Effects of Backbone Planarity and Tightly Packed Alkyl Chains in the

Article pubs.acs.org/Macromolecules

Effects of Backbone Planarity and Tightly Packed Alkyl Chains in the Donor−Acceptor Polymers for High Photostability Hyo Sang Lee,†,‡ Hyeng Gun Song,∥ Hyeseung Jung,# Myung Hwa Kim,# Changsoon Cho,& Jung-Yong Lee,& Sungnam Park,‡,§ Hae Jung Son,†,‡ Hui-Jun Yun,∥ Soon-Ki Kwon,∥ Yun-Hi Kim,*,⊥ and BongSoo Kim*,% †

Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea Green School and §Department of Chemistry, Korea University, Seoul 02841, Republic of Korea ∥ School of Materials Science and Engineering, Engineering Research Institute, and ⊥Department of Chemistry and RINS, Gyeongsang National University, Jinju 52828, Korea # Department of Chemistry and %Department of Science Education, Ewha Womans University, Seoul 03760, Republic of Korea & Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Graphene Research Center, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ‡

S Supporting Information *

ABSTRACT: The photostability of donor−acceptor (D−A) polymers remains a critical issue despite recent improvements in the power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells. We report the synthesis of three highly photostable polymers (PDTBDT-BZ, PDTBDTBZF, and PDTBDT-BZF2) and their suitability for use in high-performance OPV cells. Under 1 sunlight of illumination in air for 10 h, these polymer films demonstrated remarkably high photostability compared to that of PTB7, a representative polymer in the OPV field. While the PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2 polymer films maintained 97, 90, and 96% photostability, respectively, a PTB7 film exhibited only 38% photostability under the same conditions. We ascribed the high photostability of the polymers to both the intrinsically photostable chemical moieties and the dense packing of alkyl side chains and planar backbone polymer chains, which prevents oxygen diffusion into the PDTBDT-BZ films. This work demonstrates the high photostability of planar PDTBDT-BZ series polymers composed of photostable DTBDT and BZ moieties and suggests a design rule to synthesize highly photostable photovoltaic materials.

1. INTRODUCTION Organic photovoltaic (OPV) cells are important future energy sources for portable photovoltaic devices and building- or device-integrated photovoltaic applications.1−4 OPV cells boast many advantages over traditional technologies, including highthroughput, low-cost fabrication, and high degrees of flexibility and mechanical stability.5−7 Recently several donor−acceptor (D−A) type polymers, composed of electron-rich donor (D) and electron-deficient acceptor (A) moieties, have yielded dramatic enhancements in power conversion efficiencies (PCEs), reaching levels of over 8%.8−14 To achieve high performance, D−A polymers require the following key characteristics: (i) good spectral overlap with the solar spectrum, (ii) adequate energy level alignment of their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels with respect to n-type organic molecules, like a fullerene derivative of [6,6]-phenyl-C71butyric acid methyl ester (PC71BM) or a non-fullerene molecule of hPDI415 (a factor that determines high opencircuit voltage (Voc) and high exciton separation at the © XXXX American Chemical Society

polymer/fullerene interface), (iii) high hole mobility and its balance with electron mobility of n-type molecules in the photoactive layer, and (iv) the formation of nanoscale domains through phase separation between p-type D−A polymers and n-type molecules, which overcomes the limited exciton diffusion length (10−20 nm) and results in a high interfacial area in OPV cells.2,16 However, despite the high PCEs attained in polymer solar cell (PSC) devices employing excellent D−A polymers, the photostability of D−A polymers is far from ideal for use in potential OPV applications.17−22 Most high-performance D−A polymers are highly susceptible to photo-oxidation when exposed to sunlight and air. PSC devices incorporating such photoactive materials generally need to be produced in an inert atmosphere and require expensive barrier films that prevent the ingress of oxygen in order to maintain long-term stability. Received: July 22, 2016 Revised: October 2, 2016

A

DOI: 10.1021/acs.macromol.6b01580 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Synthetic Routes of PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2 Polymers

reached by Soon et al.,21 with a more detailed description of triplet formation and singlet oxygen generation in OPV polymer films. With previous studies in mind, we designed and synthesized three highly photostable D−A polymers, PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2, based on planar dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT) as a donor moiety for all three polymers and 2,1,3-benzothiadiazole (BZ), 5-fluoro-2,1,3-benzothiadiazole (BZF), and 5,6-difluoro2,1,3-benzothiadiazoles (BZF2) as the respective acceptor moieties. By combining photostable donor and acceptor moieties, our design yielded polymers with highly planar backbones. DTBDT is a five-membered fused ring system and was selected as the donor since it promotes intermolecular packing while maintaining a low HOMO level.27,28 These fused aromatic moieties are also resistant to photo-oxidation.18,21,29 The BZ moiety was selected as the acceptor since BZFn units containing different numbers of fluorine atoms (n = 0, 1, 2) have been shown to be good acceptors.9,30−32 The appended fluorine atoms impose planarity to the polymer backbone and lower both the HOMO and LUMO levels, which is favorable for high performance in PSCs.31,33 High photostability has also been reported in fluorinated aromatic molecules.34,35 By connecting DTBDT and BZ moieties, a highly planar backbone conformation can be achieved through S···N and S···F interactions.33,36 Importantly, the highly planar backbone facilitates the packing of polymer chains, which reduces the chance of triplet formation and impedes the diffusion of photooxidizing chemicals, such as oxygen or photoinduced radicals, into the stacked polymer chains.18,21

Proper commercialization of OPVs requires the development of D−A polymers that provide both high performance and high photostability. To achieve this goal, photostability studies on highperformance photovoltaic polymers have been conducted and photostable polymer design guidelines have been suggested. Manceau et al.20 have surveyed the photostability of several conjugated polymers and identified chemical subunits in the polymer chain that are susceptible to photochemical degradation. This work provides a good framework for selecting photostable chemical moieties that will be useful in the design of high performance D−A polymers. The photodegradation mechanism of conjugated polymers has been elucidated for various PSCs.18,19,21,23 In the case of poly(3-hexylthiophene) (P3HT), the α-methylene unit in the alkyl chain of P3HT is degraded through oxygen-mediated radical attacks under light illumination in the presence of oxygen. The conjugated backbone of P3HT is also broken down by a [4 + 2]cycloaddition reaction between thiophene subunits and singlet oxygen that is generated by energy transfer from the photoexcited P3HT triplet state to the ground state of triplet oxygen.24 The well-known poly(thieno[3,4-b]thiophene-cobenzo[1,2-b:4,5-b′]dithiophene) (PTB) series polymers have also shown serious photochemical instabilities.23,25,26 The main source of photodegradation in PTB polymers is again attributed to the [4 + 2]-cycloaddition reaction between its π-systems (either thieno[3,4-b]thiophene (TT) or benzo[1,2-b:4,5-b′]dithiophene (BDT) units) and singlet oxygen.23 Recently, Mateker et al. 18 screened several OPV materials and determined that highly crystalline organic materials in films are resistant to photo-oxidation. Similar conclusions were B


Article

Macromolecules

Figure 1. UV−vis absorption spectra of PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2 polymers in (a) solution and (b) film states. The concentrations of polymer solutions were fixed at 0.02 mg/mL. The onsets were determined from the crossing points of dotted lines.

