Effect of Fluorine Substitution on Photovoltaic Properties of

Oct 30, 2015 - Two new small molecules, C3T-BDTP and C3T-BDTP-F with alkoxyphenyl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) and meta-fluorina...
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Effect of Fluorine Substitution on Photovoltaic Properties of Alkoxyphenyl Substituted Benzo[1,2-b:4,5-b′]dithiophene-Based Small Molecules Beibei Qiu,† Jun Yuan,† Xuxian Xiao,† Dingjun He,† Lixia Qiu,† Yingping Zou,*,†,‡ Zhi-guo Zhang,*,§ and Yongfang Li*,§ †

College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, China § Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡

S Supporting Information *

ABSTRACT: Two new small molecules, C3T-BDTP and C3T-BDTP-F with alkoxyphenyl-substituted benzo[1,2-b:4,5b′]dithiophene (BDT) and meta-fluorinated-alkoxyphenylsubstituted BDT as the central donor blocks, respectively, have been synthesized and used as donor materials in organic solar cells (OSCs). With the addition of 0.4% v/v 1,8diiodooctane (DIO), the blend of C3T-BDTP-F/PC71BM showed a higher hole mobility of 8.67 × 10−4 cm2 V−1 s−1 compared to that of the blend of C3T-BDTP/PC71BM. Two types of interlayers, zirconium acetylacetonate (ZrAcac) and perylene diimide (PDI) derivatives (PDINO and PDIN), were used to further optimize the performance of OSCs. With a device structure of ITO/PEDOT:PSS/donor:PC71BM/PDIN/Al, the OSCs based on C3T-BDTP delivered a satisfying power conversion efficiency (PCE) of 5.27% with an open circuit voltage (Voc) of 0.91 V, whereas the devices based on C3T-BDTP-F showed an enhanced PCE of 5.42% with a higher Voc of 0.97 V. KEYWORDS: organic solar cells, small molecule photovoltaic materials, alkoxyphenyl, meta-fluorinated-alkoxyphenyl, interlayer

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INTRODUCTION

design new molecules to delve into the correlation between molecular structure and device performance. During the period of gradually improving performance of SM-OSCs, a variety of structures have been designed and studied, and some of them have been recognized as model backbone structures.12 Among them, an effectively long conjugated acceptor-π-donor-π-acceptor (A-π-D-π-A) backbone, consisting of an electron-donating unit (D) as the central building block, two electron-accepting units (A) as end groups, and two oligothiophene fragments as π-conjugation bridges, has been widely used and investigated.9,15−18 In the context of the electron-donating unit, two-dimensional (2D) benzo[1,2-b:4,5-b′]dithiophene (BDT), and alkylthiophenesubstituted BDT in particular, has been deeply studied by many groups and plays a key role in improving the PCEs of both PSCs and SM-OSCs.19−26 Our group has synthesized an alkoxyphenyl-substituted BDT unit that has shown desirable photovoltaic properties when being applied to PSCs with an attractive PCE of 6.2%, but it has not been investigated in SM-

During the past few decades, organic solar cells (OSCs), including polymer solar cells (PSCs) and small molecular OSCs (SM-OSCs), have attracted significant attention because of their attractive superiorities in fabricating lightweight, low-cost, large-area, flexible devices through simple solution processes.1−4 Thanks to the innovation of material structure and device optimization, remarkable improvements have been made, especially in the most important parameter, power conversion efficienciy (PCE), which has increased rapidly from below 1% in the 1990s to >10% for bulk heterojunction (BHJ) PSCs and >9% for BHJ SM-OSCs.5−11 Compared with small molecules, which now are well-known with critical properties of a well-defined molecular structure, high purity, and good reproducibility, polymers possess high absorption coefficients, a wide range of light absorption, and good film-forming qualities but also have relatively poor performance reproducibility resulting from the batch-to-batch variation issue caused by intrinsic polydispersity characteristics.12−14 Although the present result of SM-OSCs is fairly satisfactory, the structure−property relationship of SM-OSCs has not been investigated as widely as PSCs; therefore, it is still necessary to © 2015 American Chemical Society

Received: August 1, 2015 Accepted: October 30, 2015 Published: October 30, 2015 25237

