Dimeric Porphyrin Small Molecules for Efficient Organic Solar Cells

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Dimeric Porphyrin Small Molecule for Efficient Organic Solar Cells with High Photo-Electron Response in Near Infrared Region Tianqi Lai, Liangang Xiao, Ke Deng, Tianxiang Liang, Xuebin Chen, Xiaobin Peng, and Yong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15506 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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ACS Applied Materials & Interfaces

Dimeric Porphyrin Small Molecule for Efficient Organic Solar Cells with High Photo-Electron Response in Near Infrared Region Tianqi Lai,§ Liangang Xiao,§ Ke Deng, Tianxiang Liang, Xuebin Chen, Xiaobin Peng,* Yong Cao State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, 381 Wushan Road, Guangzhou 510640, P. R. China. E-mail: [email protected] Abstract: Small molecules (SMs) with elongated backbones are promising for achieving higher photovoltaic performance. Herein, a dimeric porphyrin small molecule ZnP2-DPP consisting of two porphyrin units linked with an ethynylene as the core and two diketopyrrolopyrrole (DPP) units as the arms is designed and synthesized as an electron donor for solution-processed bulk-heterojunction (BHJ) organic solar cells (OSCs). And a significantly enhanced power conversion efficiency of 8.45% with an impressive short-circuit current density (JSC) up to 19.65 mA cm-2 is achieved for the BHJ OSCs based on ZnP2-DPP under AM 1.5G irradiation (100 mW cm-2) compared to the OSCs based on the dimeric porphyrin linked with bis-ethynylenes reported previously. Furthermore, the devices show broad photo-electron response up to 1000 nm with high near infrared external quantum efficiency up to 66% at 780 nm. This is the first report that SM OSCs show such a large JSC about 20 mA cm-2 simultaneously with a considerably high and deep photo-electron response up to 1000 nm. Keywords:

small

molecules,

organic

solar

cells,

dimeric

porphyrin,

diketopyrrolopyrrole, near infrared Introduction: Bulk-heterojunction organic solar cells (BHJ OSCs) are promising for fabricating lightweight and flexible devices via the low-cost and high-throughput roll-to-roll 1

ACS Paragon Plus Environment

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production process.1-3 Polymer solar cells have been widely studied and power conversion efficiencies (PCEs) up to 13% have been achieved for single-junction devices with BHJ architectures,4,5 which are contributed by the low band-gaps and the inter-region connectivity for the efficient charge transport among domains induced by the long electron-delocalized polymer chains. 6-11 On the other hand, small molecules (SMs) show some advantages including well-defined chemical structures, almost no batch-to-batch variations, the lower entropic barriers and the lack of chain entanglement to form ordered aggregations in solid states compared to polymers.12 Although less investigated, SM OSCs have also reached 10% PCE in the last few years.13 In view of the advantages of polymers and SMs, the SMs with elongated backbones are promising for achieving higher organic photovoltaic (OPV) performance, which have been demonstrated by recent reports. For examples, Chen reported a series of high performance SMs based on oligothiophenes,13,14 and Wang found that some multi-fluorine substituted oligomers showed much higher PCEs than the polymers with the same backbones.15 Porphyrins

show

high

absorption

coefficients,

and

their

photo-

and

electro-chemical properties can be easily tuned through the functionalization of the peripheries. Inspired by the photosynthesis in nature, porphyrins and their derivatives have been explored as organic semiconductors for several decades.16,17 However, the performance of porphyrin-based OSCs was low. Recently, after ethynylene bridges have been introduced between a porphyrin core and electron acceptor units to construct conjugated donor-acceptor (D-A) porphyrin SMs.18-21 the PCEs of porphyrin-based OSCs have been significantly enhanced up to 9% through molecular design and device optimization,22,23 indicating that porphyrin unit is an excellent building block for OSCs. In order to investigate the photovoltaic properties of porphyrins with elongated backbones, we designed and synthesized three dimeric porphyrins BHJ OSCs very recently.24 Though the PCEs are much higher than those dimeric porphyrins without electron acceptor units, they are still moderate with a maximum PCE of only 6.42% 2


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after device optimizations. Considering that the linkage between the two porphyrin units impacts the intra-molecular charge transfer (ICT),24 herein, we design a new dimeric porphyrin ZnP2-DPP with two porphyrin units bridged by only one ethynylene bridge as the core and two diketopyrrolopyrrole (DPP) units as the arms for solution-processed

SM OSCs.

