Indenothiophene-based asymmetric small molecules for organic solar cells

Abstract

The development of organic semiconductors is of key importance in order to improve the performance of organic solar cells (OSCs). Three indenothiophene (IT)-containing small molecules (IT3T, ITFBT and IT2FBT) were designed and synthesized for small molecule OSCs. The thermal, optical, and electrochemical properties of the molecules were investigated. The optical bandgaps of the three small molecules are ranged from 1.80 to 2.20 eV depending on different terminal groups flanked on the IT. We study the photovoltaic performances of the three molecules by fabricating OSCs with PC₇₁BM as an electron acceptor. Among the three molecules, ITFBF exhibited the best power conversion efficiency of 4.57% with a high open circuit voltage (V_OC) of 0.98 V. We also briefly discuss structure–property guidelines for small molecules used for OSCs. The results demonstrate that IT-based small molecules are promising for small molecule OSCs with large V_OCs.

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1. Introduction

Small molecule organic solar cells (OSCs), in which the photoactive layer commonly consists of a p-type semiconducting small molecule and a n-type fullerene derivative, have made significant progress in the past decade.^1–4 In comparison with polymers, semiconducting small molecules for OSCs have the advantages of well-defined structures, high purity, and superior batch-to-batch reproducibility.^5–7 So far, the power conversion efficiency (PCE) of single-junction small molecule OSCs has exceeded 11%.³

For small molecule OSCs, the photoactive layer plays an important role not only in the open circuit voltage (V_OC), but also in the current density, both of which affect the PCE of the resulting device. Therefore, the innovations in active layer materials are attracting more and more attention. Although a large number of small molecule donors have been used for OSCs, the building blocks for efficient small molecule donors are limited to few symmetric units such as benzodithiophene, dithienosilole, indacenedithiophene, and diketopyrrolopyrrole.^3,8–11 For example, dithienosilole-based (DTS) derivatives were used for small molecule OSCs as donor units, leading to PCEs in the range from 5.8% to over 9%.⁹ Wang et al. introduced indacenedithiophene-based (IDT) derivatives as donor units to produce a series of high performing small molecules for OSCs.^10d Deng et al. reported benzodithiophene-based small molecules with PCEs over 11%.³ All the small molecules mentioned above have symmetric structures. At the same time, some examples of asymmetric small molecules were also reported for OSCs by Sharma et al.¹² However, the number of asymmetric semiconducting molecules is much less than that of the symmetric counterparts due to less availability of asymmetric π-conjugated cores.⁴ Recently, our group first introduced asymmetric indenothiophene (IT) to construct donor–acceptor copolymers for polymer solar cells with PCEs over 9%.¹³ However, the indenothiophene has never been used for constructing small molecule donors for OSCs.

In this context, three asymmetric indenothiophene-cored small molecules (IT3T, ITFBT and IT2FBT), were designed, synthesized, characterized, and used for the fabrication of OSCs. Three different terminal groups were flanked on the IT core for constructing the target small molecules in order to study the structure–property relationships of the IT-based small molecules. The chemical structures of the target small molecules are shown in Scheme 1. Among them, the devices based on ITFBT exhibited the best PCE of 4.57% and a V_OC of 0.98 V under AM 1.5 G irradiation (100 mW cm⁻²). IT3T and IT2FBT gave low PCEs of 0.71% and 1.92%, respectively.

Scheme 1 Synthesis of IT3T, ITFBT and IT2FBT: (i) 2-bromothiophene, Pd₂(dba)₃, P(o-tolyl)₃, aqueous solution, aliquat 336, toluene, reflux, 24 h; (ii) NBS, CH₂Cl₂, 20 °C; (iii) Pd(PPh₃)₄, toluene, reflux, 24 h.

2. Experimental section

2.1 Materials

All reagents were purchased from commercial suppliers, and used as received without further purification unless otherwise stated. Column chromatography was conducted with silica gel (300–400 mesh).

