2·V-1·s-1, and PESt59 exhibits the smallest charge mobility of 7.91´10-16 m-2·V-1·s-1. Negative correlation exists between charge mobility and breakdown strength, as higher mobility leads to smaller breakdown strength.30 The electrical insulating performance is modulated by manipulating deep traps, controlled by interfacial structure. The generated deep traps suppress charge migration to nearly 1/35 of PE in mobility, which suppress charge acceleration under extreme voltages, leading to the greatly enhanced dielectric and electrical insulating capability of PESt, especially PESt37 and PESt59.
Generally, the dielectric strength of an organic polymer is dependent on its molecular and condensed matter structures. Normally following the same sample preparation procedure, the resistivity and breakdown strength of a copolymer are between its polymeric monomers. However, PESt shows abnormally enhanced breakdown performance. The large area two-phase nano-scale interfacial region in PESt is morphologically like that in nano-doped polymers.20,32-34. Typically, nano SiO2/TiO2 is added into dielectric polymers, introducing deep traps at interactive area around the nano particles, and consequently results in 10-20% increase in breakdown strength.35-38 Based on the experimental results in Fig. 2, it is confirmed that large amount of interfacial deep traps is formed in gradient structure of PESt, showing the nano-enhanced effect of insulating performance. Thus, it could expect gradient polymerization as an advanced effective technique with the potential in replacing nano-doping in enhancing the dielectric performance, as it exhibits the advantages of solution processing and larger insulating performance enhancement towards 50%.
As the dielectric constant and breakdown strength of PESt are synchronously enlarged compared to either PE or s-PS, the energy storage characteristics are further investigated, and compared with typical commercially applied dielectric films, as demonstrated in Fig. 3d-f.
For linear polymers, the energy density is expressed as, Ue = 0.5e0erEb2, where e0 is the vacuum dielectric constant, er is the dielectric constant of polymer, Eb is the electrical breakdown strength. Based on equation above, the energy densities of PESt15, PESt37 and PESt59 are calculated. For thickness of 150 mm samples, due to the enhanced dielectric constant and breakdown strength, energy density of PESt59 is reaching 7.7 J·cm-3, which is far larger than other dielectric polymers at room temperature, including the polymeric monomers of PE (2.1 J·cm-3) and s-PS (3.1 J·cm-3).
Power capacitor usually works in elevated temperatures as a result of severe heating during operation. In universal standard, the maximum working temperature of the dielectric film in power capacitors is 70 oC, however during extreme conditions, the temperature could reach 100 oC.39,40 Here, electrical breakdown and energy density of the polymers are investigated at elevated temperatures of 40 oC, 70 oC and 100 oC, as demonstrated in Fig. 3c and Fig. 3d, the detailed breakdown strengths in Weibull distributions are demonstrated in Fig. S4. The breakdown strengths of PESt are linearly decreasing with increased temperature, while PE and s-PS show rapid exponential decrease at 40 oC and 70 oC correspondingly. PESt shows stronger anti-thermal performance as both of PESt37 and PESt59 maintain high breakdown strengths at extreme temperature of 100 oC. In Fig. 3d, the energy densities of all five polymers are decreasing with raised temperature due to the decreased breakdown strength. However, PESt59 keeps high energy storage performance above 5 J·cm-3 at 70 oC and 100 oC, which is comparatively larger than the widely applied BOPP of < 2 J·cm-3 at thickness of 150 mm.41-43
To further investigate the energy storage property of PESt, PESt59 of the best performance is taken as an example and compared with typical linear polymers applied in power capacitors. To be noticed that the breakdown strength of a dielectric is strongly dependent on film thickness. Thinner film exhibits larger breakdown strength as a result of limited space charge accumulation and thus suppressed inner electric field distortion.29 Therefore, the thickness dependent electrical breakdown of PESt59 is experimentally investigated before calculating the energy density, as demonstrated in Fig. 3e. As the statistical data of linear polymers in Fig. 3f are all in thickness of 2~10 mm, PESt59 is controlled to 5 mm in thickness, measured by step profilers, and its dielectric strength is increased to 890.1 kV/mm, leading to the energy density of 22.9 J·cm-3. In Fig. 3f, it is clearly demonstrated that PESt59 exhibits a tremendous improvement as compared to the typical polymeric materials applied in power capacitor such as BOPP, PET and PC. Moreover, PESt59 is solution processable, which is favorable for large-area rapid manufacturing, showing advantages in both film processing and energy storage density.
