Impact of Low Molecular Weight Poly(3-hexylthiophene)s as

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Impact of Low Molecular Weight Poly(3-hexylthiophene)s as Additives in Organic Photovoltaic Devices Zach D. Seibers,† Thinh P. Le,§ Youngmin Lee,§ Enrique D. Gomez,§,∥ and S. Michael Kilbey, II*,‡ †

Department of Energy Science & Engineering and ‡Departments of Chemistry and Chemical and Biomolecular Engineering University of Tennessee at Knoxville, Knoxville, Tennessee 37996, United States § Department of Chemical Engineering and ∥Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: Despite tremendous progress in using additives to enhance the power conversion efficiency of organic photovoltaic devices, significant challenges remain in controlling the microstructure of the active layer, such as at internal donor−acceptor interfaces. Here, we demonstrate that the addition of low molecular weight poly(3-hexylthiophene)s (low-MW P3HT) to the P3HT/fullerene active layer increases device performance up to 36% over an unmodified control device. Low MW P3HT chains ranging in size from 1.6 to 8.0 kg/mol are blended with 77.5 kg/mol P3HT chains and [6,6]phenyl C61 butyric acid methyl ester (PCBM) fullerenes while keeping P3HT/PCBM ratio constant. Optimal photovoltaic device performance increases are obtained for each additive when incorporated into the bulk heterojunction blend at loading levels that are dependent upon additive MW. Small-angle X-ray scattering and energy-filtered transmission electron microscopy imaging reveal that domain sizes are approximately invariant at low loading levels of the low-MW P3HT additive, and wide-angle X-ray scattering suggests that P3HT crystallinity is unaffected by these additives. These results suggest that oligomeric P3HTs compatibilize donor−acceptor interfaces at low loading levels but coarsen domain structures at higher loading levels and they are consistent with recent simulations results. Although results are specific to the P3HT/PCBM system, the notion that low molecular weight additives can enhance photovoltaic device performance generally provides a new opportunity for improving device performance and operating lifetimes. KEYWORDS: bulk heterojunction morphology, organic photovoltaics, P3HT/PCMB, dispersity, thin films efficiency;2,3 however, even these champion devices fall short of the 15% panel efficiencies that economic studies label as commercially competitive.4 Numerous studies consistently demonstrate that electronic processes that give rise to functioning OPV devices are sensitively linked to the structure at length scales spanning from the nanoscale, where molecular orientation at donor−acceptor interfaces and π-stacking interactions manifest, to the mesoscale, where domain connectivity and phase segregation effects come into play. For example, transmission electron microscopy (TEM) studies reveal the formation of smaller, purer crystalline phases interconnected by long polymer chains in P3HT−PCBM BHJs.5 Although these crystalline domains are small, they feature increased planarity and π-stacking, which increase interchain charge transfer6 at the expense of donor− acceptor interfacial area.7 Increasing the molecular weight of regioregular (rr) P3HT is known to increase the conductivity of these crystalline regions until Mw ≈ 70 kDa, at which point the

1. INTRODUCTION Organic photovoltaic (OPV) devices are a promising alternative to traditional silicon solar cell technology because of their compatibility with high-throughput film processing methods and use of Earth-abundant materials. Overcoming limitations in device performance and lifetime are considered to be pinnacle challenges that, if solved, will enable the widespread deployment of OPV devices for sustainable energy generation. The leading concept in OPV systems is the bulk heterojunction (BHJ) architecture, wherein the donor and acceptor materials are blended, cast, and processed, which under optimized conditions, leads to a bi-continuous network of donor- and acceptor-rich domains that maximize the interfacial area. To understand connections between composition, processing, and performance, mixtures consisting of poly(3-hexylthiophene) (P3HT) as the donor and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as the acceptor have been extensively studied. They are often considered a benchmark system for OPVs, and laboratory-scale devices based on a BHJ blend of P3HT and PCBM can exceed 4.5% power conversion efficiency (PCE).1 The development of new materials and advanced device fabrication procedures has led to OPV devices exceeding 10% © XXXX American Chemical Society

