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Dec 21, 2016 - Technology, 1 Jinji Road, 541004, Guilin, Guangxi People,s Republic of China. •S Supporting Information. ABSTRACT: For the given orga...
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Dramatically Boosted Efficiency of Small Molecule Solar Cells by Synergistically Optimizing Molecular Aggregation and Crystallinity Xiaoling Ma,† Fujun Zhang,*,† Qiaoshi An,† Qianqian Sun,† Miao Zhang,† and Jian Zhang*,‡ †

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Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, 100044, Beijing, People’s Republic of China ‡ Department of Material Science and Technology, Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, 1 Jinji Road, 541004, Guilin, Guangxi People’s Republic of China S Supporting Information *

ABSTRACT: For the given organic donor and acceptor materials, optimizing molecular aggregation and crystallinity for appropriate phase separation plays a crucial role in achieving high performance solar cells. In this study, the power conversion efficiency (PCE) of DR3TSBDT:PC71BM based small molecule solar cells (SMSCs) was markedly raised from 7.25% to 9.48% for active layers processed with 0.2 vol % 1,8-diiodooctane (DIO), resulting from the improved fill factor (FF) and short circuit current density (JSC). The performance improvement may be attributed to an appropriate DR3TSBDT molecular aggregation and crystallinity, as well as the optimized phase separation. The influence of DIO concentrations on DR3TSBDT molecular aggregation can be confirmed from the red-shifted absorption and photoluminescence peaks of films along with increase of DIO concentrations. Meanwhile, the ratio of hole/electron mobility approached 1.06 in the optimized SMSCs well according with the highest FF of the corresponding SMSCs. The morphology characterizations indicate that DR3TSBDT molecular aggregation and crystallinity could be finely adjusted by doping appropriate DIO additive. KEYWORDS: Small molecule solar cells, Power conversion efficiency, Crystallinity, Phase separation, Additive

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optimizing phase separation degree.16 Recently, several efficient strategies have been developed to tailor phase separation degree by employing mixed solvents, hot solution, thermal annealing (TA), solvent vapor annealing (SVA), and methanol treatment.17−20 Apart from the aforementioned strategies, solvent additives have been widely adopted to adjust phase separation during the time scale of film formation, which is a particularly welcomed strategy when TA treatment cannot be implemented due to the natural properties of used materials, especially for highly efficient narrow band gap polymer materials.21 Small molecule solar cells (SMSCs) have attracted more research interest because of small molecule intrinsic advantages, such as a well-defined chemical structure, facile synthesis, uniform batch-to-batch, easy purification, and allowing easier energy level control.22−24 Solvent additive 1,8-diiodooctane (DIO) has been commonly used to optimize phase separation for obtaining high performance solar cells due to the selective solubility for PC71BM and its high boiling point of 332 °C.25 The explanations of DIO on performance improvement of OSCs can be summarized as (i) DIO can enlarge domain size and purity as well as molecular crystallinity, which are favorable to increase charge carrier transport and decrease charge carrier

INTRODUCTION Organic solar cells (OSCs) have been considered as an attractive alternative due to their unique advantages of light weight, flexibility, and environmental friendliness.1−3 The performance of OSCs have made great strides during the past few decades, with the champion power conversion efficiency (PCE) more than 11%.4 There are some crucial factors underlying such breakthroughs, including high efficient photovoltaic materials, phase separation optimization, and novel device structure.5−11 It is universally known that the degree of phase separation governs photovoltaic processes, such as exciton diffusion and dissociation, charge carrier transport, and collection.12 Bulk heterojunction (BHJ) composite films were commonly obtained from spin coating donor (D)/ acceptor (A) blend solutions. When donor and acceptor materials are finely intermixed on the scale of molecular size, most of excitons will be dissociated into free charge carrier owing to the sufficient D/A interfacial area; charge carrier transport may be hindered due to the shortage of bicontinuous channels connecting the electrodes. In another case, large phase size in active layers is propitious to charge carrier transport; meanwhile, exciton dissociation may be depressed because most of excitons must undergo a longer diffusion range to reach the limited D/A interfaces.13−15 The balanced excitons and charge carrier dynamic processes can be achieved by finely © 2016 American Chemical Society

Received: November 20, 2016 Published: December 21, 2016 1982

DOI: 10.1021/acssuschemeng.6b02808 ACS Sustainable Chem. Eng. 2017, 5, 1982−1989

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) DR3TSBDT, PC71BM. and DIO molecular structures. (b) Device diagrammatic sketch. (c) Energy levels of used materials.

