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Aqueous Soluble Fullerene Acceptors for Efficient Eco-Friendly Polymer Solar Cells Processed from Benign Ethanol/Water Mixtures Youngkwon Kim,†,⊥ Joonhyeong Choi,†,⊥ Changyeon Lee,† Youngwoong Kim,† Changkyun Kim,† Thanh Luan Nguyen,‡ Bhoj Gautam,§ Kenan Gundogdu,∥ Han Young Woo,*,‡ and Bumjoon J. Kim*,†

Chem. Mater. 2018.30:5663-5672. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 10/21/18. For personal use only.

†

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, South Korea ‡ Department of Chemistry, Korea University, Seoul 136-713, South Korea § Department of Chemistry and Physics, Fayetteville State University, Fayetteville, North Carolina 28301, United States ∥ Department of Physics and Organic and Carbon Electronics, North Carolina State University, Raleigh, North Carolina 27695, United States S Supporting Information *

ABSTRACT: We present a new series of fullerene derivatives that exhibit solubility in ethanol/water solvent mixtures and implement these materials to fabricate polymer solar cells (PSCs) using environmentally benign solvents. In order to simultaneously optimize the processability of the fullerenes in ethanol/water solvent mixtures and device performance, different fullerene mono-adducts were designed by introducing oligoethylene glycol side chains with different lengths and number of branches. As a result, we achieved power conversion efficiencies up to 1.4% for PSCs processed from benign ethanol/water mixtures in air. Significantly, the new alcohol/water-soluble fullerene derivatives displayed electron mobilities up to 1.30 × 10−4 cm2 V−1 s−1, 150 times higher than those of a previously reported alcohol-soluble fullerene bis-adduct, owing to efficient packing of the fullerenes. Femtosecond transient absorption spectroscopy revealed the acceptor side chain to markedly impact geminate and/or nongeminate charge recombination in the PSCs. In addition, side chain optimization of these fullerenes produced well-intermixed morphologies with high domain purity when blended with p-type polymer to provide hole and electron transport pathways. Our results provide important guidelines for the design of electroactive materials for safe and environmentally benign fabrication of PSCs and other organic electronic devices.

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safely dispose of these solvents render them unsustainable.14 Therefore, it is important to develop the environmentally benign solvent-based platform for sustainable, large-scale device manufacturing of the eco-friendly PSCs (eco-PSCs). In particular, water and ethanol can be considered as one of the most attractive candidates for processing eco-PSCs by removing environmental and health concerns associated with the use of conventionally employed organic solvents. Engineering the side chains in conjugated materials (i.e., tuning the polarity, chain length, number of branches, and branch point composition) represents a compelling approach to develop aqueous-solvent processable materials, as side chains determine the solubility, crystallinity, and charge mobility of organic semiconductors.15−24 In particular, since the solubility and crystalline interchain packing of materials are

INTRODUCTION Polymer solar cells (PSCs) have attracted significant attention as future power generators owing to their solution-processability, band-gap tunability, and flexibility.1−5 However, the harmful halogenated processing solvents, such as chloroform and chlorobenzene, currently used in many lab-scale studies are the critical hurdles impeding the large-scale mass productions of PSCs. Despite the significance of and increasing interest in developing materials processable with “green” solvents, there has been limited success to date.6−9 Recent efforts include the discovery and optimization of new processing conditions that employ more benign organic solvents (e.g., toluene, xylene, anisole) compared to halogenated solvents, and/or tailoring conjugated materials to be compatible with such solvents.10−12 However, most of even nonhalogenated aromatic organic solvents may pose environmental and health hazards, and more importantly, their use is strictly prohibited in the semiconductor industry.13 In addition, the environmental toxicity and high energy cost to © 2018 American Chemical Society

