A facile method to synthesize [A′(D′AD)₂]-based push–pull small molecules for organic photovoltaics

Abstract

An efficient route to synthesize, for the first time, a series of small molecules based on the [A′(D′AD)₂] architecture was developed using selective direct heteroarylation of the C–H bond with a Pd(AcO)₂/Bu₄NBr simple catalytic system. The C–H arylation of the unsymmetrical compound 4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-7-(thiophen-2-yl)benzo[c][1,2,5] thiadiazole (5) showed that the (C5) in the ethyldioxythiophene moiety is more reactive towards C–H arylation than its counterpart in the thiophene unit. The new small molecules bear two different acceptors and/or donors, which led to decrease the electron band gap and improve the strength of the push–pull system as well as the amount of intramolecular charge transfer from donor to acceptor units. Lactams and imide containing acceptors are used as second electron withdrawing units due to their well conjugated structures, strong π–π interactions and high electron affinity as well as their potential as electron withdrawing units in photovoltaic conjugated materials. All small molecules showed broad absorption spectra with optical band gaps, which were estimated to be in the range of 1.72–1.29 eV. From cyclic voltammetry, the highest occupied molecular orbital (HOMO) energy level could be tuned by changing the second acceptor, suggesting high open circuit voltage (V_oc). The EHID(EDBTT)₂:PC₇₁BM-based solar cells reached a maximum PCE of 3.24%.

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Introduction

The development of solution processing of organic solar cells has received considerable attention recently. Small organic molecules have unique advantages such as low cost, flexibility, light weight, and potential for large area device fabrication.^1–4 Moreover, small molecules offer potential advantages over conjugated polymers such as their defined molecular structure, high purity, ease of purification, ease of large scale production and good batch-to-batch reproducibility. These advantages encouraged many scientists to design a perfect small molecule chemical structure to improve the optical and electrical properties as well as the power conversion efficiency^5–8 that recently reached up to 10%. The first bulk heterojunction solar cell was realized in 1995.⁹ This design improved the charge extraction and enhanced both light absorption and photocurrent density. Ten years later, the first solution-processed small molecules were introduced as donor materials in the molecular bulk heterojunction (BHJ). These materials based on oligothiophenes, which were investigated as novel electron donors and blended with PCBM, showed a power conversion efficiency in the range of 0.2–0.8%.^10,11

Recently, a series of conjugated donor–acceptor (push–pull) chemical structures were developed. This helps to broaden the absorption window, increase the efficiency of intramolecular charge transfer (ICT)^12a,b along the backbone structure, and enhance the net short circuit current (J_sc)^12c–15 in device applications. Among these structures, a series of conjugated (D–A–D) and (A–D–A) small molecules based on 2,1,3-benzothiadiazole,^16–19 thieno[3,4-c]pyrrole-4,6-dione,²⁰ diketopyrrolo-pyrrole,^21–24 and isoindigo^25–27 as electron accepting units were integrated into two low-energy gap oligothiophenes. These materials showed broad absorption bands. Insertion of a second donor into the (D–A–D) small molecule structure offered a variety of architectures such as [A(D–D′)₂]²⁸ and [D′(D–A–D)₂]^29–31 building blocks.

In parallel, most organic small molecules are synthesized via Stille and/or Suzuki cross-coupling reactions.^32,33 Despite the high selectivity of these synthesis methodologies, they have some disadvantages such as contamination by organometallic intermediates. Recently, we succeeded in the synthesis of isoindigo-based small molecules³⁴ via Stille and Suzuki cross-coupling methodologies, which needed organometallic monomer derivatives that contaminated several byproducts in the net product. To avoid the difficulty faced during the synthesis of some stannyl and/or boronic esters, catalyzed C–H direct heteroarylation was preferred.

In this study, a Pd(OAc)₂-catalyzed C–H direct arylation methodology^35–37 was used to synthesize a new series of [A′(D′AD)₂] constituted small molecules. Compound (5), 4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (EDBTT), was the key structure of this class of materials (Scheme 1), which contains ethylene dioxythiophene (EDOT) and thiophene as electron rich cores, offering strong ICT with acceptors and extend the low energy optical transitions.³⁶ In addition, 2,1,3-benzothiadiazole (BT) afforded high electron affinity, which increased across the backbone structure through the introduction of another electron accepting core such as 2,5-bis(2-ethylhexyl)-3,6-di(thien-2-yl)pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione (DTDPP) (7), N,N′-bis(2-ethylhexyl)isoindigo (EHID) (9) and/or 5-hexylthieno[3,4-c]pyrrole-4,6-dione (HTPD) (11) between the two arms of EDBTT to obtain our targets A′(EDBTT)₂ (Scheme 1). This shows that the CH-arylation methodology is successful in the synthesis of such complicated small molecules in a minimal number of steps.

