Stacked All-Polymer

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Organic Electronic Devices

Doping-Dedoping Interplay to Realize Patterned/ Stacked All-Polymer Optoelectronic Devices Juhee Kim, Mingyun Kang, Jangwhan Cho, Seong Hoon Yu, and Dae Sung Chung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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ACS Applied Materials & Interfaces

Doping-Dedoping Interplay to Realize Patterned/Stacked All-Polymer Optoelectronic Devices Juhee Kim, Mingyun Kang, Jangwhan Cho, Seong Hoon Yu, and Dae Sung Chung* Department of Energy Science & Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Republic of Korea KEYWORDS doping, dedoping, polymer semiconductor, transistor, photodiode

ABSTRACT One of the remaining keys to the success of polymer electronics is the ability to systematically pattern/stack polymer semiconductors with high precision. This paper reports on the precise patterning and stacking of various polymer semiconductors with the assistance of a molecular oxidizing agent and reducing agent, for donor and acceptor semiconductors, respectively. Such doping-induced solubility control methods have been previously well-developed; however, practical application to various optoelectronic devices have been limited. To pattern/stack

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various polymers in various dimensions, it is important to carefully design not only the doping method for desolubilizing polymer semiconductors but also the dedoping method for recovering the genuine characteristics of each polymer semiconductor. Based on a systematic approach for such doping-dedoping interplay, various high-performance optoelectronic devices are demonstrated: 1) all-polymer complementary inverter pattern with high gain of 176, 2) allpolymer planar heterojunction photodiode with green-selective nature and high specific detectivity over 1012 Jones, and 3) all-polymer ambipolar transistor pattern with balanced hole and electron mobilities. The results of the study indicate the potential of practical application of the doping-dedoping interplay to lateral/vertical patterning, of different polymer semiconductors, with high precision. INTRODUCTION Research on polymer semiconductors has advanced dramatically in the last two decades mainly due to their advantages such as mechanical flexibility, light weight, low-temperature processability, and various optical/physical properties that can be tailored by tuning the molecular structures.1-7 Thanks to such intensive research, polymer semiconductors have achieved remarkable success in many applications, including photovoltaic cells with a power conversion efficiency over 10 %,8,9 thin film transistors with a charge carrier mobility higher than 10 cm2 V-1 s-1,10 and transparent electrodes with a high conductivity, comparable to that of indium tin oxide (ITO).11,12 More recently, in addition to enhancing the performance of the polymer semiconductor itself, there is a growing interest in developing processing methods that enable more precise patterning or multi-layer stacking of polymer semiconductors. This is primarily due to the incompatibility of polymer semiconductors with conventional


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photolithography. There have been many successful research demonstrations of fine-patterning methods of polymer semiconductors, such as the modified photolithography method that uses fluorinated photoresist or a supercritical carbon dioxide (CO2) photoresist,13,14 the stamping method that uses poly(dimethylsiloxane) (PDMS) as a mold,15-17 scanning probe lithography,18,19 and the doping-induced solubility control (DISC) method.20 The patterning methods described above have their own advantages and disadvantages in terms of pattern precision, pattern spacing, and throughput. Among those, the DISC method is the most attractive from the viewpoint of multi-layer stacking, because it enables well-preserved neat properties of the pre-deposited layer when processing the upper-deposited layer. The DISC method utilizes reversibly tunable solubility of π-conjugated polymer semiconductors by controlling their nature of being doped and dedoped. Because molecular dopants can oxidize or reduce polymer semiconductors, the resulting electrically charged polymers have extremely low solubility in common nonpolar organic solvents.20,21 Therefore, subsequent deposition of the second layer from the same processing solvent with that of the first layer rarely alters the morphological/structural features of the pre-deposited first layer. This enables multi-layer, heterogeneous stacking of polymer semiconductors.

Moreover,

after

completing

multi-layer

stacking,

the

pre-deposited

semiconductor layer can recover its pristine electrical/optical characteristics by exposing it to adequate dedoping process. Although there are several other methods of forming vertically stacked heterogeneous polymer semiconductor junctions based on either orthogonal solvents or cross-linking methods, 22-26 there are limitations to these methods. The use of orthogonal solvents is limited to the specific combinations of polymer semiconductors and the use of cross-linking can perturb the genuine molecular crystalline stacking of pristine polymer semiconductors. Therefore, patterning/stacking multi-component polymer semiconductors using DISC methods


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and realizing various optoelectronic devices can be a meaningful experiment for future polymer electronics. Nonetheless, the actual application of DISC method has been limited, and only simple transistor performance has been demonstrated to confirm recovery of semiconductor properties

of

poly(3-hexylthiophene-2,5-diyl)

