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

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

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Synthesis and photovoltaic properties of a low bandgap donor–acceptor alternating 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, ROC Department of Chemical and Materials Engineering, Cheng Shiu University, Kaohsiung County 833, Taiwan, ROC c Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, ROC b

a r t i c l e i n f o

abstract

Article history: Received 28 April 2011 Received in revised form 11 July 2011 Accepted 16 July 2011 Available online 5 August 2011

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.

Keywords: Donor–acceptor alternating copolymer Polymer solar cells Low bandgap Annealing Optical spacer

1. Introduction In the recent decade, polymer solar cells (PSCs) based on conjugated polymers have been extensively studied because of their potential use for future cheap and renewable energy production [1–3]. Polymer-based solar cells have unique advantages over traditional silicon-based solar cells, such as low cost, light weight, and potential use in flexible devices [4–6]. 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 polymer, while the acceptor is generally a type of organic or inorganic molecule. At present, one of the most successful donor polymers is regioregular poly(3-hexylthiophene) (P3HT). A bulk heterojunction photovoltaic device combining regioregular P3HT as the electron donor with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as the electron acceptor achieves power conversion efficiencies (PCEs) of 4–5% by device optimization [7–9]. In order to further improve the PCE of the PSCs, much research work has been devoted to find new conjugated polymer donor

n

Corresponding author. Tel.: þ886 7 5919278; fax: þ886 7 5919277. E-mail address: [email protected] (T.-L. Wang).

0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.07.021

materials aiming at broader absorption, lower band gap, higher hole mobility, and suitable electronic energy levels. However, the performance of the photovoltaic cells with these conjugated polymers is considerably limited by their relatively large band gaps, 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 [10–13] Many D–A type copolymers have been used in PSCs to achieve PCEs above 5% with extensive device engineering efforts [10,14–16]. Using the fused thiophene family as the donor is an attracting 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 polymer backbone more rigid and coplanar, therefore enhancing effective p-conjugation, lowering bandgap, and extending absorption. Introduction of thienothiophene units tends to stabilize the quinoid structure in the polymer chain and thus enhances the planarity along the polymer backbone. The high power conversion efficiency can be attributed to the rigidity and planarity of

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the polymer backbone, leading to a high hole mobility of the copolymer. Dithieno[3,2-b:20 ,30 -d]thiophene (DTT) is well known as an important building block in the synthesis of high mobility materials for organic field-effect transistors (OFETs) [20,21]. Recently, synthesis of DTT-containing copolymers and their applications in OFETs [22,23] and PSCs [24,25] have been reported. Therefore, it should be possible to obtain novel D–A type copolymers with excellent performance by introducing DTT moieties into polymer backbones. On the other hand, 2,1,3-benzothiadiazole (BT) is an electronaccepting (A) molecule that has been utilized to construct some n-type semiconducting polymers showing high electron mobility [26–28]. Recently, BT has also been used as the acceptor unit in cooperation with varieties of electron-donating (D) units as low bandgap donors in bulk heterojunction polymer solar cells [25,29–32]. 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. Consequently, 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 experiments [33,34]. The coated large area devices successfully demonstrated the photovoltaic efficiencies up to 0.6%. In this report, some characteristics and optoelectronic properties of the fabricated PSCs were investigated. The effect of thermal annealing, the solvent additive, and the optical spacer on the PCEs of solar cells is also reported.

and isopropyl alcohol sequentially. The ITO surface was spincoated 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 the active layer was continued to be spin-coated. The PDTTBT:PCBM blend solutions were prepared with 1:1 weight ratio (10 mg/mL PDTTBT) in 1,2-dichlorobenzene (DCB) as the active layer. Devices were fabricated by spin-coating at 800 rpm for 30 s on top of the PEDOT:PSS layer. The obtained thickness for the blend film of PDTTBT:PCBM 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 are annealed directly on top of a hot plate in the glove box, and the temperature is monitored using a thermocouple touching the top of the substrates. After removal from the hotplate, the substrates are immediately put onto a metal plate at the room temperature. Ultraviolet–visible (UV–vis) spectroscopic analysis was conducted on a Perkin-Elmer Lambda 35 UV–vis spectrophotometer. Photoluminescence (PL) spectrum was recorded on a Hitachi F-7000 fluorescence spectrophotometer. The film topography images of active layers were recorded with a Digital Instruments Dimension 3100 atomic force microscope (AFM) in tapping mode under ambient conditions. The J–V curves were measured using a Keithley 2400 source meter, under illumination from a solar simulator. The intensity of solar simulator was set with a primary reference cell and a spectral correction factor to give the performance under the AM 1.5 (100 mW/cm2) global reference spectrum (IEC 60904-9).

