Synergetic Solvent Engineering of Film Nanomorphology to Enhance

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Synergetic Solvent Engineering of Film Nanomorphology to Enhance Planar Perylene Diimide Based Organic Photovoltaics Jialin Wang, and Ziqi Liang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08284 • Publication Date (Web): 11 Aug 2016 Downloaded from http://pubs.acs.org on August 14, 2016

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ACS Applied Materials & Interfaces

Synergetic Solvent Engineering of Film Nanomorphology to Enhance Planar Perylene Diimide Based Organic Photovoltaics Jialin Wang and Ziqi Liang* Department of Materials Science, Fudan University, Shanghai 200433, China

ABSTRACT: Solvent additive has proven as a useful protocol for improving the film nanomorphology of polymer donor (D) : fullerene acceptor (A) blends in bulk heterojunction (BHJ) photovoltaic cells. By contrast, the effect of such solvent additive on nonfullerene BHJ cells based on perylene diimide acceptor, for instance, is less effective owing to their highly planar structure and strong π-aggregation in solid state. Here we choose N,N′-bis(1-ethylpropyl)perylene-3,4,9,10-tetracarboxylic

diimide

(PDI)

and

thieno[3,4-b]thiophene-alt-

benzodithiophene (PTB7) as a model D:A blend system to investigate how solvent engineering strategy synergistically impacts the blend film nanomorphology. Based on the differences of solvent volatility and solubility, various host solventschloroform (CF) and chlorobenzene (CB) and solvent additiveschloronaphthalene (CN) and 1,8-diiodooctane (DIO) are selected for comparative studies. It is found that the π-aggregation of PDIs can be largely suppressed by using low-boiling point (Tb) CF solvent, yet enlarged by using high-Tb CB. Moreover, CN additive provides good solubility of PDI molecules and hence reduce large PDI aggregates in CB

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system, while DIO exhibiting poor solubility works oppositely. By contrast, DIO that presents larger Tb difference with CF prolongs the film-forming, which assists to optimize the PDI aggregation and increase the intermixed PTB7:PDI phases more significantly than CN in CF system, yielding the finest phase-separation morphology and balanced charge mobility. Consequently, the inverted BHJ cells based on CF-processed PTB7:PDI blend film with 0.4 vol% DIO exhibit the highest PCE of 3.55% with a fill factor of 56%, both of which are among the best performance for such a paradigm PTB7:PDI blend based BHJ cells.

KEYWORDS: organic photovoltaics, nonfullerene, perylene diimide, aggregation, solvent engineering

1. INTRODUCTION Recent years have witnessed rapid advances of perylene diimides (PDIs) based nonfullerene acceptors for solution-processed organic photovoltaic (OPV) cells.1−4 PDIs are considered as promising candidates yet encounter the issue of large π-stacked aggregation, especially in planar PDI molecules, resulting in micrometer-sized crystals and thus incomplete exciton dissociation.5−10 Bulk-heterojunction (BHJ) OPV cells based on the blends of planar PDI-type electron acceptors and polymer donors usually yielded poor power conversion efficiencies (PCEs) with low fill factor (FF) and short-circuit current density (Jsc), even when they are paired with those champion donors which exhibited record PCE in fullerene-based solar cells.11−14 Although twisted or bulky, 3D structured PDI molecules have been largely made to suppress their aggregations, which involves enormous synthetic effort and are not desired for low-cost,


