TPD-based polythiophene derivatives with higher V_oc for polymer solar cells

Abstract

New D–A copolymers, PTPD-DT and PTPD-DFDT, based on a thieno[3,4-c]pyrrole-4,6-dione (TPD) acceptor unit and 2,2′-bithiophene (DT) or 3,3′-difluoro-2,2′-bithiophene (DFDT) donor units, were designed and synthesized for application as donor materials in polymer solar cells (PSCs). A control polymer PTPD-DT with a similar structure but without fluorine substitution on the 2,2′-bithiophene (DT) unit was also synthesized for comparison. Compared with PTPD-DT, the polymer PTPD-DFDT with fluorine substitution on the DT unit shows a lower HOMO energy level of −5.55 eV, more broad absorption in the wavelength range from 300 to 700 nm, greater coplanarity and crystalline structure. The PSCs based on PTPD-DFDT/PC₇₁BM demonstrated a power conversion efficiency of 5.52%, with a higher open-circuit voltage of 0.96 V. Furthermore, PTPD-DFDT exhibits a simpler molecular structure and easier synthesis steps, which is beneficial for mass production in future.

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1. Introduction

In recent years, power conversion efficiencies (PCEs) over 10% have been achieved in single bulk-heterojunction (BHJ) polymer solar cells (PSCs).^1–6 The molecular design of active-layer materials^7–17 and morphology control of the blend films^18–24 are the two main approaches for improving the photovoltaic performance of the PSCs.

As is well-known, open-circuit voltage (V_oc) is an important parameter in improving the performance of PSCs, which is directly proportional to the offset between the highest occupied molecular orbital (HOMO) level of the donor material and the lowest unoccupied molecular orbital (LUMO) level of the acceptor material (such as PCBM (phenyl-C₆₁-butyric acid methyl ester) or PC₇₁BM (phenyl-C₇₁-butyricacid methyl ester)).^7,25

Donor–acceptor (D–A) copolymerization and side chain engineering are two main strategies in the design and synthesis of high performance conjugated polymer donor materials in PSCs.^7,26–28 Among various acceptor units used in D–A copolymers, thieno[3,4-c]pyrrole-4,6-dione (TPD), as a relatively weak electron-withdrawing unit, often leads to a deep-lying HOMO energy level of TPD-based D–A copolymers,^29–34 which could result in high V_oc of PSCs with the polymer as donor. On the other hand, fluorination is an effective strategy to down-shift the HOMO energy level of conjugated polymers and thus improve their photovoltaic properties.¹⁶ In particular, 3,3′-difluoro-2,2′-bithiophene (DFDT), with a simple structure, has been used as a donor unit in high performance D–A copolymer donor materials.^1c,35 In addition, the DFDT unit shows a higher coplanarity due to the intramolecular S⋯F interactions, which is beneficial in obtaining a rigid molecular backbone and higher hole mobility for D–A copolymers.

Taking into account the above discussions, we designed and synthesized a new D–A copolymer, PTPD-DFDT, which is based on the TPD acceptor and the DFDT donor unit. As a control polymer for studying the effect of fluorine substitution on the DT unit, a D–A copolymer PTPD-DT, based on the TPD acceptor unit and the 2,2′-bithiophene (DT) donor unit was also synthesized. Compared with PTPD-DT, the polymer PTPD-DFDT, with fluorine substitution on the DT unit, showed a lower HOMO energy level of −5.55 eV, broader absorption in the wavelength range from 300 to 700 nm, greater coplanarity and crystalline structure. As a result, PSCs with PTPD-DFDT as the donor and PC₇₁BM as the acceptor demonstrated a higher V_oc of 0.96 V and a PCE of 5.52%. In addition, PTPD-DFDT exhibits a simpler structure and easier synthesis compared to other high-efficiency photovoltaic materials, which is beneficial for low cost and mass production.

2. Experimental section

All chemicals and solvents were reagent grade and purchased from Aldrich, Alfa Aesar and TCI Chemical Co. The synthetic route and molecular structures of the D–A copolymers PTPD-DT and PTPD-DFDT are shown in Scheme 1, and the detailed synthetic procedures for the polymers are as follows. Detailed characterization procedures and conditions are described in the ESI.†

Scheme 1 Synthetic route and molecular structures of PTPD-DT and PTPD-DFDT.

