Article pubs.acs.org/cm
Doping Poly(3-hexylthiophene) Nanowires with Selenophene Increases the Performance of Polymer-Nanowire Solar Cells Han Yan, Jon Hollinger, Colin R. Bridges, George R. McKeown, Tamara Al-Faouri, and Dwight S. Seferos* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada S Supporting Information *
ABSTRACT: Developing novel materials analogous to poly(3hexylthiophene) (P3HT) with increased absorption range and good one-dimensional self-assembly properties should increase photovoltaic performance while taking advantage of the well-established structure−property relationships developed for P3HT. Herein, we have fabricated novel polymer nanowires composed of P3HT doped with varying amounts of selenophene. Doping is accomplished by statistical polymerization and results in the incorporation of selenophene into the P3HT crystal lattice. Selenophene doping increases optical absorption far beyond what can be achieved by simply blending two materials. Polymer nanowire solar cells using selenphene-doped P3HT outperform native P3HT nanowire and corresponding ternary blend solar cells, reaching an overall maximum performance of >4% PCE. These are some of the highest values of any polymer nanowire solar cells and show that the selenophene-doping strategy is important for achieving high-performance devices.
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INTRODUCTION With the continuous increase of power conversion efficiency (PCE), organic solar cells (OSCs) are a promising solution to the traditional compromise between the simultaneous goals of high efficiency and low cost.1−6 Aside from the optimization of frontier orbital energy levels, the performance of OSCs generally depends on the optical absorption range7,8 and bulk heterojunction (BHJ) film morphology, which should ideally contain a bicontinuous donor and acceptor network at a length scale commensurate with exciton diffusion (typically 10−20 nm).9,10 Of the numerous semiconducting polymers, poly(3hexylthiophene) (P3HT) continues to be one of the most interesting candidates due largely to its well-known semicrystalline properties, which leads to quite a high current despite limitations in optical absorption and frontier orbital levels.11−14 An interconnected network of self-assembled domains creates continuous percolation pathways that contribute to a high current when P3HT is paired with a common electron acceptor such as [6,6]-phenyl C61/C71-butyric acid methyl ester (PCBM), which is largely excluded from the semicrystalline P3HT domains.15−20 Blends of P3HT:PCBM have provided a thorough understanding of structure− property−function relationships in OSCs.10,21,22 Unfortunately, generalizations derived from P3HT:PCBM cannot be broadly applied to novel materials.23−27 Thus, developing novel materials analogous to P3HT with increased absorption range and good one-dimensional (1D) self-assembly properties is an important line of research that should increase PCE while taking advantage of the well-established structure−property− function relationships developed for P3HT. © 2014 American Chemical Society
In 2007, a selenium analogue of P3HT, regioregular poly(3hexylselenophene) (P3HS), was reported.28 This polymer has an extended optical absorption range (up to 760 nm) and similar charge mobility as regioregular P3HT. Although anticipated as a good candidate for polymer solar cells, P3HS has not achieved better photovoltaic performance than P3HT thus far.29 Although substitution of sulfur atoms in P3HT by heavy heteroatoms increases the tendency of the polymers to form better ordered phases, interestingly, the overall portion of the ordered phase is lower than that of P3HT-based films.30 The decreased purity of the P3HS phases is considered to be the main reason for lower PCE values.31 Moreover, although the optical absorption range red shifts to 760 nm, there is a corresponding reduction of absorption of green light.32 The limitations of single polymer devices can be overcome by introducing another polymer in the active layer which has a complementary absorption.33,34 Although