Novel solution-processible small molecules based on benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene for effective organic photovoltaics with high open-circuit voltage

Abstract

Two novel A–D–A small molecules D1 and D2, containing benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene as the central electron-donating unit, 3-ethylrhodanine as end-capped electron-withdrawing units, and two thiophenes or three thiophenes as conjugated π-bridges, were designed and synthesized. The effects of the conjugated π-bridges on the photophysical, electrochemical and photovoltaic properties as well as the aggregation structure, were fully investigated. Compared with D2, D1 shows stronger packing capability, deeper HOMO energy levels, higher hole mobility, and more appropriate microphase separation with the fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), which lead to better photovoltaic performance with higher short-circuit current density (J_sc) and open-circuit voltages (V_oc). Bulk-heterojunction (BHJ) organic photovoltaic (OPV) devices were fabricated using a blend of the as-synthesized small molecules and PC₆₁BM in different solvents. The D1-CF device prepared from chloroform solution with a weight ratio of 1 : 0.5 exhibited a power conversion efficiency (PCE) of 2.10% and an amazingly high V_oc of 1.10 V under AM 1.5G (100 mW cm⁻²) illumination.

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Introduction

In the last decades, organic photovoltaics (OPVs) have attracted extensive attention due to their easy fabrication, low cost and light weight.^1–3 In the course of high-performance OPVs development, bulk-heterojunction (BHJ) polymer solar cells have achieved an impressive power conversion efficiencies (PCE) of 10.6%.⁴ Besides, solution-processable small-molecule BHJ devices have been also widely studied in recent years, owing to the advantages of definite structure, facile purification, high purity and batch-to-batch reproducibility,^5,6 and the PCEs of small-molecule BHJ devices have reached over 9%.^7,8

So far, it has been considered as the most successful design strategy that the small molecules are constructed from an electron-donating unit (D) as the core and electron-withdrawing units (A) as the arms.^9,10 For example, star-shaped small molecules with triphenylamine as the core have been prudently studied by several groups,^11–17 and the photovoltaic performance could be effectively improved by engineering the structure of the arms. Typically, S(TPA-BT-HTT) with benzo[1,2,5]thiadiazole and thiophene derivatives as arms achieved a PCE of 4.3%.¹⁶ Meanwhile, a series of A–D–A linear small molecules based on dithieno[(3,2-b:2′,3′-d)]silole (DTS) as D unit and thiadiazole[3,4-c]pyridine as A units have been reported by Bazan and co-workers, and the SM BHJ device based on DTS(PTTh₂)₂ achieved a high PCE of 6.7%.¹⁸ Subsequently, p-DTS(FBTTh₂)₂, using 5-fluorobenzo[1,2,5]thiadiazole as A unit to instead of thiadiazolo[3,4-c]pyridine, was reported and achieved a PCE over 8.0%.^19,20 Simultaneously, Chen and co-workers designed and synthesized a series of A–D–A small molecules by introducing oligothiophene derivates as donor units and different electron-withdrawing units as A units, their study indicated that the position and density of different alkyl side or end groups could dominate not only the solubility of materials in organic solvent but also their packing structure and solid-state miscibility with fullerenes.^21,22 When the central oligothiophene unit was replaced by a more electron-rich and planar structure such as benzo[1,2-b:4,5-b′]dithiophene (BDT)²³ and DTS,²⁴ the small molecules end-capped with electron-withdrawing alkyl cyanoacetate groups showed an improved mobility and absorption which led to enhanced PCEs, so the device based on DCAO3TSi showed a PCE of 5.8%, with a V_oc of 0.8 V and fill factor (FF) of 0.64%.²⁴ They also reported a series of A–D–A small molecules comprising BDT unit as D unit and various electron-withdrawing terminal units, the small molecules end-capped with 3-ethylrhodanine showed better photovoltaic performance than that end-capped with alkyl cyanoacetate and malononitrile.^23,25–27 Typically, the OPV device based on DR3TDOBDT achieved a PCE value of 8.26%.²⁷ Clearly, the chemical structure of the electron-donating core, the electron-withdrawing tail and the π-conjugated units would extremely impact aggregation structure, hole mobility, and photovoltaic performance.

