End group tuning in small molecule donors for non-fullerene organic solar cells

Journal Pre-proof End group tuning in small molecule donors for non-fullerene organic solar cells Jie Guo, Dmitry O. Balakirev, Chengjun Gu, Svetlana M. Peregudova, Sergei A. Ponomarenko, Zhitian Liu, Yuriy N. Luponosov, Jie Min, Aiwen Lei PII:

S0143-7208(19)32444-1

DOI:

https://doi.org/10.1016/j.dyepig.2019.108078

Reference:

DYPI 108078

To appear in:

Dyes and Pigments

Received Date: 17 October 2019 Revised Date:

14 November 2019

Accepted Date: 26 November 2019

Please cite this article as: Guo J, Balakirev DO, Gu C, Peregudova SM, Ponomarenko SA, Liu Z, Luponosov YN, Min J, Lei A, End group tuning in small molecule donors for non-fullerene organic solar cells, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.108078. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

End group tuning in small molecule donors for non-fullerene organic solar cells Jie Guo, a‡ Dmitry O. Balakirev,b‡ Chengjun Gu,

c,d‡

Svetlana M. Peregudova,e Sergei

A. Ponomarenko,b,f Zhitian Liu, d Yuriy N. Luponosov, b,f* Jie Min,c,g,h,* Aiwen Lei a,*

a

College of Chemistry and Molecular Sciences, the Institute for Advanced Studies

(IAS), Wuhan University, Wuhan, Hubei 430072, China b

Enikolopov Institute of Synthetic Polymeric Materials of the Russian Academy of

Sciences, Profsoyuznaya st. 70, Moscow, 117393, Russia c

The Institute for Advanced Studies, Wuhan University, Wuhan 430072, China

d

Hubei Engineering Technology Research Center for Optoelectronic and New Energy

Materials, School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430205, P. R. China e

Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,

Vavilova St. 28, Moscow, 119991, Russia f

Chemistry Department, Moscow State University, Leninskie Gory 1-3, Moscow

119991, Russia g

Key Laboratory of Materials Processing and Mold (Zhengzhou University), Ministry

of Education, Zhengzhou, 450002 China h

Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic

Chemistry, Chinese Academy of Sciences, China *Corresponding author. ‡ The first three authors contributed equally to this paper. E-mail addresses: [email protected]

(Yu.N.

Luponosov),

[email protected] (Aiwen Lei)

[email protected]

(Jie

Min),

Abstract Two novel small molecules, which consist of electron donating benzodithiophene core bridged through bithiophene π-spacer with terminal either dicyanovinyl (DCV-Me) or n-butyl cyanoester (CNAB) electron-withdrawing groups, were designed and used as donor materials for all-small molecule OSCs (all-SM-OSCs) with a non-fullerene acceptor (IDIC). The novel donor oligomers were firstly characterized by thermal gravimetric analysis, differential scanning calorimetry, UV-Vis spectroscopy, and cyclic voltammetry as well as studied by density functional theory calculations. The simple change of the DCV-Me to CNAB group leads to more pronounced crystallinity, higher solubility and higher energy levels in the donor BDT-2T-CNAB. The photovoltaic devices based on the BDT-2T-CNAB:IDIC blend exhibit higher short-circuit current (Jsc) and fill factor, and thus much higher power conversion efficiency (PCE) of 6.17 % than those of BDT-2T-DCV-Me:IDIC devices (1.56 %). Compared to the BDT-2T-DCV-Me system, the BDT-2T-CNAB based device shows smoother film surface morphology, and superior exciton dissociation, charge generation and charge carrier mobilities as well as lower non-geminate recombination losses. The results clearly demonstrate that the design of new small molecule donors for high-performance all-SM-OSCs should aim to choose suitable end acceptor units, among which the alkyl cyanoester groups are one of the most promising.

