A new V-shaped triphenylamine/diketopyrrolopyrrole containing donor material for small molecule organic solar cells

Abstract

A new V-shaped small molecule, TPA(DPPT2)2 is designed and synthesized with triphenylamine (TPA) as the core, diketopyrrolopyrrole (DPP) as the arm and bithiophene as end-groups. The compound shows broad absorption, a low optical band gap of 1.61 eV and hole mobility of 1.99 × 10⁻⁴ cm² V⁻¹ s⁻¹. By blending with a [6,6]-phenyl-C₇₁-butyric acid methylester (PC₇₁BM) acceptor, an optimized power conversion efficiency (PCE) of 3.81% is achieved. The best efficiency was obtained via adding remarkably small percentages of solvent additives (0.1% v/v of 1,8-diiodooctane, DIO) to form well-defined structures and improved phase-separated film morphology in the bulk-heterojunction layer.

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Introduction

Organic bulk-heterojuction (BHJ) solar cells have attracted intense investigation in both academia and industry owing to their predominant features such as low cost, flexibility, light weight and large area mass production.^1–8 By blending electron donor materials (polymer or small molecules) with electron acceptor materials (typically fullerene derivatives), such a BHJ structure provides efficient charge separation of the photogenerated excitons and charge transport at the donor/acceptor interfaces.⁹ To date, state-of-the-art single-layer BHJ polymer solar cells has been achieved in polymer/fullerene blends with power conversion efficiencies (PCE) approaching 10%.^10–13 On the other hand, great strides have been made in small molecular organic solar cells (OSCs) with PCE over 9%.^14,15 However, OSCs based on small molecular materials exhibited distinct advantages, such as more straightforward synthesis and purification, versatile chemical modifications, well-defined structures as well as less batch-to-batch variation.^16–19 Therefore, it is believed that new higher device performance are allowed to be achieved in solution-processable small molecular organic solar cells. Thus, much effort has been attracted into the development of small molecules OSCs.

It is reported that both the triphenylamine (TPA) and diketopyrrolopyrrole (DPP) are promising building blocks for small molecular donor materials in OSCs.^20–24 TPA backbone possessing special propeller starburst-like molecular structure generally shows good electron-donating and high charge-transporting properties.²⁵ PCE of 4.3% has been achieved in star-shaped donor material S(TPA-BT-HTT), which is based on TPA as the core and electron donating unit, benzothiadiazole (BT) as bridge and electron accepting unit, and oligothiophene as the arm and donor unit.²⁶ On the other hand, the DPP unit exhibits the feature of well conjugated structure, strong electron-withdrawing property, high light absorption and easy chemical modification, which makes it one of most popular electron-withdrawing building blocks for electron donor materials in OSCs.²⁷ DPP usually coupled with a variety of electron-rich groups to form low band gap donor materials. For example, solution-processed small-molecule OSCs based on BDT-2DPP with two-dimensional conjugated benzodithiophene (BDT) as the donating core and DPP as the accepting arm exhibited PCE over 5.3%.^28,29 However, the covalently combination of two famous building blocks have received litter attention and their device performances are still poor, with PCE less than 3%.^30–33 Therefore, there is still great room for designing new materials based on TPA/DPP for high performance organic solar cells.^33,34

Herein, we report the design and synthesis of a new V-shaped small molecule, TPA(DPPT2)2 by using TPA as the core, DPP as arm and bithiophene as end-groups. The V-shaped compound is different from previously reported star-shaped and linear structured materials. By introducing bithiophene end-group to the electron-withdrawing DPP bridge, the conjugation length of backbone can be enlarged, resulting in the enhancement of intermolecular π–π interactions and improvement of molecular ordering and charge carrier mobility. In addition, the extending of conjugated bithiophene broadens the absorption band, yielding enhanced light-harvesting ability, which is beneficial to improve the performance of solar cells. Therefore, a PCE of 3.81% has been achieved in optimized device with TPA(DPPT2)2:PC₇₁BM blends by adding 0.1 vol% of DIO, which is among the highest values for small molecules based on both TPA and DPP for organic solar cells.

Experimental section

Materials

All reagents were purchased from Energy Chemical, Suna Tech Inc, Stream, Sigma-Aldrich and used without further purification. Reagent grade solvents used in this study were freshly dried using standard distillation methods.

