Bispentafluorophenyl-Containing Additive: Enhancing Efficiency and

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Bispentafluorophenyl-Containing Additive: Enhancing Efficiency and Morphological Stability of Polymer Solar Cells via Hand-GrabbingLike Supramolecular Pentafluorophenyl:Fullerene Interactions Kai-En Hung, Che-En Tsai, Shao-Ling Chang, Yu-Ying Lai, U-Ser Jeng, Fong-Yi Cao, Chain-Shu Hsu, Chun-Jen Su, and Yen-Ju Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13426 • Publication Date (Web): 22 Nov 2017 Downloaded from http://pubs.acs.org on November 23, 2017

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ACS Applied Materials & Interfaces

Bispentafluorophenyl-Containing Additive: Enhancing Efficiency and

Morphological

Stability

of

Polymer

Solar

Cells

via

Hand-Grabbing-Like Supramolecular Pentafluorophenyl:Fullerene Interactions

Kai-En Hung,a Che-En Tsai,a Shao-Ling Chang,a Yu-Ying Lai,b U-Ser Jeng,c,d Fong-Yi Cao,a Chain-Shu Hsu,a Chun-Jen Su,c and Yen-Ju Chenga * a

National Chiao Tung University, Department of Applied Chemistry, 30010 University Road, Hsinchu,

Taiwan. b

Institute of Polymer Science and Engineering, National Taiwan University, No.1, Sec.4, Roosevelt Rd,

Taipei, 10617, Taiwan. c

National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu

30076 Taiwan. d

National Tsing Hua University, Department of Chemical Engineering, 101, Section 2, Kuang-Fu Road,

Taiwan, 30013, Hsinchu Taiwan. KEYWORDS: additives, fullerene aggregation, morphological stability, supramolecular interactions, polymer solar cells. E-mail: [email protected]

ACS Paragon Plus Environment


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Abstract A new class of additive materials bis(pentafluorophenyl) diesters (BFEs) where the two pentafluorophenyl (C6F5) moieties are attached at the both ends of a linear aliphatic chain with tunable tether lengths (BF5, BF7, and BF13) were designed and synthesized. In the presence of the BF7 to restrict the migration of fullerene by hand-grabbing-like supramolecular interactions induced between the C6F5 groups and the surface of fullerene, the P3HT:PC61BM:BF7 device showed stable device characteristics after thermal heating at 150 oC for 25 h. The morphologies of the active layers were systematically investigated by OM, GISAXS, and AFM. The tether length between the two C6F5 groups plays a pivotal role in controlling the intermolecular attractions. BF13 with long and flexible tether might form a BF13-fullerene sandwich complex that fails to prevent fullerene’s movement and aggregation, while BF5 with too short tether length decreases the possibility of interactions between the C6F5 groups and the fullerenes. BF7 with the optimal tether length has the best ability to stabilize the morphology. In sharp contrast, the non-fluorinated BP7 analogue without C6F5-C60 physical interactions does not have the capability of morphological stabilization, ambiguously revealing the necessity of the C6F5 group. Most importantly, the function of BF7 can be also applied to the high-performance PffBT4BT-2OD:PC71BM system which exhibited an original PCE of 8.80%. After thermal heating at 85oC for 200 h, the efficiency of the PffBT4BT-2OD:PC71BM:BF7 device only decreased slightly to 7.73%, maintaining 88% of its original efficiency. To the best of our knowledge, this is the first time that the thermal-driven morphological evolution of the high-performance PffBT4BT-2OD polymer has been investigated and its morphological stability in the inverted device can be successfully preserved by the incorporation of BF7. This research also demonstrates that BF7 is not only effective to PC61BM but also to PC71BM.


