Interplay of Intramolecular Noncovalent Coulomb Interactions for

Aug 19, 2015 - Four different kinds of photovoltaic polymers were synthesized by controlling the intrachain noncovalent coulomb interactions through t...
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Interplay of Intramolecular Noncovalent Coulomb Interactions for Semicrystalline Photovoltaic Polymers Mohammad Afsar Uddin,†,‡ Tack Ho Lee,§ Shuhao Xu,‡ Song Yi Park,§ Taehyo Kim,§ Seyeong Song,§ Thanh Luan Nguyen,†,‡ Seo-jin Ko,§ Sungu Hwang,∥ Jin Young Kim,*,§ and Han Young Woo*,†,‡ †

Department of Chemistry, Korea University, Seoul 136-713, Republic of Korea Department of Cogno-Mechatronics Engineering, Pusan National University, Miryang 627-706, Republic of Korea § School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea ∥ Department of Nanomechatronics Engineering, Pusan National University, Miryang 627-706, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Four different kinds of photovoltaic polymers were synthesized by controlling the intrachain noncovalent coulomb interactions through the incorporation of alkoxy- or alkylthiosubstituted phenylene, 4,7-di(furan-2-yl)benzothiadiazole, and 4,7di(thiophen-2-yl)benzothiadiazole as a building block. Fine modulation of the interplay of dipole−dipole, H-bond, and chalcogen−chalcogen interactions (O···S, O···H, S···S, S···F, etc.) along the polymeric backbone influenced the chain planarity, interchain organization, film morphology, and electrical and photovoltaic properties significantly. By replacing the alkoxy substituents with alkylthio groups, the torsional angle increased (136−168°) due to the absence of an O···S attractive coulomb interaction (and/or increased S···S steric hindrance), enhancing the amorphous nature with hindered interchain packing. The alkoxy-substituted polymers exhibited nanofibrillar structures, showing strong interlamellar scattering peaks up to (300) with tight face-on π−π stacking in grazing incidence X-ray scattering. The measured carrier mobility of the alkoxy-containing polymers was 1−2 orders of magnitude higher than that of the alkylthio-containing polymers. The incident-light-intensity-dependent photovoltaic characteristics clearly suggested efficient charge generation/extraction with less charge recombination for the alkoxycontaining semicrystalline polymers. The resulting photovoltaic energy conversion efficiency of the PPDT2FBT, PPDF2FBT, PPsDF2FBT, and PPsDT2FBT blended devices with PC70BM was measured to be 8.28%, 5.63%, 5.12%, and 0.55%, respectively. This study suggests an important molecular design guideline for the further optimization of photovoltaic polymers and devices by finely controlling the interplay of the weak noncovalent coulomb interactions. (JSC), open-circuit voltage (VOC), and fill factor (FF). These parameters are closely related to light absorption in the photoactive layer, energy level matching among the donor, acceptor and electrodes, interfaces control, carrier transport, and BHJ film morphology, etc. Ideal photovoltaic polymers should have a broad absorption range, low band gap with a deep highest occupied molecular orbital (HOMO), well-matched frontier orbital levels with electron acceptors (i.e., fullerene derivatives), and/or metal electrodes for efficient charge generation and transport. The semicrystalline BHJ film morphology also needs to be controlled carefully for the optimization of charge transport with minimal electron−hole recombination. The coexistence of crystalline domains (in the pure donor and acceptor phase) and intermixed amorphous phase is often emphasized to maximize

1. INTRODUCTION Bulk heterojunction (BHJ) polymer solar cells (PSCs) based on a blend of electron-donor polymers and electron-accepting fullerene derivatives have attracted considerable interest due to recent achievement of power conversion efficiencies (PCEs) of 9−10% with many advantages, such as mechanical flexibility and low-cost solution processing on a plastic substrate for flexible and portable photovoltaic devices.1−4 Recently, remarkable and rapid progress has been made in PSC technology via polymer structural elucidation,5−9 morphology control,10−14 and device architecture optimization, etc.15−20 Understanding the structure−property relationships for the optimization of plastic semiconductors has become an important topic in the field of soft matter physics. Still, there is room for improving the photovoltaic molecular structure and the device architecture for further improvements in the device efficiency and stability for future industrial applications.21,22 The device characteristics of PSCs depend on several fundamental parameters, such as short-circuit current density © 2015 American Chemical Society

