Facile preparation of small molecules for bulk heterojunction solar cells

Abstract

Two solution-processable diketopyrrolopyrrole (DPP)–pyrene–DPP small molecules PYDPP-EH and PYDPP-BO with 2-ethylhexyl (EH) and 2-butyloctyl (BO) side chains at DPPs, respectively, are synthesized for organic photovoltaic devices with only two step reactions from commercially available materials without using toxic stannyl intermediates and potentially dangerous lithiation reactions. Compared with PYDPP-BO or the pyrene–DPP–pyrene type SMs with very long N-alkyl substitutes to ensure their solubility reported by other groups, the shorter but more 2-ethylhexyl groups endow PYDPP-EH with not only enough solubility to be solution-processed but also stronger π–π interactions, more ordered self-aggregation as well as better intermolecular charge transport in films, which induce the blend films with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) with higher hole mobility and the solar cells with more efficient photocurrent response and a higher power conversion efficiency of 4.64%. This study manifests that subtly changing molecular structure with shorter but more alkyl chains can enhance photovoltaic performance.

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Introduction

The past decades have witnessed the increasing interest in bulk heterojunction (BHJ) organic solar cells (OSCs) as a promising technology, which can potentially solve the growing global energy and environmental issues due to their cost-effective, solution-processable, lightweight and flexible features.^1–6 To date, the power conversion efficiencies (PCEs) of small molecule (SM) and polymer solar cells have been improved up to 10% with the aid of the synergistic optimizations of chemical structure design and device engineering.^7–10 Recently, SM OSCs have been the subject of intense study because of the advantages of SMs including well-defined chemical structure and weight, easy purification and high reproducibility almost with no batch to batch variation in comparison with their polymer counterparts,^3,5 which makes SM OSCs more potential in commercial applications. However, it should be noted that many step reactions are usually involved to prepare the high performance donors, sometimes even using toxic stannyl intermediates or potentially dangerous lithiation reactions, which goes against the low cost and the green technology of organic photovoltaics. Therefore, the reaction as less and safe as possible should be employed to prepare high performance donor materials.

Over the past decades, designing electron donor–acceptor (D–A) molecular structures is the most effective strategy to obtain donor materials for high-efficiency OSCs.^3,4,11–13 Also, the type, position and length of alkyl side chains play important roles in determining their solubility and finely tuning the properties including the intermolecular stacking, film morphology and charge transport and recombination related to the donor materials.^7,14–16 Equally important with molecule design, device engineering, such as ternary blending,^17–20 tandem OSCs,^21,22 interfacial modification,^10,23–25 etc., has recently attracted tremendous attentions in academics in order to improve the photovoltaic performance significantly. And SMs with high absorption coefficient and mobility have great potential in obtaining high performance ternary and tandem solar cells. Among the often used acceptor units, diketopyrrolopyrrole (DPP) is very promising due to its rigid and planar structure, desirable optical properties, sufficient solubility, good photochemical stability and facile synthesis.^15,26–36 For examples, a ground-breaking achievement was obtained by Nguyen using the benzofuran-substituted DPP as donor material that showed 4.4% PCE with phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) in BHJ devices.²⁷ Yao and Zhan et al. almost simultaneously synthesized a linear D–A SMs based on 5-alkylthiophene-2-yl-substitued benzo[1,2-b:4,5-b′]dithiopphene (BDT) as core and DPP units as arms, revealing PCEs as high as 5.29% and 5.79%, respectively.^33,34 We also designed and synthesized a serial of porphyrin small molecules with DPP as the acceptor unit, leading to PCEs from 4.78% to 9.06%.^15,26,35,36 On the other hand, pyrene, which is not only commercially available but also cheap, is a frequently applied electron-rich unit for organic electronic devices. And pyrene and DPP were chosen as the donor and acceptor units, respectively, by Lee first to prepare pyrene–DPP–pyrene SMs, and the PCEs were dramatically enhanced from 0.7% to 4.1% by changing the site of pyrene substitution from C1 to C2 due to the more planar conformation of C2-pyrene based SMs.²⁹ We noted that the N-alkyl substitutes in these SMs should be longer than 2-hexyldecyl to ensure the SMs to be solution-processed since there is no alkyl substituent at pyrene unit and there are only two N-alkyl substitutes. However, it is well-known that the materials with too long alkyl chains are not favourable for forming ordered self-aggregation, leading to poor performance for the OSCs. Therefore, the molecules with shorter alkyl chains usually show high possibility to achieve high performance if the materials show enough solubility to be solution-processed. Taking these considerations into account, in this study we design two DPP–pyrene–DPP type SMs at C2 positions of pyrene since two DPP units can provide four N-alkyl substituents and therefore shorter alkyl chains can be employed to ensure the material's solubility, and two target materials PYDPP-EH and PYDPP-BO with 2-ethylhexyl and 2-butyloctyl, respectively, are synthesized as shown in Scheme 1 with only two step reactions without using toxic stannyl intermediates or potentially dangerous lithiation reactions. And the BHJ OSCs based on PYDPP-EH and PYDPP-BO show PCEs of 4.64% and 2.86%, respectively.

