Every Atom Counts: Elucidating the Fundamental Impact of Structural

Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

pubs.acs.org/cm

Every Atom Counts: Elucidating the Fundamental Impact of Structural Change in Conjugated Polymers for Organic Photovoltaics Chi Kin Lo,† Bhoj R. Gautam,‡,⊥ Philipp Selter,§ Zilong Zheng,† Stefan D. Oosterhout,∥ Iordania Constantinou,¶,# Robert Knitsch,§ Rylan M. W. Wolfe,† Xueping Yi,¶ Jean-Luc Brédas,† Franky So,¶ Michael F. Toney,∥ Veaceslav Coropceanu,† Michael Ryan Hansen,§ Kenan Gundogdu,‡ and John R. Reynolds*,† †

School of Chemistry and Biochemistry, Center for Organic Photonics and Electronics, Georgia Tech Polymer Network, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡ Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, United States ⊥ Department of Chemistry and Physics, Fayetteville State University, Fayetteville, North Carolina 28301, United States § Institute of Physical Chemistry, Westfälische Wilhelms-Universität Münster, Corrensstr. 28/30, D-48149 Münster, Germany ∥ SLAC National Accelerator Laboratory, Stanford University, Menlo Park, California 94025, United States ¶ Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States S Supporting Information *

ABSTRACT: As many conjugated polymer-based organic photovoltaic (OPV) materials provide substantial solar power conversion efficiencies (as high as 13%), it is important to develop a deeper understanding of how the primary repeat unit structures impact device performance. In this work, we have varied the group 14 atom (C, Si, Ge) at the center of a bithiophene fused ring to elucidate the impact of a minimal repeat unit structure change on the optical, transport, and morphological properties, which ultimately control device performance. Careful polymerization and polymer purification produced three “one-atom change” donor−acceptor conjugated alternating copolymers with similar molecular weights and dispersities. DFT calculation, absorption spectroscopy, and high-temperature solution 1H nuclear magnetic resonance (NMR) results indicate that poly(dithienosilole-alt-thienopyrrolodione), P(DTS-TPD), and poly(dithienogermole-alt-thienopyrrolodione), P(DTG-TPD) exhibit different rotational conformations when compared to poly(cyclopentadithiophene-alt-thienopyrrolodione), P(DTC-TPD). Solid-state 1H MAS NMR experiments reveal that the greater probability of the anticonformation in P(DTS-TPD) and P(DTG-TPD) prevail in the solid phase. The conformational variation seen in solution and solid-state NMR in turn affects the polymer stacking and intermolecular interaction. Twodimension 1H-1H DQ-SQ NMR correlation spectra shows aromatic−aromatic correlations for P(DTS-TPD) and P(DTG-TPD), which on the other hand is absent for P(DTC-TPD). In a thin-film interchain packing study using grazing incidence wide-angle X-ray scattering (GIWAXS), we observe the π-face of the conjugated backbones of P(DTC-TPD) aligned edge-on to the substrate, whereas in contrast the π-faces of P(DTS-TPD) and P(DTG-TPD) align parallel to the surface. These differences in polymer conformations and backbone orientations lead to variations in the OPV performance of blends with the fullerene PC71BM, with the device containing P(DTC-TPD):PCBM having a lower fill factor and a lower power conversion efficiency. Ultrafast transient absorption spectroscopy shows the P(DTC-TPD):PCBM blend to have a more pronounced triplet formation from bimolecular recombination of initially separated charges. With a combination of sub-bandgap external quantum efficiency measurements and DFT calculations, we present evidence that the greater charge recombination loss is the result of a lower lying triplet energy level for P(DTC-TPD), leading to a higher rate of recombination and lower OPV device performance. Importantly, this study ties ultimate photovoltaic performance to morphological features in the active films that are induced from the processing solution and are a result of minimal one-atom differences in polymer repeat unit structure.

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INTRODUCTION

The performance of bulk heterojunction (BHJ) organic photovoltaic (OPV) materials has seen significant enhancements over the past five years with both molecule and polymer containing devices (with fullerene and nonfullerene acceptors) © XXXX American Chemical Society

Received: February 7, 2018 Revised: April 4, 2018

A

DOI: 10.1021/acs.chemmater.8b00590 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

Scheme 1. Stille Cross-Coupling Polymerization and Structures of the Fused-Dithiophene-co-thienopyrrolodione Polymers, P(DTC-TPD), P(DTS-TPD), and P(DTG-TPD)

methods and architectures. Similar one-atom change investigations have been conducted, but the polymers prepared in these previous studies had variations in the synthetic methods employed leading to disparate molecular and macromolecular structures.13,44 Here, we used parallel polymer syntheses and purifications carried out as identically as possible to obtain polymers with similar molecular weights and dispersities, along with chemical and structural purities, allowing us to isolate the effect of changing a single atom on photovoltaic properties. Solutionstate properties are examined with proton nuclear magnetic resonance (NMR) and UV−vis-NIR spectroscopy. The solidstate organization was probed via solid-state NMR and grazing incidence wide-angle X-ray scattering (GIWAXS), redox and electronic properties using steady-state and transient absorption spectroscopies, along with electrochemistry, and electronic structures studied using density functional theory (DFT) calculations to reveal the effect of intermolecular packing between polymer chains on OPV device performance. We found that longer C−Si and C−Ge bonds compared to the C− C bond at the one-atom change center led to different polymer backbone conformations in both solution and in the solid state, which impacted the backbone orientations against substrates upon which the polymers were processed. Transient absorption spectroscopy results indicate a higher bimolecular recombination rate between separated charges in the DTC polymer due to a more stable triplet energy level, which was confirmed by theoretical calculations. Finally, we combine these results to demonstrate how the optoelectronic properties and photovoltaic performance is linked to morphological features in the active films. We demonstrate how they are induced from aggregates that form in the processing solution, which can be correlated with the minimal one-atom differences in polymer repeat unit structure.