Scheme 1 Synthetic route for DTDPP(EDBTT)₂, EHID(EDBTT)₂ and HTPD(EDBTT)₂ via C–H arylation reaction.

All the previous acceptors have high electron affinities that help to lower the HOMO levels and maintain high oxidation potentials. In addition, the aliphatic chains attached to these acceptor moieties induced solubility of the obtained small molecules in most organic solvents,² which assists in solution processable solar cells.

Experimental

Materials

3,4-Ethylenedioxythiophene (Sigma-Aldrich, 97%), thiophene-3,4-dicarboxylic acid (Frontier Scientific), 2,1,3-benzothiadiazole (Sigma-Aldrich, 98%), and 2-(tributylstannyl)-thiophene (Sigma-Aldrich, 97%) were used as received. The materials 4,7-dibromo-2,1,3-benzothiadiazole (1),^38,39 2,5-bis(2-ethylhexyl)-3,6-di(5-bromothien-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-(2H,5H)-dione (7),⁴⁰ 6,6′-dibromo-N,N′-bis(2-ethylhexyl)-isoindigo (9),⁴¹ 2-tributylstannyl-3,4-ethylene-dioxythiophene (4),⁴² and 1,3-dibromo-5-hexylthieno[3,4-c]pyrrole-4,6-dione (HTPD) (11)⁴³ were synthesized by literature procedures.

Characterization

¹H and ¹³C NMR spectra were obtained with a Varian spectrometer (400 and/or 300 MHz). The UV/Vis absorption spectra were obtained with a Varian Cary UV/Vis/NIR-5000 spectrophotometer on the pure samples. Differential scanning calorimetry (DSC) was performed with a TA instrument (DSC-TA Q-20) under a nitrogen atmosphere at a heating rate of 10 °C min⁻¹. Cyclic voltammetry (CV) measurements were performed with a B-class solar simulator: potentiostat/galvanostat (SP-150 OMA company); the supporting electrolyte was tetrabutylammonium-hexafluorophosphate (Bu₄NPF₆) in acetonitrile (0.1 M) and the scan rate was 50 mV s⁻¹. A three-electrode cell was used; a Pt wire and silver/silver chloride [Ag in KCl (0.1 M)] were used as the counter and reference electrodes, respectively. The HOMO and LUMO energy levels (E_HOMO and E_LUMO) of the materials were determined from the following relationships: E_HOMO = −E_ox − 4.71 and E_LUMO = −E_red − 4.71 (in eV), where E_ox and E_red are the onset oxidation and reduction potentials, respectively, of the material vs. the Ag/AgCl reference electrode. The small molecule thin films for electrochemical measurements were drop-coated from a material/chloroform solution on ITO glass slides. 10 mg mL⁻¹ XRD experiments were performed with a Bruker D8 advanced model diffractometer and with Cu-Kα radiation (λ = 1.54 Å) at a generator voltage of 40 kV and a current of 40 mA.

Fabrication and characterization of photovoltaic devices

The bulk heterojunction organic photovoltaic devices were fabricated with a structure of ITO/PEDOT:PSS/organic material:PC₇₁BM/Ca/Al. The ITO substrates were cleaned by ultrasonication using a detergent, deionized water, acetone and isopropyl alcohol. Surface treatment was performed by exposing ITO to UV/ozone treatment. PEDOT:PSS (Clevios P VP AI4083) solution was spin coated at 5000 rpm for 20 s on an ITO substrate, serving as an anode. The coated substrate was dried at 150 °C for 10 min in air and was transferred into a glove box filled with N₂. The small molecule : PC₇₁BM (1 : 1, w/w), dissolved in chlorobenzene with a concentration of 20 mg mL⁻¹, was spin cast on the PEDOT:PSS layer and dried at 100 °C for 5 min (except for the first small molecule). The cathode, Ca (10 nm) and Al (100 nm), was deposited in a vacuum chamber (10⁻⁶ Torr). The active area was 4.64 mm². The current–voltage (J–V) characteristics were measured under air mass (AM) 1.5 G illumination with light-source operation at 100 mW cm⁻². The incident photon-to-current efficiency (IPCE) measurements were performed using a QE-IPCE 3000 spectral response/QE/IPCE measurement system (Titan Electro-Optics Co. Ltd).

