9.73% Efficiency Nonfullerene All Organic Small Molecule Solar Cells

Mar 21, 2017 - In the last two years, polymer solar cells (PSCs) developed quickly with n-type organic semiconductor (n-OSs) as acceptor. In contrast,...
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9.73% Efficiency Nonfullerene All Organic Small Molecule Solar Cells with Absorption-Complementary Donor and Acceptor Haijun Bin,⊗,†,‡ Yankang Yang,⊗,†,‡ Zhi-Guo Zhang,*,† Long Ye,§ Masoud Ghasemi,§ Shanshan Chen,∥ Yindong Zhang,⊥,# Chunfeng Zhang,⊥,# Chenkai Sun,†,‡ Lingwei Xue,† Changduk Yang,∥ Harald Ade,*,§ and Yongfang Li*,†,‡,∇ †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China § Department of Physics and Organic and Carbon Electronics Lab (ORaCEL), North Carolina State University, Raleigh, North Carolina 27695, United States ∥ Department of Energy Engineering, School of Energy and Chemical Engineering, Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, South Korea ⊥ National Laboratory of Solid State Microstructures, School of Physics, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China # Synergetic Innovation Center in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China ∇ Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, China S Supporting Information *

ABSTRACT: In the last two years, polymer solar cells (PSCs) developed quickly with n-type organic semiconductor (n-OSs) as acceptor. In contrast, the research progress of nonfullerene organic solar cells (OSCs) with organic small molecule as donor and the n-OS as acceptor lags behind. Here, we synthesized a D−A structured medium bandgap organic small molecule H11 with bithienylbenzodithiophene (BDTT) as central donor unit and fluorobenzotriazole as acceptor unit, and achieved a power conversion efficiency (PCE) of 9.73% for the all organic small molecules OSCs with H11 as donor and a low bandgap n-OS IDIC as acceptor. A control molecule H12 without thiophene conjugated side chains on the BDT unit was also synthesized for investigating the effect of the thiophene conjugated side chains on the photovoltaic performance of the p-type organic semiconductors (p-OSs). Compared with H12, the 2D-conjugated H11 with thiophene conjugated side chains shows intense absorption, low-lying HOMO energy level, higher hole mobility and ordered bimodal crystallite packing in the blend films. Moreover, a larger interaction parameter (χ) was observed in the H11 blends calculated from Hansen solubility parameters and differential scanning calorimetry measurements. These special features combined with the complementary absorption of H11 donor and IDIC acceptor resulted in the best PCE of 9.73% for nonfullerene all small molecule OSCs up to date. Our results indicate that fluorobenzotriazole based 2D conjugated p-OSs are promising medium bandgap donors in the nonfullerene OSCs.

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an appropriate phase separation with donor materials.2 High power conversion efficiency (PCE) of 10−11% has been realized in recent years, by designing new polymer donor.3 However, the intrinsic drawbacks of the fullerene acceptors, such as weak absorption in visible region, high production costs and poor morphology stability, make it a problematic material for future application in OSCs.

INTRODUCTION

In the past decades, solution-processed organic solar cells (OSCs) have been widely investigated in an effort to produce renewable energy with advantages of semitransparency, lightweight and flexibility.1 Generally, OSCs adopt a bulk heterojunction (BHJ) structure with a conjugated polymer or a solution-processable p-type organic semiconductor (p-OS) as donor and fullerene derivatives as the electron acceptor. The advantages of the fullerene derivative acceptors are high electron mobility, high electron affinity and easy formation of © 2017 American Chemical Society

Received: December 17, 2016 Published: March 21, 2017 5085

DOI: 10.1021/jacs.6b12826 J. Am. Chem. Soc. 2017, 139, 5085−5094

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Figure 1. (a) Chemical structures of small molecule donors H11 and H12 and low bandgap n-OS acceptor IDIC. (b) Device structure of the all small molecule OSCs.

