Aggregation Strength Tuning in Difluorobenzoxadiazole-Based

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Aggregation Strength Tuning in Difluorobenzoxadiazole-Based Polymeric Semiconductors for High-Performance Thick-Film Polymer Solar cells Peng Chen, Shengbin Shi, Hang Wang, Fanglong Qiu, Yuxi Wang, Yumin Tang, Jian-Rui Feng, Han Guo, Xing Cheng, and Xugang Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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ACS Applied Materials & Interfaces

Aggregation Strength Tuning in Difluorobenzoxadiazole-Based Polymeric Semiconductors for High-Performance Thick-Film Polymer Solar cells Peng Chen,∇ Shengbin Shi,∇ Hang Wang, Fanglong Qiu, Yuxi Wang, Yumin Tang, Jian-Rui Feng, Han Guo,* Xing Cheng, and Xugang Guo* Department of Materials Science and Engineering and The Shenzhen Key Laboratory for Printed Organic Electronics, Southern University of Science and Technology (SUSTech), No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055, China

KEYWORDS: difluorobenzoxadiazole-based polymer, π-spacer, temperature-dependent aggregation, polymer solar cell, thick active layer

ABSTRACT High-performance polymer solar cells (PSCs) with thick active layers are essential for large-scale

production.

Polymer

semiconductors

exhibiting

temperature-dependent

aggregation property offer great advantages towards this purpose. In this study, three difluorobenzoxadiazole

(ffBX)-based

donor

polymers,

PffBX-T,

PffBX-TT,

and

PffBX-DTT, were synthesized, which contain thiophene (T), thieno[3, 2-b]thiophene (TT), and

dithieno[3,2-b:2’,3’-d]thiophene

(DTT)

as

the

π-spacers,

respectively.

Temperature-dependent absorption spectra reveal that the aggregation strength increases in the order of PffBX-T, PffBX-TT, and PffBX-DTT as the π-spacer becomes larger. PffBX-TT with the intermediate aggregation strength enables well controlled disorder-order 1

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transition in the casting process of blend film, thus leading to the best film morphology and the highest performance in PSCs. Thick-film PSC with an average power conversion efficiency (PCE) of 8.91% and the maximum value of 9.10% is achieved using PffBX-TT:PC71BM active layer with a thickness of 250 nm. The neat film of PffBX-TT also shows a high hole mobility of 1.09 cm2 V−1 s−1 in organic thin-film transistors. When PffBX-DTT and PffBX-T are incorporated into PSCs utilizing PC71BM acceptor, the average PCE decreases to 6.54% and 1.33%, respectively. The performance drop mainly comes from reduced short-circuit current, as a result of non-optimal blend film morphology caused by a less well controlled film formation process. A similar trend was also observed in non-fullerene type thick-film PSCs using IT-4F as the electron acceptor. These results show the significance of polymer aggregation strength tuning towards optimal bulk heterojunction film morphology using ffBX-based polymer model system. The study demonstrates that adjusting π-spacer is an effective method, in combination with other important approaches such as alkyl chain optimization, to generate high-performance thick-film PSCs which are critical for practical applications.

INTRODUCTION Polymer solar cells (PSCs) with a bulk heterojunction (BHJ) structure are highly promising towards large-scale and cost-effective solar energy harvesting through solution-based device fabrication techniques, such as roll-to-roll coating and inkjet printing.1-4 Synergistic efforts in both semiconductor materials development and device engineering are 2


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rapidly advancing the device performance of the PSCs.5-15 To date, the highest power conversion efficiencies (PCEs) have now surpassed 13% in the state-of-the-art single-junction and 14% in multi-junction PSCs.11,

12, 16-19

However, most of these high PCE values are

obtained from PSCs having thin active layer with a thickness around 100 nm, which are troublesome for the high-throughput manufacturing process.7, 9, 13, 14, 20 The performance of thick-film PSCs (active layer >200 nm) is often inferior to their thin-film analogues in terms of fill factors (FFs) and/or short-circuit currents (Jscs), mainly due to the increased charge recombination probability in thick films.21 As a result, there are a limited number of thick-film PSCs that can achieve PCEs over 9% at current stage.22-42 Interestingly, majority of these results are attained using donor polymers exhibiting strong temperature-dependent aggregation (TDA) property, 6, 8, 23-29, 31-33, 35-38 which provides a feasible approach to effectively tune the BHJ film morphology through control of the disorder-order transition of the donor polymer during the film casting process.43 The importance of the TDA property was well demonstrated by Yan et al. in the polymer:fullerene system using a model polymer based on the difluorobenzothiadiazole (ffBT) moiety.23, 44 Later on, this strategy leads to successful development and application of a wide range of high-performance donor polymers not only based on ffBT,8,

23-27, 45, 46

but also on

difluorobenzotriazole (FTAZ),47, 48 naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NT),23, 31-33, 49 benzodithiophene (BDT),11,

16, 37, 50, 51

etc. These donor polymers work well with both

fullerene and non-fullerene type acceptor materials. Recently, a new building block difluorobenzoxadiazole (ffBX)28 was developed in 3