2.1 Synthesis of the polymers: PTPD-DT and PTPD-DFDT

Monomer TPD (0.3 mmol), monomer DT or DFDT (0.3 mmol), dry toluene (10 mL) and dry DMF (2 mL) were added to a 25 mL double-neck round-bottom flask. The NMR spectra of the monomers are shown in Fig. S1–S4 in the ESI.† The reaction container was purged with Ar for 20 min to remove O₂, and then Pd(PPh₃)₄ (21 mg) was added. After another flushing with Ar for 20 min, the reactant was heated to reflux for 12 h. The reactant was poured into MeOH (100 mL), and the precipitates were collected by filtration and then washed with MeOH. The solid was dissolved in chloroform (CHCl₃) (100 mL) and passed through a column packed with silica gel. The column was eluted with CHCl₃. The combined polymer solution was concentrated and poured into 100 mL MeOH. Then the precipitates were collected and dried under vacuum overnight to get the polymers.

For PTPD-DT. Yield: 210 mg (71%). GPC: M_w = 93.7 K; M_n = 47.2 K; M_w/M_n = 1.98. Elemental analysis for C₆₂H₇₈N₂O₂S₆ (%): C, 70.61; H, 8.48; N, 1.42; found (%): C, 71.47; H, 8.96; N, 2.10. ¹H NMR: (400 MHz, CDCl₃), δ (ppm): 7.80–7.51 (m, 2H), 7.25–6.80 (m, 4H), 3.56–3.40 (m, 3H), 2.80–2.52 (m, 6H), 1.42–1.11 (m, 54H), 0.94–0.91 (m, 12H). The ¹H NMR spectrum of PTPD-DT is shown in Fig. S5 in the ESI.†

For PTPD-DFDT. Yield: 230 mg (75%). GPC: M_w = 236.3 K; M_n = 83.7 K; M_w/M_n = 2.82. Elemental analysis for C₆₂H₇₈N₂O₂S₆ (%): C, 68.12; H, 7.98; N, 1.37; found (%): C, 66.43; H, 7.42; N, 1.61. ¹H NMR: (400 MHz, CDCl₃), δ (ppm): 7.78–7.35 (m, 2H), 7.22–6.75 (m, 2H), 3.70–3.27 (m, 3H), 3.00–2.52 (m, 6H), 1.47–1.11 (m, 54H), 1.09–0.60 (m, 12H). The ¹H NMR spectrum of PTPD-DFDT is shown in Fig. S6 in the ESI.†

3. Results and discussion

3.1 Synthesis and characterization of the polymers

The synthesis route and molecular structures of PTPD-DT and PTPD-DFDT are shown in Scheme 1. The polymers were synthesized by the Stille coupling reaction under the action of Pd(PPh₃)₄. The polymers possess good solubility in o-dichlorobenzene (o-DCB), chlorobenzene and chloroform (CHCl₃). The molecular weights of the polymers were estimated by high temperature gel-permeation chromatography (GPC) using 1,2,4-trichlorobenzene as the eluent at 160 °C. The number-average molecular weight (M_n) of PTPD-DT and PTPD-DFDT were 47.23 kDa and 83.69 kDa, with PDI of 1.98 and 2.82, respectively.

Thermogravimetric analysis (TGA) was performed to investigate the thermal stability of PTPD-DT and PTPD-DFDT under a N₂ atmosphere. The onset decomposition temperature of 5% weight-loss was 337 °C and 429 °C for PTPD-DT and PTPD-DFDT, respectively, as shown in Fig. 1(a). The result of TGA indicates that the two polymers have a high thermal stability for application in PSCs. Furthermore, the higher 5% weight-loss decomposition temperature of PTPD-DFDT compared to PTPD-DT indicates a higher thermal stability for PTPD-DFDT. Fig. 1(b) shows the DSC thermograms of the two polymers; there are apparent endothermic peaks at 192 °C and 259 °C during increasing temperature and exothermic peaks at 162 °C and 241 °C during decreasing temperature for PTPD-DT and PTPD-DFDT, respectively, which indicates stronger crystallization of PTPD-DT and PTPD-DFDT. The crystallization characteristics of the two polymers may be ascribed to the larger π-conjugated structure and higher coplanarity of PTPD-DT and PTPD-DFDT, which could favor tighter intermolecular π–π stacking. In addition, the polymer PTPD-DFDT shows a higher melting peak and crystallization peak compared to PTPD-DT, which reveals that a stricter rigid molecular skeleton and tighter intermolecular interactions exist in PTPD-DFDT, related to the intramolecular (thienyl) S⋯O (carbonyl on TPD) and (thienyl) F⋯S (thienyl) interactions.

