Efficient polymer solar cells processed by environmentally friendly halogen-free solvents

Abstract

The use of environmentally friendly halogen-free organic solvents for the fabrication of polymer solar cells will be of great importance for future practical applications. In this work, a new alternative conjugated polymer with 3,4-bis(octyloxy)-phenyl substituted benzo[1,2-b:4,5-b]dithiophene as the donor unit and benzo[c][1,2,5]thiadiazole as the acceptor unit was synthesized and used as the donor material for polymer solar cells. This polymer showed good solubility in halogen-free solvents such as toluene, o-xylene and so on. The blend film morphology, charge mobility and photovoltaic performance were investigated in halogen-free solvents. The photovoltaic devices fabricated from o-xylene with N-methyl-2-pyrrolidone as additive provided the best power conversion efficiency of 4.57%, comparable to that fabricated from halogenated solvents such as 1,2-dichlorobenzene/1,8-diiodooctane with a power conversion efficiency of 4.33%. Our results demonstrate that halogen-free solvents are promising for the fabrication of high efficiency polymer solar cells.

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Introduction

Bulk heterojunction (BHJ) organic solar cells (OSCs) have experienced rapid progress in the past decade and a power conversion efficiency (PCE) higher than 9% has been achieved for both single junction and tandem OSCs.^1–11 The main advantage of OSCs is that they can be fabricated by low cost solution processing technology.^12–18 The most frequently used solvents for the fabrication of OSCs are halogenated organic solvents such as chlorobenzene (CB) and o-dichlorobenzene (DCB) that possess high boiling points, adequate viscosity, and superior solubility for nonpolar conjugated oligomers/polymers and fullerene derivatives. Unfortunately, these halogenated solvents are detrimental to human health and the environment, thus their use should be carefully considered from the viewpoints of large-scale roll-to-roll production and practical application.^19–21 Increasing attention has been paid to the use of halogen-free organic solvents, such as toluene, o-xylene, 1,2,4-trimethylbenzene, 1,2-dimethylnaphalene, anisole and so on, to replace halogenated ones for fabricating PSCs devices.^22–34 For example, Jen et al. used toluene solvents to fabricate devices based on PIDII-DFBT : PC₇₁BM blends and PCE of 7.26% has been achieved. Li et al. fabricated the devices based P3HT : ICBA blend by halogen-free solvents and achieved a comparable PCE that is similar to the devices prepared from DCB. Hou et al. modified conjugated polymers and fullerene derivatives by triethylene glycol monoether functional group and a PCE of 5.23% was obtained by processing the active layer with halogen-free solvents. Wang et al. attached a polar and inert phosphonate moiety to the side chain of conventional photovoltaic polymers and a PCE of 2.11% was achieved by using anisole as the processing solvent. It is still challenging to fabricate PSCs using halogen-free organic solvents to achieve efficiency as high as that fabricated using halogenated solvents. Nevertheless, the use of environmentally friendly halogen-free organic solvents for the fabrication of PSCs will be of great importance for future practical applications.

Non-halogenated organic solvents, such as toluene, o-xylene and so on are biodegradable that produce little negative effect on environment and human health. In comparison with halogenated organic solvents, halogen-free organic solvents are usually poor solvents for fullerene derivatives such as PC₇₁BM. When halogen-free organic solvents were used as processing solvent, during the drying of active layer fullerene derivatives will first precipitate to form large aggregates, leading to unfavorable morphology and low efficiency.^35–37 It is possible to adjust the solubility of PC₇₁BM in halogen-free organic solvents by using the additive. N-Methyl-2-pyrrolidone (NMP), which has a boiling point of 202 °C and is a good solvent for PC₇₁BM (48 mg mL⁻¹), can be used as additive for halogen-free organic solvents to tune the morphology of active layer.³⁸ In addition, NMP is minimally flammable, of low toxicity and environmentally friendly. Encouraged by these ideas, we designed and synthesized bis(alkoxy)phenyl substituted benzo[1,2-b:4,5-b]dithiophene (BDT) based polymers (PBBDTBT), since BDT based polymers have been proved to be excellent donor materials.^5,11,39,40 The chemical structure of PBBDTBT is shown in Scheme 1. As expected, PBBDTBT showed a good solubility in halogen-free organic solvents such as toluene and o-xylene. After the addition of NMP as additive, the solubility of PBBDTBT in the solvent mixture decreased, leading to proper phase separation for the blend films and enhanced photovoltaic performance for the devices. The initial test revealed that the photovoltaic performance of PBBDTBT : PC₇₁BM based devices fabricated with o-xylene/NMP as the processing solvent produced a PCE of 4.57%, higher than the PCE of 4.33% for devices fabricated from halogenated solvents (DCB/DIO).