Received: August 29, 2017 Accepted: December 18, 2017

A

DOI: 10.1021/acsami.7b13078 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

and alters PCBM distribution across the depth of the BHJ film.45 Devices containing different amounts of low-MW P3HTs are fabricated and tested under simulated light conditions. The characteristic domain size and crystallinity of P3HT in each film are investigated by wide-angle and small-angle X-ray scattering (WAXS and SAXS, respectively). Energy-filtered transmission electron microscopy (EFTEM) imaging is used to qualitatively assess the intermixing and domain size of each sample.

ability of long, semiflexible P3HT chains to crystallize is inhibited by chain entanglement.8 Investigations of P3HT/ PCBM BHJ films reveal that optimal performance in single-cell devices is achieved at blend compositions that are 1:1 (by weight) P3HT/PCBM.9 However, neutron reflectivity studies conducted by Chen et al. show that the miscibility limit of PCBM in P3HT is far lower: PCBM is miscible at PCBM volume fractions, ϕPCBM, up to ϕPCBM ≈ 20%.10 Because of this miscibility limit, phase segregation occurs upon thermal annealing, which also drives preferential migration of PCBM to the BHJ/anode or BHJ/cathode interfaces.11−17 It also has been suggested that crystallization of P3HT drives this phase segregation: PCBM fullerenes are expunged from growing crystallites, forcing PCBM to the amorphous electrode/BHJ interfaces.10,18−20 This migration is undesirable, as it is often accompanied by increases in P3HT crystallinity and aggregation of PCBM, which degrades device performance.21−24 A variety of polymer25−34 and copolymer30,35−40 additives have been used to improve the morphology and performance of P3HT−PCBM BHJ films. Historically, polymer additives have been successfully used to inhibit macrophase separation in thermoplastic blends with loading levels reaching up to 10% by weight.41−43 In OPV devices, polymeric or copolymeric additives are generally selected or designed to increase OPV device performance by altering BHJ film morphology or by tuning electronic properties of the device, such as work function or conductivity. Table S1 (in the Supporting Information) summarizes numerous polymeric and copolymeric additives based on thiophenes that have been used in P3HT/PCBM BHJ devices and their impact on PCE. Two significant traits emerge from this list: (1) The thienylcontaining additives are generally smaller in size compared to the P3HT donor polymer, and (2) PCE tends to increase at low additive loading, reach a maximum, and then decrease as additive loading level is further increased. The fact that device performance improves despite significant differences in the chemical nature and molecular design of these additives is somewhat remarkable. This recognition motivates several important questions: Is there a common mechanism that drives these performance improvements? Under what conditions do they persist, and how do those additives affect film morphology? Understanding these effects and tradeoffs is crucial for advancing the development of OPV devices, as it would enable control over morphology or guide the design of new and useful additives that improve device performance. In this article, we examine the impact of low molecular weight poly(3-hexylthiophene) additives (low-MW P3HT) on morphology of the active layer and photocurrent generation in P3HT/PCBM BHJ devices. Because these oligomeric additives are chemically similar to the P3HT donor polymers, differing only in their MW, these studies provide insight into the impact of additive size and loading level on BHJ device performance and film morphology. Beyond the broad set of experimental studies showing that P3HT-containing additives improve device performance, this effort is motivated by recent results from large-scale molecular dynamics (MD) simulations, showing that BHJs comprising oligomeric P3HTs (degree-ofpolymerization ≈10) and PCBM form a layered/smectic morphology upon thermal annealing44 and petascale MD simulations (in tandem with neutron reflectometry experiments) demonstrating that small amounts of low-MW P3HT modifies interfaces between donor- and acceptor-rich domains