Fig. 1 (a) TGA plot of PTPD-DT and PTPD-DFDT with a heating rate of 10 °C min⁻¹ under nitrogen atmosphere. (b) DSC thermograms of PTPD-DT and PTPD-DFDT with a scan rate of 10 °C min⁻¹ under nitrogen atmosphere.

3.2 Absorption spectra and electrochemical properties

UV-vis absorption spectra of the two polymers in dilute o-DCB solution and in solid thin film are shown in Fig. 2(a) and (b). The optical parameters are listed in Table 1 for a clear comparison. The polymer films show a maximum absorption peak at 553 nm and 579 nm for PTPD-DT and PTPD-DFDT, respectively, red-shifted by 28 nm and 29 nm compared with the absorption of the polymer solutions. PTPD-DFDT displays a sharp vibration absorption peak at ca. 660 nm in comparison with that of PTPD-DT at about 628 nm in solid film, indicating that PTPD-DFDT is more effectively aggregated in the solid state due to the strong intermolecular interaction related to the good planarity and stronger interchain interaction of PTPD-DFDT, benefitting from its fluorine substitution. Furthermore, PTPD-DFDT possesses a maximum absorption coefficient (ε) of 0.42 × 10⁵ cm⁻¹ at 579 nm, which is slightly larger than that of PTPD-DT film, with ε of 0.38 × 10⁵ cm⁻¹ at 553 nm. The onset absorption edges of PTPD-DT and PTPD-DFDT films are at 672 nm and 710 nm, corresponding to an optical bandgap of 1.84 eV and 1.75 eV, respectively.

Fig. 2 UV–vis absorption spectra of PTPD-DT (a) and PTPD-DFDT (b) in o-DCB solution and solid film; (c) cyclic voltammograms of the polymer films on a platinum electrode measured in 0.1 mol L⁻¹ Bu₄NPF₆ acetonitrile solution at a scan rate of 50 mV s⁻¹. (d) X-ray diffraction patterns of the polymer films cast from o-DCB onto Si substrates.

Table 1 Optical properties and molecular energy levels of the polymers

Polymers	Optical properties of the films				ϕox (V)	HOMO (eV)	ϕred (V)	LUMO (eV)
	λmax (nm)	λedge (nm)	E^optg (eV)	ε (10^5 cm^−1)
PTPD-DT	553	672	1.84	0.38	0.71	−5.42	−1.60	−3.31
PTPD-DFDT	579	710	1.75	0.42	0.84	−5.55	−1.16	−3.55

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HOMO and LUMO energy levels of the polymers were measured by cyclic voltammetry. Fig. 2(c) shows the cyclic voltammograms of the two polymers. The electronic energy levels were calculated according to the equations:³⁶ HOMO = −e(φ_ox + 4.71) (eV); LUMO = −e(φ_red + 4.71) (eV), where the unit of the onset oxidation potential (φ_ox) and the onset reduction potential (φ_red) is V vs. Ag/Ag⁺. The φ_ox values of PTPD-DT and PTPD-DFDT from the cyclic voltammograms are 0.71 V and 0.84 V vs. Ag/Ag⁺, corresponding to HOMO energy levels of −5.42 eV and −5.55 eV, respectively; the φ_red values of PTPD-DT and PTPD-DFDT are −1.60 V and −1.16 V vs. Ag/Ag⁺, corresponding to LUMO energy levels of −3.31 eV and −3.55 eV, respectively. In comparison with PTPD-DT, PTPD-DFDT with fluorine substitution on the DT unit possesses a lower HOMO energy level at −5.55 eV, which is beneficial for achieving a higher V_oc in PSCs with PTPD-DFDT as the donor.