DOI: 10.1021/acsami.5b07066 ACS Appl. Mater. Interfaces 2015, 7, 25237−25246

Research Article

ACS Applied Materials & Interfaces OSCs.27 Moreover, fluorination of polymers and small molecules has become a popular way to adjust the structures of optoelectronic materials to obtain superior performances due to its advantage of increasing the open circuit voltage (Voc), which should be attributed to the deeper highest occupied molecular orbital (HOMO) energy level, and in some cases, other photovoltaic parameters (such as short circuit density (Jsc), fill factor (FF), etc.), which could be explained by several proposed mechanisms to a certain extent.19,28−32 Recently, fluorinated alkoxyphenyl-substituted BDT-based polymers have been synthesized by our group and Yang’s group with high PCE values of 8.0 and 7.02% for PBO-m-FPO and PBDTPF-DTBT, respectively.33,34 However, in terms of SM-OSCs, the fluorine atom was always introduced to modify the electron-accepting moiety.35−38 It will be interesting to see how the SM-OSCs’ properties will change upon replacing the electron-donating unit with a fluorine-substituted one. In addition, considering the remarkable ability to improve the stability and performance of OSCs, various cathode interlayers, including inorganic (transition metal oxides and transition metal chelates) and organic interlayers (polymer-based and small molecule-based interlayers), have been designed and play a significant role in promoting the development of OSCs.39−41 Considering all of the reasons mentioned above, we designed and synthesized two new small molecules with an A-π-D-π-A linear framework based on an alkoxyphenyl-substituted BDT unit (BDTP) and a meta-fluorinated one (BDTP-F) containing octyl cyanoacetate as terminal acceptor unit, named C3TBDTP and C3T-BDTP-F, respectively, as shown in Scheme 1.

an enhanced PCE of 5.42% with a high Voc of 0.97 V. This Voc is, to the best of our knowledge, one of the highest open voltages reported to date for devices prepared from A-π-D-π-A small molecules with 1,8-diiodooctane (DIO) doping.

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RESULTS AND DISCUSSION Synthesis and Thermal Properties. The synthetic routes of the two A-π-D-π-A type small molecules, named C3T-BDTP and C3T-BDTP-F, are depicted in Scheme 2. The two central

Scheme 2. Synthetic Routes to C3T-BDTP and C3T-BDTPFa

Scheme 1. Molecular Structures of C3T-BDTP and C3TBDTP-F

a

Reaction conditions: (i) Mg, THF, then SnCl2/HCl/H2O; (ii) nBuLi, THF, then Sn(CH3)3Cl; (iii) Mg, THF, then SnCl2/HCl/H2O; (iv) LDA, THF, then Sn(CH3)3Cl; (v) Pd(PPh3)4, toluene, reflux; (vi) CHCl3, triethylamine, RT, 24 h; (vii) Pd(PPh3)4, toluene, reflux; (viii) CHCl3, triethylamine, RT, 24 h.

We investigated the effects of the introduction of a fluorine atom into the donor unit of a small molecule on absorption features, energy levels, hole mobilities, and photovoltaic properties of the two molecules. Because of the abilities of maximizing PCE of cathode interlayers, two types of interface layers, zirconium acetylacetonate (ZrAcac) and perylene diimide (PDI) derivatives (PDINO and PDIN), were used to further optimize the performance of the SM-OSCs.41 With a device structure of ITO/PEDOT:PSS/molecule:PC71BM/ PDIN/Al, the OSCs based on C3T-BDTP delivered a satisfying PCE of 5.27%, and the devices based on C3T-BDTP-F showed

building blocks, 2,6-bis(trimethyltin)-4,8-bis(4-ethylhexyloxy-1phenyl)-benzo[1,2-b:4,5-b′]-dithiophene (4) and 2,6-bis(trimethyltin)-4,8-bis(4-ethylhexyloxy-1-mata-fluorophenyl)benzo[1,2-b:4,5-b′]-dithiophene (7), of the two small molecules were synthesized from benzo[1,2-b:4,5-b′]dithiophene4,8-dione (1) by Grignard reaction and lithium reagent successively according to previous literature.27,33 Subsequently, the two central units were reacted with 5-bromo-3,3-dioctyl2,2′:5,2″-terthiophene-2-carbaldehyde (8) via Stille coupling reaction to afford CHO3T-BDTP (9) and CHO3T-BDTP-F 25238

