Effect of fluorination and symmetry on the properties of polymeric photovoltaic materials based on an asymmetric building block

Abstract

Conjugated polymers based on an asymmetric dithieno[3,2-b:2′,3′-d]pyran (DTPa) donor and benzothiadiazole (BT), mono-fluorinated benzothiadiazole (fBT) or di-fluorinated benzothiadiazole (ffBT) acceptors were designed and synthesized. The introduction of fluorine substituents in the BT unit could not only enhance the electronegativity of the acceptors, but also change the symmetry of the BT derivatives, and thus affect the optical, electrochemical, and optoelectronic performance of the final polymers. With the increase of fluorine atoms in the BT unit, the peaks of the absorption spectra for these three polymers hypsochromic shift gradually, combined with the decrease of the HOMO energy levels. The asymmetric structure of fBT results in more complex multichromophore systems and consequently shows a broader absorption FWHM (full-width-at-half-maximum) of 229 nm in chloroform, as well as low absorption intensity and charge carrier mobility for polymer PDTPa-fBT. Finally, polymer solar cells based on these polymers demonstrate power conversion efficiency varying from 4.01% for PDTPa-BT to 3.70% for PDTPa-fBT and to 5.26% for PDTPa-ffBT. These results indicate that the symmetry of both electron-donating and electron-accepting building blocks in conjugated polymers could evidently influence the optical and photovoltaic properties, which might pave the way for the further development of novel photovoltaic polymers based on asymmetric building blocks.

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Introduction

Polymer solar cells (PSCs) have attracted much interest in both scientific research and industrial applications, due to their solution processability and mechanical flexibility. These properties allow PSCs to be manufactured using high-throughput, low-cost processes, such as roll-to-roll printing and inkjet printing.¹ The development of both electron-donating and electron-accepting building blocks for donor–acceptor (D–A) type polymeric materials has contributed to significant improvements in the photovoltaic performance of PSCs.^2,3 In principle, building blocks in conjugated polymers could be divided into symmetric and asymmetric units on the basis of geometrical features, and the symmetric structures could be further divided into centrosymmetric and axisymmetric, as shown in Fig. 1. Typical centrosymmetric building blocks in high performance photovoltaic polymers include thieno[3,2-b]thiophene (TT), benzo[1,2-b:4,5-b′]dithiophene (BDT), indacenodithiophene (IDT), diketopyrrolopyrrole (DPP) and isoindigo (IID) etc. One the other hand, thiophene (T), cyclopenta[2,1-b:3,4-b]dithiophene (CPDT), dithieno[3,2-b:2′,3′-d]silole (DTS), dithieno[3,2-b:2′,3′-d]germole (DTG), dithieno[3,2-b:2′,3′-d]pyrrole (DTP), carbozole (Cz), benzothiadiazole (BT), and thieno[3,4-c]pyrrole-4,6-dione (TPD) etc. are typical axisymmetric building blocks to construct D–A type photovoltaic polymers.

Fig. 1 Absorption spectra of DTPa and BTs based polymers in chloroform solution (left) and solid film on a quartz plate (right).

In the initial period, borrowed from the research of poly(3-hexylthiophene-2,5-diyl) (P3HT),^4–6 chemists firmly believed that promising photovoltaic polymers should be regioregular and thus symmetric building blocks were predominate in the design and synthesis of D–A copolymer photovoltaic materials. In recent years, asymmetric building blocks have attracted much attention and 3-fluorothieno[3,4-b]thiophene-2-carboxylate (fTT)-based polymers have become one kind of promising photovoltaic materials and the power conversion efficiencies (PCE) have exceeded 10%.^7–13 However, quite few asymmetric building blocks, particularly for electron-donating units, have been used in the design of conjugated photovoltaic polymers. As a result, the influence of the symmetry of both donor and acceptor units on the properties of photovoltaic polymers was rarely investigated (Scheme 1).

Scheme 1 Structures and symmetry of typical building blocks in high performance conjugated polymeric photovoltaic materials.

