Quinoxaline-Based Wide Band Gap Polymers for Efficient

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Quinoxaline-Based Wide Band Gap Polymers for Efficient NonFullerene Organic Solar Cells with Large Open-Circuit Voltages Jie Yang, Mohammad Afsar Uddin, Yumin Tang, Yulun Wang, Yang Wang, Huimin Su, Rutian Gao, Zhi-Kuan Chen, Junfeng Dai, Han Young Woo, and Xugang Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04432 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 18, 2018

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ACS Applied Materials & Interfaces

Quinoxaline-Based

Wide

Band

Gap

Polymers

for

Efficient

Non-Fullerene Organic Solar Cells with Large Open-Circuit Voltages Jie Yang,†, || Mohammad Afsar Uddin,‡, || Yumin Tang,† Yulun Wang,† Yang Wang,† Huimin Su,† Rutian Gao,§ Zhi-Kuan Chen,§ Junfeng Dai,† Han Young Woo,*,‡ Xugang Guo*,† †

Department of Materials Science and Engineering and The Shenzhen Key Laboratory

for Printed Organic Electronics, South University of Science and Technology of China, No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055, China. ‡

Department of Chemistry, Korea University, Seoul 136-713, South Korea.

§

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials

(IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, P. R. China.

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

Abstract We present here a series of wide band gap (Eg: > 1.8 eV) polymer donors by incorporating thiophene-flanked phenylene as electron-donating unit and quinoxaline as electron-accepting co-unit to attain large open-circuit voltages (Vocs) and short-circuit currents (Jscs) in non-fullerene organic solar cells (OSCs). Fluorination was utilized to fine-tailor the energetics of polymer frontier molecular orbitals (FMOs) by replacing a variable number of H atoms on the phenylene moiety with F. It was found that fluorination can effectively modulate polymer backbone planarity through intramolecular noncovalent S…F and/or H…F interactions. Polymers (P2–P4) show improved molecular packing with favorable face-on orientation compared to their non-fluorinated analogue (P1), which is critical to charge carrier transport and collection. When mixed with IDIC, a non-fullerene acceptor, P3 with two F atoms achieves a remarkable Voc of 1.00 V and a large Jsc of 15.99 mA/cm2, simultaneously, yielding a power conversion efficiency (PCE) of 9.7%. Notably, the 1.00 V Voc is among the largest values in IDIC-based OSCs, leading to a small energy loss (Eloss: 0.62 eV) while maintaining a large PCE. The P3:IDIC blend shows efficient exciton dissociation through hole transfer even under a small energy offset of 0.16 eV. Further fluorination leads to the polymer P4 with increased chain twisting and mismatched FMO levels with IDIC, showing the lowest PCE of 2.93%. The results demonstrate that quinoxaline-based copolymers are promising donors for efficient OSCs and the fluorination needs to be fine-adjusted to optimize the interchain packing and physicochemical properties of polymers. Additionally, the structure-property correlations from this work provide useful insights

2


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for developing wide band gap polymers with low-lying HOMOs to minimize Eloss and to maximize Voc in non-fullerene OSCs for efficient power conversion. KEYWORDS: polymer semiconductors, fluorination, non-fullerene organic solar cells, energy losses, open-circuit voltages.

Introduction Organic solar cells (OSCs) have gained considerable attention owing to their distinctive advantages, including light weight, potential usage in flexible/stretchable devices, and solution-based device fabrication.1, 2 To date, the most promising OSCs feature a bulk heterojunction (BHJ) structure, which incorporates an electron donating semiconductor and an electron accepting one to yield a bicontinuous interpenetrating network spontaneously.1 Over the last two decades, the electron acceptor materials have been mainly dominated by fullerenes and their derivatives, such as [6,6]-phenyl-C61 (or C71)-butyric acid methyl ester and indene-C60 bisadduct.1,

3

However, these fullerene

derivatives are plagued by several intrinsic drawbacks, including limited materials library and FMO energy level tailorability, poor light absorption, and weak endurance against thermal stress and solar irradiation,1, 4 and hence the highest power conversion efficiencies (PCEs) are limited to < 12% for the conventional polymer:fullerene OSCs.5 Recently, the pioneering works by Zhan and co-workers created great opportunities for the OSC field by developing a series (ITIC series) of non-fullerene narrow band gap fused-ring electron acceptor (NBG-FREA) materials, which show substantial electron mobilities along with widely tunable optical absorption and FMO levels.6,

3


7

The


invention of the ITIC derivatives in combination with the device engineering has greatly improved the PCEs of OSCs. To date, polymer donor:non-fullerene acceptor solar cells have attained the PCEs surpassing ~13%.8, 9 In order to maximize the PCEs of OSCs, it is critical to develop well-matched polymer donors for different acceptors. Among various donor semiconductors, polymers with wide band gap (WBG, Eg > 1.8 eV) and deep FMO levels are beneficial to attain high open-circuit voltages (Vocs).10 However, owing to the reduced absorption, their short-circuit currents (Jscs) are limited, typically < 16 mA/cm2, when combined with the fullerene acceptors in OSC devices.10 Intriguingly, when blended with emerging NBG-FREAs like ITIC series or IDTBR-series,7, 11, 12 the OSCs can achieve high Vocs > 0.9 V along with large Jscs and fill factors (FFs) at the same time, which is attributed to their complementary absorption, well-matched FMO levels, and optimized film morphology.13,