Figure 2. (a) J−V curves of SMSCs for active layers processed with different DIO concentrations. (b) EQE spectra of corresponding SMSCs. (c) Absorption spectra of DR3TSBDT:PC71BM films processed with different DIO concentrations. (inset) Absorption spectra of blend film and solution without DIO. (d) PL spectra of DR3TSBDT:PC71BM films processed with different DIO concentrations under 610 nm light excitation. (inset) PL spectra of pure DR3TSBDT film and solution.

recombination;26−28 (ii) DIO can suppress molecular aggregation to optimize D/A redistribution for sufficient exciton dissociation.29−33 However, DIO is not a panacea for PCE improvement of OSCs. For example, the PCE of OSCs with PIDTDTQx:PC71BM as active layer was markedly decreased from 4.57% to 1.96% by adding 4 vol % DIO and then was recovered from 1.96% to 3.71% by spin-coating ethanol on the active layer.34 Therefore, the underlying reasons and working mechanisms of DIO on the performance of OSCs are still being debated. In this work, the influence of DIO concentrations on phase separation of DR3TSBDT:PC71BM blend films was carefully investigated to achieve high performance SMSCs. The PCEs of SMSCs were significantly increased up to 9.48% from 7.25% for active layers processed with 0.2 vol % DIO, corresponding to an approximate 30.8% PCE improvement. The PCE improvement was mainly due to appropriate phase separation dependent on DR3TSBDT molecular aggregation

and crystallinity driven by DIO. The molecular structures, energy levels of used materials, and device diagrammatic sketch are exhibited in Figure 1. Detailed experimental procedures are described in the Supporting Information.

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RESULTS AND DISCUSSION The current density dependent on voltage (J−V) characteristics curves of SMSCs were measured under AM 1.5G irradiation with 100 mW/cm2 light intensity, as exhibited in Figure 2a. The sole difference is DIO concentrations which are less than 0.4 vol % in blend solutions. Obviously, JSC and FF values of SMSCs were increased along with DIO concentrations up to 0.2 vol % and decreased for doping more DIO. Meanwhile, the VOC of SMSCs were slightly decreased from 0.90 to 0.89 V and then to 0.85 V dependence on DIO concentrations. The underlying reason for the decreased VOC may be attributed to the elevated the highest occupied molecular orbital (HOMO) 1983

DOI: 10.1021/acssuschemeng.6b02808 ACS Sustainable Chem. Eng. 2017, 5, 1982−1989

Research Article

ACS Sustainable Chemistry & Engineering Table 1. Key Parameters of SMSCs Processed with Different DIO Concentrationsa DIO (vol %) 0 0.1 0.2 0.3 0.4 a

JSC (mA/cm2) 13.78 14.07 14.84 13.97 12.73

(13.53 (13.89 (14.75 (13.82 (12.54

± ± ± ± ±

0.26) 0.18) 0.15) 0.16) 0.22)

cal JSC (mA/cm2) 12.84 13.25 14.04 13.16 11.90

VOC (V) 0.90 0.89 0.89 0.85 0.85

(0.90 (0.89 (0.89 (0.85 (0.85

± ± ± ± ±

FF (%)

0.01) 0.01) 0.01) 0.01) 0.02)

58.45 65.84 71.77 69.55 64.66

(59.53 (65.45 (70.89 (68.62 (63.85

± ± ± ± ±

PCE (%) 1.32) 0.55) 0.97) 0.93) 1.25)

7.25 8.25 9.48 8.26 7.00

(7.12 (7.95 (9.29 (7.68 (6.78

± ± ± ± ±

0.13) 0.32) 0.27) 0.58) 0.25)

Rs (Ω cm2)

Rsh (Ω cm2)

