Enhanced and Tunable Open-Circuit Voltage using Dialkylthio Benzo

Jun 2, 2012 - Asymmetric Electron-Donating 4-Alkyl-8-alkoxybenzo[1,2-b:4,5-b′]dithiophene Unit for Use in High-Efficiency Bulk Heterojunction Polyme...
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Enhanced and Tunable Open-Circuit Voltage using Dialkylthio Benzo[1,2-b:4,5-b′]dithiophene in Polymer Solar Cells Doyun Lee, Emir Hubijar, Grace Jones D. Kalaw, and John P. Ferraris* Department of Chemistry and The Alan G. MacDiarmid Nanotech institute, The University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080-3021, United States S Supporting Information *

ABSTRACT: In this study, we explore the effect of the dialkoxy and dithioalkoxy side chains on the benzo [1,2-b:4,5b′]dithiophene (BDT) unit by comparing the O-BDT homopolymer (O-PBDT), S-BDT homopolymer (S-PBDT), and S-BDTalt-O-BDT copolymer (SO-PBDT) by computational calculations and experimental results. The polymers were prepared by Pdcatalyzed Stille coupling. Additionally, hole mobility and film morphology were studied by fabricating organic field effect transistors (OFETs) and using TappingMode AFM, respectively. The photovoltaic properties of the polymers were measured from fabricated PSC devices. The replacement of the alkoxy (−OR) groups with thioalkoxy (−SR) groups lowered the HOMO energy level of the conjugated polymers from 5.31 to 5.41 eV, and consequently enhanced Voc, while still preserving the excellent properties offered by the BDT-based polymers. Especially, high Voc of 0.99 V was achieved from S-PBDT polymer based PSC with up to 4.0% of PCE, which is one of the highest efficiencies reported from a homopolymer-based PSC without thermal/ solvent annealing or incorporated additives. KEYWORDS: S-BDT, S-PBDT, dithioalkoxy benzodithiophene

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INTRODUCTION Polymer-based solar cells (PSCs) have attracted considerable attention as a promising long-term solution for clean, renewable energy due to advantages including simple and low cost fabrication, flexibility and lightweight.1−13 Bulk heterojunction solar cells (BHSCs), with an active layer comprising an interpenetrating network of conjugated polymers and fullerene derivatives, have been extensively studied and display the highest efficiencies to date in PSCs. The conducting polymers in these devices operate as an electron donor while the fullerene derivatives act as an electron acceptor.4,7,12,14−21 The most common polymer used as a donor material in PSCs is poly (3-hexylthiophene) (P3HT),19,22−28 which has afforded power conversion efficiencies (PCE) up to ∼5% when combined with a soluble fullerene derivative, (6,6)-phenyl C61-butyric acid methyl ester (PCBM), as the acceptor.8,25 Nevertheless, even though P3HT-based PSCs have shown promising results, it has proven difficult to further improve the efficiency of P3HT-based PSCs. This is primarily due to its intrinsic limited absorption in the solar spectrum (energy band gap of 1.9 eV) and the relatively small energy difference between its highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PCBM. Both factors combine to limit the overall performance in terms of the short-circuit current (Jsc) and open-circuit voltage Voc.25,29−33 The power conversion efficiency (PCE) in PSCs is © 2012 American Chemical Society

defined as the ratio of power out (Pout) to power in (Pin) and is directly related with product of the fill factor, FF, the shortcircuit current density, Jsc, and the open-circuit voltage, Voc. Hence, to obtain high performance polymer solar cells, it is imperative to design new electron-donating polymer materials that possess (1) strong and broad absorption to harvest more sunlight, (2) high charge mobilities, and (3) low-lying HOMO energy levels, while keeping the band gap between 1.2 and 1.9 eV.1,31,32,34,35 In recent years, numerous p-type, low-band gap polymers that harvest more light by fine-tuning their absorption properties to better match the solar spectrum have been reported. These polymers typically employ a donor−acceptor (D−A) approach with alternating electron-rich and electrondeficient moieties along their backbone.1,32,36−39 The band gaps of these donor−acceptor p-type polymers can be easily tuned by controlling intramolecular charge transfer (ICT) between the donor and acceptor moieties.32,34,40−45 A common feature of many of these efficient low band gap donor−acceptor type conjugated polymers in PSCs is the employment of the benzo [1,2-b: 4,5-b′] dithiophene (BDT) unit.33,35,46−60 This family of polymers based on BDT, specifically dialkoxy substituted moieties, attracted our interest because of their exceptional Received: April 9, 2012 Revised: June 1, 2012 Published: June 2, 2012 2534