DOI: 10.1021/acsami.5b07066 ACS Appl. Mater. Interfaces 2015, 7, 25237−25246

Research Article

ACS Applied Materials & Interfaces

lower HOMO levels.42 These results indicate that the introduction of fluorine in the donor unit of the small molecule could result in deeper HOMO levels, which may be conducive to higher Voc. The energy level diagrams of the two small molecules and other materials used in SM-OSCs are shown in Figure 2d. For BHJ OSCs, as is well-known, the band offset determined by the LUMO of donor materials and PCBM must be greater than the exciton binding energy.43 Compared with the energy level of PC71BM, the LUMO energy level offsets (ΔELUMO) of C3TBDTP/PC71BM and C3T-BDTP-F/PC71BM are 0.66 and 0.57 eV, respectively, which were large enough to offer driving force for charge separation and transfer, indicating that the energy levels of both molecules reflect their suitability for application as photovoltaic donors with PC71BM as the acceptor in SMOSCs. Photovoltaic Properties of SM-OSC Devices. The general BHJ OSCs were fabricated with a device structure of ITO/PEDOT:PSS/active layer/interlayer/Al to investigate the photovoltaic properties of the two similar small molecules. Moreover, three cathode interlayers, including ZrAcac, PDINO, and PDIN, were employed to further optimize the performance. Recently, the commercially available ZrAcac has been used as an efficient cathode interfacial layer.39,44,45 As a thickness-insensitive electron extraction layer (EEL), PDINO has been used in perovskite hybrid solar cells and effectively improved the performance.46 PDIN, as another thicknessinsensitive EEL, has been used to improve PCEs of fullerenefree PSCs.47 The device architecture and chemical structures of the three interlayers are sketched in Figure 2c. Figure 3 shows the typical current density−voltage (J−V) curves of the devices based on C3T-BDTP and C3T-BDTP-F under one sun illumination. Table 2 displays the corresponding device performances, including Voc, Jsc, FF, and PCE. As shown in Tables S1 and S2, for both C3T-BDTP and C3T-BDTP-F, an optimal donor/acceptor weight ratio of 1:1 was obtained. The device for C3T-BDTP shows a moderate PCE of 3.66% with a Voc of 0.94 V, a Jsc of 5.96 mA cm−2, and a FF of 0.65. A slightly higher PCE of 3.79% with a higher Voc of 0.99 V, a Jsc of 5.85 mA cm−2, and a FF of 0.66 was obtained for the device based on C3T-BDTP-F. To obtain desirable morphology, we investigated solvent additive processing, which has been recognized as an effective method to optimize device properties, to determine its impact on the resulting performances of SM-OSCs. As shown in Figure 3a and Figures S5 and S6, using ZrAcac as the cathode interlayer, when 0.4% v/v of the solvent additive 1,8diiodooctane (DIO) was added to the chloroform (CHCl3) casting solution, much better PCEs of 4.70 and 4.83% for C3TBDTP and C3T-BDTP-F, respectively, were obtained with significantly improved Jsc and a slight reduction of Voc. For C3T-BDTP, when replacing the ZrAcac layer with PDI-based interlayers, due to enhanced Jsc, higher PCE values of 5.11 and 5.27% were obtained for the PDINO and PDIN devices, respectively. For C3T-BDTP-F, a similar result was obtained that the peak PCEs of PDINO and PDIN devices were also increased to 5.09 and 5.42%, respectively, with a high Voc of 0.97 V. The Voc of 0.97 V is much higher than that of alkylthiophene-substituted BDT-based small molecules using DIO as the solvent additive.42 For both C3T-BDTP and C3TBDTP-F, in comparison with ZrAcac, devices with PDINO or PDIN as the cathode layer show slightly lower series resistance (Rs), which may explain the preferable performance. Though

(10) in the modest yield (50−55%). Finally, C3T-BDTP and C3T-BDTP-F, which were fully characterized by 1H NMR, 13C NMR, and elemental analysis, were obtained from aldehyde compounds 9 or 10 with 2-octyl cyanoacetate via Knoevenagel condensation reactions. Thermal stabilities of the two small molecules were measured by thermogravimetric analysis (TGA) under a nitrogen atmosphere at a heating rate of 10 K min−1, and the decomposition temperatures (Td, defined as the temperature corresponding to >5% mass loss) for C3T-BDTP and C3TBDTP-F were determined to be 373 and 356 °C, indicating their high thermal stability, as shown in Figure 1.

Figure 1. TGA curves of the two small molecules C3T-BDTP and C3T-BDTP-F.

Optical and Electrochemical Properties. The absorption spectra for C3T-BDTP and C3T-BDTP-F in CHCl3 solution and thin film are shown in Figure 2a at room temperature, and the corresponding absorption parameters are summarized in Table 1 for comparison. In CHCl3 solution, the two small molecules exhibit almost coincident profiles, whereas in solid state, the absorption profile of C3T-BDTP-F is red-shifted ∼40 nm absorption compared to that of C3T-BDTP, indicating that the stacking behavior of the small molecule could be influenced distinctly by introducing a F atom into the donor unit. In solution, the two molecules both show a broad absorption band range from 350 to 600 nm with a maximum at 495 nm. In the film state, because of stronger intermolecular interactions, the maximum absorption peaks of the two molecular films show obvious bathochromic shifts of 35 and 77 nm for C3T-BDTP and C3T-BDTP-F, respectively. The optical bandgaps (Egopt) estimated from the UV−vis absorption onsets (693 nm for C3T-BDTP and 706 nm for C3T-BDTP-F) in the solid state are determined to be 1.79 eV for C3T-BDTP and 1.75 eV for C3T-BDTP-F. To investigate the electrochemical properties of the two molecules, we apply cyclic voltammetry (CV) measurements to measure the HOMO levels of the two molecules. From Figure 2b, the oxidation onset potentials of C3T-BDTP and C3TBDTP-F are 0.778 and 0.727 V, respectively. The HOMO energy levels of the molecules could be calculated as HOMO = 25 ox −e(Eox on + 4.4) (eV), where the unit Eon is in V vs Ag/AgCl. The HOMO and LUMO levels of C3T-BDTP and C3TBDTP-F are −5.13 eV/−3.34 eV and −5.18 eV/−3.43 eV, respectively. Compared with the alkylthiophene-substituted BDT-based molecule with similar structure, the two alkoxyphenyl-substituted BDT-based small molecules have slightly 25239