Table 1. Summary of Optical and Electrochemical Properties of the PDTBDT Series Polymers UV−vis absorption

cyclic voltammetry film

solution

film

polymers

λmax (nm)

λonset (nm)

λmax (nm)

λonset (nm)

Eopt g (eV)

a Eox onset (V)

HOMO (eV)b

c Ered onset (V)

LUMO (eV)d

e EEC g (eV)

PDTBDT-BZ PDTBDT-BZF PDTBDT-BZF2

655 654 642

706 700 685

675 668 654

750 724 710

1.65 1.71 1.75

0.5 0.6 0.6

−5.3 −5.4 −5.4

−1.6 −1.5 −1.5

−3.2 −3.3 −3.3

2.1 2.1 2.1

The electrochemical oxidation onset potential in films with respect to the ferrocene/ferrocene+ (Eox = −4.8 eV). bHOMO is calculated by −(Eox onset + 4.8 eV). cThe electrochemical reduction onset potential in films with respect to the ferrocene/ferrocenium. dHOMO is calculated by −(Eox onset + 4.8 eV). eElectrochemical bandgap, i.e. (LUMO−HOMO). a

This report describes the development of three polymer variations demonstrating excellent photostability, a characteristic attributed to the conformation of the planar backbone, and long linear alkyl chains that promote strong interchain polymer packing. Moreover, we manufactured PSC devices based on these polymers with different architectures and demonstrated their high photovoltaic performance.

respectively (Figure S8d−f). These data indicate that all of the polymers were thermally stable. The light absorption properties of the polymers were monitored by UV−vis absorption spectroscopy. Absorption spectra of each polymer in a chlorobenzene solution and in the solid state are shown in Figure 1. In the solvated state, three major light absorption bands were observed around 350, 425, and 650 nm. The absorptions below 500 nm were ascribed to π−π* or n−π* transitions. The strong absorption peaks observed at 655, 654, and 642 nm for PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2, respectively, originate from the HOMO−LUMO transition38 and indicate a blue-shift with an increasing number of appended fluorine atoms. This result is similar to that reported for other fluorine-containing polymers.25,31 The absorption maxima of PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2 films were red-shifted to 675, 668, and 654 nm, respectively, a shift of 12−20 nm compared to the corresponding solvated polymers. These considerable red-shifts suggest significant interchain stacking of aromatic backbones during film formation. The optical band gaps were determined from the absorption edges to be 1.65, 1.71, and 1.75 eV for PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2, respectively. In addition, it is interesting to note that no substantial changes in light absorption were observed for all three polymers, even after elevating the temperature of the polymer solutions up to 110 °C (Figure S9). This phenomenon appears to be unique to our polymers relative to traditional D−A polymers.8,33,39 This suggests that the polymer backbones remain planar and rigid in solution, which may facilitate tight interchain packing in the film state.40 The optical properties, electrochemical properties, and band gaps for each polymer are summarized in Table 1.

2. RESULTS AND DISCUSSION The synthetic routes to PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2 are depicted in Scheme 1. Molecule 1 was synthesized according to a previously published report.28,37 Compounds 2−4 were purchased from SunaTech Inc. (Jiangsu, P. R. China). The polymers were synthesized using a Stille cross-coupling reaction and purified by subsequent Soxhlet extraction using methanol, acetone, hexane, and chloroform. The chloroform fractions were collected to study the photostability and photovoltaic properties of the polymers. The weight-average molecular weights of PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2 were 28, 70, and 89 kDa, respectively, with corresponding polydispersity indices (PDIs) of 2.43, 3.33, and 3.87. Characterizational data from the PDTBDT-BZ series polymers, including gel-permeation chromatograms (GPC) and 1H NMR data, are provided in the Supporting Information (Figures S5−S7). Differential scanning calorimetry (DSC) and thermogravimetric analyses (TGA) were used to examine the thermal properties of the PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2 polymers. DSC thermograms showed no transitions up to 240 °C (Figure S8a−c). The TGA data yielded 5% weight loss temperatures of 400, 410, and 373 °C for PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF 2 C


Article

Macromolecules

Figure 2. Frontier molecular orbitals, their energy levels, and energy minimum molecular geometries of (DTBDT-BZ)3, (DTBDT-BZF)3, and (DTBDT-BZF2)3 model compounds.

Figure 3. GIXD images of (a) PDTBDT-BZ, (b) PDTBDT-BZF, (c) PDTBDT-BZF2, (d) PDTBDT-BZ:PC71BM, (e) PDTBDT-BZF:PC71BM, and (f) PDTBDT-BZF2:PC71BM films.

similar to the 4,8-di(thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT) moiety. The dependence of the optical and electrochemical properties on the number of pendant fluorine atoms can be interpreted by quantum mechanical density functional theory (DFT) calculations. DFT calculations on the three repeat units of each polymer ((DTBDT-BZ)3, (DTBDT-BZF)3, and (DTBDT-BZF2)3) generated electronic structures and molecular geometries. To save calculation cost and time, DFT calculations were performed on gas-phase molecules consisting of three repeat units and shortened alkyl chains. Frontier molecular orbitals, their energy levels, and the energyminimized molecular geometries of the model compounds

Cyclic voltammetry (CV) was used to monitor the electrochemical behavior of the polymer films. HOMO and LUMO levels were determined from the onset potentials of the oxidation and reduction curves, respectively. The HOMO levels of PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2 polymer films were −5.3, −5.4, and −5.4 eV, respectively, while the LUMO levels of the corresponding polymers were −3.2, −3.3, and −3.3 eV (Figure S10). The addition of highly electronegative fluorine atoms into the polymer backbone led to a lowering of HOMO and LUMO levels.25,31 In addition, comparisons of the UV−vis absorption spectra and CV data of the PDTBDT-BZ series against previously reported polymers9,30 show that the DTBDT donor is electronically D


Article

Macromolecules

Figure 4. UV−vis absorption spectra of (a) PDTBDT-BZ, (b) PDTBDT-BZF, (c) PDTBDT-BZF2, and (d) PTB7 polymers in chlorobenzene solutions that were exposed to 1 sunlight illumination for a given hour. Chlorobenzene was anhydrous and used as received. The percentage of the optical absorption maintenance is shown in parentheses.

