High-mobility low-bandgap conjugated copolymers based on
indacenodithiophene and thiadiazolo[3,4-
c]pyridine units for thin film
transistor and photovoltaic applications
Ying Sun,
abShang-Chieh Chien,
acHin-Lap Yip,
aYong Zhang,
aKung-Shih Chen,
aDavid F. Zeigler,
dFang-Chung Chen,
cBaoping Lin*
band Alex K.-Y. Jen*
aReceived 12th April 2011, Accepted 30th June 2011 DOI: 10.1039/c1jm11564b
Two new semiconducting polymers based on indacenodithiophene and thiadiazolo[3,4-c]pyridine units were synthesized via Stille coupling polymerization. The polymers, PIDTPyT and PIDTDTPyT, exhibited main absorption bands in the range of 550–800 nm while their absorption maxima were located at around 700 nm in films. With two additional thiophene spacers, PIDTDTPyT showed a broader absorption band but a 20 nm blue-shifted maximum peak compared to that of PIDTPyT. Both of the polymers possess low bandgaps (1.6 eV) and deep energy levels for both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Organic field-effect transistors (OFETs) device measurements indicate that PIDTPyT and PIDTDTPyT have high hole carrier mobilities of 0.066 and 0.045 cm2V1s1, respectively, with the on/off ratio on the order of 106. Bulk heterojunction photovoltaic devices consisting of the copolymers and PC
71BM gave
power conversion efficiencies (PCE) as high as 3.91% with broadband photo-response in the range of 300–800 nm. The relationships between the photovoltaic performance and film morphology, energy levels, hole mobilities are discussed.
1.
Introduction
Polymer solar cells (PSCs) are considered as one of the most promising alternative to conventional solar cells due to their unique advantages of low cost, light weight, flexibility and processability in large-area substrates.1–4So far, the bulk heter-ojunction (BHJ) structure, which comprises a blend of an elec-tron-rich polymer as donor and an electron-deficient fullerene derivative as acceptor, is the most efficient architecture for PSCs.5–8In the past decade, a remarkable amount of efforts to improve the power conversion efficiency (PCE) of PSCs has been devoted to the development of novel conjugated donor poly-mers.9–16It has been demonstrated that the ideal p-type polymer in the BHJ layer should simultaneously possess a strong absorption overlap with the solar spectrum, suitable HOMO– LUMO energy levels to facilitate efficient exciton dissociation in the BHJ while maintaining high open-circuit voltage (Voc), good
film morphology and high hole mobility.2,17–19 Up to now, significant progress has been made for BHJ polymer solar cells with a PCE as high as 7–8%.7,8
Recently, indacenodithiophene (IDT) has emerged as a prom-ising donor unit for constructing donor–acceptor (D–A) alter-nating conjugated copolymers for PSCs.20–25 The structure is characterized as two thiophene rings rigidified together with a central phenyl ring, which can provide strong intermolecular interactions for ordered packing to improve the charge carrier mobility.21Previously, a series of random copolymers based on indacenodithiophene, benzothiadiazole (BT) and thiophene units were synthesized.24 These polymers showed broad absorption bands, reasonable field-effect hole mobilities (104to 103cm2 V1s1) and promising OPV performance. Recently, Ting et al.23 and Chen et al.22have reported that an alternating copolymer based on IDT and BT units exhibits a high hole mobility with the PCE over 6%, which is higher than the analogous random copolymers. The PCE of IDT-based polymers can be potentially improved by further deepening the HOMO level to increase the Voc and narrowing the bandgap to enhance light absorption.
However, the trade-off between the bandgap and the HOMO– LUMO relationships of the polymer donor and fullerene acceptor makes it difficult to simultaneously optimize the energy levels and bandgap. So further development of new IDT based polymers is very essential to achieve more understanding of the structure–performance relationships.
aDepartment of Materials Science and Engineering, University of
Washington, Seattle, Washington, 98195, USA. E-mail: ajen@ u.washington.edu; Fax: +1 206 543 3100; Tel: +1 206 543 2626
bSchool of Chemistry and Chemical Engineering, Southeast University,
Jiangning District, Nanjing, Jiangsu Province, P.R. China 211189. E-mail: [email protected]; Fax: +86-25-52090616; Tel: +86-25-52090616
cDepartment of Photonics & Display Institute, National Chiao Tung
University, Hsinchu, 30010, Taiwan
dDepartment of Chemistry, University of Washington, P.O. Box 351700,
Seattle, Washington, 98195, USA
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 13247
www.rsc.org/materials
PAPER
Thiadiazolo[3,4-c]pyridine (PyT), which is expected to be a stronger electron acceptor compared to 2,1,3-benzothiadiazole due to the more electron-deficient property of the pyridine, has been less explored in D–A copolymers. Blouin et al.26 incorpo-rated thienyl-flanked PyT with the carbazole unit to yield a low molecular weight copolymer showing limited mobility and effi-ciency. They demonstrated that the asymmetric structure of the PyT unit may diminish the structural organization of the poly-mer and consequently decreased the device performance. Recently, a series of polymers copolymerized with various donor moieties and an alkylated thienyl-flanked PyT unit were reported by Zhou et al.27 Encouragingly, these polymers showed good solubility, reduced bandgaps, deeper HOMO–LUMO levels and better performance than their corresponding 1,4-dithien-2-yl-2,1,3-benzothiadiazole (DTBT) based polymers. In spite of the achieved high efficiency based on PyT units, the relationships between the structure, deep energy levels, charge mobility, and device performance still need to be further explored.
