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Development of Spiro[cyclopenta[1,2-b:5,4-b#]dithiophene-4,9#fluorene]-Based A-#-D-#-A Small Molecules with Different Acceptor Units for Efficient Organic Solar Cells Wengong Wang, Ping Shen, Xinning Dong, Chao Weng, Guo Wang, Haijun Bin, Jing Zhang, Zhi-Guo Zhang, and Yongfang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14114 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 22, 2017

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ACS Applied Materials & Interfaces

Development of Spiro[cyclopenta[1,2-b:5,4-b′]dithiophene-4,9′-fluorene]-Based A-π-D-π-A Small Molecules with Different Acceptor Units for Efficient Organic Solar Cells

Wengong Wanga,b, Ping Shena*, Xinning Dongb, Chao Wenga, Guo Wanga, Haijun Binb, Jing Zhangc*, Zhi-Guo Zhangb, and Yongfang Lib*

a

College of Chemistry and Key Laboratory of Environmentally Friendly Chemistry and

Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, China. E-mail: [email protected] b

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

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: [email protected] c

School of Materials Science & Engineering, Jiangsu Collaborative Innovation Central of

Photovoltaic Science & Engineering, Changzhou University, Changzhou213164, China E-mail: [email protected]

ABSTRACT: Three acceptor-π-donor-π-acceptor (A-π-D-π-A) small molecules (STFYT, STFRDN, and STFRCN) with spiro[cyclopenta[1,2-b:5,4-b′]dithiophene-4,9′-fluorene] (STF) as the central donor unit,

terthiophene

as

the

π-conjugated

bridge,

indenedione,

3-ethylrhodanine,

or

2-(1,1-dicyanomethylene)rhodanine as the acceptor unit are designed, synthesized and characterized

ACS Paragon Plus Environment


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as electron donor materials in solution-processing organic solar cells (OSCs). The effects of the spiro STF-based central core and different acceptors on the molecular configuration, absorption properties, electronic energy levels, carrier transport properties, the morphology of active layers, and photovoltaic properties are investigated in detail. The three molecules exhibit desirable physicochemical features: wide absorption bands (300-850 nm) and high molar absorption coefficients (4.82 × 104-7.56 × 104 M-1 cm-1) and relatively low HOMO levels (-5.15 − -5.38 eV). Density functional theory calculations reveal that the spiro STF central core benefits to reduce the steric hindrance effect between the central donor block and terthiophene bridge and suppress excessive intermolecular aggregations. The optimized OSCs based on these molecules deliver power conversion efficiencies (PCEs) of 6.68%, 3.30%, and 4.33% for STFYT, STFRDN, and STFRCN, respectively. The higher PCE of STFYT-based OSCs should be ascribed to its better absorption ability, higher and balanced hole and electron mobilities and superior active layer morphology compared with the other two compounds. So far, this is the first example of developing the A-π-D-π-A type small molecules with a spiro central donor core for high-performance OSC applications. Meanwhile, these results demonstrate that using spiro central block to construct A-π-D-π-A molecule is an alternative and effective strategy for achieving high-performance small molecule donor materials. KEYWORDS: spiro[cyclopenta[1,2-b:5,4-b′]dithiophene-4,9′-fluorene], A-π-D-π-A small molecule, organic solar cells, photovoltaic properties, acceptor unit

1. INTRODUCTION In recent years, organic solar cells (OSCs) have emerged as one of highly promising technologies


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of converting solar energy into electricity, due to their numerous advantages, like low cost, light weight, high flexibility and facile solution-processability.1-4 Up to now, the bulk-heterojunction (BHJ) OSCs constructed from a photoactive layer of a p-type donor material (π-conjugated polymer or small molecule (SM)) and a fullerene derivative acceptor (PC61BM or PC71BM) have been proven to deliver high power conversion efficiencies (PCEs).5,6 Currently, BHJ OSCs with conjugated polymers as donor materials exhibited PCEs over 10%.7-16 In parallel, solution-processable OSCs based on small molecules have been intensively investigated and rapidly developed because of their prominent advantages, that is, (i) low cost fabrication by solution-processing; (ii) facilely tunable optical properties and electronic energy levels via chemical functionalizations; (iii) high carrier mobility and open-circuit voltage (Voc); (iv) well-reproducible photovoltaic performances due to the definite molecular structure. Nowadays, small molecule OSCs (SMOSCs) exhibited comparably high PCEs to polymer-based OSCs, which demonstrates an enormous potential for research and development of this type OSCs.17-21 The rapidly improved PCEs greatly benefited from the remarkable development of novel SM materials, device architectures as well as fabrication technologies. Basically, the most important is the design and synthesis of novel solution-proccessable organic SM materials. Currently, the acceptor-π-donor-π-acceptor (A-π-D-π-A) skeleton is one of the most promising and effective molecular design motifs, in which an electron-rich donor unit (D) as the central core, is covalently linked with two electron-deficient terminal acceptor units (A) by two π-conjugated bridges (Figure 1a).22,23 The absorption spectra, electronic energy levels, carrier mobility and structural ordering of these SM can be easily tailored by various selection of the three parts of D, A and π-bridge. As a result, these SM materials displayed broad and strong absorptions, low-lying the highest occupied