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large-scale production.15−20 Moreover, they break the PDI coplanarity, which weakens longrange π-ordering and hence reduces electron mobility.21 Solvent engineering is one of the paramount protocols in improving the device performance, which is based on the solubility difference of donor and acceptor components in solvent additive and the difference of boiling point (Tb) between host solvent and solvent additive. Such two factors can synergistically impact the film-forming kinetics, including the crystallization and aggregation of both donors and acceptors along with their phase separation, which have been well-studied in polymer:fullerene blends.22−28 It has however not yet been extended to nonfullerene acceptors based BHJ blends due to the striking difference between spherical fullerene and planar π-structural PDI acceptors. More importantly, a good understanding is needed towards the parameters that dictate the morphology of nanostructured polymer:PDI blend films. While numerous efforts have been focused on the effect of solvent additive on the film morphology of PDI based molecules in a blend with donor polymers11,13,29 or small molecules,30,31 the impact of a combination of different host solvent and solvent additive, which enables a fine-tuning of the nanomorphology of the blend film, has yet to be largely explored. In order to clarify the synergistic effects of host solvent and solvent additive on the nonfullerene based BHJ nanomorphology, we choose the benchmark thieno[3,4-b]thiophene-altbenzodithiophene (PTB7) as polymer donor and 2,9-di(pent-3-yl)-anthra[2,1,9-def:6,5,10d'e'f']diisoquinoline-1,3,8,10-tetrone (PDI) planar molecule as acceptor to construct inverted BHJ photovltaic cells as shown in Figure 1, and employ two sets of host solvents with different Tb, i.e., chlorobenzene (CB) and chloroform (CF), and solvent additives of chloronaphthalene (CN) and 1,8-diiodooctane (DIO) with different PDI solubility. Table S1 in the Supporting Information lists Tb of these host solvents and solvent additives, and their respective solubility of



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PTB7 donor and PDI acceptor. When using high-Tb CB solvent, planar PDI molecules tend to aggregate easily, while the use of volatile CF solvent can suppress PDI aggregation efficiently yet is not beneficial for the crystallization of polymer donor. Thus, by adding CN (good solubility) in CB and adding DIO (poor solubility) in CF systems, respectively, the size of PDI aggregates is effectively reduced, which therefore increases PTB7:PDI intermixed phases. The resulting PTB7:PDI based inverted BHJ solar cells show the best PCE of 3.55% when using CF as host solvent and 0.4 vol% DIO as solvent additive.

Figure 1. (a) Chemical structures of PTB7 and PDI. (b) Schematic of inverted BHJ photovoltaic cells.

2. EXPERIMENTAL SECTION 2.1. Materials. All the chemicals and solvents were purchased from Aldrich / J&K Scientific, Ltd. (China) and used without further purification. All the solvents used for the 4 Environment ACS Paragon Plus

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synthesis were HPLC grade. PDI acceptor was purchased from Derthon Optoelectronic Materials Science & Technology Co., Ltd. PTB7 was obtained from Ossila Ltd. (Mn = 10.5 kDa, Mw = 18.0 kDa). Both of them were used as received and stored in the dark in N2 atmosphere glove box. 2.2. Characterization. Optical absorption spectra were acquired on Agilent 8453 UVVisible spectrophotometer. Photoluminescence (PL) spectra were obtained on Horiba FluoroMax-4 spectrofluorometer with the intensity corrected by absorbed photon numbers at the exciting light wavelength. Grazing-incidence wide-angle X-ray scattering (GIWAXS) experiments were conducted at the beamline BL14B1 of Shanghai Synchrotron Radiation Facility (SSRF). The intensity distributions are presented as a function of the scattering vector q = (4π/λ) sin(2θ/2) where 2θ is the scattering angle, while domain sizes are determined by the Scherrer equation from the peak full width at half maximum (FWHM).10 Film topography was imaged with atomic force microscopy (AFM) in the tapping mode on a Bruker Dimension Edge scanning probe microscope. Transmission electron microscopy (TEM) imaging was performed on Tecnai G2 F20 S-Twin microscope at an accelerating voltage of 200 kV. 2.3. Device Fabrication and Measurements. The devices were fabricated and measured according to the methods and procedures shown in our previous work.32 A blend solution of PTB7:PDI (3:7 by weight) in anhydrous chlorobenzene (20 mg/mL) or chloroform (10 mg/mL) was spin-coated onto ZnO-coated ITO glass slides at 1000 rpm, yielding the active layer of ~100 nm thick.

