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Cooperatively Tuning Phase Size and Absorption of Near IR Photons in P3HT:Perylene Diimide Solar Cells by Bay-Modifications on the Acceptor Xinliang Zhang,†,‡ Bo Jiang,† Xin Zhang,† Ailing Tang,† Jianhua Huang,† Chuanlang Zhan,*,† and Jiannian Yao*,† †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China S Supporting Information *

ABSTRACT: P3HT is a widely used and commercial polymer donor, but it cannot absorb near IR solar photons. Chemical accessibility to tune the frontier molecular orbits of π-conjugated small molecule acceptors increases the possibility to improve their near IR absorption, which is complementary to P3HT. Taking the aggregation tendency of the planar π-system into account, we herein use the traditional n-type organic semiconductor of perylene diimide (PDI) as the model backbone, showing a molecular way to cooperatively tune the aggregation tendency and absorption of the near IR photons. Practically, we replace the 2-methoxylethoxyl units from the mother PDI monomer (OPDI-O), one-by-one, with the 4,8-bis(2-(2-ethylhexylthienyl) benzo[1,2-b′:4,5-b′]dithiophene (BDT) moieties, giving two other PDI monomers of B-PDI-O and BPDI-B. Because of the photoinduced intramolecular charge transfer transition from the BDT unit to the PDI core, B-PDI-B exhibits a broad absorption shoulder beyond 600 nm in the dilute solution, and beyond 650 nm in the solid film. This red-shifted absorption enhances usage of the near IR photons of the solar emission when using P3HT as the donor. The steric effects between the PDI and BDT planes produce twisted conformations, which effectively suppress aggregation tendency. The domain size decreases from >0.5 μm (O-PDI-O) to ∼100 nm (B-PDI-O) and then ∼20 nm (B-PDI-B) with using 4% DIO as additive. Consistently, the short-circuit current density, open-circuit voltage, and efficiency of the optimal P3HT:PDI best cells all increase as BDT unit is introduced on the bay-region. clene-fused trisimide,10 diketopyrrolopyrrole,11 and indenedione.12 Perylene diimide derivatives (PDIs), as one kind of the earliest employed and widely used organic semiconductors, exhibit superior optical and electric properties, such as strong absorption in the visible region of 450−650 nm,13 excellent electron affinity with tunable low-lying lowest-unoccupied molecular orbit (LUMO, about −4.0 eV),14 promising electron mobility of 101−10−3 cm2 V−1 s−1,15−17 and excellent chemical, thermal, and photochemical stabilities. 18 Despite these promising properties, efficient PDI-based solar cells have not been shown in the past several decades. One main bottleneck is possibly related to the strong aggregation tendency, which is ascribed to the large π-system of perylene chromophore. The formed large aggregates (normally over 100 nm) in the blend film may limit the exciton diffusion and charge separation, which is unfavorable for the performance of PDI-based solar cells.

1. INTRODUCTION Alternatively to the silicon-based counterparts, solutionprocessed bulk-heterojunction organic photovoltaic devices (BHJ OPVs) have attracted extensive attention because of their lightweight, simple-processing, low-cost, and flexibility advantages. A BHJ OPV cell uses the donor and acceptor blend to harvest solar energy, generating photovoltaic response. The acceptor materials are of the same importance as the donors, while progress devoted to acceptors is relatively limited.1−4 Currently, fullerene derivatives still dominate the acceptors in efficient BHJ OPVs. Organic small molecule acceptors are being considered as potential alternatives to replace the fullerene ones in the past decades. One of the advantages of using organic small molecule as the acceptor may lie in the rich source and chemical multiplicity of organic small molecules, both of which increase the probability to diversify combinations of donor/acceptor (D/A) pairs with matched energy levels and broad solar spectral coverage. To date, several types of small molecule acceptors have been reported, for example, the derivatives of bifluorenylidene,5 benzothiadiazole,6 naphthalene diimide,7 quinacridone,8 fluoranthene-fused imide,9 decacy© XXXX American Chemical Society

Received: May 5, 2014 Revised: September 30, 2014

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dx.doi.org/10.1021/jp5093674 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Scheme 1. Chemical Structures of the Acceptors

unit has been widely used as a potential electron-donor unit in efficient polymer and small molecule donors.34,35 On the basis of our previous work on O-PDI-O (Scheme 1),36 we replace the bay-substituted 2-methoxylethoxyl (O) groups with the BDT (B) units, one-by-one, to generate B-PDI-O and B-PDIB, respectively. After replacing the two 2-methoxylethoxyl substituents, B-PDI-B exhibits a broad ICT absorption in the near IR region, extending to 750 nm in solution and 800 nm in film. The near IR absorption enhances usage of the near IR solar photons. Moreover, the steric effects between the PDI and BDT units produce twisted conformations, effectively suppressing the aggregation tendency. When using P3HT as the donor, the domain size decreases from >0.5 μm (O-PDI-O) to ∼100 nm (B-PDI-O) and then ∼20 nm (B-PDI-B). As a result, the short-circuit current density (Jsc), open-circuit voltag (Voc), and power conversion efficiency (PCE) of the optimal cells all increase with the blend acceptor going from O-PDI-O to BPDI-O and then B-PDI-B.