Scheme 1 Synthetic routes for PBBDTBT.

Results and discussion

Material synthesis and characterization

Synthetic routes for the monomer and polymer are shown in Scheme 1. Compound 1,⁴¹ 2 ⁴² and 5 ^43,44 were synthesized according to the literature procedure. The treatment of compound 2 with n-BuLi by lithium halogen exchange reaction afforded lithium intermediate, which was quenched with commercially available benzo[1,2-b:4,5-b′]dithiophene-4,8-dione to afford the intermediate diol. The diol was converted to compound 3 in a yield of 85% by the reaction with SnCl₂·2H₂O in dilute hydrochloride acid. Abstraction of the two active hydrogen atoms of compound 3 by n-BuLi was followed by reaction with trimethyltin chloride to furnish monomer 4 in a yield of 87%. Stille polycondensation of monomer 4 and compound 5 afforded copolymer PBBDTBT as a black-blue solid. The number average molecular weights (M_n) and polydispersity (PDI) of PBBDTBT were determined by gel permeation chromatography (GPC) using THF as an eluent and narrowly distributed polystyrenes as calibration standards to be 43 kg mol⁻¹ and 3.0, respectively. The related data are summarized in Table 1. PBBDTBT was fully soluble in common organic solvents such as dichloromethane (DCM), chloroform (CF), o-xylene and DCB at room temperature, and could be fully dissolved in toluene at about 50 °C. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) experiments were carried out under nitrogen atmosphere at a heating rate of 10 °C min⁻¹ to investigate thermal properties of PBBDTBT. As shown in Fig. S3 and S4,† the TGA curve demonstrated that PBBDTBT is of good thermal stability with a 5% decomposition temperature up to 333 °C. DSC measurement showed no obvious glass transition in the range of 60 to 250 °C, reflecting PBBDTBT could be amorphous. X-ray diffraction (XRD) experiments were carried out to investigate the packing properties of PBBDTBT in the solid state. As shown in Fig. S5,† PBBDTBT presents two diffraction peaks, the first peak at 3.67° in the small angle region could be ascribed to the distance between polymer chains separated by flexible side chains and the second broad peak centered at 22.55° in the wide angle region could be ascribed to the π–π stacking distance of polymer backbones. According to the Bragg equation, these distances were calculated to be 24.09 and 3.91 Å, respectively. Such a broad diffraction peak in the wide angle region indicated that this polymer is amorphous.

Table 1 Physical, electronic, and optical properties of PBBDTBT

Polymer	Td^a (°C)	λ (nm) solution	λ (nm) film	Eg,opt^b (eV)	HOMO (eV)	LUMO (eV)
a Td is the 5% weight-loss temperature under N2 atmosphere.b Calculated from the onset absorption of PBBDTBT, Eg,opt = 1240/λonset.
PBBDTBT	333	419, 574	433, 590, 627	1.76	−5.43	−3.67

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Optical and electrochemical properties

