Ring-protected small molecules for organic photovoltaics

Ring-protected small molecules for organic photovoltaics

Jianyuan Sun,a Lianjie Zhang,

a Ashishi Dubey,

b Swaminathan Venkatesan,

b Ting-Yu Lin,

c Logan

Sanow,a Yu-Chueh Hung,

c Andrew Sykes,

d Hongshan He,

e Qiquan Qiao,

b and Cheng Zhang

a*

aDepartment of Chemistry and Biochemistry, South Dakota State Univ, Brookings, SD 57007-0896.

bDepartment of Electrical Engineering and Computer Science, South Dakota State Univ.

cInstitute of Photonics Technologies, National Tsing Hua University, Taiwan, ROC.

dDepartment of Chemistry, University of South Dakota, Vermillion, SD57069

eDepartment of Chemistry, Eastern Illinois University, Charleston IL 61920-3099

* [email protected]

ABSTRACT

Recently, excellent solar cell device performances have been achieved with solution-processed small-molecule donor

materials. Small molecules have well defined structures and thus allow better control of self-assembly in the solid state.

However, the easy formation of H-type aggregates and lack of strong interactions between nanodomains could limit

charge transport, device performance, and long-term stability. We have recently explored the synthesis of ring-protected

small molecules (with rings surrounding the center of the molecules), studied the intermolecular interactions in solution

and solid state, and conducted preliminary solar cell device fabrications. It has been found that the molecules behave

very differently from conventional flat small molecules in both solution and solid states. Proton NMR study of solutions

of different concentrations revealed the presence of strong intermolecular interactions as a result of absence or shortage

of open-ended alkyl side chains; however, such strong interactions do not lead to precipitation of the molecules even at

high concentrations. Excellent films are routinely obtained from the neat small molecules despite the much reduced

number of solubilizing groups. The New findings strongly suggest that ring protection is an effective strategy to avoid H-

aggregation and maintain strong pi-pi interactions simultaneously. Such materials are expected to form head-tail self-

assemblies that will open new possibilities for small molecule organic materials. Conceptually, thin films of such

materials are potentially more isotropic in charge transport than conventional small molecule and polymer films, a

property desirable for photovoltaics and some other optoelectronic applications.

Keywords: Chromophore, self-assembly, single crystal, dimer formation, herringbone, solar cell, hole mobility, time of

flight, small molecule.

1. INTRODUCTION

Solution-processed small molecule-based solar cells (SMSCs)1 are emerging as a competitive alternative to their polymer

counterparts. A photoelectric conversion efficiency of 7.38 % was reported in 2012 for a benzo[1,2-b:4,5-b]dithiophene-

based symmetrical chromophore.2 Earlier this year (2013), a new record of 8.1% was created for small molecule donor-

based bulk heterojunction solar cells.3 Despite their promising advantages including well-defined structure with less

batch-to-batch variation,4,5

versatile molecular structure, and easier band structure control,6,7

molecular chromophores do

have a common problem: poor film-forming property. This problem is rarely mentioned in published papers on small

molecule photovoltaics, but is routinely encountered in our device research. Our recent work on ring-protected

chromophores8 has shown that a ring-protected dipolar chromophore (RPC1, Figure 1), which was designed for electro-

optic (EO) applications, is capable of forming homogeneous films while the films of a conventional chromophore CC18-9

(Figure 1e) are full of micron-sized crystallites. STEM image of a sub-monolayer film of RPC1 showed that RPC1

Please verify that (1) all pages are present, (2) all figures are correct, (3) all fonts and special characters are correct, and (4) all text and figures fit within the redmargin lines shown on this review document. Complete formatting information is available at http://SPIE.org/manuscripts

Return to the Manage Active Submissions page at http://spie.org/app/submissions/tasks.aspx and approve or disapprove this submission. Your manuscript willnot be published without this approval. Please contact [email protected] with any questions or concerns.