To reduce their aggregation tendency, two approaches have been conducted to design PDI-based small molecule acceptors. One is to construct PDI dimers, in which two PDI units are covalently linked through an aromatic/single bond bridge on the bay-region or through a single bond on the imide-N position.19−24 The steric effects from the covalent neighboring aromatics generate twisted conformations, which have proven effective in suppressing aggregation tendency of PDI derivatives. Using the commercially poly(3-hexylthiophene) (P3HT) as the donor, the reported efficiency from the state-ofthe-art cells is approaching or exceeding 2%.19−22 When utilizing the conjugated polymer donor of PBDTTT-C-T, the efficiency has reached 4% recently.23,24 The other is to functionalize PDI monomer on the imide-position using alkyl-chains, and/or the bay-region using aliphatic or aromatic substituents. When using P3HT as the donor, the resulted PDI acceptors only give a very low efficiency, typically below 0.5%.25−27 When using another promising donor, for example, DTS(FBTTh2)2, an efficiency of 3% has been reported recently.28 Can the efficiency be improved by using P3HT as the donor, and PDI monomer as the acceptor? P3HT is a commercial and most widely studied polymer donor. It has potential in the commercial future of BHJ OPVs. P3HT in solution shows a wide absorption band in the wavelength range from 300 to 550 nm. When forming a solid film and after thermal annealing, it gives an absorption band, typically between 400 and 650 nm (Supporting Information Figure S1). Considering the fact that ca. 50% of photons in the solar spectrum have energies corresponding to a wavelength of 600−1000 nm,29 increasing absorption of these near IR photons is important to improving the P3HT-based solar cells. To this end, one way to improve the performance of P3HT-based nonfullerene cells is to synthesize small molecule acceptors, which have enhanced complementary absorption in the near IR region, for example, beyond 650 nm in film. PDI derivatives normally show a strong absorption band between 400−600 nm in solution. One way to red-shift the absorption is to extend the π-system.30−33 Another simple way may be to use intramolecular charge transfer (ICT) transition through introduction of covalent electron-donating units. Covalently introducing the electron donor onto the imideposition has been proven negligible in influencing the absorption because of the nodes of the highest occupied molecular orbital (HOMO) and LUMO on the imide nitrogen position.18 Thus, the choice is to functionalize the bay-region by using electron donors. In this Article, we select a highly conjugated aromatic unit of 4,8-bis(2-(2-ethylhexylthienyl) benzo[1,2-b′:4,5-b′]dithiophene (BDT) as the electron-donating bay-substituents. The BDT

2. RESULTS AND DISCUSSION Synthesis and Thermal Stability. Synthesis of B-PDI-O and B-PDI-B is depicted in Scheme 2. O-PDI-O was Scheme 2. Synthetic Routes toward B-PDI-O and B-PDI-B

B



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Figure 1. Ground-state geometries of methyl analogues of O-PDI-O, B-PDI-O, and B-PDI-B, as calculated from density functional theory. (a) The dihedral angles between two naphthalene planes of the PDI core, all viewing along the N−N direction. (b) The dihedral angles of B-PDI-B between BDT and PDI units viewing along the 1,6-direction (the left), and between two BDT units viewing along the 1,7-direction (the right).

Figure 2. GIWAXS profiles of pristine films of O-PDI-O (a), B-PDI-O (b), and B-PDI-B (c).

Table 1. Optoelectronic Properties of O-PDI-O, B-PDI-O, and B-PDI-B λmax [nm] PDI

εmax [M−1 cm−1]a

ε650nm [M−1 cm−1]b

Amax [/100 nm]

sol.

film

Egopt [eV]c

LUMO [eV]d

HOMO [eV]d

EgCV [eV]d

LUMO [eV]e

HOMO [eV]e

Egcal [eV]e

O-PDI-O B-PDI-O B-PDI-B

5.4 × 104 3.1 × 104 3.5 × 104

0.5 × 103 2.9 × 103 13.0 × 103

0.18 0.068 0.070

566 558 554

531 539 562

2.06 1.82 1.63

−3.78 −3.81 −3.95

−5.93 −5.68 −5.57

2.15 1.87 1.62

−3.35 −3.26 −3.14

−5.24 −5.23 −5.49

1.89 1.97 2.53

film

Molar extinction coefficient at λmax in solution. bMolar extinction coefficient at 650 nm in solution. cOptical band gap estimated from the formula of 1240/λedge. dElectrochemical band gap and energy levels estimated from the cyclic voltammetry data. eObtained from the Gaussian program.