The

optimized BHJ

OSCs

based

on

ZnP2-DPP:PC61BM (PC61BM: [6,6]-phenyl-C61-butyric acid methyl ester) show broad photo-electron response up to 1000 nm with high near infrared external quantum efficiencies up to 66% and 32% at 780 and 900 nm, respectively, in near infrared region, and an enhanced PCE of 8.45% is achieved with an impressive short-circuit current density (JSC) up to 19.65 mA cm-2 under AM 1.5G irradiation (100 mW cm-2).

3



C2H5

C4H9

C2H5

Page 4 of 19

C4H9

S S N

N TIPS

Zn N

N N

Br

N Zn

TIPS N

N S

C4H9

S

Pd

2 db

a3 ,

C2H5

TIPS-ZnP-E

P(o -

tol

C4H9

yl)

3,

CH2Cl2/pyridine CuCl, TMEDA, dry air, 25 oC C2H5

C4H9

TH F/E

C2H5

TIPS-ZnP-Br t3 N ,6

0

o

C

C4H9

C2H5

C4H9

C2H5

C2H5

C4H9

S S

N N

N

N Zn

N

N

R

N

Zn

R

S

S

N

N

N

N

N

Zn

R N

N S

S

N

Zn N

R

S

S

C4H9

C2H5

C4H9

C2H5

C4H9

C2H5

C4H9

C2H5

R = TIPS, ZnP2-TIPS, yield 89%

R = TIPS, (TIPS-ZnP-E)2, yield 85%

TBAF, THF, 0 oC

TBAF, THF, 0 oC

R=H, ZnP2-E

R=H, (H-ZnP-E)2 C2H5 C2H5

C4H9

C4H9 Br

N S

O S

O

N

Br

N

S

O

Pd(PPh3)4, CuI THF/Et3N, 60 oC

O

S N

Pd(PPh3)4, CuI THF/Et3N, 60 oC

C4H9

C4H9

C2H5 C2H5

DPP-Br

DPP-Br C2H5

C4H9

C2H5

C4H9

S

O

N S

S N

N

N

N

N

Zn N

N

Zn N

O C4H9

C4H9

N S

O

C2H5 C4H9

C2H5

N

N

N

N

N

Zn

S N

N

N

C4H9

N N

Zn N

S

O

O

C4H9

S

O S

N

C4H9

S

C2H5

C2H5 C2H5

N

S

C4H9

S

C2H5

C4H9

O S

O S

S

C2H5

C2H5 N

C2H5

C4H9

S

S

C2H5 C4H9

C2H5

C4H9

C2H5

C4H9

C2H5

(DPP-ZnP-E)2, yield 85%

C4H9

C2H5

C4H9

ZnP2-DPP, yield 77%

Scheme 1. Synthetic routes of (DPP-ZnP-E)2 and ZnP2-DPP.

Results and Discussions: TIPS-ZnP-E and (DPP-ZnP-E)2 were synthesized according to the reported procedures.24 As shown in Scheme 1, the intermediate dimeric porphyrin ZnP2-TIPS was synthesized through by the reaction between TIPS-ZnP-Br and TIPS-ZnP-E. Finally, after the de-protection of the terminal TIPS groups of ZnP2-TIPS with tetrabutylammonium fluoride (TABF) to afford ZnP2-E, the target molecule ZnP2-DPP

was

synthesized

by

a

coupling

reaction

4


of

ZnP2-E

with

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3-(5-bromo-thiophene-2-yl)-2,5-bis-(2-ethyl-hexyl)-6-thiophene-2-yl-2,5-dihydro-pyr rolopyrrole-1,4-dione (DPP-Br).25 The molecular structure and purity were verified by 1

H NMR spectroscopy, elemental analysis and MALDI-TOF spectrometry.

