Molecular Orientation and Performance of Nanoimprinted Polymer

Dec 4, 2014 - a typical donor material.6,7 Reorientation of the polymer backbone ... master and nanostructured P3HT/PCBM films imprinted at. 100 and 1...
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Molecular Orientation and Performance of Nanoimprinted PolymerBased Blend Thin Film Solar Cells Xinhui Lu,*,†,§ Htay Hlaing,†,∥ Chang-Yong Nam,‡ Kevin G. Yager,‡ Charles T. Black,‡ and Benjamin M. Ocko*,† †

Condensed Matter Physics and Materials Science Department, and ‡Center for Functional Nanomaterials, Brookhaven National Laboratory, Brookhaven National Laboratory, Upton, New York 11973, United States § Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong ∥ Department of Physics and Astronomy, State University of New York, Stony Brook, New York 11794, United States S Supporting Information *

ABSTRACT: In this work, we have used synchrotron-based grazing incidence X-ray scattering to measure the molecular orientation and morphology of nanostructured thin films of blended poly(3-hexylthiophene)/[6,6]-phenyl C61-butyric acid methyl ester blends patterned with nanoimprint lithography. Imprinting the blend films at 150 °C results in significant polymer chain orientational anisotropy, in contrast to patterning the film at only 100 °C. The temperaturedependent evolution of the X-ray scattering data reveals that the imprint-induced polymer reorientation remains at high temperatures even after the patterned topographic features vanish upon melting. Photovoltaic devices fabricated from the blend films imprinted at 150 °C exhibit a ∼21% improvement in power conversion efficiency compared to those imprinted at 100 °C, consistent with a polymer chain configuration better suited to charge carrier collection.

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INTRODUCTION Organic photovoltaic (OPV) devices hold promise because of their potential advantages of low cost manufacturing and lightweight and flexible form factors. Record OPV power conversion efficiencies (PCE) have steadily increased to now more than 10%.1 These high-performing OPV devices often employ a bulk heterojunction (BHJ) architecture, in which electron donor and acceptor materials are highly intermixed in order to increase their interfacial area and decouple the photoexcited exciton diffusion length from the active layer thickness. Isolated and disordered domains present in the BHJ can contribute to unproductive charge recombination in the device. A nanointerdigitated donor−acceptor structure, composed of interconnected electron donor−acceptor phases with well-defined interfaces, has been proposed to overcome these limitations,2−4 and simulations suggest that this structure can greatly improve the device’s internal and external quantum efficiencies, thus allowing increased active layer thickness for greater light absorption and better photovoltaic performance.2 Nanoimprint lithography (NIL) has proven to be an effective approach for high resolution patterning of organic thin films with good fidelity.5 This method can also increase the molecular ordering and crystallinity in polymer thin films5 and induce a reorientation in poly(3-hexylthiophene) (P3HT), a typical donor material.6,7 Reorientation of the polymer backbone chains can influence the electrical performance of © XXXX American Chemical Society

devices because charge transport is known to be highly anisotropic in these materials.8 Nearly all structural studies of imprinted OPV materials have been on the pure donor polymers,5−7,9 and only a few studies have been on the direct imprinting of donor and acceptor blends.10,11 In these studies, device performance improvement has been attributed to the enhanced light trapping associated with the shallow surface features created by the imprinting, and the lack of structural information did not allow correlations between molecular orientation and performance. In addition, nanostructuring the electrodes was also found to improve the device performance due to enhanced charge collection and conductivity.12,13 In this work, we have studied the influence of the imprint process on the molecular structure of blended P3HT/[6,6]phenyl C61-butyric acid methyl ester (PCBM) patterned thin films. Here, we have used a silicon imprint master with 140 nm grating period and 50 nm feature height (Figure 1a) to pattern the organic materials. Through synchrotron-based grazing incidence X-ray scattering measurements, we find that the nanoimprinting process induces the polymer chain reorientation in the BHJ blend. Compared to a pure P3HT film without PCBM, the blend films show greater overall P3HT chain Received: August 11, 2014 Revised: December 4, 2014

