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Tuning the Organic Solar Cell Performance of Acceptor 2,6Dialkylaminonaphthalene Diimides by Varying a Linker between the Imide Nitrogen and a Thiophene Group Roshan Fernando, Zhenghao Mao, Evan Muller, Fei Ruan, and Geneviève Sauvé* Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, United States S Supporting Information *

ABSTRACT: Core-substituted naphthalene diimides (NDI) are promising candidates as acceptors for organic solar cells. To study their structure−property relationships, a series of 2,6-dialkylamino-NDI compounds with various substituents were synthesized, characterized, and tested in bulk heterojunction solar cells by blending with regioregular poly(3hexylthiophene) (P3HT). The imide substituents consisted of a linker connected to a thiophene group, where the linker was phenyl, methyl, or ethyl. The core substituents were cyclohexylamino or 2-ethylhexylamino. While the various substituents had little effect on the optoelectronic properties in solution, they strongly affected device performance and blend morphology. Under the conditions studied, the best performance was obtained with the methyl linker combined with the cyclohexylamino core substituent, with a power conversion efficiency of 0.48% and a high open circuit voltage of 0.97 V. For blends of P3HT with modified NDI non-fullerene acceptors, the methyl linker promoted larger phase-separated domains than the ethyl or phenyl linkers. DFT calculations showed that the linker determines the orientation of the thiophene conjugated plane with respect to the NDI conjugated plane. That angle was 114°, 45°−61°, and 8° for the methyl, phenyl, and ethyl linkers, respectively. Using thiophene at the end of the imide substituent adds a unique dimension to tune morphology and influence the molecular heterojunction between donor and acceptor.

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INTRODUCTION Plastic solar cells have received a lot of attention due to their potential for providing inexpensive solar energy on a large scale.1−4 This field has seen tremendous growth since the discovery of bulk heterojunction solar cells, with power conversion efficiencies (PCE) quickly climbing to nearly 10% just in the past decade.5 Most of the growth results from improvements in device engineering and in synthesis and optimization of the conjugated polymer (or molecule) electron donor when paired with a fullerene derivative as the acceptor. Although fullerene derivatives have been highly successful, they have some disadvantages, including poor absorption of visible light and restricted tuning of energy levels due to poor electronic communication between the fullerene and its substituents.6−10 As we reach the predicted limit for PCE with single BHJs based on fullerene acceptors,11 interest in developing alternative acceptors has resurged, and reported PCEs are now reaching ∼5%, illustrating the potential of nonfullerene acceptors in OPV.12 Promising candidates include perylene diimide derivatives,13−17 fluoranthene-fused imides,18 conjugated polymers,12,19−21 block copolymers of conjugated polymers,22,23 and others.24−30 However, very little is known about their structure-property relationships. Core-substituted naphthalene diimides (NDI) are good candidates as non-fullerene acceptors for organic photovoltaic (OPV) because they have high electron affinity due to the imide groups, are planar conjugated molecules that can π-stack © XXXX American Chemical Society

with each other to give excellent electron transport properties,21 and their optical and electrochemical properties are easily tunable through substitution at the core positions.31 Colorful NDI-based materials can be obtained either by (1) extending the conjugation system through the core positions or (2) using electron-donating substituents groups to create a push−pull system with the electron-poor NDI core. While efforts in using the first strategy have shown great promise for the design of non-fullerene acceptors for organic photovoltaic (OPV),12,19,32,33 efforts in exploring the second strategy have been limited, and we are only aware of our report, where we demonstrated the potential of 2,6-dialkylaminonapthalene diimides as electron acceptors for OPV.34 We became interested in 2,6-dialkylamino-NDI because they exhibit a strong absorption band in the visible region, intense red emission, and well-behaved electrochemistry with two reversible reductions and two reversible oxidations in solution. While core substituents are well-known to tune energy levels of NDI, imide substituents have negligible effects on the energy levels of NDI. Instead, imide substituents are typically used to tune solubility and molecular packing in films35−37 and must therefore be considered when evaluating the potential of 2,6dialkylamino-NDI for device applications. In our previous Received: November 20, 2013 Revised: January 22, 2014

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dx.doi.org/10.1021/jp411432a | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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cyclohexylamino (RF1, RF3, RF5) or 2-ethylhexylamino (RF2, RF4, RF6). All these substitutions were expected to affect blend morphology and device performance while keeping the optical and electrochemical properties of the acceptor constant. All compounds were synthesized, characterized, and tested as electron acceptors in organic solar cells. Performance parameters varied depending on the combination of substituents used, and the methyl linker combined with the cyclohexylamino core substituent gave the best performance under the conditions studied. DFT calculations indicate that the linker determines the orientation of the thiophene conjugated plane with respect to the NDI conjugated plane while still allowing the NDI cores to π-stack with each other.

report, we used either a phenyl or 4-(thiophene-2-yl)phenyl as the imide substituent, shown in Figure 1. We also used two

