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Nonfullerene Acceptor with “Donor-Acceptor Combined #Bridge” for Organic Photovoltaics with Large Open-Circuit Voltage Yang Yang, Jiacheng Wang, Han Xu, Xiaowei Zhan, and Xingguo Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04541 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Applied Materials & Interfaces

Nonfullerene Acceptor with “Donor-Acceptor Combined π-Bridge”

for

Organic

Photovoltaics

with

Large

Open-Circuit Voltage Yang Yang,1 Jiacheng Wang,1 Han Xu,1 Xiaowei Zhan2* and Xingguo Chen1*

1

Hubei Key Laboratory on Organic and Polymeric Opto-electronic Materials, College

of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China. Email: [email protected] 2

Department of Materials Science and Engineering, College of Engineering, Peking

University, Beijing 100871, China. Email: [email protected]. KEYWORDS: molecular design, D-A combined π-bridge, nonfullerene fused-ring electron acceptor, high open-circuit voltage, organic solar cell ABSTRACT

In this work, a kind of “D-A (Donor-Acceptor) combined π-bridge” based on the regioselective reactivity of monofluoro-substituted benzothiadiazole (FBT) to link a thiophene ring has been designed to construct a new A-π-D-π-A type small molecular acceptor (IDT-FBTR) with indacenodithiophene (IDT) as a central core (D) and 3-octyl-2-(1,1-dicyanomethylene)rhodanine as an electron-withdrawing terminal group (A). Due to the strong intramolecular push-pull electron effect, IDT-FBTR shows a strong and broad intramolecular charge transfer (ICT) absorption band in the 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

range of 500-750 nm. Especially, as an electron-deficient FBT unit (A’) and an electron-rich thiophene ring (D’) in “D-A combined π-bridge” exert an “offset effect” to regulate the HOMO-LUMO energy levels of the molecule, a relatively high LUMO energy level can be maintained for IDT-FBTR that is helpful to enhance the open circuit voltage (Voc) for highly efficient OSCs. Therefore, the optimized OSC device based on IDT-FBTR as the acceptor and PTB7-Th as the donor shows a much high Voc of 1.02 V with a relatively low Eloss of 0.56 eV and best PCE of 9.14%.

INTRODUCTION

Organic solar cells (OSCs) have received much attention as a kind of clear energy device due to the advantages of low-cost, solution processability, flexibility, visible transparency and large-area fabrication etc.1-5 Benefiting from the rational molecular design of new excellent electron-donating materials or effective strategy for device fabrication, the high power conversion efficiency (PCE) up to 11% and even 12% has been achieved using the fullerene derivatives (such as PC61BM and PC71BM) as the electron-accepting materials.6-10 As known, the fullerene electron-accepting materials have some excellent characteristics such as high electron mobility and isotropic electron transporting ability.11,12 Despite that, they also have some obvious drawbacks, such as weak absorption in the visible solar spectrum region, difficulty for tuning the electronic structure and poor solubility as well as high production cost.2,13,14 To overcome these drawbacks of the fullerene electron-accepting materials, one of the effective strategies is to develop 2 ACS Paragon Plus Environment

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the novel non-fullerene accepting materials, especially the small molecular acceptors (SMAs), since SMAs have many advantages, such as good structural modification, easy synthesis, good solubility, strong absorption complementary to that of the electron-donating material in the absorption region.15-17 At present, there are two categories of promising SMAs that has been widely studied and achieved rapid development in recent years. One is the perylene diimide (PDI) based SMAs 18-20

, and the other is fused-ring electron acceptors (FREAs)

21-45

. Because of the

appealing features like strong electron affinity, good photochemical stability and easy functionalization, PDI unit has been widely used as building block to construct the SMAs. Many PDI-based SMAs with novel structural features such as the three-dimensional (3D) or helical-type structures have been designed to avoid the strong aggregation tendency, and the corresponding OSCs devices exhibited excellent photovoltaic performances with high PCE of over 9%.18-20 At the same time, the FREAs have received the more worldwide attention since the first