Received: June 15, 2015 Revised: August 19, 2015 Published: August 19, 2015 5997

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Chemistry of Materials Scheme 1. (a) Synthetic Scheme and (b) Molecular Structures of Polymers

charge generation, transport, and extraction.23,24 The molecular design of crystalline low band gap polymers with good solution processability and the appropriate miscibility with fullerene acceptors is still challenging. Several approaches to designing crystalline photovoltaic polymers have been reported, such as incorporating fused aromatic structures in a polymer backbone.25−27 On the other hand, these approaches often increase the chain rigidity too much, reducing the solubility and creating poor miscibility with fullerenes. A series of highly efficient semicrystalline photovoltaic polymers were recently designed and synthesized by the fine-modulation of intra- and/or interchain noncovalent coulomb interactions, such as dipole− dipole interactions and hydrogen bonding.28 This leads to a highly ordered nanofibrillar film morphology with [6,6]-phenylC71-butyric acid methylester (PC70BM), showing the PCE of up to 9.39% in a ∼ 300 nm thick conventional single-cell device structure. Noncovalent attractive interactions via O···S, S···F, and C−H···N minimize the torsional angle along the polymer backbone, thereby improving the planarity of the polymer chain and interchain ordering without losing solution processability. Fluorination is also an effective way of decreasing the HOMO and lowest unoccupied molecular orbital (LUMO) levels of the donor polymer with strengthening of the intra/interchain dipole−dipole interactions.9,28−31 Noncovalent coulomb interactions have been emphasized to play an important role in controlling the crystalline film morphology to increase the carrier mobility, JSC, and FF.32−34 In this contribution, a series of thiophene (or furan)- and difluorobenzothiadiazole (2FBT)-based donor (D)−acceptor (A) type low band gap polymers were designed and synthesized by the systematic modification of intramolecular noncovalent coulomb interactions. In the same molecular framework, a range of dipole−dipole, H-bond, and/or chalcogen−chalcogen interactions (O···S, O···H, S···S, S···F, etc.) were considered by incorporating alkoxy and alkylthio substituents as the side-chain in a polymeric backbone. Detailed optical, electrochemical, conformational analysis, interchain ordering, BHJ morphology, and the resulting photovoltaic characteristics of four types of polymers were studied and analyzed in terms of the molecular structures with different noncovalent interactions. The structure−property relationship revealed here suggests an

important molecular design guideline for the further optimization of photovoltaic polymers and devices.

2. RESULTS AND DISCUSSION 2.1. Molecular Design and Synthesis. Four different types of D−A copolymers were designed on the basis of electron-rich thiophene (or furan) and electron-poor 5,6difluoro-benzo[c][1,2,5]thiadiazole alternating structures. This study considered a series of noncovalent interactions in the molecular design of polymers between S (in thiophene) and O (in alkoxy groups), between S (in alkylthio substituent) and O (in furan), and between S (in alkylthio substituent) and S (in thiophene) in a polymeric backbone. The dialkoxyphenylene, bis(alkylthio)phenylene, and 2FBT-based monomers were prepared by modifying the procedures reported in previous studies.28,35,36 Two different kinds of dibrominated monomers, 1,4-dibromo-2,5-bis(2-hexyldecyloxy)benzene (M1) and 1,4dibromo-2,5-bis(2-hexyldecylthio)benzene (M2), were reacted with two different 2FBT-based monomers, 4,7-bis(5-trimethylstannylfuran-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (M3) and 4,7-bis(5-trimethylstannylthiophen-2-yl)-5,6-difluoro-2,1,3benzothiadiazole (M4), to yield four D−A copolymers, poly[(2,5-bis(2-hexyldecylthio)phenylene)-alt-(5,6-difluoro4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPsDT2FBT), poly[(2,5-bis(2-hexyldecylthio)phenylene)-alt(5,6-difluoro-4,7-di(furan-2-yl)benzo[c][1,2,5]thiadiazole)] (PPsDF2FBT), poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt(5,6-difluoro-4,7-di(furan-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDF2FBT), and poly[(2,5-bis(2-hexyldecyloxy)phenylene)alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) via Stille cross-coupling with Pd2(dba)3 as a catalyst in chlorobenzene using a microwave reactor (67−86% yield). Scheme 1 presents the chemical structures and synthetic scheme for the copolymers. The detailed synthetic procedures for the monomers are described in the Supporting Information (Scheme S1). The numberaverage molecular weight and molecular weight distribution for PPsDT2FBT, PPsDF2FBT, PPDF2FBT, and PPDT2FBT were measured to be 31 (polydispersity index, PDI = 1.7), 19 (2.6), 40 (1.8), and 40 kDa (2.1), respectively. 2.2. Structural Analysis by Density Functional Theory. The torsional profiles, energy minimum conformation, and the 5998

