An extremely low bandgap donor-acceptor copolymer for panchromatic solar cells

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An extremely low bandgap donor–acceptor copolymer for panchromatic

solar cells

Tzong-Liu Wang

a,n

, Chien-Hsin Yang

a

, Yeong-Tarng Shieh

a

, Ya-Chun Chen

a

, Tsung-Han Ho

b

,

Chin-Hsiang Chen

c

a

Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan, ROC

b

Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, ROC

c

Department of Electronics, Cheng Shiu University, Kaohsiung 833, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 10 April 2012 Received in revised form 24 June 2012

Accepted 29 June 2012 Available online 20 July 2012 Keywords:

Donor–acceptor copolymer Polymer solar cells Low bandgap Annealing

a b s t r a c t

An extremely low bandgap donor–acceptor copolymer has been designed and synthesized as the donor material of the polymer solar cells via Stille coupling reaction. The donor–acceptor alternating structure consisted of 4,4-diethylhexyl-cyclopenta[2,1-b:3,4-b0_{]dithiophene (CPDT) donor unit and} 2,3-bis(2-ethylhexyl)thieno[3,4-b]pyrazine (TP) acceptor unit. Since both units have attached branched alkyl chains, the polymer was well dissolved in common organic solvents. UV–vis spectrum of the polymer film exhibited a panchromatic absorption ranging from 280 to 1285 nm and a low bandgap of 1.20 eV. Compared to that in solution, solid-state UV–vis absorption spectrum of the polymer showed a strong bathochromic shift, indicating more efficientp-stacking and stronger intermolecular interactions. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the polymer were estimated to be 5.07 and 3.87 eV, respectively. Based on the ITO/PEDOT:PSS/ Polymer:PC61BM/Al device structure, the power conversion efficiency (PCE) under the illumination of AM 1.5 (100 mW/cm2_{) was 0.149%. The effects of annealing temperature (100–200 1C) for 30 min on} the device performance were studied as well. It was found that PCE of 0.264% could be acquired under the annealing condition at 175 1C 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 increase in external quantum efficiency.

1. Introduction

In recent decades, polymer solar cells (PSCs) based on con-jugated polymers have attracted considerable attention because of their potential use for future cheap and renewable energy production[1–3]. Polymer-based solar cells have the potential to fabricate them onto large areas of lightweight flexible substrates by solution processing at a low cost [4–6]. Efficient polymer-based solar cells utilize donor–electron acceptor bulk heterojunc-tion (BHJ) films as active layers[1,2]. The donor is typically a kind of conjugated polymer, while the acceptor is generally a type of organic or inorganic molecule. 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.

In order to improve the power conversion efficiency (PCE) of the PSCs, much research work has been devoted to finding new conjugated polymer donor materials aiming at broader absorption, lower bandgap, higher hole mobility, and suitable electronic energy levels. In order to achieve the above requirement simulta-neously, the most promising strategy is the donor–acceptor (D–A) route because of the vast possibility in the unit combinations to tailor the energy levels of conjugated polymer[7–10]. Through the introduction of push-pull driving forces to facilitate electron delocalization and the formation of quinoid mesomeric structures over the conjugated main chain, the bond length alternation (BLA) can be significantly reduced. The bandgap decreases linearly as a function of the increasing quinoid character with concomitant decreasing BLA value[11,12]. The electronic and optoelectronic properties of donor–acceptor copolymers could be efficiently manipulated by controlling intramolecular charge transfer (ICT) which is correlated with the high-lying HOMO of the donor unit and the low-lying LUMO of the acceptor unit[12,13]. Many D–A type copolymers have been used in PSCs to achieve PCEs above 5% with extensive device engineering efforts[7,14–16].

Contents lists available atSciVerse ScienceDirect

journal homepage:www.elsevier.com/locate/solmat

Solar Energy Materials & Solar Cells

n

Corresponding author. Tel.: þ886 7 5919278; fax: þ886 7 5919277. E-mail address: tlwang@nuk.edu.tw (T.-L. Wang).

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Using the fused thiophene family as the donor is an attractive approach due to its stable quinoid form resulting in a low bandgap accompanied by good electrochemical stability[17–19]. Molecules containing fused-ring systems can make the poly-mer backbone more rigid and coplanar, therefore enhancing effective

p

-conjugation, lowering bandgap and extending absorp-tion. The carbon-bridged 4,4-diethylhexyl-cyclopenta[2,1-b:3,4-b0_{]dithiophene (CPDT) has attracted considerable research}

inter-est due to its potential to serve as the donor building block for low bandgap polymers[20–24]. The more extended conjugation and stronger intermolecular interaction due to the forced coplanarity of the two thienyl subunits result in the low energy gaps of the D–A copolymers based on this donor unit. In addition, the bridging carbon at the 4-position of CPDT can be readily functionalized by alkyl groups to increase solubility without causing additional twisting of the repeating units in the resulting polymers. Parti-cularly, in a previous report, poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b0

]dithiophene)-alt-4,7-(2,1,3-benzothiadia-zole)] (PCPDTBT) comprising of CPDT and benzothiadiazole (BT) unit was demonstrated to be one of the most promising low bandgap polymers for the use in PSCs[21].

