Solar Energy Materials & Solar Cells 95 (2011) 3295–3302

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Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

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

Synthesis and photovoltaic properties of a low bandgap donor–acceptoralternating copolymer with benzothiadiazole unit

Tzong-Liu Wang a,n, An-Chi Yeh b, Chien-Hsin Yang a, Yeong-Tarng Shieh a,Wen-Janq Chen a, Tsung-Han Ho c

a Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan, ROCb Department of Chemical and Materials Engineering, Cheng Shiu University, Kaohsiung County 833, Taiwan, ROCc Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, ROC

a r t i c l e i n f o

Article history:

Received 28 April 2011

Received in revised form

11 July 2011

Accepted 16 July 2011Available online 5 August 2011

Keywords:

Donor–acceptor alternating copolymer

Polymer solar cells

Low bandgap

Annealing

Optical spacer

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.solmat.2011.07.021

esponding author. Tel.: þ886 7 5919278; fax

ail address: tlwang@nuk.edu.tw (T.-L. Wang).

a b s t r a c t

A new donor–acceptor alternating copolymer as the donor material of the active layer in polymer solar

cells has been synthesized. The alternating structure consisted of dithieno[3,2-b:20 ,30-d]thiophene

(DTT) donor unit and 5,6-bis(tetradecyloxy)benzo-2,1,3-thiadiazole (BT) acceptor unit. Both units were

confirmed by 1H NMR and elemental analysis. Since the BT unit has long alkyoxyl side chains, the

polymer was soluble in common organic solvents. Optoelectronic properties of the copolymer (PDTTBT)

were investigated and observed by UV–vis, photoluminescence (PL) spectra, and cyclic voltammogram

(CV). UV–vis spectrum exhibited a broad absorption band in the range of 300–750 nm and a low

bandgap of 1.83 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular

orbital (LUMO) levels of PDTTBT could be determined from the data of CV and UV–vis spectrum. Based

on the ITO/PEDOT:PSS/PDTTBT:PCBM/Al device structure, the power conversion efficiency (PCE) under

the illumination of AM 1.5 (100 mW/cm2) was 0.113%. It was found that PCE of 0.301% could be

acquired under the annealing condition at 150 1C for 30 min. In addition, solar cells fabricated with the

1,8-octanedithiol (OT) additive in the mixture solvent or adding TiOx optical spacer show efficiencies

significantly improved over 15%.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

In the recent decade, polymer solar cells (PSCs) based onconjugated polymers have been extensively studied because oftheir potential use for future cheap and renewable energy produc-tion [1–3]. Polymer-based solar cells have unique advantages overtraditional silicon-based solar cells, such as low cost, light weight,and potential use in flexible devices [4–6]. Efficient polymer-basedsolar cells utilize donor–electron acceptor (D–A) bulk heterojunction(BHJ) films as active layers [1,2]. The donor is typically a kindof conjugated polymer, while the acceptor is generally a type oforganic or inorganic molecule. At present, one of the most success-ful donor polymers is regioregular poly(3-hexylthiophene) (P3HT).A bulk heterojunction photovoltaic device combining regioregularP3HT as the electron donor with [6,6]-phenyl C61 butyric acidmethyl ester (PCBM) as the electron acceptor achieves powerconversion efficiencies (PCEs) of 4–5% by device optimization [7–9].

In order to further improve the PCE of the PSCs, much researchwork has been devoted to find new conjugated polymer donor

ll rights reserved.

: þ886 7 5919277.

materials aiming at broader absorption, lower band gap, higher holemobility, and suitable electronic energy levels. However, the per-formance of the photovoltaic cells with these conjugated polymersis considerably limited by their relatively large band gaps, whichresult in the mismatch of the absorption spectrum of the active layerand the solar emission, especially in the red and near-infraredranges. Therefore, the development of the low bandgap donorpolymers is of crucial importance for increasing the efficiency. Oneof the most promising strategies to tailor the energy levels ofconjugated polymer is the donor–acceptor route because of the vastpossibility in the unit combinations [10–13] Many D–A typecopolymers have been used in PSCs to achieve PCEs above 5% withextensive device engineering efforts [10,14–16].