3. RESULTS AND DISCUSSION 3.1. Optical Properties. Optical absorption spectra were characterized and compared for the PTB7:PDI blend films along with PTB7 and PDI neat films as spin-coated from six different



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combinations of host solvents (CF and CB) and solvent additives (CN and DIO), that is, CB, CF, CB/CN, CF/CN, CB/DIO, CF/DIO systems. The spectra of the blend and neat films are displayed in Figure 2 and Figure S1, respectively. Note that we have systematically investigated the effects of different volume ratios of CN and DIO additives on the photovoltaic performance. As a result, the optimal ratios in PTB7:PDI blend are set as 0.4 vol% of CN and DIO for CF solvent, and 0.75 vol% of CN and DIO for CB solvent, respectively. As seen from Figure 2, optical absorption of the PTB7:PDI blend is found to depend strongly upon host solvent and solvent additive. All the blend films except for CF-processed one present a wide absorption range of 400–750 nm with four main bands around 500, 550, 595 and 680 nm, respectively. The first two bands originate from individual PDI chromophores,13,30 which correspond to its 0−1 and 0−0 transitions, respectively, while the latter two mainly contribute from PDI aggregates and PTB7 crystallites,13 respectively. Notably, the CF-processed blend film shows the absence of absorption band at 595 nm and significantly weaker peaks at both 550 and 680 nm, suggesting that π-stacks of both PDI and PTB7 can be largely suppressed by using volatile CF solvent due to the rapid film-formation. By contrast, the CB-processed blend film exhibits remarkably enhanced absorption bands and red-shifted absorption edge, suggesting the stronger crystallization of both PTB7 and PDI in nonvoltaile CB solvent.11,29 After adding DIO or CN solvent additive, both optical absorption intensity and edge of blend films are increased and red-shifted, respectively, in either CB or CF system. Among them, the CF-processed blend film with DIO shows a higher degree of enhanced and bathochromicshifted absorption spectrum than that with CN, which is the similar to the phenomena of CB system, indicative of stronger aggregation due to its less solubility and weaker intermolecular interaction between DIO and PTB7 or PDI molecules.11,29


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Figure 2. UV-Vis absorption spectra of PTB7:PDI blend films in pure CF, CB solvents and with DIO and CN solvent additives, respectively.

3.2. Molecular Packing. Next, we investigated the changes of molecular packing in the blend films caused by different host solvents and solvent additives. Grazing-incidence wideangle X-ray scattering (GIWAXS) experiments were performed on the PTB7:PDI blend films without and with additives, respectively. Figure 3 presents GIWAXS patterns of the blend films as a function of host solvent and solvent additive, showing distinct packing behaviours of PTB7 and PDI molecules. Likewise, in order to differentiate these diffraction peaks, both PTB7 and PDI neat films were also investigated as shown in Figure S2. The neat PTB7 exhibits predominantly amorphous characteristics in either pure CB or CF system, as evidenced by the weak peaks of (100) lamellae at 0.32 Å−1 and (010) π-π stacking at 1.65 Å−1, respectively, which are in agreement with previous reports.33−35 By comparison, the neat PDI exhibits a significantly higher degree of crystallinity and more notable π−π stacking characteristics in pure CB than CF system. Its crystal size is estimated as 60.7 and 92.5 nm for CF and CB systems, respectively, from the full-width-half-maximum of the diffraction peaks.10,34 In the blend films, all samples exhibit reduced PDI crystallinity than their corresponding neat PDI films, which is caused by the presence of amorphous PTB7.10 However, the crystalline packing of PDI molecules dominates in



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the blend film, irrespective of processing from either CF or CB solution. Table S2 lists the crystal size of PTB7 and PDI domains in different blends. In CF system, the (100) diffraction band characteristic of PTB7 is found at 0.33 Å−1 in the in-plane direction, while the (200) diffraction peak typical of PDI appears at 0.61 Å−1 (Figure 3a).10,31 In CB system, by contrast, the (100) peaks of both PTB7 and PDI can be seen at 0.34 and 0.37 Å−1, respectively (Figure 3b).10,31 This difference can be explained by the fact that highTb CB solvent has sufficient time to facilitate PTB7 crystallization as well as PDI aggregation, while CF solvent functions reversely owing to its high volatility. This is also evidenced by their stronger peak intensity and increased domain size in CB than CF system as shown in Figure S2 and Table S2, respectively. It can be therefore concluded that CF is a favourable host solvent for planar PDI based blend films. Upon adding solvent additive, the blend film exhibits different molecular packing, which is more remarkable in CF than CB system, as shown in Figure 3c–f. Note that the (200) peak of PDI molecules is weakened in CF system, and the (100) peak of PTB7 in the blend becomes noticeable in the out-of-plane direction with either CN or DIO additive (Figure 3c,d). These results suggest that introducing solvent additive balances the crystallinity of PTB7 phases and aggregation of PDI domains.30 By contrast, when processing from CB solution with additive, the blend film shows diffraction peaks of PTB7 and PDI, which are similar to those without additive, yet with little change of peak intensity (Figure 3e,f). Note that in the CB system the intensity is higher with DIO than with CN because DIO is a poor solvent for both PTB7 and PDI. Thus, the film casting process allows PDI molecules slowly to aggregate,30,31 especially in the presence of DIO which aggravates it.36,37 However, when using CN additive, the aggregated PDI molecules are re-dissolved, thus inhibiting the formation of large scale PDI aggregates and creating more