FREA

(namely

ITIC)

based

(indacenodithieno[3,2-b]thiophene,

on

a

IT)

bulky

fused-ring

end-capped

core with

2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) unit has been designed and synthesized by Zhan’s group.21 Compared with fullerene based acceptors, FREAs exhibited many advantages, including high electron mobility, strong and broad absorption in the visible and near-infrared region, tunable and appropriate energy levels and easy synthesis and purification. Thus, great efforts have been focused on designing new kind of FREAs through the structural modification of the 3 ACS Paragon Plus Environment


electron-withdrawing terminal group22-25 or fused-ring electron-donating core22,26-34 to tailor the electronic and optical properties. Up till now, the corresponding single-junction OSCs based on some FREAs have exhibited high power conversion efficiency (PCE) over 13 % which have greatly exceeded the fullerene based OSCs.34,35

As known, benzothiadiazole (BT) unit as an electron-deficient unit has been applied to build p-type D-A copolymers as the electron-donating materials with the PCBM as an electron-accepting material for the OSCs with high PCEs.6,8 Besides, the BT unit has also been used to design new kind of SMAs for the high performance OSCs. Due to its moderate electron-withdrawing capability, BT-based SMAs generally show relatively high LUMO (lowest unoccupied molecular orbital) energy. For example, BT unit as the second electron-accepting unit has been acted as a “π-bridge” to link the central electron-donating unit (D) and the terminal electron-accepting group (A1) for constructing the SMAs with the linear structure of A1-A2-D-A2-A1.36-50 The firstly reported BT based A1-A2-D-A2-A1 type SMA was called FBR that was designed and synthesized by McCulloch and co-workers, in which the fluorene unit was chosen as the central donor (D). The OSCs based on FBR as an electron-accepting material to match with P3HT as an electron-donating material exhibited a high PCE of over 4.11%.36 Later, the further structural modification for FBR was performed through the substitution of fluorene core by some other fused conjugated heterocyclic aromatic hydrocarbons, such as indacenodithiophene with different side-chains including alkylphenyl chain 4 ACS Paragon Plus Environment

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(IDT-2BR)37 or long alkyl chain (O-IDTBR)38, 3D spirobiﬂuorene core39 and angular-shaped dithienonaphthalene (DTNR)40. And these corresponding SMAs based OSCs exhibited very excellent photovoltaic performances with the large Voc in the binary and ternary solar cells41-43. In addition, the BT derivatives, such as BTA44,45, TbT46 or FBT47,48, has also been introduced as the second acceptor (A2) to effectively regulate the optical and electronic properties of BT based A1-A2-D-A2-A1 type SMAs with TD unit49 or IC unit50,51 as the first acceptor (A1).

As known, two bromo groups at 4,7-positions of BT unit shows the regioselective reactivity after introducing a fluorine atom (F) at its 5-position, which has been widely used to design the regionregular polymers and small molecular donors.52 Moreover, the F atom has little steric hindrance effect that does not destruct the molecular planarity of conjugation backbone. Furthermore, through constructing the noncovalent F-S of F-H bond, it can greatly promote and strengthen the intermolecular interaction.22,

53

Thus, the introduction of F

atom into conjugation backbone is much favorable to the improvement of the charge transport and the short circuit current density (Jsc). However, due to its strong electron-withdrawing ability, the fluorination at the molecular backbone in SMAs could lower the HOMO-LUMO energy levels. The reduction of the LUMO level often means a drop of the open circuit voltage (Voc) in the OSCs 22, thus hinders the further improvement of the photovoltaic performances. In order to increase the Voc, one of the strategies is to uplift the LUMO energy levels of the acceptors. Generally, 5 ACS Paragon Plus Environment


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the introduction of electron-rich group into molecular conjugation backbone can raise the HOMO-LUMO energy levels, so it has been proven to be an effective approach in the molecular design. For example, the electron-rich thiophene ring as a π-bridge had been

introduced

to

connect

the

central

electron-donating

core

and

the

electron-withdrawing terminal group, which can effectively enhance the Voc and improve the PCE.54, 55 Therefore, it is very important to search a balance between the Voc and Jsc for improving the photovoltaic performance through rational molecular design to regulate the HOMO-LUMO energy levels of the SMAs. Considering that an electron-deficient accepting unit (A’) and electron-rich donating unit (D’) show an “offset effect” to regulate the HOMO-LUMO energy levels, we propose a new molecular design strategy of “ Donor-Acceptor (D-A) combined π-bridge” to construct the A-π-D-π-A type small molecular acceptor. Herein, a kind of “D-A combined π-bridge” has been designed based on the regioselective reactivity of monofluoro-substituted benzothiadiazole (FBT) unit (A’) to connect the thiophene ring (D’). And it has been applied as a π-bridge to link indacenodithiophene

