Benzooxadiazole-based donor/acceptor copolymers imparting
bulk-heterojunction solar cells with high open-circuit voltages
Jian-Ming Jiang, Po-An Yang, Shang-Che Lan, Chia-Ming Yu, Kung-Hwa Wei
*Department of Materials Science and Engineering, National Chiao Tung University, 300 Hsinchu, Taiwan
a r t i c l e i n f o
Article history:
Received 21 August 2012 Received in revised form 2 November 2012 Accepted 14 November 2012 Available online 20 November 2012 Keywords:
Polymer solar cell Suzuki coupling
Donor/acceptor conjugated polymers
a b s t r a c t
In this study we used Suzuki cross-coupling to synthesize three new donor/acceptor copolymersdPFTBO, PAFTBO, and PCTBOdfeaturing soluble alkoxy-modified 2,1,3-benzooxadiazole (BO) moieties as acceptor units and electron-rich building blocksddialkyl fluorene (F), alkylidene flu-orene (AF), and carbazole (C), respectivelydas donor units. These polymers, which we characterized using gel permeation chromatography, thermogravimetric analysis, NMR spectroscopy, UVeVis absorption spectroscopy, and electrochemical cyclic voltammetry, exhibited good solubility, low-lying energy levels for their highest occupied molecular orbitals, excellent thermal stability, and air stability. Using these polymers, we fabricated bulk-heterojunction solar cell devices having the structure indium tin oxide/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate/polymer:[6,6]-phenyl-C61-butyric acid
methyl ester (PC61BM) (1:1, w/w)/Ca/Al. Under AM 1.5G illumination (100 mW cm2), the solar cell
incorporating PFTBO exhibited a high value of Vocof 1.04 V and that based on PCTBO provided a power
conversion efficiency of 4.1% without the need for any post treatment.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Polymer solar cells (PSCs) are attracting growing interest as a potential renewable energy technology because they can be manufactured at low cost with the capability of being used in flexible large-area devices [1e3]. To date, bulk-heterojunctions (BHJs), in which the active layer consists of a blend of electron-donating conjugated polymers and electron-accepting fullerene derivatives, have been the most prevalent active layer structures in polymer solar cells exhibiting high power conversion efficiencies (PCEs). Several conjugated polymers have been developed featuring electron donor/acceptor (D/A) units in main chaine conjugated configurations[4e15]and side chaineattached archi-tectures[16e20]. Recently, BHJ solar cells based on blends of some D/A low-band gap polymers and [6,6]-phenyl-C61-butyric acid
methyl ester (PC61BM) or PC71BM have been investigated
exten-sively, providing PCEs as high as 7%[21e27].
The PCE of a solar cell device is essentially determined by short-circuit current density (Jsc), the fill factor, and the open-circuit
voltage (Voc). The relatively low open-circuit voltage (ca. 0.6 V)
obtained in some thiophene-polymer based BHJ devices will limit
their PCEs. In a BHJ-structured active layer, the open-circuit voltage is typically 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, although some other characteristics of the device structure (e.g., the type of cathode material, the active layer morphology, or exciton non-radiative recombination) can also affect the values of Vocof BHJ
PSCs[28e31]. Therefore, the value of Voccan be increased either by
elevating the LUMO energy level of the fullerene or depressing the HOMO energy level of the polymer while keeping its counterpart unchanged. Low-band gap polymers that provide efficient absorp-tion of the solar spectrum, however, tend to have high-lying HOMOs and low-lying LUMOs; the difference in the energy levels between the low-lying LUMOs of the polymers and the LUMO of the fullerene frequently result in inefficient charge separation, leading to a smaller enhancement of Jsc. On the other hand, the combination of
a high-lying HOMO in a low-band gap polymer and afixed LUMO in fullerene will also provide a lower value of Voc. Therefore, fine
tuning of the band gap and the energy levels such as lowering the HOMO and LUMO of the polymer simultaneously but with a larger decrease in the LUMO while maintaining its value 0.3 eV above that of the fullerene is required to obtain BHJ PSCs with high values of Vocand Jsc[32e34]. Currently, the highest open-circuit voltages
obtained from BHJ PSCs (ca. 1 V) have required polymers possessing medium-sized band gaps (ca. 2 eV)[35e38].