ZnP2-DPP is thermally stable with less than 5% weight loss at 408 °C under nitrogen. As shown in Figure 1 and listed in Table 1, in solution, ZnP2-DPP exhibits four absorption peaks with the maximum Soret and Q bands at 502 and 789 nm, respectively. Compared with (DPP-ZnP-E)2, ZnP2-DPP exhibits a red-shifted and intensified Q band, suggesting the facilitated ICT in ZnP2-DPP. In thin film, ZnP2-DPP shows absorption up to 1000 nm, and the two main peaks are red-shifted to 509 and 868 nm, respectively. The significantly red-shifted and intensified near-infrared band 868 nm can be ascribed to the strong intermolecular π-π stacking in solid state. Similar to the case in solution, ZnP2-DPP also shows an intensified ICT band than (DPP-ZnP-E)2 in film, further supporting that the ICT is improved in ZnP2-DPP after the replacement of the bis-ethynylene in (DPP-ZnP-E)2 with a mono-ethynylene in ZnP2-DPP. The optical band gap (Egopt) is calculated to be 1.28 eV for ZnP2-DPP from the absorption onset in film, which is slightly smaller than that of 1.36 eV for (DPP-ZnP-E)2.

5



Figure 1. UV-Vis-NIR absorption spectra of ZnP2-DPP in THF solution (a) and thin film (b), and (c) device structure of the solar cells, and (d) the energy diagram of the OSC materials.

Cyclic voltammetry (CV) in thin film was measured (Figure S1) to estimate the energy level of the highest occupied molecular orbital (HOMO) (EHOMO) from the onset oxidation potential (φox) according to the equation of EHOMO=−e(φox+4.8−φFc/Fc+) (eV), where φFc/Fc+ is the redox potential of ferrocene/ferrocenium couple and the energy level of Fc/Fc+ was taken as 4.8 eV below vacuum.26,27 Since φFc/Fc+ was measured to be 0.44 V versus Ag/AgCl, the EHOMO from the onset oxidation potential is calculated to be −5.13 eV, which is slightly deeper than (DPP-ZnP-E)2 (−5.09 eV). And the energy level of the lowest unoccupied molecular orbital (LUMO) (ELUMO) is calculated to be −3.85 eV from the equation of Egopt = ELUMO−EHOMO. 6


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Table 1. Optical and electrochemical data of ZnP2-DPP. λmax solution

λmax film

EHOMO

ELUMO

Egopt

(nm)

(nm)

(eV)

(eV)

film (eV)

502, 789

509, 868

−5.13

−3.85

1.28

Compound ZnP2-DPP

As shown in Figure 1c, the solution-processed BHJ devices were fabricated with a convention device structure of glass/ITO/PEDOT:PSS/ZnP2-DPP:PC61BM/PFN/Al (PFN:

poly[9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene]),28

and

the

schematic energy diagram of the OSC materials is seen in Figure 1d. The devices were optimized by varying thermal annealing (TA) temperature, TA time and solvent vapor annealing (SVA) time of chloroform (CF), and the device fabrication conditions are detailed in SI. Figure 2 shows the current density-voltage (J-V) and the external quantum efficiency (EQE) curves of the devices upon different post-processing, and the corresponding photovoltaic parameters are summarized in Table 2. Without any post-treatment, the devices show a low PCE of 1.82% with an open-circuit voltage (Voc) of 0.81 V, a short-circuit current (Jsc) of 7.16 mA cm-2 and a small fill factor (FF) of 31.31%. Upon thermal annealing, the PCE is significantly enhanced to 6.86% with remarkably improved Jsc and FF values to 18.90 mA cm-2 and 54.19%, respectively, though the Voc is reduced to 0.67 V. It should be noted that the OPV performance is remarkably improved in comparison with (DPP-ZnP-E)2-based devices upon TA (PCE=4.50%, Jsc = 14.37 mA cm-2, Voc = 0.68V, and FF = 46.04%).22 Furthermore, though SVA has been demonstrated to be effective for improving the performance of many OSCs,29-33 it has no positive effects on (DPP-ZnP-E)2-based ones. On the contrary, chloroform SVA treatment further improves the PCE of ZnP2-DPP-based devices up to 8.45% with simultaneously enhanced Jsc and FF values up to 19.65 mA cm-2 and 66.15%, respectively. It is noted that the Jsc of 19.65 mA cm-2 is the highest for BHJ SM OSCs to date. According to the absorption spectra and mobility measurements reported below, the facilitated ICT of ZnP2-DPP plays a very 7



important role for the much improved values of PCE and Jsc for the devices based on ZnP2-DPP than (DPP-ZnP-E)2.