(IDT)

central

core

(D)

and

3-octyl-2-(1,1-dicyanomethylene)rhodamine (ORCN) terminal group (A) to design a new A-π-D-π-A type small molecular acceptor (IDT-FBTR), in which an electron-rich thiophene ring between FBT and ORCN can upshift the LUMO energy level of the molecule and offset the downshift of LUMO energy level with the introduction of a F atom at BT unit. In this work, the synthesis and structural characterization for IDT-FBTR has been presented, and the photophysical and 6 ACS Paragon Plus Environment

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electrochemical properties of IDT-FBTR has been studied. At the same time, the photovoltaic performances of the OSCs with IDT-FBTR as the acceptor and PTB7-Th as the donor have been detailed investigated. The device exhibits a maximum PCE of 9.14% with a relatively high open circuit voltage (Voc) of 1.02 V due to the relatively high LUMO energy level of the acceptor.

EXPERIMENTAL

Synthetic procedures

All of the chemicals were purchased from commercial source and used as received unless otherwise stated.

Synthesis

of

7,7'-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithioph ene-2,7-diyl)bis(4-bromo-5-fluorobenzo[c][1,2,5]thiadiazole) (III)

4,7-dibromo-5-fluorobenzo[c][1,2,5]thiadiazole

(I)

(444

mg,

1.42

mmol),

(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7diyl)bis(trimethylstannane) (II) (439.2 mg, 0.36 mmol) and Pd(PPh3)4 (21 mg, 0.018 mmol) were dissolved in toluene (10 mL) under argon atmosphere. The reaction mixture was heated to reflux for 48 h. After cooling to the room temperature, the solvent was removed under vacuum. The mixture was purified by column chromatography over silica gel using petroleum ether/CH2Cl2 (1.5:1) as eluent to give compound (III) as a reddish brown solid (434 mg, 89.0 %). 1H-NMR (400 MHz, 7 ACS Paragon Plus Environment


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CDCl3, δ ppm): 7.89 (s, 2H), 7.48 (s, 2H), 7.44 (d, J = 12 Hz, 2H), 7.19 (d, J = 8 Hz, 8H), 7.03 (d, J = 8 Hz, 8H), 2.49 - 2.45 (m, 8H), 1.49 - 1.47 (m, 8 H), 1.23 - 1.17 (m, 27 H), 0.77 - 0.74 (m, 12 H). 13C-NMR (100 MHz, CDCl3, δ): 161.99, 159.49, 157.01, 154.27, 154.19, 154.04, 148.72, 144.89, 141.86, 141.46, 139.94, 139.91, 135.58, 128.57, 127.94, 124.61, 118.11, 115.08, 114.77, 95.72, 95.48, 63.22, 35.63, 31.76, 31.43, 29.20, 22.65, 14.16. MS (MALDI-TOF): calculated for C76H74Br2F2N4S4, 1368.3; found: 1368.0 (M+).

Synthesis

of

5,5'-((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiop hene-2,7-diyl)bis(5-fluorobenzo[c][1,2,5]thiadiazole-7,4-diyl))bis(thiophene-2-car baldehyde) (V)

(III) (140 mg, 0.10 mmol), 5-(tributylstannyl)thiophene-2-carbaldehyde (IV) (164 mg, 0.41 mmol) and Pd(PPh3)4 (6 mg, 0.005 mmol) were dissolved in toluene (10 mL) under argon atmosphere. The reaction mixture was heated to reflux for 48 h. After cooling to the room temperature, the solvent was removed under vacuum. The mixture was purified by column chromatography over silica gel using CHCl3 as eluent to give (V) as a dark brown solid (122 mg, 85.2 %). 1H-NMR (400 MHz, CDCl3, δ ppm): 10.01 (s, 2H), 8.35 (d, J = 4 Hz, 2H), 8.10 (s, 2H), 7.86 (d, J = 4 Hz, 2H), 7.78 (d, J = 12 Hz, 2H), 7.57 (s, 2H), 7.26 (d, J = 8 Hz, 8H), 7.13 (d, J = 8 Hz, 8H), 2.60 2.56 (m, 8H), 1.62 - 1.57 (m, 18 H), 1.34 - 1.26 (m, 26 H), 0.88 - 0.85 (m, 12 H). Anal. Calcd for C86H80F2N4O2S6: C, 72.13; H, 5.63; N, 3.91. Found: C, 72.15; H, 5.64; N, 8 ACS Paragon Plus Environment

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4.12. MS (MALDI-TOF): calculated for C86H80F2N4O2S6, 1430.5; found: 1430.8 (M+).