8830 - 96 V. 3 (p.1 of 10) / Color: No / Format: Letter / Date: 7/29/2013 4:34:24 PM

SPIE USE: ____ DB Check, ____ Prod Check, Notes:

molecules self-assemble into string-like structures, instead of H-aggregate, as a result of the dominance of the head-tail

interaction over the antiparallel interaction (Figure 2). Head-tail self-assembly of dipolar chromophores is not only

desirable in EO materials, but may also lead to homogeneous films with good carrier mobility (via strong head-tail pi

interaction) and no domain boundaries. With judicious designs to tailor optical and nonlinear optical properties, we

believe ring-protected small molecules will find applications in OPV, as well as other optoelectronic devices such as

OFET devices.

We are also interested in investigating self-assembly of chromophores with ring protection for one side of pi system.

Benzo[1,2-b:4,5-b]dithiophene (BDT) has been chosen as a central unit as it has been widely used in high performance

OPV molecules and polymers. One-side protected BDT has been synthesized and is being used to build a series of ring-

protected BDT-based chromophores, both symmetrical and nonsymmetrical (thus dipolar). In this paper, we present the

results of two ring-protected chromophore SC352 and SC354 (Scheme 1) in comparison with their ringless literature

analogs2.

Figure 1. a. Structure of RPC1, b. RPC1 3D structure, c. A conventional dipolar chromophore (CC1), d. Optical image of a neat

RPC1 film (~2 m thick), e. Optical image of a neat CC1 film (~2 m thick).

RPCs Antiparallel Head-Tail

+

Donor

Acceptor

or

Figure 2. Two interaction modes of chromophores. Head-tail can become dominant for RPCs.

2. MATERIAL SYNTHESIS

4,8-(1,12-Dodecylenedioxy)Benzo[1,2-b:4,5-b]dithiophene (5). A 2L 3-neck flask, equipped with a thermometer, was

flushed with N2 gas, charged with 1 (59.51g, 137.05 mmol) and diethyl ether (595mL), and cooled in a –60 oC bath.

BuLi (2.31 mol eq, 125.2 mL) was added when the internal temperature was lowered to –35 oC. Addition rate was kept

slow so that the temperature dropped to –65 oC at the end of addition. No precipitate was observed. The reaction was

monitored by NMR to assure that the dilithiation reaction was complete. 1,2-Bis(2,2-diethoxyethyl)disulfide10

(2.93 eq,

120g, 96.5%, homomade) was added to the reaction solution (at –65 oC) over 1h. The reaction was allowed to warm up

to room temperature. Ether was removed by rotary evaporation under 500 mbar. Hexanes (500mL) and water (100mL)

were added to dissolve the residue. The aqueous phase was separated and extracted with hexanes (50 mL). Hexane

extracts were combined and frozen for 1h to remove hexane-insoluble impurities. The clear solution was decanted into a

2L flask and condensed by rotary evaporation at 250 mbar to afford yellow oil (141.5g). Without purification, the crude

product 3 was hydrolyzed to remove the protecting groups to give dialdehyde (4): A mixture of crude 3, acetone (2100 g),

water (360 g) and 2N HCl (360 g) was stirred at 57 oC for 40 min. It was then neutralized with NaHCO3 powder. After

acetone was removed by rotary evaporation, the crude oil was taken up by hexanes. The hexane solution was dried with





MgSO4 and condensed by rotary evaporation under 250 mbar. The residue was vacuumed with an oil pump at 90 oC to

remove volatile impurities. 1H NMR (CDCl3): 0.94 (m, 8H), 1.04-1.24 (m, 4H), 1.24-1.48 (m, 4H), 1.5-1.67 (m, 2H),