3.3 X-ray diffraction analysis

X-ray diffraction (XRD) measurement was used to investigate the crystalline properties of the two polymers. As shown in Fig. 2(d), PTPD-DT exhibits broad diffraction peaks at 2θ = 3–10°. PTPD-DFDT displays two slightly obvious diffraction peaks at 2θ = 4.7° (100) and 9.4° with a d-spacing value of 18.8 Å, which corresponds to the interchain distance separated by the side chains on the TPD units of PTPD-DFDT. Furthermore, PTPD-DT and PTPD-DFDT show a π–π stacking diffraction peak (010) at 2θ = 23.6° and 24.8°, corresponding to a π–π stacking d-spacing of 3.77 Å and 3.59 Å respectively. PTPD-DFDT shows more compact π–π stacking than PTPD-DT, which should be ascribed to the higher coplanarity and stronger interchain interaction of PTPD-DFDT benefitting from fluorine substitution.

3.4 Theoretical calculations

For further investigation of the energy levels and electronic properties of PTPD-DT and PTPD-DFDT, theoretical DFT calculations using the B3LYP/6-31G(d, p) basis set were performed to predict the polymers’ properties. In order to avoid excessive calculation demand and make computation easier, we chose three TPD-DT/DFDT repeating units as a simplified model of the molecule, and the alkyl side chains in the molecule were replaced by methyl groups. Optimized molecular geometries and orbital distribution of the HOMOs and LUMOs of the polymers are shown in Fig. 3.

Fig. 3 The frontier molecular orbitals of the polymers obtained from DFT calculations at the B3LYP/6-31G (d, p) level, (a) HOMO and LUMO energy levels, and the molecular plane by side view of PTPD-DT; (b) HOMO and LUMO energy levels, and the molecular plane by side view of PTPD-DFDT; (c) calculated dihedral angles between two adjacent units in the polymers’ molecular backbone.

The calculated LUMO energy levels of PTPD-DT and PTPD-DFDT are −2.66 eV and −2.76 eV, respectively. The HOMO energy levels are −4.80 eV and −4.88 eV for PTPD-DT and PTPD-DFDT, respectively. The changing tendency of the HOMO and LUMO energy levels in the DFT theoretical calculations are consistent with the values obtained by CV measurements. As shown in Fig. 3(a) and (b), the molecular orbital distributions indicate that the HOMOs of the polymers are delocalized along the whole π-conjugated backbone of the molecule while their LUMOs are mainly localized on their TPD-based acceptor segments. The PTPD-DFDT molecule shows a higher coplanarity in comparison with PTPD-DT by side view, due to the intramolecular (thienyl) S⋯O (carbonyl on TDP) interactions and (thienyl) S⋯F (thienyl) interactions in the polymer PTPD-DFDT. Furthermore, in the PTPD-DFDT molecule, the dihedral angle between two adjacent units, especially between the TPD unit and the adjacent thiophene and between the adjacent two fluoro-thienyl groups in the three repeat units are found to be below 0.01° (as shown in Fig. 3(c)), suggesting a strict coplanar conformation. The theoretical calculation results for the two polymers are well consistent with the observations in optical, electrochemical and thermal analysis measurements.

3.5 Photovoltaic properties

We measured the photovoltaic properties of the two polymers by fabricating PSCs with a conventional structure of ITO/PEDOT:PSS/active layer/Ca (20 nm)/Al (80 nm), where ITO is the abbreviation of indium tin oxide; PEDOT:PSS is the abbreviation of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate). The device fabrication processes are described in the ESI.† The active layers were spin-coated from o-DCB blend solutions of PTPD-DT or PTPD-DFDT and PC₇₁BM. Fig. 4 shows the current density–voltage (J–V) characteristics and external quantum efficiency (EQE) plots of the optimized PSCs based on PTPD-DFDT and PC₇₁BM blend films, and Table 2 lists the photovoltaic performance parameters of the PSCs.

Fig. 4 (a) J–V characteristics and (b) EQE curves of the PSCs based on PTPD-DFDT:PC₇₁BM blend film with different conditions.