Received: May 17, 2018 Revised: July 26, 2018 Published: July 27, 2018 5663

DOI: 10.1021/acs.chemmater.8b02086 Chem. Mater. 2018, 30, 5663−5672

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Chemistry of Materials

Scheme 1. Synthetic Route to the Ethanol/Water-Soluble Oligoethylene Glycol (OEG)-Substituted Fullerene Derivatives

phases in phase-separated blends with p-type polymers, decreasing electron transport and increasing charge recombination losses.34−36 Indeed, bis-C60-A was measured to exhibit very low electron mobility (8.8 × 10−7 cm2 V−1 s−1) as compared to the reasonably high hole mobility measured for PPDT2FBT-A (2.0 × 10−4 cm2 V−1 s−1). As such, new fullerene derivatives that confer solubility in polar solvents and yield high electron mobility are of paramount importance.33 Therefore, efficient and aqueous-processable fullerene acceptors should be designed to satisfy the conditions of (1) monoadducts type, (2) nonionic side chains, and (3) side chains with optimized length and branch structure to achieve both high solubility in aqueous solvent systems and yield crystalline assemblies of polymers conducive to electron transport in thin film. In this work, we developed a series of fullerene monoadducts with different OEG side chains, phenyl-C61-butyric acid 3,4,5-tris(2,5,8-trioxatridecanyloxy)benzyl ester (PCBO12), phenyl-C61-butyric acid 3,4,5-tris(2,5,8,11-tetraoxatridecan-13-yloxy)benzyl ester (PCBO15), and phenyl-C61butyric acid 3,4,5-tris(1,3-bis(3, 6, 9-trioxadecanyl)glycerol)benzyl ester (PCBO27), which enable fabrication of eco-PSCs processed from benign solvent mixtures (ethanol/water). These fullerene mono-adducts (PCBOs) incorporate linear or branched OEG side chains onto C60 derived from phenylC61-butyric acid methyl ester (PCBM) via ester linkage to achieve sufficient solubility in ethanol and water. We modulate the length and bulkiness of the OEG solubilizing groups to investigate the impact of side chain design on the molecular packing of the resulting fullerenes processed in ethanol/water solvent mixtures. The resulting PCBOs dissolved readily in

inversely related, sophisticated side chain engineering is vital to the design of PSCs with both high photovoltaic performance and eco-friendly processability.25−27 To this end, the development of polar side chains that impart solubility in alcohols and water without adversely impacting PSC performance is critically needed. Two classes of polar side chains in conjugated materials include those with ionic28−31 and nonionic moieties.32,33 Unfortunately, ionic groups in the photoactive layers typically act as charge traps, hindering charge transport by quenching charge carriers and altering the energy levels of the polymers, resulting in extremely low power conversion efficiencies (PCEs < 0.04%) and stability.28,29 Utilization of conjugated polymers with nonionic hydrophilic side chains can avoid this detrimental charge-trapping issue. In our previous work, eco-PSCs composed of p-type poly[(2,5bis(1,3-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)propan-2yloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT-A) and n-type fullerene bis-adduct bis-C60-A with polar oligoethylene glycol (OEG) side chains processed in ethanol afforded PCE of 0.7%.33 Despite the successful fabrication of eco-PSCs in benign ethanol solvents, the PCE remains low compared to conventionally processed PSCs, and there is significant room to improve the design of new aqueous-processable electroactive materials. In particular, the previously developed, ethanolsoluble bis-C60-A should be redesigned. Its bulky bis-adduct structures significantly hinder the formation of a well-defined crystalline nanostructure. More importantly, the presence of numerous regioisomers of the bis-adduct fullerenes both reduces the crystallinity and limits the percolation of fullerene 5664

DOI: 10.1021/acs.chemmater.8b02086 Chem. Mater. 2018, 30, 5663−5672

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Figure 1. (a) Device structure of inverted PSC and chemical structures of donor and acceptor used in this study; (b) normalized UV−visible absorption spectra in film; (c) the energy level diagram of the ethanol/water-soluble polymer donor (PPDT2FBT-A) and PCBO12, PCBO15, and PCBO27.