ternary blends are a reasonable approach to achieve higher efficiency for organic photovoltaic devices, the morphology optimization is even more challenging than polymer/fullerene binary blends. A parallel strategy has been reported by our group previously which involves directly adding a sensitizer into the polymer backbone through chain-growth statistical copolymerization.35 These polymers have a narrow dispersity, have widened optical absorption, and avoid the complex phase behavior that occurs when blending three materials, and thus performance can be Received: May 31, 2014 Revised: July 18, 2014 Published: July 24, 2014 4605
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mmol) were added to a Schlenk flask and placed under a vacuum for at least 20 min. Dry THF (4.7 mL) was added to the flask followed by iPrMgCl (0.84 mL, 1.68 mmol) at room temperature. After the reaction had proceeded for 1 h, Ni(dppp)Cl2 (5.1 mg, 0.011 mmol, 0.67 mol %) was added and the reaction heated to 40 °C for 20 min, then quenched with 5% HCl solution. The polymer was precipitated in methanol and filtered through a Soxhlet thimble, followed by extractions using methanol, hexanes, and chloroform. The chloroform fraction was collected, concentrated, and passed through a short silica gel column using chloroform as the elutant. The solvent was removed under reduced pressure to give the polymer as a purple solid (175 mg, 59%). 1H NMR (CDCl3, 400 MHz): δ [ppm] 7.19 (s, 0.15H), 7.13 (s, 0.04H), 6.98 (s, 0.67H), 6.93 (s, 0.16H), 2.85−2.73 (m, 2H), 1.75− 1.65 (m 2H), 1.48−1.25 (m, 6H), 0.95−0.85 (m, 3H). GPC (1,2,4trichlorobenzene, 140 °C): Mn = 46.6 kDa, Mw = 66.2 kDa, Đ = 1.42. P3HT-stat-P37S (57:43). 2,5-Dibromo-3-heptylselenophene (150 mg, 0.388 mmol) and 2,5-dibromo-3-hexylthiophene (126 mg, 0.388 mmol) were added to a Schlenk flask and placed under a vacuum for 20 min. Dry THF (5.5 mL) was added to the flask followed by iPrMgCl (0.39 mL, 0.775 mmol) at room temperature. After the reaction had proceeded for 1 h, Ni(dppp)Cl2 (2.8 mg, 0.0052 mmol, 0.67 mol %) was added and the reaction heated to 40 °C for 20 min, then quenched with 5% HCl solution. The polymer was precipitated in methanol and filtered through a Soxhlet thimble, followed by extraction using methanol, hexanes, and chloroform. The chloroform fraction was collected, concentrated, and passed through a short silica gel column using chloroform as the elutant. The solvent was removed under reduced pressure to give the polymer as a purple solid (100 mg, 75%). 1H NMR (CDCl3, 400 MHz): δ [ppm] 7.19 (s, 0.25H), 7.13 (s, 0.19H), 6.98 (s, 0.34H), 6.93 (0.24H), 2.85−2.70 (m, 2H), 1.76−1.65 (m, 2H), 1.49−1.20 (m, 7H), 0.97−0.83 (s, 3H). GPC (1,2,4trichlorobenzene, 140 °C): Mn = 42.5 kDa, Mw = 51.2 kDa, Đ = 1.21. P3HT-stat-P37S(20:80). 2,5-Dibromo-3-heptylselenophene (376 mg, 0.971 mmol) and 2,5-dibromo-3-hexylthiophene (79.0 mg, 0.243 mmol) were added to a Schlenk flask and placed under a vacuum for at least 20 min. Dry THF (12.8 mL) was added to the flask followed by i-PrMgCl (0.61 mL, 1.21 mmol) at room temperature. After the reaction had proceeded for 1 h, Ni(dppp)Cl2 (4.4 mg, 0.081 mmol, 0.67 mol %) was added and the reaction heated to 40 °C for 20 min, then quenched with 5% HCl solution. The polymer was precipitated in methanol and filtered through a Soxhlet thimble, followed by extractions using methanol, hexanes, and chloroform. The chloroform fraction was collected, concentrated, and passed through a short silica gel column using chloroform as the elutant. The solvent was removed under reduced pressure to give the polymer as a purple solid (154 mg, 60%). 