On the other hand, thieno[3,4]pyrazine (TP) is a narrow-bandgap heterocycle, which is useful in constructions of low bandgap polymers[25]. TP is advantageous as an acceptor unit because it allows modiﬁcation of the pyrazine ring without introducing steric effects directly into the polymer backbone. Thus, alkyl and aryl groups can be added at positions 2 and 3 to tune structural and electronic properties of the acceptor. Recently, TP has also been used as the acceptor unit in cooperation with varieties of electron-donating units as low bandgap donors in bulk heterojunction polymer solar cells [23,26–29]. Extremely low bandgap and wide optical absorption band could be achieved for the D–A type TP-containing polymers. However, although a D–A type conjugated polymer using CPDT as the donor and 2,3-di-n-octylthieno[3,4]pyrazine as the acceptor showed a much lower bandgap of 1.20 eV, the PCE of this device only reached 0.1%[23]. More recently, a random type copolymer comprising of CPDT, TP, and thiophene units also exhibited a low bandgap of 1.20 eV[24]. Nonetheless, this device only led to a PCE of 0.6%. Therefore, in this regard, it still needs to make more effort to raise the PCE of D–A type TP-containing type copolymers for applica-tions in PSCs.

As stated above, the D–A type copolymers consisting of alter-nating CPDT and TP units should be a kind of promising donor material for the active layers of solar cells. However, to fulﬁll the expectation of high performance PSCs, a judicious ﬁne-tuning of the HOMO and LUMO energy levels of the TP unit in the conjugated polymer is of critical importance in the present case to obtain a low bandgap polymer with high hole mobility and strong absorption ability. Herein, we have synthesized a new D–A type copolymer consisting of alternating CPDT and TP units via the molecular engineering approach, where both the CPDT and TP unit have attached branched alkyl chains. The optoelectronic properties and the PCE of the fabricated PSCs were investigated. The effect of thermal annealing on the PCEs of solar cells is also reported.

2. Experimental 2.1. Materials

3-Bromothiophene (Acros), 3-thiophenaldehyde (TCI), 2-ethyl-hexyl bromide (Acros), lithium bromide (Acros), butyllithium (Acros), oxalyl chloride (Acros), hydrazine monohydrate (Alfa Aesar), copper(I) bromide (Alfa Aesar), pyridinium chlorochromate (PCC, TCI), trimethyltin chloride (Acros), N-bromosuccinimide (NBS, Acros),

bis(triphenylphosphine)palladium(II) dichloride (Alfa Aesar), poly (3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS, Aldrich) and phenyl-C61-butyric acid methyl ester (PC61BM, FEM

Tech.) were used as received. All other reagents were used as received.

2.2. Synthesis

The donor material, 4,4-diethylhexyl-cyclopenta[2,1-b:3,4-b0_{]dithiophene (CPDT), was prepared according to a reported}

literature method[20]. The acceptor material, 2,3-bis(2-ethyl-hexyl)thieno[3,4]pyrazine (TP) was prepared according to the published procedures [24,30]. The copolymer poly[2,6-(4,4-diethylhexyl-4H-cyclopenta[2,1-b;3,4-b0

]dithiophene)-alt-5,7-(2,3-bis(2-ethylhexyl)thieno[3,4]pyrazine)] (PCPDTTP) was synthesized via Stille coupling reaction of the donor unit of 2,6-bis-trimethylstannanyl(4,4-diethylhexyl-cyclopenta[2,1-b: 3,4-b0_{]dithiophene) with the acceptor unit of}

5,7-dibromo-2,3-bis(2-ethylhexyl)thieno[3,4]pyrazine.

