Adding Amorphous Content to Highly Crystalline Polymer Nanowire Solar Cells Increases Performance.

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Adding Amorphous Content to Highly Crystalline Polymer Nanowire Solar Cells Increases Performance Han Yan, Yin Song, George R. McKeown, Gregory D. Scholes,* and Dwight S. Seferos* Polymer solar cells are based on blends of a conjugated polymer (donor) and a fullerene derivative (acceptor),[1,2] yet the morphology of these two components is not fully understood. It is typical to describe the morphology as a bulk heterojunction (BHJ) that contains donor–acceptor interfaces for exciton dissociation, along with pure interconnected donor and acceptor phases for charge transport.[3,4] In this regard, a comblike structure is the envisioned ideal morphology; however, recent reports have questioned whether this pure phase model is appropriate. In polymer solar cells comprised of polymer/ fullerene active layers, polymer donors can crystallize into pure domains, but never completely, such that an amorphous polymer phase is present and mixes with fullerene. This necessitates the inclusion of a mixed phase when describing the morphological of the BHJ structure.[5–25] Although the mixed phase has been identified within the BHJ, a recent debate on the mixed morphology has arisen. Specially, there is almost equal evidence to support both a two-phase model (pure polymers or pure fullerenes with polymer/fullerene mixed phases)[5–13] and a three-phase model (pure polymers and pure fullerenes with polymer/fullerene mixed phases).[14–25] The concept of the mixed phase challenges to some extent the notions regarding the operation of organic solar cells. It is generally hypothesized that excitons originating in a pure phase diffuse to a donor–acceptor interface and split into separated charges.[26,27] Mixed phases can improve exciton splitting.[24] Since the amorphous materials within mixed phases are expected to have a larger band gap than pure phases, this creates an energetic driving force for carriers to migrate away from mixed regions.[8,28,29] On the other hand, the mixed phase negatively affects charge transport within the solid. Impurities can affect charge carrier mobility by orders of magnitude, and consequently, modulate nongeminate charge recombination.[19,30–34] For these reasons, the role that mixed phases have on the device performance remains poorly understood. Obtaining the answers to the fundamental questions mentioned above is only possible with rational manipulation of phase purity within the active layer. Due to the self-assembled Dr. H. Yan, Y. Song, G. R. McKeown, Prof. D. S. Seferos Department of Chemistry University of Toronto 80 St. George Street, Toronto, Ontario M5S 3H6 Canada E-mail: [email protected] Prof. G. D. Scholes Department of Chemistry Princeton University Washington Road, Princeton, NJ 08544, USA E-mail: [email protected]

DOI: 10.1002/adma.201501065

Adv. Mater. 2015, DOI: 10.1002/adma.201501065

nature of the BHJ, the morphology has only been controlled indirectly by external parameters such as the solvent, annealing conditions, additives, and the materials themselves. This leads to materials that are usually far from equilibrium. Herein, we utilize directed self-assembly to control the morphology of a typical BHJ. By self-assembling polymers into nanowires (NWs) and mixing them with unassembled polymers and fullerenes, we are able to control the relative composition of pure and mixed phases. We use a variety of morphological, photophysical, and electronic measurements to identify the relationships between specific structures and their solar cell properties. Previously, we have synthesized novel selenophene– thiophene copolymer nanowires and demonstrated their improved conversion efficiency in polymer nanowire solar cells.[35] Despite the increased short-circuit current, the reduced open-circuit voltage limits their further success in photovoltaic devices. In this study, we choose a statistical copolymer with a previously optimized thiophene to selenophene ratio of 80:20 (Figure 1a; detailed information can be found in the Experimental Section and Figure S1, Supporting Information), and combine this copolymer with Indene-C60 bisadduct (ICBA) to increase the open-circuit voltage of the cell.[36,37] This polymer was allowed to self-assemble into nanowires by an ultrasonicassisted method.[38] We choose toluene as a marginal solvent and obtain polymer nanowires by subtly controlling the sonication and aging time as well as the temperature. The process is monitored by the color change in the solution and absorption spectroscopy (Figure 1a). After self-assembly, the film absorption peak shifts from 535 to 570 nm with significantly increased vibrational peaks. Nanowire formation is further confirmed by atomic force microscopy (AFM) (Figure S2, Supporting Information). To manipulate the phase purity, we blended various amounts of unassembled polymer with ICBA, and then added this mixture to the assembled polymer nanowires. The control that this procedure lends to the film microstructure can be visualized by AFM (Figure 1b–e). Compared with topography (Figure S3, Supporting Information), more details are provided by phase information. For the polymer/fullerene system the brighter domain is assigned to polymer crystallites.[39] For all films, we observe distinct polymer nanowires uniformly dispersed in ICBA. Phase contrast changes with unassembled polymer incorporation and varies the phase purity. To clearly demonstrate the phase evolution, we derive cross-section profiles from the phase images (Figure 1f). Sharp phase boundaries with large peak-to-valley contrast are present in (100% NWs + 0% Poly), revealing very pure donor and acceptor domains. The sectional profile of (90% NWs + 10% Poly) shows broader peaks with smaller phase contrast, and we can infer that some amorphous polymer aggregates beside the NWs. Increasing the unassembled content to 25% further decreases the phase

