A triple bond side-chained 2D-conjugated benzodithiophene based photovoltaic polymer

Abstract

A series of triple bond side-chained benzodithiophene copolymers, derived from 4,8-bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-benzo[1,2-b:4,5-b′]dithiophene, were synthesized by Stille coupling reactions. Several electron acceptors are introduced into the polymer backbone to tune the photo-electronic properties of the copolymers. The obtained copolymers are readily soluble in common organic solvents, such as toluene, THF and chloroform. The number average molecular weights of these polymers were determined by GPC using a polystyrene standard, ranging from 15 400 to 28 300 which are decent results for polymers with a rigid triple bond side chain. These polymers show high decomposition temperature (403–419 °C), narrow band gap (1.27–1.89 eV), and low-lying HOMO energy level (−5.4 to −5.6 eV). The best polymer solar cell (PSC) device based on the copolymer PBDTPA-TPD and PC₆₁BM showed a power conversion efficiency (PCE) of 0.93% with a V_oc = 0.96 V, J_sc = 1.9 mA cm⁻², and FF = 51%, which is one of the highest V_oc values achieved by a benzodithiophene polymer.

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1. Introduction

Polymer solar cells (PSCs) have attracted increasing interest due to their light-weight, large-area, and flexibility through solution processing.^1–4 PSCs usually adopt a bulk-heterojunction (BHJ) structure, and the photoactive layer consists of an interpenetrating network of π-conjugated polymer donors and soluble fullerene acceptors.^5–7 Among the different polymer donor units, the benzodithiophene (BDT) unit shows good planarity due to its symmetrical molecular structure, meanwhile BDT polymer also exhibits high carrier mobility due to strong intermolecular π–π stacking interactions.^8–10 A 4,8-dialkoxy substituted BDT based donor–acceptor (D–A) type conjugated polymer, has shown an excellent maximum power conversion efficiency (PCE) of 9.35% (ref. 11) which is the highest PCE for PSCs. However, modification and optimization of BDT based polymers are still hot topics for higher efficiency PSCs. For example, by replacing strong electron-donating alkoxy chains with a weak electron donor, the HOMO energy level of the polymer is reduced, which can increase the open circuit voltage (V_oc) of PSCs devices.¹² By the concept of two dimensional (2D) conjugated BDT, dialkyl-thiophene substituted BDT is synthesized, which to some extent reduces the HOMO energy level and extends the absorption spectrum of the polymer, compared to dialkoxy substituted BDT.^13–17 Besides the thiophene side chain of BDT, other conjugated side chains, such as thiophene[3,2-b]thiophene, benzene etc.,¹⁸ have also been tried to build a more efficient BDT photovoltaic polymer.

According to the sp hybrid in the hybrid orbital theory, the triple bond in the alkynyl group is a weak electron withdrawing group, meanwhile performs high rigidity which is good for intermolecular stacking.¹⁹ By introducing alkynyl group into BDT side chain, the HOMO energy level of polymer materials can be lowered, due to enhanced π–π stacking. PCE of (tri-isopropyl-silyl) acetylene based BDT polymer have reached 5.76%,²⁰ but strong rigidity of triple bond leads to poor solubility of the polymer, which make it inconvenient for PSCs devices processing.

In this paper, to make more soluble alkynyl substituted BDT donor materials, we have designed and synthesized two-dimension conjugated unit of dialkoxy-phenylacetylene substituted BDT, 4,8-bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-benzo[1,2-b:4,5-b′]dithiophene (Scheme 1). Copolymerized with the acceptor TPD, DPP and TT (Scheme 2), three BDT based copolymers are obtained, and their optical properties, electrical properties and photovoltaic properties are studied.

Scheme 1 Synthetic routes of monomers. Reaction conditions: (a) BBr₃, methylene chloride, 0 °C; (b) bromooctane, K₂CO₃, DMF, 60 °C; (c) ethynyl-trimethyl silicon, CuI, trans-dichlorobis(triphenyl-phosphine)palladium(II), triethylamine, 80 °C; (d) KOH, methylene chloride; (e) n-BuLi, THF, 0 °C, SnCl₂, reflux; (f) n-BuLi, Me₃SnCl, THF, 0 °C.

Scheme 2 Synthetic routes of polymers.

