Wide Bandgap and Highly Conjugated Copolymers Incorporating 2

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Wide Bandgap and Highly Conjugated Copolymers Incorporating 2-(Triisopropylsilylethynyl)thiophene Substituted Benzodithiophene for Efficient Nonfullerene Organic Solar Cells Lixin Wang, Haifen Liu, Zhaoxiang Huai, and Shaopeng Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09253 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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ACS Applied Materials & Interfaces

Wide Bandgap and Highly Conjugated Copolymers Incorporating 2-(Triisopropylsilylethynyl)thiophene Substituted Benzodithiophene for Efficient Nonfullerene Organic Solar Cells Lixin Wang,* Haifen Liu, Zhaoxiang Huai, and Shaopeng Yang* Hebei Key Laboratory of Optic-Electronic Information Materials, College of Physics Science and Technology, Hebei University, Baoding 071002, P. R. China KEYWORDS: wide bandgap polymers, trialkylsilylethynyl, benzodithiophene, polymer solar cells, high open-circuit voltage

ABSTRACT : Recent years have seen a rapid progress in the power conversion efficiencies (PCEs) of nonfullerene polymer solar cells (NF PSCs). However, the donor materials accordingly used are typical low- or medium bandgap polymers, some of which possess badly overlapped absorption spectra relative to the low bandgap n-type acceptors, for example, ITIC. To obtain polymers simultaneously owning a wide bandgap, a highly extended π-conjugation system, and a low-lying highest occupied molecular orbital (HOMO), a polymer (PBDTSi-TA) incorporating 2-(triisopropylsilylethynyl)thiophene substituted benzodithiophene (BDTSi) and

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fluorinated benzotriazole (FTAZ) units was designed and synthesized. PBDTSi-TA (Egopt = 1.92 eV) exhibits strong molecular aggregation properties and a lower-lying HOMO energy level compared to its structural analogues. When blended with ITIC and after device optimization with solvent vapor annealing in combination with a developed PDIN/BCP/Ag cathode structure, PSCs yielded a PCE of 7.51%, with Voc = 0.96 V. Moreover, a rather small energy loss (Eloss) of 0.60.63 eV would be determined. As comparison, another polymer (PBDTSi-Qx) with a more electron-deficient quinoxaline-based acceptor unit was also synthesized and applied to NF PSCs. Charge generation rate, exciton dissociation probabilities, dark leakage current, nanoscale morphology and charge carrier mobilities have been evaluated to probe the reasons for the differentiated performances. The results suggest that PBDTSi-TA is a promising donor material for NF PSCs, and the molecular design strategy demonstrated here would be helpful for pursuing high-performance polymers for PSCs.

INTRODUCTION The past decade has witnessed a noticeable rise in the power conversion efficiencies (PCEs) of polymer solar cells (PSCs), and increasing attention to PSCs has been triggered due to their salient advantages of low cost, light weight, flexibility and large-area solution processability. Until now, PCEs exceeding 12% have been achieved for PSCs1-3, which promise a bright prospect for their

commercialization. Universally, fullerene derivatives are applied as an

electron acceptor in the well-known bulk heterojunction (BHJ) architecture4. However, fullerene acceptors would suffer drawbacks including weak absorption in the visible region, poor tunability of energy levels, tedious purification and bad morphological stability, which hinder PSCs in their performance and industrial potential5-7. Encouragingly, non-fullerene (NF)


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acceptors with easily tunable intrinsic properties have been successfully explored in recent years, and the performances of NF-based PSCs are rivalling their fullerene counterparts8-9. Focusing on NF PSCs, intensive efforts have been directed to the synthesis of novel NF acceptors, which are often combined with available fullerene-compatible high-performance polymers for PSCs use1013

. However, the existing polymers intended for fullerene-polymer system may not match well

with NF molecules14-17. So exploring new efficient polymer donors and demonstrating a meaningful molecular design strategy are of vital importance18-21. Broad and strong light absorption ability is a prerequisite for PSCs to afford satisfactory shortcircuit current density (Jsc). So it is preferable for the active layer absorption to match the solar spectrum. Individually, both the polymer donor and the nonfullerene acceptor possess relatively narrow absorption spectra, thus it is necessary for the two components to have complementary absorption properties, so as to efficiently absorb solar photons and generate more excitons22. In other words, for low bandgap acceptors, wide bandgap donor materials are needed to realize high-performance NF PSCs23. Since Zhan et al. reported the highly efficient nonfullerene acceptor of ITIC (Egopt = 1.59 eV)24, a lot of known polymers have been blended with ITIC for PSCs investigation, and impressive PCEs have been achieved25-27. However, the polymer donors accordingly used are typical those whose optical bandgaps are smaller than 1.8 eV, in other words whose absorption spectra greatly overlap with that of ITIC24-26, 28. Therefore, developing efficient wide bandgap polymer donors to complement the absorption spectra of the highperformance low bandgap acceptors (e.g., ITIC) is necessary and urgent19, 29-30. On the other hand, it is well acknowledged that NF-based PSCs are superior to their fullerene counterparts in attaining high open-circuit voltages (Voc) by enlarging the energy level offsets between the HOMO of the donor and the LUMO of the acceptor, because a 0.3-0.5 eV driving force for


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exciton dissociation is not a prerequisite in NF-based PSCs31-32. So enlarging Vocs of ITIC-based NF PSCs to the most possible extent is feasible. Benzo[1,2-b:4,5-b’]dithiophene (BDT) is an important building block to construct highly efficient photovoltaic polymers. In 2008, Hou et al. precedently applied the BDT unit into photovoltaic polymers33, and in the next year, Yu et al. combined it with thieno[3,4-b]thiophene (TT) to demonstrate more intriguing properties of materials and device34. Thereafter, hundreds of BDT-containing copolymers have been developed for PSCs. In particular, Hou et al. synthesized 2D-conjugated BDT-containing copolymers by substituting alkoxy units with alkylthienyl units (BDTT)35, and finally they found that the hole mobility and the overall PCEs could be improved greatly. As for constructing wide bandgap copolymers based on BDT unit, You et al. alternated alkyl-substituted BDT unit with fluorinated 2-alkyl-benzo[d][1,2,3]triazoles (FTAZ) via a thiophene spacer36. The lone pair on the nitrogen atom is more basic and is more easily delocalized into the triazole ring, which would cause the involving polymers to be more electron rich and feature a higher LUMO energy level36. Moreover, functional alkyl chains could be incorporated onto the N atom of TAZ, and smaller F atoms may promote molecular interactions through F atom-indued non-covalent interactions, which is beneficial for improving charge carrier mobility36-37. Recently, Li et al. reported a number of 2D-conjugated BDT and FTAZ based wide bandgap copolymers and investigated their photovoltaic properties in NF PSCs32, 38. For example, they directly anchored trialkylsilyl to the lateral thiophene ring of BDTT unit, and as such they fabricated a polymer named J7138. Their results indicate that substituting Si atom for C atom, between the lateral alkyl and thienyl, could strengthen interchain interactions and deepening the HOMO level of the materials.