11.4 6.5 4.6 6.3 8.9

500 602 1085 909 841

Average values for each parameter were calculated according to 20 cells.

level of donor induced by its molecular aggregation.35−37 To give the more solid evidence, ultraviolet photoemission spectroscopy (UPS) of pure DR3TSBDT films processed with different DIO concentrations was measured, as shown in Figure S1. It is known that HOMO level can be calculated by following equation: EHOMO = hυ − (EOnset − Ecutoff), where hυ is the incident photon energy, Ecutoff is defined as the lowest kinetic energy of the measured electrons, Eonset is referred to the high kinetic energy onset.38,39 It is apparent that the Ecutoff monotonously shifts to low energy region along with the increase of DIO concentrations and the Eonset is almost kept constant. According to UPS spectra of pure DR3TSBDT films processed with different DIO concentrations, the EHOMO levels of DR3TSBDT can be monotonously elevated along with the increase of DIO concentrations, which can well explain the decreased VOC of SMSCs. The elevated EHOMO levels of DR3TSBDT may be attributed to the enhanced DR3TSBDT molecular aggregation or crystallinity, which can be further confirmed from the next sections. The slightly decreased VOC can be completely compensated with the increase of FF and JSC, leading to the improved performance of SMSCs. The champion PCE of SMSCs approached 9.48% for active layers processed with 0.2 vol % DIO, along with a VOC of 0.89 V, a FF of 71.77%, and a JSC of 14.84 mA/cm2. It is highlighted that the rather low DIO concentration takes crucial role in improving the performance of SMSC. The external quantum efficiency (EQE) spectra of all SMSCs were characterized in air conditions, as exhibited in Figure 2b. It is evident that EQE spectra were enhanced by doping appropriate DIO in the whole spectral zone, meanwhile, EQE spectral range was slightly extended toward long wavelength. For SMSCs processed with 0.2 vol % DIO, EQE values were larger than 75% from 470 to 600 nm, and the best EQE value approached 79% at 385 nm. The calculated JSC values can be obtained by integrating the corresponding EQE spectra, which are slightly less than the measured values due to the cells without encapsulation for EQE spectra measurement in air conditions. All the key parameters and calculated JSC values are listed in the Table 1. The improvement of EQE spectra should mainly be ascribed to the optimized photon harvesting, exciton dissociation, charge carrier transport, and collection for active layers processed with appropriate DIO. To investigate the influence of DIO concentration on photon harvesting, absorption spectra of blend films were measured, as exhibited in Figure 2c. The absorption spectra of pure DR3TSBDT and PC71BM films are exhibited in Figure S2. Evidently, photon harvesting ability can be markedly enhanced for active layers processed with DIO. It is worth pointing out that red-shifted absorption peaks can be clearly observed from 570 to 598 nm for active layers processed with different DIO concentrations. A shoulder absorption peak about 650 nm can be clearly observed for active layers processed with more DIO, which may originate from the more ordered DR3TSBDT

