Through manipulation of the solubilizing side chains, we were able to dramatically improve the molecular weight
Over the past few years considerable attention has been drawn to the field of organic photovoltaics (OPVs) [
These devices employ a bulk heterojunction (BHJ) architecture consisting of an active (absorbing) layer, comprised of a phase-separated network of a polymer donor (p-type) phase and a fullerene acceptor (n-type) phase sandwiched between an anode and cathode. Ideally, the domain sizes within the active layer are less than 10 nm, ensuring that excitons generated within the bulk of a phase are able to diffuse to the donor-acceptor interface, where charge separation occurs. It is also necessary that the network is bicontinuous to ensure that the separated charges have unobstructed paths to their respective electrodes. Improving the active layer, both its components (donor and acceptor) and its morphology are the subject of the majority of OPV research and accounts for the recent upsurge in PCEs. Contributing to these improvements are (1) new processing methods that allow for more ideal phase separation and ordering within the absorbing layer [
Hundreds of low bandgap polymers have now been reported for BHJ solar cells [
With this in mind, we were interested in improving the molecular weight of a copolymer (
In this paper, we detail the syntheses, characterization, and photovoltaic properties of copolymers comprised of 4,8-dialkoxybenzo[1,2-b:4,5-b′]dithiophene (BDT) and 2,1,3-benzothiadiazole, where dodecoxy (
Side chain variation in 4,8-dialkoxybenzo[1,2-b:4,5-b′]dithiophene (BDT) 2,1,3-benzothiadiazole copolymers.
3-Thiophenecarboxylic acid (
Flash chromatography was performed on a Biotage Isolera Flash Purification System using Biotage SNAP Flash Purification Cartridges as the stationary phase. Microwave-assisted polymerizations were carried out using a CEM Discover Microwave reactor. 1H and 13C NMR spectra were recorded on a Bruker Avance DPX-300 NMR spectrometer. 119Sn NMR spectra were recorded on a Bruker Avance DRX-500 instrument. UV-vis absorption spectra were recorded on an Agilent 8453 diode-array spectrophotometer operating over a range of 190–1100 nm. GC-MS were recorded on an Agilent 6850 Series GC system coupled to an Agilent 5973 mass selective detector run in electron impact mode. Infrared spectra were recorded over the 450–4000 cm−1 region using on a Perkin Elmer Spectrum 100 spectrophotometer with an ATR sampling accessory equipped with a diamond anvil. Gel permeation chromatography (135°C in 1,2,4-trichlorobenzene) was performed by American Polymer Standards (Ohio).
3-Thiophenecarboxylic acid (20.0 g, 0.156 mol) (
3-Thiophenecar-bonyl chloride (13.2 g, 0.09 mol) was dissolved in methylene chloride (45 mL) and added dropwise at 0°C to a solution of diethylamine (25 mL) in methylene chloride (25 mL). The solution was stirred at 0°C for 2 hours and then at RT overnight. Diethylamine (10 mL) was added and the mixture was stirred for another 2 hours. The diethylamine hydrochloride precipitate was filtered off and the organic phase was extracted with water. The solution was dried over magnesium sulfate and evaporated to give a brown oil (14.1 g, 85%). Characterization of the product was consistent with the published data.
Compound
Compound
Compounds
Under ambient atmosphere in a 5 mL microwave tube equipped with a stir bar was added the bis(trimethyltin) monomer (0.5 mmol) along with the dibromo monomer (0.485 mmol) and 2 mL of chlorobenzene. The mixture was stirred for 5 minutes and
(Yield from CHCl3 extract, 5–30%). FT-IR: 2917 (s), 2849 (s), 1577 (m), 1524 (m), 1489 (m), 1450 (s), 1400 (m), 1354 (s), 1273(m), 1177 (s), 1038 (br), 908 (m), 854 (m), 816 (s), 750 (w), 718 (m), 689 (m). GPC (TCB, 135°C):
(Yield from CHCl3 extract, 5–20%). FT-IR: 2951 (m), 2916 (s), 2850 (s), 1573 (m), 1523 (m), 1487 (m), 1447 (s), 1397 (m), 1351 (s), 1257 (w), 1175 (s), 1030 (br), 906 (m), 852 (w), 817 (s), 715 (w), 688 (w). GPC (TCB, 135°C):
(Yield from CHCl3 extract, 40–56%.) FT-IR: 2951 (m), 2916 (s), 2852 (s), 1575 (m), 1523 (m), 1486 (m), 1448 (s), 1396 (m), 1337 (s), 1257 (w), 1178 (s), 1019 (br), 906 (m), 853 (w), 818 (s), 715 (w), 688 (w). GPC (TCB, 135°C):