14

Many studies have shown that WBG polymer:NBG-FREA blend

systems can operate efficiently through photogenerated hole transfer channels under a smaller driving force with a ∆EHOMO (the offset between highest occupied molecular orbitals (HOMOs) of donors and acceptors) < 0.3 eV, which was early considered to be the threshold value for efficient exciton separation at donor/acceptor (D/A) interfaces in fullerene-based OSCs.12, 15, 16 This progress is of great importance for maximizing Vocs without sacrificing Jscs and FFs in OSCs. Additionally, in this type of blend systems with Eg (D) > Eg (A), the solar absorption mainly originates from the narrow band gap (NBG) acceptor and the WBG donor affords the complementary absorption, resulting in greatly improved Jscs in non-fullerene solar cells. Therefore, designing and synthesizing

4


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WBG polymer semiconductors with low-lying HOMOs to match NBG acceptors should be an effective approach to further increase PCEs.17 As the weak electron donor unit with a large aromatic resonance energy, the incorporation of phenylene moiety leads to polymer semiconductors with typically suppressed HOMOs and hence large Vocs.18 Recently, Yan et al. reported the polymer PffBT4T-B by incorporating phenylene moiety into the polymer PffBT4T, affording PffBT4T-B with a broadened band gap of 1.80 eV, a suppressed HOMO of 5.61 eV, and an improved Voc of 0.97 V when combined with the NBG acceptor ITIC-Th.19 The phenylene moiety offers four positions for further structural modification and physicochemical property optimization. Among various strategies, fluorination has been proved to be an effective one in lowering materials’ HOMOs,20-22 owing to the highest electron negativity of F atoms.23-27 Moreover, the F incorporation can generate various intramolecular non-covalent coulombic interactions, including F…H and F…S interactions, locking the chain conformation to achieve self-planarized backbone and compact packing of polymer chains in film state, leading to large Jscs and FFs in OSCs.28, 29

Hence, it is highly feasible to optimize the polymers’ FMO levels and self-assembly

properties through fluorinating polymer chain. Among various electron deficient units, quinoxaline has been used to construct polymer semiconductors for solar cell applications owing to its appropriate electronic structure and solubilizing ability enabled by the attached side chains.30-34 For instances, quinoxaline-benzodithiophene based copolymers show complementary absorption and well-aligned FMO levels with ITIC-based acceptors, achieving high PCEs >9%.15,

35-37

5


On the other hand, most


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efficient quinoxaline-based polymers reported to date often have medium band gap and show Voc ~0.9 V, while further increasing polymers’ band gap or decreasing HOMO to obtain larger Voc often leads to dramatically decreased Jsc and PCE. As a result, few quinoxaline-based WBG donor polymers have achieved large Voc and high device performance simultaneously in OSCs.31, 34, 38 In consideration of that the solar absorption mainly originates from the NBG acceptor and the WBG donor affords the complementary absorption in the blend system with the Eg(D) > Eg(A), photogenerated hole transfer can operate efficiently under a small driving force, and constructing polymer semiconductors with low-lying HOMOs by incorporating both quinoxaline and phenylene moieties in the backbone have not been systematically studied in non-fullerene OSCs, we present here the design and synthesis of a series of WBG polymer donors based on our recent work,39 by changing the number of F atoms on the phenylene unit (P1-P4 with F = 0, 1, 2, 4). The four polymers show gradually enlarged Egs from 1.87 to 2.00 eV having decreased HOMOs from -5.17 to -5.68 eV as the number of F atoms is increased. When mixed with IDIC,40,

41

a

non-fullerene acceptor, the OSCs having a device structure of ITO (indium tin oxide)/PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate))/polymer:IDIC/PDINO (perylene diimide functionalized with amino N-oxide)/Al42 achieve improved Vocs from 0.86 to 1.06 V gradually and decreased Eloss from 0.76 to 0.56 eV. Among them, P3 containing two F atoms on the phenylene moiety attains a PCE of 9.70% with a large Jsc of 15.99 mA/cm2, a FF of 60.89%, and a remarkable Voc of 1.00 V, simultaneously. This

6


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Voc is among the largest values in IDIC-based non-fullerene OSCs showing a small Eloss of 0.62 eV and a remarkably high PCE of 9.7%. This PCE is also among the highest in quinoxaline-based WBG donor polymers.31 The results indicate that the quinoxaline and thiophene-flanked

phenylene

copolymers

are

promising

donors

for

efficient

non-fullerene OSCs and the fluorination can afford polymer semiconductors with optimized electronic, morphological and physicochemical properties.

