Synthesis and photovoltaic properties of narrow band gap copolymers of dithieno[3,2-b:2′,3′-d]thiophene and diketopyrrolopyrrole

Abstract

Two new sets of donor–acceptor and donor–π–acceptor type copolymers with a narrow band-gap, PDTTDPP and PTDTTTDPP, based on dithieno[3,2-b:2′,3′-d]thiophene (DTT) and dithieno[3,2-b:2′,3′-d]thiophene bridged with thiophene (TDTTT) as a short π-conjugated spacer with diketopyrrolopyrrole (DPP), were successfully synthesized by Stille polymerization. Incorporation of linear long chain alkyl (decanyl) groups in the DTT core and branched alkyl (ethylhexyl) groups in the DPP unit improved the solubility and processability of the resulting co-polymers. UV-vis absorption spectroscopy and cyclic voltammetry results demonstrated that introduction of thiophene as a shorter conjugated spacer between the donor and acceptor units in the PTDTTTDPP copolymer facilitated tuning of the absorption capability between the donor and acceptor resulting in a red-shifted absorption as compared to the PDTTDPP copolymer. Thermogravimetric analysis of the copolymers showed high thermal decomposition temperatures of 375 °C for PDTTDPP and 370 °C for PTDTTTDPP. The hole mobility of the copolymers, measured using an organic thin film transistor device, was 1.0 × 10⁻⁶ cm² for PDTTDPP and 4.5 × 10⁻³ cm² for PTDTTTDPP. Photovoltaic properties of PDTTDPP and PTDTTTDPP with PC₆₁BM were evaluated using different device fabrication conditions and photovoltaic devices based on the polymer : PC₆₀BM (1 : 1 ratio (w/w)) bulk hetero-junction demonstrated a maximum PCE of 1.39% and 0.29% for PTDTTTDPP:PC₆₁BM and PDTTDPP:PC₆₁BM, respectively, in chlorobenzene.

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Introduction

In the past decade, polymer solar cells (PSCs) based on conjugated polymers as electron donors with fullerene derivatives as acceptors have received tremendous attention for their unique advantages over traditional silicon-based solar cells, which include low cost solution fabrication processing, light-weight, large area, and flexible panels, as well as potential contributions to clean and renewable energy.¹ In a typical bulk hetero-junction (BHJ) device structure, the most widely used configuration, a blend of an electron-donor (p-type conjugated polymer) and an electron-acceptor (n-type polymer or [6,6]-phenyl-C₆₁-butyric acid methyl ester, PC₆₁BM), is positioned as a photoactive layer between two conducting electrodes.² Among the various conjugated polymers, regioregular poly(3-hexylthiophene) (rrP3HT) is one of the most promising donor materials, with a power conversion efficiency (PCE) of PSCs based on its thin film blend with PC₆₁BM reaching ∼3 to 6%.³ However, further enhancement in PCE with P3HT is limited for lower photocurrent generation due to its spectral mismatch with solar photon flux and its highest occupied molecular orbital (HOMO) level (∼−5.0 eV), which are strongly related to the short circuit current (J_sc) and open circuit voltage (V_oc), respectively.⁴

To achieve high performance BHJ type PSCs, electron-donating conjugated polymers are required to possess strong and broad absorption of solar light, good hole mobility, and reasonable HOMO levels to maximize the J_sc and V_oc, respectively. Various design strategies have been pursued to fulfil this requirement. One popular and feasible approach in macromolecular systems is to design and synthesize copolymers containing alternating electron-rich donor (D) and electron-poor acceptor (A) monomeric units on a conjugated molecular backbone.⁵ Many studies on the design and synthesis of D–A conjugated co-polymers, which can efficiently harvest a majority of the energy in the solar spectrum, have reported effective ways to obtain low band gap polymers.^6,7 In these studies, a great deal of attention was paid to D–A conjugated polymers whose optical and electronic properties could be tuneable through the intramolecular charge transfer (ICT) from D to A. Low band gap D–A copolymers should possess two features in order to achieve high efficiency for device performance. First, the LUMO energy level of the polymer (D) should be positioned above the LUMO of the fullerene derivative (A) by at least 0.2–0.3 eV to ensure efficient electron transfer,^2b,c and at the same time, the energy gap between the HOMO of the electron donating polymer (D) and the LUMO of the electron acceptor (A) should be maximized in order to increase the open circuit voltage (V_oc) of the device.⁸ The other perhaps more important feature is to have not only a high charge carrier mobility of holes and electrons for efficient charge extraction and low series resistance giving a good device fill factor (FF), but also a moderate miscibility with the electron acceptor (e.g., PCBM) to form an interpenetrating network. Well-chosen D and A groups are particularly desirable for low band gap polymers due to a significant enhancement of ICT intensity and conjugation length, which lead to better extended absorption and a higher absorption coefficient. By using the aforementioned D–A concept, various groups have synthesized diketopyrrolopyrrole (DPP)-based low band gap copolymers, which have emerged as extremely attractive materials for solar cell devices that have achieved power conversion efficiencies above 5% with extensive device engineering.⁹ Furthermore, the planarity of the DPP skeleton and its ability to accept hydrogen bonds due to the electron deficient nature of the DPP core result in copolymers that encourage π–π stacking.