Application in field-effect transistors and memories
Insulating polymers with varied dielectric performances lead to regulated OFET performance. Here, PESt was applied as gate dielectric and p-channel semiconductor 2,7-di dodecyl benzothieno[3,2-b] benzothiophene (C12-BTBT) was served as the charge transfer layer to fabricate bottom-gate top-contact OFET, of which the structure is demonstrated in Fig. 4a. The transfer characteristics of C12-BTBT/PESt transistors are first investigated, as shown in Fig. 4b. The on current of C12-BTBT/PESt15 transistor is 1.43´10-4 A, while those of C12-BTBT/PESt37 and C12-BTBT/PESt59 transistors are 2.77´10-4 A and 6.73´10-4 A respectively. As the off-state currents of these 3 types of OFETs are nearly the same at 10-10 A level, the on/off ratios are 2.1´105, 4.1´105 and 1.0´106, and the threshold voltages are -34 V, -32 V and -24 V, respectively, in orders of PESt15, PESt37 and PESt59 devices. Thus, C12-BTBT/PESt59 transistor exhibits the best OFET performance, as the largest on-state current, highest on/off ratio and smallest threshold voltage are synergistically achieved. The field-effect mobility of OFET is expressed as,44
where Id is the drain current, W is the channel width, L is the channel length, Ci is the capacitance of the dielectric layer, Vg is the source-gate voltage, and Vth is the threshold voltage. By applying Equation 2, the field-effect mobility is obtained, and is demonstrated in Fig. 4c. msat of OFET is dependent with Vg in index-like profile. C12-BTBT/PESt59 OFETs exhibit the highest mobility reaching 14.2 cm2·V-1·s-1 in saturation region, while those of C12-BTBT/PESt15 and C12-BTBT/PESt37 OFETs are 9.8 cm2·V-1·s-1 and 5.7 cm2·V-1·s-1 respectively. As compared to the widely applied PS gate dielectric, C12-BTBT/PESt59 OFETs show competitive intrinsic performance, with slight difference in on/off ratio, but larger maverage based on statistical data of 10 devices, as demonstrated in Table S3 in Supplementary information.
The output characteristics of OFETs with PESt15, PESt37 and PESt59 gate dielectrics are shown in Fig. S5. All three output characteristics are typical without nonlinearity in small source-drain voltage (Vd) region, indicating the contact resistance is well suppressed. The maximum on-state current in Fig. S5 is in accordance with transfer characteristics in Fig. 4b for each device, indicating stable operation of these OFETs.
Positive gate stress promotes electron injection into the polymeric dielectric in OFET, which induces strong hole transport in p-type semiconductors, and results in increased mobility. In this work, ±80 V gate voltages are applied with Vd = 0 V before operation, the field-effect mobility of OFETs is demonstrated in Fig. 4c. Different from the proportional relations in as-prepared OFETs, gate voltage dependent field-effect mobility of floating-gate OFETs experiences a linear increase at small Vg, followed by a quasi-platform at elevated voltages. Thus, OFETs can work with enhanced mobility at smaller voltages towards Vg = 0 V. However, for PESt15 and PESt37 OFETs, the field-effect mobility is smaller when compared to the as-prepared devices in saturation region (Vg = -60 V). Uncommonly, due to the large trap density as shown in Fig. 2h, PESt59 OFET exhibits a continuous increase in mobility after a short quasi-platform, and reaches 27.4 cm2·V-1·s-1 at elevated Vg, which is 2 times of the as-prepared devices.