2. EXPERIMENTAL METHODS 2.1. Synthesis of Low MW P3HT Additives. Low MW P3HTs were synthesized via Grignard metathesis polymerization of 2,5dibromo-3-hexylthiophene (Puyang Huicheng Chemical Co., Ltd.) using the method reported by McCollough et al.46 with adaptations described by Kochemba et al. to ensure end-group fidelity.47,48 In short, freshly distilled monomer is converted to the regiospecific monomer with isopropyl magnesium chloride (i-PrMgCl) in dry tetrahydrofuran (THF) at reflux conditions and titration is used to pinpoint the actual concentration of i-PrMgCl.46,48 After 6 h, the conversion to the active and inactive species is monitored periodically by gas chromatography−mass spectrometry to ensure monomer is not overconverted and the reaction is allowed to cool to room temperature. The degree-of polymerization is controlled by adjusting the molar ratio of active monomer species to the amount of [1,3bis(diphenylphosphino)propane]-dichloronickel(II) catalyst added. Each Kumada catalyst transfer polycondensation (KCTP) reaction was allowed to proceed for 15 min at room temperature before quenching with a 2× molar excess (relative to the Ni catalyst) of 4 M HCl (obtained from Fisher and diluted appropriately). After adding HCl, the mixture was stirred for 2 min and then the polymer was precipitated into cold methanol. The P3HT polymer was recovered by filtration and subjected to a series of Soxhlet extractions in methanol, acetone, and chloroform, except for the 1.6k P3HT, which was recovered by filtering from acetone only. The molecular weight of each purified P3HT was determined relative to PS standards by gel permeation chromatography (GPC) using THF as the mobile phase. The number-average molecular weight, Mn, and regioregularity also were estimated by 1H NMR spectroscopy (Varian, 300 MHz NMR) in CDCl3. All solvents used were obtained from Fisher. 2.2. Photovoltaic Device Performance. Photovoltaic devices were fabricated and tested as previously described.49,50 Briefly, indium tin oxide (ITO)-coated glass substrates were cleaned by sequential rinsing with 1% Aquet detergent solution and water, acetone, and isopropyl alcohol, and dried using filtered N2. The substrates then were cleaned via UV ozonolysis for 30 min. After cleaning, anode buffer layers consisting of a 1:6 blend of poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) at 1.3 wt % in H2O were spin coated onto the ITO-coated glass for 1 min at 1000 rpm. These substrates were then thermally annealed at 110 °C for 15 min to remove any residual water within the polymer thin film. The samples were transferred to a nitrogen-filled glovebox, where P3HT and PCBM were deposited by spin coating from a chlorobenzene solution at 1000 rpm for 3 min. The ratio of P3HT and PCBM was held constant at 1:1 for all BHJ thin films, and the lowMW P3HT additives were added to the matrix P3HT on the basis of the mass of the total P3HT content. A 96% regioregular P3HT with a weight-average molecular weight of 77.5 kg/mol and a dispersity, Đ = 1.9 purchased from Merck was used as the matrix P3HT. The device fabrication was completed by applying Al cathode layers (nominally 1 μm thick) atop each active layer film via metal vapor deposition. While remaining in the N2-filled glovebox, each device (0.16 cm2 in size) was annealed at 165 °C for 15 min and tested under AM 1.5 G 100 mW/ cm2 illumination. Current−voltage characteristics were measured and recorded using a Keithley 2636A source meter. 2.3. Small- and Wide-Angle X-ray Scattering. Prior to film deposition, all silicon substrates were cleaned by sequentially rinsing each substrate with chloroform, acetone, and methanol and then B


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ACS Applied Materials & Interfaces drying with a filtered stream of N2. Films mimicking BHJ blends were cast on silicon substrates in a N2 filled glovebox. After casting, films were annealed at 165 °C for 15 min and allowed to cool to room temperature. X-ray diffraction measurements were collected at Lawrence Berkeley National Laboratory using beamline 7.3.3. 2.4. Energy-Filtered Transmission Electron Microscopy. Samples for EFTEM were prepared by spin-coating the P3HT/ PCBM active layer with different amounts of low-MW P3HT additives on top of PEDOT:PSS-coated silicon substrates (Silicon Sense). Films were then floated off in distilled water and collected on TEM grids. Samples were vacuum-dried overnight. Thermal annealing of TEM samples was carried out in a N2 glovebox at 165 °C for 15 min. EFTEM were conducted on an FEI TITAN G2 at the Materials Research Institute, The Pennsylvania State University. The standard three-window method was employed to obtain sulfur elemental maps.