Synthesis of monomers and small molecules

Synthesis of 4-bromo-7-(thiophen-2-yl)-2,1,3-benzothiadiazole 3. 4,7-Dibromo-2,1,3-benzothiadiazole (1.00 g, 3.40 mmol) and tributyl(thiophen-2-yl)-stannane (1.39 g, 3.70 mmol) were dissolved in anhydrous toluene (50 mL) and dichlorobis(triphenylphosphine)-palladium(II) (0.120 g, 0.170 mmol) was added at room temperature and degassed with argon for 20 min. The mixture was refluxed at 110 °C for 24 h under a nitrogen atmosphere. After being allowed to cool to room temperature, the reaction mixture was extracted with EtOAc and the organic layers were separated, washed thoroughly with water, and finally dried with anhydrous Na₂SO₄. The solvent was evaporated under vacuum and the crude product was purified by neutralized silica gel flash column chromatography (hexane) to generate a yellow solid (0.670 g, 66% yield) of 3. ¹H NMR (400 MHz, CDCl₃), δ (ppm): 7.99 (d, J = 4.03 Hz, 1H, CH, thiophene), 7.75 (d, 1H, CH, benzothiadiazole), 7.60 (d, 1H, CH, benzothiadiazole), 7.38 (d, J = 4.03 Hz, 1H, CH, thiophene), 7.12 (dd, 1H, CH, thiophene); ¹³C NMR (100 MHz, CDCl₃), δ (ppm): 153.68, 151.90, 138.41, 128.08, 127.99, 127.23, 127.00, 125.76, 112.28; MALDI TOF [M + 1]⁺ found: 297.17 (calcd: 297.19); anal. calcd for C₁₀H₅BrN₂S₂: C, 40.41; H, 1.70; Br, 26.89; N, 9.43; S, 21.58; found: C, 40.39; H, 1.70; Br, 26.77; N, 9.49; S, 21.51.

Synthesis of 4-(3,4-ethylenedioxythiophene-7′-yl)-7-(thiophen-2-yl)-2,1,3-benzothiadiazole (5) (EDBTT). A mixture of 3 (3.00 g, 10.0 mmol), 2-tributylstannyl-3,4-ethylenedioxythiophene (4) (4.50 g, 11.0 mmol) and Pd(PPh₃)₂Cl₂ (0.350 g, 0.50 mmol) was dissolved in THF (50 mL) and degassed with nitrogen for 20 min. The reaction mixture was stirred and heated under reflux at 73 °C for 16 h. After having been allowed to cool to room temperature, the reaction mixture was extracted with ethyl acetate and the organic layers were separated, washed thoroughly with water, and finally dried with anhydrous Na₂SO₄. The solvent was evaporated under vacuum and the crude product was purified by silica gel flash column chromatography (n-hexane/EtOAc, 10 : 1) to afford 5 (1.25 g, 34% yield) as an orange-red solid. ¹H NMR (400 MHz, CDCl₃), δ (ppm): 8.33 (d, J = 4.00 Hz, 1H, CH, thiophene), 8.07 (s, 1H, CH, benzothiadiazole), 7.86 (s, 1H, CH, benzothiadiazole), 7.41 (d, J = 4.00 Hz, 1H, CH, thiophene), 7.19 (dd, 1H, CH, thiophene), 6.57 (s, 1H, CH, EDOT–thiophene), 4.38 (t, J = 12.00 Hz, 2H, CH₂, EDOT), 4.30 (t, J = 12.00 Hz, 2H, CH₂, EDOT); ¹³C NMR (100 MHz, CDCl₃), δ (ppm): 152.46, 152.28, 141.60, 140.46, 139.56, 127.90, 127.12, 126.11, 113.44, 102.39, 64.99, 64.28; MALDI TOF [M + 1]⁺ found: 358.44 (calcd: 358.46); anal. calcd for C₁₆H₁₀N₂O₂S₃: C, 53.61; H, 2.81; N, 7.81; O, 8.93; S, 26.84; found: C, 53.60; H, 2.80; N, 7.79; O, 8.90; S, 26.87.