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agreement with the measured optical and electrochemical spectra (as will be discussed in the following section). In PPsDF2FBT, oxygen (in furan) and sulfur (in the methylthio side-chain) exhibit little attractive O···S electrostatic interaction, showing a torsional angle of ∼168°. In the PPDF2FBT polymer, the furan and dimethoxybenzene showed a planar conformation (∼180° torsional angle) via two intramolecular O···H hydrogen bonding interactions. Interestingly, the dialkoxyphenylene−furan linkage in PPDF2FBT showed an unsymmetrical conformational profile with one main energy minimum conformation (Figure S1). This suggests that the O··· O interaction in PPDF2FBT is repulsive due to the partial negative charges on both oxygen atoms. The Mulliken charge distribution was also calculated, which showed that the O atom in furan is partially negative (−0.5), and O in the alkoxy sidechain is also partially negative (−0.5) (Figure S2). From the torsional profiles and Mulliken charge distribution, the alkylthio-substituted PPsDT2FBT also showed S···S repulsion and substantial steric hindrance, which induced a twisted polymer chain. A hydrogen bonding interaction between C−H (in phenylene) and O (in furan) is possible in PPsDF2FBT (see Figure 1), which decreases chain twisting. The alkoxysubstituted PPDF2FBT and PPDT2FBT show an almost planar conformation, which may strongly influence the interchain ordering and resulting BHJ film morphology. 2.3. Optical, Electrochemical, and Thermal Properties. Figure 2a,b shows the UV−vis absorption spectra of the

frontier orbital structures were calculated by density functional theory (DFT; Jaguar quantum chemistry software, B3LYP/631G** level) (Figure 1 and Figures S1, S2, and S4). For

Figure 1. Minimum energy conformations (yellow, sulfur; red, oxygen; green, fluorine; blue, nitrogen).

simplicity, methyl substituents were considered for the calculation instead of hexyldecyl side-chains. The torsional profiles were calculated by considering (1) the tendency to minimize the torsional angle to maximize the π-delocalization of the structure, (2) the steric repulsion, and (3) dipole−dipole interactions, such as electrostatic and/or hydrogen bonding interactions. Figure S1 shows the torsional profiles for the constituting building blocks. The linkage between thiophene (or furan) and 2FBT showed a planar conformation (dihedral angle: ∼180°) due to the small size of a fluorine atom and the additional stabilization via possible intramolecular hydrogen bonding between hydrogen (in thiophene or furan) and the nitrogen in 2FBT. In PPDT2FBT, the dimethoxyphenylene− thiophene bond shows the energy minimum conformation at a torsional angle of ∼17° or ∼153° via the electrostatic coulomb attraction between Sδ+ (in thiophene) and Oδ‑ (in methoxy substituents) atoms or C−H···O hydrogen bonding interactions.28,37 We guess these two conformations are randomly distributed along the polymeric backbone. Although one noncovalent coulomb interaction (such as S···O) is weak in the range 0.4−0.5 kcal/mol, it can contribute significantly to planarization of the polymer backbone if these noncovalent attractive interactions accumulate and interplay together along a polymeric backbone. In contrast, the torsional angle of the bis(methylthio)phenylene−thiophene linkage in PPsDT2FBT was calculated to be ∼136°. The alkylthio side-chain has been used in organic semiconductors, which exhibited some unique optoelectronic properties and well-ordered intermolecular packing through S···S chalcogen−chalcogen intra/intermolecular interactions with an improvement in the solubility and electrochemical stability (deeper HOMO) by replacing the alkoxy substituents with alkylthiol chains.38−43 Other research groups also showed that alkylthio side-chains exhibited steric hindrance, which significantly affects the absorption and electronic properties of the conjugated polymers.44,45 The role of the S···S interaction seems to be quite sensitive to the molecular structure, as mentioned above. In PPsDT2FBT, the S···S interaction was found to be repulsive with a large torsional angle, showing good

Figure 2. Normalized UV−vis absorption spectra in chloroform (a) and in film (b). Cyclic voltammograms (c) and thermogravimetric analysis data (d) of polymers.

polymers in chloroform and in film. All the polymers exhibited a broad absorption in the range 350−700 nm, with two distinct high- and low-energy bands assigned to the π−π* transition (approximately 400 nm) and intramolecular charge transfer (ICT) transition (500−700 nm), respectively. The measured absorption maxima of PPsDT2FBT, PPsDF2FBT, PPDF2FBT, and PPDT2FBT were λabs = 500, 610, 642, and 642 nm in solution, and λabs = 550, 617, 643, and 652 nm in film, respectively (Table 1). The polymers show the maximum molar absorption coefficient (εmax) on the same order, 4.5 × 104 to 8.9 × 104 M cm−1 in chloroform. With the exception of PPsDT2FBT, all the polymers showed a strong vibronic shoulder peak, indicating prominent interchain aggregation in both the solution and film. The spectra of PPsDT2FBT were featureless and substantially blue-shifted compared to those of 5999