3. Results and discussion 2. Experimental

3.1. Material synthesis and structural characterization

2.1. Materials

Since 2,1,3-benzothiadiazole (BT) is an electron-accepting heterocycle showing high electron mobility, BT has been recently used as the acceptor unit (A) in cooperation with varieties of electron-donating units (D) as low bandgap donors in BHJ PSCs. On the other hand, the incorporation of linearly symmetrical and coplanar thienothiophene unit DTT in the conjugated polymers is predicted to facilitate high power conversion efficiencies. Therefore, it is expected that wide sunlight absorption band and high power conversion efficiency could be achieved for the D–A type copolymer using DTT as the donor and BT as the acceptor. The synthetic route of the copolymer PDTTBT is shown in Scheme 1. The polymer was synthesized via Stille coupling reaction of the donor unit with the acceptor unit. To increase the solubility of the copolymer, long alkyoxyl chains were attached onto the benzothiadiazole unit, while the donor part still retains its planarity. The structures of both monomers and copolymer were confirmed by 1H NMR and elemental analysis (analytical data is given in the Supporting Information). The polymer is well dissolved in common organic solvents such as chloroform, 1,2-dichlorobenzene, THF, and toluene. Molecular weight of the polymer determined by gel permeation chromatography showed a low Mn value of 9100, which might be due to the steric hindrance from 5,6-dialkyoxyl substituents on the BT unit. In addition, the polymer exhibited a high glass transition temperature (Tg) of 131 1C as a result of the rigid donor and acceptor units.

2,3-Dibromothiophene (Alfa Aesar), bis(phenylsulfonyl) sulfide (Acros), selenium (Acros), copper(II) chloride (Acros), butyllithium (Acros), catechol (Lancaster), 1-bromotetradecane (Alfa Aesar), tin(II) chloride (Alfa Aesar), N-thionylaniline (TCI), trimethyltin chloride (Acros), bis(triphenylphosphine)palladium(II) dichloride (Alfa Aesar), poly(3,4-ethylenedioxythiophene)-poly (styrenesulfonate) (PEDOT:PSS, Aldrich), and phenyl-C61-butyric acid methyl ester (PCBM, FEM Tech.) were used as received. All other reagents were used as received. 2.2. Synthesis The donor and acceptor materials, dithieno[3,2-b:20 ,30 -d]thiophene (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-5,6bis(tetradecyloxy)benzo-2,1,3-thiadiazole-4,7-diyl) (PDTTBT) was synthesized via Stille coupling reaction of the donor unit of 2,6-bistrimethylstannanyl-dithieno[3,2-b:20 ,30 -d]thiophene with the acceptor 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 this study is ITO/PEDOT:PSS/PDTTBT:PCBM/Al. PDTTBT acts as the p-type donor polymer and PCBM 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,

3.2. Optical properties The normalized UV–vis absorption spectra of the PDTTBT copolymer in THF solution and the film cast from THF are presented in Fig. 1(a). The optical absorption threshold at 706 nm from the spectrum of the film corresponds to the bandgap (Eg) of

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Scheme 1. Synthesis of PDTTBT copolymer.

PDTTBT film PDTTBT in THF

Intensity

Absorbance (a.u.)

PDTTBT film PDTTBT inTHF

500 300

400

500

600

700

800

Wavelength (nm)

0.07 0.06

(αh ν)

0.05 0.04 0.03 0.02 0.01 0.00 1.7

1.8

1.9

700

800

Fig. 2. Photoluminescence spectra of PDDTBT in dilute THF solution and thin film with excitation at 400 nm.