1.68-1.84 (m, 2H), 3.49 (dd, J=15.4 & 2.9Hz, 2H), 3.65 (dd, J=15.4 & 3.5Hz, 2H), 6.86 (s, 2H), 9.54 (t, 2H). Ring-

closure of 4: This reaction was performed in relatively small scales. A typical run: A mixture of crude 4 (14 g),

chlorobenzene (1.5 L) and Amberlyst15 (300 mL) was boiled under reflux with N2 protection for 2h. The solvent was

removed by rotary evaporation. Crude products of all runs were combined and purified using silica gel column

chromatography with EtOAc/Hexanes (1/64) as the eluent. The product crystallized from the residue solutions after

rotary evaporation. Yield 8.52g, 16% (for four steps). 1H NMR (CDCl3): 0.71 (m, 8H), 0.97 (m, 4H), 1.27 (m, 4H),

1.64 (m, 4H), 4.42 (m, 2H), 4.64 (m, 2H), 7.35 (d, J=5.6Hz), 7.51 (d, J= 5.6Hz). 13

C NMR (CDCl3): 24.22, 27.16,

27.24, 27.81, 28.75, 71.96, 120.60, 125.73, 129.46, 131.24, 143.60 ppm.

2,6-Bis(trimethyltin)-4,8-(1,12-dodecylenedioxy)benzo[1,2-b:4,5-b’]dithiophene (6). In N2 glovebox, 2.5 M n-BuLi

(2.1 eq, 10.6 mL) was added in one shot to a solution of 5 (4.893 g, 12.59 mmol) in THF (150 mL) at RT. The solution

turned cloudy immediately. Two minutes later, 2.12 eq, trimethylstannyl chloride solid (26.69 mmol, 5.319 g) was

added at once. The solution became clear in 10s. The reaction flask was taken out of glovebox. Saturated aqueous

NaHCO3 (3mL) was added to the reaction mixture. The solvent was removed by rotary evaporation at 100 mbar. The

residue was dissolved by 100 hexanes. The hexane solution was dried with anhydrous Na2CO3, and condensed, and

vacuum dried to afford 9.03 g. Purity by 1H NMR spectral analysis: ≥76%. Purification was attempted using silica gel

chromatography with success. 1H NMR (CDCl3): 0.44 (s, 18H), 0.66 (m, 4H), 0.74 (m, 4H), 0.97 (m, 4H), 1.28 (m,

4H), 1.64 (m, 4H), 4.42 (m, 2H), 4.70 (m, 2H), 7.56 (s, 2H).

Compound SC352. A mixture of 6 (2.816 mmol, 2.012 g), 711

(5.628 mmol, 2 g), Pd(PPh3)4 (0.056mmol, 64 mg) and

THF (~3mL) in pressure reaction tube was stirred at 120C for 30 hours. The crude product was purified on a silica gel

column using chloroform as eluent. Crystallization of the product from EtOAc afforded 2.13g (yield 80%). Purity

estimated from NMR analysis: 97%. 1H NMR (CDCl3): 0.89-0.71 (m, 8H), 1.03 (m, 4H), 1.33 (m, 4H), 1.71 (m, 4H),

4.44 (m, 2H), 4.66 (m, 2H), 7.16 (d, 2H, J=3.9Hz), 7.17 (d, 2H, J=3.8 Hz), 7.23 (d, 2H, J=3.8Hz), 7.24 (d, 2H, J=4.1Hz),

7.29 (d, 2H, J=3.8Hz), 7.54 (s, 2H), 7.67 (d, 2H, J=3.9Hz), 9.86 (s, 2H).

