Synthesis and Photovoltaic Properties of Alternating Conjugated Polymers Derived from Thiophene-Benzothiadiazole Block and Fluorene/Indenofluorene Units

Abstract

A new donor-accepter-donor-accepter-donor (D-A-D-A-D) type 2,1,3-benzothiadiazole-thiophene-based acceptor unit 2,5-di(4-(5-bromo-4-octylthiophen-2-yl)-2,1,3-benzothiadiazol-7-yl)thiophene ($DTBTTBr_2$) was synthesized. Copolymerized with fluorene and indeno[1,2-b]fluorene electron-rich moieties, two alternating narrow band gap (NBG) copolymers PF-DTBTT and PIF-DTBTT were prepared. And two copolymers exhibit broad and strong absorption in the range of 300-700 nm with optical band gap of about 1.75 eV. The highest occupied molecular orbital (HOMO) energy levels vary between -5.43 and -5.52 eV and the lowest unoccupied molecular orbital (LUMO) energy levels range from -3.64 to -3.77 eV. Potential applications of the copolymers as electron donor material and $PC_{71}BM$ ([6,6]-phenyl-$C_{71}$ butyric acid methyl ester) as electron acceptors were investigated for photovoltaic solar cells (PSCs). Photovoltaic performances based on the blend of PF-DTBTT/$PC_{71}BM$ (w:w; 1:2) and PIF-DTBTT/$PC_{71}BM$ (w:w; 1:2) with devices configuration as ITO/PEDOT: PSS/blend/Ca/Al, show an incident photon-to-current conversion efficiency (IPCE) of 2.34% and 2.56% with the open circuit voltage ($V_{oc}$) of 0.87 V and 0.90 V, short circuit current density ($J_{sc}$) of $6.02mA/cm^2$ and $6.12mA/cm^2$ under an AM1.5 simulator ($100mA/cm^2$). The photocurrent responses exhibit the onset wavelength extending up to 720 nm. These results indicate that the resulted narrow band gap copolymers are viable electron donor materials for polymer solar cells.

Introduction

Polymer solar cells (PSCs) with bulk-heterojunction (BHJ) structure have received considerable attention due to its potential applications in making large area, flexible solar panels through roll-to-roll process.1 Owing to the electron acceptor materials fullerenes and their derivatives (PCBM) exhibiting weak absorption in the visible range, the absorption of polymer:PCBM based solar cell is mainly from the electron donor materials. As a result, numerous efforts have focused on the development of the electron donor materials. To achieve high performance PSCs, the ideal polymer should have a broad and strong absorption band, suitable energy levels, high solubility and so on.2 Hence, great progress devoted to develop new narrow band gap (NBG) conjugated polymers, which not only can better match the solar irradiance spectrum for increasing short-circuit current density (Jsc) but also tune energy levels for high open-circuit voltage (Voc) for the high power incident photon-to-current conversion efficiency (IPCE) of PSCs, have been made.3-8

To the best of our knowledge, the most successful strategy to obtain the NBG conjugated polymers is the introduction of donor-acceptor (D-A) alternating repeating units into the backbone of the polymer, which can effectively broaden the absorption and tune energy levels through the intramolecular charge transfer (ICT) from electron-rich unit to electrondeficient moiety. Among the D-A conjugated polymers, plenty of the electron-donating moieties, such as fluorene (F),9 indeno[1,2-b]fluorene (IF),10 carbazole (Cz),11 indeno[3,2-b]carbazole (ICz),12 silafluorene (SiF),13 cyclopenta[2,1-b:3,4-b]dithiophene (CPDT),14 dithieno[3,2-b:2,3-d]silole (DTS),15 dithieno[3,2-b:2,3-d]pyrrole (DTP)16, benzo[1,2-b:4,5-b]dithiophene (BDT),17,18 dithieno[2,3-d: 2,3-d]benzo-[1,2-b:4,5-b]dithiophene (DTBDT),19 naphthadithiophene (NDT)20 and so on have been investigated. Yet, there are only limited number of electron-withdrawing moieties, such as substituted thieno[3,4-b]thiophene18,21 with electron-withdrawing group, thieno[3,4-b]pyrazine (TPz),22 di-ketopyrrolo-[3,4-c]pyrrole-4,6-dione (DPP),23 N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD),24 isoindigo (ID),25 2,1,3-benzothiadiazole (BT)26 have been reported. Amongst them, owing to stronger electron-withdrawing ability, relatively high oxidation potential and good stability in air, the BT and its derivative were investigated widely and exhibited promising performance, which were regarded as one of the effective building blocks to date for tuning the energy levels and broadening the absorption spectrum of the resulting copolymer.27-29 It was noted that BT copolymerized with rich electron unit can reduce the band gap by lowering the lowest unoccupied molecular orbital (LUMO) energy level but the highest occupied molecular orbital (HOMO) energy level have little change.17

