Quinoxaline-containing non-fullerene small molecular acceptors with

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Quinoxaline-containing non-fullerene small molecular acceptors with linear A2-A1-D-A1-A2 skeleton for poly(3-hexylthiophene)-based organic solar cells Bo Xiao, Ailing Tang, Jing Yang, Asif Mahmood, Xiangnan Sun, and Erjun Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00216 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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ACS Applied Materials & Interfaces

Quinoxaline-containing non-fullerene small molecular acceptors with linear A2-A1-D-A1-A2 skeleton for poly(3-hexylthiophene)-based organic solar cells Bo Xiao1,2, Ailing Tang1, Jing Yang1,2, Asif Mahmood1,2, Xiangnan Sun1, Erjun Zhou*1

1

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence

in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China. 2

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

E-mail: [email protected] Abstract We used the quinoxaline (Qx) unit to design and synthesize two non-fullerene small molecular acceptors (NFSMAs) of Qx1 and Qx1b with A2-A1-D-A1-A2 skeleton, where indacenodithiophene (IDT), Qx and rhodanine (R) were adopted as the central donor (D), bridge acceptors (A1) and terminal acceptors (A2). Qx1 and Qx1b contain the different side chains of 4-hexylphenyl and octyl in the central IDT segment to modulate the properties of final small molecules. Both small molecules show good thermal stability, high solubility, strong and broad absorption spectra with optical bandgaps of 1.74 eV and 1.68 eV, respectively. Qx1 and Qx1b exhibit the complementary absorption spectra with the classic poly(3-hexylthiophene) (P3HT) and the high-lying lowest unoccupied molecular orbital (LUMO) energy levels of -3.60 and -3.66 eV respectively. Polymer solar cell (PSC) based on P3HT:Qx1 showed a high open-circuit voltage (Voc) of 1.00 V and a power conversion efficiency (PCE) of 4.03%, while P3HT:Qx1b achieved a Voc of 0.95 V and a PCE of 4.81%. These results demonstrate that Qx unit is also an effective building block to construct promising n-type non-fullerene small molecules to realize a relatively high Voc and PCE for P3HT-based solar cell.

Keywords Non-fullerene acceptor; P3HT; photovoltaic cells; polymer solar cells; quinoxaline

1. Introduction 1

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In recent two decades, polymer solar cells (PSCs) have attracted extensive attention because of the advantages of low-cost, light weight and flexibility1-7. Especially in last three years, tremendous improvement has been witnessed due to substantial efforts of researchers and the power conversion efficiencies (PCEs) of PSCs have increased up to 13%8-12, where the development of non-fullerene (NF) electron-acceptor materials plays a vital role. In comparison with n-type polymer acceptors13-15, small molecular acceptors (SMAs) show unique advantages such as well-defined molecular structure, easy modification of absorption and energy levels, facile synthesis and purification16. Thus, the development of non-fullerene small molecular acceptors (NFSMAs) becomes one of the hottest field in PSCs and attract much attentions in both academy and industry. In order to commercialize the PSCs technology, besides the efficency and stability, the cost of photovoltaic materials should also be considered as an important factor. Among a large number of p-type polymers, regio-regular poly(3-hexylthiophene) (P3HT)

17-19

is a good

candidate because it is relatively stable, low-cost, easy synthsized and available in quantities over 10 kg20. Thus, the PSCs based on P3HT in conjunction with NFSMAs is an important research direction. And among them, n-type SMAs containing the strong electron-accepting segment of benzothiadiazole (BT)21-25 showed the high PCEs of 4.1-6.4% with adjustable open-circuit voltage (Voc) of 0.72~0.97 V. Because P3HT has its instinct characteristic of the high-lying lowest unoccupied molecular orbital (LUMO) level of ~3.1 eV, the design of NFSMAs with up-shifted LUMO level could be a good strategy to improve the Voc. According to our previous works, we utilized a weak electron-accepting building block of benzotriazole (BTA)26-27 to construct NFSMAs and the Voc could be further improved to 1.02 V for P3HT: BTA1 and 1.22 V for P3HT: BTA2 combination. Thus, the introduction of weaker electon-deficient segment is a convenient strategy to design NFSMAs with high-lying LUMO energy levels, which could improve the Voc of P3HT-based solar cells. It is well known that quinoxaline (Qx) is other important weak electron accepting building block and it has aroused widespread concern28. Qx unit has been copolymerized with many electron-donating units to obtain donor-acceptor (D-A) alternative p-type polymers, in which the highest PCE of 8.6%29 and 11.3%30 have been achieved with fullerene acceptor and non-fullerene acceptor, respectively. Furthermore, Qx was also used to construct p-type small 2