In the design of D–A conjugated polymers, a useful strategy is to introduce a more rigid and planar monomeric unit with quinoidal character into the conjugated system, which can efficiently reduce the band gap and enhance π–π stacking. Fused thiophene ring systems are well known to stabilize the quinoidal structure.¹⁰ Recently, solar cell devices with efficiencies greater than 7% have been demonstrated using fused thiophene (thieno[3,4-b]thiophene) conjugated polymers through a systematic tuning of the band gap, absorption and V_oc, and relevant device parameters.^1b In our search for new electron-rich monomers and taking into account these recent results, we became interested in the dithieno[3,2-b:2′,3′-d]thiophene (DTT) unit,¹¹ an important building block for a wide variety of functional organic materials. The planarity and S–S interaction of the fused DTT structure promote highly ordered π-stacking,^13a and high hole mobility,^13a which are predictors for high charge transport in devices.^13b Several groups have reported the synthesis of DTT derivatives for applications in organic thin film transistors (OTFTs).^12,14 It is interesting to note that despite all of these promising features, to the best of our knowledge, there have been a number of reports on the photovoltaic properties of DTT-containing D–A type copolymers which delivered poor device performance.¹⁵ Xiaowei Zhan and co-workers reported DTT-based donor–acceptor polymers consisting of alternating perylenediimides (PDI)–dithienothiophen unit as an acceptor and bis(thienylvinylene)-substituted polythiophene as a donor with high electron mobility and power conversion efficiency (PCE) up to 1% under simulated AM 1.5, 100 mW cm⁻² conditions.^15a,d Moreover, this group also reported porphyrene based DTT-π-conjugated alternating copolymers^15b and substituted-DTT alternating with thiophene copolymers^15c for photovoltaic applications. However the device performance of these two examples was poor. Recently, we rationally designed a copolymer, PTDTTTQX based on DTT and adopted a simple D–π–A structure with thiophenes as a shorter conjugated spacer between the electron donor and electron acceptor to facilitate the electronic coupling, wavelength tuning, and absorption capability between the donor and acceptor, resulting in a red-shifted absorption with good power conversion efficiency (PCE).¹⁶ Therefore, encouraged by this strategy, herein we report the synthesis and photovoltaic properties of two novel sets of narrow band gap and solution processable alternating copolymers, PDTTDPP and PTDTTTDPP, based on DTT containing D–A and thiophene-bridged DTT containing D–π–A polymer backbones, comprised of 3,5-didecanyldithieno[3,2-b:2′,3′-d]thiophene (DTT) and 2,6-bis(thiophen-2-yl)-3,5-didecanyldithieno[3,2-b:2′,3′-d]thiophene (TDTTT) as the donor and 3,6-bis(5-bromo-2-thienyl)-2,5-dihydro-2,5-diethylhexylpyrrolo[3,4-c]pyrrole-1,4-dione (DPP) as the acceptor units, respectively. Introduction of thiophene units to the 2- and 6-positions of the DTT core in TDTTT can reduce steric hindrance, extend conjugation, enhance absorption, and improve charge transport properties. However, further increasing rigidity with fused thiophenes is known to decrease their solubility, solution processability, and environmental stability.^14b Since alkoxy groups have much stronger electron donating effects than alkyl groups, conjugated donor polymers with alkoxy groups as substituents usually exhibit higher HOMO levels than their alkyl-substituted counterparts.¹⁷ Therefore, in order to modulate the molecular energy levels to give deeper HOMO levels, we introduced linear long chain alkyl (decanyl) groups at the 3- and 5-positions of the DTT core and branched alkyl groups to the DPP unit, which consequently improved the solubility and processability of the resulting co-polymers. Furthermore, the design of materials containing DTT and thiophene-bridged DTT derivatives has led to a novel family of polymeric semiconductors with potential use in optoelectronic applications as polymer backbones with good π–π stacking combined with good solubility and stability.

Experimental

Materials

All reagents, unless otherwise specified, were obtained from Aldrich and were used as received. 3,5-Didecanyldithieno[3,2-b:2′,3′-d]thiophene (1),^11c2,6-dibromo-3,5-didecanyldithieno[3,2-b:2′,3′-d]thiophene (2),¹⁶2,6-bis(tributylstannanyl)-3,5-didecanyldithieno[3,2-b:2′,3′-d]thiophene (3),^15e and 3,6-bis(5-bromo-2-thienyl)-2,5-dihydro-2,5-diethylhexylpyrrolo[3,4-c]pyrrole-1,4-dione (DPP) (6)^18a,b were synthesized by adapting methods reported in the literature. Reaction solvents were distilled prior to use (THF was distilled from sodium/benzophenone); other solvents were used as received. Column chromatography was carried out on silica gel (300–400 mesh).

Characterization

¹H and ¹³C NMR spectra were measured using a JEOL or a Bruker spectrometer operating at 100 MHz for ¹³C and 400 MHz or 300 MHz for ¹H, respectively, in deuterated chloroform solution with TMS as an internal reference standard for chemical shifts. Molecular weights of polymers were determined using the gel permeation chromatography (GPC) method with polystyrene standards. GPC analysis was performed with polymer/chloroform (HPLC grade) solution at a flow rate of 1 mL min⁻¹ at 25 °C, on a Futecs NP-4000 chromatography instrument with Shodex LF-804 (Shodex Co., Japan) connected to a refractive index detector. Thermogravimetric analysis (TGA) was performed using a TA Instruments 2050 series instrument. The temperature of degradation (T_d) corresponded to a 5% weight loss. Elemental analyses were carried out using a CE Instruments Flash EA 1112 series. UV-vis absorption spectra were recorded using a Shimadzu UV-2550 UV-vis spectrophotometer with 1 cm path length quartz cells for solution and spin-coated films on quartz plates for solid state measurements. The optical band gap was determined from the onset of the absorption band edge. Cyclic voltammetry (CV) data were measured on a BAS CV-50W voltammetric analyzer in dry acetonitrile (electrochemical grade, Fisher Scientific) using tetrabutylammonium hexafluoroborate (Bu₄NBF₆) as the electrolyte and indium tin oxide (ITO) and Ag/Ag+ as the working and reference electrodes, respectively, at a scan rate of 100 mV s⁻¹ at room temperature under argon. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the polymer were calculated using a ferrocene value of −4.8 eV below the vacuum level as the internal standard from CV anodic and cathodic scans, respectively. AFM measurements were performed using a Digital Instruments Multimode atomic force microscope controlled by a Nanoscope IIIa scanning probe microscope controller. All characterizations were performed under ambient conditions without a protective atmosphere.