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Figure 1. Seventy degree cross-sectional SEM images of (a) the silicon master with grating pattern and a full cross-section (inset); (b) the P3HT/ PCBM film imprinted at 100 °C; and (c) the P3HT/PCBM film imprinted at 150 °C. Corresponding GISAXS patterns for P3HT/PCBM films imprinted at (d) 100 °C and (e) 150 °C.

spaced vertical scattering features, known as Bragg rods (BRs), and each of these rods exhibits a modulation that originates from interference between X-rays scattered from pattern top and bottom interfaces. The BR spacing, which determines the pitch of the imprinted pattern, is 0.00453 Å−1 for blend films imprinted at both 100 and 150 °C. This value corresponds to a 1387 Å feature pitch, consistent with the dimension of the silicon master. The modulation in scattering intensity along the BRs defines the imprinted trapezoid’s depth, which is 454 Å for the blend film imprinted at 100 °C and slightly shallower, 385 Å, for the blend film imprinted at 150 °C. The phase shift between different orders of BRs determines the pattern average sidewall angle: for a sidewall angle of γ, the same order diffraction maxima on each BR form a straight line with a slope of (90 − γ)°.15 GISAXS analysis was performed by fitting the first order and second order Bragg peaks along the BRs assuming a simple trapezoidal form factor model (Supporting Information, Figure S1). The average sidewall angle of the grating pattern imprinted at 100 °C is 45°, while patterns imprinted at 150 °C have a slightly steeper sidewall angle of 53° (Supporting Information, Figure S1). The differences in the imprinted structures at the two temperatures may be caused by the formation of different PCBM crystalline domain sizes at the two imprinting temperatures, as evidenced by the qualitatively rougher appearance of 150 °C imprinted blends in SEM (Figure 1c). Nevertheless, imprinting the blend at both temperatures transfers the profile of the Si master with good

realignment and a degree of azimuthal alignment. We correlate the changes in blend internal molecular reorientation to the materials’ photovoltaic performance by fabricating and measuring nanoimprinted P3HT/PCBM BHJ solar cells.

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RESULTS AND DISCUSSION

We investigated the structural profiles of the silicon imprint master and nanostructured P3HT/PCBM films imprinted at 100 and 150 °C by employing both scanning electron microscopy (SEM) (Figure 1a−c) and grazing incidence small-angle X-ray scattering (GISAXS) (Figure 1d,e). (Details of the imprinting procedure are presented in the Methods section and in our previous studies of imprinted P3HT7 and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS).14) While SEM gives a direct view of the local structure in real space, GISAXS provides complementary information in reciprocal space over macroscopic regions. The cross-sectional SEM image of the Si master (Figure 1a inset) shows a trapezoid-like grating cross-section with rounded tops, whereas the imprinted blends (Figure 1b,c inset) show inverted cross-sections with rounded grooves and flat grating tops, indicating a reasonable degree of transfer fidelity. The P3HT/PCBM is patterned to a depth of about 45 nm, with an underlying unpatterned residual layer of about 50 nm. Analysis of GISAXS patterns provides quantitative information on the average feature sizes in the imprinted P3HT/ PCBM films. The GISAXS patterns (Figure 1d,e) exhibit evenly B

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Figure 2. 2D GIWAXS images of P3HT/PCBM blend films imprinted with grating patterns at (a,b) 100 °C and (c,d) at 150 °C. The incident X-rays are parallel to the grating grooves (ϕ = 0°) in panels a,c and perpendicular (ϕ = 90°) in b,d. (e) Schematic P3HT backbone orientations from three different perspectives. (f−h) Schematic P3HT molecular orientations view in the grating groove direction (left) and perpendicular to gratings (right) for (f) “face-on,” (g) “edge-on,” and (h) “vertical” oriented P3HT domains. The scattering data for 150 °C is consistent with a combination of the structures shown in f and g.