Characterization

¹H NMR and ¹³C NMR spectra were measured on a Bruker DRX-300 and DRX-400 spectrometer. Elemental analyses of carbon, hydrogen, and nitrogen were performed on a Vario EL III microanalyzer. Mass spectra were measured on Bruker autoflex matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF). The ultraviolet-visible (UV-vis) absorption spectra were recorded on a Shimadzu UV-2500 recording spectrophotometer. Thermogravimetric analysis (TGA) was undertaken with a NETZSCH STA 449C instrument. The thermal stability of the samples under a nitrogen atmosphere was determined by measuring their weight loss while heating at a rate of 20 °C min⁻¹ from 25 to 600 °C. Differential scanning calorimetry (DSC) was performed on a NETZSCH DSC 200 PC unit at a heating rate of 10 °C min⁻¹ from −50 to 300 °C under nitrogen. Cyclic voltammetry (CV) was measured on a CHI660D electrochemical workstation with a solution of 0.1 mol L⁻¹ tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) in dichloromethane, and a Pt plate as the working electrode, a platinum wire as auxiliary electrode, and an Ag wire as pseudo-reference electrode with ferrocenium–ferrocene (Fc⁺/Fc) as the internal standard. Atomic force microscopy (AFM) was conducted on SPA300HV in tapping mode using an SPI3800 controller, Seiko Instruments Industry, Co., Ltd. The organic field effect transistor (OFET) device characteristics were measured using an Agilent 4155B semiconductor parameter analyzer. The mobility was determined in the saturation regime by using the equation I_DS = (μWC_i/2L)(V_G − V_T)², where I_DS is the drain–source current, μ the field-effect mobility, W the channel width, L the channel length, C_i the capacitance per unit area of the gate dielectric layer and V_T the threshold voltage. J–V curves was measured with a computer-controlled Keithley 236 Source Measure Unit. A xenon lamp coupled with AM1.5 solar spectrum filters was used as the light source, and the optical power at the sample was 100 mW cm⁻². The incident photon to converted current efficiency (IPCE) spectrum was measured by Stanford Research Systems model SR830 DSP lock-in amplifier coupled with WDG3 monochromator and 500 W xenon lamp.

OFET fabrication

Devices were fabricated in a top-gate, bottom-contact (TG-BC) configuration on glass substrate (Corning 7059). The source and drain electrodes were defined by photolithography. The concentration of active materials is about 5 mg mL⁻¹ in o-dichlorobenzene (DCB) solvent and the solution was spin-coated at 2000 rpm. Films were annealed in glovebox for 10 min at a temperature of 120 °C. Gold source and drain electrodes were evaporated through a shadow mask with a channel length L = 20 μm and width W = 1 mm.

OSC fabrication

Photovoltaic cells were fabricated with a structure of ITO/PEDOT:PSS/BDT-2DPP:PC₇₁BM/Ca/Al. The patterned indium tin oxide (ITO) glass (sheet resistance = 30 Ω⁻¹) was pre-cleaned in an ultrasonic bath of acetone and isopropanol, and treated in ultraviolet–ozone chamber (Jelight Company, USA) for 30 min. A thin layer (30 nm) of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Baytron PVP AI 4083, Germany) was spin-coated onto the ITO glass and baked at 150 °C for 30 min. Samples were transferred to a N₂-filled glovebox. The chloroform solution of active layers containing donor and acceptor PC₇₁BM (20 mg mL⁻¹) was subsequently spin-coated on PEDOT:PSS layer to form a photosensitive layer. Calcium (ca. 20 nm) and aluminium (ca. 80 nm) layers were subsequently evaporated onto the surface of the photosensitive layer under vacuum (ca. 10⁻⁵ Pa) to form the negative electrode. The active area of the device was 5 mm².

