Manipulating the photovoltaic properties of small-molecule donor materials by tailoring end-capped alkylthio substitution

Abstract

The engineering of alkylthio side chains is a smart and effective strategy to improve the photovoltaic properties of donor materials for organic solar cells (OSCs). In order to further exploit the capability of alkylthio chains in tuning the physicochemical and photovoltaic properties of small-molecule donor materials, two small molecules DTS-BTT-S and BDTT-BTT-S with alkylthio end groups were designed and synthesized. The DFT calculation results reveal that introducing alkylthio end groups in DTS-BTT-S and BDTT-BTT-S effectively down-shifted the HOMO energy levels in comparison with their analogous molecules with alkyl end groups. The DTS-BTT-S based OSC device exhibited a higher V_oc of 0.840 V, with a PCE of 3.65% without the need of extra-treatments of the active layer. The optimized device based on BDTT-BTT-S showed a higher PCE of 5.21%, with V_oc = 0.823 V, J_sc = 10.93 mA cm⁻² and FF = 57.2%.

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Introduction

Solution-processed organic solar cells (OSCs) have been regarded as promising next-generation energy sources owing to the unique advantages of light weight, easy fabrication, and the capability to be fabricated into the large flexible devices.^1–6 Typically, bulk heterojunction (BHJ) OSCs possess a sandwiched structure composed of a photoactive blend layer between an anode and a cathode (at least one of them should be transparent).^7–9 During the past two decades, the innovation of photoactive materials (particularly donor materials) mainly dominated the development of the organic photovoltaic field.^10–18 Generally, donor materials are classified as conjugated polymers and small molecules. Compared to their polymeric counterparts, small molecules have attracted considerable attention due to the unique features of defined molecular structure, defined molecular weight and good batch-to-batch reproducibility.^19–24 Much progress of OSCs devices based on small-molecule donors and fullerene acceptors has been made with power conversion efficiency (PCE) of over 9%.^20–24 High efficiency small-molecule donors should be fulfilled the essential requirement of broad and strong absorption spectra to match the solar spectrum for increasing short-circuit current density (J_sc), high hole mobility for increasing J_sc and fill factor (FF) values, and low-lying HOMO (highest occupied molecular orbital) energy level to maximize open-circuit voltage (V_oc). Since π-conjugated backbones determine the optoelectronic properties of the resulting molecules, rational construction of molecular frameworks is a promising strategy to design small molecules with tailored optical, electronic and physical properties for high efficiency donor materials. Much progress has been achieved by employing various well known electron-rich and electron-deficient units to address this issue.^{4,15,22,25–27} Furthermore, properly selecting the side chains is critical not only in improving the solubility of donor materials for solution-processed device fabrication, but also in charge carrier mobilities of the devices and thus the OSC device performance.^28–31 Recently, we strategically employed alkylthio side chains on the thiophene conjugated side chains of the two-dimension (2D)-conjugated copolymer.^32–34 Owing to the fact that the sulfur atom functions as a moderate π-acceptor group at the first position of the conjugated bridge,^35,36 the introduction of alkylthio side chains effectively down-shifted the HOMO energy level, slightly red-shifted the absorption, and enhanced the hole mobility of the corresponding copolymers, leading to an overall improvement of photovoltaic performance.^32,33 By consistently introducing alkylthio side chain into the A–D–A (A = acceptor, D = donor) structured small molecule (namely BDTT-S-TR), lower-lying HOMO energy level and higher hole mobility of BDTT-S-TR (compared to the alkyl side chain-based corresponding molecule) were observed.²³ In particular, the device based on BDTT-S-TR demonstrated a high PCE of 9.20% without any extra treatment, indicating that the engineering of alkylthio side chain is in favor of the appropriate aggregation and formation of ideal nanoscaled D/A interpenetrating network morphology of the blend film without the need of extra treatment.²³ The results mentioned above reveal that the engineering of alkylthio side chain can be considered as a smart and effective strategy to improve the photovoltaic properties of either polymer or small molecule donor materials.