through polyvinylidene difluoride membranes. Subsequently, other fullerene byproducts were removed by silica gel column chromatography, eluting with DCM and methanol. Finally, dialysis was conducted in DI water containing a small amount of ethanol to remove residual ions such as K+, Na+, and Zn2+. We note that final step is crucial since OEG chains commonly have a strong affinity for moisture and ionic impurities, which can adversely affect the charge transport and overall performance of PSCs.43 After purification, 1H NMR spectroscopy and matrix-assisted laser desorption ionization time-of-flight mass spectrometry were conducted to determine the structures of the fullerene mono-adducts (Figures S1 and S2). All of the OEG-substituted PCBOs were readily soluble in common organic solvents (e.g., DCM, chloroform (CF), toluene, etc.) and in ethanol/water mixtures. We measured the solubility of the PCBOs in 88:12 (v/v) ethanol/water mixture, which was found to be the optimal solvent for fabricating eco-PSC devices. The solubility increased gradually with the length of the hydrophilic OEG side chains, in the order of PCBO12, PCBO15, and PCBO27 showing solubilities of 11 ± 1, 83 ± 4, and 197 ± 13 mg mL−1, respectively. Figure 1a,b shows the chemical structures and normalized UV−vis absorption spectra of the three fullerene acceptors and the PPDT2FBT-A donor polymer.33 PPDT2FBT-A was measured to have a number-average molecular weight (Mn) and a weight-average molecular weight (Mw) of 20.5 kDa and 51.5 kDa, respectively, relative to polystyrene standards by size exclusion chromatography. The adsorption spectra of the three PCBOs were similar; however, the overall absorbance of PCBO12 was slightly higher than those of PCBO15 and PCBO27 in the visible region owing to the larger fullerene fraction in the PCBO12 film, which is anticipated to provide improved light absorption in the PSCs44 (Figure S3). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of PPDT2FBTA and the fullerene derivatives were measured by ultraviolet photoelectron spectroscopy (UPS) and cyclic voltammetry (CV) (Figures 1c, S4 and S5). The HOMO/LUMO levels of PPDT2FBT-A were −5.19/−3.46 eV, as estimated from the UPS measurements and optical band gap. The HOMO/ LUMO levels of the PCBOs from CV increased gradually with side chain length, from −5.71/−3.86 for PCBO12, to −5.69/− 3.83 for PCBO15 and −5.67/−3.76 eV for PCBO27, and were higher than those of the PCBM.37 We confirmed that the HOMO levels of PCBOs from CV showed good agreements with those from the UPS measurements (Figures S4 and S5).

ethanol/water mixtures while maintaining effective electronic properties, enabling the fabrication of efficient PSCs from ecofriendly aqueous solvent mixtures. Eco-PSCs employing PCBO12 as the acceptor and PPDT2FBT-A as the donor in the photoactive layer afforded a PCE of 1.40%, which, to the best of our knowledge, is the highest reported PCE value for water/ethanol-processed PSCs. The favorable performance of these materials was shown to originate mainly from the improved short circuit current (JSC) and fill factor (FF) due to the high electron mobility of the mono-adduct PCBO12 (1.30 × 10−4 cm2 V−1 s−1), which is comparable to that of PCBM37 and >150 times increase compared to previously used bis-C60A (8.80 × 10−7 cm2 V−1 s−1).33 Decreasing the OEG side chain length reduced the solubility in ethanol/water and produced more highly ordered intermolecular fullerene packing structures and bulk-heterojunction morphologies with the polymer donor with higher domain purity, thus leading to improved charge generation, JSC, and PCE of these eco-PSCs.

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RESULTS AND DISCUSSION A new series of aqueous-processable fullerene mono-adducts (PCBOs), PCBO12, PCBO15, and PCBO27, named according to the number of oxygen atoms in OEG side chains, were synthesized by incorporating three different OEG solubilizing groups onto PCBM as shown in Scheme 1. Detailed synthetic routes for the three OEG-substituted fullerenes are provided in the Supporting Information (Schemes S1−S3). The new fullerene mono-adducts are derived from the PCBM, having an asymmetric solubilizing group known to provide fullerenes with excellent solubility and intermolecular packing.38 We selected the asymmetrically tethered hydrophilic OEG side chains to minimize their influence on the π−π stacking interactions of the fullerenes, so as to preserve their effective charge transport properties in PSCs. The PCBOs were synthesized by N,N′-dicyclohexylcarbodiimide (DCC)-mediated esterification reactions between three different OEG-substituted benzyl alcohols (4a−c) and [6,6]-phenyl-C61-butyric acid (PCBA).39,40 Briefly, PCBA was synthesized by the acidification of PCBM with HCl and acetic acid, and 4a−c were prepared according to previously reported methods.40−42 After the DCC coupling reactions, unreacted 4a−c were separated from the fullerene mixtures by suspension in 1:10 (v/v) methanol/deionized (DI) water, followed by centrifugation. Unreacted PCBA was removed by precipitation into dichloromethane (DCM)/isopropyl alcohol mixtures, and dicyclohexylurea byproducts were removed by filtration 5665

DOI: 10.1021/acs.chemmater.8b02086 Chem. Mater. 2018, 30, 5663−5672

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Figure 2. (a) J−V curves under the illumination of AM 1.5G, 100 mW cm−2; (b) EQE plots; (c) light intensity-dependent JSC; and (d) VOC of PPDT2FBT-A:PCBOs devices.