Synthesis

2,5-bis(2-Ethylhexyl)-3-(5′′-hexyl-[2,2′:5′,2′′-terthiophen]-5-yl)-6-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPPT2). A mixture of compound 3-(5-bromo-2-thienyl)-2,5-bis(2-ethylhexyl)-2,5-dihydro-6-(2-thienyl)pyrrolo[3,4-c]pyrrole-1,4-dione (2.00 g, 3.13 mmol), 2-(5′-hexyl-[2,2′-bithiophen]-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1.37 g, 3.64 mmol), and 2 M K₂CO₃ (1.44 g, 10.4 mmol) were dissolved in 50 mL of toluene followed by Pd(PPh₃)₄ (90 mg, 0.08 mmol) and degassed for 30 min. The resulting mixture was then stirred under reflux for 24 h. The mixture was extracted by CH₂Cl₂, washed with brine, and dried over anhydrous MgSO₄. Then the solvent was evaporated under vacuum. The crude product was purified by chromatography (SiO₂; CH₂Cl₂) to yield a purple solid (2.09 g, 83%). ¹H NMR (300 MHz, CDCl₃): δ (ppm) 8.96 (d, J = 4.2 Hz, 1H), 8.89 (d, J = 2.4 Hz, 1H), 7.62 (d, J = 5.4 Hz, 1H), 7.28 (m, 2H), 7.22 (d, J = 3.9 Hz, 1H), 7.05 (t, J = 4.2 Hz, 2H), 6.71 (d, J = 3.6 Hz, 1H), 4.03 (m, 4H), 2.81 (t, J = 7.5 Hz, 2H), 1.92 (m, 2H), 1.32 (m, 24H), 0.90 (m, 15H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm): 161.80, 161.58, 146.48, 142.48, 139.99, 139.72, 139.00, 136.95, 135.10, 134.01, 132.33, 130.22, 129.96, 128.57, 128.42, 127.80, 125.87, 125.04, 124.43, 124.02, 123.84, 108.21, 108.08, 45.93, 39.28, 39.10, 31.56, 31.53, 30.39, 30.23, 29.70, 28.75, 28.57, 28.37, 23.70, 23.57, 23.12, 23.07, 22.57, 14.09, 14.08, 14.02, 10.57, 10.51. MS (MALDI-TOF): calcd for C₄₄H₅₆N₂O₂S₄, 772.32. Found m/z 773.06 (M + H⁺).

3-(5-Bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)-6-(5′′-hexyl-[2,2′:5′,2′′-terthiophen]-5-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (BrDPPT2). N-Bromosuccinimide (NBS, 0.28 g, 1.58 mmol) was added in portions to a solution of DPPT2 (1.00 g, 1.32 mmol) in N,N-dimethylformamide (DMF) (50 mL) in a two-necked flask at 0 °C. The mixture was warmed to room temperature and stirred overnight in the dark. Dichloromethane was added and then the solution was washed with brine. The organic phase was dried (MgSO₄) and the solvent was evaporated under reduced pressure. The crude product was purified chromatographically (SiO₂; hexane–dichloromethane, 1 : 1) to yield a purple solid (0.70 g, 62%). ¹H NMR (300 MHz, CDCl₃): δ (ppm) 8.97 (d, J = 7.54, 2 Hz, 1H), 8.62 (d, J = 7.54, 2 Hz, 1H), 7.28 (m, 1H), 7.21 (m, 2H), 7.04 (t, 3.9 Hz, 2H), 6.71 (t, J = 7.5 Hz, 1H), 4.00 (m, 4H), 2.80 (m, 2H), 1.94 (m, 2H), 1.34 (m, 24H), 0.91 (m, 15H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 161.70, 161.31, 146.54, 143.14, 140.43, 139.15, 138.18, 137.27, 134.90, 133.92, 131.39, 127.67, 125.98, 125.06, 124.46, 124.06, 123.85, 118.38, 108.38, 108.43, 107.91, 46.01, 39.27, 39.14, 31.94, 31.56, 31.54, 30.37, 30.23, 29.71, 29.33, 28.76, 28.56, 28.35, 23.69, 23.60, 23.12, 23.05, 22.70, 22.57, 14.09, 14.03, 10.56, 10.51. MS (MALDI-TOF): calcd for C₄₄H₅₅BrN₂O₂S₄, 852.23. Found m/z 851.42 (M − H⁺).