polycondensation (KCTP). We believe this is due to head-tohead coupling of the first two regiospecific monomers to create the higher oxidation state catalyst needed for the KCTP cycle.51 Because morphology and electronic properties of additivecontaining BHJ films vary with loading level, each of the oligomeric additives were incorporated into BHJ films at 0.1, 0.5, 1.0, 5.0, and 10.0% by weight while maintaining the ratio of (total) P3HT to PCBM constant at 1:1. 3.1. Device Performance Testing. Device performance characteristics as a function of the loading level of each oligomeric P3HT additive are summarized in Table 2. All values, including data for control devices containing no additives, represent the average of six identically prepared devices. Remarkably, an increase in power conversion efficiency (PCE) relative to control devices is observed except for devices having 5 and 10% loading of the 1.6k and 4.4k P3HT additives. These devices exhibit a significant decrease in PCE due to decreases in short circuit current (Jsc) and fill factor (FF). The general behavior that PCE improves up to a certain loading level and decreases thereafter is a pattern that is consistent with several reports that use different P3HT-containing additives; each is purported to act through different mechanisms.29,36 In addition to this general behavior, all of the devices featuring BHJ blends containing low-MW P3HT additives show open circuit voltages that are equal to or greater than the VOC of control devices. Current density−voltage curves for the best performing device from each additive and the control device under light and dark conditions are shown in Figure 1. The shallower, more rounded profile of the dark curves indicates that the BHJ films containing the oligomeric additive have a lower overall shunt resistance in comparison to that of the control devices. Particularly noteworthy is the 0.01 mA/cm2 diode current of the device having the 4.4k additive at 0.5% loading. Among the additive-modified devices tested here, this device displays the highest (average) PCE of 3.40%, which perhaps is due to its low shunt resistance. This performance is followed closely by that of the devices made with the 8.0k P3HT additive at 10 wt %, which have an average PCE of 3.37%. The control devices

3. RESULTS AND DISCUSSION The weight-average and number-average molecular weight, Mw and Mn, respectively, of dispersity and regioregularity of each low-MW P3HT additive used in this study are listed in Table 1. Table 1. Molecular Characteristics of Low MW P3HT Additives Used in This Studya GPC b

additive MW 1.6k 4.4k 8.0k

NMR

Mn

Đ

Mn

% R.R.

1610 4460 8090

1.12 1.15 1.11

920 2490 3590

93.5 97.5 92.0

a

1.6k, 4.4k, and 8.0k are used throughout the text to identify each additive. bAdditives are referred to by their nominal Mn.

Oligomers of three different molecular weights, nominally referred to as 1.6k, 4.4k, and 8.0k, were examined to understand how the chain length of the polymer additive affects the behavior of the BHJ film. Although the synthesis of the P3HT oligomers followed protocols known to give excellent control of end groups,47 NMR spectroscopy shows that the regioregularity of these oligomers is slightly less than 98.5%, which is often obtained for low-MW P3HTs synthesized via the Kumada catalyst transfer

Table 2. Performance Characteristics for BHJ Devices Containing Low-MW P3HT Additives at Different Levels as Well as Control Devices additive control 1.6k

4.4k

8.0k

loading (wt %) 0.1 0.5 1.0 5.0 10 0.1 0.5 1.0 5.0 10 0.1 0.5 1.0 5.0 10 30 50

Jsc (mA/cm2) 7.83 8.70 8.49 8.37 8.03 7.58 7.90 9.53 9.34 8.09 8.40 8.45 7.80 8.30 8.98 9.42 8.28 8.57

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

VOC (V)

0.25 0.28 0.31 0.43 0.15 0.29 0.10 0.26 0.66 0.31 0.43 0.14 0.27 0.13 0.21 0.92 0.34 0.43

0.58 0.59 0.60 0.59 0.58 0.59 0.59 0.61 0.58 0.59 0.59 0.61 0.61 0.61 0.62 0.60 0.60 0.66 C

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.25 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

FF 0.54 0.59 0.59 0.60 0.48 0.42 0.55 0.56 0.50 0.49 0.41 0.57 0.53 0.57 0.57 0.57 0.45 0.31

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

PCE (%) 0.01 0.02 0.02 0.03 0.03 0.02 0.02 0.01 0.03 0.07 0.02 0.01 0.01 0.01 0.02 0.04 0.01 0.01

2.54 3.18 3.14 3.16 2.37 1.94 2.67 3.40 2.91 2.36 2.12 3.03 2.61 3.02 3.26 3.37 2.37 1.87

