Nonfullerene-Acceptor All-Small-Molecule Organic Solar Cells Based

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Non-fullerene-acceptor all-small-molecule organic solar cells based on highly twisted perylene bisimide with efficiency over 6% Rui Xin, Jiajing Feng, Cheng Zeng, Wei Jiang, Lei Zhang, Dong Meng, Zhongjie Ren, Zhaohui Wang, and Shouke Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13721 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on December 29, 2016

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ACS Applied Materials & Interfaces

Non-fullerene-acceptor all-small-molecule organic solar cells based on highly twisted perylene bisimide with efficiency over 6% Rui Xin,† Jiajing Feng,‡ Cheng Zeng,‡ Wei Jiang,‡,* Lei Zhang,

†

Dong Meng,‡ Zhongjie Ren,†,*

Zhaohui Wang,‡ and Shouke Yan† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical

Technology Beijing, 100029, China *

E-mail: [email protected]

‡

Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Sciences,

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China *

E-mail: [email protected]

Keywords:

non-fullerene

all-small-molecule

organic

solar

cells,

perylene

bisimide,

perpendicular twist, non-fullerene acceptor, solvent-vapor annealing

ACS Paragon Plus Environment

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ABSTRACT: Two twisted singly-linked perylene bisimide (PBI) dimers with chalcogen bridges in the PBI cores, named as C4,4-SdiPBI-S and C4,4-SdiPBI-Se, were synthesized as acceptors for non-fullerene all-small-molecule organic solar cells (NF all-SMSCs). A moderate-bandgap small molecule DR3TBDTT used as the electron donor displayed complementary absorption with C4,4-SdiPBI-S and C4,4-SdiPBI-Se. It was found that solvent-vapor annealing (SVA) played a critical role on photovoltaic performance in NF all-SMSCs, which improves the crystallinity of donor and acceptors, promotes the proper phase segregation domain size and therefore enhances the charge transport. The PCEs of NF all-SMSCs devices based on DR3TBDTT: C4,4-SdiPBI-S and DR3TBDTT: C4,4-SdiPBI-Se increased from 2.52% to 5.81% (JSC = 11.12 mA cm−2, VOC = 0.91 V, and FF = 57.32%) and 2.65% to 6.22% (JSC = 11.55 mA cm−2, VOC = 0.92 V, and FF = 58.72%), respectively, after exposure to chloroform vapor. The best efficiency of 6.22 % is one of the highest PCEs for NF all-SMSCs based PBI acceptor so far. The studies illustrate that high efficient NF all-SMSCs can be achieved by using perylene bisimide acceptor with a suitable SVA process.


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INTRODUCTION

Organic solar cells (OSCs) are thought to be a promising replacement for silicon based solar cells owing to their satisfying advantages, such as light weight, low cost, flexibility and so on.1 In the past decades, fullerene derivatives as electron acceptors have been extensively investigated for the efficient solution-processed bulk heterojunction (BHJ) OSCs due to their high electron mobility, strong electron affinity and reversible electrochemical reduction, etc.2-7 However, there are also some disadvantages for fullerene materials, such as weak light absorption, poor band gap variability, high production cost, morphological instability and complex synthetic procedures.8,9 Compared to the widely investigated fullerene derivatives, particular attention has been recently paid to create solution-processed non-fullerene (NF) acceptors.8-18 Indeed, recent advances have led to new non-fullerene (NF) acceptors that show impressive device performance. For example, NF polymer solar cells (NF-PSCs) with a polymer donor have been well-developed with the power conversion efficiencies (PCEs) over 12% by the continuous material evolution and device optimization, which is over the PCEs of fullerene acceptor based OSCs.12 In contrast, NF all-small-molecule solar cells (NF all-SMSCs) with a small molecule donor have presented relatively low performance,13-18 with the PCEs of 6.03%,13 7.14%.19 However, many obvious disadvantages of polymer materials restrict their application in NFPSCs, such as wide molecular weight distribution and variable batch quality.17 Compared with polymers, small molecule materials are promising for OSCs in terms of their attractive advantages, such as well-defined chemical structure, easy-tunable light absorption and energy