To understand these relationships, two new D–A copolymers (PIDTPyT and PIDTDTPyT) were synthesized based on PyT and thienyl-flanked PyT acceptor moieties copolymerized with the IDT donor monomer. After introducing the more electron-deficient PyT unit, both PIDTPyT and PIDTDTPyT showed reduced bandgaps (1.60 eV for PIDTPyT and 1.62 eV for PIDTDTPyT) and deep HOMO energy levels compared to their BT conterparts.22OFET measurements showed that the poly-mers had excellent charge transporting properties with hole mobilities as high as 0.066 cm2V1s1and an on/off ratio of 106. BHJ solar cells based on the blend of the polymers and PC71BM
showed the PCEs up to 3.41% and 3.91% for PIDTPyT and PIDTDTPyT, respectively. The main limitation on device performance was the fill factor (FF) though both devices exhibited superior broadband photo-response spectra in the range of 300–800 nm. The small energy offset between the LUMO levels of the polymer and PC71BM as well as the inferior
vertical mobility may be the limitation factors for the efficiency.
2.
Experimental section
Materials
All chemicals were purchased from Aldrich and TCI without further purification. Diethyl 1,4-bis(thiophen-2-yl)-2,5-benzene-dicarboxylate was prepared as reported.28Toluene was distilled from sodium and benzophenone ketyl under nitrogen prior to use.
Measurement and characterization
UV-Vis spectra were recorded using a Perkin-Elmer Lambda-9 spectrophotometer. The 1H NMR and13C NMR spectra were collected on a Bruker AV 300 or 500 spectrometer operating at 300 or 125 MHz in deuterated chloroform solution with TMS as reference. ESI-MS spectra were recorded on a Bruker APEX Qe 47e Fourier transform (ion cyclotron resonance) mass spec-trometer. The molecular weight was measured by a Waters 1515 gel permeation chromatograph (GPC) with a refractive index detector at room temperature (THF as the eluent). Cyclic vol-tammetries of polymer films were conducted on a BAS CV-50 W voltammetric system with a three-electrode cell in acetonitrile
with 0.1 M of tetrabutylammonium hexafluoro-phosphate using a scan rate of 100 mV s1. ITO, Ag/AgCl and Pt mesh were used as working electrode, reference electrode and counter electrode, respectively. The differential scanning calorimetry (DSC) was performed using a DSC2010 (TA instruments) under a heating rate of 10C min1and a nitrogen flow of 50 mL min1. The AFM images under tapping mode were taken from the actual devices fabricated for photovoltaic measurement on a Veeco multimode AFM with a Nanoscope III controller.
Synthesis of monomers
Synthesis of compound 1. To a solution of 1-bromo-4-hex-ylbenzene (3.0 g, 12.5 mmol) in THF (25 mL) was added drop-wise n-BuLi (5.5 mL, 2.5 M in hexane, 13.75 mmol) at78C. The resulting solution was stirred for 1 h and then quenched with a solution of diethyl 1,4-bis (thiophen-2-yl)-2,5-benzene-dicar-boxylate (1.0 g, 2.59 mmol) in THF (15 mL). And the reaction was kept at78C for 1 h, slowly warmed to room temperature and stirred overnight. Water was added to the solution and the mixture was extracted with ethyl acetate. The combined organic layers were dried with Na2SO4and then solvent was removed by
rotary evaporation. The resulting white solids were added to acetic acid (30 mL). After the addition of concentrated H2SO4
(1 mL), the reaction was allowed to reflux for 5 h and then quenched with water (150 mL). The mixture was extracted with CH2Cl2and the combined organic layer was washed with water
for 3 times and dried over Na2SO4. Solvent was removed under
reduced pressure and the residue was purified by silica gel chromatography (eluent: hexane/dichloromethane ¼ 10/1) to afford a white solid (1.3 g 67%).1H NMR (CDCl
3, ppm): 7.44 (s, 2H), 7.26 (d, J¼ 4.89 Hz, 2H), 7.15 (d, J ¼ 8.28 Hz, 8H), 7.08 (d, J¼ 8.31 Hz, 8H), 7.02 (d, J ¼ 4.89 Hz, 2H), 2.57 (t, 8H), 1.58 (m, 4H), 1.31 (m, 28H), 0.88 (t, 12H). 13C NMR (CDCl 3, ppm): 156.05, 153.60, 142.27, 141.57, 141.47, 135.30, 128.48, 128.10, 127.59, 123.33, 117.69, 62.85, 35.78, 31.94, 31.57, 29.37, 22.82, 14.32. HRMS (ESI): (M+, C 64H74S2), calcd: 906.5232; found: 906.5201.