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Herein, we report a new wide bandgap copolymer incorporating the BDT donor and the fluorinated TAZ acceptor units (denoted as PBDTSi-TA, Chart 1). Structurally, we connected triisopropylsilylethynyl (TIPS) to the lateral thiophene ring of 2D-conjugated BDTT. TIPS had previously been directly linked to the BDT unit to construct copolymers for fullerene based PSCs39-40, but the Vocs are relatively low. However, relative to alkoxy substituents, TIPS is more effective in decreasing the frontier molecular orbital energy levels of the polymers41. As stated earlier and as revealed by PTB7-Th compared to PTB7, changing the alkoxy groups into aromatic alkylthienyl groups can also decrease the HOMO level as well as increase hole mobility and improve overall performance. Thus, inserting a thienyl group between the TIPS and BDT units, or namely extending π-π conjugation of thienyl-substituted BDT via TIPS (Chart 1), synergetic effects from TIPS and thienyl substitution, such as lowering HOMO level for attainable high Vocs and extending conjugation for strong intermolecular interactions, may be expected. As comparison, a quinoxaline-based acceptor unit with more strongly electrondeficient character is also copolymerized with the newly developed donor unit (PBDTSi-Qx, Chart 1). When blended with ITIC for PSCs, results of PCE = 6.49%, Voc = 0.97 V are achieved based on PBDTSi-TA after solvent vapor annealing (chlorobenzene). Moreover, an even higher Chart 1. Chemical Structures of PBDTSi-TA, PBDTSi-Qx and ITIC


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Voc of 0.99 V with PCE = 4.86% could be obtained without any additive or post-treatment. To our best knowledge, such Vocs represent ones of the highest values ever reported for ITICcontaining NF PSCs. Furthermore, a PCE of 7.51% could be achieved by courtesy of the improvements of Jsc and FF, when incorporating a thin layer of bathocuproine (BCP, 5nm) between the N,N’-bis(propylenedimethylamine)-3,4,9,10-perylenediimide (PDIN) cathode buffer and the metal electrode to prevent holes from interfacial carrier recombination, and to decrease the contact resistance at the cathode.

RESULTS AND DISCUSSION Material Synthesis and Characterization The syntheses of the target polymers are depicted in Scheme 1. Detailed synthetic procedures and characterization are followed in the Experimental Part. Starting from commercially available 2-bromothiophene and triisopropylsilylacetylene, compound 1 was prepared via a Sonogashira coupling reaction. Compound 2 was synthesized consulting similar literature procedures35. Compound 2 was lithiated with LDA at low temperature and then quenched with trimethyltin chloride to yield the donor unit BDTSi-Sn. The target polymers were obtained via a Stille polycondensation

reaction

between

corresponding

building

blocks

using

tris(dibenzylideneacetone)dipalladium (Pd2(dba)3) and tri(o-tolyl)phosphine (P(o-tol)3) as the catalysts in xylene at 120 oC. The polymers were purified through a Soxhlet extractor by successive eluting with methanol, acetone, hexane, and chloroform. All polymers can be well soluble in chlorobenzene (CB), chloroform, and dichlorobenzene (DCB), at warm temperature. Molecular weights and polydispersity indices (PDIs) were estimated by gel permeation chromatography (GPC) method using an eluent of THF and a polystyrene internal standard.


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PBDTSi-TA exhibits a number-average molecular weight (Mn) of 48.8 kg/mol with a PDI of 1.20, and PBDTSi-Qx shows a Mn of 49.2 kg/mol with the PDI of 1.35 (Table 1). It is recognized that molecular weights of the polymers may affect the device performance via modulating the aggregation behavior42. Herein, two polymers possess similar molecular weights, which will exclude the influence of Mns when comparing the structure-properties relationship. Scheme 1. Synthetic Routes of PBDTSi-TA and PBDTSi-Qxa

a

Reagents and conditions: i) Pd(PPh3)2Cl2, CuI, THF, NEt3, reflux. ii) (a) n-BuLi, THF, 0 oC, then benzo[1,2-b:4,5-b']dithiophene-4,8-dione, 50 oC; (b) SnCl/HCl/H2O, 50 oC. iii) LDA, inert atmosphere, THF, -78 oC, then (CH3)3SnCl. iv) Pd2(dba)3, P(o-tolyl)3, xylene, reflux. Table 1. Molecular Weights, Optical and Electrochemical Properties of the Polymers Mn [kg/mol]a

PDI

maxsoln

maxfilm [nm]

Egopt [eV]b

HOMO [eV]c

LUMO [eV]c

PBDTSi-TA

48.8

1.20

540

545

1.92

-5.54

-2.97

PBDTSi-Qx

49.2

1.35

593

595

1.73

-5.42

-2.95

Polymer

[nm]

a

Determined by GPC using the THF eluent and the polystyrene standard. bCalculated from the absorption band onset of the polymer film, Egopt = 1240/λ onsetfilm. cEnergy levels evaluated by cyclic voltammetry measurements, EHOMO/LUMO = - (φox/φred + 4.70) (eV).