Benzo[1,2-b:3,4-b′:6,5-b′′]trithiophene (BTT1), a planar electron-donating unit, was introduced into OPVs by Nielsen et al. for its high hole mobility in 2011.²⁸ Subsequently, the conjugated polymers based on BTT1 units have been reported and shown good photovoltaic performance,^29–33 and the OPV device based on PBTT4BT showed a PCE value of 5.6%.³³ As an isomer of BTT1, benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene (BTT2) possesses similar structure and deeper HOMO energy level in theory.³⁴ Xiao et al. synthesized three star-shaped small molecules adopting BTT2 as the core and benzo[1,2,5]thiadiazole as the arms, but the OPV devices showed an unsatisfactory photovoltaic performance because of the weak π–π stacking of the star-shaped small molecules, the highest PCE was only 0.74% (V_oc = 0.69 V, J_sc = 2.93 mA cm⁻², FF = 0.37).³⁵

Herein, BTT2 unit has been introduced to linear A–D–A small molecules. Compared with BTT1 as electron-donating core, BTT2 unit would lead to deeper HOMO energy level, which is beneficial to obtaining high V_oc OPV devices. Synchronously, the linear A–D–A structure could result in an enhanced π–π stacking and higher crystallinity which is in favor of its hole mobility. Thus, two A–D–A small molecules D1 and D2 (Fig. 1) were designed and synthesized with BTT2 as the D unit, 3-ethylrhodanine as the end-capped A units, and two thiophenes or three thiophenes as conjugated π-bridge, respectively. Furthermore, the effects of the difference of conjugated π-bridge on the aggregation structure as well as photophysical, electrochemical, and photovoltaic property have been carefully studied. To our knowledge, it is the first time to introduce BTT2 as the electron-donating unit of linear A–D–A small molecules.

Fig. 1 Chemical structure of the small molecules D1 and D2.

Experimental

Materials

All the chemicals were purchased from commercial suppliers (Aldrich, Energy Chemical, Alfa, etc.). THF and toluene were refluxed over sodium and benzophenone, then distilled. DMF was dried and distilled under reduced pressure. All other commercially available materials were used without further purification unless noted otherwise. Column chromatography was carried out on silica gel (Qingdao Haiyang Chemical Co., LTD., 200–300 mesh). Benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene (BTT2) and compound 10 were synthesized according to the literature procedures.^23,36

Measurement and characterization

¹H NMR and ¹³C NMR spectra were recorded using Bruker AVANCE 400 spectrometer. UV-vis spectra and photoluminescence (PL) spectra of the as-synthesized small molecules were measured on Perkin-Elmer Lamada 25 spectrometer and Perkin-Elmer LS-50 luminescence spectrometer, respectively. Thermogravimetric analyses (TGA) were performed under nitrogen at a heating rate of 20 °C min⁻¹ with Netzsch TG 209 analyzer. X-ray diffraction (XRD) spectra were tested on Bruker D8 Advance. Cyclic voltammetry (CV) was conducted on an electrochemistry workstation (ZAHNER ZENNIUM) with the small molecule film on Pt plate as the working electrode, Pt wire as the counter electrode, and saturated calomel electrode (SCE) as a reference electrode in a 0.1 M tetra-n-butyl-ammonium hexafluorophosphate acetonitrile solution at a scan rate of 100 mV s⁻¹.

Photovoltaic devices fabrication

The BHJ solar cells were fabricated in the traditional sandwich structure, which adopted indium tin oxide (ITO) glass as substrates. The ITO glass substrates were cleaned by ultrasonic treatment in acetone and isopropyl alcohol. MoO₃ was deposited on the cleaned ITO glass substrate with a thickness of about 10 nm which was measured by Ambios Technology XP-2 surface profilometer. Subsequently, the photoactive layer was prepared by spin-coating the solution of small molecules and PC₆₁BM on the top of MoO₃. Finally, 0.5 nm LiF as an electron extraction layer and an aluminum cathode (100 nm) were deposited on the top of active layer in a vacuum of 5 × 10⁻⁴ Pa, producing active area of 3.8 mm² for each cell.

Synthesis of compounds

The synthesis routes of D1 and D2 are shown in Scheme 1 and S1.† Typically, a solution of the intermediate 11 (or 12) (0.28 mmol) and 3-ethylrhodanine (0.45 g, 2.8 mmol) in chloroform (20 mL) was evacuated and filled with N₂ three times, followed by the addition of piperidine (0.5 mL). The mixture was refluxed and stirred overnight under N₂. The cooled reaction mixture was poured into water, extracted with chloroform. Then the organic layer was dried over anhydrous magnesium sulfate and concentrated to afford the crude product. Purification by column chromatography (chloroform/petroleum ether = 1/1, v/v) to yield a product. The yield and NMR results of the polymers were as follows.