Table 1. Photovoltaic Properties and SCLC Electron Mobilities VOC (V)

JSC (mA cm−2)

FF

PCEmaxb (PCEavg)b (%)

μe,maxc (μe,avg)c (cm2 V−1 s−1)

PPDT2FBT-A: PCBO12

0.61

4.78

0.48

PPDT2FBT-A: PCBO15

0.64

3.69

0.47

PPDT2FBT-A: PCBO27

0.71

1.24

0.42

PPDT2FBT-A: Bis-C60-Aa

0.81

2.36

0.39

1.40 (1.20 ± 0.17) 1.11 (0.96 ± 0.08) 0.37 (0.35 ± 0.04) 0.75

1.30 × 10−4 (1.08 ± 0.13 × 10−4) 1.02 × 10−4 (9.20 ± 0.08 × 10−5) 6.30 × 10−6 (5.34 ± 0.05 × 10−6) 8.80 × 10−7

PPDT2FBT-A: PCBO

a

Parameters reported in our previous work, in which devices were processed from ethanol. bValues in parentheses are averages and standard deviations of 10 devices. cElectron mobilities of PCBO pristine films prepared from 88:12 (v/v) ethanol/water mixture measured by the SCLC method under ambient condition. The average values and standard deviations were calculated from 8 separate devices. 33

photovoltaic device performance is summarized in Table 1. The PPDT2FBT-A:PCBO12 and PPDT2FBT-A:PCBO15 devices yielded the highest maximum PCE (PCEmax) values of 1.40% (VOC = 0.61 V; JSC = 4.78 mA cm−2; FF = 0.48) and 1.11% (VOC = 0.64 V; JSC = 3.69 mA cm−2; FF = 0.47), respectively, whereas the PPDT2FBT-A:PCBO27 device afforded a lower PCEmax of 0.37% (VOC = 0.71 V; JSC = 1.24 mA cm−2; FF = 0.42). PPDT2FBT-A:PCBO12 exhibited the lowest VOC among the three blends, primarily due to the low LUMO level of PCBO12. Therefore, its high PCE of PPDT2FBT-A:PCBO12 is due to the enhanced JSC and FF values. Accordingly, the EQE curves of Figure 2b show PPDT2FBT-A:PCBO12 devices to generate the most photocurrent in the range of 350−750 nm. The integrated JSC values from the EQE spectra matched well with the measured JSC values, with less than 5% error. To investigate the effects of the structural modifications of the mono-adduct fullerenes on their charge transport properties, the space-charge-limited current (SCLC) method was

The increased HOMO/LUMO energy levels of the PCBOs were mainly attributed to the introduction of the OEG groups, which act as electron-donating groups.45,46 Therefore, we expect that all of the three PCBOs exhibited favorable energylevel matching with PPDT2FBT-A to facilitate efficient interfacial exciton dissociation.47,48 To examine the effects of the length and bulkiness of the OEG-based solubilizing groups on device performance, ecoPSCs were fabricated with the inverted structure, ITO/ZnO/ PPDT2FBT-A:PCBOs/MoO3/Ag. The entire device fabrication process was performed under ambient conditions (i.e., outside a glovebox) with ethanol and water as the processing solvents. In order to improve the film uniformity, both the ZnO coated ITO substrate and the blend solutions were heated to 80−90 °C before spin-coating the blends on the ZnO layer. The detailed device fabrication procedures are described in the Experimental Section. The current density− voltage (J−V) and external quantum efficiency (EQE) characteristics are shown in Figure 2a,b, respectively, and the 5666

DOI: 10.1021/acs.chemmater.8b02086 Chem. Mater. 2018, 30, 5663−5672

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Figure 3. Room-temperature transient absorption spectra (TAS) for (a) PPDT2FBT-A:PCBO12, (b) PPDT2FBT-A:PCBO15, and (c) PPDT2FBT-A:PCBO27 blend films acquired with a pump energy of 1.91 eV.