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EXPERIMENTAL SECTION Materials. 1,4,5,8-Naphthalenetetracarboxylic dianhydride (Aldrich), 2-ethylhexylamine (Aldrich), 4-(thiophen-2-yl)aniline (Aldrich), 2-thiophenemethylamine (Alfa Aesar), and 2-(2-thienyl)ethylamine (TCI America) were used as received. All other reagents and solvents were used as received unless otherwise specified. 2,6-Dibromo-1,4,5,8-naphthalenetetracarboxylic dianhydride (26BrNDA) was synthesized from 1,4,5,8naphthalenetetracarboxylic dianhydride (NDA) following a literature procedure and used without any purification.38 General Procedure for the Synthesis of cNDI Molecules. To a glacial acetic acid (40 mL) suspension of crude 26BrNDA (1.1 g/2.6 mmol, quantitatively) was added 6 mol equiv of the respective amines for the imidization. The reaction mixture was heated to reflux under nitrogen (refluxing time: 10 min for 4-(thiophen-2-yl)aniline and 1 h for both 2thiophenemethylamine and 2-(2-thienyl)ethylamine). The mixture was then cooled and poured into methanol (250 mL) to precipitate the product. The precipitate was collected by vacuum filtration and washed thoroughly with acetone until a colorless acetone filtrate was obtained (the color of the precipitate was reddish-brown with 4-(thiophen-2-yl)aniline and yellowish-orange with both 2-thiophenemethylamine and 2-(2-thienyl)ethylamine). After drying under vacuum for 24 h, this crude precipitate was refluxed either with cyclohexylamine (20 mL) or with 2-ethylhexylamine (20 mL) for 1 h under nitrogen to get the desired products. Products were collected by precipitation in methanol (200 mL) and vacuum filtration. The product was washed with methanol and acetone until the red filtrate became colorless. The crude deep blue products were dried under vacuum for 24 h prior to purification by column chromatography on silica gel using chloroform as the eluent. N,N′-Di((thiophene-2-yl)methyl)-2,6-bis(N-cyclohexyl)amino-1,4,5,8-naphthalenetetracarboxydiimide (RF1). A violet-blue solid. Overall 25% yield from NDA. 1H NMR (600 MHz, CD2Cl2, δ, ppm): 9.43 (d, 2H), 8.17 (s, 2H), 7.23− 7.21 (m, 4H), 6.94 (t, 2H), 5.48 (s, 4H), 3.81 (m, 2H), 2.14 (m, 4H), 1.85 (m, 4H), 1.75−1.67 (m, 2H), 1.56−1.45 (m, 8H), 1.39−1.32 (m, 2H). 13C NMR (600 MHz, CD2Cl2, δ, ppm): 25.23, 26.20, 33.79, 38.41, 51.70, 102.17, 119.25, 121.81, 126.09, 126.45, 126.93, 128.56, 139.64, 148.97, 163.19, 166.25. MALDI-TOF-MS: m/z calcd for C36H36N4O4S2 652.22, found 651.54. Mp = 316−318 °C. TGA: Td = 379 °C at 5% weight loss. Elemental analysis: Anal. Calcd for C36H36N4O4S2: C, 66.23; H, 5.56; N, 8.58. Found: C, 65.45; H, 5.38; N, 8.48. N,N′-Di((thiophene-2-yl)methyl)-2,6-bis(N-2-ethylhexyl)amino-1,4,5,8-naphthalenetetracarboxydiimide (RF2). A violet-blue solid. Overall 25% yield from NDA. 1H NMR

Figure 1. 2,6-Dialkylamino-NDI molecules, illustrated in blue, from our previous publication.34

alkylamino core substituents, cyclohexylamino and 2-ethylhexylamino, since different solubilizing groups also affect solubility and film properties. As expected, the imide substituent and alkylamino core substituents used did not affect the optoelectronic properties of the core-substituted NDI in solution. On the other hand, these substitutions greatly affected device performance when tested in organic solar cells using regioregular poly(3-hexylthiophene) (P3HT) as the electron donor. The core-substituted NDI with 4-(thiophen2-yl)phenyl imide substituents (5a) showed the best PCE with a high open circuit voltage (Voc) while the same molecule without the thiophene groups (4a) resulted in very poor device performance. We hypothesized that the thiophene groups in 5a were important to obtain a better blend morphology in films, perhaps by improving miscibility with P3HT. To better understand the role of the linker between the imide nitrogen and thiophene group, we explored a series of core-substituted NDI where the linker is phenyl, methyl, or ethyl, shown in Figure 2. The core substituent was also varied, either