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molecules31-34, although the photovoltaic performance was not inferior with the highest PCE of 5.09%31. However, utilizing Qx to construct photovoltaic acceptor materials is rarely reported, and it was only used to modify the fullerene35. Actually, Qx unit has weaker electron-deficient ability in comparison with BT unit and the structure of Qx can also be easily fine-tuned. For example, by introducing the alkyl chain on 2, 3-positions and/or the fluorine atoms on 7, 8-positions, the solubility, energy levels and crystallinity could be well modified. Therefore, we designed two Qx-based SMAs by adopting the classic A2-A1-D-A1-A2 structure (there is a C=C double bond between A2 and A1), where indacenodithiophene (IDT), Qx and rhodanine (R) were used as central donor (D), bridge acceptor (A1) and terminal acceptor (A2), respectively. Meanwhile, to control the molecular packing and morphology of blending film, two kinds of side chains, 4-hexylphenyl and octyl, are attached in the central segment of IDT. The two SMAs, named as Qx1 and Qx1b, were synthesized and used in PSCs to combine with P3HT. The effect of side chains in IDT unit on the electronic and optoelectronic properties was also systematically investigated. Fullerene-free PSCs showed a high Voc of 1.00 V and a PCE of 4.03% for P3HT:Qx1, while a Voc of 0.95 V and a PCE of 4.81% for P3HT:Qx1b. These results indicate that Qx unit is an effective building block to construct promising n-type non-fullerene small molecular acceptors. 2. Results and discussion 2.1 synthesis and characterization of Qx1 and Qx1b

3



Scheme 1. Synthetic route of two Qx-based small molecule acceptors.

The chemical structures and synthetic methods of Qx1 and Qx1b are shown in scheme 1. The important intermediate of 8-bromo-2,3-diphenylquinoxaline-5-carbaldehyde (compound 2) was synthesized by converting one bromine in 5,8-bromo-2,3-diphenylquinoxaline (compound 1) to aldehyde group. Then by Stille coupling reaction of compound 2 and compound 3 (or 3b), compound 4 (or 4b) were obtained with two aldehydes as the end groups. And followed by Knoevenagel condensation using 3-ethylrhodanine, Qx1 and Qx1b could be obtained with 64% and 67% yields, respectively. The corresponding intermediates and final Qx1 and Qx1b were clearly characterized by 1H-NMR and MALDI-TOF MS, which were shown in Figure S1. Qx1 and Qx1b are stable with decomposition temperatures (with 5% weight loss) at 394 and 399 °C under nitrogen, as shown in Figure S2. And they exhibit high solubility in common organic solvents, such as chloroform (CF), toluene, chlorobenzene (CB) and o-dichlorobenzene (DCB). These characteristics make Qx1 and Qx1b as the suitable candidates for solution-processed photovoltaic devices. Density functional theory (DFT) calculations were carried out on the molecular structures by using Gaussian at the B3LYP/6-31G level of theory. As shown in Figure 1, these two molecules possess a linear molecular backbone. However, the phenyl groups on the Qx units exhibit a large steric hindrance with a dihedral angle of 56° or 54° for Qx1 and Qx1b, respectively, which 4


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slightly distort the plane between central IDT and end-capped units. Furthermore, Qx1b with four alkyl groups on the central IDT unit displays a less steric hindrance, which decrease the dihedral angles between Qx and IDT from 15o for Qx1 to 12o for Qx1b. Thus, this two Qx-based small molecules might demonstrate inferior electron transport properties in comparison with BT-based

22-24

and BTA-based small molecules26-27, 36, which possess a totally planar structure.