breaking the 10% barrier in power conversion efficiencies (PCEs). An effective strategy in polymer donor design involves employing a conjugated framework with multiple fused rings where a ladder-type building block forces the backbone into planarity, extends the conjugation length, improves electron delocalization, reduces rotational disorder, and enhances physical, chemical, and mechanical stabilities of the polymers.1 Common multiple-ring electron-rich moieties include benzodithiophene (BDT),2−5 dithienosilole (DTS),6−8 dithienogermole (DTG),9−12 and cyclopentadithiophene13−15 (CPDT, also called DTC in this paper to be consistent with acronyms DTS and DTG). In the case of electron-deficient moieties,16 thienopyrrolodione (TPD),17−19 isoindigo (iI),20−24 diketopyrrolopyrrole (DPP),25,26 and benzothiadiazole (BTD)27−29 all consist of fused structures. Developing a deep understanding of the structure−property relationships in OPV materials is necessary if researchers are to continue enhancing device performances; however, many high performing polymers with power conversion efficiencies (PCEs) of >8% have quite different molecular structures, frontier energy levels, and physical properties.9,12,30−40 Even for polymers with close repeat unit structural similarities, direct comparison may be difficult due to batch-to-batch variances (e.g., average molecular weight, dispersity, end group identity, etc.), an intrinsic drawback of polymeric materials.41,42 Multiple reports43−45 have attempted to elucidate the impact of minimal change along the polymer’s repeat unit structure on device performance. In one example using two copolymers consisting of BTD and DTS or DTG, Kim et al. conclude that the longer C−Ge bond length allows for greater backbone planarity and crystallinity, leading to an improvement in structural order and charge carrier mobility, ultimately resulting in a slight PCE enhancement from 4.23% to 4.56% due to a 34% enhancement of short-circuit current.44 However, the two polymers in the study have significantly different molecular weights and dispersities, and they are synthesized by different cross-coupling polymerizations (Suzuki and Stille). In this report, we employ a family of photoactive conjugated polymers with only a minimal “one-atom” change where each alternating copolymer has a repeat unit containing a TPD acceptor and a bithiophene donor bridged by a Group 14 center atom. Previous studies (summarized in Table S1) have shown that DTC-TPD, DTS-TPD, and DTG-TPD polymers in fullerene-blend BHJ devices lead to PCE as high as 6.4%, 8.1%, and 8.5%, respectively, showing the substantial impact of a minimal change in repeat unit structure on OPV device performance. However, it is important to recognize that a direct comparison of photovoltaic performance from different studies is not sufficient to isolate and elucidate the structural impact on material and device properties because these materials have different synthetic approaches, molecular weights, and purities, and the devices are constructed using different processing

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RESULTS AND DISCUSSION Polymer Synthesis: Control of Molecular Weight, Dispersity, and Purity. The Stille reaction (Scheme 1) was chosen as the cross-coupling polymerization technique due to its high yield, molecular weight, versatility, and stability for a wide range of functional groups. The purified palladium catalyst, tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) was handled inside an argon-filled glovebox and stored at −20 °C under inert atmosphere. This was crucial in achieving high molecular weight polymers by avoiding catalyst decomposition46,47 and Pd(0) nanoparticle formation, which can lead to incorrect catalyst loading and homocoupled fragments along the backbones. For details on ensuring the purity of the Pd catalyst and the purification route, the authors recommend an article written by Zalesskiy et al.48 To ensure the three polymers resulted in similar molecular weights, B


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Chemistry of Materials Table 1. Physical Properties, Elemental Accuracies, and Residual Impurity Content of the Polymers Elemental Analysis Actual (Theoretical) P(DTC-TPD) P(DTS-TPD) P(DTG-TPD)

b

ICP-MS

a

c

Yield (mg)

GPC Mn(kDa)/ĐM

C (%)

H (%)

N (%)

S (%)

P (ppm)

Pd (ppm)

Sn (ppm)

375 (59%) 423 (67%) 483 (73%)

26.4/1.4 24.5/1.7 20.8/1.7

70.54 (70.33) 67.25 (66.91) 63.06 (62.81)

8.14 (8.32) 7.85 (8.13) 7.20 (7.63)

2.16 (2.10) 2.10 (2.05) 2.03 (1.93)