Table 2 Photovoltaic performance parameters of the PSCs based on PTPD-DFDT/PC₇₁BM (1 : 1.5, w/w) under illumination of AM 1.5G, 100 mW cm⁻²

D/A ratio	Voc (V)	Jsc (mA cm^−2)	Jsc^a (mA cm^−2)	FF (%)	PCE (%)
a Integrated from EQE.b With 3% DPE as solvent additive.c With 3% DPE as solvent additive and PFN as buffer layer.
1 : 1.5	0.98	6.03	5.68	47.4	2.80
1 : 1.5^b	0.96	10.25	9.81	54.0	5.31
1 : 1.5^c	0.96	11.18	10.50	51.4	5.52

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Firstly, the photovoltaic performances of the polymers were optimized by using different donor/acceptor (D/A) weight ratios of 1 : 1, 1 : 1.5, and 1 : 2. PTPD-DT and PTPD-DFDT exhibit best PCEs of 2.32% and 2.80%, respectively, when the D/A ratio is 1 : 1.5. It’s worth mentioning that PTPD-DFDT shows an impressive high V_oc of 0.98 V in all three different D/A ratios (as shown in Fig. 4(a), Table 2, Fig. S7 and Table S1 in the ESI†). The V_oc value is slightly higher than that of the PTPD-DT-based devices with a V_oc of 0.94 V (as shown in Fig. S8 and Table S2†). The higher V_oc values of the PSCs result from the lower HOMO energy levels of the polymer donors (−5.42 eV for PTPD-DT and −5.55 eV for PTPD-DFDT).

In order to enhance the photovoltaic performance of PTPD-DFDT-based PSCs, a solvent additive treatment was performed in the fabrication of the devices. Here diphenyl ether (DPE) was selected as the solvent additive, because it can effectively improve the morphology of the blend film.^37,38 We have tested DIO, CN, NMP and DPE as additives when fabricating the devices. The PSCs based on PTPD-DT or PTPD-DFDT as donor material show the best photovoltaic performances when DPE was used as an additive (as shown in Tables S3 and S4 in the ESI†), so we chose DPE as an additive in the process of optimizing the devices. An optimized PCE of the PSC based on PTPD-DFDT/PC₇₁BM (1 : 1.5, w/w) with 3% DPE additive treatment reached 5.31% with a V_oc of 0.96 V, J_sc of 10.25 mA cm⁻², and a FF of 54%. The PCE was further improved to 5.52% by introducing a thin PFN cathode buffer layer in the PSCs,^39,40 due to the increased J_sc (as shown in Fig. 4(a) and Table 2). PFN is the abbreviation of (poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]).

However, the PCE of the PSCs based on PTPD-DT was not improved by the solvent additive treatment. The PTPD-DFDT-based device with 3% DPE additive treatment exhibits a maximum PCE value of only 2.18% due to the decreased FF. When PFN was used as a cathode buffer layer, a slightly improved PCE of 2.48% was achieved, which is ascribed to the increased V_oc and J_sc (as shown in Fig. S8 and Table S2†). It should be mentioned that the larger J_sc (11.18 mA cm⁻²) of the PSCs based on PTPD-DFDT compared to that (6.15 mA cm⁻²) of the PTPD-DT-based PSCs, could be partially due to the larger M_n of PTPD-DFDT, since the molecular weight of the polymer donors often influence the J_sc of the PSCs.^41–43

Fig. 4(b) shows the EQE curves of the PSCs. The current densities calculated from the EQE curves under the standard solar spectrum (AM 1.5G) are consistent with the J_sc values obtained from the J–V measurement with deviation less than 5% (see Table 2, Fig. S7(b) and S8(b) in the ESI†), indicating that the J–V measurements in this work are reliable.