Synthesis of compound 2. n-BuLi (1.1 mL, 2.5 M in hexane) was added dropwise to a solution of compound 1 (1 g, 1.1 mmol) in THF (20 mL) at78C. The mixture was kept at78C for 30 min and then warmed to room temperature for another 30 min. After cooling to 78C again, trimethyltin chloride (2.5 mL, 1 M in hexane) was added. The reaction was stirred overnight at room temperature and then quenched with water, extracted with hexane, and dried over Na2SO4. After removal of the solvent,
ethanol was added to the mixture and the precipitate was collected as a white solid (1.1 g, 82%).1H NMR (CDCl
3, ppm): 7.40 (s, 2H), 7.15 (d, J¼ 8.04 Hz, 8H), 7.07 (d, J ¼ 8.28 Hz, 8H), 7.03 (s, 2H), 2.56 (t, 8H), 1.58 (m, 4H), 1.31 (m, 28H), 0.89 (t, 12H), 0.35 (s, 18H).13C NMR (CDCl 3, ppm): 157.72, 153.77, 147.55, 142.66, 141.35, 141.15, 134.93, 130.81, 128.39, 128.20, 117.96, 62.33, 35.79, 31.95, 31.55, 29.39, 22.82, 14.32, 7.82. HRMS (ESI): (M+, C 70H90S2Sn2), calcd: 1234.4528; found: 1234.4480.
Synthesis of compound 3. To a solution of 3,4-diaminopyridine (3.93 g, 36 mmol) in 50 mL 48% aqueous hydrobromic acid was
added slowly 6 mL bromine and the mixture subsequently was allowed to reflux overnight. The mixture was cooled down to room temperature and then filtered. The resultant precipitation was washed sequentially with aq. Na2CO3, aq. Na2S2O3 and
water. The crude product was further refluxed in a 10% solution of Na2CO3for 1 h and then obtained by filtration. Purification by
silica gel chromatography (eluent: dichloromethane/ethyl acetate¼ 10/1) gave 3 (3.5 g, 37%) as a yellow solid.1H NMR (CDCl3, ppm): 7.89 (s, 1H), 4.49 (s, 2H), 3.68 (s, 2H).
Synthesis of compound 4. To a stirred, cooled solution of 3,4-diamino-2,5-dibromopyridine (1 g, 3.78 mmol) in pyridine (12 mL) at 0C was added dropwise SOCl2(0.4 mL). Stirring was continued
for an additional 2 h and then the mixture was poured into water. The crude product was collected by filtration and washed with water. Further purification by silica column chromatography (eluent: hexane/dichloromethane¼ 4/1) afforded 4 as a pale yellow solid (0.62 g, 56%).1H NMR (CDCl
3, ppm): 8.57 (s, 1H).13C NMR
(CDCl3, ppm): 155.20, 150.22, 145.23, 136.65, 111.68. HRMS
(ESI): (M+, C
5HN3SBr2) calcd: 292.8258; found: 292.8341.
Synthesis of compound 5. A mixture of compound 3 (0.292 g, 1 mmol) and 2-(tributylstannyl)thiophene (0.970 g, 2.6 mmol) in THF (15 mL) was degassed several times over 1 h. Pd2(dba)3
(37 mg) and P(o-tol)3 (73 mg) were added and the resultant
mixture was heated at reflux for 15 h and then allowed to cool. Water was added to the mixture and the aqueous phase was extracted with dichloromethane for three times. The combined organic extracts were dried over Na2SO4, filtered and the solvent
was removed under vacuum. The crude was further purified with silica column chromatography (eluent: hexane/dichlorome-thane¼ 2/1) to give compound 5 as a red solid (0.241 g, 80%).1H NMR (CDCl3, ppm): 8.88 (s, 1H), 8.74 (dd, J1¼ 3.81 Hz, J2¼ 1.12 Hz, 1H), 8.14 (dd, J1¼ 3.66 Hz, J2¼ 1.1 Hz, 1H), 7.64 (dd, J1¼ 4.80 Hz, J2¼ 1.1 Hz, 1H), 7.52 (dd, J1¼ 5.10 Hz, J2¼ 1.1 Hz, 1H), 7.30 (dd, J1¼ 5.01 Hz, J2¼ 3.8Hz, 1H), 7.27 (dd, J1¼ 4.80 Hz, J2 ¼ 3.7 Hz, 1H).13C NMR (CDCl3, ppm): 154.94, 148.03, 146.48, 141.79, 140.80, 136.55, 131.89, 130.54, 128.89, 128.15, 127.87, 127.25, 120.50. HRMS (ESI): (M+, C 13H7N3S3) calcd: 300.9802; found: 300.9875.