1. Introduction Solution-processed organic solar cells (OSCs) based on bulk heterojunction (BHJ) have attached lots of attention due to their advantages of light weight, low-cost, flexible substrates, and semitransparency.1-4 The OSCs generally contain a BHJ blend active layer of p-type and n-type organic semiconductors as electron donor and electron acceptor materials, respectively. In the past years, the acceptors based on fullerene derivative such like [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), have achieved high device

performance in OSCs.5, 6 Despite the success of fullerene acceptors, the shortcomings such like limited visible light absorptions, difficult to tune energy levels, and weak morphological instability limiting further research and applications of OSCs.7, 8 To date, researchers have designed and synthesized amounts of novel non-fullerene acceptors (NFAs).9-11 Compared to fullerene acceptors, NFAs have some advantages, such like easily tunable molecular energy levels, excellent optical absorption properties, good batch reproducibility, and potentially low cost.2, 12 Nowadays, with well-designed molecular structure, the performance of non-fullerene OSCs has exceeded that of fullerenes single OSCs.13, 14 In recent years, along with the well-developed conjugated polymers, small-molecule (SM) OSCs have attracted significant attention due to their advantages such as well-defined molecular structure, well reproducible synthesis, easy purification, and easy tuning of energy levels.15 However, the PCEs of SM-based OSCs is still lower than the polymer OSCs based on their polymeric counterparts. The highest PCE of SM OSCs is slightly over 12 % and only a few research groups reported PCEs over 10 %.2, 5, 8, 16 Compared with their polymer counterpart, the poor photovoltaic performance of SM OSCs probably due to the small molecule donors have unsuitable molecular energy levels, inappropriate ability to crystallization, narrow and weak absorption spectrum for solar light, high non-radiative recombination, etc.2 There are tens of works devoted to a study of acceptor groups effect on properties and photovoltaic performance in small molecule fullerene solar cells..2,17-24 By modifying the donor molecule with different acceptor end units, the device performance of small molecule fullerene solar cells have been greatly improved due to increased solutility, enhanced blend morphology and adjusted molecular energy level.5,20,25,26,37 However, the works studding the effect of acceptor groups on performance of NFA SM OSCs are rare8. Therefore, there is an urgent need in such researches to develop a guideline for chemists having deal with the molecular design of small donor molecules for NFA OSCs and to boost further their efficiency.

In this work, we designed two solution processable A-π-D-π-A small molecules, BDT-2T-DCV-Me

with

methyl

dicyanovinyl

end

group

(DCV-Me)

and

BDT-2T-CNAB with n-butyl cyanoester end group (CNAB), as depicted in Scheme 1a. The design of the molecules are rather similar to the other published A-π-D-π-A molecules based on benzodithiophene donor unit.38 However, it can be considered as a more simple molecular design, since we did not use any side alkyls groups at β-position of thiophenes and used short bithiophene π-bridge instead of terthiophene one. Thus, these molecules can be considered as the examples, which have more appropriate parameters of synthetic complexity for commercialization..39 After study the chemical and physical properties of the molecules, we found that BDT-2T-CNAB shows better advantages than the BDT-2T-DCV-Me in solubility, crystallinity, charge carrier mobilities and blend morphology. After device optimization, the PCEs of OSCs are 1.56 % for BDT-2T-DCV-Me:IDIC and 6.17 % for BDT-2T-CNAB:IDIC, respectively. The large difference of the photovoltaic performance of these two all-SM systems have been identified by the measurements of physical dynamics and blend morphology.