2.2 Synthesis of 5-hexyl-[2,2′;5′,2′′]terthiophene (2)¹⁴

2-(5′-Hexyl-[2,2′-bithiophen]-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1) (2.02 g, 5.4 mmol), 2-bromothiophene (4.34 g, 26.6 mmol), potassium phosphate tribasic (2 M in H₂O, 10.0 mmol) and 1 drop of aliquat 336 were added to 50 mL of toluene. After the mixture was degassed, tri(o-tolyl)phosphine (P(o-tol)₃) (20 mg, 0.066 mmol) and tris(dibenzylideneacetone)dipalladium(0) (Pd₂(dba)₃) (10 mg, 0.011 mmol) were added. Then the reaction mixture was stirred vigorously at 110 °C for 24 h. After cooling to room temperature, the mixture was poured into methanol. The crude product was collected by filtration and purified by column chromatography using hexane as eluent to afford compound 2 as a yellow solid (1.06 g, 59%). ¹H NMR (CDCl₃, 400 MHz, ppm): 7.22 (d, J = 4.8 Hz, 1H), 7.18 (d, J = 3.6 Hz, 1H), 7.08 (d, J = 3.6 Hz, 1H), 7.05–7.00 (m, 3H), 6.71 (d, J = 3.6 Hz, 1H), 2.82 (t, J = 7.6 Hz, 2H), 1.70 (m, 2H), 1.44–1.32 (m, 6H), 0.92 (t, J = 6.8 Hz, 3H).

2.3 Synthesis of 5′′-bromo-5-hexyl-[2,2′;5′,2′′]terthiophene (3)¹⁴

N-Bromosuccinimide (NBS) (0.54 g, 3.0 mmol) was added slowly to a solution of compound 2 (1.01 g, 3.0 mmol) in 30 mL of CH₂Cl₂. After stirring the mixture at room temperature for 2 h, 10 mL of water was added, and the mixture was then extracted with dichloromethane (2 × 50 mL). The organic phase was washed with water twice, brine solution, then dried over anhydrous MgSO₄. After removing the solvent, the crude product was purified by column chromatography to give a yellow solid (0.87 g, 70%). ¹H NMR (CDCl₃, 400 MHz, ppm): 7.02–6.98 (m, 4H), 6.92 (d, J = 3.6 Hz, 1H), 6.71 (d, J = 3.6 Hz, 1H), 2.81 (t, J = 7.6 Hz, 2H), 1.70 (m, 2H), 1.44–1.31 (m, 6H), 0.92 (t, J = 6.8 Hz, 3H).

2.4 Synthesis of IT3T

Compound 3 (0.50 g, 1.2 mmol), IT-Sn (0.33 g, 0.4 mmol) and 20 mL of dry toluene were added into a 50 mL two-neck flask. The solution was degassed by bubbling with nitrogen for 0.5 h. 10 mg of Pd(PPh₃)₄ was then added and the mixture was heated at 110 °C for 24 h. After the solvent was removed, the crude product was purified by column chromatography to give a red solid (0.31 g, 66%). ¹H NMR (CDCl₃, 400 MHz, ppm): 7.54 (d, J = 7.6 Hz, 2H), 7.37 (d, J = 8.0 Hz, 1H), 7.27–7.25 (m, 1H), 7.18 (d, J = 3.6 Hz, 1H), 7.14–7.08 (m, 7H), 7.04–7.01 (m, 4H), 6.72 (d, J = 3.6 Hz, 2H), 2.82 (t, J = 7.6 Hz, 4H), 1.99 (br, 4H), 1.75–1.67 (m, 4H), 1.45–1.32 (m, 12H), 1.06–0.85 (m, 22H), 0.79–0.59 (m, 14H); ¹³C NMR (CDCl₃, 100 MHz, ppm): 156.24, 154.04, 154.00, 145.71, 144.12, 140.23, 139.24, 139.22, 139.19, 137.97, 137.10, 137.07, 136.95, 136.26, 136.21, 136.17, 135.90, 135.87, 135.68, 135.30, 135.27, 135.23, 134.46, 130.54, 124.91, 124.76, 124.70, 124.67, 124.63, 124.32, 124.22, 124.15, 123.87, 123.64, 123.48, 123.30, 120.50, 119.03, 54.37, 43.54, 35.06, 34.26, 33.97, 31.65, 30.28, 28.87, 28.59, 27.50, 22.93, 22.67, 14.18, 14.12, 10.74. HRMS (MALDI-DHB, m/z): calcd for C₆₇H₇₈S₈ [M]⁺ 1138.3869; found 1138.3864. Elemental analysis (%) calc. for C₆₇H₇₈S₈: C, 70.60; H, 6.90; found: C, 70.41; H, 6.91.