Taking into mentioned-above excellent characteristic of BT, herein, a new donor-accepter-donor-accepter-donor (D-A-D-A-D) building block based on BT and thiphene moieties, 2,5-di(4-(5-bromo-4-octyl-thiophen-2-yl)-2,1,3- benzothiadiazol-7-yl)thiophene (DTBTTBr2) was synthesized and characterized. DTBTT has a higher BT content with the ratio of BT/thiophene (2/3) than one of DTBT (1/2). Selecting the fluorene and indeno[1,2-b]fluorene as the inferiordonor, via Suzuki coupling reaction, two NBG alternating copolymers poly {9,9-dioctylfluorene-2,7-diyl-alt- 2,5-di(4-(4-octylthiophen-2-yl)-2,1,3-benzothia-diazol-7-yl)- thiophene-5,5-diyl} and poly {6,6,12,12-tetra-octylindeno-[1,2-b]fluorene-2,8-diyl-alt-2,5-di(4-(4-octyl-thiophen-2-yl)-2,1,3-benzothiadiazol-7-yl)thiophene-5,5-diyl}, namely PFDTBTT and PIF-DTBTT, were synthesized and characterized by elemental analyses, gel permeation chromatrography (GPC), cyclic voltammetry (CV), thermal gravimetric analysis (TGA) etc. Both copolymers showed the broad absorption from 300 to 700 nm, showing the optical band gap of about 1.75 eV, and excellent thermal stabilities. The highest occupied molecular orbital (HOMO) energy levels vary between −5.43 and −5.52 eV. Potential applications of the copolymers to be as electron donor material in BHJ PSCs were investigated, showing the relatively higher Voc about 0.9 V.

 

Experimental

General Methods. 1H NMR spectra were recorded on a Bruker DRX 400 spectrometer. GC-MS and FAB-MS were obtained on TRANCE2000, Fiunigan. Co., and VG ZABHS, respectively. Thermal gravimetric analysis (TGA) was conducted on a TGA 2050 thermal analysis system (TA instruments) under a heating rate of 10 ℃ min−1 with a nitrogen flow rate of 20 mL min−1. Analytical GPC was obtained using a Waters GPC 2410 in tetrahydrofuran (THF) via a calibration curve of polystyrene standards. Elemental analyses were performed on a Vario EL Elemental Analysis Instrument (Elementar Co.). UV-Visible absorption spectra were measured on a UV-2550 spectrophotometer (Shimadzu Co.). PL spectra in solution were taken by Spectro fluorophotometer 970CRT. The microwave assisted polymerization reactions were carried on a mono-microwave reactor system (NOVA-II, Shanghai Preekem Co.). Cyclic voltammetry (CV) was measured on a CHI electrochemical workstation (Shanghai Chenhua Co.) at a scan rate of 50 mV s−1 with a nitrogen-saturated solution of 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in acetonitrile (CH3CN) with platinum and Ag/AgCl as the working and reference electrodes, respectively.

