A Wide Band Gap Polymer with a Deep Highest Occupied Molecular

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Article Cite This: J. Am. Chem. Soc. 2018, 140, 7159−7167

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A Wide Band Gap Polymer with a Deep Highest Occupied Molecular Orbital Level Enables 14.2% Efficiency in Polymer Solar Cells Sunsun Li,†,∥ Long Ye,‡ Wenchao Zhao,†,∥ Hongping Yan,§ Bei Yang,†,∥ Delong Liu,† Wanning Li,†,∥ Harald Ade,‡ and Jianhui Hou*,†,∥

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†

State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Department of Physics and Organic and Carbon Electronics Lab (ORaCEL), North Carolina State University, Raleigh, North Carolina 27695, United States § Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States ∥ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: To simultaneously achieve low photon energy loss (Eloss) and broad spectral response, the molecular design of the wide band gap (WBG) donor polymer with a deep HOMO level is of critical importance in fullerene-free polymer solar cells (PSCs). Herein, we developed a new benzodithiophene unit, i.e., DTBDT-EF, and conducted systematic investigations on a WBG DTBDT-EF-based donor polymer, namely, PDTB-EF-T. Due to the synergistic electron-withdrawing effect of the fluorine atom and ester group, PDTB-EFT exhibits a higher oxidation potential, i.e., a deeper HOMO level (ca. −5.5 eV) than most well-known donor polymers. Hence, a high open-circuit voltage of 0.90 V was obtained when paired with a fluorinated small molecule acceptor (IT-4F), corresponding to a low Eloss of 0.62 eV. Furthermore, side-chain engineering demonstrated that subtle side-chain modulation of the ester greatly influences the aggregation effects and molecular packing of polymer PDTB-EF-T. With the benefits of the stronger interchain π−π interaction, the improved ordering structure, and thus the highest hole mobility, the most symmetric charge transport and reduced recombination are achieved for the linear decyl-substituted PDTB-EF-T (P2)-based PSCs, leading to the highest short-circuit current density and fill factor (FF). Due to the high Flory−Huggins interaction parameter (χ), surface-directed phase separation occurs in the P2:IT-4F blend, which is supported by X-ray photoemission spectroscopy results and cross-sectional transmission electron microscope images. By taking advantage of the vertical phase distribution of the P2:IT4F blend, a high power conversion efficiency (PCE) of 14.2% with an outstanding FF of 0.76 was recorded for inverted devices. These results demonstrate the great potential of the DTBDT-EF unit for future organic photovoltaic applications.

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INTRODUCTION Polymer solar cells (PSCs) are a promising renewable energy technology due to their unique advantages of being lightweight, exhibiting mechanical flexibility, and having potential for lowcost fabrication.1−4 At present, the molecular design of photoactive layer materials is one of the most important topics in PSC research.5−10 During the past three years, with the benefit from the acceptor−donor−acceptor (A−D−A)-type small molecule acceptors (SMAs),9−22 the power conversion efficiencies (PCEs) of fullerene-free PSCs have reached up to 13%.22−25 Very recent studies indicated that enhancing the intramolecular charge transfer (ICT) effect in these SMAs by introducing an electron-withdrawing unit12,26 or/and functional group15,20−22,27−30 benefits the extension of their absorption to the near-infrared region and the increase of the short-circuit current density (Jsc).20−22,31 However, the open-circuit voltages (Voc) of these devices are relatively low because of the low-lying © 2018 American Chemical Society

lowest unoccupied molecular orbital (LUMO) levels of the SMAs.21,32 To improve the Voc and maintain the high Jsc, donor polymers with wide band gaps (WBGs)33 and deeper highest occupied molecular orbital (HOMO) levels are needed to pair with these SMAs. To date, although a few WBG polymers have enabled high-efficiency polymer:SMA devices,24,25,34−39 the HOMO levels of most of the polymers are still too high to yield high Voc in PSCs based on SMAs with very low LUMO levels. Therefore, the design of efficient WBG donor polymers with deep HOMO levels is of critical importance for making further breakthroughs in the PCE. Recently, benzo[1,2-b:4,5-b′]dithiophene (BDT)-based polymers have been shown to exhibit superior photovoltaic properties in fullerene-free PSCs.14−32,36−40 With addition of Received: March 9, 2018 Published: May 8, 2018 7159