2.5 Synthesis of ITFBT

Following the same procedure as that used for IT3T, ITFBT was obtained as a dark red solid (0.12 g, 44%) through a Stille coupling reaction between 7-bromo-5-fluoro-4-(5′-hexyl-[2,2′]bithiophenyl-5-yl)-benzo[1,2,5]thiadiazole (0.31 g, 0.6 mmol), and IT-Sn (0.17 g, 0.2 mmol). ¹H NMR (CDCl₃, 400 MHz, ppm): 8.19–8.12 (m, 4H), 7.78–7.67 (m, 3H), 7.64–7.55 (m, 1H), 7.48–7.41 (m, 2H), 7.21–7.17 (m, 2H), 7.13–7.10 (m, 2H), 6.72–6.71 (m, 2H), 2.81 (t, J = 7.2 Hz, 4H), 2.16–2.03 (m, 4H), 1.74–1.67 (m, 2H), 1.42–1.17 (m, 14H), 1.14–0.78 (m, 24H), 0.70–0.61 (m, 12H); ¹³C NMR (CDCl₃, 100 MHz, ppm): 160.17, 157.64, 156.61, 156.60, 154.60, 153.29, 149.67, 147.36, 147.33, 146.08, 143.84, 140.42, 140.37, 140.29, 138.19, 136.91, 134.55, 131.14, 131.08, 131.03, 130.98, 130.86, 129.58, 129.54, 126.23, 126.12, 124.98, 124.94, 123.88, 123.70, 123.23, 123.06, 120.93, 120.91, 119.71, 119.50, 116.17, 115.86, 115.68, 110.85, 110.82, 110.66, 54.53, 43.70, 35.14, 34.10, 34.08, 31.60, 30.26, 28.82, 28.40, 27.53, 22.86, 22.60, 14.15, 14.03, 10.68. HRMS (MALDI-DHB, m/z): calcd for C₇₁H₇₆F₂N₄S₈ [M]⁺ 1278.3804; found 1278.3798. Elemental analysis (%) calc. for C₇₁H₇₆F₂N₄S₈: C, 66.63; H, 5.99; N, 4.38; found: C, 66.70; H, 6.12; N, 4.13.

2.6 Synthesis of IT2FBT

Following the same procedure as that used for IT3T, IT2FBT was obtained a dark red solid (0.14 g, 54%) through a Stille coupling reaction between 4-bromo-5,6-difluoro-7-(5′-hexyl-[2,2′]bithiophenyl-5-yl)-benzo[1,2,5]thiadiazole (0.31 g, 0.6 mmol) and IT-Sn (0.16 g, 0.2 mmol). ¹H NMR (CDCl₃, 400 MHz, ppm): 8.35–8.23 (m, 4H), 7.73–7.70 (m, 2H), 7.56–7.52 (m, 2H), 7.27–7.25 (m, 2H), 7.20–7.17 (m, 2H), 6.79–6.75 (m, 2H), 2.85 (t, J = 7.6 Hz, 4H), 2.20–2.06 (m, 4H), 1.77–1.70 (m, 2H), 1.46–1.23 (m, 14H), 1.10–0.75 (m, 24H), 0.71–0.61 (m, 12H); ¹³C NMR (CDCl₃, 100 MHz, ppm): 155.85, 155.83, 154.83, 150.91, 150.83, 148.75, 148.73, 148.66, 148.60, 148.24, 148.17, 146.45, 145.41, 141.36, 141.28, 141.20, 138.24, 138.20, 134.26, 132.11, 132.09, 132.02, 131.75, 131.71, 131.68, 131.66, 131.59, 131.16, 131.10, 129.97, 129.91, 129.87, 129.83, 126.13, 125.25, 125.05, 124.08, 124.04, 123.14, 120.85, 119.77, 111.21, 54.51, 43.64, 35.16, 34.26, 34.15, 31.65, 30.30, 28.88, 28.40, 27.50, 22.92, 22.66, 14.17, 14.10, 10.75. HRMS (MALDI-DHB, m/z): calcd for C₇₁H₇₄F₄N₄S₈ [M]⁺ 1314.3615; found 1314.3610. Elemental analysis (%) calc. for C₇₁H₇₄F₄N₄S₈: C, 64.80; H, 5.67; N, 4.26; found: C, 65.02; H, 5.71; N, 4.19.