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level, controllable crystalline properties, high purity and stable batch quality, etc.16,17 Therefore, NF all-SMSCs, featuring both p-type and n-type conjugated small molecules, indeed possess the admirable characteristics due to the unique properties of small-molecular constituents. Perylene bisimides (PBIs) and their derivatives were used as one of the most promising non-fullerene

acceptors

with

remarkable

electro-optical

properties

and

fine-tunable

structures.7,8,20-29 Several kinds of twisted PBI oligomers connected either by head-to-head pattern via imide-position or through bay-region with/without linkers have been developed as high performance acceptors.23-27 Very recently, we have focused on exploring the structures and functionality of PBI-based photoactive materials used in PSCs,23,29 in which a PCE up to 9.28% has been achieved for triperylene hexaimide derivative (TPH-Se) and PDBT-T1 blend film.29 Nevertheless, NF all-SMSCs based on PBI acceptors were relatively rarely studied and PCEs over 3% have seldom been reported.15,16,18,30-33 Therefore, it still has a space to improve the performance of PBI-based NF all-SMSCs by modifying the molecular architecture of PBIs, choosing a suitable donor and optimizing the processing procedures of the device. Herein, two new twisted singly-linked PBI by introducing chalcogen bridges into the PBI core were synthesized as electron acceptors, namely C4,4-SdiPBI-S and C4,4-SdiPBI-Se. Both C4,4-SdiPBI-S and C4,4-SdiPBI-Se display slightly up-shifting LUMO levels by the introduction of chalcogen bridges to the PBI bay-regions, thus the open-circuit voltage (VOC) of solar cells would be improved. Because two PBI rings are twisted and nearly perpendicular to each other with the dihedral angle of about 80°,27,28 C4,4-SdiPBI-S and C4,4-SdiPBI-Se are less crystalline and can prevent the forming of large aggregates when blended with donors to fabricate NF all-SMSCs. This potentially assures high exciton dissociation efficiency and large JSC in the device. In addition, because of the difficult thermal crystallization of C4,4-SdiPBI-S


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and C4,4-SdiPBI-Se, solvent-vapor annealing (SVA) was used to reconstruct the active layer film morphology in OSCs,34-38 which has rarely been reported in the NF all-SMSCs. In this study, a well-established small molecule named DR3TBDTT with a benzo[1,2-b:4,5b’]dithiophene (BDT) block was used for electron donor.39 After the proper optimization of device fabrication and SVA, a maximum PCE of 6.22% with a JSC of 11.55 mA cm-2, and a fill factor (FF) of 58.72% for the NF all-SMSCs based on C4,4-SdiPBI-Se:DR3TBDTT blends was achieved, which is one of the highest values reported in non-fullerene all-SMSCs for PBI families. RESULTS AND DISCUSSION Optical and electronic properties

Figure 1. Chemical structures and the side views of the optimized geometries by using DFT calculations at the B3LYP/6-31G(d) level of a) C4,4-SdiPBI-S and b) C4,4-SdiPBI-Se. c) Chemical structure of the donor DR3TBDTT. d) UV-vis absorption spectra of the thin films. e) Energy level diagrams of C4,4-SdiPBI-S, C4,4-SdiPBI-Se and DR3TBDTT.


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C4,4-SdiPBI-S, C4,4-SdiPBI-Se and DR3TBDTT are synthesized according to the literatures. Their chemical structures are shown in Figure 1. These compounds with branched alkyl chains are soluble in common solvents and are unambiguously characterized by MALDITOF, NMR et al. The UV-vis absorption spectra of C4,4-SdiPBI-S and C4,4-SdiPBI-Se are shown in Figure 1d and Figure 2a. The C4,4-SdiPBI-S film exhibits three main absorption peaks at 446, 472, and 506 nm. Interestingly, C4,4-SdiPBI-Se displays two peaks of 476 and 512 nm with slight red-shifts compared with C4,4-SdiPBI-S. The optical band gaps (Eg) of C4,4SdiPBI-S and C4,4-SdiPBI-Se assessed from the film absorption edges are 2.20 and 2.22 eV, respectively. Meanwhile, their absorption spectra in chloroform solutions present the almost similar peaks with that in the film state, suggesting the weak intermolecular aggregation of C4,4SdiPBI-S and C4,4-SdiPBI-Se in the solid states. It should be noted that the principal absorption spectra of acceptors and donor are quite complementary with the coverage of 350-750 nm, which is benificial for light harvesting in the range of solar spectrum. Table 1. Optical and electronic properties of C4,4-SdiPBI-S and C4,4-SdiPBI-Se.