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Chemistry of Materials Table 1. Summary of Optical, Electrochemical, and Thermal Properties of Polymers polymer PPsDT2FBT PPsDF2FBT PPDF2FBT PPDT2FBT

λabs (in CHCl3) [nm] 500 610 642 642

a

(4.5) (7.5) (5.1) (8.9)

λabs (in film) [nm]

Egopt [eV]b

HOMO [eV]

LUMO [eV]

Td [°C]e

1.87 1.86 1.78 1.76

−5.76 /−5.25 −5.61/−5.10 −5.44/−4.97 −5.41/−5.09

−3.89 /−2.69 −3.75/−2.61 −3.66/−2.53 −3.65/−2.60

359 378 393 402

550 617 643 652

c

d

c

d

Maximum molar absorption coefficient (×104 M cm−1). bOptical band gap estimated from the onset point in film. cHOMO levels were measured by cyclic voltammetry, and LUMO levels were estimated from HOMO and the corresponding optical band gap. dHOMO and LUMO energy levels were calculated by DFT. eDecomposition temperature (Td) was determined by TGA with 5% weight loss.

a

thiophene or furan moiety in a polymeric chain were small despite the stronger electron-donating ability of thiophene relative to furan. The thermal stability of the polymers was analyzed by thermogravimetric analysis (TGA) at a heating rate of 10 °C/ min under nitrogen. The alkylthio-substituted PPsDT2FBT and PPsDF2FBT showed a lower decomposition temperature with 5% weight-loss (Td = 359 and 378 °C) probably due to the weak C−S bond in the alkylthio side-chain. The alkoxysubstituted PPDF2FBT and PPDT2FBT showed a Td at 393 and 402 °C, respectively (Figure 2d). 2.4. Photovoltaic Characteristics. To determine how the intrachain noncovalent interactions can affect the photovoltaic characteristics, conventional single-cell devices were fabricated with the configuration, ITO/PEDOT:PSS/polymer:PC70BM/ Al (PEDOT:PSS, poly(3,4-ethylenedioxythiophene):polystyrene sulfonic acid). Figure 3a shows the energy level diagram of

the other polymers. This must be closely related to the broken chain planarity via S···S repulsion in the alkylthio side-chains and thiophene moieties in the polymer backbone, showing good agreement with DFT structural analysis. The alkoxysubstituted PPDF2FBT and PPDT2FBT show a strong vibronic shoulder peak at ∼642 nm and a red-shifted ICT peak relative to those of the alkylthio-substituted structures, indicating more pronounced interchain packing. PPDT2FBT shows a slightly red-shifted spectrum compared to PPDF2FBT, which may be due to the stronger electron-donating ability of thiophene than furan. The absorption of PPsDF2FBT was blueshifted compared to that of PPDF2FBT, which is related to the larger torsional angle in PPsDF2FBT and the weaker electronreleasing effect of the alkylthio groups compared to the alkoxy substituents. This suggests that the alkylthio substituents interrupt the planar conformation and the degree of intra/intermolecular orbital overlap with decreased effective conjugation in the polymer chain. The spectra in the film were red-shifted substantially compared to those in solution for all polymers. In particular, PPsDT2FBT showed a ∼50 nm red-shift in the absorption in film form, indicating increased effective πconjugation via interchain aggregation. The UV−vis spectra were also measured in solution by changing the solution temperature (25−80 °C, Figure S3). With increasing temperature, the vibronic shoulder peak decreased gradually for PPsDF2FBT and PPDT2FBT, confirming that the shoulder peak originated from interchain aggregation. PPDF2FBT showed a strong shoulder peak, even at 80 °C, suggesting the strongest intermolecular packing among the four copolymers. The frontier orbital structures and their energy levels were calculated (on the basis of a repeat unit) by DFT (B3LYP/631G** level), and the data are summarized in Table 1. The alkylthio-substituted PPsDT2FBT and PPsDF2FBT showed deeper HOMO and LUMO levels than the alkoxy-substituted ones. The HOMO and LUMO electronic structures of the polymers were also calculated (Figure S4). In all polymers, the wave functions in the LUMO are localized mainly on the 2FBT moiety. In contrast, the wave functions in the HOMO are delocalized on a whole polymeric backbone except for PPsDT2FBT. The HOMO is localized mainly on the dialkylthiophenylene moiety due to the twisted structure of PPsDT2FBT, suggesting good agreement with the measured UV−vis spectra.46 The HOMO and LUMO energy levels of the polymers were also determined experimentally by cyclic voltammetry (CV) (Figure 2c). The alkylthio-substituted polymers showed deeper HOMO levels (−5.76 to −5.61 eV) than the alkoxy-substituted structures (−5.44 to −5.41 eV). Similar trends were also observed in the LUMO energy levels. According to the CV and DFT data, the HOMO and LUMO levels are strongly dependent on the alkoxy or alkylthio substituents on the phenylene unit. The effects of the