The dielectric characterization in Fig. 2 has already discussed molecular dependent charge trapping capability of PESt15, PESt37 and PESt59, and they are further analyzed in non-volatile OFET memory applications in Fig. 4d-f. After +80 V gate-stress, the on-state current of C12-BTBT/PESt15 device increases to 3.11´10-4 A in Fig. 4d, with off-state current slightly dropping, leading to the enlarged on/off ratio. The threshold voltage decreases to -19 V and the subthreshold swing (SS) characteristic is significantly improved. On the contrary, the threshold voltage enlarges to -43 V as the transfer curve shifts towards the direction of negative Vg after -80 V pre-treatment. As memory window is defined as the discrepancy of Vth between programmed and erased states, it is DVth = 24 V for C12-BTBT/PESt15 OFETs.
In C12-BTBT/PESt37 OFETs in Fig. 4e, the shift in transfer characteristics is rather obvious compared to C12-BTBT/PESt15 device. After positive programming, Vth shifts to -12 V from - 32 V, while -80 V gate stress triggers negative shift towards Vth = -46 V, resulting in a memory window of 34 V. Compared with that of PESt15 of 24 V, the memory window is enlarged by 50% with the replacement of gate dielectric polymer. As following the same treatment on C12-BTBT/PESt15 and C12-BTBT/PESt37 OFETs, the memory window of C12-BTBT/PESt59 device is 47 V in Fig. 4f, showing the best memory characteristics among the three PESt devices. This enlargement of DVth is contributed by better shift performance after both positive programming (19 V) and negative erasing (28 V). Previous investigations on model gate dielectric PS show DVth = 33 V in C12-BTBT/PS OFET with the statistical data of 10 devices.13 Thus, PESt37 shows comparable memory performance against PS based OFETs, and PESt59 shows 42% enhancement in memory window.
Mechanistically, memory window is controlled by space charge accumulation in gate dielectric, which is modulated by charge trap characteristics as analyzed in Fig. 2h. Trap density defines the capability of immobilized charges, which determines the electret intensity. Larger electret intensity leads to stronger compensation electric field that modulates charge transport of semiconducting layer, which eventually results in larger shifts of transfer characteristics as demonstrated in Fig. 4f. As the broad nano-scale interface in PESt contributes to the enriched charge traps compared with simply PE and s-PS, and PESt59 exhibits the largest trap density among the three copolymers (Fig. 2h). Consequently, it results in enlarged memory window in OFET memories.
The trap depth modulates the average immobilized duration of trapped charges, which determines stability of a polymer electret. Here, in order to systematically investigate the long-term operation of PESt based OFET memories, retention characteristic is obtained with a time interval of 104 s in ambient conditions at 45% relative humidity. As demonstrated in Fig. 4g, the initial programmed current largely varies with polymer dielectric, and PESt59 OFET exhibits the largest programmed current of 1.2´10-3 A, while that of PESt15 OFET is only 2.7´10-5 A, showing 2 orders decrease. The initial erased currents of the three OFETs are nearly the same, all at 10-10 A level. Thus, PESt59 OFET realizes the best initial performance with largest on-state current and on/off ratio.
In case of decay rate, PESt59 OFET shows superb memory characteristics with less than 1 order decay in programmed current and 2 orders decay in erased current at the end of 104 s. Comparatively, PESt15 and PESt37 OFETs exhibit observable quicker decay in both programmed and erased states. To be noticed that for all the three devices, the decay in erased current is more violent, which indicates that electrons are more firmly trapped than holes in PESt.
Distinctive retention of OFET memory is dependent on long-term stability of polymeric electret, which is closely related to relaxation time, controlled by trap depth. For PESt59 with trap depth of 1.6 eV (calculated from TSC results, shown in Fig. 2h), the trapped charges are more difficult to detrap, leading to the stable memory effect for both programmed and erased states. As PESt59 exhibits larger and deeper traps than s-PS, and s-PS typically owns larger trap density than the widely applied atactic polystyrene (a-PS) due to the enriched crystal-amorphous region, PESt59 is considered as an advanced gate dielectric for OFET memories.