Chromophore SC354. A mixture of SC352 (0.47 mmol, 440 mg), 812

(1.17 mmol, 162.6 mg), chloroform (8 mL) and

triethylamine (39 mg) in a 32-mL vial was heated with stirring in a 60 oC oil bath for 16 h. The crude product was

purified on a silica gel column using chloroform as eluent. Crystallization of the product from EtOAc afforded 460 mg,

yield: 79 %. 1H NMR (CDCl3): 0.62-0.83 (m, 8H), 0.95-1.05 (m, 10H including a triplet at 0.91 of 6H – two CH3), 1.2-

1.5 (m, 16H including 12H from underlined CH2 groups in two CH2CH2CH2CH2CH2CH3), 1.5-2.0 (m, 8H including 4H

from underlined CH2 groups in two CH2CH2CH2CH2CH2CH3), 4.23 (m, 4H – OCH2), 4.39 (m, 2H), 4.61 (m, 2H), 7.05

(d, J=3.8Hz), 7.13 (d, 2H, 3.8Hz), 7.15 (d, 2H, 4.1Hz), 7.23 (d, 2H, J=3.8Hz), 7.34 (s, 2H), 7.59 (d, 2H, J=4.0Hz), 8.16

(s, 2H). 13

C NMR (CDCl3): 162.97, 148.92, 145.80, 142.99, 139.05, 138.93, 137.24, 136.18, 135.69, 134.41, 134.28,

132.20, 128.63, 127.29, 125.95, 125.10, 124.83, 124.21, 116.61, 116.04, 97.55, 72.04, 66.60, 31.44, 28.85, 28.55, 27.80,

27.32, 27.22, 25.52, 24.33, 22.56, 14.03. Proton and carbon NMR spectra are in Figure 3.

Chromophores SC370 and SC371. Synthesized according to a literature procedure.2

3. RESULTS AND DISCUSSTIONS

3.1 Chromophore synthesis





The synthetic route for ring-protected BDT is shown in Scheme 1. Compound 1 was developed in the synthesis of

RPC1.8 Cyclization of compound 4 was performed in diluted solution as the ring-protected DBT (5) is more reactive

than 4 toward nucleophilic species. The crude 6 could not be purified by column chromatography or crystallization, and

was used directly in Stille coupling with 7 to give chromophore SC352. Solubility of SC352 is ~4 mg/mL in CHCl3, and

is much lower in ethyl acetate (< 1mg/100mL). Knovenagel condensation of SC352 and hexyl cyanoacetate led to

chromophore SC354 (1H NMR spectrum in Figure 3), which is much more soluble in chromophore (>30mg/mL at RT).

The synthesis of the ringless analogs of SC352 and SC354, i.e., SC370 and SC371 (Scheme 1), follows procedure in the

literature.2 SC371 is only slightly different from the literature chromophore

2 in that SC371 has two hexyl groups at the

two ends, while the literature chromophore has two octyl groups.

S

S

S

S

S

S

O

O

Sn Sn

S

S

O

O

C8H17

C8H17

C8H17

C8H17

Br

S S

S O

C8H17

C8H17

S

S

O

O

S

S

O

O

S S

S

SS

S

CN

OO

NC

OO

C6H13

C6H13

C8H17C8H17

C8H17C8H17

S

S

O

O

C12H24

O

O

C12H24

O

O

C12H24

S

EtO OEtS

EtO OEtS

EtO OEt

S

EtO OEt

Br

Br

O

O

C12H24

S

OS

O

S

S

O

O

C12H24

SS SS

S

S

O

O

C12H24

Sn Sn

S

S

OO

BrS

S

S

O

S

S

O

O

C12H24

S S

S

SS

S

CN

OO

NC

OOC6H13

C6H13

CN

OOC6H13

NEt3

n-BuLi

+H3O

+

acetone

Amberlyst 15 n-BuLi

Me3SnCl

(PPh3)4Pd

1 2 3 4 5 6 7

SC352 SC354

(PPh3)4Pd

8 9 SC370 SC371

CN

OOC6H13

NEt3

Scheme 1. Synthesis of Chromophores SC352, SC354, SC370 and SC371.

Figure 3. 1H NMR spectrum of chromophore SC354.