Figure 3. Photovoltaic characteristics of polymer:PC70BM BHJ solar cells: (a) device architecture and energy level diagram, (b) J−V characteristics, and (c) EQE spectra of optimized BHJ PSCs.

the PSC devices. To determine the optimized fabrication conditions, the devices were tested under a range of conditions, such as different blend ratios, thermal treatments, and processing additives. For this series of polymers, the PSCs made from PPsDT2FBT, PPsDF2FBT, PPDF2FBT, and PPDT2FBT could be optimized at polymer:PC70BM blend ratios of 1:1, 1:1, 1:1.5, and 1:1.5, respectively (Table S1). Chlorobenzene (CB) was used as a solvent with processing additives of 2 vol % diphenylether (DPE), 2 vol % 1,8diiodooctane (DIO), 3 vol % DPE, and 3 vol % DPE for PPsDT2FBT, PPsDF2FBT, PPDF2FBT, and PPDT2FBT, respectively. Figure 3b presents the J−V characteristics of the optimized PSCs. PPDT2FBT showed the best photovoltaic 6000

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Chemistry of Materials Table 2. Summary of Photovoltaic Parameters

a

polymer

JSC [mA cm−2]

JSC (calc)a [mA cm−2]

VOC [V]

FF

PPsDT2FBT PPsDF2FBT PPDF2FBT PPDT2FBT

1.01 10.2 11.5 15.4

1.26 10.24 11.09 15.81

1.04 0.90 0.80 0.80

0.52 0.56 0.62 0.67

PCE [%] 0.55 5.12 5.63 8.28

[0.41 [4.98 [5.43 [8.04

± ± ± ±

0.17]b 0.16] 0.18] 0.34]

JSC (calc), calculated JSC from EQE spectra. bAverage value of PCE ± standard deviation for 10 different measurements.

Figure 4. Morphology of polymer:PC70BM BHJ films: (a−d) AFM topographical images and (e−h) phase images for PPsDT2FBT (a, e), PPsDF2FBT (b, f), PPDF2FBT (c, g), and PPDT2FBT (d, h) BHJ films with PC70BM deposited on ITO/PEDOT:PSS at the same film fabrication condition with the optimized BHJ PSC devices.

Figure 5. GIWAXS morphological data of (a−d) pristine polymers and (e−h) polymer:PC70BM blended films for PPsDT2FBT (a, e), PPsDF2FBT (b, f), PPDF2FBT (c, g), and PPDT2FBT (d, h). (i) In-plane and (j) out-of-plane line-cuts of GIWAXS measurements. BHJ films were prepared at the same condition for the optimized photovoltaic devices.

6001

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Å) in both xy and z directions. It is noteworthy to mention that PPDT2FBT shows the shorter interlamellar packing distance in pristine and blended films, when compared to PPDF2FBT. Although PPDF2FBT showed the stronger tendency for interchain aggregation in UV−vis and AFM measurements, the GIWAXS data compares the fine structure of the packed polymer chains, showing the shorter lamellar distance and stronger π−π stacking in the z direction for PPDT2FBT. This may be related to the different chain curvature of the polymers.47,48 The smaller bond angle (∼125°) of a phenylene−furan−2FBT linkage (compared to a phenylene− thiophene−2FBT bond of ∼148°) induces a more curved structure which hinders tight interchain packing. The interchain noncovalent O···S and S···F interactions may contribute to the tighter packing in the PPDT2FBT film.28 In PPDF2FBT with a furan moiety in the backbone, no intra- and interchain O···S and S···F coulomb interactions are expected. Pronounced faceon π−π stacking peak was also measured for the alkoxysubstituted polymer blends, showing a d-spacing of 3.5−3.7 Å in the out-of-plane direction. In the BHJ PSC device architecture, charge transport in the vertical direction is strongly dependent on the face-on interchain π−π stacking. The pronounced face-on π−π stacking of the alkoxy-containing PPDF2FBT and PPDT2FBT devices supports the higher carrier mobility (with lower charge recombination), leading to better charge extraction with superior photovoltaic properties compared to their alkylthio-containing counterparts. 2.6. Charge Carrier Mobility. The charge carrier transport characteristics were examined by measuring the carrier mobility via the fabrication of polymer field-effect transistor (PFET) (Figures S5 and S6). The detailed conditions for device fabrication are described in the Experimental Section (channel length L = 50 μm, channel width W = 2950 μm). The topcontact bottom gate PFET devices were fabricated using the polymers as the active layer, and the hole mobility was calculated from the source−drain current versus gate voltage curves (Ids vs Vgs). The calculated field-effect hole mobility (in the saturation regime) for the PPDF2FBT and PPDT2FBT pristine films was 9.26 × 10−3 and 1.32 × 10−2 cm2 V−1 s−1, respectively. For the alkylthio-substituted polymers, ∼100 times lower hole mobility was measured: 1.45 × 10−4 and 5.64 × 10−5 cm2 V−1 s−1 for PPsDT2FBT and PPsDF2FBT, respectively. The PFET carrier mobility data also support the different film morphology for the two types of polymers. Table 3 summarizes the carrier mobility data.