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molecular orbital (HOMO) levels, high charge carrier mobilities as well as desirable molecular stacks. The A-π-D-π-A concept first originated from A-D-A type organic molecules reported by Peter Bäuerle group.24-26 However, the early A-D-A molecules showed poor solubility in organic solvent, therefore, vacuum-deposition technique was typically used to fabricate OSCs based on these molecules with relatively low PCEs (< 5%).24 In 2011, Li group pioneered star-shaped molecules with triphenylamine core and dicyanovinyl end-groups for solution-proccessable OSCs.27 Thereafter, solution-proccessable OSCs based on organic SMs have received more and more attention due to the continuously improved PCEs. Now Chen and other groups did outstanding research work in solution-proccessable SMOSCs based on A-π-D-π-A molecules, in which they demonstrated that donor and acceptor units as well as π-conjugated bridges all have a great impact on the molecular configuration and physicochemical properties of the resulting materials.22,23 So far, the popular and effective donor units include benzo[1,2-b:4,5-b′]dithiophene (BDT),18-21,28-34 dithienosilole (DTS),35-38 cyclopenta[2,1-b;3,4-b′]dithiophene (CPDT),35 indacenodithiophene (IDT),39,40 and porphyrin.41 Obviously, these donor units share a common feature that all of them possess a rigid coplanar structure. This structure would enhance the coplanarity of the corresponding molecules, which benefits to form compact configuration, long-range π-π stacks, and improve the charge mobility. However, it is more likely accompanied by improper aggregation behavior and difficult solution processability for this kind of rigid molecules, and thus reduced the photovoltaic performance.42,43 Therefore, it is urgently necessary to propose an approach to properly twist conformation and reduce the coplanarity of the conjugated SM backbone. This conformational twisting SM must be judiciously designed to achieve the desired aggregation, light absorption, solution-processability and film-morphology criteria for obtaining efficient SMOSCs. Meanwhile,


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regarding

terminal

acceptor

units,

2-(1,1-dicyanomethylene)rhodanine

indenedione

(RCN),20

alkyl

(YT),31

ethylrhodanine

cyanoacetate

and

their

(RDN),29,30,34 corresponding

derivatives44-46 have been popularly used in the A-π-D-π-A molecular system. And furthermore, for A-π-D-π-A SMs, a suitable electron-deficient end group can successfully and effectively tune the absorption and energy levels and achieve high performance.47-49 In this contribution, we developed and characterized three solution-processable A-π-D-π-A SMs (STFYT,

STFRDN

and

STFRCN,

as

shown

in

Scheme

1)

with

spiro[cyclopenta[1,2-b:5,4-b′]dithiophene-4,9′-fluorene] (STF, as shown in Figure 1b) as the central donor unit, terthiophene group as the π-conjugated bridge, indenedione (YT), 3-ethylrhodanine (RDN), or 2-(1,1-dicyanomethylene)rhodanine (RCN) as the end acceptor group. First, STF as a spiro-annulated building block features a non-planar structure, in which two π-conjugated systems of a fluorene (FL) moiety and a CPDT unit are connected through a common sp3-hybridized carbon (see Figure 1b). Obviously, this spiro conformation is significantly different from the above mentioned planar donor units, such as BDT, DTS, CPDT, IDT, and porphyrin. Molecular materials containing a spiro-configured central core result in a kind of spiro-steric hindrance effect, which usually benefits to achieve improved solubility, high phase transition temperature as well as extraordinary optoelectronic properties. Furthermore, it is expected that the twisted conformation can reduce excessive aggregation, therefore reducing excimer formation and leading to a nanoscale domain formation when these SMs are blended with an acceptor material (e. g. PC71BM). Nowadays, STF based molecular materials had been applied in solid-state heterojunction perovskite solar cells,50,51 organic heterojunction diode52 and dye-sensitized solar cell.53 However, it has seldom been applied in polymer or SM based OSCs. Second, as the end-capped groups YT, RDN, and RCN have been



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proved as effective terminal acceptor units for building high-performance SMOSCs.20,30,31 More importantly, the integration of STF with the different acceptor units (YT, RDN, and RCN) can tune the absorption ability, bandgaps and energy levels of the resulting SMs. Finally, terthiophene also functioned as a unique and effective π-conjugated bridge. In the previous reports SMs with the terthiophene bridge exhibited various promising physicochemical characteristics, such as broad absorption spectra and reduced bandgaps due to the extended conjugated length, and improved π-π intermolecular interactions.22,29,30,49 In these three STF-based SMs, the defined molecular structure with the same backbone but different acceptor units provide an opportunity to systematically investigate the influences of the spiro central core and various acceptor groups on the molecular configuration, physicochemical and photovoltaic properties. These STF-based materials exhibit excellent solubility in chloroform (CHCl3), dichloromethane and tetrahydrofuran (THF) due to the four n-octyl groups and spiro molecular structures, which ensures a desirable solution-processability and excellent film-forming property of these materials. The BHJ-OSC devices based on STFYT with YT as the terminal acceptor group using PC71BM as the electron acceptor material, depicted a high PCE of 6.68% with DIO as processing additive, due to a broad and strong absorption, and a high hole mobility. In parallel, the devices based on the other two molecules of STFRDN and STFRCN demonstrated relative lower PCEs of 3.30% and 4.33%, but high Voc of 0.87 and 0.88 V, respectively. This study gave a good example to investigate the effect of the intensity of acceptor units in A-π-D-π-A framework. Meanwhile, as far as we know, this is the first instance of developing the A-π-D-π-A type SMs with the spiro STF as the central donor unit for high-efficiency SMOSCs applications.


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Figure 1. (a) The schematic diagram of A-π-D-π-A structure; (b) Illustration of the construction of STF skeleton.

