Letter pubs.acs.org/macroletters
Surface Energy Modification of Semi-Random P3HTT-DPP Jenna B. Howard, Seyma Ekiz, Sangtaik Noh, and Barry C. Thompson* Department of Chemistry, Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, California 90089-1661, United States S Supporting Information *
ABSTRACT: Alkyl solubilizing side chains on conjugated polymers can serve as a handle for modifying polymer properties. Recently, oligo-ether and semifluoro alkyl side chains were utilized to tune the surface energy of random P3HT-based polymers without changing the optical and electronic properties. Here, this method is applied to semirandom poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP) and the subsequent polymer device, optical, electronic, structural, and thermal properties are characterized. P3HTMETT-DPP, bearing oligo-ether side chains, exhibited higher crystallinity, closer lamellar packing, and lower temperature thermal transitions. P3HTFHTT-DPP, featuring semifluoro alkyl side chains, presented reduced crystallinity, greater lamellar packing distances, and higher temperature thermal transitions. P3HTMETT-DPP performed similarly to P3HTT-DPP under identical processing conditions, whereas P3HTFHTT-DPP had greatly reduced JSC due to lower polymer concentration necessitated by solubility.
B
model which suggests polymer-compatibility and intimate mixing of polymer donors is essential to facilitate VOC tuning via an averaging of frontier orbitals, thereby minimizing holetrapping effects in higher lying HOMO.9,17,21,22 Surface energy, γ, of polymers has been indicated as a potential key figure of merit15,17 and easily measured physical parameter for identifying potential pairs of polymer donors for ternary blends. More recently, we demonstrated a modular method for modifying the surface energy of an existing polymer system (P3HT) by randomly incorporating monomers with the same conjugated backbone but different solubilizing side chains featuring oligo-ether or semifluoro alkyl moieties compared to alkyls.23 Modified polymers exhibited increased or decreased surface energy by incorporation of oligo-ether or semifluoro alkyl chains, respectively, while maintaining desirable optical and electronic properties; thus, providing a potential handle for controlling compatibility of known state-of-the-art polymers. P3HT is a simple conjugated polymer, which served well for close analysis of the overall effects of heteroatom alkyl chains. However, this method has not yet been studied in higher performing polymers of interest for ternary blends, nor have the solution processing methods for optimum device performance been explored. Here we present a new family of semi-random poly(3hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTTDPP) polymers with modified surface energy profiles utilizing the previously described method (Scheme 1). Semi-random
ulk heterojunction (BHJ) organic solar cells incorporating conjugated polymers are a promising platform for harvesting energy with the promise of low cost and short pay back time.1,2 Commonly, a binary system is utilized in the active layer of BHJ solar cells, comprising a polymer donor and fullerene acceptor.3 Today, efficiencies for single-layer BHJ devices have exceeded 10%, reaching near-theoretical limits.4 Higher efficiencies may be attained with multiple active layers, or tandem solar cell devices, whereby open circuit voltage (VOC) is additive across the junctions when connected in a series fashion. However, short circuit current (JSC) is limited to the lowest current of the junctions.5 Single layer devices offer more simple fabrication methods compared to their tandem cell counterparts. In the past five years, a new class of BHJ solar cells, ternary blends have gained rapid interest and widespread research effort due to unique device properties.6,7 Specifically, ternary blends incorporating two polymer donors and one fullerene acceptor have been shown to have cumulative JSC and compositiondependent VOC, reflective of the relative content of the polymer donors.8−13 Initial consideration of the respective frontier orbital levels would suggest VOC should be pinned to the lowest possible value between the two possible polymer/fullerene pairs. While this behavior is observed in some cases,14−16 many recent cases have demonstrated unpinned VOC and ultimately a ternary blend device with higher JSC and overall improved efficiency compared to the binary cases.12,13,15,17 Trending, or tuning, of VOC in ternary blends is intriguing and has become the center of extensive study and theory to elucidate electronic and structural evidence to expand upon working hypotheses.18−20 Our recent work has pointed to an organic-alloy © 2016 American Chemical Society