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Scheme 1. Synthesis Route of O-PBDT, S-PBDT, and SO-PBDT Polymers

homopolymer (S-PBDT) and S-BDT-alt-O-BDT copolymer (SO-PBDT) by computational calculations and experimental results. The polymers were all prepared by Pd-catalyzed Stille coupling reactions shown in Scheme 1. Additionally, hole mobility and film morphology were studied by fabricating organic field effect transistors (OFETs) and using TappingMode AFM, respectively. The photovoltaic properties of the polymers were measured from fabricated PSC devices. The replacement of the alkoxy (−OR) groups with thioalkoxy (−SR) groups lowered the HOMO energy level of the conjugated polymers, and consequently enhanced Voc, while still preserving the excellent properties offered by the BDTbased polymers.

performance as a common unit in PSCs that can achieve PCEs up to ∼8%.41,47,52,53 Since BDT has a symmetrical and large planar conjugated structure that provides for easier π−π stacking, it was expected to have a relatively high hole mobility.38,49,61,62 Indeed, it is been reported that BDTthiophene based polymers achieved a hole mobility of 0.25 cm2 V−1 s−1, which is among the highest values for the polymerbased organic field effect transistors (OFETs).49,61,62 Nevertheless, even though some dialkoxy-substituted BDT based polymers exhibit PCEs up to 7.73%, the majority of them have relatively high HOMO levels (in the range of −4.90 to −5.22 eV), leading to open-circuit voltage (Voc) values ranging from 0.56 to 0.76 V.46,49,52,63 Although, these Voc values are higher than for P3HT polymers, there is still significant room for improvement, which could afford even higher efficiencies in PSCs. Since the Voc is directly proportional to the difference between the HOMO energy level of the electron donor and the LUMO energy level of the electron acceptor, it is important to find ways to lower the HOMO level of the electron donor.34,42,43 Recently, we reported64 the first synthesis of a novel dithioalkoxy-substituted benzo [1,2-b:4,5-b′]dithiophene (BDT) based donor−acceptor conjugated polymers, which would operate as an electron donor unit in BHSCs, and the homopolymer of dithioalkoxy-BDT (S-PBDT) was successfully synthesized via a Pd catalyzed Stille coupling reaction and fully characterized including promising preliminary photovoltaic performance. The highest PCE of up to 2.75% was achieved using the polymer and PCBM blend. The most encouraging result from the previous work was that Voc of the devices reached up to 0.99 V, resulting from the deeper HOMO level of the polymer with Jsc of 5.45 mA/cm2 and FF of 0.51. Although the dialkoxy BDT based homopolymer was reported,49 photovoltaic performance of the homopolymer has not been reported due to its poor solubility. It encouraged us to investigate the effect of side chain on BDTs, dithioalkoxyBDT (S-BDT) and dialkoxy-BDT (O-BDT), and how it related to the HOMO and LUMO levels of the polymers and to the Voc in PSC application. In this report, we explore the effect of the dialkoxy and dithioalkoxy side chains on the BDT unit by comparing the O-BDT homopolymer (O-PBDT), S-BDT