3.2 Chromophore Self-Assembly in Single Crystal XRD and Chloroform Solution

ppm0.01.02.03.04.05.06.07.08.0





SC352 single crystals were obtained from one of a series of SC352 solutions in chlorobenzene (CB) of different degree

of saturation. SC352 crystallizes in the monoclinic system (Figure 4a), space group P 21/c, with cell dimensions: a

19.7883(17), b 10.4567(9), c 20.9060(18) Å, 95.94o, and Z = 4. There is some fuzziness to the atoms in the aliphatic

ring, which is shown in Figure 4a. SC352 molecules first assemble into dimers (Figure 4b) of which every pair of

molecules stack face-on-face with an offset of one carbon atom along the molecular axis. The dimer assembly is tighter

at the center (with a distance of 3.745 Å) and less tight at the ends (with a distance of 4.019 Å). A dimer resembles a

chromophore with ring protecting both sides of the pi system. The dimers further pack into a herringbone structure

(Figure 4c) with no face-face interaction between neighboring dimers, unlike the common herringbone structures of

atomatic molecules.

Crystal growth of SC354 has been tested in different solvents: chlorobenzene, mixture of chlorobenzene and ether or

pentane. Transparent needle crystals have been obtained from its chlorobenzene solution. The sizes of crystals are still

small (~0.02 mm). The needles are currently being used as seeds. Hopefully, larger needles will be obtained in the near

future. Crystal growth of SC370 and SC371 have also been explored. Mixtures of CB and hexanes and mixtures of

chloroform and hexanes have been used for SC370 and C371 respectivcely. Both chromophores only gave fluffy

precipitate, with no sign of crystal formation.

Figure 4. a. Unit cell of SC352 crystal, b. Structure of dimer in the crystal, and c. Herringbone packing in the crystal.

It was noticed that 1H NMR aromatic peaks of SC352 shifted from sample to sample by as much as 0.1 ppm, a

phenomenon never observed in our lab where above one thousand aromatic compounds (including two hundreds of

chromophores) had been synthesized. Figure 5 shows that the aromatic region of 1H NMR spectra of SC352 solutions at

different concentrations. All peaks shifted to the higher field as concentration was increased. Magnitudes of shift are

larger for protons at or near the center of chromophore, smaller for protons located at the termini. For examples, when

concentration was changed from 1.98 mM to 12.3 mM, the singlet peak at 7.5 ppm (from the two identical aromatic

protons on the BDT unit) shifted by as much as 0.084 ppm, while the doublet at 7.6 ppm (from the terminal Hs on the

terminal thiophene units) only shifted by 0.036 ppm. The trend in peak shift magnitude matches the trend of separation

distance between the dimeric chomophores in SC352 single crystal (shorter at the center, larger at the ends). The result

suggests that SC352 forms thigh aggregates (presumably, dimers) even in chloroform solution. The concentrations of

SC352 used were limited by its low solubility (~4 mg/mL CHCl3 for 99% pure sample).





Similar phenomenon was observed for SC354 (Figure 6). From 13.2 to 74.9 mM, the BDT proton peak (at 7.5ppm)

shifted by 0.115 ppm and the olefinic proton peak (at 8.2 ppm) shifted by 0.054 ppm, to the higher field. The magnitudes

of shift are larger than those of SC354. It is very likely that SC354 also forms dimers in solution, and possibly with

shorter separation distance. Despite the strong interaction in solution, solubility of SC354 is excellent (greater than 30

mg/mL CHCl3).

SC370, the ringless version of SC352, showed much less peak shift (Figure 7). From 0.809 mM to 11.3 mM (an increase

of 14 fold, larger than that of SC352, 6.2 fold), peak shifts of only 0.024 and 0.014 ppm were observed for BDT protons

and terminal thienyl protons, respectively. Presumably, the presence of six free-ended side chains does not allow SC370

to interact as strongly as SC352 and SC354. However, when condition is right for aggregation, they fall out of solution

much faster than their ring-protected counterparts and are thus very difficult to form large single crystals (or crystals of

any size).

Figure 5. 1H NMR spectra of chromophore SC352 at different concentration in CDCl3.

