Conjugated random copolymers of benzodithiophene
–
benzooxadiazole
–diketopyrrolopyrrole with full visible
light absorption for bulk heterojunction solar cells
Jian-Ming Jiang, Hsiu-Cheng Chen, His-Kuei Lin, Chia-Ming Yu, Shang-Che Lan, Chin-Ming Liu and Kung-Hwa Wei*We have used Stille coupling polymerization to synthesize a series of new donor–acceptor (D–A) conjugated random copolymers—PBDTT-BO-DPP—that comprise electron-rich alkylthienyl-substituted benzodithiophene (BDTT) units in conjugation with electron-deficient 2,1,3-benzooxadiazole (BO) and diketopyrrolopyrrole (DPP) moieties that have complementary light absorption behavior. These polymers exhibited excellent thermal stability with thermal degrading temperatures higher than 340C. Each of these copolymers exhibited (i) broad visible light absorption from 400 to 900 nm and (ii) a low optical band gap that is smaller than 1.4 eV and a low-lying highest occupied molecular orbital that is deeper than5.22 eV. As a result, bulk heterojunction photovoltaic devices derived from these polymers and fullerenes provided a high short-circuit current density that is larger than 12 mA cm2. In particular, a photovoltaic device prepared from the PBDTT-BO-DPP (molar ratio, 1 : 0.5 : 0.5)/PC71BM (w/w, 1 : 2) blend system with 1-chloronaphthalene (1 volume%) as an additive exhibited excellent photovoltaic performance, with a value of Voc of 0.73 V, a high short-circuit current density of 17 mA cm2, a fill factor of 0.55, and a promising power conversion efficiency of 6.8%, indicating that complementary light-absorption random polymer structures have great potential for increasing the photocurrent in bulk heterojunction photovoltaic devices.
Introduction
Polymer solar cells (PSCs) have attracted considerable attention as promising energy resources because they allow the produc-tion of low-cost, light-weight, large-area, exible devices through ink-jet printing and roll-to-roll solution processing.1
Tremendous efforts have been made toward improving the power conversion efficiencies (PCEs) of bulk heterojunction (BHJ) devices that incorporate conjugated polymers and fullerene derivatives as their electron-donating and -accepting components, respectively.2The PCE of a solar cell device is the
product of its short-circuit current density (Jsc), open-circuit
voltage (Voc), and ll factor (FF). In a device having a
BHJ-structured active layer, the open-circuit voltage is typically linearly proportional to the difference in energy between the highest occupied molecular orbital (HOMO) of the polymer and the lowest unoccupied molecular orbital (LUMO) of the fullerene; therefore, the value of Voccan be increased either by
elevating the LUMO energy level of the fullerene or by decreasing the HOMO energy level of the polymer.3Increases in
FFs can occur mainly through improving active layer
morphologies for balanced charge transport and carrying out extensive device optimization.4,5 Whereas, the value of J
sc is
determined by the amount of absorbed light that results from the band gap of the polymer, the layer thickness and the breadth of the absorption as well as the active layer morphology that dictates the transport of electrons and holes to the cathode and the anode, respectively. Because conjugated polymers typically absorb in only a limited region of the solar spectrum, many PSCs do not exhibit high PCEs. The fabrication of tandem BHJ solar cells and the use of low-band gap polymers blended with fullerenes as the active layer are two main approaches that have been used to enhance the absorption of solar light. Tandem solar cells usually comprise multiple single-BHJ cells stacked in series, with each layer featuring a different absorp-tion band; the resulting combined absorpabsorp-tion covers a broader region of the solar spectrum.6The fabrication of a tandem solar
cell, however, is more complicated than that of a single solar cell because not only proper processing conditions must be tailored to all cells that are in series but also additional inter-facial layers between cells are usually required. For single cells, the development of new broadly absorbing conjugated poly-mers is a necessary approach toward achieving high-efficiency PSCs. The donor–acceptor (D–A) approach, in which a perfectly alternating pattern of covalently bound electron-rich and -poor chemical units comprises the backbone, is frequently adopted
Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu, 30050, Taiwan, ROC. E-mail: khwei@mail.nctu.edu. tw; Fax: +886-3-5724727; Tel: +886-3-5731771
Cite this:Polym. Chem., 2013, 4, 5321
Received 25th January 2013 Accepted 27th February 2013 DOI: 10.1039/c3py00132f www.rsc.org/polymers
Chemistry
PAPER
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to obtain conjugated polymers exhibiting low band gaps as a result of the internal charge transfer between the D and A units.7
Although tuning the band gap of a D–A conjugated polymer while maintaining a low-lying HOMO for a high value of Voccan
be carried out by adopting a weak electron donor conjugated with a strong electron acceptor, it has limited effect on broad-ening the absorption of the solar spectrum by the D–A
conju-gated polymer. One approach toward broadening the
absorption of the solar spectrum involves the use of a random D–A conjugated polymer exhibiting complementary light absorption from different D units in conjugation with various A units.8 For this approach, one architecture is concerned with
using two D units for copolymerizing with one A unit.8a–dThe
other architecture involves the copolymerization of two different A units with one D unit to form random polymer structures.8e–h The latter polymer architecture will result in
consistent values of Vocfor the photovoltaic devices because its
HOMO and LUMO energy levels are largely localized on the D and A units, respectively, from theoretical studies. These random copolymers exhibit considerably broadened absorp-tions and/or two distinct absorption peaks in the short and long wavelength regions; accordingly, random D–A polymer struc-tures with complementary light absorption units have great potential for increasing the photocurrent in PSCs.
Among the various low-band gap conjugated polymers, D–A conjugated polymers adopting diketopyrrolopyrrole (DPP), which possesses a lactam structure, as a strong electron-acceptor unit have emerged as interesting materials in PSC applications;9a,bthey possess planar and well-conjugated
skel-etons that give rise to strong p–p interactions and result in absorptions in the near-infrared (NIR) region, 600–900 nm, as well as high carrier mobility.9c–eBHJ PSCs based on conjugated
polymers containing DPP units have been reported by several research groups to exhibit PCEs of 4–6.5%.9f–j On the other
hand, benzooxadiazole (BO), which adopts a quinoid structure, is also a strongly electron-accepting moiety that has been used in conjugated polymers for PSCs exhibiting PCEs of up to 5–6%,10,11c with two strong absorption peaks near 400 and
600 nm, respectively.
Herein, we report the synthesis of new conjugated random copolymers featuring diketopyrrolopyrrole (DPP) and benzooxa-diazole (BO) as electron-acceptor units in conjugation with electron-donating thienyl-substituted benzodithiophene (BDTT)11units that exhibit full-range absorption of visible light
and, thereby, provide high-efficiency BHJ solar cells.
Experimental section
Materials and synthesis2,6-Bis(trimethyltin)-4,8-bis(5-ethylhexyl-2-thienyl)benzo[1,2-b:4, 5-b0]dithiophene (M1),11
4,7-bis(5-bromothien-2-yl)-5,6-bis(octy-loxy)benzo[c][1,2,5]oxadiazole (M2),10and
3,6-bis(5-bromothien-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (M3)9a
were prepared according to the reported procedures. [6,6]-Phenyl-C61-butyric acid methyl ester (PC61BM) and
[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were purchased from
Nano-C. All other reagents were used as received without further purication, unless stated otherwise.
General procedure for the synthesis of PBDTT-BO-DPP through Stille coupling
Alternating polymer PBDTT-BO-DPP (1 : 0.3 : 0.7), P1. A solution of M1 (100 mg, 0.11 mmol), M2 (23.1 mg, 0.033 mmol), M3 (52.7 mg, 0.077 mmol), and tri(o-tolyl)phosphine (2.6 mg, 8.0 mol%) in dry chlorobenzene (4 mL) was degassed for 15 min. Tris(dibenzylideneacetone)dipalladium (2.0 mg, 2.0 mol%) was added under N2and then the reaction mixture was
heated at 130C for 48 h. Aer cooling to room temperature, the solution was added dropwise into MeOH (100 mL). The crude polymer was collected, dissolved in CHCl3, and reprecipitated
in MeOH. The solid was washed with MeOH, acetone, and CHCl3 in a Soxhlet apparatus. The CHCl3 solution was
concentrated and then added dropwise into MeOH. The solids were collected and dried under vacuum to give PBDTT-BO-DPP (1 : 0.3 : 0.7), P1 (140 mg, 80%). Anal. calcd: C, 69.70; H, 7.14; N, 2.41. Found: C, 67.85; H, 7.02; N, 2.53%.