Scheme 1 Synthesis of D1 and D2.

D1. Red black solid (0.32 g, 84.2% yield). ¹H NMR (400 MHz, CDCl₃): δ 7.73 (s, 2H), 7.52 (s, 1H), 7.51 (s, 1H), 7.23 (s, 1H), 7.18 (s, 2H), 7.13 (s, 2H), 4.29–4.09 (m, 4H), 3.00 (t, J = 7.36 Hz, 2H), 2.95–2.87 (t, J = 7.60 Hz, 4H), 2.82 (t, J = 7.50 Hz, 4H), 1.89–1.81 (m, 2H), 1.80–1.73 (m, 4H), 1.73–1.67 (m, 4H), 1.54–1.42 (m, 10H), 1.42–1.33 (m, 14H), 1.33–1.25 (m, 16H), 1.00–0.83 (m, 15H). ¹³C NMR (100 MHz, CDCl₃): δ 191.89, 167.13, 146.52, 140.89, 140.83, 139.54, 137.20, 134.90, 134.75, 134.53, 133.47, 132.35, 131.72, 131.26, 131.11, 130.72, 130.31, 129.63, 124.68, 120.34, 119.79, 119.69, 118.61, 39.84, 31.88, 31.72, 31.37, 30.81, 30.40, 30.03, 29.69, 29.54, 29.41, 29.30, 22.71, 22.68, 14.17, 14.11, 12.29. MALDI-TOF MS (C₇₂H₈₈N₂O₂S₁₁) m/z: calcd for 1364.38, found 1364.44. m.p. 202 °C. Elemental analysis: calcd for C, 63.30; H, 6.49; N, 2.05; S, 25.82; found C, 63.06; H, 7.04; N, 2.03; S, 25.50.

D2. Black solid (0.24 g, 81.4% yield). ¹H NMR (400 MHz, CDCl₃): δ 7.74 (s, 2H), 7.51 (s, 1H), 7.49 (s, 1H), 7.23 (s, 1H), 7.19 (d, J = 3.38 Hz, 2H), 7.16 (d, J = 3.32 Hz, 2H), 7.08 (s, 2H), 4.26–4.09 (m, 4H), 3.01 (t, J = 7.30 Hz, 2H), 2.88 (t, J = 7.62 Hz, 4H), 2.80 (t, J = 7.64 Hz, 4H), 1.90–1.82 (m, 2H), 1.80–1.72 (m, 4H), 1.71–1.65 (m, 4H), 1.53–1.32 (m, 10H), 1.33–1.24 (m, 30H), 1.03–0.86 (m, 15H). ¹³C NMR (100 MHz, CDCl₃): δ 191.78, 166.97, 145.94, 140.52, 140.45, 140.22, 139.76, 138.25, 138.20, 136.99, 135.10, 134.69, 134.47, 134.26, 133.77, 130.91, 130.59, 130.20, 126.91, 126.62, 124.46, 123.64, 119.96, 118.36, 39.79, 31.96, 31.91, 31.86, 31.78, 29.63, 29.54, 29.52, 29.46, 29.38, 22.88, 22.84, 22.76, 22.75, 14.35, 14.29, 14.23, 14.22, 14.18, 12.30. MALDI-TOF MS (C₈₀H₉₂N₂O₂S₁₃) m/z: calcd for 1528.35, found 1528.41. m.p. 72 °C. Elemental analysis: calcd for C, 62.78; H, 6.06; N, 1.83; S, 27.24; found C, 62.49; H, 6.61; N, 2.32; S, 26.78.

Result and discussion

Synthesis and thermal stability

Compound 3 was prepared from 1 using Friedel–Crafts reaction and Wolff–Kishner–Huang reduction. The important intermediate compound 4 was obtained through lithiation of 3 with n-BuLi and subsequent quenching with trimethyltin chloride. Compound 9 was synthesized through a four-step reaction from 2-bromo-3-hexyl-5-thenaldehyde (5) using Stille coupling and bromination reactions. Compounds 11 and 12 were synthesized from a Pd(PPh₃)₄-catalyzed Stille coupling reaction between 4 and 10 or 9. The targeted molecules were then prepared by Knoevenagel condensation. The molecular structures of D1 and D2 were confirmed by ¹H, ¹³C NMR and MALDI-TOF MS. The thermal properties of D1 and D2 were investigated by thermogravimetric analysis (TGA). TGA curves were recorded at a heating rate of 20 °C min⁻¹ under nitrogen atmosphere (Fig. 2). The results revealed the degradation temperature of D1 and D2 are both above 400 °C, which indicate that the thermal stability of the two small molecules is adequate for photovoltaic applications.