Figure 6. 1H NMR spectra of chromophore SC354 at different concentrations in CDCl3.





Figure 7. 1H NMR spectra of chromophore SC370 at different concentrations in CDCl3.

3.3 Time-of-flight mobility measurement of SC354 and SC371 films

TOF devices fabricated for time-of-flight hole mobility measurement had a configuration of indium tin oxide

(ITO)/SC354 (1500 nm) or SC371 (450 nm)/N,N’-bis(naphthalen-1-yl)-N,N’-bis(phenyl)-benzidine (NPB) (100 nm)/Al

(40 nm), where NPB layer was used as a charge generation layer. SC354 and SC371 were dissolved in o-

dichlorobenzene (15 g/L) and were spin-coated on ITO with 600 rpm for 60 seconds and then dried for 10 minutes. The

same process was repeated a few times to obtain desired sample thickness. Then, NPB and Al electrode were vapor

deposited at a rate of 2 Å /s. The devices were encapsulated by covering with a clean glass and sealing with epoxy resin

to avoid the influence of oxygen and moisture.

Figure 8. Left: Hole mobility of SC354 and SC371 films as measured by the time-of-flight method. Right: Normalized UV-vis

absorption spectra of SC354 and SC371 in chloroform solution and film. Right: Films of SC371 spun-coated from chloroform (a)

and chlorobenzene (b) solutions, and SC354 from chlorobenzene solution (c).

In TOF measurement, an optical pulse incident on the material creates photo-generated carriers, which drift across the

sample to the charge-collecting electrode under an applied electric field.13

A digital oscilloscope (Tektronix TDS3032B)

was used to capture the voltage across a current sensing resistor to obtain the transient curve. The typical transient curves

of TOF under applied biases were plotted to extract the transit time. Hole mobility in chromophore film can be obtained

from the transit time, which is determined by the onset of the tail. The obtained mobility valeus of two films of SC354





and two films of SC371 were plotted as functions of square root of applied field (Figure 8). The average hole mobility of

SC354 films is higher than that of SC371 films by one order of magnitude. The poor charge transport in SC371 films

may be attributed to the lower film quality due to its high crystallization tendency.

3.4 Optical characterization and solar cell devices from SC354 and SC371

Normalized UV-vis absorption spectra of SC354 and SC371 in

chloroform solution and film are shown in Figure 9. In solution,

the max of SC371 (490 nm) is shorter than that of SC354 (515 nm),

presumably due to the twisting effect of four octyl groups on the

thiophene units in SC371. Similar effect has been observed for

alkylated oligothiophenes14,15,16

and nonalkylated oligothiophenes17

in solution. The peak extinction coefficient of SC354 (11984 cm-

1M

-1) is also higher than that of SC371 (89171 cm

-1M

-1), in

consistency with the better conjugation in SC354. However, in

solid state, the max of SC371 (553 nm) is red shifted as compared

to that of SC354 (535 nm). This is due to stronger

interchromophore interaction in the solid state. SC371 aggregated

much faster than SC354, and only gave good films when a low

boiling point solvent like chloroform was used. When chlorobenzene was used to spin-coat SC371, the chromophore

aggregated, leaving almost no color on the substrate. In sharp contrast, better films could be obtained from SC354 using

chlorobenzene. The above results demonstrate once again the effectiveness of ring structure in reducing

interchromophore interaction and improving solution processability. Interestingly, much higher mobility (the vertical

direction, i.e., across the thickness of film) is obtained for SC354. Presumably, SC354 films are more uniform and have

less grain boundaries.

Figure 10. Left: J-V curves of devices of SC354 and PC60BM (1:1 wt, 15mg/ml in dichlorobenzene). Right: The results of the

best SC371 device (SC371:PC60BM = 8mg:4mg in 1ml CHCl3, 1700rpm, ramp 1s).

Figure 9. Normalized UV-vis absorption spectra of

SC354 and SC371 in chloroform solution and film.