UV-vis absorption characteristics of PBBDTBT were investigated in different solvents such as toluene, o-xylene and DCB (1 × 10⁻⁵ M) at room temperature and as films. As shown in Fig. 1, in toluene, o-xylene and DCB solutions PBBDTBT exhibited similar absorption spectra with a weak absorption peak at 419 nm and an intense absorption peak at 574 nm and a should peak at about 612 nm, with a molar extinction coefficient of 50.34 L g⁻¹ cm⁻¹ in o-xylene. The first peak at 419 nm could be ascribed to the π–π* transition and the second peak and the shoulder could be attributed to the intramolecular charge transfer interaction (ICT) between BDT unit and benzo-2,1,3-thiadiazole unit.⁷ In going from solution to film, PBBDTBT displayed a broader spectrum with two red-shifted peaks at 433 and 590 nm and a shoulder peak at 625 nm, indicating the formation of ordered stacking in some extent as films. The absorption data are also summarized in Table 1. The optical band gap (E_g,opt) of PBBDTBT was evaluated from the film absorption onset to be 1.76 eV. Upon blending with PC₇₁BM, films spin coated from toluene and o-xylene solutions revealed similar absorption properties; whereas films spin coated from DCB solutions exhibited a slightly broad absorption peak with the shoulder peak at 630 nm became more intense, indicating that polymer chains could form more ordered packing when spin coated from DCB solutions. The absorption valley of polymers in the short wavelength region was compensated by the absorption of PC₇₁BM.

Fig. 1 UV-vis absorption spectra of (a) PBBDTBT in diluted toluene, o-xylene and DCB solutions and as films; (b) PBBDTBT : PC₇₁BM (1 : 1.5 by weight) blending films spin coated from toluene, o-xylene and DCB solutions; (c) molar extinction coefficient of PBBDTBT in diluted o-xylene.

Electrochemical properties of PBBDTBT were investigated by cyclic voltammetry (CV) with a standard three-electrode electrochemical cell. As shown in Fig. 2, the oxidation onset of PBBDTBT film was determined to be 0.72 V and the HOMO level of PBBDTBT was calculated to be −5.43 eV using the equation: E_HOMO = −e(E_ox + 4.71) (eV). The LUMO level was calculated to be −3.67 eV according to the equation E_LUMO = E_HOMO + E_g,opt. The results are also summarized in Table 2.

Fig. 2 Cyclic voltammogram of PBBDTBT as films on a platinum electrode in 0.1 mol L⁻¹ Bu₄NPF₆ acetonitrile solution at a scan rate of 100 mV s⁻¹.

Table 2 Summary of the photovoltaic properties of the solar cells

Solvent	Solvent additive	Voc (V)	Jsc (mA cm^−2)	FF	PCE (%) best/ave^a	d (nm)	Hole mobility (cm^2 V^−1 s^−1)
a The average PCEs were based on ten devices.
Toluene	No	0.77	2.77	0.34	0.73/0.61	194	4.15 × 10^−6
	3% NMP	0.71	5.80	0.51	2.10/1.96	171	1.05 × 10^−5
o-Xylene	No	0.78	3.86	0.45	1.35/1.24	128	8.73 × 10^−6
	4% NMP	0.73	8.94	0.70	4.57/4.50	138	1.48 × 10^−5
DCB	No	0.76	6.62	0.64	3.22/3.11	124	1.18 × 10^−5
	3% DIO	0.71	8.97	0.68	4.33/4.25	137	1.52 × 10^−5