General procedure for thermal direct Pd-catalyzed CH-arylation cross-coupling

Under dry conditions, a mixture of 5 (0.360 g, 1.00 mmol), desired dibromo-derivative (7, 9 and/or 11) (0.480 mmol), potassium acetate (0.590 g, 6.00 mmol), tetrabutylammonium bromide (TBAB) (0.640 g, 2.00 mmol) and palladium acetate (0.040 g, 0.200 mmol) in dry DMF (15 mL) was degassed with nitrogen for 20 min then heated under reflux for 24 h at 80 °C. The reaction mixture was cooled to room temperature followed by addition of a mixture of CH₂Cl₂ (50 mL) and water (25 mL). The organic layer was separated and dried with anhydrous Na₂SO₄, then filtered and concentrated under reduced pressure. The crude product was purified by flash chromatography on silica gel, eluted by n-hexane/EtOAc (10 : 3) to afford the corresponding product derivative.

Synthesis of DTDPP(EDBTT)₂. Compound 5 (0.186 g, 0.52 mmol) reacted with 2,5-bis(2-octyldodecyl)-3,6-di(5-bromothien-2-yl)pyrrolo-[3,4-c]pyrrole-1,4-(2H,5H)-dione (7) (0.143 g, 0.21 mmol) to afford 8 (0.148 g, 60% yield) as a deep violet solid. ¹H NMR (400 MHz, CDCl₃), δ (ppm): 9.17 (d, J = 3.30 Hz, 2H, 2 × CH, DPP–thiophene), 8.40 (d, J = 5.1 Hz, 2H, 2 × CH, benzothiadiazole), 8.06 (s, 2H, 2 × CH, benzothiadiazole), 7.83 (d, 2H, 2 × CH, thiophene), 7.36 (d, 2H, 2 × CH, DPP–thiophene), 7.12 (d, J = 4.66 Hz, 2H, 2 × CH, thiophene), 6.89 (d, J = 4.30 Hz, 2H, 2 × CH, thiophene), 4.91–4.82 (m, 8H, 4 × CH₂, EDOT), 3.05 (m, 4H, 2 × CH₂–, hexyl-C1), 1.68 (m, 2H, 2 × CH–, hexyl-C2), 1.19–1.33 (m, 16H, 2 × (CH₂)₃, hexyl-C3-C5 & 2 × CH₂, ethyl–), 0.740–0.790 (m, 12H, 4 × CH₃–, ethylhexyl–); MALDI TOF [M + 1]⁺ found: 1237.65 (calcd: 1237.66); anal. calcd for C₆₂H₅₆N₆O₆S₈: C, 60.17; H, 4.56; N, 6.79; O, 7.76; S, 20.73 found: C, 60.11; H, 4.50; N, 6.78; O, 7.69; S, 20.67.

Synthesis of EHID(EDBTT)₂. Compound 5 (0.186 g, 0.520 mmol) reacted with 6,6′-dibromo-N,N′-bis(2-ethylhexyl)-isoindigo 9 (0.152 g, 0.230 mmol) to afford 10 (0.101 g, 57% yield) as a dark blue solid. ¹H NMR (400 MHz, CDCl₃), δ (ppm): 9.10 (d, J = 8.40 Hz, 1H, indole), 9.02 (d, J = 8.40 Hz, 1H, indole), 8.41 (d, J = 4.60 Hz, 2H, 2 × CH, thiophene), 8.10 (s, 2H, 2 × CH, benzothiadiazole), 7.86 (s, 2H, 2 × CH, benzothiadiazole), 7.43 (dd, 2H, 2 × CH, thiophene), 7.31 (d, J = 4.00 Hz, 2H, 2 × CH, thiophene), 7.26 (dd, J₁ = 7.55 Hz, J₂ = 2.30 Hz, 2H, 2 × CH, indole), 6.83 (d, J = 1.87 Hz, 2H, 2 × CH, indole), 4.49–4.45 (m, 8H, 4 × CH₂, EDOT), 3.66 (m, 4H, 2 × CH₂–, hexyl-C1), 1.84 (m, 2H, 2 × CH–, hexyl-C2), 1.35–1.32 (m, 16H, 2 × (CH₂)₃, hexyl-C3-C5 & 2 × CH₂, ethyl–), 0.920–0.890 (m, 12H, 4 × CH₃–, ethylhexyl–); MALDI TOF [M + 1]⁺ found: 1199.56 (calcd: 1199.57); anal. calcd for C₆₄H₅₈N₆O₆S₆: C, 64.08; H, 4.87; N, 7.01; O, 8.00; S, 16.04; found: C, 64.06; H, 4.87; N, 6.99; O, 8.01; S, 15.91.