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analogous to the ffBT unit by replacing the sulfur atom in benzothiadiazole with an oxygen atom having a higher electronegativity, targeted on gaining larger open-circuit voltages (Vocs) in PSCs by downshifting the energy levels of frontier molecular orbitals (FMOs) of the donor polymers. Electron-rich building blocks, such as quarterthiophene (T4),28 BDT,29 and terthiophene (T3),52 have been utilized to couple with ffBX to construct new donor polymers, which enable PSCs with a Voc of ca. 0.1-0.2 V larger than its ffBT-based polymer analogues,28, 52

resulting in encouraging PCE values (>9%) for PSCs with thick active layer.28, 29 In addition,

the ffBX-based polymers have also been applied in organic thin-film transistors (OTFTs) with remarkable mobilities up to 2.92 cm2 V−1 s−1 and tunable charge carrier polarity.53 These results inspire us to further explore the potentials of ffBX-based polymers towards high-performance thick-film PSCs. In this work, we report three new donor polymers based on ffBX with two flanking 4-(2-decyltetradecyl)thiophene units (ffBX-2TDT), which are then connected with three different

π-spacers

including

thiophene

(T),

thieno[3,2-b]thiophene

(TT),

and

dithieno[3,2-b:2′,3′-d]thiophene (DTT) to afford polymers PffBX-T, PffBX-TT, and PffBX-DTT, respectively. All three polymers display temperature-dependent aggregation properties as expected and their aggregation strength increases with the number of fused thiophene rings in the π-spacers, in the order of T, TT and DTT. The strategy using these π-spacers has been reported before, but in NDI-based n-type semiconductors for applications in all-polymer solar cells.54-56 Among these three polymers, PffBX-TT with the intermediate aggregation strength shows the most promising solar cell performance, enabling an average 4


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power conversion efficiency (PCE) of 8.91% and the maximum value of 9.10% with the polymer:PC71BM active layer thickness of 250 nm. This PCE is comparable to the highest reported values for PSCs using ffBX-based donor polymers.28,

29, 52

In addition, the neat

PffBX-TT film shows a large average hole mobility (µh) of 0.63 cm2 V−1 s−1 in OTFTs under the optimal annealing condition. For PffBX-T with the weakest aggregation strength in this series, the OTFTs show a smaller average µh of 0.44 cm2 V−1 s−1. Its incorporation into PSCs results in a greatly degraded PCE of 1.33%. Although PffBX-DTT exhibits the highest average µh of 1.61 cm2 V−1 s−1 in the series, the PCE is decreased to 6.54%. The PCE drop is mainly due to the suppressed Jsc values as a result of non-optimal polymer:PC71BM blend film microstructure revealed by film morphology characterization. An additional study using non-fullerene type acceptor revealed a similar trend in thick-film PSC performance, with PffBX-T:IT-4F blend showing the highest PCE of 8.02% within this series of donor polymers. The large modulation of blend film morphology and its clear correlation to polymer aggregation property imply the importance of backbone structure fine-tuning in polymer semiconductor for applications in PSCs. This specific study on the effects of π-spacer units leads to a direct linkage between the degree of ring fusion and the polymer aggregation strength, which provides a valid approach to use in combination with alkyl chain modification8,

23, 29

for the optimization of polymer structures to reach higher device

performance of PSCs. RESULTS AND DISCUSSION Polymer Synthesis. The key monomer, dibromo ffBX-2TDT-Br, was synthesized in high

purity

from

dibrominated 5

difluorobenzoxadiazole


and


2-tri(n-butyl)stannylated-4-(2-decyltetradecyl)thiophene procedure.28,

53

thiophene

(T-Sn),

according

Page 6 of 39

to

the

published

The dibromo monomer was copolymerized with trimethylstannylated trimethylstannylated

thieno[3,2-b]thiophene

(TT-Sn),

and

trimethylstannylated dithieno[3,2-b:2′,3′-d]thiophene (DTT-Sn) using Pd-mediated Stille coupling under microwave irradiation (Scheme 1). After polymerization, the polymer chains are end-capped with mono-functionalized thiophene to afford the target polymers PffBX-T, PffBX-TT, and PffBX-DTT. The polymers were then purified via Soxhlet extraction to remove low molecular weight fractions and impurities (Supporting Information). All three polymers show similar molecular weights as measured by high-temperature gel permeation chromatography (GPC) at 150 ºC using 1,2,4-trichlorobenzene as the eluent. Their number-averaged molecular weights (Mns) are in the range of 32-35 kDa with comparable polydispersity index (PDI) values between 1.7-2.0 (Table 1). The polymer PffBX-T with thiophene π-spacer is readily soluble in chloroform and chlorobenzene at room temperature, while PffBX-TT and PffBX-DTT with more extended π-spacers become less soluble at this temperature, but can be well dissolved in warm 1,2-dichlorobenzene (o-DCB), which enables their facile incorporation into OTFT and PSC devices. Scheme 1. Synthetic routes to the polymers PffBX-T, PffBX-TT, and PffBX-DTT.

6


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Table 1. Molecular weights, optical and electrochemical properties of the polymers PffBX-T, PffBX-TT, and PffBX-DTT. Polymer

a)

Mn (kDa)

PDI

λmaxa)

λonseta)

EHOMOb)

ELUMOc)

Egopt d)

(nm)

(nm)

(eV)

(eV)

(eV)

PffBX-T

33.2

1.7

619

734

−5.69

−4.00

1.69

PffBX-TT

34.5

1.9

696

758

−5.47

−3.83

1.64

PffBX-DTT

32.8

2.0

703

767

−5.43

−3.81

1.62

As-cast polymer films from o-DCB solutions;

b)

EHOMO = −(Eoxonset + 4.80) eV, Eoxonset is determined

electrochemically using Fc/Fc+ internal standard; c) ELUMO = EHOMO + Egopt; d) Calculated from onset of the as-cast polymer films, Egopt = 1240/λonset (eV).