efficiency of PCE = 8.28% with a VOC of 0.8 V, JSC of 15.4 mA/ cm2, and FF of 67%. The PCE was in the order PPDF2FBT (5.63%), PPsDF2FBT (5.12%), and PPsDT2FBT (0.55%). PPsDT2FBT showed the lowest PCE but the highest VOC (1.04 V). Table 2 lists the detailed photovoltaic parameters. The measured VOC follows the HOMO energy levels of the polymers, showing 1.04, 0.9, 0.8, and 0.8 V for the PPsDT2FBT-, PPsDF2FBT-, PPDF2FBT-, and PPDT2FBTbased BHJ devices, respectively. Although PPsDT2FBT:PC70BM showed the highest VOC, the tilted molecular structure decreased the J SC and FF values significantly. The tilted polymeric backbone increases the band gap with an excessively deep HOMO and decreases light absorption and charge carrier mobility. The external quantum efficiency (EQE) spectra are consistent with the measured photovoltaic characteristics of the polymers. 2.5. Characterization of Film Morphology. To further understand the photovoltaic properties of the polymers, the BHJ film morphologies of four polymers were analyzed by atomic force microscopy (AFM) in tapping-mode. The AFM images of the polymer:PC70BM BHJ films for the alkythiosubstituted (PPsDT2FBT, PPsDF2FBT) and alkoxy-substituted structures (PPDF2FBT, PPDT2FBT) show a different nanoscale morphology. In Figure 4, the blended BHJ film of PPsDT2FBT showed a featureless and smooth surface with a root-mean-square (RMS) roughness of 0.44 nm, revealing a typical amorphous morphology. In contrast, the BHJ films of PPDF2FBT and PPDT2FBT showed the crystalline film morphology with nanofibrillar structures and a RMS roughness of 2.47 and 2.31 nm, respectively. The PPsDF2FBT:PC70BM film showed an intermediate morphology between the amorphous and crystalline morphologies (RMS roughness = 1.69 nm). The crystalline interchain ordering in the BHJ film leads to efficient charge transport with minimal electron−hole recombination. The AFM morphologies also show good agreement with DFT analysis, and optical and photovoltaic characteristics. PPDF2FBT showed the most pronounced nanofibrillar structures (Figure 4c,g), suggesting good consistency with the UV−vis measurements versus temperature (Figure S3). Detailed nanostructures related to interchain packing in the pristine and polymer:PC70BM blended films were also characterized by grazing incidence wide-angle X-ray scattering (GIWAXS, Figure 5). The alkoxy-substituted PPDF2FBT and PPDT2FBT pristine films showed stronger intermolecular ordering relative to alkylthio-substituted polymers, exhibiting interlamellar scattering up to (300) and a strong π−π stacking peak (010) in the out-of-plane direction. From the in-plane and out-of-plane interlamellar diffraction peaks, the lamellar dspacing of 21−24 and 22−27 Å was measured in the xy and z directions for the alkoxy-substituted polymers in the pristine films. Interestingly, PPDT2FBT shows the tighter lamellar stacking (shorter d-spacing) in both directions, when compared to PPDF2FBT. The π−π stacking distance for PPDF2FBT and PPDT2FBT was 3.5−3.7 Å. The π−π stacking was negligible in both the in-plane and out-of-plane directions for PPsDT2FBT and PPsDF2FBT. The blended BHJ films also showed a similar trend. The well-resolved interlamellar scattering was measured up to (300) in the z direction for both PPDF2FBT:PC70BM and PPDT2FBT:PC70BM blended films. The tighter interlamellar packing (∼22 Å) was measured for PPDT2FBT:PC70BM relative to PPDF2FBT:PC70BM (24−27