For the molecular frontier orbitals of Qx1 and Qx1b, the highest occupied molecular orbital (HOMO) is mainly distributed on the IDT unit, while the LUMO is delocalized on the whole backbone, which should be beneficial to electron transport. The calculated LUMOs and HOMOs are -3.11 and -5.16 eV for Qx1, while -3.14 and -5.18 eV for Qx1b, respectively. These calculated LUMOs are obviously higher than that of BT-based small molecules IDT-2BR (-3.62 eV) and O-IDTBR (-3.67 eV) 25, which could improve the Voc of P3HT-based solar cells.

Figure 1. (a) (b) Side view and top view of optimized geometries for Qx1 and Qx1b; (c) (d) optimized molecular orbitals of Qx1 and Qx1b. 5



Figure 2. (a) UV-vis absorption spectra of Qx1, Qx1b and P3HT in film; (b) the energy levels of P3HT, Qx1 and Qx1b.

The UV-vis absorption spectra of Qx1 and Qx1b in CHCl3 solution (~10-5 mol·L-1) are shown in Figure S3. Both the two small molecules demonstrate a broad and strong absorption with two absorption peaks at 403 and 610 nm for Qx1 with a maximum molar extinction coefficient (ε) of 0.83 × 105 L·mol-1·cm-1, while 403 and 631 nm for Qx1b with ε of 0.95 × 105 L·mol-1·cm-1. From Qx1 to Qx1b, the redshift of absorption peaks at long-wavelength and the improvement of ε indicate the smaller side chain in IDT unit could improve the absorption properties for photovoltaic application. However, the values of ε are slightly lower than those of BT-based (1.3× 105 L·mol-1·cm-1 for IDT-2BR22) and BTA-based small molecules (1.3× 105 and 1.1× 105 L·mol-1·cm-1 for BTA126 and BTA2

27

, respectively) which should come from the

inferior molecular planarity. The photoluminescence (PL) spectra of Qx1 and Qx1b solutions in CHCl3 are shown in Figure S3b, which are excited at the wavelength corresponding to the maximum absorption peaks. The PL peak wavelength of the Qx1 and Q1b are 700 nm and 720 nm, respectively, but the PL intensity of latter decreases obviously. The UV-vis absorption spectra of Qx1, Qx1b and P3HT in film, are shown in Figure 2a. Compared with the peaks in the solution, the solid films of Qx1 and Qx1b exhibit red-shifted peaks at 629 and 660 nm, implying a self-organization behavior exists in the thin film. The redshift of UV-vis absorption 6


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and PL spectra from Qx1 to Qx1b indicate the modification of side chain in the D part for A2-A1-D-A1-A2 type molecules could largely affect the optical properties, which could be also found in the other type small molecules24. Because the absorption range of P3HT film is from 400 to 650 nm, the Qx1 and Qx1b films can form a relatively complementary absorption with P3HT. The optical bandgaps (Eg) of Qx1 and Qx1b thin film are estimated to be 1.74 eV and 1.68 eV from the absorption edge (714 nm and 738 nm), respectively. The electrochemical properties of Qx1 and Qx1b films on a platinum plate were studied by cyclic voltammetry (CV). Qx1 and Qx1b display one reversible oxidation and one quasi-reversible reduction waves, as shown in Figure S4. Both the LUMO and HOMO energy levels are estimated according to the equations of LUMO = - e (φred + 4.8) (eV) and HOMO = - e (φox + 4.8) (eV), where φred and φox are the onset of reduction and oxidation potentials vs FeCp20/+ (absolute energy level is 4.8 eV below vacuum). As shown in Figure 2b, the LUMO and HOMO energy levels of Qx1 are calculated to be -3.60 eV and -5.42 eV. Qx1b shows slightly lower LUMO energy level (-3.66 eV) and higher HOMO energy level (-5.35 eV), which results in the decrease of bandgap. As LUMOs level of both small molecules are higher than that of PC61BM (-3.91eV), which should achieve an improved Voc relative to P3HT:PC61BM system. The detailed optical and electrochemical properties are summarized in Table 1.

