Synthesis of new D-A1–D-A2 type low bandgap terpolymers based on different thiadiazoloquinoxaline acceptor units for efficient polymer solar cells

Abstract

Two low bandgap D-A1–D-A2 conjugated copolymers, namely denoted as P1 (non-fluorine substituted thiadiazoloquinoxaline A2) and P2 (fluorine substituted thiadiazoloquinoxaline A2) with the same D (thiophene) and A1 (benzothiadiazole) groups were synthesized in order to investigate the effect of fluorine atoms on the photovoltaic performance of polymer solar cells. The electrochemical properties demonstrate that the highest occupied molecular orbital (HOMO) energy level lowered from −5.08 eV (for P1) to −5.16 eV (for P2), whereas the lowest unoccupied molecular orbital (LUMO) energy levels remain nearly the same. These copolymers showed strong absorption in the wavelength range 300–1100 nm and have a bandgap of around 1.08 and 1.11 eV for P1 and P2, respectively. After the optimization of the weight ratio and concentration of solvent additives 1-chloronaphathalene (CN), the highest power conversion efficiencies of bulk heterojunction polymer solar cells achieved were up to 5.30% and 7.21% for P1 and P2 as donor and PC₇₁BM as acceptor. The enhanced V_oc and J_sc for the P2 based device can be mainly ascribed to the lower HOMO energy levels and higher hole mobility and better morphology of the fluorinated copolymer P2 : PC₇₁BM blend.

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Introduction

In the last decade, polymer solar cells (PSC) with bulk heterojunctions (BHJ) have attracted significant attention from researchers as renewable energy sources, because of their unique advantages such as low cost, manufacturability, scalability and the possibilities they offer for flexible large-area device manufacturing.^1–6 To date, PSC efficiencies have reached impressive levels exceeding 10%.^7–10 Such great progress has been achieved not only through better understanding and control of charge generation transport, but mainly through the development of a wide range of new conjugated polymers. At present the research work is in progress with a goal of a power conversion efficiency of more than 15%.¹¹ Therefore, new approaches and new highly conjugated polymers are in demand.

To date, breakthrough in PCE rapid increase was mainly achieved via D–A approach with strict electron-donor and electron-acceptor units alternation in the polymer chain which is the most attractive and successful strategy for controlling the energy levels and optical bandgap of copolymers via intramolecular charge transfer. While D–A polymers bandgap and energy levels can be controlled by combination of different electron-donor and electron-acceptor structures, nevertheless the conventional D–A copolymers still doesn't provide sufficient opportunities for obtaining suitable energy levels and broad solar radiation absorption, since D–A possess rather narrow absorption bands with 200 nm half-maximum width. Improving PSC efficiency via common D–A copolymers structure modification of D and A units combination now seems to have reached a plateau. In order to overcome these limitations of D–A polymers we follow a straightforward extension approach to develop new regular alternating “D-A1–A2-D” polymer structure. A lot of important factors for highly efficient electron donors such as light absorption, HOMO/LUMO levels, hole mobility and D–A copolymers solubility of can carefully varied through second component introduction via a D-A1–D-A2 approach. Unlike conventional D–A polymers containing only one electron-acceptor block in repeating unit, introducing two electron-acceptor units with different absorption capacity into alternating D-A1–D-A2 copolymer structure will provide broader absorption spectrum than its components. Using additional electron-acceptor block as the second A2 unit in D–A block copolymer can broaden the absorption spectrum up to near-infrared region due to appearance of new intramolecular charge transfer ICT bands, and also to cause a synergistic effect in optical, electrical and structural properties of copolymers. Thus, by applying this approach, we can utilize advantages reported in earlier experiments and D and A units to develop their new combinations to meet conjugated polymers desired properties. One of main reasons for using multiple semiconductor components in polymers synthesis is that it can lead to absorption broadening to favor photons absorption in photovoltaic cells and molecular packing configuration by introducing third component that facilitates stacking.

Recently Wang et al. have shown the advantages of the D-A1–D-A2 strategy to improve the PCE of its D-A1 and D-A2 components.¹² Janssen et al. developed isomeric random and regular alternating conjugated terpolymers containing two electron-acceptors diketopyrrolopyrrole (DPP), thienopyrrolidone (TPD) and one electron donor, to study monomer units sequential distribution effect on semiconducting properties.¹³ It has been shown that the optical properties and electrochemical characteristics of photovoltaic regular and random terpolymers significantly differ from each other. Randomly spaced donor and acceptor structure along the polymer chain has a significant negative impact on the photovoltaic performance resulting in PSC efficiency for random terpolymers to reach just 1.0%, while reaching more than 5.3% for regular terpolymers.

Li recently developed semi-crystalline conjugated “D-A1–D-A2” polymer structure on basis of two electron-acceptor pentacycliclactam and DPP units for use in field effect transistors and polymer solar cells.¹⁴ Polymer absorbs light over a wide range of solar spectrum from 350 to near-infrared up to 900 nm and exhibits high hole mobility of 0.81 cm² V⁻¹ s⁻¹ and PSC efficiency of 4.7%.

Fluorine introduction into polymer chain is considered as one of the most effective ways for simultaneous reducing HOMO and LUMO levels, while maintaining the bandgap almost constant in comparison to their corresponding non-fluorinated analogues, which significantly increase V_oc and PSC efficiency as a result, and also increase of intra-intermolecular interaction of polymers due to strong induced C–F bond that leads to high charge mobility and transport. Furthermore strong F⋯H/F⋯S interaction and well-resolved fluoropolymers fibril structure can make significant contribution to active level morphology and to suppress charge recombination. In this regard, fluoride utilization can lead to synergism of the resulting material optoelectronic characteristics.