Figure 2. (a) J-V and (b) EQE curves of ZnP2-DPP-based devices without post-treatment, with TA and with TA&SVA.

As shown in Figure 2b, ZnP2-DPP-based devices exhibit much higher and broader EQE response region than (DPP-ZnP-E)2 with the response edge up to 1000 nm upon TA and TA&SVA, which can be ascribed to the red-shifted absorption of ZnP2-DPP than (DPP-ZnP-E)2. Upon TA treatment, while the two maximum EQE peaks are 55% and 49% at 490 and 770 nm, respectively, for (DPP-ZnP-E)2-based OSCs, they are enhanced to 64% and 66% at 500 and 780 nm, respectively, for ZnP2-DPP-based ones. Upon the further SVA, the EQE response region for ZnP2-DPP-based OSCs is further broaden. This is the first report that the EQE response is to 1000 nm with high EQE values more than 50% at a wavelength longer than 850 nm. The broader EQE response region and much higher EQE values contribute to the much larger Jsc for ZnP2-DPP-based OSCs upon TA and SVA, which are also consistent with the conclusion that the ICT in ZnP2-DPP is facilitated after the replacement of the bis-ethynylene with one ethynylene. Since the photo response regions of most highly-efficient OSCs are confined to visible region or their EQE values in NIR region usually are small, our results indicate that dimeric porphyrins may be promising for achieving high performance tandem and ternary 8


Page 8 of 19

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organic solar cells. Table 2. Device parameters for the OSCs based on ZnP2-DPP.

a

Treatment

Jsc (Jcal)a(mA cm-2)

Voc (V)

FF (%)

PCE (avg)b(%)

CAST

7.16 (7.14)

0.81

31.31

1.82 (1.68 ± 0.14)

TA

18.90 (18.27)

0.67

54.19

6.86 (6.78 ± 0.08)

TA&SVA

19.65 (19.01)

0.65

66.15

8.45 (8.34 ± 0.11)

Jcal: Integrated from the EQE spectrum, baverage value PCE with standard deviation

of 10 different devices.

To investigate the effects of TA and SVA on the device performance, we measured the

absorption

spectra,

hole

mobilities

and

surface

morphology

of

ZnP2-DPP:PC61BM blend films under different processing conditions. As showed in Figure 3a, due to the presence of PC61BM, the absorption peaks assigned to ZnP2-DPP for the as-cast blend films are seen at 506 and 859 nm, which are slightly different from those of ZnP2-DPP pure film. Upon thermal annealing, the 859 nm peak of the as-cast ZnP2-DPP:PC61BM blend films significantly blue-shifts to 771 nm with a very weak shoulder at ca. 850 nm. Since the near IR absorption peak of ZnP2-DPP in solution is at 789 nm, the even shorter wavelength 771 nm peak in the blend films upon TA can be ascribed to the intermolecular π-π stacking of more ordered H-aggregations, which could be beneficial for improving the OPV performance.34,35 For the films with further SVA treatment, the 771 nm peak decreases with almost no peak shift, but the intensity of 850 nm shoulder slightly increases, indicating the self-aggregation of ZnP2-DPP is slightly changed upon the further SVA treatment. Hole mobilities were measured using the space-charge limited current method (SCLC)

with

the

hole-only

device

structure

of

ITO/PEDOT:PSS/active

layer/MoO3/Al. The J-V plots are provided in Figure 3b and the corresponding data are summarized in Table 3. For the devices without any post-processing, the hole mobility is1.82×10-5 cm2 V-1 s-1, which is improved to 3.96×10-5 cm2 V-1 s-1 upon TA 9



Page 10 of 19

and further to 1.44×10-4 cm2 V-1 s-1 upon TA&SVA. The improved mobilities upon TA and TA&SVA contribute to the enhanced performance than those with no post-treatment. However, while the PCEs of the OSCs based on ZnP2-DPP upon TA and TA&SVA are significantly higher than that of (DPP-ZnP-E)2-based upon TA, the hole mobilities of ZnP2-DPP-based devices upon TA and TA&SVA are lower than and similar to, respectively, that of (DPP-ZnP-E)2-based ones(1.46×10-4 cm2 V-1 s-1). It is possible that the facilitated ICT in ZnP2-DPP contributes significantly to the improved photocurrent generation, leading to significantly enhanced Jsc values and PCEs for ZnP2-DPP-based devices than (DPP-ZnP-E)2-based ones upon TA and TA&SVA.