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Optical and Electrochemical Properties The normalized absorption spectra of PBDTSi-TA and PBDTSi-Qx as well as ITIC in solution state and as a thin film are shown in Figure 1, and the corresponding data are listed in Table 1. Both film and solution spectra of PBDTSi-TA exhibit well-defined absorption peaks with a clear vibronic shoulder in the longer wavelength region, and the solution spectra well resemble those of the film. All these profiles indicate that strong molecular self-organization behavior exists even in solution. To conform this, we examined temperature-dependent absorption properties of PBDTSi-TA in dilute CB solution. As revealed by Figure S1a, heating the solution (10-5 M) from 20 to 80 oC, the absorbances of the shoulder (580 nm) decreased. Furthermore, when the concentration of the solution was diluted one order of magnitude, the shoulder peak decreased again, although the main absorption peaks at the shorter wavelength range almost unchange. Besides, a thermochromic effect from red to slight red was observed with the heating process. As for PBDTSi-Qx, the shoulder peak is not clear. However, its solution and film absorption spectra are rather similar, and the maximum absorbances are broad, so we also supposed its strong interchain aggregation in solution. Indeed, when the dilute solution (10-6 M) of PBDTSi-Qx was heated from 20 to 80 oC, the absorbance around 630 nm decreased greatly and thermochromism from blue to red was also observed (Figure S1b). The strong aggregation behaviors of PBDTSiTA and PBDTSi-Qx may be related to the extended π-π conjugation area realized via connecting a TIPS group to the lateral thienyl group of BDTT. For PBDTSi-TA, the roles of fluorine atoms as described earlier should also be included. In comparison with J71 (Egopt = 1.96 eV), a polymer based on trialkylsilyl-substituted BDTT and fluorinated TAZ units38, PBDTSi-TA exhibits similar absorption profiles and a slightly lower bandgap due to the incorporation of conjugated ethynyl motifs. Obviously, compared to PBDTSi-Qx, the absorption of PBDTSi-TA


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efficiently complement that of ITIC (Figure 1b), which is favorable for light harvesting. Moreover, the maximum extinction coefficient of PBDTSi-TA solution (540 nm, 0.9 × 10-5 M-1 cm-1) is slightly higher than that of PBDTSi-Qx (593 nm, 0.8 × 10-5 M-1 cm-1). Optical bandgaps were determined from absorption onsets of the films, that is, 1.92 eV for PBDTSi-TA and 1.73

1.0

(a)

Solution

0.8 0.6 0.4

PBDTSi-TA PBDTSi-Qx ITIC

0.2 0.0

400

500

600

700

800

Normalized absorbance (a. u.)

eV for PBDTSi-Qx. Normalized absorbance (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0

(b)

Film

0.8 0.6 0.4

PBDTSi-TA PBDTSi-Qx ITIC

0.2 0.0

400

Wavelength (nm)

500

600

700

800

Wavelength (nm)

Figure 1. (a) UV-vis absorption spectra of PBDTSi-TA, PBDTSi-Qx and ITIC in CHCl3 solution (10-5 M). (b) Absorption spectra of PBDTSi-TA, PBDTSi-Qx and ITIC in thin film. Cyclic voltammetry (CV) was employed to evaluate molecular energy levels of the polymers. As depicted in Figure 2a, the onset oxidation/reduction potentials (Eox/Ered) of PBDTSi-TA are 0.84/-1.73 V (vs. Ag/Ag+). FeCp20/+, whose absolute energy level is 4.8 eV below vacuum, was used as the internal standard and its redox potential (φFe/Fe+, vs. Ag/Ag+) retained 0.1 V. Thus, the HOMO and LUMO levels of PBDTSi-TA were determined to be -5.54 and -2.97 eV, respectively, following the equations of EHOMO/LUMO = - (φox/φred + 4.70) (eV). Similarly, the EHOMO and ELUMO of PBDTSi-Qx were calculated to be -5.42 and -2.95 eV, respectively. Furthermore, the energy levels of the acceptor, ITIC, were also measured under the same conditions. Thus energy levels of the three samples could be calibrated and adjusted by the internal standard Fe/Fe+ to eliminate all the measurement shift. As for comparison, the energy


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level alignments of the components including the acceptor ITIC are shown in Figure 2b. It is worth noting that the HOMO energy levels of PBDTSi-TA and PBDTSi-Qx are both deeper than some reported polymers based on BDT and with the similar acceptor units28, 32, 38, which may benefits from the structural modification of BDTT via TIPS. Therefore, attainable high Vocs would be anticipated. Specially, the energy offset between the HOMO of PBDTSi-TA and the LUMO of ITIC is close to the electrochemical bandgap of ITIC. As for Jsc, the excitons dissociation process should be considered. It is well acknowledged that the molecular energy offsets of the active layer components should be larger than 0.3 eV so as to promote exciton dissociation19. On the other hand, some works indeed indicate that decreased or reversed driving force, denoted by a rather small or an opposite energy offset, could be unfavorable for exciton dissociation and increase in Jsc19, 43. So in view of the relatively small △EHOMO offsets between the two polymers and ITIC, hole transfer ability from ITIC to the donors should be especially evaluated. As shown in Figure S2, the photoluminescence (PL) emission peaks of PBDTSi-TA and PBDTSi-Qx were efficiently quenched by the introduction of ITIC to the blend film, suggesting effective electron transfer from the polymers to ITIC. However, the PL emission of ITIC (700-900 nm)24 could not be efficiently quenched by PBDTSi-TA or PBDTSi-Qx, implying that indeed the hole transfer from the ITIC phase to the polymer phase has been affected negatively.


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(a)

e-

(b)

0/+

FeCp2

- 2.95 - 2.97

-3.0 -3.5

PBDTSi-Qx

ITIC

-4.5

- 4.7

-5.0

ITO

-5.5 -6.0

- 5.2

PEDOT: PSS - 5.42- 5.54

-6.5 -7.0 -2.0 -1.6 -1.2 -0.8 -0.4

0.0

0.4

0.8

1.2

1.6

h

+

- 3.94

- 3.72

- 3.5 - 4.3 (Al)

ITIC

Energy (eV)

-4.0

PBDTSi-TA

Current (a. u.)