Fig. 2 TGA curves of D1 and D2 with a heating rate of 20 °C min⁻¹ under nitrogen atmosphere.

Photophysical properties

UV-vis absorption spectra (Fig. 3) of the small molecules were measured in dilute chloroform solutions (10⁻⁵ M) and thin films on quartz plates. The optical absorption data are summarized in Table 1. In dilute chloroform solution, D1 shows two distinct absorption bands with the peak wavelengths (λ_maxs) of 370 and 500 nm. The former is due to π–π* transitions and the latter is attributed to the intramolecular charge transfer (ICT) from BTT2 unit to the end electron-withdrawing units. D2 shows the similar absorption spectrum in solution with the λ_maxs at 395 and 505 nm. Compared with D1, D2 with one more thiophene between BTT2 core and 3-ethylrhodanine terminal unit is slightly redshifted in solution, which may be caused by the extending conjugation length. In addition, the molar extinction coefficient of D1 (93 500 L M⁻¹ cm⁻¹) at 500 nm is slightly higher than that of D2 (90 000 L M⁻¹ cm⁻¹) at 505 nm.

Fig. 3 UV-vis absorption spectra of D1 and D2 in chloroform (1 × 10⁻⁵ M) and thin films.

Table 1 Optical and electrochemical properties of D1 and D2

	Solution	Film	Energy levels		Band gaps
	λmax (nm)	λmax (nm)	HOMO (eV)	LUMO (eV)	E^ecg (eV)	E^optg (eV)
D1	370, 500	396, 531, 602	−5.59	−3.55	2.04	1.87
D2	395, 505	425, 552	−5.51	−3.59	1.92	1.83

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The absorption spectra of the films are obviously broadened and the absorption peaks are redshifted about 31 nm for D1 and 47 nm for D2, compared with them in solution. It is noticed that D1 film obviously appears an absorption peak at 602 nm, indicating a more orderly aggregation. However, D2 film only shows a very weak shoulder peak around 602 nm. Therefore, D1 could form stronger π–π interchain association and π–π stacking than D2 in solid state.^26,37 The optical band gaps (E^opt_g) of D1 and D2 estimated by the onset absorption of the thin film are 1.87 eV, 1.83 eV, respectively.

To investigate the charge transfer from the small molecules to the PC₆₁BM, the photoluminescence (PL) spectra of D1 film, D2 film, and their corresponding composite thin films with PC₆₁BM were performed and shown in Fig. 4. D1 film with a thickness of 32 nm shows a strong PL emission peak at 661 nm, whereas D2 film with a thickness of 30 nm only exhibits a weak emission peak at 698 nm. It is noticed that the PL emission intensity of D1 is about four times higher than D2, which indicates D2 possesses stronger fluorescence quenching. Simultaneously, the emission peaks of the small molecules-PC₆₁BM composite films are nearly disappeared, indicating an effective charge transfer from them to PC₆₁BM.³⁸

Fig. 4 Photoluminescence spectra of D1, D2, D1/PC₆₁BM and D2/PC₆₁BM composite films.

X-ray diffraction and theory calculation

To investigate the effect of conjugated π-bridge on the aggregation structure, X-ray diffraction (XRD) was performed on thin films. As shown in Fig 5, D1 film exhibits obvious diffraction peaks at 2θ of 4.74°, 5.30°, corresponding to the lamellar distance of 18.65 Å, 16.65 Å. Simultaneously, D1 shows strong intense diffraction peaks at 2θ of 23.41°, 24.59°, corresponding to the π–π stacking distance of 3.8 Å, 3.63 Å. We presume that there exist two packing mode in D1 film which lead to two lamellar distances and π–π stacking distances. Compared with D1 film, D2 film only shows diffusion halo peaks at 2θ = 4.86° (the lamellar distance of 18.15 Å) and 2θ = 23.83° (the π–π stacking distance of 3.72 Å). According to the XRD data, we can know that D1 possess higher degree of crystallinity than D2. The lamellar distance (16.65 Å) and the π–π stacking distance (3.63 Å) corresponding to the main diffraction peaks for D1 film are both smaller than those for D2 film. It indicates that D1 possess stronger π–π stacking than D2, which is beneficial to higher hole mobility and consistent with the optical results.

Fig. 5 XRD patterns of D1 and D2 films.