Inspired by these results, we have prepared two “D-A1–D-A2” structure copolymers P1 and P2 fluorinated analogue by adding two fluorine atoms to thiadiazoloquinoxaline fragment (Scheme 2). In this work, two thiadiazoloquinoxaline (TDQ) and benzodithiazole (BT) electron-acceptor units were included into conjugated P1 and P2 polymers for PSC applications. TDQ is a strong electron acceptor due to its rigid planar structure and the presence of four imino groups with unique properties such as high charge carrier mobility, extending absorption up to near infrared region and ease of functionalization with various aryl and alkyl groups for improving solubility. TDQ-based conjugated polymers demonstrated conversion efficiency of more than 3.58% with photoresponse to 1200 nm.¹⁵ BT-based polymers exhibit broad absorption up to near-infrared region up to 1000 nm with efficiency of more than 5–6%.^16–18 It was interesting to investigate D–A conjugated polymers containing both TDQ and BT units. As usual, two thiophene groups are introduced on the both sides of the BT group to reduce the steric hindrance between two acceptor units, which also affects the properties such as charge carrier mobility, optical absorption and energy levels of the resulting copolymers. Novel P1 and P2 polymers (Scheme 1) have narrow bandgaps. PSCs based on them have a strong tendency to form crystalline structures with photoresponse in thin films up to 1100 nm. Bulk heterojunction polymer solar cells were fabricated using these copolymers as donors along with PC₇₁BM as acceptor to investigate the effect of substitution of fluorine atoms in TDQ acceptor units of polymer backbone. After the optimization of weight ratios and solvent concentration, P2 possesses superior photovoltaic characteristics with PCE of 7.21% compared to its non-fluorinated analog P1 with PCE of 5.30%. Our results show that conjugated polymers with electron-acceptor TDQ and BT units may demonstrate narrow bandgap and strong semi-crystallinity and therefore can potentially be used as electron donor in high performance organic electronic devices.

Scheme 1 Synthetic route of monomers M2.

Scheme 2 Synthetic route copolymers P1 and P2.

Experimental details

Instruments and characterization methods

The ¹H and ¹³C NMR spectra of the initial compounds and polymers were recorded on a Bruker Avance 400 spectrometer operating at 400.13 and 100.62 MHz, respectivelyTGA was performed on a Perkin_Elmer TGA-7 instrument at a heating rate of 20 K min⁻¹. The molecular weight distribution was analyzed by GPC using a Bruker LC21 liquid chromatograph with refractometric and UV detectors. Chromatography conditions: methylene chloride, 1 mL min⁻¹, λ = 390 nm. Calibration was performed using PS standards. The absorption spectra of thin film and solution were recorded on a PC2000 fiber optical spectrophotometer. Cyclic voltammetry measurements were performed on a potentiostat-galvanostat AUTOLAB Type III equipped with standard three-electrode scheme in an acetonitrile solution of 0.1 mol L⁻¹ tributylammonium hexa-fluorophosphate (n-Bu₄NPF₆) at a potential scan rate of 50 mV s⁻¹. Films of the investigated polymers were deposited on a glassy carbon electrode surface and used as working electrode. Ag/Ag⁺ and platinum were used as reference and counter electrodes, respectively. XRD measurements were recorded on a Bruker D8 Advanced model diffractometer with Cu Kα radiation (λ = 1.542 Å) at a generator voltage of 40 kV.

Synthesis of copolymers P1

The synthesis of monomers is described in ESI.†

The polymerization was performed by a Stille coupling reaction. In a 50 mL flask, M1 (0.6738 g, 0.5 mmol) and M3 (0.3130 g, 0.5 mmol) were dissolved in 15 mL toluene, and the solution was flushed with argon for 15 min, then 27 mg Pd(PPh₃)₄ was added into the solution. The mixture was again flushed with argon for 20 min. The reaction mixture was heated to reflux for 48 h. The reaction mixture was cooled to room temperature and added dropwise to 400 mL methanol. The precipitate was collected and further purified by Soxhlet extraction with methanol, hexane, and chloroform in sequence. The chloroform fraction was concentrated and added drop-wise into methanol. Finally, the precipitates were collected and dried under vacuum overnight to get polymer P1 as a black solid (0.52 g, yield 71%). ¹H NMR (CDCl₃, 400 MHz): δ (ppm) 8.00–7.40 (br, 20H, ArH), 2.20–1.95 (br, 8H, CH₂), 1.90–0.70 (br, 92H, CH₃, CH₂). Elem. anal. for (C₉₆H₁₂₀N₆S₄)_n calc.: C, 77.58; H, 8.14; N, 5.65; S, 8.63. Found: C, 77.19; H, 8.18; N, 5.34; S, 8.19.

Synthesis of copolymers P2

Copolymer P2 as synthesized by following the same procedure for P1 but using M2 instead of M1. M2 (0.6919 g, 0.5 mmol) and M3 (0.3130 g, 0.5 mmol mg) were used as starting materials in the polymerization. Finally, P2 was obtained as a black solid (0.57 g, yield 75%). ¹H NMR (CDCl₃, 400 MHz): δ (ppm) 7.80–7.40 (br, 18H, ArH), 2.25–1.90 (br, 8H, CH₂), 1.90–0.70 (br, 92H, CH₃, CH₂). Elem. anal. for (C₁₂₀H₁₁₈F₂N₆S₄)_n calc.: C, 75.73; H, 7.81; N, 5/52; S, 8.42; found: C, 75.44; H, 7.69; N, 5.12; S, 8.01.

Device fabrication and characterization

The BHJ PSCs with conventional structure of ITO/PEDOT : PSS/active layer/Al were fabricated as follow: the indium tin oxide coated (ITO) glass substrate was cleaned subsequently by ultrasonic treatment for 10 min in detergent, de-ionized water, acetone and isopropyl alcohol and then dried in ambient conditions. A thin layer of PEDOT : PSS (∼40 nm) was then spin coated on the pre-cleaned ITO glass at 3500 rpm for 40 s and annealed at 120 °C for 20 min. Then the active layer consist of a blend of copolymer and PC₇₁BM was spin coated onto the top of PEDOT : PSS coated ITO coated glass at room temperature. We have used CF and CN (different concentration)/CF as solvent for the blend. The thickness of the all active layer was kept constant i.e. ∼90–95 nm. After the active layer was dried completely aluminum (Al) electrode was thermally deposited under a pressure of 10⁻⁶ Torr. The current density–voltage (J–V) characteristics were measured by a Keithley 4200 source-meter under AM 1.5 G (100 mW cm⁻²) simulated by solar simulator. The incident photon to current conversion efficiency (IPCE) values were measured short circuit current state with illumination of monochromatic light and the current was measured with Keithley electrometer. The photovoltaic properties are reported on average values of 5 devices.