DOI: 10.1021/jacs.8b02695 J. Am. Chem. Soc. 2018, 140, 7159−7167

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Journal of the American Chemical Society Chart 1. (a) Chemical Structures of DTBDT-EF Unit and (b) PDTB-EF-T Polymer Derivatives

alkyl chain structure of the ester. In the case of the estermodified polythiophene derivatives, although the branched alkyl chains endow them with good solubility,49,50,52 the bulky side-chain structure on the main chain is supposed to be detrimental to compact molecular packing. Hence, precise modulation of the molecular aggregation and crystalline properties of polymer PDTB-EF-T by tuning the side-chain on the ester needs to be carried out. In this contribution, a series of PDTB-EF-T polymer derivatives, named P1, P2, and P3 with different side-chains (n-octyl, n-decyl, and 3,7-dimethyloctyl), were designed and synthesized (Chart 1b). With the benefits from the synergistic electron-withdrawing effect of the fluorine atom and ester group, all three polymers show deep HOMO levels of ca. −5.5 eV (estimated by utilizing cyclic voltammetry method, and with ferrocene/ferroncenium redox couple utilized as the internal standard), and therefore, the Voc of PSCs based on polymer PDTB-EF-T and a fluorinated A−D−A-type SMA (IT-4F)22 (see Scheme S1 in the Supporting Information) could reach ca. 0.90 V, corresponding to a low photon energy loss (Eloss = Eg − eVoc) of 0.62 eV. Further systematic investigations demonstrated that linear side-chains are conducive to realizing stronger interchain π−π interactions and a distinctly increased π−π coherence length (Lc) for polymer PDTB-EF-T, which is consistent with our hypothesis. Owing to the stronger molecular aggregations, improved structural order, and thus higher hole mobility of n-decyl-substituted PDTB-EF-T (P2), the highest Jsc and fill factor (FF) were obtained in P2:IT-4Fbased PSCs. Due to the high Flory−Huggins interaction parameter (χ), surface-directed phase separation occurs in the P2:IT-4F blend, which is supported by X-ray photoemission spectroscopy results and cross-sectional transmission electron microscope images. Further utilization of the vertical D/A phase distribution through device engineering led to a high PCE of 14.2% for the PSC cast with P2:IT-4F. Accordingly, the DTBDT-EF unit is demonstrated to be a promising alternative for future organic photovoltaic applications.

the thiophene unit both onto BDT side positions and adjacent to BDT, the π-overlap could be extended and the coplanarity of the polymer chain improved,40−42 which contribute to the significantly enhanced PCEs for the polymer:SMA devices.36,40,43,44 This result suggests that 2,4,6,8-tetra(thiophen-2yl)benzo[1,2-b:4,5-b′]dithiophene (DTBDT) is a successful unit for constructing well-matched donor polymers for A−D− A-type SMAs. However, owing to the electron-donating nature of the thiophene unit, the HOMO levels of the WBG DTBDTbased polymers are relatively high.42−45 To downshift the HOMO level while keeping the large Eg, introducing a suitable electron-withdrawing substituent onto the DTBDT unit is considered a more effective strategy compared to incorporating a strong electron-deficient conjugated building block. Among the varied functional groups, fluorine atom is regarded as a smart choice for lowering the energy levels without introducing undesirable steric hindrance,46 and recent works have also demonstrated the excellence of fluorination in fullerene-free PSCs.22−24,37 Another functional group that deserves attention is the ester group. Instead of introducing the ester on the quinoidal unit, such as thieno[3,4-b]thiophene,47 several polymers with ester groups on the main chains show large Eg and low-lying HOMO levels,39,48−51 and a typical example is ester-modified polythiophene (i.e., PDCBT).49,50 More importantly, we found that the intra- and interchain nonbonding interactions induced by the ester groups in the polymer PDCBT help to form an ideal polymer:SMA blend morphology, thus achieving significantly improved PCE.50 Considering the above, we propose introducing the fluorine atoms on the thiophene side groups of DTBDT and the ester on the thiophene units on both sides of DTBDT (DTBDT-EF, Chart 1a) to synergistically deepen the HOMO level of the DTBDT unit, and it has been validated by the density functional theory (DFT)-based theoretical calculations (Figure S1a), indicating that DTBDT-EF unit is quite suitable for constructing WBG polymers with deep HOMO levels. Apart from the energy level modulation, optimizing the aggregation effects and molecular packing properties of the donor polymer via tailoring the molecular structure is particularly important in fullerene-free PSCs because the donor polymer plays a dominant role in controlling the polymer:SMA morphology.7,8,34,40,50,51 For the DTBDT-EF unit, the steric hindrance caused by the alkyl chains on the ester is unfavorable for backbone coplanarity and interchain π−π stacking. To avoid the distortion of the main chain, here a single thiophene unit was inserted between two DTBDT-EF units. According to the DFT-based theoretical calculation results, the new DTBDT-EF-based polymer, namely, PDTBEF-T (Chart 1b), shows a planar geometry, and the adjacent ester groups are located on different sides of the main chain (Figure S1b). Another point that needs to be emphasized is the