Photovoltaic device fabrication and characterization

BHJ polymer solar cells, with a structure of ITO/PEDOT:PSS (40 nm)/polymer:PC₆₁BM (90 nm)/LiF/Al, were prepared on commercial ITO-coated glass substrates with a sheet resistance of 7 Ohms sq⁻¹. Prior to use, substrates were cleaned with deionized water, then sonicated in acetone followed by isopropanol. A thin film of PEDOT:PSS (40 nm) (AI 4083, H. C. Starck) was spin-coated (3000 rpm, 40 s) onto the substrate and was dried at 140 °C for 20 min. The active layer was prepared, with a thickness of approximately 90 nm onto the surface of PEDOT:PSS, by spin coating with different polymer to PCBM (Nano-C, USA) ratios (from 1 : 1 to 1 : 4 w/w) in chlorobenzene, then drying at RT for 10 min under nitrogen and filtering through a 0.45 μm poly(tetrafluoroethylene) (PTFE) filter and spin coating at 700 rpm for 40 s. Devices were completed by deposition of a 0.5 nm layer of LiF and a 150 nm layer of Al. These layers were thermally evaporated at a pressure of 1 × 10⁻⁶ Torr at room temperature. The active area was 0.9 cm². The current–voltage (I–V) characteristics of the photovoltaic devices in the dark and under white light illumination were measured at AM 1.5 G solar simulator (Newport) and 100 mW cm⁻² conditions, adjusted with a standard PV reference cell (2 cm × 2 cm), a mono-crystalline silicon solar cell, calibrated at NREL, Colorado, USA, with a Keithley 2400 source-measure unit. The external quantum efficiency (EQE) was performed using a Polaronix K3100 spectrometer.

Synthesis of monomers and polymers

Synthesis of 2,6-dithienyl-3,5-didecanyldithieno[3,2-b:2′3′-d] thiophene (4). 2,6-Dibromo-3,5-didecanyldithieno[3,2-b:2′3′-d] thiophene (2) (2.0 g, 3.15 mmol), thienyl-2-boronic acid (1.0 g, 7.87 mmol, 2.5 eq.) and Pd(PPh₃)₄ (91 mg, 2.5 mol%) were added to a 50 mL Schlenk flask and were subjected to three vacuum/argon fill cycles. Argon degassed THF (20 mL) and 2 M aqueous K₂CO₃ (5 mL) were added, three more vacuum/argon cycles were repeated to ensure oxygen was excluded, and the mixture was stirred for 20 min under argon. The reaction mixture was heated to reflux for 24 h and was monitored by TLC. After completion of the reaction, THF was removed on a rotary evaporator, and the product was extracted into chloroform, then successively washed with water, and dried over MgSO₄. Removal of the solvent afforded the crude product, which was purified using column chromatography (silica gel, n-hexane as eluent) to give a pale yellow powder (1.21 g, 60%). ¹H NMR (400 MHz, CDCl₃, ppm, Fig. S1†) δ: 7.35 (d, J = 6.2 Hz, 2H), 7.18 (d, J = 4.8 Hz, 2H), 7.10 (m, 2H), 2.93 (t, 4H), 1.8 (m, 4H), 1.41–1.26 (m, 28H), 0.89 (t, 6H). ¹³C NMR (100 MHz, CDCl₃, ppm, Fig. S1†) δ: 142.51, 136.33, 132.80, 131.09, 128.43, 127.51, 126.24, 125.83, 31.91, 29.64, 29.59, 29.56, 29.34, 29.06, 28.74, 22.68, 14.11. Calculated for C₃₆H₄₈S₅: C, 67.45; H, 7.55; S, 25.01; found: C, 67.52; H, 7.68; S, 25.10%.

Synthesis of 2,6-bis(thiophen-2-yl)-3,5-didecanyldithieno[3,2-b:2′,3′-d]thiophene distannane (5). To a stirred solution of 4 (0.64 g, 1.0 mmol) in THF (20 mL) was added TMEDA (0.33 mL, 2.2 mmol) at −50 °C, and then 1.6 M n-butyllithium in hexane (2.2 mmol, 1.37 mL) was added dropwise at −78 °C. After 30 min, the reaction mixture was refluxed for 3 h. Then the reaction mixture was cooled to −78 °C, and a 1.0 M solution of trimethyltin chloride in THF (0.5 mL, 2.4 mmol) was added in one portion. The reaction mixture was warmed to room temperature and was stirred for 6 h. The reaction was quenched with 20 mL of water and was extracted with diethyl ether. The organic extract was dried with anhydrous MgSO₄ and was evaporated in vacuo. Recrystallization of the residue from methanol furnished 0.58 g (60%) of the title product as a colorless semisolid. ¹H NMR (300 MHz, CDCl₃, ppm, Fig. S2†) δ: 7.28 (d, J = 3.4 Hz, 2H), 7.21 (d, J = 3.3 Hz, 2H), 2.94 (t, 4H), 1.80–1.73 (m, 4H), 1.42–1.26 (m, 28H), 0.89 (t, 6H), 0.41 (s, 18H). ¹³C NMR (80 MHz, CDCl₃, ppm, Fig. S2†) δ: 142.13, 138.77, 136.43, 135.51, 132.84, 132.31, 131.47, 128.40, 127.47, 127.23, 126.22, 125.74, 31.91, 29.63, 29.60, 29.58, 29.33, 29.01, 28.81, 22.67, 14.08%.

Synthesis of copolymer PDTTDPP

2,6-Bis(tributylstannanyl)-3,5-didecanyldithieno[3,2-b:2′,3′-d]thiophene (3) (290 mg, 0.275 mmol; 1.1 eq.) and 3,6-bis(5-bromo-2-thienyl)-2,5-dihydro-2,5-diethylhexylpyrrolo[3,4-c]pyrrole-1,4-dione (DPP) (6) (170.6 mg, 0.25 mmol; 1.0 eq.) were dissolved in anhydrous chlorobenzene (15 mL). The solution was flushed with argon for 10 min, and then Pd₂dba₃ (11.5 mg, 5 mol% with respect to monomer) and P(o-tolyl)₃ (7.5 mg, 10 mol%) were added to the flask. The flask was purged three times with successive vacuum and argon fill cycles. The polymerization reaction was heated at 110 °C for 24 h under an argon atmosphere. The reaction mixture was cooled to room temperature and was slowly poured into 100 mL of well-stirred methanol containing 10 mL of hydrochloric acid. The precipitated dark solid was subjected to sequential Soxhlet extraction with methanol, acetone, and hexane to remove the low molecular weight fraction of the material. The residue was extracted with chloroform and was dried under vacuum for 12 h to obtain 105 mg of a black solid (42%). ¹H NMR (400 MHz, CDCl₃, ppm, Fig. S3†) δ: 9.1–8.8 (m, br, 2H), 7.60–6.90 (m, br, 2H), 3.99 (d, br, 4H), 2.9–2.5 (t, br, 4H), 1.80–0.91 (m, br, 50H), 0.91–0.80 (t, br, 18H). Calculated for C₅₈H₈₀N₂O₂S₅: C, 69.69; H, 8.27; N, 2.80; S, 16.04; found: C, 69.75; H, 8.40; N, 2.60; S, 16.10%. Number average molecular weight (M_n) = 5.215 × 10³; weight-average molecular weight (M_w) = 5.730 × 10³; polydispersity index (PDI) = 1.09 (M_w/M_n).