The hole-only and electron-only devices with ITO/PEODT : PSS/copolymer : PC₇₁BM/Au and Al/copolymer : PC₇₁BM/Al architectures were also fabricated in an analogous way, in order to measure the hole and electron mobility, respectively. The impedance data were measured with an impedance analyser (frequency 1 mHz and 1000 kHz), under illumination.

Results and discussion

Synthesis of monomers and polymers

We have synthesized new thiadiazoloquinoxaline-containing condensed M1 and M2 planar monomers as effective “strong acceptor” for narrow-gap structures “D-A1–D-A2” copolymers P1 and P2. Dibromo (monomer M1) was prepared according to the published ref. 18. Target monomer 4,9-dibromo-6,7-bis(9,9-didodecyl-7-fluoro-9H-fluoren-2-yl)[1,2,5]thiadiazolo[3,4-g]quinoxaline (M2) was obtained in several synthetic steps shown in Scheme 1. In the first step esterification of ortho-bromobenzoic acid (1) with isopropyl alcohol was performed with almost quantitative yield, in the presence of thionyl chloride. The obtained isopropyl 2-bromobenzoate (2) was introduced into the cross-coupling reaction with p-fluorophenylboronic acid under Suzuki reaction conditions in the presence of Pd(PPh₃)₄, which allowed to obtain biphenyl derivative 3 with a yield of 85% (after chromatographic purification). As a result of alkaline hydrolysis of the ester group in compound 3, followed by acidification of the reaction mixture, para-fluorobiphenyl-2-carboxylic acid (4) was obtained with a yield of about 95%, which was used without further purification in the reaction with thionyl chloride in refluxing chloroform and converted to the corresponding acid chloride 5 in quantitative yield. Acid chloride 5 under the action of anhydrous AlCl₃ in dichloromethane undergoes intramolecular cyclization to give the 2-fluoro-9H-fluoren-9-one (6) with a yield of 87%. Reduction of the carbonyl group in compound 6 in the Wolff–Kishner reaction conditions leads to the quantitative formation of the corresponding 2-fluoro-9H-fluorene (7) and subsequent alkylation of the latter with 1-bromododecane in the presence of lithium diisopropylamide leads to the formation of dialkyl derivative 8 with a yield of about 95%. In the next step compound 8 was acylated with oxalyl chloride in the presence of AlCl₃ to form the diketone 9 with a moderate yield of 38%. At the final stage of the synthesis material 9 is reacted with 4,7-dibromo-2,1,3-benzothiadiazole-5,6-diamine (10) under heating in a solvent mixture AcOH–dioxane to form the desired monomer M2 which was isolated after chromatographic purification with yield 67%.

The composition and structure of intermediate products 2–9 and target compound M2 were confirmed by elemental analysis, ¹H, ¹³C and ¹⁹F NMR spectroscopy. In particular, there are three multiplets in the proton spectrum of M2 at δ 7.02–7.10 (m, 4H), 7.70–7.62 (m, 4H), 7.80–7.90 (m, 4H), related to the 12 aromatic protons, triplet at δ 1.86 (t, J = 8.19 Hz, 8H), related to the methylene groups directly adjacent to the fluorene moiety, whereas in the interval of δ 1.40–0.60 ppm there are signals related to the other 92 protons of the alkyl substituents. Although the ¹H NMR spectrum is rather complicated, the ratio of the intensity values of the aromatic part of the spectrum to aliphatic corresponds well to the proposed structure (Fig. S13 and ESI†). The carbon spectrum of M2 contains 10 signals in the range of 113–165 ppm related to 10 different aromatic quaternary carbons. Signals of the atoms C3, C1 and C6 are doublets with J_C–F = 247, 7.6 and 1.6 Hz, respectively, due to the spin–spin interaction with F atom. In the region of 108–132 ppm there are 6 signals related to six tertiary carbon atoms, wherein the signals of atoms C5, C4 and C2 are doublets with J_C–F = 9.0, 23.0, 23.0 Hz, respectively, due to the spin–spin interaction with a fluorine atom. In the region of 55.46 and 14.11 ppm there are characteristic signals related to the carbon atom of cyclopentadiene moiety of fluorine and terminal CH₃ groups of the alkyl substituents, respectively. In the region of 41–22 ppm there are signals related to the rest of the aliphatic carbon atoms, which once again confirms the proposed structure (Fig. S13b, ESI†). The ¹⁹F NMR spectra contain a singlet signal at −112.94 ppm relating to fluorine atom of M2 (Fig. S13c, ESI†).

The synthetic routes for copolymers are shown in Scheme 2 and the detailed synthetic process and characterization are described in the ESI.† Copolymer P1 and P2 was synthesized in good yields of 71–76% via Stille cross-coupling reaction of corresponding dibromo monomers (M1 and M2) with bisstanyl monomers M3 using toluene as the solvent and Pd(PPh₃)₄ catalyst at 110 °C. The copolymers were purified by Soxhlet extraction with methanol, hexane and chloroform to remove small molecular and residual catalyst. Chloroform fraction was precipitated in methanol again and dark purple polymer was obtained after dried in a vacuum oven. Copolymeric were characterized by ¹H NMR spectroscopy (Fig. S14a and b, ESI†) and elemental analysis. Both the copolymers are readily soluble in common organic solvents such as chloroform, chlorobenzene and o-dichlorobenzene. The number-average molecular weights (M_n), weight-average molecular weight (M_w) and polydispersity indices (PDIs) of copolymers were determined by gel permeation chromatography (GPC) analysis with a polystyrene standard calibration and chloroform as the eluent. P1 and P2 display high M_n 14.7 and 18.6 kDa, and PDI 1.93 and. 1.81, relatively (Table 1). The molecular weight of conjugated copolymers plays a key role in determining their optical, electrical properties and charge carrier mobilities. A high molecular weight is required for the polymer to ensure the long conjugation length and promote the intermolecular charge transport and consequently enhance the device efficiency.