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RESULTS AND DISCUSSION Synthesis of the Polymers. The synthesis routes of the polymers are presented in Scheme 1. 2,6-dibromo-4,8-bis(2ethylhexyloxy) benzo [1,2-b:4,5-b′] dithiophene (1) and 2,6bis(trimethylstannyl)-4,8-bis(2-ethylhexyltoxy) benzo [1,2b:4,5-b′] dithiophene (2) were synthesized as reported,49 and the detailed synthesis of 2,6-dibromo-4,8-bis(2-ethylhexylthio) benzo [1,2-b:4,5-b′] dithiophene (3), and 2,6-bis(trimethylstannyl)-4,8-bis(2-ethylhexylthio) benzo [1,2-b:4,5b′] dithiophene (4) were reported in our recent work.64 The homopolymers were prepared via a Pd-catalyzed Stille-coupling reaction between dibromo-monomers (1 and 3) and distannylmonomers (2 and 4), and the SO-PBDT alternating copolymer was prepared by reacting the monomers 2 and 3 shown in Scheme 1. A detailed synthesis procedure and yield for the polymers are stated in Experimental Section. The molecular weight and polydispersity index (PDI) of the polymers measured by GPC were Mn = 10.9K and PDI = 2.96 from O-PBDT polymer, Mn = 9.3K and PDI = 2.98 from SO-PBDT polymer and Mn = 16.1K and PDI = 3.81 from S-PBDT polymer. Despite repeated polymerization attempts using different reaction conditions in which reaction time and the amount of catalyst were varied, these are highest the molecular weight polymers that we could obtain for these polymers with the ethylhexyl substituent. Thermal Stability. Thermal gravimetric analysis (TGA) of the polymers shown in Figure 1 revealed a 5% weight loss for 2535

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related with the difference between the HOMO of electron donor, conjugated polymer, and the LUMO of electron acceptor, fullerene derivatives. Since Voc is one of main contributors in estimating the PCE, S-PBDT polymer is expected to yield higher efficiency among the polymers in PSCs application. Furthermore, the LUMO of the polymers were similarly changed along with the HOMO levels, indicating that the lowered LUMO level of S-PBDT will have a smaller difference with the LUMO of electron acceptor (PCBM), and thus, the excited electrons could be efficiently transferred with less energy loss. Finally, the electrochemical band gap was narrower from S-PBDT (2.15 eV) compared with O-PBDT (2.37 eV) due to the greater decrease of the LUMO level (2.94 eV from O-PBDT and 3.26 eV from S-PBDT). Moreover, the HOMO and LUMO energy levels of SO-PBDT were placed between those of O-PBDT and S-PBDT polymers and means that tunable HOMO and LUMO levels are possible by managing the ratio between O-BDT and S-BDT units in the polymer backbone. DFT Calculation. To predict the electronic properties and energy levels, theoretical calculations were performed by using the density functional theory (DFT) with the B3LYP/6-31G* basis set using Spartan (Wave function Inc.). Calculations were performed on tetramers as simplified models. The frontier molecular orbitals and optimized molecular geometries of all polymers are illustrated in Figure 4. According to these model calculations, both the HOMO and LUMO levels could be lowered by replacing oxygen (HOMO = 5.11 eV) with sulfur (HOMO = 5.39 eV). The calculated HOMO levels of the tetramers are close to the experimental values. However, the calculated LUMO levels of the tetramers are much higher than that of the experimental values of the polymers, indicating that the LUMO levels are more sensitive to chain length in the model compounds. Since we are primarily interested in the HOMO level in the polymer, DFT calculations for the shorter models are still informative. The trend of the HOMO and the LUMO levels reflected the experimental results where S-PBDT polymer has the deepest HOMO level, O-PBDT polymer has the highest HOMO level with the HOMO level of SO-PBDT polymer placing between the S-PBDT and O-PBDT polymers. Therefore, all the results above indicate that the polymers will exhibit different Voc when used in PSC devices and the Voc could be varied by controlling the amount of the two components (O-BDT and S-BDT).

Figure 1. TGA plot of the polymers with a heating rate of 10 °C/min under inert atmosphere.