Using the fused thiophene family as the donor is an attractingapproach due to its stable quinoid form resulting in a low band-gap accompanied by good electrochemical stability [17–19].Molecules containing fused-ring systems can make the polymerbackbone more rigid and coplanar, therefore enhancing effectivep-conjugation, lowering bandgap, and extending absorption.Introduction of thienothiophene units tends to stabilize thequinoid structure in the polymer chain and thus enhances theplanarity along the polymer backbone. The high power conver-sion efficiency can be attributed to the rigidity and planarity of

T.-L. Wang et al. / Solar Energy Materials & Solar Cells 95 (2011) 3295–33023296

the polymer backbone, leading to a high hole mobility of thecopolymer.

Dithieno[3,2-b:20,30-d]thiophene (DTT) is well known as animportant building block in the synthesis of high mobility materi-als for organic field-effect transistors (OFETs) [20,21]. Recently,synthesis of DTT-containing copolymers and their applications inOFETs [22,23] and PSCs [24,25] have been reported. Therefore, itshould be possible to obtain novel D–A type copolymers withexcellent performance by introducing DTT moieties into polymerbackbones.

On the other hand, 2,1,3-benzothiadiazole (BT) is an electron-accepting (A) molecule that has been utilized to construct somen-type semiconducting polymers showing high electron mobility[26–28]. Recently, BT has also been used as the acceptor unit incooperation with varieties of electron-donating (D) units as lowbandgap donors in bulk heterojunction polymer solar cells[25,29–32]. High hole mobility and wide optical absorption bandcould be achieved for the D–A type BT-containing polymers.Hence, this category of polymer donors has been extensivelystudied and has shown outstanding photovoltaic performances.

Consequently, the copolymer consisting of alternating DTT andBT units, where DTT and BT are adopted as the donor and acceptorsegments, should be a promising material for the active layer ofsolar cells. Recently, this copolymer has been prepared andexplored in roll-to-roll coating experiments [33,34]. The coatedlarge area devices successfully demonstrated the photovoltaicefficiencies up to 0.6%. In this report, some characteristics andoptoelectronic properties of the fabricated PSCs were investi-gated. The effect of thermal annealing, the solvent additive, andthe optical spacer on the PCEs of solar cells is also reported.

2. Experimental

2.1. Materials

2,3-Dibromothiophene (Alfa Aesar), bis(phenylsulfonyl) sul-fide (Acros), selenium (Acros), copper(II) chloride (Acros), butyl-lithium (Acros), catechol (Lancaster), 1-bromotetradecane (AlfaAesar), tin(II) chloride (Alfa Aesar), N-thionylaniline (TCI), tri-methyltin chloride (Acros), bis(triphenylphosphine)palladium(II)dichloride (Alfa Aesar), poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS, Aldrich), and phenyl-C61-butyricacid methyl ester (PCBM, FEM Tech.) were used as received. Allother reagents were used as received.

2.2. Synthesis

The donor and acceptor materials, dithieno[3,2-b:20,30-d]thio-phene (DTT) and 5,6-bis(tetradecyloxy)benzo-2,1,3-thiadiazole (BT),were prepared according to the published literatures [35,36].The copolymer poly(dithieno[3,2-b:20,30-d]thiophene-2,6-diyl-alt-5,6-bis(tetradecyloxy)benzo-2,1,3-thiadiazole-4,7-diyl) (PDTTBT) wassynthesized via Stille coupling reaction of the donor unit of 2,6-bis-trimethylstannanyl-dithieno[3,2-b:20,30-d]thiophene with the accep-tor unit of 4,7-dibromo-5,6-bis(tetradecyloxy)benzo-2,1,3-thiadiazole(see Supporting Information).