Thermal, Optical and Electrochemical Properties of Polymers. Thermogravimetric analysis (TGA) shows that all three polymers have good thermal stability with the thermal decomposition temperatures (Tds) all above 360 ºC (Figure S1a) defined by the point of 5% weight loss. Further evaluation with differential scanning calorimetry (DSC) measurement (Figure S1b) reveals that PffBX-T polymer exhibits weak endothermal and exothermal transition peaks at ~228 ºC and 213 ºC during the heating and cooling cycle, respectively. However, no pronounced thermal transitions were observable for both PffBX-TT and 7



PffBX-DTT within the measurement range up to 320 ºC, which likely suggest stronger inter-chain interaction of these two polymers, leading to the transition temperature out of the range. Optical properties of three polymers were studied by ultraviolet-visible (UV-vis) measurement. The normalized UV-vis absorption spectra of the polymer solutions and films at room temperature are plotted in Figure 1a and 1b, respectively. All three polymers are pre-aggregated in diluted 1,2-dichlorobenzene (o-DCB) solutions (concentration: 10−5 M), which show strong 0-0/0-1 optical transitions located around 600-750 nm, indicative of their planar backbone structures promoted by the small steric hindrance between neighboring arenes and the non-covalent F…S or F…H interactions, and the strong inter-chain attractions. The absorption peaks are gradually red-shifted following the order of PffBX-T, PffBX-TT, and PffBX-DTT, which indicates conjugation length extension by the incorporation of fused and more extended π-spacers. When going from solution to film state, there is a small bathochromic shift (~5 nm) in the absorption maximum (λmax) for PffBX-T and PffBX-TT films, suggesting slightly improved molecular ordering in solid state. In contrast, PffBX-DTT film exhibited a

8


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Figure 1. UV-vis absorption spectra of the ffBX-based polymers in (a) diluted o-DCB solutions (concentration: 10−5 M) and (b) pristine films at room temperature; (c, d, e) Temperature-dependent absorption spectra of polymers in diluted o-DCB solutions. (f) Relative aggregation strength curves of the ffBX-based polymers, calculated using the relative intensity of 0-0 peaks normalized to that at 30 ºC. blue-shift (~5 nm), which is related to its strongest aggregation in solution (vide infra).28 Please note that the absorption profiles are more different in solution than in film state for these polymers, reflecting the effects of π-spacers on aggregation. As the π-spacer varies from T to TT and to DTT, its electron donating ability is increased, which leads to enhanced intermolecular coulombic interaction from polymer PffBX-T to PffBX-TT and to PffBX-DTT. In addition, as the π-spacer becomes larger, the polymer solubility is decreased, 9



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which also results in gradually increased aggregation in solution from PffBX-T to PffBX-TT and to PffBX-DTT. However, in film state, all polymers are in condensed state and there are no solvation effects, which make the difference less significant in film than in solution. In fact, PffBX-DTT displayed the highest aggregation strength in solution among three polymers, as revealed by the temperature dependent UV-vis absorption spectra measured between 30-100 ºC (Figure 1c-1e). Large fraction of the PffBX-DTT polymer is still aggregated even at elevated temperature of 100 ºC in diluted solution (Figure 1e), while PffBX-TT polymer is fully disaggregated around 100 ºC (Figure 1d) and PffBX-T polymer is fully disaggregated at even lower temperature of 70 ºC (Figure 1c). The λmaxs of disaggregated polymers are all blue-shifted to around 550-570 nm compared to the aggregated polymers, implying that the polymer chains are distorted from the planar conformation in the disaggregated state at elevated temperature.57 Figure 1f plots the order-disorder transition as a function of temperature for all three polymers using the relative intensity of 0-0 peaks normalized to that at 30 ºC. It shows that the polymer aggregation strength can be systematically modulated by selecting appropriate building blocks in this case as the π-spacers, and similar effects have been observed by adjusting the branching position or the length of the alkyl chains.23,

58

Referring to the

blue-shift of PffBX-DTT film (versus solution), explanation was given in the previous study on the polymer PffBX4T-2DT with the similar behavior.28 The aggregation is too strong for PffBX-DTT. Therefore, when casting from warm solution in a disordered state, polymer chains quickly become kinetically quenched before they could reach the ordered and thermal 10


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equilibrium state that is only observable in diluted solution. The strong aggregation tendency of PffBX-DTT would lead to difficulty for device fabrication and optimization due to the narrowed processing window, with respect to the selection of casting solvent, casting temperature etc. Similar consideration applies when the polymer aggregation is too weak; the temperature window in which the disorder-order transition happens become too narrow, hence, the well-controlled crystallization process might also be an issue. Electrochemical properties of these ffBX-based polymers were investigated by cyclic voltammetry