Result and Discussion Polymer Synthesis Synthesizing these quinoxaline-based copolymers P1-P4 (Figure 1) with varying number of F atoms on phenylene moiety is straightforward (see Supporting Information for details). Monomers M1-M5 (Supporting Information) were prepared according to the published procedures.18, 39 P1-P4 were synthesized via Stille polycondensation between the brominated monomer M1 and the corresponding tin monomers (M2-M5) using tris(dibenzylideneacetone)dipalladium and tri(o-tolyl)phosphine as the Pd catalyst and the ligand, under microwave irradiation (Supporting Information). After purification via Soxhlet extraction, polymerizations afford the product polymers in decent yields of 76-82% (the detailed synthesis shown in SI). All the quinoxaline-based polymers show sufficient solubility in chloroform, chlorobenzene, and o-dichlorobenzene, enabled by four solubilizing chains on the quinoxaline moiety. Polymer molecular weights (Mns) and polydispersity index (PDIs) were characterized by high temperature (80 °C) gel permeation

chromatography

(GPC)

using

polystyrene

as

the

standard

and

o-dichlorobenzene as the eluent. For minimizing polymer molecular weight effects, each

7



polymerization was carried out under the same conditions and all polymers show comparable Mns (24~38 KDa) and PDI values (Table 1).

Figure 1. Chemical structures of polymers P1-P4 and the non-fullerene acceptor IDIC.

Density Functional Theory-Based Calculations To study the effects of the phenylene fluorination on polymer backbone conformations and FMO energy levels, density functional theory (DFT)-based calculations were conducted at the B3LYP/6-31G* level with the Gaussian 09 program. For simplicity, three repeating units of polymers are used for calculation (Figure 2). In addition, the calculation is based on gas state without considering the intermolecular interaction, which makes the values different from those derived from CV measurement (vide infra). In spite of such difference, DFT calculation shows good internal consistency and offers useful insights into materials geometry and optoelectronic properties for these polymer. First, the relative conformation with regard to quinoxaline-thiophene was determined on the basis of the torsional energy as a function of dihedral angle of the thiophene with respect to the quinoxaline (Figure S1). The DFT calculation revealed that 8


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the conformation with sulfur and the fluorine atoms on the opposite sides has the lowest energy, therefore this particular conformation was selected for the following calculations. For polymer P1 without F atom, the dihedral angles between phenylene and neighboring thiophenes were found to be 19.7o and 16.0o (Figure 2) in the energy-minimized structure. After introducing one F atom, the dihedral angles are reduced to 19.3o and 14.2o and the polymer P2 shows improved backbone planarity, which is enabled by the intramolecular non-covalent S…F or H…F coulombic interactions.43-45 For polymer P3 with two F atoms on the phenylene moiety, the dihedral angels are further reduced to 6-8o and P3 shows the highest backbone planarity in this series of polymers. The improved backbone planarity should benefit the packing of polymer chains and facilitate charge transport. However, further F addition affords the tetrafluorinated P4 having a slightly more twisted backbone (versus P3), which is correlated to the increased steric hindrance due to the larger F van der Waals radius compared to H radius. Owing to its high electronegativity, the F addition results in the triad of P1-P4 repeating units with gradually decreased HOMOs from -4.81 to -5.08 eV (Figure 2), and the low-lying HOMOs should be beneficial to the Vocs in OSCs.

Figure 2. Energy minimized geometries of triads of P1-P4 repeating units. The DFT 9



calculation conducted at B3LYP/6-31G* level and the side chains truncated for simplicity.

Polymer Optoelectronic Properties Optical properties of P1-P4 were characterized by measuring their UV-vis absorption spectra in solution and film states, and the spectra are illustrated in Figure 3a and 3b. All polymers show comparable absorption spectra from 350 to 650 nm, complementary to absorption of the NBG non-fullerene acceptor IDIC (Figure 3b), which should be beneficial to the Jsc in OSCs. From solution to film state, the spectra of four polymers exhibit minimal red-shift, which is attributed to the strong polymer pre-aggregation in solution. Among four polymers, P1 shows the least structured absorption profile, indicative of its lowest interchain aggregation in solution. Such absorption characteristics are attributed to its highest backbone torsion in the series (Figure 2), due to the absence of the F-induced non-covalent coulombic interactions. Therefore, fluorinating polymer backbone facilitates intramolecular non-covalent interactions and/or interchain coulombic attractions, which should be beneficial to polymer chain ordering and charge carrier transport. Extinction coefficients (α) of polymer films were measured to be 3.3×104 (P1), 5.4×104 (P2), 6.1×104 (P3), and 4.7×104 cm-1 (P4), indicating that the addition of F atoms leads to increased polymer extinction coefficients compared with P1 without F atoms on phenylene, which could be ascribed to more planar backbone and increased interchain interaction in the film state.44, 46-48

Further F addition leads to a smaller extinction coefficient of P4 (versus P3) and the

trend is consistent with the polymer backbone planarity. The optical band gap (Eopt g ) was calculated using the equation of E opt = 1240/λonset (eV), where λonset is the onset g 10