Scheme 1 The synthetic routes of PYDPP-EH and PYDPP-BO.

Experimental

Materials

All chemical reagents, unless otherwise stated, were purchased from Sigma-Aldrich, Acros, and TCI Chemical Co., Ltd., and used without further purification. Tetrahydrofuran (THF) and toluene were distilled from sodium-benzophenone under nitrogen, and dichloromethane (CH₂Cl₂) and chloroform (CHCl₃) were distilled from CaH₂ prior to being used. All reactions involving air-sensitive reagents were carried out in nitrogen (N₂) or argon (Ar) atmosphere. 3-(5-Bromo-thiophen-2-yl)-2,5-bis-(2-ethylhexyl)-6-thiophene-2-yl-2,5-dihydro-pyrrolo[3,4-c] pyrrole-1,4-dione (Br-DPP-EH, 2a) and 3-(5-bromo-thiophen-2-yl)-2,5-bis-(2-butyloctyl)-6-thiophene-2-yl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione (Br-DPP-BO, 2b) were purchased from Suna Tech Inc. in China.

Synthesis

2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyrene (1). A 50 mL two-necked round-bottom flask was charged with pyrene (233.0 mg, 1.15 mmol), bis(pinacolato)diboron (644.5 mg, 2.54 mmol), and cyclohexane (7 mL) and the mixture was deoxygenated with Ar for 30 min before di-μ-methoxobis(1,5-cyclooctadiene)diiridium(I) ([Ir(OMe)COD]₂) (38.1 mg, 0.06 mmol) and 4-4′-di-tert-butyl-2-2′-bipyridine (dtbpy) (30.8 mg, 0.12 mmol) were added. Then the mixture was stirred at 80 °C for 16 h under the protection of Ar. After cooled to room temperature, the reaction was quenched by water. And then the mixture was extracted by CH₂Cl₂ and washed by water, dried with anhydrous sodium sulfate (Na₂SO₄) and concentrated. Finally, the residue was recrystallization from CH₂Cl₂/acetonitrile solvent, yielding a white solid (402.0 mg, 77%). ¹H NMR (500 MHz, chloroform-d (CDCl₃)): δ (ppm) 8.61 (s, 4H), 8.08 (s, 4H), 1.46 (s, 24H) (Fig. S1, ESI†).