(P3HT),

after

doping

and

subsequent

dedoping.27,28 This work demonstrates that realizing doping-dedoping interplay with 2,3,5,6-tetrafluorotetracyanoquinodime-thane (F4TCNQ) and 1,8-diazabi-cyclo[5.4.0]undec-7-ene (DBU) as an oxidizing and reducing dopant,27,29 respectively, enables 1) well-patterned all-polymer complementary inverter, 2) well-stacked all-polymer PN junction bilayer diode, and 3) wellpatterned all-polymer bulk heterojunction (BHJ) ambipolar transistor. Details of dopingdedoping interplay mechanism are fully described in conjunction with the supporting analyses such as UV-Vis-NIR absorption spectra and two-dimensional grazing incidence X-ray diffraction (2D-GIXD) patterns. RESULTS AND DISCUSSION All-polymer complementary inverter. In previous studies, the fine-patterning ability of DISC method was well-described with a single polymer component; however, patterning with multi-components has not been demonstrated. One of the representative optoelectronic application that requires multi-component patterning is a complementary inverter, where p-type and n-type field-effect transistors (FETs) are independently patterned. Previous works on organic complementary inverter have been based on inkjet printing,30-34 modified photolithography,35,36 and electrohydrodynamic nanowire printing.37 However, there has been no research effort to introduce the DISC method. In the case of lateral patterning of multi-components for realizing complementary inverter, unexpected issues of mismatch in the output current level of two


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transistors can occur due to multiple doping and dedoping procedures, which can further reduce the inverter gain significantly. Therefore, the fine patterning of two different polymer semiconductors and maintenance of high mobility nature of pristine polymers are both keyfactors of realizing high-performance complementary inverter from doping-dedoping interplay. For this purpose, polymer field-effect transistors (PFETs) consisting of p-type P3HT and ntype

poly[(N,N’-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5’-

(2,2’-bithiophene)] (PNDI2OD-T2) were fabricated and patterned via doping-dedoping interplay. In this research, for p-type PFET, we chose P3HT considering that it has both high doping efficiency and high charge carrier mobility. To dope P3HT with a p-dopant, we introduced the ambient pressure evaporation of molecular dopants rather than a high vacuum method, as used in previous DISC studies to highlight high throughput patterning. First, the P3HT solution in chloroform was cast onto a self-assembled-monolayer-treated SiO2/Si++ dielectric/gate, and then F4TCNQ was thermally evaporated onto the pre-deposited P3HT film through the patterned shadow mask. This was followed by washing with chloroform to develop the isolated pattern of P3HT.38 Next, chlorobenzene solution of PNDI2OD-T2, which is also known as a high mobility n-type polymer, was spin-cast in the vicinity of pre-patterned P3HT and thermally annealed at 170 ℃ for 10 min. To immobilize PNDI2OD-T2, the deposited film was exposed to thermal evaporation of DBU, followed by developing a pattern with chloroform. The chemical structures of the used polymers and molecular dopants are shown in Figure S1. As a final step, patterned P3HT and PNDI2OD-T2 films were simultaneously exposed to p-dedoping solution, propylamine in cyclohexanone (1:9 v/v), mainly to eliminate F4TCNQ. To finish the dedoping procedure of DBU, thermal annealing was performed at 150 ℃ for 20 min for all the devices. Figure 1 schematically presents the overall experimental procedure to develop the patterned all-


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polymer complementary inverter via doping-dedoping interplay, with the actual pattern image taken by optical microscope.

Figure 1. Schematic description of the fabrication process for all-polymer complementary inverter using the suggested doping-dedoping interplay. (a) Si/SiO2 substrate treated with octyltrichlorosilane (OTS) self-assembled-monolayer. (b) Spin-coating of P3HT solution onto the surface of OTS-treated substrate. (c) Vapor-phase doping with F4TCNQ at 210 ℃ for 15 min