Alternating polymer PBDTT-BO-DPP (1 : 0.5 : 0.5), P2. Using a procedure similar to that described above for P1, a mixture of M1 (100 mg, 0.11 mmol), M2 (38.4 mg, 0.055 mmol), and M3 (37.5 mg, 0.055 mmol) in dry chlorobenzene (4 mL) was poly-merized to give P2 (120 mg, 70%). Anal. calcd: C, 69.33; H, 7.18; N, 2.53. Found: C, 67.98; H, 7.57; N, 2.97%.
Alternating polymer PBDTT-BO-DPP (1 : 0.7 : 0.3), P3. Using a procedure similar to that described above for P1, a mixture of M1 (100 mg, 143 mmol), M2 (53.7 mg, 0.077 mmol), and M3 (22.5 mg, 0.033 mmol) in dry chlorobenzene (4 mL) was poly-merized to give P3 (122 mg, 70%). Anal. calcd: C, 69.18; H, 7.09; N, 2.52. Found: C, 68.01; H, 7.39; N, 2.85%.
Measurements and Characterization
1H NMR spectra were recorded using a Varian UNITY 300-MHz
spectrometer. Thermogravimetric analysis (TGA) was per-formed using a TA Instruments Q500; the thermal stabilities of the samples were determined under N2 by measuring their
weight losses while heating at a rate of 20 C min1. Size exclusion chromatography (SEC) was performed using a Waters chromatography unit interfaced with a Waters 1515 differential refractometer; polystyrene was the standard; the temperature of the system was set at 45C and CHCl3was the eluent. UV-Vis
spectra of dilute dichlorobenzene (DCB) solutions (1 105M) were recorded at approximately 25C using a Hitachi U-4100 spectrophotometer. Solid lms for UV-Vis analysis were obtained by spin-coating polymer solutions onto quartz substrates. Cyclic voltammetry (CV) of the polymer lms was performed using a BAS 100 electrochemical analyzer operated at a scan rate of 50 mV s1; the solvent was anhydrous MeCN containing 0.1 M tetrabutylammonium hexauorophosphate (TBAPF6) as the supporting electrolyte. The potentials were
measured against a Ag/Ag+(0.01 M AgNO3) reference electrode;
ferrocene/ferrocenium ion (Fc/Fc+) was used as the internal standard (0.09 V). The onset potentials were determined from the intersection of two tangents drawn at the rising and
background currents of the cyclic voltammogram. HOMO and LUMO energy levels were estimated relative to the energy level of the ferrocene reference (4.8 eV below vacuum level). Topo-graphic and phase images of the polymer:fullerene lms (surface area: 5 5 mm2) were recorded using a Digital Nano-scope III atomic force microNano-scope operated in the tapping mode under ambient conditions. The thicknesses of the active layers in the devices were measured using a VeecoDektak 150 surface proler.
Fabrication and Characterization of Photovoltaic Devices Indium tin oxide (ITO)-coated glass substrates were cleaned stepwise in detergent, water, acetone, and isopropyl alcohol (ultrasonication; 20 min each) and then dried in an oven for 1 h; subsequently, the substrates were treated with UV ozone for 30 min prior to use. A thin layer (ca. 20 nm) of poly-ethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS, Baytron P VP AI 4083) was spin-coated (5000 rpm) onto the ITO substrates. Aer baking at 140 C for 20 min in air, the
substrates were transferred to a N2-lled glove box. The polymer
and PCBM were co-dissolved in DCB at various weight ratios, but with axed total concentration (40 mg mL1). The blend solution was stirred continuously for 12 h at 90C and then ltered through a PTFE lter (0.2 mm); the photoactive layer was obtained by spin coating the blend solution onto the ITO/ PEDOT:PSS surface at a rate between 600 and 2500 rpm for 60 s. The thicknesses of the photoactive layers were approximately 80–105 nm. The devices were nished for measurement aer thermal deposition of a 30 nm lm of Ca and then a 100 nmlm of Al as the cathode at a pressure of approximately
Scheme 1 Synthesis of the copolymers P1, P2, and P3.
