Effect of Side Groups on the Photovoltaic Performance Based on

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Organic Electronic Devices

Effect of Side Groups on the Photovoltaic Performance based on Porphyrin – Perylene Bisimide Electron Acceptors Yiting Guo, Yanfeng Liu, Qinglian Zhu, Cheng Li, Yingzhi Jin, Yuttapoom Puttisong, Weimin Chen, Feng Liu, Fengling Zhang, Wei Ma, and Weiwei Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10955 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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ACS Applied Materials & Interfaces

Effect of Side Groups on the Photovoltaic Performance based on Porphyrin – Perylene Bisimide Electron Acceptors Yiting Guo,†,± Yanfeng Liu,‡ Qinglian Zhu,§ Cheng Li,† Yingzhi Jin,‡ Yuttapoom Puttisong,‡ Weimin Chen,‡ Feng Liu,*,⊥ Fengling Zhang,*,‡ Wei Ma,*, § and Weiwei Li*,† †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 10090, China. ‡

Biomolecular and Organic Electronics, Department of Physics, Chemistry and Biology, Linköping University, SE-581 83, Linköping, Sweden. §

State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, P. R. China. ⊥

College of Chemistry and Environmental Science, Hebei University, Baoding 071002, People’s Republic of China. ±

University of Chinese Academy of Sciences, Beijing 10049, People’s Republic of China.

KEYWORDS. Non-fullerene organic solar cells, porphyrin, perylene bisimide, organic semiconductor, side-chain engineering ABSTRACT. In this work, we developed four porphyrin-based small molecular electron acceptors for non-fullerene organic solar cells, in which different side groups attached to porphyrin core were selected in order to achieve optimized performance. The molecules contain

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porphyrin as core, perylene bisimides as end groups and ethynyl unit as linker. Four side groups, from

2,6-di(dodecyloxy)-phenyl

to

(2-ethylhexyl)thiophen-2-yl,

pentadecan-7-yl

and

3,5-di(dodecyloxy)-phenyl unit, were applied into the electron acceptors. The new molecules exhibit broad absorption spectra from 300 nm to 900 nm and high molar extinction coefficients. The molecules as electron acceptors were applied into organic solar cells, showing increased power conversion efficiencies from 1.84% to 5.34%. We employed several techniques, including photoluminescence spectra (PL), electroluminescence spectra (EL), atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray (GIWAXS) to probe the blends to find the effects of the side groups on the photovoltaic properties. We found that, the electron acceptors with 2,6-di(dodecyloxy)-phenyl units show high-lying frontier energy levels, good crystalline properties and perform low non-radiative recombination loss, resulting in possible large phase separation and low energy loss, which is responsible for the low performance. Our results provide a detailed study about the side groups of non-fullerene materials, demonstrating that porphyrin can be used to design electron acceptors toward near-infrared absorption.

1. INTRODUCTION With the potential for the light weight, flexible, and low cost fabrication of large-area devices, organic solar cells (OSCs) have attracted extensive interest as a promising technology for green energy alternatives.1-5 Most of OSC devices are based on bulk-heterojunction (BHJ) structure,6 that is, a blend of an electron donor and an electron acceptor. Fullerene derivatives (e.g., PC61BM and PC71BM) as electron acceptor paired with polymers as electron donor were mainly employed for a long period, and power conversion efficiencies (PCEs) achieved over 11%.7 Due to the breakthrough of molecular design in non-fullerene electron acceptors8-14 that possess the