a

synthesized according to a reported procedure.36 B-PDI-O and B-PDI-B were synthesized through coupling 2-trimethyltin BDT (BDT-SnMe3) with compounds 1 and 2, respectively, using Pd(PPh3)4 as catalyst. The final products were fully characterized by 1H NMR, 13C NMR, MALDI-TOF MS, and elemental analysis (Experimental Section). All acceptors are readily soluble in commonly used organic solvents such as dichloromethane, chloroform, and o-dichlorobenzene (o-DCB) at room temperature, due to the solubilizing 2-ethylhexyl and/ or 2-methoxylethoxyl chains. Their thermal properties were investigated by thermogravimetric analysis (TGA). All three acceptors exhibit excellent thermal stability with the decomposition temperature (at 5% weight loss) at 397.5, 424.6, and 443.0 °C for O-PDI-O, BPDI-O, and B-PDI-B, respectively (Supporting Information Figure S2). The acceptor exhibits higher thermal stability after BDT replaces 2-methoxylethoxyl unit, which may be reasoned from the replacement of the carbon−oxygen bonds with the stronger carbon−carbon bonds.37 Optimal Conformations. Optimal conformations of the acceptors were calculated on Gaussian 09 at the B3LYP/6-31G level of theory in the gas phase. As shown in Figure 1a, the

dihedral angles between two naphthalene planes of the PDI core are ∼14°, ∼20°, and ∼21° for O-PDI-O, B-PDI-O, and BPDI-B, respectively. For the former two molecules, the 2methoxylethoxyl unit stretches out in a dihedral angle of 5−7°, with respect to the attached naphthalene plane (Supporting Information Figure S3 and Table S1). The dihedral angles between the BDT and PDI planes for B-PDI-O and B-PDI-B are ∼53°, and the dihedral angles between two BDT planes for B-PDI-B are ∼74° (Figure 1b). The twisted conformation is expected to reduce the aggregation tendency of the PDI derivatives.19−23 Crystallinity of the Acceptors. The influence of twisted conformations on the crystallinity was investigated by using 2D grazing-incidence wide-angle X-ray scattering (GIWAXS) (Figure 2). The relative linecut profiles are shown in Supporting Information Figure S4. All of the films were spun-cast from the corresponding o-DCB solutions atop a silicon wafer. As shown in Figure 2a, the pristine O-PDI-O film exhibits a strong (100) reflection positioning on the meridional plane at qz = 0.48 Å−1 (corresponding to a d-spacing of 13.0 Å), and a weak reflection (010) emerges on the equatorial plane at a qxy value of 1.81 Å−1 (d = 3.5 Å), suggesting the crystalline C



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Figure 3. Absorption spectra of O-PDI-O, B-PDI-O, and B-PDI-B: (a) in dilute chloroform solutions (1 × 10−6 M) and (b) in films.

Figure 4. Energy level diagram of the materials in the inverted (a) and conventional (b) photovoltaic device.

nature with π-stacking along the in-plane direction. The OPDI-O film also shows two reflections at q = 0.34−0.36 Å−1 (corresponding to a d spacing of 18.5 and 18.0 Å),38,39 which are also evidence of the crystalline nature. For B-PDI-O (Figure 2b) and B-PDI-B (Figure 2c), no obvious (100) and (010) reflections were recorded, suggesting suppressed crystallinity by replacing the 2-mothoxylethoxyl with the BDT unit. Optical and Electrochemical Properties. The molecular optoelectronic properties of the three PDI derivatives were characterized using UV−visible absorption spectrum and cyclic voltammetry (CV). Table 1 shows the results, and Figure 3 gives the absorption spectra of the three acceptors in dilute chloroform solutions and in solid films. In solution, the mother PDI of O-PDI-O displays an absorption maximum (λmax) at 566 nm with a shoulder at 527 nm. When the BDT replaces the 2-methoxylethoxyl substituent, the resulted B-PDI-O and BPDI-B exhibit an absorption peak at 558 and 554 nm, respectively. The maximum absorption coefficients (εmax) of the three acceptors are in the range of (3.1−5.4) × 104 M−1 cm−1. Because of the photoinduced ICT transition from the BDT unit to the PDI core, B-PDI-O and B-PDI-B both exhibit a broadening absorption shoulder in the near IR region beyond 600 nm. As compared to O-PDI-O (605 nm), the absorption onset extends to 670 nm (B-PDI-O) and 750 nm (B- PDI-B), respectively. The absorptivity values at 650 nm increase from 0 (O-PDI-O) to 2.8 (B-PDI-O) and 13.0 M−1 cm−1 (B-PDI-B). Another broad absorption band emerges in the wavelength region of 300−400 nm when BDT replaces 2-methoxylethoxyl unit. These near UV peaks locate at 310 and 375 nm, which can be ascribed to the characteristic absorption of the BDT unit.40 Each acceptor affords a broader and red-shifting absorption band as it goes from the dilute solution to film. The λmax locates at 531, 539, and 562 nm for O-PDI-O, B-PDI-O, and B-PDIB, respectively. The λmax of the former two acceptors is blueshifted by 20−30 nm, which might be a result of formation of H-aggregates. For B-PDI-B, the λmax is red-shifted by ∼10 nm as compared to that in solution, which might be due to the formation of the J-aggregates.41 P3HT exhibits absorption from 400 to 650 nm in the film. B-PDI-B shows an additional complementary absorption in the short wavelength range from