Synthesis of compound 6. To a solution of compound 4 (0.2 g, 0.664 mmol) in 5 mL chloroform was added the n-bromosucci-nimide (0.26 g, 1.462 mmol) and the mixture was stirred in the dark for 24 h. The reaction was quenched with water and the water layer was extracted with chloroform. The combined organic layer was dried over Na2SO4 and the solvent was
removed under vacuum. Purification by the silica column chro-matography (eluent: hexane/dichloromethane ¼ 2/1) gave the compound 6 as a dark red solid (0.27 g, 86%).1H NMR (CDCl
3, ppm): 8.73 (s, 1H), 8.43 (d, J¼ 4.05 Hz, 1H), 7.83 (d, J ¼ 3.99 Hz, 1H), 7.22 (d, J¼ 4.08 Hz, 1H), 7.18 (d, J ¼ 3.99 Hz, 1H). HRMS (ESI): (M+, C 13H5Br2N3S3) calcd: 456.8012; found: 456.8095. Synthesis of polymers
Synthesis of polymer PIDTPYT. Compounds 2 (123.4 mg, 0.1 mmol) and 4 (29.3 mg, 0.1 mmol) were charged in a 25 mL
flask under N2protection. Toluene (4 mL) and DMF (0.4 mL)
were added and the mixture was degassed following the quick addition of Pd2(dba)3(4 mg) and P(o-tol)3(10 mg). The mixture
was heated at reflux for 2 days, cooled to RT and then poured into methanol. The precipitate was collected by filtration and washed by Soxhlet extraction with acetone and hexane for 12 h respectively. Then the resulting solid was dissolved in chloroform and the solution was poured into hexane to afford the polymer PIDTPYT (85 mg, 74%).1H NMR (500 MHz, CDCl
3, d) 8.80
(br, 1H), 8.63 (br, 1H), 8.10 (br, 1H), 7.57 (dd, 2H), 7.12 (m, 16H), 2.58 (br, 8H), 1.57 (m, 4H), 1.29 (m, 28H), 0.88 (m, 30H). Elemental analysis: calcd C, 79.57; H, 7.16; N, 4.03; found C, 78.81; H, 6.81; N, 3.73%. Mn ¼ 115 kDa, Mw ¼ 267 kDa,
PDI¼ 2.32.
Synthesis of polymer PIDTPYT. Compounds 2 (123.4 mg, 0.1 mmol) and 6 (45.7 mg, 0.1 mmol) were charged in a 25 mL flask under N2protection. Toluene (5 mL) and DMF (0.5 mL) were
added and the mixture was degassed following the quick addition of Pd2(dba)3(4 mg) and P(o-tol)3(10 mg). The mixture was heated
at reflux for 2 days, cooled to RT and then poured into methanol. The precipitate was collected by filtration and washed by Soxhlet extraction with acetone and hexane for 12 hours respectively. Then the resulting solid was dissolved in chloroform and the solution was poured into hexane to afford the polymer PIDTDTPYT (67 mg, 56%).1H NMR (500 MHz, CDCl
3, d) 8.79 (br, 1H), 7.72
(m, 4H), 7.55 (m, 4H), 7.23 (dd, 8H), 7.14 (dd, 8H), 2.60 (br, 8H), 1.70 (m, 4H), 1.37 (m, 28H), 0.92 (m, 30H). Elemental analysis: calcd C, 76.70; H, 6.52; N, 3.48; found C, 75.45; H, 6.16; N, 3.44%. Mn¼ 32 kDa, Mw¼ 87 kDa, PDI ¼ 2.73.
Fabrication of the OFET device. The Field-effect transistors were fabricated with a bottom-gate, top-contact configuration. The heavily n-doped silicon substrates with a 300 nm thick thermally grown SiO2dielectric (from Montco Silicon
Technol-ogies, Inc.) were first cleaned by sonicating in acetone and iso-propanol and then exposed to air plasma. The cleaned substrates were then treated with hexamethyldisilazane (HMDS) through vapor phase deposition in a vacuum oven (200 mTorr, 100C, 3 h). Subsequently, the semiconductor polymer films were spin-coated in a glove box from their 10 mg mL1chloroform solu-tions which were stirred overnight and filtered with a 0.2 mm PTFE filter. Interdigitated source and drain electrodes (L¼ 1000 mm, W ¼ 12 mm) were deposited by evaporating a 50 nm thick gold film and defined with a shadow mask. The transfer and output characteristics were measured in a glove box using an Agilent 4155B semiconductor parameter analyzer. The satura-tion field-effect mobility (m) was calculated from the following equation:
Ids¼ m(W/2L)Ci(Vgs Vth)2
where W and L are the channel width and length, respectively. Ci
is the capacitance of the insulating SiO2layer per unit area, Vgs
and Vthare the gate voltage and the threshold voltage,
respec-tively. The threshold voltage (Vth) was obtained as the x-axis
intercept of the linear section of the plot of (Ids)1/2vs. Vgs. The
subthreshold swing was estimated by taking the inverse of the slope of Idsvs. Vgsin the region of exponential current increase.
