Photovoltaic properties and annealing effects of a low bandgap copolymer containing dithienothiophene and benzothiadiazole units

1. Introduction

Over the past decades, polymer solar cells (PSCs)

based on conjugated polymers have attracted

con-siderable attention because of their potential use for

future cheap and renewable energy production [1–

3]. In particular, the polymer solar cell has the

advantage over all photovoltaic technologies that

the possible manufacturing speed is very high and

the thermal budget is low because no high

tempera-tures are needed [4]. Efficient polymer-based solar

cells utilize donor–electron acceptor (D–A) bulk

heterojunction (BHJ) films as active layers [1, 2].

The donor is typically a kind of conjugated

poly-mer, while the acceptor is generally a type of

organic or inorganic molecule. The most exploited

donor polymers is regioregular

poly(3-hexylthio-phene) (P3HT), while the acceptor materials are

generally the fullerene derivatives such as

[6,6]-phenyl C

butyric acid methyl ester (PCBM). A

bulk heterojunction photovoltaic device combining

regioregular P3HT as the electron donor with

func-tionalized fullerenes as the electron acceptor has

demonstrated power conversion efficiencies (PCEs)

up to 7% [5, 6].

Photovoltaic properties and annealing effects of a low

bandgap copolymer containing dithienothiophene and

benzothiadiazole units

T. L. Wang

_{, Y. T. Shieh}

_{, C. H. Yang}

_{, T. H. Ho}

_{, C. H. Chen}

1_{Department of Chemical and Materials Engineering, National University of Kaohsiung, 811 Kaohsiung, Taiwan,}

Republic of China

2_{Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, 807}

Kaohsiung, Taiwan, Republic of China

3_{Department of Electronic Engineering, Cheng Shiu University, 833 Kaohsiung, Taiwan, Republic of China}

Received 13 June 2012; accepted in revised form 29 August 2012

Abstract. A conjugated alternating copolymer as the donor material of the active layer in polymer solar cells has been

designed and synthesized via Stille coupling reaction. The alternating structure consisted of 3,5-didecanyldithieno[3,2-b:2!,3!-d]thiophene (DDTT) donor unit and 5,6-bis(tetradecyloxy)benzo-2,1,3-thiadiazole (BT) acceptor unit. Since both units have been attached pendant chains, the polymer was soluble in common organic solvents. UV-vis spectrum exhibited a broad absorption band in the range of 270–780 nm and a low bandgap of 1.83 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the polymer were estimated to be –5.10 and –3.27 eV, respectively. Based on the ITO/PEDOT:PSS/PDDTTBT:PCBM/Al device structure, the power conversion effi-ciency (PCE) under the illumination of AM 1.5 (100 mW/cm2_{) was 0.127%. The effects of annealing temperature (50–}

150°C) for 30 min on the device performance were studied. It was found that PCE of 0.292% could be acquired under the annealing condition at 50°C for 30 min. The improved device efficiency under the optimal condition was confirmed by the higher light harvest in UV-vis spectra, the enhanced quenching of photoluminescence (PL) emission, and the improved nanoscale morphology by atomic force microscopy (AFM) examination.

Keywords: nanomaterials, polymer solar cells, low bandgap, annealing

However, the performance of the photovoltaic cells

with these conjugated polymers is considerably

limited by their relatively large bandgaps, which

result in the mismatch of the absorption spectrum of

the active layer and the solar emission, especially in

the red and near-infrared ranges. Therefore, the

development of the low bandgap donor polymers is

of crucial importance for increasing the efficiency.

One of the most promising strategies to tailor the

energy levels of conjugated polymer is the

donor-acceptor route because of the vast possibility in the

unit combinations [7–11] Many D–A type

copoly-mers have been used in PSCs to achieve PCEs

above 5% with extensive device engineering efforts

[7, 12–14].

For the D–A type copolymers, much research work

has been devoted to using the fused thiophene

fam-ily as the donor due to its stable quinoid form

resulting in a low bandgap accompanied by good

electrochemical stability [15–17]. Molecules

con-taining fused-ring systems can make the polymer

backbone more rigid and coplanar, therefore

enhanc-ing effective !-conjugation, lowerenhanc-ing bandgap and

extending absorption. Introduction of

thienothio-phene units tends to stabilize the quinoid structure

in the polymer chain and thus enhances the

pla-narity along the polymer backbone. The high power

conversion efficiency can be attributed to the

rigid-ity and planarrigid-ity of the polymer backbone, leading

to a high hole mobility of the copolymer. In the case

of fused-ring systems,

dithieno[3,2-b:2!,3!-d]thio-phene (DTT) is well known as an important

build-ing block due to its high mobility [18, 19]. Recently,

organic field-effect transistors (OFET) [20, 21] and

PSCs [22, 23] containing

dithieno[3,2-b:2!,3!-d]thio-phene (DTT) building block in the D–A type

copoly-mers have been reported.

Recently, 2,1,3-benzothiadiazole (BT) has been

uti-lized to construct some n-type semiconducting

polymers showing high electron mobility [24–26].

It has also been used as the acceptor unit in

cooper-ation with varieties of electron-donating (D) units

as low bandgap donors in bulk heterojunction

poly-mer solar cells [23, 27–30]. High hole mobility and

wide optical absorption band could be achieved for

the D–A type BT-containing polymers. Hence, this

category of polymer donors has been extensively

studied and has shown outstanding photovoltaic

performances.