- - - -Eg:1.83eV

1.6

600 Wavelength (nm)

2.0

2.1

2.2

Photoenergy hν (eV) 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.

the PDTTBT copolymer. Hence, the estimated optical bandgap is 1.76 eV. To obtain a more accurate optical band gap of PDTTBT, the fundamental equation ahn ¼ B(hn–Eopt)n developed in Tauc relation [37] was used. The optical bandgap calculated by this equation is 1.83 eV, smaller than that (1.9–2.0 eV) of the widely used regioregular poly(3-hexylthiophene) (P3HT), as shown in Fig. 1(b). As seen from Fig. 1(a), the UV–vis absorption spectrum of the copolymer in dilute THF solution exhibited two absorption peaks positioned at about 343 and 490 nm. The peak at 343 nm is probably due to the p–pn transition of the dithienothiophene moiety [38], while the peak in the visible region is assigned to the intramolecular charge transfer (ICT) between the donor and the acceptor [25,39]. Similarly, the absorption spectrum of PDTTBT film also shows two peaks, one at 361 nm and the other at

514 nm. In contrast to the spectrum in solution, both peaks show small red-shifts, indicating more efficient p-stacking and stronger intermolecular interactions in the solid state. In particular, the broadened absorption spectrum ranging from 300 to 750 nm indicates a low bandgap polymer has been obtained, as evident from the Eg of PDTTBT. It is apparent that the ICT interaction between donor and acceptor moieties in the D–A copolymers is a practical approach to lower the bandgap and broaden the absorption bands across the entire visible wavelength region of conjugated polymers. Hence, our successful synthesis of a low bandgap D–A type copolymer is further confirmed. The photoluminescence (PL) emission spectra of PDTTBT in dilute THF solution and thin film are shown in Fig. 2. Both the fluorescence spectra exhibit the vibronic structure with a maximum at 559 and 579 nm. As seen from the figure, both spectra show only one emission peak, indicating that an effective energy transfer from the DTT segments to the BT unit occurs. The red-shift in the spectrum of the PDTTBT film is probably due to the lowering of the bandgap of copolymer by more efficient p-stacking in the solid state. 3.3. Electrochemical properties Cyclic voltammogram (CV) is a preliminary characterization technique to determine the redox properties of organic and polymeric materials. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of PDTTBT could be determined from the E1/2 (Fig. 3) and the onset absorption wavelength (Eg, energy bandgap) in Fig. 1. As seen in Fig. 3, the oxidation onset potential for PDTTBT has been determined as 0.62 V vs. Ag/Ag þ . The external ferrocene/ ferrocenium (Fc/Fc þ ) redox standard E1/2 is 0.27 V vs. Ag/Ag þ . Assuming that the HOMO energy for the Fc/Fc þ standard is 4.80 eV with respect to the zero vacuum level, the HOMO energy

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0.0020

Table 1 Photovoltaic characteristics of the devices under different annealing temperatures for 30 min.

Current Density/Acm

0.0015

0.0010

Voc (V) Jsc (mA/cm2) FF (%) Z (%)

0.0005

0.0000

-0.0005

-0.0010 -1.0

-0.5

0.0

0.5

1.0

1.5

2.0

E/V vs. Ag/Ag Fig. 3. Cyclic voltammograms of PDDTBT film on an ITO substrate in CH3CN/AcOH (V/V¼ 7/1) containing 0.1 M tetrabutylammonium perchlorate at a scan rate of 50 m Vs  1.

3.32 4.3

4.3 AI

PDTTBT

4.8 ITO

PCBM

5.0 PEDOT:PSS 5.15 6.1

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

for PDTTBT has been evaluated to be 5.15 eV. Hence the LUMO level determined from its HOMO and bandgap (Eg) is 3.32 eV. Fig. 4 shows the schematic diagram representing the potential metrically determined HOMO and LUMO energy of PDTTBT and PCBM relative to the work function of the electrodes. 3.4. Photovoltaic properties At present, bulk heterojunction structures are the main candidates for high-efficiency polymeric solar cells. The bulk heterojunction solar cells based on PDTTBT in combination with PCBM have been prepared and investigated. The employed device structure was ITO/PEDOT:PSS/PDTTBT:PCBM/Al. The blend solutions (in DCB) of PDTTBT:PCBM were prepared with 1:1 weight ratio as the active layer. The current–voltage (J–V) curve for the as-prepared blend film cast at room temperature (RT) under illumination from solar simulator at 100 mW/cm2 light intensity is shown in Fig. 7, and the corresponding open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and power conversion efficiency (PCE,Z) are listed in Table 1. The power conversion efficiency of solar cell using the as-prepared blend film as the active layer is 0.113%. In a previous work by Bundgaard et al. [33,34], the device efficiencies for the untreated and annealed film based on this material (Mn ¼ 12,300 g/mol) are 1.7% and 2.2%,