300 to 400 nm and in the long wavelength region of 650−800 nm, both to the donor P3HT. Measurements by CV provide the electrochemical band gaps of 2.15 (O-PDI-O), 1.87 (B-PDI-O), and 1.62 eV (B-PDI-B), which is close to the corresponding optical band gap value. The LUMO/HOMO energy levels are −3.78/−5.93 eV, −3.81/− 5.68 eV and −3.95/−5.57 eV, respectively, for O-PDI-O, BPDI-O, and B-PDI-B (Figure 4 and Supporting Information Figure S5). The LUMO energy is lowered, whereas the HOMO energy is raised when BDT replaces the 2-methoxylethoxyl units, one-by-one. The change of the LUMO/HOMO energy is likely related to the replacement of the bridged oxygen atom with the π-conjugated BDT moiety, leading to a change of the electron-conjugation going from the π−n (PDI-O) to the π−π (PDI-BDT) mode. Under the same conditions, we also measured the energy levels of P3HT, whose LUMO and HOMO energy levels are of −2.94 and −4.94 eV.21 The HOMO and LUMO of the PDI acceptors can match those of P3HT. Photovoltaic Properties. BHJ OPV devices using P3HT as the donor and each of O-PDI-O, B-PDI-O, and B-PDI-B as the acceptor were fabricated and characterized iniatially with a conventional device configuration of ITO/PEDOT:PSS/ P3HT:PDI/Ca/Al under the illumination of AM 1.5G at 100 mA cm−2. Here, PEDOT:PSS is poly(3,4ethylenedioxythiophene):poly(styrenesulfonate). Figure 5 shows the current density (J)−voltage (V) curves of the best

Figure 5. J−V curves of the best devices with 4% DIO as the additive. D



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and B-PDI-B-based cells was higher than that in the B-PDI-O device. The higher FF from O-PDI-O may be reasoned from the high crystallinity of the acceptor, which is shown in Figure 2. The higher FF achieved from B-PDI-B may be attributed to the improved film morphology (vide post).42 The external quantum efficiency (EQE) of the conventional and inverted devices was tested to confirm the accuracy of the photovoltaic measurements and shown in Figure 6. In the

devices. Table 2 summarizes the cell paprameters of opencircuit voltage (Voc), short-circuit current-density (Jsc), fill factor (FF), and PCE of the best devices. Table 2. Photovoltaic Performance Based on P3HT:PDI Blend device conv.a

conv.b

inv.b a

acceptor

PCE (avg)c [%]

O-PDI-O B-PDI-O B-PDI-B O-PDI-O B-PDI-O B-PDI-B O-PDI-O B-PDI-O B-PDI-B

0.004 0.30 (0.28) 0.85 (0.84) 0.004 0.35 (0.32) 1.25 (1.21) 0.005 0.44 (0.43) 1.66 (1.65)

Jsc [mA cm−2] Voc [V] 0.02 1.5 3.5 0.02 1.6 3.5 0.02 1.7 5.3

FF [%]

0.36 0.44 0.58 0.33 0.57 0.61 0.44 0.56 0.61

51 44 41 53 39 58 53 45 51

Without DIO. bWith 4% DIO. cAverage value from 10 devices.