Based on this vision, the copolymer consisting of

alternating DTT and BT units, where DTT and BT

are adopted as the donor and acceptor segments,

should be a promising material for the active layer

of solar cells. Recently, this copolymer has been

prepared and explored in roll-to-roll coating

experi-ments [31–33]. However, the acquired PCEs of

photovoltaic devices based on this polymer are still

low. It may be helpful to raise the PCE via the

bandgap engineering strategy. Since only alkyloxy

side chains were attached on the BT unit in this

copolymer, the highest occupied molecular orbital

(HOMO) and lowest unoccupied molecular orbital

(LUMO) energy levels of the polymer may be

mod-ified if pendant chains are attached to both the

donor and acceptor units . Herein, we have

synthe-sized a new D-A type copolymer consisting of

alter-nating DTT and BT units, where the DTT and BT

unit has pendent alkyl chains and alkyloxy chains,

respectively. The optoelectronic properties, PCE

and the effect of thermal annealing of the fabricated

PSCs were investigated.

2. Experimental

2.1. Materials

Tetrabromothiophene (Alfa Aesar, USA), undecanal

(Alfa Aesar, USA), ethyl mercaptoacetate (Acros,

Belgium), n-butyllithium (Acros, Belgium), lithium

hydroxide (Alfa Aesar, USA), tin(II) chloride (Alfa

Aesar, USA), sodium bichromate (Showa Chemical

Co., Japan), potassium carbonate (Showa Chemical

Co., Japan), triethylamine (Acros, Belgium),

N-thionylaniline (TCI, Japan), trimethyltin chloride

(Acros, Belgium), bis(triphenylphosphine)

palla-dium(II) dichloride (Alfa Aesar, USA),

poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)

(PEDOT:PSS, Aldrich, USA) and phenyl-C

-butyric acid methyl ester (PCBM, FEM Tech.,

Ger-many) were used as received. All other reagents

were used as received.

2.2. Synthesis

The donor material,

3,5-didecanyldithieno[3,2-b:2!,3!-d]thiophene (DDTT), was prepared

accord-ing to a reported literature method [34]. The

accep-tor material,

5,6-bis(tetradecyloxy)benzo-2,1,3-thiadiazole (BT) was prepared according to the

published procedures [35]. The copolymer

poly(3,5-

didecanyldithieno[3,2-b:2!,3!-d]thiophene-2,6-diyl-

alt-5,6-bis(tetradecyloxy)benzo-2,1,3-thiadiazole-4,7-diyl) (PDDTTBT) was synthesized via Stille

coupling reaction of the donor unit of

2,6-bis-trimethylstannanyl-3,5-didecanyl dithieno[3,2-b:

2!,3!-d]thiophene with the acceptor unit of

4,7-

dibromo-5,6-bis(tetradecyloxy)benzo-2,1,3-thiadi-azole.

2.2.1.

2,6-Bis-trimethylstannanyl(3,5-didecanyldithieno[3,2-b:2!3!-d]thiophene)

To a solution of 3,5-didecanyldithieno[3,2-b:2!,3!-d]

thiophene (1.12 mmol) in THF (40 mL) was added

dropwise n-BuLi (2.5 mmol, 1.6 M in hexane) at

–78°C under argon. The reaction was keep at –78°C

for 2 h. Then trimethylchlorostannane (2.5 mmol)

ent at 364 and 446 nm, the obtained EQE is only

10.4 and 6.2%, correspondingly. After thermal

treat-ment for the blend films, the EQE is significantly

improved with minor changes in the peak position.

Among the EQE spectra taken at different

anneal-ing temperatures, the film annealed at 50°C almost

demonstrates the highest EQE in the most

illumi-nated regions by possessing the EQE of two

domi-nant bands, at 377 and 460 nm, respectively,

reach-ing ca. 20.2 and 9.3%. Consequently, the highest

power conversion efficiency (0.292%) has been

achieved by this blend film. However, the EQE

val-ues are still small compared to those of high

per-formance PSCs. The low EQE may be attributed to

the high recombination rate of charge carriers in the

PDDTTBT/PC61BM blend system, which results

in the low photocurrent.

4. Conclusions

The D–A type copolymer PDDTTBT based on

DDTT and BT units has been synthesized and

employed as the donor material in the active layer

of BHJ-type polymer solar cells. UV-vis absorption

spectra indicated that a low bandgap polymer with a

wide absorption band has been obtained. After

annealing treatment, an irregular absorption trend

in UV-vis spectra was observed due to both donor

and acceptor segments possessing pendent side

chains. When the blend film was treated at an

opti-mum condition (50°C/30 min), the PV cell

per-formance was dramatically improved and the power

conversion efficiency of device reached to 0.292%

under white light illumination (100 mW/cm

_{). We}

attribute the higher efficiency to enhanced 3-D

interpenetrating networks in the active layer,

increase of light absorption, and improved carrier

mobility.

Acknowledgements

We gratefully acknowledge the support of the National Sci-ence Council of Republic of China with Grant NSC 99-2221-E-390 -001-MY3.

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