Synthesis of PYDPP-EH (3a). A 50 mL two-necked round-bottom flask was charged with 1 (81.0 mg, 0.178 mmol), 2a (323.3 mg, 0.535 mmol), toluene (20 mL), anhydrous potassium carbonate (2 M in H₂O, 0.35 mL, 0.712 mmol) and ethanol (0.35 mL). Then the mixture was deoxygenated with Ar with 30 min before Pd(PPh₃)₄ (10.3 mg, 0.0089 mmol) was added. The mixture was stirred at 90 °C for 72 h under the protection of Ar. After cooled to room temperature, the reaction was quenched by water and the reactant was extracted with CHCl₃, and then washed with water, dried with anhydrous sodium sulfate (Na₂SO₄) and concentrated. Finally, recrystallization from CHCl₃/THF solvent was used to afford 3a as a dark red solid (180 mg, 81%). ¹H NMR (500 MHz, CDCl₃): δ (ppm) 9.02 (d, 2H), 8.92 (d, 2H), 8.38 (s, 4H), 8.11 (s, 4H), 7.73 (d, 2H), 7.62 (d, 2H), 7.28 (s, 2H), 4.10 (s, 4H), 4.06 (d, 4H), 2.00 (m, 2H), 1.90 (m, 2H), 1.53–1.18 (m, 32H), 1.02–0.80 (m, 24H) (Fig. S2, ESI†). Mass (MALDI-TOF): Obs. 1246.53 (Fig. S3, ESI†); calcd for C₇₆H₈₆N₄O₄S₄, 1246.55.

Synthesis of PYDPP-EH (3b). Reaction conditions and workup were the same as that for 3a, except 1 (34.5 mg, 0.076 mmol), 2b (163.5 mg, 0.228 mmol), toluene (8.5 mL), anhydrous potassium carbonate (2 M in H₂O, 0.15 mL, 0.29 mmol), ethanol (0.15 mL), and Pd(PPh₃)₄ (4.4 mg, 0.004 mmol) were used. Recrystallization from THF solvent gave 3b as a dark red solid (58 mg, 52%). ¹H NMR (500 MHz, CDCl₃): δ (ppm) 9.00 (s, 2H), 8.88 (s, 2H), 8.41 (s, 4H), 8.11 (s, 4H), 7.75 (s, 2H), 7.62 (s, 2H), 7.28 (s, 2H), 4.09 (dd, 8H), 1.94 (m, 4H), 1.49–1.16 (m, 64H), 0.86 (tt, 24H) (Fig. S4, ESI†). Mass (MALDI-TOF): Obs. 1470.87 (Fig. S5, ESI†); calcd for C₉₂H₁₁₈N₄O₄S₄, 1470.80.

Instruments

¹H NMR spectra were measured on a Bruker AVANCE Digital 500 MHz spectrometer in deuterated chloroform using tetramethylsilane as an internal standard. Matrix-assisted laser desorption/ionization time-of-flight mass spectra were acquired on a Bruker Daltonics BIFLEX III MALDI-TOF analyzer. UV-vis absorption spectra were recorded using a Shimadzu UV-3600 spectrophotometer. A CHI660A electrochemical workstation equipped with platinum electrodes was used to collect the cyclic voltammetry (CV) data at a scanning rate of 50 mV s⁻¹ in the nitrogen-saturated solution of 0.1 M tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) in acetonitrile (CH₃CN). A glassy carbon electrode coated with a thin PYDPP-EH or PYDPP-BO film, a Pt wire and an Ag/AgCl (0.1 M) electrode were used as the working electrode, the counter electrode and the reference electrode, respectively. The surface morphology of active layer was characterized by atomic force microscopy (AFM) measurement on a Nanoscope NS3A system.

Fabrication and characterization of OSCs and hole-only devices

Solution-processed BHJ solar cells were fabricated as follows: indium tin oxide (ITO) coated glass substrates were successively cleaned by sonication in acetone, detergent, distilled water, and isopropyl alcohol prior to device fabrication. After treated with an oxygen plasma for 5 min, 40 nm-thick poly(styrene sulfonate)-doped poly(ethylene-dioxythiophene) (PEDOT:PSS) (Bayer Baytron 4083) layer was spin-coated onto the ITO-coated glass substrates at 2500 rpm for 30 s. Subsequently, the substrates were dried at 130 °C for 20 min in air and then transferred into a glove box under the protection of N₂ flow. Then, a CHCl₃ solution of PYDPP-EH or PYDPP-BO/[6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) with or without 1,8-diiodo-octane (DIO) was spin-coated onto the surface of the PEDOT:PSS layer to form the active layers, which were dried under inert atmosphere overnight. The thicknesses of the active layers were about 90 nm as measured by a profilometer. The ultra-thin poly[(9,9-bis(30-(N,N-dimethylamino)-propyl)-2,7-fluorene)]-alt-2,7-(9,9-dioctylfluorene) (PFN) layer was deposited by spin casting at 2000 rpm for 30 s from a 0.02% (w/v) methanol solution. Finally, Al (∼100 nm) was vacuum-deposited with a shadow mask as the top electrode and the effective area was measured to be 0.16 cm². The current density–voltage (J–V) characteristics were measured under AM 1.5 solar simulator (Japan, SAN-EI, XES-40S1) at 100 mW cm⁻², and the data were collected using a Keithley 2400 digital source meter. The spectral response was measured with a DSR100UV-B spectrometer with a SR830 lock-in amplifier. A calibrated Si photodiode was used as a reference before each measurement.