Reducing the energy loss (E_loss) is very important in achieving a high V_oc. E_loss is defined as E_loss = E_g − eV_oc, where E_g is the optical bandgap of the narrow bandgap polymer donor in the fullerene-based PSCs. In order to obtain the maximum V_oc in PSCs, the minimum E_loss was suggested to be 0.6 eV.^44,45 However, for most high-performance PSCs, E_loss is typically 0.7–1.1 eV.^44,46 According to the energy loss equation mentioned above, the corresponding E_loss of the PSCs based on PTPD-DT and PTPD-DFDT are 0.95 eV and 0.80 eV, respectively. In order to investigate the effect of the dielectric constant of the polymers on the E_loss, we measured the dielectric constant of the two polymers, as shown in Fig. S10 and S11.† The dielectric constants of PTPD-DT and PTPD-DFDT are 3.24 and 3.15, respectively, which is not high and a common value in comparison with other conjugated polymers.⁴⁷ The results indicate that the fluorine substitution didn’t result in a higher dielectric constant for PTPD-DFDT, and the high photovoltaic performance of PTPD-DFDT can’t be ascribed to the influence of the dielectric constant.

3.6 Morphology study

In order to investigate the effect of the solvent additive treatment on the active layer morphology, transmission electron microscopy (TEM) and tapping-mode atomic force microscopy (AFM) were used to measure the bulk and surface morphologies of the blend films.

As shown in Fig. 5(a), the large-scale phase-separated morphology of PC₇₁BM can be clearly seen in the TEM image for the blend film of PTPD-DT:PC₇₁BM with a D : A weight ratio of 1 : 1.5. Although the morphology of PC₇₁BM was improved when treated by DPE as a solvent additive, the polymer PTPD-DT was still excessively aggregated due to strong intermolecular interactions (Fig. 5(e)). In both cases the large-scale phase-separated morphology may cause more geminate recombination and bimolecular recombination, and thus results in lower J_sc and FF of the PSCs. Phase separation size becomes even larger when the D/A ratio was 1 : 2 (as shown in Fig. S9 in the ESI†).

Fig. 5 TEM images and tapping-mode AFM images of PTPD-DT:PC₇₁BM blend films with D/A ratio of 1 : 1.5 (top) and with 3% DPE as solvent additive (bottom). TEM images (a and e), AFM (all 5 × 5 μm) height images (b and f), three-dimensional height images (c and g) and topography images (d and h) of the blend films. The R_q of the blend films are 4.37 nm (b) and 2.35 nm (f).

The effect of the DPE solvent additive treatment on the active layer morphology was also studied by AFM measurements. Fig. 5(f)–(h) shows the AFM images of the PTPD-DT/PC₇₁BM blend film (1 : 1.5, w/w) with (f–h) or without (b–d) DPE additive. It can be seen that the surface of the blend film processed with the DPE additive became more smooth than that of the film without using the additive, and the roughness of the blend film decreased from 4.37 to 2.35 nm with DPE additive treatment (see Fig. 5(f) and (g)). As shown in Fig. 5(d) and (h), the blend film demonstrated more uniform morphology (h) than the film without DPE additive (d).

For the blend film of PTPD-DFDT and PC₇₁BM with D/A weight ratio of 1 : 1.5, a relatively large-scale phase-separated morphology was also observed in the TEM image (as shown in Fig. 6(a)) due to its higher coplanarity and crystalline nature. When 3% DPE was added as the additive, a fibrillar interpenetrating network is clearly observed with a size of ca. 10–20 nm, which is beneficial for charge separation and transport (Fig. 6(e)). Therefore, the performance of the device was greatly improved. The formation of the fibrillar interpenetrating network should be ascribed to the appropriate interchain interaction of the polymer benefitting from both the effects of its fluorine substitution¹⁶ and the treatment of the DPE solvent additive.

Fig. 6 TEM images and tapping-mode AFM images of PTPD-DFDT:PC₇₁BM blend films with D/A ratio of 1 : 1.5 (top) and with 3% DPE as solvent additive (bottom). TEM images (a and e), AFM (all 5 × 5 μm) height images (b and f), three-dimensional height images (c and g) and topography images (d and h) of the blend films. The R_q of the blend films are 2.00 nm (b) and 2.67 nm (f).