Table 1 Molecular weights and thermal properties of the polymer
Polymer Mwa Mna PDIa Tdb P1 75.1k 20.3k 3.7 408 P2 87.7k 35.1k 2.5 357 P3 101.5k 37.6k 2.7 345 PBDTTBO 145.8k 52.1k 2.8 333 PBDTTDPP 57.3k 18.5k 3.1 452 aValues of M
n, Mw, and PDI of the polymers were determined through GPC (in CHCl3 using polystyrene standards). bThe 5% weight-loss temperatures (C) in the air.
Fig. 1 TGA thermograms of the copolymers P1, P2, and P3, recorded at a heating rate of 20C min1under a N2atmosphere.
1 106mbar. The effective layer area of one cell was 0.1 cm2. The current density–voltage (J–V) characteristics were measured using a Keithley 2400 source-meter. The photocurrent was measured under simulated AM 1.5 G illumination at 100 mW cm2 using a Xe lamp-based Newport 66902 150-W solar simulator. A calibrated silicon photodiode with a KG-5lter was employed to check the illumination intensity. External quantum efficiencies (EQEs) were measured using an SRF50 system (Optosolar, Germany). A calibrated mono-silicon diode exhibiting a response at 300–1000 nm was used as a reference. For hole mobility measurements, hole-only devices were
fabricated having the structure
ITO/PEDOT:PSS/poly-mer:PCBM/Au. The hole mobility was determined bytting the dark J–V curve to the space-charge-limited current (SCLC) model.12
Results and discussion
Synthesis and characterization of the polymers
Scheme 1 outlines the general synthetic strategy that we used to obtain the random polymers. To ensure good solubility of the BO derivative M2, we positioned two octyloxy chains on the BO ring. We synthesized M1, M2, and M3 using the reported methods.9a,10,11Aer Stille coupling using Pd
2dba3as the
cata-lyst in chlorobenzene at 130C for 48 h, we obtained the poly-mers P1, P2, and P3 in yields of 70–80%. We determined the solubilities of these copolymers in various solvents at a concentration of 5 mg mL1. PBDTTBO, P2, and P3 were completely soluble in CHCl3, chlorobenzene, and DCB at room
temperature, whereas P1 and PBDTTDPP were soluble only aer heating at 50C. We determined the weight-average molecular weights (Mw) of these polymers (Table 1) through gel
perme-ation chromatography (GPC), against polystyrene standards, with CHCl3as the eluent.
Thermal stability
We used TGA to determine the thermal stabilities of the poly-mers (Fig. 1). In air, the 5% weight-loss temperatures (Td) of P1,
Fig. 2 UV-Vis absorption spectra of the polymers PBDTTBO, PBDTTDPP, P1, P2, and P3 as (a) dilute solutions (1 105M) in DCB and (b) solidfilms.
Table 2 Optical properties of the polymers
lmax,abs(nm) FWHM (nm)
lonset (nm)
Eoptg (eV) Solution Film Solution Film Film
P1 752 680, 740 184 232 950 1.31
P2 677, 736 664, 737 236 264 920 1.34
P3 640, 722 630, 722 244 258 845 1.46
PBDTTBO 590, 620 590, 630 150 160 695 1.78
PBDTTDPP 764 750 170 212 950 1.31
Fig. 3 Cyclic voltammograms of solidfilms of the copolymers P1, P2, and P3.
Table 3 Electrochemical properties of the copolymers P1, P2, and P3
Eoxonseta(V) Eredonseta(V) HOMOb(eV) LUMOb(eV) Eoptg (eV)
P1 0.42 1.27 5.22 3.53 1.69
P2 0.52 1.26 5.32 3.54 1.78
P3 0.56 1.27 5.36 3.53 1.83
aThe potentials were measured against a Ag/Ag+ (0.01 M AgNO 3) reference electrode; ferrocene/ferrocenium ion (Fc/Fc+) was used as the internal standard (0.09 V). bHOMO and LUMO energy levels determined using the equations HOMO¼ (4.8 + Eox) eV and LUMO ¼ (4.8 + Ered) eV.