RT

100 1C

125 1C

150 1C

175 1C

200 1C

0.316 1.255 28.46 0.113

0.272 1.549 30.15 0.127

0.265 1.617 30.28 0.130

0.336 2.898 30.87 0.301

0.593 1.551 29.42 0.271

0.235 2.008 28.77 0.136

respectively. Compared to the device efficiency achieved by them, the low efficiency of our device may be due to the lower molecular weight of polymer (Mn ¼9100 g/mol), preparation techniques for the device architecture, different thicknesses of the active layer, glove box and vapor deposition equipment, etc. To further improve the device efficiency based on this material, it is considered that bandgap tuning of conjugated polymers via molecular design is a direct and efficient approach. The general strategies have been suggested and discussed in the several literatures [40–42]. In the present case, it would be helpful to fine-tune the bandgap of the D–A type copolymer through either of these approaches: (1) attaching the electron donating groups to raise the HOMO or attaching the electron withdrawing groups to lower the LUMO, (2) introduction of a methine group between the donor and the acceptor units to give a more flat structure, and (3) attaching an atom of high electron affinity (such as fluorine) in selected locations of fused rings [41]. Although chemical synthesis can raise the HOMO level or lower the LUMO level of a conjugated polymer, the former action produces a detrimental reduction of Voc, whereas the latter could push the DELUMO (the energy difference between the LUMOs of donor and acceptor) away from the minimum energy offset, which should be larger than 0.3 eV for efficient charge separation. As a result, lowering the bandgap of a conjugated polymer to absorb visible and nearinfrared radiation, while providing efficient charge separation and high 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 solar cells, we have studied photovoltatic properties under different annealing temperatures. The effect of annealing temperature on the UV–vis absorption spectra for the thin films of PDTTBT:PCBM (1:1 weight ratio) spun cast on quartz substrates is shown in Fig. 5. These films were annealed under nitrogen atmosphere inside the glove box at atmospheric pressure. The annealing time was kept 30 min for all of the annealing temperatures. The absorption spectra show a considerable change after thermal annealing of the films. The first band is attributed to the p–pn transition of DTT segments, whereas the last band is due to the ICT interaction as stated above. At annealing temperature of 100 1C, the intensities of both bands increase without change in the position. An increase in the absorption strength after heat treatment normally means increased packing of the PDTTBT domains. The film heat-treated at 125 1C shows a similar behavior. The maximum absorption is observed for the film annealed at 150 and 175 1C, indicating an enhanced conjugation length and the more ordered structure of PDTTBT. Since the thickness of all the films is similar (ca. 110 nm), the increase in the peak absorption intensity during thermal annealing may be attributed to the lowering of the bandgap between p and pn, the increase of the optical p–pn transition, and the increased interchain interaction among the PDTTBT. After thermal annealing, the PDTTBT molecules afford higher energy and can move more easily. Consequently, the polymer chains become mobile and self-organization can occur to form ordering.

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Fig. 5. UV–vis absorption spectra of PDDTBT:PCBM blend films after annealing at different temperatures for 30 min. 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.

RT 100°C 125°C 150°C 175°C

Intensity

200°C

400

500

600

Wavelength (nm) Fig. 6. Photoluminescence spectra of PDDTBT:PCBM blend films after annealing at different temperatures for 30 min with excitation at 375 nm.

Therefore, the peak intensity increases in the more ordered films due 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, however, results in a decrease in the intensities of both the bands. From the above results, the optimum annealing temperatures for PDTTBT and PCBM may be around 150–175 1C. Fig. 6 shows the PL intensity for blend films annealed at different temperatures. The PL is due to photogenerated excitons in PDTTBT that do not take part in the charge separation. The phenomenon of PL quenching can be attributed to the interfacial charge transfer. Normally, PL quenching increases with the increase of interfacial area between donor and acceptor materials in the active layer. PL quenching provides direct evidence for exciton dissociation, and thus efficient PL quenching is necessary to obtain efficient organic solar cells. As shown in the figure, it can be seen that the PL intensity decreases with the increase of annealing temperature. The PL intensity shows a minimum at thermal annealing of 200 1C. This significant reduction in the PL intensity is attributed to efficient photoinduced charge separation