employed to measure the electron mobilities of the PCBO neat films (Table 1 and Figure S6). The PCBO neat films were prepared in 88:12 (v/v) ethanol/water mixture under ambient condition, which was the same processing solvent of fabricating eco-PSC devices. Markedly higher maximum SCLC electron mobilities (μe) of 1.30 × 10−4 and 1.02 × 10−4 cm2 V−1 s−1 were observed for the PCBO12 and PCBO15 films, respectively, whereas PCBO27 afforded the lowest electron mobility (μe,max = 6.30 × 10−6 cm2 V−1 s−1), over an order of magnitude less than PCBO12 and PCBO15 films. This significant reduction in the electron mobility is attributed to the longer, branched OEG chains of PCBO27 interrupting the formation of ordered fullerene phases. Importantly, although PCBO27 showed the lowest electron mobility among the mono-adduct PCBOs, all μe values measured for the PCBOs were significantly higher than that obtained from bis-C60-A (μe,max = 8.8 × 10−7 cm2 V−1 s−1). The μe of the PCBO12 film was measured to be ca. 150 times higher than that of bis-C60-A (Table 1). Further, the higher μe of PCBO12 is comparable with high hole mobility (μh = 2.0 × 10−4 cm2 V−1 s−1) of PPDT2FBT-A, resulting in more balanced electron and hole mobilities; μh/μe ratios of the PCBO12, PCBO15, and PCBO27 active layer blends were measured to be 1.5, 2.0, and 31.7, respectively. Therefore, the PPDT2FBT-A/PCBO12 active layer exhibits optimum charge transport characteristics for efficient charge extraction and reduced recombination.49 The charge carrier recombination was next examined by measuring the J−V characteristics as a function of the light intensity (P). Figure 2c shows the JSC of the PPDT2FBTA:PCBO devices plotted as a function of P on a log scale, which describes the extent of bimolecular recombination from the photocurrent loss. In general, the relationship between JSC and P can be expressed with a power-law, where JSC ∝ Pα. If the photocurrent loss due to bimolecular recombination is negligible, α approaches 1.50,51 The α values of the PPDT2FBT-A:PCBO12 (1.02) and PPDT2FBT-A:PCBO15 blends (0.96) were closer to unity than that of the PPDT2FBT-A:PCBO27 blend (0.94), indicating the higher incidence of bimolecular recombination in the PPDT2FBTA:PCBO27 blend. The change in the P-dependent VOC with different OEG chains was more pronounced (Figure 2d). The slope (S) gives kBTq−1, where kB is Boltzmann constant, T is an absolute temperature, and q is elementary charge, and decreased considerably from 1.60 for PPDT2FBTA:PCBO27 films to 1.28 for PPDT2FBT-A:PCBO15 films and 1.16 for PPDT2FBT-A:PCBO12 films, indicating that monomolecular and/or trap-assisted charge recombination was significantly suppressed in the PPDT2FBT-A:PCBO12 blend.52,53

To gain a deeper understanding of the impact of acceptor side chain modification on charge generation and recombination, we used femtosecond transient absorption spectroscopy in PPDT2FBT-A:PCBO12, PPDT2FBT-A:PCBO15, and PPDT2FBT-A:PCBO27 blends to monitor exciton separation and charge recombination dynamics. Figure 3 shows the transient absorption spectra (TAS) in the near-infrared (IR) region at different time delays following exposure of the blend films to pump pulses at 1.91 eV (650 nm). Donor polymer excitons were generated predominantly at this excitation energy (Figure S7). The pump excitation intensity was kept low (∼3 μJ cm−2) to avoid possible exciton−exciton annihilation and nonlinear effects. Two absorption peaks were observed for all three blends at 0.95 and 1.3 eV, corresponding to the excited state absorption of polymer excitons and polarons, respectively. These assignments are consistent with previously reported TAS of polymer blends,54−56 and were further confirmed by comparison with the TAS of the neat polymer donor film (Figure S7). In the TAS of the neat polymer film, we observed the dominant singlet exciton peak at ∼0.95 eV and a weak signal at ∼1.3 eV due to polaron states. To investigate the time evolution of excitons and polarons in the different blends, we isolated individual contributions of these species by deconvolution of TAS at each delay using Gaussian fitting.56,57 To compare the exciton dynamics in the different blends, we plotted the time evolution of the peak at ∼0.95 eV (Figure 4a). Exciton decay profiles of neat and blend films were fitted with double exponential functions. The neat polymer donor exciton dynamics was monitored, and the exciton lifetime was estimated to be 38 ps. In contrast, the exciton peak decayed at a much faster rate in all of the PPDT2FBT-A:PCBO12, PPDT2FBT-A:PCBO15, and PPDT2FBT-A:PCBO27 blends, yielding average decay times of 6 ps. The similar exciton decay times observed for all three blends suggested that the charge transfer from donor to acceptor is similarly efficient in all of the blends and is not limited by the acceptor side chain modification. Next, we monitored the polaron dynamics in the three blend films, varying excitation fluence to study geminate and nongeminate bimolecular recombination kinetics (Figure 4b−d). Whereas polarons in the PPDT2FBT-A:PCBO12 and PPDT2FBT-A:PCBO15 blends exhibited faster recombination with increasing excitation intensity, those in the PPDT2FBTA:PCBO27 blend were less sensitive to excitation fluence. Existence of a strong correlation between the recombination rate and excitation intensity indicates enhanced nongeminate bimolecular recombination at higher excitation fluence, due to more efficient charge generation in the PPDT2FBT5667