strongest lamellar peaks, while the PDTBDT-BZF and PDTBDT-BZF2 films exhibited lower lamellar peak intensities. In contrast, along the in-plane direction, the PDTBDT-BZ film showed the weakest lamellar peak, while PDTBDT-BZF and PDTBDT-BZF 2 exhibited stronger peaks. Along the qz direction, well-established π−π stacking peaks were observed for the PDTBDT-BZF and PDTBDT-BZF2 films while a relatively weak but discernible peak was present in the PDTBDT-BZ film sample. No π−π peaks were evident along the qy direction. The enhanced π−π stacking and face-on backbone orientation of the PDTBDT-BZF and PDTBDTBZF2 polymer chains are consistent with the results of previous studies, demonstrating the important role of fluorine atoms in determining π−π stacking and backbone orientation.31,41,42 Research into PSC materials has recently been focused on improving the photostability of semiconducting polymers. To address this concern, we examined the photostability of our synthesized polymers by adding each to UV quartz cuvettes containing dilute (0.02 mg/mL) chlorobenzene solution and exposed them to 1 sunlight of illumination. The UV−vis absorption spectra for the dissolved PDTBDT-BZ series polymers and poly[2-ethylhexyl4-(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophen-2-yl)-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7)43 polymer after photodegradation for a given period of time are shown in Figure 4. As the number of fluorine atoms in the backbone increased, so did the photostability of the solution-phase polymers. Note that all of the PDTBDT-BZ series polymers displayed greater photostability than the PTB7 polymer. After 5 h of photodegradation, the PTB7, PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2 polymers maintained 7, 40, 58, and 64% of their initial light absorption, respectively. Next we tested the PDTBDT-BZ series polymer films when exposed to 1 sunlight illumination in air. For comparison, we included the well-known polymer films

are summarized in Figure 2. The overall electronic distributions of the frontier orbitals were similar among the model compounds, except that the appended fluorine atoms’ orbitals in (DTBDT-BZF)3 and (DTBDT-BZF2)3 participated in the molecular orbitals. Surface plots of HOMO and LUMO levels indicate that the two middle repeat units are in high electronic communication. The HOMO−1 and LUMO+1 surface plots reveal the basic character of the electron-rich unit (DTBDT) and the electron-deficient unit (BZ), respectively. Combining the orbitals of fluorine atoms with those of the (DTBDT-BZ)3 molecule resulted in slightly lower HOMO and LUMO levels while increasing the planarity of the molecules. The HOMO and LUMO levels of (DTBDT-BZ)3 were −4.84 and −2.83 eV, respectively, while the analogous levels of (DTBDT-BZF)3 were −4.95 and −2.92 eV, respectively. All dihedral angles of the model compounds are below 5°, which implies that all of the polymer chains were geometrically flat, which is favorable for interchain stacking and consistent with the features observed in UV−vis absorption spectra. The crystalline structures of the PDTBDT-BZ series polymer films were determined from grazing-incidence X-ray diffraction (GIXD) data (Figure 3a−c). Peaks corresponding to a lamellar phase appeared at nearly identical positions around qz = 0.3 Å−1 (d-spacing (d) = 21 Å) for the PDTBDT-BZ series polymers. Considering the fully stretched polymer chain width, i.e., the sum of the length of two long −C10H23 chains and the width of the aromatic backbone, the d-spacing values were smaller, implying that the linear alkyl chains were interdigitated. Peaks corresponding to π−π stacking were observed around qz = 1.65 Å−1 (d = 3.8 Å) in the out-of-plane (qz) direction for all three polymer films. For a more detailed analysis, line-cut profiles, shown in Figures S11a,b, were obtained from GIXD images along the out-of-plane and the in-plane (qy) directions. Along the out-of-plane direction, the PDTBDT-BZ film showed the E


Article

Macromolecules

Figure 5. UV−vis absorption spectra of (a) PDTBDT-BZ, (b) PDTBDT-BZF, and (c) PDTBDT-BZF2 polymer films as a function of photodegradation under 1 sunlight illumination in air for a given exposure time. (d) A summary of light absorption intensity changes at the peak of each polymer, i.e., 675, 668, and 654 nm respectively for PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2 polymers.

Figure 6. FTIR spectra of (a) PDTBDT-BZ, (b) PDTBDT-BZF, and (c) PDTBDT-BZF2 polymer films. Raman spectra of (a) PDTBDT-BZ, (b) PDTBDT-BZF, and (c) PDTBDT-BZF2 polymer films. All the film spectra were taken after photodegradation under 1 sunlight illumination in air for a given exposure time.

polymer films under 1 sunlight illumination in a solar simulator. Light was incident on the polymer films in air through the glass/ITO side, which makes the light spectrum incident on the films identical to that in OPV cells. UV−vis absorption spectra of PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2 polymers as a function of 1 sunlight illumination time are shown in Figures 5a−c. Figure 5d displays a summary of light absorption intensity changes for each polymer, with values of 675, 668, and

poly(3-hexylthiophene) (P3HT),18,21,44 PTB7,43 and poly[2,6(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT)32 as controls. All the polymer films were prepared by spincoating a chlorobenzene solution containing each polymer (10 mg/mL concentration) onto precleaned ITO/glass substrates. Polymer film thicknesses were approximately 70 nm. Photodegradation experiments were conducted by placing the F


Article

Macromolecules

molecules. As demonstrated above, the photostabilities of all the PDTBDT-BZ polymer films were pronounced, while the solution-phase PDTBDT-BZ polymers were not quite as photostable. This suggests that photostability cannot simply be attributed to the structure of the polymer backbone. Interchain stacking also plays a critical role in improving the photostability of conjugated polymers. The large difference in polymer photostability observed between the solvated and film states can be attributed to the accessibility of oxygen (a main cause of photodegradation) to the conjugated backbone in the solvated state. In the film state, tightly stacked polymer chains prevent oxygen penetration into the polymer films. The PDTBDT-BZ series polymers are highly planar with very low dihedral angles, suitable for interchain π−π stacking. Consistent with this report, it has previously been shown that highly planar backbone polymers are often highly photostable.18,21 For example, regioregular P3HT polymer films, which are more crystalline than regiorandom P3HT polymer films, are much more resistant to photodegradation. Likewise, planar PBDTTPD polymer films were more photostable than less planar PBDFTPD polymer films.18 Lastly, tight packing of alkyl chains promotes photostability in polymer films. It is interesting that the planar-backboned PTB7 polymer is still highly susceptible to photodegradation even in the film state (Figure 4 and Figures S13−S14). FTIR is a simple and powerful tool used to examine the conformation and packing of alkyl chains. In general, there are four intense peaks, corresponding to alkyl chain stretching, between 2800 and 3000 cm−1. Poly(3-butylthiophene) (P3BT) polymer films containing less ordered alkyl side chains displayed peaks at 2861, 2870, 2932, and 2956 cm−1, which correspond to the symmetric CH2 (νs(CH2)), symmetric CH3 (νs(CH3)), asymmetric CH 2 (ν a s (CH 2 )), and asymmetric CH 3 (νas(CH3)) stretching peaks, respectively.45 As the length of alkyl chains attached to the polythiophene backbone increases, the alkyl chains adopt trans zigzag conformations and peaks corresponding to stretching vibrations shift toward lower frequencies with increased intensities of the νs(CH2) and νas(CH2) peaks and reduced intensities of the νs(CH3) and νas(CH3) peaks. The PDTBDT-BZ series polymer films yielded FTIR spectra that contained stretching bands corresponding to long, linear alkyl chains (Figures S14a−c). Peaks corresponding to alkyl side chain stretching (νs(CH2), νs(CH3), νas(CH2), and νas(CH3)) appeared at 2854, 2871, 2928, and 2957 cm−1, respectively. In contrast, FTIR spectra of the PTB7 polymer displayed peaks at 2863, 2875, 2933, and 2961 cm−1, corresponding to the νs(CH2), νs(CH3), νas(CH2), and νas(CH3) vibrations of branched alkyl side chains, with stronger peaks at 2875 and 2961 cm−1 (Figure S14d). These peaks of PTB7 films are at even higher frequencies than the corresponding stretching modes of P3BT. This suggests that the branched short 2-ethylhexyl chains in the PTB7 are not well-ordered, whereas the linear alkyl side chains of the PDTBDT-BZ series polymers are well-aligned, straight, and interdigitated, consistent with the GIXD data. Therefore, long, linear alkyl chains substituted onto DTBDT moieties effectively support high photostability by forming a dense barrier against oxygen diffusion. In summary, the photostability of conjugated polymers can be significantly improved by (i) incorporating photochemically stable chemical moieties and (ii) the preventing oxygen infiltration into the polymer chains. The latter requires tight