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INTRODUCTION Solution-processed polymer solar cells (PSCs) with advantageous features of light weight and flexible substrates are promising for renewable energy.1-5 Bulk heterojunction (BHJ) solar cells with maximum donor–acceptor interfacial area are the widely adopted configuration for efficient charge separation. Although extensive efforts have been made on nanostructure engineering to tailor the morphology of active layers,6-11 maintaining the optimized morphology under the long-term thermal heating is rather challenging. Poly(3-hexylthiophene) (P3HT) as the electron donor and [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM) as the electron acceptor are the most investigated system in BHJ solar cells. (Figure 1)12-14 Mild thermal annealing of P3HT:PC61BM blend induces the higher crystallinity of P3HT for efficient charge transport.15-18 Nevertheless, if the thermal heating is applied persistently, the spherical PC61BM is prone to diffuse out of the P3HT domain and form large PC61BM clusters.19-21 Such progressive evolution gives rise to the micrometer-sized phase segregation, dramatically reducing D-A interfacial area and device performance.22-23 Long-term sunlight irradiation inevitably accumulates heat that adversely deteriorates the optimized morphology.24-25 Several strategies such as reducing crystallinity of polymers26-27 or fullerenes28-30, incorporating a compatibilizer31, and crosslinking between components in active layers were described to stabilize the morphology of active layers. 32-35 Restricting PC61BM from the severe diffusion is the most effective approach to fix the optimal morphology. In situ crosslinking reaction between fullerene-based materials has effectively suppressed the thermal-driven phase separation.35 The optimized morphology of the active layer can be thus preserved through the covalent fixation. However, precisely controlling the temperature to trigger the occurrence of chemical crosslinking is difficult. In comparison with the covalent bonding strategy to fix the morphology, utilization of noncovalent interactions emerges as a simpler and smarter alternative. Introducing additives to achieve the optimal morphology of active layers has been widely implemented.36-47 Despite the additive strategies have gained substantial success in improving efficiency, the capability of preserving the optimized morphology against thermal heating is imperative but still lacking. Benzene and hexafluorobenzene spontaneously assemble into an alternating face-to-face stacking in order to form the complementary quadrupole-quadrupole attractions.48-57 Similar favorable interactions have been demonstrated between a pentafluorophenyl (C6F5) motif and the surface of



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a fullerene.58 Recently, we developed a PC61BM-based material PC61BPF attaching a pentafluorophenyl moiety.59 Through the intermolecular attractions between the fullerene materials and the C6F5 moiety of PC61BPF to suppress the morphological evolution, the P3HT/PC61BM-based devices incorporating PC61BPF as the additive successfully exhibited stable device characteristics against thermal treatment.59 By smartly adopting the concept of C6F5-C60 interactions, we further target at developing synthetically more accessible and cost-effective materials. To this end, we have developed a new class of bis(pentafluorophenyl) diesters (BFEs) where a linear aliphatic chain is end-capped by two C6F5 groups via ester linkages. The presence of two C6F5 groups in BFEs is intended to induce hand-grabbing-like interactions to physically hold fullerenes from migration-driven aggregation via the C6F5-C60 interactions. The length of the carbon-chain tether connecting two C6F5 groups in BFEs could sterically affect the miscibility and interactions with the photo-active materials, thus determining the effectiveness of morphological stabilization. To systematically investigate the length-dependent effect, three symmetrical BFEs, bis(pentafluorophenyl)glutarate with 5-carbon chain (denoted as BF5), bis(pentafluorophenyl)pimelate (BF7) with 7-carbon chain and bis(pentafluorophenyl)brassylate (BF13) with 13-carbon chain were designed.

RESULTS AND DISCUSSION Synthesis of BFEs. The synthesis of BFEs is simple and straightforward. As depicted in Scheme 1, condensation of glutarate acid, pimelate acid and brassylate acid with 2 equivalent pentafluorophenol in the presence of dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) led to the formation of BF5, BF7 and BF13, respectively, with moderate yields. In a similar manner, bis(phenyl)pimelate (BP7) with two nonfluorinated phenyl groups was also synthesized as a reference compound for comparison.

Figure 1.

The polymer structures used in this research.