Figure 3. a) UV-Vis-NIR absorption spectra of ZnP2-DPP:PC61BM blend films upon different treatment; b) J-V characteristics under dark for hole-only devices based on ZnP2-DPP:PC61BM (1:1).

Table 3. Hole mobilities based on ZnP2-DPP:PC61BM (1:1) blend films.

µh(cm2 V-1 s-1)

As-Cast

TA

TA&SVA

1.82 × 10-5

3.96 × 10-5

1.44 × 10-4

The surface morphology of ZnP2-DPP:PC61BM blends was investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). As shown in Figure 4a, b and c, all the films show quite smooth surfaces with root-mean-square 10


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(RMS) roughness values of 0.68, 0.71 and 0.91 nm for the as-cast, TA and TA&SVA-treated films, respectively, indicating the good miscibility of the donor with PC61BM the acceptor. Figure 4d, e and f show the TEM images of the blend films. Compared with the as-cast film, better phase separation could be found after TA process. With further SVA treatment, more bi-continuous interpenetrating networks can be clearly observed, which contributes to higher charge transport efficiency, leading to the improved Jsc and FF values.36

Figure 4. AFM height images (a, b and c) and TEM images (d, e and f) of ZnP2-DPP:PC61BM blend films upon different post-treatment.

We further measured the photocurrent density (Jph) versus the effective voltage (Veff) to investigate the charge generation, dissociation and collection properties of the related OSCs (Jph = JL−JD, JL and JD are the current densities under illumination and in the dark, respectively. And Veff = V0−Va, V0 is the voltage when Jph = 0, Va is the applied voltage.). The saturated Jph (Jsat) value depends on the maximum exciton generation rate (Gmax) and can be estimated using Jsat = qGmaxL (q: elementary charge, L: active layer thickness), and the larger Jph/Jsat ratio usually indicates higher charge dissociation and collection efficiencies.28,37-39 As shown in Figure 5, the devices 11



based on ZnP2-DPP:PC61BM with TA and TA &SVA show quite similar Jsat values of 203 and 211 A m-2 and the Gmax values are calculated to be 1.27×1028 and 1.32×1028 m-3 s-1, respectively. However, the Jsat and Gmax values for the (DPP-ZnP-E)2-based devices with TA treatment are only 174 A m-2 and 1.09×1028 m-3 s-1, respectively (Figure S8).24 The larger Gmax values for ZnP2-DPP-based devices are consistent with the wider absorption range of ZnP2-DPP. At a Veff of 0.5 V, the Jph values of ZnP2-DPP-based devices with TA and TA&SVA treatment are almost saturated with Jph/Jsat values of 89.9% and 92.8%, respectively.38 On the other hand, the Jph/Jsat value of 90% for (DPP-ZnP-E)2-based devices with TA is at a much higher Veff of 1.2 V and the Jph/Jsat at Veff of 0.5 V is only 75%, suggesting that the charge dissociation and collection efficiencies for (DPP-ZnP-E)2-based devices is not so effective compared to ZnP2-DPP-based devices with TA and TA&SVA treatment.

Figure 5. Net J-V characteristics of ZnP2-DPP:PC61BM based devices with different treatments under constant incident light intensity (AM 1.5G, 100 mW cm-2).

Conclusions: In summary, a new dimeric porphyrin ZnP2-DPP consisting of two porphyrin units linked with an ethynylene bridge as the core is designed and synthesized as the electron donor for BHJ solar cells. Replacing the bis-ethynylene linkage in (DPP-ZnP-E)2 with a mono-ethynylene between two porphyrin units can enhance the 12