Compound

λfilmmax (nm)

λsol.max a (nm)

εa (M cm-1)

EHOMO b (eV)

ELUMO c (eV)

C4,4-SdiPBI-S

446, 472, 506

447, 471, 504

106215

-6.07

-3.87

2.20

C4,4-SdiPBI-Se

476, 512

476, 510

100230

-6.08

-3.86

2.22

-1

Eg d (eV)

a

Measured in dilute CHCl3 solution (1.0 × 10-5 M). b HOMO (eV) calculated according to EHOMO = (ELUMO - Eg) eV. LUMO (eV) estimated by the onset of the reduction peaks and calculated according to ELUMO = -(4.8 + Eonsetre) eV. d Calculated by the onset of absorption in film according to Eg (eV) = (1240/λonsetabs.). c

In addition, the LUMO levels of acceptors are estimated to be about 3.86 or 3.87 eV from cyclic voltammetry (CV) measurements (Figure 2b and Table 1). The energy offset between the HOMO of donor (-5.02 eV) and the LUMO of acceptors is about 1.16 or 1.15 eV, and thus a


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high VOC would be anticipated. The LUMO gaps of ca. 0.6 eV are higher than the empirical value needed for the effective exciton dissociation (0.3 eV).40

Figure 2. a) UV-vis absorption spectra of C4,4-SdiPBI-S and C4,4-SdiPBI-Se in CHCl3, and b) Cyclic voltammograms of C4,4-SdiPBI-S and C4,4-SdiPBI-Se in CH2Cl2. Non-fullerene all-SMSCs Solution-processed NF all-SMSC devices were fabricated using DR3TBDTT as the electron donor and C4,4-SdiPBI-S or C4,4-SdiPBI-Se as the electron acceptor with the conventional single-junction device structure of ITO/PEDOT:PSS/donor:acceptor/ETL/Al, in which the active layers was spin-coated from chloroform solution.37 The weight ratio of donor and acceptor (D:A) was primarily optimized and presented in the supporting information (Table S1). It is found that DR3TBDTT:C4,4-SdiPBI-S and DR3TBDTT:C4,4-SdiPBI-Se devices with the weight ratio of 1:1 without solvent annealing exhibit the best efficiencies of 2.52 and 2.65% with VOC of about 0.95 and 0.96 V, respectively. And then, five annealing solvents with different boiling points were selected for SVA, including carbon disulfide, chloroform, tetrahydrofuran, toluene, and odichlorobenzene. Our results indicate that chloroform is the best annealing solvent to improve the device performance among the selected five solvents (Table S2, S3). When 50 µl chloroform


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was used and the annealing time was set 50s, the PCEs were increased to 5.29% and 5.35% for DR3TBDTT:C4,4-SdiPBI-S and DR3TBDTT:C4,4-SdiPBI-Se, respectively. Compared with the untreated devices, the devices with SVA exhibited the remarkable improvement in JSC and FF, although the VOC was slightly decreased. It should be noted that the DR3TBDTT:C4,4-SdiPBI-S devices processed with SVA (Vsol. = 50 µl, Time = 60 s) have the best photovoltaic response with a PCE of 5.81%, a VOC of 0.91 V, a JSC of 11.12 mA/cm2 and a FF of 57.32%. Under the same condition, the DR3TBDTT:C4,4-SdiPBI-Se devices also exhibited outstanding photovoltaic performances with a PCE of 6.22% , a VOC of 0.92 V, a much higher JSC of 11.55 mA/cm2 , and a higher FF of 58.72% (Table 2). It is apparent that the SVA process played a critical role on improving photovoltaic performance and the PCEs are 2-3 times higher than that of before SVA. In particular, there were significant increases of JSC and FF as well. Table 2.

Photovoltaic performances of NF all-SMSCs based on DR3TBDTT:C4,4-SdiPBI-S

and DR3TBDTT:C4,4-SdiPBI-Se blend films with and without SVA.