2. Experimental section

2.1. Materials

Unless otherwise stated, all reagents and starting materials were used as commercially purchased without further purification. The TPD, DPP and TT comonomers were purchased from Derthon Optoelectronic Materials Science Technology Co Ltd. Tetrahydrofuran (THF) and toluene were distilled from sodium with benzophenone as indicator and N,N-dimethylformamide (DMF) was distilled from CaH₂ under argon atmosphere before use. All air and water sensitive reactions were performed under argon or nitrogen atmosphere.

2.2. Materials characterization

The nuclear magnetic resonance (NMR) spectra were recorded on a BrukerAdvanceIII 600 (600 MHz) using CDCl₃ as the solvent. UV-vis absorption spectra were measured with a Hitachi U-4100 spectrophotometer. The molecular weight and polydispersity were determined by gel permeation chromatography (GPC) analysis using an ELEOS System with polystyrenes as the reference standard and THF as an eluent. Thermogravimetric analysis (TGA) was performed on a TA-Q600 analyzer with a heating rate of 20 °C min⁻¹ under nitrogen. Electrochemical cyclic voltammetry (CV) measurements were conducted on an electrochemistry workstation (CHI660D) with a glassy carbon as working electrode, a platinum wire as counter electrode and an Ag/AgCl as reference electrode under nitrogen in a solution of Bu₄NPF₆ (0.1 M).

2.3. Device fabrication

Photovoltaic devices were fabricated on pre-patterned indium tin oxide (ITO) coated glass substrates with a layered structure of ITO/PEDOT:PSS/donor:PC₆₁BM/Ca (10 nm)/Al (100 nm). The ITO coated glass substrates were cleaned in ultrasonic bath in acetone, toluene, methanol and isopropyl alcohol sequentially. After a twenty-minute oxygen plasma treatment, a thin layer of PEDOT:PSS (30 nm) was spin-coated onto the ITO anode and then dried at 120 °C for 20 min. The photosensitive layer was prepared by spin-coating a blend solution of the polymers and PC₆₁BM with a weight ratio of 1 : 1.5 in deoxygenated anhydrous o-dichlorobenzene on the ITO/PEDOT:PSS substrate and then annealed at 150 °C for 10 min in a glove box. 4% of DIO was added as processing additive to the blend solutions. The active layer thickness was around 90 nm. Finally, Ca (10 nm) and aluminum (100 nm) were thermally evaporated at a vacuum of ∼2 × 10⁻⁴ Pa on top of active layer. The photovoltaic performance was measured under illumination at 100 mW cm⁻² AM 1.5G irradiation using a Xe arc lamp in an argon atmosphere (<0.1 ppm H₂O and O₂), and the current density–voltage (J–V) curves was obtained by Keithley 2400. The external quantum efficiency (EQE) was obtained by a source meter, silicon photodiode and a computer-controlled light source-monochromator-lock-in system.

2.4. Synthesis

1-Bromo-3,5-di-hydroxyphenyl (M1). 3,5-Dimethoxybromobenzene (10.0 g, 46.2 mmol) was dissolved in 100 ml dry CH₂Cl₂. The solution of 3,5-dimethoxybromobenzene was cooled to −78 °C, and 1 M CH₂Cl₂ solution of BBr₃ (100 ml, 100.0 mmol) was added slowly. Then, the solution was allowed to warm to ambient temperature overnight. The stirred solution was cooled to 0 °C, and then quenched with H₂O. The aqueous phase was extracted with Et₂O. The combined organic layers were washed with H₂O, dried over MgSO₄. Solvent was removed, and the resulting white solids were used for the next step reaction without further purification.

1-Bromo-3,5-bis(octyloxy)phenyl (M2). 1-Bromo-3,5-di-hydroxyphenyl (3 g, 15.9 mmol) and anhydrous K₂CO₃ (16.21 g) was diluted with 90 ml of N,N-dimethylformamide, and the mixture was stirred at ambient temperature for 30 min. After the addition of bromooctane (12.4 g, 64.4 mmol), solution was heated at 60 °C overnight. The cooled reaction mixture was filtered through Celite, and concentrated to yield the crude product which was purified by column chromatography (hexane eluent) to obtain 1-bromo-3,5-bis(octyloxy)phenyl 4.4 g (10.5 mmol, 66.8%). ¹H NMR (CDCl₃, 600 MHz), δ (ppm): 6.63 (d, 2H), 6.36 (t, 1H), 3.89 (t, 4H), 1.75 (m, 4H), 1.42 (m, 4H), 1.29 (m, 16H), 0.89 (t, 6H) (Fig. S1†).