Cyclic voltammetry (CV) was carried out using a computer-controlled Pine Model AFCBP 1 Bi-Potentiostat with PineChem software in a standard single-compartment, three electrode cell. The working electrode was glassy carbon, while the counter electrode was a platinum wire. The pseudoreference electrode was a silver wire and was calibrated against (Fc/Fc+). The polymer was drop-cast onto glassy carbon from a 2.5 mg/mL solution in chlorobenzene. All measurements were carried out in degassed solutions of acetonitrile with tetrabutylammonium hexafluorophosphate (0.1 M, electrochemical grade) as the supporting electrolyte. The scan rate used was 100 mVs−1. The electrochemical onsets were determined as the position at which the current differed by 2
X-Ray powder diffraction data were collected at room temperature using a BRUKER P4 general-purpose four-circle X-ray diffractometer modified with a GADDS/Hi-Star detector at 40 kv and 30 mA for Cu K
Bottom-contact thin-film transistors (TFTs) were fabricated by spin-coating (1000 rpm) polymer solutions (0.75 wt% in 1,2-dichlorobenzene) over the 1.2 cm × 1.2 cm substrate with prefabricated device structures containing a Si gate electrode, 300 nm SiO2 gate dielectric and 5 nm Ti/45 nm Au source-drain contacts with channel length
The transistors were characterized by measuring the drain current (
ITO-coated glass slides (Delta Technologies) were cleaned by sonication in acetone and isopropanol for 20 minutes each, followed by ozone cleaning for 30 minutes. A solution of PEDOT : PSS (Clevios P—H.C. Starck) at 1 : 1 PEDOT : PSS solution to deionized water was spin-coated onto the slides at 4000 RPM (resulting in a 40 nm layer). The films were then baked in a vacuum oven at 100°C for 30 minutes. Polymer solutions were then spin-coated onto the PEDOT : PSS-coated substrates. Solutions were prepared by mixing polymer and 1,2-dichlorobenzene (8 mg/mL for
The films were set to dry for 30 minutes and then placed under vacuum for thermal deposition of electrodes (2 nm LiF/100 nm aluminum). The active area of the device was 38 mm2. All film thicknesses were determined via a JEOL JSPM-5200 operating in AFM tapping mode.
The current-voltage (
The synthetic routes to the monomers and polymers are shown in Scheme
Synthesis of 4,8-dialkoxybenzo[1,2-b:4,5-b′]dithiophene (BDT) 2,1,3-benzothiadiazole copolymers.
The polymers
Molecular weight of the polymers.
4.8 | 6.3 | 1.33 | |
2.9 | 3.4 | 1.19 | |
27.1 | 68.8 | 2.54 |
Thermogravimetric analysis (TGA) was performed on polymers
TGA curves of
UV-visible absorption spectra (normalized by area) for polymers
UV-Vis spectra of
Cyclic voltammetry measurements were made on polymer films of
Summary of optical and electrochemical properties.
598 | 1.69 | 0.15 | −4.95 | −1.72 | −3.08 | 1.87 | |
579 | — | 0.13 | −4.93 | −1.62 | −3.18 | 1.75 | |
637 | 1.69 | 0.12 | −4.92 | −1.59 | −3.21 | 1.71 |
Cyclic voltammograms of
To give insight into the internal packing structure of the polymers, powder X-ray diffraction (PXRD) was used. Samples were mounted on a loop and two frames were measured at
(a) Integrated powder diffraction patterns for
The peaks at low angle (
Thin-film transistors were fabricated from the three polymers to assess charge transport. Figure
Electrical characteristics of
The photovoltaic characteristics of the polymers were determined using a standard bulk heterojunction device architecture—ITO/PEDOT : PSS/
Photovoltaic characteristics of best devices prepared from
FF | PCE (%) | |||
---|---|---|---|---|
0.69 | 1.22 | 0.42 | 0.33 | |
0.71 | 0.81 | 0.33 | 0.19 | |
0.73 | 8.93 | 0.46 | 2.99 |
Current-voltage characteristics of bulk heterojunction solar cells based on
Consistent with the HOMO levels determined by cyclic voltammetry, the open circuit voltages (
The EQE spectra for the
External quantum efficiency spectra of bulk heterojunction solar cells based on
We prepared and characterized two new low bandgap copolymers based on BDT and BT that employ branched side chains at the 4 and 8 positions, poly[(4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-2,1,3-benzothiadiazole-4,7-diyl] (
This work was supported by grants from FiberCell Inc. and the Department of Energy (DOE DE-FG02-07ER46428).