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Proceedings of Engineering and Technology Innovation , vol. 2, 2016, pp. 14 - 16 

14 

Synthesis and Annealing Study of Silicon- and  

Fluorine-containing Low Bandgap Conjugated Polymers  

for Solar Cell Applications 

Cheng-Hsien Wu, Cian-Hao Huang, Pei-Ni Song, Tzong-Liu Wang
*
 

Department of Chemical and Materials Engineering, National University of Kaohsiung,  

Kaohsiung, Taiwan. 

Received 07 March 2016; received in revised form 12 April 2016; accept ed 09 May 2016 

 

Abstract 

Two donor-acceptor alternating conjugated 

copolymers were synthesized as the donor materi-

als of the active layer in polymer solar cells via 

Stille coupling reaction. Both copolymers con-

sisted of dithienosilole as the donor unit and non-

fluorinated 2,1,3-benzooxadiazole or fluorinated 

2,1,3-benzooxadiazole as the acceptor unit. The 

nonfluorinated polymer and fluorinated polymer 

were designated as P1 and P2, respectively. The 

structures of copolymers were confirmed by FT-IR, 
1
H NMR, and 

13
C NMR. Optoelectronic properties 

of the polymer were investigated and observed by 

UV-vis spectrum, photoluminescence spectrum, 

and cyclic voltammetry. Both polymers exhibited 

a panchromatic absorption ranging from 300 nm to 

1100 nm and displayed low band gaps of 1.46 eV 

and 1.42 e V, respectively. Both of the synthesized 

materials were used as the donor material in the 

bulk heterojunction (BHJ) solar cells and then 

power conversion efficiency (PCE) measurements 

were conducted in different weight ratios of the 

polymer:PC61BM blends . 

Compared to the nonfluorinated  one, the 

fluorinated polymer exhibited a higher PCE of 

2.67%  at room temperature under the illumin ation 

of AM 1.5 (100 mW/cm
2
). Study of the effect of 

annealing on the performance of the P1/PC61BM 

devices using a 1:2 blend ratio of P1:PC61BM 

exhibited the highest short-circuit current density 

of 5.88 mA/cm
2
 and a power conversion efficiency 

of 2.76% at annealing temperature of 125
o
C. The 

bulk heterojunction polymer solar cell employing 

P2 and PC61BM at a blend ratio of 1:2 exhibited 

the highest short-circuit current density of 6.44 

mA/cm
2
 and a power conversion efficiency of 

3.54% at annealing temperature of 150 
o
C. 

Keywor ds : donor-acceptor, solar cell, dithi-

enosilole, 2, 1, 3-benzooxadiazole, 

low band gap, power conversion 

efficiency 

1. Introduction 

Except the “weak donor–strong acceptor” 

concept for the design of D-A  alternating  co-

polymers with low band gaps and high Vocs for 

solar cell applications  [1], adding elec -

tron-withdrawing groups to the polyme r back-

bone is also considered as an effic ient approach. 

Recently, fluorine has attracted much attention 

as an electron-withdrawing group in the synth e-

sis of conjugated polyme rs for h igh -efficiency  

solar cells [2-3]. According to a  theoretical study 

by Brédas et a l., the introduction of fluorine  

atom as a  substituent of the conjugated polymer 

would lo wer both of the HOM O and LUM O 

energy levels of the synthesized polymer [4]. 

On the  othe r hand , s ilo le-cont ain ing  a ro-

mat ic  mo iet ies hav e  attracted  mu ch  attent ion  

as ne w bu ild ing  units  o f con jugated  po ly me rs  

because of the ir un ique  optoe lect ron ic p ro p -

ert ies. As rega rds  the ir ca rbon ana logues, 

silo le-conta in ing po ly me rs have quit e  

lo w-ly ing  LUM O leve ls, wh ich are  att ributed  

to the fact that the  -orb ita l of the Si-C bond  

can e ffect ive ly  interacts with  the -orb ita l o f 

the butad iene frag ment [ 5]. By  trans forming  

PCPDT BT  to the s ilo le-conta in ing po ly me r, 

poly{[4,4'-b is(2-ethy lhe xy l)d ith ieno (3,2-b; 2',

3'-d )silo le ]-2,6-d iy l-a lt-(2,1,3-ben zoth id ia zo l

e)-4,7-d iy l} h as been re cent ly rea lized and a  

PCE of as high as 5.6%  has been ach ieved  [6]. 