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Photovoltaic properties

A conventional device structure of ITO/PEDOT : PSS/PBBDTBT : PC₇₁BM/LiF/Al was adopted to investigate the photovoltaic performance of PBBDTBT : PC₇₁BM based PSCs fabricated from halogen-free solvents such toluene and o-xylene. Devices fabricated from DCB solutions were also tested for a comparison. All devices with varied D/A ratios exhibited in Table S1,† and the best photovoltaic performance in a donor/acceptor weight ratio of 1 : 1.5. J–V curves of optimized devices measured under simulated solar illumination (AM1.5G, 100 mW cm⁻²) are shown in Fig. 3a and the device parameters are summarized in Table 2. For plain devices fabricated with toluene and o-xylene as the processing solvents, inferior PCEs of 0.73% and 1.35% were achieved; whereas a PCE of 3.22% was obtained using DCB as the processing solvent. The different photovoltaic performance might be due to the inferior solubility of PC₇₁BM in toluene or o-xylene to DCB. The boiling point and evaporation speed of processing solvent have a significant influence on the morphology of blend films. Active layers spin-coated from toluene and o-xylene solutions exhibited a severe phase separation with larger spherical grains.³⁰ The unfavorable film morphology is responsible for the poor photovoltaic performance of devices fabricated with toluene or o-xylene as the processing solvent. To tune the morphology of blend films different additives were utilized. When 3% NMP (by volume) was used as additive in the toluene solution, an improved PCE of 2.10% with V_oc of 0.71 V, J_sc of 5.80 mA cm⁻² and FF of 0.51 was achieved. Similarly, higher PCE of 4.57% with V_oc of 0.73 V, J_sc of 8.94 mA cm⁻² and FF of 0.70 was obtained for devices fabricated from o-xylene with 4% NMP (by volume) as additive. The increased J_sc and FF contributed to the overall enhanced photovoltaic performance. In contrast, when DCB containing 3% DIO (by volume) was used as the processing solvent, a PCE of 4.33% was achieved. Notably, optimized devices fabricated from o-xylene/NMP solutions showed superior performance to that fabricated from DCB/DIO solutions, demonstrating that halogen-free solvent o-xylene is promising for future commercial application.

Fig. 3 (a) The J–V characteristics of devices based on PBBDTBT : PC₇₁BM under the illumination of AM1.5G (100 mW cm⁻²); (b) EQE curves of solar cells fabricated under different conditions.

External quantum efficiencies (EQEs) were measured to examine the accuracy of J_sc from the J–V measurements. As shown in Fig. 3b, EQE spectra of the devices processed by toluene, o-xylene, and DCB are in the range of 350 to 700 nm. The results demonstrated that devices fabricated with additives have higher EQE values than that fabricated without using additives, indicating the use of additive is favorable to form appropriate morphology for charge separation and transport. The J_sc calculated from the integration of EQE curves agreed well with that obtained from J–V measurement, verifying the accuracy of the PCE obtained from J–V measurements.

Hole and electron mobilities

Hole mobilities of blend films were measured with space charge limited current (SCLC) method and the results are summarized in Table 2. Devices were fabricated from different solvents with a typical structure of ITO/PEDOT : PSS/PBBDTBT : PC₇₁BM (1 : 1.5 w/w)/Au. Electron mobilities of blend films were measured and the results are summarized in Table S2.† Devices were fabricated from different solvents with a typical structure of FTO/PBBDTBT : PC₇₁BM (1 : 1.5 w/w)/Al. Dark J–V experiments were carried out and the hole mobility of PBBDTBT was calculated from J = 9ε_oε_rμV²/8L³. The curves are shown in Fig. S6 and S7.† PBBDTBT : PC₇₁BM based plain devices fabricated from toluene, o-xylene and DCB exhibited hole mobilities of 4.15 × 10⁻⁶, 8.73 × 10⁻⁶, and 1.18 × 10⁻⁵ cm² V⁻¹ s⁻¹, respectively. Hole mobilities were improved to 1.05 × 10⁻⁵, 1.48 × 10⁻⁵, 1.52 × 10⁻⁵ cm² V⁻¹ s⁻¹ for devices fabricated from toluene containing 3% NMP, o-xylene containing 4% NMP, and DCB containing 3% DIO, respectively. The improvement of hole mobilities, which matched better the electron mobility, could be ascribed to the optimization of film morphology after using solvent additive. More balanced charge mobility is beneficial for the charge transport in the devices, leading to higher J_sc and FF.