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Figure 1. a) Chemical structure of P37S-stat-P3HT (80:20) used in this study and film absorption spectrum of assembled and unassembled polymers. The inset shows the color change in solution. b–e) Phase images obtained by tapping-mode AFM (2 µm × 2 µm) with different polymer phase purity: b) 100% NWs + 0% Poly; c) 90% NWs + 10% Poly; d) 75% NWs + 25% Poly; e) 50% NWs + 50% Poly. f) Corresponding cross-sections of (b–e). g) WAXD scattering pattern of (100% NWs + 0% Poly). The inset shows the corresponding 2D-WAXD patterns. h) Relative photoluminescence quenching extent for polymer/ICBA blend films with various phase purities.

contrast, and we attribute this to the increased polymer in the ICBA matrix. When adding equal amounts of unassembled and assembled polymer (50% NWs + 50% Poly), the phase contrast reverses due to irregular polymer aggregates. Based on these results, we can draw certain conclusions about the morphology evolution in our samples. The morphology undertakes four stages: pure NWs + pure ICBA for (100% NWs + 0% Poly); pure NWs + a thin polymer sheath layer + pure ICBA for (90% NWs + 10% Poly); pure NWs + mixed ICBA for (75% NWs + 25% Poly); pure NWs + mixed ICBA + irregular donor aggregates for (50% NWs + 50% Poly). To lend support to these assignments, the morphologies of polymer nanowires with the same various unassembled contents but lacking ICBA were also prepared and analyzed (Figures S4 and S5, Supporting Information), and the results are consistent with the ICBA-containing films. Further support for this morphology evolution is discussed below.

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To examine the crystallinity and molecular orientation, we performed two-dimension wide angle X-ray diffraction (2DWAXD) measurements on all films (Figure 1g for 100% NWs + 0% Poly; Figure S6, Supporting Information for other samples). The 2D diffraction patterns suggest that polymer chains preferentially pack in an “edge-on” orientation, but these peaks are spread into a partial arc shape indicating there is a distribution of molecular orientations. The diffraction peak at 2θ ≈ 5.4° was integrated to further determine the polymer orientation distribution. The full width at half maxima (FWHM) and central peak position of the diffraction (Figure S7 and Table S1, Supporting Information) show that all polymers have similar orientation, ≈89° to the substrate. Grain size analysis (Table S2, Supporting Information) from the FWHM shows that all samples have similar [100] coherence length, approximately 15 nm. From this, we conclude that the unassembled polymers are




Materials

Jsc Voc [mA cm−2] [V]

FF [%]

PCEmax [%]

PCEavga) Long rise time [%] [ps]

100% NWs + 0% Poly

9.66

0.81 61.34

4.8b)

4.6 ± 0.1

38

90% NWs + 10% Poly

10.62

0.80 61.05

5.2b)

5.0 ± 0.1

35

75% NWs + 25% Poly

10.57

0.79 54.28

4.5

4.3 ± 0.2

25

2.9 ± 0.1

15

50% NWs + 50% Poly

7.24

0.77 55.75

3.1

b)

a) Average PCE of 15 devices fabricated under identical conditions ±1 standard deviation (σ); b)Maximum lies outside ±1σ.