Solar cell devices have been fabricated from SC354 and SC371 with a configuration of Al (80-100 nm)/Ca

(20nm)/chromophore:PC60BM/PEDOT:PSS/ITO glass. For SC354 of 96% purity, good films were obtained using O-

dichlorobenzene (DCB) as solvent. The best results were obtained from films prepared from a solution of 15 mg SC354

and 15 mg PC60BM in 1mL DCB. Different spin rates were used. The best efficiency was obtained from a film spin-

coated at a relatively high rpm (2600, Figure 10). It has been found that the device performance is highly dependent on

SC354 purity. When purity is 99%, device efficiency dropped to <0.5% even using chlorobenzene as solvent. The J-V

curve of the best SC371 device (SC371:PC60BM = 8:4 mg, 1mL CHCl3, 1700 rpm) and a table of results of SC371

devices fabricate using different solvents are shown also in Figure 10. In consistence with the observation of film quality

dependence on solvent, a small amount of high boiling point solvent caused efficiency to drop dramatically. Given the

fact that SC371 is even more crystalline, the high purity of SC371 used in this work is likely the reason for the lower

device performance as compared to 4.56% efficiency in the literature.2

4. SUMMARY AND CONCLUSIONS

Ring-protected BDT-based chromophores have been synthesized. The ring structure proves to be much more effective

than conventional side chains in improving solubility. Ring-protected chromophore SC352 formed dimer in crystal and

the dimer further pack into herringbone structure. Dimer formation even happens in chloroform solution producing

unusually large NMR peak shifts for the aromatic protons in both SC352 and SC354 as concentration was increased.

SC354 displays higher hole mobility in thick films (measured by the time of flight method) than SC371, and also

performs better in solar cell devices. Both ring-protected chromophores (SC352 and SC354) can grow into single

crystals, while the two ringless chromophores have not shown any sign of crystal formation.

In contract to the herringbone structures formed from conventional aromatic molecules, there is no face-on-face

interaction among SC352 dimers. It is a direction of our future effort on structure optimization to encourage face-on-face

interaction/packing through fine tuning of geometry and polarity of chromophores. The ultimate goal is to establish

dominance of head-tail interaction in the solid state so that good charge mobility can be obtained in amorphous and

isotropic films.

Acknowledgement: This material is based upon work supported by the State of South Dakota Board of Regents

(SDBOR) Competitive Research Grant SDBOR/SDSU 2013-10-06, the National Science Foundation/EPSCoR Grant No.

0903804, and SDSU chemistry department start-up funds.

REFERENCES

[1] Lin, Y.; Zhan, X., “Organic solar cells based on small molecules,” Int. Organic Optoelectronics 375 (2013).

[2] Zhou, J.; Wan, X.; Liu, Y.; Zuo, Y.; Li, Z.; He, G.; Long, G.; Ni, W.; Li, C.; Su, X.; Chen, Y., “Small Molecules

Based on Benzo[1,2-b:4,5-b’]dithiophene Unit for High-Performance Solution-Processed Organic Solar Cells,” J.

Am. Chem. Soc. 134, 16345 (2012).

[3] Zhou, J.; Zuo, Y.; Wan, X.; Long, G.; Zhang, Q.; Ni, W.; Liu, Y.; Li, Z.; He, G.; Li, C.; Kan, B.; Li, M.; Chen, Y.,

“Solution-Processed and High-Performance Organic Solar Cells Using Small Molecules with a Benzodithiophene

Unit,” J. Am. Chem. Soc. 135, 8484-8487 (2013).

[4] Shang, H. X.; Fan, H. J.; Liu, Y.; Hu, W. P.; Li, Y. F.; Zhan, X. W., “A Solution-Processable Star-Shaped Molecule

for High-Performance Organic Solar Cells,” Adv. Mater. 23, 1554-1557 (2011).