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advantages of strong absorption in the visible and near-infrared (NIR) region, the PCEs of devices based on these acceptors were enhanced to 14%.15-17 Porphyrin derivatives have unique optical and electronic properties due to their eighteen delocalized π-electrons.18 In addition, the absorption of porphyrins lies in both the blue (Soret band) and red (Q band) parts of the visible spectrum, and can be easily extended into the NIR region by using donor-acceptor design.19-25 These properties enable porphyrins to be successfully applied in photovoltaic devices, such as dye-sensitized solar cells with PCEs over 13%.26-27 Porphyrin-based semiconductors have also been widely reported as electron donors in OSCs and obtained the impressing PCEs,21, 28-32 but these materials have been rarely used as the electron acceptors.33-37 In the our previous work, a new star-shape porphyrin-based electron acceptor has been successfully developed and a promising PCE of 7.4% was obtained.34 The result indicated that porphyrin has the great potential to build new non-fullerene acceptors for high performance OSCs. Side chains play crucial impact on the performance of BHJ devices,38-41 such as electronic properties, crystallinity and microphase separation in photoactive layers. For example, Hadmojo et al. reported the first application of porphyrin-based donor in non-fullerene solar cells, in which they found the side chains of the donors could influence the molecular orientation in thin films and resulted in distinct photovoltaic performance.32 In this work, we designed and synthesized four porphyrin-based electron acceptors with different side chains to study their effect on the photovoltaic performance. The new electron acceptors use linear skeletons containing porphyrin as a core unit and perylene bisimides as end groups, in which side chains attached to the porphyrin core varies from 2,6-di(dodecyloxy)-phenyl (o-PBIPor), (2-ethylhexyl)thiophen-2-yl (T-PBIPor), pentadecan-7-yl (HD-PBIPor) to 3,5-di(dodecyloxy)-phenyl (m-PBIPor) (Scheme 1).


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The new electron acceptors were applied into OSCs, in which the PCEs varied from 1.84% to 5.34%. We further perform several techniques, including atomic force microscopy (AFM), grazing-incidence wide-angle X-ray (GIWAXS), photoluminescence spectra (PL) and electroluminescence spectra (EL), to study the relation between the chemical structures and photovoltaic performance. The results indicate that it is important to select side units for non-fullerene electron acceptors in order to realize high performance OSCs.

Scheme 1. Chemical structures of the non-fullerene electron acceptors based on porphyrin and PBIs and their synthetic routes. (i) Sonogashira coupling with Pd(PPh3)4/CuI in THF/triethylamine, 80 °C, 12 h.

2. RESULTS AND DISCUSSION Synthesis of the acceptors The synthetic route for the PBIPor acceptors were present in Scheme 1, and the detailed procedures were summarized in the Experimental Section. All the four acceptors were synthesized via Sonogashira coupling reaction by using diethylnylporphyrin with different side chains and monobromo perylene bisimide with the yield of 56% - 70%. These electron acceptors can be easily purified via chromatography and show good solubility in common solvent, such as


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CH2Cl2, CHCl3 and chlorobenzene (CB) (> 15 mg/ml), which ensure the application in solution-processed OSCs. Optical and electrochemical properties The UV−vis absorption spectra of o-, T-, HD- and m-PBIPor were performed in chlorobenzene (CB) solution and thin films. In solution, all of the acceptors show a similarly strong absorption in the range of 400 – 570 nm with molar extinction coefficients from 2.3×105 to 2.8×105 M−1 cm−1 at maximum absorption peaks (Figure 1a), which related to the Soret-band of the porphyrin and PBI units.34 Another absorption region between 650 – 850 nm can also be found in these acceptors, which is due to the intramolecular charge transfer between the D/A units.42 However, they show different absorption onset. HD-PBIPor shows slightly blue-shift absorption in this region, which may be due to the non-aromatic side units. m-PBIPor performs red-shift absorption compared to other acceptors due to the aromatic side units with two alkyloxy chains far away from porphyrin core (Figure 1a). These electron acceptors show red-shift absorption spectra in thin films (Figure 1b). It is very interesting to observe that HD-PBIPor has the smallest band gap, indicating that the alkyl side chains are helpful for the aggregation of porphyrin in thin films compared to other aromatic units. The optical band gaps (Egs) of o-, T-, HD- and m-PBIPor are calculated as 1.38 eV, 1.40 eV, 1.32 eV and 1.35 eV (Table 1). It is desired to mention that the absorption band in the NIR region of these acceptors show relatively low intensity compared to that in visible light region, which may cause the low quantum efficiency in NIR region in solar cells.


5


5

o-PBIPor T-PBIPor HD-PBIPor m-PBIPor

5

2.5x10 -1

5

-1

2.0x10

5

1.5x10

5

1.0x10

4

5.0x10

300

400

500 600 700 800 Wavelength (nm)

900

1000

Normalized Absorbance

(b)

(a) 3.0x10

ε / M 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

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1.0 0.8 0.6 0.4 0.2 0.0 300

400

500

600 700 800 Wavelength (nm)

900

1000

Figure 1. Absorption spectra of the PBIPor electron acceptors (a) in CB and (b) in thin films.