1H NMR (CDCl3, 400 MHz): δ [ppm] 7.19 (s, 0.16H), 7.13 (s, 0.65H), 6.98 (s, 0.04H), 6.93 (0.16H), 2.85−2.70 (m, 2H), 1.75−1.65 (m 2H), 1.49−1.20 (m, 7H), 0.97−0.83 (s, 3H). GPC (1,2,4-trichlorobenzene, 140 °C): Mn = 26.4 kDa, Mw = 35.1 kDa, Đ = 1.33. Device Fabrication and Testing. Indium−tin oxide (ITO)coated glass substrates (Colorado Concept Coatings LLC) were cleaned successively with aqueous detergent, deionized water, methanol, and acetone for 5 min each and then treated in an oxygen-plasma cleaner for 15 min. PEDOT:PSS was filtered through a 0.45 μm syringe filter, spin coated at 3000 rpm, and then annealed at 150 °C for 10 min in an ambient atmosphere. Homopolymer, statistical copolymer, and polymer blends with different ratios were dissolved in 1,2,4-trichlorobenzene (TCB) with total polymer concentrations equal to 20 mg/mL and heated to 120 °C for 3 h. The solution was slowly cooled to room temperature at a rate of 20 °C per hour, placed in a glovebox for aging (5−7 days) to induce nanowire formation, and finally mixed with PC71BM. For homopolymers and blend polymers, the ratio of polymer to PC71BM was 1:1; while for statistic copolymers, the ratio was 1:1.5. Each represents the empirically determined optimized donor−acceptor ratio. The mixed solutions were stirred for another 2 h at room temperature before use. The solar cell devices were fabricated by spin-coating (800 rpm) the active layer on PEDOT:PSS coated ITO in a N2-filled glovebox. All samples were quickly dried on a hot-plate at 150 °C after spin-coating.
optimized by the same processes as a two component system such as P3HT:PCBM. Despite these advantages, however, we were not able to achieve higher PCE than P3HT, likely due to morphology issues. Herein, we report novel selenophene−thiophene copolymer nanowires and their use in polymer nanowire solar cells. Unlike BHJ solar cells, polymer nanowire solar cells are relatively rare.16−19 The nanowire solar cell meets device morphology requirements by polymer self-assembly. The nanowires reduce the distance that excitons must diffuse to half of the width of the fiber. They also guide charge transport along their long axis. Linear conjugated polymers tend to form long nanowires by π−π interactions. This directs charge transport along the favored π−π direction and exciton transport along the favored backbone direction. Another advantage of nanowire solar cells is that self-assembly improves phase purity, thus resulting in well-defined donor and acceptor-rich domains. Selenophene− thiophene copolymers are very important materials for polymer nanowire solar cells because they have certain properties that are better than either homopolymer. Here, we show that when selenophenes are incorporated into the P3HT chain, they serve to dope the crystalline P3HT nanowires, improving their optical absorption and leading to the highest PCE when mixed with PC71BM. This is not the conventional redox doping that is typically associated with conjugated polymers. We consider that this is more similar to doping crystalline Si, and we have chosen to use the term doping to describe crystalline P3HT nanowires that have various selenophene contents.
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EXPERIMENTAL SECTION
General Considerations. All reagents were used as received unless otherwise noted. 2,5-Dibromo-3-heptylselenophene was prepared as previously reported.35 2,5-Dibromo-3-hexylthiophene, iPrMgCl (2.0 M in THF), and Ni(dppp)Cl2 were purchased from Sigma-Aldrich. THF was dried using an Innovative Technology solvent purification system. P3HT and PC71BM were purchased from American Dye Source. Instrumentation. 1H NMR was performed using a Varian Mercury 400 (400 MHz) spectrometer. GPC measurements were carried out using a Malvern 350 HT-GPC system at 140 °C with 1,2,4trichlorobenzene (stabilized with butylated hydroxytoluene). Molecular weights were determined using narrow dispersity polystyrene standards. The absorption spectrum was recorded on a Varian Cary 5000 spectrometer. Wide angle X-ray diffraction measurements were carried out using a Bruker AXS D8 Discovery with GADDS area detector. AFM imaging was carried out on a Veeco Dimension 3000 microscope. Materials. Poly(3-septylselenophene) (P37S). A flame-dried Schlenk flask containing 2,5-dibromo-3-septylselenophene35 (587 mg, 1.52 mmol) in dry THF (14 mL) was sparged with argon. Isopropylmagnesium chloride (0.72 mL, 1.3 M in THF, 1.44 mmol) was added dropwise to the solution and stirred for 1 h at ambient temperature before being transferred to a flask containing [N,N′dimesityl-2-3-(1,8-naphthyl)-1,4-diazabutadiene]dibromonickel 42 (9.7 mg, 0.015 mmol). The solution was stirred at ambient temperature for 15 min before quenching with 5% hydrochloric acid. The solution was precipitated into methanol and purified by Soxhlet extraction using methanol, hexanes, and chloroform. The chloroform fraction was concentrated then passed through a short silica gel column using chloroform as the elutant to afford the polymer as a purple solid (242 mg, 70%). 