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intermixed phases. Moreover, the addition of high-Tb additive will prolong the film plasticization, which allows for structural rearrangement to enhance π-stacking. It is thus speculated that favourable packing can be obtained when CN additive is paired with high-Tb CB solvent, and DIO with low-Tb CF.

Figure 3. GIWAXS diffractograms for PTB7:PDI blend films spin-coated from (a) CF, (b) CB, (c) CF with 0.4 vol% CN (d) CF with 0.4 vol% DIO, (e) CB with 0.75 vol% CN, and (f) CB with 0.75 vol% DIO, respectively.

3.3. Film Morphology. Tapping-mode atomic force microscopy (AFM) and high-resolution transmission electron microscope (HR-TEM) imaging were combined to visualize these morphology changes of the PTB7:PDI blend films, which are caused by the above solvent engineering. As shown in Figure 4a, the CB-processed blend film exhibits a typical grain-like PDI aggregates with domain size around 1 µm,38 exhibiting a large root mean square (RMS) surface roughness of 19.8 nm (Figure 4b), while Figure 4c shows a negligible phase-segregated film morphology with some long yet scattered fibrillar PDI crystals in CF-processed film,



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yielding a largely reduced roughness down to 9.56 nm (Figure 4d). These phenomena indicate that the PDI aggregations can be effectively suppressed by replacing high-Tb CB with low Tb CF solvent, which agrees well with the above GIWAXS and optical absorption results.

Figure 4. HR-TEM (a,c) and tapping-mode AFM (b,d) images of the PTB7:PDI blend films processed from pure CB and CF solvents, respectively.

Upon adding CN additive, the size of grain-like PDI domains becomes smaller and continuous in CB/CN system (Figure 5a,b), while the fibrillar PDI crystals disappear, thus forming more homogeneous yet distinctive phase-separated film morphology in CF/CN system (Figure 5c,d). The reasons are that the strong intermolecular interaction of high-Tb CN and PDI can re-dissolve PDI bundles in both CB and CF systems.13 In addition, the film roughness is found slightly decreased in CB/CN system yet increased in CF/CN compared to their pure solvent systems, which are 16.8 and 11.6 nm, respectively. Such a difference is due to the larger Tb difference of CN with CF than CB. When adding DIO additive, in stark contrast, large sizes of PDI domains are observed in CB/DIO system (Figure 5e,f), while finely phase-separated film morphology with greatly reduced PDI domain size (below 30 nm) are formed in CF/DIO system


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as revealed in Figure 5g,h. Such morphological evolutions can be interpreted as follows. Compared with CN additive, DIO present higher Tb and poor solubility for both PTB7 and PDI molecules, which result in stronger aggregations and higher crystallization. Therefore, the size of PDI aggregates in CB/DIO-processed film is further enlarged, which is also supported by its highest roughness of 43.4 nm.

On the other hand, small PDI aggregates and low PTB7

crystallization in CF/DIO-processed film are rearranged by addition of DIO due to its higher-Tb and large solubility difference of PTB7 and PDI components, showing an appropriate film roughness of 7.36 nm. Therefore, among of six solvent combinations, CF/DIO-processed film exhibits the finest nanomorphology with optimal PDI aggregation, enhanced PTB7 crystallization and improved PTB7:PDI intermixed phases.