Table 1 Molecular weights and thermal properties and yield of copolymers P1and P2

Copolymer	Yield (%)	Mn (kDa)	Mw (kDa)	PDI Mw/Mn	Td (°C)
P1	71	14.7	28.3	1.93	336
P2	75	18.6	33.7	1.81	347

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Thermal analysis

The thermal properties of the copolymers were determined by thermogravimetric analysis (TGA), as shown in Fig. 1 and summarized in Table 1. Copolymers exhibited good thermal stability with 5% decomposition temperature up to of 336 °C for P1 and 347 °C for P2 in nitrogen. The high thermal stability of the copolymers prevents the deformation of the morphology and is important for OPV device application. These results demonstrate that both copolymers exhibit high thermal stability, which is important for the application of PSC and other optoelectronic devices.

Fig. 1 TGA curves of the copolymers P1 and P2 with the scan of 20 °C min⁻¹.

Optical properties

The UV-vis-NIR absorption spectra of P1 and P2 copolymers were investigated in chloroform solution at and in thin film are shown in Fig. 2, and the corresponding absorption properties are summarized in Table 2. Both copolymers exhibit similar absorption spectra, resulting from similar copolymer backbone, with two absorption bands between 300 and 700 nm, and a second band between 700 and 1100 nm. The first band can be attributed to the π–π* transition, while the band with lower energy is due to the intramolecular charge transfer (ICT) between the electron-rich and electron deficient monomers. The main absorption peak becomes broader and the maximum shifts toward longer wavelength at 956 nm for P1 and 952 nm for P2. Fluorinated copolymer P2 has a slightly higher absorption coefficient in film than the non-fluorinated counterpart. In the solid state, this large red shift of ∼54 nm from solution to solid state means more coplanar structure and stronger inter chain π–π stacking in the solid state. It was observed that upon fluorination, P2 exhibited a vibronic feature in the ICT band in thin film absorption spectra. Introduction of fluorine atoms is advantageous to form an aggregation of the conjugated backbones through some supramolecular interactions such as F–H, and F–S interactions^19,20 and π–π stacks that favors the charge transfer. The vibrational shoulder peak observed in the absorption spectra of P2 thin film also confirms this type of aggregation. The electron withdrawing nature of the fluorine atoms may lead to a permanent shift of π-electrons and weaken the conjugation effect, which slight blue shift in the UV-visible spectrum. The optical bandgap (E^opt_g) of the two copolymers were estimated from the absorption edges of the polymer films according to E^opt_g = 1240/λ_edge. The measured E^opt_g of P2 than P1 films were 1.09 and 1.11 eV, respectively. It could infer that the introduction of fluorine atoms on the core increased the energy bandgap between HOMO and LUMO by reducing the HOMO levels.

Fig. 2 Normalized absorption spectra of P1 and P2 in solution and thin films.

Table 2 Optical electrochemical data of P1 and P2 copolymers

Copolymer	λ^solvmax (nm)	λ^filmmax (nm)	E^opg (eV)	E^oxonset (V)	E^redonset (V)	HOMO (eV)	LUMO (eV)	E^ecg (eV)
P1	313, 400, 904	313, 400, 952	1.08	0.60	−0.67	−5.08	−3.81	1.27
P2	313, 400, 908	313, 398, 946	1.11	0.68	−0.68	−5.16	−3.80	1.36

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Electrochemical properties

Cyclic voltammetry (CV) was employed to examine the electrochemical properties and determine HOMO energy levels and LUMO energy levels. The measured cyclic voltammograms are illustrated in Fig. 3 and the results are summarized in Table 2. The HOMO, LUMO, and electrochemical band gaps (E^ec_g) can be calculated from the value of onset oxidation (E^ox) and onset reduction (E^red) potentials, respectively of the copolymers according to the equations:

E_HOMO = −q (E^ox_onset + 4.48) eV

E_LUMO = −q (E^red_onset + 4.48) eV

E^elec_g = E_HOMO − E_LUMO

Fig. 3 Cyclic voltammograms of P1 and P2 films.

The calculated E_HOMO and E_LUMO energy levels from the onset oxidation and reduction potential of P1 are −5.08 and −3.81 eV, respectively. After adding two fluorine atoms, P2 shows a much deeper HOMO level of −5.16 eV, whereas the LUMO level (−3.80 eV) is almost unchanged. Evidently, the electron-withdrawing nature of the fluorine atom lowers the HOMO energy level of the fluorinated polymer compared with that of the non-fluorinated analog. As expected, this indicated that the introduction of two fluorine atoms onto the thiadiazoloqunoxaline unit can effectively lower the HOMO energy level of the copolymer by ∼0.08 eV, which leads to an increase in V_oc of BHJ devices fabricated by using these fluorinated materials and to ensure better environmental stability of the material. On the other hand, LUMO energy levels of P1 and P2 were calculated to be −3.81 eV and −3.80 eV, respectively. Thus, one can find that the LUMO energy levels were almost not affected by the introduction of fluorine atoms. Both LUMO energy levels are 0.3 eV higher than that of the acceptor PC₇₁BM (−4.2 eV), ensuring energetically favorable electron transfer from the polymer donor to the PC₇₁BM acceptor in PSCs. Electrochemical band gaps are then calculated to be 1.27 and 1.36 eV for P1 and P2 respectively. On account of the larger influence on the HOMO energy level in comparison with the LUMO, the electrochemical bandgap (E^elec_g) slightly increases upon fluorination. The difference between the optical and electrochemical band gap could be explained by the exciton binding energy of the copolymers and/or the interfacial barriers for charge injection.^21,22