* Corresponding author.
E-mail address:khwei@mail.nctu.edu.tw(K.-H. Wei).
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In recent years, 9,9-dialkylfluorenes have emerged as attractive donor candidates for D/A polymer photovoltaics because of their good processability, high absorption coefficients, and considerable values of Voc[39,40]. By changing the sp3-hybridized carbon atom
at the 9-position of 9,9-dialkylfluorene to an sp2-hybridized atom,
the resulting alkylidenefluorene permits the alkyl chains to adopt a coplanar conformation relative to the polymer backbone, thereby facilitating cofacial
p
ep
stacking, which can lead to very short intermolecular distances (<4 A) in crystalline or liquid crystalline states and, accordingly, enhanced charge carrier transportation[41]. Unlike a C-bridged fluorene, the corresponding N-bridged carbazole moiety is fully aromatic, providing superior chemical and environmental stability. Poly(N-alkyl-2,7-carbazole) derivatives have been applied successfully in polymer light emitting diodes
[42]and organicfield-effect transistors[43], demonstrating good p-type transport properties.
On the other hand, BHJ devices based on main chain D/A polymers containing alkoxy benzooxadiazole (BO) units as acceptors and several thiophene-based building blocks as donors have exhibited relatively high values of Voc [44,45]; therefore,
combining a strongly electron-withdrawing acceptor with a weakly electron-donating donor can be a very effective means of lowering the HOMO energy level in the D/A polymer and, ulti-mately, enhancing the value of Vocof the resulting PSC[46]. Those
studies inspired us to further explore the possibility of copoly-merizing alkoxy-modified BO derivatives with weakly electron-donating units to synthesize copolymers exhibiting high values of Voc. In this study, we prepared a series of new D/A alternating
polymersdPFTBO, PAFTBO, and PCTBOdbased on 9,9-dialkylfluorene (F), alkylidene fluorene (AF), and N-alkyl-2,7-carbazole (C) units, respectively, as weak electron donors and alkoxy-modified BO (BO) units as electron-deficient acceptors; conjugation of the electron-withdrawing BO units to the weakly electron-donating units provided polymers with deep HOMO energy levels and medium-sized band gaps. These desirable features provided PFTBO, PAFTBO, and PCTBO with good hole mobilities and high values of Voc, making them suitable for
photovoltaic applications. 2. Experimental section 2.1. Materials and synthesis
The synthesis of 4,7-bis(5-bromothiophen-2-yl)-5,6-bisoctyloxybenzo[c][1,2,5]oxadiazole (M1)[44]has been reported elsewhere.
4,4,5,5-Tetramethyl-2-[2-(4,4,5,5-tetramethyl-1,3-dioxolan-2-yl)-9,9-dioctyl-9H-fluoren-7-yl]-1,3-dioxolane (M2) [47], 2-[9-(hepta- decan-9-ylidene)-2-(4,4,5,5-tetramethyl-1,3-dioxolan-2-yl)-9H-fluoren-7-yl]-4,4,5,5-tetramethyl-1,3-dioxolane (M3) [41], and 9-(heptadecan-9-yl)-2,7-bis(4,4,5,5-tetramethyl-1,3-dioxolan-2-yl)-9H-carbazole (M4) [48] were prepared according to reported procedures. PC61BM was purchased from Nano-C. All other reagents
were used as received without further purification, unless stated otherwise.