(a) S

S

S

S

NC

O

S

O

CN

S

S

S

S

S

NC

CN

S O

NC

CN S

S

S

O S

S

BDT-2T-CNAB

BDT-2T-DCV-Me

O

S

O

(b)

B O

O

S S

2

R

or O

S

S B O

S

S

S

O

S

S

S

S

S

R

O H

S

3

4 or 6

Br

Br S S

1

Pd(PPh3)4; aq. 2M Na2CO3 toluene / ethanol Reflux 55-85% R1 =

O

O

*

4

aq. 1M HCl THF; reflux 85%

*

5

BDT-2T-DCV-Me

H

6

aq. 1M HCl THF; reflux 87%

O

O *

O

O

R2 =

NC

CN

MW heating pyridine; reflux 36%

*

H

7

O NC O

BDT-2T-CNAB

MW heating pyridine; reflux 81%

Scheme 1. (a) Chemical structures of SM donors. (b) Synthetic routes of BDT-2T-DCV-Me and BDT-2T-CNAB donors

2. Results and discussion 2.1 synthesis and chemical characterization The synthetic routes of donor molecules BDT-2T-DCV-Me and BDT-2T-CNAB are outlined in Scheme 1b. The first synthetic step was the preparation of protected thiophene-substituted benzodithiophene (BDT) precursors (4) or (6) via Suzuki cross-coupling reactions between the brominated BDT core (1) and boronic acid pinacol esters (2)22 or (3)37 in 55-85 % isolated yields, respectively. Then, dioxane protective groups, which are necessary at the stage of organoboronic esters preparation, were removed by reflux of compounds 4 or 7 in THF solutions with 1M HCl to give corresponding ketone (5) or aldehyde (8) in 85 % and 87 % isolated yields, respectively. Finally, Knoevenagel condensation reaction in pyridine, which

was used both as a base and a solvent, between the ketone (5) and malononitrile resulted to the final product in case of BDT-2T-DCV-Me molecule in 36 % isolated yield. At the same time, the condensation between the aldehyde (7) and n-butyl cyanoacetate resulted to BDT-2T-CNAB molecule in 81 % isolated yield. The lower yield of BDT-2T-DCV-Me can be explained by its insufficient solubility, leading to some losses during purification by column chromatography. In both cases, the final stage was carried out under a microwave irradiation in accordance with a previously developed synthetic approach. 40 The chemical structure and purity of all obtained compounds were confirmed by NMR spectroscopy, mass spectrometry and elemental analysis (see Experimental part and SI). Both BDT-2T-DCV-Me and BDT-2T-CNAB molecules are soluble in common organic solvents, such as chloroform (CF), tetrahydrofuran (THF) and chlorobenzene. The measured solubility of BDT-2T-CNAB in CF was found to be 7.9 g/L, which is 4.7 times higher as compared to BDT-2T-DCV-Me (1.7 g/L) (Table 1).

2.2 Thermal properties The thermal properties of BDT-2T-DCV-Me and BDT-2T-CNAB were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The relevant curves of TGA and DSC measurements are shown in Fig. 1 and Fig. 2, respectively, and their results are summarized in Table 1. TGA reveals

that

BDT-2T-DCV-Me

shows

a

reasonably

high

thermal

and

thermal-oxidative stability with an onset decomposition temperature (at 5 % weight-loss (Td)) at 370 ºC in air and 402 ºC in nitrogen, respectively. However, its analog with the CNAB end acceptor unit shows slightly lower thermal and thermal-oxidative stability with Td at 339 ºC in air and 363 ºC in nitrogen, respectively. The lower stability of BDT-2T-CNAB can be ascribed to a less stability of the CNAB unit as compared to DCV-Me. This fact can be explained by a decomposition of the ester end group26 in BDT-2T-CNAB molecule.

Fig. 1. Thermal analysis data: TGA curves of BDT-2T-DCV-Me and BDT-2T-CNAB molecules in air (a) and under inert atmosphere (b) Fig. 2 shows the DSC scans for the molecules under discussion. Both materials as received are crystalline with rather high values of melting temperature (Tm) and similar values of melting enthalpy (∆Hm). However, subsequent cooling revealed that crystallization from the melt is significantly hindered for BDT-2T-DCV-Me. Therefore, on the second heating DSC scan the melting of a crystalline phase at 230 ºC followed by a cold crystallization (220-240 ºC) are observed before the isotropization at 245 ºC. In contrast to BDT-2T-DCV-Me, the BDT-2T-CNAB has a distinct crystallization at the cooling process and similar values of Tm and ∆Hm for the first and second heating scans. Thus, comparing the phase behavior of two materials one can conclude that BDT-2T-CNAB demonstrates much higher tendency to crystallize both from a solution and melt. Thus, BDT-2T-CNAB combines better solubility with a higher crystallinity. (b)20 HeatFlow, J/g*K

