Efficient and Air‐Stable Aqueous‐Processed Organic Solar Cells …ohgroup.snu.ac.kr/data/file/br_21/1825640006_neE9wmlq... · 2018-10-31 · Wonho Lee, Bhoj Gautam, Xiang Liu,

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1802674 (1 of 12) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Efficient and Air-Stable Aqueous-Processed Organic Solar Cells and Transistors: Impact of Water Addition on Processability and Thin-Film Morphologies of Electroactive Materials

Changyeon Lee, Hae Rang Lee, Joonhyeong Choi, Youngkwon Kim, Thanh Luan Nguyen, Wonho Lee, Bhoj Gautam, Xiang Liu, Kai Zhang, Fei Huang, Joon Hak Oh, Han Young Woo,* and Bumjoon J. Kim*

DOI: 10.1002/aenm.201802674

transistors (OFETs), organic light emit-ting diodes and polymer solar cells (PSCs), developing ecofriendly processing methods suitable for industrial fabrication has become an important topic of recent investigations.[1,2] To this end, a great deal of research efforts have been devoted to phasing out halogenated solvents such as chloroform and chlorobenzene, which have been prevalent for optimizing the fabrication of lab-scale OFET and PSC devices, because they are detrimental to both human health and the environment. In this context, a wide range of halogen-free solvents including toluene, xylenes, and trimethylbenzenes have been recently proposed as greener solvent alternatives and some notable progress has been made.[3–11] However, it remains in ques-tion whether those halogen-free solvents, which are often claimed to be “green sol-vents,” are indeed practically applicable at an industrial-scale for sustainable manu-facturing of organic electronics due to the

serious health hazards and harmful environmental impacts that they can pose (e.g., median lethal dose LD50 for chlorobenzene: 1110 mg kg−1, toluene: 5580 mg kg−1, water: >90 000 mg kg−1, and De minimis % limit allowed to release for chlorobenzene

The authors report the development of a desirable aqueous process for ecofriendly fabrication of efficient and stable organic field-effect transistors (eco-OFETs) and polymer solar cells (eco-PSCs). Intriguingly, the addition of a typical antisolvent, water, to ethanol is found to remarkably enhance the solubility of oligoethylene glycol (OEG) side chain-based electroactive materials (e.g., the highly crystalline conjugated polymer PPDT2FBT-A and the fullerene monoadduct PC61BO12). A water–ethanol cosolvent with a 1:1 molar ratio provides an increased solubility of PPDT2FBT-A from 2.3 to 42.9 mg mL−1 and that of PC61BO12 from 0.3 to 40.5 mg mL−1. Owing to the improved processability, efficient eco-OFETs with a hole mobility of 2.0 × 10−2 cm2 V−1 s−1 and eco-PSCs with a power conversion efficiency of 2.05% are successfully fabricated. In addition, the eco-PSCs fabricated with water–ethanol processing are highly stable under ambient conditions, showing the great potential of this new process for industrial scale application. To better understand the underlying role of water addition, the influence of water addition on the thin-film morphologies and the performance of the eco-OFETs and eco-PSCs are studied. Additionally, it is demonstrated that the application of the aqueous process can be extended to a variety of other OEG-based material systems.

Dr. C. Lee, J. Choi, Y. Kim, Dr. W. Lee, Prof. B. J. KimDepartment of Chemical and Biomolecular EngineeringKorea Advanced Institute of Science and Technology (KAIST)Daejeon 34141, Republic of KoreaE-mail: [email protected]. R. LeeDepartment of Chemical EngineeringPohang University of Science and Technology (POSTECH)77 Cheongam-ro, Pohang, Gyeongbuk 37673, Republic of KoreaProf. J. H. OhSchool of Chemical and Biological EngineeringInstitute of Chemical ProcessesSeoul National University1, Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea

Organic Electronics

Dr. T. L. Nguyen, Prof. H. Y. WooDepartment of ChemistryKorea UniversitySeoul 02841, Republic of KoreaE-mail: [email protected]. B. GautamDepartment of Chemistry and PhysicsFayetteville State UniversityFayetteville, NC 28301, USAX. Liu, Dr. K. Zhang, Prof. F. HuangInstitute of Polymer Optoelectronic Materials and DevicesState Key Laboratory of Luminescent Materials and DevicesSouth China University of TechnologyGuangzhou 510640, P. R. China

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201802674.

1. Introduction

With the growing demand for environmentally sustainable man-ufacturing of optoelectronic devices such as organic field-effect

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and toluene: 1.0%).[12–14] Furthermore, complex and costly man-agement systems required for the production, use, and waste recovery of these halogen free solvents encourages research into suitable replacements.[15,16] In recent years, alcohol or aqueous processing based on the nanoparticle dispersion of photoactive materials has been suggested as an alternative route for the green fabrication of organic electronic devices.[17–20] However, this process also involves a large volume of toxic halogenated/organic solvents during the nanoparticle synthesis, which should be ultimately removed.

In the pursuit of achieving a truly ecofriendly processing method, an essential solution is considered to develop effective photoactive materials that are alcohol- or ultimately water-soluble. The most effective strategy to produce efficient alcohol/water-soluble organic semiconductors is to select the main semi-conducting backbone from a pool of previously reported prom-ising photoactive materials and replace the original hydrophobic side chains with hydrophilic side chains.[12,21–33] Recently, we developed an ethanol-soluble PPDT2FBT-A polymer, poly[(2,5-bis(1,3-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)propan-2-yloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thi-adiazole)], based on nonionic and hydrophilic oligoethylene glycol (OEG) side chains.[12,34] The use of the resulting PPDT2FBT-A polymers enabled the production of OFETs and PSCs by solution processing in ethanol (eco-OFETs and eco-PSCs).