14.57 (14.44) 14.47 (14.10) 13.42 (13.24)

bdl bdl bdl

25 31 38

26 26 57

a

Yields from chloroform Soxhlet fractions. bMolecular weights and dispersities were determined by gel permeation chromatography with polystyrene as the calibration standard and 1,3,4-trichlorobenzene at 140 °C as the eluent. cPhosphorus levels for all polymer samples were below detection limit (bdl).

dispersities, and purities, the polymerizations were conducted in parallel and identically end-capped with thiophene, followed by Soxhlet extractions, catalyst scavenging, and removal of inorganic impurities using column chromatography. All three polymers have similar number-average molecular weights (Mn) and dispersities (ĐM) at 20−27 kDa and 1.4−1.7, as shown in Table 1 with the GPC chromatograms presented in Figure S1. All chromatograms showed monomodal distribution indicating all polymers were sufficiently soluble in hot trichlorobenzene, which helped confirm the polymer molecular weights obtained truly reflected their physical attributes. To further understand the chemical and structural purities of our polymers, elemental analysis was performed indicating the C, H, N, and S contents were within 0.4% of calculated values. Inductively coupled plasma mass spectrometry (ICP-MS) was used to confirm residual phosphorus, palladium, and tin content within the polymer matrices. To fully digest the polymer matrix, microwave-assisted acid digestion in sulfuric acid and nitric acid was performed (the detailed method can be found in the Supporting Information). Previous elemental investigations by ICP-MS on conjugated materials synthesized by cross-coupling reactions such as Stille and direct arylation polymerizations have shown residual Sn, Pd, and P contents as high as 2 orders of magnitude greater than our analyses.49,50 The low impurity content in our polymers confirms that the purification process through end-capping, reprecipitation, palladium scavenging, and column chromatography allow us to obtain three highly pure polymers, and thus reduce the number of potential charge trapping sites within the polymer matrix.51,52 Electronic Structure: More Stable Syn-conformers. The excited states of the polymers were modeled using oligomers with five DTX-TPD (X = C, Si, or Ge) repeat units (∼5 nm) (in these calculations, the n-octyl side chains were replaced with methyl groups). In addition to the calculations performed in the ground-state geometry, corresponding to the syn-conformation (torsion angle α = 0° between DTC and TPD units, see Figure S2 and Figure S3), the calculations were also performed for the anticonformation (torsion angle α = 180°). The derived energies of the lowest excited singlet (S1) and triplet (T1) states of P(DTC-TPD), P(DTS-TPD), and P(DTG-TPD) are shown in Table 2. The results indicate that in all three systems the synconformers have a lower energy of ∼2 kcal/mol. As shown in Table 2 and Tables S2−4, the dihedral angle, however, has a modest effect on the frontier orbital and excited-state energies (up to 0.1 eV) and a negligible effect on the energy splitting between the S1 and T1 states. Note that when the polymer T1 state is lower than the lowest charge-transfer (CT) triplet states in the blend with fullerenes, then the T1 states can contribute to charge recombination. Based on the results shown in Table 2, it

Table 2. TD-DFT Energies (eV) of the Lowest Excited Singlet and Triplet States of (DTC-TPD)n=5, (DTS-TPD)n=5, and (DTG-TPD)n=5 Oligomersa S1 (DTC-TPD)n=5 (DTS-TPD)n=5 (DTG-TPD)n=5

T1

anti

syn

anti

syn

1.74 1.82 1.82

1.68 1.73 1.73

1.36 1.43 1.43

1.31 1.36 1.37

a

The calculations are performed at the tuned-ωB97XD/PCM level with the 6-31G(d) basis set.

might be expected that this effect is more noticeable in P(DTCTPD). Optoelectronic Properties: Difference in Solid-State Aggregation. Ionization potentials (IPs) and electron affinities (EAs) of the polymers were estimated by electrochemistry using differential pulse voltammetry (Figure S5) and are given in Table 3. The thin-film onsets of light absorption for all the polymers are ∼730 nm (Figure 1). The absorption profiles of P(DTS-TPD) and P(DTG-TPD) are similar with λmax at ∼671 nm. P(DTC-TPD), on the other hand, shows a stronger absorption profile between 430 and 680 nm. The TDDFT calculations (Table S3) are in line with these experimental results, indicating that the first optical bands in all three polymers have similar energies. The analysis of the first 0‑1 absorption band indicates that the ratio I0‑0 A /IA (Table 3) of the 0-0 (∼675 nm) and 0-1 (∼600 nm) vibrational peaks is smaller for P(DTC-TPD) in both thin film (Figure 1) and solution (Figure S6). This ratio decreases by ∼25% for all three systems in comparing the solution and solid-state results. This change in 53 0‑1 I0‑0 In order A /IA may be a signature of polymer aggregation. to shed more light on this issue, we estimated the vibrational couplings and related relaxation energies of the S1 state; these 0‑1 quantities define I0‑0 A /IA for the isolated polymer chains. The computed relaxation energies are given in Table 4 whereas the vibrational couplings are shown in Figure S4. As seen from Table 4, the relaxation energies computed by two approaches are in good agreement. In the anticonformation, they are similar for all three systems. However, in the case of the synconformation, the relaxation energy of P(DTC-TPD) is significantly larger than those for P(DTS-TPD) and P(DTGTPD). These results are thus in agreement with the trend 0‑1 found for I0‑0 A /IA ratios assuming that the syn-conformation is significantly populated for all three systems. Given that the 0‑1 solid-state I0‑0 ratio is smaller than the solution-state A /IA absorption, as well as the fact that the computed relaxation energies of both conformers are similar, we conclude that P(DTC-TPD) has a predominately H-type aggregation 0‑1 characteristic. Similar results for I0‑0 A /IA were also found for C