2.2. General procedure for Suzuki polymerization: alternating polymer PFTBO
A mixture of M1 (105 mg, 0.150 mmol), M2 (96.3 mg, 0.150 mmol), Aliquat 336 (ca. 20 mg), K2CO3(aq)(2 M, 1.5 mL), and
chlorobenzene (CB) 4 mL were degassed under N2 at 60C for
15 min. Pd(PPh3)4 was added to the mixture, which was then
heated at 130 C for 48 h. Phenylboronic acid (49.9 mg, 0.300 mmol) was added and then the mixture was stirred 6 h.
Subsequently, bromobenzene (0.03 mL, 0.3 mmol) was also added to the mixture, which was stirred for another 12 h. After cooling to room temperature, the solution was added dropwise into MeOH (100 mL). The crude polymer was collected, dissolved in CHCl3, and
reprecipitated from MeOH. The solid was washed with MeOH, acetone, and CHCl3in a Soxhlet apparatus. The CHCl3solution was
concentrated and then added dropwise into MeOH. The precipitate was collected and dried under vacuum to give PFTBO (100 mg, 72%).1H NMR (300 MHz, CDCl3):
d
8.54e8.31 (m, 2H), 8.05e7.88(m, 2H), 7.80e7.55 (m, 6H), 4.25 (br, 4H), 2.41 (br, 4H), 1.78e1.25 (m, 48H), 0.91 (s, 12H). Anal. Calcd: C, 76.25; H, 8.68; N, 3.01. Found: C, 75.18; H, 8.55; N, 3.15.
2.2.1. Alternating polymer PAFTBO
Using a polymerization procedure similar to that described above for PFTBO, a mixture of M1 (105 mg, 0.15 mmol) and M3 (98.1 mg, 0.15 mmol) in dry CB (4 mL) was polymerized to give PAFTBO (71 mg, 52%).1H NMR (300 MHz, CDCl3):
d
8.51e8.29 (m,2H), 8.17e7.98 (m, 2H), 7.78e7.51 (m, 6H), 4.22 (br, 4H), 2.81 (br, 4H), 1.98e1.56 (m, 48H), 0.83 (s, 12H). Anal. Calcd: C, 76.55; H, 8.57; N, 2.98. Found: C, 74.98; H, 8.42; N, 2.77.
2.2.2. Alternating polymer PCTBO
Using a polymerization procedure similar to that described above for PCTBO, a mixture of M1 (105 mg, 0.15 mmol) and M4 (98.6 mg, 0.15 mmol) in dry CB (4 mL) was polymerized to give PCTBO (120 mg, 85%).1H NMR (300 MHz, CDCl 3):
d
8.85e8.57 (m, 2H), 8.06e7.83 (m, 2H), 7.68e7.42 (m, 6H), 4.32 (br, 4H), 3.98 (s, 1H), 2.13 (br, 4H), 1.67e1.28 (m, 48H), 0.91 (s, 12H). Anal. Calcd: C, 75.03; H, 8.64; N, 4.45. Found: C, 73.15; H, 8.47; N, 4.56.2.3. Measurements and characterization
1H NMR spectra were recorded using a Varian UNITY 300-MHz
spectrometer. Thermogravimetric analysis (TGA) was performed using a TA Instruments Q500 apparatus; the thermal stabilities of the samples were determined under a N2 atmosphere by
measuring their weight losses while heating at a rate of 20C 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; THF was the eluent. UVeVis spectra of dilute samples (1 105M) in dichlorobenzene (DCB) were recorded at room temperature (ca. 25 C) using a Hitachi U-4100 spectrophotometer. Solid films for UVeVis spectroscopic analysis were obtained by spin-coating the poly-mer solutions onto a quartz substrate. Cyclic voltammetry (CV) of the polymerfilms 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 hexa-fluorophosphate (TBAPF6) as the supporting electrolyte. The
potentials were measured against a Ag/Agþ (0.01 M AgNO3)
reference electrode; the ferrocene/ferrocenium ion (Fc/Fcþ) pair 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 voltammograms. HOMO and LUMO energy levels were estimated relative to the energy level of the ferrocene reference (4.8 eV below vacuum level). Topographic and phase images of the polymer/PC61BM
films (surface area: 5 5
m