Despite the first successful demonstration, the figures of merit for eco-OFETs and eco-PSCs were still low when com-pared to conventionally processed devices, having a hole mobility and power conversion efficiency (PCE) of only 1.0 × 10−2 cm2 V−1 s−1 and 0.75%, respectively. This is due to the relatively poor processability of the OEG-based materials in eth-anol as the PPDT2FBT-A polymers still showed low solubility in ethanol (≈2 mg mL−1 at room temperature). In the case of the fullerene acceptor, the solubility issue was much more serious: the OEG-based fullerene monoadduct was completely insoluble in ethanol and only the fullerene-bisadduct, which prevents efficient molecular packing and electron transport, could be used as the active material. The lack of solubility of the active materials not only impeded the optimization of the active layer morphologies, but also limited the opportunities for future development of efficient ethanol-processable materials. In order to improve the ethanol solubility from a material design stand-point, longer or bulkier solubilizing groups would need to be introduced. However, the crystallinity of the resulting materials is predicted to decrease in such cases, which in turn sacrifices the optimal electronic properties. It is therefore highly desir-able to develop new green solvent processing systems, but it also represents a significant challenge as there are no examples of solvents which are capable of improving the processability while still retaining the ecofriendliness of ethanol.

Herein we report novel processing conditions, utilizing water–ethanol cosolvents that can simultaneously improve pro-cessability and device performance. Interestingly, we found that the water–ethanol cosolvents can provide significantly improved solubility for OEG-based conjugated materials, in stark contrast to what is achieved with pure ethanol or pure water. The PPDT2FBT-A and the newly synthesized [6,6]-phenyl C61 butyric acid [3,4,5]-tri(ethylene glycol)-benzoic acid methyl ester (PC61BO12) fullerene monoadduct are nearly insoluble in either

ethanol or water; however, the solubilities of PPDT2FBT-A and PC61BO12 increased up to 42.9 and 40.5 mg mL−1, respec-tively, at a 25:75 (by vol%) water–ethanol composition. In order to gain a deeper understanding of the impacts of the water addition, the morphology-device performance correlation was investigated in PSCs and OFETs as a function of the water–eth-anol composition (from 0:100 to 100:0 by volume). From these experiments, it was determined that a water–ethanol process with an optimal water content produced highly ordered edge-on crystallites of PPDT2FBT-A polymers in thin film states, resulting in a high OFET mobility of 2.0 × 10−2 cm2 V−1 s−1. In the case of PSCs, the enhanced solubility of PC61BO12 in the water–ethanol cosolvents prevented the formation of large aggregates of fullerenes in the photoactive layers. Additionally, this allowed for the formation of more optimized bulk-hetero-junction (BHJ) morphologies, due to the improved miscibility of the fullerene adduct with the fibrils of the PPDT2FBT-A polymers. Benefitted from the favorable features facilitated by the water–ethanol process, a PCE of 2.05% was achieved by the resulting eco-PSC. To the best of our knowledge, the per-formances achieved for both the eco-OFETs and eco-PSCs are the highest among the devices fabricated with alcohol- or water-soluble conjugated materials. Importantly, all of the solution processing for these devices was performed in air, and the air stability of the eco-PSCs was unexpectedly high. The eco-PSCs stored in ambient air without encapsulation can retain 95% of the initial PCEs during a long exposure time of over 270 h.

2. Results and Discussion

The chemical structures of the model p-type ethanol-process-able and highly crystalline PPDT2FBT-A polymer and n-type PC61BO12 fullerene used for this study are depicted in Figure 1. The PPDT2FBT-A polymer donor was prepared by following the reported procedures,[12] and the number-average molecular weight and dispersity (Đ) of PPDT2FBT-A were determined to be 20.5 kg mol−1 and 2.51, respectively, by size exclusion chro-matography (SEC) calibrated with polystyrene standards. The PC61BO12 fullerene monoadduct was synthesized by following the reported literature procedure.[35]

One intriguing experiment provided an insight into new water–ethanol ecofriendly processing method: a highly alco-holic rum, Bacardi 151° (made by Bacardi Limited, 75.5% eth-anol and 24.5% water), significantly improved the solubility of the PPDT2FBT-A polymer (Figure 1 and Figure S1, Supporting Information). This result was surprising as water itself is an antisolvent for OEG-based conjugated materials, but the mixture of water/ethanol can improve the solubility remarkably compared to ethanol itself. Therefore, the solubilities (mg mL−1) of PPDT2FBT-A and PC61BO12 in various water–ethanol cosol-vents were first evaluated to better understand the effects of the addition of water on the solubility of OEG-based materials in the cosolvent system (Figure 2; Table 1). The water–ethanol composition was varied from 0:100 to 100:0 (vol%:vol%). Here-after, the mixed solvents are denoted as “Water-x,” where x represents the volume amount of water in the cosolvent. The mole percent of each solvent is also provided in Table 1 to facilitate the comparison of the actual compositions of water




and ethanol molecules at various Water-x conditions. It was found that Water-0 to Water-25 are ethanol-dominant when it comes to molar ratio, while Water-25 to Water-100 are water-dominant (aqueous) by molar ratio. The concentration limits of PPDT2FBT-A and PC61BO12 were determined using UV–vis spectroscopy,[36,37] and the detailed procedures for the solubility tests are given in the Supporting Information. For PPDT2FBT-A, the solubility in ethanol was 2.3 mg mL−1 at room temperature, which indicates that ethanol itself is not a good solvent for OEG-based PPDT2FBT-A (Figure 2a). However, it was found that the solubility can be remarkably improved in a broad range of the water–ethanol compositions (Water-5 to Water-40). For example, only 5 vol% of water in a solvent mixture afforded three times enhancement in the solubility limit (7.8 mg mL−1 at Water-5) in comparison to that in Water-0. The solubility of PPDT2FBT-A further increased along with a gradual increase of the water fraction, reaching a maximum value of 42.9 mg mL−1 in Water-25. When considering the typical fabrication conditions

of organic electronics, this solubility is sufficiently high to produce uniform active layers and to vary the film thickness as necessary. Water-40 is still a good solvent for PPDT2FBT-A (23.2 mg mL−1) but after this point, the solubility of PPDT2FBT-A begins to drop significantly with the solubility limit of Water-50 at only 2.7 mg mL−1 and nearly insoluble in the higher water percentage solvent systems.