Scheme 1. Synthesis Routes of Small Molecule Donors H11 and H12

encouraging result was obtained by Deng et al. for achieving a record high efficiency of 11.07%,11g which is comparable with its polymer counterpart. In considering the potential advantage of the p-OS, attempts have been pursued to extend its application into the nonfullerene OSCs with the p-OS as donor and n-OS as acceptor,12 and a PCE of 5−9%13,14 were realized. But at present this nonfullerene OSCs show inferior photovoltaic performance in comparison with the nonfullerene PSCs, thus needing further exploration. In exploring polymer donor materials for nonfullerene PSCs, we previously developed a series of two-dimension (2D)conjugated D−A copolymers based on bithienylbenzodithiophene (BDTT) as donor unit and fluorobenzotriazole (FBTA) as acceptor unit.9b,15 The polymers possess medium bandgaps, suitable highest occupied molecular orbital (HOMO) energy levels, good crystallinity and higher hole mobilities, and afforded high effciency of 9−11% in nonfullerene PSCs in combination with low bandgap n-OS acceptors.9b,15 These prominent photovoltaic properties of the polymers are benefitted from the 2D-conjugated structure,16 fluorination effect of the backbone and the proper electron deficient nature of the benzotriazole units.17 On the basis of the success of the BDTT-alt-FBTA D−A copolymers, herein we designed and synthesized a new solution-processable D−A structured organic molecule H11 (see Figure 1a) with BDTT as central donor

Thus, great efforts have been recently devoted to developing the nonfullerne acceptors with the advantages of strong light absorption, easily tunable energy levels and potentially low-cost production.4−8 Various types of n-type organic semiconductor (n-OS) acceptors,5 such as acceptor−donor−acceptor (A−D− A) structured n-OSs,6 rylene diimides7 and orgianc dyes,8 are successfully designed. The most noticeable nonfullerene acceptors are low bandgap n-OSs such as ITIC.6b Through rational design of absorption-complementary medium bandgap polymer donors, PCEs of the nonfullerene polymer solar cells (PSCs) based on the low bandgap n-OS acceptors have reached to 11−12%.9 In addition, McCulloch et al. found that nonradiative recombination losses can be suppressed in nonfullerene PSCs with the n-OS acceptor,10 which is very important for achieving high open circuit voltages (Voc) thus high efficiency in the photovoltaic cells. In comparison with polymer donors, the p-OSs (or called organic small molecules) donors possess advantages of welldefined molecular weight, easy purification and good reproducibility of photovoltaic performance (small batch-tobatch variation). Therefore, the solution-processable p-OS donor materials have also attracted great attentions for the application in OSCs.11 Especially, the linear p-OS donor materials with D1−A−D2−A−D1 strcuture11d or with A−D− A structure11b,f,g demonstrated good photovoltaic performance in the OSCs with PC71BM as acceptor. Recently, an 5086

DOI: 10.1021/jacs.6b12826 J. Am. Chem. Soc. 2017, 139, 5085−5094

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Figure 2. (a) Absorption spectra of H11, H12 and J52 in thin film. (b) Cyclic voltammograms of H11 and H12 films on a platinum electrode measured in 0.1 mol L−1 Bu4NPF6 acetonitrile solutions at a scan rate of 20 mV s−1.

Figure 3. (a) J−V curves of the nonfullerene OSCs based on the p-OS/IDIC (2:1, w/w) with or without (as cast) thermal annealing at 120 °C for 10 min, under the illumination of AM1.5G, 100 mW cm−2. (b) The IPCE spectra of the optimized devices with thermal annealing at 120 °C for 10 min.