Received: June 6, 2016 Accepted: August 1, 2016 Published: August 2, 2016 977
DOI: 10.1021/acsmacrolett.6b00436 ACS Macro Lett. 2016, 5, 977−981
Letter
ACS Macro Letters Scheme 1. Stille Polycondensation of (a) P3HTFHTT-DPP, (b) P3HTMETT-DPP, and (c) P3HTT-DPP
Table 1. Molecular Weights (Đ), Electrochemical HOMO Values, Optical Band Gaps, and SCLC Mobilities of P3HTT-DPP, P3HTMETT-DPP, and P3HTFHTT-DPP Polymers polymer P3HTT-DPP P3HTMETT-DPP P3HTFHTT-DPP
Mn (Đ)a (kDa) 13.5 (5.2) 16.2 (3.1) 10.9 (3.1)
HOMOb (eV; film)
Egc (eV)
5.23 5.21 5.27
1.53 1.50 1.53
μhd (cm2 V−1 s−1) −4
1.47 × 10 1.43 × 10−4 0.64 × 10−4
Tm; Tc (°C)
γe (mN/m)
214; 210 209; 190 255; 238
20.6 25.7 15.0
a Determined by SEC with polystyrene standards and o-DCB eluent. bCyclic voltammetry (vs Fc/Fc+) in acetonitrile, 0.1 M TBAPF6. cCalculated from the absorption band edge in thin films, Eg = 1240/λedge. dMeasured for neat, as-cast polymer films. eMeasured for neat, as-cast polymer films.
without modification.23 Proton NMR spectra of polymers reflects the monomer feed ratios and composition (Figures S1− S3). Previously described P3HTT-DPP utilizes an 80% feed ratio of 3-hexylthiophene.24 Surface energies of the resulting polymers were determined with a contact angle goniometer by measuring contact angles of water on pristine as-cast polymer films. Consistent with the P3HT model, 23 P3HTT-DPP, P3HTMETT-DPP, and P3HTFHTT-DPP surface energies were determined to be 20.6, 25.7, and 15.0 mN/m, respectively (Table 1). P3HTMETT-DPP is particularly interesting due to its increased surface energy, which approaches those of fullerenes such as PCBM. While surface energy measurements are not
P3HTT-DPP comprises mostly 3HT monomer with small feed ratios of DPP acceptor, thus, maintaining several desirable P3HT properties, but demonstrates a higher photovoltaic efficiency due to improved optical absorption.24 Previous investigations of ternary blend devices have frequently incorporated P3HTT-DPP, making it an ideal target for surface energy modification and subsequent characterization.8,9,15,22 In the present case, new semi-random polymers P3HTMETTDPP and P3HTFHTT-DPP were synthesized, in which the monomer feed ratios were 40% 3-hexyl thiophene, 10% thiophene, 10% DPP, and 40% 3-methoxyethoxythiophene (MET) or 3-nonafluoroheptylthiophene (FHT), respectively, via Stille polycondensation (Scheme 1, Mn and Đ reported in Table 1). Monomers were prepared via literature procedures 978
DOI: 10.1021/acsmacrolett.6b00436 ACS Macro Lett. 2016, 5, 977−981
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ACS Macro Letters reported for many conjugated polymers, most of the known values generally range from 20.0 to 30.0 mN/m.25−30 Electronic and optical properties of the modified polymers were found to be virtually identical to P3HTT-DPP. Film CV measurements indicated HOMO levels of P3HTMETT-DPP and P3HTFHTT-DPP to be within experimental error of P3HTT-DPP (Table 1). Hole mobility measurements of neat polymer films were consistent with previously established values for P3HTT-DPP.24 P3HTMETT-DPP hole mobility is similar to its aliphatic parent, while P3HTFHTT-DPP is slightly lower. Hole and electron mobility measurements were also performed on polymer/fullerene blends (Table S2). Holeto-electron mobility ratios are similar for P3HTT-DPP and P3HTMETT-DPP, whereas P3HTFHTT-DPP was somewhat lower due to its smaller hole mobility, as electron mobilities for all blends are similar. UV−vis spectra of the polymers exhibited band gaps ranging from 1.50 to 1.53. Notably, the absorptivity of the intramolecular charge transfer (ICT) band of P3HTMETT-DPP is more broad and intense than P3HTTDPP, while the π−π band is less intense (Figure 1).