2.2.1. Synthesis of D–A type copolymer (PCPDTTP)

In a 100 mL ﬂask, the two monomers (1 mmol of each), 2,6-bis-trimethylstannanyl(4,4-diethylhexylcyclopenta[2,1-b:3,4-b0_]

dithiophene) and 5,7-dibromo-2,3-bis(2-ethylhexyl)thieno[3,4]-pyrazine were dissolved in 40 mL of dry DMF and then ﬂashed by argon for 10 min. Following that, 0.03 mmol of Pd(PPh3)2Cl2

was added, and the reactant was purged by argon for another 20 min. The reaction mixture was then heated at 120 1C for 48 h under the protection of argon. The sticky, deep green solution was cooled and poured into 100 mL of methanol, where the crude polymer was precipitated and collected as dark green powder, which was then subjected to Soxhlet extraction with methanol, hexane, and THF. The polymer was recovered from the THF fraction by rotary evaporation. The synthetic route is shown in

Scheme 1. Yield: 35%. 1HnmR (500 MHz, CDCl3,

d

ppm): 6.95

(s, 2H), 2.74 (m, 4H), 2.39 (m, 4H), 1.78 (m, 4H), 1.56 (m, 8H), 1.31 (m, 24H), 0.87 (m, 24H) Anal. Calcd for (C47H70N2S3)n: C, 74.34; H,

9.29; N, 3.69. Found: C, 73.25; H, 9.68; N, 3.37. GPC (THF): Mn ¼13,000 g/mol, Mw ¼19,100 g/mol, PDI¼ 1.47.

2.3. Device fabrication and characterization

The device structure of the polymer photovoltaic cells in this study is ITO/PEDOT:PSS/PCPDTTP:PC61BM/Al. PCPDTTP acts as the

p-type donor polymer and PC61BM acts as the n-type acceptor in the

active layer. Before device fabrication, the glass substrates coated with indium tin oxide (ITO) were ﬁrst cleaned by ultrasonic treatment in acetone, detergent, de-ionized water, methanol and isopropyl alcohol sequentially. The ITO surface was spin coated with ca. 80 nm layer of poly(3,4-ethylene dioxythiophene): poly(styrene) (PEDOT:PSS) in the nitrogen-ﬁlled glove-box. The substrate was dried for 10 min at 150 1C and then continued to spin coating the active layer. The PCPDTTP:PC61BM blend solutions were prepared

with 1:1 weight ratio (10 mg/mL PCPDTTP) in 1,2-dichlorobenzene (DCB) as the active layer. This solution blend was spin-cast onto the PEDOT:PSS layer at 800 rpm for 30 s. The obtained thickness for the blend ﬁlm of PCPDTTP:PC61BM was ca. 110 nm. The devices were

completed by evaporation of metal electrodes Al with area of 6 mm2

deﬁned by masks.

The ﬁlms of active layers were annealed directly on top of a hot plate in the glove box, and the temperature was monitored by using a thermocouple touching the top of the substrates. After removal from the hotplate, the substrates were immediately put onto a metal plate at room temperature. Ultraviolet–visible (UV–vis) spectro-scopic analysis was conducted on a Perkin-Elmer Lambda 35

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To deal with the above issues, it may be helpful to adopt the following strategies. Since a facile planarization of polymer back-bone may enhance the hole-transporting mobility, the EQE of the pristine polymer (PCPDTTP) may be elevated by inserting a double bond between the CPDT and TP unit. This modification may also raise the extinction coefficient of polymer in both of the visible and NIR region. On the other hand, incorporation of electron-donating or electron-withdrawing substituents directly onto the aromatic unit in the main chain represents another effective way to fine-tune the HOMO and LUMO level. In general, donating groups raise the HOMO energy, while electron-withdrawing groups lower the LUMO energy. Recently, fluorine substituted conjugated polymers have also been stated as pro-mising polymers for PSCs[37,38]. Since fluorine is a small atom in size, it can be introduced onto the polymer backbone without any deleterious steric effects. According to density functional theory (DFT) calculations, there will be a decrease in the HOMO energy level by adding the fluorine atom to the acceptor unit. Hence, both ethyhexyl side chains in the TP unit may be replaced by one or two fluorine atoms to lower the HOMO level of the pristine polymer and increase the Voc. In contrast, although the

morphol-ogy of bicontinuous and nanoscale phase separation of the blend film will facilitate the carriers transportation and lead to a reduction of loss and recombination, the optimized phase mor-phology may not be easily attained because the film-forming state is difficult to manipulate. Considering the above description and discussion, fine-tuning the structures of the donor and acceptor units of the conjugated polymer via the above-men-tioned strategies may be the most effective way to achieve high performance PSCs.

4. Conclusions

The D–A type copolymer PCPDTTP based on CPDT and TP 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 panchro-matic absorption band has been obtained. Through the annealing treatment at an optimum condition (175 1C/30 min), the photo-voltaic performance was signiﬁcantly improved and the power conversion efﬁciency of the device reached 0.264% under white light illumination (100 mW/cm2_{). We attribute the higher}

efﬁ-ciency to the improved morphology in the active layer, increase of light absorption, and higher carrier mobility. In conclusion, to achieve high performance solar cells, the most effective way may be to ﬁne-tune the structures of donor and acceptor units of the conjugated polymer via the strategies of bandgap engineering.

Acknowledgements

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

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