Cyclic voltammetry (CV) was employed to investigate the electrochemical properties of o-, T-, HD- and m-PBIPor thin films (Table 1 and Figure S1). o-PBIPor shows the deep highest occupied molecular orbital (HOMO) level of -5.54 eV and the lowest unoccupied molecular orbital (LUMO) level of -3.53 eV, while LUMO levels of other three acceptors switch to low-lying positions. Since the wide band gap polymer PBDB-T43 (Scheme S1) that will be used as electron donor in this work has LUMO levels of -3.57 eV (Figure S1e), this means that the LUMO offset between PBDB-T and o-PBIPor is negligible, which may severely limit the exciton separation into free charges. We also perform density functional theory (DFT) calculation to analyze the energy levels of the electron acceptors, as shown in Table S1. All the molecules show localized HOMO on porphyrin and LUMO on PBI units. o-PBIPor has high-lying LUMO level of -3.42 eV, which is consistent with the trend from CV measurement. Table 1. Optical and Electrochemical Properties of the Electron Acceptor PBIPors. Acceptor

Egsol (eV)

o-PBIPor

1.48

Egfilm (eV) 1.38

EHOMO (eV)a

ELUMO (eV)a

-5.54

-3.53


6

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a

T-PBIPor

1.52

1.40

-5.51

-3.68

HD-PBIPor

1.46

1.32

-5.44

-3.63

m-PBIPor

1.49

1.35

-5.50

-3.74

Determined using a work function value of -4.8 eV for Fc/Fc+.

Crystalline properties The crystalline properties of these electron acceptors were studied by using GIWAXS,44 as shown in Figure 2 and the data was summarized at Table S2. All the molecules show good crystalline properties due to their strong (100) and (010) diffraction peaks. o-PBIPor and m-PBIPor have two peaks in the region of 0.3 – 0.5 Å-1, corresponding to the d-spacings around 20.0 Å and 15.0 Å. The diffraction peaks around 0.3 Å-1 is similar to T- and HD-PBIPor, which can attribute to lamellar stacking of alkyl side chains. The other peaks around 0.5 Å-1 may be caused by the crystallization of alkoxyl phenyl units, as also observed in other alkoxyl phenyl-contained materials.45-46 o-PBIPor and m-PBIPor also show strong (010) diffraction peaks around 1.62 Å-1, that is corresponding to strong π-π stacking of conjugated backbone with d-spacing around 3.85 Å, which represents a predominant “face-on” orientation. The results indicate that by using alkyloxy phenyl side chains, the crystallinity of the acceptors can be improved and the orientation of the acceptors can be changed, which will influence the performance in organic solar cells.


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Figure 2. GIWAXS images of (a-d) the PBIPor thin films and (e) the out-of-plane and in-plane cuts of the corresponding GIWAXS patterns. The thin films were spin-coated from CB/pyridine (10%).

Solar cells performance The BHJ OSCs were fabricated by spin-coating PBDB-T:acceptor (1:1 w/w) solution in CB with 10 vol% pyridine as an additive in an inverted cell structure with ITO/ZnO bottom and MoO3/Ag top electrodes. The wide band gap polymer PBDB-T (Scheme S1) was synthesized according to the literature procedure.43 The processing conditions for the active layer, such as the amount of pyridine as solvent additive, the donor to acceptor ratio and the thickness of the active layer were systematically test to achieve optimized performance. Results are summarized in


8

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Tables S3-5. The characteristics of solar cell, including short-circuit current densities (Jsc), open-circuit voltage (Voc), Fill Factor (FF) and PCEs, were measured under the AM1.5G spectrum from solar simulator and then determined by the external quantum efficiencies (EQE) with the AM1.5G (100 mW cm-2) spectrum. The best results of the optimized solar cells are shown in Figure 3 and Table 2.