2.3. Device fabrication and characterization

The device structure of the polymer photovoltaic cells in thisstudy is ITO/PEDOT:PSS/PDTTBT:PCBM/Al. PDTTBT acts as thep-type donor polymer and PCBM as the n-type acceptor in theactive layer. Before device fabrication, the glass substrates coatedwith indium tin oxide (ITO) were first cleaned by ultrasonictreatment 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 dioxythiophe-ne):poly(styrene) (PEDOT:PSS) in the nitrogen-filled glove-box.The substrate was dried for 10 min at 150 1C and then the activelayer was continued to be spin-coated. The PDTTBT:PCBM blendsolutions were prepared with 1:1 weight ratio (10 mg/mLPDTTBT) in 1,2-dichlorobenzene (DCB) as the active layer. Deviceswere fabricated by spin-coating at 800 rpm for 30 s on top of thePEDOT:PSS layer. The obtained thickness for the blend film ofPDTTBT:PCBM was ca. 110 nm. The devices were completed byevaporation of metal electrodes Al with area of 6 mm2 definedby masks.

The films of active layers are annealed directly on top of a hotplate in the glove box, and the temperature is monitored using athermocouple touching the top of the substrates. After removal fromthe hotplate, the substrates are immediately put onto a metal plateat the room temperature. Ultraviolet–visible (UV–vis) spectroscopicanalysis was conducted on a Perkin-Elmer Lambda 35 UV–visspectrophotometer. Photoluminescence (PL) spectrum was recordedon a Hitachi F-7000 fluorescence spectrophotometer. The filmtopography images of active layers were recorded with a DigitalInstruments Dimension 3100 atomic force microscope (AFM) intapping mode under ambient conditions. The J–V curves weremeasured using a Keithley 2400 source meter, under illuminationfrom a solar simulator. The intensity of solar simulator was set witha primary reference cell and a spectral correction factor to give theperformance under the AM 1.5 (100 mW/cm2) global referencespectrum (IEC 60904-9).

3. Results and discussion

3.1. Material synthesis and structural characterization

Since 2,1,3-benzothiadiazole (BT) is an electron-acceptingheterocycle showing high electron mobility, BT has been recentlyused as the acceptor unit (A) in cooperation with varieties ofelectron-donating units (D) as low bandgap donors in BHJ PSCs.On the other hand, the incorporation of linearly symmetrical andcoplanar thienothiophene unit DTT in the conjugated polymers ispredicted to facilitate high power conversion efficiencies. There-fore, it is expected that wide sunlight absorption band and highpower conversion efficiency could be achieved for the D–A typecopolymer using DTT as the donor and BT as the acceptor.

The synthetic route of the copolymer PDTTBT is shown inScheme 1. The polymer was synthesized via Stille coupling reactionof the donor unit with the acceptor unit. To increase the solubilityof the copolymer, long alkyoxyl chains were attached onto thebenzothiadiazole unit, while the donor part still retains its planar-ity. The structures of both monomers and copolymer were con-firmed by 1H NMR and elemental analysis (analytical data is givenin the Supporting Information). The polymer is well dissolved incommon organic solvents such as chloroform, 1,2-dichlorobenzene,THF, and toluene. Molecular weight of the polymer determined bygel permeation chromatography showed a low Mn value of 9100,which might be due to the steric hindrance from 5,6-dialkyoxylsubstituents on the BT unit. In addition, the polymer exhibited ahigh glass transition temperature (Tg) of 131 1C as a result of therigid donor and acceptor units.

3.2. Optical properties

The normalized UV–vis absorption spectra of the PDTTBTcopolymer in THF solution and the film cast from THF arepresented in Fig. 1(a). The optical absorption threshold at 706 nmfrom the spectrum of the film corresponds to the bandgap (Eg) of

300 400 500 600 700 800

Abso

rban

ce (a

.u.)

Wavelength (nm)

PDTTBT film PDTTBT inTHF

1.6 1.7 1.8 1.9 2.0 2.1 2.20.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

(αhν

)

Photoenergy hν (eV)

- - - -Eg:1.83eV

Fig. 1. (a) UV–vis absorption spectra of PDDTBT in dilute THF solution and thin

film, (b) plot of (ahn)2 vs. hn for PDDTBT film.

500 600 700 800In

tens

ityWavelength (nm)

PDTTBT film PDTTBT in THF

Fig. 2. Photoluminescence spectra of PDDTBT in dilute THF solution and thin film

with excitation at 400 nm.

Scheme 1. Synthesis of PDTTBT copolymer.