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Scheme 1. Synthesis of BF5, BF7 and BF13 and BP7

Thermal properties. Differential scanning calorimetry (DSC) measurements showed that the melting point of BFEs decreases as the tether length increases (63, 53, and 40 oC for BF5, BF7, and BF13 respectively, Figure 2a). BF13 has the lowest melting point due to its highest structural flexibility. Meanwhile, the melting point of the non-fluorinated BP7 is at ca. 47 oC. On the other hand, the neat PC61BM showed a clear crystallization point Tc at 242 oC with the heat of crystallization (∆Hc) of 12.29 J/g during the cooling of the DSC measurement. To investigate the influence of the additive on the thermal transition of PC61BM, the 5 wt% BFEs were added into PC61BM for the DSC measurements (Figure 2b). After the incorporation of BF5, BF7, and BF13, the Tc of PC61BM gradually decreased to 222, 218 and 203 oC, while the heat of crystallization (∆Hc) also decreased to 8.37, 8.23 and 6.28 W/g, respectively. The reduced crystallization



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ability of PC61BM is associated with the presence of C6F5-C60 interactions. Moreover, the longer tether chain of the BFEs results in the lower Tc, broader crystallization peak and smaller ∆Hc.

Figure 2. (a) Differential scanning calorimetry heating curves of BF5, BF7, and BF13 and BP7 at a ramping rate of 10 oC/min; (b) differential scanning calorimetry cooling curves of neat PC61BM and PC61BM with 5 wt% BF5, BF7, and BF13 with a cooling rate of 10 oC/min.

Charge mobility measurements. To investigate the influence of BFEs on the charge transporting characteristics of the P3HT:PC61BM blend, the space-charge-limited-current (SCLC) measurements using ITO/PEDOT:PSS/active layer/Au and ITO/Al/active layer/Al device structures were employed to evaluate the hole mobility (µh) and electron mobility (µe), respectively. The results are summarized in Table 1. After introducing 7 wt% BF5, BF7 and BF13, the µh changed only marginally, while the µe decreased slightly as increasing the tether length of the BFEs from 1.08×10−4 to 7.88×10−5, 5.09×10−5 and 3.08×10−5 for P3HT:PC61BM, P3HT:PC61BM:BF5, P3HT:PC61BM:BF7 and P3HT:PC61BM:BF13 blends, respectively. These results indicate that the incorporation of the C6F5-containing BFEs only slightly affects the electron mobility of the P3HT:PC61BM system (Table 1). Table 1. SCLC hole and electron mobility of P3HT:PC61BM blends without or with 7 wt% BF5, BF7, and BF13. No additive

With BF5

With BF7

With BF13

(cm2s−1V−1)

(cm2s−1V−1)

(cm2s−1V−1)

(cm2s−1V−1)


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Hole mobility µh

5.96×10−5

3.93×10−5

5.01×10−5

4.34×10−5

Electron mobility µe

1.08×10−4

7.88×10−5

5.09×10−5

3.08×10−5

Thermal stability of the P3HT:PC61BM-based system. Traditional P3HT:PC61BM-based devices with ITO/PEDOT:PSS/P3HT:PC61BM/Ca/Al with configuration were used to evaluate the ability of the C6F5-containing BFEs for morphological stabilization against thermal treatment. BF5, BF7 and BF13 (7 wt%) additives were doped into the P3HT:PC61BM (1:1 in wt) blend to form ternary-blend active layers (P3HT:PC61BM:BF5, P3HT:PC61BM:BF7, P3HT:PC61BM:BF13, respectively) which were further heated at 150 °C for 0, 5, 10, 15, 20, and 25 h before depositing top electrodes to complete the devices. Under the identical device conditions, the reference P3HT:PC61BM devices without introducing any additive were also fabricated for comparison. The J–V curves of all the devices and their corresponding photovoltaic parameters (PCE, Voc, Jsc, and FF) as a function of heating time (5 h, 10 h, 15 h, 20 h and 25 h) at 150 °C are shown in the Figure 3a-e and Table S1-S5 (see the supporting information, SI). Table 2 only summarizes the device data before and after heating at 150 °C for 25 h. Before the thermal treatment, the devices using the P3HT:PC61BM, P3HT:PC61BM:BF5, P3HT:PC61BM:BF7, and P3HT:PC61BM:BF13 blends showed similar efficiencies of 3.81%, 3.73%, 3.70%, 3.53%, respectively, indicating that the incorporation of 7 wt% C6F5-containing additives does not influence the device performance (Table 2). The efficiency of the P3HT:PC61BM device degraded gradually from 3.81% to 0.57% after heating at 150 °C for 25 h (Figure 3a), which represents an 85% loss in efficiency. The dramatically reduced Jsc but unchanged Voc confirm that the performance deterioration is mainly attributed to the thermal-driven phase separation and PC61BM aggregation. However, in the presence of 7 wt% BF5, the P3HT:PC61BM:BF5 device showed a 35% efficiency loss from 3.73% to 2.43% after heating at 150 °C for 25 h (Figure 3b), indicating that BF5 additive already exerts a pronounced effect on alleviating the morphological degradation. Most importantly, the efficiency of P3HT:PC61BM:BF7 device only decays slightly (8% loss) from 3.70% to 3.43% after 25 h heating (Figure 3c), which suggests that BF7 with longer tether length has better capability of stabilizing the morphology. Interestingly, when the tether length increases to 13 carbons in BF13, the ability of morphological stabilization becomes totally invalid (Figure 3d). An extremely low efficiency of 0.31% was obtained for the P3HT:PC61BM:BF13 device after 25 h heating. These results manifest that the tether length