There fo re , the  d ithieno[3,2-b :2',3'-d]-s ilole  

(DT S) un it  has  frequent ly  used  as th e  s i-

*Corresponding aut hor. Email: t lwang@nuk.edu.t w 



Proceedings of Engineering and Technology Innovation , vol. 2, 2016, pp. 14 - 16 

15 Copyright ©  TAETI 

lo le -conta in ing  donor un it in  the  deve lop ment  

of high-efficiency PSCs . 

Here in, we have investigated the effect of 

silole-containing and fluorination on the pho-

tovoltaic performances of the D -A conjugated 

copolymers. T wo D-A copoly mers co mprising  

DTS unit and nonfluorinated and fluorinated 

2,1,3-benzoo xad iazo le (BO) unit  has been syn-

thesized to study the effect of silicon and fluo-

rine substitution. The effect of annealing on the 

device performance of the poly mer solar ce lls  

fabricated fro m the nonfluorinated and fluori-

nated polymers has been investigated in detail. 

2. Method 

2.1. Synthesis of Polymers 

In this study, two D-A type copolymers were  

synthesized via Stille coupling reaction. For 

convenience, they are designated as P1 and P2. 

The synthetic routes are shown in Fig. 1. 

 

 
Fig. 1 Polymer synthesis  

2.2. Annealing Study 

The  dev ic e st ruct ure o f PSCs fo r cu rrent  

density -vo ltage  (J -V) me asu re ments  is  

IT O/ PEDOT : PSS/Po ly me r: PC61BM/ LiF/A l. 

The films of act ive layers we re ann ea led d i-

rect ly  on top  o f a hot  p late  in  the  glove  bo x,  

and the te mpe ratu re was mon ito red by  using a  

thermo couple touch ing the  top o f the  su b-

strates.  A fte r re mova l fro m the  hotp late , the  

substrates were immed iat e ly put onto a  meta l 

plate  at the  roo m te mp eratu re . Ult rav io -

let -v isib le  (UV-Vis ) spectroscopic  ana lysis  

was conduct ed on  a  Perkin– Elme r La mbda  35 

UV-Vis  spectrophoto mete r.  The  J-V cu rves  

we re  measured  using  a  Ke ith ley  2400 sourc e  

mete r, unde r illu min at ion fro m a  sola r simu -

lato r.  The  intensity  o f so la r s imu lato r was set  

with  a  prima ry re fe rence  c e ll and  a  spectra l 

correct ion facto r to g ive  the pe rformanc e u n -

der the  AM  1.5 (100 mW/c m2) g loba l re fe r-

ence spectrum (IEC 60904-9). 

3. Results and Discussion 

 
Fig. 2 J-V characteristics of devices for P1 a fter 

annealing at different te mperatures for 30  

min 

 
Fig. 3 J-V characteristics of devices for P2 a fter 

annealing at different te mperatures for 30  

min 

Figs. 2 - 3 show the J-V characteristics under 

white light illu mination (100 mW/c m
2
) for 

photovoltaic devices subjected to thermal an-

nealing. For the P1 poly mer, the device annealed 

at 125 °C shows the best efficiency of 2.76% , 

while the P2 poly mer shows the optimu m effi-

ciency of 3.54%  at 150 °C. Thus, it is c lear that 



Proceedings of Engineering and Technology Innovation , vol. 2, 2016, pp. 14 - 16 

16 Copyright ©  TAETI 

annealing treatment affects the device perfo r-

mance  significantly and that there e xists opt i-

mu m range of the annealing te mperature. It is  

noteworthy that the Voc values of P2 a re higher 

than those of the P1 polymer because the intro-

duction of F into the conjugated backbone has 

lowered the HOM O and LUMO energy levels of 

the copolymer. Due to the values of Voc a re  

re markable for the P2 poly me r, the power con-

version efficiencies of the solar ce ll for P 2 a re  

much improved. 

4. Conclusions 

Two D–A type copolymers based on DTS as 

the donor unit and nonfluorinated and fluori-

nated BO as the acceptor unit have been syn-

thesized and e mp loyed as the donor materia ls  in  

the active layer of BHJ -type polyme r solar cells.  

As a result of annealing treatment at an optimu m 

condition (125 °C/30 min ), the PV cell perfor-

mance of P1 was dra matica lly enhanced and the 

power conversion efficiency of device reached 

to 2.76%. Through the modification by fluori-

nation on the BO units, the photovoltaic pe r-

formance of P 2 at  150 °C was much imp roved 

due to the increased Jsc and enhanced Voc and the 

power conversion efficiency of the device 

reached 3.54%  under wh ite light illu mination  

(100 mW/cm
2
). 

Acknowledgement 

We gratefully acknowledge the support of the 

Ministry of Science and Technology in Taiwan 

through Grant NSC 99-2221-E-390-001-M Y3. 

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