T.-L. Wang et al. / Solar Energy Materials & Solar Cells 95 (2011) 3295–3302 3297

the PDTTBT copolymer. Hence, the estimated optical bandgap is1.76 eV. To obtain a more accurate optical band gap of PDTTBT, thefundamental equation ahn¼ B(hn–Eopt)

n developed in Tauc rela-tion [37] was used. The optical bandgap calculated by this equationis 1.83 eV, smaller than that (1.9–2.0 eV) of the widely usedregioregular poly(3-hexylthiophene) (P3HT), as shown in Fig. 1(b).

As seen from Fig. 1(a), the UV–vis absorption spectrum of thecopolymer in dilute THF solution exhibited two absorption peakspositioned at about 343 and 490 nm. The peak at 343 nm isprobably due to the p–pn transition of the dithienothiophenemoiety [38], while the peak in the visible region is assigned to theintramolecular charge transfer (ICT) between the donor and theacceptor [25,39]. Similarly, the absorption spectrum of PDTTBTfilm also shows two peaks, one at 361 nm and the other at

514 nm. In contrast to the spectrum in solution, both peaks showsmall red-shifts, indicating more efficient p-stacking and strongerintermolecular interactions in the solid state. In particular,the broadened absorption spectrum ranging from 300 to 750 nmindicates a low bandgap polymer has been obtained, as evidentfrom the Eg of PDTTBT. It is apparent that the ICT interactionbetween donor and acceptor moieties in the D–A copolymers is apractical approach to lower the bandgap and broaden the absorp-tion bands across the entire visible wavelength region of conju-gated polymers. Hence, our successful synthesis of a low bandgapD–A type copolymer is further confirmed.

The photoluminescence (PL) emission spectra of PDTTBTin dilute THF solution and thin film are shown in Fig. 2. Boththe fluorescence spectra exhibit the vibronic structure witha maximum at 559 and 579 nm. As seen from the figure, bothspectra show only one emission peak, indicating that an effectiveenergy transfer from the DTT segments to the BT unit occurs.The red-shift in the spectrum of the PDTTBT film is probably dueto the lowering of the bandgap of copolymer by more efficientp-stacking in the solid state.

3.3. Electrochemical properties

Cyclic voltammogram (CV) is a preliminary characterizationtechnique to determine the redox properties of organic andpolymeric materials. The highest occupied molecular orbital(HOMO) and lowest unoccupied molecular orbital (LUMO) energylevels of PDTTBT could be determined from the E1/2 (Fig. 3) andthe onset absorption wavelength (Eg, energy bandgap) in Fig. 1.

As seen in Fig. 3, the oxidation onset potential for PDTTBT hasbeen determined as 0.62 V vs. Ag/Agþ . The external ferrocene/ferrocenium (Fc/Fcþ) redox standard E1/2 is 0.27 V vs. Ag/Agþ .Assuming that the HOMO energy for the Fc/Fcþ standard is4.80 eV with respect to the zero vacuum level, the HOMO energy

-1.0 -0.5 0.0 0.5 1.0 1.5 2.0-0.0010

-0.0005

0.0000

0.0005

0.0010

0.0015

0.0020

Cur

rent

Den

sity

/Acm

E/V vs. Ag/Ag

Fig. 3. Cyclic voltammograms of PDDTBT film on an ITO substrate in CH3CN/AcOH

(V/V¼7/1) containing 0.1 M tetrabutylammonium perchlorate at a scan rate

of 50 m Vs�1.

PDTTBT PCBM

4.8

ITO5.0

PEDOT:PSS

5.15

3.32

4.3

6.1

4.3

AI

Fig. 4. Energy level diagram of the components in the polymer solar cell.

Table 1Photovoltaic characteristics of the devices under different annealing temperatures

for 30 min.