To investigate the effect of alkylthio end groups on the physicochemical properties and thus further exploit the potential of alkylthio side chains in tuning the photovoltaic properties of small-molecule donor materials, in this work, we design and synthesize two small molecules with D2–A–D1–A–D2 molecular frameworks and alkylthio end-capped bithiophene as D2 unit (namely DTS-BTT-S and BDTT-BTT-S, as shown in Scheme 1). As expected, the theoretical calculation and molecular simulation with density functional theory (DFT) results reveal that the two molecules exhibited lower-lying HOMO energy levels than that of the terminated alkyl chains counterparts, indicating that the engineering of alkylthio chains in small molecules as end groups also effectively down-shifted the HOMO energy levels.

Scheme 1 Synthetic routes and chemical structure of DTS-BTT-S and BDTT-BTT-S.

Experimental section

Synthesis of materials

4,4-Bis(2-ethylhexyl)-2,6-bis(trimethylstannyl)-4H-silolo[3,2-b:4,5-b′]dithiophene (2)³⁷ and (4,8-bis(5-((2-ethylhexyl)thio)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(trimethylstannane)³² were synthesized by the literature methods.

4-Bromo-5-fluoro-7-(5′-(hexylthio)-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole (1). Compound 2 was prepared by methods similar to those reported in the literature.²⁶

¹H NMR (400 MHz, CDCl₃): δ (ppm): 8.03–8.02 (d, J = 4 Hz, 1H), 7.69–7.67 (d, J = 8 Hz, 1H), 7.22–7.21 (d, J = 4 Hz, 1H), 7.16–7.15 (d, J = 4 Hz, 1H), 7.04–7.03 (d, J = 4 Hz, 1H), 2.87–2.83 (t, J = 8 Hz, 2H), 1.69–1.30 (m, 8H), 0.91–0.87 (t, 3H). ¹³C NMR (100 MHz, CDCl₃), δ (ppm): 164.52, 161.19, 156.36, 150.98, 142.23, 141.51, 138.00, 135.97, 131.93, 129.27, 129.14, 128.74, 117.94, 98.47, 41.07, 33.54, 31.60, 30.35, 24.75, 16.24.

7,7′-(4,4-Bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-(hexylthio)-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (DTS-BTT-S). In a 100 mL flask, compound 1 (0.764 g, 1.49 mmol) and 2 (0.5 g, 0.676 mmol) were dissolved in 40 mL of toluene with 85 mg [Pd(PPh₃)₄] as the catalyst. The solution was heated to reflux and stirred for 48 h under argon protection. Then, the reactant was poured into water (100 mL) and extracted with chloroform (50 mL × 3). The combined organic layer was washed with water three times and then dried over MgSO₄. After removal of solvent, the crude product was purified by column chromatography on a silica gel using petroleum/chloroform (1 : 2) as eluent to obtain DTS-BTT-S (390 mg, yield 45%) as a metallic purple solid.

¹H NMR (400 MHz, CDCl₃): δ (ppm): 8.32–8.25 (t, 2H), 7.92–7.89 (d, J = 12 Hz, 2H), 7.62–7.59 (d, J = 12 Hz, 2H), 7.15–7.09 (m, 4H), 7.00–6.99 (d, J = 4 Hz 2H), 2.86–2.82 (t, J = 8 Hz, 4H), 1.68–1.59 (m, 4H), 1.43–1.09 (m, 34H), 0.90–0.84 (m, 18H). MALDI-TOF MS: calcd for C₆₄H₇₂F₂N₄S₁₀Si m/z = 1284.02; found 1282.31.

7,7′-(4,8-Bis(5-((2-ethylhexyl)thio)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-(hexylthio)-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (BDTT-BTT-S). BDTT-BTT-S (yield 53%) was synthesized with similar method as described above for DTS-BTT-S except that compound 3 was used instead of compound 2.

¹H NMR (400 MHz, CDCl₃): δ (ppm): 8.51 (s, 2H), 7.73–7.71 (d, J = 8 Hz, 2H), 7.55–7.54 (d, J = 4 Hz, 2H), 7.49–7.47 (d, J = 8 Hz, 2H), 7.01–7.00 (d, J = 4 Hz, 2H), 6.87–6.85 (d, J = 8 Hz, 2H), 6.78–6.76 (d, J = 8 Hz, 2H), 6.69–6.68 (d, J = 4 Hz, 2H), 2.99–2.92 (t, 4H), 2.58–2.42 (m, 4H), 1.87–1.06 (m, 34H), 0.91–0.85 (m, 18H). MALDI-TOF MS: calcd for C₇₄H₇₆F₂N₄S₁₄ m/z = 1056.21; found 1056.39.