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.10 0.18 0.26 0.11 0.17 0.17 0.14 0.11 0.22 0.32 0.18 0.08 0.19 0.13 0.16 0.17 0.14 0.05


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Figure 1. J−V curves under simulated light (a) and dark conditions (b) of devices made without low molecular weight P3HT (control) and the best performing devices for each additive MW.

having no additives have PCEs of 2.54%, which is typical and consistent with benchmark devices tested in our laboratory.49,52 Thus, these values represent performance improvements in PCE of ∼36% due to incorporation of the low-MW P3HT additives. Although we refrain from speculating as to whether these performance improvements are linked to beneficial morphological changes until results of film metrology are presented, previously reported petascale simulations of Carrillo et al. demonstrate that low-MW P3HT chains effectively migrate to donor−acceptor interfaces, increasing interfacial area and altering domain sizes.44,45 To address whether devices made with the 8.0k P3HT additive would show a decrease in performance at even higher loading levels, an additional set of devices having 30 and 50% loading of the 8.0k P3HT additive was made and tested. These devices showed decreases in performance, exhibiting power conversion efficiencies of 2.37% (±0.14%) and 1.87% (±0.14%), as shown in Table 2. The open-circuit voltage of devices incorporating 50% of the 8.0k P3HT shows an increase in the VOC to 0.66 V and a drop of the fill factor to 0.31, suggesting space charge limitations that enhance the VOC due to recombination.53 3.2. Grazing Incidence (GI)-SAXS. GI-SAXS measurements were used to gain insight into whether the low-MW P3HT additives bring about changes in domain size or spacing or affect donor−acceptor phase separation upon thermal annealing. As seen in Figure 2, horizontal linecuts along qy show two features, the first being a broad shoulder at q ≈ 0.02 and the second being a smaller shoulder at 0.06 Å−1 (where q = 4π sin(θ)/λ), both of which can be linked to characteristic length scales of donor- and acceptor-rich domains within the BHJ thin films.54−56 The characteristic domain spacing, d, can be calculated from the local maxima of each feature by d = 2π/ q. Using this analysis, the features at q = 0.021 and 0.062 in the films with no additives (control films) correspond to domain spacings of 30 and 10 nm, respectively. A separation distance of 30 nm is consistent with the distance between P3HT fibrils,20,49,57 whereas the scattering feature at 10 nm has not been indexed conclusively; we speculate that it may correspond to the size or separation distance associated with fullerene clusters.58,59 Although the features at 30 and 10 nm are apparent in all samples, clear changes in the intensities at low q are observed with addition of low-MW P3HT. The increase in scattering intensities near q = 0.005 Å−1 is attributed to a coarsening of donor or acceptor domains (size and spacing >30 nm). A comparison of the three plots reveals that this increase in domain sizes occurs at different loading levels, depending on

Figure 2. GI-SAXS results for annealed spin cast films of P3HT and PCBM featuring 0.1−10 wt % loadings of 1.6k (a), 4.4k (b), and 8.0k (c) P3HT additives. For comparison purposes, the scattering from an unmodified (control, pink line) BHJ blend is shown in each figure.

additive MW. When the MW of the additive is 1.6k, a higher slope at low q is visible at 10% additive loading. With the larger additives, the upturn at low q occurs at lower loading levels: the system with the 4.4k additive shows an upturn at low q at 5%, whereas the upturn at low q is observed at 0.5 wt % additive in the case of the 8.0k additive. Figure 3a captures a correlation between the molecular weight of the additive and the concentration at which the D


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Figure 3. (a) Loading level at which low-MW P3HT additives induce phase separation, as determined by GI-SAXS, decreases as the MW of the additive increases. (b) Comparison of the average power conversion efficiency (PCE) for devices made with each P3HT additive as a function of additive loading level. The horizontal lines indicate the PCE of a control device without additives.