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Chemistry of Materials Table 3. Optical and Electrochemical Properties of Polymer Films Absorptiona P(DTC-TPD) P(DTS-TPD) P(DTG-TPD)

λmax (nm)

λonset (nm)

615, 671 613, 672 617, 678

724 725 730

Electrochemistryd

b Eopt gap

(eV)

1.71 1.71 1.69

0‑1c I0‑0 A /IA

IP (eV)

EA (eV)

Eechem (eV) gap

1.05 (1.33) 1.26 (1.55) 1.30 (1.63)

−5.60 −5.67 −5.60

−3.47 −3.53 −3.61

2.13 2.14 1.99

c Thin films spin-coated on glass slides from 5 mg/mL solution in chloroform. bEopt g = 1240/λonset,film. Vibrational peak ratio of thin-film absorption d (solution in parentheses). Oxidation and reduction potentials were estimated by thin-film electrochemistry using different pulse voltammetry scan. IP and EA values were calculated by assuming saturated calomel electrode (SCE) to be 4.74 eV vs vacuum and Fc/Fc+ to be +0.38 eV with respect to SCE.54 a

Figure 1. (a) Normalized absorption spectra of polymer thin-films. (b) Carbon-to-center atom bond lengths calculated at the DFT/B3LYP/631G** level.

P(DTC-TPD) showed one multiplet at ∼8.1 ppm, whereas P(DTS-TPD) and P(DTG-TPD) displayed two additional peaks at ∼7.5 and ∼8.5 ppm. On the basis of supplementary DFT calculations for two different polymer conformations, (see Supporting Information Figure S7 for details), we assign the appearance of two aromatic 1H resonances to the formation of an intramolecular hydrogen−carbonyl interaction. A similar situation has recently been studied by Chaudhari et al. for diketopyrrolopyrrole-dithienylthieno[3,2-b]thiophene (DPPDTT) polymers.55 Thus, the two additional peaks at ∼7.5 and ∼8.5 ppm can be attributed to the syn- and anticonformers (Figure 3) in the polymer aggregates. This confirms the presence of the two conformers, as they are hypothesized above in the absorption and DFT results. To resolve the polymer aggregates that lead to the two conformers and allow the backbones to overcome the rotational barrier, elevated-temperature 1H NMR was performed at 110 °C. At this temperature, the two additional 1H peaks observed at room temperature began to resolve into one, single peak at ∼8.1 ppm for P(DTS-TPD) and P(DTG-TPD), which is similar to the chemical shift for P(DTC-TPD) aromatic proton. This suggests that parts of the polymer backbone have overcome the rotational barrier and can rotate freely. Moreover, because the solution concentrations (∼10 mg/mL) for the NMR investigations are comparable to the processing solutions for OPV device fabrication, we expect that the hydrogen− carbonyl interaction persists and will affect the intra- and intermolecular interactions and thus, the aggregation behavior in their corresponding thin films. It is important to note that, even at 110 °C, there are multiple aromatic proton peaks for P(DTS-TPD) and P(DTG-TPD), indicating the presence of multiple conformers. As will be shown later in the OPV device section, polymer:fullerene blends were cast at 80 °C. It can thus be expected that there are multiple polymer conformers in the

Table 4. DFT Estimates of the Relaxation Energy (in eV) of the S1 State in P(DTC-TPD), P(DTS-TPD) and P(DTGTPD) Obtained from the Adiabatic Potential (AP) Energy Surface of the S1 State and Normal-Mode (NM) Analysisa

(DTC-TPD) (DTS-TPD) (DTG-TPD) a

Conformation

By contribution of vibration

By DFT total energy difference

syn anti syn anti syn anti

0.254 0.271 0.116 0.293 0.103 0.289

0.249 0.264 0.091 0.262 0.084 0.263

These calculations were performed at the B3LYP/6-31G(d) level.

the P(DTS-TPD) and P(DTG-TPD). However, for these 0‑1 systems the smaller solid-state I0‑0 A /IA ratios are likely a result of both aggregation and an increase in population of the anticonformers because these conformers are found to have larger relaxation energies than syn-conformers. The difference in solution and solid-state aggregating interactions in this family of polymers was further studied by nuclear magnetic resonance and grazing incidence wide-angle X-ray scattering. Temperature-Dependent Solution 1H NMR: Revealing Differences in Polymer Conformation. Solution 1H NMR spectra were collected in chlorobenzene-d5 to initially confirm the polymer structures; however, the resulting 1H NMR spectra at 50 °C included unexpected results, which turned out to be related to the differences in polymer conformations. Due to the symmetry of the polymer repeat unit structure, one expects a single aromatic proton signal from the two aromatic protons on the bridged bithiophene moiety. However, as shown in Figure 2, the room-temperature 1H NMR spectra revealed different characteristic aromatic peaks between the three polymers. D


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Figure 2. Solution 1H NMR spectra of the polymers in chlorobenzene-d5 at 50 °C showing multiple aromatic peaks for P(DTS-TPD) and P(DTGTPD) compared to P(DTC-TPD). At the elevated temperature of 110 °C, the aromatic 1H signals from of P(DTS-TPD) and P(DTG-TPD) resolve, indicating different energies associated with the rotational barriers for the three polymers.