(CV)

measurement

of

the

polymer

films

(Figure

S2)

using

ferrocene/ferrocenium (Fc/Fc+) as the internal standard, and the results are summarized in Table 1. On the basis of the CV data, the highest occupied molecular orbital levels (EHOMOs) and the lowest unoccupied molecular orbital levels (ELUMOs) were calculated to be −5.69/−4.00, −5.47/−3.83, and −5.43/−3.81 eV for PffBX-T, PffBX-TT, and PffBX-DTT, respectively, in which EHOMOs are electrochemically determined while ELUMOs are derived from EHOMOs and the optical band gaps (Egopts) of the polymer films using the equation: ELUMO = EHOMO + Egopt. The deep-lying HOMO levels are beneficial for obtaining large Vocs in PSCs.59 On the other hand, the polymer ELUMO/EHOMO levels are gradually increased in the order of T, TT, and DTT as the π-spacer is extended.60 However, from PffBX-TT to PffBX-T, the energy levels of FMOs are increased by bigger margins than those from PffBX-DTT to PffBX-TT. The results are associated with the weaker intermolecular interaction and lower degree of crystallinity of PffBX-T, which can limit conjugation length and result in larger bandgap and elevated HOMO and LUMO levels. Notably, the ELUMOs are relatively deep but 11



still could provide enough energy offset for exciton dissociation in PSCs when using appropriate fullerene derivatives as acceptor material.61-64 Theoretical Calculation. Chain conformations of these ffBX-based polymers were studied by performing density functional theory (DFT)-based calculation at B3LYL level using the 6-31G basis set. Trimers of the polymer repeating units were used and the alkyl chains were replaced with methyl groups to simplify the calculation. The optimized geometries are depicted in Figure 2, showing highly planar backbone for all three polymers. Distortion from planarity mainly exists between the π-spacer and the adjacent alkyl-substituted thiophenes, which shows an average S-C-C-S dihedral angle of 179.6°, 177.5° and 173.9° for the T, TT, and DTT π-spacer, respectively. EHOMO/ELUMO at optimized geometries are −5.06/−3.51, −5.06/−3.54 and −5.03/−3.54 eV for the PffBX-T, PffBX-TT, and PffBX-DTT trimers (Table S1), respectively, the trend of which in principle matches with the experimental data. Similar for all three polymers, the HOMO orbitals are delocalized across the entire polymer chain while the LUMO orbitals are mainly localized onto the strong electron-withdrawing ffBX units (Figure S4).

12


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Figure 2. DFT optimized conformation of the trimers of the repeating units of the polymers (a) PffBX-T; (b) PffBX-TT; (c) PffBX-DTT. The torsion angles are marked in the top views.

Organic Thin-Film Transistors and Polymer Solar Cells. Top-gate/bottom-contact (TGBC) OTFTs with a device structure of glass/Au/semiconductor/CYTOP/Al were fabricated to investigate the charge transport properties of these ffBX-based polymers, and the fabrication and characterization details are included in the Supporting Information. All three polymers show p-type dominating transport characteristics in OTFTs and their mobilities are highly dependent on the annealing temperature of the semiconductor layers (Table S2-S4). The optimal annealing temperature is found to be 250, 150, and 200 ºC for PffBX-T, PffBX-TT, and PffBX-DTT, respectively, with the representative I-V curves illustrated in Figure 3 and transistor performance parameters summarized in Table 2. Under the optimal annealing conditions, the

13



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Figure 3. Transfer and output characteristics of polymers (a, b) PffBX-T, (c, d) PffBX-TT, and (e, f) PffBX-DTT-based top-gate/bottom-contact OTFTs under the optimal annealing conditions. OTFT dimensions: L = 20 µm and W = 5 mm.

maximum µh is 0.67, 1.09, and 2.17 cm2 V−1 s−1 with the average µh of 0.44, 0.63, and 1.61 cm2 V−1 s−1 for PffBX-T, PffBX-TT, and PffBX-DTT, respectively, which is approaching the highest reported µh for PffBT and PffBX-based polymer semiconductors.44, 53, 65 All these ffBX-based polymers show large threshold voltages (VTs: -42 14


_

-47 V) in OTFTs, which are

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attributed to their low-lying HOMOs, leading to great hole injection barriers. Due to the residual weak n-type transport, relatively high off-currents (Ioffs) of 10−7-10−8 A are discovered for all three polymers in the saturation regime, and hence the current on/off ratios (Ion/Ioffs) are limited to the level of 104-105, which is commonly seen in ambipolar OTFTs.66 The optimal annealing temperature for PffBX-T is just above its thermal transition temperature measured by DSC, suggesting that the relatively weak polymer aggregation enables film microstructure improvement through the glass transition. This leads to highly −3 2 improved µh of 0.44 cm2 V−1 s−1 compared to that of the pristine PffBX-T film (3.2×10 cm

V−1 s−1). The charge carrier mobility is critical for charge carrier extraction and short-circuit currents (Jscs) in solar cells. Therefore, the results indicate that these ffBX-based polymer semiconductors are promising candidates to build high-performance thick-film PSCs, in view of their high charge carrier mobility, especially for PffBX-TT and PffBX-DTT polymers, which exhibit substantial mobilities (> 0.2 cm2 V−1 s−1) even in pristine film (Table S3 and S4).

Table 2. Device performance parameters of polymers PffBX-T, PffBX-TT, and PffBX-DTT-based top-gate/bottom-contact OTFTs under the optimal annealing conditions. Polymer

a)

Tanneal

µh a)

µhmax

VT,avg (V)

Ion/Ioff

0.67

−43 ± 7

4×103

0.63 ± 0.36

1.09

−42 ± 11

1×104

1.61 ± 0.67

2.17

−47 ± 10

8×104

2

−1 −1

2

(°C)

(cm V s )

(cm V−1 s−1)

PffBX-T

250

0.44 ± 0.14

PffBX-TT

150

PffBX-DTT

200

Average mobilities and standard deviations are based on at least 5 devices.