Blends

DR3TBDTT:C4,4SdiPBI-S

DR3TBDTT:C4,4SdiPBI-Se

SVA

VOC [V]

JSC [mA/cm2]

FF [%]

PCEc [%]

Na

0.95 ± 0.006

6.59 ± 0.31

40.34 ± 2.1

2.52 (2.46)

Yb

0.91 ± 0.003

11.12 ± 0.22

57.32 ± 1.5

5.81 (5.73)

Na

0.96 ± 0.007

7.16 ± 0.28

38.46 ± 2.3

2.65 (2.53)

Yb

0.92 ± 0.003

11.55 ± 0.18

58.72 ± 1.9

6.22 (6.11)

a

Without SVA. b With SVA by CHCl3 (Vsol. = 50 µl, Time = 60 s). c The best PCE values are given, followed by the average PCE values, calculated from more than five devices.


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Figure 3. a) current density-voltage (J-V), and b) the EQE curves of DR3TBDTT:C4,4-SdiPBI-S and DR3TBDTT:C4,4-SdiPBI-Se devices with or without SVA. Figure 3a shows the current density-voltage (J-V) curves of the DR3TBDTT:C4,4-SdiPBIS and DR3TBDTT:C4,4-SdiPBI-Se devices processed with/without SVA under simulated AM 1.5 sun illumination. The external quantum efficiency (EQE) spectra of the blends treated with/without SVA are displayed in Figure 3b. The JSC value calculated from the EQE spectrum based on C4,4-SdiPBI-S and C4,4-SdiPBI-Se with SVA are 10.33 and 11.13 mA/cm2, respectively, which are comparable to the JSC values from the corresponding J-V curves with 7.1% and 3.6% error. The significant improvement of photovoltaic performance could be attributed to the increased crystallinity, the proper phase segregation domain size and the great charge transport properties by the SVA process. Absorption spectra


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Figure 4. UV-vis absorption spectra of a) the DR3TBDTT:C4,4-SdiPBI-S and b) the DR3TBDTT:C4,4-SdiPBI-Se blend films with and without SVA. The DR3TBDTT:SdiPBI-S and DR3TBDTT:SdiPBI-Se blend films with or without SVA by CHCl3 were also measured by UV-vis absorption spectra. As shown in Figure 4, the obvious increased absorption intensity and red-shifts in range of 550 to 750 nm were observed, which might lead to enhancement of Jsc. In addition, the remarkable increased absorbance at 660 nm, which is assigned to the π-π stacking of DR3TBDTT, indicates that SVA process can promote ordered arrangement of the molecules and increase molecular interactions of the donor molecules in the blend films, and thus improve hole transport mobility. The results are consistent with the trend of J-V and EQE. Charge transport Table 3. SCLC mobility data of DR3TBDTT:C4,4-SdiPBI-S and DR3TBDTT: C4,4-SdiPBI-Se blend films.


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DR3TBDTT:C4,4-SdiPBI-S (cm2 V-1 S-1)

DR3TBDTT:C4,4-SdiPBI-Se (cm2 V-1 S-1)

µh

µe

µh

µe

Without SVA

1.16×10-4

2.91×10-5

1.71×10-4

3.48×10-5

SVA

3.81×10-4

5.22×10-5

4.78×10-4

6.67×10-5

To further illuminate the distinguished device performances with/without SVA, we firstly performed the space-charge limited current (SCLC) model to evaluate charge transport properties of DR3TBDTT:C4,4-SdiPBI-S and DR3TBDTT:C4,4-SdiPBI-Se blend films before and after SVA (Figure S5, S6 and Table 3). The hole mobility was higher than the electron mobility for both DR3TBDTT:C4,4-SdiPBI-S and DR3TBDTT:C4,4-SdiPBI-Se blend films, illustrating slightly unbalanced charge transport. However, after SVA process, both the hole and electron mobilities were obviously enhanced，which may explain the improved performances achieved in DR3TBDTT:C4,4-SdiPBI-S and DR3TBDTT:C4,4-SdiPBI-Se all-SMSCs (Table 3). In addition, DR3TBDTT:C4,4-SdiPBI-Se blend films with and without SVA exhibited slightly higher hole and electron mobilities than those of DR3TBDTT:C4,4-SdiPBI-S blend films, resulting in its much high JSC and PCE. The enhanced mobility is possibly attributed to the improved interactions between Se-Se atoms of C4,4-SdiPBI-Se because of the loose electron cloud of the farthest p orbital of selenium.41 Film morphology


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Figure 5. AFM height images of DR3TBDTT:C4,4-SdiPBI-S blend film with (a) and without (b) SVA and their corresponding phase images (e and f), respectively. AFM height images of DR3TBDTT:C4,4-SdiPBI-Se blend film with (c) and without (d) SVA and their corresponding phase images (g and h), respectively. The area is 2×2 µm2.