Synthesis of HTPD(EDBTT)₂. Compound 5 (0.186 g, 0.520 mmol) reacted with 1,3-dibromo-5-hexyl-5H-thieno[3,4-c]pyrrole-4,6-dione 11 (0.093 g, 0.230 mmol) to afford 12 (0.128 g, 37% yield) as a dark brown solid. ¹H NMR (400 MHz, CDCl₃), δ (ppm): 8.45 (d, J = 4.05 Hz, 2H, 2 × CH, thiophene), 8.14 (s, 2H, 2 × CH, benzothiadiazole), 7.91 (s, 2H, 2 × CH, benzothiadiazole), 7.68 (d, J = 4.00 Hz, 2H, 2 × CH, thiophene), 7.45 (dd, 2H, 2 × CH, thiophene), 4.53–4.51 (m, 8H, 4 × CH₂, EDOT), 3.66 (m, 2H, CH₂–, hexyl-C1), 1.55 (m, 2H, CH₂–, hexyl-C2), 1.31 (m, 6H, 3 × CH₂, hexyl-C3-C5), 0.870 (t, 3H, CH₃–, hexyl-C6); MALDI TOF [M + 1]⁺ found: 950.07 (calcd: 950.20); anal. calcd for C₄₄H₃₁N₅O₆S₇: C, 55.62; H, 3.29; N, 7.37; O, 10.10; S, 23.62; found: C, 55.98; H, 3.29; N, 7.37; O, 10.13; S, 23.55.

Results and discussion

Synthesis and characterization

As shown in Scheme 1, 4,7-dibromo-2,1,3-benzothiadiazole (1) was coupled with 2-(tributylstannyl)thiophene (2) under Stille coupling conditions to afford 4-bromo-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (3) in a good yield. Compound 3 was then reacted with 2-tributylstannyl-3,4-ethylenedioxy-thiophene (4) to obtain co-monomer 5. To optimize the synthesis of new small molecules using direct C–H arylation, several reaction conditions were utilized, as shown in Table S1 (ESI†). The optimal reaction condition was achieved by using Pd(OAc)₂ as a catalyst, and terabutylammonium bromide (TBAB) and KOAc as a base. The temperature, solvent, and concentration were held constant for this study, based on general procedures reported in the literature.^35–37 Furthermore, the Pd(OAc₂)(O-Tol)³³ catalyst was not sufficient for this reaction even when combined with TBAB and/or tris(2-methoxyphenyl)phosphine (L2) (Table S1, ESI†). By using a one-step, highly selective, Pd-catalyzed direct arylation coupling reaction, compound 5 coupled with a series of electron withdrawing moieties, 7, 9 and/or 11, afforded DTDPP(EDBTT)₂ (8), EHID(EDBTT)₂ (10) and HTPD(EDBTT)₂ (12), respectively (Scheme 1).

The selectivity resulted from the difference in reactivity between (C5) in the EDOT moiety and its counterpart in the thiophene unit. The (C5) in the EDOT moiety is more active due to the effect of the cyclic ether oxygen atom, which acts as a directing group. This cyclic ether oxygen attracts the Pd catalyst by a coordination bond and makes the catalyst much nearer to position C5 in the EDOT moiety, see the proposed reaction mechanism pathway (Scheme S1, ESI†). This simple catalytic system decreases the by-products obtained and avoids the use of organometallic reagents, as well as helps to obtain a reasonable product yield.³³

All the new material structures were thermally stable and were confirmed with NMR spectroscopy, FT-IR and elemental analysis. The FT-IR spectra provided evidence for the presence of the lactam and imide groups by showing two peaks assigned to the free carbonyl (–C=O) stretching vibration in the range of 1682–1800 cm⁻¹ (Fig. S1, ESI†).