15



PSCs with the inverted structure of ITO/ZnO/polymer:PC71BM/MoO3/Ag were fabricated to evaluate the photovoltaic properties of these ffBX-based polymers under the standard AM 1.5G illumination (100 mW cm−2).67 After systematic optimization under various conditions (Table S5-S9), it was found that the best-performing PSC devices were made using active layers spin-casted from polymer:PC71BM blend with a weight ratio of 1:1.5 in chloroform for PffBX-T or in the CB/o-DCB mixed solvents (volume ratio 1:1) for PffBX-TT and PffBX-DTT. The addition of 1,8-diiodooctane (DIO)68 as the processing additive (1% volume ratio) can further improve the solar cell performance. The optimal active layer thickness is all above 200 nm for PSCs made from these polymers, and it is as high as ca. 250 nm for the PffBX-TT and PffBX-DTT-based PSCs. It should be noted that PSCs with thicker active layers are more feasible to be employed in the high-throughput manufacturing process for the large-scale production.29 The current density-voltage (J-V) curves of the best-performing PSCs are plotted in Figure 4a and their corresponding device performance parameters are summarized in Table 3. Among these polymers, PffBX-T as the donor offers the highest Voc of 0.99 eV in PSCs owing to its deepest HOMO level,69 along with a good FF of 69.2%. However, the Jsc is surprisingly low, only 2.0 mA cm−2, and therefore the average PCE can only reach 1.33%. Interestingly, PffBX-TT as the donor greatly enhances the average PCE to 8.91% with the maximum value of 9.10%, mainly attributed to the much improved Jsc of 15.3 mA cm−2, despite the slightly lower Voc of 0.85 eV and the reduced FF of 66.2%. This Jsc is among the highest values for ffBX-based PSC devices.28, 29, 52 An average PCE of 6.54% is found for the 16


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PffBX-DTT-based PSC device with a Jsc of 12.8 mA cm−2, a FF of 62.2%, and a Voc of 0.82 eV. As the π-spacer becomes larger, the Voc is reduced gradually, which is consistent with the trend in the EHOMOs of the three polymers. The Jsc values obtained in PSCs were verified by external quantum efficiency (EQE) measurement using the same device (Figure 4b). The integrated values from the EQE curves are in good agreement with the PSC device results with an error of 1-4%, indicating the good reliability of the solar cell performance. The EQE curves further reveal that the photon-to-current conversion is most efficient around 600-700 nm for all three polymers within the response range of 300-810 nm, which coincides with the absorption maxima of these donor polymers. The maximum EQE value in these response curves is 11%, 77%, 53% for PffBX-T, PffBX-TT, and PffBX-DTT-based PSC devices, respectively. The best-performing PffBX-TT:PC71BM blend film shows EQEs more than 60% over almost the entire absorption range, hence further improvement of EQE in the short wavelength region is expected to result in even higher PCE in PSCs.

Figure 4. (a) J-V curves, (b) EQE spectra of the best-performing inverted PSCs with the polymer:PC71BM active layers.

17



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Table 3. Device performance parameters of PSCs containing polymer:PC71BM blend as the active layer.

Polymer

a)

Thickness

Voc a)

Integrated

Jsc a) -2

Jsc b)

FF a)

PCE a)

PCEmax

(%)

(%)

(%)

(nm)

(V)

(mA cm )

PffBX-T

200

0.99 ± 0.01

2.0 ± 0.1

1.9

69.2 ± 6.7

1.33 ± 0.12

1.48

PffBX-TT

250

0.85 ± 0.01

15.3 ± 0.8

15.8

66.2 ± 2.0

8.91 ± 0.21

9.10

PffBX-DTT

260

0.82 ± 0.01

12.8 ± 0.2

12.5

62.2 ± 1.3

6.54 ± 0.10

6.66

(mA cm-2)

The average values with standard deviations are based on 10 devices; b) Integrated from EQE curves.

The generality of this correlation between donor polymer and thick-film PSC performance was further verified using non-fullerene type acceptor IT-4F.11 Device fabrication details were provided in the Supporting Information, along with the device performance parameters and J-V curves of the best-performing cells shown in Table S10 and Figure S5a, respectively. Note that a similar trend in PSC performance was observed, with PffBX-TT as the donor polymer displaying the highest PCE of 8.02% within this series of polymers. The average PCE of PffBX-TT:IT-4F blend is 7.47%, and it decreased to 6.19% and 1.44% for PffBX-DTT:IT-4F and PffBX-T:IT-4F blend respectively. The excellent agreement between the results obtained with PC71BM and IT-4F acceptors demonstrated the importance of donor polymer selection. Given the abundance of the non-fullerene type acceptor.19 there certainly is possibility for further enhancement in PSC performance but it is beyond the scope of current study. In view of the slightly better performance of polymer:PC71BM blends, they were chosen as representative examples for following characterizations. 18