Material characterizations

NMR spectra were measured in CDCl₃ on Bruker AV 400 MHz FT-NMR spectrometer and chemical shifts are quoted relative to tetramethylsilane for ¹H and ¹³C nuclei. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer TGA-7. UV-Vis absorption spectra were obtained on an Agilent Technologies Cary Series UV-Vis-NIR Spectrophotometer. Cyclic voltammetry was performed on a Zahner IM6e electrochemical workstation with a three-electrode system in a solution of 0.1 M [Bu₄N]PF₆ acetonitrile solution at a scan rate of 100 mV s⁻¹. Glassy carbon disc coated with sample film was used as the working electrode. A Pt wire was used as the counter electrode and Ag/Ag⁺ was used as the reference electrode. Ferrocene/ferrocenium redox couple (Fc/Fc⁺) was used as the external standard and its redox potential is 0.06 V vs. Ag/Ag⁺. The HOMO and LUMO (lowest unoccupied molecular orbital) energy levels were calculated from the onset oxidation potential (φ_ox) and the onset reduction potential (φ_ox) of the molecules using the following equations: HOMO = −e(φ_ox + 4.74) (eV); LUMO = −e(φ_red + 4.74) (eV).

Device fabrication and characterization

The BHJ OSCs were fabricated with a configuration of ITO coated glass/PEDOT:PSS/active layer/Ca/Al. The ITO glass was pre-cleaned and modified with a thin layer of PEDOT:PSS which was spin-cast from a PEDOT:PSS aqueous solution (Clevios P VP AI 4083 H. C. Stark, Germany) on the ITO substrate, and the thickness of the PEDOT:PSS layer is about 30 nm. The D/A materials were dissolved at a total concentration of 3.5% (35 mg mL⁻¹) in chlorobenzene and spin-coated on the ITO/PEDOT:PSS electrode. Finally, the Ca/Al cathode was deposited on the active layer by vacuum evaporation at 1 × 10⁻⁴ Pa. The effective area of each device is about 4 mm². The current density–voltage (J–V) measurement of the devices was conducted on a computer-controlled Keithley 236 Source Measure Unit in a dry box under an inert atmosphere. A xenon lamp with an AM 1.5 filter was used as the white light source, and the optical power at the sample was 100 mW cm⁻². The external quantum efficiency (EQE) was measured by a solar cell spectral response measurement system QE-R3011 (Enli Technology Co., Ltd.). The light intensity at each wavelength was calibrated with a standard single-crystal Si photovoltaic cell. All the measurements were performed under an ambient atmosphere at room temperature.

Mobility measurement

The hole and electron mobilities were calculated by using the space-charge-limited current (SCLC) method.³⁸
where ε is the dielectric constant of the polymers, ε₀ is the permittivity of the vacuum, μ₀ is the zero-field mobility, E₀ is the characteristic field, J is the current density, and L is the thickness of the film. The J–V data for the mobility measurements were obtained from a hole only device for the hole mobility measurement and from an electron-only device for the electron mobility measurement.

Results and discussion

Physicochemical properties

Both DTS-BTT-S and BDTT-BTT-S possess good solubility in common organic solvents, such as chloroform and chlorobenzene. Thermogravimetric analysis (TGA) results suggest that the two molecules are stable enough for the application as donor materials in OSCs with the decomposition temperature (T_d) at 5% weight-loss at approximately 371 °C and 344 °C, respectively (as shown in Fig. 1).

Fig. 1 TGA plots of DTS-BTT-S and BDTT-BTT-S with a heating rate of 10 °C min⁻¹ under a N₂ atmosphere.

Fig. 2 shows the absorption spectra of the two molecules in chlorobenzene solution and in thin film. The DTS-BTT-S solution exhibits an absorption maximum peak at 589 nm with an extinction coefficient (ε) of 7.72 × 10⁴ M⁻¹ cm⁻¹. The absorption band of DTS-BTT-S film is red-shifted significantly with its absorption edge extending from 672 nm in solution to 814 nm, corresponding to a bandgap (E_g) of 1.52 eV. The absorption maximum peak of BDTT-BTT-S in solution is 555 nm, with an extinction coefficient of 6.74 × 10⁴. The absorption edge of BDTT-BTT-S film is red-shifted to 741 nm, corresponding to a bandgap of 1.67 eV. The thin film absorption spectra of DTS-BTT-S and BDTT-BTT-S both exhibit vibronic structure, suggesting the existence of strong π–π stacking between the molecule backbones.