1-(Trimethylsilyl)ethynyl-3,5-bis(octyloxy)phenyl (M3). 1-Bromo-3,5-bis(octyloxy)benzene (4.3 g, 10.5 mmol), copper iodide (27.3 mg) and bis(triphenylphosphine)dichloridepalladium (336 mg) was dissolved in 42 ml of dry triethylamine under nitrogen. After the mixture was heated to 50 °C, trimethylsilylacetylene (1.1 g, 11.4 mmol) was added. The solution was heated at 80 °C for 24 h. The cooled reaction mixture was quenched with 2 M HCl solution, washed with H₂O, dried over MgSO₄, and concentrated to yield crude product, which was purified by column chromatography (hexane eluent) to obtain 3.2 g of light yellow powder (7.3 mmol, 70%). ¹H NMR (CDCl₃, 600 MHz), δ (ppm): 6.62 (d, 2H), 6.46 (t, 1H), 3.94 (t, 4H), 1.78 (m, 4H), 1.46 (m, 4H), 1.33 (m, 16H), 0.92 (t, 6H), 0.27 (s, 9H) (Fig. S2†).

1-Ethynyl-3,5-bis(octyloxy)phenyl (M4). 1-(Trimethylsilyl)ethynyl-3,5-bis(octyloxy)benzene (3.2 g, 7.3 mmol) was dissolved in 35 ml of dichloromethane, and 2.3 g of potassium hydroxide was dissolved in 35 ml of methanol. These two solutions were mixed and stirred at ambient temperature for 3 h. The reactant was poured into water and extracted by dichloromethane. The organic layer was dried over anhydrous MgSO₄. After removing solvent, the crude product was purified by column chromatography (hexane eluent) to obtain 2.4 g of light yellow powder (7.68 mmol, 99%). ¹H NMR (CDCl₃, 600 MHz), δ (ppm): 6.65 (d, 2H), 6.49 (t, 1H), 3.94 (t, 4H), 3.04 (s, 1H), 1.78 (m, 4H), 1.46 (m, 4H), 1.33 (m, 16H), 0.92 (t, 6H) (Fig. S3†).

4,8-Bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-benzo[1,2-b:4,5-b′]dithiophene (M5). 1-Ethynyl-3,5-bis(octyloxy)benzene (2 g, 6.4 mmol) was dissolved in 70 ml of dry THF under nitrogen. Solution was cooled to 0 °C, and then 4 ml of n-BuLi (6.5 mmol, 1.6 M) was added dropwise. The mixture was stirred at 0 °C for 30 min, then benzo[1,2-b:4,5-b′]dithiophene-4,8-dione (0.67 g, 3.1 mmol) was added. The stirred mixture was heated at reflux for 1 h, then cooled to 0 °C again. 3.3 g of tin chloride dihydrate dissolved in 50 ml 20% HCl solution was added, and the reaction was heated at reflux for 2 h. The cooled reaction mixture was extracted with ether. The organic layer was washed with H₂O several times, dried over MgSO₄, and concentrated to yield crude product, which was purified by column chromatography (hexane–dichloromethane (8 : 1, v/v) eluent) to obtain 1.3 g of light yellow powder (1.47 mmol, 45%). ¹H NMR (CDCl₃, 600 MHz), δ (ppm): 7.74 (d, 2H), 7.62 (d, 2H), 6.84 (d, 4H), 6.55 (t, 2H), 4.02 (t, 8H), 1.84 (m, 8H), 1.51 (m, 8H), 1.36 (m, 32H), 0.93 (t, 12H) (Fig. S4†).