Fabrication of the photovoltaic and hole only device. To fabri-cate conventional configuration solar cells, ITO-coated glass substrates (15 U sq1.) were first cleaned with detergent, de-ionized water, acetone, and isopropyl alcohol. Subsequently, a PEDOT:PSS (Baytron PVP AI 4083, filtered at 0.45 mm) layer (45 nm) was spin-coated onto the cleaned ITO-coated glass substrates at 5000 rpm and then annealed at 120C for 30 min under ambient conditions. After that, the substrates were loaded into a nitrogen-filled glove-box. Following that, the active layer was spin-coated onto the PEDOT:PSS layer from a homo-geneous blending solution of polymer:PC71BM. The solution
was prepared by dissolving the polymer and PC71BM with
a certain blending weight ratio in o-dichlorobenzene (o-DCB) overnight and filtered with a 0.2 mm PTFE filter. Finally, the substrates were transferred into the evaporator with shadow masks to define the active area of the devices (10.08 mm2) and pumped under high vacuum (<2 107Torr). Then calcium (30 nm) and aluminium (100 nm) were thermally evaporated onto the active layer sequentially. The un-encapsulated solar cells were measured in glove box conditions using a Keithley 2400 SMU source measurement unit and an Oriel Xenon lamp (450 W) with an AM1.5 filter as the solar simulator. A reference silicon solar cell with a KG5 filter, which has been previously standardized by the National Renewable Energy Laboratory, was used to calibrate the light intensity to 100 mW cm2. To fabricate the hole only device, the same procedure as that used for the photovoltaic device was followed except that MoO3was
used to replace the calcium.
3.
Results and discussion
Synthesis partGeneral synthetic routes of the monomers are described in Scheme 1. The synthesis of compound 1 involved the double addition of the 1-hexylbenzene lithium to the di-ester compound to give the corresponding alcohol, which subsequently under-went an acid-mediated intramolecular cyclization reaction.28 Lithiation of the compound 1 followed by quenching with tri-methyltin chloride afforded the functionalized IDT based monomer 2 in a good yield (82%). Compound 3 was synthesized according to previously reported procedures.26The conversion of the compound 3 to the target monomer 4 was conducted through the ring closure reaction with thionyl chloride. To avoid the
undesired product with the bromine atom at the 4-position replaced by chlorine atom, pyridine was employed to dissolve the amine compound and absorb the hydrochloric acid. The amount of thionyl chloride was controlled to be around 1.4–1.5 eq. and the reaction was carried out at low temperature. Under these conditions, the pure dibromo functionalized monomer 4 was obtained with a reasonable yield (56%). The purity of the monomer was confirmed by GC-MS and NMR. The Stille coupling reaction between 4 and tributyl (2-thienyl) stannane yielded compound 5 and subsequent bromination by n-bromo-succinimide gave the target monomer 6, which was carefully purified using silica column chromatography.
Scheme 2 depicted the synthetic methods for the polymers. It is worth noting that the polymer structure is regiorandom due to the asymmetric PyT unit. Copolymers PIDTPyT and PIDTDTPyT were then synthesized by Stille cross-coupling reaction of the dibromo monomers (4 and 6) with the bis-stannyl compound 2, using tris(dibenzylidene-acetone)dipalladium(0) (Pd2dba3) as catalyst and tri(o-tolyl) phosphine (P(o-tol)3) as the
corresponding ligand in toluene/DMF solution. The polymeri-zation was taken at 110C under nitrogen atmosphere for 48 h. The resulting polymers were collected by directly precipitating the solution in methanol followed by filtration. After Soxhlet extraction with acetone and hexane, the final polymers were further purified by dissolving in CHCl3and then precipitating in
hexane. The two polymers have good solubilities in chlorinated solvents such as chloroform, chlorobenzene and dichloroben-zene. The molecular weights of the two polymers were measured by GPC with polystyrene as standard and THF as eluent. The number-average molecular weight (Mn) of PIDTPyT and
PIDTDTPyT is 111.0 kDa and 37.7 kDa, with polydispersity indices (PDI) of 2. 32 and 2.73 (listed in Table 1) respectively. The thermal properties were investigated with differential scan-ning calorimetry (DSC) but no thermal transition signal was observed in the range from 20 to 350C for both of the polymers. Optical and electrochemical properties
The UV-vis absorption spectra of the two polymers in CHCl3
solutions and films are shown in Fig. 1 and the summarized data are listed in Table 1. Two obvious absorption bands can be observed for the two polymers both in films and solutions. The shorter wavelength absorbance can be assigned to the delocalized p–p* transition in the polymer chains while the absorption band
Scheme 1 Synthetic routes for monomers.
Scheme 2 Synthetic routes for polymers (regiorandom polymer structure).