The use of water–ethanol cosolvents also showed striking impacts in the solubility of the PC61BO12 fullerene monoadduct (Figure 2b). As seen in the inset pictures of Figure 2b, PC61BO12 was insoluble in either pure ethanol or water, likely due to the strong aggregation characteristics of the C60 fullerene cores. How-ever, PC61BO12 became soluble by the incorporation of a small amount of water (5 vol%) into the ethanol and can be dissolved in a range of cosolvent mixtures (5–35 vol% water). As observed for the PPDT2FBT-A polymer, PC61BO12 exhibited the highest solu-bility (40.5 mg mL−1) in the Water-25 solvent. This solubility value is almost a hundred times higher than that in pure ethanol. In


Figure 1. a) PPDT2FBT-A solutions in ethanol and water–ethanol. Chemical structures of PPDT2FBT-A and PC61BO12. b) PPDT2FBT-A pristine films fabricated from ethanol and water–ethanol. Device architectures of eco-OFETs and eco-PSCs.

Figure 2. Solubility of a) PPDT2FBT-A and b) PC61BO12 in water–ethanol cosolvents.



addition to PPDT2FBT-A and PC61BO12, the influence of water addition on the solubility of other conjugated organic materials with OEG side chains was examined and it was found that all of the materials had significantly enhanced solubilities in water–ethanol cosolvents (Figure S2, Supporting Information).

The aggregation behavior of PPDT2FBT-A in water–ethanol cosolvents with different water contents was further investigated by monitoring the optical properties by UV–vis spectroscopy (Figure S3, Supporting Information). The UV–vis spectra of PPDT2FBT-A in Water-0 showed a pronounced absorption peak (λmax) at ≈650 nm, indicating that the polymer chains of PPDT2FBT-A were preaggre-gated in the poor-solvent, ethanol.[12] As the fraction of water in the cosolvent mixture increased, the vibronic feature resulting from aggregation was gradually decreased. For instance, a sig-nificant blue-shift of ≈35 nm in the absorption maximum wave-length (λmax) was observed when the solvent was changed from Water-0 to Water-25. This hypsochromic shift in λmax indicates that the PPDT2FBT-A polymer chains became disaggregated in the mixed solvents due to the improved solubility from the addi-tion of water. In contrast, further addition of water (Water-35 and Water-50) resulted in a red-shift of λmax, which is attributed to the intermolecular aggregation resulting from the reduced solubility.

The observations from the UV–vis experiments are in line with the changes in the solubilities for PPDT2FBT-A and PC61BO12.

To better understand the origin of the solubility improve-ment with water addition, the radius of interaction (Ra) between the OEG-based materials and the water–ethanol cosol-vents were estimated using the Hansen solubility parameters (HSPs).[36,38–45] The similarity between two substances can be assessed from Ra, which is calculated from the differences in three HSP components

a D2 D12

P2 P12

H2 H12δ δ δ δ δ δ( ) ( ) ( )= − + − + −R a b c (1)

where δD, δP, and δH are the London dispersion interaction, permanent dipole-permanent dipole interaction, and hydrogen bonding interaction, respectively, and a, b, and c are weighting factors (a = 4 and b = c = 1, empirically set by Hansen). The Ra term means the distance between two molecules in the Hansen solubility space in which the three HSPs are coordinated on the x, y, and z axes. Evaluation of the Ra distances may help provide a plausible explanation for the enhanced solubility of the OEG-based photoactive materials in the water–ethanol mixed solvents. For example, the smaller Ra values that were obtained, the more favorable interaction and the better solubility are predicted to be observed.[41,42,45] A detailed description for the estimation of the Ra values is provided in the Supporting Information (Tables S1–S3, Supporting Information). Using the estimated HSPs and Equation (1), the Ra values were obtained and plotted as a function of the water volume fractions. As shown in Figure 3 and Table S1 (Supporting Information), progressive decreases in the Ra values were observed upon increasing water fraction for both PPDT2FBT-A and PC61BO12 until minimum Ra values were reached with Water-15–Water-25. Subsequently, the Ra values increased monotonically with the further increase in the water fraction. It should be noted that results of the Ra calculations were highly correlated with the observations in the trends of the solubility limits of PPDT2FBT-A and PC61BO12. While the trends in the Ra values of the PPDT2FBT-A and PC61BO12 with respect to the water content are clear, it should be noted that the change in the absolute Ra values was small. It is speculated that the fast diffusion of extremely small water molecules could be another important parameter for the improved solubility of OEG-based conjugated materials in the water–ethanol cosolvents.[38]

The use of a water–ethanol cosolvent during processing can have critical influences on the film deposition and resulting


Table 1. Measured solubility of PPDT2FBT-A and PC61BO12 in different water–ethanol cosolvents.

Water in cosolvent [vol%]

Water in cosolventa) [mol%]

Solubility [mg mL−1]

PPDT2FBT-A PC61BO12

0 0 2.3 0.3

5 15 7.8 2.7

10 26 9.3 11.7

15 36 17.2 27.9

25 52 42.9 40.5

35 63 29.8 26.0

40 68 23.2 <0.003

50 76 2.7 <0.003

55 80 0.4 <0.003

70 88 <0.007 <0.003

100 100 <0.007 <0.003

a)Density and molar mass used for this calculation were 0.789 g cm−3 and 46.07 g mol−1 for ethanol, 0.997 g cm−3 and 18.02 g mol−1 for water, respectively.