Figure 4. Two-dimensional GI-WAXS detector images for BHJ films featuring the 1.6k (top), 4.4k (middle), and 8.0k (bottom) P3HT additives is shown as a function of additive loading level (increasing from left to right, according to labels along the top edge of the images). The scattering from an unmodified control sample is shown at the right. The q scales along the abscissa and ordinate of the control image are common to each detector image.

upturn at low q is apparent. From this data, it is clear that the loading limit exhibits an inverse relationship with the molecular weight of the low-MW P3HT. In our thin film samples and as inferred from the increase in the intensity at low q seen in the GI-SAXS measurements that is attributed to coarsening of the domain structure, this change is observed when additive concentration exceeded 10, 5, and 0.5% for the 1.6k, 4.4k, and 8.0k additives, respectively. These results indicate that at low loading levels, the low-MW P3HT additives do not alter the donor or acceptor domain sizes. At higher concentrations, the emergence of larger structures is evident by the increase in scattering intensities at lower q. These scattering features might originate from the growth of pure PCBM clusters or from the additives separating from the P3HT/PCBM mixture. We consider these two possibilities when discussing GI-WAXS data in the next section. As can be seen in Figure 3b, these “loading limits” where the domain structures coarsen appear to correspond to the maximum average PCE recorded for devices containing the 1.6k and 4.4k P3HT additives. Surpassing this limit adversely

affects the PCE of low-MW P3HT-modified devices, ostensibly due to an increase in domain size. This dependence of PCE on domain size is consistent with existing reports of P3HT− PCBM BHJ films.55,56,60−62 On the other hand, devices made with the 8.0k P3HT additive exhibit a different behavior: despite showing the most significant changes in the low q scattering, the performance of the devices made with this additive initially diminished to a level consistent with a control BHJ active layer but then increase as the loading level of the additive is increased from 1 to 10% (with subsequent decreases at higher levels). One possibility for the sustained performance in this range is that 8.0k chains are just long enough to be commensurate with the width of P3HT fibrils, making them more likely to be incorporated into P3HT crystals than P3HT additives of lower molecular weight. However, the incorporation of the low-MW additives could decrease the tie chain density, thereby limiting transport and device performance. Above 0.5 wt % of 8.0k additive, however, GI-SAXS indicates that coarser phase separation occurs, which may aid transport within P3HT domains and therefore counterbalance the effect E


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radially averaging the collected intensity about the center of the beam. As expected, strong peaks at q ≈ 0.4 Å−1 corresponding to the (100) diffraction peak of P3HT chains are observed, indicating that P3HT crystallization occurs in each of the films. For all samples, only the form I polymorph is apparent, even though the form II polymorph is often observed for P3HTs having molecular weights of 2−3 kDa.65 Furthermore, the intensities from control samples and those containing low-MW additives is approximately the same, suggesting that donor P3HT crystallization is roughly unaffected by the presence of the low-MW P3HT additives. In all of the diffraction patterns, no peaks corresponding to crystallization of PCBM are identified, which suggests that although PCBM aggregates, it does not crystallize. The lack of changes in the crystallization of P3HT and the absence of crystalline PCBM features63 suggests that the increases in domain size observed in the GI-SAXS measurements arise from the low-MW P3HTs “swelling” the PCBM-rich domains. The presence of these relatively short P3HT chains would prevent the formation of organized PCBM crystal structures and also account for the observed increase in domain sizes. This interpretation of these results is also consistent with molecular dynamic simulations that have shown that similar low-MW P3HTs readily migrate into the PCBMrich domains of BHJ blends. 3.4. Energy-Filtered TEM. We used energy-filtered transmission electron microscopy (TEM) to examine the morphology of P3HT/PCBM films containing low-MW P3HT additives at different loading levels. As described in the Experimental Methods section, all films were made to mimic BHJ thin films. Figure 6 shows the sulfur elemental maps of a P3HT/PCBM film containing no additives (control film) and films with 1.6k low-MW P3HT at 0.5%, 4.4k low-MW P3HT loading at 0.5%, and 8.0k low-MW P3HT at 0.5 and 10% loading. In these sulfur maps, image intensities are proportional to the sulfur concentration, and the light regions indicate P3HT-rich (sulfur-rich) domains. All samples exhibit similar morphology, suggesting the addition of low-MW P3HT has little or no impact on the overall morphology of the P3HT/ PCBM films. Given that investigations of film morphology suggest that incorporation of the low-MW additive has no effect on film morphology yet PCE tends to increase, it is reasonable to analyze the effect of the low-MW additive on the characteristics of the P3HT blend. Studies of P3HT/PCBM BHJs by Russell et al.,56 who used a series of P3HTs ranging in molecular weight from Mn = 4.6 to 48.0 kDa with narrow dispersity (1.2 ≤ Đ ≥ 1.4) and by Brabec et al.,66 who used different fractions collected after a series of Soxhlet extractions of a disperse, low molecular weight P3HT (having Mn = 11.3 kDa and Đ = 1.9) demonstrate that BHJ devices made with low molecular weight P3HTs are marked by low PCEs relative to those made with high molecular weight P3HT. While devices made only using oligomeric P3HTs (less than 5.0 kDa) suffer due to low charge carrier mobility,66 the situation for devices made with higher molecular weight P3HTs is more complex because molecular weight of the P3HT impacts kinetic processes that are dependent on chain mobility as well as thermodynamic miscibility. This interplay between kinetics and thermodynamics results in morphological differences, giving rise to an optimum PCE at intermediate molecular weights.56 However, the approach taken here, in which a low molecular weight P3HT additive of narrow dispersity is blended with a disperse,