in the longer wavelength has been attributed to intramolecular charge transfer (ICT) from the electron-rich IDT units to the electron-deficient PyT functionalities.2
The absorption maxima in chloroform solution were observed at 707 and 650 nm for PIDTPyT and PIDTDTPyT, respectively. In the solid state, PIDTPyT showed an absorption spectrum similar to that obtained in the solution with only slightly broader absorption range and a red-shifted (7 nm) absorption onset. Considering the high molecular weight and the rigid backbone of the polymer, the absence of red-shift in the absorption maxima may originate from the aggregation of the polymer chains formed in solution.29The strong internal charge transfer between IDT and PyT would facilitate the polymer backbone to adopt a more planar structure, thereby enhancing the stacking and aggregation of polymer even in solution. The absorption maxima and the onset of PIDTDTPyT were red-shifted from the solution to the solid state with a pronounced shoulder at approximately 680 nm. This indicates a higher structural organization and an ordered packing which may be related to stronger aggregation induced by intermolecular interactions in the solid state.30
According to many results published in the literature,16,31,32the bandgap of the polymer decreased when extra thiophene units were introduced between the donor and the acceptor due to the more extended and delocalizedp-electron system. It is interesting that PIDTDTPyT exhibits a broader absorption range but with blue-shifted absorption maxima and the onset both in solution and in film compared to PIDTPyT. The increased conjugation length can be indicated by the red-shifted short-wavelength absorption band of polymer PIDTDTPyT toward PIDTPyT. However, the magnitude of the red shift in the longer-wavelength absorption band induced by increasing the monomer conjuga-tion length is less pronounced. When more donor units are
introduced to the backbone, the concentration of the acceptor units decreases and consequently the accessible ICT decreases to the point where most energy transitions are localized on the donor segments.33 Therefore, by introducing one thiophene spacer between the donor and the acceptor, the ICT character-istic may be diminished in PIDTDTPyT. Another possible reason may be the less rigid and planar polymer backbone of PIDTDTPyT due to the twist of the polymer chain introduced by the thiophene units, causing reduced conjugation.34
Evaluated from the onset of the UV-vis absorption, the bandg-aps of the polymers PIDTPyT and PIDTDTPyT were determined to be 1.60 and 1.62 eV, respectively. Compared to the polymer copolymerized with IDT and BT units,22 PIDTPyT exhibited a bathochromic shift (60 nm) in the absorption maximum and a reduced optical bandgap (0.14 eV), which also indicated that the stronger electron-accepting ability of PyT induces more efficient ICT. These low bandgap polymers were expected to give more light absorption and broader photo-response.
Cyclic voltammetry (CV) was employed to evaluate the HOMO and LUMO levels of polymers which were calculated using the following equation:
HOMO¼ [Eox+ 4.80] eV
LOMO¼ [Ered+ 4.80] eV
where Eoxand Eredare the onset of the oxidation and reduction
potentials, respectively. The CV curves of polymers are shown in Fig. 2 and the detailed results are listed in Table 1. The HOMO levels of two polymers were found to be5.40 and 5.32 eV for PIDTPyT and PIDTDTPyT, respectively. The deep HOMO levels may contribute to improve oxidative stability and higher open-circuit voltage (Voc).35
The LUMO levels measured by CV were determined to be 3.51 and 3.38 eV for PIDTPyT and PIDTDTPyT, respec-tively. However, calculated from the HOMO level and the optical bandgap of the polymer films (lonsetof the absorption spectra),
the LUMO energy levels were estimated to be 3.80 and 3.70 eV for PIDTPyT and PIDTDTPyT, respectively. The close energetic proximity of the polymer and the LUMO level of PC61BM and PC71BM may diminish the efficient charge
sepa-ration and lead to increased charge recombination.36
Table 1 Physical, optical and electrochemical properties of polymers
Polymer Mn/kg mol1a PDIa UV-vis absorption Cyclic voltammetry Bandgap/ eV Mobility/
cm2V1s1 Thresholdvoltage On/offratio
In CHCl3 In film lmax/ nm lonset/ nm lmax/ nm lonset/ nm HOMO/ eV LOMO/eV PIDTPyT 111.0 2.32 707 765 707 775 5.40 3.80b/– 3.51c 1.60 d /1.89c 0.066e/ 0.062f 18.9 e /–14.9f 106 PIDTDTPyT 37.7 2.73 650 745 680 765 5.32 3.70b/– 3.38c 1.62 d /1.94c 0.037e/ 0.045f 14.5 e /–12.4f 106
aDetermined from GPC against the polystyrene standard using THF as eluent.bCalculated from the optical bandgap and HOMO level.cMeasured
from the cyclic voltammetry.dOptical bandgap.eAs cast.fAnnealing at 110C.
Fig. 1 UV-vis absorption spectra of polymers PIDTPyT and PIDTDTPyT in chloroform solution (a) and in film (b).
To further understand the geometric and electrical properties, density functional theory (DFT) theoretical calculations were performed at the B3LYP/6-31G (d) level by modeling the trimer compounds. Particularly, the hexyl-phenyl side chains were replaced with ethyl–phenyl groups for computational simplicity considering that side-chain substituents have minimal effects on the oxidative and reductive properties of the polymers. The optimized molecular geometries and HOMO and LUMO wave functions are depicted in Fig. 3. For both the polymers, the HOMO wavefunctions are delocalized over the whole conjugated polymer backbone, which indicates that the HOMO levels are determined by both donor and acceptor units. However, the LUMO wave functions for PIDTPyT are primarily localized on PyT units, while they extend onto the thiophene units for PIDTDTPyT. This implies that adding the extra thiophene units has impacts on both the HOMO and LUMO levels. These results from the calculations are in agreement with the experimental values from CV tests. As can be observed, the optimized geom-etries of the polymers revealed highly planar molecular struc-tures, which can provide more evidence for the planar structure and good packing as previously discussed.