Figure 2. GIXRD data of P3HTT-DPP (black), P3HTMETT-DPP (blue), and P3HTFHTT-DPP (red).
Figure 1. Absorption profiles of thin films of P3HTT-DPP (black), P3HTMETT-DPP (blue), and P3HTFHTT-DPP (red).
Figure 3. Photoluminescence spectra of thin films of P3HTT-DPP (black), P3HTMETT-DPP (blue), and P3HTFHTT-DPP (red).
Additionally, the peak positions of the ICT bands and absorption onset of P3HTMETT-DPP have a minor red-shift (∼6 nm) compared to both P3HTT-DPP and P3HTFHTTDPP. This suggests greater donor−acceptor electronic interactions in P3HTMETT-DPP and may also indicate a higher degree of order or closer chain packing compared to the alkyl and semifluoro alkyl polymers. Analysis of GIXRD data was consistent with UV−vis findings and our previous model study of the structural effects of incorporating heteroatom modified alkyl chains (Figure 2). Simply, the P3HT and P3HTT-DPP polymers with semifluoro alkyl chains are less crystalline compared to regular alkyl chain polymers and have greater lamellar packing distances. Conversely, oligo-ether chain incorporation yields more crystalline polymers with tighter packing. Consistent with previous photoluminescence studies of modified P3HT polymers and the described packing, P3HTFHTT-DPP is more emissive than P3HTT-DPP, whereas P3HTMETT-DPP is less emissive (Figure 3). This is attributed to either a decrease or increase, respectively, of nonradiative quenching pathways. Moreover, P3HTMETT-
DPP emission is slightly red-shifted, consistent with the UV− vis absorption patterns. Photoluminescence measurements of polymer:fullerene blends reveal a higher degree of quenching in semifluoro and oligo-ether polymers, suggesting there may be a higher degree of mixing between polymer and fullerene in these cases. However, TEM analysis of blend films exhibited little discernible difference between the blends of P3HTT-DPP and the other polymers (Figure S10). Analysis of thermal transitions of the polymers was also consistent with our previous study and general findings for semifluoro alkyl and oligo-ether side-chain polymers. P3HTFHTT-DPP exhibited and increased Tm and Tc, while P3HTMETT-DPP transitions were decreased compared to P3HTT-DPP (Table 1). Many previously studied conjugated polymers bearing oligo-ether groups have unsuitably low thermal transitions.31 Here, P3HTMETT-DPP has only marginally lower thermal transitions compared to it is aliphatic parent analog, appropriate for photovoltaic devices. Overall, this new family of semi-random P3HTT-DPP based polymers bear ideal optical, electronic, and physical properties desired for polymer solar cells. Binary BHJ solar cells (ITO/ 979
DOI: 10.1021/acsmacrolett.6b00436 ACS Macro Lett. 2016, 5, 977−981
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ACS Macro Letters
conjugated polymers will reveal new processing methods ideal for backbones, predominantly comprising hydrocarbons, featuring hydrophilic side chain moieties. These polymers are unique from nonpolar systems that are nearly ubiquitous in conjugated polymer research,31,36 and lack a more comprehensive understanding of processing methods. In summary, we have applied our previously described modular method for tuning surface energy of P3HT-based random copolymers to semi-random P3HTT-DPP. In this work, we have demonstrated that modified polymers retain desirable characteristics for photovoltaic application. P3HTFHTT-DPP suffers from lower solubility in solvents commonly used for conjugated polymers, and thus requires lower polymer concentrations in thin film processing. P3HTMETT-DPP maintains similar photovoltaic performance under identical processing conditions. Ongoing work includes surface energy modification of alternating copolymers and incorporation of the presented polymers in ternary blends to explore polymer−polymer compatibility.