Synthesis

of

2,2'-((5E,5'E)-((((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5, 6-b']dithiophene-2,7-diyl)bis(5-fluorobenzo[c][1,2,5]thiadiazole-7,4-diyl))bis(thio phene-5,2-diyl))bis(methanylylidene))bis(3-octyl-4-oxothiazolidine-5,2-diylidene) )dimalononitrile (IDT-FBTR)

(V) (122 mg, 0.085 mmol), 2-(3-octyl-4-oxothiazolidin-2-ylidene)malononitrile (VI) (94 mg, 0.34 mmol), 0.5 mL triethylamine and 20 mL chloroform were added to a 100 mL round-bottomed flask. The solution was heated to 70 °C overnight. After cooling to room temperature, the mixture was poured into water and extracted with chloroform and purified by column chromatography using petroleum ether/CHCl3 (1:2) as eluent to yield IDT-FBTR (83 mg, 50.2 %) as a dark brown powder. 1H-NMR (400 MHz, CDCl3, δ ppm): 8.36 (d, J = 4 Hz, 2H), 8.12 (s, 4H), 7.76 (d, J = 12 Hz, 2H), 7.59 (s, 2H), 7.56 (d, J = 4 Hz, 2H), 7.28 (d, J = 8 Hz, 8H), 7.14 (d, J = 8 Hz, 8H), 4.24 - 4.20 (m, 4H), 2.60 - 2.56 (m, 8H), 1.77 - 1.74 (m, 4H), 1.62 - 1.56 (m, 12 H), 1.38 - 1.25 (m, 54 H), 0.90 – 0.84 (m, 20 H). 13C-NMR (100 MHz, CDCl3, δ ppm): 166.05, 165.58, 161.42, 158.85, 157.37, 154.31, 152.90, 152.79, 149.33, 145.71, 141.93, 141.75, 141.39, 140.45, 140.42, 138.01, 137.94, 135.72, 135.42, 131.40, 131.30, 128.75, 128.62, 128.40, 127.94, 125.05, 118.21, 115.16, 114.84, 114.38, 113.29, 112.33, 109.34, 109.19, 63.24, 56.00, 45.44, 35.64, 31.76, 31.43, 29.75, 29.20, 9 ACS Paragon Plus Environment


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29.14, 29.11, 28.86, 26.00, 22.66, 22.65, 14.16. Anal. Calcd for C114H114F2N10O2S8: C, 70.19; H, 5.89; N, 7.18. Found: C, 70.07; H, 6.09; N, 7.19. MS (MALDI-TOF): calculated for C114H114F2N10O2S8, 1949.7; found: 1949.2 (M+).

RESULT AND DISCUSSION

Synthesis and Characterization

Scheme 1 describes the synthetic routes for IDT-FBTR in detail. Compound (III) was synthesized easily via Stille cross-coupling reaction of (I) and (II) in high yield of 89.0 %, in which the regioselectivity of the reaction was controlled by the different reactivity of two bromo groups at monofluoro-substituted benzothiadiazole (FBT) unit. Compound (III) was reacted with (IV) via Stille coupling to give V, in which a "D-A combined π-bridge" has been constructed with the combination of a FBT unit and a thiophene ring. Due to the great molecular coplanarity and large rigid conjugation backbone, compound (V) showed poor solubility in some common organic solvents including CHCl3, CH2Cl2 and THF etc. To ensure that the molecule has

good

solubility,

a

long-chain

alky

group

was

introduced

into

2-(1,1-dicyanomethylene)rhodanine unit to yield (VI), which was chosen as the terminal electron-withdrawing group. The target small molecular acceptor, namely IDT-FBTR, was finally obtained by Knoevenagel condensation of (V) with (VI).

10 ACS Paragon Plus Environment

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Scheme 1. The synthetic route for IDT-FBTR.