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wavelength of film absorption. On the basis of the λonset (Figure S3), the Eopt is 1.87 (P1), g 1.90 (P2), 1.92 (P3), and 2.00 eV (P4) (Table 1). Polymer semiconductors were further characterized using cyclic voltammetry (CV) for studying their electrochemical properties. Based on the CV curves (Figure 3c), the derived lowest unoccupied molecular orbital (LUMO)/HOMO energy levels are -2.90/-5.17 (P1), -2.94/-5.33 (P2), -2.98/-5.54 (P3), and -3.13/5.68 eV (P4) (Figure 3d). Please note that the band gap derived from the CV measurement is larger than the Eopt g , which is attributed to the electron-hole pair (or exciton) binding energy.49 Among the polymer series, the nonfluorinated phenylene-based P1 shows the highest-lying HOMO, and introducing different number of F atoms onto the phenylene moiety leads to gradually lowered LUMOs/HOMOs from P2 to P4. The LUMO/HOMO of IDIC was found to be –3.82/-5.70 eV, measured under the same condition for the polymers. The LUMO-LUMO offsets between polymer and IDIC are 0.92 (P1), 0.88 (P2), 0.84 (P3), and 0.69 eV (P4), indicating sufficient driving forces for exciton separation through photogenerated electron transfer from polymer to IDIC. The corresponding HOMO-HOMO offsets between the polymer and IDIC are 0.53 (P1), 0.37 (P2), 0.16 (P3), and 0.02 eV (P4). The ∆EHOMO between P1 (or P2) and IDIC is > 0.3 eV, suggesting a sufficient driving force for exciton fission via hole transfer from IDIC to polymer. While, P3 and P4 show reduced ∆EHOMOs of 0.16 and 0.02 eV, which are less than the widely accepted minimum energy offset of 0.3 eV for efficient hole transfer. However, many recent results have shown that ∆EHOMO < 0.3 eV can also lead to efficient hole transfer in non-fullerene solar cells.12,

15, 16, 50

11


Therefore, for further


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investigation of the exciton separation and hole transfer, studies on photoluminescence (PL) quenching efficiency were also conducted (vide infra). Table 1. Summary of molecular weights and optoelectronic properties of P1-P4. Polymer

Mn [KDa]a

PDI

λsol max [nm]b

λfilmax [nm]c

α [10 cm-1]d

λonset [nm]

P1

24.6

1.7

561

563

3.3

P2

32.2

2.1

591

594

P3

38.5

2.4

592

P4

29.7

2.8

576

a

a

4

Eopt g

EHOMO

ELUMO

[eV] e

[eV]

f

[eV]f

663

1.87

-5.17

-2.90

5.4

654

1.90

-5.33

-2.94

593

6.1

647

1.92

-5.54

-2.98

577

4.7

622

2.00

-5.68

b

-3.13 −5

Measured by GPC at 80 °C. Absorption of chloroform solution (2 × 10 M). c Absorption of film. d Absorption coefficient of the polymer film. e Calculated using the equation: Eopt = 1240/λonset (eV). f EHOMO = ‒ (4.8 + Eoxonset ) eV; ELUMO = ‒ (4.8 + Ered onset) determined electrochemically relative to the Fc/Fc+ reference. g

Figure 3. (a) P1-P4 absorption spectra in chloroform (2×10-5 M). (b) Absorption spectra of P1-P4 and IDIC film (spin-coated from 5 mg/mL chloroform solution); inset shows the extinction coefficients. (c) Cyclic voltammograms of P1-P4 films relative to the Fc/Fc+ reference. (d) Energy level diagram of P1-P4 and IDIC.

Polymer Photovoltaic Properties The photovoltaic characteristics of these new quinoxaline polymers were 12


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investigated by fabricating both conventional OSCs with an architecture of ITO/PEDOT:PSS/polymer:IDIC/PDINO/Al and inverted ones with an architecture of ITO/ZnO/polymer:IDIC/MoOx/Ag.25, 42 IDIC (Figure 1), a NBG acceptor with an intense absorption from 600 to 800 nm, was selected as the acceptor material, which provides a complementary absorption to various WDG donor materials and has been proved to be an excellent acceptor in many non-fullerene solar cells.40, 41, 51 In addition, compared with most other high-performance FREAs, the very deep HOMO of IDIC renders it as a promising acceptor candidate for these quinoxaline-based polymers with low-lying HOMOs. The OSCs were systematically optimized by varying the processing solvents and blend ratios of polymer:IDIC. Chloroform as the processing solvent was found typically yielding improved device performance. The ratios of polymer:IDIC greatly affect the device performance and it was found that the OSCs can attain larger PCEs with a total concentration of 12 mg/mL having a polymer:IDIC ratio of 1:1 (w/w). To further

optimize

blend

film

morphology,

various

processing

additives,

i.e.