Fabrication and Characterization of the Photovoltaic Cells. A patterned indium tin oxide (ITO) coated glass with a sheet resistance of 10-15 Ω/square was cleaned by a surfactant scrub, followed by a wet-cleaning process inside an ultrasonic bath, beginning with de-ionized water, followed by acetone and isopropanol. After oxygen plasma cleaning for 5 min, a 40 nm thick poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Bayer Baytron 4083) anode buffer layer was spin-casted onto the ITO substrate and then dried by baking in a vacuum oven at 80 ℃ overnight. The active layer, with a thickness in the 70-80 nm range, was deposited on top of the PEDOT: PSS layer by spin-casting. The concentration of the polymer/PC71BM (1:1 or 1:2, weight ratio) blend solutions used in this work for spin-coating active layer was 10 mg/mL (based on the polymer weight concentration), and CB was used as solvent, or 0.5% (v/v) diphenyl sulfide (DPS) was used as additive to improve photovoltaic performance of the devices. Then, a 8 nm calcium and a 100 nm aluminium layer were successively evaporated with a shadow mask under vacuum of 3 × 10-4 Pa. The thickness of the PEDOT: PSS and active layer were verified by a surface profilometer (DektakXT, Boyue In. Co.). The overlapping area between the cathode and anode defined a pixel size of device of 0.10 cm2. The thickness of the evaporated cathode was monitored by a quartz crystal thickness/ratio monitor (SI-TM206, Shenyang Sciens Co.). Except for the deposition of the PEDOT:PSS layers, all the fabrication processes were carried out inside a controlled atmosphere in a nitrogen dry-box (Etelux Co.) containing less than 1 ppm oxygen and moisture. The PCEs of the resulting polymer solar cells were measured under 1 sun AM 1.5G (Air mass 1.5 global, 100 mW·cm-2) condition using a solar simulator (XES-70S1, San-EI Electric Co.). The current density–voltage (J–V) characteristics were recorded with a Keithley 2410 source-measurement unit. The spectral responses of the devices were measured with a commercial EQE/ incident photon to charge carrier efficiency (IPCE) setup (7-SCSpecIII, Bejing 7-star Opt. In. Co.). A calibrated silicon detector was used to determine the absolute photosensitivity.

Materials. All reagents, unless otherwise specified, were obtained from Aldrich, Acros, and TCI Chemical Co., and used as received. All the solvents were further purified under argon flow. 4,7-Dibrom-2,1,3-benzothiadiazole,30 4-octyl-2-(tributylstannyl)thiophene,31 2,5-bis(tributylstannyl)thiophene, 32 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (FB),33 2,8-bis-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-6,6,12,12-tetra-octylindeno[1,2-b]-fluorene (IFB)34 were prepared following the procedures according to reference and characterized by 1H NMR, GC-MS before use.

Synthesis of 4-Bromo-7-(4-octylthiophen-2-yl)-2,1,3-benzothiadiazole (TBTBr). In a 250 mL three-neck flask, 500 mg Pd(PPh3)2Cl2 (0.71 mmol) was added to a stirring 150 mL THF solution of 4,7-dibromo-2,1,3-benzothiadiazole (17 g, 57.8 mmol) and 4-octyl-2-(tributylstannyl)-thiophene (23.4 g, 48 mmol) at room temperature. After stirring for 24 h, the solvent was removed under reduced pressure, then 200 mL cold petroleum ether (PE) was added into the residue and filtrated under reduced pressure. The obtained solid washed with cold PE three times and purified by chromatography using PE/dichloromthane (5:1, V/V Silica gel, 300-400 mesh), affording 13.2 g yellow solid with a yield of 67.3%. mp 45-47 ℃. 1H NMR (400 MHz, CDCl3) δ 7.95 (s,1H); 7.82 (d, J = 8.0 Hz, 1H); 7.67 (d, J = 8.0 Hz, 1H); 7.06 (s, 1H); 2.68 (t, J = 7.2 Hz, 2H); 1.67 (m, 2H); 1.38-1.27 (m, 10H); 0.88 (t, J = 6.4 Hz, 3H). Alal. Calcd for C18H8BrN2S2: C, 52.81, H, 5.17, Br, 19.52%; N, 6.84%; S, 15.66%; Found: C, 52.91%; H, 5.24%; Br, 19.29%; N, 6.07%; S,15.06%.