DOI: 10.1021/acs.chemmater.8b02086 Chem. Mater. 2018, 30, 5663−5672

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PPDT2FBT-A polymer exhibited strong crystalline characteristics with distinct scattering peaks up to fourth order reflections (400) in the out-of-plane direction (qout) and a (010) π−π stacking peak appeared in the in-plane direction (qin). For all of the fullerene derivatives (Figure S8), isotropic scattering rings were observed at q ≈ 0.71 and 1.42 Å−1, resulting from fullerene aggregates. In addition, pronounced scattering spots were observed at qin ≈ 1.42 Å−1 (near the isotropic ring patterns) and at qout ≈ 0.55 and 1.10 Å−1. The peak intensites are much stronger than those in the scattering profiles of the pristine PCBM films, suggesting that the introduction of the shorter OEG side chains promoted intermolecular interactions between the PCBOs.60−63 We observed significant differences in the scattering intensities of the PCBOs: as the length and number of OEG chains decreased, the isotropic scattering rings at q ≈ 1.42 Å−1 and the spots at qin ≈ 1.42 Å−1 became more discernible, indicating formation of strongly aggregated fullerene structures with less bulky OEG side chains. Additionally, the intensity of the scattering spots at qout ≈ 0.55 and 1.10 Å−1 decreased with the longer length and more number of OEG chains. This feature can be correlated with the effective formation of an electron transport network from PCBO12. We note that the OEG side chain dependence of the PCBO aggregation trends are similar in both blends and neat PCBO thin films. Figure 5 shows the GIXS results of PPDT2FBT-A:PCBO12, PPDT2FBTA:PCBO15, and PPDT2FBT-A:PCBO27 blend films, corresponding with optimized device fabrication conditions. Notably, while all of the three blend films displayed a diffuse halo at q ≈ 1.42 Å−1 from PCBO, the highest intensity peak was observed in the PPDT2FBT-A:PCBO12 blend. The strongest peaks (qin ≈ 1.42 Å−1 and qout ≈ 0.55 and 1.10 Å−1) from the PCBO molecules were seen in the PPDT2FBTA:PCBO12 blend film, but it is almost indistinguishable in the PCBO27 blend films. Therefore, the higher JSC and FF values of PPDT2FBT-A:PCBO12 blend films can be attributed to the formation of sufficient percolation of aggregated PCBO12 domains for better electron transport. The bulk morphologies of the three blend films were investigated by transmission electron microscopy (TEM) (Figure 6). All three blend films formed interconnected

Figure 4. Time evolution of (a) excitons and (b−d) light intensitydependent polaron decay profiles for PPDT2FBT-A:PCBO12, PPDT2FBT-A:PCBO15, and PPDT2FBT-A:PCBO27 blends.