all polymers at ∼295 °C. This degradation was also reported for similar BDT-based polymers and is acceptable for organic electronic device applications. Optical and Electrochemical Properties. The optical absorption spectra of the polymers in chloroform and as films using UV−vis spectroscopy are shown in Figure 2a and b. The polymers showed similar absorption profiles with two major peaks with slightly different absorption maxima, which could be attributed to π−π* transitions in the polymer backbone. The summary of the absorption spectra is listed in Table 1. Interestingly, the absorption onset of S-PBDT was observed at 600 nm, which is red-shifted ∼40 nm from that of O-PBDT (560 nm),. Consequently, the optical band gaps of the polymers estimated from the absorption onset were 2.07 eV (S-PBDT) and 2.21 eV (O-PBDT), respectively. This result was confirmed from the electrochemical study using cyclic voltammetry shown in Figure 3. The HOMO and LUMO levels of the polymers were estimated from the point where the current signal of oxidation and reduction potential start to deviate from the baseline. It is clearly seen on the cyclic voltamogram that the HOMO levels were decreased upon replacing the alkoxy group on O-PBDT (5.31 eV) to less electron-donating thioalkoxy group on S-PBDT (5.41 eV). The decreased or deeper HOMO levels could be expected to afford increased Voc in PSC application because the Voc is directly

Figure 2. UV−visible absorption spectra of PBDT polymers (a) in chloroform and (b) as films. 2536

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Table 1. Optical and Electrochemical Properties of the Polymers polymer

λmax (nm)

Eopt g (eV)

Eox (V)

Ered (V)

HOMO (eV)

LUMO (eV)

Eec g (eV)

O-PBDT SO-PBDT S-PBDT

483 523 502 539 505 544

2.21 2.03 2.07

0.60 0.65 0.70

−1.77 −1.60 −1.45

−5.31 −5.36 −5.41

−2.94 −3.11 −3.26

2.37 2.25 2.15

Figure 5. Current density versus voltage (J−V) curves measured under simulated 100 mW/cm2 a.m. 1.5G illumination.

Figure 3. Cyclic voltammogram of O-PBDT, SO-PBDT, and S-PBDT polymer films (cyclic voltammogram were off set along y-axis).

Table 2. Solar Cell Results of the Devices Prepared from the Polymers with PCBM (1:1 ratio by weight) in oDichlorobenzene

Photovoltaic Properties. Photovoltaic properties of the polymer were investigated in solar cell device structures of ITO/PEDOT:PSS(30 nm)/active layer/Ca(10 nm)/Al(100 nm). Figure 5 shows the J−V curves of the PSCs under AM 1.5 G illumination with 100 mW/cm2 and the results are summarized in Table 2. The results shown in Table 2 and Figure 5 are the highest performance from each polymer and detailed data are included in the Supporting Information. The highest PCE was obtained from the device of S-PBDT: PCBM (1:1 ratio by weight) blend, and the PCE reaching up to 4.0%. It is worth noting that the PCE of 4.0% is one of the highest efficiencies attained for homopolymer-based PSCs without any other common treatment such as thermal annealing or addition of additives. The photovoltaic result revealed that deeper HOMO levels of the polymers enhanced the Voc of the devices from 0.83 to 0.99 V, which is about a 20% increase. Also, the Voc from SO-PBDT based device (0.91 V) placed in the middle of O-PBDT (0.83 V) and S-PBDT (0.99 V). The final PCE from S-PBDT based devices, however, was greater than 100% higher than the PCE from O-PBDT based devices. This large difference was further investigated by looking into several factors, including the effect

O-PBDT SO-PBDT S-PBDT

thickness (nm)

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

55 65 50

0.83 0.91 0.99

4.18 4.40 7.66

0.45 0.43 0.53

1.56 1.73 4.00

of polymer solubility. The S-PBDT polymer was readily soluble in THF, chloroform, chlorobenzene, and o-dichlorobenzene at ambient temperature without heating. However, the O-PBDT polymer required heating (>50 °C) in any of the organic solvents listed above, and it was only partially soluble at room temperature. Although the SO-PBDT polymer also required heating, once it dissolved, it stayed in solution. We repeated polymerization for the polymers but we could not further improve solubility of the polymers. As a result, in the preparation of the polymer: PCBM blend, the O-PBDT polymer blend solution was stirred at 70 °C for overnight while the S-PBDT: PCBM blend was mixed at room temperature for overnight using a mechanical shaker. Another factor is the effect that the lowered LUMO level of S-PBDT on the PCE. Scharber et al. reported that the lower LUMO levels

Figure 4. HOMO and LUMO wave functions of the tetramers using the density functional theory (DFT) with the B3LYP/6-31G* basis set. 2537

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of the polymers, while maintaining the band gap, enhanced the PCE as long as the energy difference between the conjugated polymer electron donor, and the fullerene derivative electron acceptor,, is greater than 0.3 eV.34 A third factor contributing to the higher PCE is the surface morphology of spun-cast polymer:PCBM blend films. TappingMode AFM shown in Figure 6 revealed differences in the surface morphologies of the homopolymers.