6,6′-(5,5′-(((4-(sec-Butyl)phenyl)azanediyl)bis(4,1-phenylene))bis(thiophene-5,2-diyl))bis(2,5-bis(2-ethylhexyl)-3-(5′′-hexyl-[2,2′:5′,2′′-terthiophen]-5-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (TPA(DPPT2)2). A mixture of compound BrDPPT2 (151 mg, 0.18 mmol), 4-(sec-butyl)-N,N-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (50 mg, 0.09 mmol), and 2 M K₂CO₃ (36 mg, 2.6 mmol) were dissolved in 15 mL of toluene followed by Pd(PPh₃)₄ (45 mg, 0.04 mmol) and degassed for 30 min. The resulting mixture was then stirred under reflux for 24 h. The mixture was extracted in CH₂Cl₂, washed with brine, and dried over anhydrous MgSO₄, then the solvent was evaporated under vacuum. The crude product was purified by chromatography (SiO₂; CH₂Cl₂) and the product was washed with MeOH to yield a black solid (141 mg, 85%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) 9.01 (d, J = 4 Hz, 2H), 8.94 (d, J = 4 Hz, 2H), 7.57 (d, J = 8.8 Hz, 4H), 7.39 (d, J = 4 Hz, 2H), 7.28 (d, J = 4 Hz, 2H), 7.20 (d, J = 3.6 Hz, 2H), 7.14 (m, 8H), 7.03 (dd, J₁ = 6 Hz, J₂ = 4 Hz, 4H), 6.71 (d, J = 3.6 Hz, 2H), 4.41 (m, 8H), 2.80 (t, J = 3.8 Hz, 4H), 2.62 (m, 1H), 1.95 (m, 4H), 1.33 (m, 50H), 0.90 (m, 36H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 161.66, 161.61, 149.61, 147.86, 146.39, 144.42, 139.76, 139.02, 138.85, 137.24, 136.66, 134.11, 128.28, 128.03, 127.99, 127.29, 126.99, 125.74, 125.03, 124.40, 123.96, 123.81, 123.65, 123.47, 108.34, 108.02, 46.00, 41.17, 39.31, 39.26, 31.57, 31.54, 31.23, 30.41, 30.33, 30.23, 29.71, 29.33, 29.25, 28.77, 28.60, 28.54, 27.22, 23.72, 23.15, 23.13, 22.58, 21.72, 14.12, 14.09, 12.29, 10.61. MS (MALDI-TOF): calcd for C₁₁₀H₁₃₁N₅O₄S₈, 1842.80. Found m/z 1842.68 (M⁺). Anal. calcd for C₁₁₀H₁₃₁N₅O₄S₈, C 71.66; H, 7.16; N, 3.80. Found: C 71.62; H, 7.22; N, 3.72.

Results and discussion

Scheme 1 exhibited the synthetic route for the new compound TPA(DPPT2)2. As shown, DPPT2 was prepared by Pd(0)-catalyzed Suzuki coupling reaction through the starting material of monobromo-substituted DPP and the hexyl-substituted-bithiophene borate with a yield of 85%. After a simple bromination with N-bromosuccinimide (NBS), BrDPPT2 was obtained. Finally, TPA(DPPT2)2 was synthesized through Suzuki cross coupling reaction by BrDPPT2 and 4-(sec-butyl)-N,N-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline in a high yield of 83%. The final product TPA(DPPT2)2 was well soluble in common organic solvents at room temperature, including dichloromethane, chloroform, chlorobenzene and o-dichlorobenzene. And the new compound was fully characterized by ¹H NMR, ¹³C NMR, MS spectra and elemental analysis.

Scheme 1 Synthetic route of TPA(DPPT2)2.

Density functional theory (DFT) calculations were carried out with the Gaussian 09 suite at the B3LYP/6-311G level of theory to investigate ground-state geometries, electronic structures and energies of frontier orbitals for TPA(DPPT2)2. The calculations were simplified by replacing the long alkyl substituents with methyl groups. The optimized molecular structures and frontier orbitals are illustrated in Fig. 1. As shown in the front-view of Fig. 1a, the backbone of TPA(DPPT2)2 displays a wide V-shaped configuration, and the dihedral angles of θ₁ (12–34) and θ₂ (12–32′) are 47.4° and 38.5°, respectively. Compared to the bare TPA (41.3°), the dihedral angles in the new compound exhibited slight variation, due to the introduction of another two DPP arm into the main chain. From top-view, the backbone of TPA(DPPT2)2 exhibits good planarity, ensuring effective intramolecular π-delocalization and intermolecular π–π stacking, which is desirable for charge transport.^35–37 From Fig. 1b, the electron density of the highest occupied molecular orbital (HOMO) is delocalized mainly through the whole molecular skeleton, while lower electron density can be observed on the central phenyl rings and the end thiophene at each side. However, the lowest unoccupied molecular orbital (LUMO) is mainly distributed in the electron-withdrawing DPP arms. Such electronic redistribution would provide an obvious intramolecular charge separation between donor and acceptor after excitation.^38,39 The calculated LUMO and HOMO energy levels are 2.65 and 4.58 eV, respectively, which are in good agreement with other TPA/DPP derivatives.³³

Fig. 1 Optimized geometries for the new compound TPA(DPPT2)2 (a) and frontier molecular orbitals distributions (b).