Synthesis of copolymer PTDTTTDPP

2,6-Bis(thiophen-2-yl)-3,5-didecanyldithieno[3,2-b:2′,3′-d]thiophene distannane (5) (265.8 mg, 0.275 mmol; 1.1 eq.) and 3,6-bis(5-bromo-2-thienyl)-2,5-dihydro-2,5-diethylhexylpyrrolo[3,4-c]pyrrole-1,4-dione (DPP) (6) (170.6 mg, 0.25 mmol; 1.0 eq.) were dissolved in chlorobenzene (15 mL). The solution was flushed with argon for 10 min, and then Pd₂dba₃ (11.5 mg, 5 mol% with respect to monomer) and P(o-tolyl)₃ (7.5 mg, 10 mol%) were added to the flask. The flask was purged three times with successive vacuum and argon fill cycles. The polymerization reaction was heated at 110 °C for 24 h under an argon atmosphere. The reaction mixture was cooled to room temperature and was slowly poured into 100 mL of well-stirred methanol containing 10 mL of hydrochloric acid. The precipitated dark solid was subjected to sequential Soxhlet extraction with methanol, acetone, and hexane to remove the low molecular weight fraction of the material. The residue was extracted with chloroform and was dried under vacuum for 12 h to obtain 175 mg of a black solid (60%). ¹H NMR (400 MHz, CDCl₃, ppm, Fig. S4†) δ: 9.0–8.8 (m, br, 2H), 7.40–6.75 (m, br, 6H), 4.1–3.85 (d, br, 4H), 2.9–2.6 (t, br, 4H), 1.90–1.1 (m, br, 50H), 0.90–0.80 (t, br, 18H). Calculated for C₆₆H₈₄N₂O₂S₇: C, 68.11; H, 7.45; N, 2.41; S, 19.29; found: C, 68.25; H, 7.36; N, 2.30; S, 19.40%. Number average molecular weight M_n = 5.516 × 10³; M_w = 9.995 × 10³; PDI = 1.81 (M_w/M_n).

Results and discussion

Synthesis

Synthetic approaches to the monomers and the polymers are outlined in Scheme 1. Bromination of 3,5-didecanyldithieno[3,2-b:2′,3′-d]thiophene (1) with NBS gave the corresponding monomer 2 in 96%,¹⁶ which on lithiation, followed by quenching with Bu₃SnCl gave the desired monomer 3 in 80% yield.^15e Suzuki coupling of the monomer 2 with thiophene-2-boronic acid gave compound 4 in 60% yield, which on lithiation followed by quenching with Me₃SnCl gave 2,6-bis(thiophen-2-yl)-3,5-didecanyldithieno[3,2-b:2′,3′-d]thiophene distannane (5) in 60% yield. DPP (6) was synthesized by a pseudo Stobbe condensation between isopropyl succinic ester and thiophene-2-carbonitrile as reported in the literature.^18a,b Finally, Stille polymerization of monomer 6 with 3 or 5 was carried out using Pd₂(dba)₃/P(o-tolyl)₃ as the catalyst in anhydrous chlorobenzene to afford the corresponding copolymers PDTTDPP and PTDTTTDPP, respectively, which were purified by sequential Soxhlet extraction with methanol, hexane, and CHCl₃. The CHCl₃ fraction was then reduced in volume, precipitated in methanol, and collected by filtration to yield a black solid. The chemical structures of the compounds isolated at each step were confirmed by their ¹H and ¹³C NMR spectra. The structures of PDTTDPP and PTDTTTDPP were also confirmed from their ¹H NMR spectra (Fig. S3 and S4†, respectively, in the ESI). Both copolymers had good solubility in common organic solvents such as tetrahydrofuran, chloroform, chlorobenzene (CB), and o-dichlorobenzene. M_n, M_w and PDI were determined using GPC in chloroform based on a calibration with monodisperse polystyrene standards, and the results are summarized in Table 1 (also see Table S1† in the ESI). As shown in Table 1, PDTTDPP and PTDTTTDPP show very low M_n and M_w values. We believe that the low molecular weight of polymers may be due to their insufficient solubility during polymerization. Since high molecular weight is necessary to get better and reasonable device performance, we are currently being pursued to redesign the molecular structure of polymers (e.g. adding longer alkyl chains in the DPP unit) to improve their solubility and molecular weight.

Scheme 1 Synthesis of the monomers and polymers.

Table 1 Molecular weights and thermal properties of the copolymers

Polymers	M n/g mol^−1	M w/g mol^−1	PDI	T d/°C
PDTTDPP	5215	5730	1.09	375
PTDTTTDPP	5516	9995	1.81	370

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Thermal properties

The thermal stability of the two polymers was investigated by TGA in a nitrogen atmosphere (Fig. S5 and S6†). The TGA study revealed that both polymers displayed good thermal stability with thermal decomposition temperatures (T_d) with 5% weight loss of 375 °C for PDTTDPP and 370 °C for PTDTTTDPP. The glass transition temperature (T_g) of the polymers was not observed from DSC measurements. Good thermal stability of the copolymers may be favourable for their application in PSCs.