Recently, low band gap polymer based on dithieno[3,2-b:2′,3′-d]pyran (DTPa) building block (in order to distinguish from dithieno[3,2-b:2′,3′-d]pyrrole (DTP),^14–18 we used DTPa as the abbreviation of dithieno[3,2-b:2′,3′-d]pyran) was synthesized and used in polymer solar cells.^19,20 Compared to that of CPDT unit, the introduction of electron-rich oxygen atom not only increased the electron donating property of DTPa, but also endowed the building block with an asymmetric feature (Scheme 2).

Scheme 2 Structures and symmetry of DTPa and BT fluorinates.

On the other hand, benzothiadiazole derivatives (BTs) are of the most important electron-accepting building blocks in conjugated polymers for high performance PSCs, due to their excellent electrical and optical properties as well as chemical and thermal stabilities.^3,21–25 Especially, the symmetry of BT unit can be readily tuned from axisymmetric to asymmetric and then to axisymmetric by introduce one or two fluorine atoms into the 5- and 6-position of BT ring, as shown in Scheme 2. At the same time, fluorination of BT can decrease the energy levels of the corresponding polymers, as well as enhance the intra- and/or intermolecular interactions.^26,27

In this study, we designed and synthesized a series of D–A conjugated polymers based on asymmetric DTPa donor and BT fluorinates acceptors to investigate the effect of symmetry and fluorination on properties of the polymeric photovoltaic materials. The structures of the polymers are shown in Scheme 3.

Scheme 3 Synthetic routes and structures sketch of DTPa and BTs based polymers.

Experimental section

Measurements and characterization

The molecular weights of the polymers were measured by gel permeation chromatography (GPC) method. The number-average molecular weights (M_n), weight-average molecular weights (M_w) and polydispersity index (PDI, M_w/M_n) were measured on a PL-220 (Polymer Laboratories) chromatography connected to a differential refractometer with polyethylene as reference standard and 1,2,4-trichlorobenzene as an eluent (at 150 °C). Elemental analyses were performed on a Flash EA 1112 analyzer or Elementarvario EL III. UV-Vis absorption spectra were recorded on a Shimadzu spectrometer model UV-3150. Absorption spectra measurements of the polymer solutions were carried out in chloroform at room temperature. Absorption spectra measurements of the polymer films were carried out on the quartz plates with the polymer films spin-coated from the polymer solutions in dichlorobenzene. The electrochemical cyclic voltammetry was conducted on Shanghai Chenhua CHI620 electrochemical workstation with a platinum plate, platinum wire, and Ag/AgCl electrode as the working electrode, counter electrode, and reference electrode, respectively, in a 0.1 mol L⁻¹ tetrabutyl ammonium hexafluorophosphate (Bu₄NPF₆) acetonitrile solution. Polymer thin films were formed by drop-casting of polymer solutions in CHCl₃ on the working electrode and then dried in air. Atomic force microscopy (AFM) images of the thin films were obtained on a Veeco Dimension 3100 Atomic Force Microscope operating in tapping mode. X-ray diffraction (XRD) measurements of thin films were performed in reflection mode with Cu Kα radiation (λ = 0.15418 nm) using a Rigaku D/max 2500 V X-ray diffractometer.

Fabrication of photovoltaic devices

PSCs were fabricated with ITO glass as a positive electrode, Ca/Al as a negative electrode, and the blend film of the polymer/PC₇₁BM located between them as a photosensitive layer. The ITO glass was precleaned and modified by a thin layer of PEDOT:PSS, which was spin-cast from a PEDOT:PSS aqueous solution on the ITO substrate, and the thickness of the PEDOT:PSS layer was about 35 nm. The photosensitive layer was prepared by spin-coating a blend solution of polymer and PC₇₁BM (1 : 2, by weight) in o-dichlorobenzene (DCB), at concentration of 10–15 mg mL⁻¹ for polymer, on the ITO/PEDOT:PSS electrode. Then, the cathode was deposited on the polymer layer by vacuum evaporation under 3 × 10⁻⁴ Pa. The thickness of the photosensitive layer was ca. 100 nm, measured on an Ambios Tech XP-2 profilometer. The effective area of one cell was ca. 4 mm². The current–voltage (I versus V) measurement of the devices was conducted on a computer controlled Keithley 236 source measure unit. A xenon lamp with an AM 1.5 filter was used as the white-light source, and the optical power at the sample was 100 mW cm⁻².