2. RESULTS AND DISCUSSION 2.1. Molecular Design, Synthesis and Characterization. Three A-π-D-π-A SMs have a STF-based block as the central donor unit and two terthiophene-based π-conjugated bridges, with two YT, RDN, or RCN groups as terminal acceptor units. STF as a spiro skeleton consists of a fluorene (FL) moiety and a CPDT unit, which are connected by a common sp3-hybridized carbon (see Figure 1b). It has stronger electron-donating ability than 9,9′-spirobifluorene (SF) owing to the replacement of a FL moiety with a CPDT segment in SF, which helps to become a more effective donor unit. Moreover, compared with planar donor units (e.g. CPDT, BDT), A-π-D-π-A molecules with the STF-based central donor core possess many promising advantages, such as higher solubility, improved morphological stability in the solid state and reduced a tendency to form aggregates benefitting from the peculiar cross-shaped rigid structure with two perpendicularly oriented molecular halves to inhibit intermolecular interactions. All in all, we focus on evaluating the spiro STF donor unit and different A units affect the desired bulk



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properties in two aspects: (i) the spiro STF-based core to twist the resulting molecular configuration, thus improve solubility, reduce aggregation and form desired nanoscale domain; (ii) variation of electron-withdrawing ability of A units to control the light-harvesting ability, optical properties, molecular energy levels and charge carrier mobility. The synthetic route to intermediates and target SMs are demonstrated in Scheme 1. The detailed experimental methods and procedures are provided in the Supporting Information (SI). STF-based compound 5 is one of the most important intermediates, which was obtained according to the reported procedure with some modifications.54,55 The STF skeleton was constructed by a carbinol intermediate derived from a lithiated compound and an aromatic ketone derivative. Then the carbinol intermediate is engaged in an intramolecular Friedel-Crafts cyclization to produce the spiro-compound in acidic conditions. Therefore, herein the synthesis of STF-based spiro-compound started from the monobrominated bithiophene (2) whose two electron-rich α-positions were protected by trimethylsilyl. Then the silyl-protected monobrominated bithiophene (3) was reacted with n-BuLi and then treated with a slightly excessive 2,7-di-tert-butylfluorenone (1) to achieve the intermediate carbinol 4 in a high yield of 85.9%. Finally, the carbinol 4 was subjected to a Friedel-Crafts cyclization reaction in the present of BBr3 to yield STF-based compound 5 in an acceptable yield of 47.6%. One thing to note here two trimethylsilyl in bithiophene are also cleaved during the formation of compound 5, which is available for some further functionalization (hydrogen-lithium-tin exchange, compound 6, Scheme 1). Moreover, it was found that the introduction of protecting groups (silyl groups) on the two electron-rich α-positions of bithiophene in combination with the use of suitable Lewis acids (BBr3) remarkably increases the efficiency of the intramolecular cyclization process of bithiophene derivatives. This cyclization reaction condition is different from the reported standard


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acidic conditions (refluxing acetic acid and hydrochloric acid).54 Evidently, our method is simple and practicable with respect to the mild catalytic conditions, simultaneously with a comparably high yield. The other key intermediate of compound 8 was easily prepared by Stille couple reaction between the bis-stannylated intermediate 6 and momobrominated compound 7 in a yield of 75.2% with Pd(PPh3)4 as the catalyst. O Triton-B Pyridine/air

1

S S

S

Br Mg Et2O

S

S

4

TMS

S

Br

MgBr

S

5

CHO Pd(PPh3)4

ii) (CH3)3SnCl,-78°C then r.t.

S

OHC C8H17 C8H17 Toluene,120°C, 36h

S

8 +

S Piperidine CHCl3, reflux overnight

10

CHO

N

O

S

S

S

S

STFRDN

C8H17 C8H17

O

C8H17 C8H17

S S

S

S S

S

STFYT

S

S

S

S

S

C8H17 C8H17

O

O

S C8H17 C8H17

S

S

S

N

Sn

S

O

O

9 O

6

S

8

C8H17C8H17

O

S

S

S

S

Sn

S

S

S

7

S

ii) 1, Et2O, r.t.

3 Br

ii)TMS, r.t.

2 Br

TMS i) n-BuLi,-78°C, 2h

S

S

i) n-BuLi/THF, -78°C

CH2Cl2, 0°C then r,t,

S S

6 +

TMS i) LDA/THF, 0°C, 2h

S

S

Br Ni(dppp)Cl2 Et2O

BBr3

OH TMS

Br

C8H17 C8H17

S

N O

CN S

NC

CN

N

11

O

NC

S

N O

S

S

S

S

C8H17C8H17

S

S

STFRCN

CN

S

S

S C8H17 C8H17

CN

N O

Scheme 1. Synthetic routes of the target molecules of STFYT, STFRDN, and STFRCN. The three target SMs (STFYT, STFRDN, and STFRCN) were obtained by a Knoevenagel condensation reaction between the dialdehyde intermediate 8 and compounds 9, 10, and 11,



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respectively, in CHCl3 and a catalytic amount of piperidine with moderate yields (40-55%). The molecular structures of important intermediates and three target molecules were confirmed by 1H NMR,

13

C NMR, and MALDI-TOF-MS/HRMS, and the corresponding spectra are presented in the

SI (Figure S1-13). All the target molecules show excellent solubility in chloroform (CHCl3), dichloromethane and tetrahydrofuran (THF) at room temperature, owing to the alkyls of the STF-based core (two tertbutyls) and π-conjugated bridge (four n-octyls) as well as spiro molecular structures. This guarantees to be applied in BHJ solar cells by the solution-processing. The thermal stability and solid state thermal transition of these SMs were investigated by thermogravimetric analysis (TGA, Figure S14, SI) and differential scanning calorimetry (DSC, Figure S15), respectively, under N2 atmosphere. TGA results reveal that all the SMs have well thermal stability with the temperature with 5% weight loss (Td) of about 373 °C, 379 °C, and 390 °C for STFYT, STFRDN, and STFRCN, respectively. In addition, STFRDN with STF as the donor unit shows even better thermal stability than the reported analog DR3TDTC35 with the planar CPDT as the central donor core, implying the spiro central core benefits to improving the thermal stability of the resulting molecule. From DSC plots (Figure S15a,b), one can find that characteristic phase transition behaviors on the second heating scans are in the range of 160-180 °C with melting temperatures (Tm) of about 176 °C and 165 °C for STFYT and STFRDN, respectively. Whereas STFRCN exhibits a relatively complicated phase transition behavior (Figure S15c), a weak endothermic peak and clear exothermic peak appear at 154 °C and 176 °C, respectively, which corresponds to a melting temperature (Tm') and cooling crystallization temperature (Tc') result from amorphous molecules. The phenomenon of cooling crystallization raises the aggregation of both side chains and molecular backbone and as a result of another clear endothermic peak at a high