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through patterned shadow mask. (d) P3HT pattern development by washing undoped area with chloroform via spin-coating. (e) Spin-coating of PNDI2OD-T2 solution onto the patterned P3HT film followed by annealing at 170 ℃. (f) Vapor-phase doping with DBU at 80 ℃ for 3 min through patterned shadow mask. (g) PNDI2OD-T2 pattern development by washing undoped area with chloroform via spin-coating. (h) Dedoping P3HT and PNDI2OD-T2 films by pdedoping solution and thermal annealing, respectively. For the former, the doped film is exposed to the p-dedoping solution consisting of cyclohexanone and propylamine with a volume ratio of 9:1 for 30 s. Then thermal annealing process is followed for n-dedoping at 150 ℃ for 20 min. (i) Optical microscope image of the actual pattern of P3HT and PNDI2OD-T2 films for the allpolymer complementary inverter using doping-dedoping interplay. The scale bar is 2000 µm. To study the electrical, photophysical and crystalline nature of P3HT and PNDI2OD-T2 at each stage of the film deposition, doping, dedoping, and final annealing, UV-Vis-NIR absorption, 2D-GIXD, and FET studies were conducted. Figure 2(a) and (b) show the UV-Vis-NIR absorption spectra of P3HT and PNDI2OD-T2 films, respectively, with various doping-dedoping steps. In the case of P3HT, F4TCNQ-doping was observed clearly in the low energy region with the characteristic absorption features of F4TCNQ as well as the polaron band of P3HT+.28 This was followed by p-dedoping in propylamine solution; the resulting absorption spectrum closely resembled pristine P3HT film, implying efficient dedoping mechanism. The doping of PNDI2OD-T2 film with DBU rendered a decrease in both the absorption peaks (band I and II) of the pristine film accompanied by evolution of a new feature at ~ 500 nm, which indicates the formation of PNDI2OD-T2 anions.29,39 In the case of PNDI2OD-T2 in comparison to P3HT, the far less dramatic change in the absorption spectra can be ascribed to the different degrees of intermolecular charge transfer in each doping system. The film morphology and thickness of


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PNDI2OD-T2 were well-preserved during DBU-doping processes. In both cases, further heat treatments were introduced at 150 ℃ for 20 min to fully recover the pristine state of each polymer. Note that we intentionally conducted DBU-doping with a minimal amount for the easiness of dedoping. For comparison, a heavily DBU-doped film is also shown, indicating that DBU can cause a substantial change of the absorption feature of PNDI2OD-T2 film. We can speculate that doping interactions of P3HT with F4TCNQ and PNDI2OD-T2 with DBU follow two different mechanisms, ion-pair (IPA) formation and charge-transfer complex (CPX) formation, respectively.20,40 IPA formation occurs when electron transfers from HOMO of donor (P3HT) to LUMO of acceptor (F4TCNQ), and then, completely ionized dopants generate free charge carriers in polymer matrix. The features of IPA between P3HT and F4TCNQ can be observed from dopant ion absorption in low energy region (~ 770 and 880 nm) as seen in Figure 2(a). On the other hand, mechanism of DBU-doping in PNDI2OD-T2 is speculated to follow CPX formation, similar to F4TCNQ-doping in 4T, as reported previously.40 CPX formation occurs by hybridization of an occupied frontier orbital from a donor and an unoccupied frontier orbital from an acceptor, where charge carrier transfer occurs less efficiently. The absorption feature of the CPX does not coincide with the dopant ion absorption. We confirmed the formation of unpaired electrons of doped states and recovery of pristine states on dedoping steps by electron paramagnetic resonance (EPR) spectroscopy, of which the signal intensity is proportional to the number of unpaired electrons. As shown in Figure 2(c) and (d), strong paramagnetic signals are observed in doped P3HT and PNDI2OD-T2 films, while no signals appeared in dedoped states as well as pristine states. For comparison, heavily DBU-doped state is also shown.


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Figure 2. (a) UV-Vis-NIR absorption spectra of P3HT films with a series of doping-dedoping processes: pristine, doped with F4TCNQ, dedoped with p-dedoping solution, and annealed at 150 ℃. The doped states can be characterized by P3HT+ polaron band at low energy region. (b) UV-Vis absorption spectra of PNDI2OD-T2 films with a series of doping-dedoping processes: pristine, doped with DBU, dipped in p-dedoping solution, and dedoped (annealed at 150 ℃). The heavily DBU-doped state is also shown for comparison. The doped states can be characterized by a new feature at ~ 500 nm. In both cases, absorption features become practically same with


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the pristine one after dedoping and annealing (in case of DBU-doping, dedoping, and annealing are the same processes). EPR spectra of various doping-dedoping steps for (c) P3HT and (d) PNDI2OD-T2 films, of which the signal intensity is proportional to the number of unpaired electrons. Both doped states are obviously observed with stronger peaks and these features completely disappear after dedoping. Similar trends were observed from the 2D-GIXD analyses for each doping-dedoping step. As shown in Figure 3(a-f), P3HT exhibited a dramatic change in the crystalline feature as a result of each doping and dedoping process. For example, the lamella d-spacing was changed as 16.26, 19.18, 16.02 and 16.26 Å in corresponding states (pristine, doped, dedoped, and annealed at 150 ℃ for 20 min, respectively). This is in good agreement with the results of previous reports on doping mechanism of P3HT.41,42 In contrast, PNDI2OD-T2 exhibited mostly unchanged 2DGIXD patterns regardless of the doping-dedoping interplay.43 This is shown in Figure 3(g-l) and implies that the degree of charge transfer driven by DBU-doping was not as significant as that of F4TCNQ-doping, except in the case of the PNDI2OD-T2 film dipped in the p-dedoping solution, which showed slightly weakened Bragg diffraction peak intensities, implying unexpected side effects. Nonetheless, the final annealing step enabled to fully recover the original crystalline feature of PNDI2OD-T2. Furthermore, to track the electrical state of each step, FET study was conducted for each doping-dedoping stage of both polymers.