The hole mobilities of the active layers were determined by fitting the dark current to the model of space charge limited current (SCLC) in the configuration of ITO/PEDOT:PSS (40 nm)/active layer/MoO₃ (10 nm)/Ag (∼60 nm). The active layers were fabricated under the same conditions as the corresponding solar cells. The electric-field dependent SCLC mobility was estimated using the following equation:

where J is the current, μ₀ is the zero-field mobility, ε₀ is the permittivity of free space, ε_r is the relative permittivity of the materials, d is the thickness of the active layer, β is the field activation factor and E is the effective electric field.

Results and discussion

Design and synthesis

A facile preparation with only two steps is carried out to synthesize the DPP–pyrene–DPP type SMs PYDPP-EH and PYDPP-BO with 2-ethylhexyl and 2-butyloctyl chains, respectively, and the detailed synthetic routes are illustrated in Scheme 1. Pyrene is functionalized at its C2-positons with 4,4,5,5-tetramethyl-1,3,2-dioxaborolan to couple with mono-brominated DPPs with 2-ethylhexyl and 2-butyloctyl chains to afford PYDPP-EH and PYDPP-BO, respectively. There are only two step reactions to prepare the target materials from commercially available materials without using toxic stannyl intermediates and the potentially dangerous lithiation reactions. Furthermore, it also should be noted that the purification is easy even without employing column chromatography with high yields. Contributed by the four alkyl chains at each molecule, PYDPP-EH and PYDPP-BO exhibit good solubility in CHCl₃, which allows them to be readily solution-processed to form smooth and pinhole-free films for OSC devices.

Optical and electrochemical properties

The optical properties of PYDPP-EH and PYDPP-BO were investigated by UV-vis absorption spectroscopy in dilute CHCl₃ solutions and in thin films, as illustrated in Fig. 1, and the corresponding data are summarized in Table 1. In CHCl₃ solution, their absorption spectra are almost identical with the main peak at 588 nm, indicating that the molecular electronic structures are essentially independent of the alkyl substituents, and the main absorption band at 450–650 nm originates from the considerable intramolecular charge transfer (ICT) between pyrene donor core and DPP acceptor units.^27,28,33,34

Fig. 1 The normalized UV-vis absorption spectra of PYDPP-EH (a) and PYDPP-BO (b) in CHCl₃ solutions and in solid films on quartz plate.

Table 1 Optical and electrochemical data for compounds 3a and 3b obtained from optical spectroscopy and cyclic voltammetry

Molecules	λmax in solution (nm)	λmax in film (nm)	λonset in film (nm)	Eg,optical^a (eV)	Eox (V)	EHOMO^b (eV)	ELUMO^b (eV)
a Estimated from the absorption edge of the films, Eg,optical = 1240/λonset.b Calculated from the empirical formula, EHOMO = −(Eox + 4.43) eV and ELUMO = EHOMO + Eg.
3a	381, 554, 589	408, 594, 646	716	1.73	0.75	−5.18	−3.45
3b	380, 556, 590	400, 577, 626	710	1.75	0.76	−5.19	−3.44

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In film, the absorption bands of PYDPP-EH in the visible region are remarkably broadened, exhibiting bathochromic shifts of approximately 60 nm with maxima at 646 nm and a shoulder at 594 nm (Table 1). The typical red-shift accounts for the favorable π–π interactions with some ordered self-aggregation in solid film, thus giving rise to electronic delocalization. As for PYDPP-BO shown in Fig. 1b, only a new shoulder peak is seen at 628 nm with the main peak at 580 nm, indicating that the molecules in PYDPP-BO films are far more disordered caused by the bulkier alkyl substituents of 2-butyloctyls. Also, the absorption band edges extend up to 716 and 710 for PYDPP-EH and PYDPP-BO films, from which the optical bandgaps (E_g,optical) are estimated to be 1.73 and 1.75 eV (Table 1), respectively.