Figure 3. Plots of the radius of interaction and solubility for a) PPDT2FBT-A and b) PC61BO12 with respect to the volume fraction of water.



polymer morphologies, as well as the device performances, all of which may depend on the solvent mixture ratio. For instance, the different boiling points and volatilities of ethanol and water may have effects on these characteristics. To investigate how water–ethanol cosolvents impact the formation of films, the surface mor-phologies of the PPDT2FBT-A thin films, fabricated with different water–ethanol processing cosolvents, were analyzed using atomic force microscopy (AFM, Figure S4, Supporting Information). For these experiments, the same concentration (2 mg mL−1) and spin-casting condition (3000 rpm for 40 s) were used for all samples (details are provided in the Supporting Information). As shown in Figure S4 (Supporting Information), a smooth and uniform surface morphology, with a root-mean-square (RMS) roughness of 1.3 nm, was obtained with the use of the Water-5 solvent. In contrast, as the water content increased to 10 and 15 vol%, an increased RMS value of ≈2.7 nm, with a more distinct phase contrast, was observed indicating that the addition of water induced aggregation of the PPDT2FBT-A polymer during the film-forming process. This feature became more significant upon loading of a larger amount of water in ethanol (25–50 vol%); that is, largely aggregated thin film morphologies with rough topolo-gies (RMS roughness: 6–10 nm) were seen in the Water-25 and Water-50 samples. From these results, it is suggested that aqueous processing at properly chosen water contents is beneficial to form desired polymer aggregates while maintaining the homo-geneous film surfaces. However, the addition of large amounts of water can result in a loss of thin-film uniformity, most likely due to the higher boiling point and lower volatility of water than those of ethanol (water: 100 °C and 2.33 kPa, ethanol: 78 C and 5.95 kPa). Ethanol rapidly evaporates first during the spin-coating process, thus, in the cases of the water–ethanol cosolvents with large amounts of water, the residual solvent will become a water-excessive mixture during the spin-coating process. As a result,

rough thin films with large polymer aggregations are observed due to the poor solubility of PPDT2FBT-A in the water-excessive conditions. Importantly, we noted that these severely aggregated thin-film morphologies can even be observed from the Water-25 solution despite it having the best solubility of the polymer in solution. This implies that the evaporation kinetics of water during solution processing is a critical parameter in determining the thin-film morphology, and a comprehensive understanding on the interplay between the polymer solubility and the sol-vent evaporation kinetics as a function of the water content will be important to precisely control the thin-film morphologies of water–ethanol processed conjugated polymers.

Grazing incidence X-ray scattering (GIXS) analyses were conducted to investigate microstructural changes in the PPDT2FBT-A thin-films according to the water content. For the GIXS analyses, the polymer films were prepared by spin-coating of a water–ethanol cosolvent solution (≈2 mg mL−1) onto n-octadecyltrimethoxysilane (OTS)-modified SiO2/Si sub-strates. After solution processing, the films were dried in a vacuum oven to remove residual solvents. All of the 2D-GIXS images in Figure 4a exhibited a highly crystalline edge-on ori-entation of the PPDT2FBT-A films with strong Bragg diffraction peaks up to (400) along the out-of-plane direction, together with a pronounced π–π stacking peak along the in-plane direction (Figures 4b,c), even without additional treatment or annealing. These results indicate that the addition of water to the polymer solution did not significantly affect the crystalline nature and molecular orientation of the PPDT2FBT-A polymer. In addi-tion, the packing parameters of the PPDT2FBT-A films were compared depending on the volume percent of water used. As shown in Table 2, the lamellar and π−π stacking distances of all films were similar (≈22.5 and 3.6 Å, respectively). However, the coherence lengths (Lc) of the (010) π−π stacking peaks were


Figure 4. a) 2D-GIXS patterns of PPDT2FBT-A films processed with various water contents (0–50 vol%). Line cut data of 2D GIXS patterns in the b) out-of-plane and c) in-plane directions.



affected by the water contents.[46–48] The calculated Lc of the Water-0 film was 62.9 Å, but the Lc values increased to 67.4 Å for the water–ethanol cosolvents, indicating that the aqueous process produced larger crystallites with edge-on geometry.

To correlate the aforementioned thin-film morphologies with the charge transport properties of PPDT2FBT-A processed with various water–ethanol cosolvents, bottom-gate/top-contact eco-OFETs were fabricated and tested. The p-channel transfer char-acteristics of PPDT2FBT-A devices processed with various water contents are depicted in Figure 5 and the OFET performances of the PPDT2FBT-A thin-films are summarized in Table 3. The ethanol-processed PPDT2FBT-A OFETs yielded a mod-erate hole mobility of 6.4 × 10−4 cm2 V−1 s−1. However, the hole mobilities of the PPDT2FBT-A devices were greatly enhanced with the addition of water. For example, the Water-15 device showed a peak charge carrier mobility of 2.9 × 10−3 cm2 V−1 s−1, which is 4.5-times higher than that of the Water-0 device. The trends of the hole mobilities of PPDT2FBT-A OFET devices are in good agreement with the derived Lc values from GIXS analysis (Figure 5b). This suggests that the high electrical per-formance from the aqueous processing conditions is attributed to the formation of highly coherent edge-on crystals, thereby facilitating efficient transport of holes between the source and drain electrodes. The enhanced field-effect hole mobility of 2.0 × 10−2 cm2 V−1 s−1 was obtained by applying thermal annealing techniques to the aqueous-processed PPDT2FBT-A films (Figure S5, Supporting Information).