Figure 2. Series of core-substituted NDI explored in this contribution. B



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(600 MHz, CD2Cl2, δ, ppm): 9.37 (t, 2H), 8.08 (s, 2H), 7.22 (d, 4H), 6.93 (t, 2H), 5.48 (s, 4H), 3.42 (t, 4H), 1.78 (m, 2H), 1.59−1.53 (m, 4H), 1.51−1.48 (m, 4H), 1.41−1.34 (m, 8H), 1.01 (t, 6H), 0.94 (t, 6H). 13C NMR (600 MHz, CD2Cl2, δ, ppm): 11.36, 14.42, 23.62, 25.18, 29.46, 31.83, 38.29, 39.83, 46.83, 102.14, 118.84, 121.55, 126.11, 126.25, 126.90, 128.64, 139.58, 149.98, 163.00, 166.17. MALDI-TOF-MS: m/z calcd for C40H48N4O4S2 712.31, found 711.83. Mp = 220−222 °C. TGA: Td = 382 °C at 5% weight loss. Elemental analysis: Anal. Calcd for C40H48N4O4S2: C, 67.38; H, 6.79; N, 7.86. Found: C, 67.65; H, 6.95; N, 7.82. N,N′-Di(2-(thiophene-2-yl)ethyl)-2,6-bis(N-cyclohexyl)amino-1,4,5,8-naphthalenetetracarboxydiimide (RF3). A dark blue-gray solid. Overall 21% yield from NDA. 1H NMR (600 MHz, CD2Cl2, δ, ppm): 9.45 (d, 2H), 8.17 (s, 2H), 7.19 (d, 2H), 6.97−6.94 (m, 4H), 4.42 (t, 4H), 3.84−3.77 (m, 2H), 3.23 (t, 4H), 2.13 (m, 4H), 1.85 (m, 4H), 1.72−1.66 (m, 2H), 1.56−1.44 (m, 8H), 1.35−1.31 (m, 2H). 13C NMR (600 MHz, CD2Cl2, δ, ppm): could not record due to solubility issues. MALDI-TOF-MS: m/z calcd for C38H40N4O4S2 680.25, found 679.53. Mp = 304−305 °C. TGA: Td = 379 °C at 5% weight loss. Elemental analysis: Anal. Calcd for C38H40N4O4S2: C, 67.03; H, 5.92; N, 8.23. Found: C, 66.78; H, 5.81; N, 8.15. N,N′-Di(2-(thiophene-2-yl)ethyl)-2,6-bis(N-2-ethylhexyl)amino-1,4,5,8-naphthalenetetracarboxydiimide (RF4). A violet-blue solid. Overall 30% yield from NDA. 1H NMR (600 MHz, CD2Cl2, δ, ppm): 9.37 (t, 2H), 8.07 (s, 2H), 7.18 (d, 2H), 6.96−6.93 (m, 4H), 4.40 (t, 4H), 3.42 (t, 4H), 3.23 (t, 4H), 1.77 (h, 2H), 1.57−1.51 (m, 4H), 1.50−1.47 (m, 4H), 1.42−1.34 (m, 8H), 1.00 (t, 6H), 0.93 (t, 6H). 13C NMR (600 MHz, CD2Cl2, δ, ppm): 11.34, 14.42, 23.61, 25.12, 28.64, 29.48, 31.81, 39.82, 41.96, 46.76, 102.14, 118.70, 121.55, 124.32, 125.91, 126.20, 127.44, 141.65, 149.93, 163.23, 166.50. MALDI-TOF-MS: m/z calcd for C42H52N4O4S2 740.34, found 739.50. Mp = 243−245 °C. TGA: Td = 384 °C at 5% weight loss. Elemental analysis: Anal. Calcd for C42H52N4O4S2: C, 68.08; H, 7.07; N, 7.56. Found: C, 68.36; H, 6.98; N, 7.58. N,N′-Di(4-(thiophene-2-yl)phenyl)-2,6-bis(N-cyclohexyl)amino-1,4,5,8-naphthalenetetracarboxydiimide (RF5). A dark blue solid. Overall 15% yield from NDA. 1H NMR (600 MHz, CD2Cl2, δ, ppm): 9.39 (d, 2H), 8.19 (s, 2H), 7.81 (d, 4H), 7.43 (d, 2H), 7.39 (d, 2H), 7.35 (d, 4H), 7.15 (dd, 2H), 3.81−3.76 (m, 2H), 2.08 (m, 4H), 1.79 (m, 4H), 1.66−1.60 (m, 2H), 1.51−1.39 (m, 8H), 1.34−1.27 (m, 2H). 13C NMR (600 MHz, CD2Cl2, δ, ppm): 25.00, 26.12, 33.60, 51.57, 102.32, 119.46, 122.26, 124.56, 126.12, 126.95, 127.24, 128.77, 129.92, 135.34, 135.53, 143.87, 149.09, 163.68, 166.94. MALDI-TOF-MS: m/z calcd for C46H40N4O4S2 776.25, found 775.54. Mp was not detected up to 360 °C. TGA: Td = 436 °C at 5% weight loss. Elemental analysis: Anal. Calcd for C46H40N4O4S2: C, 71.11; H, 5.19; N, 7.21. Found: C, 70.83; H, 5.16; N, 7.06. N,N′-Di(4-(thiophene-2-yl)phenyl)-2,6-bis(N-2-ethylhexyl)amino-1,4,5,8-naphthalenetetracarboxydiimide (RF6). A dark blue solid. Overall 12% yield compared to NDA. 1H NMR (600 MHz, CD2Cl2, δ, ppm): 9.32 (t, 2H), 8.21 (s, 2H), 7.82 (d, 4H), 7.44 (d, 2H), 7.39 (d, 2H), 7.36 (d, 4H), 7.15 (dd, 2H), 3.42 (m, 4H), 1.71 (h, 2H), 1.51−1.45 (m, 4H), 1.44−1.40 (m, 4H), 1.35−1.28 (m, 8H), 0.93 (t, 6H), 0.87 (t, 6H). 13C NMR (600 MHz, CD2Cl2, δ, ppm): 11.28, 14.46, 23.54, 25.03, 29.49, 31.78, 39.77, 46.71, 102.50, 119.22, 122.20, 124.57, 126.13, 126.93, 127.27, 128.79, 129.96, 135.37, 135.52, 143.86, 150.25, 163.71, 167.00. MALDI-TOF-MS: m/z calcd

for C50H52N4O4S2 836.34, found 835.48. Mp was not detected up to 360 °C. TGA: Td = 432 °C at 5% weight loss. Elemental analysis: Anal. Calcd for C50H52N4O4S2: C, 71.74; H, 6.26; N, 6.69. Found: C, 71.86; H, 6.12; N, 6.71. Methods. 1H NMR spectra were recorded using a 600 MHz spectrometer. Chemical shifts are reported in parts per million relative to CD2Cl2 (5.32 ppm). 13C NMR data were collected on a 600 MHz spectrometer equipped with a broad band probe and regulated at 25 °C using a 45° pulse width, no recycle delay, and an acquisition of 1.3 s with WALTZ proton decoupling. Processing was accomplished with a line broadening of 1 Hz and referenced to CD2Cl2 at 54.0 ppm. MALDITOF MS spectra were acquired in reflective negative ion mode; samples were prepared from chloroform solutions using 2,2′:5′,2′′-terthiophene as the matrix. Thermal gravimetric analysis (TGA) was performed on a TA Q500 thermogravimetric analyzer. Differential scanning calorimetric (DSC) analysis was performed on a TA Q2000 calorimeter. Prior to this measurement, the cNDI molecules were dried overnight under vacuum at 80 °C to remove traces of solvent impurities. The samples were scanned from −50 to 350 °C (Figures S23− S28) at a rate of 10 °C/min. Melting points were measured with a MEL-TEMP apparatus from Laboratory Devices. UV− vis spectra were collected in chloroform (HPLC grade) or on glass substrates with spin-coated films using chloroform solutions. Emission spectra were collected in chloroform solutions. Cyclic voltammetry measurements were performed at room temperature using 0.1 M Bu4NPF6 in dry dichloromethane (distilled over CaH2) as electrolyte and ferrocene as internal standard. The solution was purged with dry nitrogen for 10 min prior to the measurement. Glassy carbon (GC) working electrodes were polished with 0.05 μm alumina, thoroughly cleaned, and dried. Platinum wires were used as the counter and reference electrodes. All scans were performed at a scan rate of 0.1 V/s. The E1/2 values vs ferrocene/ferrocenium (Fc/ Fc+) were calculated by setting the E1/2 of Fc/Fc+ to 0.0 V. For film measurements, two drops of a compound (RF1−RF6) in chloroform (HPLC grade) solution (filtered using 0.45 μm PTFE filters) were placed on a polished GC electrode and allowed to dry in air. Films were washed with dry acetonitrile prior to measurements. The measurement was done in an electrolyte solution of 0.1 M Bu4NPF6 in acetonitrile purged with dry nitrogen for 10 min prior to measurements. A Ag/ AgNO3 electrode was used as the reference electrode (E1/2 = 0.091 V vs Fc/Fc+), and a Pt wire was used as the counter electrode. All scans were maintained at a rate of 0.1 V/s. HOMO and LUMO energy levels were estimated from E1/2 values (for solution) and the onset (for films) of the oxidation and reduction waves, respectively, and the value of 5.1 eV vs vacuum was used for Fc/Fc+;39 i.e., HOMO = −[E1/2,ons,ox + 5.1] eV and LUMO = −[E1/2,ons,red + 5.1] eV. Density functional theory calculations were done in gas phase using Gaussian 09 software package.40 The B3LYP hybrid DFT level was used together with 6-31G(d,p) basis set for all atoms.41,42After a conformational analysis on orientations of imide substitution for each molecule, local minima were identified with energy and frequency calculations. Kohn− Sham molecular orbitals were then calculated for the local minimum of each molecule using the same level of theory and basis set. The M062X hybrid DFT level was used with 631G(d,p) basis set for optimization of dimers. The input C