[5] Loser, S.; Bruns, C. J.; Miyauchi, H.; Ortiz, R. P.; Facchetti, A.; Stupp, S. I.; Marks, T. J., “A naphthodithiophene-

diketopyrrolopyrrole donor molecule for efficient solution-processed solar cells,” J. Am. Chem. Soc. 133, 8142-

8145 (2011).





[6] Welch, G. C.; Perez, L. A.; Hoven, C. V.; Zhang, Y.; Dang, X. D.; Sharenko, A.; Toney, M. F.; Kramer, E. J.;

Nguyen, T.-Q.; Bazan, G. C., “A modular molecular framework for utility in small-molecule solution-processed

organic photovoltaic devices,” J. Mater. Chem. 21, 12700-12709 (2011).

[7] Demeter, D.; Rousseau, T.; Leriche, P.; Cauchy, T.; Po, R.; Roncali, “Manipulation of the Open-Circuit Voltage of

Organic Solar Cells by Desymmetrization of the Structure of Acceptor–Donor–Acceptor Molecules,” J. Adv. Funct.

Mater. 21, 4379−4387 (2011).

[8] Zhang, C.; Zhang, L.; Benight, S. J.; Olbricht, B. C.; Johnson, L. E.; Robinson, B. H.; Norwood, R. A.; Dalton, L.

R., “Shape engineering to promote head-tail interactions of electro-optic chromophores,” SPIE Meeting, 8827,

paper 4 (2013).

[9] Enami, Y.; Hong, J.; Zhang, C.; Luo, J.; Jen, A. K.-Y., “Optical transmission stability of hybrid sol-gel

silica/electrooptic polymer waveguide modulators,” IEEE Photonics Tech. Lett. 23 (20), 1508 (2011).

[10] Henssler, J. T.; Matzger, A. J., “Facile and Scalable Synthesis of the Fused-Ring Heterocycles Thieno[3,2-

b]thiophene and Thieno[3,2-b]furan.,” Org. Lett. 11 (14), 3144-3147 (2009).

[11] Wei, Y.; Wang, B.; Wang, W.; Tian, J., “A convenient synthesis of functional group end-capped thiophene

oligomers,” Tetrahedron Lett. 36 (5), 665-8 (1995).

[12] Hasham, A. I.; Abdel El-Azim, W. M.; El-Maadawi, N. M., “Synthesis and evaluation of some antioxidant

additives,” Egypt. J. Petrol. 16 (2), 59-80 ( 2007).

[13] Lin, T. Y.; Cheng, I. C.; Hung, Y. C., “Hole mobility characterization of DNA biopolymer by time-of-flight

technique,” Appl. Phys. Lett. 101, 153701 (2012).

[14] Sumi, N.; Nakanishi, H.; Ueno, S.; Takimiya, K.; Aso, Y.; Otsubo, T., “Synthesis and properties of a series of the

longest oligothiophenes up to the 48-mer,” Bull. Chem. Soc. Jpn. 74, 979-988 (2001).

[15] Bäuerle, P.; Fischer, T.; Bidlingmeier, B.; Stabel, A.; Rabe, J. P., “Oligothiophenes—Yet Longer? Synthesis,

Characterization, and Scanning Tunneling Microscopy Images of Homologous, Isomerically Pure

Oligo(alkylthiophene)s,” Angew. Chem. Int. Ed. Engl. 34, 303-307 (1995).

[16] Mustafa, A. H.; Shepherd, M. K., Synthesis of extended ,'-oligo(silylthiophenes) by cerium(IV) oxidative

coupling reactionsChem. Commun. 2743-2744 (1998).

[17] Zhao, M.-T.; Singh, B. P.; Prasad, P. N., “A systematic study of polarizability and microscopic third‐order optical

nonlinearity in thiophene oligomers,” J. Chem. Phys. 89, 5535-5541 (1988).





Ring-protected small molecules for organic photovoltaics

Documents

Transcript of Ring-protected small molecules for organic photovoltaics