PBDTSi-TA

PBDTSi-Qx

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


- 4.6 (Ag)

PDIN BCP

- 5.64 - 6.05

- 7.0

+

Potential (V vs Ag/Ag )

Figure 2. (a) Cyclic voltammograms of PBDTSi-TA, PBDTSi-Qx and ITIC thin films measured in 0.1 M Bu4NPF6 acetonitrile solution at a scan rate of 50 mV/s. (b) Energy level alignments of the components used in NF PSCs. Photovoltaic Properties To investigate the photovoltaic properties of the newly developed polymer donors, NF PSCs with a conventional structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)/polymer:ITIC/N,N’-bis(propylenedimethylamine)-3,4,9,10 -perylenediimide (PDIN)/Al were fabricated, wherein alcohol soluble PDIN was used as the cathode buffer layer due to its work-function tunablity upon Al44. Firstly, different solvents were examined (Table S1). Then, donor:acceptor (D/A) weight ratios, solvent vapor annealing (SVA) and solvent additive of DIO were screened (Table 2 and Table S2-S3). PBDTSi-TA and PBDTSi-Qx based devices exhibit different responses toward some optimization conditions. The as-cast devices based on PBDTSi-TA afforded an optimal PCE of 4.86%, with Voc = 0.99 V, Jsc = 11.18 mA cm-2, and FF = 0.44. When subjected to CB SVA treatment for 2 min, Jsc and FF are both improved so as to yield the results of PCE = 6.49% and Voc = 0.97 V (Table 2). However, PBDTSi-Qx based devices did not positively response to CB SVA (Table S2). On the other


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Table 2. Photovoltaic Results of PBDTSi-TA:ITIC and PBDTSi-Qx:ITIC Based PSCs under the Illumination of AM 1.5G, 100 mW/cm2 Blends PBDTSiTA:ITIC

PBDTSiQx:ITIC

D/A [wt/wt]

Jsc [mA/cm2]

Voc [V]

FF [%]

PCEmax(PCEave)f [%]

1:1.5

9.36

0.96

37.7

3.40 (3.32 ± 0.11)

1:1.2

11.18

0.99

44.0

4.86 (4.84 ± 0.11)

1:1.2a

12.67

0.97

52.8

6.49 (6.31 ± 0.22)

1:1.2b

10.96

0.97

43.5

4.63 (4.50 ± 0.13)

1:1.2c

13.22

0.97

52.9

6.79 (6.67 ± 0.12)

1:1.2d

6.72

0.96

51.6

3.35 (3.28 ± 0.10)

1:1.2a,c

14.09

0.96

55.3

7.51 (7.43 ± 0.12)

1:1

10.37

0.95

48.4

4.78 (4.67 ± 0.19)

1:1.2a,e

8.93

0.92

49.1

4.06 (3.97 ± 0.09)

1:1.2e 1:1e

9.66 9.47

0.90 0.91

49.4 50.9

4.31 (4.18 ± 0.13) 4.38 (4.28 ± 0.10)

a

Chlorobenzene vapor annealing for 2 min. bWith addition of 1% DIO (v/v). cPDIN/BCP/Ag as the cathode system. dPDIN/BCP/Al as the cathode system. eWith addition of 0.5% DIO (v/v). f Average PCEs calculated from at least eight independent cells. All blend films were processed with CB. Device structure:ITO/PEDOT:PSS/active layer/PDIN/Al unless otherwise denoted. hand, DIO additive is not beneficial for the device performance of PBDTSi-TA, but it is favorable for that of PBDTSi-Qx (Table S3). Therefore, the optimum PCE of 4.38%, with Voc = 0.91 V, Jsc = 9.47 mA cm-2, and FF = 0.509 for PBDTSi-Qx based PSCs was obtained under the conditions of with 0.5% DIO (v/v) but without SVA (Table 2). Relative to PBDTSi-Qx, the better PSCs performance of PBDTSi-TA is, at least in part, due to its more complementary absorption spectra with ITIC. The current density-voltage (J-V) curves of the optimized devices are shown in Figure 3a, and the photovoltaic performance data have been listed in Table 2. As expected, benefiting from the deeper HOMO level of PBDTSi-TA, PSCs based on PBDTSi-TA


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deliver larger Vocs not only than the devices based on PBDTSi-Qx, but also than the reported NF PSCs based on polymers with the similar backbone motifs32, 38. To the best of our knowledge, a high Voc of 0.99 V with PCE = 4.86% is rarely reported for ITIC-based NF PSCs. The Voc of the PBDTSi-TA:ITIC based PSC is impressive, but the Jsc and FF are relatively low. This may correlate with the suboptimal blend morphology that would cause serious carrier recombination in the active layer and lead to low charge carrier mobility (vide infra). Without more favorably acceptor materials in hand for morphology tuning, it should be better to improve the Jsc and FF via device optimization in order to confirm the capacity of PBDTSi-TA. Cathode buffer layers play significant roles in determing the device performances. The ideal cathode buffer should satisfy some criteria: to increase electron selectivity, i.e., blocking holes efficiently, and to reduce charge transport resistance. As for the cathode buffer layer of PDIN, its HOMO level is only 0.41 eV lower than that of ITIC (Figure 2b), which may not efficiently prevent holes, transported to the cathode, from recombination with electrons, on the other hand, the LUMO level of PDIN is rather higher than that of aluminium to reduce contact resistance (Figure 2b). So there is the chance to improve device performances based on the PDIN cathode buffer layer. Bathocuproine (BCP) with a wide bandgap and a deep HOMO level is extensively used as a buffer layer in OLEDs and planar heterojunction small molecule organic solar cells due to its pretty hole-blocking and electron-transporting ability. Although the LUMO energy of BCP indicates a large barrier to electron extraction at the cathode, transport in BCP occurs through damage-induced trap states created by the evaporation of hot metal atoms onto the BCP surface (Figure 2b)46, so as to reduce energy barrier against electron transport . Strategically, when a thin layer of BCP (5 nm) was inserted between the PDIN buffer and a silver electrode, the Jsc and FF are both improved, delivering PCEs of 7.51% and 6.79% for the PBDTSi-TA based devices with