chloronaphthalene (CN),6 1,8-diiodooctane (DIO),41 1-phenylnaphthalene (PN),52 diphenyl ether (DPE),53 N-methyl pyrrolidone (NMP),54 and 1,8-octanedithiol (ODT)55 were examined (Table S1). In addition, using the conventional architecture (Figure 4a) with an electron transport interlayer of PDINO, a perylene diimide functionalized with amino N-oxide, can further improve OSC performance, which features well-matched LUMO with IDIC, leading to ohmic contact between the blend film and cathode.42, 56-58 Table 2. Photovoltaic performance characteristics of P1–P4:IDIC conventional OSCs with an architecture of ITO/PEDOT:PSS/polymer:IDIC (1:1)/PDINO/Al fabricated under the optimal condition. 0.3 vol% DIO was added as the processing additive. The highest performance parameters are shown with the average data ± standard deviations 13



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from 10 devices included in parenthesis. Blend films

Voc (V)

Jsc (mA/cm2)

Cal. Jsc (mA/cm2)

FF (%)

PCE （%）

Eloss (eV)

P1:IDIC

0.86 (0.86±0.00)

8.53 (8.55±0.23)

8.46

66.22 (65.72±0.5)

4.86 (4.85±0.11)

0.76

P2:IDIC

0.91 (0.91±0.01)

13.15 (13.35±0.32)

12.88

64.84 (63.92±1.54)

7.75 (7.73±0.22)

0.71

P3:IDIC

1.00 (1.00±0.00)

15.99 (15.37±0.50)

60.89 (61.32±0.52)

9.70 (9.40±0.23)

0.62

P4:IDIC

1.06 (1.06±0.01)

4.82 (4.85±0.10)

57.43 (56.76±1.15)

2.93 (2.91±0.07)

0.56

16.00

4.75

Figure 4. (a) Organic solar cells with a conventional architecture. (b) J–V curves and (c) EQE spectra of P1-P4:IDIC solar cells having the highest performance. (d) Jph as a function of effective voltage. Figure 4b presents the current density-voltage (J–V) curves of the optimized P1-P4:IDIC OSCs fabricated with a 1:1 polymer:IDIC weight ratio and using 0.3% (v/v) DIO additive. The detailed performance parameters under AM 1.5 G illumination (100 mW cm−2) are summarized in Table 2. The Vocs are 0.86 (P1), 0.91 (P2), 1.00 (P3), and 1.06 V (P4). Therefore, as the F number increases, the Voc becomes larger gradually, 14


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consistent with polymer HOMOs (Table 1). Moreover, F addition leads to Jsc enhancement from P1 to P3. The Jsc of P1:IDIC-based device is 8.53 mA/cm2, while P2:IDIC and P3:IDIC-based cells show improved Jscs of 13.15 and 15.99 mA/cm2, respectively. However, the P4:IDIC-based cell exhibits a small Jsc of 4.82 mA/cm2, hence further F addition results in a greatly reduced Jsc, which is ascribed to the smallest charge motilities (vide infra) and insufficient ∆EHOMO between P4 and IDIC, leading to suppressed hole transfer from IDIC to P4. Among all polymers, it is worthy to note that P3:IDIC can attain a large Voc of 1.00 V, a high Jsc of 15.99 mA/cm2, and a good FF of 60.89%, simultaneously, yielding a remarkable PCE of 9.7%. The PCE is among the largest values of WBG polymer donor materials based on quinoxaline (Figure 6a) reported to date.31, 34, 38 The external quantum efficiency (EQE) spectra of the optimal OSCs are illustrated in Figure 4c. All solar cells show a wide photoresponse from 300 nm to 800 nm, suggesting that both the donor polymer and IDIC acceptor contribute to Jscs on the basis of their absorption characteristics. The maximum EQE value reaches 47.3% (P1), 58.7% (P2), 71.0% (P3), and 22.5% (P4), and the Jsc integrated from EQE spectrum is 8.46 (P1), 12.88 (P2), 16.00 (P3), and 4.75 mA/cm2 (P4). The P3 Jsc (16.00 mA/cm2) from EQE spectrum is higher than that (15.99 mA/cm2) shown in the J–V curves and other Jscs show a mismatch < 2.5%, indicating the good reliability of the PCEs of these cells.

Charge Generation, Recombination, and Extraction To study photophysical processes of charge generation, recombination, and extraction, photocurrent density (Jph) as a function of effective voltage (Veff) was

15



investigated and the Jph-Veff curves are included in Figure 4d.59 In P1-P3:IDIC-based PSCs, Jph is saturated at a Veff = ~2V, over which all charge carriers are expected to be collected by electrodes with negligible charge recombination. The charge collection probability (Pc) is termed as Pc = Jph/Jsat at short-circuit condition, in which Jsat is the saturated current density. The derived Pc is 93.3% (P1), 97.0% (P2), and 96.7% (P3), however, for P4:IDIC-based device, no obvious Jph saturation was observed, suggesting significant charge recombination even under high reverse bias with Veff > 2 V,39 leading to its lowest Jsc among all the polymers. In addition, it should be pointed out that that fluorination can lead to the polymer dipole moment change between the ground state and the excited state, which shows profound effects on charge carrier recombination.29 Photoluminescence (PL) spectra of neat polymer film, IDIC film, and polymer:IDIC blend film were also characterized to investigate the exciton dissociation characteristics. Figure 5a-5d show the PL spectra obtained from the pristine and blend films by exciting at 532 nm, where donor polymers have intense absorption and IDIC shows weak absorption, hence the PL emission mainly comes from the donor polymers. It was found that neat polymer films show intense PL emission in the range of 600-800 nm, while the PL in their blend films with IDIC is greatly suppressed with a PL quenching efficiency (∆PL) > 95% (Table S6), indicating that the excitons formed in the polymer efficiently dissociate into free charge carriers via photo-induced electron transfer to IDIC in polymer:IDIC blends. Figure 5e-5h show the PL emission excited at 632 nm, where IDIC has intense absorption and the polymer donors show relatively weak absorption. Hence, the PL emission mainly comes from the IDIC acceptor. The PL in the blends is