Different D/A weight ratios were first optimized, and a D/A weight ratio of 1:2 gives the best cell for each acceptor. The best O-PDI-O-based device shows an inferior PCE of 0.004%. Replacing one 2-methoxylethoxyl unit with BDT, producing BPDI-O, leads to an enhanced PCE of 0.30%. When both of the 2-methoxylethoxyl units are replaced, the performance is further improved, yielding an even higher efficiency of 0.85% with a higher Voc of 0.58 V, a higher Jsc of 3.5 mA cm−2, and an FF of 0.41. On the basis of the optimal D/A weight ratio of 1:2, the devices were further optimized by varying the 1,8-diiodooctane (DIO) content. The best performance was obtained at a DIO content of 4% (v/v) for all three acceptors. As shown in Table 2, the best efficiency increases with the blend acceptor going from O-PDI-O to B-PDI-O and then B-PDI-B. The best device based on B-PDI-B with two BDT units involved gives an improved PCE of 1.25%, with a Voc of 0.61 V, a Jsc of 3.5 mA cm−2, and an FF of 0.58. On the basis of the optimal D/A weight ratio and DIO content, the photovoltaic performances were further explored using an inverted device configuration of ITO/ZnO/ P3HT:PDI/MnOx/Al. Table 2 gives the relative results, and Figure 5 shows the J−V curves of the best cells. One can see that with respect to the conventional cell, using B-PDI-O and B-PDI-B as the blend acceptor gives better photovoltaic performance, and the PCE increases with the blend acceptor changing from O-PDI-O to B-PDI-O and then B-PDI-B. Among the three acceptors, B-PDI-B gives the best PCE of 1.66% with a Voc of 0.61 V, a Jsc of 5.3 mA cm−2, and an FF of 0.51. We speculate that this better performance achieved from an inverted device may be reasoned to the varied surface D/A compositions in the active layer. The vertical phase-separation effects in PDI-based cells have been observed in another work of our group.24 The donor material tends to segregate toward the top surface of the active layer, while the acceptor of PDI is toward the buried surface. The resulting donor-rich top and acceptor-rich bottom are beneficial for the hole and electron collection from the selective electrode when using the inverted device configuration. From Table 2, it can be seen that the Jsc value significantly increases as the blend acceptor goes from O-PDI-O to B-PDIO and then B-PDI-B. Meanwhile, the Voc is raised also in that sequence. As a result, the PCE is improved whenever using a conventional or an inverted cell structure. The FF in O-PDI-O-

Figure 6. EQE spectra of the optimal cells based on P3HT:PDI (PC61BM) blend.

optimal cells, the integrated current density values (O-PDI-O, 0 mA cm−2; B-PDI-O, 1.3 mA cm−2; B-PDI-B, 5.0 mA cm−2) from the EQE spectrum were well matched with the Jsc values (O-PDI-O, 0.02 mA cm−2; B-PDI-O, 1.7 mA cm−2; B-PDI-B, 5.3 mA cm−2) from the photovoltaic measurement, respectively. Across all wavelengths, O-PDI-O shows a very low efficiency. The device based on B-PDI-O exhibits a broad and stronger response in the wavelength range from 400 to 650 nm. The B-PDI-B-based device displays an even stronger response in this wavelength range. The maximum EQE value is of 36%. Interestingly, there appears obvious EQE responses in the wavelength range from 650 to 800 nm. A combination of the EQE responses both from 400 to 650 nm and from 650 to 800 nm contributes to the enhanced Jsc value for the B-PDI-Bbased device, with respect to B-PDI-O and O-PDI-O. The EQE response observed from 650 to 800 nm is in line with the obvious absorption in this wavelength region observed in the B-PDI-B pristine film (Figure 3b), which suggests that the EQE response between 650 and 800 nm can be attributed to the B-PDI-B acceptor. To further support this, we prepared the P3HT:PC61BM (w/w = 1:1)-based cells. The best device, which gives a PCE of 3.25%, displays a strong EQE response (61%) within 660 nm and a weak tail extending to 720 nm. This weaker tail of the EQE response (6% from the P3HT:PC61BM-based device vs 11% from the P3HT:B-PDIB-based cell) strongly supports that the EQE signals between 650 and 800 nm observed from the B-PDI-B-based device are attributed to the absorption of the nonfullerene acceptor. For B-PDI-B, the EQE response in the inverted device is higher than that in the conventional cell, which is consistent with the higher Jsc value obtained from the photovoltaic measurement. Morphologies. In BHJ solar cells, the morphological features are deeply correlated to the device performance. Atomic force microscopy (AFM) images were acquired to investigate the surface morphology. Figure 7 shows the representative phase images. Figure 7a−c are the phase images of the best cells without use of additive. The blend film from O-PDI-O has a domain E



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Figure 7. AFM phase images of the conventional cells from O-PDI-O (a and d), B-PDI-O (b and e), and B-PDI-B (c and f) and without (a−c) or with 4% DIO (d−f), respectively.