2.7 Instruments

¹H and ¹³C NMR spectra were recorded on a Bruker AVANCE-400 spectrometer operating at 400 and 100 MHz, respectively. High-resolution mass spectroscopy (HRMS) measurements were performed on an IonSpec 4.7 T spectrometer. Absorption spectra were obtained by using a spectrophotometer (Lambda 365 UV/vis). The electrochemical cyclic voltammetry measurements were carried out on a CHI 700E electrochemical workstation. OSCs were measured by a Keithley 2440 source measurement unit under AM 1.5 G irradiation (100 mW cm⁻²) on an Oriel sol3A simulator (Newport) which had been precisely calibrated with a NREL-certified silicon reference cell before testing. The external quantum efficiency (EQE) spectra were measured on a Newport EQE measuring system. Atomic force microscopy (AFM) was performed with the Bruker's Dimension FastScan at a tapping mode.

2.8 Electrochemistry

The cyclic voltammetry (CV) measurements were measured by using a three-electrode cell system (a Pt disk working electrode coated with thin films of small molecules, a Pt wire counter electrode and an Ag/AgNO₃ reference electrode). The measurements were performed in a 0.1 mol L⁻¹ anhydrous and nitrogen-saturated tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) acetonitrile solution at a scan rate of 100 mV s⁻¹. The energy level of the ferrocene was assumed to have an absolute energy level of −4.82 eV to vacuum. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were calculated from the onset oxidation potentials (E_ox) and onset reduction potentials (E_red) vs. Ag/AgNO₃ reference electrode according to the following equations:

E_HOMO = −(E_ox + 4.82) (eV)

E_LUMO = −(E_red + 4.82) (eV)

2.9 Fabrication of conventional OSCs

The OSCs were fabricated with the device structure of: indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/small-molecule:PC₇₁BM/PDIN/Al. ITO glass was cleaned by ultrasonication sequentially in detergent, water, acetone, and isopropanol for 30 min each and then dried in an oven at 80 °C overnight. After the ITO glass substrates were subjected to ultraviolet/ozone treatment for 15 min, PEDOT:PSS (Baytron PVPAI 4083) which had been filtered through a 0.45 μm filter was spin-coated on the ITO substrates at 3000 rpm for 60 s. Then the film-loaded substrates were dried at 140 °C in air for 15 min. A mixture of small-molecule : PC₇₁BM (1 : 2.2, w/w) was dissolved in chlorobenzene at a concentration of 20 mg mL⁻¹ and stirred overnight. Then the small-molecule : PC₇₁BM solution was spin-cast at 1100 rpm for 60 s to form the active layer. An electron injection interlayer was prepared by spin-coating a methanol solution of PDIN (1.5 mg mL⁻¹ containing 0.2% acetic acid) to facilitate efficient electron injection. Finally, the negative electrode was prepared by thermally depositing ∼120 nm aluminum through a shadow mask under a high vacuum of 5 × 10⁻⁵ Pa. The device areas of the OSCs were fixed at 0.06 cm².