Charge transport properties
The charge transport properties of the active materials in BHJ play an important role in the OPV performance. A good carrier mobility ensures efficient exciton dissociation, charge transfer and reduces the charge recombination.37 Top contact organic field-effect transistor (OFET) devices were fabricated to evaluate the hole mobilities of the polymers. The OFET devices without annealing exhibited the typical p-channel characteristics. Fig. 4 showed the output curves at different gate voltage (Vgs) and
transfer characteristics in the saturation regime under constant source–drain voltage (Vds ¼ 100 V) for PIDTPyT and
PIDTDTPyT. We also investigated the annealing effect on the OFET characteristics and the detailed data including mobilities, on/off ratios, and threshold voltages obtained from the OFETs based on these two polymers before and after annealing are listed in Table 1.
The hole-mobilities of PIDTPyT and PIDTDTPyT without annealing were determined to be 0.066 and 0.037 cm2 V1s1, respectively, with the on/off ratio at the order of 106. After annealing at 110C, the mobility of PIDTPyT remained essentially constant while PIDTDTPyT showed an increased mobility of 0.045 cm2V1s1. The high charge mobility indicates that there are strong intermolecular interactions induced by the aforementioned p–p stacking and donor–acceptor interactions. The fused ring aromatic structure of the IDT unit may generate better molecular organization and provide highly efficient pathways for charge carrier transport. PyT may help promote the donor–acceptor interaction and decrease the intramolecular stacking distance.36 The negligible annealing effect on the mobility for PIDTPyT also indicated this strong intermolecular interaction of polymer chains. The lower mobility of PIDTDTPyT may be ascribed to weaker intermolecular interactions due to the additional thiophene
Fig. 2 Cyclic voltammograms of PIDTPyT and PIDTDTPyT (working electrode: ITO; reference electrode: Ag/AgCl; counter electrode: Pt mesh).
Fig. 3 Molecular geometries and HOMO, LUMO wavefunctions of the trimer model of PIDTPyT and PIDTDTPyT.
Fig. 4 Output (a and c) and transfer (b and d) characteristics of PIDTPyT (a and b) and PIDTDTPyT (c and d).
spacers. And the interaction can be enhanced upon annealing, therefore promoting polymer chains form more ordered organi-zation in film to result in higher mobility.
Photovoltaic properties
The photovoltaic properties of PIDTPyT and PIDTDTPyT were investigated with the conventional device structure of ITO/ PEDOT:PSS(40 nm)/polymer:PC71BM/Ca(30 nm)/Al(100 nm),
where the device area is 10.08 mm2. PC
71BM, which features
a wider absorption region compared to PC61BM, was chosen as
the n-type acceptor in order to efficiently utilize the solar light. The BHJ active layer was prepared by spin-coating dichloro-benzene solution of the polymer:PC71BM on a PEDOT:PSS
layer. All the devices were annealed in 150C for 15 min before the electrode deposition. The variation effects of the blending ratio, which seriously influence the charge balance characteristics and consequently determine the device performance, were investigated in detail. The summarized electrical parameters of these OPV devices are listed in Table 2. Higher current density (Jsc), FF and the resulting PCE were obtained under the ratio of
1 : 3, which may be due to the optimized electron–hole charge balance and reduced charge recombination.
Photovoltaic devices based on PIDTPyT:PC71BM (1 : 3) with
the optimized thickness of 68 nm showed a PCE value up to 3.41% with a Jscof 10.03 mA cm2, a Vocof 0.82 V, and a FF of
42%. For PIDTDTPyT-based devices, a highest resulting PCE of 3.91% with the optimized thickness of 80 nm was achieved when the FF improved to 50%, and the Jscand Vocdecreased slightly to
9.89 mA cm2and 0.79 V compared to PIDTPyT. Representative J–V curves of these two devices measured under illumination (100 mA cm2, AM 1.5G) are plotted in Fig. 5(a) based on the optimized conditions. Notably, the active layer thickness of the devices is only around 60–80 nm corresponding to the photo-current of ca. 10 mA cm2, indicating that the two polymers blended with PC71BM featured good absorption properties.
Moreover, Fig. 5(b) displays the external quantum efficiency (EQE) and UV-vis spectra of the two polymers and PC71BM.
The EQE spectra were tightly correlated with the UV-vis absorption of the bulk heterojunctions. Compared to PIDTDTPyT, the EQE curve for PIDTPyT exhibited weaker photoresponse between 350 and 610 nm but increased EQE between 610 and 800 nm, which can be explained by the more red-shifted absorption of PIDTPyT. To conclude, both devices based on these two low-bandgap materials blended with PC71BM
exhibited superior broadband photo-response spectra covering 300–800 nm. The limitation to the better performance was the poor fill factor which has been found to be related to the film morphology, charge transfer and the recombination process.1,3,18
Optimal phase segregation and formation of a bicontinuous interpenetrating network between the polymer donor and PCBM acceptor are critical to achieve high device performance and largely influence the charge separation and exciton dis-socation.38–40 Tapping-mode atomic force microscopy (AFM) was used to investigate the morphology of thin films of the BHJ blends of the polymer and PC71BM. As shown in Fig. 6, the BHJ
films showed very smooth surfaces with the rms roughness around0.5 nm and no obvious large-scale phase separation was observed. This indicated that both polymers have good film-forming abilities and compatibilities with PC71BM.