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Figure 5. HR-TEM (a,c,e,g) and AFM (b,d,f,h) images of the PTB7:PDI blend films with CN and DIO additives, respectively.

Photoluminescence (PL) quenching measurements were further conducted to provide additional evidence for the above reduced PDI aggregates, which are caused by solvent engineering.39 The PL quenching spectra and the corresponding intergrated maximum PL intensity of PTB7:PDI blend films without and with different additives are displayed in Figure 6a and b, respectively. It is evidently found that the PL quenching efficiency of blend film is


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increased as the size of PDI aggregates decreased, implying more efficient photoinduced charge transfer from PTB7 to PDI. In CB system, the PL quenching is more efficient upon adding CN than DIO additive, while opposite in CF system. As a result, the CF/DIO-processed blend film show the highest PL quenching efficiency, which corresponds to its finest phase-separated film morphology with smallest PDI domain size as shown in Figure 5g,h.11

Figure 6. (a) PL quenching spectra and (b) coresponding integrated maximum PL intensity of PTB7:PDI blend films without and with different additives.

3.4. Device Performance. Lastly, we constructed inverted OPV cells of ITO/ZnO (30 nm) /PTB7:PDI (100 nm) /MoO3 (10 nm) /Al (100 nm) to establish the desirable influences on device performance, which results from significantly improved molecular ordering and phase segregation owing to synergistic effects of host solvent and solvent additive. These improvements are expected to contribute favourably to charge dissociation at the PTB7:PDI interfaces, leading to improved Jsc and FF. Figure 7 displays the representative photocurrent density−voltage (J−V) characteristics of six PTB7:PDI BHJ OPV devices under simulated AM 1.5 sun illumination and corresponding external quantum efficiency (EQE) spectra, respectively. The parameters of cell performance are summarized in Table 1. As shown in Figure 7a, the



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device processed from pure CB solution exhibits poor performance with a PCE of 1.48% owing to its unfavourable charge dissociation at the PTB7:PDI interfaces caused by large aggregation of PDI molecules, while this performance is significantly improved with a PCE of 2.42% in CFprocessed devices due to the largely reduced PDI domains. In CB system, when adding 0.75 vol% CN additive, both Jsc and FF are increased, but it turns the opposite by using the same amount of DIO additive. This is because CN additive is favourable for PDI while DIO is not, which coincides with the morphology changes and PL quenching results. After adding 0.4 vol% of CN or DIO to CF solution, the device performance are both improved, in which that with DIO is better than CN owing to its distinctive and finely phase-separated film morphology. The device processed with CN shows a moderately enhanced PCE of 3.07% with an open-circuit voltage (Voc) of 0.84 V, a Jsc of 7.03 mA/cm2, and a FF of 52%, while that with DIO exhibits the highest PCE of 3.55% with a Voc of 0.84 V, a Jsc of 7.57 mA/cm2, and a FF of 56%, which is among the best device performance in such planar PDI based photovoltaic cells.11–13 Clearly, the CF/DIO system leads to the most prounounced increase of both Jsc and FF, which is attributed to the optimal PDI aggregation, enhanced PTB7 crystallization and improved PTB7:PDI intermixed fractions.11,22 Figure 7b indicates that all CF-processed cells exhibit higher EQE values than CBprocessed ones since their smaller PDI domains in the blend film are beneficial to exciton splitting and electron transport, thus leading to higher Jsc.40,41 Further adding CN or DIO additive, the EQE values are significantly enhanced around 590 nm, which is in good agreement with their optical absorption. Finally, CF-processed cells with 0.4 vol% of DIO additive exhibit the highest EQE values throughout the full-wavelength range.