The morphologies of blend films (1:1 w/w) of the two acceptors with DR3TBDTT spincoated from CHCl3 solution with and without SVA were studied by atomic force microscopy (AFM) as presented in Figure 5 and transmission electron microscopy (TEM) shown in the supporting information (Figure S7). It can be seen from Figure 5, before solvent vapor annealing, all the blend films displayed especially smooth surface and uniform morphology with small root-mean-square (RMS) roughness of 0.71 nm for C4,4-SdiPBI-S and 0.46 nm for C4,4SdiPBI-Se, indicating two acceptors have the wonderful miscibility with DR3TBDTT. Therefore, there were no obvious phase-separation to provide the good holes and electrons transporting pathway. After SVA, all the blend films showed much more phase-separated and slightly bigger aggregation domain than that of without SVA. It is also clearly found from phase image that a good interpenetrating network was formed after SVA, with a much rougher surface structure than that of the film without SVA (Figure 5). The RMS roughness of the


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DR3TBDTT:C4,4-SdiPBI-S and the DR3TBDTT:C4,4-SdiPBI-Se blend films was 1.46 and 1.13 nm, respectively. These changes are of good consistency with the previous OPV performance measurement. As a result, the increased phase-separation and appropriate aggregation domain size of the blend films after SVA would provide continuous pathways for the charge separation and transport. Molecular packing Grazing-incidence wide-angle X-ray scattering (GIWAXS) is served as an effective tool to investigate the crystallinity and molecular packing of the films. The out-of-plane (OOP) and inplane (IP) two-dimensional (2D) GIWAXS scattering profiles were shown in Figure 6. For the pure DR3TBDTT film, the films without and with SVA all showed high crystallinity (Figures 6a and b). The major qz at 0.301 Å-1 in OOP and 1.707 Å-1 in IP, which correspond to a lamellar structure (d = 20.87 Å) and π-π stacking distance (d = 3.68 Å), demonstrate formation of mainly edge-on oriented crystallites in the film. In addition, the C4,4-SdiPBI-S and C4,4-SdiPBI-Se film showed amorphous state before SVA (Figures 6c and e), but the crystallinity was dramatically enhanced after SVA (Figures 6d and f). With SVA, the scattering peak (100) at qz= 0.372 Å-1 for C4,4-SdiPBI-S and 0.355 Å-1 for C4,4-SdiPBI-Se appeared in OOP, respectively. No obvious (010) peaks were observed, indicating an a-axis orientation with b- and c-axes arranged in film plane.


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Figure 6. 2D GIWAXS patterns (a-j) and out-of-plane (OOP) and in-plane (IP) scattering profiles (k, l) of DR3TBDTT, C4,4-SdiPBI-S, C4,4-SdiPBI-Se and related blends before and after SVA treatment (chloroform, Vsol. = 50 µl, Time = 60s). In the blend films without SVA treatment, as displayed in Figure 6, only weak (100) peaks can be found at about qz = 0.298 Å−1 (DR3TBDTT:C4,4-SdiPBI-S, d = 21.085 Å) and 0.295 Å−1 (DR3TBDTT:C4,4-SdiPBI-Se, d = 21.299 Å), indicating a poor molecular packing behavior.