(|q| = 1.70 Å−1) implies an offset of 7.1°. This is less than the ϕ width of the scattering profile. For the blend film imprinted at 100 °C, the scattering patterns obtained for the X-ray beam parallel and perpendicular to the grooves (ϕ = 0 and 90°) are nearly identical (Figure 2a,b), indicating that imprinting induces no preferential azimuthal molecular orientation at this temperature. This imprinted sample exhibits a uniform ring of scattering at 0.39 Å−1 (16.1 Å, layer spacing), indicating that randomly oriented P3HT domains are introduced by the imprinting process (Figure 2a,b). The orientation distribution here is much broader than typically seen for planar blend thin films, which concentrate along the qz axis.6 The additional scattering intensity along the qr axis arising from in-plane lamellar stacking suggests a far greater “face-on” population in the imprinted film than a typical planar film. Likewise, the scattering arc at 1.70 Å−1 is the same for both azimuthal groove directions. This arc corresponds to the π−π stacking of the conjugated P3HT backbones with a lattice constant of 3.70 Å. The P3HT π−π peak is more intense along the qz axis, spanning an angular width of ∼30° half-width-at-half-maximum (HWHM), indicating that the π−π stacking is preferentially aligned along the surface normal direction. The 100 °C results, with a predominately face-on orientation, show that the predominantly uniform “edge-on” order found in planar blend films is disrupted by the imprinting process but that the template grooves do not introduce any measurable anisotropy in the direction of the polymer backbones. Previous measurements of pure P3HT films imprinted with much deeper gratings did not resolve the “face-on” π−π peak,5,6 possibly due to the lower X-ray flux that was available in those measurements. GIWAXS images acquired from blend films imprinted at higher temperature (150 °C) show significant differences

fidelity, resulting in a surface area increase of ∼27% in both cases. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements provide detailed information on the molecular structure in the imprinted blend films. 2D GIWAXS patterns were measured simultaneously with the GISAXS images using the same sample setup. In the presented data (Figure 2), we have adjusted the image color scales so that all four images have similar backgrounds and intensities of the amorphous PCBM peak at |q| ∼ 1.4 Å −1. The scattering pattern from an as-cast blend film (Supporting Information, S2) exhibits three orders of lamellar peaks along qz, similar to previously published results6 and indicating that the lamellar stacking of P3HT in the as-cast blend is preferentially aligned along the surface normal direction. The GIWAXS patterns of imprinted P3HT/PCBM blends films (Figure 2a−d) exhibit more intense lamellar peaks and well-defined π−π peaks indicating enhanced molecular order and crystallinity. This is consistent with both imprinting temperatures falling between previously reported values of the glass transition temperature (∼50 °C) and melting temperature (∼200 °C) of P3HT/PCBM blends.16 In order to investigate the azimuthal alignment of the polymer chains, scattering experiments were performed with the direction of the incident X-ray beam parallel (ϕ = 0°, Figure 2a,c) and perpendicular (ϕ = 90°, Figure 2b,d) to the groove direction.7 Here, ϕ = 0° aligns the imprinted grooves with the incident beam plane, which is determined when the GISAXS pattern is symmetric with respect to the qz axis. It must be noted that the curvature of the Ewald sphere leads to an apparent angle offset between the direct beam and grating direction to maximize a given reciprocal-space peak. This is especially relevant at larger |q| (peaks with a large in-plane component of the Bragg reflection angle). In our experiment, a P3HT (100) peak (|q| = 0.39 Å−1) along the qr axis implies an offset of 1.6°, whereas a π−π peak C

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Figure 3. (a) Lamellar (100) ring at ϕ = 0° measured at 25 °C, 50 °C, 100 °C, and 150 °C (top to bottom panels). (b) Schematic illustrations of P3HT molecular orientation inside the melting gratings. (c) The intensity integral over the polar angles (χ) for the four increasing temperatures.