Vilsmeier−Haack reaction followed by bromination. Knoevenagel condensation of Compound 2 with 2-ethylhexyl cyanoacetate afforded the intermediate Compound 3. The target small molecules H11 and H12 were obtained respectively through a Stille coupling reaction between Compound 3 and distannyl benzodithiophene with alkylthienyl conjugated side chains or alkoxy side chains. The detailed synthesis processes and structural characterization of H11 and H12 are described in the Supporting Information (SI). Absorption Spectra and Electronic Energy Levels. Figure 2a shows the absorption spectra of the thin films of H11, H12. And the film absorption spectrum of the corresponding 2D-conjugated D−A copolymer J52 (the molecular structure of J52 is shown in Figure 1a) of H11 was also shown in Figure 2a for comparison. H11 and H12 show a very similar absorption profile with absorption edge at 663 nm, corresponding to a medium optical bandgap of 1.87 eV. The absorption bands are significantly broadened, and the absorption edge is red-shifted by 12 nm in comparison with that of the corresponding D−A copolymer J52. The broadened aborption leads to better complementary absorption with that of IDIC acceptor in the vis−NIR region as can be seen from Figure S1a in SI, which is thought to be more effective in light-harvesting. The film maximum extinction coefficient of the H11 film is 7.8 × 104 cm−1, which is higher than that (6.5 × 104 cm−1) of H12, as shown in Figure 2a. The higher absorbance of H11 should be

unit and FBTA as acceptor unit. The ethylhexyl cyanoacetate unit is used as the end group in H11 to expand the conjugation and enhance the electron-accepting ability in the small molecules,18 for obtaining comparable photovoltaic performance of the small molecules with their correponding polymers. In addition, the acceptor end-capping can also expand the LUMO (lowest unoccupied molecular orbital) wave function for better frontier orbital overlap, thereby getting extinction coefficient enhancement.19 For investigating the effect of the thienyl conjugated side chains on the photovoltaic performance of the p-OSs, a control molecule H12 (see Figure 1a) with alkoxy substituents on BDT unit was also synthesized. Photovoltaic performance of the molecules was investigated by fabricating nonfullerene OSCs with H11 and H12 respectively as the p-OS donor and a low bandgap n-OS IDIC,6c (see Figure 1a) as acceptor (see Figure 1b). Encouragingly, the OSC based on H11 demonstrated a PCE of 9.73% with a high open circuit voltage (Voc) of 0.98 V. To the best of our knowledge, the PCE of 9.73% is the highest value reported in literatures to date for the all small molecule OSCs.

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RESULTS AND DISCUSSION Materials Synthesis. The synthetic routes and chemical structures of the p-OS donors are depicted in Scheme 1. Compound 1 was converted to Compound 2 initially by 5087

DOI: 10.1021/jacs.6b12826 J. Am. Chem. Soc. 2017, 139, 5085−5094

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Table 1. Device Parameters of the Nonfullerene OSCs Based on Donors:IDIC under the Illumination of AM 1.5 G, 100 mW cm−2 donors

Voc (V)

Jsc (mA cm−2)

FF (%)

H11a H11b H12a H12b

0.988 (0.989 ± 0.003)e 0.977 (0.965 ± 0.003) 0.969 (0.972 ± 0.003) 0.955 (0.955 ± 0.004)

10.99 (10.85 ± 0.16) 15.21 (15.21 ± 0.17) 4.94 (4.83 ± 0.13) 10.51 (10.17 ± 0.19)

49.54 (49.98 ± 0.47) 65.46 (65.13 ± 0.77) 38.20 (37.69 ± 0.60) 54.89 (55.53 ± 0.46)

PCE (%) 5.37 9.73 1.83 5.51

(5.36 (9.56 (1.77 (5.39

± ± ± ±

0.02) 0.10) 0.07) 0.07)

Rsc Ω·cm2

Rpd KΩ·cm2

22.41 (21.81 ± 0.83) 9.16 (9.92 ± 0.47) 78.54 (81.57 ± 4.27) 18.48 (18.07 ± 0.32)

0.43 (0.43 ± 0.01) 0. 98 (1.06 ± 0.12) 0.45 (0.45 ± 0.00) 0.62 (0.69 ± 0.06)