m2) were obtained using a Digital Nanoscope III atomic force microscope (AFM) operated in the tapping mode under ambient conditions. The thickness of the active layer of the device was measured using a Veeco Dektak 150 surface profiler.2.4. Fabrication and characterization of photovoltaic devices Indium tin oxide (ITO)ecoated glass substrates were cleaned sequentially in detergent, water, acetone, and isopropyl alcohol (ultrasonication; 20 min each) and then dried in an oven for 1 h; the substrates were then treated with UV ozone for 30 min prior to use. An aqueous solution of poly(ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS, Baytron P VP AI 4083) was spin-coated (5000 rpm) onto the ITO substrates. After baking at 140C for 20 min in air, a thin layer (ca. 20 nm) of PEDOT:PSS was formed on the substrates; the PEDOT:PSSeoneITO samples were trans-ferred to a N2-filled glove box. The polymer and PC61BM were
co-dissolved in DCB at various weight ratios, but with afixed total concentration (40 mg mL1). The blend solutions were stirred continuously for 12 h at 90C and thenfiltered through a PTFE filter (0.2
m
m); the photoactive layers were obtained by spin-coating (600e2000 rpm, 60 s) the blend solutions onto the ITO/ PEDOT:PSS surfaces. The thickness of each photoactive layer was approximately 85e120 nm. The devices were ready for measure-ment after thermal deposition (pressure: ca. 1 106mbar) ofa 20-nm-thickfilm of Ca, followed by a 100-nm-thick Al film as the cathode. The effective layer area of one cell was 0.04 cm2. The current densityevoltage (JeV) characteristics were measured using a Keithley 2400 source meter. The photocurrent was measured under simulated AM 1.5 G illumination at 100 mW cm2using a Xe lampebased Newport 66902 150-W solar simulator. A calibrated Si photodiode with a KG-5filter was employed to confirm the illu-mination intensity. External quantum efficiencies (EQEs) were measured using an SRF50 system (Optosolar, Germany). A cali-brated mono-silicon diode exhibiting a response at 300e800 nm was used as a reference. For hole mobility measurements, hole-only devices were fabricated having the structure ITO/PEDOT:PSS/ polymer/Au. The hole mobility (
m
h) was determined byfitting thedark JeV curve into the space-charge-limited current (SCLC) model
[16], based on the equation
J ¼ 98ε0εr
m
h V2L3
where ε0 is the permittivity of free space, εr is the dielectric
constant of the material, V is the voltage drop across the device, and L is the thickness of active layer.
3. Results and discussion
3.1. Synthesis and characterization of the polymers
Scheme 1outlines our general synthetic strategy for obtaining the monomers and the polymers. To ensure good solubility of the BO derivative M1, we positioned two octyloxy chains on the BO ring, as in previous reports[44]; we synthesized M2, M3, and M4 using reported methods [41,47,48]. We performed Suzukie MiyauraeSchlüter polymerization of the monomers in a biphasic mixture of CB and aqueous K2CO3with Pd(PPh3)4as the catalyst
precursor. After polymerization for 48 h, we added phenylboronic acid and then bromobenzene (after a further 12 h) to end-cap the polymer; capping of the termini is necessary to obtain stable conjugated polymers exhibiting high photovoltaic performance
[49,50]. Accordingly, we obtained the polymers PFTBO, PAFTBO, and PCTBO as dark-red solids in yields of 50e82%. We determined the weight-average molecular weights (Mw) of these polymers Scheme 1. Synthesis and structures of the polymers PFTBO, PAFTBO, and PCTBO.