Synthesis of the polymers

The synthesis of the polymers was carried out using palladium-catalyzed Stille-coupling between monomer 4,7-dibromo-2,1,3-benzothiadiazole derivatives (1 eq.) and DTPa-distannyl (1.02 eq.), as shown in Scheme 3. Monomers, Pd₂(dba)₃ (2.5% in mol), (o-tol)₃P (20% in mol) and toluene were added in a reaction vial with a magnetic stirring bar. After being purged by three freeze–pump–thaw cycles, the mixture was heated at 100 °C for 24 hours. Then, bromobenzene (5 eq.) was added as end-cappers. After stirring for another 6 hours, the reaction solution was cooled to room temperature and then added dropwise to 120 mL ethanol. The black precipitate was filtered into a Soxhlet funnel and extracted by methanol, hexane, and chloroform successively. The polymers in chloroform solution were further purified by preparative gel permeation chromatography. The products fraction were concentrated and precipitated in ethanol to recover the polymers. After filtration by 0.45 μm nylon filters, the products were dried under vacuum over night to yield the titled polymer as black solids.

PDTPa-BT. Yield: 75%; M_n = 15.0 kDa, M_w = 32.2 kDa, PDI = 2.1; anal. calcd for (C₃₅H₄₆N₂OS₃)_n: C, 69.26; H, 7.64; N, 4.62. Found: C, 70.27; H, 7.91; N, 4.55.

PDTPa-fBT. Yield: 46%; M_n = 13.1 kDa, M_w = 21.9 kDa, PDI = 1.7; anal. calcd for (C₃₅H₄₅FN₂OS₃)_n: C, 67.27; H, 7.26; N, 4.48. Found: C, 68.21; H, 7.58; N, 4.35.

PDTPa-ffBT. Yield: 91%; M_n = 16.6 kDa, M_w = 31.5 kDa, PDI = 1.9; anal. calcd for (C₃₅H₄₄F₂N₂OS₃)_n: C, 65.38; H, 6.90; N, 4.36. Found: C, 66.07; H, 7.21; N, 4.22.

Results and discussion

Optical properties and theoretical calculations

Absorption spectra of these DTPa-based polymers in dilute chloroform solution and as spin-coated films on quartz substrates were shown in Fig. 1. Table 1 summarized the detail optical data, including the absorption peak (λ_max), full-width-at-half-maximum (FWHM), absorption onsets (λ_onset), and the optical band gap (E^opt_g). All of the absorption spectra recorded from both in solution and in film feature two absorption bands: the first one in the short wavelength region of 350–480 nm, which can be assigned to localized π–π* transitions, and the second band in the long wavelength region of 500–950 nm, corresponding to intramolecular charge transfer (ICT) between donor and acceptor units.

Table 1 Optical properties of DTPa and BTs based polymers

Polymer	In CHCl3		In films
	λmax (nm)	FWHM (nm)	λmax (nm)	λonset (nm)	E^optg (eV)
PDTPa-BT	428, ∼744, 806	185	434, ∼754, 821	906	1.37
PDTPa-fBT	420, 723, ∼792	229	427, 736, 799	898	1.38
PDTPa-ffBT	423, 724, 789	166	433, 722, 794	861	1.44

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With the increasing of fluorine atom on BT unit, from PDTPa-BT to PDTPa-fBT, and then to PDTPa-ffBT, the absorption peaks and onsets are significantly blue-shifted both in solution and in film. Compared to PDTPa-BT, the difluorinated analogue PDTPa-ffBT shows absorption spectrum with similar shape but narrow FWHM and increased intensity, which is agreement with that of symmetric donor based conjugated polymers.^23,28–30 Furthermore, there are vibronic shoulders or secondary peaks in the shorter wavelength range for three polymers both in solution and in film, except for PDTPa-fBT in solution, where the vibronic shoulder peak appears in the longer wavelength range (∼792 nm). In addition, for PDTPa-fBT in solution, the absorption FWHM also increases obviously to 229 nm and the absorption coefficient decreases dramatically by about 20% and 30%, in comparison with that of PDTPa-BT and PDTPa-ffBT, respectively. The abnormity of absorption spectrum for PDTPa-fBT should be ascribed to the asymmetric structure of mono fluorinated BT unit. The absorption peaks and edges of the three polymers in the solid film are obviously red-shifted than that in solution, indicating the existence of intermolecular interactions in the solid state. The absorption edges for solid films of PDTPa-BT, PDTPa-fBT and PDTPa-ffBT decrease from 906, 898 to 861 nm, corresponding to optical band gaps increasing from 1.37, 1.38 to 1.44 eV, respectively.