A:PCBO12 and PPDT2FBT-A:PCBO15 blend films.58,59 Conversely, the low sensitivity of polaron recombination to excitation intensity in the PPDT2FBT-A:PCBO27 blend films suggests that geminate recombination is dominant, which correlates well with the P-dependent VOC trends in Figure 2d. The predominantly geminate charge recombination observed in the PPDT2FBT-A:PCBO27 blend may be related to the bulky and branched OEG side chains impeding charge transport. To better understand the relationship between the PCBO side chain structure and electronic properties of the active layer blends, we performed grazing incidence X-ray scattering (GIXS) on PCBO pristine films, PPDT2FBT-A neat film, and the PPDT2FBT-A:PCBO blend films, prepared by aqueous solvent at the optimized device condition (88:12 (v/v) ethanol/water mixtures) (Figures 5, S8 and S9). The

Figure 6. TEM images of (a) PPDT2FBT-A:PCBO12, (b) PPDT2FBT-A:PCBO15, and (c) PPDT2FBT-A:PCBO27 blend films.

fibrillar networks of PPDT2FBT-A with a 15−20 nm fiber width, resulting from their high crystallinity due to interchain noncovalent Coulombic interactions and high planarity of the PPDT2FBT-A backbone.33,64 Interestingly, the PPDT2FBTA:PCBO12 and PPDT2FBT-A:PCBO15 blend films yielded more distinct fibrils, presumably due to stronger aggregation of PCBO molecules having fewer and shorter OEG side chains. Therefore, neither introduction of OEG side groups nor

Figure 5. 2D-GIXS patterns of (a) PPDT2FBT-A:PCBO12, (b) PPDT2FBT-A:PCBO15, (c) PPDT2FBT-A:PCBO27 blend films and the corresponding (d) in-plane and (e) out-of-plane line-cut profiles. 5668

DOI: 10.1021/acs.chemmater.8b02086 Chem. Mater. 2018, 30, 5663−5672

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CONCLUSIONS In summary, we synthesized three OEG-substituted fullerene mono-adducts (i.e., PCBOs) capable of being processed from environmentally benign ethanol/water mixtures. First, we characterized the basic optical and electrochemical properties of the fullerene derivatives PCBO12, PCBO15, and PCBO27 having OEG side chains with different structures and lengths. Next, we fabricated eco-PSCs using ethanol/water mixtures in ambient conditions. The PCBO12-based eco-PSCs yielded the highest PCE of 1.40% among the three active layer blends. Importantly, the higher device performance of the PCBO12based PSCs was mainly attributed to the high electron mobility of the mono-adduct, PCBO12, resulting from the more tightly packed PCBO domains enabled by the reduced steric hindrance imparted by the short, linear OEG side groups. Among the three blend devices, PPDT2FBT-A:PCBO12 showed the least charge recombination and superior charge transport properties. The higher domain purity of the PCBO12 and PCBO15 films afforded more efficient charge transport. The results in this work provide important guidelines for the design of aqueous-processable electroactive materials having high carrier mobilities suitable to achieve highly efficient ecoPSCs.

processing the blend solutions in ethanol/water mixtures hindered the formation of the interconnected fibrillar morphologies, which is a crucial requirement for efficient charge transport.65 Resonant soft X-ray scattering (RSoXS) measurements were conducted to gain a quantitative measure of the BHJ morphology of the PPDT2FBT-A:PCBOs blends.66−68 Figure 7 shows the Lorentz-corrected RSoXS profiles acquired at

Figure 7. RSoXS profiles of PPDT2FBT-A:PCBOs blends.

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284.4 eV, at which maximum scattering contrast between the polymer and the PCBOs domains was observed.66 Distinct peaks were observed at q = 0.021 and 0.023 Å −1 , corresponding to average domain spacings of 29.9 and 27.3 nm for PPDT2FBT-A:PCBO12 and PPDT2FBT-A:PCBO15, respectively. In contrast, the scattering intensity of PPDT2FBT-A:PCBO27 films was much lower in the q range between 0.002 and 0.06 Å−1, corresponding to domain spacings of 10−310 nm, than the other two blends, consistent with the lower aggregation tendency of PCBO27. These data indicate higher domain purity of the PPDT2FBT-A:PCBO12 and PPDT2FBT-A:PCBO15 blends than that of the PPDT2FBT-A:PCBO27 blend. To further quantify the domain purity, the relative root-mean-square composition variation, proportional to the root of integrated scattering intensity (√ISI) and related to domain purity,69−71 was determined for each of the blends. The relative √ISI increased significantly from 0.18 for the PPDT2FBT-A:PCBO27 blend to 0.97 in the PPDT2FBT-A:PCBO15 blend and 1 in the PPDT2FBTA:PCBO12 blend which was used as a reference. Thus, the √ISI variation of the PPDT2FBT-A:PCBO12 blend was 5.6 times greater than that of the PPDT2FBT-A:PCBO27 blend. We speculate that the higher domain purity in the PCBO12and PCBO15-containing blends is mainly attributed to phase separation driven by the stronger crystallization of PCBO12 and PCBO15.72−74 These results are consistent with (1) the more distinct PCBO packing structures of PPDT2FBTA:PCBO12 and PPDT2FBT-A:PCBO15 observed in the GIXS measurements and (2) the formation of more distinct PPDT2FBT-A fibrils and BHJ morphologies observed by TEM. It is commonly accepted that high domain purity is beneficial in facilitating charge transport and suppressing charge recombination, leading to higher JSC and FF values.74−77 In contrast, the weak and broad scattering of PPDT2FBT-A:PCBO27 blends indicates reduced fullerene aggregation.77 This feature suggests that there are limited charge percolating pathways and large recombination losses in the PPDT2FBT-A:PCBO27 blend, which agrees well with the results of our charge recombination studies.