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temperature of 246 °C (Tm). Clearly, STFYT and STFRCN have higher characteristic phase transition temperatures (Tm) than those of STFRND, suggesting a more evident tendency to form crystalline aggregates for the former two SMs. On the other hand, the solid state transition on the second cooling scan (Figure S15a) of STFYT is obviously prominent with a sharp recrystallization peak (Tc) at 145 °C compared to the other two SMs of STFRDN (Figure S15b, Tc = 139 °C) and STFRCN (Figure S15c, Tc = 150 °C). This result means that STFYT possesses a better crystallinity than STFRDN and STFRCN, as also evidenced by the following X-ray diffraction (XRD) results.

2.2. Optical Properties The optical properties of these SMs were characterized through UV-vis absorption spectrum. Figure 2 describes the absorption spectra of these SMs both in chloroform (CF) solution and as films. The corresponding optical data are summarized in Table 1. In dilute CF solution, these A-π-D-π-A SMs show strong and broad absorption bands over the entire visible spectrum (300-750 nm) as shown in Figure 2a. The maximum absorption peaks (λmax) of the three molecules are located at around 570, 522, and 552 nm for STFYT, STFRDN, and STFRCN, respectively, corresponding to the intramolecular charge transfer (ICT) effect between the STF-based donor unit and acceptor units (YT, RDN, and RCN). STFYT and STFRCN exhibit distinct shoulder peaks at the short wavelength direction (both at ca. 475 nm), whereas STFRDN just give a relatively faint vibronic peak in the wavelength range of 400 to 500 nm, which should be attributed to stronger intermolecular interactions and molecular aggregations even in STFYT and STFRCN solutions. This observation has been proved by DSC analysis as mentioned before. The absorption peaks of STFYT and STFRCN with YT and RCN as the A units are red-shifted obviously than that of STFRDN with RDN



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as the A unit. Moreover, STFYT exhibits a remarkably higher extinction coefficient (ε) of 7.56 × 104 M-1 cm-1 than that of STFRDN (4.82 × 104 M-1 cm-1) and STFRCN (4.91 × 104 M-1 cm-1), which is comparable with the observations of the previously reported SMs based on other central donor units, such as BDT,29,30,34 IDT,39,40 and CPDT.35 The differences in optical properties of the three molecules mainly resulted from the various electron-deficient intensity of these acceptors (YT, RDN, and RCN). The results indicate that the A group in A-π-D-π-A type SM greatly influences on the absorption property and light-harvesting ability. Additionally, three molecules as films display obvious red-shifted and widened absorptions as compared with their solutions, indicating enhanced π-π stacking interactions and some ordered structure in the solid state (Figure 2b). The λmax of three SMs generate bathochromic shift by 37-70 nm in comparison with those of the solutions and a significant bathochromic shift effect is also found in the solid state for STFRCN (622 nm) and STFYT (607 nm) compared to STFRDN (580 nm). Similar to the solution, absorption shoulders appear in the shorter-wavelength region besides maxima of the absorption peaks in STFYT and STFRCN films. These absorption shoulders should be mainly attributed to the ordered molecule π-π stacking. It means that STFYT and STFRCN possess more ordered molecular packing and stronger crystalline properties than that of STFRDN. Herein, it should be noted that most of analogous A-π-D-π-A type small molecules with planar donor units, such as BDT,21,29,34 DTS,35 CPDT,35 and IDT,40 showed serious aggregations and strong π-π stacking in solid state, reflecting distinct absorption shoulders in the absorption spectra. However, as shown in Figure 2b the three SMs display inapparent absorption shoulders in the shorter-wavelength region. This means a relatively slight aggregation and weak stacking in solid state. In this respect, we infer the spiro central donor unit did work in reducing excessive aggregation for these SMs to some extent


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as mentioned in Introduction. According to the absorption edges (λedge) of molecular films, the optical bandgaps (Egopt) are deduced to be 1.61, 1.71, and 1.57 eV for STFYT, STFRDN, and STFRCN, respectively (Table 1), indicating that the acceptor unit has an evident impact on the optical bandgap of A-π-D-π-A SMs. As mentioned earlier, for A-π-D-π-A SM, not only the terminal A unit but also the central D unit has great influence on the optoelectronic properties. A comparison of the observed absorption bands and optical bandgaps of our three SMs based on STF central core with the reported analogues with other typical donor units (BDT,29,30,34 IDT,40 and CPDT35) suggests that spiro-structured STF can effectively broaden the absorption band and reduce the bandgaps. Wide and strong absorption is an important premise to obtain a larger Jsc, and thus a higher PCE. Therefore, it can be expected that the photovoltaic performances of STFYT and STFRCN are superior to STFRDN, due to the broader and stronger absorption characteristics.

1.0

Normalized absorption (a.u.)

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STFYT STFRDN STFRCN

(a)

0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

Wavelength (nm)


800


1.0

Normalized Absorption (a.u.)