Because of the asymmetry of DTPa unit, repeating units of polymers are difficult to define due to the exist of different monomer sequences. As a consequence, multiple chromophores could be expected³¹ and the possible monomer sequences of three polymers are illustrated in Scheme 4. Because of the asymmetric structure of DTPa and symmetric structure of BT and ffBT, there are two types of chromophores with different monomer sequences in polymer PDTPa-BT and PDTPa-ffBT. By contrast, the introduction of mono fluorine atom on BT unit breaks the symmetry and thus there are four types of chromophores with different monomer sequences in polymer PDTPa-fBT.

Scheme 4 Multichromophoric structure in DTPa and BTs based polymers.

To investigate the impact of different monomer sequences caused by the asymmetric structure of the building blocks on the optoelectronic properties of the conjugated polymers, density functional theory (DFT) calculations were performed to verify stationary points as stable states for the optimized conformations and single point energies, with a molecular main chain length n = 1, at B3LYP/6-311G level of theory in vacuum using the Gaussian 09 program package.³² The final energies are calculated as the sum of single point and zero point energies. In particular, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels positions and related electron distributions are calculated and all the alkyl chains are replaced by methyl groups in the calculation to avoid excessive computation demand.

Calculated HOMO, LUMO levels and bandgaps (E^cal_g) of the DTPa–BTs dimers with different sequences are shown in Fig. 2. The results of CPDT–BT dimer is also presented for comparison. Compared the energy levels of CPDT-BT dimer and DTPa-BT dimers, it could be found that the insertion of oxygen atom into CPDT segment decreases the band gap of the dimer by both increasing the HOMO energy level and decreasing the LUMO energy level. The reduction of band gap is derived from the enhancement of intramolecular charge transfer in DTPa-BT dimers benefited from the strong electron donating property of DTPa unit, as depicted in wave functions of the frontier molecular orbital of CPDT-BT dimer and DTPa-BT dimers in Fig. 3. As can be observed, the HOMO is delocalized along the whole conjugated backbone while the LUMO is mostly concentrated on the BT acceptor. These images provide further evidence of the formation of well-defined D–A structure and the intramolecular charge transfer behavior of these dimers (i.e., the HOMO to LUMO transition is a donor to acceptor intramolecular charge transfer). Compared to that of CPDT-BT dimer, the HOMO of DTPa-BT dimers are further transferred to DTPa unit, indicating the enhancement of the electron-donating ability. In addition, from Fig. 2, we can see that sequence 1 and 2 reveal similar HOMO and LUMO energy levels, and the same phenomenon could be also found for sequence 7 and 8. In other words, for DTPa-BT and DTPa-ffBT dimers, inserting oxygen atom into CPDT in the direction of either face to or back to BT unit have a negligible impact on the energy levels of the dimers.

Fig. 2 Calculated HOMO, LUMO levels and band gaps of the DTPa–BTs dimers with different sequences.

Fig. 3 The frontier molecular orbital (LUMO, top; HOMO, bottom) of CPDT-BT dimer and DTPa-BT dimers obtained from DFT calculations. Color code: gray (C), white (H), red (O), blue (N), and yellow (S).

In contrast, the energy levels of the dimers are influenced evidently by the sequences of asymmetric fBT unit. Because of the electronic effects of halogen substituent on aromatic rings together with the large electron negativity of fluorine atom, the meta-fluorination of BT could remarkably decrease both HOMO and LUMO energy levels of the dimer (sequence 5 and 6), compared with ortho-fluorination (sequence 3 and 4). The wave functions of the frontier molecular orbital of DTPa-fBT dimers are shown in Fig. 4. The more efficient concentration of HOMO to DTPa and transformation of LUMO to ortho-fluorinated BT confirms stronger ICT in ortho-fluorinated BT based dimers, which results in lower band gaps in corresponding dimers (sequence 3 and 4).