DOI: 10.1021/acsami.5b07066 ACS Appl. Mater. Interfaces 2015, 7, 25237−25246

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ACS Applied Materials & Interfaces

Figure 2. (a) UV−vis absorption features of C3T-BDTP and C3T-BDTP-F; (b) cyclic voltammograms of C3T-BDTP and C3T-BDTP-F films on a platinum electrode in 0.1 M Bu4NPF6, CH3CN solution; (c) device architecture and chemical structures of three interlayers; and (d) schematic energy diagram of the materials involved in the OSCs.

40−50% for C3T-BDTP and C3T-BDTP-F, respectively. Different EQE photoresponse characteristics of C3T-BDTP and C3T-BDTP-F-based devices could be ascribed to the different situations of intra- and intermolecular stacking behavior and absorption features. As marked in Figure 4a and b with red dashed lines, the EQE spectra of C3T-BDTP show a convex arc in the wavelength range from 460 to 570 nm, whereas for C3T-BDTP-F, the EQE spectra show a concave arc. To gain insight into the different EQE spectra between C3T-BDTP and C3T-BDTP-F, the absorption of small molecule:PC71BM (1:1) blend films were measured for comparison. As shown in Figure 5, although the absorption of the C3T-BDTP-F:PC71BM blend is slightly red-shifted, the absorption of the C3T-BDTP:PC71BM blend in the wavelength range from 350 to 570 nm shows a higher absorbance coefficient, which may be the main reason for the different EQE curves and comparatively lower Jsc of the C3T-BDTP-F-based devices. The integral current density values deduced from the EQE curves of C3T-BDTP- and C3T-BDTP-F-based devices with DIO are fairly consistent (∼3% mismatch) with the Jsc from the J−V measurements. Morphology. As shown in Figure 6a and e, the surface of the small molecule:PC71BM (1:1) layer is relatively smooth with a root-mean-square (RMS) roughness of approximately 2.44 and 2.94 nm for C3T-BDTP and C3T-BDTP-F, respectively. With the addition of 0.4% v/v DIO, the RMS values of the blend films increase to 5.68 and 6.49 nm, respectively. As is well-known, to gain superior performance, the phase separation must be regulated to a suitable range, in

Table 1. Optical and Electrochemical Properties of C3TBDTP and C3T-BDTP-F small molecule

λmax solutiona (cm)

λmax filmb (cm)

λonset film (nm)

Egoptc (eV)

EHOMOd (eV)

ELUMOe (eV)

C3T-BDTP C3T-BDTP-F

495 495

530 572

693 706

1.79 1.75

−5.13 −5.18

−3.34 −3.43

a

Measured in chloroform solution. bCast from chloroform solution. Bandgap estimated from the optical absorption onset. dHOMO = e −e(Eox on + 4.4) (eV) using Ag/AgCl as the reference electrode. LUMO 25 optc = Eg + HOMO (eV). c

various interlayers have been demonstrated to effectively improve photovoltaic properties, there might be one or some of them that could show a relatively better effect when applied to a specific system.41 Compared with C3T-BDTP, no matter whether DIO is added or not, devices based on C3T-BDTP-F show slightly higher Voc but a little lower Jsc. The higher Voc of C3T-BDTP-F-based devices could be ascribed to the deeper HOMO level of C3T-BDTP-F. To deeply understand the reason why, under the same conditions, the Jsc of the fluorinated molecule C3T-BDTP-F-based devices is lower than that of C3T-BDTP, external quantum efficiency (EQE) and hole mobility have been studied in detail. To further understand the device performance, we show EQE curves in Figure 4a for C3T-BDTP-based devices and Figure 4b for C3T-BDTP-F-based devices. All the EQE spectra exhibit a spectral response from 300 to 700 nm with a broad plateau at approximately 370−620 nm around 40−60% and 25240

DOI: 10.1021/acsami.5b07066 ACS Appl. Mater. Interfaces 2015, 7, 25237−25246

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ACS Applied Materials & Interfaces

Figure 3. Current density−voltage (J−V) characteristics: (a) C3TBDTP:PC71BM (1:1) with 0.4% v/v DIO and (b) C3T-BDTPF:PC71BM (1:1) with 0.4% v/v DIO.