Theoretical calculations

We have additionally performed a theoretical study on the P1 and P2 molecular structures within the framework of density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The initial geometry optimization calculations were performed employing the gradient corrected functional PBE²³ of Perdew, Burke and Ernzerhof. The def-SVP basis set²⁴ was used for all of the calculations. At this stage of the calculations, to increase the computational efficiency (without loss in accuracy), the resolution of the identity method²⁵ was used for the treatment of the two-electron integrals. Subsequent geometry optimization were further performed using the hybrid exchange–correlation functional B3LYP²⁶ as well as Truhlar's meta-hybrid exchange–correlation functional M06,²⁷ and the same basis set. Tight convergence criteria were placed for the SCF energy (up to 10⁻⁷ E_h) and the one-electron density (rms of the density matrix up to 10⁻⁸) as well as for the norm of the Cartesian gradient (residual forces both average and maximum smaller than 1.5 × 10⁻⁵ a.u.) and residual displacements (both average and maximum smaller than 6 × 10⁻⁵ a.u.). Solvent effects were included for chloroform (CF) using the integral equation formalism variant of the Polarizable Continuum Model (IEFPCM), as implemented in the Gaussian package.²⁸

TD-DFT excited state calculations were performed to calculate the optical gaps of the P1 and P2 using the same functionals and basis set on the corresponding ground state structures. The UV/vis spectra were calculated using the B3LYP and M06 functionals. The first rounds of geometry optimization were performed using the Turbomole package.²⁹ All of the follow up calculations were performed using the Gaussian package.²⁸

The first rounds of calculations were the geometry optimizations of the P1 and P2 structures. To increase the computational efficiency the alkyl groups were truncated to ethyl groups. Vibrational analysis on all of the optimized structures did not reveal any vibrational modes with imaginary Eigen frequencies, i.e. the final optimized structures are true local (if not global) minima. The main part of the structures that consists of the benzothiadiazole (BT) and thiadiazoloquinoxaline (TDAQ) moieties, as well as the linking thiophenes, are planar. We have calculated the HOMO and LUMO energy levels and the optical gaps, defined here as the energetically lowest allowed vertical electronic excitation, employing the PBE, M06, and B3LYP functionals. In Table 3, in addition to the frontier orbitals' energy levels, we also provide the optical gap, the main contributions to the first excitation, as well as the wavelength of the first excitation and of the excitations with the largest oscillator strengths.

Table 3 Calculated properties of P1 and P2. Specifically HOMO and LUMO energies (eV), HOMO–LUMO gap (eV), HL, optical gap (eV), OG, with corresponding oscillator strengths, f, the wavelengths of the first excitation and excitations with the largest oscillator strengths, the main contributions to the first excited state, and the dipole moment (D), μ

	HOMO (eV)	LUMO (eV)	HL (eV)	OG (eV)	λ1st/max (nm)	f	Main contributions	μ (D)
a Values when solvent effects are taken into account for chloroform.
P1
PBE	−4.61	−3.67	0.93	1.33	931	0.45	H → L (89%), H → L+1 (6%)	4.76
	−4.74^a	−3.79^a	0.95^a	1.27^a	975^a	0.62^a	H → L (94%)^a	6.24^a
B3LYP	−5.16	−3.26	1.89	1.68	736/581/530/401/349	0.62	H → L (99%)	4.53
	−5.29^a	−3.37^a	1.91^a	1.63^a	762/580/550/390/351^a	0.75^a	H → L (99%)^a	5.69^a
M06	−5.44	−3.19	2.25	1.77	700/546/496/336/324	0.63	H → L (98%)	4.67
	−5.59^a	−3.31^a	2.28^a	1.72^a	721/546/511/378/337^a	0.76^a	H → L (98%)^a	5.88^a
P2
PBE	−4.67	−3.74	0.92	1.32	936	0.44	H → L (89%), H → L+1 (6%), H−1 → L (5%)	3.48
	−4.76^a	−3.82^a	0.94^a	1.27^a	978^a	0.61^a	H → L (94%)^a	4.79^a
B3LYP	−5.22	−3.34	1.88	1.67	741/579/517/411/352	0.61	H → L (99%)	2.97
	−5.31^a	−3.41^a	1.91^a	1.62^a	764/578/533/351/341^a	0.75^a	H → L (99%)^a	3.72^a
M06	−5.50	−3.27	2.24	1.76	705/544/487/394/337	0.62	H → L (98%)	3.09
	−5.63^a	−3.34^a	2.29^a	1.73^a	717/541/494/367/337^a	0.78^a	H → L (98%)^a	3.23^a

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In addition to the B3LYP functional we have also performed our calculations employing the M06 functional. The M06 meta-hybrid functional was chosen since it provides leveled performance over transition types.^30,31 We provide results using all three functionals, which can additionally be used for comparison with the literature. The HOMO–LUMO (HL) gap of each structure calculated using the hybrid B3LYP functional is notably smaller, by ∼0.35 eV, than that using the meta-hybrid M06 functional however the calculated optical gaps are only marginally smaller, with a difference ∼0.1 eV. In Table 3 we also provide the character of the first allowed excitations only for contributions larger than 4%. The first excitation, as calculated by each of the functional for all three structures, clearly exhibits a single-configuration character.

In Fig. 4, we have plotted the isosurfaces (isovalue = 0.02) of the HOMO and LUMO for all three structures. For both of the structures the HOMO extends over the main body and the LUMO is delocalized mainly over the TDAQ moieties. To quantify the contributions of the moieties to the frontier orbitals we have calculated the total and partial density of states (PDOS). The PDOSs for P1 and P2 are shown in Fig. S15.† We partition all of the structures into the linking thiophenes, the TDAQ and BTDA moieties and aliphatic groups for both structures, and additionally to fluorene (FL) for P1 and di-fluoro fluorene (FFL) for P2. Both structures have high contributions to the HOMO from the TDAQ, and BTDA, specifically at 27.9% and 24.6% for P1, respectively, and at 26.4% and 25.5% for P2, respectively. The linking thiophenes also have high contributions to the HOMO, at 46.5% for P1 and at 45.5% for P2, which results in an extensive delocalization along the main chain of the structure. The situation is different for the LUMO of the structures, which in both cases extends almost exclusively over the TDAQ moiety, at 72.9% for P1, and at 74.2% for P2. The remaining minor contributions are from the BTDA and linking thiophene moieties at 11.2% and 10.6% for P1, and 11.4% and 10.3% for P2. These are in agreement with our earlier observations on the orbital delocalization.