In addition, it can be seen from AFM images with 3% DPE as solvent additive (Fig. 6(f)–(h)) that the blend film demonstrated more uniform morphology than that of the film without DPE additive (b–d). The roughness of the blend film increased from 2.00 nm to 2.67 nm with DPE as a solvent additive. The AFM results are consistent with the TEM results. These results indicate that morphology control using an additive is an effective approach in improving the photovoltaic performance of PSCs. The results also demonstrated that the active layer morphology has a great influence on the performance of the devices.

3.7 Hole and electron mobilities

In order to explore the charge transport properties of the active layers with different fabrication conditions, the space charge limited current (SCLC) method was used to measure the hole and electron mobilities, as shown in Fig. 7. The detailed data of the mobilities are summarized in Table 3. The hole mobilities of PTPD-DT and PTPD-DFDT are 2.42 × 10⁻⁴ cm² V⁻¹ s⁻¹ and 3.65 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively. The larger hole mobility of PTPD-DFDT compared to PTPD-DT is mainly due to the stronger intermolecular interaction in PTPD-DFDT with fluorine substitution. The blend film of PTPD-DFDT/PC₇₁BM (1 : 1.5, w/w) demonstrates a best hole mobility of 7.22 × 10⁻⁵ cm² V⁻¹ s⁻¹ and a best electron mobility of 1.01 × 10⁻⁴ cm² V⁻¹ s⁻¹. The hole and the electron mobilities of the active layer with the treatment of 3% DPE additive were further improved to 9.94 × 10⁻⁵ cm² V⁻¹ s⁻¹ and 2.97 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively, which is consistent with the higher J_sc of the PSCs with D/A weight ratio of 1 : 5 and 3% DPE as additive.

Fig. 7 Plots of ln JL³V⁻² vs. (V/L)^0.5 of the polymers and blend films of the polymer and PC₇₁BM: (a) and (c) hole-only devices (ITO/PEDOT:PSS/polymer or polymer:PC₇₁BM/Au); (b) and (d) electron-only devices (ITO/ZnO/polymer:PC₇₁BM/Ca/Al).

Table 3 Hole and electron mobilities of the pure polymers and polymer:PC₇₁BM blend films with different conditions

D/A ratio	PTPD-DT		PTPD-DFDT
	Hole mobility (cm^2 V^−1 s^−1)	Electron mobility (cm^2 V^−1 s^−1)	Hole mobility (cm^2 V^−1 s^−1)	Electron mobility (cm^2 V^−1 s^−1)
a With 3% DPE as a solvent additive.
1 : 1	3.85 × 10^−5	3.98 × 10^−6	3.88 × 10^−5	4.73 × 10^−5
1 : 1.5	6.60 × 10^−5	1.34 × 10^−5	7.22 × 10^−5	1.01 × 10^−4
1 : 2	2.60 × 10^−5	1.32 × 10^−5	3.24 × 10^−5	1.86 × 10^−5
1 : 1.5^a	8.56 × 10^−5	5.80 × 10^−5	9.94 × 10^−5	2.97 × 10^−4
Pure	2.42 × 10^−4	—	3.65 × 10^−4	—

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4. Conclusions

Two new D–A copolymers, PTPD-DT and PTPD-DFDT, based on the TPD acceptor unit and DT or DFDT donor units, were designed and synthesized for application as donor materials in PSCs. The polymer PTPD-DFDT shows a relatively broad absorption in the wavelength range from 400 to 700 nm, deeper HOMO energy levels, and higher coplanarity and crystallinity compared to PTPD-DT, which is ascribed to the fluorine substitution and intramolecular F⋯S interactions of PTPD-DFDT. A PSC based on the PTPD-DFDT/PC₇₁BM blend film exhibits a higher V_oc of 0.96 V and a moderated PCE of 5.52%. In addition, PTPD-DFDT has a simpler structure and easier synthesis steps, which is beneficial for low cost and mass production in future.

Acknowledgements

This work was supported by National Natural Science Foundation of China (NSFC) (No. 91333204, 51203168, 51422306, 51503135, 51573120 and 201502134), the Priority Academic Program Development of Jiangsu Higher Education Institutions, Jiangsu Provincial Natural Science Foundation (Grant No. BK20150332), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 15KJB430027), the Postdoctoral research start-up funding of Soochow University (32317366, 32317400), and the Jiangsu Postdoctoral Grant (7131707314).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10481a

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