Figure 4. External quantum efficiency (EQE) spectra of (a) C3TBDTP:PC71BM (1:1) with 0.4% v/v DIO and (b) C3T-BDTPF:PC71BM (1:1) with 0.4% v/v DIO.

which case charge carrier transport and exciton dissociation and charge separation could be balanced.48,49 From the phase images in Figure 6b and f, the aggregation size of the blend without DIO is too small to get efficient EQEs.50−52 High RMS and larger aggregation size with the addition of DIO reveal that the arrangement of molecules is much more ordered. Compared with the as-cast film, better phase separation could be found with the addition of DIO, which is consistent with the increased Jsc and enhanced EQE. X-ray Characterization. To investigate the relationship between the photovoltaic properties and additives, we measured the crystallinities of C3T-BDTP/PC71BM and C3T-BDTP-F/PC71BM with and without DIO by X-ray diffraction (XRD) analysis for thin films spin-coated from CHCl3 solution onto ITO glass substrates. As shown in Figure 7a and b, the two molecules both exhibited strong (100) reflection peaks at approximately 2θ = 4.0° for the C3T-BDTP and C3T-BDTP-F/PC71BM blended film, corresponding to a d100 spacing value of approximately 22.1 Å. The peak at small angle suggests that d100 spacing is between the side chain rather

Figure 5. Absorption coefficient spectra of the spin-coated small molecule:PC71BM (1:1) films with 0.4% v/v DIO.

than π−π stacking, which indicates that the packing is dominated by side chains.24,53 The similar d100 spacing value of C3T-BDTP/PC71BM and C3T-BDTP-F/PC71BM may be due to a quite small van der Waals radius of 1.35 Å of the F

Table 2. Summary of the Photovoltaic Characteristics of the Small Molecule:PC71BM (1:1) Blend Films with Different Interlayers active layer C3T-BDTP:PC71BM (1:1) with 0.4% DIO

C3T-BDTP-F:PC71BM (1:1) with 0.4% DIO

a

interlayer

Voc (V)

Jsc (mA cm−2)

FF (%)

PCEmax (%)

PCEavg (%)a

Rs (Ω cm2)b

ZrAcac PDINO PDIN ZrAcac PDINO PDIN

0.909 0.910 0.908 0.970 0.974 0.968

8.40 8.81 9.65 8.14 8.10 8.80

61.5 63.8 60.1 61.2 64.4 63.6

4.70 5.11 5.27 4.83 5.09 5.42

4.63 4.99 5.16 4.74 5.00 5.30

17.6 15.9 16.1 15.5 15.3 15.1

PCEavg is the data obtained from 8 devices. bRs is defined from the J−V curves at V = Voc. 25241

DOI: 10.1021/acsami.5b07066 ACS Appl. Mater. Interfaces 2015, 7, 25237−25246

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ACS Applied Materials & Interfaces

Figure 6. AFM height images (size: 5 × 5 μm2): C3T-BDTP:PC71BM (1:1) without DIO (a) and with DIO (c); C3T-BDTP-F:PC71BM (1:1) without DIO (e) and with DIO (g). AFM phase images (size: 5 × 5 μm2): C3T-BDTP:PC71BM (1:1) without DIO (b) and with DIO (d); C3TBDTP-F:PC71BM (1:1) without DIO (f) and with DIO (h). Root-mean-square (RMS) roughness values are given to describe the smoothness of the morphology.

the addition of 0.4% v/v DIO, the reflection intensity of the diffraction peak was significantly enhanced, indicating that the degree of crystallinity was further improved, which is in

atom; therefore, undesired steric hindrance could be avoided, and thus, molecular size will not be distinctly changed. For both C3T-BDTP/PC71BM and C3T-BDTP-F/PC71BM blends, with 25242

DOI: 10.1021/acsami.5b07066 ACS Appl. Mater. Interfaces 2015, 7, 25237−25246

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ACS Applied Materials & Interfaces

two small molecules showed similar absorption characteristics in the wavelength range from 300 to 600 nm in CHCl3 solution, whereas in solid state, C3T-BDTP-F showed redshifted absorption compared to C3T-BDTP. The blend of C3T-BDTP-F/PC71BM possessed a higher hole mobility of 8.67 × 10−4 cm2 V−1 S1− compared with the blend of C3TBDTP/PC71 BM. With a device structure of ITO/PEDOT:PSS/donor:PC71BM/PDIN/Al, the PSCs based on C3T-BDTP delivered a satisfying PCE of 5.27%, and the devices based on C3T-BDTP-F showed an enhanced PCE of 5.42% with a high Voc of 0.97 V. These results indicate that the introduction of a fluorinated group to the donor unit of a small molecule may slightly reduce the Jsc due to the different absorption profiles but could effectively lower the HOMO level and improve Voc; thus. higher PCE could be realized to some extent.