In order to estimate their frontier orbital energy levels, CV measurements were carried out and the energy level of Ag/AgCl electrode was calibrated with the ferrocene/ferrocenium ion system to be 4.43 eV. As displayed in Fig. S6,† the onset oxidation potentials (E_ox) of PYDPP-EH and PYDPP-BO are 0.75 and 0.76 V, from which the HOMO energy levels (E_HOMO) can be calculated to be −5.18 and −5.19 eV, respectively, according to the empirical formula shown in Table 1. Additionally, the corresponding LUMO energy levels (E_LUMO) of PYDPP-EH and PYDPP-BO estimated from E_LUMO = E_HOMO + E_g, are −3.45 and −3.44 eV, respectively, indicating that the different N-alkyl substitutes show almost no impacts on the electron distribution at their HOMOs and LUMOs. The optimum geometry and electron-state-density distribution is schematically illustrated in Fig. S7 (ESI†).

Photovoltaic properties

Single solution-processed BHJ OSCs were fabricated utilizing PC₆₁BM as the electron acceptor and PYDPP-EH or PYDPP-BO as the electron donor under a conventional device structure of ITO/PEDOT:PSS/active layer/PFN/Al. The weight ratio of the SM donors to PC61BM was optimized to be 1 : 1 and the active layers were spin-cast using CHCl₃ solution with or without DIO additives. The current density–voltage (J–V) curves of the optimized BHJ solar cells based on PYDPP-EH and PYDPP-BO are shown in Fig. 2a, and the key device parameters are listed in Table 2. As for PYDPP-EH, the as-cast devices display a PCE of 3.14%, with a short-circuit current density (J_SC) of 7.60 mA cm⁻², an open-circuit voltage (V_OC) up to 0.89 V and a fill factor (FF) of 50.83%. Upon thermal annealing (TA) at 100 °C for 10 min, the J_SC is enhanced to 9.10 mA cm⁻² with unchanged V_OC and FF, resulting in a PCE of 4.13%. The devices fabricated in the presence of 0.4 vol% DIO show a further enhanced PCE to 4.64%, J_SC to 9.66 mA cm⁻², and FF to 55.17% with a slightly declined V_OC to 0.87 V. Compared to PYDPP-EH, the photovoltaic properties of PYDPP-BO based OSCs are clearly inferior, and the PCE of as-cast devices is only 0.81% with a rather low J_SC of 1.94 mA cm⁻², a V_OC of 0.88 V and a FF of 47.76%. After thermal annealing at 100 °C for 10 min, the overall performance is still very poor with a PCE of 1.30% in spite of the effectively improved J_SC and FF. Encouragingly, an significant performance enhancement is achieved but still much poorer than PYDPP-EH-based devices when the active layers were fabricated in the presence of 0.4 vol% DIO with improved PCE to 2.86% mainly contributed by the enhanced J_SC to 6.20 mA cm⁻².

Fig. 2 (a) Illuminated J–V curves and (b) corresponding EQE response for optimized BHJ solar cells based on PYDPP-EH and PYDPP-BO.