-2

10 5

(b) 0.6


0.4

0

0.3

-5

0.2

-10

0.1

-15 -1.0

-0.5

0.0 Voltage (V)


0.5

EQE

(a) 15 Current density (mA cm )

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


0.5

0.0 300

1.0

400

500 600 700 800 Wavelength (nm)

900

1000

Figure 3. (a) J-V characteristics in dark (dashed line) and under white light illumination (solid line). (b) EQE of the optimized PBDB-T:PBIPor (1:1) solar cells. Table 2. Characteristics of Optimized Solar Cells of the Donor Polymer PBDB-T with the PBIPor Electron Acceptors. Acceptor

a

Jsc a

Voc

(mA cm−2)

(V)

FF

Jsc EQE

PCE (%)

Eloss

−2 Best avg b (mA cm ) (eV)

o-PBIPor

4.10 ± 0.01 0.88 ± 0.01 0.49 ± 0.01 1.87 1.78

3.85

0.50

T-PBIPor

9.56 ± 0.12 0.79 ± 0.00 0.50 ± 0.02 3.79 3.75

9.26

0.61

HD-PBIPor 9.48 ± 0.06 0.80 ± 0.01 0.53 ± 0.02 4.23 3.99

9.01

0.52

m-PBIPor

10.47

0.54

11.02 ± 0.64 0.81 ± 0.01 0.58 ± 0.01 5.34 5.07

PBDB-T:PBIPor (1:1) fabricated from CB with 10% pyridine.

b

Average PCEs are obtained

from 8 devices.


9


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PBDB-T:o-PBIPor cells show the lowest PCE of 1.87% with Jsc of 4.10 mA cm-2, Voc of 0.88 V and FF of 0.49. The energy loss (Eloss) calculated as the difference between Eg and Voc is 0.50 eV, which is much lower than 0.60 eV.47 This is consistent with the negligible LUMO offset between PBDB-T and o-PBIPor. The PCEs were then enhanced to 3.75%, 4.23% and 5.34% for T-, HD- and m-PBIPor due to the increased Jsc and FF (Table 2). Solar cells based on the three acceptors show relatively low Vocs of 0.79 V, 0.80 V and 0.81 V, so that Elosss are enhanced to 0.61 V, 0.52 V and 0.54 V. The EQE spectra of the optimized devices are shown in Figure 3b, and Jscs from EQE spectra were consistent with those measured from AM1.5G (Table 2). All the cells show broad photoresponse in the range of 300 nm – 900 nm. The maximum EQE values of o-, T-, HD- and m-PBIPor based devices are 0.20, 0.41, 0.45 and 0.51, which is consistent with their Jscs. EQE spectra in the NIR region is relatively low compared to that in visible light region, which is due to their relatively low absorption coefficient in NIR region. We also test the solar cell stability under thermal stress, as shown in Figure S2. PBIPor-based non-fullerene solar cells exhibit a sharp decreased PCE after 10 h thermal annealing, and then the cells were relatively stable. In comparison, fullerene-based solar cells with PBDB-T as electron donor performed continuously decreased PCEs under the same condition. This also indicates that our non-fullerene electron acceptors can provide better thermal stability. In order to study the origination of PCEs difference in these cells, we firstly performed space charge limited current (SCLC) measurement to study the charge transport properties, as shown in Table S6. Except for o-PBIPor, the other acceptor-based blends fabricated from CB with 10% pyridine showed enhanced electron mobility (µe) of 10-5 cm-2 V-1 s-1 compared to 10-6 cm-2 V-1 s-1 in blends without pyridine. This indicates that PBIPor acceptors have better crystallinity for improved charge transport when adding pyridine into solution, which is


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consistent with the observation in literature.33 Therefore, the ratio of hole and electron mobility was reduced in solar cells based on T-, HD- and m-PBIPor, explaining their relatively high FF compared to PBDB-T:o-PBIPor cells (Table S6). We further use the charge dissociation probability P(E, T) that is determined according to the reported method48 to study charge recombination in these cells. The photocurrent density (Jph, defined by JL – JD, JL and JD photocurrent densities under illumination and in the dark) versus effective voltage (Veff, defined by V0 – V, V0 is the voltage where Jph = 0) curves for the solar cells used in this work were shown in Figure 4a and 4b. By using the Jph (Veff = 2V) and Jsc, the P(E, T) can be calculated as 79.2%, 85.3%, 88.4% and 94.5% for o-, T-, HD- and m-PBIPor-based solar cells when the devices were fabricated from CB/pyridine solution, while the P(E, T) were 82.6%, 80.0%, 87.5% and 90.6% when the films were fabricated from CB. From these results, we can conclude that pyridine as additive indeed can reduce the charge recombination due to the increased P(E, T). In addition, PBDB-T:o-PBIPor showed the lowest P(E, T), while m-PBIPor based devices had the highest P(E, T). This is also consistent with their photovoltaic performance. (a)