Figure 8. Fluorescence spectra from the pure film of P3HT, PDI, and their blends, respectively. The fluorescence was obtained upon excitation at the λmax of the acceptor in the solid film, that is, at 530 nm (a), 540 nm (b), and 560 nm (c), respectively. The blend films were prepared according to the optimum cell conditions.

size of 0.5−1 μm, which is much larger than the effective exciton diffusion length of typically 5−20 nm.43 As the BDT replaces the 2-methoxylethoxyl unit, the domain size is effectively reduced. For B-PDI-O, the domain size dramatically reduces to ∼150 nm. Using B-PDI-B as the acceptor, narrow strips of domains with a size of ∼30 nm are observed. Figure 7d−f shows the phase images using 4% DIO as additive. For O-PDI-O, use of DIO does not lead to an obvious reduction of the domain size. For B-PDI-O and B-PDI-B, the domain size reduces to ∼100 and ∼20 nm, respectively. Simultaneously, the morphology changes from rode-like into fibril for the B-PDI-O blend, and the fibrils become spherical domains for the B-PDI-B blend after use of 4% DIO. The rootmean-square (rms) roughness values are 31.0, 1.37, and 0.84 nm, respectively. The relatively smoother surface and smaller domain size observed from B-PDI-B relative to those from the other two acceptors indicate better compatibility with P3HT for this acceptor. Supporting Information Figure S6 shows the phase images of the best inverted devices, each of which is similar to that from the best conventional cell with the same PDI as the blend acceptor. Both of the decreasing tendencies of the phase size and rms roughness are consistent with the observation of the increasing trend of the Jsc, Voc, and PCE as the acceptor changes from OPDI-O to B-PDI-O and then B-PDI-B. The smaller domains in P3HT:B-PDI-B blends allow for a higher D−A interfacial

volume density, which is one of the factors for the enhanced Jsc value.44 The reduced rms roughness may decrease the contact resistance, also leading to an increase in the Jsc value. Voc is complicatedly related to several factors, for example, to the energy level and volume density of charge transfer (CT) state or to the metal−organic interfacial contacts.45,46 As reflected from the AFM data, the variations in the phase size and surface roughness upon changing the blend acceptor may relate to the increase in Voc.46 The decrease of the domain size from O-PDI-O to B-PDI-O and B-PDI-B is reasoned from the reduced aggregation, as shown by the optimal conformations and GIXRD characterizations: the twisted conformation suppresses the aggregation tendency, and thereby decreases the domain size. Photoluminescence (PL) measurements provide additional evidence for the reduction of the domain size. Figure 8 shows the PL spectra of the pristine film of the donor and acceptor and their blends, respectively. In the pristine films, P3HT and O-PDI-O fluoresce obviously (Figure 8a), while the fluorescence of B-PDI-O (Figure 8b) or B-PDI-B (Figure 8c) is very weak. In the blend, the PL of P3HT is quenched. P3HT has a characteristic emission peak at 645 nm, at which the fluorescence of the acceptor is, however, very weak. We therefore utilized the relative intensity at 645 nm from the blend as compared to that from the pristine P3HT film as a parameter to describe the PL quenching. This value decreases F