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After annealing, the scattering peaks were obviously enhanced in both DR3TBDTT:C4,4SdiPBI-S and DR3TBDTT:C4,4-SdiPBI-Se blend films, which indicated that the SVA treatment effectively improved the molecular order and crystallinity. As a consequence, with SVA, the improvement of molecular order and crystallinity of acceptor and donor facilitated to form the appropriate phase-separation and aggregation domain size, and thus increased charge transport, JSC and FF, which corresponds well with AFM topographic images (Figure 5). CONCLUSIONS In this work, we designed a kind of NF all-SMSCs by integrating two highly twisted PBI dimers (C4,4-SdiPBI-S and C4,4-SdiPBI-Se) as fullerene-free acceptors and DR3TBDTT with a moderate-band-gap as electron donor. The introduction of sulphur and selenium atoms into PBI cores led to good opto-electrical properties and charge transport characteristics. Moreover, molecular arrangement and crystallinity, surface morphology, phase segregation and molecular packing of the blend films were studied by AFM, TEM and GIWAXS. SVA was employed to improve the crystallinity and phase segregation domain, leading to a significant improvement of photovoltaic properties. After annealed by chloroform vapor, PCEs up to 5.81 and 6.22% were obtained for DR3TBDTT:C4,4-SdiPBI-S and DR3TBDTT:C4,4-SdiPBI-Se blend films, respectively. This result is one of the highest PCEs in NF all-SMSCs based PBI acceptor reported. These findings are useful for the further expansion of non-fullerene OSCs based on PBI acceptor and demonstrate that non-fullerene all-SMSC systems are promising for competing with other OSC systems. EXPERIMENTAL SECTION Synthesis of C4,4-SdiPBI-S and C4,4-SdiPBI-Se


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C4,4-SdiPBI-S and C4,4-SdiPBI-Se were synthesized using slightly modified procedures reported (ESI†).14 C4,4-SdiPBI-S: 1H NMR (400 MHz, CDCl3): δ = 9.60-9.57 (d, J = 12.0 Hz, 2H), 9.449.41 (d, J = 12.0 Hz, 2H), 8.79 (s, 2H), 8.14-8.07 (m, 4H), 5.33-5.10 (m, 4H), 2.30-2.21 (m, 8H), 1.89-1.79 (m, 8H), 1.35-1.25 (m, 32H), 0.88-0.74 (m, 24H);

13

C NMR (100 MHz, CDCl3): δ =

165.2, 164.3, 163.9, 163.2, 140.8, 139.0, 138.4, 133.3, 132.9, 132.2, 131.9, 131.6, 130.8, 128.0, 126.9, 126.5, 126.3, 125.2, 124.2, 123.5, 55.3, 54.9, 32.1, 32.0, 29.2, 29.0, 22.6, 22.5, 14.0, 14.0, 13.9; HRMS (MALDI(N)), 100%): calcd (%) for C84H86N4O8S2, 1342.5893; found 1342.5890. C4,4-SdiPBI-Se: 1H NMR (400 MHz, CDCl3): δ = 9.60-9.56 (d, J = 16.0 Hz, 2H), 9.459.41 (d, J = 16.0 Hz, 2H), 8.71-8.68 (m, 2H), 8.26-8.24 (m, 2H), 8.15 (m, 2H), 5.30-5.10 (m, 4H), 2.20 (m, 8H), 1.88-1.80 (m, 8H), 1.33-1.26 (m, 32H), 0.86-0.74 (m, 24H);

13

C NMR (100

MHz, CDCl3): δ = 165.0, 164.4, 163.9, 163.2, 141.5, 140.8, 134.8, 134.4, 133.5, 132.9, 132.3, 131.1, 130.4, 129.8, 126.8, 126.7, 126.7, 126.6, 125.6, 122.5, 55.2, 54.8, 32.0, 32.0, 29.2, 29.0, 22.6, 22.5, 14.0, 14.0, 13.9; HRMS (MALDI(N)), 100%): calcd (%) for C84H86N4O8Se2, 1438.4809; found 1438.4810. Fabrication of Organic Photovoltaic Devices Photovoltaic

devices

were

manufactured

with

normal

device

structure

of

ITO/PEDOT:PSS/donor:acceptor/ETL/Al.37 The ITO-coated substrates (1.5 × 1.5 cm) were washed in liquid detergent, deionized water, acetone, and isopropyl alcohol under ultrasonic treatment for 20 minutes each part and dried in an overnight. Then the substrates were dealt with UV-Ozone for 20 minutes. PEDOT:PSS (Clevios P VP AI 4083, filtered at 0.45 µm) was spincoated at 3000 rpm for 30 s onto ITO surface, and the substrates were heated at 150 °C for 20