Fig. 2 Normalized UV-vis absorption spectra of DTS-BTT-S and BDTT-BTT-S in chlorobenzene solution and thin film on quartz.

The electronic energy levels of the two molecules were determined by cyclic voltammetry (CV) (Fig. 3). The onset oxidation potentials (φ_ox) of DTS-BTT-S and BDTT-BTT-S are 0.49 V and 0.57 V vs. Ag/Ag⁺, respectively, while the onset reduction potentials (φ_red) are −1.24 V and −1.25 V vs. Ag/Ag⁺, respectively. Thus, the HOMO and LUMO energy level values are −5.23 eV and −3.50 eV for DTS-BTT-S, and −5.31 eV and −3.49 eV for BDTT-BTT-S, respectively, as listed in Table 1. The lower HOMO energy of DTS-BTT-S (−5.23 eV) than that of the alkyl end-capped p-DTS(FBTTh₂)₂ (−5.12 eV)²⁶ indicates that the introduction of alkylthio end groups effectively down-shifted the HOMO energy levels in comparison with the analogous molecules with alkyl end groups.

Fig. 3 Cyclic voltammograms of DTS-BTT-S and BDTT-BTT-S films on a glassy carbon electrode in 0.1 mol L⁻¹ Bu₄NPF₆ acetonitrile solution at a scan rate of 100 mV s⁻¹.

Table 1 Absorption spectral properties and electronic energy levels of the organic molecule materials

Organic molecule materials	λmax (nm)			E^optg [eV]	HOMO [eV]	LUMO [eV]	E^cvg [eV]
	Solution	Film
DTS-BTT-S	589	626	683	1.52	−5.23	−3.50	1.73
BDTT-BTT-S	555	591	650	1.67	−5.31	−3.49	1.82

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Theoretical calculation was performed by density functional theory (DFT) at the B3LYP/6-31G (d, p) level to provide further insight into the fundamental aspects of molecular architecture of the two molecules. In particular, the HOMO and LUMO levels and related electron distributions of the two molecules and their corresponding analogous molecules with alkyl end groups were calculated (Fig. 4). The DFT results reveal that the DTS-BTT-S and BDTT-BTT-S with alkylthio chains as end-groups possess lower HOMO energy levels (lower by 0.12 eV, respectively) than their analogous molecules with alkyl end-groups. In our previous studies, we have demonstrated that the sulfur atom has some π-acceptor capability which can down-shift the HOMO levels when engineering alkylthio chains in conjugated side chains of the polymers.^32,33 Benefited from the unique π-accepting properties of the sulfur atom in alkylthio chains, in this case, the introduction of alkylthio chains as end-groups in small molecules also effectively down-shifted the HOMO energy levels in comparison with their analogous molecule with alkyl end-groups. Obviously, the low-lying HOMO energy level can potentially guarantee a higher V_oc of the OSCs with the molecules as donor, since the V_oc value of the device is proportional to the energy offset between the LUMO of the acceptor and the HOMO of the donor.³⁹

Fig. 4 LUMO and HOMO energy levels and molecular geometry of the small molecules calculated by DFT/B3LYP/6-31G (d, p) with methyl groups in replacing alkyl substituents to simplify the calculations.

Photovoltaic performance

A traditional OSC architecture of glass/ITO/PEDOT:PSS/active layer/Ca/Al was fabricated to evaluate the photovoltaic performances of DTS-BTT-S and BDTT-BTT-S. Fullerene derivative PC₇₀BM was employed as acceptor (A) material to blend with DTS-BTT-S or BDTT-BTT-S as donor (D) in chlorobenzene. The fabrication conditions were optimized by varying D/A weight ratio, active layer thickness, and processing additives. The optimal thickness of the active layer is 80–90 nm. The optimal D/A ratio is 4 : 6 (w/w) for DTS-BTT-S : PC₇₀BM and 5 : 4 (w/w) for BDTT-BTT-S : PC₇₀BM based devices.