Synthsis of 2,5-Di(4-(4-octylthiophen-2-yl)-2,1,3-benzothiadiazol-7-yl)thiophene (DTBTT). In a 250 mL threeneck flask containing 2 g, (4.9 mmol) 4-bromo-7-(4-octylthiophen-2-yl)-2,1,3-benzothiadiazole and 1.62 g (2.44 mmol) 4-octyl-2-(tributylstannyl)thiophene in 100 mL THF solution, 200 mg Pd(PPh3)2Cl2 (0.28 mmol) was added and refluxed for 24 h. After cooled to room temperature, the solvent was removed under reduced pressure. The crude product purified by chromatography using PE/dichloromethane (4:1, V/V; Silica gel, 300-400 mesh), affording 1.2 g purple solid with a yield of 66.7%. 1H NMR (400 MHz, CDCl3) δ 8.20 (s, 2H), 7.99 (d, J = 1.2 Hz, 2H), 7.95 (s, J = 8.0 Hz, 2H), 7.85 (s, J = 8.0 Hz, 2H), 7.05 (s, 2H), 2.70 (t, J = 7.6 Hz, 4H), 1.71 (m, 4H), 1.31-1.25 (m, 20H), 0.89 (t, J = 6.8 Hz, 6H). Alal. Calcd for C40H44N4S5: C, 64.82, H, 5.98, N, 7.56, S, 21.63; Found: C, 64.89, H, 5.74, N, 7.91, S, 21.4.

Synthesis of 2,5-Di(4-(5-bromo-4-octylthiophen-2-yl)-2,1,3-benzothiadiazol-7-yl)thiophene (DTBTTBr2). In a 500 mL three-neck flask, 7 g, (9.46 mmol) 2,5-di(7-(4-octylthiophen-2-yl)-2,1,3-benzothiadiazol-4-yl)thiophene was dissolved into the mixture of 300 mL chloroform and 50 mL ice acetic acid. Then 3.7 g (20.8 mmol) N-bromosuccinimide (NBS) was added slowly and stirred at room temperature for 24 h under dark. After cooled to room temperature, the react mixture was poured into 200 mL water and extracted with CHCl3. Then washed with brine and distilled water with three times, respectively. The combined organic layers were dried over MgSO4, and the solvents were removed by rotary evaporation. Finally, the crude compound was purified by column chromatography (Silica gel 300-400 mesh; eluent: PE/dichloromthane 2.5: 1, V/V) and isolated as a purple solid. Yield: 7.3 g (86.1%). The 1H NMR data was not obtained due to its poor slobulity. Alal. Calcd for C40H42Br2N4S5: C, 53.44, H, 4.71, Br, 17.78, N, 6.23, S, 17.84; Found: C, 53.21, H, 4.59, Br, 17.94, N, 6.36, S, 17.25.

Synthesis of the Copolymer PF-DTBTT. Carefully purified monomers FB (128.4 mg, 0.2 mmol), DTBTTBr2 (179.2 mg, 0.2 mmol) and Pd(PPh3)4 (5.0 mg), a mixture of toluene (5 mL) and aqueous solution of 2M Na2CO3 (2 mL) and 5 drops R336 was added into a 55 mL microwave reactor tube under argon. The tube was putted into a mono-microwave reaction system with vigorous stirring. The program of the microwave reaction system is followed as 130 ℃ for 10 min, then 150 ℃ for another 20 min. At the end of polymerization, the polymer was end-capped with phenylboronic acid first, then bromobenzene in mono-microwave reactor to remove bromine and boronic ester end groups, respectively. After the polymerization, the mixture was then poured into 300 mL methanol, and the precipitated material was collected and extracted with ethanol, acetone, hexane and toluene in Soxhlet, respectively. The solution of the copolymer in toluene was condensed to 6 mL and then poured into methanol (300 mL). The precipitation was collected and dried under vacuum overnight (yield: 70%). Mn = 23,300 g/mol(polydispersity index 1.73). 1H NMR (400 MHz, CDCl3) δ 8.27 (m, 2H), 8.24 (br, 2H), 8.00-7.77 (m, 4H), 7.68 (m, 2H), 7.52-7.37 (m, 4H), 3.72 (m, 4H), 2.81 (br, 2H), 2.07 (br, 4H), 1.76 (br, 4H), 1.45-1.10 (m, 36H), 0.89-0.79 (m, 18H).