We now examine the scattering peaks along the qr axis and their azimuthal variance with respect to the groove orientation. The π−π peak along the qr axis clearly exhibits much stronger intensity at ϕ = 90° (Figure 2d) than at ϕ = 0°. This indicates a preferential in-plane π−π stacking along the grating direction. This might either originate from “edge-on” oriented domains with the backbone perpendicular to the groove direction (illustrated in Figure 2g) or from “vertically” oriented domains with the side chain perpendicular to the groove direction (illustrated in Figure 2h). We find that the former is more likely since the latter orientation would produce a more intense lamellar peak along the qr axis than observed in Figure 2d. Accordingly, we will assume negligible contribution of the vertical orientation in the following discussion. The temperature-dependent evolution of the GIWAXS patterns for the P3HT/PCBM films imprinted at 150 °C provides additional information on the nature of the molecular orientation. We heated the 150 °C imprinted sample to three elevated temperatures and waited for 15 min at each temperature before making an X-ray scattering measurement. Figure 3a shows the low-q region of the scattering pattern, in the vicinity of (100) lamellar peak, at ϕ = 0°. The corresponding polar scattering profiles, obtained at the lamellar peak position, shown in Figure 3c, provides information on the polar angular distribution of the lamellar stacking. At 25 and 50 °C, the lamellar intensity peaks at χ ≈ 80°, and this corresponds to the “face-on” oriented regions with the polymer backbone parallel to the groove direction. A similar orientation was also observed in the imprinting of pure P3HT.7 With increasing temperature, the direction of this lamellar peak rotates toward vertical direction. At 100 °C, the peak center has moved to about χ = 50°, and by 150 °C, the peak has moved close to the vertical axis. The GISAXS measurements for the temperature evolution of imprinted films (Supporting Information, Figure S3) indicated the melting process primarily begins at a temperature between 50 and 100 °C through gradual disappearance of higher order scattering peaks, similar

between incident X-rays parallel (ϕ = 0°) and perpendicular (ϕ = 90°) to the grating groove direction (Figure 2c,d). We observe both azimuthal and meridional variations in the scattering intensity, revealing that the P3HT domains reorient and preferentially align both in-plane (relative to the grating direction) and out-of-plane (relative to the substrate normal). Because 150 °C is closer to the P3HT melting temperature,16 there is more thermal energy for polymer chain reorientation. We first present an analysis of the scattering peaks along the qz axis, a direction which is sensitive to P3HT regions with molecular orientations normal to the substrate. Similar to the results in the 100 °C imprinted sample, for both values of ϕ, the presence of the π−π peak along the qz axis suggests the existence of “face-on” oriented domains. The “face-on” arrangement implies that the lamellar peak of these regions must be along the qr axis. We observe the lamellar peak for both ϕ = 0 and 90°, an indication that there is no net azimuthal orientation of P3HT backbones with a “face-on” orientation. Figure 2e and f illustrate two possible azimuthal orientations for the “face-on” domains with polymer backbones parallel or perpendicular to the grooves. The GIWAXS pattern also shows features along the qz axis consistent with P3HT lamellar stacking normal to the substrate, suggesting the coexistence of “edge-on” and “face-on” orientations after imprinting. Although one would normally expect the lamellar peak intensity to be invariant of ϕ rotation, this is only strictly the case when the same point in reciprocal space is probed as we rotate ϕ. However, due to the curvature of the Ewald sphere and nonzero incident angle, the qz axis we measured is always slightly titled from the “true” qz axis. Our results show that the lamellar peak intensity at ϕ = 90° is stronger than the corresponding intensity at ϕ = 0°, suggesting different polar widths for different ϕ directions. Indeed, inspection of Figure 2c and d clearly shows that the polar widths of the (100), (200), and (300) peaks are narrower at ϕ = 90°, consistent with the conserved integrated intensity. D

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Figure 4. (a) Schematic of the imprinted P3HT/PCBM solar cell. (b) Representative J−V characteristics of control devices annealed at 100 °C (blue) and 150 °C (red), and devices imprinted at 100 °C (green) and 150 °C (cyan). Light current is shown as solid curves, and dark current is shown as dashed curves).