3. Results and discussion

3.1 Synthesis and characterization

The synthetic routes for the target molecules (IT3T, ITFBT and IT2FBT) are shown in Scheme 1 and the detailed synthetic procedures are shown in the Experimental section. The stannylized IT-Sn was prepared according to the procedures we reported previously.^13a The synthesis of compound 3 started from compound 1, which was reacted with 2-bromothiophene to afford compound 2 in 59% yield. Bromination of compound 2 afforded compound 3 in 70% yield. The Stille reaction between compound 3 and IT-Sn using Pd(PPh₃)₄ as a catalyst gave compound IT3T in 66% yield. Similarly, ITFBT and IT2FBT were obtained by Stille coupling reactions between IT-Sn and the two benzothiadiazole-based bromides 4, respectively. The chemical structures and the purity of all new compounds were characterized by NMR, elemental analysis and high resolution mass spectrometry (HRMS).

3.2 Thermal properties

The thermal stability of the three small molecules was investigated by thermogravimetric analysis (TGA) at a heating rate of 10 °C min⁻¹ under N₂. IT3T, ITFBT and IT2FBT showed sufficiently high decomposition temperatures (T_d) of 403, 405, and 398 °C, respectively (Fig. S1† and Table 1).

Table 1 Summary of the intrinsic properties of IT3T, ITFBT and IT2FBT

	λmax (nm)		Eg,opt (film)^a (eV)	EHOMO (eV)	ELUMO (eV)	Eg,CV (film)^b (eV)	Td^c (°C)
	In CH2Cl2	Film
a Estimated from the onset of the absorption spectra of thin films.b Estimated from cyclic voltammetry.c Decomposition temperature at 5% weight loss.
IT3T	525	565	2.20	−5.29	−2.97	2.32	403
ITFBT	621	686	1.81	−5.35	−3.56	1.79	405
IT2FBT	609	673	1.84	−5.46	−3.51	1.95	398

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3.3 Optical properties

The absorption spectra of IT3T, ITFBT and IT2FBT in solution or in solid state are shown in Fig. 1, and the related parameters are summarized in Table 1. Due to the identical acceptor–donor–acceptor (A–D–A) structured backbone, ITFBT and IT2FBT exhibited analogous absorption bands both in the solution and in the thin film. For both ITFBT and IT2FBT, the shorter wavelength absorption bands originate from the π–π* transition and the longer ones are from the intramolecular charge transfer (ICT) between donor and acceptor units. However, IT3T exhibited only the π–π* transition band in solution or in thin film, which agrees with the donor–donor configuration of its molecule backbone. In going from the solution to the thin film, the absorption bands of all the three compounds become broadened and red-shifted. However, in comparison with IT3T, both IT2FBT and ITFBT exhibited a more red-shifted absorption in going from solution to thin film indicating a possible stronger π–π interaction for latter two. The optical bandgaps of IT3T, ITFBT, and IT2FBT are 2.20, 1.81 and 1.84 eV, respectively. Among the three molecules, ITFBT possesses the smallest band gap of 1.81 eV, which is beneficial for enhanced light harvesting.

Fig. 1 Normalized absorption spectra of the small molecules in dichloromethane (a) and as-cast films (b).

3.4 Electrochemical properties

To determine the HOMO and the LUMO energy levels of IT3T, ITFBT and IT2FBT, cyclic voltammetry (CV) measurement was performed and the results are shown in Fig. 2 and Table 1. The HOMO energy levels for IT3T, ITFBT and IT2FBT were estimated to be −5.29, −5.35 and −5.46 eV, respectively. Both ITFBT and IT2FBT exhibit deeper HOMO energy levels in comparison with IT3T suggesting that the A–D–A structured backbone is favorable to down-shift the HOMO level of the resulting molecule. Compared to ITFBT, IT2FBT contains two additional electron withdrawing fluorine atoms thereby resulting in a deeper HOMO energy level of −5.46 eV. The electrochemical bandgaps of the three small molecules are also estimated and they share the order of IT3T > IT2FBT > ITFBT, which agrees with their optical bandgap trend. The electrochemical properties of these molecules suggest that molecules with A–D–A structured backbone are beneficial for OSCs with enhanced light absorption as well as large V_OCs.