EXPERIMENTAL SECTION

General Procedure To Synthesize PCBOs. PCBA (5) (500 mg, 1 equiv) was added to o-dichlorobenzene (ODCB, 250 mL) under N2 in 500 mL round-bottom flasks, sonicated for 10 min, and stirred at 100 °C for 30 min to ensure complete dissolution. The mixture was cooled to room temperature, and 4-dimethylaminopyridine (0.4 equiv) and 3,4,5-OEG-benzyl alcohol (4a−c) (1.5 equiv) were added and stirred for 15 min. Next, DCC (1.2 equiv) was dissolved in ODCB (3 mL) and injected into the reaction mixture. The reaction was stirred for 3 d; then the ODCB was removed under reduced pressure by evaporation. The PCBOs were purified as follows: (1) Unreacted 4a−c were removed by centrifugation at 8000 rpm for 1 h in a 1:10 (v/v) methanol/water mixture; (2) residual PCBA was precipitated by centrifugation at 5000 rpm for 15 min in a DCM/ isopropyl alcohol (IPA) mixture; (3) the supernatant containing the desired products was filtered through a polyvinylidene difluoride (PVDF) membrane (0.22 μm pore size, Ks BIO) to remove dicyclohexylurea byproduct; (4) PCBOs were separated from fullerene byproducts by silica gel column chromatography using DCM/methanol mixtures as eluents; (5) PCBOs were dialyzed in pure DI water containing a small amount of ethanol to remove residual ions such as K+, Na+, and Zn2+. All purification steps, except chromatography, were conducted at least three times. Detailed synthesis procedures of PCBOs and their characterization data can be found in the Supporting Information. Fabrication of eco-PSCs. Eco-PSCs were fabricated with an inverted architecture (ITO/ZnO/active layer/MoO3/Ag) using ethanol and water mixtures as follows. All procedures except for thermal depositions of MoOx/Ag were carried out under ambient condition. ITO-coated glass substrates were sonicated in acetone, DI water, and IPA, dried for several hours at 80 °C, and treated with UVozone (10 min) prior to spin-coating ZnO layers. The ZnO layer was deposited by spin-coating onto the ITO substrates using ZnO solution, and the coated substrates were then annealed at 215 °C for 15 min under ambient conditions. The polymer donor PPDT2FBT-A and the PCBO12, PCBO15, and PCBO27 were dissolved in 88:12 (v/v) ethanol/water mixture. The concentrations of the blend solutions were 4−5 mg mL−1 for PPDT2FBT-A, and the optimized PPDT2FBT-A:PCBOs ratios were found to be 1:2.5 (w/w) for PPDT2FBT-A:PCBO12 and 1:2 (w/w) for PPDT2FBT-A:PCBO15 and PPDT2FBT-A:PCBO27. The blend solutions were stirred for 1 h at 80−90 °C. The ITO/ZnO substrates were then heated for 5−10 min in the range of 80−90 °C before spin-coating the blend solutions 5669

DOI: 10.1021/acs.chemmater.8b02086 Chem. Mater. 2018, 30, 5663−5672

Article

Chemistry of Materials

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at 3000 rpm for 40 s to yield active layers with 70−90 nm thick under ambient conditions. Finally, the substrates were placed in an evaporation chamber under high vacuum (