Optical and electrochemical properties

The photophysical characteristics of the copolymers were investigated by UV-vis absorption spectra in CB solution and in solid films spin-coated on quartz substrates. Fig. 1 shows the UV-vis absorption spectra of PDTTDPP and PTDTTTDPP in CB solution and in the film state. Both polymers gave rise to broad wavelength absorption in the 400 to 850 nm range, which is an indication of a relatively rigid and planar polymer backbone with a high degree of conjugation between the donor and acceptor units within the polymer chain.

Fig. 1 UV-vis absorption spectra of (a) PDTTDPP and (b) PTDTTTDPP in chlorobenzene solution and in thin film.

As shown in Fig. 1a, an absorption maximum for PDTTDPP in CB solution was observed at 660 nm. Compared to the weak absorption band at around 490 nm in solution, the intense absorption at 660 nm, originating from an intramolecular charge transfer (ICT) between the DTT and DPP segments, is indicative of the strong electron withdrawing effect of the DPP group. The absorption maximum of PDTTDPP was observed at around 660 nm in thin film, which is almost the same as that in solution except longer wavelength absorption over 800 nm was also observed. This is attributable to some amount of aggregation of the polymer chains and orderly π–π stacking in the solid state. The optical band gap, obtained from the onset of the absorption spectrum in film, was estimated to be 1.51 eV. Similarly, as shown in Fig. 1b, the absorption maximum of the PTDTTTDPP co-polymer in CB solution was observed at 674 nm and shows almost identical absorption maxima in the solution and film states. However, the slight red shift of the absorption with a shoulder at around 750 nm in the film indicates that the conformation of PTDTTTDPP changed little from the solution to the film state and also that aggregates formed in the solid state, which are beneficial for improving the charge mobility of the resulting film. The optical band gap for PTDTTTDPP, obtained from absorption spectra in the film, was found to be 1.47 eV. A comparison of the polymers reveals that PTDTTTDPP has a more broad absorption with vibronic features and a lower optical band gap (∼0.04 eV) than PDTTDPP, which should be due to the introduction of two thiophene units as a short π-conjugated spacer between the dithienothiophene donor and diketopyrrolopyrrole acceptor units. These results are summarized in Table 2.

Table 2 Optical and electrochemical properties of the copolymers

Polymers	Solution (CB)			Film (CB)			E ox,onset/V	HOMO/eV	LUMO/eV
	λ max/nm	λ onset/nm	E g/eV	λ max/nm	λ onset/nm	E g/eV
PDTTDPP	660	825	1.50	635	825	1.51	0.84	−5.24	−3.76
PTDTTTDPP	674	830	1.49	670	840	1.47	0.80	−5.20	−3.40

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In order to investigate the electrochemical properties of the copolymers and estimate their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, cyclic voltammetry (CV) was carried out in a three-electrode cell, using 0.10 M n-Bu₄NBF₆ as the supporting electrolyte in acetonitrile (Fig. 2). In anodic scans, the onset oxidation potential (E_ox,onset) of the PDTTDPP and PTDTTTDPP copolymers occurred at 0.84 and 0.80 eV, respectively. The HOMO energy levels of PDTTDPP and PTDTTTDPP were estimated to be −5.24 and −5.20 eV, respectively, according to the empirical formula, HOMO = −(E_ox,onset + 4.71) (eV).¹⁹ Based on the correlation between the HOMO energy level and optical band gap (E_g) estimated from the UV-vis absorption spectrum of both polymers (E_g = E_HOMO − E_LUMO), the LUMO energy levels of PDTTDPP and PTDTTTDPP were calculated to be −3.76 and −3.40 eV, respectively. The electrochemical results summarised in Table 2 show that the two copolymers exhibit almost identical HOMO levels but PTDTTTDPP has a higher LUMO level.

Fig. 2 Cyclic voltammograms of (a) PDTTDPP and (b) PTDTTTDPP in 0.1 mol L⁻¹n-Bu₄NPF₆acetonitrile solution.

Organic field-effect transistor properties

For applications of polymers in organic electronic devices, it is important to know whether they have hole- and/or electron-transport properties with high charge-carrier motilities. Since hole mobility in the donor polymer is an important parameter that can affect the performance of solar cells, the hole mobilities of PDTTDPP and PTDTTTDPP were measured using organic field-effect transistor (OFET) devices with a bottom gate, top contact device configuration built on an n-doped octadecyltrichlorosilane-treated silicon wafer. Fig. 3 shows the typical transfer characteristic curves of the OFET based on the polymers at room temperature and with thermal annealing at 100 °C. The device performance of PDTTDPP exhibits a hole mobility of 1.0 × 10⁻⁶ cm² V⁻¹s⁻¹ at room temperature (Fig. 3a), and no enhancement in hole mobility was observed after thermal annealing at 100 °C for 5 minutes (Fig. 3b). The OFETs based on PTDTTTDPP show a hole mobility of 1.7 × 10⁻³ cm² V⁻¹s⁻¹ at room temperature (Fig. 3c), whereas thermal annealing at 100 °C for 5 minutes (Fig. 3d) led to improved performance with a mobility of 4.5 × 10⁻³ cm² V⁻¹s⁻¹. The OFET device performance based on PTDTTTDPP at room temperature and with thermal annealing at 100 °C showed a significant enhancement in hole mobility, almost three orders of magnitude greater than that of PDTTDPP, which may be favorable for enhancement of efficiency in solar cell devices.^18c

Fig. 3 Typical transfer characteristic curves at constant V_D = −80 V for FET devices with PDTTDPP (a) before annealing; (b) after annealing at 100 °C; and PTDTTTDPP (c) before annealing; (d) after annealing at 100 °C.

Photovoltaic properties

Based on the optical and electrochemical properties of PDTTDPP and PTDTTTDPP, an energy band diagram of photovoltaic devices fabricated using blends of the polymers and PC₆₁BM is presented in Fig. 4. According to the diagram, the energy differences in the LUMO levels of the polymers and PC₆₁BM were 0.51 eV for PTDTTTDPP and 0.15 eV for PDTTDPP. This suggests that electron transfer efficiency from the polymer donor PTDTTTDPP to the acceptor PC₆₁BM is more effective than that of PDTTDPP. It has been proposed that the difference in LUMO energy levels of the polymer (D) and fullerene derivative (A) should be 0.3–0.5 eV, the exciton binding energy, to ensure efficient electron transfer.^2b,c

Fig. 4 Energy diagram of ITO, PEDOT:PSS, PDTTDPP, PTDTTTDPP and PC₆₁BM.