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RESULTS AND DISCUSSION Material Synthesis and Optoelectronic Properties. All PDTB-EF-T polymer derivatives were synthesized by Stille coupling reaction of two monomers, i.e., 4,8-bis(5-((2ethylhexyl)thio)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl bis(trimethylstannane) (BDT-SF) and dialkyl 5,5″-dibromo-[2,2′:5′,2″-terthiophene]-3,3″-dicarboxylate (TerT-E). Detailed synthetic procedures of the TerT-E units with different alkyl chains and polymer PDTB-EF-T can be found in the Supporting Information (SI). The synthesis of the monomer BDT-SF was similar to the method reported in our previous works.22 Because P1 with linear octyl side-chains shows limited solubility in chlorobenzene (CB), to guarantee 7160

DOI: 10.1021/jacs.8b02695 J. Am. Chem. Soc. 2018, 140, 7159−7167

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Journal of the American Chemical Society

Table 1. Molecular Weight, Optoelectronic and Molecular Packing Properties, and Carrier Mobility of PDTB-EF-T Polymer Derivatives polymer P1 P2 P3

Mn [kDa] 25.7 39.9 37.2

PDI 2.6 4.1 5.2

λmaxsoln [nm]

λmaxfilm [nm]a

540 544 536

548 (7.9 × 10 cm ) 548 (8.1 × 104 cm−1) 544 (7.2 × 104 cm−1) 4

−1

Eg [eV]

dlamellar [Å]b

Lc [Å]c

μh [cm2 V−1 s−1]

1.93 1.93 1.94

23.5 25.6 24.2

27.7 27.9 21.6

1.0 × 10−4 1.2 × 10−4 7.3 × 10−5

Data in parentheses are the maximum extinction coefficients of the polymers in the solid film state. bd-spacing corresponds to the lamellar packing in the IP direction. cCoherence length obtained using Scherrer’s equation (Lc = 2π/fwhm) for the π−π stacking peak in the OOP direction. a

strong optical absorption from 400 to 600 nm in the solid film state (Figure 1b), which is complementary to that of IT-4F. Hence, the PDTB-EF-T polymers were considered to be properly matched with IT-4F. Compared with P1 and P2, P3 with branched alkyl chains exhibits a slightly blueshifted absorption spectrum (Figure 1b), corresponding to a slightly larger Eg of 1.94 eV. In addition, the absorption shoulder of P3 is also weaker than those of the other two polymers. This result implies that polymer PDTB-EF-T with linear side-chains shows a stronger intermolecular π−π interaction in the solid film state, which also contributes to the higher maximum extinction coefficients (Table 1). The ultraviolet−visible (UV−vis) absorption spectra of the three polymers in diluted CB solution reveal a trend similar to that in the thin film state (Figure 1c), and no obvious shoulder feature can be observed for P3. Although P1 and P2 exhibit almost the same absorption spectra in the thin film state, the absorption spectrum of P2 in diluted solution is slightly red-shifted relative to P1, leading to the largest maximum absorption wavelength (λmaxsoln) of 544 nm. We ascribed this to the higher molecular weight of P2 (Figure S3d). Aggregation Effects and Molecular Packing of Polymers. To investigate the aggregation behaviors of PDTB-EF-T polymers with different alkyl chains, temperature-dependent absorption measurements were carried out (Figure S3).8,53 For all three polymer CB solutions, the absorption spectra exhibit significant redshifts as the temperature drops, signifying the strong interchain aggregation of polymer PDTB-EF-T at room temperature. At a high temperature of 110−120 °C, the three polymers possess similar absorption spectra, with the peak absorption at approximately 480 nm, indicating that the polymer chains are fully disaggregated. As the temperature decreases from 110 to 80 °C, the absorption spectrum of P2 shows a substantial redshift (Figure 1d), which means that the polymer chains become gradually more coplanar during the cooling process. When the temperature is lower than 80 °C, a new main peak with a shoulder peak emerges, corresponding to the existence of aggregates with strong π−π stacking in P2 solution.53 However, for P1 and P3, distinct redshifts of the absorption spectra can be observed when the temperature is below 100 °C, and they also have a lower transition temperature of 60 °C. A similar trend could also be observed for the P2 with lower molecular weight (Figure S3d). Hence, P2 exhibits the strongest aggregation effect compared to the other two polymers due to its linear side-chains and higher molecular weight. Grazing-incidence wide-angle X-ray scattering (GIWAXS) was also employed to investigate the influence of side-chain substitutions on the molecular packing properties of the variants of PDTB-EF-T.54 Figure S4 clearly shows the π−π stacking (010) diffraction peak at q = 1.74 Å−1 only in the outof-plane (OOP) direction and the prominent lamellar packing