Table 1. Averaged Device Performance Metrics with Standard Deviationa Jsc (mA/cm2) 100 100 150 150 a

°C °C °C °C

control imprint control imprint

4.3 3.8 3.9 4.7

± ± ± ±

0.2 0.2 0.1 0.4

Voc (V) 0.58 0.58 0.55 0.58

± ± ± ±

Rs (Ωcm2)

FF

0.01 0.01 0.01 0.01

0.61 0.61 0.55 0.60

± ± ± ±

0.02 0.01 0.01 0.03

16 18 22 18

± ± ± ±

2 2 3 4

PCE (%) 2.2 1.9 1.7 2.3

± ± ± ±

0.2 0.1 0.1 0.2

For 100 and 150 °C control samples, the device characteristics are averaged over 3 devices. For imprinted samples, they are averaged over 5 devices.

In contrast, imprinting the P3HT/PCBM blend at 150 °C improves the average device power conversion efficiency by ∼21% on average (from 1.9% to 2.3%) compared to that of the device imprinted at 100 °C, primarily by increasing the Jsc by ∼24% (from 3.8 to 4.7 mA/cm2) (Table 1). Annealing the control devices in air at 150 °C noticeably degrades their performance by reducing the Voc and fill factor (Figure 4b) and is accompanied by micron-scale PCBM crystallization that we do not observe in devices imprinted at the same temperature (Supporting Information, Figure S4). Analysis of the device dark I−V characteristics shows that despite the topographical similarity between devices imprinted at 100 and 150 °C, the 150 °C imprinted devices appear to have more deep trap states18,19 most likely related with ambient oxygen20,21 in the active layer along with potentially increased carrier recombination, as indicated by the diode ideality factors over 2 and higher reverse-bias dark currents (Supporting Information, Figure S5). Despite these deficiencies, the 150 °C imprinted device produces more photocurrent and has higher efficiency, suggesting that improved carrier extraction within the blend by the molecular reorientation compensates these shortcomings. This improvement, not observed in similarly structured blends imprinted at 100 °C, cannot result from the topographic changes but rather must originate from either differences in the internal P3HT/PCBM phase separation or the measured differences in P3HT molecular orientation. Although the Xray scattering results do not provide direct information regarding the extent of phase separation, the overall higher lamellar peak intensity for the 150 °C imprinted blend over the 100 °C one suggests a higher degree of phase separation at 150 °C. This is consistent with the conclusion of a previous study that 150 °C is close to the optimal annealing temperature for P3HT/PCBM blends.16 For future work, it will be helpful to measure the hole mobility via hole-only diode in order to quantify the effect of polymer alignment. However, deducing the charge carrier mobility from such measurements is