To explore the potential utility of the copolymers for solar cells, their photovoltaic properties were investigated by fabricating PSCs based on PDTTDPP and PTDTTTDPP as the electron donors and PC₆₁BM as the electron acceptor in a BHJ device with a general structure of ITO/PEDOT:PSS (40 nm)/polymer:PC₆₁BM (90 nm)/LiF/Al. A LiF layer was deposited between the active layer and the Al electrode, as this layer has been reported to cause a shift of the vacuum level at the LiF/Al interface, which changes the effective work function of the electrode resulting in improved electron injection.^20a To understand the role of the ratio between the polymers (donor) and PCBM (acceptor) on the photovoltaic properties of BHJ solar cells, devices with different weight ratios were fabricated with fixed thicknesses (90 nm) of the active layer and tested. Photovoltaic properties of the solar cell devices such as short circuit current (J_sc), open circuit voltage (V_oc), fill factor (FF), and power conversion efficiency (PCE) are summarized in Table 3. It was observed that in the case of amorphous donor polymers, the active layer consisting of an electron-donor polymer with a higher content of PCBM usually yields better device performance due to more efficient charge separation from excitons.^20b However, in our study, devices with a blend ratio of 1 : 1 (w/w) gave better solar cell performance with both polymers, which may be due to a greater charge balance occurring between the donors and acceptors.^20c The current density–voltage (J–V) characteristic curves, which showed the best device performance of PDTTDPP and PTDTTTDPP with PC₆₁BM under light illumination at AM 1.5 G (100 mW cm⁻²), are shown in Fig. 5. As shown in Fig. 5a, the PDTTDPP device delivered lower performance, a J_sc of 1.47 mA cm⁻², a V_oc of 0.65 V, a FF of 0.31, and a PCE of 0.29%, while the PTDTTTDPP device, parameters shown in Fig. 5(b), had an enhanced J_sc of 5.15 mA cm⁻², a V_oc of 0.60 V, a FF of 0.45, and a PCE of 1.39%. The lower J_sc and FF values of the PDTTDPP devices indicate that the photovoltaic properties of the polymer may be limited because of a lower charge separation and inefficient electron transfer^20d due to an insufficient energy difference in the LUMO levels between PDTTDPP and PC₆₁BM. Similarly, as aforementioned, we speculate that the three orders of magnitude higher hole mobility of PTDTTTDPP contributed to its superior performance compared to PDTTDPP.

Table 3 Photovoltaic properties of polymer solar cell devices based on the copolymers PDTTDPP and PTDTTTDPP with different ratios of PC₆₁BM in CB

Polymer ^a	J sc/mA cm^2	V oc/V	FF	PCE (%)
a Ratio with PCBM.
PDTTDPP (1 : 1)	1.47	0.65	0.31	0.29
PDTTDPP (1 : 2)	0.77	0.67	0.31	0.13
PTDTTTDPP (1 : 1)	5.15	0.60	0.45	1.39
PTDTTTDPP (1 : 2)	2.67	0.57	0.24	0.36
PTDTTTDPP (1 : 4)	2.76	0.54	0.57	0.85

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Fig. 5 Current density–voltage (J–V) curves of photovoltaic devices fabricated using (a) PDTTDPP and (b) PTDTTTDPP with PC₆₁BM (1 : 1 w/w) under white light illumination at AM 1.5.

The morphology of the film is also very important for the performance of polymer/PCBM BHJ solar cells. In order to investigate the relationship between solar cell performance and morphology, we studied the morphology of polymer:PCBM films (1 : 1 w/w ratio), using tapping-mode atomic force microscopy (AFM). The AFM image of PTDTTTDPP in Fig. 6(b) shows a smooth surface and small phase segregation indicating good film formation of the materials, whereas the AFM image of PDTTDPP in Fig. 6 (a) shows relatively larger domains which are not favourable for charge separation and the formation of percolation paths, and directly affect the device performance.^20b

Fig. 6 AFM topography images of PDTTDPP (a) and PTDTTTDPP : PC₆₁BM (1 : 1 w/w) (b) films.

The external quantum efficiency (EQE), which is used to evaluate the photoresponse of fabricated solar cells with respect to wavelength, is shown in Fig. 7 for both polymer solar cell devices. The shape of the EQE plot is similar to the absorption spectrum of the device, indicating that the excitons produced in the active layers due to absorption of photons are efficiently dissociated into free charge carriers at the interface of the polymer and PCBM. The EQEs are also consistent with the solar cellJ–V curves as shown in Fig. 5. From Fig. 7, it can be seen that the PTDTTTDPP device has a more efficient photon to current conversion efficiency, which is greater than 20% from 350 to 500 nm and almost 20% from 600 to 700 nm, while the PDTTDPP device has less than 10% efficiency over the entire absorption wavelength range.

Fig. 7 EQE spectra of photovoltaic devices fabricated using (a) PDTTDPP and (b) PTDTTTDPP with PC61BM (1 : 1 w/w) under white light illumination at AM 1.5.

It is noteworthy that PTDTTTDPP exhibited higher PCEs compared to PDTTDPP, despite their very similar HOMO levels and broad absorption spectra extending into the near-IR region, which could be attributed to several factors such as higher molecular weights of the copolymer, higher hole mobility, better charge separation, and more efficient electron transfer between PTDTTTDPP and PC₆₁BM. Further, we envision that, like other DPP based copolymers,^9a,g,18b the solar cell performance of this type of copolymer could be further improved from the standpoint of polymer structural modifications (e.g. adding longer alkyl chains in the DPP unit to improve their solubility and molecular weight), device fabrication conditions and control of phase morphology using co-solvents and processing additives. These areas of research are currently being pursued in our laboratory (see Table S2 in the ESI†).