Table 3. Summary of Charge Carrier Mobility by PFET and SCLC Measurements polymer PPsDT2FBT PPsDF2FBT PPDF2FBT PPDT2FBT

μh(PFET) [cm2 V−1 s−1] 1.45 5.64 9.26 1.32

× × × ×

−4

10 10−5 10−3 10−2

μh(SCLC) [cm2 V−1 s−1] 3.01 1.16 6.45 4.11

× × × ×

−5

10 10−5 10−4 10−3

μe(SCLC) [cm2 V−1 s−1] 2.31 1.31 3.54 2.83

× × × ×

−5

10 10−5 10−4 10−3

μh/μe 1.30 0.89 1.85 1.45

The vertical carrier mobility was also investigated using the space charge limited current (SCLC) method, which shows a direct relationship with the photovoltaic device characteristics. Hole-only (ITO/PEDOT:PSS/polymer:PC70BM/Au) and electron-only (fluorine-doped tin oxide (FTO)/polymer:PC70BM/Al) devices were fabricated under the same 6002

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Figure 6. J−V characteristics of (a) hole- and (b) electron-only devices based on polymer:PC70BM BHJ films. Hole and electron mobilities were obtained by fitting the curves according to the Mott−Gurney equation.

fabrication conditions for the optimized polymer:PC70BM BHJ devices. The potential loss due to the series resistance (VSR) of the ITO and the built-in potential (Vbi) were considered carefully to ensure accuracy in the measurements. The J−V characteristics follow the Mott−Gurney relationship as follows:

JSCLC =

Figure 7. Charge recombination and transport characteristics: (a) light intensity dependence of JSC, (b) light intensity dependence of VOC, (c) J−V characteristics in the dark, and (d) photocurrent measurement for PPsDT2FBT, PPsDF2FBT, PPDF2FBT, and PPDT2FBT BHJ PSCs.

9ε0εr μV 2 8L3

(1)

VOC =

Here ε0 is the free-space permittivity, εr is the dielectric constant of semiconductor, μ is the mobility, V is the applied voltage, and L is the thickness of the BHJ films. The J−V characteristics showed a quadratic dependence on the voltage over a range of several volts (Figure 6). The amorphous PPsDT2FBT:PC70BM and PPsDF2FBT:PC70BM showed lower hole and electron mobility (μh and μe) values of ∼10−5 cm2 V−1 s−1. In contrast, the crystalline PPDF2FBT and PPDT2FBT devices showed a 1−2 order higher μh, which is consistent with the AFM and GIWAXS analysis data. Interestingly, in contrast to the PFET data of pristine films, the PPDT2FBT:PC70BM device shows 10 times higher μh and μe values than PPDF2FBT:PC70BM. The stronger face-on π−π stacking and shorter interlamellar d- spacing in PPDT2FBT:PC70BM supports the higher vertical carrier mobility compared to that of PPDF2FBT:PC70BM. 2.7. Charge Recombination and Transport Characteristics. To examine the charge recombination characteristics, the incident-light-intensity-dependent JSC and VOC characteristics were measured under short-circuit and open-circuit conditions (Figure 7). Figure 7a shows a log−log plot of JSC as a function of the light intensity. The curve was fitted according to the power-law dependence of JSC on the light intensity

JSC ∝ I α

Egap q

−

2 kT ⎛ (1 − P)γNc ⎞ ⎟ ln⎜ q ⎝ PG ⎠

(3)