mainly amorphous and do not alter the crystal orientation or grain size. It is most likely an amorphous deposition layer that associates with the NWs. The intermixing of polymer and ICBA is another important issue to depict the blend film morphology. Photoluminescence (PL) experiments were conducted to further understand mixing in the samples prepared here. Due to the charge transfer process at the donor–acceptor interface, PL quenching can be employed to estimate the relative extent of mixing between the polymer and ICBA. The PL spectra of pure polymer with different assembled to unassembled ratios and the corresponding ICBA/blend samples were collected (Figure S8, Supporting Information). In each case, the films are excited at 570 nm. The PL intensity decreases linearly until the content of unassembled polymer reaches 25% (Figure 1h). We conclude that at these concentrations the unassembled polymers uniformly mix with ICBA. At greater than 25% unassembled NWs the curve deviates from linearity. This suggests that after a certain point the unassembled polymer can no longer uniformly mix with ICBA, and pure polymer aggregation takes place. The PL results are consistent with AFM observations discussed above and our proposed morphology evolution and their mixed phases in these samples. After manipulating and characterizing the morphology of these samples, we then fabricated photovoltaic devices to test whether the different morphologies lead to different performances. The photovoltaic performance data, which include the short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE) values, were determined (Table 1). All the devices show typical rectifying curves

under 100 mW cm−2 illumination (Figure 2a). The (100% NWs + 0% Poly) device has 4.8% PCE, which is higher than our previous report of 4.1% when blending with phenyl-C71-butyric acid methyl ester (PC71BM). The Jsc here is 9.66 mA cm−2, which is smaller than with PC71BM (11.47 mA cm−2), but the Voc of 0.81 V is much larger than with PC71BM (0.55 V) and this leads to an overall improved PCE.[35] Importantly, the ICBA-based device also has a high FF of 61.34%, similar to PC71BM. This reveals a significant advantage for nanowiretype solar cells. They work well with novel fullerene derivatives, which are somewhat different than amorphous narrow bandgap polymers. The most significant finding in the present work is that incorporation of 10% unassembled polymer is more efficient than a two-phase structure, which contains pure polymer nanowires and pure ICBA. The best conversion efficiency is 5.2%. Jsc increases from 9.66 to 10.62 mA cm−2; at the same time, the Voc only decreases from 0.81 to 0.80 V. With further addition of unassembled content, the PCE value drops to 4.5%, and finally to 3.1%. The trend continues further and we observe a 2.2% PCE when 100% of the polymers are unassembled (Figure S9, Supporting Information). The PCE decay is mainly ascribed to the loss of FF and Jsc. We observe similar trends in the more classic poly(3-hexylthiophene)/PC71BM (P3HT/ PCBM) nanowire solar cells (Table S3, Supporting Information), where 10% unassembled leads to the best performance. This illustrates the generality of this important finding. To confirm the Jsc variation, we measured the external quantum efficiency (EQE) spectrum to study the spectral response. The EQE curves cover a broad wavelength range from 300 to 750 nm. With increasing unassembled content, a blueshifted peak position and decreased relative intensities of vibration peaks are observed. From these EQE plots, the calculated current densities are 9.59 mA cm−2 for (100% NWs + 0% Poly), 10.55 mA cm−2 for (90% NWs + 10% Poly), 10.52 mA cm−2 for (75% NWs + 25% Poly), and 7.19 mA cm−2 for (50% NWs + 50% Poly), which are consistent with the J–V curves. The effects of a mixed phase on device performances originate from the photophysical dynamics and this was studied next. To understand the photoexcited dynamics, we performed broadband pump-probe measurements on all samples. We used an excitation pulse centered at ≈600 nm with a spectral range from 540 to 710 nm. The incidence fluence was

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Table 1. Photovoltaic performances of polymer nanowire solar cells containing various mixed phases with ICBA and the corresponding long rise time from pump–probe measurement.

Figure 2. Photovoltaic performances of polymer nanowire solar cells composed of different ratios of assembled to unassembled polymers blended with ICBA: a) J–V curves and b) corresponding EQE plots.




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Figure 3. Transient absorption measurements of (90% NWs + 10% Poly) blends and corresponding results of global and target analysis. a) Transient absorption spectra as a function of pump-probe time delay and probe wavelength. b) Global fits of the transient spectra at select pump-probe time delays. c) Species-associated spectra. d) Time evolution of the corresponding species-associated spectra. Legends in (c) and (d): Ex @ D/A and Ex @ D denote the exciton photogenerated at the donor/acceptor (or polymer/ICBA) interface and inside the donor (or polymer) domain, respectively. CT denotes the charge-transfer product.