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associated with the structural flexibility is an important structural factor for the morphological stabilization. The BF13 with the longest aliphatic length might be too flexible to restrict the migration of fullerenes. On the contrary, the 5-carbon length in BF5 might be too short for the two C6F5 moieties to easily interact with the fullerenes. The 7-carbon tether in BF7 turns out to have the optimal distance and suitable flexibility to efficiently exert the C6F5-C60 attractions in the bulk. Considering that BF7 is the most effective additive material, we intentionally synthesized a non-fluorinated analogue of BF7, bisphenyl pimelate (BP7), where two nonfluorinated phenyl groups are connected by a 7-carbon tether. In sharp contrast, the device using 7 wt% BP7 showed much lower efficiency of 1.4% after 25 h heating (Figure 3e). These experimental comparisons verify that the existence of pentafluorophenyl groups in BF7 for inducing the C6F5-C60 attractions is imperative to fix the morphology. The PCE decay of P3HT:PC61BM devices without and with 7 wt% BF5, BF7, BF13, BP7 as a function of heating time at 150 oC is shown in Figure 4. Table 2 Characteristics of the ITO/PEDOT:PSS/P3HT:PC61BM with 7 wt% or without BF5, BF7, BF13, and BP7 additive/Ca/Al devices where the active layers were isothermally heated at 150 ˚C for 25 h.

heating time

Voc

Jsc

FF

PCEmax.

PCEavg.a

(h)

(V)

(mA/cm2)

(%)

(%)

(%)

0

0.60

8.90

71.3

3.81

3.67 ± 0.13

25

0.62

2.05

44.6

0.57

0.50 ± 0.05

0

0.60

8.71

71.4

3.73

3.56 ± 0.16

25

0.60

6.17

63.6

2.43

2.31 ± 0.10

0

0.60

8.65

71.2

3.70

3.66 ± 0.02

25

0.62

8.16

67.7

3.43

3.40 ± 0.01

0

0.60

8.86

66.4

3.53

3.18 ± 0.29

additive

No

BF5

BF7

BF13


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BP7

a

25

0.60

1.18

44.1

0.31

0.25±0.03

0

0.60

8.45

68.6

3.48

3.08 ± 0.35

25

0.62

3.93

57.5

1.40

1.18 ± 0.20

average PCE obtained from 10 devices

Figure 3. J-V curves of the ITO/PEDOT:PSS/P3HT:PC61BM/Ca/Al devices with thermal heating at 150 oC for different time. (a) without additive; (b) with 7 wt% BF5; (c) with 7 wt% BF7; (d) with 7 wt% BF13; (e) with 7 wt% BP7.