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and without SVA, respectively (Table 2, Figure 3a). These PCEs are largely higher than those from devices incorporating the same polymer moieties32, but with only alkyl side chains on BDT rather than TIPS. The gain in FF should be related to the reduced series resistance (Rs), which is reduced from 16.32 to 8.83 Ω cm2 that could be reflected by the enlarged slope of the J-V curve around Voc (Figure 3a). Because the Rs largely depends on the contact resistance that derives from the interface between the active layer and the electrode, and mismatched energy levels may cause a large Rs value, the decrease in Rs implies less interface barrier for carrier extraction and reduced contact resistance that is favorable for FF increase47. On the other hand, the gain in Jsc for the case with PDIN/BCP/Ag should be related with the hole-blocking effect of BCP that could be revealed by the relatively low leakage current across the reverse bias and a high rectification ratio (Figure 3d). It is worth noting that Ag metal electrode works better than Al (Table 2), likely due to the special electron transport mode of BCP, i.e., involving hot metaldeposition-induced damage. More details about the underlying work mechanisms, about the extensive choose of alcohol-soluble interlayer, and about the compatibility of such an improved cathode system with more material systems, are under research and will be reported elsewhere in due time. Minimizing the energy loss (Eloss) of the PSCs is a crucial consideration in pursuing high PCEs. Eloss is defined as Eloss = Eg − eVoc, where Eg is the smallest optical band gap of the active layer components. Considering the Egopt of 1.59 eV for ITIC, Eloss in the optimum devices based on PBDTSi-TA is 0.63 eV, which is approaching the empirically low threshold of 0.6 eV. Such small Eloss is related to the minimal △ EHOMO in the corresponding devices. External quantum efficiency (EQE) curves of the optimized devices are demonstrated in Figure 3b. Across the whole response range, the device based on PBDTSi-TA with the PDIN/Al cathode system


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exhibits higher EQE values than the device incorporating PBDTSi-Qx, which agrees well with their Jsc and PCE results. As for the PBDTSi-TA based device with the PDIN/BCP/Ag cathode system, EQEs are higher than those of the device with PDIN/Al, across the wavelength region between 440 and 800 nm. In contrast, the PDIN/BCP/Ag based device shows lower EQEs below 440 nm due to the light absorption of BCP (Figure S3). To get more insights into the different device performance between PBDTSi-TA and PBDTSi-Qx, we evaluated the maximum exciton generation rate (Gmax) and exciton dissociation probabilities (P (E, T)) via probing the photocurrent density (Jph) versus effective voltage (Veff ) characteristics (Jph is determined as JL - JD, where JL is the current density under illumination and and JD is in the dark. Veff is determined as Veff = Vo - Va, where Vo is the voltage at which Jph = 0 and Va is the applied bias). The plots of Jph versus Veff are presented in Figure 3c. For PBDTSiTA-based device, Jph reaches saturation at a relatively low Veff (Veff > 3 V, wherein saturation current density Jsat was obtained), upon which point photo-induced excitons completely dissociated into charge carriers and Jsat is exclusively dependent on the maximum exciton generation rate (Gmax). Accordingly, Jsat = qLGmax, where q is elementary charge and L is the thickness of the active layer. However, for the case of PBDTSi-Qx, Jph does not get saturation until Veff > 5 V, which may suggest more significant geminate and/or bimolecular recombination45. Gmax values were determined to be 1.21 × 1028 m-3 s-3 for PBDTSi-TA and 8.91 × 1027 m-3 s-3 for PBDTSi-Qx. The increased Gmax for PBDTSi-TA is due to the complementary absorption with ITIC in addition to its higher absorption coefficient. P(E,T) is determined from the ratio of Jph/J

sat,

and the P(E,T) values at short-circuit condition are 85.6% for PBDTSi-TA

and 73 .9% for PBDTSi-Qx, indicating the overall charge dissociation probabilities of PBDTSiTA-based blend are higher than that of PBDTSi-Qx. Nevertheless, Jph values of the PBDTSi-TA


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based device are lower than those of the device based on PBDTSi-Qx when logarithmic Veff is below 0.1 V, which may indicate relatively significant charge carrier recombination with respect to PBDTSi-TA, so that more carriers could not be swept to the desired electrodes. Indeed, as shown in Figure S4, Jscs of the devices based on PBDTSi-TA and PBDTSi-Qx both show a linear dependence on the light intensity in logarithmic coordinates with slopes (S) significantly deviating from 1.0 (S = 0.81 for PBDTSi-TA, S = 0.86 for PBDTSi-Qx), wherein the Jsc can be correlated to light intensity (I) by Jsc ∝ IS and any deviation from S ≈ 1 indicates bimolecular recombination. Thus the carrier recombination loss is serious for the two ITIC-blending polymers and it is even worse for PBDTSi-TA under low Veff conditions. Such effects may be related to the suboptimal nanostructures of the blends (vide infra) and would affect the Jsc. Device stability is of importance for PSCs, and it may involve with the active layer materials. With worry about the susceptibility of triple bond against oxidation, oxidative stability of the device based on PBDTSi-TA with the developed bilayer cathode buffer was carried out. The stability results as a function of storage time under different O2 conditions are shown in Figure S5. The PCE of the device storaged in a N2-filled glovebox (H2O < 0.2 ppm, O2 < 1 ppm) gradiently decayed to 80% of its initial efficiency (PCE = 6.61%) after 14 days. For the device storaged in a simple dry cabinet (RH < 20%) which was connected to air via a CaCl2 (d = 2 cm) drying tube and opened several times every day, the PCE decayed to 73% of the initial value (PCE = 6.79%) after 14 days. So the results as compared indicate that the polymer of PBDTSiTA is rather less sensitive to air oxygen, and the triple bond would not necessarily lead to awful oxidative instability. It is well known that regular-structured PSCs with PEDOT:PSS as the anode buffer layer usually exhibit seriously degraded device lifetime. The PCE degradation, even


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if under an inert atmosphere, is ascribed to the fact that the acidic PEDOT:PSS could etch the ITO and lead to interface instability through indium diffusion into the active layer polymer.