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Space charge limited current (SCLC) measurement was subsequently conducted to understand the charge transport properties of the polymer blend films. The active layers were prepared under the same conditions for the best-performing PSC devices. Hole-/electron-only devices were used to characterize the hole/electron mobility (µh,SCLC/µe,SCLC), respectively. The mobility values are summarized in Table S11 and the J-V plots are given in the Figure S6. Differing from the trend in OTFT devices, the PffBX-TT:PC71BM blend exhibits the highest hole mobility (3.27×10−3 cm2 V-1 s-1) in the out-of-plane direction while that (1.32×10−4 cm2 V-1 s-1) of PffBX-T:PC71BM is still the lowest. Furthermore, the large electron mobility (1.82×10−3 cm2 V-1 s-1) of PffBX-TT:PC71BM offered the most balanced hole and electron mobility within the series (µh,SCLC/µe,SCLC = 1.80), partially explaining its highest Jsc in PSCs. The high µe,SCLC (1.02×10−3 cm2 V-1 s-1) of PffBX-T:PC71BM is likely attributed to the presence of large PC71BM domains in the blend film due to unfavorable phase separation (vide infra), resulting in a µh,SCLC/µe,SCLC ratio of only 0.13. The highly unbalanced transporting property of PffBX-T:PC71BM film likely contributes to its lowest Jsc value in the PSCs.21 For PffBX-DTT:PC71BM blend, we obtained an intermediate mismatch between hole and electron mobility (µh,SCLC/µe,SCLC = 4.68), agreeing well its Jsc values among the three blends. Film Morphology and Its Correlations to Device Performance. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to study the film morphology of the polymer:PC71BM blends. Figure 5 illustrates the AFM and TEM images of the blend films prepared under the same conditions for the best-performing PSC devices. In 19



addition, the blend films without DIO additive were also studied in parallel to gain insights into the film morphology evolution, and the images are shown in Figure S7 and Figure S8 in the Supporting Information and the corresponding data are summarized in Table S12.

Figure 5. (a-c) AFM height images and (d-f) TEM images of the polymer:PC71BM blend films prepared under the optimal conditions for the PSC devices.

For the PffBX-T:PC71BM blend films without DIO, large granular structures with sizes at the micrometer level are present in the film as can be seen in both the AFM and TEM images. The low miscibility issue between PffBX-T and PC71BM still exists after the addition of the high boiling point processing additive DIO in the solvent. Even though the root-mean-square (RMS) roughness of the AFM images is significantly reduced from 11.31 to 0.80 nm after the DIO addition (Table S12), granular structures with size of more than 100 nm are still pronounced in the TEM image (Figure 5d), which is much larger than the desirable 20


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domain size of ~20 nm for bulk heterojunction PSCs.70, 71 Therefore, the majority of the excitons would recombine before they can diffuse to the nearest donor:acceptor interfaces to form free charge carriers in the PffBX-T:PC71BM blend films, which leads to the low Jsc and EQE in the PSC devices.72 For PffBX-TT:PC71BM and PffBX-DTT:PC71BM blends, the donor and acceptor materials are mixed relatively well, especially for PffBX-TT. The phase separation in the PffBX-TT:PC71BM blend is considered to be good even without using DIO additive, leading to an encouraging PCE of 5.38% in the PSC device (Table S5). Upon the DIO addition, similar morphology is seen in the AFM image but with a slightly increased surface roughness, and the TEM image (Figure 5e) reveals a refined phase separation with more homogeneous microstructure, in contrast to the fibril structure observed in the as-cast blend film. The fibril structure is more pronounced in the PffBX-DTT:PC71BM blend film especially before the DIO addition (Figure S8c), likely suffering from the strong aggregation strength and highly crystalline nature of the polymer PffBX-DTT. The highest crystallinity of PffBX-DTT within these three polymers is unambiguously shown by the strongest π-π stacking peaks in the x-ray diffraction measurement of the neat films (Figure S9a). The π-π stacking distance of PffBX-DTT is extracted to be 3.51 Å, which is consistent with the previously reported PffBX polymers.28, 29 After thermal annealing, the pristine PffBX-DTT film exhibits a stronger (100) lamellar stacking, and (200) and even (300) peaks become observable (Figure S9b), at the meantime, PffBX-DTT shows a suppressed (010) π-π stacking peak. The results indicate an increased film crystallinity and a higher degree of edge-on orientation after thermal treatment. As a result, the highest hole mobility of 21



2.17 cm2 V-1 s-1 is observed for the neat PffBX-DTT film in OTFTs. Upon blending with PC71BM, the edge-on packing structure was greatly suppressed for all three polymers (Figure S9c), indicative of increased polymer face-on orientation in the blend, which is beneficial for charge extraction in vertical direction and hence solar cell performance. The blend film morphology strongly correlates with the PSC performance in particular the Jsc values in all cases, again emphasizing the importance of controlled phase separation and the formation of interpenetrating networks within the bulk heterojunction blend film. Aggregation strength of the donor polymer plays a key role in the film formation process, and the optimal aggregation strength of the PffBX-TT polymer leads to the best blend film morphology and the highest Jsc value in PSC devices, which also shows sufficiently high FF and large Voc to afford the highest PCE value (9.10%) within this series of ffBX-based donor materials. CONCLUSIONS In summary, we have synthesized three difluorobenzoxadiazole-based donor polymers including PffBX-T, PffBX-TT, and PffBX-DTT, to systematically investigate the effects of π-spacer units on the polymer aggregation strength and the resulting thick-film PSC device performance. The polymer aggregation strength increases in the order of PffBX-T, PffBX-TT, and PffBX-DTT as the size of π-spacer becomes larger. The intermediate aggregation strength of PffBX-TT enables well controlled disorder-order transition during the blend film casting process, thus leading to the optimal film morphology and the highest performance in PSCs. For PffBX-TT, the highest PCE of 9.10% with an average value of 8.91% was attained from thick-film PSCs using the polymer:PC71BM active layer with a thickness of 250 nm. 22