3.6 × 1013 photons cm−2. Note that broadband pump pulse used in our experiments only partially overlaps with the absorption spectrum of the blends. Thus, the absorbed fluence in our experiments is ≈0.7 × 1013 photons cm−2, which is below the threshold for exciton-charge/hole annihilation.[40,41] The transient absorption spectra of four different blends and two neat polymer films are summarized in Figure 3a and Figures S10–S12, Supporting Information. The transient absorption spectra exhibit two positive-going peaks at 578 and 630 nm, which correspond to the 0–1 and 0–0 transitions in the absorption spectrum, respectively. A broad negative-going feature ranging from 670 to 700 nm (at long wavelengths, limited by the spectral range of the probe pulse) grows in from 10 to 100 ps. The negative-going feature is attributed to the photoinduced absorption (PA) of the polymer hole polaron (i.e., a charge-transfer product) on the basis of previous studies.[42–44] This observation indicates that electron transfer occurs on the time scales of several tens of picoseconds. Compared with the transient absorption spectra of neat films (Figures S13 and S14, Supporting Information), another noticeable feature in the transient absorption spectra of (90% NWs + 10% Poly) is that the positive-going features from 670 to 700 nm are reduced to almost zero in the first picosecond. Since the PA of the hole polaron and stimulated emission overlaps within this spectral range, we attribute the reduction of the positive signal to the fast electron transfer from excitons photogenerated near ICBA.

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To more clearly distinguish different pathways of polaron formation, we performed a combined global and target analysis of the transient data. As discussed already, the polymer/fullerene blends are comprised of both a pure polymer phase as well as a mixed polymer/fullerene phase. Therefore, we used a two-step kinetic model (Scheme 1). Specifically, we divide the initially photogenerated excitons into two groups: i) excitons photogenerated near ICBA (i.e., mostly in the mixed phase) and ii) excitons photogenerated inside polymer domains. Figure 3b–d displays global fits of the transient spectra at selected population times, along with the species associated spectra and their time evolutions. The fits show that the species-associated spectrum corresponding to the charge-transfer product has two rise time constants: 910 fs and 35 ps. The short time constant is on the same time scale as charge transfer to the acceptor in polymer/ fullerene blends,[45–50] whereas the long time constant is on the same time scale as exciton diffusion.[47,48,51] Thus, we assign the short time constant to charge transfer by excitons from group (i) and the long-time constant to exciton diffusion limited charge transfer, characteristic of group (ii) excitons. To understand how the morphology affects photoexcited dynamics, we compare the global target analysis of the four different samples (Table S4, Supporting Information and Table 1). The time constant of the fast process ranges from ≈500 to ≈900 fs. However, the time constants of the slow process increase with the percentage of nanowires (from 50% to 100%). The slow time constants observed in the pump-probe




experiment reflect the average diffusion length of the exciton before meeting the ICBA. Thus, the difference of slow time constants might indicate that the average diffusion length decreases with additional fractional percentage of mixed phase. The reduced diffusion length is highly consistent with the mixing behavior of unassembled polymer with ICBA, and may potentially be beneficial for charge separation before recombination. The recombination of photogenerated charges in the active layer reduces the Jsc and FF. To better understand the Jsc and FF variation with the addition of unassembled polymers, we measure how the J–V characteristics depend on light intensity.[52–54] We tested all four samples with illumination

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Scheme 1. Kinetic model used in global and target analysis.

intensities varying from 20 to 100 mW cm−2 by a step of 10 mW cm−2 (Figure S15, Supporting Information). Insight into the recombination mechanism can be obtained by measuring Jsc as a function of light intensity. Jsc is proportional to Iα, where I is the light intensity and 0 < α < 1. Under an ideal condition, the nongeminate recombination should be minimized (α ≈ 1), any deviation of α from 1 implies that nongeminate recombination has taken place. Under the open-circuit condition, 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. A slope of thermal voltage (kT/e) shows nongeminate recombination as opposed to a higher slope (2kT/e), which is assigned to a geminate recombination. A similar Jsc slope is obtained for (100% NWs + 0% Poly) and (90% NWs + 10% Poly) (Figure 4a,b and Table 2). The addition of unassembled polymer decreases the Jsc slope to 0.69, and then to 0.64. At the same time, the Voc slope decreases from 1.22 kT/e to 1.05 kT/e with incorporation of 10% unassembled polymer, and saturates with more content. Based on these results, we conclude that the initial PCE increase can be attributed to the suppression of geminate recombination. While the PCE decrease with further addition of unassembled polymer is due to the nongeminate recombination. The essence of nongeminate recombination is the competition between carrier sweep-out by the internal field and the loss of photogenerated carriers by recombination.[52] Carrier sweep-out is proportional to the magnitude of the internal field and limited by the carrier mobility. To better understand the adverse effect of excess mixed phase on device performance, we measure the hole mobility and calculate the corresponding charge sweep-out time. The hole mobility in blends with ICBA