654 nm for PDTBDT-BZ, PDTBDT-BZF, and PDTBDTBZF2 polymers, respectively. Even after 10 h of illumination by 1 sunlight in air, the PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2 films maintained 97, 90, and 96% of their initial light absorption characteristics. To further confirm our results, we tested the photostability characteristics of each polymer by Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy (Figure 6). All the spectral features of the original molecular vibrations in the polymer films were preserved, even after a 10 h sunlight exposure. On average, the PDTBDT-BZ series polymers performed better when compared to the photodegradation of P3HT, PTB7, and PCPDTBT films under the same conditions. UV−vis absorption spectra of P3HT, PTB7, and PCPDTBT polymers as a function of 1 sunlight illumination time up to 20 h in air are shown in Figures S12a−c. Figure S12d displays a summary of intensity changes at the absorption maximum for each polymer (510, 672, and 765 nm for P3HT, PTB7, and PCPDTBT polymers, respectively). PTB7 exhibited the lowest photostability and PCPDTBT was the most stable among these three polymers. After photodegradation for 10 h, the light absorption ratios between each hour (t) and at the initial film state (t = 0) were 79, 38, and 88% for P3HT, PTB7, and PCPDTBT, respectively. FTIR spectra of photodegraded PTB7 films further supported the severe photodegradation. In particular, the growing formation of carbonyl-related products was clear (Figure S13). Moreover, we have previously reported the photostability of three diketopyrrolopyrole (DPP)-based polymer films (PDPPBDT, PDPPDTT, and PDPPDTBDT), which retained 79, 96, and 90% of their initial light absorption characteristics.29 Using the photostability results for the PDTBDT-BZ series and other polymers (P3HT, PTB7, PCPDTBT, PDPPBDT, PDPPDTT, and PDPPDTBDT) that were tested under the same conditions, we propose some chemical design rules to the synthesis of photostable semiconducting polymers. First, aromatic chemical structures that constitute the polymer backbone are paramount to high photostability. The order of photostability, from lowest to highest, for the following donor moieties was BDT, 4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1b;3,4-b′]dithiophene (CPDT), DTBDT, and dithieno[3,2b:2′,3′-d]thiophene (DDT). This suggests that the incorporation of fused aromatic systems with fewer alkyl chains would increase photostability. Consistent with previous reports, the BZ acceptor is quite photostable, similar to the DPP moiety.20,21 According to solution-phase photostability tests, the fluorination of the BZ moiety further increased its intrinsic photostability. This implies that the electronic stabilization of aromatic backbones by electronegative fluorine atoms plays a large role in improving photostability, which is also in agreement with previous studies.34,35 However, it is interesting that all the PDTBDT-BZ polymer films displayed high photostabilities, despite not all having fluorine atoms. This discrepancy can be explained by the packing of polymer chains. Interchain stacking, facilitated by a planar polymer backbone, combined with the high linearity of the alkyl chains, enhanced photostability to a greater degree than the electronic stabilization of reactive polymer chains through their interactions with fluorine atoms. This phenomenon is discussed in greater detail below. The planarity of polymer backbones plays a significant role in generating local interchain stacking of aromatic moieties. The dense packing of polymer chains protects them from oxygen G


Article

Macromolecules

Figure 7. (a) J−V characteristics of normal type PSCs based on PDTBDT-BZ series polymers, measured under AM 1.5G, 100 mW/cm2 illumination. (b) The corresponding EQE spectra of normal type PSCs based on PDTBDT-BZ series polymers.

Table 2. Summary of Photovoltaic Parameters of PSCs Based on PDTBDT-BZ Series Polymersa device type

active layer

inverted with a V-groove film

PDTBDT-BZ:PC71BM PDTBDT-BZF:PC71BM PDTBDT-BZF2:PC71BM PDTBDT-BZ:PC71BM PDTBDT-BZF:PC71BM PDTBDT-BZF2:PC71BM PDTBDT-BZF:PC71BM

normal

inverted

a

Jsc (mA/cm2)

Voc (V) 0.83 0.92 0.91 0.83 0.88 0.90 0.88

(0.83 (0.92 (0.91 (0.83 (0.88 (0.90 (0.88

± ± ± ± ± ± ±

0.01) 0.01) 0.01) 0.01) 0.01) 0.01) 0.01)

8.81 11.84 9.59 10.28 11.47 10.02 12.65

Jsc,EQE (mA/cm2)

(8.5 ± 0.4) (11.5 ± 0.3) (9.2 ± 0.3) (9.5 ± 0.8) (11.4 ± 0.3) (9.7 ± 0.5) (12.6 ± 0.3)

9.59 11.34 9.24 9.62 11.11 9.38 11.60

FF (%) 59.2 65 57.7 64.9 73.5 60.5 72.6

(58 (64 (59 (62 (73 (61 (72

± ± ± ± ± ± ±

PCE (%) 1) 1) 1) 3) 3) 4) 3)

4.33 7.06 5.04 5.54 7.42 5.46 8.08

(4.1 (6.7 (4.9 (4.9 (7.0 (5.1 (7.8

± ± ± ± ± ± ±

0.2) 0.3) 0.2) 0.7) 0.3) 0.4) 0.3)

Averages and standard deviations are provided in addition to data from the best cell of each type.

GIXD data (Figures 3d−f) acquired from polymer:PC71BM blend films were analyzed in order to assess the differences in photovoltaic performance among the PDTBDT-BZ series PSCs. Interestingly, the PDTBDT-BZ:PC 71 BM and PDTBDT-BZF2:PC71BM films yielded more ordered lamellar stacking when compared to the corresponding polymer-only films. In the line-cut profile along the qz direction (Figures S11c,d), sharp (100) and (200) peaks were observed, and both the polymer’s lamellar spacing distances were 20 Å, nearly identical to the corresponding lamellar spacing values of the polymer-only films. The increased crystallinity in the blend films is somewhat puzzling. It is expected that a simple blending of p-type organic materials and PC71BM molecules typically results in decreased crystallinity. This unusual behavior might be attributed to the effect of the high boiling point of the solvent additive (1,8-diiodooctaine (DIO)).48 DIO additives in polymer:PC71BM blend solutions create longer drying times, which allows adequate time for the polymer chains to align themselves between the PC71BM molecules. While the PC71BM molecules formed nanoscale crystalline domains with no preferred direction, the polymer chains assembled themselves with nanoscale segregation from the PC71BM domains49 (Figure 3d,f). Note that the GIXD data obtained from PDTBDT-BZ:PC71BM and PDTBDT-BZF2:PC71BM films indicate that a large phase separation occurred in the film state. This was confirmed by transmission electron microscopy (TEM) of the polymer:PC71BM blend films (see below). The high-performing PDTBDT-BZF:PC71BM blend film yielded a diffractogram that differed significantly from those of the other polymer:PC71BM blend films. Considering the PDTBDT-BZF polymer-only film, crystallinity was improved to a lesser degree than in the PDTBDT-BZ and PDTBDT-BZF2 blended polymer films, and the π−π ordered stacking of PDTBDTBZF polymer chains was still observed along the qz direction