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EXPERIMENTAL SECTION

Materials. 4-Bromophenol, 4-bromo-2-fluorophenol, lithium diisopropylamide (LDA), n-butyllithium (n-BuLi), tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4), and trimethyltin chloride (Sn(CH3)3Cl) were obtained from J&K and Alfa Asia Chemical Co. Toluene was dried over Na/benzophenone and freshly distilled prior to use. All other reagents and solvents were purchased commercially as ACS-grade quality and used without further purification. Synthesis. 1-Bromo-4-(2′-ethylhexyloxy)benzene (2), 4,8-bis(4ethylhexyloxy-1-phenyl)-benzo[1,2-b:4,5-b′]-dithiophene (3), 2,6-bis(trimethyltin)-4,8-bis(4-ethylhexyloxy-1-phenyl)-benzo[1,2-b:4,5-b′]dithiophene (4), 4-bromo-1-(2′-ethylhexyloxy)-2-fluorobenzene (5), 4,8-bis(4-ethylhexyloxy-1-meta-fluorophenyl)-benzo[1,2-b:4,5-b′]-dithiophene (6), 2,6-bis(trimethyltin)-4,8-bis(4-ethylhexyloxy-1-metafluorophenyl)-benzo[1,2-b:4,5-b′]-dithiophene (7), and 5-bromo-3,3dioctyl-2,2′:5,2″-terthiophene-2-carbaldehyde (8) were prepared according to previous literature.27,33,54 Synthesis of CHO3T-BDTP (9). A solution of 4 (0.44 g, 0.76 mmol) and 8 (0.25 g, 0.25 mmol) in dry toluene (12 mL) was degassed several times with argon followed by the addition of Pd(PPh3)4 (15 mg, 0.01 mmol). After stirring at 100 °C for 24 h under argon, the mixture was poured into water and extracted with CHCl3. The organic layer was washed with water, dried over Na2SO4, and evaporated. The residue was purified by silica gel chromatography using a mixture of petroleum and dichloromethane ether (1:2) as eluent to afford compound CHO3T-BDTP as a red solid (0.22 g, 55%). 1 H NMR (500 MHz, CDCl3): δ 9.83 (s, 2H), 7.64 (d, 4H), 7.59 (s, 2H), 7.35 (s, 2H), 7.24 (d, 2H), 7.14 (d, 4H), 7.10 (d, 2H), 7.08 (s, 2H), 4.02−3.96 (m, 4H), 2.86−2.71 (m, 8H), 1.85−1.78 (m, 2H), 1.72−1.63 (m, 8H), 1.55−1.22 (m, 56H), 1.00 (t, 6H), 0.95 (t, 6H), 0.91−0.83 (m, 12H). Synthesis of CHO3T-BDTP-F (10). CHO3T-BDTP-F was synthesized similarly to that described above for 9, and a red solid was obtained (0.21 g, 52%). 1 H NMR (500 MHz, CDCl3): δ 9.83 (s, 2H), 7.60 (s, 2H), 7.47− 7.40 (m, 4H), 7.32 (s, 2H), 7.24 (d, 2H), 7.20 (t, 2H), 7.12 (d, 2H), 7.10 (s, 2H), 4.08−4.03 (m, 4H), 2.86−2.72 (m, 8H), 1.91−1.83 (m, 2H), 1.72−1.64 (m, 8H), 1.53−1.23 (m, 56H), 1.02 (t, 6H), 0.96 (t, 6H), 0.90−0.85 (m, 12H). Synthesis of C3T-BDTP. CHO3T-BDTP (0.16 g, 0.10 mmol) was dissolved in a solution of dry CHCl3 (30 mL); three drops of triethylamine and then octyl cyanoacetate (0.3 mL, 1.60 mmol) were added, and the resulting solution was stirred for 24 h under argon at room temperature. The reaction mixture was then extracted with CH2Cl2, washed with water, and dried over Na2SO4. After removal of solvent, it was purified by chromatography on silica gel using a mixture of dichloromethane and petroleum ether (3:2) as eluent to afford C3T-BDTP as a black solid (120 mg, 60% yield).

Figure 7. X-ray diffraction (XRD) analysis of a small molecule/ PC71BM film with and without 0.4% v/v DIO for (a) C3T-BDTP/ PC71BM and (b) C3T-BDTP-F/PC71BM.

agreement with AFM results that the RMS roughness was larger and the phase separation was enhanced. The enhanced crystallinity with the addition of DIO is beneficial to the carrier transport, resulting in an improved Jsc. Hole Mobility. Besides the absorption profiles and energy levels, the hole transport abilities of donor materials also have great effects on the resulting performance, especially on the Jsc and FF of BHJ solar cells.45 To verify the positive effect of a small amount of DIO on charge transport, we also explored the hole mobilities of C3T-BDTP/PC71BM (1:1) and C3T-BDTPF:PC71BM (1:1) with and without 0.4% v/v DIO for comparison. The hole mobilities for devices fabricated with C3T-BDTP and C3T-BDTP-F are 1.65 × 10−4 cm2 V−1 S−1 and 1.92 × 10−4 cm2 V−1 S1−, respectively, whereas they were increased to 5.42 × 10−4 cm2 V−1 S−1 and 8.67 × 10−4 cm2 V−1 S−1 with the addition of 0.4% v/v DIO, which is consistent with the increased Jsc. Compared with C3T-BDTP, the devices based on C3T-BDTP-F showed higher hole mobility, which is in accordance with many literature studies that discuss the effects of fluorination of small molecules. The results indicate that the lower Jsc of the C3T-BDTP-F-based devices should not be attributed to the hole mobility but to the absorption characteristics.