Jscf (mA cm−2) 15.48 10.83

As-cast film. bThermal annealed at 120 °C for 10 min. cCalculated from the inverse slope at V = Voc in J−V curves under illumination. dCalculated from the inverse slope at V = 0 in J−V curves under illumination. eThe values in parentheses are average values from 10 devices. fCalculated from the IPCE spectra of the best OSCs. a

benefitted from its 2-D conjuaged structure.20 In addition, the extinction coefficient of H11 is also slightly higher than that (7.3 × 104 cm−1) of its corresponding D−A copolymer J52, confirming the success of our design strategy for the 2Dconjugated structure of H11. The HOMO and LUMO energy levels of H11 and H12 were measured by electrochemical cyclic voltammetry with Ag/ AgCl as reference electrode and ferrocene/ferrocenium (Fc/ Fc+) redox couple (4.8 eV below vacuum) as the internal calibration.21 From the onset oxidation/reduction potentials (φox/red) in the cyclic voltammograms (as shown in Figure 2b), the HOMO/LUMO energy levels of H11 and H12 were calculated to be −5.31/−3.03 eV, −5.28/−3.01 eV respectively, according to the equations of EHOMO/LUMO = −e(φox/red − φ1/2(Fc/Fc+) + 4.8) (eV) where φ1/2(Fc/Fc+) is the redox potential of Fc/Fc+ vs. Ag/AgCl in the measurement system. The electronic energy levels of H11 and H12 as donors match well with that of the IDIC acceptor (EHOMO and ELUMO are located at −5.65 and −3.97 eV respectively).6c Photovoltaic Properties. The nonfullerene OSCs were fabricated with a conventional device structure of ITO/ PEDOT:PSS ((poly(3,4-ethylenedioxythiophene): poly(styrene-sulfonate))/active layer (H11 or H12 blended with IDIC)/PDINO (perylene diimide functionalized with amino N-oxide)22/Al to investigate the photovoltaic properties of the p-OSs of H11 and H12. Photovoltaic performance of the OSCs was optimized by changing weight ratios of donor:acceptor from 1:1 to 3:1, and our optimized weight ratio is 2:1 as shown in Table S1 and Table S2 in SI. Figure 3a shows the current density−voltage (J−V) curves of the OSCs based on p-OS/ IDIC with the optimized donor/acceptor weight ratio of 2:1, and the detailed photovoltaic performance data are collected in Table 1 for a clear comparison. It can be concluded that all the OSCs show high Voc of 0.95−0.99 V, which benefits from relatively low-lying HOMO energy levels of the p-OS donors of H11 and H12. Notably, the thermal annealing treatment at 120 °C for 10 min is critically important for achieving high device performance. For the OSCs based on H11, PCE is greatly increased from 5.37% for the as-cast device to a considerably high value of 9.73% for the thermal annealing device, which is mainly due to the significantly increased Jsc (from 10.99 to 15.21 mA cm−2) and FF (from 49.54% to 65.46%). For the H12-based device, PCE is also increased from 1.83% for the ascast device to 5.51% for the thermal annealed device. Nevertheless, the optimized PCE of the H12-based OSC is much lower than that of the optimized H11-based OSCs, indicating that thienyl conjugated side chains in the 2Dconjugated H11 play an important role in achieving high photovoltaic performance. It is noteworthy that the PCE of