Fig. 4 The frontier molecular orbital (LUMO, top; HOMO, bottom) of DTPa-fBT dimers obtained from DFT calculations. Color code: gray (C), white (H), red (O), blue (N), yellow (S) and turquoise (F).

In CPDT and BT based conjugated polymers, mono fluorination of BT units did not influence the absorption spectra and energy levels of the polymers too much.^27,33,34 In other words, sequences of meta-fluorinated BT and ortho-fluorinated BT connected to CPDT shared quite similar absorption peaks and energy levels. Compared to that of CPDT, owing to the inserting of oxygen atom, the strong electron-donating property of DTPa unit projected the difference of fluorination position of BT units. In virtue of the energy difference among different sequences, the more isomers would result in the more energy diffusion. As a result, the polymer PDTPa-fBT based on asymmetric DTPa and asymmetric fBT displays broader absorption band and lower absorption intensity than the other two polymers.

Electrochemical properties. Cyclic voltammetry was employed to study the electrochemical properties of these DTPa and BTs based conjugated polymers. From the onset oxidation and reduction potentials in the cyclic voltammogram, the HOMO and LUMO energy levels could be readily estimated, which are corresponding to ionization potential (IP) and electron affinity (EA), respectively. Cyclic voltammograms of the polymer films are shown in Fig. 5. HOMO levels, LUMO levels and electrochemical band gaps (E^CV_g) of the polymers are calculated and summarized in Table 2, together with the onset oxidation potential (E_ox) and onset reduction potential (E_red). The introduction of fluorine atoms on BT unit significantly decreased the HOMO energy levels of the polymers from −4.91 eV for PDTPa-BT to −5.09 eV for PDTPa-fBT and then to −5.24 eV for PDTPa-ffBT. However, the LUMO energy levels of the polymers decrease quite slightly, just from −3.50 to −3.58 eV. The electrochemical band gaps of the polymers are well matched with their optical band gaps within the experimental error, as shown in Tables 1 and 2.

Fig. 5 Cyclic voltammograms of the polymer films on Pt electrode in 0.1 mol L⁻¹ Bu₄NPF₆, CH₃CN solution with a scan rate of 100 mV s⁻¹.

Table 2 Electrochemical properties of the polymers based on DTPa and BTs

Polymer	Eox	HOMO (eV)	Ered	LUMO (eV)	E^CVg (eV)
PDTPa-BT	0.57	−4.91	−0.84	−3.50	1.41
PDTPa-fBT	0.75	−5.09	−0.77	−3.57	1.52
PDTPa-ffBT	0.90	−5.24	−0.76	−3.58	1.66

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Photovoltaic properties. To investigate the effects of fluorination and symmetry of building blocks on the photovoltaic properties of the DTPa–BTs polymers, bulk heterojunction polymer solar cells (PSCs) were fabricated. Fig. 6 shows the current density–potential characteristic of PSCs based on the blends of polymer:PC₇₁BM under 100 mW cm⁻² illumination (AM 1.5G). The open-circuit voltage (V_oc), short-circuit current (J_sc), fill factor (FF) and PCE of the PSCs are summarized in Table 3.

Fig. 6 Current density–voltage characteristics of PSCs based on the DTPa–BTs polymers.

Table 3 Photovoltaic properties of the PSCs based on the DTPa–BTs polymers

Polymer	Voc (V)	Jsc (mA cm^−2)	FF	PCE	Mobility (cm^2 V^−1 s^−1)
PDTPa-BT	0.584	11.95	0.574	4.01%	4.5 × 10^−4
PDTPa-fBT	0.688	9.57	0.562	3.70%	2.4 × 10^−5
PDTPa-ffBT	0.710	11.62	0.638	5.26%	2.6 × 10^−4