Thermal stability

The DSC curve exhibited clear melting peaks and phase transitions of crystallization of the A′(D′AD)₂-based small molecules, DTDPP(EDBTT)₂, EHID(EDBTT)₂ and HTPD(EDBTT)₂ (Fig. 1b). The melting (T_m), crystallization (T_c) and decomposition (T_d) temperatures are summarized in Table S2 (ESI†). As it is known, the molecular weights, intermolecular interactions and alkyl chain affect the thermal properties. The melting point of DTDPP(EDBTT)₂ increased from 157 to 174 °C in the case of EHID(EDBTT)₂ due to the highly planar structure and the rigidity of isoindigo unit. Furthermore, DTDPP(EDBTT)₂ showed a slight phase transition signal observed at around 101 °C, supposed to be a result of a twisted structure for the present molecule due to the steric hindrance at the linkage between the EDOT and benzothiadiazole units.⁴⁴

Fig. 1 (a) TGA thermograms of DTDPP(EDBTT)₂, EHID(EDBTT)₂ and HTPD(EDBTT)₂ and (b) differential scanning calorimetry (DSC) curves of DTDPP(EDBTT)₂, EHID(EDBTT)₂ and HTPD(EDBTT)₂ under a nitrogen atmosphere at a heating rate of 10 °C min⁻¹.

HTPD(EDBTT)₂ showed a low T_m at 85 °C due to the attachment of the hexyl chain to the small sized TPD unit. The thermal stability of all the small molecules was observed by TGA at a heating rate of 100 °C min⁻¹ under a nitrogen atmosphere (Fig. 1a). If the threshold for thermal decomposition is defined as a mass loss of 5%, all the materials possess good thermal stability with decomposition temperatures above 350 °C.

X-ray diffraction analysis

To study the crystalline structure and the planarity of the obtained materials 8, 10 and 12, X-ray diffraction (XRD) measurements were obtained (Fig. S2, ESI†). New small molecules exhibited two important diffraction peaks, the first one at approximately 2θ = 5.79° (DTDPP(EDBTT)₂), 6.64° (EHID(EDBTT)₂) and 5.45° (HTPD(EDBTT)₂), which correspond to a d-spacing of 15.4, 14.0 and 17.1 Å, respectively. These spacings refer to the interchain spacing between the planes of the main conjugated backbone of these small molecules separated by the corresponding ethylhexyl- and/or hexyl-chains. The high intensities observed in the diffraction peaks of DTDPP(EDBTT)₂ and EHID(EDBTT)₂ proved to have a better ordering in the solid state, which would lead to a good film morphology for BHJ solar cells. The second peak assigned at 2θ = 21.46° (DTDPP(EDBTT)₂) and 21.30° (HTPD(EDBTT)₂) with a d-spacing of 4.1 and 4.3 Å, respectively, demonstrates the π–π stacking between the small molecule backbones and implies long range ordering and a highly organized assembled structure in the solid state. Fig. S3 (ESI†) showed the proposed packed structures in the solid state for DTDPP(EDBTT)₂, EHID(EDBTT)₂ and HTPD(EDBTT)₂.

Optical properties

The optical properties were investigated using UV-visible absorption spectroscopy (Fig. 2), and the data are summarized in Table 1. Considering that the EDBTT unit is fixed, the changes in absorption maximum depend on the active conjugated length induced by the introduction of second acceptors. In general, absorption in the range of 423–468 nm is ascribed to ICT between the EDOT and thiophene donors and the acceptors. In film, in the case of DTDPP, small molecule 8 exhibited a broad absorption with onset absorption (λ_onset) at 962 nm. DTDPP is considered as a (D–A–D) unit due to the presence of two thiophene units around the DPP core, which extends the effective conjugation length and bathochromically shifts the wide absorption band associated with the DPP core and thiophene rings.³⁴

Fig. 2 UV-Vis spectra of small molecules (a) in CHCl₃ solution and (b) in films.

Table 1 Optical and electrochemical properties of the materials in this study

Materials	UV/Vis absorption				Cyclic voltammetry
	Solution^a	Film			EHOMO (eV)	ELUMO (eV)	E^elcg^c (eV)
	λmax (nm)	λmax (nm)	λonset (nm)	E^opg^b (eV)
a Measured in CHCl3.b Estimated using the onset of the UV-Vis spectrum (E^opg = 1240/λonset).c Calculated from E^elcg = ELUMO − EHOMO.d Calculated by the equation ELUMO = EHOMO + Eg,opt.
DTDPP(EDBTT)2	(630)(704)	836	962	1.29	−5.49	−3.80(−4.20)^d	1.69
EHID(EDBTT)2	586	641	822	1.51	−5.52	−3.89(−4.01)^d	1.63
HTPD(EDBTT)2	496	524	721	1.72	−5.55	−3.87(−3.83)^d	1.68

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EHID(EDBTT)₂ showed a λ_max value of 641 nm. The lowest λ_max value was 524 nm in the case of HTPD(EDBTT)₂, which was expected to have the lowest ICT. In contrast, DTDPP(EDBTT)₂ showed two λ_max peaks in solution at 630 and 704 nm, which combined into one peak at 836 nm in the film. All the small molecules have two different high energy absorption bands in the range from 300 to 450 nm, which have been attributed to a more localized π–π* transition due to the presence of two different electron accepting units along the backbone structure.