molecular aggregation and crystalline degree during DIO slow volatilization from active layers. To further verify this speculation, the absorption spectra of blend DR3TSBDT:PC71BM solution and film were measured, as exhibited in illustration of Figure 2c. There is an obvious redshift of absorption peak for the blend films compared with the corresponding solutions, which should be ascribed to DR3TSDBT molecular aggregation in solid state.32 Therefore, the varied absorption spectra of active layers should be mainly ascribed to DR3TSBDT molecular aggregation driven by DIO slow volatilization, which also affects phase separation between DR3TSBDT and PC71BM. To further check this phenomenon, PL spectra of blend films processed with different DIO concentrations were measured, as exhibited in Figure 2d. The PL emission intensity was markedly enhanced for blend films processed with more DIO, indicating that DR3TSBDT domain size may be enlarged. Meanwhile, PL emission peak of DR3TSBDT was also red-shifted from 700 to 713 nm along with DIO concentration increase. To confirm the red-shift of PL emission peak induced by DR3TSBDT molecular aggregation, PL spectra of pure DR3TSBDT solution and film were measured, as exhibited in illustration of Figure 2d. As the illustration shown, PL emission peaks of pure DR3TSBDT solution and film locate at 670 and 720 nm, respectively. The apparent red-shift of PL emission peaks of DR3TSBDT should owe to the molecular aggregation in solid film.8 The PL emission peaks of DR3TSBDT are approximately 700 nm in blend films without DIO, 704 nm in blend films processed with 0.2 vol % DIO, 708 nm in blend films processed with 0.3 vol % DIO, 713 nm in blend films processed with 0.4 vol % DIO, and 720 nm in pure DR3TSBDT films. Therefore, the gradual redshift of PL emission peaks is mainly ascribed to the variation of DR3TSBDT molecular aggregation degree, which agrees with the observed phenomenon from absorption spectra of blend films. Therefore, phase separation of active layers can be optimized during DR3TSBDT molecular packing. According to the performance of SMSCs, phase separation degree should approach an optimal state for the active layers processed with 0.2 vol % DIO. To deeply investigate the influence of DIO concentrations on photovoltaic dynamic processes, photocurrent density (Jph) versus effective voltage (Veff) curves of SMSCs were calculated and are exhibited in Figure 3. Jph is defined as Jph = JL − JD, where JL and JD are current density under standard solar simulated light source and in dark, respectively. The term Veff is defined as Veff = V0 − Va, where V0 is the voltage at Jph = 0 and Va is the applied voltage.40 Assuming exciton dissociation efficiency (ηdiss) and charge carrier collection efficiency (ηcoll) are sufficient at Veff = 2 V, Jsat essentially depends on the maximal exciton generation rate (Gmax).35 At Veff = 2 V, Jsat ≈ qLGmax, where q is elementary charge and L is active layer thickness. The SMSCs processed with 0.2 vol % DIO exhibited the highest Gmax of 0.81 × 1028 m−3 s−1. The calculated Gmax 1984

DOI: 10.1021/acssuschemeng.6b02808 ACS Sustainable Chem. Eng. 2017, 5, 1982−1989

Research Article

ACS Sustainable Chemistry & Engineering

active layer and contact resistance between active layer and electrodes. The decreased Rs is beneficial to charge carrier transport in the active layer and collection at the interfaces. The Rsh mainly depends on the loss of charge carrier due to the defects in active layers, such as pin holes and traps.42 The increased Rsh indicates that the defects in active layers have been decreased by doping appropriate DIO. The minimum Rs and the maximum Rsh of optimized SMSCs further indicate that the efficient charge carrier transport and collection, as well as the less loss of charge carrier can be obtained in the active layers processed with 0.2 vol % DIO, leading to the highest FF value.43 To investigate charge carrier transport in active layers processed with different DIO concentrations, J−V curves of all SMSCs were measured under different light intensities from 3 mW/cm2 to 100 mW/cm2. The J−V curves of SMSCs processed with 0.2 vol % DIO are exhibited in Figure 4a and those of other SMSCs are exhibited in Figure S3. Based on J−V curves of the optimized SMSCs, JSC dependence on light illumination intensity is described in Figure 4b. In general, a power law dependence of JSC on light intensity is expressed as JSC ∝ Iα.44 All SMSCs exhibited an almost linear relationship of photocurrent density dependence on light intensity in double logarithmic coordinates. At short-circuit condition, photogenerated charge carrier can be efficiently swept out from active layers under the relatively large Veff. If the slope (α) value is close to 1, bimolecular recombination in active layers could be negligible.45 The slope (α) values are slightly increased from 0.973 to 0.990 and then decreased to 0.972 along with the increase of DIO concentrations from 0 to 0.2 vol % and 0.4 vol %, respectively. For SMSCs processed with 0.2 vol % DIO, the largest α value of 0.990 implies that photogenerated charge carrier should be efficiently collected by an individual electrode. The large deviation of α from unit suggests that more photogenerated charge carrier will be recombined in active layers. To further study the influence of DIO concentrations on charge carrier transport in active layers, hole-only and electrononly devices consisting of ITO/PEDOT:PSS (40 nm)/active layers (120 nm)/MoO3 (8 nm)/Ag (100 nm) and Al (100 nm)/LiF (0.8 nm)/active layers (120 nm)/LiF (0.8 nm)/Al (100 nm) were fabricated under the same conditions, respectively. The J−V curves of hole/electron-only devices were measured to characterize charge carrier transport along vertical direction by space charge limited current (SCLC) method, as exhibited in Figure S4.46 The J dependence on V can be expressed as the Mott−Gurney equation:

Figure 3. Jph−Veff curves of SMSCs processed with different DIO concentrations.

should be the effective exciton generation rate (the excitons can be completely converted into free charge carriers), which is different from that derived from all of photogenerated excitons according to absorption spectra of active layers. Only the dissociated excitons have contribution on Jsat. It is inevitable that part of photogenerated excitons cannot be dissociated into free charge carrier due to inappropriate phase separation degree. Photon harvesting of blend films processed with 0.2 vol % DIO is not the best one according to the absorption spectra of blend films, as shown in Figure 2c. This phenomenon further indicates that most of photogenerated excitons can be efficiently converted into free charge carrier in the SMSCs processed with 0.2 vol % DIO, suggesting the more appropriate phase separation in the active layers. To further investigate the effect of DIO concentrations on ηdiss and ηcoll, the ηdiss and ηcoll could be assessed by the Jph/Jsat values at short-circuit condition or maximal power output condition. The Jph/Jsat at short-circuit condition approached 95.1% for active layers processed with 0.2 vol % DIO, indicating the efficient exciton dissociation.41 The Jph/Jsat value was then decreased to 91.7% for active layers processed with 0.4 vol % DIO, indicating that exciton dissociation efficiency was decreased due to excessive phase separation or molecular aggregation. The Jph/Jsat at maximal power output condition exhibited the largest value of 81.1% for active layers processed with 0.2 vol % DIO, indicating the efficient charge carrier collection in the SMSCs. The detailed Jph, Jsat, Gmax, and Jph/Jsat values at short-circuit conditions and maximal power output condition were calculated, as listed in Table 2. As a result, the best values were obtained in the SMSCs processed with 0.2 vol % DIO, indicating optimal phase separation formed in the corresponding active layer. The series resistances (Rs) and shunt resistances (Rsh) of SMSCs were calculated according to the J−V curves, as listed in Table 1. It is apparent that the SMSCs processed with 0.2 vol % DIO exhibit the minimum Rs of 4.6 Ω cm2 and the maximum Rsh of 1085 Ω cm2. It is known that Rs mainly includes bulk resistance of

J=

9 V2 εrε0μ 3 8 L

Where ε0 is the vacuum permittivity, εr is the dielectric permittivity, μ is charge carrier mobility, V is the applied

Table 2. Jph, Jsat, Gmax, and Jph/Jsat Values of SMSCs Processed with Different DIO Concentrations

a

DIO (vol %)

Jpha (mA/cm2)

Jphb (mA/cm2)

Jsat (mA/cm2)

0 0.1 0.2 0.3 0.4

13.78 14.06 14.84 13.97 12.74

11.04 11.97 12.65 11.73 9.88

14.65 14.89 15.60 15.24 13.89

Gmax (m−3s−1)

Jph/Jsata (%)

Jph/Jsatb (%)

× × × × ×

94.1 94.4 95.1 91.7 91.7

75.4 80.4 81.1 77.0 71.1

0.76 0.78 0.81 0.79 0.72

1028 1028 1028 1028 1028

Short-circuit condition. bMaximal power output condition. 1985

DOI: 10.1021/acssuschemeng.6b02808 ACS Sustainable Chem. Eng. 2017, 5, 1982−1989

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ACS Sustainable Chemistry & Engineering

Figure 4. (a) J−V curves of SMSCs processed with 0.2 vol % DIO under different light illumination intensities (100, 80, 50, 40, 28, 16, 8, 3 mW/ cm2). (b) JSC dependence on light intensity curves for all SMSCs.