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It is clear that with the increase of the fluorine atoms on BT unit, the V_oc of the devices improve from 0.584 V for PDTPa-BT to 0.688 and 0.710 V for PDTPa-fBT and PDTPa-ffBT based PSCs, respectively, which comes from the decrease of HOMO energy levels. The PDTPa-BT gives a PCE of 4.01%, with a J_sc of 11.95 mA cm⁻² and a FF of 57.4%. Under the same conditions, the device based on PDTPa-ffBT shows a comparable J_sc of 11.62 mA cm⁻² and an enhanced FF of 63.8%, leading to an overall PCE of 5.26%. However, the polymer PDTPa-fBT exhibited a J_sc of 9.57 mA cm⁻² and PCE of 3.7%, which are inferior to those of both BT and ffBT based polymers. In fact, for some D–A type photovoltaic polymers comprising of symmetric donor and BT acceptor, mono fluorination in BT unit always improve the photovoltaic performance by increasing all the V_oc, J_sc and FF.^30,31,33 In some cases, fBT based polymers even show superior PCE to those of ffBT based polymers, especially in J_sc.^35–37 The results indicate the combination of both asymmetric electron-donating building block and asymmetric electron-accepting unit might be unadvisable to construct D–A type photovoltaic polymers.

The photovoltaic performance of the polymers could be further investigated by the external quantum efficiency (EQE) spectra of the devices. As shown in Fig. 7, the flat EQE values indicate the balanced contribution from polymer and PC₇₁BM in both of PDTPa-BT and PDTPa-ffBT based devices. PDTPa-BT based device shows broad EQE up to about 950 nm, but lower intensity in the range of 600–820 nm, compared to that of PDTPa-ffBT. The photoresponse of solar cells based on PDTPa-BT and PDTPa-ffBT are consistent with their absorption characteristic. The effect of fluorination on the photovoltaic properties of polymers based on asymmetric DTPa donor and symmetric BT and ffBT acceptors are similar to that of symmetric donors.^29,35 However, for the PDTPa-fBT device, EQE values decrease significantly in all measured range, especially from 600 to 900 nm stemmed from the polymer, which further verifies the reason of low J_sc compared with the other two polymers. The calculated short-circuit currents from the integration of the EQE values were 11.36, 9.17, and 11.10 mA cm⁻², which agree well with corresponding J–V characteristics of the PSCs.

Fig. 7 EQE spectra of the PSCs based on the polymer:PC₇₁BM blends.

To further study the performance of the PSCs devices, the hole mobilities in the photosensitive layers were measured by the space charge limited current (SCLC) method using devices with structure of ITO/PEDOT:PSS/polymer:PC₇₁BM/Au. For unipolar transport in a trap-free semiconductor with an ohmic injecting contact, the SCLC can be approximated by the Mott–Gurney equation:³⁸

where J is the current density, ε_r is the dielectric constant of the polymer, ε₀ is the free-space permittivity (8.85 × 10⁻¹² F m⁻¹), μ₀ is the charge mobility at zero field, γ is a constant, L is the thickness of the blended film layer, V = V_appl − V_bi, V_appl is the applied potential, and V_bi is the built-in potential which results from the difference in the work function of the anode and the cathode (in this device structure, V_bi = 0.2 V). Fig. 8 displays ln(JL³/V²) versus (V/L)^0.5 curve for the measurement of the hole mobility of the blends by the SCLC method. The calculated mobilities using eqn (1) are summarized in Table 3.

Fig. 8 ln(JL³/V²) versus (V/L)^0.5 plots for the measurement of hole mobility in the polymer:PC₇₁BM blends by the SCLC method.

As can be observed, PDTPa-BT and PDTPa-ffBT based active layers show comparable mobilities at 10⁻⁴ cm² V⁻¹ s⁻¹ orders of magnitude. While PDTPa-fBT with both asymmetric building blocks only realizes a hole mobility of 2.4 × 10⁻⁵ cm² V⁻¹ s⁻¹, one order of magnitude lower than that of the other two polymers. The obviously low mobility in PDTPa-fBT:PC₇₁BM blends should be an important reason for the inferior J_sc in PDTPa-fBT based solar cell.

In order to investigate the effect of fluorination and symmetry of the polymers on the morphologies and microstructures of the polymer:PC₇₁BM blends and consequently on the photovoltaic performance, atom force microscopy (AFM) and X-ray diffraction (XRD) were employed to analyze active layers of the polymer solar cells. As shown in Fig. 9, both PDTPa-BT:PC₇₁BM and PDTPa-fBT:PC₇₁BM blends displayed smooth surface with root-mean-square roughness (RMS) values of 0.73, 0.56 nm, respectively. By contrast, RMS value of PDTPa-ffBT:PC₇₁BM obviously increases to 2.88 nm, indicating the raise of larger aggregation domain in the blend film.