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products were dried and reacted with cyclohexylamine or 2ethylhexylamine to substitute the core bromines. The final products were purified by column chromatography and isolated as bright violet-blue powders, except for RF3, which was dark blue-gray. The identity and purity of the products were confirmed by NMR, MALDI-TOF-mass spectroscopy, and elemental analysis. The overall yields for the conversion of 1,4,5,8-naphthalenetetracarboxylic dianhydride to the desired core-substituted NDIs varied between 12% and 30%. The overall yields observed for RF1−RF4 were between 21% and 30%, close to the purity of 26BrNDA, implying that the overall yield for RF1−RF4 was limited by the yield for 26BrNDA and that both the imidization and nucleophilic substitution reactions proceeded in relatively high yields. On the other hand, overall yields for RF5 and RF6 were significantly lower, 12% and 15%. These lower yields resulted from selectivity issues during the imidization step. Although the imidization step was performed in a protic solvent to prevent nucleophilic substitution of core bromines,43,44 some nucleophilic substitution side reactions were observed for imidization reactions with the aromatic amine, 4-(thiophene-2-yl)aniline. In an attempt to minimize these side reactions, the imidization was terminated earlier, thus reducing the overall yield for RF5 and RF6. Note that the synthesis method described here is different from how RF5 (called 5a) was synthesized in our previous report;34 Previously, we first installed 4-bromophenyl groups at the imide positions followed by Suzuki coupling with thiophene-2boronic acid pinacol ester, whereas here we directly react 4(thiophen-2-yl)aniline with the imide. The new method used in this study thus eliminates the Suzuki coupling step and its associated use of water during the reaction but resulted in a lower yield (12%) than the previous method (29%). The coresubstituted NDI molecules were soluble in a variety of organic solvents, including dichloromethane, THF, toluene, chlorobenzene, and chloroform. RF3 was found to have the lowest solubility, suggesting a strong tendency to aggregate. Thermal Analysis. To evaluate the thermal stability of our molecules, we performed TGA analysis, shown in the Supporting Information. All core-substituted NDI molecules showed excellent thermal stability, with decomposition temperatures (Td) at 5% weight loss above 379 °C, summarized in Table 1. The core-substituted NDI molecules with an alkyl linker (RF1−RF4) had a similar Td around 380 °C, whereas the molecules with a phenyl linker had a higher Td of 436 and 432 °C for RF5 and RF6, respectively. This is in contrast to our previous report, where RF5 (5a) had a Td of only 215 °C.34 We surmise that the higher Td using this new synthesis approach presumably results from avoiding the use of water. Melting point temperatures (Table 1) were obtained by differential scanning calorimetric (DSC) analysis and confirmed by traditional melting point measurement. The molecules with a phenyl linker, RF5 and RF6, did not show any phase transition over the studied temperature range. These results suggest strong intermolecular forces in the solid state for RF5 and RF6. The core-substituted NDI with alkyl linkers had measurable melting points that appear to depend on the alkylamino core substituent: the core-substituted NDI with cyclohexylamino core substituents had a melting point of 316 and 305 °C for RF1 and RF3, respectively, whereas the core-substituted NDI with 2-ethylhexylamino core substituents had lower melting points of 222 and 244 °C for RF2 and RF4, respectively. These results suggest that core-substituted NDI with cyclohexylamino

geometries of dimers were constructed using optimized monomer geometries. Photovoltaic Devices. A blend solution P3HT:RFx (x = 1−6) (1:2.5 ratio by weight, with a total concentration of 20 mg/mL) in o-dichlorobenzene was prepared. ITO-coated glass (R = 15 Ω/sq) substrates were cleaned stepwise in detergent, water, acetone, and isopropanol under ultrasonication for 15 min each and then treated by UV-ozone for 15 min. A thin PEDOT:PSS layer was spin-coated onto the ITO glass, followed by drying at 150 °C for 15 min in air. Photoactive layers with a thickness of ∼70 nm of P3HT:RFx were spincoated from the blend solution. The optimal spin-coating condition we found was 500 rpm for 1 min followed by another step of 1000 rpm for 3 s. Ca (25 nm) and Al (100 nm) were deposited in sequence under vacuum pressure of 2 × 10−6 Torr using a Angstrom Engineering Evovac Deposition System. Solar cells were measured inside the glovebox (Pure LabHE) under 1.5 AM solar illumination at intensity of 100 mW/cm2 using Oriel Sol2A solar simulator and Keithley 2400 IV station. The total effective area of the cell was 0.17 cm2. A total of 8−10 devices were analyzed. The hole-only devices were fabricated following the same procedure presented above except the top electrode was replaced with molybdenum trioxide (MoO3; 10 nm) and silver (Ag; 80 nm). For the electron-only devices, an aluminum (Al; 60 nm) layer was deposited on ITO glass by thermal evaporation followed by spin-coating the blend solution. An aluminum (Al; 100 nm) top contact was deposited to make an ITO/Al/P3HT:RF1/Al structure. The IPCE spectra were measured with a QEX10 Quantum Efficiency Measurement System. AFM measurement was performed on an Agilent Technologies 5500 instrument in tapping mode. Samples for AFM were the surface of actual photovoltaic devices.