The blend film morphology

The morphology of the active layer has an obvious influence on the photovoltaic performance of devices. Therefore, atomic force microscopy (AFM) was used to investigate the morphology of blend films spin coated from different solvents. As shown in Fig. 4, the blend films spin coated from toluene and o-xylene solutions exhibited a relatively rough surface topography with large spherical aggregates and the root-mean-square (R_a) values are 16.6 and 12.0 nm, respectively. Such high roughness values can be attributed to low solubility of PC₇₁BM in toluene and o-xylene. Interestingly, after NMP was used as the additive in toluene and o-xylene, the blend films showed distinct differences in morphologies from the system without NMP. Both films became smooth, and the roughness values were decreased to be 9.29 and 3.53 nm, respectively. The solubility of conjugated polymers and PC₇₁BM has a significant influence on the morphology of blend films. PBBDTBT exhibited a good solubility in toluene, o-xylene and DCB; whereas the solubility of PC₇₁BM in toluene and o-xylene is poorer than that in DCB. When toluene or o-xylene is used as the processing solvent, PC₇₁BM will firstly precipitate from the solution to form large aggregates during drying of the active layer. After adding NMP, the solubility of polymer in blend solutions will decrease, leading to the formation of polymer networks, which can prevent the formation of large spherical PC₇₁BM aggregates during the drying of blend films. As for blend films spin-coated from DCB solutions, the morphology became rougher and the R_a value increased from 1.65 to 2.29 nm after using DIO as additive. Obviously, the addition of a small amount of polar solvent as the additive to non-halogenated solvents such as toluene and o-xylene can significantly promote micro-phase separation for the blend films and have a profound effect on the photovoltaic performance.

Fig. 4 AFM height images of PBBDTBT : PC₇₁BM (1 : 1.5, w/w) blend films spin-coated from different solvents.

Conclusions

3,4-Bis(octyloxy)-phenyl substituted benzo[1,2-b:4,5-b]dithiophene (BDT) based polymer (PBBDTBT) was designed and synthesized. PBBDTBT showed a good solubility in halogen-free solvents such as toluene, o-xylene and so on. The influence of non-halogenated solvent on the morphology, charge mobility and photovoltaic performance was thoroughly investigated. PBBDTBT : PC₇₁BM based PSCs processed from non-halogenated solvent o-xylene/NMP produced the best PCE of 4.57% with a V_oc of 0.73 V, a J_sc of 8.94 mA cm⁻² and an FF of 0.70, higher than the devices fabricated using halogenated solvents such as DCB/DIO with a PCE of 4.33%. Our results have demonstrated that non-halogenated solvents are promising for the fabrication of high efficiency PSCs.

Experimental part

4,8-Bis(1,2-bis(octyloxy)-4-phenyl)-benzo[1,2-b:4,5-b′]-dithiophene (3)

Under protection of nitrogen, 2.4 M n-butyllithium (n-BuLi) solution in n-hexane (3.05 mL) was slowly added to 4-bromo-1,2-bis(octyloxy)benzene (3 g, 7.25 mmol) in THF (50 mL) at −78 °C, and the mixture was kept at −78 °C for 1 h. Benzo[1,2-b:4,5-b′]dithiophene-4,8-dione (0.76 g, 3.45 mmol) was added, and the mixture was stirred overnight at the room temperature. Then SnCl₂·2H₂O (1.8 g) in 20% HCl (25 mL) was added and the mixture was stirred for 5 h at the room temperature. The mixture was poured into ice water and extracted by dichloromethane (DCM) three times. The combined organic phase was dried over magnesium sulfate and evaporated to afford the crude product. Further purification was carried out on a silica gel column, eluting with pure petroleum ether. White crystals were obtained (1.92 g, 65%). ¹H NMR (400 MHz, CDCl₃): δ 7.58 (d, J = 5.6 Hz, 1H), δ 7.50 (s, 2H), δ 7.09 (d, J = 5.6 Hz, 1H), δ 7.00 (d, J = 8.6 Hz, 1H), δ 3.97–3.92 (m, 4H), δ 1.89–0.99 (m, 30H). ¹³C NMR (100 MHz, CDCl₃): δ 149.11, 148.99, 138.23, 136.13, 131.92, 130.21, 127.11, 123.18, 121.86, 114.91, 113.67, 69.28, 31.87, 31.84, 29.45, 29.42, 29.33, 29.29, 26.13, 26.05, 22.71, 22.69, 14.13. Anal. calcd for C₅₄H₇₈O₄S₂: C 75.83; H 9.19. Found: C 75.87; H 9.52.