to our previous studies of imprinted P3HT films.6 The reorientation toward the edge-on orientation with increasing temperature (see Figure 3a) appears to follow the concomitant morphological changes of the imprinted blends. This supports our conclusion that the face-on orientation is induced by the imprinting process which creates a considerable side-wall interfacial area. It is likely driven by the favorable energy configuration of the low-energy hexyl groups residing at the interface. It is worth noting that at 150 °C there is still a signature of a lamellar peak along the qr axis. This suggests the remnants of “face-on” oriented domains. These domains might locate within the gratings but not at the side-wall interface as illustrated in Figure 3b, thus not reorienting with the flattening of the grating profiles. Note here, for both 100 and 150 °C imprinted films, the amorphous PCBM peak, located at |q| ∼ 1.4 Å, appears ring-like and smooth, indicating that no preferential orientation of PCBM crystalline domains is formed during imprinting. We measure the combined effects of nanostructuring the P3HT/PCBM blend and changing the internal P3HT molecular orientation on photovoltaic performance by fabricating BHJ solar cells using films imprinted at 100 and 150 °C (schematic in Figure 4a and also Methods). The devices were characterized under simulated AM 1.5G solar illumination. Figure 4b shows representative device characteristics for each type, and the average performance values and standard deviations are shown in Table 1. Imprinting the blend active layer at 100 °C provides no PV performance benefit, decreasing slightly the short circuit current (Jsc) (from to 4.3 to 3.8 mA/ cm2) and overall power conversion efficiency (from 2.2% to 1.9%) compared to those of unimprinted devices with blended active layers processed identically (Figure 4b and Table 1). These measurements suggest that any performance gain in an imprinted device due to P3HT reorientation to a face-on configuration is offset by less absorption due to an effectively thinner active layer after imprinting. E

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Chemistry of Materials complicated by the nonplanar (imprinted) device geometry and would necessitate more sophisticated device modeling in order to infer mobility values. Here, we notice that there is no significant change in open circuit voltage Voc or series resistance Rs (Table 1), which indicates that the hole mobility is not the limiting resistance in the device structure. Another important possibility, based on our X-ray scattering analysis, is the improved hole extraction due to the imprinting-induced preferential polymer backbone alignment perpendicular to the grating wall. The increase of crystallinity could be another possible reason for the improved performance, indicated by the relatively higher lamellar peak intensity of the 150 °C imprinted film. Note, it is nontrivial to quantify the crystallinity of blend films imprinted at the two temperatures from scattering patterns due to the anisotropic molecular packing of the film. Nevertheless, the similar intensities of the π−π scattering peak suggest no significant difference in absorption. The relatively low PCE of our devices is in part due to the limitation of our ambient conditions in the thermal imprinting setup. This leads to unavoidable air contamination during our device fabrication procedure. Further improvement of the device performance is foreseeable by performing the imprinting procedure in a glovebox. Besides, it is also worthwhile to apply this idea to more promising BHJ systems.22,23

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METHODS

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ASSOCIATED CONTENT

Article

Poly(3-hexylthiophene) (P3HT) (American Dye Source, Mw ≈ 20000−70000 g·mol−1) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (American Dye Source, Mw ≈ 910.88 g·mol−1) were separately dissolved in chlorobenzene at a weight ratio of 2%. The solutions were filtered through 0.45 μm PTFE filters after being stirred at 70 °C for 4 h and then mixed at a weight ratio of 1:1 to form a 2 wt % blend solution. The blend solution was spin-cast at 1000 rpm for 45 s on an ITO glass coated with a 20 nm TiO2 by atomic layer deposition (ALD), which results in a 120 nm thick P3HT/PCBM film. The film was then imprinted with a silicon master with grooves of a 140 nm pitch and a 50 nm depth purchased from Light Smyth at 100 or 150 °C for 15 min and cooled to room temperature before stamp removal. The master was coated with a low energy layer (perfluorodecyltrichlorosilane) for easy lifting. The resultant P3HT/ PCBM gratings were covered by spin-casting a volume ratio of 1:2 of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (Sigma-Aldrich) and isopropanol at 3000 rpm for 45 s. Then, the film was placed on a hot plate at 150 °C for 1 min to dry the PEDPT:PSS layer. Finally, a 100 nm Au contact was deposited at a base pressure of 2 × 10−6 Torr using a thermal evaporator. Devices were illuminated by a 150 W solar simulator (Newport) equipped with an AM 1.5G filter, and the J−V characteristics were determined by using an Agilent 4156C precision semiconductor parameter analyzer. The light intensity was calibrated for the 70 mW/cm2 (0.7 SUN) condition by using a calibrated KG6 color filtered Si reference solar cell (Newport) and a spectrometer calibrated for absolute irradiance measurement (Ocean Optics). The GIWAXS and GISAXS experiments were carried out at the X9 undulator beamline at the National Synchrotron Light Source, Brookhaven National Laboratory, where the 13.5 keV photons (λ = 0.9184 Å) were focused to a spot with a height of 80 μm and a width of 200 μm at the sample position. The scattering/sample chamber vacuum was maintained at ∼10−2 Torr in order to reduce beam damage, diffuse scattering, and X-ray absorption. The incident angle is set to ∼0.15° to ensure full penetration of the sample. During the GISAXS measurement, the grating was rotated over a range of azimuthal angle ϕ between ±3° at a constant speed so that the Ewald sphere will sweep over most of the BR plane,24 where ϕ is defined by the angle between the grating grooves and the plane of incident beam and surface normal.