Table 2 lists the main photovoltaic performance parameters of the OSCs based on DTS-BTT-S and BDTT-BTT-S under the illumination of AM 1.5 G at 100 mW cm⁻². Fig. 5 show current density–voltage (J–V) curves of the OSCs based on DTS-BTT-S : PC₇₀BM (4 : 6, w/w) and BDTT-BTT-S : PC₇₀BM (5 : 4, w/w).

Table 2 Photovoltaic properties of the OSCs based on DTS-BTT-S : PC₇₀BM (4 : 6, w/w) and BDTT-BTT-S : PC₇₀BM (5 : 4, w/w) under the illumination of AM 1.5 G at 100 mW cm⁻²

Active layer	DIO [%]	Voc [V]	Jsc [mA cm^−2]	FF [%]	PCE^b [%]
a The active layer was processed with methanol.b The values in square brackets are the average PCE obtained from 12 devices.
DTS-BTT-S : PC70BM = 4 : 6	0	0.840	9.92	43.7	3.65 [3.59]
DTS-BTT-S : PC70BM = 4 : 6^a	0	0.805	9.12	43.7	3.21 [3.19]
DTS-BTT-S : PC70BM = 4 : 6	0.3	0.809	9.40	39.6	3.01 [2.93]
DTS-BTT-S : PC70BM = 4 : 6^a	0.3	0.777	7.85	42.5	2.59 [2.48]
DTS-BTT-S : PC70BM = 4 : 6	0.4	0.795	9.00	39.0	2.79 [2.75]
DTS-BTT-S : PC70BM = 4 : 6^a	0.4	0.762	8.42	42.1	2.70 [2.56]
BDTT-BTT-S : PC70BM = 5 : 4	0	0.866	4.07	62.4	2.20 [2.16]
BDTT-BTT-S : PC70BM = 5 : 4^a	0	0.822	3.72	60.1	1.84 [1.58]
BDTT-BTT-S : PC70BM = 5 : 4	1	0.825	3.25	49.4	1.33 [1.25]
BDTT-BTT-S : PC70BM = 5 : 4^a	1	0.795	2.35	42.2	0.79 [0.76]
BDTT-BTT-S : PC70BM = 5 : 4	2	0.849	3.99	54.4	1.85 [1.83]
BDTT-BTT-S : PC70BM = 5 : 4^a	2	0.812	9.04	58.5	4.30 [4.10]
BDTT-BTT-S : PC70BM = 5 : 4	3	0.860	4.84	56.4	2.34 [2.28]
BDTT-BTT-S : PC70BM = 5 : 4^a	3	0.823	10.93	57.2	5.14 [4.97]
BDTT-BTT-S : PC70BM = 5 : 4	4	0.858	4.86	55.9	2.33 [2.31]
BDTT-BTT-S : PC70BM = 5 : 4^a	4	0.801	9.32	59.4	4.44 [4.38]

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Fig. 5 J–V curves of the OSCs based on (a) DTS-BTT-S (4 : 6, w/w) and (b) BDTT-BTT-S (5 : 4, w/w) under the illumination of AM 1.5 G at 100 mW cm⁻².

Attributing to the low-lying HOMO energy level, the device based on DTS-BTT-S : PC₇₀BM (4 : 6, w/w) exhibited a higher V_oc of 0.840 V, with J_sc = 9.92 mA cm⁻² and FF = 43.7%, leading to a PCE of 3.65% without the need of extra-treatments of the active layer. It is worth to point out that the device based on the analogue of DTS-BTT-S (namely p-DTS(FBTTh₂)₂) only showed a PCE of 1.8%, with a lower V_oc = 0.780 V, J_sc = 6.6 mA cm⁻² and FF = 0.36 without any extra-treatments of the active layer.²⁶ Considering the fact that the device performance could be potentially enhanced by treatment with methanol in some cases, we used methanol to act as solvent soaking treatment following spin-coating the active layer. The devices exhibited lower V_oc = 0.805 V and J_sc = 9.12 mA cm⁻², with a lower PCE of 3.21%. By using small quantities (0.3% and 0.4%) of diiodooctane (DIO) as a solvent additive during the film-casting step, lower V_oc (0.809 and 0.795 V, respectively) and FF values (39.6% and 39%, respectively) were obtained, resulting in the lower PCE of 3.01% and 2.79%, respectively. Much lower V_oc and J_sc were measured after the dual treatments of DIO (0.3% and 0.4% vol) and methanol soaking, leading to an overall decrease of PCEs of the devices. The results indicate that the optimized morphology of the DTS-BTT-S : PC₇₀BM (4 : 6, w/w) blend film is realized without the need of the DIO and methanol soaking treatments. The device based on BDTT-BTT-S : PC₇₀BM (5 : 4, w/w) showed a high V_oc of 0.866 V, with a PCE of 2.20% without any extra-treatment. After treatment of methanol soaking, poorer photovoltaic performance with PCE of 1.84% was obtained. The devices that the active layer was processed with various amount of DIO as solvent additive have been constructed and compared (as shown in Table 2), and the optimized processed condition is using of 3% DIO additive, which gives a slightly higher PCE of 2.34%. Further optimization by employing methanol as solvent soaking treatment, the device showed the highest PCE of 5.21%, with V_oc = 0.823 V, J_sc = 10.93 mA cm⁻² and FF = 57.2%.