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Figure 4. The PCE decay of P3HT:PC61BM devices without and with 7 wt% BF5, BF7, BF13, and BP7 as a function of heating time at 150 oC. Optical Microscopy. Optical microscopy (OM) was utilized to investigate the morphological change of the P3HT:PC61BM, P3HT:PC61BM:BF5, P3HT:PC61BM:BF7, P3HT:PC61BM:BF7, P3HT:PC61BM:BF13 thin films which were prepared identically to those for the device fabrication. The OM images before and after the heating at 150 °C for 25 h are shown in Figure 5. Similar to the prior reports19, the homogeneity of the P3HT:PC61BM blend was altered subsequent to the heating and numerous long string-like PC61BM aggregates were observed (Figure 5a). When 7 wt% BF5 was introduced, the PC61BM aggregates in the P3HT:PC61BM:BF5 film were greatly reduced after heating (Figure 5b), revealing that BF5 has certain capability of preventing PC61BM aggregation. Moreover, the PC61BM aggregates in the P3HT:PC61BM:BF7 blend can be completely suppressed after thermal treatment (Figure 5c), indicating that BF7 stabilizes the morphology most effectively. When BP7, a nonfluorinated version of BF7, was employed, the thermal-driven aggregation of PC61BM was observed again in the P3HT:PC61BM:BP7 OM image after heating, demonstrating that the C6F5 groups unambiguously play a key role in slowing down the morphological evolution (Figure 5e).


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Figure 5. The OM images of P3HT:PC61BM films before/after 150 ˚C heating for 25 h (a) without additive (b) with 7 wt % BF5, (c) with 7 wt % BF7, (d) with 7 wt % BF13 and (e) with 7 wt % BP7.

In contrast, P3HT:PC61BM:BF13 blend exhibited severe aggregation after heating, confirming that too long tether length fails to suppress the morphological evolution of the P3HT:PC61BM (Figure 5d). The OM images are fairly consistent with the device-performance results. (The OM images of P3HT:PC61BM film with different heating time are shown in Figure S1).

Computation of the Optimized Geometries of BFEs and Interaction with C60. To understand the tether length effect of BFEs and elucidate why BF13 does not show the ability of morphology stabilization, density-functional-theory (DFT) calculations were performed. Geometry optimizations were all carried out with the Gaussian 09 package at the wB97XD/6-311G(d,p) level of theory. The minimal nature of stationary points was confirmed by frequency analysis. The utilization of the wB97XD functional lies on the fact that it includes empirical correction for dispersion energy, which could describe the weak intermolecular interactions more accurately than traditional DFT methods.60 Pristine C60 was employed as a simplified



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model for the fullerene acceptors so as to reduce the computational cost. The optimized geometries of stretching BF5, BF7, and BF13 are depicted in Figure 6a, of which all CH2 moieties adopt staggered conformation. The distances between the two oxygens of O-C6F5 are 7.22, 9.78, and 17.44 Å for BF5, BF7, and BF13, respectively. It was found that the molecular length of BF13 enables its two pentafluorophenyl groups to encapsulate a C60 to form a host-guest sandwich complex. Given that the calculated diameter of C60 is ca. 7 Å and the π stacking distance is within the range of 3–4 Å, the length of a tether must be at least 13 Å long (7+3+3 Å) to form such a host-guest sandwich complex. Among the three BFEs, only BF13 meets this structural condition. This possibility is further supported by a local minimum of the potential energy surface for the C60-BF13 sandwich complex whose optimized geometry is illustrated in Figure 6b. One C6F5 group is at a distance of 3.50 Å from C60 and the other one is 3.52 Å. The stabilization energy for the formation of the C60-BF13 sandwich complex is estimated by its association energy to be −30.35 kcal mol−1, which is much larger than the previously calculated intermolecular strength (−10.58 kcal mol−1) between C60 and methyl pentafluorobenzonate (Figure 6c) under the same level of theory59, suggesting that the formation of the C60-BF13 sandwich complex is much more thermodynamically favorable than that of the non-sandwich complex. The OM images in Figure 5d clearly reveal that the P3HT:PC61BM blends with 7 wt% of BF13 subsequent to thermal treatment exhibited the large-area spherulite-like crystallization, while others showed scattered the axialite-like crystallization. The existence of the C60-BF13 sandwich complex might be responsible for this disparity. In the sandwich configuration, the two C6F5 groups of BF13 interact with the same fullerene, thus losing the capabilities of hindering fullerenes from aggregation. Based on the OM images, it facilitated the crystallization of PC61BM instead, resulting in inadequate morphology for PSC applications. It should be also noted that the electron mobility of P3HT:PC61BM:BF13 drops almost by an order of magnitude compared to the P3HT:PC61BM blend presumably due to the formation of PC61BM:BF13 sandwich complex having an especially large disruption.