0.8 0.7 0.6

10 8

0.4

6

0.3

0.4

0.6

0.8

0.0

1.0

400

Voltage (V)

10 2

PBDTSi-TA:ITIC (SVA) PBDTSi-Qx:ITIC (DIO)

0.1 1

Current density (mA/cm 2)

(d)

0.1

500

600

2

700

0 800

Wavelength (nm)

(c)

1

4

PBDTSi-TA (SVA+PDIN/BCP) PBDTSi-TA (SVA) PBDTSi-Qx (DIO)

0.1 0.2

12

0.5

0.2

0.0

14

PBDTSi-TA (SVA+PDIN/BCP) PBDTSi-TA (SVA) PBDTSi-Qx (DIO)

-2

(b)

PBDTSi-TA:ITIC (SVA+PDIN/BCP) PBDTSi-TA:ITIC (SVA) PBDTSi-Qx:ITIC (DIO)

Integrated Jsc (mA cm )

6 4 2 0 -2 -4 -6 -8 -10 -12 -14 -16 -0.2

EQE(%)

Current density (mA cm

-2

)

(a)

Jph (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 10

PBDTSi-TA:ITIC (SVA+PDIN/BCP) PBDTSi-TA:ITIC (SVA)

1 0.1 0.01 1E-3 1E-4 1E-5 -3

-2

-1

0

1

2

3

Voltage (V)

Veff (V)

Figure 3. (a) Current–voltage (J-V) characteristics of the PSCs based on PBDTSi-TA (open circle-PDIN/BCP/Ag；solid circle-PDIN/Al) and PBDTSi-Qx (PDIN/Al), under simulated AM 1.5G illumination (100 mW cm-2). (b) External quantum efficiency curves and integrated Jscs of the corresponding devices. (c) Photocurrent density (Jph) versus effective voltage (Veff) curves of the optimized devices based on PBDTSi-TA and PBDTSi-Qx with the PDIN/Al cathode system. (d) J-V curves of the specified devices based on PBDTSi-TA in the dark. Morphology and Charge Carrier Mobility


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To further illustrate the differentiated performances, morphologies of the blend films were investigated by tapping-mode atomic force microscopy (AFM) and transmission electron microscopy (TEM), and hole (μh) and electron (μe) mobilities were calculated via the space charge limited current (SCLC) method with structures of ITO/PEDOT:PSS/blend/MoO3/Ag and ITO/PDIN/blend/BCP/Al, respectively. The electron and hole mobilities for PBDTSi-TA:ITIC without SVA are 1.01 × 10-5 and 1.21 × 10-5 cm2 V-1 s-1, respectively, which are accordingly increased to 1.06 × 10-5 and 1.73 × 10-5 cm2 V-1 s-1 for the film with SVA (Figure S6 and Table S4). In view of the comparable balance (μh/μe), these mobility changes can partly accounts for the performance gain with SVA. As for PBDTSi-Qx:ITIC, the electron and hole mobilities of the film with DIO are also higher than that without DIO (electron: 1.49 × 10-5 cm2 V-1 s-1 (without DIO), 3.09 × 10-5 cm2 V-1 s-1 (0.5% DIO); hole: 1.03 × 10-5 cm2 V-1 s-1 (without DIO), 2.98 × 10-5 cm2 V-1 s-1 (0.5% DIO)). For the pristine blends, the hole mobility of PBDTSiTA:ITIC is higher than that of PBDTSi-Qx:ITIC, which may be ascribed to the F atom induced intermolecular interactions as for PBDTSi-TA. For the lower hole mobility of PBDTSi-TA:ITIC with SVA as compared to that of PBDTSi-Qx:ITIC with DIO, it may be the fact that different processing methods bring bulk morphology changes to varied extent (Figure 5a versus 5c). Overall, the mobilities for the blends of the two polymers are all relatively low, which partly accounts for their suboptimal Jsc and FF. Charge carrier mobilities usually correlate with the materials crystallinity. As illustrated in Figure S7, X-ray diffraction measurements of the two polymers and their blends with ITIC all reveal no defined diffraction peaks, and thus such amorphous properties as indicated agree with the low charge transport ability. As shown in Figure 4a and Figure 4b, the blend film of PBDTSi-TA:ITIC with SVA and without SVA both exhibits clear nanoscale phase separation, but the case with SVA is more uniform and the


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domain sizes are more finer, which is beneficial for exciton dissociation and charge transport. On the other hand, the surface of the blend film with SVA is smoother than that without SVA (rootmean-square roughness: 0.96 nm vs. 1.30 nm). As for PBDTSi-Qx:ITIC, the film with 0.5% DIO displays more cognizable phase separation relative to that without DIO (Figure 4c vs. Figure 4d), which indicates more better intermixing of the two components as revealed by a slightly rougher surface (RMS: 0.85 nm vs. 0.58 nm). As shown by TEM images in Figure 5 that reveals the internal nanostructures of the blend films, the as-cast blend films of PBDTSi-TA:ITIC and PBDTSi-Qx:ITIC both exhibit clear phase separation. But the domain sizes for both of the two blends are beyond 100 nm that is seriously larger than the exciton diffusion lengths (typically below 15 nm), and this would be unfavorable for charge carrier generation and transport. Under such conditions, the Jscs would be decreased and the FF could be reduced due to the chances for exciton or carrier recombination. In contrast, for the blend films of PBDTSi-TA:ITIC with SVA and PBDTSi-Qx:ITIC with DIO, the morphologies are both significantly improved as revealed by the smaller domains and the well interpenetrating networks, which echoes the trend in device performances.


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Figure 4. AFM phase images of the blend films of PBDTSi-TA:ITIC (a, b) and PBDTSiQx:ITIC (c, d) and their corresponding height images (e-h for a-d, respectively). The scan size is 1 μm × 1 μm for PBDTSi-TA:ITIC and 0.5 μm × 0.5 μm for PBDTSi-Qx:ITIC.

a

With SVA

500 nm

b

W/O SVA

500nm nm 500

c

With DIO

500 nm nm 500

d

W/O DIO

500 nm

500 nm

Figure 5. TEM images of the blend films of PBDTSi-TA:ITIC with (a) and without (b) chlorobenzene solvent vapor annealing, and the films of PBDTSi-Qx:ITIC with (c) and without (d) the addition of DIO.