0

-6

10

0

-10 Endo

-12 Endo

HeatFlow, J/g*K

(a)6

100

150

200 Temperature, oC

250

-20 100

150

200 Temperature, oC

250

Fig. 2. Thermal analysis data: DSC heating-cooling-heating cycle curves of

BDT-2T-DCV-Me (a) and BDT-2T-CNAB (b); red line (first heating), blue line (cooling), black line (second heating). Table 1. Thermal properties of BDT-2T-DCV-ME and BDT-2T-CNAB. Compounds

First heating

Solubility

Second heating

TGA

TGA

(air)

(N2)

Tm, °C

∆Hm, J/g

Tm, °C

∆Hm, J/g

Td, °C

Td, °C

in CF, g/L BDT-2T-DCV-Me

1.7

247

37

245

23

370

402

BDT-2T-CNAB

7.9

237

35

234

33

339

363

2.3 Optical and electrochemical properties UV-Vis absorption spectra of BDT-2T-DCV-Me and BDT-2T-CNAB in diluted chloroform solutions and in thin solid films are shown in Fig. 3 and the results are summarized in Table 2. The shapes of absorption spectra of both molecules are similar both in solution and in film. The low intensive absorption bands in solution at about 300-375 nm can be ascribed to π-π* transition of the conjugated backbone, whereas the intensive absorption band peaking at 516-520 nm can be ascribed to the intramolecular charge transfer transition. The main difference between the two molecules in solutions is that the BDT-2T-DCV-Me has both a bit red-shifted maximum of absorption (λmax) and absorption edge (λonset).The values of optical frontier orbital energy gap (Egopt) are 2.09 eV and 2.12 eV for BDT-2T-DCV-Me and BDT-2T-CBAB, respectively. In comparison to solutions, the absorption spectra of both molecules in films were found to be broadened and red-shifted, extending the absorption maximum up to 555 nm and exhibiting a new band peaking at 600 nm. The bathochromic shifts and novel absorption peaks suggest the presence of strong intermolecular interaction and aggregation in the solid film.41 Table 2. Optical and electrochemical properties of the molecules. Compounds

UV-Vis absorption

Cyclic Voltammetry

BDT-2T-DCV-Me BDT-2T-CNAB

Solution λmax λonset λonset (nm) (nm) (nm) 520 593 2.09 516 584 2.12

Film λmax λonset (nm) (nm) 555/600 651 555/600 651

Egopt (eV) 1.90 1.90

HOMO (eV) -5.56 -5.54

LUMO (eV) -3.35 -3.31

Fig. 3. Normalized UV-Vis absorption spectra of BDT-2T-DCV-Me and BDT-2T-CNAB molecules (a) in chloroform solution (b) in thin film.

The electrochemical properties of the molecules were further studied using cyclic voltammetry (CV) relatively to a saturated calomel electrode. The oxidation of the both molecules occurs in one irreversible stage. Comparing to irreversible one-step reduction of BDT-2T-CNAB the reduction process of BDT-2T-DCV-Me takes place in two stages, where the first stage is quasi reversible, and the second is irreversible. The quasi reversibility of the first reduction stage can be explained by a stabilization of an anion radical by the methyl group at DCV fragment. 40 The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies were calculated using the first standard formal oxidation and reduction potentials (Table 2). The molecules have similar values of HOMO energy level since it is mainly localized on the donor fragment, which are identical for both molecules. The difference in LUMO level is a bit more pronounced, since the acceptor fragments are not the same. BDT-2T-CNAB has higher LUMO as compared to BDT-2T-DCV-Me (-3.31 eV vs. -3.35 eV), which leads to a wider electrochemical bandgap (EgEC) for the former (2.23 eV vs. 2.21 eV) and in agreement

EgEC (eV) 2.21 2.23

with the optical absorption data.