2,6-Bis(trimethyltin-4,8-bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-benzo[1,2-b:4,5-b′]dithiophene) (M6). 4,8-Bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-benzo[1,2-b:4,5-b′]bithiophene (BDTPA) (0.6 g, 0.68 mmol), N,N,N,N-tetramethylethylenediamine (0.28 ml, 1.8 mmol) was dissolved in 30 ml of dry THF under nitrogen. The mixture was cooled to 0 °C, and 1.2 ml of n-BuLi (1.8 mmol, 1.6 M) was added dropwise. After being stirred at 0 °C for 4 h, 2.2 ml of 1 M trimethyltin chloride was added slowly. The stirred solution was warm up to ambient temperature for overnight. After the reaction, the solution was poured into water and extracted by ether. The organic layer was dried over anhydrous MgSO₄. After removing solvent under vacuum, the residue was recrystallized from isopropanol to obtain 0.67 g of product (0.544 mmol, 80%). ¹H NMR (CDCl₃, 600 MHz), δ (ppm): 7.76 (t, 2H), 6.86 (d, 4H), 6.55 (t, 2H), 4.03 (t, 8H), 1.84 (m, 8H), 1.51 (m, 8H), 1.36 (m, 32H), 0.93 (t, 12H), 0.52 (t, 18H) (Fig. S5†). ¹³C NMR (CDCl₃, 150 MHz), δ (ppm): 160.1, 144.7, 143.6, 139.1, 130.9, 124.2, 110.2, 110.0, 102.9, 98.7, 85.6, 68.3, 31.8, 29.4, 29.2, 26.0, 22.7, 14.1, −8.2 (Fig. S6†). Anal. calcd for C₆₄H₉₄O₄S₂Sn₂: C, 62.55; H, 7.71; S, 5.22. Found: C, 62.73; H, 7.62; S, 5.15. HRMS (APCI) m/z: calcd 1229.4632 [M + 1]⁺, found: 1229.4740.

PBDTPA-TPD. 2,6-Bis(trimethyltin)-4,8-bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-benzo[1,2-b:4,5-b′]dithiophene (184.4 mg, 0.15 mmol), 1,3-dibromo-5-(2-ethylhexyl)-thieno[3,4-c]pyrrole-4,6-dione (TPD) (63.5 mg, 0.15 mmol), (o-tolyl)phosphine (7.3 mg, 16%) and tris(dibenzylideneacetone)dipalladium (2.8 mg, 2%) was dissolved in 5 ml dry toluene under nitrogen. The mixture was heated to 70 °C for 4 h. Then, the stirred solution was heated to 110 °C for 48 h. After the reaction, the cooled mixture was dropped into methanol. The precipitate was collected and Soxhlet-extracted in sequence with methanol, hexane and chloroform. The product was dried under vacuum for 1 day to obtain the target polymer PBDTPA-TPD as a dark red solid. ¹H NMR (CDCl₃, 600 MHz), δ (ppm): 9.5–7.5 (m), 7.2–5.5 (m), 4.5–3.3 (m), 2.3–0.3 (m) (Fig. S7†). GPC (THF, polystyrene standard): M_n = 23.9 kDa, M_w = 57.7 kDa, PDI = 2.41.

PBDTPA-TT. 2,6-Bis(trimethyltin)-4,8-bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-benzo[1,2-b:4,5-b′]dithiophene (122.9 mg, 0.1 mmol), 2-ethylhexyl-4,6-dibromo-thieno[3,4-b] thiophene-2-carboxylate (TT) (48.6 mg, 0.1 mmol), tris(o-tolyl)phosphine (4.9 mg, 16%) and tris(dibenzylideneacetone)dipalladium (1.8 mg, 2%) was dissolved in 5 ml of dry toluene under nitrogen. The other procedures are just as PBDTPA-TPD polymer. Finally, the target polymer PBDTPA-TT is obtained as a dark solid. ¹H NMR (CDCl₃, 600 MHz), δ (ppm): 8.5–7.5 (m), 7.2–6 (m), 4.5–3.2 (m), 2.2–0.3 (m) (Fig. S8†). GPC (THF, polystyrene standard): M_n = 15.4 kDa, M_w = 81.2 kDa, PDI = 5.25.

PBDTPA-DPP (OD). 2,6-Bis(trimethyltin)-4,8-bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-benzo[1,2-b:4,5-b′]dithiophene (61.4 mg, 0.05 mmol), 3,6-bis(5-bromo-thiophen-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione (DPP(OH)) (50.9 mg, 0.05 mmol), tris(o-tolyl)phosphine (2.4 mg, 16%) and tris(dibenzylideneacetone)dipalladium (0.9 mg, 2%) was dissolved in 5 ml dry toluene under nitrogen. The other procedures are just as PBDTPA-TPD polymer. Finally, the target polymer PBDTPA-DPP(OD) is obtained as a dark green solid. ¹H NMR (CDCl₃, 600 MHz), δ (ppm): 9.0–8.5 (m), 7.1–6.2 (m), 4.0 (s), 2.1–1.2 (m) (Fig. S9†). GPC (THF, polystyrene standard): M_n = 28.3 kDa, M_w = 275.6 kDa, PDI = 9.72.