Density functional theory (DFT) calculation is also performed to investigate the optimal molecular structure of D1 and D2 using the Gaussian 09 program suite at B3LYP/6-31G(d) level. As shown in the Fig. 6 and S1,† D1 maintains a nearly linear structure, and D2 shows an obvious banana structure caused by the stronger twisting in the ground state. The calculation results indicate D1 can form stronger packing than D2, which is benefit to higher crystallinity. The result is consistent with the XRD and optical results.

Fig. 6 Optimized molecular geometries calculated from DFT.

Electrochemical properties

The electrochemical properties of the small compounds were investigated by cyclic voltammetry (CV) in 0.1 M tetra-n-butyl-ammonium hexafluorophosphate acetonitrile solution at a potential scan rate of 100 mV s⁻¹. As shown in Fig. 7, the redox potential of the ferrocene/ferrocenium (Fc/Fc⁺) is 0.60 eV vs. SCE. The HOMO and LUMO energy levels can be estimated from the onset oxidation and reduction potentials of cyclic voltammetry.³⁹ The HOMO and LUMO energy levels of the small compounds were calculated according to the following equations:

LUMO = −e(E_red + 4.80 − E_{1/2,(Fc/Fc⁺)})

HOMO = −e(E_ox + 4.80 − E_{1/2,(Fc/Fc⁺)}).

Fig. 7 Cyclic voltammogram curves of D1 and D2 films on a platinum electrode in a 0.1 M Bu₄NPF₆ CH₃CN solution at a scan rate of 100 mV s⁻¹.

The results of the electrochemical measurement and calculated energy levels of the small molecules are listed in Table 1. As shown in Table 1 and Fig. 7, the onset reduction of D1 and D2 are −0.65 V and −0.61 V, corresponding to the LUMO energy levels of −3.55 eV and −3.59 eV, respectively. Both of them are positioned above the LUMO energy level of PC₆₁BM over 0.3 eV, which can ensure efficient charge separation.^18,40 According to the onset oxidation potential, the HOMO energy levels are calculated to be of −5.59 eV for D1 and −5.51 eV for D2. The HOMO of D2 is 0.08 eV higher than that of D1, which could be attributed to the aggrandizement of electron delocalization. Generally, the lower the HOMO level of the donor, the higher the V_oc of the OPV device.⁴¹ Since the HOMO energy levels of D1 and D2 are both lower than that of BTT1 based small molecule BT4OT (HOMO = −5.48 eV), so these small molecules may favor for higher V_oc than BT4OT (V_oc = 0.88 V).⁴² According to the difference between the HOMO and LUMO energy levels, the band gaps (E^ec_g) are determined to be 2.04 eV for D1 and 1.92 eV for D2.

Morphology

It is well known that the morphology of the active layer is a key factor to decide the photovoltaic performance of OPVs. Therefore, the morphology of the active layer was examined by atomic force microscopy (AFM) and shown in Fig. 8. When the active layer was prepared with chlorobenzene as solvent and a donor–acceptor weight ratio of 1 : 2, the D1-CB film (Fig. 8a) and the D2-CB film (Fig. 8b) show a similar texture, so the photovoltaic performance of them would mainly be governed by their inherent photophysical properties. After being prepared by chloroform solution with a weight ratio of 1 : 0.5, the D1-CF film (Fig. 8c) shows the smallest domain sizes among these films, the desirable texture can reduce the charge recombination and improve the J_sc value of the D1-CF device.^43,44 On the contrary, the D2-CF film (Fig. 8d) shows obvious phase separation, which would result in a very low generation rate of free charge carriers and a very high recombination rate of free charge carriers.^43,44

Fig. 8 AFM height images of (a) D1-CB, (b) D2-CB, (c) D1-CF, (d) D2-CF films.