9.73% is the newly achieved record value reported in literature to date for the all small molecule OSCs. Figure 3b displays the incident photon to converted current efficiency (IPCE) spectra of the optimized devices with thermal annealing at 120 °C for 10 min. The H11-based devices exhibit higher IPCE value of over 60% in the wavelength region from 450 to 740 nm. The Jsc values integrated from the IPCE spectra are 15.48 mA cm−2 for the H11-based device and 10.83 mA cm−2 for the H12-based device, which agree well and are 1.7% and 3.0% respectively higher than those values obtained from the J−V tests, indicating that the measurement is highly reliable for the 9.73% PCE of the H11-based OSCs. The inverted OSCs with a device structure of ITO/ZnO/ H11:IDIC (2:1, w/w) /MoO3/Al was also fabricated with the same active layer preparation conditions as that of the optimized conventional structured OSCs. The device showed a PCE of 8.17% with a Voc of 0.96 V, a Jsc of 15.30 mA cm−2 and a FF of 55.59%, as shown in Figure S2 in SI. The slightly lower PCE of the inverted device may be related to the concentration distribution of the donor and acceptor materials in the vertical direction, and further device optimization is underway. To investigate the possibility for future application of the all small molecule nonfullerene OSCs, we fabricated the larger active area (100 mm2) OSCs based on H11:IDIC (2:1, w/w) with the conventional device structure. Figure S3 in SI shows J−V curve of the 100 mm2 OSCs based on H11:IDIC (2:1, w/ w) with thermal annealing at 120 °C for 10 min, under the illumination of AM 1.5G, 100 mW cm−2. The device exhibited a PCE of 8.52%, with a Voc = 0.975 V, Jsc = 15.52 mA cm−2 and FF = 56.34%. The PCE of 8.52% is a relatively high value for the OSCs with device area of 100 mm2, which implies that the all small molecule nonfullerene OSCs with H11 as donor are promising for future applications. The effect of thermal annealing treatment on improving the photovoltaic performance of the OSCs based on the p-OS/ IDIC was investigated by the analysis of the absorption spectra, charge carrier mobility and morphology before and after thermal annealing. Figure S1b,c,d in SI shows the absorption spectra of the blend films before and after thermal annealing. It can be seen that thermal annealed blend films show stronger absorption peaks in the wavelength range of 350−620 nm, especially for the H11 based blend film. The stronger absorption could be ascribed to the more ordered structure of H11 in the blend films as confirmed below, and it should be beneficial to the increase of Jsc and IPCE values for the OSCs. The dependence of Jsc and Voc of the OSCs on light intensity (Plight) were measured (see Figure S4 in SI) to understand the charge recombination behavior in the OSC devices.23 The relationship of Jsc and the light intensity (Plight) can be 5088

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Figure 4. AFM height images of the as-cast (a, c) and thermal annealed films (b, d) and their corresponding phase images (e, g, f, h) for active layers.

described by the formula of Jsc∝(Plight)α, where the exponential factor α indicates the extent of bimolecular recombination. The α factor should be 1 if there is no bimolecular recombination and it is smaller than 1 for the devices with the bimolecular charge recombination. Figure S4(a) in SI shows the plots of log Jsc vs. log Plight, and the α factors can be obtained from the slopes of the straight lines in the figure. The α values are 0.999 and 0.953 for the OSCs with H11 and H12 as donor, respectively. The α value of the H11-based OSC is very close to 1, which indicates the bimolecular recombination of the charge carriers is suppressed in the device. Figure 4(b) in SI displays the plots of Voc vs. natural logarithm of light intensity (ln Plight) for the OSCs. If bimolecular recombination is the sole loss mechanism, the slope of Voc vs ln Plight lines should be close to kT/e (where k is the Boltzmann constant, T is Kelvin temperature, e is the elementary charge)23 The slopes for the OSCs based on H11 and H12 are 0.923 kT/e and 1.004 kT/e respectively, which are quite close to kT/e, indicating that there is very weak other recombination in the devices. Combining the results of the light intensity dependence of Jsc and Voc, there is very weak recombination in the nonfullerene OSCs, especially for the OSCs based on H11:IDIC. The hole mobility and electon mobility were measured using the space charge limited current method. The plots of the J−V curves of the hole-only and electron-only devices are provided in Figure S5a and Figure S5b in SI, respectively. The hole (μh) and electron (μe) mobilities of the as-prepared H11:IDIC film are calculated to be 0.14 × 10−4 cm2 V−1 s−1 and 2.06 × 10−4cm2 V−1 s−1, respectively. For the thermal-annealed film, μh of 0.77 × 10−4 cm2 V−1 s−1 and μe of 5.13× 10−4 cm2 V−1 s−1 were recorded, showing a higher and more balanced charge carrier mobility. The thermal annealing effect on the blend films of H12:IDIC also showed the same trend, μh of 0.39 × 10−4 cm2 V−1 s−1 and μe of 0.73 × 10−5cm2 V−1 s−1 were observed for the as-cast film; and μh of 0.79 × 10−4 cm2 V−1 s−1 and μe of 1.18 × 10−5cm2 V−1 s−1 for the thermal-annealed film. The increased and more balanced charge carrier mobilities after