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RESULTS AND DISCUSSION Synthesis. The six core-substituted NDI molecules RF1− RF6 were synthesized as described in Scheme 1. First, 2,6-

Scheme 1. Synthesis of Core-Substituted Naphthalene Diimides

dibromo-1,4,5,8-naphthalenetetracarboxylic dianhydride (26BrNDA) was synthesized according to the literature.38 The purity of 26BrNDA was estimated by proton NMR spectroscopy in DMSO and was about 35%. Crude 26BrNDA was then refluxed in glacial acetic acid with the respective amines to install substituents at the imide positions. The D



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Table 1. Thermal and Optical Properties film

solution RF1 RF2 RF3 RF4 RF5 RF6

Td (°C)

Tm (°C)

λems max (nm)

379 382 379 384 436 432

316 222 305 244 >360 >360

657 656 656 652 656 653

−1 λabs cm−1) max, ε (nm, kM

630, 628, 629, 626, 628, 627,

22.6 22.4 22.5 22.9 24.0 24.9

λabs onset (nm)

Eg,sol (eV)

λabs max (nm)

λabs onset (nm)

Eg,film (eV)

664 663 663 660 664 659

1.87 1.87 1.87 1.88 1.87 1.88

625 614 630 621 626 611

675 685 685 694 680 677

1.84 1.81 1.81 1.79 1.82 1.83

maximum around 655 nm in chloroform. In thin film (Figure 3b), the visible absorption band was slightly broader than in solution for all molecules studied, resulting in a red-shift of the absorption onset and a decrease in the energy gap from 1.9 to 1.8 eV. The broadening was greater for the compounds with 2ethylhexylamino core substituents (RF2, RF4, RF6) than for the molecules with the cyclohexylamino core substituents, consistent with the alkylamino core substituents impacting solid-state molecular arrangements. Electrochemical Properties. In solution, these molecules show reversible to quasi-reversible electrochemical behavior (Figure 4a and Table 2), consistent with the literature for 2,6dialkylamino-NDI.34,45 They all had two reversible reduction peaks with E1/2 at −1.4 V and −1.8 V vs Fc/Fc+, showing their potential as n-type material. Additionally, RF1−RF4 showed two reversible oxidation peaks at 0.6 and 1.0−1.1 V vs Fc/Fc+, whereas RF5 and RF6 showed one reversible oxidation peak at 0.6 V vs Fc/Fc+ and one quasi-reversible peak at higher voltages. The latter shows a much higher current upon oxidation, consistent with oxidative polymerization via the terminal thiophene group, as observed and described in our previous publication.34 This electropolymerization is more prominent in RF5 and RF6 because the phenyl linker can stabilize radicals on the thiophene by resonance. The HOMO and LUMO energy levels in solution were estimated from the E1/2 values, assuming Fc/Fc+ at 5.1 eV below vacuum,39 and were −5.7 and −3.7 eV for the HOMO and LUMO energy levels, respectively. In thin film, the electrochemical behavior of these molecules was not as well-behaved (Figure 4b). The HOMO and LUMO energy levels were nevertheless estimated from the onset of oxidation and reduction, respectively. The HOMO energy levels were estimated between −5.5 and −5.7 eV, all below the −5.2 eV threshold for electrochemical stability in air,46 whereas the LUMO energy levels were estimated between −3.7 and −3.9 eV. When compared with the prototypical conjugated polymer donor P3HT, LUMO = −3.2 eV and HOMO = −5.2 eV (estimated by CV in our laboratory for P3HT film), we find that the RF1−RF6 molecules have the appropriate energy levels to act as electron acceptor for excitons created in P3HT and as hole donor for excitons created in the RF1−RF6 molecules. Photovoltaic Properties. To compare the photovoltaic properties of the six molecules, we first performed some basic optimization of solvent, spin-coating conditions, total concentration, annealing conditions, and materials ratio using RF1 as electron acceptor blended with P3HT as electron donor. All device fabrication and current−voltage curve measurements were done under inert atmosphere. The best photocurrent and overall power conversion efficiency were obtained for a 1:2.5 ratio of P3HT:RF1, annealed at 100 °C for 1 h. The best conditions for RF1 were then used to compare the photovoltaic properties of the RF1−RF6 acceptors. The film thicknesses

core substituents are more crystalline than with the branched 2ethylhexylamino core substituents. Optical Properties. The absorption spectra of the molecules in chloroform solution and in thin films are shown in Figure 3, and the results are summarized in Table 1. All six

Figure 3. Absorption spectra in chloroform solution and thin films (spin-coated): (a) RF1, RF3, and RF5 (molecules with cyclohexylamino core substituents); (b) RF2, RF4, and RF6 (molecules with 2-ethylhexylamino core substituents). Spectra were normalized to 1 at the λmax of the lowest energy absorption band in order to better compare the low-energy absorption bands. Solution concentrations were 9.1 × 10−6−1.1 × 10−5 M.

molecules had an absorption spectra in solution similar to previously reported 2,6-dialkylamino-NDI molecules,34,45 as expected since the various substituents studied here should not affect the electronic structure of the core-substituted NDI. The main absorption features include a strong band (500−660 nm) with a maximum near 630 nm (ε = 22−25 kM−1 cm−1) due to the push−pull system between the donating alkylamino core substituents and the withdrawing imides and a band in the 350−360 nm region corresponding to absorption of the naphthalene core. This latter absorption band is stronger in intensity for RF5 and RF6 (ε = 69 kM−1 cm−1 for RF5 and RF6 versus 34−37 kM−1 cm−1 for RF1−RF4) because the absorption of the (4-thiophen-2-yl)phenyl imide substituents overlaps with that of the naphthalene core. All molecules had an absorption onset around 660 nm in solution, corresponding to an optical energy gap of 1.87 eV. Emission properties in solution were also controlled by 2,6-dialkylamino-NDI, with a E



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Figure 4. Cyclic voltammograms of RF1−RF6 in (a) dichloromethane solution and (b) drop-cast film on a glassy carbon electrode.