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

(b)

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STFYT STFRDN STFRCN

0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

800

900

Wavelength (nm)

Figure 2. UV-vis absorption spectra of STFYT, STFRDN, and STFRCN in (a) chloroform solution and (b) thin film on quartz.

Table 1. Optical and electrochemical properties of STFYT, STFRDN, and STFRCN λmaxa) Compound (nm) STFYT STFRDN STFRCN a)

570 523 552

εb) (M-1 cm-1) 7.56×104 4.82×104 4.91×104

λmaxc) λedge Egopt d) HOMOe) LUMOe) (eV) (nm) (nm) (eV) (eV) 607 580 622

769 724 790

1.61 1.71 1.57

-5.38 -5.25 -5.15

-3.71 -3.61 -3.68

Measured in dilute chloroform solution. b)Maximum absorption coefficients in solution. c)Measured

on a quartz plate by polymers cast from chloroform solution. d)Estimated from the onset wavelength of the absorption spectra: Egopt = 1240/λedge. e)Calculated according to the equation: HOMO/LUMO = -e(Eox/red + 4.41) (eV).

2.3. Electronic Energy Levels. The electronic energy levels, especially for the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of conjugated polymers or SMs are very important for application in BHJ OSCs. Electrochemical cyclic voltammetry (CV) is usually


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employed to determine HOMO and LUMO levels. In CV measurement, ferrocene/ferrocenium (Fc/Fc+) redox couple was employed as internal calibration and its energy level was taken as -4.80 eV below the vacuum level. In our experiment, the onset oxidation potential (Eox) of Fc/Fc+ is observed as 0.39 V versus Ag/AgCl. Accordingly, from CV curves of the three SMs films (Figure 3a-c), we can easily obtain the Eox and reduction potentials (Ered) of these SMs and further obtain the HOMO and LUMO levels using the equation: HOMO/LUMO = -e(Eox/red + 4.41) (eV), where the onset potentials (Eox and Ered) are versus Ag/AgCl. From the above equation, the calculated HOMO/LUMO levels are -5.38/-3.71 eV for STFYT, -5.25/-3.61 eV for STFRDN, and -5.15/-3.68 eV for STFRCN (Table 1). Among these SMs, STFYT gives the deepest HOMO level and STFRCN has the highest one, which is consistent with our theoretical calculation results as we will discuss below. Moreover, compared to the reported analogues with other typical donor units (BDT,29,30,34 IDT,40 and CPDT35), these STF-based molecules have obviously lower-lying HOMO and LUMO levels, implying that spiro STF core benefits to lowering HOMO and LUMO levels simultaneously. At the same time, one can find that all the compounds exhibit similar LUMO levels (-3.61 to -3.71 eV). The results indicate that the end acceptor group has a greater impact on HOMO level than that of LUMO one. In consideration of HOMO and LUMO levels of PC71BM (-3.91 and -5.87 eV),56 all the compounds have proper HOMO/LUMO levels for the utilization as electron donor materials in the BHJ-OSCs with PC71BM as acceptor. Moreover, the deep-lying HOMO levels are favorable to obtain a high open-circuit voltage (Voc) of the OSCs with these SMs as donor materials. These optical and electrochemical results indicated that the absorption spectra, bandgap and electronic energy levels of these STF-based compounds can be manipulated effectively by variation



in terminal acceptor units.

(a)

(b) STFYT Current

Current -1.0

-0.5

0.0

0.5

1.0

1.5

-1.5

STFRDN

-1.0

-0.5

0.0

0.5

1.0

Potential (V vs Ag/AgCl)


(c)

Current

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STFRCN

-1.0

-0.5

0.0

0.5

1.0


Figure 3. Cyclic voltammograms of the three SMs films on the platinum electrode in an acetonitrile solution of 0.1 mol L-1 Bu4NPF6 with a scan rate of 0.1 V s-1, (a) STFYT; (b) STFRDN; (c) STFRCN.

2.4. Density Functional Theory Calculations To study the influence of the spiro STF-based central core and the different acceptor units on the molecular structures and electronic energy levels of these SMs, theoretical calculation was performed and evaluated using the Gaussian 09 with the B3LYP functional and the 6-31G* basis set. Frontier molecular orbitals and molecular geometries displayed here were drawn by using the GaussView 5.0 molecular visualization and manipulation program. To reduce the time of calculation, the alkyls (octyl and ethyl) of the molecules were replaced by methyl groups (Table 2).


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The orbital electron distributions of their HOMO/LUMO levels, including the calculated HOMO/LUMO levels are given in Table 2. For all the three molecules, the HOMO level is primarily delocalized from the STF-based central donor unit to some part of the acceptor unit (YT, RDN, and RCN), whereas the LUMO level is delocalized on the entire conjugated main chain. The HOMO/LUMO levels of STFYT, STFRDN, and STFRCN were calculated to be -4.94/-2.74, -4.89/-2.67, and -4.87/-2.68 eV, respectively. Clearly, the trend of variation for energy levels is agreed with the results gained from the electrochemical CV measurements (see Table 1). Table 2. Electron density distributions of the HOMO and LUMO and molecular energy levels of the three SMs by DFT with the B3LYP/6-31G* basis set. Compound

HOMO

LUMO

-4.94 eV

-2.74 eV

-4.89 eV

-2.67 eV

-4.87 eV

-2.68 eV

STFYT

STFRDN

STFRCN

The optimized molecular geometries of the SMs are shown in Figure 4. Herein, the dihedral angel of θ1 is defined to be the conjugation effect of the central STF-based donor unit and the terthiophene π-conjugated bridge and θ2 determines whether effective conjugation of the different terminal acceptor groups and the π-conjugated bridge. As shown in Figure 4, we can find that all molecular backbones display similar geometries and all of the three molecules have good planar structures from the side view, which will be beneficial for the molecular stacking. The different terminal acceptor