between electron-donating (PDTTBT) and electron-accepting (PCBM) molecules. This may be attributed to the higher charge carrier mobility or higher interfacial area between D–A molecules compared 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 PL intensity of the blend film annealed at 150 1C is a little higher in comparison with that of the film annealed at 175 and 200 1C, the highest power conversion efficiency (0.301%) has been achieved by this blend film as shown in Fig. 7 and Table 1. As we will discuss in the following part, it seems that the higher value of roughness and higher degree of nanoscale phase separation in the blend film annealed at 150 1C enhance the transport rate of charge carriers to the metal electrode and reduce the charge recombination of the excitons. Based on these studies, we conclude that there may be three reasons to explain why the film annealed at 150 1C exhibits the optimal PCE. It is probably because (1) the increase of optical absorption in the visible light region, (2) the improved charge carrier mobility in both donor and acceptor phases after thermal annealing, and (3) the increased interfacial area between the donor and acceptor phases offset the former effect (PL quenching) and result in an overall improvement in the device performance. Therefore, the morphology of PDTTBT/PCBM blend film before and after annealing should be studied. Since the morphology of the heterojunction plays an important role in the performance of polymer solar cells, we studied the topography of the blend films of PDTTBT:PCBM (1:1, w/w) by AFM. Although the AFM images of film surfaces at different annealing temperatures have been taken, only three representative images are shown in Fig. 8 for comparison. The values of average roughness and root-mean-square roughness for the blend films are shown in Table 2. It is clear that the images for both the as-prepared film and the film annealed at 200 1C look relatively smooth. It is evident that a more rough surface observed in the film annealed at 150 1C increases the contact area between the active layer and the metal electrode. Hence, the transport rate of the charge carriers to the metal electrode is higher and the recombination 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 annealed at 175 1C.

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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.

3.6. Effects of solvent mixture and optical spacer The mixed solvent approach has been demonstrated as a promising method to modify solar cell morphology and improve device performance. Alkanethiol has been found effective to achieve higher PCEs for low bandgap polymer solar cells [10,43]. In our study, we added 1 vol% 1,8-octanedithiol (OT) to the PDTTBT:PCBM solution (in DCB). The as-prepared film after being processed with OT additive exhibited 16% increase in the PCE as shown in Fig. 9 and Table 3.

On the other hand, it has been indicated that the introduction of an optical spacer fabricated from titanium suboxide (TiOx) led to the higher device efficiency [44,45]. The TiOx optical spacer deposited between the active layer and the aluminum electrode redistributes the light intensity to optimize absorption and charge separation in the BHJ layer. As a result of the increased absorption in the BHJ layer, the efficiency significantly improved. Moreover, the TiOx optical spacer breaks the symmetry and functions as an electron collecting and hole blocking layer. Therefore, we also compared the device efficiency by inserting TiOx layer in the cell

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Table 2 Surface roughness of PDTTBT:PCBM blend films obtained from AFM after annealing at different temperatures for 30 min. Annealing temp.

RT

100 1C

125 1C

150 1C

175 1C

200 1C

Average roughness (nm) Root mean square (nm)

1.02 1.42

1.46 2.04

4.40 5.60

4.69 6.12

4.26 5.78

3.40 4.33

3301

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

Acknowledgments We gratefully acknowledge the support of the National Science Council of Republic of China with Grant NSC 99-2221-E390-001-MY3.

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2011.07.021.

References

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 3 Photovoltaic characteristics of the devices processed from pure DCB solvent, solvent mixture (DCB with 1 vol% OT), and TiOx optical spacer.

Voc (V) Jsc (mA/cm2) FF (%) Z (%)

DCB

DCB þOT

TiOx

0.316 1.255 28.46 0.113

0.470 1.037 26.95 0.131

0.471 1.029 26.87 0.130

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

4. Conclusions The D–A type copolymer PDDTBT based on DTT 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. Through the annealing treatment at an optimum condition (150 1C/30 min), the PV cell performance was dramatically improved and the power conversion efficiency of the device reached to 0.301% under white light illumination (100 mW/cm2). We attribute the higher efficiency to enhanced 3-D interpenetrating networks in the active layer, increase of light absorption, and improved carrier mobility. Using the OT additive and optical spacer architecture, the photovoltaic performances increased more than 15% compared to the unannealed PV cell. In contrast, the improvement of performance for

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