CONCLUSIONS In summary, two highly conjugated copolymers based on a triisopropylsilylethynyl (TIPS) substituted BDTT donor unit and a fluorinated benzo[d][1,2,3]triazoles (FTAZ) or a quinoxaline (Qx) based acceptor unit have been designed and synthesized as the donor materials for nonfullerene PSCs. Among them, the polymer PBDTSi-TA containing the FTAZ acceptor unit possesses a wider bandgap, and its absorption spectra well complement those of ITIC, which is demonstrated to be favorable for photoexciton generation and Jsc improvement. Incorporating a conjugated TIPS group onto the lateral thiophene rings of BDTT could promote the intermolecular π-π interactions, red-shift absorption spectra, and lower the HOMO energy levels of the polymers. Initial PSCs based on PBDTSi-TA:ITIC afford a PCE of 4.86% and an


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impressively high Voc of 0.99 V, which could be the hitherto reported highest Voc of ITIC-based NF PSCs, with a PCE of such level. Under such conditions, an Eloss of 0.6 eV would be determined which is identical to the empirically low threshold. Through solvent vapor annealing (SVA) with chlorobenzene, and in combination with a developed PDIN/BCP/Ag cathode structure, an intriguing PCE of 7.51% with Voc = 0.96 V could be obtained. In view of the negligible △EHOMO between PBDTSi-TA and ITIC, which has somewhat hindered the hole transfer process, more higher PCEs could be obtained if some acceptor with slightly lower HOMO than that of ITIC was matched, and attempts toward this goal are currently underway. It is also believed that this work would hint a new approach to design highly conjugated copolymers for high-performance PSCs and provide an effective alternative to optimize PSCs performance in view of the universal use of alcohol-soluble conjugated polyelectrolytes as cathode interlayers. EXPERIMENTAL SECTION Materials and Methods All commercially available chemicals were used as received unless otherwise specified. Tetrahydrofuran (THF) was distilled over sodium and benzophenone under nitrogen atmosphere. 4,7-bis(5-bromothiophen-2-yl)-5,6-difluoro-2-(2-hexyldecyl)-2H-benzo[d][1,2,3]triazole (FTAZ) , 5,8-Bis-(5-bromothiophen-2-yl)-2,3-bis-(3-octyloxyphenyl)quinoxaline (Qx) and ITIC were purchased from Solarmer Materials Inc. PDIN was purchased from Suna Tech Inc. The NMR spectra were collected on a Bruker AVANCEⅢ 600 MHz spectrometer with tetramethylsilane (TMS; δ 0 ppm) as an internal standard. The mass spectra (FT-MS) were conducted on a Thermo Fisher Scientific LTQ FT Ultra mass spectrometer. UV-vis absorption spectra were recorded using a Hitachi U-3000 spectrometer. Photoluminescence (PL) spectra of


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thin films on a quartz substrate were measured by a Hitachi-F4500 spectrofluorometer. The electrochemical measurements were performed in a deoxygenated solution of tetra-nbutylammonium hexafluorophosphate (0.1 M) in CH3CN with a CHI604E electrochemical workstation, wherein a Pt plate working electrode coated with samples, a Pt wire counter electrode, and a Ag/AgNO3 reference electrode were applied. AFM images of the active layers were obtained using a Nanoscope IV instrument at a tapping mode. TEM images were obtained by a FEI Tecnai G2 F 20 S-TWIN with an accelerating voltage of 200 kV. X-ray diffraction patterns of thin films drop-casted on a quartz substrate were acquired with a Bruker D8 ADVANCE diffractometer at 40 kV and 40 mA with a nickel-filtered Cu-Ka beam. Device Fabrication and Characterization PSCs with a conventional structure of ITO/PEDOT:PSS/blend/PDIN/Al were fabricated. ITO glass (10 Ω/sq) was ultrasonically cleaned with detergent, deionized water, acetone, and isopropanol in sequence, and then subjected to ultraviolet/ozone treatment for 20 min. Poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, Baytron P VP AI 4083) was spin-cast at 3500 rpm for 60 s to form a thin layer of 30 nm thickness. Afterward, the ITO glass with PEDOT:PSS was baked at 140 oC for 20 min. In a N2-filled glovebox, the BHJ layer was spin-deposited from a solution of active layer components with different controls (20 mg/ml in total, 1000-1500 rpm). The optimum blend thicknesses for PBDTSi-TA:ITIC and PBDTSiTA:ITIC are 70 and 90 nm, respectively. The active layer thickness was determined by a Veeco Dektak 150 profilometer. Afterwards, a PDIN methanol solution with 0.2 vol % AcOH (1.5 mg ml-1) was spin-coated at 2500 rpm for 30 s to form the cathode buffer layer. Finally, the alumium electrode (~ 100 nm) was evaporated onto the top of the devices under a pressure below 1 × 10-5 Pa. As for the devices with SVA, active layers were kept in a glass Petri dish containing 0.5 ml


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chlorobenzene for 1-3 min, and for the cases with a PDIN/BCP/Ag (or Al) cathode structure, 5 nm BCP was evaporated in the middle of the deposition of PDIN and 100 nm Ag (or Al), under high vacuum at the rate of 0.1 nm/s. The active area of the devices was 6 mm2. The J-V curves were measured by a computer-controlled Keithley 2400 source-measure unit under AM 1.5 G, 100 mW cm-2 illumination which was provided by a SAN-EI ELS 155 (XE) solar simulator equipped with a standard monocrystalline silicon solar cell to calibrate the light intensity. The EQE data was measured by a solar cell spectral response measurement system (Crowntech Inc., USA). The light intensity at each wavelength was calibrated using a standard crystal Si photovoltaic cell.