thermal annealing could help the generated carriers to be quickly collected by the electrode and lead to the improvements of Jsc and FF in the devices. It should be mentioned that when IDIC blended with different donors of H11 or H12, the blend films show different electron mobilities (for the annealed film, 5.13 × 10−4 cm2 V−1 s−1 for the film blending with H11 versus 1.18 × 10−5 cm2 V−1 s−1 for that blending with H12). Recent results from Yan and co-workers on the fullerene system showed preaggregation behavior of the donor component leading to different fullerene acceptor aggregation/crystallinity, which affected the electron mobilties of the blend film.3b The big difference of the electron mobilities of the two blend films in our case, could result from the different aggregation behavior of the IDIC component in the two blend films. The charge transport properties of the two donors were also investigated by the organic field effect transistor (OFET) method with the results shown in Figure S6 in SI. The hole motilities from OFET measurements are 1.38 × 10−1 cm2 V−1 s−1 for H11 and 0.52 × 10−2 cm2 V−1 s−1 for H12. The results indicate that the hole mobilities in the parallel direction to substrate measured by OFET method are much higher than those in vertical direction measured by SCLC method for the two molecules, and H11 shows higher hole mobility than H12 in parallel directions. Morphological Characterization. Atomic force microscopy (AFM) with tapping-mode was utilized to investigate the effect of thermal annealing on the morphology of the active layers of the OSCs. The surface morphologies of the as-cast and thermal annealed films are shown in the Figure 4. A smooth surface morphology with relatively small root-mean-square (RMS) roughness of 1.07 nm for the H11 based blend film (Figure 4a) and 1.77 nm for the H12 based blend film (Figure 4c) are observed for the as-cast blend films. After thermal annealing, the RMS roughness of the blend films dramatically increased to 6.94 and 2.63 nm for the blend films based on H11 and H12 respectively (Figure 4b, d). The RMS values of the allsmall molecule blend films are generally larger than those of the 5089

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(300) peak at 0.840 Å−1 and (400) Å−1 peak at 1.195 Å−1. Along with the laminar peaks, π−π stacking at 1.773 Å−1 (corresponding to a π−π stacking distance of 3.54 Å) can be clearly seen in the in-plane (IP) direction. The result indicates that H11 is more prone to adopt a predominantly edge-on crystalline orientation relative to the substrate. H12 adopts a similar edge-on crystalline feature with (100) peak at 0.287 Å−1, (200) peak at 0.586 Å−1, (300) peak at 0.885 Å−1 and (400) Å−1 peak at 1.187 Å−1 in the OOP direction and π−π stacking at 1.782 Å−1 (corresponding to a π−π stacking distance of 3.52 Å) in the IP direction (see Figure S8 in SI). In contrast, n-OS acceptor IDIC tends to form a face-on orientation relative to the substrate as evidenced by the (100) peak at 0.270 Å−1 in the IP direction and strong π−π peak in the OOP direction (Figure 5c). Its strong and defined characters are associated with its strong aggregation behavior in the solid, which are a result from its more planar structure relative to other n-OS acceptors. For a suitable p-OS donor that matches well with IDIC, one primary requirement is to suppress its strong aggregation. However, the strong aggregation behavior of the p-OS itself (due to the lower entropic barrier relative to its polymer conterpart) creates a challenge to find an ideal pair of p-OS donor and n-OS acceptor that can form bicontinuous networks in their blends. This is one of the reasons for the low efficiency of common nonfullerene OSCs. When IDIC was blended with H11, it is of interest to note that GIWAXS plots exhibit more random orientation and disorded microstructural features with relatively weak peak intensities, and these diffraction patterns are arised from a combination of H11 and IDIC (Figure 5d). The diffraction patterns along with a smooth AFM surface suggests a compatible feature of p-OS donor and n-OS acceptor in their blend. Quantitatively, the full width at half-maximum (fwhm) values of (010) peaks in the IP and OOP directions are 0.130 and 0.144 Å−1, respectively corresponding to an IP π−π coherence length of 4.34 nm for H11 and an OOP π−π coherence length of 3.93 nm for IDIC in the as-cast films. Upon thermal annealing, significantly stronger peaks with more narrow width accompanied by the appearance of new peaks, such as the secondary laminar peaks at 0.560 Å−1 and third laminar peaks at 0.840 Å−1 for the H11 component, are clearly discernible (Figure 5e). Clear textures comprising of face-on and edge-on crystallites (bimodal) were observed in the annealed films with much higher IP π−π coherence length (6.49 nm) and OOP π−π coherence length (5.12 nm). Furthermore, such special face-on and edge-on crystalline orientations indicate the coexistence of vertical and parallel charge transportation channel in a sandwich device structure. Similar texture feature is also disclosed by a high efficiency nonfullerene OSC.14 Very recently, Yang et al. reported that mixed edge-on and face-on orientations can enable high efficiency PSCs because of three-dimensional (3-D) charge pathways.26 Thus, for our case, the higher crystalline characteristics of the blend films with 3-D charge pathways should promote intermolecular charge transfer and eventually result in the excellent Jsc and FF in the OSCs. While for the H12 based blend films (see Figure S8 in SI), thermal annealing has the same effect as that with the H11 based blend films as described above. When compared with the optimized blend films for H11 and H12, it can be observed that the H12-based blend films are more prone to adopt a predominant edge-on crystalline orientation, which suggests that it possesses less 3-D charge