Table 1
Molecular weights, thermal properties, and hole mobilities of the polymers. Polymer Mwa(kDa) Mna(kDa) PDIa Tdb(C) Mobility (cm2V1s1) PFTBO 67.8 45.2 1.5 316 1.2 104
PAFTBO 34.6 18.2 1.9 300 5.1 104 PCTBO 64.8 46.3 1.4 300 6.9 104 aValues of M
n, Mwand PDI of the polymers were determined through GPC (polystyrene standards; THF).
b The 5% weight-loss temperature in air.
Fig. 1. TGA thermograms of the polymers PFTBO, PAFTBO, and PCTBO, recorded at a heating rate of 20C min1under a N2atmosphere.
(Table 1) through SEC, against polystyrene standards, in THF as the eluent.
3.2. Thermal stability
We used TGA to determine the thermal stability of the polymers (Fig. 1). In air, the 5% weight-loss temperatures (Td) of PFTBO,
PAFTBO, and PCTBO were 316, 300, and 300C, respectively. Thus, they all exhibited good thermal stability against O2dan important characteristic for device fabrication and application. No clear glass transitions were evident from 25 to 300C in the DSC curves of the second heating and cooling runs (20C min1) of these polymers. 3.3. Optical properties
We recorded the normalized optical UVeVis absorption spectra of the polymers as dilute DCB solutions at room temperature and as spin-coated films on quartz substrates. Fig. 2a displays the
absorption spectra of PFTBO, PAFTBO, and PCTBO in DCB at room temperature;Table 2summarizes the optical data, including the absorption peak wavelengths (
l
max,abs), absorption edgewave-lengths (
l
edge,abs), and optical band gapsðEgoptÞ. All of the absorptionspectra recorded from dilute DCB solutions featured two absorption bands: one at 330e430 nm, which we assign to localized
p
ep
* transitions, and another, broader band from 445 to 610 nm in the long wavelength region, corresponding to intramolecular charge transfer (ICT) between the acceptor (BO) and donor (9,9-dialkylfluorene, alkylidene fluorene, and N-alkyl-2,7-carbazole) units. The absorption spectra of the three polymers in the solid state were similar to their corresponding solution spectra, with slight red-shifts (ca. 20e40 nm) of their absorption maxima, indi-cating that some intermolecular interactions existed in the solidFig. 2. UVeVis absorption spectra of the polymers PFTBO, PAFTBO, and PCTBO as (a) dilute solutions in DCB (1 105M) and (b) solidfilms.
Table 2
Optical properties of the polymers.
lmax,abs(nm) lonset(nm) Eoptg (eV)
Solution Film Film
PFTBO 525 540 630 1.96
PAFTBO 538 554 640 1.93
PCTBO 525 550 630 1.96
Fig. 3. Cyclic voltammograms of solidfilms of the polymers PFTBO, PAFTBO, and PCTBO.
Table 3
Electrochemical properties of the polymers. Eox
onset(V) Eredonset(V) HOMOa(eV) LUMOa(eV) Eecg (eV)
PFTBO 0.73 1.69 5.53 3.11 2.42
PAFTBO 0.61 1.73 5.41 3.07 2.34
PCTBO 0.70 1.69 5.50 3.13 2.37
aHOMO and LUMO energy levels estimated from oxidation and reduction peaks, respectively, in cyclic voltammograms.
state. The absorption edges for PFTBO, PAFTBO, and PCTBO (Table 2) corresponded to optical band gapsðEopt
g Þ of 1.96, 1.93, and
1.96 eV, respectively.