Figure 3. (a) Syn- and anticonformations of the polymer repeating unit. 1H MAS NMR spectra of (b) P(DTC-TPD), (c) P(DTS-TPD), and (d) P(DTG-TPD). Two aromatic peaks, one corresponding to the hydrogen−carbonyl interaction (∼8 ppm, yellow) and the other to the anticonformation (∼7 ppm, blue) DTX (X = C, Si, Ge) protons are visible and illustrated by the chemical structures in panel a. The ratios between the two proton species have been determined by spectral deconvolution (see Figure S7).

NMR measurements were performed on all three polymers. Each polymer exhibits two aromatic proton signals of different intensities with ∼1 ppm difference in 1H chemical shift in close analogy to the solution 1H NMR data. This observation demonstrates that the hydrogen−carbonyl interaction observed in solution 1H NMR is also present in the solid phases of all three polymers. The two peaks can therefore be clearly

casting solutions, which likely affects the degree of aggregation in solution and intermolecular interaction in the thin films. Solid-State NMR: Hydrogen−Carbonyl Interaction Affects Molecular Packing and Final Film Morphology. To further characterize the influence of the hydrogen−carbonyl interaction between the DTX (X = C, Si, Ge) and the TPD units on the polymer conformation in the solid phase, 1H MAS E


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Figure 4. (a) Schematic illustration of the three possible donor−acceptor−donor conformations for the DTX-TDP polymers. (b) Three examples of the resulting change in the overall DTX-TPD polymer backbone conformation and orientation (linear vs helical) as generated via simple force-field energy minimization using the MM2 method in the Chem3D software package. The aliphatic units are omitted for clarity.

Figure 5. (a) Top view of two DTC-TPD polymer fragments with different donor−acceptor−donor conformations along the polymer backbone, illustrating the possible intrachain 1H-1H DQ correlations between pairs of DTC protons. (b) Side view showing two possible interchain 1H-1H DQ correlations between pairs of DTC protons along the π−π-stacking direction. Green and red arrows correspond to internuclear distances below and above 4 Å, leading to detectable and nondetectable 1H-1H DQ correlations, respectively. The aliphatic side chains have been omitted or reduced to methyl groups for clarity.

Further information about the intermolecular packing for the three different polymers can be achieved from 2D 1H-1H double-quantum single-quantum (DQ-SQ) NMR correlation spectroscopy.58,59 In contrast to recent studies of other πconjugated copolymers taking advantage of this approach,56,60,61 the present study of the P(DTX-TPD) polymers only includes two types of aromatic protons on the DTX units due the presence of the hydrogen−carbonyl interaction as identified above, making the information available from the 2D 1 H-1H DQ-SQ MAS NMR experiment very selective. Thus, to highlight pairs of spatially close and more remote aromatic protons, the 2D NMR spectra were recorded using one and four rotor periods of DQ excitation (back-to-back),62,63 respectively, in combination with signal suppression of the intense aliphatic signal.64 The spatial proximities that can be detected via 1H-1H DQ correlations are illustrated in Figure 5a,b, assuming complete planar geometries for the P(DTXTPD) polymers and disregarding the influence of molecular motion on the 1H-1H dipolar coupling.65 Note that the observed DQ intensity strongly depends on the internuclear distance, as well as the number of proton pairs within a sample.66−68 For a short DQ-excitation time of one rotor period under moderately fast MAS, 1H-1H DQ correlations of up to 4 Å69 are possible, which is well within the range of average π−π-stacking distances. For longer DQ excitation times, the DQ intensity is often severely hampered by T2 relaxation effects, making it difficult to predict a maximum detectable internuclear 1H-1H distance; however, the possible

attributed to two different conformational states of the TPD and DTX groups due to formation of an intramolecular hydrogen−carbonyl interaction between the DTX and TPD unit (see inset of Figure 3a and the corresponding colorcoding). The ratio of the two conformers (syn:anti) varies between 50:50 for P(DTC-TPD) and 40:60 for P(DTS-TPD) and P(DTG-TPD) as determined from spectral deconvolution of the 1H MAS NMR spectra of Figure 3 (also see Supporting Information Figure S7). As recently shown for other donor− acceptor polymers,56,57 the ratio of the two conformational states have an impact on the long-range polymer chain orientation (Figure 4), even when the precise distribution (alternating, random, clustering, etc.) of these states along the polymer backbone cannot be easily determined. The conformation without the intramolecular hydrogen−carbonyl interaction present (the anticonformation, marked in the 1H NMR spectra of Figure 3 and Figure 6 with blue dots, also see Figure S8) should lead to a straight polymer chain, whereas the presence of the hydrogen−carbonyl interaction introduces a “bend” in the polymer chain (the syn-conformation, marked in the 1H NMR spectra of Figure 3 and Figure 6 with yellow dots). This in turn affects the polymer stacking and ultimately the polymer morphology when considering the likelihood of forming edge-on or face-on polymer backbone structures, which is of particular importance for the later thin film applications. F