(b)

10 -2

-2

Jph (mA cm )

10

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


o-PBIPor, P(E,T) = 82.6% T-PBIPor, P(E,T) = 80.0% HD-PBIPor, P(E,T) = 87.5% m-PBIPor, P(E,T) = 90.6%

1

0.0

0.5

1.0

1.5

Veff (V)

2.0

2.5

1

0.1

o-PBIPor, P(E,T) = 79.2% T-PBIPor, P(E,T) = 85.3% HD-PBIPor, P(E,T) = 88.4% m-PBIPor, P(E,T) = 94.5%

Veff (V)

1

Figure 4. Photocurrent density Jph versus effective voltage Veff based on solar cells that were fabricated from (a) CB and (b) CB/pyridine (10%).


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In addition, the Jsc under different light intensities were determined to study the charge recombination according the literature.49 The relationship between Jsc and light intensity (P) can be described by the formula of Jsc ∝ PS, and the best fit for the data is that a straight line with the S ~ 1. As shown in Figure S3, all solar cells exhibit S = 0.97, 0.96, 0.99 and 0.99 for o-, T-, HD- and m-PBIPor when using the CB/pyridine as solvent. High S in PBDB-T:m-PBIPor cells also indicates the low charge recombination compared to o-PBIPor based cells. We also performed AFM and GIWAXS measurement for the blended thin films, as shown in Figure 5. All the four blends were found to show fibril structures in AFM phase images with similar roughness (Figure 5a-d). From GIWAXS measurement, the thin films of PBDB-T with T-, HD- and m-PBIPor have very similar crystalline patterns with strong (100) and (010) diffraction peaks. Beside these diffraction peaks, PBDB-T:o-PBIPor shows an extra diffraction peak at 0.42 Å-1 corresponding to d-spacing of 15.0 Å (Table S2). This is similar with the pure o-PBIPor thin film, indicating that crystallization of o-PBIPor in blended thin films is not disturbed, which may result in large domain and is partially responsible for the low PCE compared to the other three acceptors.


12

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Figure 5. (a-d) AFM phase images (1 µm × 1 µm), (e-h) GIWAXS images and (i) the out-of-plane and in-plane cuts of the corresponding GIWAXS patterns of blended thin films of PBDB-T:PBIPors (1:1) fabricated from CB/pyridine (10%).

PL and EL investigation From photovoltaic performance, an issue is about the highest Voc of the PBDB-T:o-PBIPor cells that is further studied by PL and EL measurement, as shown in Figure 6 and Figure 7. All blended thin films show suppressed PL, indicating the charge transfer between PBDB-T and PBIPors. The efficiency of the charge transfer can be estimated by the quenching efficiency of PBDB-T, which can be calculated from the ratio of PL intensity of different PBDB-T:PBIPor blend film to that of the pure PBDB-T film. The quenching efficiency of PBDB-T:m-PBIPor blend is 91.0%, which is the highest value comparing to its counterparts (89.3% for PBDB-T:T-PBIPor, 86.7% for PBDB-T:HD-PBIPor and 85.9% for PBDB-T:o-PBIPor). The


13


highest quenching efficiency in PBDB-T:m-PBIPor suggest more efficient charge separation and transfer in the bulk, corresponding to its highest Jsc and FF. o-PBIPor PBDB-T:o-PBIPor

6

(b) 1.6

905 nm

3

PL Intensity (× 10 )

PBDB-T

5 4 3 2

PBDB-T

1.2

936 nm

1.0 0.8 0.6 0.4 0.2

1

0.0

0 600

700

(c) 1.6

800 900 Wavelength (nm)