packing of not only the planar aromatic backbone but also the alkyl side chains. To study the photovoltaic properties of our highly photostable PDTBDT-BZ series polymers, we fabricated PSC devices based on these polymers with the structure of ITO/ PEDOT:PSS/polymer:PC71BM/TiO2/Al, where the weight ratio of polymer:PC71BM was 1:1.5. Here the TiO2 layer plays an important role in blocking recombination at the interface between the cathode and the photoactive layer.46,47 The photovoltaic properties of these devices were measured under AM 1.5G and 1 sunlight illumination after masking each cell. Figure 7a show the photocurrent density (J)−voltage (V) characteristics of normal type PSCs based on PDTBDT-BZ series polymers. The photovoltaic parameters of these PSCs are summarized in Table 2, and their PCEs, in increasing order, were 4.33, 5.04, and 7.06% respectively for PDTBDT-BZ, PDTBDT-BZF2, and PDTBDT-BZF. Each type of more than 8 devices with an active area of 20 mm2 was fabricated. The PDTBDT-BZF and PDTBDT-BZF2 devices displayed higher open-circuit voltages (Voc) of ∼0.9 V compared to the PDTBDT-BZ devices. This can be attributed to the deeper HOMO levels of the fluorine-bearing polymers. The PDTBDTBZF devices showed the highest short-circuit currents (Jsc = 11.84 mA cm−2) and the highest fill factors (FF = 64.8%) among the PDTBDT-BZ series devices. Figure 7b show the external quantum efficiency (EQE) spectra of the PDTBDT-BZ series PSCs. The photocurrent generation coverage of each polymer device corresponds to the light absorption range of the polymers, as shown in their UV−vis absorption spectra. Consistent with Jsc, the EQEs of the PDTBDT-BZF devices were the highest among the PDTBDT-BZ series PSCs. The mismatch of Jsc values estimated from EQE (Jsc,EQE) spectra were 3−8% (see Table 2). H


Article

Macromolecules

Figure 8. TEM images of polymer:PC71BM blend films: (a) PDTBDT-BZ:PC71BM, (b) PDTBDT-BZF:PC71BM, and (c) PDTBDT-BZF2:PC71BM. AFM images of polymer:PC71BM blend films: (d) PDTBDT-BZ:PC71BM, (e) PDTBDT-BZF:PC71BM, and (f) PDTBDT-BZF2:PC71BM.

Figure 9. (a) J−V characteristics of inverted type PDTBDT-BZ series polymer solar cells, measured under AM 1.5G, 100 mW/cm2 illumination. (b) The corresponding EQE spectra of normal type PDTBDT-BZ series polymer solar cells. (c) J−V characteristics of the inverted type PDTBDT-BZF polymer solar cell, measured under AM 1.5G, 100 mW/cm2 illumination. (d) The corresponding EQE spectra of the inverted type PDTBDT-BZF polymer solar cell.

showed distinct polymer fibers, approximately 22 nm wide, arranged in an irregular pattern. The PDTBDT- BZF:PC71BM blend film had less distinct, narrower polymer fibers (5−10 nm wide), suggesting that the polymer chains were better mixed with the PC71BM molecules. These distinct fibers were also seen in the PDTBDT-BZF2:PC71BM blend film but were more rod-like and uniform in size (∼10 nm). Considering the photovoltaic properties of the PDTBDT-BZ series PSCs and their GIXD data, these TEM results suggest that thick or rodlike fibers in the PDTBDT-BZ:PC71BM and PDTBDT-

(Figure 3e). These observations suggest that the PDTBDTBZF polymer was more miscible with PC71BM than the other polymers and maintained a face-on orientation. Both of these features in PDTBDT-BZF:PC71BM films were beneficial in bulk heterojunction type PSCs, as they resulted in the highest photovoltaic performance among the three PSCs tested. TEM and atomic force microscopy (AFM) were used to observe the morphological features of the PDTBDT-BZ series PSCs. TEM micrographs of polymer:PC71BM blend films are shown in Figures 8a−c. The PDTBDT-BZ:PC71BM blend film I


Article

Macromolecules

Figure 10. UV−vis absorption spectra of (a) PDTBDT-BZ:PC71BM, (b) PDTBDT-BZF:PC71BM, and (c) PDTBDT-BZF2:PC71BM blend films as a function of photodegradation time under 1 sunlight illumination in air for a given exposure time. J−V characteristics of the encapsulated inverted PSCs based on (a) PDTBDT-BZ:PC71BM, (b) PDTBDT-BZF:PC71BM, and (c) PDTBDT-BZF2:PC71BM blend films as a function of photodegradation time under 1 sunlight illumination in air for a given exposure time.

BZF2:PC71BM films do not allow for optimal contact between the polymer chains and PC71BMs and do not create the efficient vertical charge transport (Figure S15). This would result in the low Jsc and low FF values that were observed with PDTBDT-BZ:PC71BM and PDTBDT-BZF2:PC71BM blend PSCs. The higher miscibility of the PDTBDT-BZF polymer with PC71BM may originate from the relatively large degree of structural disorder in the polymer backbone, depending on the irregular position of fluorine atoms on the main chain.50−52 As determined by AFM, the PDTBDT-BZ:PC71BM, PDTBDTBZF:PC71BM, and PDTBDT-BZF2:PC71BM blend surfaces have root-mean-square (rms) roughness values of 5.32, 3.86, and 6.28 nm, respectively (Figures 8d−f). The higher rms values corresponded to the blend films with more fibrous polymers. This suggests that surface roughness is caused by low miscibility between the polymers and the PC71BMs, as the PDTBDT-BZF:PC71BM film had the lowest rms value and the higher miscibility in the blend. To further improve the photovoltaic performance of the device, inverted type PDTBDT-BZ series PSCs were also fabricated with a device structure of ITO/TiO2/polymer:PC71BM/TiO2/V2O5/Ag. The photoactive films were formed in the same fashion as the normal type PSCs. Voc values for all of the inverted type PSCs were nearly identical to those for the normal cells, but Jsc and FF values were improved (Figure 9a). The maximum PCE value (7.42%) was achieved with the PDTBDT-BZF:PC71BM-based PSC. We were further able to improve this device’s performance by using light trapping V-groove films that were patched on the glass side of the device.53 As reported previously, the light trapping effect of the V-groove film was pronounced (Figure 9c). The Jsc and PCE values were enhanced from 11.47 to 12.65 mA cm−2 (by 10.3% increase) and from 7.42 to 8.08% (by 8.8% increase), respectively, while the Voc remained the same. EQE spectra confirmed a light collection improvement between 400 and 700 nm (Figure 9d).