Figure 7. UV−vis absorption spectra of thin film of O-PBDT (55 nm) and S-PBT (45 nm) spin-coated from chloroform.

obtained from O-PBDT, which is an order of magnitude lower than the one from S-PBDT. It can be ascribed that the poorer solubility of O-PBDT than S-PBDT played an important role for polymer film formation and their morphologies. Also, the lower molecular weight of O-PBDT may contribute to the lower hole mobility because of shorter length of conjugated polymer backbone. Here, the low hole mobility of S-PBDT may also be due to its low molecular weight. In addition, the polymers exhibited comparable hole mobilities with polymers reported in the literature, which were measured via space charge limited current (SCLC).56,65

Figure 6. Height and phase AFM images (2 μm × 2 μm) of O-PBDT/ PCBM (1:1) blend film as-cast (a, d), SO-PBDT/PCBM (1:1) blend film as-cast (b, e), S-PBDT/PCBM (1:1) blend film as-cast (c, f). Panels a, b, and c are height images and panels d, e, and f are phase images (scale bar height =10.0 nm and phase =20.0°).

The height images (Figure 6a, b, c) of the polymers revealed that O-PBDT/PCBM blend film (Figure 6a) showed a higher surface roughness than the S-PBDT/PCBM blend film (Figure 6c). This indicates that greater aggregation may have occurred on the O-PBDT/PCBM film (Figure 6a) possibly due to the limited solubility of the O-PBDT polymer. In addition, the phase images confirm the observations from the height images such that the O-PBDT/PCBM film (Figure 6d) have larger domains compared with those in the S-PBDT/PCBM film (Figure 6f). Another possible factor for the observed PCE increase may be the polymer’s optical properties, such as broadness of its absorption spectrum or its absorption coefficient. To explore the absorption coefficients of these polymers, various thicknesses of polymer films were prepared by spincasting and the absorption of these films were measured over the range of 300 to 800 nm. Figure 7 displays the absorption coefficient of O-PBDT (55 nm) and S-PBDT (45 nm) polymer film with similar thickness. The thickness dependent UV−vis absorption spectra of the two polymers are presented in the Supporting Information. The absorption profile of the polymers were almost identical but the absorption coefficient of S-PBDT polymer reached up to 2.20 × 105 cm−1 at its absorption maximum, while that of the O-PBDT polymer only reached up to 1.67 × 105 cm−1. This indicates that the S-PBDT polymer not only covers a somewhat broader range of the spectrum but also possesses a higher absorption coefficient, which consequently could lead to enhanced Jsc. Finally, we studied the hole mobility of the polymers by fabricating bottom-gate bottom-contact organic field effect transistor (OFET) and details are presented in the Supporting Information. The hole mobility of 1.3 × 10−5 (cm2/(V s)) was obtained from S-PBDT polymer while the mobility of 1.1 × 10−6 (cm2/(V s)) was