Additionally, the difference in the LUMO energy level between the acceptor and donor is one another of the critical factors determining the exciton dissociation rate and charge recombination.18,41 From prior discussion on device perfor-mance, the FF is the main limitation on device performance. We speculate that a certain degree of electron–hole recombination would occur in these blending systems, thereby lowering the FF. From the energy level point of view, the LUMO levels of PIDTPyT and PIDTDTPyT are respectively 3.80 eV and 3.70 eV, which may be too close to the LUMO level of PC71BM
(4.3 eV). Thus, the exciton may not efficiently dissociate into
Table 2 Summarized performances of devices containing PIDTPyT and PIDTDTPyT with different polymer:PC71BM blending ratios
Polymer Blend ratio Voc/v Jsc/mA cm2 FF (%) PCE (%)
PIDTPyT 1 : 2 0.80 8.86 39 2.73 1 : 3 0.82 10.03 42 3.41 1 : 4 0.76 9.35 40 2.87 PIDTDTPyT 1 : 2 0.77 8.88 44 3.03 1 : 3 0.79 9.89 50 3.91 1 : 4 0.62 3.65 45 1.03
Fig. 5 Current–voltage characteristics (a) and EQE wavelength depen-dencies (b) of photovoltaic devices based on PIDTPyT and PIDTDTPyT with the blending ratio as 1:3 under illumination with 100 mW cm2(AM
1.5G).
Fig. 6 AFM images (1mm 1 mm) for the films of polymer:PC71BM
(1 : 3): (a) PIDTPyT and (b) PIDTDTPyT as cast from o-DCB solutions.
separated free charges and the charge recombination rate may potentially increase. The higher FF obtained from PIDTDTPyT based devices might be correlated with its higher LUMO level compared to the PIDTPyT-based device.
Although the results based on a field-effect transistor indicated that both PIDTPyT and PIDTDTPyT feature superior lateral hole carrier-transport abilities, the vertical hole carrier-transport behaviours of the polymers blended with the fullerene play an essential role in the photovoltaic devices performance.42Herein, hole-only devices based on ITO/PEDOT:PSS/BHJ/MoO3/Al
were fabricated to measure the vertical hole mobility. MoO3was
used as an interfacial layer to suppress electron injection from Al so that the hole-only current was measured.43Fig. 7 shows the current density and voltage curve (J–V) of the hole-only devices containing polymer and PC71BM with the blending ratio of 1 : 3.
The space charge limited current (SCLC) model was employed to investigate the vertical hole mobilities using the following equa-tion, J¼ 9303rmV2/8L3, where J is the current density (mA cm2),
303ris the permittivity of the polymer, m is the carrier mobility,
and L is the active layer thickness. The mobilities were extracted by modelling the dark current in the SCLC region. The calcu-lated vertical hole-transport mobilities of PIDTPyT and PIDTDTPyT are 6.34 105 and 7.97 105(cm2 V1s1), respectively, which are lower than the FET mobilities by three orders of magnitude. This discrepancy indicates that the vertical hole-transporting ability was greatly inferior to the lateral mobility, which might cause charge recombination issues, thereby leading to lower FF and thinner optimized device thickness.41
4.
Conclusion
To conclude, we have synthesized two new low bandgap polymers (PIDTPyT and PIDTDTPyT) through the Stille polymerization between the IDT donor and PyT as well as thienyl-flanked PyT acceptor monomers. The stronger acceptor character of the PyT unit broadened the absorption spectrum of the polymers to longer wavelengths and resulted in deep energy levels. The OFET results showed that the two polymers have high lateral hole mobilities as high as 0.066 cm2V1s1. However, the vertical mobilities of the polymers as measured by the SCLC
method were determined as three orders of magnitude lower than the FET mobilities. The photovoltaic properties of the polymers were investigated and the highest achieved PCEs for PIDTPyT and PIDTDTPyT were 3.41% and 3.91%, respectively. The deep LUMO level of the polymers combined with the low vertical mobilities may not allow efficient charge dissociation and sepa-ration, which may give rise to the limitation on photovoltaic performance.
Acknowledgements
This work is supported by the National Science Foundation’s NSF-STC program under Grant No. DMR-0120967, the AFOSR (FA9550-09-1-0426), the Office of Naval Research (N00014-11-1-0300), and the World Class University (WCU) program through the National Research Foundation of Korea under the Ministry of Education, Science and Technology (R31-21410035). A.K.-Y.J. thanks the Boeing-Johnson Foundation for financial support. Y. Sun thanks the State-Sponsored Scholarship for Graduate Students from China Scholarship Council. S. C. Chien thanks the National Science Council of Taiwan (NSC98-2917-I-009-112) for supporting the Graduate Students Study Abroad Program.
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