P2, and P3 were 408, 357, and 345C, respectively. Thus, they all exhibited good thermal stability—an important characteristic for device fabrication and application.
Optical properties
Fig. 2a and b display the absorption spectra of PBDTTBO, PBDTTDPP, and P1–3 in solution (DCB) and in the solid state, respectively; Table 2 summarizes the optical data, including the absorption peak wavelengths (lmax,abs), absorption edge
wavelengths (ledge,abs), full widths at half maximum (FWHMs),
and optical band gaps (Eoptg ). The absorption peaks of
PBDTTBO and PBDTTDPP were located at 590 and 760 nm, respectively, whereas thelms of the copolymers P1–3 exhibi-ted double absorption peaks in the range 300–1000 nm. The relative position and intensity of the absorption peaks of the copolymers P1–3 were effectively tuned by their composition; for example, the main absorption peaks for the PBDTT-BO-DPP copolymers having compositions of 1 : 0.3 : 0.7, 1 : 0.5 : 0.5, and 1 : 0.7 : 0.3 were 680/740, 642/737, and 612/721 nm, respectively; that is, they shied to a shorter wavelength upon increasing the content of BO units. The absorption spectra of
the lms of each of the copolymers P1–3 featured two
absorption bands: one at 300–500 nm, which we assigned to localizedp–p* transitions, and the other, broader band in the long wavelength region, from 500 to 950 nm, corresponding to the intramolecular charge transfer (ICT) between the acceptor BO or DPP units and the donor BDTT units. The absorption spectra of the three polymers in the solid state were similar to their corresponding solution spectra, with slight red-shis (ca. 10–40 nm) of their absorption onsets, indicating that some intermolecular interactions existed in the solid state. The absorption peak near 600 nm underwent a relative red-shi to 750 nm upon increasing the content of DPP units; the absorption spectra of the polymers were readily tuned by varying the molar ratio of BO units and DPP units. The FWHMs of these random PBDTT-BO-DPP copolymers having compositions of 1 : 0.3 : 0.7, 1 : 0.5 : 0.5, and 1 : 0.7 : 0.3 were 232, 264, and 258 nm, respectively, approximately 60–100 nm broader than those of PBDTTBO and PBDTTDPP, implying that the random copolymers would absorb more of the solar spectrum.
The absorption edges for P1–3 (Table 2) correspond to optical band gaps (Eoptg ) of 1.31, 1.34 and 1.46 eV, respectively.
Fig. 4 DarkJ–V curves for the hole-dominated carrier devices incorporating the polymers blend with PC71BM [blend ratio, 1 : 2 (w/w)], and the P2/PC71BM blend prepared in the presence of CN (1 vol%).
Fig. 5 J–V characteristics of PSCs incorporating copolymer:PC61BM blends, copolymer:PC71BM blends, and the P2:PC71BM blend prepared in the presence of CN (1 vol%); each blend ratio, 1 : 2 (w/w).