Table 1. The optoelectronic properties of Qx1 and Qx1b. Acceptor

ε 5 (×10 L·mol-1·cm-1) Qx1 0.83 Qx1b 0.95 [a] [b] solution; thin film; [c] HOMO voltammetry measurements carried electrolyte acetonitrile.

Eg opt. HOMO[c] LUMO[c] λmax[a] λmax[b] (nm) (eV) (eV) (eV) (nm) 610 629 1.74 -5.42 -3.60 631 660 1.68 -5.35 -3.66 and LUMO energy levels were estimated from the cyclic out on the as-cast thin film with 0.1 mol·L-1 Bu4NPF6

2.2 Photovoltaic properties of Qx1 and Qx1b as non-fullerene acceptor

The photovoltaic performances of Qx1 and Qx1b, combining with P3HT as electron donor, were studied by bulk heterojunction (BHJ) PSCs with a traditional device structure of ITO/PEDOT:PSS/active

layer/Ca/Al,

together

with

P3HT:PC61BM

7


for

comparison.


P3HT:PC61BM system shows a normal PCE of 3.67% (Voc = 0.61 V, Jsc = 9.86 mA·cm-2, FF = 0.61), where the active layer was fabricated from ODCB solution with D/A ratio of 1:0.8 and thermal annealed at 150 °C for 10 min. For P3HT: Qx1 combination, after the optimization of solvents, D/A ratios and annealing temperatures, the highest PCE of 3.20% (Voc = 0.99 V, Jsc = 4.83 mA·cm-2, FF=0.67) was achieved by using ODCB as spin-coating solvent with 1:0.6 (w/w) D/A ratio and annealing temperature at 100 °C for 10 min. The detail conditions were shown in Figure S5 and Table S1. In the end, 1-chloronaphthalene (CN) was tested as solvent additive and the PCE was improved to 4.03%, which is slightly higher than that of P3HT: PC61BM system. The 1,8-diiodooctane (DIO) as additive was also tested and it had only slightly positive effect on the PCE of the devices. On the other hand, for P3HT:Qx1b system, after the same optimization procedure, using ODCB as solvent, D/A ratio of 0.8:1 (w/w) and annealing temperature at 100 °C for 10 min were the best condition. Due to the wider absorption spectrum and lower LUMO energy level compared with Qx1, Qx1b-based solar cell shows a higher Jsc of 6.41 mA·cm-2 and a slightly lower Voc of 0.96 V, which result in an improved PCE of 4.37%. Furthermore, CN was also used as additive to optimize the devices of P3HT: Qx1b combination. When 0.5% CN was used in ODCB solvent, the PCE was improved from 4.37% to 4.81% with an enhanced Jsc of 7.34 mA·cm-2. Actually, the average PCE of P3HT: Qx1b combination is 4.64%, obvious higher than that of P3HT: Qx1 system (3.70%), which indicates the different side chains in the central IDT segment could affect the photovoltaic performance of Qx-based small molecular acceptors. As expected, the devices based on Qx1 and Qx1b yield higher Voc (0.95-1.00 V) than that of PCBM-based devices (0.61 V), which mainly come from the higher LUMO levels of Qx1 and Qx1b. Table 2 summarizes the device parameters including Voc, Jsc, FF, and PCE at different conditions and Figure 3a displays the J-V curves of the devices under the illumination of AM 1.5G, 100 mW·cm-2. The external quantum efficiency (EQE) plots of the optimized devices are illustrated in Figure 3b. The device of P3HT: Qx1b shows a broader photocurrent response from 350-750 nm than that of P3HT: Qx1 (350-720 nm), which is in accordance with the UV-vis absorption spectra of the blend films in Figure S6. The maximum EQE values are 35% at 520 nm and 38% at 600 nm for P3HT: Qx1 and P3HT: Qx1b, respectively. The Jsc calculated from integration of 8


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the EQE spectra with the AM 1.5 G reference spectrum are 6.16 and 7.83 mA·cm-2, respectively, which match well with the values obtained from J-V measurement. The maximum EQE values are obviously lower than other non-fullerene electron acceptors, such as O-IDTBR (55%)24, BTA1 (49%)26, BTA2 (50%)27, BT2b (56%)25, which result in the inferior Jsc for this two Qx-based small molecules. We speculate that the four bulky phenyl groups in Qx segment affect the electron transport in the photovoltaic devices, and further molecular modification to remove the bulky phenyl groups could increase the EQEs and PCEs.