3.4. Electrochemical properties
Electrochemical cyclic voltammetry has been employed widely to investigate the redox behavior of polymers and to estimate their HOMO and LUMO energy levels.Fig. 3displays the cyclic voltam-mograms of PFTBO, PAFTBO, and PCTBOfilms on a Pt electrode in a solution of TBAPF6(0.1 mol L1) in MeCN;Table 3summarizes the
relevant data. Irreversible n-doping/dedoping (reduction/re-oxidation) processes occurred for these polymers in the negative potential rangedexcept for PCTBO, which underwent a partially reversible reduction. In addition, reversible p-doping/dedoping (oxidation/re-reduction) processes occurred in the positive poten-tial range for each of these polymers. The onset oxidation potenpoten-tials (Eox
onset, vs. Ag/Agþ) for PFTBO, PAFTBO, and PCTBO were 0.73, 0.61,
and 0.70 V, respectively; their onset reduction potentialsðEred onsetÞ
were1.69, 1.73, and 1.69 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 vacuum level) [51]. The HOMO energy levels of PFTBO,
PAFTBO, and PCTBO were5.53, 5.41, and 5.50 eV, respectively. The low-lying HOMO energy levels for these BO copolymers suggest that they are oxidatively stable hole-transporting materials
[52,53]. In addition, low-lying HOMO energy levels are desirable for BHJ solar cells as an approach to maximize the values of Voc. The
LUMO energy levels of PFTBO, PAFTBO, and PCTBO were all located within a reasonable range (from3.07 to 3.13 eV,Fig. 4) and were significantly greater than that of PC61BM (ca.4.1 eV); therefore,
we expected efficient charge transfer/dissociation to occur in their corresponding devices [54,55]. In addition, the electrochemical band gapsðEec
gÞ of PFTBO, PAFTBO, and PCTBO, estimated from the
difference between the onset potentials for oxidation and reduc-tion, were in the range 2.34e2.42 eV; that is, they were slightly greater than the corresponding optical band gaps (1.93e1.96 eV). The discrepancy between the electrochemical and optical band gaps presumably resulted from the exciton binding energies of the polymers and/or the interfacial barriers for charge injection[56]. 3.5. Hole mobility
Fig. 5displays the hole mobilities of devices incorporating the pristine polymers and the polymer/PC61BM blends at a blend ratio of
1:1 (w/w). The hole mobilities of the pristine PFTBO, PAFTBO, and PCTBO were 1.2 104, 5.1 104, and 6.9 104cm2V1s1, respectively, while those of the PFTBO, PAFTBO, and PCTBO blends with PC61BM were 3.1105, 8.7 105, and 1.8 104cm2V1s1,
respectively.
3.6. Photovoltaic properties
We investigated the photovoltaic properties of the polymers in BHJ solar cells having the sandwich structure ITO/PEDOT:PSS/pol-ymer:PC61BM (1:1, w/w)/Ca/Al, with the photoactive layers having
been spin-coated from DCB solutions of the polymer and PC61BM.
The optimized weight ratio for the polymer and PC61BM was 1:1. Fig. 5. Dark JeV curves for the hole-dominated carrier devices incorporating the
pristine polymers and the blendfilms prepared at a blend ratio of 1:1 (w/w).
Fig. 6. JeV characteristics of PSCs incorporating polymer/PC61BM blends [blend ratio, 1:1 (w/w)].
Table 4
Photovoltaic properties of PSCs incorporating BO-based polymers. Polymer/PC61 BM (1:1) (w/w) Voc (V) Jsc (mA cm2) FF (%) PCE (%) Mobility (cm2V1s1) Thickness(nm) PFTBO 1.04 5.4 47 2.6 3.1 105 99 PAFTBO 0.97 7.1 50 3.4 8.7 105 105 PCTBO 0.98 7.2 58 4.1 1.8 104 101
Fig. 6presents the JeV curves of these PSCs;Table 4summarizes the data. The devices prepared from the polymer/PC61BM blends of
PFTBO, PAFTBO, and PCTBO exhibited high open-circuit voltages of 1.04, 0.97, and 0.98 V, respectively. Such high values of Vocare
consistent with these polymers having low-lying HOMO energy levels; notably, these open-circuit voltages are similar to the anticipated values. The short-circuit current densities of the devices incorporating PFTBO, PAFTBO, and PCTBO were 5.4, 7.1, and 7.2 mA cm2, respectively. Fig. 7displays the EQE curves of the devices incorporating the polymer/PC61BM blends at weight ratios
of 1:1. The theoretical short-circuit current densities obtained from integrating the EQE curves of the PFTBO, PAFTBO, and PCTBO blends were 5.2, 6.8, and 7.0 mA cm2dvalues that agree reason-ably with the measured (AM 1.5 G) values of Jsc, with discrepancies
of less than 5%. We attribute the higher values of Jscof PAFTBO and
PCTBO to their higher absorption coefficients (Fig. 2b); consistently, their EQE curve also featured higher responses at 400e650 nm. Therefore, more of the available photons from the solar radiation were absorbed by PAFTBO and PCTBO, leading to their devices exhibiting greater photocurrents.