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Figure 7. (a) J-V characteristics and (b) EQE curves of inverted BHJ OPV cells fabricated from the PTB7:PDI blends without or with CN and DIO additives under AM 1.5G light irradiation at an intensity of 100 mW cm−2. Table 1. Device performance parameters of PTB7:PDI based inverted BHJ cells and the hole / electron mobilities of PTB7:PDI blends as a function of various host solvents and solvent additives Jsc

Voc

FF

PCE

µh

µe

(mA/cm2)

(V)

(%)

(%)

(cm2 V–1 s–1)

(cm2 V–1 s–1)

CB

3.60

0.80

51.3

1.48

3.45 × 10−5

8.82 × 10−7

CF

6.94

0.82

42.5

2.42

1.46 × 10−5

2.15 × 10−6

CB + 0.75 vol% CN

4.60

0.82

53.3

2.01

7.03 × 10−5

7.34 × 10−6

CF + 0.4 vol% CN

7.03

0.84

52.0

3.07

8.32 × 10−5

9.32 × 10−6

CB + 0.75 vol% DIO

3.28

0.80

48.6

1.28

8.51 × 10−6

4.79 × 10−7

CF + 0.4 vol% DIO

7.57

0.84

55.9

3.55

9.89 × 10−5

6.85 × 10−5

Solvent

Meanwhile, space charge limited current (SCLC) measurements were conducted to examine the influences of morphological changes caused by solvent engineering on charge transport. The hole- and electron-only devices were made with a structure of ITO/PEDOT:PSS/PTB7:PDI/Au and ITO/ZnO/PTB7:PDI/Ca/Al, respectively. Dark J–V curves of devices are fitted to the SCLC model in the voltage range of 0–7 V using the Mott–Gurney equation, as shown in Figure S4, and the calculated hole (µh) and electron (µe) mobilities are summarized in Table 1. Both µh and



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µe are notably improved in CF system compared with those in CB without or with solvent additive, suggesting favorable charge transport properties as the PDI domain size decreases and the number of PTB7:PDI phases increases.10 Among all six systems, CF/DIO-based device shows the most efficient and balanced charge transport, in good agreement with its highest Jsc in cells.11 Table 2. The degree of PTB7 crystallization, PDI aggregation, PTB7:PDI intermixed fractions and its phase separation in the different solvent combinations Influencing factors

CB

CF

pure

CN

DIO

pure

CN

DIO

PTB7 crystals

high

N/A

↑

low

↑

↑

PDI aggregation

high

↓

↑

low

↑

↑

PTB7:PDI phases

low

↑

↓

high

↓

N/A

Phase separation

high

↓

↑

negligible

↑

↑

4. CONCLUSIONS In summary, we have shown that different host solvents and solvent additives can synergistically fine-tune the nanomorphology of the polymer:nonfullerene blend film. For simplicity, the main results are summarized in Table 2. Based on the blend of planar PDI acceptor and donor polymer PTB7, it is found that PDI aggregations can be largely reduced by using low-Tb CF solvent while they are enlarged by using high-Tb CB. CN additive which has strong molecular interaction with PDI aids in reducing the large PDI aggregates in CB host solvent, while poor solvent DIO works worse. After adding DIO or CN additive into CF, the PTB7 crystallization is greatly improved. More importantly, DIO with higher-Tb and poorer solubility improves intermixed PTB7:PDI phases more significanlty than CN. Consequently, the


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inverted BHJ cells based on CF-processed PTB7:PDI blend film with 0.4 vol% DIO exhibit the best PCE of 3.55%. This study demonstrates that film-morphology such as the phase size of planar PDI acceptor and the crystallinity of the polymer donor in the blend film can be efficiently tuned by regulating the matched host solvent and solvent additive. Of more importance is such a straetgy of solvent engineering can be extended to other nonfullerene based organic solar cells. ASSOCIATED CONTENT Supporting Information UV-Vis absorption spectra, integrated GIWAXS patterns and SCLC fitted J–V curves. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Z.L. acknowledges the support of National Natural Science Foundation of China (NSFC) under grant No. 51473036.

REFERENCES (1) Sonar, P.; Lim, J. P. F.; Chan, K. L., Organic Non-Fullerene Acceptors for Organic Photovoltaics. Energy Environ. Sci. 2011, 4, 1558–1574. (2) Li, C.; Wonneberger, H., Perylene Imides for Organic Photovoltaics: Yesterday, Today, and Tomorrow. Adv. Mater. 2012, 24, 613–636.



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TOC GRAPHICS


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Synergetic Solvent Engineering of Film Nanomorphology to Enhance

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