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CONCLUSIONS In summary, two new small molecules, C3T-BDTP and C3TBDTP-F with alkoxyphenyl-substituted BDT and a metafluorinated one as the donor blocks, respectively, have been synthesized and used as the donor materials in SM-OSCs. The 25243

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ACS Applied Materials & Interfaces H NMR (500 MHz, CDCl3): δ 8.20 (s, 2H), 7.65 (d, 4H), 7.59 (s, 2H), 7.35 (s, 2H), 7.29 (d, 2H), 7.15 (d, 4H), 7.11 (d, 2H), 7.08 (s, 2H), 4.28 (t, 4H), 4.03−3.94 (m, 4H), 2.87−2.69 (m, 8H), 1.85−1.79 (m, 2H), 1.78−1.72 (m, 4H), 1.71−1.62 (m, 8H), 1.55−1.22 (m, 76H), 1.00 (t, 6H), 0.96 (t, 6H), 0.92−0.83 (m, 18H). 13C NMR (126 MHz, CDCl3): δ 163.14, 159.48, 145.91, 141.61, 141.21, 140.71, 140.57, 138.43, 138.14, 137.19, 137.12, 135.98, 134.25, 132.99, 130.89, 130.42, 130.03, 129.55, 128.28, 128.24, 126.25, 118.97, 116.00, 114.95, 97.79, 77.23, 70.55, 66.57, 39.54, 31.89, 31.85, 31.79, 30.64, 30.41, 30.23, 29.68, 29.61, 29.51, 29.45, 29.39, 29.37, 29.26, 29.24, 29.19, 29.16, 28.57, 25.81, 23.97, 23.12, 22.68, 22.66, 14.15, 14.11, 11.23. Calculated analysis for C118H156N2O6S8 (%): C, 72.49; H, 8.04; N, 1.43. Actual analysis: C, 72.31; H, 8.22; N, 1.51. Synthesis of C3T-BDTP-F. C3T-BDTP-F was synthesized in a similar manner to that described above for C3T-BDTP. The crude solid was recrystallized from hexane and CHCl3 mixture to afford C3T-BDTP-F as black solid (100 mg, 50% yield). 1 H NMR (500 MHz, CDCl3): δ 8.20 (s, 2H), 7.59 (s, 2H), 7.48− 7.40 (m, 4H), 7.33 (s, 2H), 7.30 (d, 2H), 7.20 (t, 2H), 7.13 (d, 2H), 7.10 (s, 2H), 4.29 (t, 4H), 4.10−4.02 (m, 4H), 2.87−2.72 (m, 8H), 1.93−1.84 (m, 2H), 1.77−1.72 (m, 4H), 1.71−1.63 (m, 8H), 1.52− 1.22 (m, 76H), 1.02 (t, 6H), 0.96 (t, 6H), 0.92−0.85 (m, 18H). 13C NMR (126 MHz, CDCl3): δ 163.11, 145.89, 141.52, 141.29, 140.64, 138.29, 138.05, 137.58, 137.06, 135.66, 134.39, 133.06, 130.33, 128.63, 128.44, 128.28, 126.38, 125.20, 118.51, 117.21, 117.06, 115.97, 114.99, 97.90, 71.97, 66.57, 39.52, 31.87, 31.84, 31.77, 31.59, 30.51, 30.42, 30.22, 29.66, 29.60, 29.49, 29.43, 29.37, 29.35, 29.25, 29.22, 29.18, 29.15, 28.57, 25.81, 23.89, 23.08, 22.65, 22.64, 14.11, 14.08, 11.16. Calculated analysis for C118H154F2N2O6S8 (%): C, 71.18; H, 7.80; N, 1.41. Actual analysis: C, 71.02; H, 7.97; N, 1.48. Measurements and Instruments. A Bruker DMX-400 or DMX500 spectrometer was used to record 1H and 13C NMR spectra in CDCl3 at 293 K, respectively, with tetramethylsilane (TMS) as an internal standard (δ 0.00 ppm). Thermogravimetric analysis (TGA) was performed on a PE TGA-7 at a heating rate of 10 K min−1 in a nitrogen atmosphere. Elemental analyses were carried out using a FLASH EA1112 elemental analyzer. A Hitachi U-2450 UV−vis spectrophotometer was applied to record UV−vis absorption spectra. For solid state measurements, a small molecular solution in chloroform was cast on quartz plates. The CV experiments were conducted on a Zahner IM6e electrochemical workstation. CV was carried out under argon atmosphere at room temperature in an anhydrous and argonsaturated solution of 0.1 mol L−1 tetrabutyl ammonium hexafluorophosphate (Bu4NPF6) in acetonitrile at a scan rate of 50 mV s−1. A platinum disk covered by the small molecule film, Pt wire, and a Ag/ AgCl electrode were used as the working, counter, and reference electrodes, respectively. XRD measurements of the thin films prepared on ITO substrates were carried out with a 2 kW Rigaku D/max-2500 X-ray diffractometer. The morphologies of small molecules:PC71BM blend films were obtained using a Veeco’s Dimension V atomic force microscope (AFM) in tapping mode. Fabrication and Characterization of Small Molecular Organic Solar Cells. The SM-OSCs were fabricated in the traditional sandwich structure with an indium tin oxide (ITO) glass positive electrode and a metal negative electrode. Patterned ITO glass with a sheet resistance of 10 Ω sq−1 was purchased from CSG Holding Co. Ltd. (China). The ITO glass was cleaned by sequential ultrasonic treatment in detergent, deionized water, acetone, and isopropanol, and then treated in an ultraviolet-ozone chamber (Ultraviolet Ozone Cleaner, Jelight Company, USA) for 20 min. The PEDOT:PSS (poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate)) (Baytron P 4083, Germany) was filtered through a 0.45 mm filter and spin-coat at 4000 rpm for 30 s on the ITO electrode. Subsequently, the PEDOT:PSS film was baked at 150 °C for 15 min in air to give a thin film with a thickness of ∼30 nm. A blend of the small molecule and PC71BM was dissolved in chloroform (CHCl3) and spin-coat at 3000 rpm for 30 s onto the PEDOT:PSS layer. The thickness of the photoactive layer is in the range of 90−110 nm. A thin layer of interlayer in methanol with different concentrations (1 mg mL−1 for ZrAcac and 1.5 mg mL−1 for PDINO and PDIN) was spin-coat atop 1