1H NMR (CDCl3, 400 MHz): δ [ppm] 7.12 (s, 1H), 2.73 (t, 2H), 1.74−1.64 (m, 2H), 1.42−1.31 (m, 8H), 0.90 (t, 3H). GPC (1,2,4-trichlorobenzene, 140 °C): Mn = 21.1 kDa, Mw = 31.6 kDa, Đ = 1.50. P3HT-stat-P37S (80:20). 2,5-Dibromo-3-heptylselenophene (130 mg, 0.34 mmol) and 2,5-dibromo-3-hexylthiophene (438 mg, 1.34 4606
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No further annealing steps were used. LiF (0.8 nm) and Al (100 nm) were thermally evaporated using an Angstrom Engineering (Kitchener, ON) Covap II metal evaporation system at 1 × 10−6 Torr. The device area is 0.07 cm−2 as defined by a shadow mask. I−V curves were obtained with a Keithley 2400 source meter under simulated AM 1.5G conditions with a power intensity of 100 mW/cm2. The mismatch of the spectrum was calibrated using a Si diode with a KG-5 filter. Lightintensity dependent I−V curves were carried out by adjusting samplelight source distance and calibrated with the Si detector. EQE measurements were recorded using a 300 W xenon lamp with an Oriel cornerstone 260 1/4 m monochromator and compared with a Si reference cell that is traceable to the National Institute of Standards and Technology.
can effectively red-shift and broaden absorption. Simply blending the two polymers together does not produce this result and only changes the maximum absorption peak from 467 to 481 nm with increasing P37S content (Figure 1d). At each ratio, the absorption peak of the blend of the two polymers is 10−20 nm blue-shifted compared to the corresponding statistical copolymers. This highlights one of the fundamental consequences of statistical copolymerization of π-conjugated units. Previous work in our group has shown that thermal/solvent annealing does not increase the photovoltaic performance of polyselenophene:PC71BM.32,35 We thus turned to solventinduced nanowire formation to optimize the morphology of the devices reported here. We choose 1,2,4-trichlorobenze (TCB) as a marginal solvent and obtained polymer nanowires by subtly controlling the aging time. In a marginal solvent, polymers undergo a conformation transition from a random twisted chain to a more planar and ordered state, followed by the onedimensional aggregation. The process was monitored by the color change in solution, and atomic force microscopy (AFM) was used to visualize self-assembly of each polymer. The statistical polymers form overlapping nanowires (Figure 2c and Figure S1), which are identical to P3HT and P37S (Figure 2a and b, respectively). It should be noted that the nanowires are not monodisperse; they tend to form branched nanowire networks (Figure S2). The widths of nanowires from statistical analysis of AFM images (100 measurements per sample) are 45 ± 8 nm for P3HT, 48 ± 10 nm for P37S, 45 ± 6 nm for P3HT-
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RESULTS AND DISCUSSION For this study, a series of novel statistical copolymers of P3HTstat-P37S (poly(3-hexylthiophene)-stat-poly(3-heptylselenophene)) with various thiophene to selenophene ratios were synthesized by controlled chain-growth methods (Figure 1a;
Figure 1. (a) Full and a representative partial structure of P3HT-statP37S used in this study. (b) 1H NMR spectra of the aromatic region of statistical copolymers. (c) Solution absorption spectra of P3HT, P37S, and statistical copolymers. (d) Solution absorption spectra of P3HT, P37S, and blend polymers with the same unit ratio as corresponding statistical copolymers. (black square, P3HT; red circle, unit ratio of 80:20; blue up-triangle, unit ratio of 57:43; olive down-triangle, unit ratio of 20:80; purple diamond, P37S).