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Figure 6. (a) Optimized geometries of stretching BF5, BF7, and BF13; (b) C60-BF13 sandwich complex; (c) C60 and pentafluorophenyl complex.59

Morphological Stabilization of the PffBT4BT-2OD:PC71BM-Based System. Up to date, researches on the morphological stabilization in the literature have only focused on the traditional P3HT:PC61BM system. Given that a variety of high-performance p-type polymers have been developed for BHJ solar cells, further efforts should be directed toward maintaining their optimized morphology/performance against heating. A donor-acceptor copolymer PffBT4BT-2OD with strong intermolecular aggregation recently emerges as a high

profile

p-type

material.61-64

(Figure1)

The

inverted

devices

(ITO/ZnO/PffBT4BT-2OD:PC71BM/MoO3/Ag) using PffBT4BT-2OD as the donor and PC71BM as the acceptor have successfully accomplished high efficiencies.61-64 To further evaluate the morphological stability of the PffBT4BT-2OD:PC71BM blend, we applied isothermal annealing at 85 °C for 0, 50, 100, 200 h to the PffBT4BT-2OD:PC71BM blends and monitored the change of efficiency. Unlike the heating conditions for the P3HT:PC61BM system, decreasing the heating temperature to 85 oC but prolonging the heating time to 200 h were adopted in order to avoid degradation at interfaces of the inverted devices during thermal treatment.24 However, the conditions are already harsh enough to mimic the real circumstance of sunlight irradiation. The J-V characteristics and curves of the PffBT4BT-2OD:PC71BM films with different



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heating time are shown in Table 3 and Figure 7. The device initially exhibited a PCE of 8.61% before thermal treatment. The device efficiencies degraded gradually as increasing the heating time. After 200 h heating, the PCE decreased dramatically to 4.04% which is a 53% reduction of its pristine value (Figure 7a). Similar

to

the

P3HT:PC61BM

system,

we

have

demonstrated

that

the

highly

crystalline

PffBT4BT-2OD:PC71BM blend does not have good morphological stability against thermal heating. To test the effectiveness of our C6F5-containing additive in improving morphological stability of the PffBT4BT-2OD:PC71BM system, 10 wt% BF7 was doped to form ternary PffBT4BT-2OD:PC71BM (1:1 wt):BF7 blends which were isothermally heated at 85 °C for 0, 50, 100, 200 h, respectively (Table 3). In the presence of BF7, the pristine device without thermal heating achieved a PCE of 8.80% which is even higher than the device without adding BF7 (8.61%). Therefore, BF7 can simply serve as an additive to modulate the interactions between the polymer and fullerene, leading to the optimized morphology and performance. More importantly, the degradation rate of the device efficiency as a function of heating time is greatly decelerated in the presence of BF7. After 200 h heating (over 8 days), the device still maintained a high efficiency of 7.73%, preserving 88% of its original efficiency, demonstrating that BF7 remains effective in improving the morphological stability of the PffBT4BT-2OD:PC71BM system (Figure 7b). It should be noted that the efficiencies of the PffBT4BT-2OD:PC71BM:BF7 device are almost unchanged in the range of 100-200 h heating. Figure 8 shows the PCE decay of the devices as a function of heating time. These results suggest that if the heating is applied persistently over 200 h, the device characteristics of PffBT4BT-2OD:PC71BM:BF7 is prone to be stable, whereas the PffBT4BT-2OD:PC71BM device without BF7 tends to decay continuously.


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Figure 7. J-V curves of the ITO/ZnO/PffBT4BT-2OD:PC71BM/MoO3/Ag devices thermally annealed at 85 o

C for 50, 100 and 200 h (a) without additive and (b) with 10 wt% BF7.

Table 3. Characteristics of the ITO/ZnO/PffBT4BT-2OD:PC71BM (1:1.2 in wt%) without or with 10 wt% BF7/MoO3/Ag devices where the active layers were isothermally heated at 85˚C for various time. additive

No

BF7

a

heating time

Voc

Jsc

FF

PCEmax.