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CONCLUSIONS We have produced high quality and high fidelity P3HT and PCBM blend nanogratings using nanoimprint lithography. Synchrotron based GIWAXS measurements were used to investigate the molecular orientation of imprinted films. Successful imprinting was achieved at temperatures as low as 100 °C. At this temperature, the azimuthal polymer reorientation was not yet established with respect to the grating profile. Such reorientation was observed when the imprinting was performed at 150 °C. Examination of X-ray scattering patterns shows the coexistence of “face-on” and “edge-on” oriented P3HT domains in the blend gratings and that “vertical” oriented P3HT domains must be less populated. At the side-wall interfaces, the polymer backbone of the “faceon” oriented domain is aligned along the grating direction. This is evident by the evolution of the lamellar peak from in-plane to out-of-plane with the melting of the imprinting profiles. There are still remnants of the “face-on” oriented domain with no azimuthal preference after the melting of the grating profiles. It is likely that this population comes from the domains originally far away from the side-wall interfaces. In the “edge-on” oriented domains, the scattering pattern suggests a preferential alignment of the polymer backbone perpendicular to the grating direction. The inverted blend solar cell device fabricated using the 150 °C imprinted blend layers exhibits ∼35% and ∼21% improvement in PCE compared to those of the unimprinted control (annealed at 150 °C) and the 100 °C imprinted devices, respectively. These overall improvements in PV performance coincide with the observed polymer chain reorientation, a configuration favorable for efficient hole extraction, suggesting another important factor that can induce enhancement in PV performance of BHJ solar cells by the nanoimprinting process. This work provides a correlation between polymer packing orientation and OPV device performance. It emphasizes a good native packing structure as an important criteria besides optimal band gap and morphologies in the search of future successful semiconducting polymers.

S Supporting Information *

Intensity profiles and first and second order peak positions; 2D GIWAXS pattern of an as-cast P3HT/PCBM blend film; GISAXS patterns for a 150 °C imprinted film heated at elevated temperatures; SEM image of the imprint boundary of the 150 °C imprinted sample; and log I vs V plots for the control and imprinted devices. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(X.L.) E-mail: [email protected]. *(B.M.O.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the U.S. Department of Energy, Basic Energy Sciences, by the Materials Sciences and Engineering Division (to X.L, H.H., and B.M.O.), which is supported under Contract No. DE-AC02-98CH10886, and through the Center for Functional Nanomaterials (to C.-Y.N., C.T.B., and K.G.Y), which is supported under Contract No. F

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DE-AC02-98CH10886. This work was partially supported by Energy Laboratory Research and Development Initiative at Brookhaven National Laboratories. X.L. also acknowledges the financial support from CUHK Focused Innovation Scheme B (No. 1902034), Theme-based Research Scheme (No. T23407/13-N), and CUHK Direct Grant (No. 4053075).

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REFERENCES

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dx.doi.org/10.1021/cm502950j | Chem. Mater. XXXX, XXX, XXX−XXX