Fig. 4. (a) Cyclic voltammograms of BDT-2T-DCV-Me and BDT-2T-CNAB films and (b) Energy levels diagrams of BDT-2T-DCV-Me and BDT-2T-CNAB in comparison to IDIC.

2.4 Molecular simulations and optimized geometries We also investigated BDT-2T-DCV-Me and BDT-2T-CNAB by the density functional theory (DFT) calculations using B3LYP/6-31G(d, p) method, where alkyl groups length was reduced to methyl groups to simplify the calculations (see Fig. S12-13). The optimized molecular geometries and frontier molecular orbitals are illustrated in Fig. S12 and Fig. S13 in SI. Both BDT-2T-DCV-Me and BDT-2T-CNAB exhibit a linear backbone formation from the side view, suggesting that they have good planarity. In addition, the dihedral angle between the alkyl side chain and the BDT unit is 56.98o for BDT-2T-DCV-Me and 56.92o for BDT-2T-CNAB (Fig. S12). For both molecules, the LUMO orbitals are mainly located at the terminal electron-withdrawing groups and the bithiophene π-bridges. The HOMO orbitals of BDT-2T-DCV-Me and BDT-2T-CNAB are located at central BDT donor backbone and the bithiophene π-bridges. The theoretical calculation and molecular simulation with DFT revealed that the change of the DCV-Me to CNAB terminal acceptor group in such molecular systems leads to slight upshifts of LUMO and HOMO levels by 0.07 eV and 0.02 eV, respectively, which is in accordance with CV data.

2.5 Photovoltaic properties of BHJ devices Photovoltaic properties of BDT-2T-DCV-Me and BDT-2T-CNAB were tested using IDIC as the electron acceptor with a conventional device structure of ITO/PEDOT:PSS/BDT-2T-DCV-ME(or

BDT-2T-CNAB):IDIC/PDINO/Al.42

The

donor/acceptor ratios (D/A, w/w) of the active layer was adjusted under the solar simulator under AM 1.5G illumination. The corresponding parameters of the devices with optimized blending ratios and active layer thickness are provided in Fig. S7-S8 and summarized in Table S1-S2. The current density-voltage (J-V) curve of best devices of BDT-2T-DCV-Me and BDT-2T-CNAB are shown in Fig. 5(a). As shown in Table 3 both BDT-2T-DCV-Me and BDT-2T-CNAB devices show high open-circuit voltage (Voc) of over 1.0 eV. At the optimal donor/acceptor weight ratio of 1:1, the OSC device fabricated as-cast without annealing based on BDT-2T-DCV-Me:IDIC demonstrates a short-circuit current (Jsc) of 5.09 mA/cm2, a Voc of 1.09 V, a fill factor (FF) of 28.08 %, and a PCE of 1.56 %. As compared to the BDT-2T-DCV-Me:IDIC system, The device based on BDT-2T-CNAB:IDIC (2:1) with a Voc of 1.03 V achieved a much better PCE performance of 6.17 %, due to much higher FF of 59.08 % and much higher short-circuit current (Jsc) of 10.11 mA/cm2. The external quantum efficiency (EQE) of the corresponding devices is shown in Fig. 5(b), both the devices based on BDT-2T-DCV-Me and BDT-2T-CNAB have a similar broad photo response wavelength range of 300-700 nm. While in the whole range, the EQE values of BDT-2T-CNAB is much higher than BDT-2T-CNAB. The EQE peak of BDT-2T-CNAB is about 48 % at around 676 nm, while the EQE value of BDT-2T-DCV-Me below 15 % at all wavelength, which lead to the poor performance of the device. Jsc values calculated from the EQE spectra are 3.36 mA/cm2 for BDT-2T-DCV-ME and 9.89 mA/cm2 for BDT-2T-CNAB, respectively.