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groups are nearly coplanar with the linked conjugated terthiophene bridge (θ2 close to 0° for all molecules), while there is a twist angle of -8.3°, -11.2°, and 3.2° for STFYT, STFRDN, STYRCN, respectively, between the central STF-based donor unit and the π-conjugated bridge, suggesting STFYT and STFRCN have better coplanarity than STYRDN. Such noncoplanar features would lead to less effective stacking of STYRDN in the solid state. This difference, although quite small, might result in different molecular packing as observed by absorption spectroscopy, but also in different device performance as discussed below. Moreover, compared with the reported analogues with other donor units (BDT,29,30,34 IDT,40 and CPDT35), these STF-based SMs have obviously better planarity, implying that spiro STF core benefits to reduce the steric hindrance effect between the central donor unit and terthiophene bridge. The reduced steric hindrance effect is favorable to enhancing ICT transition effect between the STF-based donor and acceptor groups. These observations are consistent with the results as mentioned earlier in the optical properties (broaden the absorption band and reduce the band gaps, see Figure 2). Finally, from side view of the optimized molecular geometries (Figure 4), the STF core in all the three molecular models is perfectly cruciform with two orthogonal conjugated backbones (CPDT and FL). The cruciform X-shape of these molecules can help to avoid excessively strong crystalline packing.57

Compound

Top view

STFYT

STFRDN


Side view

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STFRCN

Figure 4. The optimized molecular geometries of STFYT, STFRDN, and STFRCN by DFT calculation.

2.5. Photovoltaic Performance To explore the effect of the spiro center D unit and different A units on the photovoltaic properties of these SMs, solution-processable BHJ SMOSCs were prepared employing STFYT, STFRDN, and STFRCN as donor materials and PC71BM as an electron acceptor of the three SMs with a typical device structure of ITO/PEDOT:PSS/donor:PC71BM/PDINO/Al. Here PDINO is a perylene diimide with N-oxide as the terminal groups, which had been proved as an effective electron transport layer material for high performance OSCs.58,59 The relevant experimental details of the device fabrication process are involved in the Experimental Section in SI. The photovoltaic performances were optimized by scanning the weight ratio of SM donor and PC71BM acceptor and the processing additive content of DIO. The related performance data are listed in Table 3. The representative current density-voltage (J-V) curves of the optimized devices are displayed in Figure 5. As shown in Table 3, both the donor/acceptor weight ratio and processing additive (DIO) have great effects on solar cell photovoltaic performance. For example, the device based on STFYT/PC71BM (1:1 by weight) without the addition of DIO as additive shows a very low power conversion efficiency (PCE) of 1.78%, corresponding to a Voc of 0.82 V, a short-circuit current density (Jsc) of 4.21 mA cm-2 and a fill factor (FF) of 51.8%. When 3% DIO (v/v) is used as the processing additive, the device performance is markedly improved with a PCE up to 6.68%, a



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slightly decreased Voc value of 0.79 V, a significantly increased Jsc value of 12.88 mA cm -2 and a FF of 65.8%. Obviously, the influence of the two aspects on photovoltaic performance is mainly showed in the change of Jsc and FF. For all the three SMs, the best PCE is gained with a donor/acceptor weight ratio of 1:1 and 3% DIO (v/v) as the processing additive. Under this optimization conditions, STFYT-, STFRDN-, and STFRCN-based devices exhibit the highest PCE of 6.68%, 3.30%, and 4.33%, respectively (see Figure 5). STFYT with YT as the acceptor unit shows a comparably high PCE with other A-π-D-π-A molecule based on BDT central unit with the same acceptor group (PCE = 6.75% ).31 Moreover, STFRDN exhibits significantly higher PCE than that of the analogue with CPDT as the central donor unit (PCE = 0.75%).35 From the data given in Table 3, one can see that the Voc values of STFYT-based devices are lower than those of STFRDN- and STFRCN-based devices. As we all know, the Voc of BHJ OSCs is directly related to the deviation between the HOMO level of a donor and the LUMO level of an acceptor material.60 Therefore, STFYT is supposed to deliver the highest Voc and STFRCN would show the lowest Voc among the three SMs on the basis of the electrochemical results as shown in Table 1. However, the experimental observations were completely opposite, STFYT exhibits the lowest Voc of 0.79 V, whereas STFRDN and STFRCN show the relatively high Voc with the similar value of 0.87 V. Up to now, the issue of the origin of Voc remains a source of debate in the scientific community. Recent researches demonstrate that Voc is determined by some other factors besides just the electronic energy levels (HOMO/LUMO) of electron donor materials (p-type polymers and SMs) and acceptors (e. g. PC71BM),61,62 such as interchain distances, side-chain bulkiness and active layer morphology.63 Especially, the previous reports has proven that the intermolecular interaction or electronic coupling between the electron donor and acceptor materials in the BHJ system has a huge impact on the reverse saturation current