RT 100 1C 125 1C 150 1C 175 1C 200 1C

Voc (V) 0.316 0.272 0.265 0.336 0.593 0.235

Jsc (mA/cm2) 1.255 1.549 1.617 2.898 1.551 2.008

FF (%) 28.46 30.15 30.28 30.87 29.42 28.77

Z (%) 0.113 0.127 0.130 0.301 0.271 0.136

T.-L. Wang et al. / Solar Energy Materials & Solar Cells 95 (2011) 3295–33023298

for PDTTBT has been evaluated to be 5.15 eV. Hence the LUMOlevel determined from its HOMO and bandgap (Eg) is 3.32 eV.Fig. 4 shows the schematic diagram representing the potentialmetrically determined HOMO and LUMO energy of PDTTBT andPCBM relative to the work function of the electrodes.

3.4. Photovoltaic properties

At present, bulk heterojunction structures are the main candi-dates for high-efficiency polymeric solar cells. The bulk hetero-junction solar cells based on PDTTBT in combination with PCBMhave been prepared and investigated. The employed devicestructure was ITO/PEDOT:PSS/PDTTBT:PCBM/Al. The blend solu-tions (in DCB) of PDTTBT:PCBM were prepared with 1:1 weightratio as the active layer. The current–voltage (J–V) curve for theas-prepared blend film cast at room temperature (RT) underillumination from solar simulator at 100 mW/cm2 light intensityis shown in Fig. 7, and the corresponding open-circuit voltage(Voc), short-circuit current (Jsc), fill factor (FF), and power conver-sion efficiency (PCE,Z) are listed in Table 1. The power conversionefficiency of solar cell using the as-prepared blend film as theactive layer is 0.113%. In a previous work by Bundgaard et al.[33,34], the device efficiencies for the untreated and annealedfilm based on this material (Mn¼12,300 g/mol) are 1.7% and 2.2%,

respectively. Compared to the device efficiency achieved by them,the low efficiency of our device may be due to the lowermolecular weight of polymer (Mn¼9100 g/mol), preparationtechniques for the device architecture, different thicknesses ofthe active layer, glove box and vapor deposition equipment, etc.To further improve the device efficiency based on this material, itis considered that bandgap tuning of conjugated polymers viamolecular design is a direct and efficient approach. The generalstrategies have been suggested and discussed in the severalliteratures [40–42]. In the present case, it would be helpful tofine-tune the bandgap of the D–A type copolymer through eitherof these approaches: (1) attaching the electron donating groups toraise the HOMO or attaching the electron withdrawing groups tolower the LUMO, (2) introduction of a methine group between thedonor and the acceptor units to give a more flat structure, and(3) attaching an atom of high electron affinity (such as fluorine) inselected locations of fused rings [41]. Although chemical synth-esis can raise the HOMO level or lower the LUMO level of aconjugated polymer, the former action produces a detrimentalreduction of Voc, whereas the latter could push the DELUMO (theenergy difference between the LUMOs of donor and acceptor)away from the minimum energy offset, which should be largerthan 0.3 eV for efficient charge separation. As a result, loweringthe bandgap of a conjugated polymer to absorb visible and near-infrared radiation, while providing efficient charge separation andhigh Voc in a polymer solar cell, is still a big challenge.

3.5. Effect of annealing

Since thermal annealing has great influence on the PCE of solarcells, we have studied photovoltatic properties under differentannealing temperatures. The effect of annealing temperature onthe UV–vis absorption spectra for the thin films of PDTTBT:PCBM(1:1 weight ratio) spun cast on quartz substrates is shown inFig. 5. These films were annealed under nitrogen atmosphereinside the glove box at atmospheric pressure. The annealing timewas kept 30 min for all of the annealing temperatures. Theabsorption spectra show a considerable change after thermalannealing of the films. The first band is attributed to the p–pn

transition of DTT segments, whereas the last band is due to theICT interaction as stated above.

At annealing temperature of 100 1C, the intensities of bothbands increase without change in the position. An increase inthe absorption strength after heat treatment normally meansincreased packing of the PDTTBT domains. The film heat-treatedat 125 1C shows a similar behavior. The maximum absorption isobserved for the film annealed at 150 and 175 1C, indicating anenhanced conjugation length and the more ordered structure ofPDTTBT. Since the thickness of all the films is similar (ca. 110 nm),the increase in the peak absorption intensity during thermalannealing may be attributed to the lowering of the bandgapbetween p and pn, the increase of the optical p–pn transition, andthe increased interchain interaction among the PDTTBT. Afterthermal annealing, the PDTTBT molecules afford higher energyand can move more easily. Consequently, the polymer chainsbecome mobile and self-organization can occur to form ordering.