To examine the photovoltaic performance of the BHJ eco-PSCs as a function of the water–ethanol cosolvent composition, the PPDT2FBT-A:PC61BO12 devices were fabricated using a

standard device architecture of indium tin oxide (ITO)/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/active layer/PNDIT-F3N/Ag. The water-soluble PEDOT:PSS hole-transport layer was crosslinked by the addition of a trace amount (0.1 vol%) of (3-glycidyloxypropyl)trimethoxysilane (GOPS) crosslinker, which allowed for the water–ethanol processing of the active layer onto the PEDOT:PSS.[49,50] Poly[(9,9-bis(3ʹ-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-5,5ʹ-bis(2,2ʹ-thiophene)-2,6-naphthalene-1,4,5,8-tetracaboxylic-N,Nʹ-di(2-ethylhexyl)imide] (PNDIT-F3N) was used as an effi-cient electron-transport layer.[51] The water content was varied from 10 to 35 vol%. The donor:acceptor (D:A) ratio and the overall concentration in each solvent were optimized at 1:2.5 and 14 mg mL−1 (D: 4 mg mL−1, A: 10 mg mL−1), respectively, producing photoactive layers with a thickness of 60–70 nm. All of the device fabrication procedures, except for the depo-sition of the Ag electrode, were performed under ambient conditions. Representative current density–voltage (J–V) and external quantum efficiency (EQE) curves of the eco-PSCs are plotted in Figure 6, and the photovoltaic parameters are sum-marized in Table 4. There was a remarkable difference in the solar cell performances depending on the added water content in the cosolvent used for processing (Figure 6a). A functioning device could not be fabricated using Water-0 due to the very poor solubility of the PC61BO12 material in pure ethanol. The device fabricated with Water-10 had a low PCE value of 0.83%, but when the water content of the cosolvent was increased to 15 vol%, the PCE value of the PPDT2FBT-A:PC61BO12 eco-PSC was significantly enhanced to 2.05%, yielding an open-circuit voltage (Voc) of 0.76 V, short-circuit current (Jsc) of 5.08 mA cm−2, and fill factor (FF) of 0.53. Further addition of water gradu-ally reduced the device performance due to the decrease of Jsc and FF values. The EQE data confirm this trend (Figure 6b). The Water-15 device generated a substantially higher photocur-rent in the entire wavelength range of 400–700 nm as compared to those of the other devices, with a maximum EQE value of 34.2%. These trends in the PCE values and the EQE responses are correlated with those for the concentration limits of PPDT2FBT-A and PC61BO12. This PCE represents around threefold enhancement from our previously developed PPDT2FBT-A:Bis-C60-A eco-PSCs.[12] It was noted that the space-charge-limited current (SCLC) electron mobility of the PC61BO12 pristine film was 3.46 × 10−4 cm2 V−1 s−1, which was two orders of magnitude


Table 2. Crystallographic parameters of PPDT2FBT-A films processed from various water–ethanol cosolvents.

Water content [vol%] Lamellar spacing π–π spacing

qz [Å−1] d [Å] qxy [Å−1] d [Å] Lc [Å]

0 0.280 22.5 1.757 3.58 62.9

10 0.280 22.5 1.761 3.57 65.6

15 0.278 22.6 1.757 3.58 67.4

25 0.284 22.2 1.756 3.58 64.4

50 0.281 22.4 1.754 3.58 64.0

Figure 5. a) Transfer characteristics of PPDT2FBT-A films processed from water–ethanol solvents with various water contents. b) coherence length and hole mobility relationships of PPDT2FBT-A polymers as a function of the water content.



higher than that of the previously used bis-C60-A fullerene (1.33 × 10−6 cm2 V−1 s−1) with bis-OEG substituents (Figure S6; Table S4, Supporting Information). This is a key factor in the significantly improved performance (≈2% PCE) of the new eco-PSCs.

The effects of the water content of the processing cosolvent on the morphological properties of the BHJ active layers in the eco-PSCs were investigated by both AFM and transmis-sion electron microscopy (TEM) measurements. The AFM sur-face topologies of the eco-PSCs (Figure 7a), fabricated using Water-10 through Water-35, demonstrate that the water con-tent has a critical effect on the thin-film uniformity of the BHJ active layer. In agreement with the device performance data, the PPDT2FBT-A:PC61BO12 blend film which was deposited from Water-15 cosolvent exhibited the most uniform surface morphology, with a low RMS of 2.5 nm, followed by Water-10 (5.9 nm), Water-25 (12.0 nm), and Water-35 (21.6 nm). These different film morphologies are attributed to the different dis-solution behaviors of PPDT2FBT-A and PC61BO12 in each of the water–ethanol cosolvents during the solution pro-cessing. For example, the solubility of PC61BO12 in Water-10 is 11.7 mg mL−1, which is close to the concentration used during device fabrication (10 mg mL−1). This concentration near the saturation point led to stronger fullerene aggregation, and con-sequently, higher roughness in the BHJ active layer than that observed for the Water-15 sample. In the case of Water-25 and Water-35 processed samples, a much higher RMS roughness of the blend film was observed despite the fact that the solvent

mixture should provide enough solubility for both PPDT2FBT-A and PC61BO12. This is likely the result of the slower evapo-ration rate of water compared to ethanol, which is in good agreement with the trend observed in the pristine PPDT2FBT-A films fabricated from different water–ethanol compositions (Figure S4, Supporting Information).