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Figure 6. 2D 1H-1H DQ-SQ MAS NMR correlation spectra of (a and d) P(DTC-TPD), (b and e) P(DTS-TPD), and (c and f) P(DTC-TPD). The spectra in panels a, b, and c were recorded using one rotor period of DQ excitation; spectra in panels d, e, and f employed four rotor periods of DQ excitation. Selective presaturation of the aliphatic protons was used to suppress the otherwise intense aliphatic signal.64 The assignment and position of the 1H-1H DQ-SQ correlation signals follow the color scheme given in Figure 3.

of the TPD groups in successive polymer layers, possibly in alternate fashion as recently identified for copolymers based on DTC and benzothiadiazole (BTD) groups.70−72 In contrast, the intense aromatic−aromatic DQ-SQ correlations observed for both P(DTS-TPD) and P(DTG-TPD) indicate a different polymer stacking as a result of a more pronounced intramolecular interactions (i.e., syn-conformation) involving the aromatic protons on the DTS or DTG and the carbonyls on the TPD groups. Moreover, taking into account the fact that the ratio between the anti- and the syn-conformations in the solid state is not 1:1 for P(DTS-TPD) and P(DTG-TPD) as it is for P(DTC-TPD (Figure 3), this implies that the multiple conformations imposed by the formation of a hydrogen− carbonyl interaction between DTX (X = C, S, G) and the TPD groups are already present in solution (Figure 2), which in turn has a profound effect on the final solid-state morphology. Combining this information with the above-mentioned geometric considerations for the donor−acceptor−donor conformation along the polymer chains as illustrated in Figure 4, it is clear that the different DTX conformations, in addition to their distribution and specific ratios already present in solution, influence the thin-film intermolecular packing of the three polymers. Grazing Incidence Wide-Angle X-ray Scattering: Edge-On for P(DTC-TPD) and Face-On for P(DTS-TPD) and P(DTG-TPD). To investigate the intermolecular interactions and polymer packing, grazing incidence wide-angle Xray scattering (GIWAXS) measurements were performed on thin films of the pristine polymers and the polymer:fullerene blends. The scattering was used to determine the polymer intermolecular packings and orientations relative to the silicon substrates, with (010) planes corresponding to π−π stacking and (100) planes to lamellar order. The scattering images revealed differences in polymer orientations for P(DTC-TPD) compared to the Si and Ge

DQ contacts observed using shorter DQ excitation times become more pronounced. Taking these points into consideration, the closest intramolecular 1H-1H DQ correlation for P(DTX-TPD) is expected between two DTX units across a TPD unit at an internuclear distance of approximately 4.1 to 4.5 Å (shown in Figure 5a). Any intermolecular 1H-1H correlations will occur between stacked DTX groups, depending on the specific stacking geometry (shown in Figure 5b). From the resulting 2D 1H-1H DQ-SQ MAS NMR spectra, it is clear that no pure aromatic−aromatic DQ-SQ correlations, as marked by a dashed circle in Figure 6, is observed for P(DTCTPD), whereas these are clearly seen for P(DTS-TPD) and P(DTG-TPD). However, for P(DTS-TPD) and P(DTGTPD), the aromatic−aromatic DQ-SQ correlation signals are observed as both 1H-1H autocorrelation and cross-correlation signals with different intensities, depending on the DQ recoupling time. The intense 1H-1H autocorrelation signals from pairs of the DTS and DTG protons are centered at the diagonal of the 2D spectrum at δSQ = ∼7 ppm and δDQ = ∼14 ppm (marked with two blue dots). Increasing the DQ recoupling time leads to the appearance of low-intensity 1 H-1H cross-correlation peaks at δSQ = ∼7 and ∼8 ppm and δDQ = ∼15 ppm (blue and yellow dots in Figure 6c,e,f), corresponding to a proximity of the protons from the syn- and anticonformations, respectively, for P(DTS-TPD) and P(DTGTPD). Note that no clear 1H-1H autocorrelation signal between the protons with hydrogen−carbonyl interaction (syn-conformation) of the DTS and DTG groups at ∼8 ppm (yellow dots) is observed. Thus, taken together, these observations indicate a strong difference in the stacking of polymer backbones in P(DTC-TPD) as compared to P(DTS-TPD) and P(DTG-TPD). The absence of any of the aromatic− aromatic 1H-1H DQ-SQ correlation signals necessitates a polymer stacking for P(DTC-TPD), where the DTC groups are arranged in such a way that these groups are shifted on top G


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Figure 7. GIWAXS patterns for pristine polymer thin-films (top) and polymer:PC71BM blends (bottom).