Figure 4. a,b) Measured Jsc and Voc of polymer nanowire solar cells with various contents of mixed phase plotted against light intensity. c) Scattered plot of hole-mobility values for all material systems. d) Evolution of charge sweep-out time as a function of the content of amorphous polymer.




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www.MaterialsViews.com Table 2. Recombination kinetics of polymer nanowire solar cells containing various mixed phases with ICBA. Materials

Slope of Jsc

Slope of Voc [kT e−1]

Hole mobilityavga) [cm2 V−1 s−1]

100% NWs + 0% Poly

0.76

1.22

1.10 × 10−2

8.40

90% NWs + 10% Poly

0.77

1.05

7.53 × 10−3

12.64

75% NWs + 25% Poly

0.69

1.04

4.01 × 10−3

24.11

50% NWs + 50% Poly

0.64

1.07

1.84 × 10−3

56.98

Sweep-out time [ns]

a)

Average hole mobility of 50 devices fabricated under identical conditions.

(Figure 4c) was measured using hole-only space-charge limited current (SCLC) diode mobility measurements. The (100% NWs + 0% Poly) film has a high average hole mobility of 1.10 × 10−2 cm2 V−1 s−1, with the addition of unassembled polymer in blend films, the average mobility decreases to 7.53 × 10−3 cm2 V−1 s−1 for (90% NWs + 10% Poly), 4.01 × 10−3 cm2 V−1 s−1 for (75% NWs + 25% Poly), and finally 1.84 × 10−3 cm2 V−1 s−1 for (50% NWs + 50% Poly) (Table 2). The corresponding sweep-out time, τsw, is given by τsw = d2/(2µVint), where d is the film thickness, µ is the average mobility, and Vint is the internal potential.[52] Based on the measured mobility, τsw starts from 8.40 ns for (100% NWs + 0% Poly) and continuously increases to 56.98 ns for (50% NWs + 50% Poly) (Figure 4d and Table 2). The increased sweep-out time demonstrates that the carriers recombine in the active layer prior to sweep-out to the external circuit with increasing unassembled content, leading to low Jsc and FF. Therefore, the optimization of a mixed phase in a blend film is actually a compromise between charge-separation and charge-transport. In conclusion, we have fabricated novel polymer solar cell materials where the phase purity of the donor and the acceptor have been systematically varied. This is used to better understand the importance of the mixed phase in these devices. Photovoltaic tests demonstrate that devices with ≈10% of mixed phases outperform pure phase devices. With the aid of photophysical studies, we answer four basic questions about the role of mixed phase: first, for optimized morphology, three-phases outperform two-phases; second, the mixed phase reduces the average exciton diffusion length, which can improve the charge separation; third, the mixed phase suppresses geminate recombination and increases the Jsc and FF. Finally, more than optimal mixed phase has an adverse effect on device performance by inducing nongeminate recombination. Besides clarifying these important issues, combining assembled and unassembled polymers is a novel route to optimize the device performance and is thus a promising strategy for design and manipulation of the active layer of organic electronic materials in general.