It should be emphasized that the regiochemistry of (III) is confirmed by the 2D NMR spectroscopy as shown in the Figure S1 in supporting information (SI). It can be seen that two protons at FBT unit and thiophene ring of IDT unit show a double peak with a chemical shift of 7.49 ppm and a single peak with a chemical shift of 7.97 ppm, respectively.

IDT-FBTR can be easily soluble in many common solvents such as CHCl3, CH2Cl2, chlorobenzene and o-dichlorobenzene at room temperature. The good solubility is very important for solution-processed fabrication of photovoltaic devices. In addition, the thermal stability of IDT-FBTR is determined by thermogravimetric analysis (TGA). It shows much high thermal stability with 5% weight loss beyond 390 °C as shown in Figure S2. Also, the differential scanning calorimetry (DSC) is 11 ACS Paragon Plus Environment


performed as shown in Figure S3 and the glass transition temperature (Tg) of IDT-FBTR is observed at about 153 °C.

Photophysical and electrochemical properties

The normalized absorption spectra of IDT-FBTR in the solution and IDT-FBTR, PTB7-Th as well as IDT-FBTR:PTB7-Th (the weight ratio = 1.5:1) blend in the film are shown in Figure 1a. As can be seen, IDT-FBTR solution displays two main absorption regions in 300-500 nm and 500-700 nm, respectively. The weak absorption bands observed in 300-500 nm are correlated to the localized π-π* transition, and the very strong and broad absorption band in 500-700 nm with a maximum at 626 nm can be ascribed to the intramolecular charge transfer (ICT) effect. Compared with the absorption in solution, IDT-FBTR exhibits a more broadened ICT absorption band with a large red-shift, and the absorption onset of IDT-FBTR is estimated to be about 740 nm corresponding to optical gap (Egopt) of 1.67 eV. Moreover, an obvious absorption shoulder around 620 nm is observed at the absorption maximum of 664 nm, indicating that there is strong intermolecular interaction and significant molecular self-organization in the IDT-FBTR crystallite.21,

56

For IDT-FBTR:PTB7-Th blend

film, a very strong and broad absorption is observed in the range of 500-800 nm, which almost covers the absorption spectra of IDT-FBTR and PTB7-Th, indicating that it can efficiently absorb the sunlight in this range.

The charge transporting property of the pure film of IDT-FBTR measured by the space-charge limited current (SCLC) method is shown in Figure S4. It exhibits a high 12 ACS Paragon Plus Environment

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electron mobility of 2.7×10-4 cm2 V–1 s–1 that can ensure effective charge carrier transport in the blend film.

The cyclic voltammetry (CV) was performed to estimate the energy levels of IDT-FBTR. As shown in Figure 1b, the initial oxidation and reduction potential are measured to be 1.13 V and -0.69 V versus Ag/AgCl, respectively. The corresponding HOMO and LUMO energy levels determined from the initial oxidation and reduction potential are about -5.47 eV and -3.65 eV, respectively, which still maintains at the relatively high levels that can be comparable to IDT-2BR with the HOMO/LUMO energy levels at -5.52/-3.69 eV37, although a very strong electron-withdrawing dicyanomethylene

group

at

the

rhodanine

unit

and

another

strong

electron-withdrawing F atom at the BT unit are simultaneously introduced into the conjugation backbone of IDT-FBTR.

As known, the introduction of strong electron-deficient groups into the conjugation backbone can significantly reduce the HOMO-LUMO energy levels of the molecule. Thus, in A1-A2-D-A2-A1 type acceptors, when the rhodanine terminal unit is replaced by a 2-dicyanomethylenerhodanine unit, the HOMO-LUMO energy levels always move down a lot, especially for LUMO energy levels, which has been demonstrated by our previously work57 as well as some other examples, such as IDT-BT-R and IDT-BT-R-CN acceptors58 and BTA1-BTA3 series acceptors45. Generally, the deeply lower LUMO energy level of the acceptor is much harmful to the enhancement of the open circuit voltage (Voc) for the OSCs. Therefore, in the 13 ACS Paragon Plus Environment


molecular design of IDT-FBTR, an extra electron-rich thiophene ring (D) is introduced to build a “D-A combined π-bridge” with FBT unit (A) since the electron-rich group can uplift the HOMO-LUMO energy levels. Benefited from the “offset effect” of D and A units in “D-A combined π-bridge” to regulate the HOMO-LUMO energy levels, IDT-FBTR shows a relatively high LUMO energy level, which can guarantee the OSCs with a high open circuit voltage confirmed by the later device tests.