its solubility for solution processing, a short polymerization time of 12 h was employed, yielding P1 with a moderate number-average molecular weight (Mn) of 25.7 kDa and a polydispersity index (PDI) of 2.6 (Table 1). P2 and P3 with longer alkyl chains possess higher Mn values of 39.9 and 37.2 kDa, respectively. All three polymers exhibit similar solubility in commonly used processing solvents, such as o-dichlorobenzene and CB, which is beneficial for making a proper comparison. Through electrochemical cyclic voltammetry (CV) measurements, the HOMO levels of the three PDTB-EF-T polymer derivatives were determined to be ca. −5.5 eV (Figure S2), which are distinctly deeper than those of several recently reported high-performing donor polymers (for example, J71, PTFB-O, PBDTS-TDZ, and PBDB-T-SF).22,25,34,36 DFT-based theoretical calculation results indicate that the HOMO level of the DTBDT-EF unit (−5.50 eV) is dramatically decreased by 0.34 eV relative to that of the DTBDT unit (−5.16 eV), which is equal to the sum of the HOMO level downshift induced by each type of electron-withdrawing groups (Figure S1a). In addition, the variation of molecular electrostatic potential (ESP) distributions is also a direct consequence of both the fluorine atom and the ester group. All of these indicate that the introduction of the fluorine atom and the ester group on the DTBDT unit has a synergistic effect on lowering the HOMO level, which contributes to the quite deep HOMO level of polymer PDTB-EF-T. Although polymer PDTB-EF-T exhibits a low-lying HOMO level, the gap between the HOMO levels of PDTB-EF-T and IT-4F could still ensure efficient exciton dissociation (Figure 1a). Moreover, all three polymers possess

Figure 1. (a) Schematic energy-level diagrams of PDTB-EF-T polymer derivatives and IT-4F. (b) Normalized UV−vis absorption spectra of PDTB-EF-T polymer derivatives and IT-4F in the solid film state. (c) Normalized absorption spectra of three polymers in diluted CB at 20 °C, and (d) the redshift of λmax with decreasing temperature. 7161