TEM images of the PPDT2FBT-A:PC61BO12 blend films at different magnifications were acquired to gain further insight into the blend morphologies of the eco-PSCs (Figure 7b,c), and show the significant difference in the phase separation behavior of the active layer for the different cosolvents used. In Figure 7b, the Water-10 blend film showed microphase sepa-ration with very pronounced and large dark clusters, which corresponds to fullerene agglomerates.[52,53] In contrast, these fullerene agglomerates almost disappeared by the addition of 15 vol% water. Again, it was observed that the fullerenes were progressively segregated with higher water content (Water-25 and Water-35). The varied phase-separation of the PPDT2FBT-A:PC61BO12 films at different cosolvent water contents is one of the most important factors to determine the PCE value of the eco-PSCs, i.e., the smallest domain sizes in the Water-15 device should be beneficial to enhancing the charge generation efficiency at the D/A interface and, thus, achieving the highest Jsc and FF values of the eco-PSCs.[54–56] In contrast, the larger and/or isolated fullerene domains in the other blend films may lead to inefficient charge generation/transport with consider-able exciton recombination.[57,58]

Aqueous processing of the BHJ active layers also plays a criti cal role in the formation of nanostructures in the eco-PSCs. As shown in the TEM images (Figure 7c), all of the blend films contained fibrillar nanostructures of the PPDT2FBT-A polymer, but their features varied based on cosolvent composition. When using Water-10 and Water-15 solvents, well-defined fibrils of PPDT2FBT-A, with widths of ≈10 nm, were clearly observed. In the Water-15 sample, which yielded the best photovoltaic per-formance, more distinct and longer nanofibers were observed compared to those of the Water-10 sample. However, the self-assembled fibrillar structures of the PPDT2FBT-A polymer were progressively collapsed by the presence of additional water (i.e., Water-25 and Water-35 samples) and well-developed fibrils were less common in the Water-35 sample. This discern-ible difference in the BHJ morphology depending on the water content of the processing cosolvent was also evidenced by the results from resonant soft X-ray scattering (RSoXS) (Figure S7,


Table 3. OFET performances of PPDT2FBT-A polymers.

Device a) µh,maxb) [cm2 V−1 s−1] µh,avg

c) [cm2 V−1 s−1] Ion/Ioff VT [V]

Water-0 6.4 × 10−4 5.3 × 10−4 (±8.63 × 10−5)d) >104 3.8

Water-10 2.0 × 10−3 1.4 × 10−3 (±3.84 × 10−4) >104 0.8

Water-15 2.9 × 10−3 1.8 × 10−3 (±7.72 × 10−4) >104 −8.0

Water-25 2.3 × 10−3 1.4 × 10−3 (±5.28 × 10−4) >104 −16.4

Water-50 1.7 × 10−3 1.3 × 10−3 (±3.69 × 10−4) >103 −0.6

Water-25e) 2.0 × 10−2 6.5 × 10−3 (±1.90 × 10−3) >105 2.3

a)OFET performances of more than 10 devices measured under a nitrogen atmos-phere; b)Maximum; c)Average hole mobility of the OFET devices (L = 50 µm and W = 1000 µm); d)Standard deviation of the obtained mobilities; e)The devices were annealed at 180 °C for 20 min.

Figure 6. a) J–V curves and b) EQE responses of PPDT2FBT-A:PC61BO12 BHJ eco-PSCs processed from different water–ethanol cosolvents.



Supporting Information).[58–60] All of the blend films exhibited distinct scattering peaks at q = 0.03 Å−1, corresponding to an average domain spacing of ≈21 nm. This length scale is well-correlated with the average distance between the well-defined PPDT2FBT-A polymer fibrils identified from the TEM images. The most intense scattering was observed in the Water-15 blend, suggesting the presence (or the purest domains) of well-developed polymer fibrils. In contrast, as the water content increased to 25 and 35 vol%, the contrast at q = 0.03 Å−1 was decreased. Instead, the broad peak at low q region (<0.018 Å−1)

appeared, indicating that large length-scale phase separation between the PPDT2FBT-A and PC61BO12 domains occurred. An excess water content caused heavily aggregated morphologies with large domains and a poor surface uniformity.

The molecular packing structures in the PPDT2FBT-A:PC61BO12 blends were also investigated by GIXS meas-urements (Figure 7d; Figure S8, Supporting Information). First, it was observed that the water content of the processing solvent had a significant impact on the crystalline/aggre-gated behaviors of PC61BO12. While all the films showed an isotropic arc-pattern at q ≈ 1.42 Å−1, which is likely attrib-uted to the randomly oriented PC61BO12 crystallites,[61–63] large contrast in the dot patterns around the arc-patterns at q ≈ 1.42 Å−1 was observed depending on the water con-tent of the processing solvent. The pronounced dot pat-terns of the PC61BO12 aggregates appeared in the Water-10, Water-25, and Water-35 blend films, but the dot pattern was suppressed in the Water-15 film. These results agree well with the other morphological data (AFM, TEM) that showed the largely phase-separated morphologies of PPDT2FBT-A:PC61BO12 blend films with aggregated PC61BO12 domains (for the Water-10, Water-25, and Water-35 samples). Also, the


Table 4. Photovoltaic performance of PPDT2FBT-A:PC61BO12 BHJ eco-PSCs processed from different water–ethanol cosolvents.

Device Voc [V] Jsc [mA cm−2] FF PCEmax (PCEavg

a)) [%]Calculated Jsc

[mA cm−2]

Water-10 0.76 2.47 0.44 0.83 (0.62) 2.48

Water-15 0.76 5.08 0.53 2.05 (1.86) 5.00

Water-25 0.74 4.80 0.49 1.74 (1.36) 4.63

Water-35 0.73 3.86 0.49 1.39 (1.16) 3.77

a)Average PCEs of the eco-PSCs derived from more than ten devices.