Fig. 4 Frontier orbitals of P1, and P2 (left) LUMO, and (right) HOMO.

In Fig. 5, we show the UV/Visual absorption spectra of the P1 and P2 structures calculated at the TD-DFT/M06 level of theory, both accounting for solvent effects for CF and in gas phase. The spectra have been produced by convoluting Gaussian functions with HWHM = 0.22 eV centered at the excitation wavenumbers. In Fig. S16 (see ESI†) we also provide the corresponding spectra calculated using the B3LYP functional which is in good agreement with the spectra using the M06 functional and only slightly overestimate in the long wavelength region by ∼30 nm and in the short wavelength region ∼10 nm.

Fig. 5 Theoretical UV/vis absorption spectrum of (a) P1, and (b) P2 (calculated using the M06 functional).

The calculated absorption spectra of P1 and P2 have significant similarities; they exhibit three main bands of high absorbance; one centered at large wavelengths ∼720 nm, as well as two centered at smaller wavelengths at 340 nm and ∼520 nm. In the ESI† we provide all of the high intensity peaks with the corresponding oscillator strengths (Table S1 and S2†).

Photovoltaic properties

Solution processed polymer solar cells were fabricated using P1 and P2 as electron donor along with PC₇₁BM with conventional structure ITO/PEDOT : PSS/P1 or P2 : PC₇₁BM/Al. First of all we have optimized the donor to acceptor ratio and the active layer was deposited using chloroform as solvent. For both copolymers the best photovoltaic performance was obtained with the 1 : 2 ratio and a total donor/acceptor concentration of 14 mg mL⁻¹. Different concentration of solvent additives i.e. 1-chloronaphathalene (CN) was added to the host solvent and found that 3% v gave the best photovoltaic performance. The current –voltage characteristics of the optimized BHJ are shown in Fig. 6a and corresponding photovoltaic parameters are complied in Table 4. The optimized BHJ PSC based on P1 cast from chloroform solvent exhibit a PCE of 3.50% with J_sc = 8.32 mA cm⁻², V_oc = 0.79 V and FF = 0.54. The introduction of fluorine atom on the fluorene units (copolymer P2) leads to better photovoltaic performance 4.70% (J_sc = 9.54 mA cm⁻², V_oc = 0.84 V and FF = 0.56). The higher value of V_oc for the P2 based PSC may be attributed to the deeper HOMO energy level of P2 as compared to P1. The improvement in FF and J_sc is mainly due to the better active layer morphology and appropriate orientation mode from the insertion of fluorine atoms into the fluorene units. Moreover, from the similarity in fluorine and hydrogen atomic sizes no additional steric hindrance is expected to be introduced, and its strong electron negativity could increase the intra- and intermolecular interactions through F–H interaction which may be favorable for copolymer self assembly and improve the crystallinity.

Fig. 6 (a) Current density–voltage characteristics under illumination of AM1.5G, 100 mW cm² and (b) IPCE spectra of the PSCs based on copolymer : PC₇₁BM (1 : 2).

Table 4 Photovoltaic parameters of PSCs with optimized P1 : PC₇₁BM and P2 : PC₇₁BM blended films processed with and without solvent additives

Active layers	Jsc (mA cm^−2)	Voc (V)	FF	PCE (%)	Jsc^c (mA cm^−2)
a As cast from CF.b CN/CF.c Estimated from the IPCE spectra.
P1 : PC71BM (1 : 2)^a	8.32	0.84	0.54	3.50	8.24
P2 : PC71BM (1 : 2)^a	9.54	0.79	0.56	4.70	9.43
P1 : PC71BM (1 : 2)^b	10.86	0.80	0.66	5.30	10.69
P2 : PC71BM (1 : 2)^b	12.34	0.74	0.68	7.21	12.26

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X-ray diffraction (XRD) measurements were carried out for two copolymer films in order to characterize their structure, such as crystallite size, crystallite orientation and intermolecular distance. The XRD profiles of P1 and P2 films processed with CF solvent were shown in Fig. 7a and shows the semi-crystalline nature of the copolymers. Both the polymers show pronounced (100) reflection peaks, indicating the ordered lamellar stacking of the copolymer chains in solid films. The (100) reflection peaks at 2θ = 4.78° and 4.96° for P1 and P2, respectively, corresponds to lamellar spacing of 19.38 Å and 18.46 Å, respectively. The lamellar spacing of P1 is slightly larger than P2 attributed to the fact that F atom in the fluorene units extends the length of side chains in the P2 film so as to stretch the lamellar distance of polymer chains in P2 film. Both the (010) reflection peaks of P1 and P2 films were located at 2θ = 23.38° which corresponds to the π–π stacking distance of 4.46 Å in both P1 and P2 films. The π–π stacking distance of two copolymers are almost the same, suggesting that the introduction of F atom in fluorene unit on copolymer side chains has little influence on π–π stacking in copolymer film. However, the intensity of the both (010) and (100) reflection peaks of P2 film is stronger than that of P1 film, indicating a more ordered π–π stacking and crystallinity of P2 as compared to P1. This indicates that the introduction of fluorine atoms into copolymer backbones has a significant effect on the crystalline property of copolymers as well as the resulting device performance.

Fig. 7 X-ray diffraction pattern of (a) pristine P1 and P2 and (b) P2 : PC₇₁BM films with and without CN additives.