Table 2 Photovoltaic properties of BHJ solar cell devices having the configuration of ITO/PEDOT:PSS/SMs:PC₆₁BM/PFN/Al

Materials	Processing	JSC/mA cm^−2	VOC/V	FF/%	PCE^a/%
a The average values of PCE with standard deviation are obtained from at least 10 devices.
PYDPP-EH	As-cast	7.60	0.89	50.83	3.41 ± 0.12
	TA	9.10	0.89	50.59	4.13 ± 0.08
	DIO	9.66	0.87	55.17	4.64 ± 0.15
PYDPP-BO	As-cast	1.94	0.88	47.76	0.81 ± 0.09
	TA	2.43	0.91	58.63	1.30 ± 0.20
	DIO	6.20	0.90	51.28	2.86 ± 0.13

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On the one hand, the enhanced performance for all the above OSCs fabricated with thermal annealing or DIO additive can be contributed by the more ordered molecular self-assembly and better nano-phase separation and D/A networks and therefore more effective charge separation and transportation,^{15,26,36–39} which will be further confirmed by AFM measurements below. On the other hand, compared to PYDPP-BO, the much better performance of PYDPP-EH-based devices regardless of the fabrication conditions is closely related to the stronger intermolecular interactions, more ordered self-assembly and better film morphology induced by the less bulkier 2-ethylhexyl substituents, which can be demonstrated by the absorption spectra of the neat and blend films shown in Fig. 1 and 3.

Fig. 3 UV-vis absorption spectra of the as-cast and optimized blend films of (a) PYDPP-EH:PC₆₁BM and (b) PYDPP-BO:PC₆₁BM (1 : 1 w/w).

In addition, as illustrated in Fig. 2b, the external quantum efficiency (EQE) curves of the optimized devices by DIO additives reveal a broad response from 350 to 700 nm. However, the devices based on PYDPP-BO display much poorer photocurrent response with a maximum monochromatic EQE of only 36% at 560 nm, while the corresponding value of PYDPP-EH-based device is increased by ∼44% up to 52% at 580 nm. Integration under the entire EQE spectra yields the calculated J_SC of 9.33 and 6.05 mA cm⁻², which is in good accord with the measured J_SC in Fig. 2a for PYDPP-EH and PYDPP-BO, respectively.

Optical properties of blend films

Fig. 3 delineates the UV-vis absorption spectra of the blend films as-cast and processed with DIO additive. As shown in Fig. 3a, the main absorption peaks of the as-cast PYDPP-EH blend film are observed at 334, 590, and 644 nm, and the main peak at 644 nm indicates the predominant slipped-stacking J-aggregates in the PYDPP-EH based thin film.^31,40 DIO has little impact on the absorption in ultraviolet region, but induces a slightly bathochromic shift and increased intensity at ∼647 nm in the visible region, which can be attributed to the more ordered J-aggregates of PYDPP-EH. Therefore, improved photovoltaic performance can be achieved for the devices processed with DIO as shown in Table 2.

In contrast, the predominant absorption peak in visible region is seen at 579 nm with only a weak shoulder at 632 nm for PYDPP-BO blend films (Fig. 3b). More interestingly, as for PYDPP-BO blended films fabricated with DIO additive, the intensity of the band at 632 nm decreases while that at 579 nm increases.^31,35,41 Therefore, it can be speculated that the different alkyl substitutes have strong impacts on not only intermolecular interactions, but also the self-organizing behaviors of the donor materials in both pure and BHJ thin films, thus leading to different photovoltaic performance.

As displayed in Fig. S9,† thermal annealing has trifling impact on the light harvest for PYDPP-EH/PC₆₁BM blend film. However, the influence of thermal annealing on the absorption spectra is slightly more pronounced for PYDPP-BO-based blend, suggesting that the stronger intermolecular packing of PYDPP-EH than PYDPP-BO due to the smaller alkyl chains is more difficult to be affected by the post treatment of TA. Therefore, only 21% but 60% PCE enhancement (calculated from Table 2) are seen for PYDPP-EH and PYDPP-BO-based OSCs, respectively.

Since DIO additive is a pre-treatment before the formation of strong intermolecular packing, it is slightly more efficient to change the intermolecular packing of the donor materials than the post treatment TA after the formation of strong intermolecular packing, leading to more enhanced performance for DIO treated organic photovoltaics.