Fig. 6 shows the external quantum efficiency (EQE) of the optimal devices based on DTS-BTT-S : PC₇₀BM (4 : 6, w/w) and BDTT-BTT-S : PC₇₀BM (5 : 4, w/w). The EQE curve of the DTS-BTT-S based device covers a wavelength range from 300 to 770 nm, with the highest EQE value of 51% at 609 nm. The BDTT-BTT-S based device showed the highest EQE value of 59% at 511 nm. The J_sc values of the devices integrated from the EQE curves are rather consistent (less than 4% mismatch) with the values obtained from J–V measurements.

Fig. 6 EQE curves of the best OSCs based on DTS-BTT-S : PC₇₀BM (4 : 6, w/w) and BDTT-BTT-S : PC₇₀BM (5 : 4, w/w).

Charge carrier mobilities of the active layers

High charge carrier mobility and balanced charge-carrier transport are critical for high performance photovoltaic materials in OSCs. To investigate the charge-carrier mobilities of the blend films, the hole-only device with a structure of ITO/PEDOT:PSS/DTS-BTT-S : PC₇₀BM (4 : 6, w/w) or BDTT-BTT-S : PC₇₀BM (5 : 4, w/w)/MoO₃/Au was fabricated for hole mobility measurements of the blend film, and the electron-only device with a device structure of ITO/ZnO/DTS-BTT-S : PC₇₀BM (4 : 6, w/w) or BDTT-BTT-S : PC₇₀BM (5 : 4, w/w)/Al was fabricated for electron mobility measurement by space charge limited current (SCLC) method. As shown in Fig. 7, the DTS-BTT-S : PC₇₀BM (4 : 6, w/w) blend film exhibited a hole mobility of 2.08 × 10⁻⁴ cm² V⁻¹ s⁻¹ and an electron mobility of 3.59 × 10⁻⁴ cm² V⁻¹ s⁻¹, while the hole and electron mobilities of the BDTT-BTT-S : PC₇₀BM (5 : 4, w/w) blend film are 9.18 × 10⁻⁴ and 1.17 × 10⁻³ cm² V⁻¹ s⁻¹, respectively. The relatively high and hole and electron mobilities of the BDTT-BTT-S : PC₇₀BM film results in the high FF values of the corresponding OSCs obtained.

Fig. 7 J^0.5 vs. (V_app − V_bi) plots of (a) hole only devices based on DTS-BTT-S : PC₇₀BM (4 : 6, w/w) and BDTT-BTT-S : PC₇₀BM (5 : 4, w/w), and (b) electron only devices based on DTS-BTT-S : PC₇₀BM (4 : 6, w/w) and BDTT-BTT-S : PC₇₀BM (5 : 4, w/w).