The charge transport property in the PSCs is generally one of the important factors for determining the J_sc and FF. The hole mobility was measured in the hole only devices using space charge limited current (SCLC) method (Fig. 8). The hole mobility of P1 and P2 in the BHJ active layers are 2.78 × 10⁻⁵ cm² V⁻¹ s⁻¹ and 5.76 × 10⁻⁵ cm² V⁻¹ s⁻¹, respectively. However, the electron mobility in the BHJ active layer based on P1 and P2 are almost same (2.46 × 10⁻⁴ cm² V⁻¹ s⁻¹). The higher PCE for PSC based on P2 than P1, may be attributed to the larger hole mobility in the P2 : PC₇₁BM as compared to that for P1 : PC₇₁BM, leading the better balanced charge transport. The higher value of hole mobility is consistent with the more crystalline nature of P2 as inferred from the XRD data.

Fig. 8 Current–voltage characteristics of hole only devices in dark using different active layers.

The devices based on these two devices exhibited low PCE values, which are due to the low J_sc and FF values. This might be attributed to poor nanophase morphology and poor exciton dissociation and charge transport. We have used the solvent additive method to improve the PCE of the PSCs. Therefore, PSCs based on P1 and P2 were fabricated with the same blend ratio and different amounts of CN additive were tried and we found that a 3% by volume gave the best photovoltaic response. The J–V characteristics of the devices under illumination are shown in Fig. 6a. The PCE of the devices significantly improved with the introduction of the solvent additive, leading to enhancement of the FF and J_sc at a slight loss in the V_oc (for P1 J_sc = 10.86 mA cm⁻², V_oc = 0.74 V, FF = 0.66, PCE = 5.30%) and (for P2 J_sc 12.34 mA cm⁻², V_oc = 0.80 V, FF = 0.68, PCE = 7.21%). These values are higher than that reported in literature using similar copolymers.^15–17 The increase in J_sc and FF is associated with loss in V_oc, mainly because CN possesses a high boiling point and is able to solvate the PC₇₁BM leading to ample impact on J_sc by providing a more optimal morphology for facilitating charge transportation.^32,33 However the loss of V_oc can be attributed to the lowering of charge-separated and charge-transfer-state energies upon additive addition.³⁴ Simultaneously, we speculate that the addition of CN decreases the size of fullerene domains and facilitates the formation of a bicontinuous inter-penetrating donor–acceptor network (as discussed in the later part of the discussion).

In order to get the accuracy of the photovoltaic performance, incident photon to current conversion efficiency (IPCE) spectra of the PCEs with P1 and P2 were measured and shown in Fig. 6b. The IPCE spectra cover a broad band spectral response range from 300 nm to 300 nm in agreement with the corresponding absorption spectra of active layer (Fig. S16, ESI only for P2 : PC₇₁BM†). The PSC based on P2 showed higher values of IPCEs than the P1, which is consistent with the higher value of J_sc for P2 based device. The J_sc values estimated from the integration of the IPCE spectra (as shown in Table 4) agree well with those obtained from J–V characteristics.

To get information about the effect of the CN additive on the hole and electron transport properties of the active layers, we also investigated the hole and electron mobilities of P1 : PC₇₁BM and P2 : PC₇₁BM blend films by SCLC method (Fig. 8 for hole only device). After the CN additive, both P1 : PC₇₁BM and P2 : PC₇₁BM blended films exhibited an increased hole mobility of 8.83 × 10⁻⁵ cm² V⁻¹ s⁻¹ and 1.45 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively. However, the electron mobility of the active layers does change much after using the CN additive. The increase in hole mobility and similar value of electron mobility leads to a more balanced charge transport within the active layer and is one of the main reasons for enhanced J_sc, FF and PCE of the corresponding PSCs.

We have used impedance spectroscopy (IS) to investigate the electrical characteristics of donor/acceptor interfaces, active layer/electrode interfaces and series resistance using an equivalent circuit models in PSCs.^35,36 Fig. 9a shows the Nyquist plots of IS for the device for P2 : PC₇₁BM active layer processed with and without CN additives and fitted with the equivalent circuit model (Fig. 9b). The corresponding device parameters are derived by fitting the experimental Nyquist plots with equivalent circuit. The shunt pair R₁ and C₁ corresponds to the D/A interfaces within the BHJ active layer film whereas, the other shunt pair of R₂ and C₂ corresponds the interfaces of BHJ active layer with anode (PEDOT : PSS) and cathode (Al). R₃ corresponds to the resistance of electrodes and wires. The PSC based on the approach without CN additives exhibits higher R₁ (1038 Ω) and R₂ (206 Ω) values and lower C₁ (6.81 × 10⁻⁸ F) and C₂ (3.46 × 10⁻⁷ F) values than that when using CN additives (R₁ = 834 Ω, R₂ = 158 Ω, C₁ = 6.81 × 10⁻⁸ F and C₂ = 6.38 × 10⁻⁸ F). These differences can be attributed to the variation of interface area between the P2 and PC₇₁BM in the active layer processed with and without CN. The better nanoscale morphology of the active layer processed with CN additive corresponds to the smaller R₁ and higher C₁ compared to that for without CN additive counterpart. Moreover, the value of R₃ is lower for the active layer processed with CN as compared to the approach without additive. These findings from the IS measurements provide strong evidence that favorable nanoscale morphology in the active layer processed with CN additive give rise to the observed improvement in the J_sc and FF in PSCs. The BHJ active layer can be considered as a shunt pair of a resistance and capacitance in equivalent circuit, the average carrier transition time at shunt conditions corresponds to the carrier lifetime (τ in the active layer). The longer τ means a lower recombination rate and a better chance that carriers could reach the electrodes. The τ values are 2.8 × 10⁻⁴ s and 7.06 × 10⁻⁵ s (τ = R₁C₁) for the devices processed with and without CN additives, respectively. The longer value of τ for the device processed with CN additive than without additive suggests that the charge carriers in the former blend have a greater opportunity to reach the respective electrodes resulting in higher J_sc and FF.

Fig. 9 (a) Nyquist plots of P2 : PC₇₁BM PSCs and (b) the fitting equivalent circuit model.