Lastly, the photostability of PDTBDT-BZ series polymer:PC71BM blend films was also investigated. Upon photooxidation of the blend films under 1 sunlight illumination, PDTBDT-BZ:PC71BM blend film changed little, while PDTBDT-BZF:PC71BM and PDTBDT-BZF2:PC71BM blend films displayed more changes; the more F atoms are attached, the higher absorption decrease was observed. And main spectral changes occurred in the polymer’s main absorption region (500−700 nm). The origin of the spectral change appears not to be simply related with crystallinity or interchain packing in that both PDTBDT-BZ:PC71BM and PDTBDT-BZF2:PC71BM blend films showed high crystalline lamellar peaks of the polymers and the π−π stacking degree of PDTBDT-BZ series polymers is in a different order from the photodegradation order. It is likely that the narrower polymer fibers in the PDTBDT-BZF:PC71BM and PDTBDT-BZF2:PC71BM blend films may have more chance to interact with oxygen. Moreover, the photooxidation characteristics in the blend films might be related to the bimolecular carrier recombination at the polymer:PC71BM interface because the photostabilities of PDTBDT-BZ series polymer blend films decreased compared to those of the pristine PDTBDT-BZ polymer films. Rao et al. have proposed that the bimolecular carrier recombination might generate triplet states, which then produce singlet oxygen from triplet oxygen,54 and the yield of the bimolecular recombination can thus be related with chemical structure or molecular energy levels. A definitive answer for this issue warrants further investigation. In addition, the photostability of the encapsulated PDTBT-BZ series polymer devices was monitored under 1 sunlight illumination. After 300 h light exposure to PSC devices, the photovoltaic property of PDTBDT-BZ, PDTBDT-BZ, and PDTBDT-BZF2 polymer devices changed to 70, 41, and 34% PCE of the initial PCEs, respectively (detailed photovoltaic parameters are summarized in Table S1). This behavior of device performance degradation followed the same trend of the photodegradation in the photoactive layers. Therefore, the photostability of photoactive J


Article

Macromolecules films is a key factor for attaining high photostability of PSC devices, in addition to the morphological change of photoactive layer or interfacial issue.19

(2) Huang, Y.; Kramer, E. J.; Heeger, A. J.; Bazan, G. C. Bulk Heterojunction Solar Cells: Morphology and Performance Relationships. Chem. Rev. 2014, 114, 7006−7043. (3) Li, G.; Zhu, R.; Yang, Y. Polymer Solar Cells. Nat. Photonics 2012, 6, 153−161. (4) Krebs, F. C.; Espinosa, N.; Hösel, M.; Søndergaard, R. R.; Jørgensen, M. 25th Anniversary Article: Rise to Power − OPV-Based Solar Parks. Adv. Mater. 2014, 26, 29−39. (5) Kim, T.; Kim, J.-H.; Kang, T. E.; Lee, C.; Kang, H.; Shin, M.; Wang, C.; Ma, B.; Jeong, U.; Kim, T.-S.; Kim, B. J. Flexible, Highly Efficient All-Polymer Solar Cells. Nat. Commun. 2015, 6, 8547. (6) Kim, Y.; Ryu, T. I.; Ok, K.-H.; Kwak, M.-G.; Park, S.; Park, N.-G.; Han, C. J.; Kim, B. S.; Ko, M. J.; Son, H. J.; Kim, J.-W. Inverted Layerby-Layer Fabrication of an Ultraflexible and Transparent Ag Nanowire/Conductive Polymer Composite Electrode for Use in High-Performance Organic Solar Cells. Adv. Funct. Mater. 2015, 25, 4580−4589. (7) Sun, K.; Xiao, Z.; Lu, S.; Zajaczkowski, W.; Pisula, W.; Hanssen, E.; White, J. M.; Williamson, R. M.; Subbiah, J.; Ouyang, J.; Holmes, A. B.; Wong, W. W. H.; Jones, D. J. A Molecular Nematic Liquid Crystalline Material for High-Performance Organic Photovoltaics. Nat. Commun. 2015, 6, 6013. (8) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-Efficiency Polymer Solar Cells. Nat. Commun. 2014, 5, 5293. (9) Subbiah, J.; Purushothaman, B.; Chen, M.; Qin, T.; Gao, M.; Vak, D.; Scholes, F. H.; Chen, X.; Watkins, S. E.; Wilson, G. J.; Holmes, A. B.; Wong, W. W. H.; Jones, D. J. Organic Solar Cells Using a HighMolecular-Weight Benzodithiophene−Benzothiadiazole Copolymer with an Efficiency of 9.4%. Adv. Mater. 2015, 27, 702−705. (10) Chen, J.-D.; Cui, C.; Li, Y.-Q.; Zhou, L.; Ou, Q.-D.; Li, C.; Li, Y.; Tang, J.-X. Single-Junction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency. Adv. Mater. 2015, 27, 1035−1041. (11) Zhao, W.; Ye, L.; Zhang, S.; Sun, M.; Hou, J. A Universal Halogen-Free Solvent System for Highly Efficient Polymer Solar Cells. J. Mater. Chem. A 2015, 3, 12723−12729. (12) Ye, L.; Zhang, S.; Huo, L.; Zhang, M.; Hou, J. Molecular Design toward Highly Efficient Photovoltaic Polymers Based on TwoDimensional Conjugated Benzodithiophene. Acc. Chem. Res. 2014, 47, 1595−1603. (13) Kim, J.-H.; Park, J. B.; Jung, I. H.; Grimsdale, A. C.; Yoon, S. C.; Yang, H.; Hwang, D.-H. Well-Controlled Thieno[3,4-c]Pyrrole-4,6(5H)-Dione Based Conjugated Polymers for High Performance Organic Photovoltaic Cells with the Power Conversion Efficiency Exceeding 9%. Energy Environ. Sci. 2015, 8, 2352−2356. (14) Yao, H.; Ye, L.; Zhang, H.; Li, S.; Zhang, S.; Hou, J. Molecular Design of Benzodithiophene-Based Organic Photovoltaic Materials. Chem. Rev. 2016, 116, 7397−7457. (15) Zhong, Y.; Trinh, M. T.; Chen, R.; Purdum, G. E.; Khlyabich, P. P.; Sezen, M.; Oh, S.; Zhu, H.; Fowler, B.; Zhang, B.; Wang, W.; Nam, C.-Y.; Sfeir, M. Y.; Black, C. T.; Steigerwald, M. L.; Loo, Y.-L.; Ng, F.; Zhu, X. Y.; Nuckolls, C. Molecular Helices as Electron Acceptors in High-Performance Bulk Heterojunction Solar Cells. Nat. Commun. 2015, 6, 8242. (16) Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, 6736−6767. (17) Kong, J.; Song, S.; Yoo, M.; Lee, G. Y.; Kwon, O.; Park, J. K.; Back, H.; Kim, G.; Lee, S. H.; Suh, H.; Lee, K. Long-Term Stable Polymer Solar Cells with Significantly Reduced Burn-in Loss. Nat. Commun. 2014, 5, 5688. (18) Mateker, W. R.; Heumueller, T.; Cheacharoen, R.; SachsQuintana, I. T.; McGehee, M. D.; Warnan, J.; Beaujuge, P. M.; Liu, X.; Bazan, G. C. Molecular Packing and Arrangement Govern the PhotoOxidative Stability of Organic Photovoltaic Materials. Chem. Mater. 2015, 27, 6345−6353. (19) Jørgensen, M.; Norrman, K.; Gevorgyan, S. A.; Tromholt, T.; Andreasen, B.; Krebs, F. C. Stability of Polymer Solar Cells. Adv. Mater. 2012, 24, 580−612.