the photoactive layer at a spin coating rate of 3000 rpm. Finally, the metal cathode Al was thermally evaporated under a shadow mask with a base pressure of approximately 10−5 Pa. The photoactive area of the device is 5 mm2. Device characterization was carried out under AM 1.5 G irradiation with an intensity of 100 mW cm−2 (Oriel 67005, 500 W) calibrated by a standard silicon cell. J−V curves were recorded with a Keithley 2450 digital source meter. Fabrication of Hole-Only Devices. The configurations of the hole-only devices were ITO/PEDOT:PSS/small molecule:PC71BM/ Au. The hole mobilities were determined by fitting the plots of the dark J−V curves for single-carrier devices according to the spacecharge-limited current (SCLC) model. The SCLC is described by modified Mott−Gurney law55 J = (9/8)ε0εr μ(V 2/d3)exp[0.89β(V /d)0.5 ] Thus, we obtain the formula

ln(Jd3/V 2) ≈ 0.89β(V /d)0.5 + ln(9ε0εr μ/8) where εr is the relative dielectric constant of the transport medium (assuming that 3.0), ε0 is the permittivity of free space (8.85 × 10−12 C V−1 m−1), μ is the carrier mobility, β is the field activation factor, and d is the device thickness. The results are plotted as ln(Jd3/V2) versus (V/ d)0.5, as shown in Figure 8.

Figure 8. C3T-BDTP- and C3T-BDTP-F-based devices for the measurement of hole mobility by the space-charge-limited current (SCLC) method (a) without and (b) with 0.4% v/v DIO.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07066. NMR spectra of the two small molecules are shown in Figures S1−S4; current density−voltage (J−V) characteristics of small molecule:PC71BM (1:1) with the device 25244

DOI: 10.1021/acsami.5b07066 ACS Appl. Mater. Interfaces 2015, 7, 25237−25246

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ACS Applied Materials & Interfaces

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structure of ITO/PEDOT:PSS/active layer/Ca/Al are shown in Figure S5; photovoltaic characteristics of the small molecule:PC71BM blend films with the device structure of ITO/PEDOT:PSS/active layer/Ca/Al are summarized in Table S1; current density−voltage (J−V) characteristics of small molecule:PC71BM with different ratios are shown in Figures S6 and S7; photovoltaic characteristics of small molecule:PC71BM blend films are summarized in Tabled S2 and S3. J−V characteristics of devices based on small molecule/PC71BM (0.4% v/v DIO) with different interlayers in the dark are shown in Figures S8 and S9. Current−voltage data for the measurement of hole mobility with and without 0.4% v/v DIO are shown in Figures S10 and Figure S11. TEM images of small molecule/PC71BM (1:1) without and with 0.4% DIO are shown in Figures S12 and S13 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (Nos. 51173206, 91433117, and 21506258) and an Innovation Foundation for Postgraduate award (2014zzts158).

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