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density in dark and hence the Voc.61-64 Weakening the intermolecular interaction or electronic coupling between the donor and acceptor molecules can decrease the reverse saturation current density in dark and hence enhance the Voc. Based on the optimized structures of STFRDN and STFRCN (see Figure 4), the two ethyls on rhodanine and 2-(1,1-dicyanomethylene)rhodanine units are arranged on the two sides of the π-D-π (terthiophene-STF-terthiophene) plane, which may shorten the distance with the acceptor molecule (PC71BM), therefore reduce the interactions with the acceptor in this regard of steric hindrance effect.63,64 The different terminal groups end-capped terthiophene-STF-terthiophene backbone will induce the variation of dipole moment, which has an effect on the packing arrangement of molecular films and thus indirectly impact on the electronic couplings with the acceptor, thus worked on the Voc.64,65 According to the above points, among these three SMs STFRDN and STFRCN should have weaker interactions with the acceptor (PC71BM) than that of STFYT, therefore resulting in the larger Voc. Table 3. Photovoltaic properties of the OSCs based on the three molecules with DIO as a processing additive. DIO (%) Voc (V) Jsc (mA cm-2) FF (%) PCE (%)

Active layer STFYT:PC71BM=1:0.8

3

0.79

12.19

65.0

6.27

STFYT:PC71BM=1:1

3

0.79

12.88

65.8

6.68

STFYT:PC71BM=1:1.2

3

0.78

12.33

63.2

6.06

STFYT:PC71BM=1:1

0

0.82

4.21

51.8

1.78

STFRDN:PC71BM=1:0.5

3

0.58

2.77

28.1

0.46

STFRDN:PC71BM=1:1

3

0.87

7.21

52.5

3.30

STFRDN:PC71BM=1:2

3

0.88

4.97

48.6

2.13

STFRDN:PC71BM=1:1

0

0.86

6.00

38.2

1.97

STFRCN:PC71BM=1.5:1

3

0.82

4.50

37.6

1.39

STFRCN:PC71BM=1:1

3

0.88

10.08

48.9

4.33



STFRCN:PC71BM=1:1.5

3

0.86

10.10

48.4

4.20

STFRCN:PC71BM=1:1

0

0.86

3.76

38.5

1.25

On the other hand, the devices based on STFYT and STFRCN display higher Jsc of 12.88 mA cm-2 and 10.08 mA cm-2 than that of the STFRDN-based device (Jsc = 7.21 mA cm-2), which can be attributed to the broader absorption in the UV-vis region (see Figure 2b) and higher absorption coefficient of the former (see Table 1). Moreover, as compared to STFRDN, the higher Jsc values (12.88 mA cm-2 for STFYT-based device and 10.08 mA cm-2 for STFRCN-based device) are confirmed by EQE measurements for the best devices (Figure 6). As shown in Figure 6, the devices based on STFYT and STFRCN exhibit broader photo-to-current responses from 300 to 800 nm and higher EQE values relative to STFRDN-based devices, which is consistent with the absorption spectra of these SMs films (see the inserted image in Figure 6). In addition, the increasing FF of the STFYT device with respect to STFRDN and STFRCN ones may relate to the superior charge carrier mobility and the active layer morphology, as discussed below.

0

Current Density (mA/cm2)

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STRYT STFRDN STFRCN

-4

-8

-12

0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

Figure 5. Typical J-V curves of photovoltaic devices based on STFYT, STFRDN, and SFTRCN with the best fabricated conditions.


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60

STFYT STFRDN STFRCN

50 40 30

1.0

Normalized Absorption (a.u.)

EQE (%)

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20 10 0 -10 300

(b)

STFYT STFRDN STFRCN

0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

800

900

Wavelength (nm)

400

500

600

700

800

Wavelength (nm)

Figure 6. EQE curves of photovoltaic devices based on STFYT, STFRDN, and STFRCN with the best fabricated conditions; the absorption spectra of three SMs films are collocated for a comparison. As for these three compounds, to further understand the differences in Jsc and FF values and inspect the influence of the different terminal A units on the transport properties of carriers, the hole and electron mobilities of the blend films are evaluated through the space charge-limited current (SCLC) method with a hole-only device (ITO/PEDOT:PSS/SM:PC71BM/Au) or an electron-only device (ITO/Al/SM:PC71BM/Al). Figure S16 in SI presents the current-voltage plots for the carrier mobility measurements of SM:PC71BM blend films with the best fabricated conditions and the detailed hole and electron mobilities are listed in Table S1. From Figure S16a, the hole mobilities (µh) of STFYT, STFRDN, and STFRCN blend films are deduced as 1.57 × 10-4, 3.15 × 10-5 and 1.26 × 10-4 cm2 V-1 s-1, respectively, which can be comparable to the analogous A-π-D-π-A compounds with other central donor units (e.g. BDT,29,30,34 IDT,40 CPDT35). On the other hand, the electron mobilities (µe) are determined to be 6.96 × 10-5, 2.08 × 10-6 and 4.24 × 10-5cm2 V-1 s-1 for STFYT, STFRDN, and STFRCN, respectively, as shown in Figure S16b. From these results, some conclusions can be drawn: (i) the hole mobility is higher than the electron mobility for each of SMs; (ii) the change trend



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of the hole mobilities is agreed well with that of the electron mobilities as well as the highest Jsc values of solar cells based on the three SMs (Table 3); (iii) the different A unit has great impact both on hole and electron transportation of A-π-D-π-A molecules. Previous results have suggested that the introduction of the highly electron-deficient terminal A group into A-π-D-π-A molecules would increase the ground-state dipole moment and enhance intermolecular electronic coupling, which will therefore reduce the molecular carrier reorganization energy.18,47 As a result, efficient charge transport is anticipated in OSCs based on the materials. For our three molecules, the electron-withdrawing strength of YT and RCN groups are larger than that of RDN.64 Accordingly, STFYT and STFRCN with YT and RCN as the terminal A groups, respectively, exhibit not only higher hole mobilities but also higher electron mobilities than STFRDN with RDN as A unit. More importantly, the hole mobility to electron mobility (µh/µe) of STFYT and STFRCN is 2.26 and 2.97, respectively, which is obviously lower than that of STFRDN (15.24) (see Table S1). The relatively high and balanced hole and electron mobilities of STFYT and STFRCN blend films could improve charge carrier transportation and decrease charge recombination, hence resulting in a higher Jsc and FF as well as better photovoltaic performance of the related devices.