detailed information can be found in the Experimental Section). Polyselenophene with a seven carbon-chain side group (P37S) was used because it has a similar crystal packing structure and hence and better solid-state miscibility with P3HT.32,35 The structure of the statistical polymers and monomer incorporation was confirmed by 1H NMR spectroscopy by integrating the distinct 1H NMR resonance of thiophene (6.93 and 6.98 ppm) and selenophene (7.13 and 7.19 ppm) units (Figure 1b). The thiophene:selenophene ratios are 80:20, 57:43, and 20:80. The four-peak pattern confirms the statistical nature of the polymer (a block copolymer only has one peak for each block). When statistically combining selenophene and thiophene monomers into one polymer, we expect a red-shifted and broadened optical absorption range (Figure 1c) compared to P3HT. Indeed the maximum absorption peaks red shift from 474 to 498 nm with increasing selenophene content (Figure 1c). Compared with P3HT (463 nm) and P37S (498 nm), even a relatively small amount of selenophene doping in P3HT
Figure 2. Topography images obtained by tapping-mode AFM (5 μm × 5 μm): (a) P3HT, (b) P37S, (c) P3HT-stat-P37S (57:43). (d) XRD patterns of P3HT, P37S, and P3HT-stat-P37S (50:50). (e) Thin film absorption spectra of polymer nanowires composed of P3HT, P37S, and statistical polymer. (f) Thin film absorption spectra of P3HT, P37S, and blend polymers with the same unit ratio of corresponding statistical copolymers. (black square, P3HT; red circle, unit ratio of 80:20; blue up-triangle, unit ratio of 60:40; olive down-triangle, unit ratio of 20:80; purple diamond, P37S). 4607
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stat-P37S (80:20), 44 ± 7 nm for P3HT-stat-P37S (57:43), and 44 ± 7 nm for P3HT-stat-P37S (20:80). The width of all wires are thus very similar. The crystalline structure of the selfassembled nanowires is further confirmed by X-ray diffraction (XRD) characterization. The self-assembly of P3HT and P37S mainly occurs in two orientations, namely, the side chain stacking direction ([100]) and the π−π stacking direction ([020]). Here, we can only observe (100) and corresponding high-order reflections for P3HT, P37S, and all three statistical polymers (Figure 2d and Figure S3). The absence of a (020) reflection can be explained by the diffraction geometry. We can only detect the signals in-plane, which are from the crystal faces parallel to the substrate, and although other faces also generate signals, we cannot record them with our experimental setup. The absence of a (020) reflection also demonstrates molecular packing orientation. All polymers adopt an edge-on configuration relative to the substrate (with the side chain vertical to the substrate and the π−π stacking parallel to the substrate). Nanowires grow along the π−π stacking direction (inset of Figure 2d). The lattice constant of P3HT-stat-P37S nanowires (57:43) is 16.44 Å, which is between the constant of P3HT nanowires (15.68 Å) and P37S nanowires (16.75 Å). This observation supports our thesis that these nanowires have selenophene units doped within the nanowire crystal lattice. Another significant of advantage of self-assembled nanowires is their ability to red shift and increase the absorption range. The P3HT and P37S nanowire films have absorption peaks at 524 and 591 nm, with strong shoulder peaks at 610 and 690 nm (Figure 2e). By doping the 37S (3-heptyselenophene) unit into the P3HT backbone, the statistical copolymers have absorption peaks at 560, 585, and 594 nm (Figure 2e). Similar shoulder peaks in the statistical copolymers also indicate significant selfassembly and effectively broaden the absorption range. Simply blending P37S with P3HT does not induce the same shift in absorption peak at low content (20%), and even at a high content the maximum absorption peak only shifts to 553 nm, which is 41 nm lower than the corresponding statistical polymer with the same selenophene content (Figure 2f). The doped nanowire strategy also increases the absorption range. Compared with polymer blends, the fwhm (full widths at half maxima) of statistical polymers are enlarged by 13, 12, and 20 nm at a 80:20, 57:43, and 20:80 thiophene:selenophene ratios, respectively. Blending the polymers together only induces a small increase in red-light absorption and does not significantly increase the absorption range. It is worth noting