(a)

4

40

P3HT:PC61BM

0 -2

(b)

35

P3HT : Qx1 ODCB ODCB:CN(99:1 v/v) P3HT : Qx1b ODCB ODCB:CN(99.5:0.5 v/v)

2

30 EQE (%)

Current density (mA.cm-2)

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

25 20 15

P3HT : Qx1 P3HT : Qx1b

-6

10 -8

5

-10

0 -0.2

0.0

0.2

0.4 0.6 Voltage (V)

0.8

1.0

400

500 600 Wavelength (nm)

700

800

Figure 3. (a) J-V curves of P3HT:Qx1 and P3HT:Qx1b devices; (b) The EQE spectra of P3HT:Qx1 and P3HT:Qx1b solar cells. Table 2. Photovoltaic performance of the PSCs based on the P3HT:Qx1 and P3HT:Qx1b under the illumination of AM 1.5 G, 100 mW·cm-2. Acceptor

D/A

Solvent

ratio (w/w)

1:0.6

Voc

Jsc

(V)

(mA.cm-2)

0.99

4.83

0.67

3.20%

(0.99±0.00)

(4.58±0.19)

(0.66±0.01)

(3.00±0.14)

1.00

6.02

0.67

4.03%

FF

µ h / µe

PCEmax

µh

µe

(ave.)a

cm2·V-1·s-1

cm2·V-1·s-1

3.1 × 10-7

1.2 × 10-7

2.6

5.2× 10-7

7.5 × 10-7

0.70

4.1 × 10-6

1.1 × 10-6

3.7

3.3 × 10-5

1.1 × 10-5

3.0

ODCB

Qx1 ODCB:CN 1:0.6 (99:1 v/v)

0.8:1

(1.00±0.00)

(5.68±0.60)

(0.65±0.02)

(3.70±0.48)

0.96

6.41

0.71

4.37%

ODCB (0.96±0.00)

(6.08±0.41)

(0.70±0.01)

(4.11±0.33)

0.95

7.34

0.69

4.81%

Qx1b ODCB:CN 0.8:1 (99.5:0.5 v/v) (0.96±0.01)

(7.15±0.27)

(0.68±0.02)

9


(4.64±0.14)


PCBM

a

1:0.8

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0.61

9.86

0.61

3.67%

(0.61±0.01)

(9.76±0.23)

(0.59±0.01)

(3.47±0.11)

-

ODCB

-

The average PCE values are calculated from eight devices.

2.3 Morphology of P3HT:Qx1 and P3HT:Qx1b films

Figure 4. AFM height images (5µm × 5µm) of (a) (b) active layers based on P3HT:Qx1 without and with CN additive; (c) (d) active layers based on P3HT:Qx1b without and with CN additive. To understand the effects of solvent additive of CN on photovoltaic performance, film morphology study of the active layers was carried out by tapping-mode atomic force microscopy (AFM). It is well known that a nanoscale phase separation and bicontinuous interpenetrating network in the active layer are necessary to efficiently separate the exciton and form percolating channels for the transport of both holes and electrons. The surface morphologies of P3HT:Qx1 (1:0.6 w/w) and P3HT:Qx1b (0.8:1 w/w) films with and without CN additive were characterized, as shown in Figure 4. The blend film of P3HT:Qx1 with annealing at 100 °C shows relatively 10


-

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rough surface with a root mean square (RMS) roughness of 13.7 nm, which is bigger than that of blend film (11.7 nm) with CN additive. But the values of RMS for P3HT:Qx1 are still larger than that of most reported literature (RMS ≤ 3 nm) with high PCEs