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Figure 3. 2D-GIXD images and section profiles of P3HT films corresponding to the various doping-dedoping steps: (a) pristine, (b) doped with F4TCNQ, (c) dedoped with p-dedoping solution, and (d) annealed at 150 ℃. (e-f) The corresponding GIXD profiles along with (e) inplane and (f) out-of-plane direction. Both the pristine P3HT and the finally annealed P3HT film showed the same (100) d-spacing of 16.26 Å, implying that crystalline property was recovered well. PNDIO2OD-T2 films were analyzed in similar way: (g) pristine, (h) doped with DBU, (i) dipped in p-dedoping solution, and (j) dedoped (annealed at 150 ℃). (k-l) The corresponding 2D-GIXD diffraction profiles along with (k) in-plane and (l) out-of-plane direction. There were no big changes in values of (100) d-spacing in the continuous processes. As summarized in Figure S2(a-d), the FET mobilities of P3HT were changed dramatically for each doping-dedoping step, in correspondence to photophysical/structural analyses. At the F4TCNQ-doping step, current level increases significantly due to high charge carrier density. After the final annealing procedure, P3HT recovered its pristine characteristics with a field-effect


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mobility of 1.64×10-3 cm2 V-1 s-1. Changes in FET characteristics of PNDI2OD-T2 PFETs (Figure S2(e-h)) are closely related with changes in the crystalline features in Figure 3. After DBU-doping, current level definitely increased, but not as much as that of P3HT after F4TCNQdoping, which can be explained by CPX formation that does not produce fully separated charge carriers. A sudden drop in mobility with more than an order of magnitude, which was resulted from dipping PNDI2OD-T2 films in p-dedoping solution, can be related with the change in crystalline feature shown in Figure 3(i). However, well-recovered mobility after thermal annealing with a value of 4.74×10-3 cm2 V-1 s-1 implies that the complete dedoping of DBUdoped PNDI2OD-T2 was possible via thermal treatment. The field-effect mobility value of each step is summarized in Table S1. As such, we could fabricate well-defined complementary inverter patterns with high throughput. After patterning both P3HT and PNDI2OD-T2 FETs, Au source and drain electrodes were deposited via thermal evaporation with channel length and width of 150 μm and 1500 μm, respectively. Nineteen inverter devices were independently fabricated, and the performances of the devices were highly reproducible, which implies that the introduced doping-dedoping patterning method is reliable. Figure 4(a) shows a microscopic image of the actually fabricated complementary inverter.44 The inverter exhibited typical voltage inversion characteristics at various supply voltages (VDD) of 20 V, 30 V, 40 V, and 50 V, as demonstrated in Figure 4(b). The inversion voltage (VM) can be defined as the characteristic voltage of Vin = Vout, where Vin and Vout are the input and output voltages, respectively. As shown in Figure 4(b), VM was closely located at the middle of the VDD range, which can be greatly beneficial for tolerance against disturbing noise. The gain values derived from dVout/dVin at VM, are summarized in Figure 4(c). The maximum gain was obtained as 176 at a VDD of 50 V, which is the highest value compared to other solution-processed polymer inverters.30-34,45,46 Each p-type


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and n-type transfer curve and the value of mobility are summarized in Figure S3 and Table S2, respectively. The statistical summary of the inverter performances for all nineteen independently fabricated/patterned all-polymer inverters is presented in Figure 4(d), showing excellent reproducibility of the suggested method. We want to emphasize that this is the first demonstration of fully solution-processed/patterned all-polymer inverters without any lithography processes.