Synthesis of the Copolymer PIF-DTBTT. Carefully purified monomers IFB (191 mg, 0.2 mmol), DTBTTBr2 (179.2 mg, 0.2 mmol) and Pd(PPh3)4 (5.0 mg), a mixture of toluene (5 mL) and aqueous solution of 2 M Na2CO3 (2 mL) and 5 drops R336 was added into a 55 mL microwave reactor tube under argon. The tube was putted into a monomicrowave reaction system with vigorous stirring. The program of the microwave reaction system is followed as 130 ℃ for 10 min, then 150 ℃ for another 20 min. At the end of polymerization, the polymer was end-capped with phenylboronic acid first, then bromobenzene in monomicrowave reactor to remove bromine and boronic ester end groups, respectively. After the polymerization, the mixture was then poured into 300 mL methanol, and the precipitated material was collected and extracted with ethanol, acetone, hexane and toluene in Soxhlet, respectively. The solution of the copolymer in toluene was condensed to 10 mL and then poured into methanol (300 mL). The precipitation was collected and dried under vacuum overnight (yield: 68%). Mn = 25,500 g/mol (polydispersity index 1.69). 1H NMR (400 MHz, CDCl3) δ 8.27 (m, 2H), 8.25 (br, 2H), 8.10 (m, 2H), 8.02 (m, 2H), 7.94 (m, 2H), 7.64 (br, 2H), 7.54 (m, 4H), 3.73 (m, 8H), 2.81 (br, 4H), 2.09 (br, 8H), 1.75 (br, 4H), 1.42-0.95 (m, 60H), 0.89-0.78 (m, 18H).

 

Results and Discussion

Synthesis and Characterization. The general synthetic routes toward the monomer DTBTTBr2 and copolymers are outlined in Scheme 1. 4,7-Dibromo-2,1,3-benzothiadiazole, 4-octyl-2-(tributylstannyl)thiophene, 2,5-bis(tri-butylstannyl) thiophene, FB and IFB were prepared following the corresponding procedures.30-34 The key monomer DTBTTBr2 was synthesized by three-step procedures. First, procedures. First, unsymmetrical TBTBr was obtained by coupling dibromobenzothiadiazole with one equiv. 4-octyl-2-(tributylstannyl) thiophene. Then, DTBTT was obtained by Stille coupling TBTBr with 2,5-bis(tributylstannyl)thiophene in a ratio of 2:1. Third, DTBTT was brominated with NBS in a 6:1 (V/V) mixture of chloroform and ice acetic acid to afford the monomer DTBTTBr2. The 1H NMR spectrum of BrTBT and DTBTT are shown in Figure S1-S2. The alternating copolymers PF-DTBTT, PIF-DTBTT from DTBTT and FB or IFB, were synthesized by the palladiumcatalyzed Suzuki coupling reaction in a mono-microwave assisted reactor system. The end-capping reactions were performed using bromobenzene and phenylboronic acid to increase the stability of the copolymer.11 Two copolymers show good solubility in common organic solvents such as chloroform, THF and toluene. The number average molecular weight of PF-DTBTT and PIF-DTBTT was determined by GPC using a polystyrene standard and THF as eluent, is about 23,300 g/ mol and 25,500 g/mol with a polydispersity index (Mw/Mn) of 1.73 and 1.69 (Table 1), respectively.

Scheme 1.The synthetic route of the monomer and copolymers.

Table 1.Molecular weight and thermal properties of the copolymers

Thermal Stabilities. Thermal stabilities of the copolymers were determined by thermogravimetric analysis (TGA) under nitrogen. As shown in Figure S3, TGA curves exhibit that both copolymers show excellent thermal stability, showing degradation temperature (Td with 5% loss) up to 416 ℃ and 393 ℃ for PF-DTBTT and PIF-DTBTT (Table 1), respectively. The degradation patterns of the copolymers are similar. The thermal stability of both polymers is adequate for the fabrication processes of PSCs and other applications in optoelectronic devices.