Morphology characterization

X-ray diffraction (XRD) was employed to obtain deep insight of the crystallinity of the two molecule films. As shown in Fig. 8a, the DTS-BTT-S and BDTT-BTT-S films both exhibit high crystallinity with strong (100) diffraction peaks at 2θ = 4.98° and 2θ = 4.94°, corresponding to a d₁₀₀-spacing of 17.72 Å and 17.86 Å, respectively. Clear diffraction (100 and 010) peaks also can be observed in the DTS-BTT-S : PC₇₀BM (4 : 6, w/w) blend film at 2θ = 4.68° and 2θ = 19.23° (as shown in Fig. 8b). The BDTT-BTT-S : PC₇₀BM (5 : 4, w/w) blend film exhibited promising strong (100) diffraction peak at 2θ = 4.81°, with clear (200 and 300) diffraction peaks at 2θ = 8.19° and 2θ = 12.55°, respectively, indicating that the lamellar structures of conjugated molecule still maintained even when blended with PC₇₀BM. The ordered structure in the active layers should be in favor of the charge transportation and the better photovoltaic performance of BDTT-BTT-S.

Fig. 8 XRD patterns of (a) pure DTS-BTT-S and BDTT-BTT-S films, and (b) the blend films of DTS-BTT-S : PC₇₀BM (4 : 6, w/w) and BDTT-BTT-S : PC₇₀BM (5 : 4, w/w).

Finally, the optimized surface morphologies of DTS-BTT-S : PC₇₀BM (4 : 6, w/w) and BDTT-BTT-S : PC₇₀BM (5 : 4, w/w) were investigated by tapping mode atomic force microscopy (AFM). The BDTT-BTT-S : PC₇₀BM blend film was processed with 3% DIO as a solvent additive and methanol solvent soaking treatment. AFM spectroscopy (Fig. 9) shows rather flat and uniform surfaces for DTS-BTT-S : PC₇₀BM and BDTT-BTT-S : PC₇₀BM films, with a root-mean-square (RMS) surface of 0.446 and 1.28 nm, respectively.

Fig. 9 AFM topography images of the blend films of (a) DTS-BTT-S:PC₇₀BM and (d) BDTT-BTT-S:PC₇₀BM; the corresponding phase images of the blend films of (b) DTS-BTT-S:PC₇₀BM and (e) BDTT-BTT-S:PC₇₀BM; 3D topography images of the blend films of (c) DTS-BTT-S:PC₇₀BM and (f) BDTT-BTT-S:PC₇₀BM. The size of the images is 5 μm × 5 μm.

The phase separation and D/A interpenetrating network in active layer determine the charge separation and transportation in bulk heterojunction OSCs. Transmission electron microscopy (TEM) was carried out to study the bulk morphology of the optimal active layer of DTS-BTT-S : PC₇₀BM (4 : 6, w/w) and BDTT-BTT-S : PC₇₀BM (5 : 4, w/w). As shown in Fig. 10, compared with the blend films of DTS-BTT-S:PC₇₀BM, the blend films of BDTT-BTT-S:PC₇₀BM gave a better bicontinuous D/A interpenetrating network, which should favor high efficiency exciton dissociation and charge transportation, and thus higher FF and J_sc values can be achieved.⁴⁰

Fig. 10 TEM images of the blend films of: (a) DTS-BTT-S : PC₇₀BM (4 : 6, w/w), and (b) BDTT-BTT-S : PC₇₀BM (5 : 4, w/w) processed with 3% DIO additive and methanol soaking treatment.

Conclusion

In summary, two small molecules DTS-BTT-S and BDTT-BTT-S with alkylthio end-groups were designed and synthesized. The absorption profile of the molecules is red-shifted to some content by tailoring alkylthio chains as end-capped substitution. Very importantly, the DFT results reveal that the engineering of alkylthio chains in DTS-BTT-S and BDTT-BTT-S as end-groups effectively down-shifted the HOMO energy levels in comparison with their analogous molecule with alkyl end-groups, respectively, which guarantee the high V_oc of the OSC devices. The DTS-BTT-S based device exhibited a higher V_oc of 0.840 V, with a PCE of 3.65% without the need of extra-treatments of the active layer. A higher PCE of 5.21% was obtained from the BDTT-BTT-S based device, with a V_oc of 0.823 V by processing with 3% DIO as a solvent additive and methanol solvent soaking treatment. The results further exploit the capability of alkylthio chains in tuning the physicochemical and photovoltaic properties of small-molecule donor materials.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (91333204, 91433117, and 51603136), Jiangsu Provincial Natural Science Foundation (Grant no. BK20150327), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant no. 15KJB430028), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Project Funded by China Postdoctoral Science Foundation (Grant no. 2015M581855).

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