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Journal of the American Chemical Society (100) diffraction peak in the in-plane (IP) direction for all three neat films. This result indicates that PDTB-EF-T polymers exhibit ordered packing with a preferential face-on orientation, which is the desired direction for charge transport.55,56 Slight changes in the side-chain length of the three polymers result in differences in the IP lamellar distance (Table 1). Among them, P1 exhibits the smallest lamellar distance of 23.5 Å, indicative of a more compact lamellar packing for polymers with linear sidechains. In addition, the values of the π−π stacking coherence length Lc determined by the full-width at half-maximum (fwhm) of the (010) peaks of P1 and P2 are significantly higher than that of P3 (Table 1), implying that a linear alkyl chain is also beneficial for more ordered π−π stacking. Therefore, polymers P1 and P2 possess distinctly higher hole mobilities (μh) of ca. 1 × 10−4 cm2 V−1 s−1 (measured using the space-charge-limited current (SCLC)57 method) compared to that of P3 (Figure S5 and Table 1). In comparison with P1, P2 shows higher scattering intensity and Lc for the face-on oriented π−π stacking; thus, the highest μh of 1.2 × 10−4 cm2 V−1 s−1 was achieved in the neat P2 film. We ascribed this partially to the stronger crystalline behavior of P2 due to its higher molecular weight.58 All results mentioned above demonstrated that subtle side-chain tuning exerts a controlled impact on the molecular aggregation and packing properties of polymer PDTB-EF-T. Photovoltaic Properties and Recombination Mechanisms. The photovoltaic properties of PDTB-EF-T polymer derivatives were studied by fabricating PSCs with a conventional structure of ITO/PEDOT:PSS/PDTB-EF-T:IT-4F/ PFN-Br/Al, which is consistent with the device structure used in our previous work.32,50,51 For all three blend systems, the optimal D/A ratios are 1:1 (w/w), and the optimized blend films were obtained by spin-coating the blend solution in CB/ 1,8-diiodooctane (1:0.005, v/v) and then thermal annealing at 150 °C for 10 min. Detailed device fabrication methods can be found in the Experimental Section in Supporting Information. The optimal current density−voltage (J−V) curves, external quantum efficiency (EQE) curves, and corresponding photovoltaic parameters are shown in Figure 2a, Figure 2b, and Table 2, respectively. As expected from the low-lying HOMO level of polymer PDTB-EF-T, high Voc of ca. 0.90 V was obtained for all three devices, and the P3-based PSC exhibits a slightly higher Voc of 0.904 V, which is in accordance with the lower HOMO level of P3. Because the Eg of the narrow band gap IT-4F is approximately 1.52 eV, PDTB-EF-T:IT-4F-based devices show a low Eloss of ca. 0.62 eV. Despite the low Eloss, a notably high Jsc of over 20 mA cm−2 can still be achieved for the PDTBEF-T:IT-4F-based devices. According to the EQE spectra, the superior Jsc mainly originates from the broad EQE response from 300 to 800 nm, and we attribute this to the complementary and strong absorption of PDTB-EF-T and IT-4F. For the P2-based device, the EQE values in the range 510−760 nm exceed 75%, and the maximum EQE recorded at 560 nm is up to 82%, which outperforms the values of the other two devices. Therefore, the highest Jsc of 20.65 mA cm−2 is obtained in the P2-based device, which is also higher than the value for the well-known PBDB-T-SF:IT-4F-based devices.22 Notably, the FF of the P2:IT-4F-based device can reach 0.70, while the FFs of the P1- and P3-based devices are 0.64 and 0.61, respectively. Benefiting from the significantly increased FF, the PSC based on P2 and IT-4F demonstrated an optimum

Figure 2. (a) J−V and (b) EQE curves of the optimized PSCs based on three PDTB-EF-T polymer derivatives as the electron donor and IT-4F as the electron acceptor under the illumination of AM 1.5G, 100 mW cm−2. (c) Pdiss versus Veff plots and (d) Jsc as a function of Plight for the three optimal devices.

PCE of 13.0%, which is comparable to the top efficiencies reported so far.22,23 To clarify the differences in the Jsc and FF of the three PDTB-EF-T:IT-4F-based PSCs, the device physics of the optimized devices were investigated. Figure 2c depicts the photogenerated current density (Jph = JL − JD, where JL and JD represent the current densities under the illumination of 100 mW cm−2 and in the dark, respectively) versus effective voltage (Veff = V0 − Va, where V0 is the voltage when Jph is zero and Va is the applied voltage) plots.59,60 Jsat. is defined as the saturation photocurrent density at a sufficiently high Veff (i.e., 3−4 V), which means that all photogenerated excitons can be dissociated into free carriers and collected at the electrodes due to the high electric field.59 Under the short-circuit condition, the P2:IT-4F-based device shows slightly higher exciton dissociation probabilities (Pdiss = Jph/Jsat) of 95% relative to the other two devices (Table 2), suggesting higher exciton dissociation and charge collection efficiencies in P2-based devices, which agrees well with the higher EQE. For the maximum power points, the Pdiss value of the P2-based device is still as high as 85%, while those of the P1- and P3-based devices drop to 77% and 72%, respectively. This result indicates that the recombination loss is much less under the maximal power output condition for the P2:IT-4F-based PSC, which partially contributes to the noticeably improved FF. We also measured the Jsc values of each optimized PSC under different illumination intensities (Plight), and Jsc is known to follow a power-law dependence with respect to Plight, which can be described as Jsc ∝ PlightS.61,62 As displayed in Figure 2d, the exponential factor (S) of the P2-based device is 1.00, while the P1- and P3-based devices show relatively lower S values of 0.94 and 0.91, respectively. Therefore, we inferred that bimolecular recombination is suppressed in the P2:IT-4F blend.61 The charge transport behavior of the three blends was further evaluated using the SCLC method, and the characteristic curves are plotted in Figure S6. Similar electron mobilities of 2−4 × 10−4 cm2 V−1 s−1 were obtained in the three electron-only devices (ITO/ZnO/PDTB-EF-T:IT-4F/Al), implying that the electron transport ability is comparable for the three blends. 7162