of loss of tie chains. Although the morphology was not examined for devices made with 8.0k P3HT above 10% loading, leaving aspects of this behavior unresolved at this point, the results are statistically significant because OPV performance at each loading level is a composite of multiple (minimum of six) devices. 3.3. GI-WAXS. To examine whether the addition of lowMW P3HT affects crystallization of P3HT or PCBM in the BHJ films, a series of grazing incidence wide-angle X-ray scattering (GI-WAXS) measurements was performed on films mimicking BHJs architecture used in PCE measurements. Figure 4 displays a matrix of two-dimensional (2D) images recorded for the annealed films containing low-MW P3HT additives at various loading levels and for the control BHJ. All of the images collected feature broad halos typically observed for P3HT−PCBM BHJs, which is a result of the abundance of amorphous material. Each image also features an amorphous halo at q ≈ 1.39 Å−1, which corresponds to scattering from PCBM-rich domains that is seen consistently in X-ray studies.63,64 Shown in Figure 5 are plots of the one-dimensional (1D) intensity versus scattering wavevector, q, which is obtained by

Figure 5. One-dimensional GI-WAXS diffraction patterns for films containing the (a) 1.6k, (b) 4.4k, and (c) 8.0k additives at different loading levels. The color scheme used for the additive loading levels is consistently applied and traces have been vertically offset for clarity and to facilitate comparison. F


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Figure 6. Sulfur elemental maps generated from energy-filtered TEM micrographs of (a) control sample and films containing (b) 1.6k at 0.5%, (c) 4.4k at 0.5%, (d) 8.0k at 0.5% and (e) 8.0k at 10.0%. Scale bar is 200 nm.

high molecular weight P3HT matrix, represents a different tack by which molecular weight is changed. To quantify the effect of the low-MW P3HT on the P3HT blend, we invoke the assumption that the matrix P3HT adheres to a Schulz−Zimm distribution and recalculate the dispersity and molecular weight of the mixture by also assuming the lowMW P3HT is monodisperse. The calculated results (see Table S3) show the main impact of the low-MW P3HT is to broaden the dispersity of the P3HT mixture by affecting Mn. Perhaps, more striking are the correlations revealed in Figure 7, which captures the interplay between PCE and the calculated characteristics of the molecular weight distribution irrespective of the low-MW P3HT additive used. Cast in this light and recalling the outcomes of morphological characterizations, Figures 3 and 7 suggest that a certain amount of dispersity in the form of low-MW additives that can populate donor−acceptor interfaces or the PCBM-rich domain is beneficial. This contention is corroborated by results from You et al.,67 who examined the molecular weight dependence of the PCE of BHJ devices made from a blend of an alternating weak-donor−strong-acceptor polymer and PCBM. In a single comparison, they demonstrate that devices made using a mixture of a high- and low-molecular weight alternating copolymers exceeded the performance (>16% increase in PCE) of devices made using a copolymer having the same characteristic Mn but performed worse than the devices made with the polymer of highest molecular weight (∼14% decrease in PCE). (Both “parent” alternating copolymers had Đ ≈ 2.1, whereas the polymer mixture had Đ ≈ 3.2.) They concluded that the low molecular weight polymer entered PCBM-rich domains and improved fundamental processes of exciton harvesting and charge transport, but those benefits were offset by a significant increase in domain size, which decreases interfacial area and detracts from device performance.67 Recognizing that all synthetic polymers