Fig. 5. UV–vis absorption spectra of PDDTBT:PCBM blend films after annealing at

different temperatures for 30 min.

006005004

Inte

nsity

Wavelength (nm)

RT

100°C 125°C

150°C 175°C

200°C

Fig. 6. Photoluminescence spectra of PDDTBT:PCBM blend films after annealing at

different temperatures for 30 min with excitation at 375 nm.

Fig. 7. J–V characteristics of devices under AM 1.5 simulated solar illumination at

an intensity of 100 mW/cm2 after annealing at different temperatures for 30 min.

T.-L. Wang et al. / Solar Energy Materials & Solar Cells 95 (2011) 3295–3302 3299

Therefore, the peak intensity increases in the more ordered filmsdue to the improved charge carrier transport in both donor(PDTTBT) and acceptor (PCBM) phases after thermal annealing.On further increasing the annealing temperature to 200 1C, how-ever, results in a decrease in the intensities of both the bands.From the above results, the optimum annealing temperatures forPDTTBT and PCBM may be around 150–175 1C.

Fig. 6 shows the PL intensity for blend films annealed atdifferent temperatures. The PL is due to photogenerated excitonsin PDTTBT that do not take part in the charge separation. Thephenomenon of PL quenching can be attributed to the interfacialcharge transfer. Normally, PL quenching increases with theincrease of interfacial area between donor and acceptor materialsin the active layer. PL quenching provides direct evidence forexciton dissociation, and thus efficient PL quenching is necessaryto obtain efficient organic solar cells. As shown in the figure, itcan be seen that the PL intensity decreases with the increase ofannealing temperature. The PL intensity shows a minimum atthermal annealing of 200 1C. This significant reduction in the PLintensity is attributed to efficient photoinduced charge separation

between electron-donating (PDTTBT) and electron-accepting(PCBM) molecules. This may be attributed to the higher chargecarrier mobility or higher interfacial area between D–A moleculescompared with those of other annealing temperatures. However,this does not necessarily mean that the stronger the PL quenching,the better the performance of the solar cells. Although the PLintensity of the blend film annealed at 150 1C is a little higher incomparison with that of the film annealed at 175 and 200 1C, thehighest power conversion efficiency (0.301%) has been achieved bythis blend film as shown in Fig. 7 and Table 1. As we will discuss inthe following part, it seems that the higher value of roughness andhigher degree of nanoscale phase separation in the blend filmannealed at 150 1C enhance the transport rate of charge carriers tothe metal electrode and reduce the charge recombination of theexcitons. Based on these studies, we conclude that there may bethree reasons to explain why the film annealed at 150 1C exhibitsthe optimal PCE. It is probably because (1) the increase of opticalabsorption in the visible light region, (2) the improved chargecarrier mobility in both donor and acceptor phases after thermalannealing, and (3) the increased interfacial area between the donorand acceptor phases offset the former effect (PL quenching) andresult in an overall improvement in the device performance.Therefore, the morphology of PDTTBT/PCBM blend film beforeand after annealing should be studied.

Since the morphology of the heterojunction plays an impor-tant role in the performance of polymer solar cells, we studiedthe topography of the blend films of PDTTBT:PCBM (1:1, w/w) byAFM. Although the AFM images of film surfaces at differentannealing temperatures have been taken, only three representa-tive images are shown in Fig. 8 for comparison. The values ofaverage roughness and root-mean-square roughness for the blendfilms are shown in Table 2. It is clear that the images for both theas-prepared film and the film annealed at 200 1C look relativelysmooth. It is evident that a more rough surface observed in thefilm annealed at 150 1C increases the contact area between theactive layer and the metal electrode. Hence, the transport rate ofthe charge carriers to the metal electrode is higher and therecombination rate of excitons is reduced. Therefore, the J–V

curve for the film annealed at 150 1C reveals an increase of Jsc

to 2.90 mA/cm2, which is almost twice of that of the film annealedat 175 1C.