Conclusions

We have designed and synthesized two new sets of narrow band gap, solution-processable, thermally stable dithieno[3,2-b:2′,3′-d]thiophene (DTT) based D–A copolymer and a D–π–A copolymer with a bridging thiophene spacer, PDTTDPP and PTDTTTDPP, respectively. These polymers show a broad absorption spectrum extending into the near-IR region. The electrochemical results show that both copolymers exhibit almost identical HOMO levels but different LUMO levels. The hole mobility of the copolymers was measured using OTFT devices which showed a substantially higher hole mobility for PTDTTTDPP as compared to PDTTDPP. The photovoltaic properties of polymer:PC₆₁BM based BHJ solar cells were evaluated under different device fabrication conditions. Solar cell devices with a 1 : 1 w/w ratio in chlorobenzene solvent demonstrated a maximum PCE of 1.39% for PTDTTTDPP and 0.29% for PDTTDPP, respectively, indicating that D–π–A type copolymers are more promising for PSC applications. More importantly, this work demonstrates that the DTT derivative bridged with a thiophene as a conjugated spacer appears to be a promising, quite simple, and useful co-monomer building block for photoactive materials as a novel family of D–π–A type polymeric semiconductors.

Acknowledgements

This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2008-314-D00107). This research was also supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (contract no. 2008-05103).