Morphology of blend films

To further gain insight into the performance enhancement upon the introduction of DIO additive, AFM images of blend films processed with or without DIO are investigated. As displayed in Fig. 4a, the as-cast PYDPP-EH/PC₆₁BM blend film is composed of networks with crystallites and phase separation with visible boundary. And the root-mean-square (RMS) value is estimated to be ∼7.42 nm. DIO induces more and larger crystallization region of PYDPP-EH with an increased RMS value of 9.97 nm. Such a microstructure is conducive to charge transport for enhanced J_SC, yielding the consistent results in Fig. 2. Quite different from PYDPP-EH blend films, typical “sea-island” surface morphology is seen for the PYDPP-BO ones with dominant size over 100 nm, which should lead to a poor exciton dissociation and poor photovoltaic performance. Unexpectedly, DIO additive inducing the PYDPP-BO blend films well-defined interpenetrating networks of donors and acceptors with a decreased RMS from 4.33 to 2.45 nm. Although such interpenetrating networks can facilitate the exciton dissociation to enhance the performance of the corresponding OSCs, the lacking of ordered self-aggregates can still be the bottleneck for efficient charge transport and high performance OSCs. On the basis of these facts, the phase separation must be regulated to a suitable range, wherein charge transportation and exciton dissociation could be balanced.

Fig. 4 Tapping mode AFM height images (5 × 5 μm) of PYDPP-EH : PC₆₁BM (1 : 1 w/w) blend films fabricated without (a) and with (b) 0.4 vol% DIO, and PYDPP-BO : PC₆₁BM (1 : 1 w/w) blend films fabricated without (c) and with (d) 0.4 vol% DIO.

Hole mobility

In order to further explore the performance difference of PYDPP-EH and PYDPP-BO based devices, hole-only devices with DIO additive were fabricated with almost the same thickness as that in the solar cells. The ln J–ln V characteristics of hole-only devices are depicted in Fig. 5. And the hole mobilities are estimated empirically by SCLC method to be 2.14 × 10⁻⁴ and 1.25 × 10⁻⁴ cm² V⁻¹ s⁻¹ for PYDPP-EH and PYDPP-BO blend films, respectively, confirming the higher J_SC for PYDPP-EH based devices shown in Fig. 2. Since the back bones of PYDPP-EH and PYDPP-BO are the same, the higher hole mobility of PYDPP-EH-based blend is ascribed to the more ordered self-aggregates in PYDPP-EH blend films (Fig. 4) induced by its smaller alkyl substitutes. And the relatively high mobility of both PYDPP-EH and PYDPP-BO can be ascribed to the conjugated chemical structure consisting of weak donor and strong acceptor moieties.

Fig. 5 The ln J–ln V characteristics of hole-only devices processed with DIO in the configuration of ITO/PEDOT:PSS/SMs:PC₆₁BM/MoO₃/Al.

Conclusions

Two A–D–A type SMs PYDPP-EH and PYDPP-BO based on pyrene as core and DPP as arms with 2-ethylhexyl and 2-butyloctyl side chains, respectively, are synthesized for organic photovoltaic devices with only two step reactions without using toxic stannyl intermediates and the potentially dangerous lithiation reactions. Compared to the pyrene-DPP-pyrene type SMs, shorter but more N-alkyl substitutes in the SMs can ensure not only the solution-processability but also strong π–π interactions with more ordered self-aggregation in films. These effects also induce the PYDPP-EH-based OSCs higher photovoltaic performance than PYDPP-BO ones with a higher PCE of 4.64%, more efficient photocurrent response with a maximum monochromatic EQE of 52% at 580 nm and a higher hole mobility of 2.14 × 10⁻⁴ cm² V⁻¹ s⁻¹ for the blend film. The work helps our understanding on the effect of alkyl side chains to donor materials' self-aggregation behaviors and therefore their photovoltaic properties, and can be a guideline for optimizing the chemical structures of organic photovoltaic materials.

Acknowledgements

This work was financially supported by grants from International Science & Technology Cooperation Program of China (2013DFG52740, 2010DFA52150), National Natural Science Foundation of China (51473053, 51073060), and the Post-doctoral Science Foundation of China (2015M580721).

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR, mass spectra, CV curves, and the optimum geometry and electron-state-density distribution of target molecules. See DOI: 10.1039/c6ra09544e

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