that the differences in absorption spectrum for statistical polymers and corresponding blends are more pronounced in the assembled nanowires than in solution, which highlights the difference between doping the crystalline nanowires and doping the polymer blend. After characterizing the molecular packing mode and optical properties of the preassembled nanowires, we then fabricated photovoltaic devices from nanowire suspensions and PC71BM in TCB. XRD patterns of nanowire:PC71BM mixtures (Figure S4) confirm the retention of similar molecular packing for the nanowires in the presence of PC71BM. All the devices show typical rectifying curves under 100 mW/cm2 illumination (Figure 3a and b). The photovoltaic performance data, which include the open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and PCE values are summarized in Table 1. For reference, P3HT:PC71BM and P37S:PC71BM nanowire devices have 3.2% and 2.9% PCE, respectively. While the performance of P3HT:PC71BM is similar to that obtained
Figure 3. (a−d) Photovoltaic performances of polymer nanowire solar cells composed of statistical copolymers and blend polymers mixed with PC71BM: (a, b) J−V curves; (c, d) corresponding EQE plots. (e) The histograms of statistical Jsc values for all material systems. (f) The histograms of statistical PCE values for all material systems.
Table 1. Photovoltaic Performances of Polymer Nanowire Solar Cells with PC71BM Used As the Acceptor materials
Jsc (mA/cm2)
Voc (V)
FF (%)
PCEmax (%)
P3HT P37S (80:20)stat (57:43)stat (20:80)stat (80:20)blend (60:40)blend (20:80)blend
8.94 9.31 11.47 12.04 12.52 8.51 6.72 8.18
0.57 0.52 0.55 0.54 0.52 0.54 0.56 0.52
62.99 60.52 64.36 59.52 57.14 63.54 55.01 60.89
3.2 2.9 4.1 3.9 3.7 2.9b 2.1 2.6b
PCEavga (%) 3.0 2.8 3.9 3.7 3.5 2.6 1.9 2.3
± ± ± ± ± ± ± ±
0.2 0.1 0.2 0.2 0.2 0.2 0.2 0.2
Average PCE of 20 devices fabricated under identical conditions ± 1 standard deviation (σ). bMaximum lies outside ±1σ.
a
for typical P3HT:PC71BM BHJs,11 the performance of P37S:PC71BM is higher here than in any previous report for P37S or P3HS BHJs, likely as a result of better morphology as a result of nanowire formation. The Jsc of P37S is 9.31 mA/cm2, which is larger than P3HT (8.94 mA/cm2), while the Voc is 0.52 V, which is smaller than P3HT (0.57 V) and is consistent with previous reports.32,35 Importantly, the P37S-based device also has a high FF of 60.52%, similar to P3HT and higher than any previous report of P37S or P3HS. We attribute this improvement to the pure and continuous P37S nanowires in the film. The most significant finding in the present work is that all three doped nanowires have better device performance than either P3HT or P37S. The best conversion efficiency achieved is 4.1% for P3HT-stat-P37S (80:20) nanowires. As expected, Jsc increases with selenophene content, from 11.47 mA/cm2 to 12.04 mA/cm2, and finally achieves 12.52 mA/cm2 with P3HT4608
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proportional to Iα, where I is the light intensity and 1 > α > 0.39,40 Under ideal conditions, the bimolecular recombination should be minimized (α ≈ 1) for maximum carrier sweep out. Any deviation of α from 1 implies that bimolecular recombination has taken place. When we plot Jsc vs I on a log−log scale, we obtain α = 0.82 and α = 0.81 for P3HT-statP37S:PC71BM and P3HT+P37S:PC71BM. The similar slope demonstrates a similar amount of bimolecular recombination in these two devices. Next, we examine the charge recombination kinetics at the Voc. At the Voc, the photocurrent is zero, and all photogenerated carriers recombine within the cell. The recombination mechanism can be verified with the dependence of Voc on the logarithm of the light intensity, which describes the order of the recombination process. A slope of thermal voltage (kT/e) features a bimolecular recombination as opposed to a higher slope (2kT/e) which is often assigned to a trap-assisted recombination (Schockley−Read−Hall recombination).37,41 We derive the Voc versus light intensity curves from the J−V curves (Figure 4d). Again, a similar dependence of Voc is observed with the slope of 1.03kT/e and 1.01kT/e for statistical copolymer and blend nanowire devices. These detailed recombination processes exclude the effect of recombination in a blend film; thus the Jsc variations between the doped and blended samples comes exclusively from photocharge generation.