PCEavg.a

(h)

(V)

(mA/cm2)

(%)

(%)

(%)

0

0.74

16.42

70.9

8.61

8.19 ± 0.35

50

0.72

13.27

65.1

6.21

5.89 ± 0.21

100

0.64

13.86

58.0

5.14

4.79 ± 0.21

200

0.58

13.43

51.9

4.04

3.76 ± 0.22

0

0.76

17.11

67.7

8.80

8.52 ± 0.26

50

0.76

16.58

66.8

8.42

8.22 ± 0.14

100

0.74

15.80

66.7

7.80

7.54 ± 0.20

200

0.76

15.85

65.9

7.73

7.19 ± 0.33

average PCE obtained from 10 devices



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Figure 8. The PCE decay of PffBT4BT-2OD:PC71BM and PffBT4BT-2OD:PC71BM:10 wt% BF7 devices as a function of heating time at 85 oC.

Small-Angle X-Ray Scattering Measurements. Grazing-incidence small-angle X-ray scattering (GISAXS) was used to monitor the thermal-induced aggregation of the fullerenes in the PffBT4BT-2OD:PC71BM thin films with or without BF7. The as-cast PffBT4BT-2OD:PC71BM thin-film exhibited prominent GISAXS intensity, which should be contributed largely by PC71BM aggregates as elaborately demonstrated previously.65-68 The radius of gyration (Rg) of the PC71BM clusters can be roughly estimated by using the Guinier approximation: I(q) = I(0)exp(-Rg2q2/3) where I(q) is the in-plane scattering intensity extracted from the corresponding 2D GISAXS pattern measured; I(0) is the zero-angle scattering intensity and q is defined as 4πλ-1sinθ in terms of the scattering angle 2θ and the wavelength λ. The GISAXS profiles I(qy) were extracted at the specular beam position within the region qz = 0.03 Å-1 (as marked in Figure S3). After thermal annealing for the PffBT4BT-2OD:PC71BM blends at 85 oC for 50, 100, and 150 h, the low-q GISAXS intensity steadily enhanced and the Rg of the PC71BM clusters estimated in the q-range of 0.01~0.04 Å-1 increases from 2.1 nm (pristine) to 6.6 nm (50 h), 7.7 nm (100 h), and 8.5 nm (150 h), indicating that the PC61BM is gradually aggregated into larger domains upon thermal treatment (Figure 9a, Table


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S6). The significantly increased intensity in the very low q-region near 0.003 Å-1 indicates a structural enhancement in an even larger length scale, presumably, from agglomeration of PC71BM aggregates.67 Nevertheless, when 10 wt% BF7 was introduced into the PffBT4BT-2OD:PC71BM blend, the intensity growth of the corresponding GISAXS profiles can be suppressed effectively in similar thermal annealing. Correspondingly, the PC71BM aggregation size similarly extracted in the q-range of 0.01~0.04 Å-1 maintains stably in the range of 3.3 to 3.6 nm (Figure 9b, Table S6) without much growth. Furthermore, relatively moderate intensity increase in the very low-q regime near 0.003 Å reveals less confined agglomeration of PCBM aggregates. These results provide direct evidence that BF7 is very efficient in suppressing the thermal-induced PC71BM aggregation in the crystalline PffBT4BT-2OD polymer during thermal treatment. Better suppression of large-scale agglomeration from PCBM aggregates, however, would require longer BFEs derivatives. (Rg of PC71BM cluster aggregates in PffBT4BT-2OD with different heating time are shown in Figure S2, Figure S3 and Table S6 Supporting Information).

Figure 9. GISAXS profiles of the as-cast and annealed PffBT4BT-2OD/PC71BM films at 85 oC for 0, 50, 100, 150 h (a) without BF7; (b) with 10 wt% BF7. The arrows in (a) mark the drastic intensity changes in the low- and high-q regimes owing to growth of individual PC71BM aggregates and their agglomeration in different length scales upon annealing.



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Atomic Force Microscopy. Atomic force microscopy (AFM) was used to observe the evolution of the surface morphology of PffBT4BT-2OD:PC71BM films before and after heating. After 200 h heating at 85 °C, the Rq value of PffBT4BT-2OD:PC71BM thin film decrease from 4.19 nm (pristine) to 3.65 nm (anneal for 200 hr). However, the surface morphology of PffBT4BT-2OD:PC71BM:BF7 thin film does not show notable change before and after 200 h heating at 85 °C, the Rq value of PffBT4BT-2OD:PC71BM:BF7 thin film decrease from 3.82 nm (pristine with BF7) to 3.55 nm (anneal for 200 hr). It again suggest that the morphology is stabilized by BF7 (Figure 10).