Here, Egap is the energy difference between the HOMO of the donor and the LUMO of the acceptor, q is the elementary charge, k is the Boltzmann constant, T is temperature, P is the dissociation probability of the bound electron−hole pairs, γ is the Langevin recombination constant, Nc is effective density of states, and G is the generation rate of bound electron−hole pairs. For free-carrier bimolecular recombination in BHJ solar cells, the semilogarithmic plot of VOC as a function of the light intensity shows a linear relationship with a slope of kT/q. If the Shockley−Read−Hall (SRH) recombination is involved, a stronger dependence of VOC on the light intensity with a slope greater than kT/q will be observed.50,51 The strongest dependence of VOC on the light intensity was measured with a slope of 1.5 kT/q for PPsDT2FBT:PC70BM, suggesting significant SRH recombination involved under the open-circuit condition. In the case of the PPsDF2FBT and PPDF2FBT devices, the slope was calculated to be 1.34 kT/q. For the PPDT2FBT device, VOC showed the weakest dependence on the light intensity, indicating that SRH recombination has the least involvement among the four polymers. The J−V characteristics in the dark were also measured for the BHJ PSCs, as shown in Figure 7c. The PPDT2FBT:PC70BM device showed the lowest leakage current (∼10−4 mA/cm2) at a reverse bias and the highest rectification ratio (∼106) compared to the other BHJ PSCs. To examine the charge generation and extraction processes, the photocurrent (Jph) versus effective voltage (Veff) was also measured for the four different PSC devices. Jph is given by Jph = JL − JD, where JL is the current density under light illumination and JD is the dark current density. Veff is given by Veff = V0 − V, where V0 is the compensation voltage where Jph = 0 mA/cm2 and V is the applied bias voltage. Figure 7d presents the measured photocurrent as a function of the effective voltage. The log− log plot of Jph versus Veff shows two regimes: a linear regime and a saturation regime, where Jph was saturated at V0 − V > ∼0.25 V for the PPsDF2FBT, PPDF2FBT, and PPDT2FBT

(2)

where I is the intensity of incident light and α is an exponent constant for PSC devices (generally 0.85−1). The relatively small α value was determined to be 0.91 for PPsDT2FBT:PC70BM, suggesting significant bimolecular recombination under short-circuit conditions.49 This can be interpreted in terms of the amorphous film morphology and the deep HOMO energy level of PPsDT2FBT, which might have an insufficient offset for hole transfer from PC70BM to the polymer. This leads to charge recombination at the interfaces between the polymer and PC70BM. Figure 7b presents a semilogarithmic plot of VOC as a function of the light intensity. The VOC of BHJ PSC is as follows: 6003

DOI: 10.1021/acs.chemmater.5b02251 Chem. Mater. 2015, 27, 5997−6007

Article

Chemistry of Materials devices. The field-independent saturation region suggests negligible trapped charges and efficient charge extraction/ collection. On the other hand, the PPsDT2FBT device showed no clear saturation in Jph with increasing Veff, indicating significant electron−hole recombination, where a stronger electric field is needed to sweep out the photogenerated charge carriers and separate the geminate electron−hole pairs.52 This is closely related to the tilted chain structure and the resulting amorphous film morphology, which hinders efficient charge transport. In addition, the generated photocurrent density of the alkoxy-substituted PPDF2FBT and PPDT2FBT devices was higher than that of the PPsDT2FBT- and PPsDF2FBTbased devices, suggesting higher charge generation and charge extraction. These results are consistent with the molecular structure analysis, morphology, and photovoltaic device characteristics.

weights of the polymers were determined by gel-permeation chromatography (GPC) with o-dichlorobenzene as the eluent on an Agilent GPC 1200 series, relative to a polystyrene standard. Cyclic voltammetry (CV) experiments were performed with Versa STAT 3 analyzer. All CV measurements were carried out in 0.1 M tetrabutylammoniumtetrafluoroborate (Bu4NBF4) in acetonitrile with a conventional three-electrode configuration employing a platinum wire as a counter electrode, platinum electrode coated with a thin polymer film as a working electrode, and Ag/Ag+ electrode as a reference electrode (scan rate: 50 mV/s). Thermogravimetric analysis (SDT Q600 V20.9 Build 20) measurements were performed at a heating and cooling rate of 10 °C/min under nitrogen. The atomic force microscopy (AFM) images (1.5 μm × 1.5 μm) were obtained using a Veeco AFM microscope in a tapping mode. PSC Fabrication and Measurement. Indium tin oxide substrates were sequentially cleaned with distilled water, acetone, and isopropanol. For BHJ devices, poly(3,4-ethylenedioxythiophene):polystyrene sulfonic acid was spin-coated onto the ITO substrate and annealed at 140 °C for 10 min. To prepare the BHJ films, polymer (14 mg/mL) was blended with PC70BM in chlorobenzene (CB) with processing additives (2 vol % diphenylether (DPE), 2 vol % 1,8diiodooctane (DIO), 3 vol % DPE, and 3 vol % DPE for PPsDT2FBT, PPsDF2FBT, PPDF2FBT, and PPDT2FBT, respectively). The optimized polymer:PC70BM blend ratio was determined to be 1:1, 1:1, 1:1.5, and 1:1.5 for PPsDT2FBT, PPsDF2FBT, PPDF2FBT, and PPDT2FBT, respectively. BHJ films were spin-cast on top of the PEDOT:PSS layer in a N2 filled glovebox. Subsequently, the device was pumped down under vacuum (