The thermal properties of V-shaped small molecule TPA(DPPT2)2 was investigated by TGA and DSC. The thermal decomposition temperature with 5% weight loss was observed at 374 °C from TGA curves (Fig. 2a), and the good thermal stability indicated the potential application of the new materials in organic solar cells. According to the DSC curves in Fig. 2b, TPA(DPPT2)2 exhibits glass transition at ca. 173 °C, endothermic melting peak at 191 °C and well-defined exothermic crystallization peak at 120 °C, indicating the crystallization ability of the compound.

Fig. 2 (a) TGA and (b) DSC curves of TPA(DPPT2)2.

Fig. 3 shows the normalized UV-vis absorption spectra of TAP(DPPT2)2 in dilute chloroform solution and thin solid film. TAP(DPPT2)2 exhibits broad absorption with two distinct band of 300–488 and 488–724 nm in solution. The former one is attributed to localized π–π* transitions and the longer wavelength bands originates from intramolecular charger transfer (ICT) from the electron-donating TPA and T2 units to the electron-withdrawing DPP arms.^40,41 Compared in solution, a significant red-shifted and broader ICT bands with two peaks at 361 and 628 nm and an obvious shoulder peak at ∼799 nm was observed at film state, suggesting strong intermolecular aggregation. The optical band gap estimated from the absorption edge of the thin film is 1.61 eV. The low band gap and broad absorption range is expected to be beneficial for light-harvesting as donor material in organic solar cells.

Fig. 3 Optical absorption spectra of TPA(DPPT2)2 in CHCl₃ solution and film state.

The electrochemical properties of TPA(DPPT2)2 were investigated by cyclic voltammetry (CV). As shown in Fig. 4, the compound exhibited both reversible oxidation and reduction behavior. The HOMO and LUMO energy levels calculated from the onset potentials of the oxidation and reduction curves were 4.89 eV and 3.44 eV, respectively. The electrochemical band gap is calculated to be 1.45 eV, which is close to the value measured from optical absorption edge. The HOMO and LUMO energy levels of TPA(DPPT2)2 are matched well with the PC₇₁BM (4.0 and 5.8 eV)⁴² acceptor, indicated it is appropriate for donor material in BHJ solar cells devices.

Fig. 4 Cyclic voltammogram curves of TPA(DPPT2)2.

To ensure efficient charge transport and reduce electron recombination in OSCs, the donor materials must be provided with high hole mobility.⁴³ Owing to the planarity of the V-shaped small molecule TPA(DPPT2)2, efficient charge transport is anticipated. The hole mobility of TPA(DPPT2)2 was measured by bottom-contact/top-gate OFET devices. In a bottom contact geometry using Au as the source and drain electrode, TPA(DPPT2)2 exhibited typical p-type semiconductor behavior. Typical output and transfer curves of the devices are shown in Fig. 5. Through thermal annealing at 120 °C, the best field-effect hole mobility was measured at ∼1.99 × 10⁻⁴ cm² V⁻¹ s⁻¹, which is among the moderate values of small molecular donors for BHJ organic solar cells.⁶

Fig. 5 (a) Typical output and (b) transfer plot (V_DS = −60 V) of organic field-effect transistors (OFETs) annealed at 120 °C from spin-casted TPA(DPPT2)2.

To investigate the performance of the new V-shaped compounds as donor material in BHJ organic solar cells, devices with a typical configuration of ITO/PEDOT:PSS/TPA(DPPT2)2:PC₇₁BM/Ca/Al were fabricated. By spin-coating TPA(DPPT2)2 : PC₇₁BM blends (1 : 1 weight ratio) from chloroform solution with a concentration of 10 mg mL⁻¹, device without any additives showed poor PCE of only 1.39%, with a short-circuit current (J_SC) of 5.01 mA cm⁻², an open-circuit voltage (V_OC) of 0.73 V, and a fill factor (FF) as low as 0.38. The J–V characteristics (a) and IPCE spectra (b) of the solar cell devices are illustrated in Fig. 6 and corresponding device performance parameters are summarized in Table 1. In general, the solvent additive used in active layer of BHJ solar cells could displayed a drastic improvement of J_SC, FF and PCE which is attributed to improved film-morphology and reduced bulk resistance of the active layer resulted in a higher mobility of carriers.^44,45 Hence high boiling point 1,8-diiodooctane (DIO) was used as the additive to optimize the film morphology of the active layer. Obviously, the introduction of low volume percent of DIO resulted in a significant increase of J_SC, FF and PCE, despite slightly loss of V_OC. The performance enhancement might be attributed from efficient percolation channels for charge transport induced by DIO. The best performance was achieved by adding 0.1 vol% of DIO, with PCE, J_SC and FF values of 3.81%, 9.48 mA cm⁻² and 0.63, respectively. By increasing the DIO content to 0.2%, the photovoltaic performance was similar to device with 0.1% of DIO, with a slightly dropped PCE of 3.70% and increased J_SC of 9.88 mA cm⁻². However, further increasing the ratio of DIO to 0.3 (Table S4†), 0.5 and 0.7% (Table S6†) resulted in a decrease of photovoltaic performance, with PCE reduced to 3.56, 3.06, 2.83%, respectively. The external quantum efficiency (EQE) curves of the optimized devices were shown in Fig. 6b, all solar cells devices with DIO exhibit relatively higher EQE (over 40%) and a broad response from 350 to 700 nm. However, the device without DIO shows the lowest value (below 25%), indicating the photo response of the device is not efficient. Compared with those very high efficiency (up to 9% of PCE) small molecular donor materials,^14,15 the moderate photovoltaic performance of the new TPA/DPP containing V-shaped material could be attribute to the relatively low EQE, which might be a sign of low light harvesting and charge collection efficiency in these devices.