Figure 7. a) AFM, b,c) TEM images, and d) 2D GIXS patterns of the PPDT2FBT-A:PC61BO12 blend films fabricated from water–ethanol cosolvents (10, 15, 25, and 35 vol% water).



aqueous process influenced the orientation of the PPDT2FBT-A polymers in their blend films. The Water-25 and Water-35 samples exhibited prominent (010) π–π stacking peaks of PPDT2FBT-A in the in-plane direction (Figure S8, Supporting Information), which indicates the presence of a substantial fraction of the edge-on oriented PPDT2FBT-A crystallites. How-ever, the orientation of the PPDT2FBT-A polymer in low water content samples (i.e., Water-10 and Water-15) was different. These films exhibited strong (010) π–π stacking peaks in the out-of-plane direction, indicating that many of the PPDT2FBT-A polymer crystallites were face-on oriented. In particular, the Water-15 sample had the most intense (010) scattering features of PPDT2FBT-A in the out-of-plane direction. Additionally, the Water-15 blend showed the smallest (010) π–π stacking distance of 3.67 Å−1, followed by Water-10 (3.70 Å−1), Water-25 (3.74 Å−1) and Water-35 (3.79 Å−1). The preferential face-on orientation of the PPDT2FBT-A polymer with a tight π–π stacking distance in the Water-15 device is a beneficial feature for producing effi-cient vertical charge transport and for the resulting photovoltaic performance of the eco-PSCs.[34,64–67]

Next, the ambient stability of the eco-PSC was examined, which is an essential consideration for possible commerciali-zation of these types of devices. It should be noted that, ben-efitted from the solvent processing of benign water/ethanol, all of the optimized eco-PSCs were produced in air without the use of a glovebox or inert atmosphere. Therefore, we tested the air stability of the best-performing PPDT2FBT-A:PC61BO12 eco-PSCs, processed from the Water-15 condition. The PCE values were monitored for devices stored in ambient air without encapsulation. Figure 8 displays a normalized PCE versus storage time plot. Surprisingly, the Water-15 eco-PSCs exhib-ited excellent air stability during long exposure time (>270 h) to ambient conditions, retaining 95% of the initial PCE. These promising results suggest that 1) the water-based ecofriendly process does not have an adverse impact on the air stability of the PSCs and 2) the OEG-based PPDT2FBT-A and PC61BO12 materials are highly stable under ambient conditions. The ben-eficial features of this new platform provide great potential for the ecofriendly and scalable manufacturing of solar cells with practical operating lifetimes. We speculate that this excellent stability could be attributed to the enhanced interfacial adhesive

properties between the hydrophilic active materials and the hole/electron transporting interlayers (i.e., PEDOT:PSS and PNDIT-F3N, respectively).[68] Further investigation of the origin of the excellent air stability in the aqueous-processed eco-PSCs is now underway.

3. Conclusion

In this manuscript, we demonstrate a simple yet effective water–ethanol cosolvent processing method for the fabrication of ecof-riendly OFETs and PSCs with high performances and stabilities. The addition of water as a cosolvent to ethanol played two crit-ical roles in improving device performance: 1) improving solu-bility of the active materials and 2) generating optimal thin-film morphologies for the active layers. Specifically, the inclusion of 25 vol% water afforded ≈20 and 135 times higher solubility (compared to that in ethanol) for the OEG-based PPDT2FBT-A polymer donor and PC61BO12 fullerene acceptor, respectively. And, the thin-film morphologies of OEG-based materials were also very sensitive to the water content in the cosolvents used for processing. Due to the much slower evaporation of water compared to ethanol, the water content of the residual solvent in the film significantly increases during film formation in the spin-coating process. This leads to rapid precipitation of the OEG-based materials, producing rough and excessively aggre-gated thin-film morphologies, even for solutions with high initial solubilities of the active materials (e.g., Water-35). There-fore, it was found that an optimized water content (15–25 vol%) was beneficial to producing uniform thin-films, tight interchain polymer packing, intermixed blend morphologies, and well-defined polymer fibril structures. Altogether, these properties contributed to yield the top-of-the-line charge-carrier mobility (2.0 × 10−2 cm2 V−1 s−1) and photovoltaic performance (2.05%) of the eco-OFETs and eco-PSCs. The findings of this model study suggest an explicit correlation between processing con-ditions, morphology, and device performance, which provide important guidelines for the further design of efficient and aqueous-processable organic optoelectronic devices. Further-more, it was demonstrated that this aqueous processing method is applicable to various types of OEG-based conjugated poly-mers, which raises the expectation for future development of efficient water–ethanol soluble electroactive materials.

4. Experimental SectionMaterials: PPDT2FBT-A and PC61BO12 were synthesized according

to the previous literature.[12,35] Detailed procedures for the synthesis of the other OEG-based polymers, poly[(4,8-bis(1,3-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)propan-2-yloxy)benzo[1,2-b:4,5-b′]dithiophene)-alt-(benzo[c] [1,2,5]thiadiazole)] (PBDTBT-A) and poly[(2,5-bis(1,3-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)propan-2-yloxy)benzene)-alt-(1,4-bis(thiophen-2-yl)-2,5-difluorobenzene) (PPDT2FP-A), were described in the Supporting Information.

Characterizations: The Mn and Đ values of polymers were determined by SEC equipped with Water 1515 Isocratic HPLC pump, temperature control module, and Waters 2414 refractive index detector. The SEC measurements were performed using o-DCB eluent at 80 °C, and calibrated with a polystyrene standard. Absorption spectra of PPDT2FBT-A in water:ethanol cosolvents were recorded by UV-1800


Figure 8. Air stability of the PPDT2FBT-A:PC61BO12 eco-PSC processed from the Water-15 cosolvent. The device was stored in ambient and dark conditions without encapsulation.



spectrophotometer (Shimadzu Scientific Instruments). TEM and AFM analyses were carried out using a JEOL 2000FX operated at 300 kV and a Veeco Dimension 3100 instrument in tapping mode, respectively. The RSoXS measurements were carried out at BL 11.0.1.2. in the Advanced Light Source (USA). For the TEM/RSoXS samples, PPDT2FBT-A:PC61BO12 active layers processed from different solvent system were directly spin-casted onto TEM copper grids and Si3N4 membranes. GIXS experiments were performed at beamlines 3C and 9A at the Pohang Accelerator Laboratory (South Korea).