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Figure 4. (a) Optical microscope image of the fabricated all-polymer complementary inverter device with patterned P3HT and PNDI2OD-T2 FETs. The channel length and width defined by Au electrodes are 150 µm and 1500 µm, respectively. The scale bar is 2000 µm. (b) Switching characteristics of the complementary inverter as a function of supply voltage, VDD. (c) The gain of the inverter calculated from the switching characteristics with high value of 176 at the supply voltage of 50 V. (d) Statistical summary of the value of gains to show remarkable reproducibility of the fabricated inverters through doping-dedoping interplay. All-polymer planar heterojunction photodiode. Another especially important advantage of DISC patterning method is the ability to fabricate multi-layered polymer semiconductor devices, while preserving each polymer’s own morphological/structural characteristics. For this purpose, we demonstrate all-polymer planar heterojunction (PHJ) PN organic photodiode (OPD). Different to the case of photovoltaics, in which the BHJ has distinct merits of efficient exciton dissociation over layered PHJ, the photodiode can reveal genuine advantages of the layered PHJ. Although limited interfacial area between p- and n-type semiconductors can disturb efficient photocurrent generation in OPDs, well-defined junction properties are beneficial for both ideal diode operation by minimizing interfacial defect states, and for suppressing unnecessary dark current injection by blocking exposure of n-type (p-type) semiconductor to low (high) work function metal. Interestingly, several research reports have focused on the overall ascendency of PHJ over BHJ in OPD application.47-49 Moreover, it should be noted that this is the first report on all-polymer PHJ OPD, which is particularly important for actual application considering its extraordinary mechanical flexibility over polymer-fullerene diode. For this purpose, all-polymer OPDs structured as ITO/ZnO/PNDI2OD-T2/P3HT/MoO3/Ag were fabricated via dopingdedoping interplay. The thickness of each constituent layer was strategically designed based on


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optical simulation that constructed green-selective OPDs, which is an important technique for realizing color-selective photodiode. The result of the optical simulation in terms of the generalized transfer matrix method (GTMM) is shown in Figure 5(a). Based on the experimentally obtained refractive indices (n, k) of each layer, the normalized absorbed photon profiles were calculated within the PHJ film. The optimized thickness of ITO/ZnO (30 nm)/PNDI2OD-T2 (70 nm)/P3HT (250 nm)/MoO3/Ag enables most of red photons (650 nm) to be quenched within front side of PNDI2OD-T2 layer without reaching PN interface, and thus also without generating charge carriers. This is because exciton diffusion length of organic semiconductors is limited to several tens of nm. In the case of blue photons (450 nm), because both PNDI2OD-T2 and P3HT have low absorption coefficients (Figure 2), most of them are not absorbed by PN junction. Therefore, in the case of only green photons (550 nm), significant amounts are absorbed at the PN interface; as a result, efficient charge separation can occur. To construct the layer-by-layer structure of PNDI2OD-T2/P3HT bilayer via the same processing solvent of chloroform, PNDI2OD-T2 was doped with DBU, and P3HT was further deposited, followed by thermal annealing to eliminate DBU within PNDI2OD-T2. Dark and illuminated JV characteristics were well-obtained as seen in Figure 5(b). It shows low dark current density of 3.7×10-8 A cm-2 at -1 V, with rectification ratio of 3.39×103 at ±1 V; this indicates that the PHJ between PNDI2OD-T2 and P3HT is well-defined. Furthermore, p-type and n-type layers faced at each charge-collecting electrode can block the undesired injection of charge carriers, implying that PHJ can lead to low dark current. With the low dark current, low noise current was obtained, as shown in Figure 5(c). The lowest detectable light power of photodiode is noise-equivalent power (NEP) which is defined as NEP =

𝑖𝑖𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛

(1)

𝑅𝑅


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where inoise is the noise current and R is the responsivity. The calculated value of NEP is 2.67×1013

W Hz-0.5. The specific detectivity is defined as 𝐷𝐷∗ =

𝑞𝑞𝑞𝑞√𝐴𝐴EQE

(2)

ℎ𝑐𝑐𝑖𝑖noise

where q is the elementary charge, λ is the illumination wavelength, A is the area of the active layer, EQE is the external quantum efficiency, h is the Planck constant, and c is the speed of light. This equation was used to obtain the green-selective detectivity spectrum with a peak value over 1.12×1012 Jones at 550 nm (Figure 5(d)). The obtained detectivity spectrum is well-matched with expected optical mechanism due to well-defined PN junction. Fast response is essential requirement for various photodiode applications. For example, a response speed above 100 Hz against various intensities of incident optical signal is highly required for video applications. Such response speeds of the photodiode can be measured by -3 dB bandwidth (f-3dB), defined as the point at which the response signal power is -3 dB compared to the response signal power in the continuous wave modulation. As seen in Figure 5(e), the -3 dB point of the optimized photodiode was measured as 2.9 kHz under a reverse bias of 0.3 V, which is sufficiently fast for photodiode applications. It is also important for a photodiode to have linear responsivity for a wide range of incident light intensities, to represent the brightness of the obtained image more accurately. The linear responsivity of a photocurrent range can be measured as a linear dynamic range (LDR), which is defined as 𝐽𝐽

(3)

LDR = 20 log( 𝐽𝐽𝑚𝑚𝑚𝑚𝑚𝑚 ) 𝑚𝑚𝑚𝑚𝑚𝑚

where Jmax and Jmin are the maximum and minimum values of the measurable current density with linearity, respectively. As seen in Figure 5(f), the measured LDR of the optimized photodiode was 107.9 dB under a reverse bias of 0.3 V. Because theoretical lower limit of LDR


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value can be estimated with the NEP, which is the lowest detectable light power, the LDR value can be obtained as 181.1 dB.