Fig. 6 (a) J–V characteristics of the solar cells based on TPA(DPPT2)2 : PC₇₁BM (1 : 1, w/w) at processing condition of without, with 0.1%, 0.2%, 0.5% (v/v) DIO; (b) the EQE spectra of corresponding solar cells.

Table 1 Photovoltaic parameters of solar cells based on TPA(DPPT2)2 : PC₇₁BM (1 : 1, w/w) blends

Devices	VOC (V)	JSC (mA cm^−2)	FF (%)	PCEmax (avg)^a (%)
a Data have been averaged for different devices (see ESI of Table S1–6).
Without DIO	0.73	5.01	38	1.39 (1.31)
0.1%	0.64	9.48	63	3.81 (3.69)
0.2%	0.64	9.88	59	3.70 (3.58)
0.5%	0.64	9.47	51	3.06 (2.88)

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Film morphologies of the V-shaped small molecule: PC₇₁BM blends were examined by tapping-mode atomic force microscopy (AFM). AFM images of blended film were carried out at the same condition as in OSC devices. Fig. 7 showed the surface topography of blended films without and with DIO (0.1, 0.2, and 0.5%), the root mean square roughness (RMS) of blended films were 0.62, 0.63, 1.81 and 1.59 nm respectively. The smooth surface ensures a well contact with the cathode and is conducive to increasing the charge collection efficiency. In addition, from the phase images (Fig. 7 bottom), blended film without DIO exhibited an underdeveloped phase separation and a less clear boundaries, leading to a relatively poor device performance. However, when DIO was used as solvent additive, comparatively obvious phase-separation with enlarged interface areas between the donor/acceptor interfaces were retained, and the obtained interpenetrated network was beneficial for exciton dissociation. Therefore, the devices with DIO showed significant enhancement of J_sc, FF and PCE.

Fig. 7 AFM topography (top) and phase (bottom) images (5 μm × 5 μm) of blend film: TPA(DPPT2)2:PC₇₁BM with (a and e) 0% DIO, (b and f) 0.1% DIO, (c and g) 0.2% DIO, (d and h) 0.5% DIO.

Conclusions

In conclusion, we have designed and synthesized a new V-shaped small molecular donor material TPA(DPPT2)2 by using TPA as the core, DPP as arm and bithiophene as end-groups. The compound exhibited good backbone planarity, with hole mobility of 1.99 × 10⁻⁴ cm² V⁻¹ s⁻¹. Photovoltaic devices based on the small molecular TPA/DPP containing V-shaped donor material exhibited significantly enhanced efficiency by adding DIO solvent additive. The best performance have been achieved at very low DIO ratio of 0.1%, with a PCE of 3.81%, J_SC of 9.48 mA cm⁻², V_OC of 0.64 V, and FF of 0.63. The improved device performance was obtained via introducing remarkably small percentages of solvent additive, which might provide high interfaces for efficient charge separation and transport in the BHJ organic solar cell devices.

Acknowledgements

We thank the National Natural Science Foundation of China (21304047), NSF of Jiangshu Province (BM2012010, BK20130919, 13KJB430017) and Synergetic Innovation Center for Organic Electronics and Information Displays for financial support.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: ¹H NMR, ¹³C NMR and MS spectra of TPA(DPPT2)2 and additional AFM images. See DOI: 10.1039/c5ra07383a

‡ These authors contributed equally to this work.

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