polymer based blend films, which may be due to the stronger self-aggregation of the OSs with the lower entropy barrier. Quantitative domain size analysis of these films was further carried out via the Fourier transform analysis of AFM phase images (Figure 4f and 4h) according to our previously established methods.24 As depicted in Figure S7 in SI, all of the power spectral densities (PSD) of the phase images show a predominant peak located at q = 0.15 ± 0.03 nm−1, which correpsonds to an average domain spacing of ∼42 nm. The AFM phase images proved that thermal annealing treatment significantly leads to a phase separation with slightly larger domains for the H11-based films, while the domains spacing remains almost the same for the H12-based films. In addition, the domain size (∼21 nm) of the annealed H11 blend films is very close to the typical exciton diffusion length, which may partially explain the best performance achieved in the H11 blends. To further understand the molecular ordering at the nanoscale, material crystallinity and molecular texture features of the neat p-OS films as well as the blends were further studied by grazing incidence wide-angle X-ray scattering (GIWAXS).25 In the out-of-plane (OOP) direction (Figure 5a and 5b), H11 shows pronounced four order (h00) laminar packing diffraction peaks with (100) peak at 0.270 Å−1, (200) peak at 0.555 Å−1,

Figure 5. (a) Line cuts of the GIWAXS images; the GIWAXS images of (b) H11, (c) IDIC, (d) H11:IDIC (as-cast) and (e) H11:IDIC (thermal annealed). 5090

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that of the H12 blend. The results indicate the molecular packing in the H11 blend is more ordered. We note that the unchanged apparent melting temperature of H11 and H12 is likely due to the fact that this temperature is very close to the eutectic temperature. Additionally, the absence of any noticible cold crystallizaiton indicates that the DSC samples are not vitrified when drop cast. Further analysis of the melting point depression data of IDIC using the Nishi−Wang formalism28 yields slightly different interaction parameters (χ), i.e. 3.78 for the H11 blend and 3.52 for the H12 blend. The higher χ, the higher phase purity and a lower molecular miscibility. Overall, the DSC results indicate the miscibility of H11:IDIC is a bit lower compared with that of H12:IDIC. Calculation of solubility parameters is also a valuable method for understanding the thermodynamic properties of organic systems, and thus offer promising capabilities for estimating the molecular miscibility. Here, Hansen solubility parameters (HSPs) of these small molecules are estimated following group additive methods29 (details are shown in the Supporting Information). According to a recent study9c in record-efficiency n-OS based PSCs, empirical HSPs allowed us to make a relatively reliable comparison of χ parameters of structurally similar materials at room temperature and understand the degree of mixing in organic blends. Following Flory−Huggins solution theory, small molecule blend systems only phaseseparate when its χ is above the critical point (χc = ∼2, see Supporting Information for details). A higher χ is able to achieve higher phase purity and lower miscibility or degree of mixing, thereby leading to a higher device FF.9c,30 In this study, the two p-OSs have the same molecular skeleton and pair with a same n-OS, so the appended side chains in the p-OSs may

pathways in the active layer. Thus, carrier collection was hampered, likely resulting in relatively low FF and Jsc. To provide deep insights on the molecular miscibility of the all small molecule blends, differential scanning calorimetry (DSC) is first employed to quantify the degree of molecular mixing in the blends.27 Figure 6 shows DSC thermograms of