2.6. Film Microstructure and Morphology Analysis X-ray diffraction (XRD) analysis was employed to investigate the effect of the spiro STF central D unit and different acceptor groups on the structural ordering and the crystallinity of SMs in the solid state. The XRD patterns of the pristine SMs films cast from CHCl3 onto silicon chip substrates are exhibited in Figure S17 (SI). As shown in Figure S17, the STFYT pristine film presents an obvious diffraction peak at around 2θ = 7.17°, corresponding to a lamellar spacing of 12.3 Å, which


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is evidently smaller than that of analogs with BDT31 and IDT40 as the central donor unit. The result indicates that the introduction of a spiro STF core in the STFYT molecule leads to reduce lamellar distances and enhance intermolecular interactions to some extent. However, the other two compounds of STFRDN and STFRCN films exhibit no distinct diffraction peak in the range of our observation, meaning that these materials are still in amorphous state and they possess a poor structural ordering and the crystallinity under our experimental conditions. This should be correlative with the cruciform X-shape of these molecules as mentioned before. The better structural ordering and higher crystallinity of STFYT relative to STFRDN and STFRCN should be favorable to charge transport, and hence increase the carrier mobilities, Jsc as well as FF in solar cells based on the former. Additionally, we note that some analogous of STFRDN and STFRCN with BDT,21,30,34 DTS35, or IDT39 as a central donor unit showed obvious (100) diffraction peaks. From the above observations, one can deduce that both the central donor unit (STF) and the acceptor groups have a great influence on the structural ordering and crystallinity of A-π-D-π-A type SMs. Atomic force microscopy (AFM) measurement was used to inspect the active layer morphologies of SM:PC71BM blend films under the best fabricated conditions, and the corresponding phase and height images of AFM are described in Figure S18 (SI). The AFM analyses reveal that the films of STFYT, STFRDN, and STFRCN blended with PC71BM exhibit root-mean-square (RMS) surface roughness values of 2.29, 3.54, and 2.19 nm, respectively. This observation indicates that the donor SMs of STFYT and STFRCN have better miscibility with the acceptor molecule (PC71BM) than STFRDN in the blends. Moreover, from the phase images one can see that the donor/acceptor interpenetrating network of STFYT based blend film is better than that of the blend films of STFRDN and STFRCN. The microstructure and morphology of blend films were further investigate



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by transmission electron microscopy (TEM), as depicted in Figure S18. The TEM results are very consistent with those seen from AFM images. In TEM images, the white and black domains are assigned to SM-rich and PC71BM-rich regions, respectively. There is a relatively strong small molecular aggregation or PC71BM aggregation in STFRDN and STFRCN samples, meaning a relatively worse donor/acceptor compatibility for STFRDN and STFRCN blend films. Furthermore, the STFYT:PC71BM blend film shows a more regular interpenetrating network structure and defined nanoscaled phase separation than STFRDN and STFRCN ones, resulting in a more efficient charge separation on the donor/acceptor interfaces. The better morphology could increase charge transport and separation efficiency on the donor/acceptor interfaces, thus leading to higher Jsc and FF. It is believed that higher PCEs could be expected through further efforts focused on device optimization.

3. CONCLUSION In summary, three A-π-D-π-A small molecules, STFYT, STFRDN, and STFRCN with the same STF central core and π-conjugated bridge, but different terminal acceptor groups (YT, RDN, RDN), have been designed, synthesized and utilized to fabricate efficient solution-processable SMOSCs as donor materials. All of these SMs exhibit a strong and broad absorption characteristics as well as relatively deep-lying HOMO levels. Both terminal acceptor groups and the spiro STF-based central core have great impacts on absorption properties, HOMO/LUMO energy levels, molecular configurations, hole mobilities and microstructure of the blend films of the molecules. As a result, BHJ photovoltaic devices based on these SMs and PC71BM exhibit a dramatic difference in the photovoltaic performance. In particular, the STFYT-based device exhibits the highest hole mobility and better active layer morphology, leading to the highest PCE of 6.68% with Jsc = 12.88 mA cm-2


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and FF = 65.8%. The results indicate that the spiro STF core, as BDT unit, can be an attractive and effective central building block for developing highly efficient A-π-D-π-A type SM donor materials. Though rhodanine (RDN) and 2-(1,1-dicyanomethylene)rhodanine (RCN) have been proved as highly efficient acceptor units, in this study we found that indenedione (YT) is a more efficient acceptor unit as compared with RDN and RCN in this molecular motif with STF group as the central donor unit. The results reveal that not only the central donor unit but also the terminal acceptor group plays an important role in improvement of the optoelectronic properties and device performance of A-π-D-π-A type SM donor materials. More importantly, the optimized combination of D and A units should be considered carefully for designing high performance A-π-D-π-A type SM donor materials.

Supporting Information The detailed experimental methods and procedures, 1H NMR,

13

C NMR and HRMS spectra, TGA,

DSC, current-voltage plots for the measurements of hole and electron mobilities, XRD patterns, AFM phase images and TEM images, data of the hole and electron mobilities.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (Grant No. 21574111), Natural Science Foundation of Hunan Province, China (Grant no. 2015JJ3019), the Key Research Foundation of Education Bureau of Hunan Province, China (Grant



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No. 14A147), and the Program for Innovative Research Cultivation Team in University of Ministry of Education of China (Grant No. 1337304).

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Development of Spiro[cyclopenta[1,2-b:5,4-b ... - ACS Publications

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