16


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substantially quenched (relative to the neat IDIC PL) with a ∆PL of 94.9% (P1:IDIC), 86.1% (P2:IDIC), and 91.1% (P3:IDIC), suggesting favorable exciton dissociation via photo-induced hole transfer from IDIC to the donor polymers. However, the P4:IDIC blend still maintains a strong PL emission, showing a low ∆PL of 57.7%, due to the insufficient hole transfer from IDIC to P4, which is attributed to the small ∆EHOMO between P4 and IDIC.

Figure 5. Photoluminescence spectra of (a) P1 and P1:IDIC, (b) P2 and P2:IDIC, (c) P3 and P3:IDIC, (d) P4 and P4:IDIC films excited at 532 nm. (e) IDIC and P1:IDIC, (f) IDIC and P2:IDIC, (g) IDIC and P3:IDIC, (h) IDIC and P4:IDIC films excited at 632 nm. On the basis of Vocs and the band gap of IDIC, the photon energy losses (Elosss) are calculated according to the equation: Eloss=Eg - eVoc, when Eg is the smaller one between polymer and IDIC band gaps, therefore the Eg (1.62 V) of IDIC was used.60 The calculated Eloss is 0.76 (P1), 0.71 (P2), 0.62 (P3), and 0.56 eV (P4). Among the series, the P3:IDIC-based cells show a relatively small Eloss of 0.62 eV and the highest PCE of 9.7%. It is remarkable to note that this Eloss value is among the smallest ones achieved for IDIC-based OSCs and the cells attaining a high PCE of 9.7% (Figure 6b). Although

17



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the energy loss (0.56 eV) for P4:IDIC is the smallest in the series, the performance is highly limited by the small Jsc (4.82 mA/cm2), leading to the lowest PCE of 2.93%. The small Jsc is mainly attributed to the minimal ∆HOMO of 0.02 eV between P4 and IDIC, leading to an insufficient driving force for exciton dissociation via hole transfer.

41, 51,

61-63

Figure 6. (a) PCE vs E opt of quinoxaline-based WBG polymer donors from the references31, 34, 38 and this work. (b) PCE vs Eloss of IDIC-based OSCs from the references41, 51, 61-65 and this work. g

Polymer Charge Transport Properties The method of space charge limited current (SCLC) was employed using hole-only devices

with

a

structure

of

ITO/PEDOT:PSS/polymers:IDIC/MoOx/Ag66

and

electron-only devices with a structure of ITO/ZnO/ polymers:IDIC/PDINO/Al50 to characterize the hole (µh,sclc) and electron mobilities (µe,sclc) of the polymer:IDIC blends, which are critical for charge extraction and Jscs. The corresponding J1/2-Vappl characteristics are presented in Figure 7a and 7b and the derived µh,sclcs and µe,sclcs of the four polymer:IDIC blends are summarized in Table 3. The resulting µh,sclc/µe,sclcs are 2.25×10-4/1.08×10-4 (P1), 4.32×10-4/1.46×10-4 (P2), 5.53×10-4/1.78×10-4 (P3), and 1.01×10-4/1.37×10-5 cm2 V–1 s–1 (P4). By the addition of one or two F atoms in the

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polymer backbone, both hole and electron motilities increase substantially in the P2:IDIC and P3:IDIC blend films, which are accord with the measured Jsc variation. However, P4:IDIC blend film shows the smallest and most imbalanced µh,sclc and µe,sclc, which limited its Jsc in solar cells. Such µh,sclc and µe,sclc in combination with the insufficient HOMO offset between P4 and IDIC resulted in the smallest Jsc of 4.75 mA/cm2 among all polymers.