4,8-Bis(1,2-bis(octyloxy)-4-phenyl)-benzo[1,2-b:4,5-b′]-dithiophene-2,6-bis(trimethyltin) (4)

To a solution of compound 3 (0.5 g, 0.585 mmol) in THF (40 mL) at −78 °C was slowly added 2.4 M n-butyllithium solution in n-hexane (0.61 mL). After the addition, the solution was warmed slowly to room temperature and kept for 1 h. After cooling down to the −78 °C, 1.0 M trimethyltin chloride solution in THF (1.29 mL) was added. The mixture was stirred overnight at the room temperature and quenched with water. The mixture was extracted with diethyl ether three times and dried by over MgSO₄, the solvent was removed and the residue was recrystallized from hexane to afford the compound 4 (0.62 g, 90%). ¹H NMR (400 MHz, CDCl₃): δ 8.04 (s, 1H), δ 7.70–7.68 (m, 2H), δ 7.04 (d, J = 8.04 Hz, 1H), δ 4.00–3.93 (m, 4H), δ 1.84–1.01 (m, 30H). δ 0.39–0.25 (m, 9H). ¹³C NMR (100 MHz, CDCl₃): δ 148.93, 148.78, 142.47, 141.78, 136.99, 132.53, 131.00, 128.64, 121.80, 115.08, 113.51, 69.23, 69.18, 31.87, 31.84, 29.46, 29.42, 29.33, 29.32, 29.26, 26.15, 26.07, 22.72, 22.69, 14.14, 14.12. Anal. calcd for C₆₀H₉₄O₄S₂Sn₂: C 61.02; H 8.02. Found: C 61.07; H 7.86.

PBBDTBT

4 (120 mg, 0.1016 mmol) and 5 (72.62 mg, 0.1016 mmol) were dissolved in toluene (5 mL) and DMF (1 mL) in a 50 mL double-neck round-bottom flask. The reaction container was carefully degassed before and after the addition of Pd(PPh₃)₄ (5.0 mg, 4.32 μmol). The mixture was stirred and refluxed under nitrogen atmosphere for 24 h. Then the mixture was cooled to room temperature and poured into methanol (150 mL), the precipitates was subjected to Soxhlet extraction with methanol, hexane and DCM. The polymer was recovered as a solid from the DCM fraction by natural sedimentation from methanol. Polymers were obtained as black-blue solid (106.5 mg, 73%). GPC (PS standards): M_n = 43 kg mol⁻¹, M_w = 130 kg mol⁻¹ and PDI = 3.0.

Acknowledgements

Financial support from the NSF of China (51003006, 21161160443), the 973 Programs (2011CB935702), Beijing Natural Science Foundation (2132042), Program for Changjiang Scholars and Innovative Research Team in University and the Fundamental Research Funds for the Central Universities are gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Experimental part including TGA curves, DSC curves, XRD curves, SCLC curves, and ¹H NMR and ¹³C NMR spectra of monomers and ¹H NMR spectra of polymers. See DOI: 10.1039/c6ra04835h

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