16

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minutes before moved into the glove box. Afterward, the photoactive layer was spin-coated from a blend chloroform solutions with different weight ratio of DR3TBDTT and C4,4-SdiPBI-S or C4,4-SdiPBI-Se. The photoactive layer thickness was about 110 nm, measured by a Bruker Dektak XT profilometer. And these substrates were placed in a glass petri dish containing different solvents such as carbon disulfide, chloroform, tetrahydrofuran, toluene and odichlorobenzene for solvent vapor annealing (SVA). Then the ETL methanol solution (0.5 mg/ml) was spin-coated on the cover of the photoactive layer at 3000 rpm for 30 s. Finally, 100 nm Al were deposited by vacuum evaporation under high vacuum (< 1.5×10-4 Pa). The active area of sole cell was 3.08 mm2, controlled by a shadow mask. The J−V curve was measured by a Precision Source/Measure Unit (B2912A; Agilent Technologies), and an AAA grade solar simulator (XES-70S1, 7 × 7 cm2 beam size; SAN-EI Electric Co. Ltd.) coupled with AM 1.5G solar spectrum filters was holden as the light source. The photocurrent was calibrated to be 100 mW cm−2, using a standard single crystal Si reference cell (SRC-1000-TC-QZ, 2 × 2 cm2; VLSI Standards Inc.). The EQE was recorded by a Solar Cell Spectral Response Measurement System (QE-R3011; Enlitech Co. Ltd.). The charge transport properties were measured using a space-charge limited current (SCLC) method. The hole mobility was measured with the device architecture of ITO/PEDOT:PSS/donor:acceptor/Au, and the electron mobility was measured with the device architecture of Al/donor:acceptor/Al. By fitting the dark J-V curves to a SCLC model, J = 9ε0εr µV2/8L3exp[0.891γ(V/L)0.5], the electron or hole mobility was calculated. Materials and Measurements


17


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Page 18 of 25

All chemicals and solvents in this work were bought from commercial suppliers. And it is used without extra purification except otherwise specified. 1

H NMR (400 MHz) and

13

C NMR (100 MHz) spectra were recorded on a Bruker

ADVANCE 400 NMR Spectrometer and tetramethylsilane (TMS) reference using the residual protonated solvent as an internal standard. Mass spectra (MALDI-TOF-MS) were determined on a Bruker BIFLEX III Mass Spectrometer. High resolution mass spectra (HRMS) were performed on IonSpec 4.7 Tesla Fourier Transform Mass Spectrometer. UV-vis spectra were recorded on Hitachi (Model U-3010) UV-vis spectrophotometer. Cyclic voltammograms (CVs) were measured with a Zahner IM6e electrochemical workstation with glassy carbon discs as a working electrode, Pt wire as a counter electrode, Ag/AgCl electrode as a reference electrode with a scan rate of 100 mV/s in the film. The supporting electrolyte, which was standardised by the ferrocene/ferroncenium (Fc/Fc+) as a redox couple, was 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in CH2Cl2 (HPLC grade). Atomic force microscopy (AFM) images of the films were obtained on a Nanoscope III Multi-Mode atomic force microscope operating

in the

tapping mode. Transmission electron microscopy

(TEM) was performed on a JEOL JEM-2100 TEM operated at 200 kV. Grazing incidence wide angle X-ray scattering (GIWAXS) measurements were completed in a Xenocs-SAXS/WAXS system by X-ray wavelength of 1.5418 Å and the samples were treated at a fixed angle of 0.2°. The samples were prepared on Si wafer substrates with the same experimental conditions for solar cells.

ASSOCIATED CONTENT


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Supporting Information Available: NMR spectra, absorption spectra, cyclic voltammograms, device performance parameters, charge transport and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Zhongjie Ren ([email protected]); Jiang Wei ([email protected]). Funding Sources This work was financially supported by the National Natural Science Foundation of China (NSFC) (No., 21274009, 51221002, 51673202, 21428304, 51473009), the 973 Program (2014CB643502), the Chinese Academy of Sciences (XDB12010400). REFERENCES (1)

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Table of Contents Graphic


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40x29mm (300 x 300 DPI)


Nonfullerene-Acceptor All-Small-Molecule Organic Solar Cells Based

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