The highest FF for the device incorporating PCTBO:PC61BM (1:1,
w/w) as the active layer was likely due to the higher hole mobility of this active layer (Fig. 5); indeed, the hole mobilities of PCTBO and PCTBO:PC61BM (1:1, w/w) were greater than those of PFTBO,
PAFTBO, PFTBO:PC61BM (1:1, w/w), and PAFTBO:PC61BM (1:1, w/w).
Moreover, when exploring the decisive factors affecting the efficiencies of PSCs, we must consider not only the absorption and energy levels of the polymers but also the surface morphologies of the polymer blends[57].Fig. 8displays the surface morphologies of our systems, determined using AFM. We prepared samples of the polymer/PC61BM blends using procedures identical to those
employed to fabricate the active layers of the devices. In each case, we observed a quite smooth morphology for PFTBO, PAFTBO and PCTBO blend, with root-mean-square (rms) roughnesses of 0.48,
1.27, and 0.57 nm, respectively. The greater phase segregation and rougher surface of the PAFTBO blend presumably arose because of poor miscibility with PC61BM; indeed, the solubility of PAFTBO was
poorer than those of PFTBO and PCTBO.
A number of other factors can influence the efficiency of a device, including its molecular weight. For example, varying the number-average molecular weight (Mn) of PCDTBT from 10 to
22 kDa caused the PCEs of its devices to vary between 2.26 and 4.15% when using PC61BM as an acceptor; the 19-kDa polymer
provided the best performance[58]. In our case, the value of Mnof
PAFTBO was lower than those of PFTBO and PCTBO; we suspect that improving the solubility and the value of Mnof PAFTBO should
result in PSCs exhibiting higher PCEs. 4. Conclusions
We have used Suzuki coupling polymerization to prepare a ser-ies of new conjugated polymersdPFTBO, PAFTBO, and PCTBOdfeaturing alternating 9,9-dialkylfluorene, alkylidene fluo-rene, and N-alkyl-2,7-carbazole units, respectively, as weakly electron-rich building blocks and TBO units as electron-deficient acceptors in their backbones. The open-circuit voltages of devices fabricated from PFTBO, PAFTBO, and PCTBO blended with PC61BM
(weight ratio, 1:1) were 1.04, 0.97, and 0.98 V, respectively; these excellent values resulted from the relatively low HOMO energy levels of these polymers. The device incorporating PCTBO and PC61BM
exhibited a high value of Vocof 0.98 V, a value of Jscof 7.2 mA cm2,
a FF of 0.58, and a PCE of 4.1% without any post treatment. Acknowledgment
We thank the National Science Council, Taiwan, forfinancial support (NSC 100-2120-M-009-006).
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![Fig. 7. EQE curves of PSCs incorporating polymer/PC 61 BM blends [blend ratio, 1:1 (w/w)].](https://thumb-ap.123doks.com/thumbv2/9libinfo/7511002.117676/5.892.80.426.814.1075/fig-curves-pscs-incorporating-polymer-blends-blend-ratio.webp)