10, 29

, which may be due to the

strong crystallinity of Qx1 in the blend film to introduce serious phase aggregations of donor and acceptor. Compared with P3HT:Qx1, the blend film of P3HT:Qx1b with CN additive becomes uniform with values of RMS from 11.2 to 9.6 nm, which is benefit to form proper size of the domains and bicontinuous interpenetrating network to be helpful to the carrier transport in the blend film. These results are consistent with the Jsc of the devices for P3HT:Qx1 and P3HT:Qx1b. 2.4 Influences of solvent additive on molecular packing

11



Figure 5. 2D GIWAXS images of (a) (d) the pristine film for Qx1 and Qx1b, (b) (c) active layers based on P3HT:Qx1 without and with CN additive, (e) (f) active layers based on P3HT:Qx1b without and with CN additive; (g) (h) In-plane line-cuts and Out-of-plane line-cuts of GIWAXS patterns for Qx1 and Qx1b in different conditions.

Since the structural ordering of the small molecule is closely related to charge transporting properties, we investigated the crystallinity of neat films of Qx1 and Qx1b and blend films of P3HT:Qx1 and P3HT:Qx1b using 2D grazing-incidence X-ray diffraction (GIWAXS), and their results are shown in Figure 5. The images and profiles represent the intensity profiles versus wave vector qz and qxy corresponding to the molecular arrangement order in out-of-plane and in-plane directions, respectively. In the out-of-plane direction, the pristine films of Qx1 and Qx1b displayed strong diffraction peaks at 1.50 Å-1 (d≈ 4.2 Å) and 1.70 Å-1 (d≈ 3.7 Å) for π-π stacking, respectively, which indicate the side chains in IDT segment have a large effect on the molecular π-π stacking distance. For P3HT:Qx1b blend film, the diffraction peak at 1.70 Å-1 disappears, which imply P3HT disturbs the π-π stacking of Qx1b. At the same time, there are some new signs of lamella peaks at 0.36 Å-1 (d≈ 17.4 Å), 0.76 Å-1 (d≈ 8.3 Å) and 1.14 Å-1 (d≈ 5.5 Å) related to (100), (200) and (300) peaks of P3HT in the out-of-plane direction, indicating that the crystallinity of P3HT is predominant in the blend film. In addition, the blend film of P3HT:Qx1b still exhibits a stronger crystallinity than that of P3HT:Qx1 and CN additive can 12


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enhance the crystallinity of the blend films further. However, in the out-of-plane direction, the blend film of P3HT:Qx1 still shows two peaks at 0.50 Å-1 and 1.50 Å-1, which is same as that in the Qx1 pristine film. It indicates the crystallinity of Qx1 sustain even when P3HT is added. This excess crystallinity of both P3HT and Qx1 may lead to the serious phase separation for the blend film, which is harm to uniform the interpenetrating network. On the other hand, in the in-plane direction, there are two stronger diffraction peaks at 0.35 and 1.13 Å-1 for Qx1b pristine film than that of Qx1 at 0.80 and 1.20 Å-1, which shows Qx1b has a stronger crystallinity than that of Qx1. When P3HT was added, the blend film of P3HT:Qx1b also shows stronger crystallinity than P3HT:Qx1 in the blend film. Above mention facts explains the best device performance obtained with CN and the results are in accordance with the results of AFM.

To investigate the influence of solvent additive on the charge carrier transport, the electron mobility (µe) and hole mobility (µh) of P3HT:Qx1 (1:0.6, w/w) and P3HT:Qx1b (0.8:1, w/w) blend films were determined by the space-charge limited current (SCLC) method, as shown in Figure S7 and Table 2. Upon annealing, blend film of P3HT:Qx1 exhibits a hole mobility (µh) of 3.1 ×10-7 cm2·V-1·s-1 and an electron mobility (µe) of 1.2 × 10-7 cm2·V-1·s-1 with µh /µe = 2.6. After adding CN as additive, both values are improved and the increase of µe is greater than µh, where µh /µe is 0.70. Compared with P3HT:Qx1, the blend films of P3HT:Qx1b show higher µh and µe (4.1 × 10-6 cm2·V-1·s-1 and 1.1 × 10-6 cm2·V-1·s-1, respectively). When CN additive was added, the values of µh and µe are improved and reached to 3.3 × 10-5 cm2·V-1·s-1 and 1.1 × 10-5 cm2·V-1·s-1, respectively. The high electron and hole transport for P3HT:Qx1b by adding CN as additive are responsible for the best photovoltaic performances.