Solubility Tests: Sufficient amounts of PPDT2FBT-A and PC61BO12 (above the solubility limit) were added to water–ethanol solvents and the resulting mixtures were heated up to 70 °C for 1 h to ensure maximum dissolution. Then, the solutions were cooled down to room temperature and filtered by polyvinyl difluoride (PVDF) syringe filters. The filtered solutions were diluted by adding known amounts of solvent, and absorbance measurements were performed for the diluted samples. The absorbance was compared to a reference curve generated using known concentrations, allowing the concentration of dissolved materials to be determined.

Fabrication and Characterization of eco-OFETs: OFETs were fabricated in a bottom-gate and top-contact configuration to characterize the electrical performance of PPDT2FBT-A using water:ethanol mixtures as processing solvents. A highly n-doped (100) Si wafer (<0.005 Ω cm) with a thermally grown 300 nm thick SiO2 layer (Ci = 11.5 nF cm−2) was utilized as the gate and dielectric. The SiO2/Si wafers were treated with an OTS self-assembled monolayer as previously reported.[69] The substrates were then washed sequentially with toluene, acetone, and isopropyl alcohol, and dried with nitrogen gas. PPDT2FBT-A was dissolved in ethanol or water–ethanol cosolvents with the concentration of ≈2 mg mL−1 and stirred at 60 °C for 5 h. The polymer thin-films were fabricated by spin-coating (3000 rpm for 50 s) of filtered polymer solution through a 0.2 µm membrane. Au electrodes (40 nm in thickness) were thermally evaporated through a shadow mask onto the semiconducting active layer. The electrical performance of the FETs was measured in a N2-filled glovebox using a Keithley 4200 semiconductor parametric analyzer. The field-effect mobility was estimated in the saturation regime (|VDS| > |VGS − VT|) using the following equation

2DSi

GS T2

IWC

LV Vµ ( )= − (2)

where IDS is the drain-to-source current, W and L are the semiconductor channel width and length, respectively, µ is the mobility, Ci is the capacitance per unit area of the dielectric, and VGS and VT are the gate and threshold voltage, respectively.

SCLC Measurements: Electron-only devices with pristine Bis-C60-A and PC61BO12 acceptors films were measured by the SCLC method using ITO/ZnO/active layers/LiF/Al device structures, respectively. The films were prepared as described in the device fabrication section. A range of 0–4 V was used for the current–voltage measurement, and the results were fitted to the Mott–Gurney equation

98SCLC 0

2

3J VL

εε µ= (3)

where ε0 is the permittivity of free space (8.85 × 10−14 F cm−1), ε is the relative dielectric constant of the active layer, µ is the charge carrier mobility, V is the potential across the device (V = Vapplied − Vbi − Vseries) corrected for potential loss due to built-in potential (Vbi) and series resistance (Vseries), and L is the active layer thickness.

Fabrication and Characterization of Normal-Type Eco-PSCs: The normal-type device with an ITO/PEDOT:PSS (+0.1 vol% GOPS)/active layer/PNDIT-F3N/Ag architecture was utilized to test the efficiencies of eco-PSCs. ITO substrates were subjected to ultra-sonication and washed with acetone, deionized water, and isopropyl alcohol solvents. After the rinsing process, the substrates were dried in an 80 °C oven for 20 min. The ITO substrates were treated with UV-ozone (10 min) before spin-coating PEDOT:PSS layers. The PEDOT:PSS solution containing 0.1 vol% of GOPS crosslinker was spin-coated onto ITO substrates at 3000 rpm for 40 s to produce a 40 nm thick PEDOT:PSS layer. The PEDOT:PSS

layers were annealed at 150 °C more than 30 min in ambient condition to fully crosslink the film, and then the devices were transferred to a N2-filled glove box. Electron donor PPDT2FBT-A was blended with PC61BO12 electron acceptor in different water:ethanol cosolvents. The D:A blend ratio was optimized to 1:2.5 (w/w), and the concentration of the polymer donor in the blend solution was 4 mg mL−1. The solutions were stirred for 1 h on a hot plate at 80 °C. The blend solution was spin-coated onto ITO/PEDOT:PSS substrates at 2000 rpm for 40 s. The resulting thickness of the eco-PSCs was measured to be 60−70 nm by AFM measurements. To remove residual solvents, the devices were dried under the high-vacuum condition overnight. PNDIT-F3N polymer was dissolved in methanol with a trace amount of acetic acid. Under ambient conditions, the solution was spin-coated onto the active layers at 2000 rpm for 60 s. The substrates were placed in an evaporation chamber under high vacuum (<10−6 Torr) for ≈1 h and the 120 nm thick Ag electrode was deposited. The active area of the fabricated device was measured as 0.164 cm2 by optical microscopy. The J–V characteristics of the eco-PSCs were collected under AM 1.5G solar irradiation (100 mW cm−2, solar simulator: K201 LAB55, McScience) and ambient conditions. This solar simulator system satisfied the Class AAA, ASTM Standards. The intensity of the solar simulator was calibrated using a standard silicon reference cell (K801S-K302, McScience). The J–V characteristics were recorded by a Keithley 2400 SMU. The EQE data were obtained using a spectral measurement system (K3100 IQX, Mc Science Inc.) with monochromatic light from a xenon arc lamp at 300 W filtered by a monochromator (Newport) and an optical chopper (MC 2000 Thor labs). The EQE data were obtained in dark conditions. The calculated JSC value was acquired by integrating the product of the EQE and the AM 1.5G solar spectrum, and showed good agreement with the measured JSC, within 5% error.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis research was supported by the National Research Foundation Grant (2017M3A7B8065584, 2012M3A6A7055540), funded by the Korean Government. H.Y. Woo is grateful for the financial support from Korea University (KU future research grant).

Conflict of InterestThe authors declare no conflict of interest.

Keywordsecofriendly solution processing, oligoethylene glycol-based conjugated materials, organic field-effect transistors, polymer solar cells, water–ethanol cosolvent

Received: August 27, 2018Revised: September 24, 2018

Published online:

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