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intramolecular charge transfer. Under AM 1.5G irradiation (100 mW cm-2), the optimized BHJ solar cells show broad photo-electron response up to 1000 nm with high near infrared EQE values up to 66% and 32% at 780 and 900 nm, respectively, leading to an impressive Jsc up to 19.65 mA cm-2. Our results demonstrate that the intramolecular charge transfer, absorption, HOMO and LUMO energy levels can be tuned to enhance the OPV performance by optimizing the bridges of dimeric porphyrins, providing a guideline for designing SM OPV materials with elongated backbones. Furthermore, the broad photo-electron response up to 1000 nm with high near infrared EQE values of the OSCs suggest that dimeric porphyrins are promising for achieving high performance tandem and ternary organic solar cells. Experimental section ZnP2-TIPS. After the mixture of TIPS-ZnP-E (510 mg, 0.53 mmol) and TIPS-ZnP-Br (384 mg, 0.37 mmol) in tetrahedrofuran (THF) (40 mL) and triethylamine (8 mL) was purged with argon for 30 min, Pd2dba3 (43 mg, 48 µmol) and P(o-tol)3 (114 mg, 37 µmol) were added. And then the mixture was stirred at 60oC for 24 h under inert atmosphere. After some routine procedures, the residue was first purified by column chromatography on silica gel then by GPC to give a dark brown solid (Yield: 89%). 1H NMR (500 MHz, CDCl3) δ: 10.17 (d, J = 4.5 Hz, 4H), 9.68 (d, J = 4.5 Hz, 4H), 9.30 (d, J = 4.5 Hz, 4H), 9.19 (d, J = 4.5 Hz, 4H), 7.73 (d, J = 3.0 Hz, 4H), 7.18 (d, J = 3.0 Hz, 4H), 3.09 (d, J = 6.5 Hz, 8H), 1.88 (m, 4H),1.70-1.32(m, 74), 1.05 (dt, 24H). Mass (MALDI-TOF): Obs. 1906.56; Calcd. for C112H134N8S4Si2Zn2: 1905.77. UV-vis (THF), λmax= 481 nm. ZnP2-E. TBAF (1 M in THF, 0.39 mmol) was added to ZnP2-TIPS (343 mg, 0.18 mmol) in 10 mL of THF at 0 °C was and then stirred for 5 min. Then, the reaction was quenched with water, and extracted with chloroform. Purification of the organic layer gives ZnP2-E product. ZnP2-DPP. After the solution of ZnP2-E (255 mg, 0.16 mmol) and DPP-Br(289 mg, 0.48 mmol) in THF (30 mL) and triethylamine(15 mL) was purged with argon for 30 min, Pd(PPh3)4 (18.5 mg, 16 µmol) and CuI (3.2 mg, 16 µmol) were added. Then 13



Page 14 of 19

the mixture was stirred at 60 oC for 24h under argon. After some routine procedures, the residue was first purified by column chromatography on silica gel then by GPC to give

a

black

solid

ZnP2-DPP

(Yield:

76.7%).

1

H

NMR

(500

MHz,

CDCl3+Pyridine-d5) δ 10.10 (d, J = 3.5 Hz, 4H), 9.28 (d, J = 4.0 Hz, 4H), 9.07-9.02 (m, 8H), 8.74(s, 2H), 8.50 (s, 2H), 7.80 (d, J = 2.0 Hz, 4H), 7.25(d, J = 3.0 Hz, 4H), 7.13(d, J = 7.5 Hz, 2H), 6.85-6.74 (m, 4H), 3.37 (s, 8H), 3.15 (d, J = 5.5 Hz, 8H)1.97-1.92 (m, 4H),1.75-1.44 (m, 36H),1.26-1.01 (m, 56H), 0.86-0.66 (m, 24H). Mass (MALDI-TOF): Obs. 2639.51; Calcd. for C154H170N12O4S8Zn2, 2639.98. UV-Vis (THF), λmax= 502, 789 nm.

Supporting information Supporting Information Available: device fabrications, 1H NMR spectrum, MALDI TOF Mass spectrum, TGA curve, Cyclic voltammogram, J-V curves and photovoltaic parameters of the BHJ solar cells. Author information Corresponding Author *E-mail: [email protected]. Author Contributions §

T.L. and L.X. contributed equally to this work.

Notes The authors declare no competing financial interest. Acknowledgements This work was financially supported by the grants from the National Natural Science Foundation of China (51473053), International Science and Technology Cooperation Program of China (2013DFG52740), the National Key Research and Development Program of China (2017YFA0206602) and the Fundamental Research Funds for the Central Universities.

References 14


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Graphical Abstract:

19


Dimeric Porphyrin Small Molecules for Efficient Organic Solar Cells

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