1.0

(a)

(b)

ferrocene

IDT-FBTR Sol IDT-FBTR Film

0.8

PTB7-Th Film

Current (a.u.)

Normalized Absorption (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

Page 14 of 32

IDT-FBTR:PTB7-Th Film

0.6 0.4

IDT-FBTR 0.2 0.0 300

400

500

600

700

-1.5

800

-1.0

-0.5

Wavelength (nm)

0.0 0.5 Potential (V)

1.0

1.5

Figure 1. The absorption spectra (a) of IDT-FBTR (black and red), PTB7-Th film (green) and IDT-FBTR:PTB7-Th blend film (pink) and CV curve (b) of IDT-FBTR.

Photovoltaic performances

The inverted bulk heterojunction OSCs with the configuration of ITO/ZnO/ IDT-FBTR:PTB7-Th/MoO3/Ag were built to evaluate the photovoltaic performances of IDT-FBTR as the electron acceptor (A) with PTB7-Th as the electron donor (D). The active layer was fabricated by spin-coating the mixture of IDT-FBTR:PTB7-Th in o-dichlorobenzene (o-DCB) with a concentration of 20 mg mL-1 in total on the ZnO. 14 ACS Paragon Plus Environment

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The optimization for the ratio of A:D and the different solvent additive have been performed to investigate the photovoltaic performances of the devices, and the corresponding data are listed in Tables S1, S2 and S3. Figure 2a and 2b show the current density-voltage (J-V) and external quantum efficiency (EQE) curves of the devices under the optimized condition, and the results are given in Table 1. As can be seen, the OSC device with the A/D of 1.5:1 exhibit the best PCE of 7.01±0.24% with Jsc of 12.95±0.39 mA cm-2, FF of 52.97±1.55%, and Voc of 1.02±0.01 V. This large Voc value is close related to the relatively high LUMO energy level of IDT-FBTR as discussed above. After addition of 5.5% 1-chloronaphthalene (CN) into the mixture under this optimal D/A ratio, the Jsc of 12.95±0.39 mA cm-2 is increased to 15.18±0.37 mA cm-2 and the FF of 52.97±1.55% is also increased to 57.55±0.88, leading to an increased PCE of 8.92±0.13%. After further optimization for the devices, the best PCE based on the IDT-FBTR:PTB7-Th reaches 9.14% with the Jsc of 15.46 mA cm-2, FF of 57.97 % and Voc of 1.02 V. This Voc is higher than that of OSCs using the PTB7-Th as the donating material and PC71BM as the accepting material, owing to the higher LUMO energy level of IDT-FBTR.40 The photon energy loss (Eloss) of the IDT-FBTR:PTB7-Th based system is 0.56 eV estimated by Eloss = Eg-eVoc, where the Eg is 1.58 eV determining from the optical bandgap of the blend film as shown in Figure 1a.

The increased Jsc with the addition of CN to the mixture could be identified by the EQE spectra. A shown in Fig. 2b, the blend film without additive shows a photo-response from 300 to 800 nm with the maximum EQE value only reaching 45% 15 ACS Paragon Plus Environment


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and 65% in the region of 300-500 nm and 500-800 nm, respectively. However, after the addition of CN, the photo-response of the blend film for EQE has been significantly enhanced in the region of 300-800 nm with the maximum EQE value reaching 74%. It should be noted that the photo-response in the region of 500-800 nm is much higher than that in the region of 300-500 nm regardless of the addition of CN, which can be ascribed to the stronger absorption of IDT-FBTR and PTB7-Th in the region of 500-800 nm. The calculated Jsc values from the integration of EQE data with the AM 1.5 G reference spectrum are about 15.69 and 13.14 mA cm-2, respectively, for the devices with and without CN additive (the errors were blow 3.5 %).

Table

1.