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OTFT characterization also reveals its intrinsic high µh of 1.09 cm2 V−1 s−1. T/DTT as the π-spacer greatly change the aggregation strength of the resulting polymers and limits the ability to control the disorder-order transition, resulting in sub-optimal blend film morphology. Such film morphology leads to suppressed Jsc values and reduced average PCEs of 1.33%/6.54% when PffBX-T/PffBX-DTT are adopted as the donor polymers in PSCs using PC71BM acceptor. A similar trend was also observed in non-fullerene type thick-film PSCs using IT-4F as the electron acceptor, with PffBX-TT as the donor polymer enabling the highest PCE of 8.02% with an average value of 7.47% within this series. This study shows that PffBX-based polymers are promising candidates to achieve high-performance, thick-film polymer solar cells. Additionally, the results indicate that polymer aggregation strength tuning is of great importance for achieving optimal bulk heterojunction film morphology and enhancing solar cell performance. EXPERIMENTAL SECTION Synthetic Procedures General Procedure for the Synthesis of Polymers: The dibromo monomer ffBX-2TDT-Br and distannylated comonomer with a molar ratio of 1:1 were combined in a oven-dried microwave tube, and then to the tube was charged with the palladium catalyst tris(dibenzylideneacetone)dipalladium (0) (Pd2(dba)3) and the ligand tris(o-tolyl)phosphine (P(o-tolyl)3) (1:8 molar ratio). The microwave tube was vacuumed and refilled with argon three times. After adding 3-4 mL anhydrous toluene under argon flow, the tube was sealed, loaded into a microwave reactor, and stirred at 80 ºC for 10 min, 100 ºC for 10 min, 145 ºC 23



for 1 h, and 140 ºC for 3 h. After the completion of polymerization, 100 uL 2-bromothiophene was injected as the end-cap and the reaction mixture was stirred at 140 ºC for 0.5 h. After cooling to room temprature, the polymer solution was dripped into HCl (12 N, 1 mL) containing methanol (100 mL), and the product polymer precipitated out. The precipitates were poured into a Soxhlet thimble. After drying in vacuum, the crude polymer was purified using sequential Soxhlet extraction. PffBX-T: The solvents used for sequential Soxhlet extraction were methanol, acetone, and hexane. After final extraction with hexane, the hexane fraction was concentrated to ~10 mL and dripped into100 mL methanol. The precipitates were filtrated and dried in vacuum to afford PffBX-T as a black solid (96 mg, 79%). 1H NMR (400 MHz, Toluene-d8, 80 ºC, ppm) δ 8.39-8.30 (m, 2H), 7.30-7.24 (m, 2H), 3.07-2.99 (m, 4H), 1.67-1.26 (m, 80H), 0.98-0.88 (m, 12H). Elem. Anal.: Calcd. for C66H102F2N2OS3 (%): C, 73.83; H, 9.58; N, 2.61; S, 8.96. Found (%): C, 73.67; H, 9.49; N, 2.54; S, 8.77. Molecular weight: Mn = 33.2 kDa, Mw = 55.1 kDa, PDI = 1.7. PffBX-TT: The solvents used for Soxhlet extraction were methanol, acetone, hexane, dichloromethane, chloroform, and chlorobezene. After final extraction with chlorobenzene, the chlorobenzene fraction was concentrated to ~10 mL and dripped into 100 mL methanol. The precipitates were filtrated and dried in vacuum to afford PffBX-TT as a black solid (68.0 mg, 85%). 1H NMR (400 MHz, 1,2-C6D4Cl2, 80 ºC, ppm) δ 8.53 (s, 2H), 7.74-7.63 (m, 2H), 3.30-3.15 (m, 4H), 2.24-2.19 (m, 2H), 2.12-1.48 (m, 80H).1.21-1.01 (m, 12H). Elem. Anal.: Calcd. For C68H102F2N2OS4 (%): C, 72.29; H, 9.10; N, 2.48; S, 11.35. Found (%): C, 72.71; H, 24


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9.07; N, 2.59; S, 11.49. Molecular weight: Mn = 34.5 kDa, Mw = 66.7 kDa, PDI = 1.9. PffBX-DTT: The solvents used for Soxhlet extraction were methanol, acetone, chloroform, and chlorobenzene. After final extraction with chlorobenzene, the chlorobenzene fraction was concentrated to ~10 mL and dripped into 100 mL methanol. The precipitates were filtrated and dried in vacuum to afford PffBX-DTT as a black solid (46.0 mg, 54%). 1H NMR (400 MHz, 1,2-C6D4Cl2, 80 ºC, ppm) δ 8.31-8.22 (m, 2H), 7.47-7.35 (m, 2H), 3.02-2.89 (m, 4H), 2.01-1.02 (m, 80H), 1.00-0.71 (m, 12H). Elem. Anal.: Calcd. for C70H102F2N2OS5 (%): C, 70.90; H, 8.67; N, 2.36; S, 13.52. Found (%): C, 70.731; H, 8.57; N, 2.37; S, 13.59. Molecular weight: Mn = 32.8 kDa, Mw = 66.1 kDa, PDI = 2.0. OTFT Fabrication and Characterization Top-gate/bottom-contact (TGBC) structure was used to characterize the OTFT performance of the three PffBX-based polymers. Borosilicate glass substrates were first cleaned using SC-1 solution at 55-65 °C (H2O:NH4OH:H2O2 = 5:1:1), then onto the substrates were patterned the source and drain electrodes (30 nm Au with 3 nm Cr adhesion layer) using standard photolithography technique. The OTFT devices feature a channel length (L) of 20, 50, or 100 µm and a channel width (W) of 5 mm, respectively. The patterned substrates were again cleaned by sonication in acetone and isopropanol for 10 min each followed by 0.5 h UV-ozone treatment. The polymer semiconductors were spin-coated from the chlorobenzene solution (PffBX-T, 5 mg mL−1) or 1,2-dichlorobenzene (o-DCB) solution (PffBX-TT and PffBX-DTT, 5 mg mL−1) and annealed at various temperatures for 20 min. The CYTOP dielectric