Figure 10. AFM images of PffBT4BT-2OD:PC71BM films: (a) height and (b) phase before heating, (c) height and (d) phase images after 200 h heating at 85 °C; AFM images of PffBT4BT-2OD:PC71BM:BF7 films: (e) height and (f) phase before heating, (g) height and (h) phase images after 200 h heating at 85 °C.

CONCLUSIONS We have developed a new class of synthetically easy and cost-effective materials bis(pentafluorophenyl) diesters (BF5, BF7, and BF13) where the two pentafluorophenyl moieties are attached at the both ends of a linear aliphatic chain with tunable tether lengths (5-carbon for BF5, 7-carbon for BF7 and 13-carbon for BF13). These materials function as additives to be incorporated into the traditional P3HT:PC61BM blend. Through the hand-grabbling-like C6F5-C60 attractions to restrict the migration of PC61BM, BF7 can


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effectively suppress the fullerene aggregation to slow down the morphological evolution. The P3HT:PC61BM:BF7 device showed very stable device characteristics after thermal heating at 150 oC for 25 h. The tether length plays a key role in controlling the intermolecular interactions. In sharp contrast, the device incorporating BF13 did not possess any morphological stability upon thermal heating due to the fact that BF13 with long and flexible tether might encapsulate a PCBM to form a BF13-fullerene complex that fails to prevent fullerene’s movement and aggregation. On the other hand, the tether length of BF5 is relatively short, which might decrease the possibility of interactions between the C6F5 groups with the fullerenes. Therefore, the efficiency of the P3HT:PC61BM:BF5 device decreased by 35% after thermal heating at 150 oC for 25 h. BF7 with the optimal tether length has the best ability to stabilize the morphology. In sharp contrast, the non-fluorinated BP7 analogue without C6F5-C60 physical interactions does not have the ability of morphological stabilization, ambiguously revealing the importance of the perfluorobenzyl

group.

Most

importantly,

BF7

is

also

effective

to

the

high-performance

PffBT4BT-2OD:PC71BM system which exhibited an original PCE of 8.80 %. After thermal heating at 85oC for 200 h, the efficiency only decreased slightly to 7.73%, maintaining 88% of its original efficiency. To the best of our knowledge, this is the first time that the thermal-driven morphological evolution of a high-performance PffBT4BT-2OD polymer other than P3HT has been investigated and its morphological stability in the inverted device can be successfully preserved by the introduction of BF7. We demonstrate that the function of BF7 is widely applicable to both n-type PC61BM and PC71BM with highly crystalline p-type polymers to achieve highly efficient and stable PSCs.

ASSOCIATED CONTENT Supporting Information. Device characteristics, OM image, X-ray scattering measurements, synthesis of BFEs, computational data, and NMR spectra. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Yen-Ju Cheng)



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Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. Funding Sources Ministry of Science and Technology, Taiwan ACKNOWLEDGMENT We thank the Ministry of Science and Technology and the Ministry of Education, Taiwan for financial support. We thank the National Center of High-Performance Computing (NCHC) in Taiwan for computer time and facilities. REFERENCES 1 Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868-5923. 2 Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Plastic Solar Cells. Adv. Funct. Mater. 2001, 11, 15-26. 3 Dennler, G.; Scharber, M. C.; Brabec, C. J. Polymer-Fullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 2009, 21, 1323-1338. 4 Thompson, B. C.; Fréchet, J. M. J. Polymer-Fullerene Composite Solar Cells. Angew.Chem., Int. Ed. 2008, 47, 58-77. 5 Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science. 1995, 270, 1789-1791. 6 Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Thermally Stable, Efficient Polymer Solar Cells with NanoscaleControl of the Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15, 1617-1621.


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Cheng,

Y.-J.;

Liao,

M.-H.;

Chang,

C.-Y.;

Kao,

W.-S.;

Wu,

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