voltage, and L is active layer thickness.47 The hole mobility, electron mobility and hole/electron mobility ratios of active layers processed with different DIO concentrations are summarized in Table S1. The hole mobility (μh) and electron mobility (μe) dependence on DIO concentrations are exhibited in Figure 5. It is apparent that μh was increased from 1.60 ×

abstracted from the 2D-GIXD patterns, as exhibited in Figure 6. The intensity of diffraction (100) direction was enhanced for

Figure 6. Out-of-plane line cuts of DR3TSBDT:PC71BM films processed with different DIO concentrations.

blend films processed with more DIO, indicating that DR3TSBDT may have more ordered molecular arrangement. The enhanced diffraction intensity indicates that the crystallinity of DR3TSBDT can be improved for efficient hole transport in the blend films. The crystallinity enhancement may be attributed to the increased number or enlarged size of crystallites.49 The crystal size can be calculated according to the Scherrer equation: L = Kλ/(βcos θ), where L is the correlation length, K is the Scherrer constant, λ is incident beam wavelength, β is full width half maximal in radians, and θ is diffraction angle.50 The correlation lengths in (100) direction was increased from 15.40 nm (without DIO) to 19.80 nm (0.4 vol % DIO), indicating the DR3TSBDT crystal size can be enlarged in active layers processed with DIO. In fact, an appropriate crystal size should play crucial role in determining exciton dissociation, charge carrier transport and collection. The more ordered DR3TSBDT molecular arrangement should be favorable to exciton diffusion and hole transport in donor domain. To further verify the effect of DIO on DR3TSBDT molecular arrangement, the GIXD on pure DR3TSBDT films processed with or without DIO was carried out, as shown in Figure S6. It is apparent that the enhanced intensity of diffraction (100) direction can be clearly observed in the pure DR3TSBDT film processed with DIO. The crystallinity of DR3TSBDT can be enhanced by doping DIO, which accords well with the observed phenomena from the blend films processed with different DIO concentrations. It means that

Figure 5. Mobilities of electron and hole in blend films processed with different DIO concentrations, Rh/e = μh/μe as marked in the figure.

10−4 cm2 V−1 s−1 (0 vol % DIO) to 6.38 × 10−4 cm2 V−1 s−1 (0.4 vol % DIO), which should be ascribed to enhanced DR3TSBDT molecular aggregation and crystallinity in active layers processed with more DIO. Meanwhile, μe was also increased from 1.13 × 10−4 cm2 V−1 s−1 (0 vol % DIO) to 3.36 × 10−4 cm2 V−1 s−1 (0.2 vol % DIO) and then decreased to 1.30 × 10−4 cm2 V−1 s−1 (0.4 vol % DIO), resulting from the variation of PC71BM distribution in active layers. The most important thing is the balance of charge carrier transport in active layers, which can be appraised by the ratios of μh to μe (Rh/e), as marked in Figure 5. The optimal Rh/e approached 1.06 for active layers processed with 0.2 vol % DIO, which can well support the highest FF and JSC for the corresponding cells. It is known that charge carrier transport strongly depends on molecular arrangement and crystallinity in blend films.48 To further confirm the influence of doping DIO concentrations on molecular arrangement and crystallinity in blend films, twodimensional grazing incidence X-ray diffraction (2D-GIXD) characterization was employed on the blend films processed with different DIO concentrations. The 2D-GIXD patterns of blend DR3TSBDT:PC71BM films processed with different DIO concentrations are exhibited in Figure S5. The out-of-plane line cuts of blend films can be 1986

DOI: 10.1021/acssuschemeng.6b02808 ACS Sustainable Chem. Eng. 2017, 5, 1982−1989

Research Article

ACS Sustainable Chemistry & Engineering

Figure 7. AFM morphology images (a−e) and phase images (f−j) of DR3TSBDT:PC71BM blend films processed with 0 (a and f), 0.1 (b and g), 0.2 (c and h), 0.3 (d and i), and 0.4 vol % (e and j) DIO.

Figure 8. TEM images of DR3TSBDT:PC71BM blend films processed with different DIO concentrations.