Synthesis Synthesis of Compound 1. To a degassed and ice-cooled solution of 2-bromothiophene (2.6 g, 15.9 mmol) in THF (40 ml) and triethylamine (40 ml) were added CuI (46 mg, 0.24 mmol), Pd(PPh3)2Cl2 (0.56 g, 0.8 mmol) and ethynyltriisopropylsilane (4.8 g, 1.65 eq.). The mixture was stirred at room temperature for 30 min and then was reflexed overnight. Upon cooling, dichloromethane was added and the resulting mixture was washed with 1 M hydrochloric acid and then with water. After drying with anhydrous MgSO4, the solvent was removed under reduced pressure. The crude product was purified by silica gel column chromatography with petroleum ether as the eluent to yield a colorless oil (3.2 g, 76%). 1H NMR (600 MHz, CDCl3): δ 7.23-7.21 (m, 2H), 6.95-6.94 (m, 1H), 1.12 (br, 18H), 1.09 (br, 3H). Synthesis of Compound 2. Under a nitrogen atmosphere, n-BuLi (6.5 ml, 16.25 mmol, 2.5 M in hexane) was slowly syringed to the THF (30 ml) solution of compound 1 (3.6 g, 13.62 mmol) at 0 oC. The mixture was kept at R.T. for 3 h and then 50 oC for 1.5 h. Upon cooling and after


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benzo[1,2-b:4,5-b’]dithiophene-4,8-dione (1.0 g, 4.54 mmol) addition, the reactants were recovered to 50 oC for another 5 h. After cooling to R.T., SnCl2·2H2O (6.6 g, 34.8 mmol) in 10% HCl (15 ml) was added and the mixture was stirred overnight. The mixture was subsequently poured into ice water and extracted with dichloromethane, dried with anhydrous MgSO4. After solvent removal, the residue was subjected to flash silica gel column chromatography eluting with ethyl acetate. The acquired solid was purified with recrystallization from ethanol/petroleum ether (5/1) to yield light yellow powders (1.8 g, 56%). 1H NMR (600 MHz, CDCl3): δ 7.61 (d, J = 5.4 Hz, 2H), 7.49 (d, J = 5.4 Hz, 2H), 7.37 (d, J = 3.6 Hz, 2H), 7.34 (d, J = 3.6 Hz, 2H), 1.171.14 (m, 42 H). 13C NMR (150 MHz, CDCl3): δ 140.80, 139.21, 136.71, 132.89, 128.09, 127.94, 124.78, 123.33, 123.06, 99.00, 97.07, 18.69, 11.35. HRMS (FT-MS): m/z calcd for C40H51S4Si2 [M + H]+ 715.2412, found 715.2405. Anal. calcd for C40H50S4Si2 (%): C, 67.17; H, 7.05. Found (%): C, 67.51; 7.02. Synthesis of BDTSi-Sn. To a solution of Compound 2 (1.2g, 1.68 mmol) in THF (30 ml) at -78 o

C was added dropwise a LDA solution (2.1 ml, 2.5 eq., 2.0 M). After being kept at -78 oC over

1 h, trimethyltin chloride (1.0 g, 5.04 mmol, dissolved in 2 ml THF) was added into the mixture at low temperature. Then the mixture was warmed to R.T. for 5 h. Finally, the reaction was quenched with water, and the mixture was extracted with dichloromethane. After solvent removal, the residue was purified with recrystallization from ethanol to obtain yellow powders (1.5 g, 86%). 1H NMR (600 MHz, CDCl3): δ 7.61 (s, 2H), 7.39 (d, J = 3.6 Hz, 2H), 7.33 (d, J = 3.6 Hz, 2H), 1.17-1.14 (m, 42H), 0.41 (s, 18H). 13C NMR (150 MHz, CDCl3): δ 143.53, 143.40, 141.67, 137.52, 133.06, 130.65, 127.89, 124.37, 121.62, 99.30, 96.76, 18.71, 11.38, -8.26. MS (FT-MS): m/z calcd for C46H65S4Si2Sn2 [M - H]+ 1041.2, found 1041.2. Anal. calcd for C46H66S4Si2Sn2 (%): C, 53.08; H, 6.39. Found (%): C, 53.64; H, 6.39.


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Synthesis of PBDTSi-TA. Monomer BDTSi-Sn (0.25 g, 0.24 mmol) and monomer FTAZ (166 mg, 0.24 mmol) were dissolved in dry xylene (25 ml) in a 50 ml schlenk flask. After purging the solution with nitrogen for 30 min, Pd(PPh3)4 (10 mg) was added. Then, the reactants were heated to 110 oC for 2 h. Afterward, the mixture was cooled and precipitated into acetone. The filtered solid was subjected to Soxhlet extraction with methanol, acetone, hexane and chloroform in sequence. Precipitation into methanol of the chloroform fractions yielded the polymer as a dark red solid (0.16 g, 53%). GPC (THF): Mn = 48.8 kDa, PDI = 1.2. 1H NMR (600 MHz, CDCl3): δ 8.52-6.09 (br, 10H), 4.66 (br, 2H), 1.47-0.82 (m, 73H). Anal. calcd for C70H85Si2S6N3F2 (%): C, 66.99; H, 6.83. Found (%): C, 66.91; H, 6.76. Synthesis of PBDTSi-Qx. PBDTSi-Qx was synthesized from the monomers of BDTSi-Sn and Qx using the method as described for PBDTSi-TA excepting that the reaction time is 1h (dark purple solid, yield 62%). GPC (THF): Mn = 49.2 kDa, PDI = 1.35. 1H NMR (600 MHz, CDCl3): δ 8.18-5.83 (br, 20H), 3.72-3.42 (br, 4H), 1.51-0.86 (br, 72H). Anal. calcd for C84H96S6Si2O2N2 (%): C, 71.34; H, 6.84. Found (%): C, 70.88; H, 6.76.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. SCLC data, absorption spectra upon temperature, PL spectra, double logarithmic Jsc-I plots, device stability, details of the influence of solvent, SVA and additive on the device properties. (PDF) AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] (L.W.) *E-mail: [email protected] (S.Y.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of Hebei Province (No. E2017201188) and the Fund of Hebei Science and Technology Bureau (No. 16211241). L.Wang thanks Prof. Qingdong Zheng and Qisheng Tu at FJIRSM for their assistance in CV measurements, and thanks Dr. Xin Wen and Yun Li at HBU for their help with GPC and PL measurements, respectively. REFERENCES 1.

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Table of Contents

-2

Current density (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 34

6 PDIN/Al 4 Si PDIN/BCP/Ag 2 S 0 PCE = 6.49% F F S S -2 S Voc = 0.97 V S n -4 N N S N -6 C8H17 -8 C6H13 Si -10 -12 PCE = 7.51% -14 Voc = 0.964 V -16 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Voltage (V)


34

Wide Bandgap and Highly Conjugated Copolymers Incorporating 2

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