DOI: 10.1021/jacs.8b02695 J. Am. Chem. Soc. 2018, 140, 7159−7167

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Journal of the American Chemical Society

Table 2. Optimal Photovoltaic Parameters of PDTB-EF-T:IT-4F-Based PSCs under the Illumination of AM 1.5G, 100 mW cm−2 Jsc [mA cm−2]a

Voc [V]a

blend d

P1:IT-4F P2:IT-4Fd P2:IT-4Fe P3:IT-4Fd

0.896 0.898 0.900 0.904

(0.892 (0.897 (0.898 (0.905

± ± ± ±

0.006) 0.007) 0.004) 0.008)

20.05 20.65 20.73 20.31

(20.13 (20.50 (20.77 (19.97

± ± ± ±

Jcalc [mA cm−2]b

0.77) 0.48) 0.22) 1.08)

19.20 20.00 19.80 19.91

FFa 0.64 0.70 0.76 0.61

(0.61 (0.70 (0.75 (0.60

± ± ± ±

PCE [%]a 0.03) 0.02) 0.01) 0.03)

11.5 13.0 14.2 11.2

(11.0 (12.9 (14.0 (10.8

± ± ± ±

0.4) 0.2) 0.1) 0.3)

Pdiss [%]c 93/77 95/85 95/86 93/72

a

Average values with standard deviations were obtained from 20 devices. bJsc integrated from the EQE spectrum. cExciton dissociation probabilities of the device under short-circuit and maximum power output conditions. dDevice with the conventional structure of ITO/PEDOT:PSS/active layer/ PFN-Br/Al. eDevice with the inverted structure of ITO/ZnO/active layer/MoO3/Al.

Figure 3. AFM height and phase images of blend films based on P1:IT-4F (a, d), P2:IT-4F (b, e), and P3:IT-4F (c, f). (g) IP and OOP line cuts of the 2D GIWAXS patterns for the three optimized blends.

information beyond surface topography, the molecular packing and microstructure in blends were characterized by GIWAXS. From the two-dimensional (2D) GIWAXS patterns shown in Figure S7, although IT-4F exhibits an amorphous feature in the neat film state, distinct (010) diffraction peaks in the OOP direction were observed in all blend films, indicative of the predominant face-on oriented π−π stacking in blends, similar to the neat PDTB-EF-T films. Note that the π−π stacking distances of P1- and P2-based blends are 3.49 Å, which is obviously smaller than that of the P3-based blend (3.61 Å) (Figure 3g). This implies that the linear side-chain-substituted polymer PDTB-EF-T is more favorable for forming close packing in the blend film due to its stronger interchain π−π interaction. In addition, the P2:IT-4F blend has the highest π−π stacking intensity among the three, indicating the higher ordering of aggregates in the blend film. The compact and highly ordered packing in the P2:IT-4F blend is conducive to efficient charge transport, which is in line with the noticeably higher carrier mobilities. Interestingly, in contrast to the other two blends, two distinct (100) peaks were observed in the IP direction for the P2-based blend (Figure 3g). As these two

Among the three hole-only devices with the ITO/ PEDOT:PSS/PDTB-EF-T:IT-4F/Au structure, a distinctly lower hole mobility was obtained in the P3:IT-4F device (6.35 × 10−5 cm2 V−1 s−1), which is consistent with the results obtained for neat films. Thanks to the highest hole mobility (3.46 × 10−4 cm2 V−1 s−1) in the P2:IT-4F blend, the most symmetric charge transport (μe/μh = 1.1) was achieved in the corresponding device (Table S1). Consequently, the superior FF in P2:IT-4F-based PSCs is the result of the reduced bimolecular recombination and balanced charge transport behavior in the blend. Morphological Characteristics of Blend Films. Considering that the device operational mechanism is closely related to the blend morphology, detailed investigations on the morphological characteristics of the blend films were carried out. Atomic force microscopy (AFM) was utilized to probe the surface morphologies of the three optimized PDTB-EF-T:IT4F blends. As depicted in Figure 3a−f, all three blend films show homogeneous surfaces with a moderate root-mean-square (RMS) roughness of ca. 2 nm, and no large aggregates can be observed from the phase images. To obtain morphological 7163