DR3TSBDT and PC71BM molecular redistribution in active layers can be simultaneously adjusted by doping DIO for better exciton dissociation and charge carrier transport and collection. Nanoscale phase separation with bicontinuous interpenetrating network play crucial role in determining the performance of solar cells.51 To verify the influence of DIO concentrations on morphology of blend films, atomic force microscopy (AFM) characterization was performed, as exhibited in Figure 7. The surface morphology and phase images of blend films processed with different DIO concentrations are exhibited in Figure 7a−e and f−j, respectively. The root-mean-square (RMS) roughness of blend films was increased from 1.21 to 3.81 nm along with the increase of DIO concentrations from 0 to 0.4 vol %. The increased surface roughness may be ascribed to donor and acceptor molecular aggregation, resulting in increased D/A phase separation degree. The variation of phase separation can also be clearly observed from AFM phase images of blend films processed with different DIO concentrations. For blend films without DIO, no obvious phase separation could be observed due to the better miscibility of DR3TSBDT and PC71BM.52 The donor or acceptor domain size was slightly increased due to molecular aggregation during DIO slow volatilization from blend films. To deeply investigate the impact of DIO concentrations on phase separation, bulk morphology of blend films processed with different DIO concentrations was characterized by transmission electron microscopy (TEM), as exhibited in Figure 8. Bright region and dark region in the TEM images correspond to donor-rich and acceptor-rich domain, respec-

tively. No featured bulk morphology could be clearly observed for active layers without DIO, also indicating finely mixed DR3TSBDT and PC71BM at molecular size scale. The nanofibril networks of active layers become more and more obvious along with DIO concentrations increase, resulting from the increased domain size due to molecular aggregation. Meanwhile, the contrast between bright region and dark region approach an optimal state for active layers processed with 0.2 vol % DIO, indicating that an appropriate phase separation may be achieved. However, bright region in TEM images was enlarged for active layers processed with more DIO, leading to the excessive phase separation. As we known, excessive phase separation is favorable to charge carrier transport, but harmful for exciton dissociation. The fine intermixing or small domain size is more conductive to exciton dissociation, but harmful for charge carrier transport.53,54 Therefore, an appropriate nanoscale phase separation is very necessary to obtain highly efficient solar cells, which can be achieved by simultaneously optimizing donor and acceptor molecular aggregation and crystallinity during the process of active layer formation.

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CONCLUSIONS The PCE of solution processed SMSCs was markedly improved from 7.25% to 9.48% by doping 0.2 vol % DIO in blend solutions, which should be attributed to the optimized phase separation in active layers. The D/A phase separation strongly depends on DR3TSBDT molecular aggregation degree, which can be confirmed from the red-shifted absorption peak and PL emission peak of blend films processed with different DIO 1987

DOI: 10.1021/acssuschemeng.6b02808 ACS Sustainable Chem. Eng. 2017, 5, 1982−1989

Research Article

ACS Sustainable Chemistry & Engineering

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concentrations. Meanwhile, DR3TSBDT molecular crystallinity can also be enhanced during DIO slow volatilization from active layers. The experimental results indicate that the rather small amount of DIO play a crucial role in adjusting DR3TSBDT molecular aggregation and crystallinity, as well as PC71BM redistribution to form an appropriate vertical phase separation, leading to an approximate 30.8% PCE improvement. The simultaneous optimization on donor and acceptor molecular aggregation and crystallinity should be an efficient strategy to achieve high performance solar cells.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02808. Detailed experimental procedures; absorption spectra of pure DR3TSBDT and PC71BM films; J−V characteristic curves of SMSCs under different light intensities; J−V characteristic curves of hole-only and electron-only devices; 2D-GIXD patterns of blend films (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: 0086-10-51684908. E-mail: [email protected] (F.Z.). *E-mail: [email protected] (J.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by NSFC (61377029, 61564003, 61675017) and FRFCU (2016YJS150). The authors gratefully acknowledge the beamline scientists at Diffuse X-ray Scattering Station (1W1A) for their kind assistance, Beijing Synchrotron Radiation Facility.

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REFERENCES

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DOI: 10.1021/acssuschemeng.6b02808 ACS Sustainable Chem. Eng. 2017, 5, 1982−1989

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DOI: 10.1021/acssuschemeng.6b02808 ACS Sustainable Chem. Eng. 2017, 5, 1982−1989