Table 4 Photovoltaic properties of PSCs incorporating PBDTT-BO-DPP copolymers
Polymer/PC61BM
(w/w; 1 : 2) Voc(V) Jsc(mA cm2) FF (%) PCE (%) Thickness (nm)
P1 0.72 10.8 55 4.3 82
P2 0.74 13.8 50 5.2 95
P3 0.76 10.7 56 4.6 89
Polymer/PC71BM (w/w; 1 : 2) Voc(V) Jsc(mA cm2) FF (%) PCE (%) Thickness (nm) Mobility (cm2V1s1)
P1 0.70 12.5 62 5.5 85 2.3 103 P2 0.72 14.7 56 6.0 88 3.3 103 P3 0.75 12 59 5.3 93 3.0 103 P2/PC71BM (w/w; 1 : 2) (CN, vol%) P2(0.5) 0.72 15.2 57 6.2 93 P2(1.0) 0.73 17 55 6.8 91 5.1 103 P2(1.5) 0.73 15.8 56 6.3 89
Electrochemical properties
We used CV to examine the electrochemical properties, including HOMO and LUMO energy levels, of the random BDTT-based copolymers. Fig. 3 displays the electrochemical properties of the polymers as solidlms; Table 3 summarizes the relevant data. Partially reversible n-doping/de-doping processes occurred for these random polymers in the negative potential range; in addition, reversible p-doping/de-doping processes occurred in the positive potential range. The onset oxidation potentials
(Eoxonset, vs. Ag/Ag+) for the copolymers P1–3 were 0.42, 0.52, and
0.56 V, respectively; in the reductive potential region, the onset reduction potentials (Eredonset) were 1.27, 1.26, and 1.27 V,
respectively. On the basis of these onset potentials, we estimated the HOMO and LUMO energy levels according to the energy level of the ferrocene reference (4.8 eV below the vacuum level).13The
HOMO energy levels of the PBDTT-BO-DPP copolymers with compositions of 1 : 0.3 : 0.7, 1 : 0.5 : 0.5, and 1 : 0.7 : 0.3 were 5.22, 5.32, and 5.36 eV, respectively, implying that they varied with respect to the modulated ICT strengths resulting from the presence of electron-acceptor units with various elec-tron-withdrawing abilities.14 The LUMO energy levels of the
copolymers P1–3 were all located within a reasonable range (from3.53 to 3.54 eV, Fig. 4) and were signicantly greater than that of PCBM (ca. 4.1 eV); thus, we would expect efficient charge transfer/dissociation to occur in their corresponding devices.15In addition, the electrochemical band gaps (Eec
g) of the
copolymers P1–3, estimated from the differences between the onset potentials for oxidation and reduction, were in the range 1.69–1.83 eV; that is, they were slightly larger than the corre-sponding optical band gaps. This discrepancy between the electrochemical and optical band gaps presumably resulted from the exciton binding energies of the polymers and/or the interface barriers for charge injection.16
Hole mobility
Fig. 4 displays the hole mobilities of the devices incorporating the polymer/PC71BM blends at a blend ratio of 1 : 2 (w/w). The
hole mobilities of the copolymers P1–3 blend with PC71BM were
2.3 103, 3.3 103, and 3.0 103cm2V1s1, respectively. When we added a small amount of 1-chloronaphthalene (CN; 1%, by volume relative to DCB) to optimize the miscibility of the PBDTT-BO-DPP (1 : 0.5 : 0.5)/PC71BM blend, the hole mobility
increased to 5.1 103cm2V1s1.
Photovoltaic properties
Next, we investigated the photovoltaic properties of the poly-mers in BHJ solar cells having the sandwich structure ITO/ PEDOT:PSS/polymer:fullerene (1 : 2, w/w)/Ca/Al, with the pho-toactive layers having been spin-coated from DCB solutions of the polymer and fullerene. The optimized weight ratio for the polymer and fullerene was 1 : 2. In this case, we added a small amount of CN (0.5–1.5%, by volume relative to DCB) to optimize the miscibility of the blends. Fig. 5 presents the J–V curves of these PSCs; Table 4 summarizes the data. The devices prepared from polymer:PC61BM blends of the copolymers P1–3 exhibited
open-circuit voltages (Voc) of 0.72, 0.74, and 0.76 V, respectively;
these values correspond to the difference between the HOMO energy level of the polymer and the LUMO energy level of PC61BM quite well.17 We suspect that the PBDTT-BO-DPP
(1 : 0.3 : 0.7) device provided the lowest value of Vocbecause of
its relatively higher-lying HOMO energy level. The Jsc of the
devices incorporating the copolymers P1–3 were 10.8, 13.8, and 10.7 mA cm2, respectively. The devices prepared from poly-mer:PC71BM blends of the copolymers P1–3 exhibited Voc of
0.70, 0.72, and 0.75 V, respectively; their Jscwere 12.5, 14.7, and
Fig. 6 EQE curves of PSCs incorporating copolymer:PC71BM blends and the P2:PC71BM blend prepared in the presence of CN (1 vol%); each blend ratio, 1 : 2 (w/w).