Photovoltaic properties

SM BHJ devices using D1 and D2 as the donors and PC₆₁BM as the acceptor were fabricated with the device configuration of ITO/MoO₃/the small molecules: PC₆₁BM/LiF/Al. All devices were tested under the illumination of AM 1.5G (100 mW cm⁻²). The representative J–V curves are shown in Fig. 9, and the corresponding photovoltaic data are summarized in Table 2. Fig. 9 presents a clear impact of the solvent and the weight ratio of the active layer on the photovoltaic performance of the devices. The D1-CB device shows a slightly higher J_sc than that of the D2-CB device, and the former achieves a V_oc of 0.99 V which is higher than that of the latter (0.80 V). As described before, the two film shows similar morphology, but the HOMO energy level of D1 (−5.59 eV) is lower than that of D2 (−5.51 eV), so the former shows higher V_oc value.⁴¹ However, no available J–V curves were obtained if the weight ratio is 1 : 0.5 with chlorobenzene as solvent (Table S1†). The D1-CF device provided a surprising V_oc of 1.10 V, a J_sc of 4.25 mA cm⁻², and FF of 0.45, resulting in a PCE of 2.10%. In contrast, the D2-CF device exhibited a worse performance with a lower V_oc of 0.62 V, a J_sc of 0.88 mA cm⁻², and FF of 0.24, yielding a PCE of 0.13%. It is well accepted that V_oc is determined by the generation rate of free charge carriers and the recombination rate of free electrons and holes.⁴⁴ Since the D1-CF film possesses a more desirable texture than the D1-CB film (Fig. 8a and c), the D1-CF device would achieved a higher generation rate of free charge carriers and a lower recombination rate of free charge carriers, which would lead to a higher J_sc and V_oc.⁴⁴ Therefore, the D1-CF device achieved an amazing V_oc value of 1.10 V which is 0.11 V higher than that of the D1-CB device. On the contrary, the D2-CF film possesses a more serious phase separation than the D2-CB film (Fig. 8b and d), so the D2-CF device would form a lower generation rate and a higher recombination rate of free charge carriers than the D2-CB device. As a result, the D2-CF device only shows a low V_oc value of 0.62 V which is 0.18 V lower than the D2-CB device.

Fig. 9 J–V curves of the OPV devices based on the small molecules: PC₆₁BM under an illumination of AM 1.5G, 100 mW cm⁻².

Table 2 Photovoltaic properties and hole mobilities of D1 and D2 composite films

Device	D : A (w:w)	Solvent	Voc (V)	Jsc (mA cm^−2)	FF	PCEave (PCEmax) (%)
D1-CB	D1(1 : 2)	CB	0.99	2.77	0.30	0.82 (0.88)
D2-CB	D2(1 : 2)	CB	0.80	2.48	0.25	0.50 (0.58)
D1-CF	D1(1 : 0.5)	CF	1.10	4.25	0.45	2.10 (2.21)
D2-CF	D2(1 : 0.5)	CF	0.62	0.88	0.24	0.13 (0.15)

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The hole mobility of the active layer was investigated by using the space charge limit current (SCLC) method⁴⁵ and the devices were fabricated with the configuration of ITO/PEDOT:PSS (25 nm)/the small molecules: PC₆₁BM/MoO₃ (20 nm)/Al (100 nm). The equation for calculating the hole mobilities is listed in the ESI,† and the corresponding current density–voltage curves are shown in Fig. S2.† The hole mobility values for D1 composite film is 2.3 × 10⁻⁵ cm² V⁻¹ s⁻¹, which is much higher than that of D2 composite film (2.5 × 10⁻⁷ cm² V⁻¹ s⁻¹). The higher hole mobility of the former is attributed to the higher crystallinity of D1 and the appropriate bicontinuous microphase separation structure of the D1-CF active layer. Therefore, the D1-CF device shows a high IPCE value of 33.6% at 520 nm (Fig. S3†). However, the D2-CF device only possesses a very low IPCE value of 6.7% at 520 nm because of its serious phase separation and low hole mobility. As a result, the J_sc (4.25 mA cm⁻²) and PCE (2.1%) of the D1-CF device are largely higher than those of the D2-CF device.

Conclusions

In conclusion, two A–D–A small molecules D1 and D2 were designed and synthesized for solution-processed OPVs by introducing benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene as electron-donating unit, 3-ethylrhodanine as electron-withdrawing units, and two thiophenes or three thiophenes as conjugated π-bridge, respectively. Compared with D2 with three thiophenes as conjugated π-bridge, D1 with two thiophenes shows more linear molecular configuration, better packing capability, higher fluorescence efficiency and lower HOMO energy level, and the corresponding composite with PC₆₁BM possesses better bicontinuous microphase separation structure and two orders of magnitude higher hole mobility. As a result, the D1-CF OPV device exhibited an amazing high V_oc of 1.10 V and a PCE value of 2.10%, under illumination of AM 1.5G (100 mW cm⁻²).

Acknowledgements

This work was financially supported by the Open Fund of Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province (nos 12K049), the Natural Science Foundation of Hunan Province of China (nos 2015JJ2141) and the Innovation Group of Hunan Natural Science Foundation (12JJ7002).

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Footnote

† Electronic supplementary information (ESI) available: Synthesis of intermediates, characterization data. See DOI: 10.1039/c4ra15721d

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