Optical Properties. The normalized UV-vis absorption spectra of the synthesized copolymers in trichloromethane (TCM) solution and thin-film states are shown in Figure 4, and the related optical data are summarized in Table 2. As shown in Figure 1, both copolymers display the broad absorption peaks in the range of 300-700 nm in the dilute TCM solution, which is slightly broader than these of PFDTBT9 and PIFDTBT.10 The polymers show two absorption bands, the peaks in the long wavelength region locate at 546 and 547 nm, the peaks in the short wavelength region lie aggregation in the solid state.35 From the onset of the thin film absorptions, according to the optical band gaps (Egopt) = at 361 and 372 nm for PF-DTBTT and PIF-DTBTT, respectively. 1240/λonset, we can estimate Egopt of 1.79 and 1.75 eV for PFDTBTT and PIF-DTBTT, respectively. The long wavelength absorption is due to strong D-A charge transfer state, which led to the extended absorption. Compared to the solution, both polymers have 14-23 nm red-shift, which is caused by the increased polymer aggregation in the solid state.35 From the onset of the thin film absorptions, according to the optical band gaps (Egopt) =1240/λonset, we can estimate Egopt of 1.79 and 1.75 eV for PF-DTBTT and PIF-DTBTT, respectively.

Figure 1.Normalized absorption and PL spectra of PF-DTBTT (a) and PIF-DTBTT (b) in solution and thin film.

Table 2.aCalculated by onset band gap wavelength of the copolymers in thin film (Egopt = 1240/λonset). bCalculated by oxidation potential of the copolymer (EHOMO = –e(Eox + 4.4)). cCalculated optical band gap and HOMO levels of the copolymers (ELUMO = EHOMO + Egopt)

Figure 2.Cyclic voltammogram of copolymers on a platinum plate in acetonitrile solution of 0.1 M [Bu4N]PF6 at a scan rate of 50 mV/s.

Electrochemical Properties. The electrochemical behaviors of the copolymers were investigated by cyclic voltammetry (CV). As shown in Figure 2, the oxidation potentials of PF-DTBTT and PIF-DTBTT were observed at around 1.03 V and 1.12 V. The HOMO levels of PF-DTBTT and PIF-DTBTT, calculated by empirical formulas: EHOMO =−e(Eox + 4.4) V, were about −5.43 and −5.52 eV, respectively. The LUMO energy levels of the copolymers were determined to be −3.64 eV for PF-DTBTT and −3.77 eV (Table 2) for PIF-DTBTT as calculated from their HOMO levles and optical band gaps (Egopt) obtained from the absorption edge of the copolymers in solid thin film.

Photovoltaic Properties of the Copolymers. The potential of the copolymers to be employed as electron donor materials for PSCs were investigated. PSCs cells based on the blend of the copolymers and PC71BM with devices configuration as ITO/PEDOT:PSS/blend/Ca/Al were fabricated. The parameters of the PSCs were shown in Table 3. Figure 3 showed a typical current density-voltage (J-V) characteristic of the devices based on blend of PF-DTBTT/PC71BM (Fig. 3(a)) and PIFDTBTT/ PC71BM (Fig. 3(b)) under the AM 1.5G illumination at an irradiation intensity of 100 mW·cm-2. The best performances were observed with devices based on the PFDTBTT/ PC71BM (w:w, 1:2) and PIF-DTBTT/PC71BM (w:w, 1:2) blend (Table 3). The devices based on PF-DTBTT/ PC71BM (w:w) blend show IPCE of 0.22%-2.34%, with the open circuit voltage of 0.86 V to 0.87 V, the short current densities of 0.86-6.02 mA/cm2 and filled factors of 30%- 44% under the AM 1.5G illumination at an irradiation intensity of 100 mW·cm−2. The devices based on PIFDTBTT/ PC71BM (W:W, 1:2) blend show PCEs of 0.19%- 2.56%, with the open circuit voltage of 0.9 V, the short current densities of 0.66-6.12 mA/cm2 and filled factors of 25%-48% under the AM 1.5G illumination at an irradiation intensity of 100 mW·cm2.

Figure 3.J/V curves the PSC based on the PF-DTBTT/ PC71BM blend (a) and PIF-DTBTT/PC71BM blend (b).