The nanoscale morphology of the active layer plays an important role in the photovoltaic performance of the device. Proper morphology is not only beneficial for exciton dissociation into free charge carriers but also necessary for charge transport to respective electrodes for efficient collection. The atomic force microscopy (AFM) images in tapping mode for the blended films cast from with and without additive are shown in Fig. 10. It can be seen from these images that the films cast from CF only showed inferior nanoscale morphology of high surface roughness (rms values are 4.02 nm and 3.74 nm for P1 and P2, respectively), which may destroy the continuous percolative pathways for hole and electron transport to the corresponding electrodes, consequently increasing the chance of recombination of charge carriers and reducing the J_sc accounting for the low PCE. Moreover, the high surface roughness may also reduce the D/A interfacial area in the active blend layer and hamper the exciton dissociation. However, the surface roughness of the blended films was significantly reduced (2.12 nm and 1.54 nm for P1 and P2, respectively), when processed with solvent additive. The reduction in the surface roughness might lead to an increase the exciton dissociation and charge transport that resulted in the improvement of both J_sc and FF.

Fig. 10 AFM images (3 × 3 μm) without (top) and with CN additives (bottom) (a and c) for P1 : PC₇₁BM and (b and d) for P2 : PC₇₁BM films.

The morphology of the active layer was further investigated using transmission electron microscopy (TEM) image from which are shown in Fig. 11. Copolymer : PC₇₁BM films without CN additive have coarse dispersive domains that can be a factor in hampering exciton diffusion and charge transport. However, the morphology of the film after adding CN was significantly changed. The large crystallites and phase separation domains disappeared and a fine nanoscale morphology within the exciton diffusion length was observed, resulting in a favorable exciton dissociation and charge transport. The film P2 : PC₇₁BM showed a much finer and denser texture and nanoscale separation between copolymer and PC₇₁BM, which could result in a high PCE with larger J_sc value.³⁷

Fig. 11 TEM images (3 × 3 μm) without (top) and with CN additives (bottom) (a and c) for P1 : PC₇₁BM and (b and d) for P2 : PC₇₁BM films. Scale bar is 200 nm.

The crystallinity of the active layer is another factor that affects the charge transport and collection, thereby disturbing the overall performance of the BHJ organic solar cell. To obtain information about the change in crystallinity of the copolymer : PC₇₁BM, we recorded the X-ray diffraction patterns of the blend films processed with CF and CN/CF solvents (Fig. 7b, only shown for P2 : PC₇₁BM). The film cast from CF showed a broad diffraction peak at 2θ = 4.96°, corresponding to the crystalline domain of P2. For the film cast from CN/CF this diffraction peak becomes narrower, that is related to the increase in the crystalline nature of P2 in the active layer that facilitated the hole transport in the active layer more efficiently.

In order to assess the effect of the solvent additive on the exciton generation, dissociation and charge transport properties, we have measured the photocurrent density (J_ph) versus the effective voltage (V_eff) of the devices and the results are shown in Fig. 12. J_ph is defined as J_ph = J_L − J_D, where J_D and J_L are the current density in dark and under illumination, respectively. V_eff is defined and V_o − V_appl, where V_o is the voltage at which the photocurrent is zero and V_appl is the external applied voltage.³⁸ The V_eff determines the electric field in the bulk region and thereby affects the carrier transport and photocurrent extraction. At low V_eff, the J_ph varies linearly with V_eff and then tends to saturate as the V_eff further increases. At high V_eff values mobile charge carriers quickly move toward the corresponding electrode with minimum recombination. At full saturation, we assume that all the excitons generated after the absorption of light by the active layer are dissociated into free charge carriers and consequently collected by the respective electrodes. The J_ph saturate at lower V_eff for the device processed with solvent additive as compared to the device processed without solvent additive, indicating that the low V_eff is needed for the exciton dissociation for former device. Therefore at high V_eff, the saturation photocurrent density (J_phsat) is limited by the total number of absorbed photons and independent of bias voltage. We have estimated the maximum generation rate of free charge carriers (G_max) according to J_phsat = qG_maxL, where q is the electronic charge and L is the thickness of the active layer.³⁹ The values of G_max for the devices based on P1 : PC₇₁BM (as cast), P1 : PC₇₁BM (SA) and P2 : PC₇₁BM (as cast) and P2 : PC₇₁BM (SA) are 8.4 × 10²¹ cm⁻³, 9.11 × 10²¹ cm⁻³, 9.68 × 10²¹ cm⁻³ and 10.25 × 10²¹ cm⁻³, respectively. G_max is related to the number of photon absorbed by the active layer and such changes in the G_max are consistent with the absorption spectra of corresponding active layers (see the Fig. S16†). The charge collection probability P_c has been estimated according to P_c = J_sc/J_phsat (ref. 40) and are 0.67, 0.78, 0.73 and 0.84 for P1 : PC₇₁BM (as cast), P1 : PC₇₁BM (SA), P2 : PC₇₁BM (as cast) and P2 : PC₇₁BM (SA), respectively. The higher value of P_c for the devices with solvent additives than the corresponding devices without solvent additives is attributed to the better phase separation, increased hole mobility and improved balance in charge transport, and enhancement in light harvesting efficiency.

Fig. 12 Variation of photocurrent density (J_ph) with effective voltage (V_eff) for the PSCs based in different active layers.

Conclusions

We have synthesized two D-A1–D-A2 copolymers P1 and P2 by Stille coupling reaction. These copolymers exhibited strong absorptions in the range 300–1100 nm and optical bandgaps of 1.09 and 1.11 eV, for P1 and P2, respectively. The structure–property correlation study reveals that substitution of fluorine atom on the accepting moiety can fine-tune the optoelectronic properties of the copolymers. The PSCs based on P1 and P2 as donors along with PC₇₁BM showed the highest PCE of 5.30% and 7.21%, respectively, after the optimization of donor to acceptor weight ratio and concentration of solvent additives. The higher value of PCE for P2 based device may be attributed to the higher semi-crystalline nature of P2 : PC₇₁BM blended film and more balanced charge transport as compared to that for P1 : PC₇₁BM.

Acknowledgements

M. L. K., S. A. K., N. A. R., A. Y. N. and. I. E. O. are thankful to the Russian Science Foundation (grant number 14-13-01444) for financial assistance.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14537j

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