Fig. 9 AFM images (5.0 μm × 5.0 μm) of the polymer:PC₇₁BM blends: (a) PDTPa-BT:PC₇₁BM; (b) PDTPa-fBT:PC₇₁BM; (c) PDTPa-ffBT:PC₇₁BM.

The similar variation trend as AFM features in the active layers is also observed in the XRD measurement (Fig. 10). It is notable that all the three blend films display Bragg peaks with maximum of 3.57, 3.67 and 3.49 nm⁻¹, corresponding to the lamellar spacing of 1.76, 1.72 and 1.80 nm, for the blends of PDTPa-BT:PC₇₁BM, PDTPa-fBT:PC₇₁BM and PDTPa-ffBT:PC₇₁BM, respectively. The film of PDTPa-BT:PC₇₁BM blend shows a very broad diffraction peak dispersed from 2.9 to about 6 nm⁻¹. By contrast, in PDTPa-fBT:PC₇₁BM and PDTPa-ffBT:PC₇₁BM blends, the fluorinated polymers present diffraction peaks without tails, which might be ascribed to the enhancement of intra- and intermolecular interaction caused by fluorine atoms. As reported in literatures, for symmetric building blocks based polymers, with the increasing of fluorine, lamellar distances of polymer:PC₇₁BM blends raised gradually, together with the enhancement of the diffraction peaks.²² In this work, the polymers based on symmetric BT and ffBT are accordance with the trend. From PDTPa-BT to PDTPa-ffBT, the lamellar distances slightly increase by 0.04 nm, while the intensity of the diffraction peaks doubles. However, the effect of fluorination in PDTPa-fBT seems more complex. The introduction of mono fluorine on BT unit not only enhances the intra- and intermolecular interactions but also results in the more complex building blocks arrangement, as stated above. The multichromophoric structures might hinder the formation of ordered array of polymer and thus decrease the hole mobility. Moreover, in PDTPa-fBT:PC₇₁BM blend, the lamellar distance of polymer is reduced to 1.72 nm, which is even smaller than the P3HT:PC₇₁BM intercalated distance (∼1.74 nm).³⁹ The results indicate that PDTPa-fBT and PC₇₁BM did not form proper micro sandwich structure (bulk heterojunction) in the active layer, which might be another important reason for the inferior short-circuit current in PDTPa-fBT:PC₇₁BM based solar cells.

Fig. 10 XRD patterns of the polymer:PC₇₁BM active layers.

Conclusion

We have designed and synthesized three D–A copolymers, combining asymmetric dithieno[3,2-b:2′,3′-d]pyran (DTPa) electron-rich unit and three kinds of benzothiadiazole (BT) electron-deficient units by Pd-catalyzed Stille-coupling reaction. With the increasing of fluorine atom on BT unit, the symmetry could be readily tuned from axisymmetric (BT) to asymmetric (fBT) and then to axisymmetric (ffBT) again. The absorption peaks blue-shift and the band gaps increase from 1.41 eV for PDTPa-BT to 1.52 eV for PDTPa-fBT and then to 1.66 eV for PDTPa-ffBT, with the down-shift of HOMO levels from −4.91 to −5.09 and then to −5.24 eV, respectively. The asymmetric structure of both DTPa and fBT lead to more complex multichromophore and consequently increased absorption FWHM as well as lower absorption intensity and charge carrier mobility for the polymer PDTPa-fBT. These results indicated that alternating D–A type conjugated copolymers with both asymmetric donor and acceptor building blocks possess quite different optical and photovoltaic properties as compared with the polymers with only one asymmetric building block. This difference may be due to the existence of complex multichromophore. Most importantly, these results provide new insights toward the design of high performance D–A type photovoltaic materials.

Acknowledgements

This work was supported by the Natural Science Foundation of China (NSFC, No. 21504019 and 51473040), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, the Natural Science Foundation of Beijing (No. 2160045), and the One Hundred Person Project of the Chinese Academy of Sciences and CAS Key Laboratory of Nanosystem and Hierarchical Fabrication.

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