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CONCLUSION In this research, the thioalkoxy-BDT (S-BDT) based homopolymer (S-PBDT), O-PBDT and SO-PBDT alternating polymers were prepared, and the optical, electrochemical and photovoltaic properties were compared. The S-BDT unit was designed and synthesized as a replacement of O-BDT unit because the promising O-BDT based donor−acceptor low band gap polymers holds the world record of 7.73% efficiency in PSC area. Even with superior Jsc and FF, its Voc reached only up to 0.76 V due to its higher HOMO level. The synthesized polymers herein displayed Voc of 0.99, 0.91, and 0.83 V for SPBDT, SO-PBDT, and O-PBDT polymers, respectively. These results can be attributed to the poorer electron-donating properties of the thioalkoxy group in S-BDT than the alkoxy group in O-BDT, which led to the decreased HOMO and LUMO energy levels of the polymers. Also, we studied solubility, absorption coefficient, film morphology, and hole mobility of the polymers. The S-PBDT based PSC presented an enhanced PCE of up to 4.0% with 7.66 mA/cm2 of Jsc and 0.53 of FF. This PCE is one of the highest efficiencies reported from a homopolymer-based PSC without thermal/solvent annealing or incorporated additives. This result suggests that the S-BDT unit could be a promising replacement of O-BDT in the OBDT based low band gap donor−acceptor polymers.47−49,52−55,57,58,60,66 Likewise, this paper reports that Voc in PSCs can be tuned by controlling of the O-BDT/S-BDT ratio in copolymers. To extend our study, we will introduce longer alkyl side chains on the BDT unit such as hexyl-decyl or butyl-octyl group to enhance the solubility of polymers that may currently limit the performance of our polymers. 2538

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(Newport M465440) was used to calibrate the solar simulator to 100 mW/cm2 AM 1.5G illumination. General Synthetic Procedure of S-PBDT, O-PBDT, and SOPBDT by Stille Reaction. The polymers were prepared by a similar procedure. Dibromo compound (0.204 mmol), bis(trimethyltin) compound (0.204 mmol), and 15 mL anhydrous toluene were added into 100 mL three-necked round-bottom flask under inert atmosphere and stirred for 20 min. Pd(PPh3)4 catalyst (7 mg) was added into the reaction mixture and flushed with nitrogen for another 20 min. The reaction temperature was increased slowly to 115 °C and stirred for overnight under inert gas atmosphere. The reaction solution was cooled to room temperature and precipitated by pouring into 300 mL of methanol. The suspension was filtered through a Soxhlet thimble, and then extracted with methanol and further with hexane for 24 h each. The residue was then extracted into chloroform. The polymer was recovered as solid from the chloroform fraction by rotary evaporation and the solid was dried under vacuum for overnight. The yield, 1H NMR, and molecular weight of polymers by GPC are as follows: S-PBDT. Yield: 74%. 1H NMR (270 MHz, CDCl3, Me4Si): δ (ppm): 7.65 (br, 2H), 2.95 (m, 4H), 1.60−1.10 (m, 18H), 0.85 (m, 12H). Mn = 16.1 K, Mw = 61.6 K, PDI = 3.81. O-PBDT. Yield: 50%. 1 H NMR (270 MHz, CDCl3, Me4Si): δ (ppm): 7.65 (br, 2H), 4.22 (m, 4H), 1.60−1.10 (m, 18H), 0.85 (m, 12H). Mn = 10.9 K, Mw = 32.3 K, PDI = 2.96. SO-PBDT. Yield: 45%. 1H NMR (270 MHz, CDCl3, Me4Si): δ (ppm): 7.65 (br, 2H), 4.22 (m, 2H), 2.95 (m, 2H), 1.60− 1.10 (m, 18H), 0.85 (m, 12H). Mn = 9.3 K, Mw = 27.8 K, PDI = 2.98.

EXPERIMENTAL SECTION

Materials. All chemicals were purchased from commercial sources (Sigma-Aldrich, Acros, Fisher-Scientific) and used without further purification unless otherwise stated. Tetrahydrofuran (THF) and acetonitrile were dried using an MBraun SP Series solvent purification system and their water contents (85% light transmission) were patterned by photolithography and cleaned in ultrasonic bath in acetone, toluene, methanol, acetone and isopropyl alcohol sequentially and UV-ozone treated for 20 min. Polyethylenedioxythiophene/polystyrenesulphonate (PEDOT/PSS, H.C. Starck) was spin-coated onto the patterned ITO substrate (21 × 21 mm2) at 3000 rpm for 40 s to create a ca. 30 nm thick layer. The substrate was dried at 150 °C for 10 min in a N2 filled glovebox. The active layer was spun cast at various spin rates onto the ITO/PEDOT: PSS substrate without further special treatments. Calcium (10 nm) and aluminum (100 nm) layers were then sequentially deposited under high vacuum (