Figure 5. (a) Simulated distribution of normalized absorbed photons within the optimized photodiode. Photons with wavelengths of 450, 550, and 650 nm represent blue, green, and red light, respectively. Most of the red photons are quenched within the PNDI2OD-T2 layer without reaching PN junction, blue photons are not absorbed because of low absorption coefficients of the polymers at this wavelength. The green photons are absorbed at the PN junction, with generating photocurrent. (b) Measured dark and illuminated (520-nm wavelength and light intensity of 78.9 µW cm-2) J-V characteristics of the optimized photodiode. (c) Noise current spectrum measured under -0.3 V. For the calculation of the specific detectivity, noise current value extracted at 48.5 Hz was used. (d) Specific detectivity spectrum of the optimized


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photodiode with green-selective nature under reverse bias of 0.3 V. (e) The Bode plot under green (520 nm) light illumination with intensity of 34.3 mW cm-2. The -3 dB point and -3 dB frequency point are specified with the dashed lines. (f) LDR of the optimized photodiode under 0.3 V. With the value from NEP, the value of LDR was obtained as 181.1 dB. All-polymer bulk heterojunction ambipolar transistor. As previously mentioned, there have been many reports on patterning a single component polymer semiconductor, including the DISC method.13-19,21,27 However, patterning polymer-polymer BHJ by DISC or other methods has not yet been demonstrated. In this case, because the active layer consists of both donor and acceptor semiconductors, both oxidizing and reducing dopants need to be employed for pattern developing. At the same time, for the recovery of the neat properties of constituting semiconductors, careful dedoping procedures need to be developed. As an application target of BHJ patterning, we chose all-polymer ambipolar transistor which can be further applied as highperformance phototransistors. For all-polymer BHJ ambipolar transistor, balanced charge carrier mobilities of hole and electron and fine patterning of polymer BHJ are important factors. Since P3HT has lower mobility than PNDI2OD-T2, of which electron mobility is generally reported as 0.2-0.85 cm2 V-1 s-1,42,50,51 we selected poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrro-le-alt5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)] (DPP-DTT) as a p-type polymer which is known to have a balanced hole mobility with PNDI2OD-T2.52,53 Although DPP-DTT has low doping efficiency in terms of solubility change, it could be well-patterned by forming BHJ with PNDI2OD-T2. In other words, the main doing mechanism of this BHJ occurs from DBU-doping. Polymer BHJ patterns for ambipolar transistor were developed via doping-dedoping interplay but with a slightly different procedure from the previously suggested one. In this case, DPPDTT:PNDI2OD-T2 (3:7, w/w) bulk solution in dichlorobenzene was cast onto SiO2/Si++


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dielectric/gate, followed by sequential evaporation of DBU and F4TCNQ onto pre-deposited BHJ film with patterned shadow mask. The next step was washing with chloroform to develop isolated pattern of BHJ. To extract F4TCNQ from the developed patterns, the substrate was exposed to the dedoping solution same as the method described in the previous inverter section. Finally, to dedope DBU-doped PNDI2OD-T2 and unnecessary residuals, thermal annealing was followed at 150 ℃ for 20 min. The actual BHJ pattern image was obtained using optical microscope, as presented in Figure 6(a). After developing the BHJ pattern, Au source and drain electrodes were deposited by thermal evaporation with a channel length and channel width of 100 μm and 1500 μm, respectively. The resulting patterned all-polymer ambipolar transistors rendered typical bipolarity according to the controlled gate biases. In the p-type measurement mode with VDS = -30 V, the hole mobility was extracted as 1.82×10-3 cm2 V-1 s-1 as showed in Figure 6(b). The electron mobility was 1.38×10-3 cm2 V-1 s-1 in the n-type measurement mode with VDS = 30 V (Figure 6(c)), which shows well-balanced hole and electron mobilities. The obtained hole and electron mobility values are similar to those of the previously reported allpolymer BHJ ambipolar transistor.54 This further implies that the neat properties of both the donor and acceptor polymers were well-preserved within the BHJ morphology after complicated doping-dedoping procedures. The statistical results of the hole and electron mobilities of the ambipolar transistors for twenty devices are shown in Figure 6(d) and (e), respectively, showing high reproducibility. This further implies that the introduced doping-dedoping interplay patterning method is reliable for constructing all-polymer BHJ devices.


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Figure 6. (a) Optical microscope image of the actual pattern of the fabricated BHJ of DPPDTT:PNDI2OD-T2 for the ambipolar transistor. The scale bar is 2000 µm. Transfer curves of the respective PFETs in (b) p-type measurement mode with VDS = -30 V and (c) n-type measurement mode with VDS = 30 V. Statistical summaries of the value of (d) hole mobilities and (e) electron mobilities to show excellent reproducibility of the fabricated ambipolar transistors with doping-dedoping interplay.