PEDOT:PSS/polymer:PC61BM/Al) were prepared using the new polymers and individually optimized (Table 2). Thin film processing conditions were carried out by simple solvent annealing of the polymer/PCBM blend after spin coating. Table 2. Photovoltaic Properties of P3HTT-DPP, P3HTMETT-DPP, and P3HTFHT-DPP polymer (polymer/PCBM ratio)
JSCc (mA/cm2)
VOC
FF
PCE
a
11.69 10.17 5.36
0.61 0.59 0.66
0.59 0.52 0.54
4.14 3.08 1.91
P3HTT-DPP (1:1.3) P3HTMETT-DPPa (1:1.3) P3HTFHTT-DPPb (1:2)
a Spin-coated from o-dichlorobenzene, dried under N2 before aluminum deposition. bSpin-coated from chloroform, dried under N2 before aluminum deposition. cMismatch corrected.
The semifluoro alkyl analog has poor solubility in o-DCB, but solubility improves with chlorobenzene, chloroform, and tetrahydrofuran. Optimal film processing conditions and device performance was observed when P3HTFHTT-DPP was solubilized in chloroform at low polymer concentrations (4 mg/mL). Understandably, the JSC for this polymer is lower than P3HTMETT-DPP due to the lower concentration of light absorbing material and relatively thinner film. Higher ratios of fullerene to polymer (2:1) were needed to optimize P3HTFHTT-DPP devices. It should be noted that the surface energy of PCBM is higher (27.6 mN/m).15 Kim and coworkers previously studied hydrophobicity tuning of o-xylene C60 bis-adduct (OXCBA) derivatives and their photovoltaic performance with P3HT.32 While the polymer/fullerene ratios were held constant for all blends, it is clear that materials with poorly matched surface energies contributed to significant differences in blend morphology and photovoltaic device performance. Here, it is likely that more fullerene was needed to promote donor−acceptor interaction and overcome selfaggregation of PCBM. Historically, fluoro-alkyl polymers are inherently interesting due to their high thermal stabilities.33,34 However, these polymers suffer from limited compatibility with higher boiling point solvents. Here, we demonstrate that random incorporation of fluoro-alkyl monomers with an overall monomer content of 40% or below is an effective method for preparing polymers with higher melting points and lower surface energy, while still achieving device performance with FF greater than 0.50. We predict that feed ratios of 50% or greater of fluoro-alkyl monomers is not advantageous, as many previously reported polymers were not sufficiently soluble for complete characterization. The oligo-ether analog is easily processed with the same conditions as P3HTT-DPP. The JSC and VOC of P3HTMETTDPP is similar to P3HTT-DPP, suggesting that incorporation of oligo-ether solubilizing groups is a viable method for modification of conjugated polymers without detriment to current or voltage of the working device. Previous work from Wang and co-workers have also utilized oligo-ether side chains on alternating polymers to achieve closer π−π packing and demonstrated remarkable improvement of device parameters via processing with a nonhalogenated solvent: methoxybenzene (MOB).35 In the present study, methoxybenzene did not sufficiently solubilize P3HTMETT-DPP for thin film processing. The fill factor suggests there is more room for improvement in processing conditions, or perhaps for a more precise tuning of the oligo-ether monomer feed ratio. We predict that continued study of oligo-ether side chains in
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00436. Syntheses of polymers, 1H NMR spectra, surface energy data, DSC, CV traces, charge carrier mobilites, device thicknesses, and TEM images (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Science Foundation (CBET Energy for Sustainability) CBET-1436875. We would like to thank Drs. Malancha Gupta and Scott Siedel for generously providing use of their Rame-Hart Goniometer.
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