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and reference electrodes, respectively. The samples were dissolved in chloroform with tetra-n-butylammonium hexafluorophosphate (Bu4NPF6, 0.1 M) as the electrolyte. 2-D grazing-incident wide-angle X-ray scattering (GIWAXS) samples of pristine films were prepared by drop-casting of solutions on silica slides, and P3HT:SM acceptors blending films were spin-coated using the same conditions of the best devices. The 2-D grazing incident X-ray diffraction was performed at beamlines 4W1C (wavelength 1.54 Å) of the Beijing Synchrotron Radiation Facility (BSRF) with an incident angle of 0.2°. X-ray photoelectron spectra were carried out on an ESCALab220I-XL photoelectron spectrometer. AFM experiments were carried out by Bruker multimode 8 in tapping mode under ambient conditions. Compounds 1 and 2 are both a mixture of the 1,6- and 1,7-regio-isomers. Synthesis and Characterizations. Compounds 1, 2, BDT-SnMe3, and O-PDI-O are synthesized according to our reported work.36,48 Synthesis of B-PDI-O. Starting materials 1 (76.7 mg, 0.1 mmol) and 2-trimethyltin-4,8-bis(5-ethylhexylthiophen-2-yl)benzo[1,2-b′:4,5-b′]dithiophene (BDT-SnMe3, 88.9 mg, 0.12 mmol) were dissolved in dry toluene and degassed with argon for 15 min. Tetratris(triphenylphosphine)palladium (Pd(PPh3)4, 30 mg, 25.8 μmol) then was added as catalyst. The mixture was stirred at 110 °C overnight under the argon atmosphere. After being cooled to room temperature, toluene was then evaporated using a rotary evaporator. The residue was purified by column chromatography on silica gel (petrol ether/ dichloromethane = 1:1) to give B-PDI-O as a ablack solid in a yield of 72%. 1H NMR (400 MHz, CDCl3), δ (ppm): 9.58− 9.60 (d, 1H), 8.75 (s, 1H), 8.66−8.68 (d, 1H), 8.37−8.48 (m, 2H), 8.16−8.18 (d, 1H), 7.88 (s, 1H), 7.66−7.67 (d, 1H), 7.51−7.53 (d, 1H), 7.32−7.33 (d, 1H), 7.23−7.24 (d, 1H), 6.86−6.87 (d, 1H), 6.79−6.80 (d, 1H), 4.62 (s, 2H), 4.08−4.18 (m, 4H), 3.99 (s, 2H), 3.58 (s,3H), 2.76−2.82 (m, 4H), 1.91− 1.99 (m, 2H), 1.58−1.70 (m, 2H), 1.24−1.42 (m, 36H), 0.80− 0.98 (m, 20H). 13C NMR (150 MHz, CDCl3), δ (ppm): 163.90, 163.81, 163.62, 163.53, 157.11, 146.07, 145.92, 145.63, 139.56, 137.17, 137.00, 136.79, 136.71, 135.62, 134.39, 133.87, 133.12, 132.35, 131.57, 130.76, 129.56, 129.37, 128.66, 128.23, 128.10, 127.80, 125.49, 125.33, 124.88, 124.30, 123.97, 123.71, 123.52, 123.33, 122.22, 121.67, 121.41, 120.97, 70.74, 69.39, 59.40, 44.32, 41.36, 38.01, 34.25, 34.20, 32.46, 30.85, 30.74, 28.86, 28.66, 25.67, 24.14, 24.05, 23.09, 22.98, 22.95, 4.12, 14.07, 10.82, 10.57, 1.03. MALDI-TOF MS (m/z) [M + H]+, 1265.8; calcd for C77H88N2O6S4 m/z, 1264.5. Anal. Calcd for C77H88N2O6S4 (%): C, 72.04; H, 7.07; N, 2.18. Found (%): C, 72.24; H, 7.14; N, 2.12. Synthesis of B-PDI-B. Starting materials 2 (77.2 mg, 0.1 mmol) and 2-trimethyltin-4,8-bis(5-ethylhexylthiophen-2-yl)benzo[1,2-b′:4,5-b′]dithiophene (BDT-SnMe3, 177.8 mg, 0.24 mmol) were dissolved in dry toluene and degassed with argon for 15 min. Tetratris(triphenylphosphine)palladium (Pd(PPh3)4, 30 mg, 25.8 μmol) then was added as catalyst. The mixture was stirred at 110 °C overnight under the argon atmosphere. After being cooled to room temperature, toluene was then evaporated using a rotary evaporator. The residue was purified by column chromatography on silica gel (petrol ether/ dichloromethane = 2:1) to give B-PDI-B as a black solid in a yield of 67%. 1H NMR (400 MHz, CDCl3), δ (ppm): 8.79 (s, 2H), 8.39−8.41 (d, 2H), 8.28−8.30 (d, 2H), 7.94 (s, 2H), 7.67−7.69 (d, 2H), 7.53−7.54 (d, 2H), 7.31−7.32 (d, 2H), 7.26−7.27 (d, 2H), 6.86−6.87 (d, 2H), 6.80−6.81 (d, 2H),

from 0.65 (O-PDI-O) to 0.22 (B-PDI-O) and 0.14 (B-PDI-B), indicating the fluorescence quenching becomes more efficient when the blend acceptor goes in that sequence. The PL quenching in the P3HT:B-PDI-B blend is the most efficient, which is consistent with the smallest domain size among the three acceptors.23 Additionally, the PL spectrum of P3HT (600−800 nm) partially overlaps with the absorption spectra of the acceptor (Supporting Information Figure S7), suggesting possible energy transfer from P3HT to the blend acceptor. The electron mobility was measured using space-charge limited current (SCLC) method with a device structure of ITO/TIPD/P3HT:PDI/Al. Supporting Information Figure S8 shows the experimental data. The O-PDI-O, B-PDI-O, and BPDI-B give an electron mobility of 6.71 × 10−2, 8.30 × 10−7, and 1.96 × 10−5 cm2 V−1 s−1, respectively. The high mobility from O-PDI-O may be related to the high crystallinity of this acceptor, as reflected from the GIWAXS data (Figure 2). From B-PDI-O to B-PDI-B, the electron mobility remarkably increases by 2 orders of magnitude, which is consistent with that increase trend of the Jsc value, as shown in Table 2.