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EXPERIMENTAL SECTION Materials. Poly(3-hexylthiophene-2,5-diyl) (P3HT) was purchased from RIEKE METALS, poly[(N,N’-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5’-(2,2’bithiophene)] (PNDI2OD-T2) was purchased from 1-Material. Poly[2,5-(2-octyldodecyl)-3,6diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno

[3,2-b]thiophene)]

(DPP-DTT)

was

purchased from Ossila. 2,3,5,6-Tetrafluoro-tetracyanoquinodimethane (F4TCNQ) was purchased from TCI chemicals. 1,8-Diazabi-cyclo[5.4.0]undec-7-ene (DBU), zinc acetate dihydrate, ethanolamine, 2-methoxyethanol, cyclohexanone, and propylamine were purchased from SigmaAldrich. All materials were used as received, without being purified. Device fabrication. For the fabrication of all-polymer complementary inverter, Si wafers with a 100-nm thick SiO2 dielectric layer on a heavily n-doped silicon were used as substrate. The substrates were cleaned in a piranha solution. Octyltrichlorosilane (OTS)-treated substrates were obtained by immersing the substrates in OTS (0.3 mL) of toluene (60 mL) for 30 min, followed by thermal annealing at 120 ℃ for 30 min. P3HT of 7 mg mL-1 was dissolved in chloroform and PNDI2OD-T2 of 10 mg mL-1 was dissolved in chlorobenzene. The P3HT solution was spincoated at 2000 rpm for 30 s onto the Si/SiO2 substrate treated with OTS. To dope with F4TCNQ, F4TCNQ (20 mg) were placed in sealed glass jar of volume ~ 200 mL. The glass jar was heated at 210 ℃ and vapor-phase doping was performed for 15 min for each sample. To develop the P3HT patterns, the doped samples were washed by spin-coating with chloroform. The PNDI2OD-T2 solution was spin-coated at 2000 rpm for 30 s. To dope with DBU, two drops of DBU were placed in a glass jar of same volume. The glass jar was heated at 80 ℃ and vaporphase doping was performed for 3 min for each sample. For heavy doping with DBU, two drops were used, and vapor-phase doping was conducted for 30 min. To develop the PNDI2OD-T2


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pattern, the doped samples were washed by spin-coated with chloroform. To dedope the doped P3HT films, the p-dedoping solution was prepared with cyclohexanone (9 mL) and propylamine (1 mL). The samples were exposed to the p-dedoping solution for 30 s. The samples were annealed at 150 ℃ for 20 min. Au electrodes were deposited onto the patterned layer by thermal evaporation at evaporating rate of 1.0 Å s-1. For the photodiode fabrication, ITO-patterned glass substrates were cleaned sequentially through sonication in a Mucasol solution, distilled water, acetone, and isopropanol for 60 min each. The substrates were dried with nitrogen flow, then treated with oxygen plasma for 10 min to induce a hydrophilic surface. For fabrication of the ZnO film, ZnO sol-gel solution was made by a mixture of zinc acetate dihydrate (1 g), ethanolamine (0.28 g), and of 2-methoxyethanol (10 mL). The ZnO films were fabricated by spin-coating onto the plasma-treated substrates at 2000 rpm for 40 s, followed by annealing at 200 ℃ for 30 min. PNDI2OD-T2 and P3HT were dissolved in chloroform with a concentration of 5 mg mL-1 and 25 mg mL-1, respectively. The PNDI2OD-T2 solution was then spin-coated at 1000 rpm for 30 s on top of the ZnO layer. DBU-doping was performed for 3 min at 80 ℃. The P3HT solution was spin-coated at 1000 rpm for 30 s onto the PNDI2OD-T2 layer. Annealing was conducted at 150 ℃ for 10 min for n-dedoping. MoO3/Ag electrodes were deposited onto the P3HT layer by performing a sequential thermal evaporation at an evaporating rate of 0.7 Å s-1 for MoO3, and 1.0 Å s-1 for Ag. For fabrication of the organic ambipolar transistor, OTS-treated Si/SiO2 substrates were prepared in the same manner as the inverter. DPP-DTT:PNDI2OD-T2 (3:7 w/w) blend solution of 15 mg mL-1 was dissolved in 1,2-dichlorobenzene with 3 vol% 1,8-diiodooctane (DIO). The blend solution was spin-coated at 2000 rpm for 30 s onto the OTS-treated Si/SiO2 substrate, followed by thermal annealing at 170 ℃ for 20 min. Vapor-phase doping of DBU and F4TCNQ was