In digital circuit applications, an inverter is realized based on two transistors (Fig. 4h). In order to achieve high performance inverter, the SS of Tdrive should be small. As demonstrated in Fig. 4f, PESt59 OFET exhibits an enhanced SS performance after a positive gate stress treatment, leading to a gain ( |dVout/dVin|) towards 15, as demonstrated in Fig. 4i. Comparatively, PS based OFETs show only a gain of 7 with the same VDD = -40 V, as demonstrated in Fig. S6. Notably, controlled by the threshold voltage of OFET, PESt59 based inverter can operate with smaller Vin = -10 V, while the smallest operation voltage of PS based inverter is -20 V.
Application in environmental filtrations
In field of environmental filtrations, corona charging is commonly applied to form electret in enhancing filtration efficiency of polymeric mask.18,19 Here, with the strong charge trapping ability and excellent electret performance, PESt is coated on the surface of nylon fiber membrane to explore its application in enhancing filtration efficiency. The sample preparation is detailed in Methods, and corona charging and surface potential measurement were performed before filtration efficiency test, as indicated in Fig. 5a. The surface morphology and pore size of nylon fiber membrane were characterized by SEM, showing a smooth morphology of the original nylon fibers as demonstrated in Fig. 5b. After PESt solution treatment, a layer of dense micropores is formed on nylon fiber surface and the pore size distribution is wide in range from 1 μm to 6 μm (Fig. 5c). The formation of micropores can be attributed to the deposition and self-assembly of PESt during chloroform evaporation. After corona charging, one layer of PESt treated nylon fiber membrane was placed on inlet of a purifier in polluted environment for 3 days, and the SEM image in Fig. 5d clearly demonstrates the adsorbed and deposited dust on surface of the nylon fibers, showing excellent filtration performance.
The filtration characteristics of PESt treated nylon fiber are further quantitatively characterized by surface potential and particulate filtration efficiency (PFE) tests. In Fig. 5e, the untreated nylon fiber shows an initial surface potential of -2627 V after 5 kV corona charging. Comparatively, the initial surface potential of PESt treated nylon fiber is -3912 V, showing 50% increase against untreated samples. As discussed in Fig. 2g and 2h, the larger deep trap density in PESt contributes to the increased surface potential, which leads to stronger electrostatic potential to adsorb particles. For as long as 4 weeks, the surface potential of PESt coated nylon fiber membrane exhibits comparatively small attenuation as compared to untreated nylon, guaranteeing the long-term stable filtration when applied as facial mask materials.
PFE test was conducted with NaCl monodisperse aerosol with the diameter range in 0.3-5 mm, the results are demonstrated in Fig. 5f. PFEs of as-prepared nylon fiber and that with PESt treatment (without charging) are nearly the same, showing ~20 % PFE for 0.3 mm particles. The PFE increases with particle size, reaching ~72 % for 5 mm particles. By conducting corona charging, the PFE of nylon fiber membrane increases dramatically, and for PESt treated fiber membrane, the increase is more significant as compared with neat nylon fibers without PESt. The nylon fibers with PESt treatment show a PFE of 47.1 % for 0.3 mm particles, and 93.1 % for 5 mm particles. In particle size range from 0.3~5 mm, corona charged PESt-coated nylon shows ~25 % increase in PFE, while that of corona charged as-prepared sample is only ~12%. PESt introduces large amount of deep traps, which eventually leads to enhanced electrostatic field in filtrating particles for medical protection.
To be noticed that PFE is strongly related to corona charging voltage, as demonstrated in Fig. S7. The larger charging voltage enhances electron emission, which modulates electrostatic potential on surface of the sample. In this investigation, 10 kV is selected as the optimized charging voltage since further increasing charging voltage contributes little in enhancing surface potential and PFE.