was

deposited

using

spin-coating

from

25


its

diluted

solution


(CYTOP-M:CT-SOLV180 = 2:1, volume ratio, Asahi Glass Co., Ltd.), then baked at 100 °C for 0.5 h. The CYTOP areal capacitance (Ci) is 4.5 nF cm−2. Finally, 50 nm Al gate electrode was deposited via thermal evaporation through shadow mask to complete the devices. The OTFTs were measured using the Keithley 4200 semiconductor characterization system. All device fabrication and characterization was performed in a nitrogen-filled glovebox. Inverted Structure PSC Fabrication and Characterization ITO-coated glass having a ~12 Ω sq−1 sheet resistance was used as the substrate, which was cleaned by sequential ultra-sonication in soap water, deionized H2O, acetone, and isopropanol for 15 min each, followed by 15 min UV-ozone treatment. The ZnO interfacial layer was spin-coated (3000 rpm, 30 s) onto the ITO-coated glass substrates and then annealed at 200 °C in air for 0.5 h. For PffBX-TT and PffBX-DTT-based solar cells, the polymer:fullerene solutions were prepared with a concentration of 8 mg mL−1 (for donor polymer) in a chlorobenzene:1,2-dichlorobenene mixture solvent (CB:o-DCB = 1:1, volume ratio) with 1% 1,8-diiodooctane (DIO) as the processing additive. The solutions were heated to

130 °C and stirred at this temperature for 12 h to reach complete dissolution. Inside of a

N2-filled glovebox, the ITO substrates were pre-heated to 130 °C on a hot plate before spin-coating the blend film at ~1200 rpm. For PffBX-T-based solar cells, the polymer:fullerene solution was prepared with a concentration of 6 mg mL−1 (for donor polymer) in chloroform (CF) with 1% DIO additive. After spin-coating, the blend films were all vacuumed for 8 h to remove the high boiling point DIO residue. Finally, 10 nm MoO3 hole injection layer and 100 nm Ag top electrode were deposited on the active layer subsequently 26


Page 26 of 39

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in high vacuum (3E−6 Torr). The solar cells with a 0.045 cm2 active area were characterized in a N2–filled glovebox using a computerized Keithley 2400 source meter and a Xeno-lamp-based solar simulator (Newport, Oriel AM1.5G, 100 mW cm−2). The light intensity was calibrated with a National Renewable Energy Laboratory (NREL)-certified monocrystalline Si diode coupled to a KG3 filter to bring the spectral mismatch to unity. External quantum efficiency (EQE) spectrum was recorded on a QE-R3011 measurement system (Enli Technology, Inc.). ASSOCIATED CONTENT Supporting Information Materials and instruments, synthesis details of the polymers, DSC and TGA plots, DFT calculation, OTFTs data, PSCs data, SCLC data, AFM and TEM images, XRD measurement, NMR spectra. AUTHOR INFORMATION Corresponding Authors * Email: [email protected] (H.G.). * Email: [email protected] (X.G.). ORCID Shengbin Shi: 0000-0003-1350-5860 Xugang Guo: 0000-0001-6193-637X Author Contributions ∇ P.C.

and S.S. contributed equally to this work. 27



Page 28 of 39

Notes The authors declare no competing financial interest. ACKNOWLEGMENTS H.G. is grateful to the Basic Research Fund of Shenzhen City (JCYJ20160530190226226). X.G. is thankful for the financial support by Shenzhen Peacock Plan project (KQTD20140630110339343),

the

Basic

Research

Fund

of

Shenzhen

City

(JCYJ20160530185244662), the Guangdong Natural Science Foundation (2015A030313900), and South University of Science and Technology of China (FRG-SUSTC1501A-72). REFERENCES 1. 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. 2. Brabec, C. J.; Gowrisanker, S.; Halls, J. J.; Laird, D.; Jia, S.; Williams, S. P. Polymer-Fullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 2010, 22, 3839-3856. 3. Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115, 12666-12731. 4. Søndergaard, R.; Hösel, M.; Angmo, D.; Larsen-Olsen, T. T.; Krebs, F. C. Roll-to-Roll Fabrication of Polymer Solar Cells. Mater. Today 2012, 15, 36-49. 5. Lu, L.; Kelly, M. A.; You, W.; Yu, L. Status and Prospects for Ternary Organic Photovoltaics. Nat. Photonics 2015, 9, 491-500. 6. Vohra, V.; Kawashima, K.; Kakara, T.; Koganezawa, T.; Osaka, I.; Takimiya, K.; Murata, 28


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