3. CONCLUSIONS A series of solution-processable perylene diimide monomerbased small molecule acceptors were successfully synthesized by functionalizing the bay-region using the highly conjugated BDT units. Replacing the 2-methoxylethoxyl (O) groups in the mother monomer of O-PDI-O, one-by-one, with BDT units yields B-PDI-O and B-PDI-B, respectively. Such structural modifications provide two sources for the enhanced photovoltaic performance. One is the enhancement of the absorption in the wavelength range from 650 to 800 nm, which is complementary to the absorption of P3HT film, improving the spectral coverage in the near IR region with the solar emission spectrum. The other is that the twisted configuration reduces the aggregation tendency, which results in reduced phase size, going from >0.5 μm to 100 nm and then 20 nm in the optimal blends. As a result, as compared to the low-performance OPDI-O-based device (PCE = 0.005%), the B-PDI-O (B-PDIB)-based device, respectively, gives an increased Jsc and Voc, and hence an enhanced PCE to 0.44% (1.7%) when using an inverted device configuration. The PCE of 1.7% is much higher than that value reported from the P3HT:PDI monomer-based solar cells and is among the top values of the P3HT-based nonfullerene cells.19−22,47 Our results show a molecular way to cooperatively tune the aggregation tendency and absorption of the near IR photons simply by chemical modification of the small molecule acceptor with the large conjugated group of BDT. 4. EXPERIMENTAL SECTION General Methods. 1H and 13C NMR spectra were recorded on a Bruker AVANCE 400 MHz spectrometer using CDCl3 as the solvent. Chemical shifts of the 1H and 13C signals were reported in ppm, referencing the signal of TMS (0 ppm) for proton, and CDCl3 (77.23 ppm) for 13C, respectively. Absorption spectra were recorded on a Hitachi U-3010 spectrometer. Fluorescence spectra were recorded on a Horiba FluoroMax-4-NIR spectrophotometer. Cyclic voltammetry was performed using a computer-controlled Zennium electrochemical workstation under argon at a scan rate of 100 mV s−1 at room temperature. A glassy carbon electrode, a Pt wire, and an Ag/AgCl electrode were used as the working, counter, G



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4.09−4.13 (m, 4H), 2.78−2.83 (m, 8H), 1.91−1.95 (m, 2H), 1.49−1.65 (m, 4H), 1.26−1.37 (m, 54H), 0.80−0.94 (m, 30H). 13 C NMR (150 MHz, CDCl3), δ (ppm): 163.70, 163.51, 146.18, 146.02, 144.81, 139.74, 139.68, 137.30, 136.98, 136.72, 136.66, 134.33, 133.41, 133.33, 130.33, 128.12, 128.06, 127.85, 125.51, 125.38, 125.02, 124.36, 123.87, 123.53, 122.38, 44.34, 41.39, 37.96, 34.23, 32.47, 30.75, 28.86, 28.69, 25.70, 24.06, 23.08, 22.98, 22.96, 14.11, 10.84, 10.65. MALDI-TOF MS (m/ z) [M + H]+, 1767.8; calcd for C108H122N2O4S8 m/z, 1766.7. Anal. Calcd for C108H122N2O4S8 (%): C, 72.60; H, 7.00; N, 1.57. Found (%): C, 72.54; H, 7.04; N, 1.53. Device Fabrication. Cell devices with a typical configuration of ITO/PEDOT:PSS/P3HT:PDI derivatives/Ca/Al were fabricated as follows. The ITO glass was precleaned with deionized water, CMOS grade acetone, and isopropanol in turn for 15 min. The organic residues were further removed by treating with UV-ozone for 1 h. The ITO glass then was modified by spin-coating PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) layer, 30 nm) on it. After the ITO glasses were dried in an oven at 150 °C for 15 min, the active layer was spin-coated on the ITO/PEDOT:PSS using a blend solution of P3HT and PDI acceptors (40 mg/mL in oDCB for PDI acceptors with donor/acceptor weight ratio 1:2, respectively). Ca (20 nm) and Al (80 nm) electrode was then subsequently thermally evaporated on the active layer under the vacuum of 1 × 10−6 Torr. For the inverted OSCs with configuration of ITO/ZnO/P3HT:PDI derivatives/MoO3/Al, the ZnO layer was fabricated as reported in the literature. After the active layer was spin-coated, MoO3 (10 nm) and Al (80 nm) were evaporated, respectively. The active area of the device was 0.06 cm2, and the thickness of the active films was ∼300 nm. The devices were characterized in nitrogen atmosphere under the illumination of simulated AM 1.5 G, 100 mW/cm2 using a xenon-lamp-based solar simulator. The current−voltage (I−V) measurement of the devices was conducted on a computer-controlled Keithley 2400 Source Measure Unit. EQE measurements were carried out on an oriel IQE 200 (Newport) with a scanning rate of 20 nm per point.

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ASSOCIATED CONTENT

Measurement details, absorption spectra, TGA curves, DFT calculations, the GIWAXS linecut profiles, CV curves, AFM images, SCLC tests, and NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Authors

*Tel.: 0086-10-82617312. Fax: 0086-10-82616517. E-mail: [email protected]. *Tel.: 0086-10-82616517. Fax: 0086-10-82616517. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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S Supporting Information *

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Article

ACKNOWLEDGMENTS

This work is financially supported by the NSFC (nos. 21327805, 91227112, and 21221002), CAS (XDB12010200), and MOST (2011CB808400 and 2012YQ120060). Beijing Synchrotron Radiation Facility (BSRF) is acknowledged for the GIXRD measurements. H



Article

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