Fig. 2 Cyclic voltammogram of IT3T, ITFBT and IT2FBT thin films.

3.5 Photovoltaic performance

The photothose of the voltaic properties of IT3T, ITFBT and IT2FBT were investigated by fabricating OSCs with the device structure of indium tin oxide (ITO)/PEDOT:PSS/donor:PC₇₁BM/PDIN/Al, where PDIN and PEDOT are 2,9-bis(3-(dimethylamino)propyl)anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10(2H,9H)-tetraone and poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate, respectively. The best performance OSCs were fabricated by using solutions with a donor (D)/acceptor (A) ratio of 1 : 2.2 (w/w), and the solutions were spin-coated at a spinning rate of 1100 rpm. The device parameters of all three small molecule-based OSCs are summarized in Table 2. The current density–voltage (J–V) curves of the OSCs are shown in Fig. 3a. It is showed that the OSC based on ITFBT:PC₇₁BM achieved the best performance among all the three small molecule-based devices. Without any additive, the device based on ITFBT:PC₇₁BM exhibited a high V_OC of 0.98 V, a J_SC of 10.24 mA cm⁻² and a FF of 0.456, all of which leads to a high PCE of 4.57%. With the same A–D–A structured backbone and similar accepter units, IT2FBT-based OSC exhibited a lower PCE of 1.92% with a decreased J_SC of 6.25 mA cm⁻², a reduced FF of 30.5%, but a larger V_OC of 1.01 V. The increased V_OC is mainly attributed to the deeper HOMO energy level of IT2FBT as determined by cyclic voltammogram. However, the device based on IT3T:PC₇₁BM showed a PCE of 0.71%, with a low J_SC of 3.29 mA cm⁻², a poor FF of 28.6%, and a low V_OC of 0.753 V. The inferior device performance for IT3T could be related to its relatively large bandgap, high HOMO energy level, as well as low carrier mobility. It should be noted that these conventional OSCs using PEDOT:PSS as anode buffer layer are relatively unstable over time in ambient conditions (Fig. S2†). However, the shelf stability of OSCs can be greatly enhanced by adopting an inverted device structure where the PEDOT:PSS is replaced by other inorganic buffer layers.¹⁵ To assure the accuracy of the PCE measurements, external quantum efficiencies (EQEs) of the OSCs were measured and shown in Fig. 3b. All the EQE curves cover a spectral response range from 300 to 700 nm. In this range, the average EQE values of IT2FBT- and ITFBT-based devices are much higher than those of the IT3T-based device. The calculated J_SC value from EQE is 10.04 mA cm⁻² for ITFBT, which is very close to that obtained from the J–V measurement (10.24 mA cm⁻²) within 2% mismatch. The calculated J_SC values from the EQE curves of IT3T- and IT2FBT-based devices are also in agreement with the corresponding measured J_SC values. In order to know the origin of the observed photovoltaic performance differences among the three molecules, we measured the hole transport properties of the blend films by using the space charge limited current (SCLC) method. Hole-only devices with the device structure of ITO/PEDOT:PSS/small molecules/MoO₃/Au were fabricated. As shown in Fig. 4, ITFBT shows the highest hole mobility of 4.8 × 10⁻⁴ cm² V⁻¹ s⁻¹ in comparison with IT3T (7.9 × 10⁻⁶ cm² V⁻¹ s⁻¹) and IT2FBT (8.4 × 10⁻⁵ cm² V⁻¹ s⁻¹). This highest hole mobility of the ITFBT-based device explains the higher J_SC and improved FF compared to the devices based on the other two molecules.