Fig. 7 Topographic AFM images of copolymer:PC71BM (1 : 2, w/w) blends incorporating (a) P1, (b) P2, (c) P3, and (d) P2 processed in the presence of CN (1 vol%).
12 mA cm2, respectively, providing a PCE of 6.0% for the device incorporating PBDTT-BO-DPP (1 : 0.5 : 0.5).
Furthermore, we have used a CN additive with different volume ratios in the solution, from 0.5% to 1.5% to process the PBDTT-BO-DPP (1 : 0.5 : 0.5)/PC71BM active layer for the
devices. When the active layer was processed with 1 vol% CN, the device incorporating the PBDTT-BO-DPP (1 : 0.5 : 0.5)/ PC71BM (1 : 2, w/w) active layer exhibited the highest value of Jsc
that represents an increase of 15% over that in the case without the CN additive (17 vs. 14.7 mA cm2), resulting in an optimal PCE of 6.8% (see Table 4). Fig. 6 displays EQE curves of the devices incorporating the polymer:PC71BM blends at a weight
ratio of 1 : 2. These devices exhibited signicantly broad EQE responses that extended from 300 to 950 nm. We attribute these EQE responses in the visible region to the corresponding absorbances of the active layers, resulting from both the intrinsic absorptions of the polymers and the presence of PC71BM, which also absorbs signicantly at 300–500 nm. The
device based on the blend of PBDTT-BO-DPP (1 : 0.5 : 0.5) and PC71BM exhibited the highest EQE response among all of our
studied systems, with a maximum value of 72% at 450 nm, consistent with its higher photocurrent. The calculated short-circuit current densities obtained from integrating the EQE curves of the devices incorporating the copolymer blends of P1– 3 with PC71BM, and that of P2 processed with CN (1 vol%) as the
additive, were 12.1, 14.1, 11.6, and 16.5 mA cm2, values that agree reasonably with the measured data (AM 1.5G; discrep-ancy: <5%).
Moreover, when exploring the decisive factors affecting the efficiencies of PSCs, we must consider not only the absorptions and energy levels of the polymers but also the surface morphologies of the polymer blends.18 Fig. 7 displays the
surface morphologies of our systems, determined using AFM. We prepared samples of the polymer/fullerene blends using procedures identical to those employed to fabricate the active layers of the devices. In each case, we observed quite smooth surfaces for the fullerene blends of the copolymers P1–3, with root-mean-square (rms) roughnesses ranging from 1.0 to 1.34 nm. The greater phase segregation and rougher surfaces of the PBDTT-BO-DPP (1 : 0.3 : 0.7) blends presumably arose because of poor miscibility with the fullerenes.
Conclusions
We have used Stille copolymerization to prepare a series of new conjugated random copolymers PBDTT-BO-DPP that absorb the full spectrum of visible light; they feature random alternating BDTT units in conjugation with electron-decient BO and DPP moieties, which have complementary light absorption behavior. These polymers possess excellent thermal stability, low-lying HOMO energy levels, and broad absorption bands that extend from the visible to the NIR—desirable properties that make these polymers promising materials for solar cell applications. A device incorporating PBDTT-BO-DPP (1 : 0.5 : 0.5) and PC71BM (blend weight ratio, 1 : 2), with CN (1 vol%) as an
additive exhibited a high value of Jscof 17 mA cm2and a PCE of
6.8%, indicating that complementary light-absorption random
polymer structures have great potential for increasing the photocurrent in bulk heterojunction photovoltaic devices.
Acknowledgements
We thank the National Science Council, Taiwan, fornancial support (NSC 101-3113-P-009-005).
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![Fig. 4 Dark J–V curves for the hole-dominated carrier devices incorporating the polymers blend with PC71BM [blend ratio, 1 : 2 (w/w)], and the P2/PC71BM blend prepared in the presence of CN (1 vol%).](https://thumb-ap.123doks.com/thumbv2/9libinfo/7530049.119719/5.892.82.431.368.648/curves-dominated-carrier-devices-incorporating-polymers-prepared-presence.webp)