Table 2. Electrochemical Properties and Estimated Frontier Molecular Orbital Energy Levels film

solution Eox 1/2 RF1 RF2 RF3 RF4 RF5 RF6

+

(V vs Fc/Fc )

0.59, 0.61, 0.57, 0.59, 0.59, 0.62,

1.07 1.11 1.03 1.06 irreversible irreversible

Ered 1/2

+

(V vs Fc/Fc )

−1.39, −1.39, −1.43, −1.43, −1.38, −1.36,

−1.76 −1.76 −1.80 −1.79 −1.78 −1.75

HOMO (eV)

LUMO (eV)

Eg (eV)

HOMO (eV)

LUMO (eV)

Eg (eV)

−5.69 −5.71 −5.67 −5.69 −5.69 −5.72

−3.71 −3.71 −3.67 −3.67 −3.72 −3.74

1.98 2.00 2.00 2.02 1.97 1.98

−5.55 −5.59 −5.48 −5.71 −5.49 −5.65

−3.78 −3.79 −3.69 −3.68 −3.89 −3.72

1.77 1.80 1.79 2.03 1.60 1.93

were about 70 nm. The best device performance parameters for as-cast and annealed films are given in Table 3, along with the average power conversion efficiency of 8−10 devices. The IV characteristics for representative annealed films are shown in Figure 5. Except for RF3, all devices improved with thermal annealing. The highest power conversion efficiency was obtained for RF1 at 0.48%, followed by RF5 at 0.43%, RF4 at 0.24%, RF6 at 0.17%, RF2 at 0.10%, and RF3 at 0.010%. The Table 3. Best Photovoltaic Performance for Blends of 1:2.5 Ratio of P3HT:Acceptor

RF1 RF2 RF3 RF4 RF5 RF6

as-cast annealed as-cast annealed as-cast annealed as-cast annealed as-cast annealed as-cast annealed

Figure 5. IV curves comparing annealed devices of RF1−RF6 under the same conditions.

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

1.2 1.4 0.33 0.45 0.063 0.055 0.92 0.93 1.0 1.4 0.43 0.72

0.76 0.97 0.89 0.77 0.84 0.50 0.80 0.81 0.81 0.92 0.67 0.67

0.33 0.35 0.20 0.30 0.23 0.34 0.28 0.31 0.25 0.34 0.29 0.35

0.31 (0.29 ± 0.02) 0.48 (0.45 ± 0.03) 0.060 (0.056 ± 0.04) 0.10 (0.097 ± 0.008) 0.012 (0.011 ± 0.01) 0.010 (0.008 ± 0.02) 0.21 (0.19 ± 0.02) 0.24 (0.23 ± 0.01) 0.25 (0.23 ± 0.01) 0.43 (0.42 ± 0.01) 0.086 (0.081 ± 0.05) 0.17 (0.15 ± 0.02)

very low efficiency of RF3 is probably due to the low solubility of RF3 in o-dichlorobenzene. To test this hypothesis, we filtered a blend solution, took an aliquot, evaporated the solvent, and weighed the dried material. Assuming that all the P3HT dissolved, we find a solubility of 0.7−1 mg/mL for RF3, suggesting that there is hardly any RF3 in the blend films under the conditions tested. Better performance could be obtained by individually optimizing the conditions for each acceptor, but such optimization is beyond the scope of this paper. The open circuit voltage, Voc, for annealed films were generally high and also followed the same trend as PCE, with a Voc = 0.97 V for RF1, 0.92 V for RF5, 0.81 V for RF4, and 0.77 F



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Figure 6. AFM height images of blend films from the actual device, and the corresponding power conversion efficiency measured (PCE).

V for RF2. Fill factors were generally low, improved with annealing, and ranged between 0.30 and 0.35 for annealed films. This may indicate either low charge carrier mobility, poor morphology, and/or the presence of high series resistances, including at interfaces. Charge carrier mobilities in blend films of P3HT:RF1 (1:2.5 ratio) were measured using the space charge limited current (SCLC) model.47−49 The estimated electron and hole mobility for as-cast film was 5.3 × 10−5 and 6.0 × 10−5 cm2 V−1 s−1, respectively, whereas the estimated electron and hole mobility of an annealed film was 8.1 × 10−5 and 9.6 × 10−5 cm2 V−1 s−1, respectively. Both as-cast and annealed films had balanced charge transport, and the mobilities were higher for the annealed film, consistent with the PCE results. While mobilities improved by about 50% upon annealing, FF increased only by 6% from 0.33 to 0.35, suggesting that other factors limit FF. Compared to an annealed P3HT:PCBM blend, the hole mobility of P3HT was similar, but the electron mobility of RF1 was about 3 times lower than that of PCBM (∼3 × 10−4 cm2 V−1 s−1).47 To gain information about morphology, we imaged the active layer film surfaces by AFM, and the height images are shown in Figure 6. The AFM image for the RF1:P3HT blend shows a combination of large and small features. Surprisingly, that surface morphology corresponded to the best PCE, suggesting that further optimization may yield better device performance. When the cyclohexylamino was changed to 2-ethylhexylamino, RF2, the feature size increased dramatically, explaining the lower PCE of 0.10% and low FF of 0.30. When the ethyl linker was used instead of methyl, smaller features were observed for both alkylamino core substituents. While the P3HT:RF3 film is predominantly P3HT with traces of RF3, the P3HT:RF4 film should have a ratio near 1:2.5 because RF4 completely dissolved in the blend solution. Core-substituted NDI with

phenyl linkers also formed a small phase separation with P3HT. It therefore appears that the ethyl and phenyl linkers promote better miscibility of core-substituted NDI with P3HT than the methyl linker. To evaluate whether the RF1−RF6 acceptors are contributing to photocurrent generation, we measured the IPCE of the as-cast devices in air (Figure 7a). The more efficient annealed devices degraded quickly when exposed to air, and their IPCE