Table

3.

Photovoltaic

BDT-2T-DCV-Me:IDIC

parameters films

and

illumination of AM 1.5G, 100 mW·cm-2.

of

the

OSCs

devices

BDT-2T-CNAB:IDIC

films

based

on

under

the

Donor

D:A

Voc

Jsc

FF

PCEmax (PCEavga)

[wt %]

[V]

[mA/cm-2]

[ %]

[ %]

BDT-2T-DCV-Me

1:1

1.06 ± 0.03

4.75 ± 0.34

29.31 ± 1.23

1.56 (1.48 ± 0.08 )

BDT-2T-CNAB

2:1

1.04 ± 0.01

10.10 ± 0.05

58.84 ± 0.24

6.17 (6.15 ± 0.02 )

the average values of PCEs calculated from over six devices.

Current density (mA cm-2)

(a)

5

(b)

BDT-2T-DCV-ME BDT-2T-CNAB

50

BDT-2T-DCV-ME BDT-2T-CNAB 40

0

EQE (%)

a

-5

30

20

10

-10

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 300

Voltage (V)

400

500

600

700

800

Wavelength(nm)

Fig. 5. (a) J–V curves and (b) EQE curves of the optimized BDT-2T-DCV-Me:IDIC and BDT-2T-CNAB:IDIC based devices. 2.6 Physical dynamics We employed photoluminescence (PL) quenching experiments to distinguish whether there is efficient exciton dissociation and charge transfer between the SM donors and IDIC in the blend films. As shown in Fig. 6(a) the quenching efficiency of the BDT-2T-DCV-Me:IDIC blend is very poor. In contrast, in Fig. 6(b) the results showed significant fluorescence quenching in the BDT-2T-CNAB:IDIC blend film, suggesting that there is effective photoinduced charge transport occurs from the BDT-2T-CNAB

to

IDIC,

BDT-2T-CNAB:IDIC blend.

promoting

the

charge

generation

in

the

(a)

(b)

BDT-2T-DCV-ME BDT-2T-DCV-ME:IDIC

PL Intensity

PL Intensity

BDT-2T-CNAB BDT-2T-CNAB:IDIC

500

600

700

800

900

1000

1100

500

600

700

800

900

1000

1100

Wavelength (nm)

Wavelength (nm)

Fig. 6. Photoluminescence spectra of (a)BDT-2T-DCV-Me BDT-2T-DCV-Me:IDIC(1:1, w/w) blend films (b)BDT-2T-CNAB BDT-2T-CNAB:ICID (2:1, w/w) blend films excited at 532 nm.

and and

Charge carrier mobility is one of the important factors in OSC device performances.43 High charge carrier mobility is preferred for efficient transportation and photocurrent collection of the photo-induced charge carriers.

23

We measured the

hole and electron mobilities using space charge limited current (SCLC) method with hole and electron only devices.26 The structure of hole only devices and electron only devices

is

ITO/PEDOT:PSS/blended

films/MoO3/Ag

and

ITO/ZnO/blended

films/Ca/Ag, respectively. The J-V characteristics of the hole only and electron only devices in dark conditions can be fit to the Mott-Gurney relation20 = ε ε μ

exp(

. √

)

(1)

Where JSCL is the current density, ε0 and εr are vacuum dielectric permittivity and relative dielectric permittivity, L is the thickness of the blend film, respectively. µ and β are carrier mobility and field activation factor, calculated from the fitting curve. The calculated hole only mobilities and electron only mobilities by six devices for BDT-2T-DCV-Me:IDIC and BDT-2T-CNAB:IDIC blend films are shown in Fig. 7. and Table 4. The hole only mobilities and electron only mobilities obtained for BDT-2T-DCV-Me:IDIC is 3.48×10-4 and 1.39×10-4, respectively. While the BDT-2T-CNAB:IDIC devices exhibited higher hole only mobilities and electron only mobilities of 3.48×10-4 and 1.39×10-4, respectively. The higher charge carrier mobility of BDT-2T-CNAB leads to the better performance of Jsc in the device.