Figure 7. J1/2-V characteristics of (a) hole-only devices and (b) electron-only devices of four polymer:IDIC blends. Table 3. Summary of SCLC mobilities of four polymer:IDIC blends. Blend films

µh,sclc (cm2 V–1 s–1)

µe,sclc (cm2 V–1 s–1)

µh,sclc/µe,sclc

P1:IDIC P2:IDIC P3:IDIC P4:IDIC

2.25×10-4 4.32×10-4 5.53×10-4 1.01×10-4

1.08×10-4 1.46×10-4 1.78×10-4 1.37×10-5

1.57 2.96 3.11 7.37

Film Morphologies and Their Correlation to Solar Cell Performance For further understanding the OSC performance of these quinoxaline-based polymers, the neat and blend film morphologies were characterized by two dimensional-grazing incidence wide angle X-ray scattering (2D-GIWAXS). All samples were prepared using the identical conditions for optimal OSCs. Figure 8 presents the 2D-GIWAXS images as well as their corresponding in-plane (IP) and out-of-plane (OOP) 19



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line-cut profiles. The extracted 2D-GIWAXS scattering parameters are compiled in Table S7 and S8. The GIWAXS data reveal low crystallinity for the neat non-fluorinated polymer P1, which corroborates its highest twisted backbone (Figure 2). With increasing the number of F substituents on the phenylene moiety from P1 to P3, polymers achieved gradually increased backbone planarity enabled by noncovalent F⋅⋅⋅H and F⋅⋅⋅S coulombic interactions, which can yield strong interchain π-π stacking having enhanced face-on (010) scattering, as shown by the OOP diffraction patterns (Figure 8i). In addition, the IP lamellar peak also becomes more pronounced with F substitution. For example, P1 shows a negligible IP (100) peak (Figure 8a) and P2 exhibits a hump-like IP (100) peak (Figure 8b). However, P3 shows a well-defined IP (100) peak accompanied by diffraction progressing to a higher order (Figure 8c), which indicates its enhanced in-plane lamellar ordering. P3 shows more compact π-π stacking having a closer stacking distance (dπ-π) of 0.35 nm than other polymers in this series (Table S8) as the result of its improved chain planarity (Figure 2). For the blend films with IDIC, the out-of-plane (010) peak becomes stronger owing to the overlapping of π-π stacking peaks of IDIC and polymer.41 Except the P1 blend, the blends of other three polymers exhibit similar peak positions for (100) lamellar and (010) π–π stacking scatterings in neat films. With increasing fluorine substitution, the enhanced in-plane (100) lamellar scattering was measured together with the strong out-of-plane (010) peak. Based on the diffraction patterns, the polymers P2-P4 mainly adopt a face-on predominant bimodal orientation in both neat and blend films; such orientation should be beneficial to charge carrier extraction in a vertical direction in solar cells. By using Scherrer equation,67,

68

the

crystal coherence lengths (CCLs) were calculated based on the full width at half maximum (FWHM) of the in-plane (100) peak and out-of-plane (010) scattering peaks and the CCL data are included in Table S7 and S8. The CCLs based on the (010)

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diffractions in the out-of-plane direction are 4.0 (P1:IDIC), 4.9 (P2:IDIC), 5.2 (P3:IDIC), and 6.1 nm (P4:IDIC), therefore, the CCLs in the blends gradually increases with increasing degree of fluorination on the phenylene moiety. Although the P4:IDIC blend shows the largest CCL (6.1 nm), its insufficient driving force for exciton separation from IDIC to P4 leads to a poor Jsc of 4.82 mA/cm2 and a PCE of 2.93%.

Figure 8. 2D-GIWAXS images of (a-d) neat polymer films and (e-h) polymer:IDIC blend films. (a) neat P1 film, (b) neat P2 film, (c) neat P3 film, (d) neat P4 film, (e) P1:IDIC blend film, (f) P2:IDIC blend film, (g) P3:IDIC blend film, and (h) P4:IDIC blend film. IP and OOP line-cut profiles of (i) neat polymer films and (j) polymer:IDIC blend films. The polymer:IDIC blend morphologies were further characterized using atomic force microscopy (AFM) in tapping mode and transmission electron microscopy (TEM). 21



Figure 9a-9d presents the AFM height images of polymer:IDIC blends and all four blends exhibit small root-mean-square surface roughness (Rq) from 1.54 nm to 2.82 nm. Among them, the P3:IDIC blend shows the highest Rq of 2.83 nm, which is likely attributed to the slightly stronger interchain aggregation and higher crystallinity of P3, relative to other polymers. The AFM phase images are

Figure 9. Tapping-mode AFM (a-d) topography and (e-h) phase images (3 µm × 3 µm) and (i-l) TEM images of (a, e, i) P1:IDIC, (b, f, j) P2:IDIC, (c, g, k) P3: IDIC, and (d, h, l) P4:IDIC blend films.

illustrated in Figure 9e-9h, among them, the P3:IDIC blend shows more structured surface features, in good agreement with its height image. Among all blends, the relatively large phase separation (Figure 9i) partially attributed to its small Jsc in P1-based OSC. Except to the P1 blend, the TEM images of all other three blends show a bicontinuous network having a phase separation at nanoscale without large aggregates,

22


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suggesting sizable D:A interfaces for efficient exciton fission and bicontinuous channels for charge transport. Although Figure 9c and 9g show the more intense aggregation for the P3:IDIC blend film, nanoscale phase separation shown in Figure 9k indicates that the aggregation does not sacrifice D/A interface for exciton separation, yielding efficient PL quenching shown in Figure 5c and 5g. The AFM height, phase and TEM images together demonstrate that by fluorination of the phenylene moiety, the enhanced intermolecular H…F and S…F attractions in P2-P4 promote polymer ordering without destructing nanoscale bicontinuous morphology, which is beneficial to charge carrier mobilities and Jscs. The fine film morphology shown in Figure 9d, 9h, and 9l also suggest that the greatly reduced performance of tetrafluorinated polymer P4 blend (versus P3 blend) likely originate from its mismatched HOMOs.