polymers. The pristine thin films of P(DTS-TPD) and P(DTGTPD) showed modest out-of-plane π−π (010) diffraction (Figure 7, top) indicating predominantly face-on orientations of the polymer backbones relative to the substrates. Their face-on orientations were retained in the polymer:PC71BM blends (Figure 7, bottom). P(DTC-TPD), on the contrary, displayed both in-plane π−π (010) and out-of-plane lamellar (100) signals that were evidence of mixed edge and face-on orientation. Thermal annealing the pristine polymer thin film also led to changes in thin film for P(DTC-TPD) (Figure S9). The diffraction pattern became more defined, sharper peaks showing improved crystallinity and a stronger orientation of polymer crystallites. We attribute this to a greater intermolecular order in the P(DTC-TPD) system, which can also be seen in the differential scanning calorimetry (DSC) study (Figure S11), where only P(DTC-TPD) displayed observable melting and crystallization phase transitions. In contrast, the annealed films of P(DTS-TPD) and, particularly, P(DTGTPD) only show slight changes, which may be an indication of limited chain mobility in the solid state. Besides the change in backbone orientation, the results for intermolecular stacking distance also show differences between P(DTC-TPD) and the other two polymers. Though all three polymers had similar π−π interplane distances of ∼3.7 Å, their lamellar distances were significantly different, with P(DTCTPD) having a significantly closer packing at 14.8 Å vs P(DTSTPD) and P(DTG-TPD) (19.6 and 18.6 Å, respectively). Figure S10 shows the integrations of the GIWAXS patterns. As mentioned earlier, we found via DFT calculation that the C−C bond length is substantially shorter than the C−Si and C−Ge bonds. This bond length difference is postulated to impact the extension and orientation of the 2-ethylhexyl chains attached to the polymer backbones, thus changing the lamellar stacking distances. That is likely why we observe the contradictive result, where P(DTS-TPD) has a slightly longer lamellar distance than P(DTG-TPD), yet the C−Si bond is slightly shorter than the C−Ge bond. The similarity in the π−π stacking distances of the three polymers contributes to the minimal difference in their

charge carrier mobility measurements (Table S1). This may seem contradictory to our results in NMR and GIWAXS, where we observed differences in intermolecular stacking and polymer structural orientation. As Noriega et al. state, the requirement for high carrier mobility in conjugated polymers is the existence of interconnected ordered domains.73 Though a polymer film can be disordered in the bulk, if the molecular weight of the polymer is sufficiently high and there are a sufficient amount of ordered domains, the polymer chains can connect these individual domains and maintain high charge mobility through the film. The similarity in their molecular weights, dispersities, and overall disorder characteristics of the polymer thin films can be the reason for the similar charge mobilities of the pristine polymers and the blended thin films with fullerene molecules added. As mentioned earlier, while P(DTC-TPD) has primarily unfavorable edge-on orientation and P(DTSTPD) and P(DTG-TPD) have primarily favorable face-on orientation, morphological directionality is mixed and, thus, P(DTC-TPD) has some face-on orientation. The P(DTCTPD):PCBM blend also shows stronger aggregation of the polymer, which may aid charge mobility in the space charge limited current (SCLC) measurement. All OPV devices have balanced SCLC hole and electron mobilities on the order of 10−3 to 10−4 cm2/(V s), which is useful for preventing charge buildup that limits photocurrent in the OPV devices. Nevertheless, the difference in backbone orientation and lamellar stacking distance between P(DTC-TPD) and P(DTS-TPD)/ P(DTG-TPD) can be correlated to the observations from theoretical calculation, optical absorption, and NMR results. It is expected that the differences in polymer conformation in solution and eventually their corresponding thin films have a strong impact on the interchain packing and backbone orientation. Organic Photovoltaic Devices: Difference in Fill Factor and Power Conversion Efficiency. Bulk heterojunction (BHJ) OPV devices were fabricated using the three polymers in the conventional device architecture (indium tin oxide (ITO)/ poly(3,4-ethylenedioxythio-phene):polystyrenesulfonate (PEH


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Figure 8. (a) Current density−voltage characteristics of OPV devices (ITO/PEDOT:PSS/Polymer:PC71BM/LiF/Al device architecture). (b) IPCE spectra of OPV devices.

Table 5. Averagea and Best (in parentheses) OPV Device Characteristics P(DTC-TPD):PC71BM P(DTS-TPD):PC71BM P(DTG-TPD):PC71BM a

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

11.3 ± 0.4 (11.7) 11.7 ± 0.4 (12.2) 12.6 ± 0.4 (13.6)

0.92 ± 0.01 (0.92) 0.88 ± 0.01 (0.91) 0.87 ± 0.00 (0.87)

54 ± 1 (55) 68 ± 1 (72) 69 ± 1 (70)

5.7 ± 0.0 (5.9) 7.0 ± 0.2 (7.4) 7.7 ± 0.3 (8.0)

Average characteristics using a minimum of 8 devices.