Figure 6. DSC thermograms of the small molecules: (a) heat-only thermograms of neat H11 and H11:IDIC blends; (b) heat-only thermograms of neat H12 and H12:IDIC.

the small molecules and their blend. The nonfullerene n-OS IDIC shows a very high melting enthalpy (ΔHm) of 77.6 J/g and a high melting point (Tm) of 284.4 °C (see Figure S9 in SI). Both small molecular donors are also semicrystalline, while with much smaller enthalpies (ΔHm < 25 J/g). After blending with small molecule donors, the melting point of IDIC in both blends is depressed to ∼275 °C, while the melting points of H11 and H12 remain unchanged. The ΔHm of H12 in the neat and blend states are almost the same, while the ΔHm of H11 in blend is reduced by a factor of 2 and its ΔHm is still higher than

Figure 7. Comparison of the dynamic curves: (a) probed at 566 nm for the neat H11 film and its blend with IDIC (2:1, w/w), (b) probed at 560 nm for H12 film and its blend with IDIC (2:1, w/w); Dynamics probed at 560 or 740 nm recorded from the blend films (2:1, w/w) of (c) H11:IDIC and (d) H11:IDIC. 5091

DOI: 10.1021/jacs.6b12826 J. Am. Chem. Soc. 2017, 139, 5085−5094

Article

Journal of the American Chemical Society

recombination.31 In our cases, the attachment of the thiophene side chains on H11 may have the same function, thus giving better device performance than that of H12 without the conjugated side chains, which is also in line with the conclusions of McNeill et al.32 and Hou et al.24a The results also rationalize the commonly used design strategy of attachment of conjugate side chains on donor or acceptor to tune their conformations.

alter the molecular miscibility of the two p-OS:IDIC blends. We found the estimated room temperature χ parameter of the H11:IDIC blend is slightly higher (∼2.52) than that of the H12:IDIC blend (∼2.35), which is very consistent with the trend as shown in the DSC measurements. The higher χ parameter estimated from both methods can well explain the observed higher FF in the H11 device and the advantage of inserting the conjugated side chain. Given that the critical χ is 2, these system would be just inside the two phase region and H11 leads to stronger phase separation than H12. Improved χ parameter and lower molecular miscibility could be new evidence accounting for the previous observations24a in high performance 2D-conjugated photovolatic polymer compared to its counterparts without conjugated side chains. Importantly, this correlation implies tuning the molecular miscibility of the nonfullerene all-small molecule donor and acceptor materials can be controlled by molecular engineering. Photophysics. Taking advantage of the complementary absorption of donors (H11 and H12) and IDIC acceptor, transient absorption experiment (TA) was performed with pump wavelength of 560 and 710 nm selected for the purpose of major excitation of donor and acceptor, respectively. Clear evidence of hole and electron transfer were observed in both of H11:IDIC and H12:IDIC (Figure S10 and S11 in SI), which provides a solid foundation for the outstanding photovoltaic performance. Detailed analysis of hole and electron transfer is similar to what we discussed before.15c Following discussion mainly focuses on the differences between H11:IDIC and H12:IDIC observed in our TA experiment. Although blends with H11 or H12 as donors show little difference in the aspect of hole transfer upon excitation at 710 nm, electron transfer from photoexcited H11 and H12 to IDIC shows clear differences (Figure 7a, b). With pump wavelength at 560 nm, donors are primarily excited and the electron transfer is manifested as a faster recombination of the major bleaching signal in both H11:IDIC and H12:IDIC. Figure 7a and Figure 7b compare temporal dynamics of the bleaching of donors (H11, H12) in blends (2:1, w/w) in comparison with that in neat donors (H11, H12). At the early stage (