DOI: 10.1021/jacs.8b02695 J. Am. Chem. Soc. 2018, 140, 7159−7167

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Journal of the American Chemical Society

Figure 4. (a) J−V curves of the P2:IT-4F-based PSCs with different device structures under the illumination of AM 1.5G, 100 mW cm−2. (b) Certified result of a P2:IT-4F-based device from NIM. (c) Statistics of the FF distribution for two types of devices. Purple and orange solid lines represent the Gaussian distribution fitting for the statistics. (d) C/S ratios and N contents for the neat films and P2:IT-4F blends obtained via XPS. Blend 1 and blend 2 represent the P2:IT-4F blend cast on PEDOT:PSS and ZnO-coated substrates, respectively. (e) Cross-sectional TEM images and EDS results for the devices.

enhancement and the better repeatability of an excellent FF by selecting the inverted architecture. The Pdiss at the maximum power point of the inverted-type device is slightly higher than that of the conventional device (Figure S10c, Table 2), implying that the use of the inverted architecture facilitates charge dissociation and collection for P2:IT-4F-based PSCs. In addition, as depicted in Figure S10d, high FFs of ca. 0.75 were achieved for both types of device under the illumination of 0.2 suns. As the illumination intensity increases, the FFs of the inverted device can be maintained above 0.75, while the PSC with the conventional architecture exhibits a distinct drop in FF, which can be attributed to charge carrier recombination losses.64,65 Hence, the inverted device architecture also helps to restrain the bimolecular recombination in the P2:IT-4F blend, leading to the higher FF. On the basis of the acquired results and previous studies,65,66 we surmised that the vertical phase distribution in the P2:IT-4F blend plays a vital role in determining the device performance. To confirm our speculation, we first measured the surface tensions of P2 and IT-4F via contact angle measurements, and the contact angles of two liquids (deionized water and glycerol) on the neat P2 and IT-4F films were measured (Figure S11 and Table S3). Note that IT-4F shows a smaller water contact angle (99.8°) compared to that of polymer P2 (107.0°), suggesting the higher hydrophilicity of IT-4F due to the several hydrophilic substituents (i.e., carbonyl and cyano). The surface tensions (γ) of P2 and IT-4F were further calculated to be 24.6 and 29.8 mN cm−1, respectively, by using the Wu method.67 Cho et al.68 and Moon et al.69 previously showed that polymer:PCBM blends with a high Flory−Huggins interaction parameter (χ) tend to trigger surface-directed vertical phase

(100) peaks correspond to the signals of neat P2 and IT-4F, respectively, we infer that the miscibility of P2:IT-4F is much lower than that of the other blends, which is also a positive factor for achieving a higher FF.63 Device Performance Comparison and Vertical Phase Distribution in the Photoactive Layer. Through fine sidechain tuning, P2 with a linear decyl on the conjugated backbone was demonstrated to be an excellent donor polymer due to its enhanced aggregation behavior and more ordered packing. Therefore, further device engineering was conducted on the P2:IT-4F blend system. The P2:IT-4F-based PSC with the inverted device structure of ITO/ZnO/active layer/MoO3/ Al could achieve a remarkably high PCE of 14.2% with a Voc of 0.900 V, a Jsc of 20.73 mA cm−2, and a FF of 0.76 (Figure 4a and Table 2). The integrated current density (Jcalc) value acquired from the EQE spectrum (Figure S8) is in good agreement with the Jsc. An optimized P2:IT-4F device was sent to the National Institute of Metrology, China (NIM), and a certified PCE of 13.9% (Voc, 0.90 V; Jsc, 20.27 mA cm−2; FF, 0.76; device area, 3.745 mm2) was recorded (Figure 4b and Figure S9). Further stability test results indicated that the PCE of the P2:IT-4F-based device was maintained at about 80% of its initial value after continuous illumination for 2000 min (Figure S10a), which is comparable to the device performance of the high-performing PBDB-T-SF:IT-4F-based PSC. In comparison with the conventional device, a significantly improved FF was obtained in the optimized inverted P2:IT-4Fbased PSC, and the similar trend could also be observed for the P1:IT-4F and P3:IT-4F-based devices (Figure S10b and Table S2). The statistics of the FF distribution of the two types of device shown in Figure 4c indicate the reliability of FF 7164