1000

600

1100

(d)

HD-PBIPor PBDB-T:HD-PBIPor

1.4

PBDB-T 3

1.2


3

T-PBIPor PBDB-T:T-PBIPor

1.4

3


(a)


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

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1.0 972 nm

0.8 0.6 0.4

700


m-PBIPor PBDB-T:m-PBIPor

2.0

PBDB-T

1.6

1000

1100

940 nm

1.2 0.8 0.4

0.2 0.0

0.0

600

700


1000

1100

600

700


1000

1100

Figure 6. (a-d) PL spectra of the neat PBDB-T, PBIPor films and PBDB-T:PBIPor blended thin films fabricated from CB with 10% pyridine. All films were excited by a light at wavelength of 532 nm.

Next, the EL measurement was performed to all pure and blends to study the charge transfer state (CT) in our system. As shown in Figure 7, all peaks from blends show little red-shift comparing to that of pure acceptors. These phenomena suggest that in this case, the CT state might be very close to the LUMO level of acceptors. Therefore, the EL emission from CT state could be overlapped by the emission from acceptors. To further track down the reason of the


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relative high Voc of PBDB-T:o-PBIPor based device, the voltage loss from non-radiative recombination pathway ( ) was calculated by the following equation (1): = −

ln ( )

(1)

where EQEEL is the external quantum efficiency of EL, which can be obtained in the Figure S4. The device based on PBDB-T:o-PBIPor shows the lowest of 0.24 V among its counterparts (0.31 V, 0.32 V and 0.31 V for T-PBIPor, HD-PBIPor and m-PBIPor based devices). It is worth noting that the difference (around 0.07 V) is quite close to the difference in Vocs (~0.08 V) between the PBDB-T:o-PBIPor based device and other devices. Therefore, the higher Voc of PBDB-T:o-PBIPor based device can be attribute to its small Voc loss from non-radiative recombination. (a)

(b) 1.0

o-PBIPor PBDB-T:o-PBIPor

1.0

Normalized EL

Normalized EL

T-PBIPor PBDB-T:T-PBIPor

0.8

0.8 0.6 0.4

0.6 0.4 0.2

0.2

0.0 0.0 600

(c)

700


1000

(d)

HD-PBIPor PBDB-T:HD-PBIPor

1.0

-0.2

1100

0.8

600

700


1000

1100

1000

1100

m-PBIPor PBDB-T:m-PBIPor

1.0 0.8

Normalized EL

Normalized EL

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


0.6 0.4

0.6 0.4

0.2

0.2

0.0

0.0 600

700


1000

1100

600

700



15


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Figure 7. Normalized EL spectra of the neat PBIPor films and PBDB-T:PBIPor blended thin films fabricated from CB with 10% pyridine. The acceptors are as following: (a) o-PBIPor, (b) T-PBIPor, (c) HD-PBIPor and (d) m-PBIPor.

3. CONCLUSION In summary, four electron acceptors, o-PBIPor, T-PBIPor, HD-PBIPor and m-PBIPor, based on a porphyrin core and PBI end groups linked via ethynyl units were synthesized and applied into OSCs to study the effect of the side chains on the device performance. These porphyrin-based electron acceptors were found to have small band gaps in films, respectively. These acceptors were applied in OSCs blended with a polymer donor PBDB-T, providing PCEs from 1.87% to 5.34%. Further investigation based on the BHJ systems of PBDB-T with PBIPors reveals that low PCE of the PBDB-T:o-PBIPor cells originate from negligible LUMO offsets, high crystallinity and low quenching efficiency. With lower non-radiative recombination loss, the performance of PBDB-T:o-PBIPor cell shows the highest Voc. Our results demonstrate that the side chain of the porphyrin-based small molecule electron acceptors have the significant influence on the performance for the OSCs, which could be a guideline for the design of new non-fullerene electron acceptors.

4. EXPERIMENTAL SECTION Synthetic procedures for the donor polymer PBDB-T can be found in Supporting Information. General Methods. All synthetic procedures were performed under argon atmosphere. Commercial chemicals were used as received. THF was distilled from sodium under an N2 atmosphere.