stat-P37S (20:80) nanowires. At the same time, the Voc decreases from 0.55 to 0.52 V, a range that is between P3HT and P37S. As a comparison, we also report the J−V curves of nanowire ternary-blend solar cells with the same P3HT to P37S ratios (Figure 3b). All ternary-blend devices show poorer photovoltaic performances compared to P3HT and P37S, mainly due to the lower Jsc that likely results from a nonideal morphology. To understand the observed Jsc increase in doped nanowire solar cells, we measured the external quantum efficiency (EQE) spectrum to study the spectral response. The EQE curves of the devices cover a broad wavelength range from 300 to 800 nm. With increasing selenophene doping content, a red-shifted response compared to P3HT is observed. From these EQE plots, the calculated current densities are 8.93 mA/cm2 for P3HT, 11.02 mA/cm2 for P3HT-stat-P37S (80:20), 11.68 mA/ cm2 for P3HT-stat-P37S (57:43), 11.91 mA/cm2 for P3HTstat-P37S (20:80), and 9.06 mA/cm2 for P37S. Although P37S also has a broad spectral response, the low EQE values limit the Jsc. When we look at the ternary blend counterparts, only those with the highest P37S content (80% P37S) have a significantly broader spectral response, and the overall intensity of the response is lower. For blends with 20% or 50% P37S, the minimal increase in spectral response in the red region cannot compensate for the decrease in the green region, thus lowering the total EQE values. By integrating the EQE spectra, the Jsc is 8.07 mA/cm2, 6.52 mA/cm2, and 7.82 mA/cm2 for 20%, 50%, and 80% P37S in P3HT ternary blends, respectively. To emphasize the difference between doped nanowire OPVs and blend nanowire OPVs, we compare the device performances of both strategies in Figure 3e and f. The nanowire doping strategy leads to better performance, and the improved PCE values due to higher Jsc. This is different from randomly copolymerizing an electron deficient group with polythiophene which leads to improvements in Voc.36 Our nanowire doping strategy increases Jsc with only a minimal loss in Voc. As well, a high FF is obtained because we have not perturbed the crystallization behavior of the polymer through this strategy. The observed device current consists of a forward current due to photogenerated charges and a loss current due to charges that are lost before collection at the electrodes. To better understand the Jsc increase for the doped nanowires, we probed the recombination kinetics by measuring how the J−V characteristics depend on light intensity.37 For this experiment, we used nanowires composed of P3HT-stat-P37S (80:20), which shows the best performance, and its corresponding blend of two polymer nanowires (P3HT+P37S) (80:20). Illumination intensities vary from 20 to 100 mW/cm2 with a step of 10 mW/ cm2. The shape of the J−V curves is strongly dependent on the recombination mechanism.38 When the circuit is open, bimolecular recombination dominates. Under short circuit conditions, bimolecular recombination is minimized. Analysis of the recombination mechanism under short circuit conditions is complicated by the simultaneous charge sweep-out during recombination. The sweep-out and recombination are approximately linearly related to light intensity. The photocurrent saturates at almost the same applied voltage for P3HT-statP37S:PC71BM as (P3HT+P37S):PC71BM. Thus, the two materials have a similar extent of sweep-out in competition with recombination, leading to similar FF, which is consistent with the device results. Insight into the recombination mechanism can therefore be obtained by measuring Jsc as a function of light intensity. Under short circuit conditions, Jsc is
Figure 4. (a, b) J−V curves of polymer nanowire solar cells composed of P3HT-stat-P37S/(P3HT+P37S) (80:20):PC71BM measured under various light intensities ranging from 20 mW/cm2 to 100 mW/cm2. (c) Measured Jsc of polymer nanowire solar cells composed of P3HTstat-P37S/(P3HT+P37S) (80:20):PC71BM plotted against light intensity on the logarithmic scale with linear fits to the data. (d) Measured Voc of polymer nanowire solar cells composed of P3HT-statP37S/(P3HT+P37S) (80:20):PC71BM plotted against light intensity on the logarithmic scale with linear fits to the data. (e, f) Phase images obtained by using tapping-mode AFM (2 μm × 2 μm): (e) P3HT-statP37S (80:20):PC71BM; (f) (P3HT+P37S) (80:20):PC71BM. 4609
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optoelectronic materials. Their improved performance over a P3HT nanowire device suggests a new strategy for the design and synthesis of polymer−nanowire solar cells.