3. CONCLUSIONS We report the synthesis, photochemical stability, and photovoltaic properties of three highly photostable D−A polymers of PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2. These polymers are based on DTBDT and BZFn (n = 0, 1, 2) units and have a highly planar backbone with linear alkyl side chains, which allow for good interchain stacking. Importantly, all of the PDTBDT-BZ series polymers displayed excellent photostability in film states when compared to other polymers. This high photostability can be attributed to both the intrinsic photochemical stability of the DTBDT and BZFn units and the high local interchain packing via the flat aromatic backbone and linear alkyl chains, the latter of which is effective at blocking oxygen diffusion into the bulk films. Furthermore, the polymers showed promising photovoltaic performance with a maximum PCE of over 8% from the PDTBDT-BZF polymer-based photovoltaic device. This work provides comprehensive insights into how both high photostability and photovoltaic performance can be attained by a careful design chemical structures of D−A polymers.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01580. Detailed experimental section of the synthesis of PDTBDT-BZ series polymers, materials and film characterizations, OPV device fabrication conditions (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*(Y.-H.K.) Tel +82 55 772 1491, Fax +82 55 772 1489; e-mail [email protected]. *(B.K.) Tel +82 2 3277 5954, Fax +82 2 3277 2369, e-mail [email protected]. Author Contributions

H.S.L. and H.G.S. equally contributed to this work. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by NRF grant funded by the Korea goverment (MSIP) (2015R1A2A1A10055620), the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2015M1A2A2056218), and by Basic Science Research Program through the NRF funded by the Ministry of Education (NRF2015R1D1A1A01058493).

■

REFERENCES

(1) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868−5923. K


Article

Macromolecules

Role of Nonbonding Interactions. J. Am. Chem. Soc. 2013, 135, 10475−10483. (37) Shin, N.; Yun, H.-J.; Yoon, Y.; Son, H. J.; Ju, S.-Y.; Kwon, S.-K.; Kim, B.; Kim, Y.-H. Highly Stable Polymer Solar Cells Based on Poly(Dithienobenzodithiophene-co-Thienothiophene). Macromolecules 2015, 48, 3890−3899. (38) Gao, L.; Zhang, Z.-G.; Xue, L.; Min, J.; Zhang, J.; Wei, Z.; Li, Y. All-Polymer Solar Cells Based on Absorption-Complementary Polymer Donor and Acceptor with High Power Conversion Efficiency of 8.27%. Adv. Mater. 2016, 28, 1884−1890. (39) Zhang, S.; Yang, B.; Liu, D.; Zhang, H.; Zhao, W.; Wang, Q.; He, C.; Hou, J. Correlations among Chemical Structure, Backbone Conformation, and Morphology in Two Highly Efficient Photovoltaic Polymer Materials. Macromolecules 2016, 49, 120−126. (40) Uddin, M. A.; Lee, T. H.; Xu, S.; Park, S. Y.; Kim, T.; Song, S.; Nguyen, T. L.; Ko, S.-j.; Hwang, S.; Kim, J. Y.; Woo, H. Y. Interplay of Intramolecular Noncovalent Coulomb Interactions for Semicrystalline Photovoltaic Polymers. Chem. Mater. 2015, 27, 5997−6007. (41) Yang, L.; Tumbleston, J. R.; Zhou, H.; Ade, H.; You, W. Disentangling the Impact of Side Chains and Fluorine Substituents of Conjugated Donor Polymers on the Performance of Photovoltaic Blends. Energy Environ. Sci. 2013, 6, 316−326. (42) Tumbleston, J. R.; Collins, B. A.; Yang, L.; Stuart, A. C.; Gann, E.; Ma, W.; You, W.; Ade, H. The Influence of Molecular Orientation on Organic Bulk Heterojunction Solar Cells. Nat. Photonics 2014, 8, 385−391. (43) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. For the Bright FutureBulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%. Adv. Mater. 2010, 22, E135−E138. (44) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15, 1617−1622. (45) Park, Y. D.; Kim, D. H.; Jang, Y.; Cho, J. H.; Hwang, M.; Lee, H. S.; Lim, J. A.; Cho, K. Effect of Side Chain Length on Molecular Ordering and Field-Effect Mobility in Poly(3-alkylthiophene) Transistors. Org. Electron. 2006, 7, 514−520. (46) Kim, Y.-S.; Kim, T.; Kim, B.; Lee, D.-K.; Kim, H.; Ju, B.-K.; Kim, K. Transient Photovoltage and Dark Current Analysis on Enhanced Open-Circuit Voltage of Polymer Solar Cells with Hole Blocking TiO2 Nanoparticle Interfacial Layer. Org. Electron. 2013, 14, 1749−1754. (47) Yoon, Y.; Kim, H. J.; Cho, C.-H.; Kim, S.; Son, H. J.; Ko, M.-J.; Kim, H.; Lee, D.-K.; Kim, J. Y.; Lee, W.; Kim, B. J.; Kim, B. Carrier Lifetime Extension via the Incorporation of Robust Hole/Electron Blocking Layers in Bulk Heterojunction Polymer Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 333−339. (48) Jung, M.; Seo, D.; Kwak, K.; Kim, A.; Cha, W.; Kim, H.; Yoon, Y.; Ko, M. J.; Lee, D.-K.; Kim, J. Y.; Son, H. J.; Kim, B. Structural and Morphological Tuning of Dithienobenzodithiophene-Core Small Molecules for Efficient Solution Processed Organic Solar Cells. Dyes Pigm. 2015, 115, 23−34. (49) Lou, S. J.; Szarko, J. M.; Xu, T.; Yu, L.; Marks, T. J.; Chen, L. X. Effects of Additives on the Morphology of Solution Phase Aggregates Formed by Active Layer Components of High-Efficiency Organic Solar Cells. J. Am. Chem. Soc. 2011, 133, 20661−20663. (50) Kim, H.; Lee, H.; Seo, D.; Jeong, Y.; Cho, K.; Lee, J.; Lee, Y. Regioregular Low Bandgap Polymer with Controlled Thieno[3,4b]Thiophene Orientation for High-Efficiency Polymer Solar Cells. Chem. Mater. 2015, 27, 3102−3107. (51) Qin, T.; Zajaczkowski, W.; Pisula, W.; Baumgarten, M.; Chen, M.; Gao, M.; Wilson, G.; Easton, C. D.; Müllen, K.; Watkins, S. E. Tailored Donor−Acceptor Polymers with an a−D1−a−D2 Structure: Controlling Intermolecular Interactions to Enable Enhanced Polymer Photovoltaic Devices. J. Am. Chem. Soc. 2014, 136, 6049−6055. (52) Wang, M.; Wang, H.; Yokoyama, T.; Liu, X.; Huang, Y.; Zhang, Y.; Nguyen, T.-Q.; Aramaki, S.; Bazan, G. C. High Open Circuit Voltage in Regioregular Narrow Band Gap Polymer Solar Cells. J. Am. Chem. Soc. 2014, 136, 12576−12579.