Fig. 8. AFM topography images (3�3 mm) of PDDTBT:PCBM blend films after annealing at different temperatures for 30 min. 2D height image for the blend film

(a) unannealed, (b) annealed at 150 1C, (c) annealed at 200 1C. Phase image for the blend film (d) unannealed, (e) annealed at 150 1C, and (f) annealed at 200 1C.

T.-L. Wang et al. / Solar Energy Materials & Solar Cells 95 (2011) 3295–33023300

3.6. Effects of solvent mixture and optical spacer

The mixed solvent approach has been demonstrated as apromising method to modify solar cell morphology and improvedevice performance. Alkanethiol has been found effective toachieve higher PCEs for low bandgap polymer solar cells[10,43]. In our study, we added 1 vol% 1,8-octanedithiol (OT) tothe PDTTBT:PCBM solution (in DCB). The as-prepared film afterbeing processed with OT additive exhibited 16% increase in thePCE as shown in Fig. 9 and Table 3.

On the other hand, it has been indicated that the introductionof an optical spacer fabricated from titanium suboxide (TiOx) ledto the higher device efficiency [44,45]. The TiOx optical spacerdeposited between the active layer and the aluminum electroderedistributes the light intensity to optimize absorption and chargeseparation in the BHJ layer. As a result of the increased absorptionin the BHJ layer, the efficiency significantly improved. Moreover,the TiOx optical spacer breaks the symmetry and functions as anelectron collecting and hole blocking layer. Therefore, we alsocompared the device efficiency by inserting TiOx layer in the cell

Fig. 9. J–V characteristics of devices processed from pure DCB solvent, solvent

mixture (DCB with 1 vol% OT), and TiOx optical spacer, under AM 1.5 simulated

solar illumination at an intensity of 100 mW/cm2.

Table 3Photovoltaic characteristics of the devices processed from pure DCB solvent,

solvent mixture (DCB with 1 vol% OT), and TiOx optical spacer.

DCB DCBþOT TiOx

Voc (V) 0.316 0.470 0.471

Jsc (mA/cm2) 1.255 1.037 1.029

FF (%) 28.46 26.95 26.87

Z (%) 0.113 0.131 0.130

Table 2Surface roughness of PDTTBT:PCBM blend films obtained from AFM after anneal-

ing at different temperatures for 30 min.

Annealing temp. RT 100 1C 125 1C 150 1C 175 1C 200 1C

Average roughness (nm) 1.02 1.46 4.40 4.69 4.26 3.40

Root mean square (nm) 1.42 2.04 5.60 6.12 5.78 4.33

T.-L. Wang et al. / Solar Energy Materials & Solar Cells 95 (2011) 3295–3302 3301

architecture. Dense TiOx films were prepared using a TiOx pre-cursor solution and spin-cast on top of the active layer, asdescribed in detail elsewhere [45]. It was found that the PCE forthe cell inserted with a layer of TiOx increases ca. 15%. However,the increase of PCE is mostly due to the improvement of Voc asshown in Fig. 9 and Table 3.

4. Conclusions

The D–A type copolymer PDDTBT based on DTT and BT unitshas been synthesized and employed as the donor material in theactive layer of BHJ-type polymer solar cells. UV–vis absorptionspectra indicated that a low bandgap polymer with a wideabsorption band has been obtained. Through the annealing treat-ment at an optimum condition (150 1C/30 min), the PV cellperformance was dramatically improved and the power conver-sion efficiency of the device reached to 0.301% under white lightillumination (100 mW/cm2). We attribute the higher efficiency toenhanced 3-D interpenetrating networks in the active layer,increase of light absorption, and improved carrier mobility. Usingthe OT additive and optical spacer architecture, the photovoltaicperformances increased more than 15% compared to the unan-nealed PV cell. In contrast, the improvement of performance for

the cell annealed at 150 1C is far more than that by the addition ofoptical layer or OT additive.

Acknowledgments

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

Appendix A. Supporting information

Supplementary data associated with this article can be foundin the online version at doi:10.1016/j.solmat.2011.07.021.

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