3. Results and discussion

3.1. Synthesis and characterization

The general synthetic routes toward the monomer and copolymers are outlined in Schemes 1 and 2 respectively. All the compounds are characterized by ¹H NMR spectra. They are in accordance with the literature²¹ for the precursors and show correctly the designed structure for polymers (see ESI Fig. S7–S9†). The obtained copolymers are readily soluble in common organic solvents, such as toluene, THF, and chloroform. The number-average molecular weights of these polymers were determined by GPC using a polystyrene standard, ranging from 15 400 to 28 300 with a polydispersity index (M_w/M_n) between 2.41 and 9.72. The molecular weights data is summarized in Table 1.

Table 1 Molecule weight and optical properties of the polymers^a,b

Polymer	Mn (k)	Mw (k)	PDI	λmax-S (nm)	λmax-F (nm)	ε (cm^−1) thin film	E^optg (eV)
a λmax-S: absorption peak in solution; λmax-F: absorption peak in film.b E^optg: optical band gap is calculated from 1240/λonset.
PBDTPA-TPD	23.9	57.7	2.41	587	613	4.1 × 10^4	1.89
PBDTPA-TT	15.4	81.2	5.25	603	624	2.4 × 10^4	1.37
PBDTPA-DPP(OD)	28.3	275.6	9.72	683	784	4.7 × 10^4	1.27

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The thermal properties of the polymer were measured with a heating rate of 20 °C min⁻¹ under inert atmosphere. As shown in Fig. 1, the decomposition temperature (T_d) at 5% weight loss of PBDTPA-TPD, PBDTPA-TT and PBDTPA-DPP(OD) was 419.7 °C, 403.7 °C and 401.7 °C respectively. The high decomposition temperature decreases the possibility of the deformation of the polymer film morphology and the degradation of the polymer active layer under applied electric fields in solar cell devices.

Fig. 1 TGA plots of the polymers with a heating rate of 20 °C min⁻¹ under an inert atmosphere.

3.2. Optical properties

The absorption spectra of the polymers were measured both in chloroform solution and in thin films (Fig. 2, Table 1). Since the different electron acceptors of copolymers lead to various absorption spectral features, PBDTPA-TPD, PBDTPA-TT and PBDTPA-DPP(OD) exhibit absorption peaks at ∼587, 603 and 683 nm respectively. Compared to the absorption in solution, the thin films show red-shifted UV-vis absorption spectrum, with absorption peaks at ∼613, 624 and 784 nm, respectively. Especially for PBDTPA-DPP(OD), the absorption spectrum becomes broader and shows a 101 nm red-shifted in thin film, which can be attributed to the planar polymer chain structure and effective inter-chain π–π stacking in the solid state. The introduction of alkynyl group enhances intermolecular interactions by extending the 2D conjugation of polymers. The absorption coefficients were tested in thin film and middle value (<5 × 10⁴ cm⁻¹) were obtained for all three polymers, which may cause limited photocurrent in solar cells.²²

Fig. 2 Normalized absorption spectra of the polymers in chloroform solution and in thin film.

3.3. Electrochemical characterization

Cyclic voltammogram (CV) (Fig. 3) was performed to evaluate the HOMO and LUMO energy levels of all the polymers. HOMO and E_LUMO were estimated from the onset oxidation potential and reduction potential of the polymer film.²³ The HOMO and LUMO levels are calculated according to the empirical formula E_HOMO = −e(E^onset_ox + 4.4) eV and E_LUMO = −e(E^onset_red + 4.4) eV. The data are listed in Table 2. The polymers show deep HOMO level around −5.6 and −5.4 eV, which may result in high open circuit voltage (V_oc) in OSC device. Meanwhile, the LUMO levels were calculated from optical band gap. The materials of PBDTPA-TT and PBDTPA-DPP(OD) exhibit low LUMO^opt value of below −4.0 eV, which may lead to insufficient exciton dissociation at donor–acceptor interfaces.²⁴

Fig. 3 Cyclic voltammogram of the polymers.