(20) Manceau, M.; Bundgaard, E.; Carle, J. E.; Hagemann, O.; Helgesen, M.; Sondergaard, R.; Jorgensen, M.; Krebs, F. C. Photochemical Stability of π-Conjugated Polymers for Polymer Solar Cells: A Rule of Thumb. J. Mater. Chem. 2011, 21, 4132−4141. (21) Soon, Y. W.; Shoaee, S.; Ashraf, R. S.; Bronstein, H.; Schroeder, B. C.; Zhang, W.; Fei, Z.; Heeney, M.; McCulloch, I.; Durrant, J. R. Material Crystallinity as a Determinant of Triplet Dynamics and Oxygen Quenching in Donor Polymers for Organic Photovoltaic Devices. Adv. Funct. Mater. 2014, 24, 1474−1482. (22) Tournebize, A.; Bussière, P.-O.; Wong-Wah-Chung, P.; Thérias, S.; Rivaton, A.; Gardette, J.-L.; Beaupré, S.; Leclerc, M. Impact of UvVisible Light on the Morphological and Photochemical Behavior of a Low-Bandgap Poly(2,7-carbazole) Derivative for Use in HighPerformance Solar Cells. Adv. Energy Mater. 2013, 3, 478−487. (23) Soon, Y. W.; Cho, H.; Low, J.; Bronstein, H.; McCulloch, I.; Durrant, J. R. Correlating Triplet Yield, Singlet Oxygen Generation and Photochemical Stability in Polymer/Fullerene Blend Films. Chem. Commun. 2013, 49, 1291−1293. (24) Hintz, H.; Sessler, C.; Peisert, H.; Egelhaaf, H. J.; Chassé, T. Wavelength-Dependent Pathways of Poly-3-hexylthiophene PhotoOxidation. Chem. Mater. 2012, 24, 2739−2743. (25) Son, H. J.; Wang, W.; Xu, T.; Liang, Y.; Wu, Y.; Li, G.; Yu, L. Synthesis of Fluorinated Polythienothiophene-co-Benzodithiophenes and Effect of Fluorination on the Photovoltaic Properties. J. Am. Chem. Soc. 2011, 133, 1885−1894. (26) Alem, S.; Wakim, S.; Lu, J.; Robertson, G.; Ding, J.; Tao, Y. Degradation Mechanism of Benzodithiophene-Based Conjugated Polymers When Exposed to Light in Air. ACS Appl. Mater. Interfaces 2012, 4, 2993−2998. (27) Wu, Y.; Li, Z.; Ma, W.; Huang, Y.; Huo, L.; Guo, X.; Zhang, M.; Ade, H.; Hou, J. Pdt-S-T: A New Polymer with Optimized Molecular Conformation for Controlled Aggregation and π−π Stacking and Its Application in Efficient Photovoltaic Devices. Adv. Mater. 2013, 25, 3449−3455. (28) Yun, H.-J.; Lee, Y.-J.; Yoo, S.-J.; Chung, D. S.; Kim, Y.-H.; Kwon, S.-K. Synthesis of Poly(Benzothiadiazole-co-Dithienobenzodithiophenes) and Effect of Thiophene Insertion for High-Performance Polymer Solar Cells. Chem. - Eur. J. 2013, 19, 13242−13248. (29) Ryu, T. I.; Yoon, Y.; Kim, J.-H.; Hwang, D.-H.; Ko, M. J.; Lee, D.-K.; Kim, J. Y.; Kim, H.; Park, N.-G.; Kim, B.; Son, H. J. Simultaneous Enhancement of Solar Cell Efficiency and Photostability via Chemical Tuning of Electron Donating Units in Diketopyrrolopyrrole-Based Push−Pull Type Polymers. Macromolecules 2014, 47, 6270−6280. (30) Yuan, L.; Zhao, Y.; Zhang, J.; Zhang, Y.; Zhu, L.; Lu, K.; Yan, W.; Wei, Z. Oligomeric Donor Material for High-Efficiency Organic Solar Cells: Breaking Down a Polymer. Adv. Mater. 2015, 27, 4229− 4233. (31) Stuart, A. C.; Tumbleston, J. R.; Zhou, H.; Li, W.; Liu, S.; Ade, H.; You, W. Fluorine Substituents Reduce Charge Recombination and Drive Structure and Morphology Development in Polymer Solar Cells. J. Am. Chem. Soc. 2013, 135, 1806−1815. (32) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C. Efficiency Enhancement in Low-Bandgap Polymer Solar Cells by Processing with Alkane Dithiols. Nat. Mater. 2007, 6, 497−500. (33) Nketia-Yawson, B.; Lee, H.-S.; Seo, D.; Yoon, Y.; Park, W.-T.; Kwak, K.; Son, H. J.; Kim, B.; Noh, Y.-Y. A Highly Planar Fluorinated Benzothiadiazole-Based Conjugated Polymer for High-Performance Organic Thin-Film Transistors. Adv. Mater. 2015, 27, 3045−3052. (34) Kim, Y.; Swager, T. M. Ultra-Photostable N-Type PPVs. Chem. Commun. 2005, 372−374. (35) Subramanian, S.; Park, S. K.; Parkin, S. R.; Podzorov, V.; Jackson, T. N.; Anthony, J. E. Chromophore Fluorination Enhances Crystallization and Stability of Soluble Anthradithiophene Semiconductors. J. Am. Chem. Soc. 2008, 130, 2706−2707. (36) Jackson, N. E.; Savoie, B. M.; Kohlstedt, K. L.; Olvera de la Cruz, M.; Schatz, G. C.; Chen, L. X.; Ratner, M. A. Controlling Conformations of Conjugated Polymers and Small Molecules: The L


Article

Macromolecules (53) Cho, C.; Kim, H.; Jeong, S.; Baek, S.-W.; Seo, J.-W.; Han, D.; Kim, K.; Park, Y.; Yoo, S.; Lee, J.-Y. Random and V-Groove Texturing for Efficient Light Trapping in Organic Photovoltaic Cells. Sol. Energy Mater. Sol. Cells 2013, 115, 36−41. (54) Rao, A.; Chow, P. C. Y.; Gelinas, S.; Schlenker, C. W.; Li, C.-Z.; Yip, H.-L.; Jen, A. K. Y.; Ginger, D. S.; Friend, R. H. The Role of Spin in the Kinetic Control of Recombination in Organic Photovoltaics. Nature 2013, 500, 435−439.

M


Effects of Backbone Planarity and Tightly Packed Alkyl Chains in the

Recommend Documents