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performed for 3 min and 15 min, respectively. The dedoping processes for F4TCNQ and DBU were conducted by p-dedoping solution and annealing at 150 ℃ for 20 min, respectively. Au electrodes were deposited onto the patterned layer by thermal evaporation at evaporating rate of 1.0 Å s-1. Device characterization. UV-Vis-NIR absorption spectra were measured using an Agilent Technologies Cary 5000 spectrophotometer. To analyze the optical simulation, the reflective index n, k values of each layer were obtained using an integrating-sphere spectrophotometer. For the EPR measurements, samples were prepared in quartz tubes by putting powders of dropcasted films that are deposited on glass substrates. The measurements were performed using a JES-FA200 (JEOL) with the frequency of 9.17 GHz. The 2D-GIXD measurements were performed using PLS-II 3C and 3D beamlines at the Pohang Accelerator Laboratory (PAL) in Korea. A digital microscope (Keyence, VHX-900F) was used to obtain images of the pattern or device. The electrical characteristics of the transistors were measured by using a parameter analyzer (Keithley, 4200A-SCS). Dark J–V characteristics were measured by a Keithley 2450 SourceMeter controlled by LabView program. The photocurrent was measured by an Oriel Cornerstone 130 1/8 m monochromator with the SourceMeter. A Stanford Research SR830 Lock-in Amplifier was used for noise current measurement, and the measured noise currents were normalized by the input bandwidth. The LDR was measured by two light sources with various optical filters: a monochromatized light (520 nm), whose light intensities were below 78.9 μW cm-2, and a laser (520 nm), whose light intensities were up to 34.3 mW cm-2. The -3 dB frequency was measured by a TDS5052 digital phosphor oscilloscope (Tektronix) with the same laser source. All these measurements were performed in a nitrogen-filled glovebox. The thicknesses of the films were measured by a profilometer (Dektak XT, BRUKER).


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CONCLUSION In this research, various all-polymer optoelectronic devices with either patterning or stacking heterogeneous semiconductor components were demonstrated by means of doping-dedoping interplay. The solubilities of polymers were controlled by using a molecular oxidizing agent (F4TCNQ) as an acceptor dopant, and molecular reducing agent (DBU) as a donor dopant, for donor and acceptor polymer semiconductors, respectively. Furthermore, to recover the neat property of each polymer, dedoping and annealing methods were designed. By means of various photophysical and structural analyses, the phenomena occurring at each experimental doping and dedoping step were clarified. Based on the suggested doping-dedoping interplay, the following aspects were demonstrated: 1) all-polymer complementary inverter pattern with high gain exceeding 170, 2) all-polymer planar heterojunction photodiode with green-selective nature as well as high specific detectivity over 1012 Jones, and 3) all-polymer ambipolar transistor pattern with balanced hole and electron mobilities. These results demonstrate the possibility of practically utilizing the doping-dedoping interplay method for lateral/vertical patterning of multicomponent polymer semiconductors with various structural/electrical characteristics.

ASSOCIATED CONTENT Supporting Information. Chemical structures, transfer characteristics of PFETs and constituting PFETs of the optimized complementary inverter. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (Grant Number NRF-2018R1A2B6003445). This research was also supported by Space Core Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant Number NRF-2014M1A3A3A02034707). REFERENCES (1) Schwartz, G.; Tee, B. C.-K.; Mei, J.; Appleton, A. L.; Kim, D. H.; Wang, H.; Bao, Z. Flexible Polymer Transistors with High Pressure Sensitivity for Application in Electronic Skin and Health Monitoring. Nat.Commun. 2013, 4, 1859. (2) Pan, H.; Li, Y.; Wu, Y.; Liu, P.; Ong, B. S.; Zhu, S.; Xu, G. Low-Temperature, SolutionProcessed, High-Mobility Polymer Semiconductors for Thin-film Transistors. J. Am. Chem. Soc. 2007, 129, 4112-4113. (3) Li, J.; Zhao, Y.; Tan, H. S.; Guo, Y.; Di, C.-A.; Yu, G.; Liu, Y.; Lin, M.; Lim, S. H.; Zhou, Y.; Su, H.; Ong, B. S. A Stable Solution-Processed Polymer Semiconductor with Record HighMobility for Printed Transistors. Sci. Rep. 2012, 2, 754. (4) He, M.; Li, J.; Sorensen, M. L.; Zhang, F.; Hancock, R. R.; Fong, H. H.; Pozdin, V. A.; Smilgies, D.-M.; Malliaras, G. G. Alkylsubstituted Thienothiophene Semiconducting Materials: Structure-Property Relationships. J. Am. Chem. Soc. 2009, 131, 11930-11938.


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TABLE OF CONTENTS


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Stacked All-Polymer

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