As the polymers have sufficient energy level offset with PC71BM to drive charge separation at the interfaces, the variations must be closely related to different morphologies in these two blend films. The morphology of films was visualized by AFM phase images (Figure 4e and f, height images in Figure S5), which can distinguish chemical components between different domains according to mechanical properties. We first investigate the P3HT-stat-P37S nanowire (80:20) PC71BM film (Figure 4e). Uniform nanowire networks are observed over all of the film. These fine nanowires with a typical width of about 20 nm can provide sufficient interfaces for charge separation before exciton recombination in polymer domains. The long nanowires arranged along the π−π stacking direction are “expressways” for charge transport. The nanowire structure can effectively suppress recombination processes, thus improving photovoltaic performance. When we look at the corresponding nanowire blend, in addition to the long nanowires, we also observe a large number of aggregates. Considering that the P3HT to P37S polymer ratio is 80:20, we infer that the aggregates are P37S. We hypothesize that the low concentration of P37S dissolves in TCB but does not form nanowires during the aging time, rather it aggregates during the spincoating procedure. To confirm our assumption, we examined the morphology of P3HT:PC71BM that contains the same concentration of P3HT as the blend solution. In this case, we do not observe aggregates (Figure S6). In another control experiment, no nanowires are observe in P37S:PC71BM when the concentration of P37S is low (Figure S7). Thus, our assumption appears rational. The P37S aggregates are detrimental to photocharge generation because they are electronically isolated and do not contribute to Jsc. This result is consistent with the EQE curves (Figure 3d) that show that adding low concentrations of P37S cannot effectively broaden the spectral response in the red region. Thus, another important benefit of the nanowire doping strategy is that preself-assembly is facile.
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ASSOCIATED CONTENT
S Supporting Information *
Topography images of polymer nanowires, XRD patterns of nanowires, AFM images of P3HT-nanowire:PC71BM, and AFM images of P37S:PC71BM. 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]. Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by NSERC, the CFI, The Ontario Research Fund, DuPont, the Connaught Foundation, and the Alfred P. Sloan Foundation (for a Research Fellowship in Chemistry).
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REFERENCES
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CONCLUSIONS In conclusion, we have used selenophene to effectively “dope” P3HT nanowires and in doing so shown for the first time that a polymer composed of both thiophene and selenophene can outperform either homopolymer in a photovoltaic device. Specifically, 3-heptylselenophene was copolymerized with 3hexylthiophene to maintain a similar crystal packing motif and thus make the selenophene more miscible with thiophene. Doping selenophene units into the polymer backbone is a much more effective way to broaden the absorption range compared with homopolymer blends that have the same composition. Preassembly overcomes the decreased phase purity and solid state limitations of poly(3-alkylselenophene) analogues, which has until now limited its success in polymer solar cells. To the best of our knowledge, this is the first report of a selenophene-containing P3HT analogue that outperforms a comparable P3HT device. The increased PCE is the result of increased Jsc. Compared with corresponding ternary blend solar cells, which is a common method to combine two polymers, the opposite trend in photovoltaic performance is observed, namely the devices perform poorer than either homopolymer. Mechanistic studies ultimately reveal that the better photovoltaic performance is attributable to a more ideal morphology as well as better spectral coverage that results when selenophene is doped into the P3HT nanowires. Doped P3HT nanowires thus appear to be a promising class of organic 4610
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