5,15-di(trimethylsilyl-ethynyl)-10,20-bis(2,6-di(dodecyloxy)-phenyl)-porphyrin


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zinc (o-Por)50 5,15-di(trimethylsilyl-ethynyl)-10,20-bis((2-ethylhexyl)thiophen-2-yl)-porphyrin zinc

(T-Por),22

(HD-Por),41

5,15-di(trimethylsilyl-ethynyl)-10,20-bis(pentadecan-7-yl)-porphyrin

zinc

5,15-di(trimethylsilyl-ethynyl)-10,20-bis(3,5-di(dodecyloxy)-phenyl)-porphyrin

zinc (m-Por)51 and 5-bromo-2,9-di(undecan-6-yl)anthra[2,1,9-def:6,5,10-d'e'f']diisoquinoline1,3,8,10(2H,9H)-tetraone (Br-PBI)34 were synthesized according to literature procedures. 1H NMR spectra were recorded at 500 MHz and on a Bruker AVANCE spectrometer with TCE-d2 as the solvent and tetramethylsilane (TMS) as the internal standard in 100 °C. Optical absorption spectra were recorded on a JASCO V-570 spectrometer with a slit width of 2.0 nm and a scan speed of 1000 nm min-1. Cyclic voltammetry was performed under an inert atmosphere at a scan rate of 0.1 V s-1 and 1 M tetrabutylammonium hexafluorophosphate in acetonitrile as the electrolyte, a glassy-carbon working electrode coated with samples, a platinum-wire auxiliary electrode, and an Ag/AgCl as a reference electrode. The Molecular weight was determined with GPC at 140 °C on a PL-GPC 220 system using a PL-GEL 13µm Olexis column and o-DCB as the eluent against polystyrene standards. GIWAXS measurements were performed at beamline 7.3.3 at the Advanced Light Source. Samples were prepared on Si substrates using identical blend solutions as those used in devices. The 10k eV X-ray beam was incident at a grazing angle of 0.12°-0.16°, selected to maximize the scattering intensity from the samples. The scattered x-rays were detected using a Dectris Pilatus 2M photon counting detector. Atomic force microscopy (AFM) images were recorded using a Digital Instruments Nano scope IIIa multimode atomic force microscope in tapping mode under ambient conditions. o-PBIPor. To a 50 mL two necked round-bottom flask were added o-Por (100 mg, 0.07 mmol), Br-PBI (118 mg, 0.15 mmol), anhydrous THF (4 mL) and triethylamine (1 mL), and the mixture was deoxygenated with argon for 30 min before Pd(PPh3)4 (8 mg, 0.007 mmol) and CuI


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(1.9 mg, 0.01 mmol) were added. Then the mixture was stirred at 80 °C for 12 h under argon. After cooled to room temperature, the mixture was washed with water and dried over anhydrous Na2SO4. Then the solvent was removed, and the residue was purified by column chromatography on silica gel to give a brown solid with yield by 60%. 1H NMR (500 MHz, CDCl2CDCl2): δ 11.07-11.06 (d, 2H), 9.89 (s, 4H), 9.56 (s, 2H), 9.15 (s, 4H), 9.02-9.00 (s, 2H), 8.82 (s, 8H), 7.83-7.81 (t, 2H), 7.15-7.13 (d, 2H), 5.32-5.27 (m 4H), 3.98 (s, 8H), 2.40-2.35 (m, 8H), 2.03-2.02 (m, 8H), 1.45-1.43, 1.12-0.54 (m, 166H). MS (MALDI-TOF): m/z: 2703.9 (M+). T-PBIPor. The same method as o-PBIPor and 70% yield. 1H NMR (500 MHz, CDCl2CDCl2) δ 10.54 (s, 2H), 9.53 (s, 4H), 9.29-9.26(t, 4H), 9.19(s, 2H), 8.76-8.74(d, 2H), 8.69-8.68(d, 2H), 7.88(s, 2H), 7.26 (s, 2H), 5.25-5.17(d, 4H), 3.16-3.15(d, 4H), 2.34-2.24 (m,

8H), 2.00-1.94 (m,

12H), 1.70-1.61 (m, 8H), 1.53-1.00 (m, 12H), 0.88 (d, 28H). MS (MALDI-TOF): m/z: 2203.6 (M+). HD-PBIPor. The same method as o-PBIPor and 56% yield.