The IPCE plots of the devices under short-circuit conditions is shown in Figure 4. The photo-current response wavelength of the devices based on PF-DTBTT and PIF-DTBTT are extending from 300 nm to 760 nm and 300 nm to 800 nm, respectively. The EQE curves of the devices based on the copolymers have two features peaks at around 395-413 nm and the other peaks around 580 nm to 605 nm for devices based on PF-DTBTT, and two features at around 410 nm and the other peaks around 615 nm to 635 nm for that of PF-DTBTT. These characteristics were closely following the trend observed in the absorption spectra of the PF-DTBTT and PIF-DTBTT (Fig. 1), indicating that the harvested photons over the entire absorption spectra contribute to the photocurrent. Optimization of PSCs based on PF-DTBTT and PIF-DTBTT got under way.

Figure 4.IPCE curves the PSC based on the PF-DTBTT (a) and PIF-DTBTT/PC71BM (b).

In order to scrutinize the deep factor to the low FF and current density, the hole mobility of the blend films of PF-DTBTT/ PC71BM and PIF-DTBTT/PC71BM (w/w, 1:2) were measured by the space-charge-limited current (SCLC) method with a device structure of ITO/PEDOT:PSS/polymers: PC71BM/Au. The field dependent SCLC behavior can be expressed as36

where εr ≈ 3 is the relative dielectric constant of the organic film, ε0 is the vacuum dielectric constant, d is the film thickness and μ0 exp[γ(E)1/2] is the electric field dependent charge carrier mobility. Here, μ0 is the mobility at E = 0 and γ denotes field activation parameter. Figure 5 presents the J – V characteristics of hole-only devices. The hole mobilities of PF-DTBTT/PC71BM and PIF-DTBTT/PC71BM blends are calculated to be 7.42 × 10−5 and 8.73 × 10−5 cm2 V−1 s−1, respectively. The results show insufficiency of holes extracted in the devices. Moreover, the electron mobility of PCBM is reported to be about 10−3 cm2 V−1 s−1. This unbalanced charge transport of charge carriers causes the low FF and current density.

Figure 5.J–V characteristics of hole-only devices with a structure of ITO/PEDOT:PSS/blends/Au.

Figure 6.AFM images: (a, b) PF-DTBTT/PC71BM without and with DPS; (c, d) PIF-DTBTT/PC71BM without and with DPS.

Surface morphologies of the blend films of PF-DTBTT/ PC71BM and PIF-DTBTT/PC71BM with and without DPS were investigated by the atomic force microscope (AFM). As shown in Figure 6, without DPS, the root-mean-square roughness (Rq) of the PF-DTBTT/PC71BM and PIF-DTBTT/ PC71BM films are 36.3 and 42.1 nm, respectively. However, as presence of the additive DPS, Rq values of the blend films dropped to 1.38 and 4.55 nm for the films of PF-DTBTT/ PC71BM and PIF-DTBTT/PC71BM, respectively. The effects of DPS treatment on morphology of the blend films are well demonstrated as shown in Figure 6. Upon addition of 0.5% DPS, a significant reduction in phase separation is observed in Figure 6(b) and 6(d). On the basis of the observations in the AFM images, it can be concluded that after adding DPS, the roughness of the blend films dropped remarkably, and the phase separation of the blends reduced distinctly.

Table 3.Photovoltaic parameters of the PSC based on the copolymers and PC71BM

We expected that IPCEs can be further improved by the optimization of electron acceptor materials, and fabrication conditions (film morphology, thickness of active layer, electrode buffer layer etc).

 

Conclusion

Two NBG conjugated copolymers PF-DTBTT and PIF-DTBTT were synthesized via Suzuki coupling between the new acceptor unit DTBTT and dioctylfluorene or tetraoctylindeno[1,2-b]fluorene units. Both copolymers showed good thermal stabilities and good solution processibilities. Potential applications of the polymers were investigated. Prototype polymer photovoltaic solar cells based on the blend of PF-DTBTT (or PIF-DTBTT) /PC71BM (w:w; 1:2) showed PCE of 2.34% and 2.56%, with the Voc of 0.87 V and 0.90 V, Jsc of 6.02 mA/cm2 and 6.12 mA/cm2 under an AM1.5 simulator (100 mW/cm2). These results indicate that the NBG copolymers are viable electron donor materials for application in photovoltaic solar cells.