Figure 7. (a) IPCE of as-cast films and (b) UV/vis of films and IPCE of P3HT:RF1. G



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Figure 8. Optimized geometries for RF1, RF3, and RF5 at the B3LYP/6-31G(d,p) level of theory.

could not be recorded accurately. The IPCE of the as-cast films peaked at 525 nm, very similar to a P3HT:PCBM solar cell, suggesting that the photocurrent is dominated by P3HT absorption. Figure 7b shows the UV−vis spectra of P3HT, RF1, and P3HT:RF1 blend (1:2.5 ratio) films. Interestingly, the absorption spectra of the blend is not a superposition of the absorption spectra of P3HT and RF1. Instead, the absorption of the blend is narrower than a superposition of both, with two maxima at ∼560 and 610 nm. This suggests some interaction between the P3HT and RF1. Comparing the IPCE with the UV−vis of the blend, we see that both have a similar onset and are both red-shifted compared to pristine P3HT, suggesting that RF1 or the P3HT:RF1 complex contribute to photocurrent generation. Unlike the UV−vis of the blend, the IPCE follows the absorption spectra of pristine P3HT between 450 and 550 nm, suggesting that pristine P3HT also contributes to light absorption. Although the actual shape of the IPCE depends on optics of the device, we believe that this data indicates that RF1 is contributing to photocurrent. Quantum Chemical Calculations. To better understand the effect of the core and imide substitutions on properties, we performed density functional theory (DFT) calculations at the B3LYP hybrid level with 6-31G(d,p) basis set. Frontier molecular orbitals for RF1−RF6, given in the Supporting Information, revealed that both HOMO and LUMO are located mainly on NDI core for all six molecules, consistent with our expectations and the literature on similar compounds. In addition, the eigenvalues for the HOMO and LUMO levels were similar for all six molecules studied, at −5.2 eV for the HOMO and at −2.8 and −2.7 eV for LUMO levels of RF1− RF4 and RF5−RF6, respectively. This further illustrates that the modifications are not affecting the lowest energy optical absorption band of isolated molecules, such as in dilute solutions. Since these calculations were performed for isolated molecules, they are not relevant to thin films. Optimized geometries of RF1−RF6, shown in Figure 8 and the Supporting Information, show that the choice of linker has a profound impact on the orientation of the thiophene group with respect to the NDI conjugated plane. In the case of methyl linker (RF1−RF2), the thiophene conjugated plane is oriented at 113.6° with respect to the NDI conjugated plane due to the sp3 hybridization of the methyl linker. For the ethyl linker

(RF3−RF4), the presence of an additional sp3-hybridized carbon allows the thiophene group to lie nearly parallel to the NDI conjugated plane, with a tilt angle of 8°. This is similar to the reported crystal structures of NDI with phenylethyl imide substituents, where the phenyl groups are almost parallel to the NDI plane.50 Finally, in the case of the phenyl linker, the phenyl plane is roughly perpendicular to the cNDI conjugated plane due to steric hindrance, consistent with published crystal structure of NDI with phenyl imide substituents.51 The thiophene lies ∼28° with respect to the phenyl linker, and as discussed above, the thiophene and phenyl groups are conjugated with each other. The overall tilt angle between the NDI and thiophene planes is 45.3° and 61.3° for RF5 and RF6, respectively. The choice of alkylamino substituent had no impact on the optimized geometry for RF1−RF2 and RF3− RF4, since the linkers are small and do not interact with the alkylamino substituents. On the other hand, a small effect of the alkylamino substituent was observed for the RF5−RF6 molecules, due to the larger phenyl linker. The angle between the c-NDI and phenyl planes was closest to 90° for RF6 (90.9°) than for RF5 (106.3°) due to steric interactions between the bulkier 2-ethylhexylamino core substituent and the phenyl linker. To gain some insight into molecular packing, we also optimized dimers in gas phase using hybrid DFT:M062X, shown in the Supporting Information. Studied core-susbstituted NDI π-stacked at a short distance of 3.2−3.4 Å, irrespective of the linker or alkylamino core-substituents. We conclude that the substitutions should not be detrimental to charge transport. Although these calculations were performed in the gas phase, we anticipate that similar geometry changes will take effect in the solid state.

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CONCLUSIONS In this study, we varied the linker between the imide and a thiophene end group as well as the alkylamino substituent. Optical and electrochemical properties were unaffected by these substitutions in solution and only slightly affected by these substitutions in films. On the other hand, power conversion efficiencies of bulk heterojunction solar cells ranged from nearly 0% to 0.5%, depending on the linker and alkylamino core substituent combination used. The best device performance was achieved using the methyl linker/cycloH