Experimental Section General Considerations: All reagents were used as received unless otherwise noted. 2,5-dibromo-3-heptylselenophene was prepared as previously reported. 2,5-dibromo-3-hexylthiophene, i-PrMgCl (2.0 M in tetrahydrofuran (THF)), and Ni(dppp)Cl2 were purchased from

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Sigma-Aldrich. THF was dried using an Innovative Technology solvent purification system. ICBA was purchased from Solarmer Energy Inc., China. P3HT was purchased from Rieke Metals and PC71BM was purchased from American Dye Source. Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS) was purchased from Heraeus (CLEVIOS P VP Al 4083). Instrumentation: 1H NMR was performed using a Varian Mercury 400 (400 MHz) spectrometer. Gel permeation chromatography (GPC) measurements were carried out using a Malvern 350 HT-GPC system at 140 °C with 1,2,4-trichlorobenzene (stabilized with butylated hydroxytoluene). Molecular weights were determined using narrow dispersity polystyrene standards. Absorption spectrum was recorded on a Varian Cary 5000 spectrometer. The photoluminescence spectrum was performed on a Photon Technology International (PTI QuantaMaster 40-F NA spectrofluorometer with a photomultiplier detector and xenon arc lamp source. AFM imaging was carried out on a Veeco Dimension 3000 microscope. 2D diffraction data were collected at the MAX Diffraction Facility at McMaster University. Pump–Probe Spectroscopy: The experimental setup was described in detail previously.[55] Briefly, an 800 nm, 100 fs laser pulse, generated by Spectra-Physics Spitfire Pro was used as the light source in the experiment with a repetition rate of 5 kHz. The 800 nm pulse was directed into a home-built NOPA to generate the broadband pulse centered at ≈600 nm with a spectral range from 540 to 710 nm for pumpprobe. The broadband pulse was then compressed by a combination of a grating compressor and a prism compressor. The pulse was compressed to less than 20 fs (estimated by a frequency-resolved optical gating (FROG) surface).[56] The compressed broadband pulse was split into two beams—pump and probe beam—and the time delay between them was controlled by the motorized translation stage. The pump pulse was chopped at a frequency of 625 Hz. Thus, the pump-probe signal captured by the camera was averaged over the signals generated by four continuous pulses. The pump and probe beams were crossed at the sample position to generate the signal. The polarizations of the pump and the probe beams were at the magic angle. The pump–probe signal was detected by a CCD Newton camera (Andor DU971N-FI Newton). The signal was balanced by a photodiode recording the probe intensity fluctuations to compensate for laser intensity fluctuations. Global target analysis was performed by using Glotaran.[57] 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 mmol) were added to a Schlenk flask and then placed under vacuum for at least 20 min. Dry THF (4.7 mL) was added to the flask followed by i-PrMgCl (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, the reaction heated to 40 °C for 20 min, and then quenched with the 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 solvent. The solvent was removed under reduced pressure to give the polymer as a purple solid (175 mg, 59%). 1 H 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 (1,2,4-trichlorobenzene, 140 °C) Mn = 20.9 kDa, Mw = 26.4 kDa, Ð = 1.26. 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. The statistical copolymer with a thiophene-to-selenophene ratio of 80:20 was dissolved in toluene with total polymer concentrations equal to 15 mg mL−1, and heated to 80 °C for 3 h. Part of the solution was then immediately transferred to an ultrasonic cleaning bath. After the solution turned dark, sonication was continued for another 10 min. The temperature of the water in the ultrasonic cleaning bath was kept




Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements D.S.S. acknowledges financial support for this work from NSERC, the CFI, DuPont, and the Alfred P. Sloan Foundation. G.D.S. acknowledges financial support for this work from the Natural Sciences and Engineering Research Council of Canada (through NSERC Polanyi Award). Received: March 3, 2015 Revised: April 2, 2015 Published online:

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constant at 15 °C during the sonication process. After sonication, the solution was aged in a glove box for another 3 h. ICBA and toluene were blended with a certain amount of unassembled polymer solution according to ratio requisition. The total concentration of ICBA in the blend solution is 50 mg mL−1. Finally, blend the polymer/ICBA solution with assembled polymer dispersion for different ratios and keep the polymer to ICBA ratio as 1:1.5. Add 3% (volume) CN in mixed solutions and stir for another 1 h at room temperature before use. The solar cell devices were fabricated by spin-coating (600 rpm) the active layer on PEDOT:PSS coated ITO in a N2-filled glove box. 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 fabrication process of P3HT/PCBM nanowire solar cells is exactly the same as the method described above. The device area is 0.07 cm2 as defined by shadow mask. I–V curves were obtained by a Keithley 2400 source meter under simulated AM 1.5G condition with power intensity of 100 mW cm−2. The mismatch of spectrum was calibrated using Si diode with a KG-5 filter. Light-intensity dependent I–V curves were carried out by adjusting sample-light source distance and calibrated with the current of 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.

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