103

103

Current density(mA/cm2)

(b) 104

Current density(mA/cm2)

(a) 104

102

101 BDT-2T-DCV-ME:IDIC BDT-2T-CNAB:IDIC Fitting line

100

10-1

102

101 BDT-2T-DCV-ME:IDIC BDT-2T-CNAB:IDIC Fitting line

100

10-1 0

1

2

3 Voltage(V)

4

5

6

0

1

2

3 Voltage(V)

4

5

6

Fig. 7. The dark J-V characteristics of devices based on BDT-2T-DCV-Me:IDIC and BDT-2T-CNAB:IDIC including hole-only (a) and electron-only (b) devices. The solid lines represent the best fitting using the SCLC modified Mott-Gurney model. Table 4. Hole and electron only mobilities of BDT-2T-DCV-Me:IDIC and BDT-2T-CNAB:IDIC blended films. D:A

µh (cm2 V-1 s-1)a

Device

µe

(cm2

V-1 µh/µe

s-1)a

BDT-2T-DCV-Me:IDIC

1:1

3.48×10-4

1.39×10-4

2.50

BDT-2T-CNAB:IDIC

2:1

4.67×10-3

8.07×10-4

57.87

Charge recombination behavior was investigated by the dependences of Jsc and Voc on light intensity (Plight). The dependence of Jsc upon I is investigated to determine whether there is second-order recombination. It can be expressed use equation

= ( )

(2)

where β is a constant and α is the exponential factor.20 As shown in fig.8(a), the fitting slopes (α) were calculated to be 0.926 for BDT-2T-DCV-ME:IDIC and 0.939 for BDT-2T-CNAB:IDIC. The value of α lower than 1 suggests that there are some bimolecular recombination exists in the BDT-2T-DCV-ME:IDIC films and BDT-2T-CNAB:IDIC films.20 The Voc is also follows a logarithmic relationship with the light intensity as =

!"# $

ln( )

(3)

where k is the Boltzmann constant, T is the temperature and q the elementary charge. As shown in fig. 8(b), the slopes of fitting line are 1.32 kT/q for BDT-2T-DCV-Me:IDIC and 1.45 kT/q for BDT-2T-CNAB:IDIC, respectively. Indicate that there is more trap-assisted recombination in BDT-2T-CNAB:IDIC.44 (b)

(a) 10

1.00 1

Voc (V)

Jsc (mA cm -2)

1.05

BDT-2T-DCV-ME (α=0.91) BDT-2T-CNAB(α=0.96) Fit Curve of Jsc Fit Curve of Jsc

0.1

0.95

BDT-2T-DCV-Me (1.32kT/q) BDT-2T-CNAB(1.43kT/q)

0.90

0.85 1

10 Light intensity (mW cm-2)

100

1

10 Light intensity (mW cm-2)

100

Fig. 8. The (a) Jsc-Light intensity curve of BDT-2T-DCV-ME:IDIC and (b) BDT-2T-CNAB:IDIC devices.

2.7 Atomic force microscopy (AFM) The morphologies of BDT-2T-DCV-ME:IDIC and BDT-2T-CNAB:IDIC blend films were characterized by atom force microscopy (AFM) and transmission electron microscopy (TEM). As shown in Fig. S11, BDT-2T-DCV-Me:IDIC blend films show better homogeneous and interpenetrating network than BDT-2T-CNAB:IDIC blend films, which indicates that BDT-2T-CNAB:IDIC blend films have more obvious phase separation, resulting in high efficiency of exciton dissociation and charge transportation.[24] The AFM images describe the high distribution on film surfaces (Fig. 9). The distinguishing profiles between bright and dark domains reveal large domain size resulted from molecule aggregation. This perishing morphology delivered by BDT-2T-DCV-Me:IDIC blend film explains well remarkable improved FF of BDT-2T-CNAB:IDIC. The obvious molecule aggregation mainly due to BDT-2T-DCV-Me rather than IDIC in a view of starkly different AFM high imagines delivered by BDT-2T-CNAB:IDIC blend film (shown in Fig. 9). The phase distribution of BDT-2T-DCV-Me:IDIC blend film reveal large domain sizes, which