Article

(4) Krebs, F. C.; Gevorgyan, S. A.; Alstrup, J. A Roll-to-Roll Process to Flexible Polymer Solar Cells: Model Studies, Manufacture and Operational Stability Studies. J. Mater. Chem. 2009, 19, 5442−5451. (5) Nelson, J. Polymer:Fullerene Bulk Heterojunction Solar Cells. Mater. Today 2011, 14, 462−470. (6) He, Y.; Li, Y. Fullerene Derivative Acceptors for High Performance Polymer Solar Cells. Phys. Chem. Chem. Phys. 2011, 13, 1970. (7) Varotto, A.; Treat, N. D.; Jo, J.; Shuttle, C. G.; Batara, N. A.; Brunetti, F. G.; Seo, J. H.; Chabinyc, M. L.; Hawker, C. J.; Heeger, A. J.; Wudl, F. 1,4-Fullerene Derivatives: Tuning the Properties of the Electron Transporting Layer in Bulk-Heterojunction Solar Cells. Angew. Chem., Int. Ed. 2011, 50, 5166−5169. (8) Kooistra, F. B.; Knol, J.; Kastenberg, F.; Popescu, L. M.; Verhees, W. J. H.; Kroon, J. M.; Hummelen, J. C. Increasing the Open Circuit Voltage of Bulk-Heterojunction Solar Cells by Raising the Lumo Level of the Acceptor. Org. Lett. 2007, 9, 551−554. (9) Rondeau-Gagne, S.; Curutchet, C.; Grenier, F.; Scholes, G. D.; Morin, J.-F. Synthesis, Characterization and DFT Calculations of New Ethynyl-Bridged C60 Derivatives. Tetrahedron 2010, 66, 4230−4242. (10) Backer, S. A.; Sivula, K.; Kavulak, D. F.; Fréchet, J. M. J. High Efficiency Organic Photovoltaics Incorporating a New Family of Soluble Fullerene Derivatives. Chem. Mater. 2007, 19, 2927−2929. (11) Scharber, M. C.; Muehlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Design Rules for Donors in Bulk-Heterojunction Solar Cells-Towards 10% Energy-Conversion Efficiency. Adv. Mater. 2006, 18, 789−794. (12) Facchetti, A. Polymer Donor−Polymer Acceptor (All-Polymer) Solar Cells. Mater. Today 2013, 16, 123−132. (13) Sonar, P.; Fong Lim, J. P.; Chan, K. L. Organic Non-Fullerene Acceptors for Organic Photovoltaics. Energy Environ. Sci. 2011, 4, 1558−1574. (14) Sharenko, A.; Proctor Christopher, M.; van der Poll Thomas, S.; Henson Zachary, B.; Nguyen, T.-Q.; Bazan Guillermo, C. A HighPerforming Solution-Processed Small Molecule:Perylene Diimide Bulk Heterojunction Solar Cell. Adv. Mater. 2013, 25, 4403−4406. (15) Zhang, X.; Lu, Z.; Ye, L.; Zhan, C.; Hou, J.; Zhang, S.; Jiang, B.; Zhao, Y.; et al. A Potential Perylene Diimide Dimer-Based Acceptor Material for Highly Efficient Solution-Processed Non-Fullerene Organic Solar Cells with 4.03% Efficiency. Adv. Mater. 2013, 5791− 5797. (16) Sharma, G. D.; Roy, M. S.; Mikroyannidis, J. A.; Justin Thomas, K. R. Synthesis and Characterization of a New Perylene Bisimide (PBI) Derivative and Its Application as Electron Acceptor for Bulk Heterojunction Polymer Solar Cells. Org. Electron. 2012, 13, 3118− 3129. (17) Hudhomme, P. An Overview of Molecular Acceptors for Organic Solar Cells. EPJ Photovoltaics 2013, 4, 40401−40411. (18) Zhou, Y.; Dai, Y. Z.; Zheng, Y. Q.; Wang, X. Y.; Wang, J. Y.; Pei, J. Non-Fullerene Acceptors Containing Fluoranthene-Fused Imides for Solution-Processed Inverted Organic Solar Cells. Chem. Commun. 2013, 49, 5802−4. (19) Earmme, T.; Hwang, Y. J.; Murari, N. M.; Subramaniyan, S.; Jenekhe, S. A. All-Polymer Solar Cells with 3.3% Efficiency Based on Naphthalene Diimide-Selenophene Copolymer Acceptor. J. Am. Chem. Soc. 2013, 135, 14960−14963. (20) Zhou, E.; Cong, J.; Wei, Q.; Tajima, K.; Yang, C.; Hashimoto, K. All-Polymer Solar Cells from Perylene Diimide Based Copolymers: Material Design and Phase Separation Control. Angew. Chem., Int. Ed. 2011, 50, 2799−2803. (21) Zhou, Y.; Yan, Q.; Zheng, Y.-Q.; Wang, J.-Y.; Zhao, D.; Pei, J. New Polymer Acceptors for Organic Solar Cells: The Effect of RegioRegularity and Device Configuration. J. Mater. Chem. A 2013, 1, 6609−6613. (22) Guo, C.; Lin, Y.-H.; Witman, M. D.; Smith, K. A.; Wang, C.; Hexemer, A.; Strzalka, J.; Gomez, E. D.; Verduzco, R. Conjugated Block Copolymer Photovoltaics with near 3% Efficiency through Microphase Separation. Nano Lett. 2013, 13, 2957−2963.

hexylamino combination, closely followed by the phenyl linker/ cyclohexylamino combination. While the cyclohexylamino core substituent gave the best PCE for methyl (RF1) and phenyl (RF5) linkers, it gave the worst efficiency for ethyl linker (RF3) because of limited solubility. Changing from the cyclic to the branched core substituent increased PCE from nearly 0% for RF3 to 0.24% for RF4, partly because of increased solubility. IPCE measurements and UV−vis absorption spectra of blend films suggest that both P3HT and RF1 contribute to photocurrent. Analysis of film surface by AFM revealed that the substitutions strongly affected the phase separation scale between P3HT and the core-substituted NDI. The phase separation with P3HT was smaller for ethyl and phenyl linkers (RF4−RF6) than with a methyl linker (RF1−RF2). It therefore appears that the methyl linker lowers the miscibility of core-substituted NDI molecules with P3HT. Calculated optimized geometries show that the angle between the NDI and thiophene planes varies with the linker used. While additional studies are required to fully understand these structure-property results and to understand why efficiencies were limited to 0.5%, this work shows that device performance and blend morphology could be tuned with the linker and alkylamino core substituent. Using thiophene at the end of the imide substituent adds a unique dimension to tune morphology and influence the molecular heterojunction between donor and acceptor.

■

ASSOCIATED CONTENT

S Supporting Information *

NMR spectra, MALDI-TOF MS spectra, UV−vis absorption and emission spectra, thermal gravimetric analysis data, differential scanning calorimetry data, additional current− voltage curves of solar cells, and computational details. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Case Western Reserve University and the National Science Foundation (CHEM 1148652) for financial support, Prof. Thomas Gray and Prof. John Protasiewicz for facilitating emission spectrophotometer and cyclic voltammetry, Dr. Dale Ray for help with NMR, and the Case High Performance Computing Cluster for computing time. This material is based upon work supported by the National Science Foundation under Grant MRI-0821515 (for the purchase of the MALDITOF/TOF).

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REFERENCES

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