Conclusion In summary, four quinoxaline and thiophene-flanked phenylene-based copolymers were synthesized and all show wide band gaps (1.87-2.00 eV). Fluorination of phenylene moiety can effectively tune the polymer FMO levels, molecular packing, and film morphology. By adding different F atom number on the phenylene moiety，the HOMO energy levels of polymer semiconductors can be effectively tuned from -5.17 eV to -5.68 eV, and the GIWAXS data show distinctive π-π staking along the out-of-plane direction. When combined with IDIC, the P1-P4:IDIC-based non-fullerene solar cells show a gradually increased Voc from 0.86 to 1.06 V and a decreased Eloss from 0.76 to 0.56 eV, depending on the number of F substitution. In particular, the difluorinated polymer P3 shows a more planar backbone compared to other three analogues and well-matched

23



FMO levels with IDIC, leading to a sizable Voc of 1 V. Notably, this Voc is one of the largest values among IDIC-based non-fullerene OSCs, leading to a small Eloss of 0.62 eV and achieving a high PCE of 9.7% under a small driving force of 0.16 eV for hole transfer. The results show that developing energetically matched WBG polymer donors for specific NBG acceptors is critical for performance enhancement and fluorination is proved to be a facile strategy to realize this goal. Moreover, the structure-property correlations provide useful insights for developing WBG polymers to minimize Elosss and to maximize Vocs in nonfullerene-based OSCs for efficient power conversion under small driving force.

Experimental Section Materials and Methods Dichloromethane, acetonitrile, toluene, and anhydrous tetrahydrofuran were dried before use according to the typical protocols. All reactions were conducted using the Schlenk line technique unless otherwise specified. The materials chemical structures are confirmed using elemental analysis (Center of Analysis, Shenzhen University) and 1H and

13

C NMR spectrometers (Bruker Ascend 400 or 500 MHz). GPC was utilized to

determine the molecular weight of polymers with the polystyrene standard and o-dichlorobenzene eluent at 80 °C. The polymer physicochemical and optoelectronic properties were characterized using DSC (Mettler, STARe, TA Instrument), TGA (Mettler, STARe, TA Instrument), UV-vis absorption spectra (Shimadzu UV-3600 UV-VIS-NIR spectrophotometer), CV (CHI660A), and two-dimensional grazing incidence wide-angle X-ray diffraction (Pohang Accelerator Laboratory).

24


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Organic Solar Cell Fabrication and Characterization Inverted Solar Cells. The substrates were cleaned and treated following our previous procedure25 and the ZnO interfacial layer was prepared and deposited according to the published procedure.69 For the deposition of the active layer, the polymer:IDIC blend solution was stirred for 12 h at room temperature to reach complete dissolution and then the solution was dropped quickly onto rotary ZnO/ITO/glass. Finally, the device was completed by evaporating MoOx (~10 nm) and Ag (~100 nm), featuring an effective area of 0.045 cm2. For fully optimizing the device performance, polymer:IDIC ratios (1.5:1-1:1.5), polymer concentrations (5, 6, or 7 mg/mL), and various solvent additives were systematically probed. Conventional Solar Cells. The substrates were cleaned and treated following our previous procedure.25 The PEDOT:PSS interfacial layer was spin-coated at a rate of 3000 rpm and annealed under 150 °C for 15 min. Inside a glove box, the polymer:IDIC active layer and the PDINO interfacial layer (~ 5nm)42 were spin-coated atop PEDOT:PSS, sequentially. The solar cells were completed through deposition of Al cathode, featuring an active area of 0.045 cm2. To optimize the solar cell performance, different solvents and interfacial layers were tested, and results showed that PDINO in ethanol solution afforded the optimal performance. ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge on the ACS Publications website. Experimental details, synthesis and characterization of intermediates and polymers, TGA and DSC data, DFT calculations, and 2D-GIXRD packing parameters.

25



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AUTHOR INFORMATION: Corresponding Author * E-mail: [email protected] * E-mail:[email protected] ORCID Han Young Woo: 0000-0001-5650-7482 Xugang Guo: 0000-0001-6193-637X Author Contributions ||

Jie Yang and Mohammad Afsar Uddin are contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS X.G. is grateful to the National Science Foundation of China (21774055), Shenzhen Peacock Plan Project (KQTD20140630110339343), Shenzhen Basic Research Fund (JCYJ20160530185244662),

Guangdong

Natural

Science

Foundation

(2015A030313900), and South University of Science and Technology of China (FRG-SUSTC1501A-72). M. A.U. and H.Y.W. acknowledge the financial support from the NRF of Korea (2015R1D1A1A09056905, 20100020209) and Korea University Grant.

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Quinoxaline-Based Wide Band Gap Polymers for Efficient

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