Photovoltaic

performance

parameters

of

the

OSCs

based

on

IDT-FBTR:PTB7-Th blend. Additive

Voc Jsc FF PCE -2 (V) (mA cm ) (%) (PCEmax)(%) None 1.02±0.01 12.95±0.39 52.97±1.55 7.01±0.24 7.31 5.5 % 1.02±0.00 15.18±0.37 57.55±0.88 8.92±0.13 CN (v/v) 9.14

µh*10−4 (cm2v−1 s−1) 3.1

µe*10−4 (cm2v−1 s−1) 0.68

3.3

1.5

The photocurrent density (Jph) versus the effective voltage (Veff) had been measured to investigate the charge generation and extraction properties as shown in the Figure 2c.59 Assuming that almost all excitons are dissociated and photogenerated charge carriers are completely collected by the electrodes at high Veff (Veff >2 V), the Jph reaches saturation (Jsat). As shown in Figure 2c, the slightly increment for the Jsat after addition of CN indicates that the charge generation may be improved by the CN treatment.30 Furthermore, the charge extraction under short-circuit condition can be 16 ACS Paragon Plus Environment

Page 17 of 32

characterized by the ratio of Jph to Jsat (Jph/Jsat).60 The calculated Jph/Jsat obtained from Figure 2c are 92.5% and 90.1% for the devices with and without CN additive, suggesting that efficient charge extraction occurs after CN treatment. We also measure the photocurrent Jsc under the different light intensity (P) to investigate the relationship between charge recombination and transmit in the photoconductive layer as shown in Figure 2d. The relationship between Jsc and P follows the formula Jsc ∝ PS. If the bimolecular recombination is retarded absolutely in a PSC device, an ideal S is equal to 1. Actually, the S value in IDT-FBTR:PTB7-Th based device with 5.5% CN is determined to be about 0.922, higher than 0.892 of the device without CN-treatment, indicating weaker bimolecular recombination takes place in the CN-treated device. The efficient charge extraction and weak bimolecular recombination are responsible for the increased Jsc and FF in the CN-treated device. b) 80

as cast w/ CN

70

0

60

as cast w/ CN

-5

50

IPCE(%)

Current Density (mAcm-2)

a)

-10

40 30 20

-15

10

-0.25

0.00

0.25

0.50

0.75

1.00

0 300

1.25

400

500

600

700

800

Wavelength(nm)

Voltage (V) c)

d)

10

as cast w/ CN

-2)

10

(mAcm

as cast w/ CN

1

sc

1

J

-2 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


0.1

0.1 0.01

0.1

1

1

10

Veff (V)

10

100

Light intensity (mW cm-2)

Figure 2. a) The J-V curves; b) The EQE curves; c) Photocurrent density (Jph) versus effective voltage (Veff) characteristics and d) Jsc versus light intensity (P) of the solar cells. 17 ACS Paragon Plus Environment


The photoluminescence (PL) spectra of pure and blend films with or without CN were measured to evaluate the exciton dissociation and charge transfer in the blend films as shown in Figure 3.61 The PL peak of IDT-FBTR is almost completely quenched by the polymer PTB7-Th without or with CN when excited at a wavelength of 640 nm. As seen in Figure 3b, the pure polymer PTB7-Th shows the strong PL peak in the range of 700-900 nm when excited at a wavelength of 680 nm. After blending with IDT-FBTR, the PL peak of PTB7-Th is about 80% and 87% quenched in the blend films without and with CN respectively. These results indicate that the exciton dissociation and charge transfer of the blend film are promoted effectively after addition of CN, which is also in good agreement with the higher Jsc and FF in the CN-treated device.

As shown in Figure 4, the charge transporting properties of the active layers was measured by the SCLC method, which can help to further understand the difference in Jsc and FF of the devices. The as-cast IDT-FBTR:PTB7-Th blend film exhibits hole mobility (µh) of 3.1 × 10-4 cm2 V–1 s–1 and electron mobility (µe) of 6.8 × 10-5 cm2 V–1 s–1 with µh/µe of 4.5. After addition of CN into the mixture, the blend film shows the similar µh to that of as-cast blended film, but a higher µe of 1.5 × 10-4 cm2 V–1 s–1 with µh/µe of 2.2 are obtained. Obviously, the larger charge mobility and much more balanced hole-electron transport property for the CN-treated device can be contributed to its higher Jsc and FF.21,61


Page 18 of 32

Page 19 of 32

a)

b)

@ 640 nm

PL intensity

PL intensity

@ 680 nm

IDT-FBTR as cast w/ CN

650

700

750

800

PCE10 as cast w/ CN

700

850

750

800

850

Wavelength (nm)