Notes and references

1. (a) G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science, 1995, 270, 1789; (b) H. Y. Chen, J. H. Hou, S. Q. Zhang, Y. Y. Liang, G. W. Yang, Y. Yang, L. P. Yu, Y. Wu and G. Li, Nat. Photonics, 2009, 3, 649; (c) J. W. Chen and Y. Cao, Acc. Chem. Res., 2009, 42, 1709; (d) Y. F. Li and Y. P. Zou, Adv. Mater., 2008, 20, 2952; (e) B. C. Thompson and J. M. J. Fréchet, Angew. Chem., Int. Ed., 2008, 47, 58; (f) Y. J. Chen, S. H. Yang and C. S. Hsu, Chem. Rev., 2009, 109, 5868.
2. (a) C. Winder and N. S. Sariciftci, J. Mater. Chem., 2007, 14, 1077; (b) M. C. Scharber, D. Mühlbacher, M. Koppe, P. Denk, C. Waldauf, A. J. Heeger and C. J. Brabec, Adv. Mater., 2006, 18, 789; (c) B. C. Thompson, Y. G. Kim, T. D. McCarley and J. R. Reynolds, J. Am. Chem. Soc., 2006, 128, 12714.
3. (a) F. Padinger, R. Rittberger and N. S. Sariciftci, Adv. Funct. Mater., 2003, 13, 85; (b) W. Ma, C. Yang, X. Gong, K. Lee and A. J. Heeger, Adv. Funct. Mater., 2005, 15, 1617.
4. K. M. Coakley and M. D. McGehee, Chem. Mater., 2004, 16, 4533.
5. E. E. Havinga, W. Hoeve and H. Wynberg, Synth. Met., 1993, 55, 299.
6. (a) J. Hou, H.-Y. Chen, S. Zhang, G. Li and Y. Yang, J. Am. Chem. Soc., 2008, 130, 16144; (b) Y. Liang, Y. Wu, D. Feng, S.-T. Tsai, H.-J. Son, G. Li and L. Yu, J. Am. Chem. Soc., 2009, 131, 56; (c) E. G. Wang, L. Wang, L. F. Lan, C. Luo, W. L. Zhuang, J. B. Peng and Y. Cao, Appl. Phys. Lett., 2008, 92, 033307; (d) J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger and G. C. Bazan, Nat. Mater., 2007, 6, 497; (e) F. Zhang, E. Perzon, X. Wang, W. Mammo, M. R. Andersson and O. Inganas, Adv. Funct. Mater., 2005, 15, 745; (f) M. Svensson, F. L. Zhang, S. C. Veenstra, W. J. H. Verhees, J. C. Hummelen, J. M. Kroon, O. Inganas and M. R. Andersson, Adv. Mater., 2003, 15, 988.
7. (a) F. Zhang, W. Mammo, L. M. Andersson, S. Admassie, M. R. Andersson and O. Inganas, Adv. Mater., 2006, 18, 2169; (b) N. Blouin, A. Michaud and M. Leclerc, Adv. Mater., 2007, 19, 2295; (c) W. Y. Wong, X. Z. Wang, Z. He, A. B. Djurisic, C. T. Yip, K. Y. Cheung, H. Wang, C. S. K. Mak and W. K. Chan, Nat. Mater., 2007, 6, 521.
8. N. Leclerc, A. Michaud, K. Sirois, J. F. Morin and M. Leclerc, Adv. Funct. Mater., 2006, 16, 1694.
9. (a) M. M. Wienk, M. Turbiez, J. Gilot and R. A. J. Janssen, Adv. Mater., 2008, 20, 2556; (b) J. C. Bijleveld, A. P. Zoombelt, S. G. J. Mathijssen, M. M. Wienk, M. Turbiez, D. M. de Leeuw and R. A. J. Janssen, J. Am. Chem. Soc., 2009, 131, 16616; (c) S. H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee and A. J. Heeger, Nat. Photonics, 2009, 3, 297; (d) Y. Y. Liang, Y. Wu, D. Q. Feng, S. T. Tsai, H. J. Son, G. Li and P. Yu, J. Am. Chem. Soc., 2009, 131, 56; (e) E. J. Zhou, Q. S. Wei, S. Yamakawa, Y. Zhang, K. Tajima, C. H. Yang and K. Hashimoto, Macromolecules, 2010, 43, 821; (f) R. S. Ashraf, Z. Chen, D. S. Leem, H. Bronstein, W. Zhang, B. Schroeder, Y. Geerts, J. Smith, S. Watkins, T. D. Anthopoulos, H. Sirringhaus, J. C. de Mello, M. Heeney and I. McCulloch, Chem. Mater., 2011, 23, 768; (g) J. C. Bijleveld, V. S. Gevaerts, S. G. J. Mathijssen, M. M. Wienk, M. Turbiez, D. M. de Leeuw and R. A. J. Janssen, Adv. Mater., 2010, 22, E242–E246; (h) H. Bronstein, Z. Chen, R. S. Ashraf, W. Zhang, J. Du, J. R. Durrant, P. S. Tuladhar, K. Song, S. E. Watkins, Y. Geerts, M. M. Wienk, R. A. J. Janssen, T. Anthopoulos, H. Sirringhaus, M. Heeney and I. McCulloch, J. Am. Chem. Soc., 2011, 133, 3272.
10. (a) F. Wudl, M. Kobayashi and A. J. Heeger, J. Org. Chem., 1984, 49, 3382; (b) M. Pomerantz, B. Chalonner-Gill, M. O. Harding, J. J. Tseng and W. J. Pomerantz, Chem. Commun., 1992, 1672.
11. (a) F. De Jong and M. J. Janssen, J. Org. Chem., 1971, 36, 1645; (b) F. Allared, J. Hellberg and T. Remonen, Tetrahedron Lett., 2002, 43, 1553; (c) J. Frey, A. D. Bond and A. B. Holmes, Chem. Commun., 2002, 2424; (d) M. He and F. Zhang, J. Org. Chem., 2007, 72, 442.
12. (a) K. H. Kim, Z. Chi, M. J. Cho, J. I. Jin, M. Y. Cho, S. J. Kim, J. S. Joo and D. H. Choi, Chem. Mater., 2007, 19, 4925; (b) Y. Sun, Y. Ma, Y. Liu, Y. Lin, Z. Wang, Y. Wang, C. Di, K. Xiao, X. Chen, W. Qiu, B. Zhang, G. Yu, W. Hu and D. Zhu, Adv. Funct. Mater., 2006, 16, 426; (c) X. C. Li, H. Sirringhaus, F. Garnier, A. B. Holmes, S. C. Moratti, N. Feeder, W. Clegg, S. J. Teat and R. H. Friend, J. Am. Chem. Soc., 1998, 120, 2206.
13. (a) X. Zhang, J. P. Johnson, J. W. Kampf and A. J. Matzger, Chem. Mater., 2006, 18, 3470; (b) J. L. Bredas, J. P. Calbert, D. A. da Silva and J. Cornil, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 5804.
14. (a) M. He, J. Li, M. L. Sorensen, F. Zhang, R. R. Hancock, H. H. Fong, V. A. Pozdin, D. M. Smilgies and G. G. Malliaras, J. Am. Chem. Soc., 2009, 131, 11930; (b) H. H. Fong, V. A. Pozdin, A. Amassian, G. G. Malliaras, D. M. Smilgies, M. He, S. Gasper, F. Zhang and M. Sorensen, J. Am. Chem. Soc., 2008, 130, 13202; (c) J. H. Gao, R. J. Li, L. Q. Li, Q. Meng, H. Jiang, H. X. Li and W. P. Hu, Adv. Mater., 2007, 19, 3008; (d) J. G. Laquindanum, H. E. Katz, A. J. Lovinger and A. Dodabalapur, Adv. Mater., 1997, 9, 36; (e) H. Sirringhaus, R. H. Friend, C. Wang, J. Leuninger and K. Mullen, J. Mater. Chem., 1999, 9, 2095; (f) M. D. Iosip, S. Destri, M. Pasini, W. Porzio, K. P. Pernstich and B. Batlogg, Synth. Met., 2004, 146, 251.
15. (a) X. Zhan, Z. Tan, E. Zhou, R. Mishra, A. Grant, B. Domercq, Z. An, X. H. Zhang, X. Zhang, S. Barlow, Y. Li, B. Kippelen and S. R. Marder, J. Mater. Chem., 2009, 19, 5749; (b) X. Huang, C. Zhu, W. Li, Y. Guo, X. Zhan, Y. Liu and Z. Bo, Macromolecules, 2008, 41, 6895; (c) S. Zhang, C. He, Y. Liu, X. Zhang and J. Chen, Polymer, 2009, 50, 3595; (d) X. Zhan, Z. Tan, B. Domercq, Z. An, X. Zhang, S. Barlow, Y. Li, D. Zhu, B. Kippelen and S. R. Marder, J. Am. Chem. Soc., 2007, 129, 7246; (e) S. Zhang, H. Fan, Y. Liu, G. Zhao, Q. Li, Y. Li and X. Zhan, J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 2843.
16. A. V. Patil, W.-H. Lee, E. Lee, K. Kim, I. N. Kang and S.-H. Lee, Macromolecules, 2011, 44, 1238.
17. C. Shi, Y. Yao, Y. Yang and Q. Pei, J. Am. Chem. Soc., 2006, 128, 8980.
18. (a) Y. Zou, D. Gendron, R. N. Plesu and M. Leclerc, Macromolecules, 2009, 42, 2891; (b) C. H. Woo, P. M. Beaujuge, T. W. Holcombe, O. P. Lee and J. M. J. Fréchet, J. Am. Chem. Soc., 2010, 132, 15547; (c) A. P. Zoombelt, M. Fonrodona, M. G. R. Turbiez, M. M. Wienk and R. A. J. Janssen, J. Mater. Chem., 2009, 19, 5336.
19. A. J. Bard and L. R. Faulkner, Electrochemical Methods: Fundamentals and Applications, Wiley, New York, 2nd edn, 2001.
20. (a) C. J. Brabec, S. E. Shaheen, C. Winder, N. S. Sariciftci and P. Denk, Appl. Phys. Lett., 2002, 80, 1288; (b) H. Hoppe, M. Niggemamn, C. Winder, J. Kraut, R. Hiesgen, A. Hinsch, D. Meissner and N. S. Sariciftci, Adv. Funct. Mater., 2004, 14, 1005; (c) C. Melzer, E. J. Koop, V. D. Mihailetchi and P. W. M. Blom, Adv. Funct. Mater., 2004, 14, 865; (d) Y. Yang, J. Zhang, Y. Zhou, G. Zhao, C. He, Y. Li, M. Andersson, O. Inganas and F. Zhang, J. Phys. Chem. C, 2010, 114, 3701.

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR data of monomers and polymers, thermogravimetric analysis of polymers in nitrogen atmosphere. See DOI: 10.1039/c1py00274k

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