In particular, when the donor is highly electropositive, the polarizability of the acceptor unit becomes a critical factor to determine the amount of ICT.^12a Therefore, using strong electron donor and strong electron acceptor units along the conjugated materials backbone decreases the electron band gap and improves the strength of the push–pull system as well as the performance of organic solar cells by controlling the photo-induced ICT from donor to acceptor units.^12b Confirming the previous deduction, all the new synthesized A′(D′AD)₂-based small molecules showed more narrow optical band gaps than their counterpart small molecules with the (D–A–D) building block, which have the same number of building units and also based on the same electron withdrawing units (DTDPP,^21–24 ID^25–27,34a and TPD²⁰), see Table 1.

Electrochemical properties

Cyclic voltammetry (CV) measurements were obtained for all materials (Fig. 3a). The HOMO and lowest unoccupied molecular orbital (LUMO) energy levels were derived from the onset of oxidation (E_ox) and reduction potentials (E_red) of the CV curves of the drop-cast films, using a platinum electrode in 0.1 M Bu₄NPF₆ acetonitrile solution at a scan rate of 50 mV s⁻¹ versus a Ag/AgCl reference electrode. According to the equations, HOMO = −(E_ox + 4.71) eV and LUMO = −(E_red + 4.71) eV. The interface barrier present between the small molecule film and the electrode surface made the estimated electronic E_g values quite higher than the optical E_g estimated in thin films. All the materials have low-lying LUMOs because of the increased electron deficiency caused by the addition of one more acceptor. DTDPP(EDBTT)₂, EHID(EDBTT)₂ and HTPD(EDBTT)₂ showed LUMO energy levels at −3.80, −3.89 and −3.87 eV, respectively (Fig. 3b).

Fig. 3 (a) Cyclic voltammograms of all small molecules in 0.1 M solution of TBAPF₆ in acetonitrile at room temperature and (b) HOMO and LUMO energy levels diagram.

DTDPP(EDBTT)₂ not only exhibited the highest lying LUMO (−3.80 eV) among the materials under study, but also showed the highest lying HOMO energy level at −5.49 eV because the presence of four thiophene units which increase the electron donating properties also increases the HOMO energy level. EHID(EDBTT)₂ and HTPD(EDBTT)₂ showed low lying HOMO energy levels at −5.52 and −5.55 eV, respectively (Fig. 3b). The E_HOMO, E_LUMO and electrochemical band gaps (E^elc_g) are summarized in Table 1.

Photovoltaic properties

To estimate the photovoltaic performances of the new synthesized small molecules, BHJ solar cells with a conventional device structure of ITO/PEDOT:PSS/organic material:PC₇₁BM/Ca/Al were fabricated. The optimized results were obtained by varying the small molecule : PC₇₁BM weight ratios, active layer thicknesses, and annealing temperature.³⁴ The optimized active layer thickness was around 80 nm for all the small molecule:PC₇₁BM blend layers.

All the device parameters are summarized in Table 2. The photovoltaic performances were significantly affected by changing the lactam- and/or imide-containing acceptor core in the small molecule structure and the EHID(EDBTT)₂:PC₇₁BM based solar cells reached a maximum PCE of 3.24% with a V_oc of 0.83 V, J_sc of 8.13 mA cm⁻², and fill factor (FF) of 0.48. The V_oc values of each optimized device decreased gradually from 0.88 to 0.75 V as the electron withdrawing ability of the accepting units TPD, DPP and ID decreased. As confirmed by the CV results, HTPD(EDBTT)₂ has the HOMO level of −5.65 eV and exhibited a high V_oc value of 0.88 V. EHID(EDBTT)₂ and DTDPP(EDBTT)₂ showed V_oc values of 0.83 and 0.75 V, respectively.