1

H NMR (500 MHz,

CDCl2CDCl2) δ 11.00-10.98 (d, 2H), 10.00 (s, 4H), 9.92 (s, 4H), 9.56 (s, 2H), 9.05-9.03 (d, 2H), 8.87-8.81 (m, 8H), 5.34-5.30 (t, 6H), 3.05-2.93 (d, 8H), 2.39-2.38 (m, 8H),

2.08- 2.05 (m, 8H),

1.76 (s, 4H), 1.45-1.19 (d, 26H), 0.99-0.95 (d, 24H), 0.82-0.78(m, 12H). MS (MALDI-TOF): m/z: 2235.7 (M+). m-PBIPor. The same method as o-PBIPor and 63% yield. 1H NMR (500 MHz, CDCl2CDCl2) δ 10.79 (s, 2H), 9.76 (s, 4H), 9.34 (s, 2H), 9.25 (s, 4H), 8.89-8.87 (d, 2H), 8.79 (s, 8H), 7.55 (s, 4H), 7.06 (s, 2H), 5.33-5.26 (d, 4H), 4.32 (s, 8H), 2.42-2.33 (m, 8H), 2.08-2.01 (m, 16H), 1.64-1.28 (m, 139H), 0.97-0.87 (m, 44H). MS (MALDI-TOF): m/z: 2703.9 (M+). Solar Cells Fabrication and Characterization. Photovoltaic devices with inverted configuration were made by spin-coating a ZnO solgel at 4000 rpm for 60 s onto pre-cleaned,


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patterned ITO substrates. The photoactive layer was deposited by spin coating a chlorobenzene solution containing PBDB-T and o-, T-, HD- and m-PBIPor and the appropriate amount of pyridine as processing additive in air. MoO3 (10 nm) and Ag (100 nm) were deposited by vacuum evaporation at ca. 4 × 10-5 Pa as the back electrode. The active area of the cells was 0.04 cm2. The J-V characteristics were measured by a Keithley 2400 source meter unit under AM1.5G spectrum from a solar simulator (Enlitech model SS-F5-3A). Solar simulator illumination intensity was determined at 100 mW cm−2 using a monocrystal silicon reference cell with KG5 filter. Short circuit currents under AM1.5G conditions were estimated from the spectral response and convolution with the solar spectrum. The external quantum efficiency was measured by a Solar Cell Spectral Response Measurement System QE-R3011 (Enli Technology Co., Ltd.). The thickness of the active layers in the photovoltaic devices was measured on a Veeco Dektak XT profilometer. PL and EL Characterization. The PL and EL spectra are recorded with an Andor spectrometer (Shamrock sr-303i-B, coupled to a Newton EMCCD detector). For the EL measurement, an external current/voltage source is employed to provide an external electric field to the pristine solar cells. EQEEL is recorded with a homemade system comprising a Hamamatsu silicon photodiode 1010B, a Keithley 485 picoammeter for measuring the emitted light intensity and a Keithley 2400 Source Meter for supplying voltages and recording injected current. Device structure for the EL and EQEEL measurement: ITO/ZnO/active layer/MoO3/Ag.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.


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Synthesis of PBDB-T, CV, DFT, crystalline properties, solar cells, EL and NMR spectra. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This study is jointly supported by MOST (2017YFA0204702) and NSFC (51773207, 21574138, 51603209, 91633301) of China. This work was further supported by the Strategic Priority Research Program (XDB12030200) of the Chinese Academy of Sciences and the Recruitment Program of Global Youth Experts of China. Wei Ma thanks for the support from Ministry of science and technology (No. 2016YFA0200700), NSFC (21504066, 21534003). X-ray data was acquired at beamlines 7.3.3 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors thank Chenhui Zhu at beamline 7.3.3 for assistance with data acquisition. Y.L and F. Z. acknowledge financial support from CSC (201706920024), the Knut and Alice Wallenberg Foundation under contract 2016.0059, the Swedish Research Council (2017-04123), and the Swedish Government Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No 200900971).


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Table of Contents Graphic


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Effect of Side Groups on the Photovoltaic Performance Based on

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