is harmful to charge transfer. On the other hand, the phase-separated interpenetrating networks in BDT-2T-CNAB:IDIC blend film may be the main reason for better photovoltaic parameters, even though the RMS values reveal a smoother surface of BDT-2T-DCV-Me:IDIC blend film (0.843 nm for BDT-2T-DCV-Me:IDIC blend film and 2.927 nm for BDT-2T-CNAB:IDIC blend film, respectively). The phase-separated interpenetrating networks increase donor-accepter interphase area, which is beneficial for exciton dissociation. The enhanced exciton dissociation undoubtedly accounts for high EQE of BDT-2T-CNAB:IDIC blend and thus improved Jsc and PCE. 28, 45

Fig. 9. The AFM images (size: 5×5 μm) of (a) BDT-2T-DCV-Me:IDIC and (b) BDT-2T-CNAB:IDIC blend films.

3 Conclusion In summary, two novel small donor molecules of A-π-D-π-A structure with different terminal acceptor groups for all-small molecule NFA OSCs were designed and synthesized. We investigated the effects of either methyl dicyanovinyl end group or n-butyl cyanoester end group on solubility, thermal properties, optical properties, charge transport, morphology and photovoltaic performance. BDT-2T-DCV-Me and BDT-2T-CNAB molecules have almost similar absorption spectra and values of HOMO energy levels, but the LUMO level of the molecule with n-butyl cyanoester groups is a bit higher.

Although the blends of both molecules with IDIC exhibited a

high hole-only and electron-only mobilities, the PCE of the device based on BDT-2T-DCV-Me is only 1.56 %. It is much lower than that of the device based on

BDT-2T-CNAB (6.17 %), having higher Jsc and FF values. The underlying main reason for this is thought to be a poor charge generation and blend morphology of BDT-2T-DCV-Me:IDIC blend. In addition, the poor photovoltaic performance of the BDT-2T-DCV-Me:IDIC device may also be explained by some other loss mechanisms, for example, a bulk recombination. Despite all this, the utility of end group acceptor unit engineering on small molecule donors to elucidate structure-property relationships will be beneficial for the design of new conjugated molecules for use in non-fullerene OSCs as well as other functional optoelectronic applications.

Acknowledgments. This work was financially supported by the National Natural Science Foundation of China (NSFC) (Grant No. 21702154 and 51773157). We also thank the support of the opening project of Key Laboratory of Materials Processing and Mold and Beijing National Laboratory for Molecular Sciences (BNLMS201905). The synthesis and characterization of the molecules were financially supported by Russian science foundation (grant number 19-73-10198). NMR and UV-vis spectra were recorded using the equipment of Collaborative Access Center ‘Center for Polymer Research’ of N. S. Enikolopov Institute of Synthetic Polymeric Materials of the Russian Academy of Sciences with the financial support from the Ministry of Science and Higher Education of the Russian Federation. The authors acknowledge P.V. Dmitryakov from National Research Centre ‘‘Kurchatov Institute’’for the help with DSC and TGA experiments. The authors acknowledge the support of the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12010200), and the National Natural Science Foundation of China (21572234, 21661132006). 4 References 1.

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Highlights •

Two new small molecule donors with terminal either dicyanovinyl (DCV-Me) or n-butyl cyanoester (CNAB) groups are reported.

•

DCV-Me:IDIC and CNAB:IDIC based devices showed the PCEs of 1.56% and 6.17%, respectively.

•

The CNAB system exhibit better morphology and physical dynamics than DCV-Me system.

Conflict of Interest: The authors declare no competing financial interest.