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Journal of the American Chemical Society separation during the film casting process. On the basis of the empirical equation χ = K( γ1 − γ2 )2 (K is a constant),69 we find that the Flory−Huggins interaction parameter χ70 between P2 and IT-4F is slightly higher than that for the 13.7% efficiency PBDB-TF:IT-4F blend, as reported in our recent study (Table S4).71 As a consequence, the large difference in surface tensions between P2 and IT-4F is likely to cause a vertical gradient in the active layer in addition to bulkheterojunction phase separation, with the lower surface energy P2 enriched on the surface. In order to acquire the accurate component distribution results of the P2:IT-4F blend, X-ray photoelectron spectroscopy (XPS) was then utilized to analyze the elemental compositions on the top surfaces of neat and blend films, and the related data are summarized in Table S5. P2 exhibits a distinctly higher peak area for S2p compared to IT-4F, which agrees well with the higher theoretical S content of P2. Thus, all C/S ratios (C1s peak area/S2p peak area) were calculated and compared (Figure 4d). The C/S ratios of blend films spincoated on PEDOT:PSS and ZnO-based substrates are 4.6 and 5.0, respectively, and both are very close to the C/S ratio of P2. A similar trend was also observed for the variations in N content. These results indicate the P2 enrichment at the top surface of the P2:IT-4F blend. Direct evidence of vertical phase segregation was further achieved by employing energy dispersive spectroscopy (EDS).72,73 Figure 4e depicts the cross-sectional transmission electron microscope (TEM) images and the corresponding line scan EDS maps of devices with conventional and inverted structures. The variation trends of the C and S elements again demonstrated that the P2enriched layer and IT-4F-enriched layer are formed near the top electrodes and bottom substrates, respectively. Hence, the vertical phase distribution in inverted-type devices is favorable for charge transport and carrier collection,74 leading to the superior FF and thus the record-high PCE in P2:IT-4F-based PSCs. Conversely, the vertical distribution is detrimental in a regular device structure. This study highlights the importance of adopting an appropriate device configuration for PSCs.

this efficiency breakthrough reveals the excellence of the DTBDT-EF unit and its great potential in future applications. We foresee further efficiency improvement through the synergistic modulation of donor and acceptor materials.

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ASSOCIATED CONTENT

S Supporting Information *

These materials are available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b02695. Experimental details, NMR spectra of compounds, DFTbased theoretical calculation results, CV curves, temperature-dependent UV−vis spectra, GIWAXS patterns, and the certificate report (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Sunsun Li: 0000-0003-3581-8358 Long Ye: 0000-0002-5884-0083 Jianhui Hou: 0000-0002-2105-6922 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.H. would like to acknowledge the financial support from NSFC (91633301, 51673201), the Chinese Academy of Sciences (XDB12030200, KJZD-EW-J01), the National Basic Research Program 973 (2014CB643501), and the CAS Croucher Funding Scheme for Joint Laboratories (CAS14601). L.Y. and H.A. gratefully acknowledge the support by ONR Grants N00141512322 and N000141712204. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-76SF00515. Bo Guan and Xiang Li are appreciated for assisting with the TEM and EDS tests. and Mingwei Wan is acknowledged for assisting with the theoretical analysis.

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CONCLUSIONS In summary, we designed and synthesized a DTBDT-EF-based polymer, namely, PDTB-EF-T, with both a large Eg of ca. 1.9 eV and a deep HOMO level of ca. −5.5 eV. We note that the HOMO level of polymer PDTB-EF-T is noticeably lower than those of other high-performing donor polymers with similar Eg values due to the synergistic electron-withdrawing effect of the fluorine atom and ester group, and therefore, a high Voc of ca. 0.90 V was obtained when blended with an A−D−A-type SMA (IT-4F) with a low-lying LUMO level. Through tuning of the alkyl chain of the ester, the aggregation effects and molecular packing properties of polymer PDTB-EF-T were finely modulated. Because the linear decyl-substituted PDTB-EF-T (P2) exhibits stronger interchain π−π interaction, more ordered π−π stacking, and thus higher hole mobility compared to its counterparts, the most symmetric charge transport and suppressed charge recombination were achieved in the blend, which contribute to the higher Jsc and significantly improved FF in the corresponding PSC. By further utilizing the vertical D/A phase distribution in the blend, the P2:IT-4F-based PSC finally demonstrated a high efficiency of 14.2% with an outstanding FF of 0.76. A certified PCE of 13.9% was also recorded, which is the top value among single-junction PSCs. The realization of

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