Wavelength (nm)

Figure 3. Photoluminescence spectra (a: excited at 640 nm; b: excited at 680 nm) of IDT-FBTR (excited at 640 nm) and PTBT-Th (excited at 680 nm) films as well as the blend films without and with CN (excited at 640 and 680 nm).

a) -30.0

b) -30.5

-30.5

as cast w/ CN

-31.0

as cast w/CN

-31.0

ln (Jd3/V2) (mAcm/V2)

ln (Jd3/V2) (mAcm/V2)

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


-31.5 -32.0 -32.5 -33.0

-31.5 -32.0 -32.5 -33.0

0

200

400

600

800

1000

0

100 200 300 400 500 600 700 800

(V/d)0.5 (V/cm)0.5

(V/d)0.5 (V/cm)0.5

Figure 4. Ln(JL3/V2) vs (V/L)0.5 of plot of the devices for the a) hole mobility measurement and b) electron mobility measurement.

Morphologies and microstructures

The morphologies of the IDT-FBTR:PTB7-Th blend film with and without CN were measured by the atomic force microscopy (AFM) and transmission electron microscopy (TEM) as shown in Figure 5. The root mean square (RMS) of the as-cast blended film is 1.084 nm, and a relative larger RMS (1.824 nm) is obtained after the 19 ACS Paragon Plus Environment


addition of CN. The larger RMS roughness means the increased contact area between the active layer and interfacial electrode, which is favorable to the charge collection.62 This is consistent with the investigation of charge extraction under short-circuit condition as just mentioned above. However, the large aggregation domains observed in the as-cast blended film (shown in Figure 5b) is harmful to exciton diffusion and separation. Compared with the as-cast blended film, the CN-treated blend film shows small and bicontinuous phase segregation. Moreover, the blend film with CN exhibits much smaller size and clearer phase separation measured by TEM (Figure 5f) than that of blend film without CN (Figure 5c), which is also in accord with the morphology observed by AFM. The bicontinuous and appropriate aggregation domains of the blend film with CN is beneficial to the exciton dissociation and charge transfer, leading to the higher Jsc, FF and PCE in the devices.

Figure 5. AFM height and phase images for IDT-FBTR:PTB7-Th blend films a, b) without additive and d, e) with CN; TEM images for PTB7-Th:IDT-FBTR blend films c) without additive and f) with CN. 20 ACS Paragon Plus Environment

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CONCLUSIONS

In this work, a strategy of “D-A combined π-bridge” has been applied to build a small molecular acceptor (IDT-FBTR), in which the combination of an electron-deficient FBT unit (A’) and an electron-rich thiophene ring (D’) has been acted as a π-bridge to link the IDT central core and ORCN terminal group. The photophysical, electrochemical and photovoltaic properties have been investigated in detail. IDT-FBTR exhibits strong ICT absorption in the range of 500-750 nm and a medium optical band gap of 1.67 eV. Because the coexistence of D and A units in “D-A combined π-bridge” exhibit an “offset effect” to regulate the HOMO-LUMO energy levels, IDT-FBTR possesses a relatively high LUMO energy level of -3.65 eV, which is the connection with a high Voc. Actually, the corresponding OSC devices based on the PTB7-Th as a donor and IDT-FBTR as an acceptor show a high Voc of 1.02 V with a relatively low Eloss of 0.56 eV. The best PCE of the devices is measured to be 9.14%. This suggests that FBT unit is an efficient electron-deficient unit to

construct a “D-A combined π-bridge” for the small molecular acceptor because of its regioselective reactivity at different positions, and further work should be done such as modifying the structure of the small molecular acceptors for high efficient OSCs.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications



website.

Details of device fabrication and optimization, fabrication of charge-only devices, 1

H-1H 2D NOE NMR spectrum of compound III, the TGA curve and DSC curve of

IDT-FBTR, tables of OPV performances based on IDT-FBTR:PTB7-Th.

ACKNOWLEDGEMENTS

We are grateful to the National Natural Science Foundation of China (Nos. 51173138) for financial support.

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5 Current Density (mAcm-2)

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PCE = 9.14% -5 -10 -15 -0.2

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0.2

0.4

0.6

0.8

Voltage (V)


1.0

for Organic Photovoltaics with Large Open-Circuit ... - ACS Publications

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