DOT:PSS)/polymer:PC71BM/lithium fluoride (LiF)/Al). The optimized processing conditions for all polymer:fullerene blends were identical, requiring 1:1.5 weight ratio of polymer to fullerene, 5 vol % of diiodooctane (DIO), and no thermal or solvent annealing. The current density−voltage curves and the summary of device performances are presented in Figure 8a and Table 5, respectively. P(DTS-TPD) and P(DTG-TPD) had open-circuit voltages (Voc) approaching 0.88 V, whereas Voc for P(DTC-TPD) was slightly higher at 0.92 V. All three devices have similar short-circuit currents (Jsc); P(DTC-TPD) and P(DTS-TPD) at ∼12 mA/cm2 and P(DTG-TPD) at ∼13 mA/cm2, which indicates that they have overall similar light absorption, charge generation, and transport properties. To confirm the Jsc results from the PCE measurements, incident photon-to-current efficiency (IPCE) spectra were collected to determine the spectral responses. As shown in Figure 8b, in the wavelength range corresponding to photocurrents from the polymers between 550 and 700 nm, P(DTG-TPD) has the highest maximum IPCE approaching 67%, followed by P(DTSTPD) at 62% and P(DTC-TPD) at 58%. The slight improvement in spectral photocurrent response for P(DTGTPD) is speculated to be resulting from higher charge generation and collection efficiencies, which also leads to a small increase in Jsc by ∼1 mA/cm2. The photocurrent values calculated from the IPCE spectral integrations over 350−900 nm for the solar devices with P(DTC-TPD), P(DTS-TPD), and P(DTG-TPD) are 11.74, 12.40, and 12.74 mA/cm2, respectively, which are consistent with the short-circuit current measurements from the current density−voltage curves. The power conversion efficiency (PCE) of the device with P(DTC-TPD) is 5.7%, significantly lower than those of P(DTS-TPD) and P(DTG-TPD) at 7.0% and 7.7%, respectively. The main difference lies in the lower fill factor (FF) of the P(DTC-TPD) device at 54% vs 68% and 69% for P(DTS-TPD) and P(DTG-TPD). FF in OPVs is influenced by many factors including electrode choice,74 geminate recombination (recombination before exciton dissociation),75,76 and competition between nongeminate bimolecular recombination and charge extraction/collection.77 It is generally postulated

that nongeminate recombination can be greatly reduced with high and balanced hole and electron mobilites,78−80 because unbalanced charge carrier mobility leads to the buildup of space charge and limits photocurrent. As presented in the previous sections, P(DTC-TPD) stands out as it possesses variations in polymer conformation leading to differences in device performance. To better understand the cause of the differences in the device FF, morphological studies, and charge mobility measurement, photophysical investigations were performed to elucidate the impact of charge carrier generation, separation, and collection in these OPV devices. Similarity in Blend Morphologies and Photoluminescence Quenching Efficiencies. Active layer blend surface morphologies, shown in Figure S12, were obtained using tapping mode atomic force microscopy and the results showed that all polymer:PCBM blends exhibited high degrees of mixing. In order to quantify the exciton dissociation efficiency, steady state photoluminescence (PL) experiments were performed (Figure S12) upon blending. The samples were illuminated using 1.97 eV (630 nm) photons in order to selectively excite the polymer donor domains. Comparison of the PL between the blend and the neat films indicates strong quenching of polymer photoluminescence in all three blends upon fullerene addition. The PL quenching efficiencies were 96.7% for P(DTC-TPD):PCBM, 99.9% for P(DTSTPD):PCBM, and 98.0% for P(DTG-TPD):PCBM blended films processed with the same conditions as the OPV devices. These observations indicate that there is a good intermixing in the blend between donor (D) and acceptor (A) materials and efficient charge transfer from polymer to fullerene upon photoexcitation. Photoluminescence Lifetime: Longer CT Excitons Lifetime for P(DTC-TPD):PCBM Blend. Time correlated single photon counting (TCSPC) was performed to measure exciton dynamics (Figure S13). The characteristic PL lifetimes, extracted from fitting single exponential functions to the transients, are 170, 267, and 246 ps for the pristine P(DTCTPD), P(DTS-TPD), and P(DTG-TPD) polymers, respectively. In the polymer:fullerene blends, PL dynamics can I


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Figure 9. Transient absorption spectra for (a) P(DTC-TPD):PCBM, (c) P(DTS-TPD):PCBM, and (e) P(DTG-TPD):PCBM. EX and P refer to polymer exciton and polymer polaron, respectively. The intensity dependent spectra at 5 ns delay are shown in panels b, d, and f. P(DTCTPD):PC71BM showed a higher triplet exciton population than polaron population, indicative of a more efficient bimolecular recombination pathway from the charge separated state to the triplet state.

excitons. Figure 9a,c,e shows the transient absorption spectra at different time delays after the samples are excited using pump pulses tuned to 1.91 eV (650 nm). At this excitation energy, excitons are predominantly created in the donor polymer. The transient absorption spectrum of each blend exhibits two features at ∼0.95 and ∼1.2 eV. On the basis of previously published results on low gap polymers, we assign these two peaks to the excited-state absorption of the polymer singlet exciton and polaron, respectively.82−84 We further confirmed this assignment by measuring the transient absorption spectra of neat polymer films (in Figure S14). In the neat donor polymer, the dominant peak is at 0.95 eV, which we attribute to a signal resulting from an exciton peak. The weak signal ∼1.2 eV is due to the polaron states generated in neat films, whereas the feature at ∼1.5 eV is due to the stimulated emission. For the polymer:fullerene blends (Figure 9), the exciton peaks deplete at a faster rate due to charge

provide information on the charge generation process. All of the blend films exhibit two-component transient PL decay dynamics. The fast component (

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