Table 2 Electrochemical properties of the polymers

Polymer	Eox (V)	Ered (V)	HOMO (eV)	LUMO (eV)	LUMO^opt^a (eV)	E^cvg^b (eV)
a LUMO^opt = HOMO − E^optg.b E^cvg = LUMO − HOMO.
PBDTPA-TPD	1.24	−1.17	−5.65	−3.24	−3.76	2.41
PBDTPA-TT	1.05	−0.85	−5.44	−3.57	−4.07	1.87
PBDTPA-DPP(OD)	1.01	−1.02	−5.42	−3.39	−4.15	2.03

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3.4. Photovoltaic performance

Polymer solar cells (PSCs) devices were fabricated with the resultant polymer as the donor and PC₆₁BM as the acceptor to investigate the photovoltaic properties of the polymer. The device structure is ITO/PEDOT:PSS/polymer:PC₆₁BM/Ca/Al. To improve the photovoltaic performance of the device, 1,8-diiodooctane (DIO) was used as the additive and the optimized D/A ratio of polymer: PC₆₁BM was 1 : 1.5 (Fig. S10†). Fig. 4 shows the current density–voltage curves (J–V) of the devices under the illumination of AM 1.5G (100 mW cm⁻²). The PBDTPA-TPD/PC₆₁BM device exhibited the best PCE of 0.93%, V_oc of 0.96 V. Due to the low-lying HOMO energy level of PBDTPA-TPD (−5.4 eV), the PSCs based on PBDTPA-TPD show nearly 1 V (0.96 V) V_oc which is one of the highest V_oc value achieved by benzodithiophene polymer. For PBDTPA-DPP(OD)/PC₆₁BM based PSCs devices, the best result was obtained with a PCE = 0.39% and a V_oc of 0.45 V. PBDTPA-TT based device performances wasn't listed in Table 3, because it does not show photovoltaic response in the ITO/PEDOT:PSS/polymer:PC₆₁BM/Ca/Al device structure. For polymers PBDTPA-DPP(OD) and PBDTPA-TT, the deep LUMO level (<−4.0 eV) may be a detrimental factor for the low PCE value, because the LUMO–LUMO offset with PC₆₁BM acceptor is less than 0.3 eV and exciton dissociation could not be sufficiently efficient. Furthermore, low value of absorption coefficient would also reduce the PCE of OSC (Table 1). The dark J–V curves were tested for polymers of PBDTPA-TPD and PBDTPA-DPP(OD) (Fig. 4). It shows good diode character.

Fig. 4 J–V curves of the PSCs based on the blend of PBDTPA-TPD or PBDTPA-DPP(OD)/PC₆₁BM under the illumination of AM 1.5, 100 mW cm⁻².

Table 3 Solar cells devices performance of PBDTPA-TPD and PBDTPA-DPP(OD)

Active layer	Voc (V)	Jsc (mA cm^−2)	FF (%)	PCE (%)
PBDTPA-TPD/PC61BM	0.96	1.90	51	0.93
PBDTPA-DPP(OD)/PC61BM	0.45	2.26	39	0.39

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The external quantum efficiency (EQE) spectra of the devices are shown in Fig. 5. EQE value of <20% were obtained for polymers PBDTPA-TPD and PBDTPA-DPP(OD). The EQE spectrum of PBDTPA-DPP(OD) shows relatively low value beyond 500 nm, while its maximum absorption in film is at about 784 nm. This mismatch reflect insignificant contribution from the major absorption of PBDTPA-DPP(OD) to the photocurrent of solar cell.²⁵ That is to say, the conversion of the low-energy excitons (from the polymer's absorption) to harvestable free charges is not efficient. This is most probably due to the poor dissociation efficiency of the low-energy excitons as the polymer-PC₆₁BM LUMO energy difference is too small (<0.3 eV).²⁶ This induces less-effective utilization of the absorption and low J_sc is obtained as a result.

Fig. 5 EQE curves of the PSCs based on the blend of PBDTPA-TPD or PBDTPA-DPP(OD)/PC₆₁BM and optical absorptions for the corresponding polymer/PC₆₁BM blend film.

4. Conclusions

In summary, a series of triple bond side-chained benzodithiophene copolymer, derived from 4,8-bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-benzo[1,2-b:4,5-b′]dithiophene, were synthesized. By introducing soluble bis(octyloxy)phenyl substituted triple band as side chain of DBT polymer, decent molecule weight (15.4 k to 28.3 k) is achieved for polymers with rigid triple band side chain. These polymers show high decomposition temperature and low-lying HOMO energy level. The best PSCs device based on the copolymer PBDTPA-TPD and PC₆₁BM showed a nearly 1 V (0.96 V) V_oc. Further work to make more efficiency triple bond side-chained benzodithiophene polymer is underway.

Acknowledgements

The authors are deeply grateful to the National Natural Science Foundation of China (Project no. 21274134, 21202181, 51173199 and 61405209), New Century Excellent Talents in University (NCET-11-0473), and Qingdao Municipal Science and Technology Program (13-1-4-200-jch) for financial support.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10154e

‡ These authors contributed equally to this work.

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