Table 2 Photovoltaic parameters of the solar cells

Materials : PC71BM (1 : 1)	Voc (V)	Jsc mA cm^−2	FF	PCE (%)
DTDPP(EDBTT)2	0.75	6.87	0.49	2.52
EHID(EDBTT)2	0.83	8.13	0.48	3.24
HTPD(EDBTT)2	0.88	3.51	0.26	0.80

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Unlike the V_oc trend, EHID(EDBTT)₂ exhibited a high J_sc value of 8.13 mA cm⁻², while HTPD(EDBTT)₂ showed low J_sc value of 3.51 mA cm⁻², due to its narrow light absorption window as well as low ICT. The J_sc value of DTDPP(EDBTT)₂ was 6.87 mA cm⁻² (Fig. 4a). To examine the accuracy of the J_sc from the J–V measurements, the corresponding external quantum efficiencies of the fabricated solar cells were measured under the illumination of monochromatic light (Fig. 4b). All the J_sc results were calculated by integrating the EQE with an AM 1.5 G reference spectrum matched with the J_sc obtained from the J–V measurements. As shown in Fig. 4b, EHID(EDBTT)₂ showed much higher EQE values of up to 45% in the broad wavelength range 400−750 nm, compared to those of HTPD(EDBTT)₂ and DTDPP(EDBTT)₂. Moreover, in spite of the broad absorption of DTDPP(EDBTT)₂, it showed low external quantum efficiencies of around 34% (Fig. 4b) reflecting some leakage in the number of charge carriers.

Fig. 4 (a) Current density–voltage (J–V) characteristics of the small molecules:PC₇₁BM solar cells and (b) corresponding external quantum efficiency (EQE) spectra measured under illumination of monochromatic light.

Morphology analysis

In order to imply the effect of the active layer morphology on the device performance, the morphologies of all the small molecule:PC₇₁BM blend films were studied by atomic force microscopy (AFM), as shown in Fig. 5.

Fig. 5 AFM images (5 × 5 μm²) of (small molecule : PC₇₁BM, 1 : 1) active layers.

HTPD(EDBTT)₂:PC₇₁BM film showed low (rms) of around 4.16 nm compared with the other small molecule active layers, but the low melting point of HTPD(EDBTT)₂ small molecule and the incompatibility with PC₇₁BM drop the J_sc to 3.51 mA cm⁻² and the net PCE to 0.80%. In spite of the high root mean-square (rms) roughness of EHID(EDBTT)₂:PC₇₁BM and DTDPP(EDBTT)₂:PC₇₁BM, determined to be 5.38 and 11.53 nm, respectively, they exhibited somewhat different AFM topology compared with the HTPD(EDBTT)₂:PC₇₁BM film. They showed a continuous network, which is good for obtaining high J_sc values of around 8.13 and 6.87 mA cm⁻², respectively. Thus, the EHID(EDBTT)₂:PC₇₁BM-based solar cell exhibited the highest photovoltaic performance among the solution-processed small molecule solar cells in this study.

Conclusions

In summary, a series of new constituted small molecules based on the [A′(D′AD)₂] composition were synthesized via a Pd(OAc)₂/Bu₄NBr catalytic system. All the new materials, DTDPP(EDBTT)₂, EHID(EDBTT)₂ and HTPD(EDBTT)₂, included two different strong acceptors and two different strong donors to induce an efficient push–pull donor–acceptor system. This new system offered high intramolecular charge transfer (ICT) and high short circuit current (J_sc) in organic photovoltaic (OPV) applications. Due to the broad absorption window, the optical band gaps of the materials were in the range of 1.29–1.72 eV; this result closely matches the ideal low band gap of small molecules, as suggested in the design criteria for organic solar cells. EHID(EDBTT)₂:PC₇₁BM-based solar cells reached a maximum power conversion efficiency (PCE) of 3.24% with an open circuit voltage (V_oc) of 0.83 V, a large short circuit current J_sc of 8.13 mA cm⁻², and exhibited a low fill factor (FF) of 0.48. New photovoltaic devices and a number of additives will be further studied to improve the FF as well as the PCE.

Acknowledgements

This study was supported by the Global Frontier R&D Program (2013-073298) in the Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning and the GIST-Caltech Research Collaboration Project through a grant provided by GIST in 2014.

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Footnote

† Electronic supplementary information (ESI) available: NMR, FT-IR, XRD characterization. See DOI: 10.1039/c5ra06660c

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