Table 2 Device parameters of OSCs based on the small molecules

Compounds	VOC (V)	JSC (mA cm^−2)	FF (%)	PCE^a (%)
a In parentheses are average values based on 8 devices.
IT3T	0.753	3.29	28.6	0.71 (0.64)
ITFBT	0.979	10.24	45.6	4.57 (4.40)
IT2FBT	1.01	6.25	30.5	1.92 (1.82)

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Fig. 3 (a) Current–voltage characteristic of organic solar cells under AM 1.5 G illumination (100 mW cm⁻²); (b) EQE curves of OSCs; (c) corresponding integrated J_SC curves of OSCs.

Fig. 4 J^0.5–V characteristics of hole-only devices based on IT3T:PC₇₁BM blends (∼119 nm thickness), ITFBT:PC₇₁BM (∼101 nm thickness), and IT2FBT:PC₇₁BM (∼87 nm thickness).

3.6 Morphology

The surface morphology of the small molecule:PC₇₁BM blends was measured by tapping mode atomic force microscopy (AFM). The topography images and corresponding phase images of the small molecule blends are shown in Fig. 5. Among the three topographic images, the ITFBT:PC₇₁BM film has the most smooth surface, with a small average root mean square (RMS) roughness value of 0.6 nm. While the other two films based on IT3T:PC₇₁BM and IT2FBT:PC₇₁BM, have larger RMS roughness values of 1.04 and 1.88 nm, respectively, demonstrating the rougher surfaces than the ITFBT:PC₇₁BM film. From the phase images, the ITFBT:PC₇₁BM film shows clear phase seperation features with domain sizes of 10–20 nm. However, the phase seperation features are relatively unclear in the case of IT3T:PC₇₁BM or IT2FBT:PC₇₁BM. At the same time, apparent aggregates with a domain size of more than 100 nm were found for the IT2FBT:PC₇₁BM film. This scale is much larger than the exciton diffusion length (<10 nm), leading to an increased recombination rate of the photo-induced excitons before reaching the interfaces. It is believed that nanoscale phase separation enables a large interface area for exciton dissociation and a continuous percolating path for hole and electron transport to the corresponding electrodes.¹⁶ The morphology difference among the three small molecule blends explains the device performance difference for the three molecules with different ending groups.

Fig. 5 Tapping mode AFM height (a–c) and phase (d–f) images of IT3T:PC₇₁BM, ITFBT:PC₇₁BM and IT2FBT:PC₇₁BM films, respectively. All the images are 500 × 500 nm.

4. Conclusion

In summary, we designed and synthesized three novel asymmetric small molecules (IT3T, ITFBT and IT2FBT) based on the indenothiophene core. Due to the electron-withdrawing ability of fluorinated benzothiadiazole units, ITFBT and IT2FBT exhibited deeper HOMO energy levels of −5.35 and −5.46 eV with smaller optical bandgaps (1.81, 1.84 eV), respectively, than those of IT3T (−5.29 eV, 2.20 eV), thus leading to the enhanced photovoltaic performance of ITFBT and IT2FBT. The best performance device based on ITFBT delivered a PCE of 4.57% with a V_OC of 0.98 V, a J_SC of 10.24 mA cm⁻² and a FF of 0.456. IT2FBT and IT3T exhibited relatively lower PCEs of 1.92% and 0.71%, respectively, which can be attributed to the unfavorable energy level, bandgap, carrier mobility or the morphology induced by the difference in their ending groups. The results suggest that the backbone configuration is an important factor determining the performance of the indenothiophene-based small molecules. Nonetheless, our results demonstrate that the asymmetric indenothiophene can be an excellent donor core for small molecule semiconductors.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. U1605241, 61325026, 51561165011), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB20000000), and the CAS/SAFEA International Partnership Program for Creative Research Teams.

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Footnotes

† Electronic supplementary information (ESI) available: Thermogravimetric analysis results, stability of the OSC based on ITFBT:PC₇₁BM, and the NMR spectra of the small molecules. See DOI: 10.1039/c7ra01902e

‡ These authors contributed equally to this work.

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