13



P3HT @520

(a)

P3HT @520 Qx1b @660 P3HT:Qx1b @520 P3HT:Qx1b @660

(b)

70

Qx1 @629

70

60

P3HT:Qx1 @520 P3HT:Qx1 @629 P3HT:Qx1 with 1% CN @520

60

50

P3HT:Qx1 with 1% CN @629

50 Intensity

Intensity

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

40 30

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P3HT:Qx1b with 0.5% CN @520 P3HT:Qx1b with 0.5% CN @660

40 30

20

20

10

10 0

0 600

650

700 750 800 Wavelength (nm)

850

600

650

700 750 800 Wavelength (nm)

850

Figure 6. Photoluminescence spectra of (a) the P3HT, Qx1 and the blend films of Qx1:P3HT (1:0.6, w/w) (excited at 520 and 629 nm, respectively) (b) the P3HT, Qx1 and the blend films of P3HT:Qx1b (0.8:1 w/w) (excited at 520 and 660 nm, respectively).

Furthermore, to understand the charge separation properties, photoluminescence (PL) emission of the pure and blend films were tested. According to the maximum absorption peaks of P3HT, Qx1 and Qx1b in film, we used the light with a wavelength at 520, 629 and 660 nm to excite the pristine and blend films. Table S2 shows the summary of the parameters of PL emission for the pure films and blend films. In Figure 6, Qx1 displays an emission peak at 748 nm and Qx1b displays an emission peak at 749 nm, which might imply the electron density of backbone of these two small molecules are similar and the influence of benzene spacer attached on the central IDT unit is weakened by the four phenyl groups on the Qx unit. Note that when the blend films of P3HT:Qx1b were excited at 520 nm, the quenching efficiency (94-96%) is more than that excited at 660 nm (quenching ≈ 85%). The results indicate that light-harvesting of donor could mainly contribute to photo-generated current. Compared with P3HT:Qx1b, the blend films of P3HT:Qx1 shows lower quenching efficiency (just 65-81%) and the blend films processed by CN additive have better quenching efficiency from 65% to 81%, which reveal that the blend film processing with CN should have an efficient charge separation at the interface. This result is consistent with the improvement of PCE from 3.20% to 4.03%.

3. Conclusions In summary, we have applied Qx unit to synthesize two n-type non-fullerene small molecular acceptors, named as Qx1 and Qx1b. Both small molecules show good thermal stability, high solubility, strong and broad absorption from 300-750 nm with an optical bandgap of 1.74 eV and 14


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1.68 eV, respectively. Qx1 and Qx1b exhibited the complementary absorption spectra and appropriate HOMO levels and LUMO energy levels matched well with P3HT. PSCs based on P3HT:Qx1 (1:0.6, w/w) showed a Voc of 1.00 V and a PCE of 4.03%, and P3HT:Qx1b achieved a Voc of 0.95 V and a PCE of 4.81%, although in an unfavorable condition with the inferior film quality and low electron mobility. These results demonstrate that Qx unit is an effective building block to construct promising n-type non-fullerene small molecule with the high Voc and PCE. Furthermore, there is a huge space to design and synthesize novel n-type small molecules containing Qx unit.

Acknowledgements The authors thank the support from National Key Research and Development Program of China (2017YFA0206600), the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant No. QYZDB-SSW-SLH033), the National Natural Science Foundation of China (NSFC, Nos. 51673048, 51473040, 21602040), the National Natural Science Foundation of Beijing (No. 2162045).

Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI:

Synthesis of materials;

1

H NMR and MALDI-TOF MS of Qx1 and Qx1b;

Thermogravimetric analysis; PL spectra, detailed photovoltaic device performance results, and current–voltage data for SCLC measurements.

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Quinoxaline-containing non-fullerene small molecular acceptors with

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