Figure 7. PCE of devices incorporating low-MW P3HT additives at various weight fractions based on Mn and dispersity calculated assuming a Schulz−Zimm distribution for the matrix P3HT and monodispersity for the low-MW additives (blue spheres). Projections (gray markers) in each of the orthogonal planes are also indicated.

are disperse and sequential Soxhlet extractions are used routinely to purify conjugated polymers and remove oligomeric and low molecular weight chains from the distribution, these works highlight not only the importance of molecular weight, but also to the significance of molar mass distribution, which collectively affect the balance between kinetic and thermodynamic factors that govern morphology and performance of BHJ devices. G


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(4) King, R. R.; Law, D. C.; Edmondson, K. M.; Fetzer, C. M.; Kinsey, G. S.; Yoon, H.; Sherif, R. A.; Karam, N. H. 40% Efficient Metamorphic GaInP/GaInAs/Ge Multijunction Solar Cells. Appl. Phys. Lett. 2007, 90, No. 183516. (5) Brinkmann, M.; Rannou, P. Molecular Weight Dependence of Chain Packing and Semicrystalline Structure in Oriented Films of Regioregular Poly(3-hexylthiophene) Revealed by High-Resolution Transmission Electron Microscopy. Macromolecules 2009, 42, 1125− 1130. (6) Zen, A.; Pflaum, J.; Hirschmann, S.; Zhuang, W.; Jaiser, F.; Asawapirom, U.; Rabe, J. P.; Scherf, U.; Neher, D. Effect of Molecular Weight and Annealing of Poly(3-hexylthiophene)s on the Performance of Organic Field-Effect Transistors. Adv. Funct. Mater. 2004, 14, 757− 764. (7) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J. S.; Fréchet, J. M. J. Controlling the Field-Effect Mobility of Regioregular Polythiophene by Changing the Molecular Weight. Adv. Mater. 2003, 15, 1519−1522. (8) Brinkmann, M.; Rannou, P. Effect of Molecular Weight on the Structure and Morphology of Oriented Thin Films of Regioregular Poly(3-hexylthiophene) Grown by Directional Epitaxial Solidification. Adv. Funct. Mater. 2007, 17, 101−108. (9) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley, D. D. C.; Cook, S.; Durrant, J. R. Composition and Annealing Effects in Polythiophene/ Fullerene Solar Cells. J. Mater. Sci. 2005, 40, 1371−1376. (10) Chen, H.; Hegde, R.; Browning, J.; Dadmun, M. D. The Miscibility and Depth Profile of PCBM in P3HT: Thermodynamic Information to Improve Organic Photovoltaics. Phys. Chem. Chem. Phys. 2012, 14, 5635−5641. (11) Liu, H.-J.; Jeng, U.-S.; Yamada, N. L.; Su, A.-C.; Wu, W.-R.; Su, C.-J.; Lin, S.-J.; Wei, K.-H.; Chiu, M.-Y. Surface and Interface Porosity of Polymer/Fullerene-Derivative Thin Films Revealed by Contrast Variation of Neutron and X-ray Reflectivity. Soft Matter 2011, 7, 9276−9282. (12) Parnell, A. J.; Dunbar, A. D. F.; Pearson, A. J.; Staniec, P. A.; Dennison, A. J. C.; Hamamatsu, H.; Skoda, M. W. A.; Lidzey, D. G.; Jones, R. A. L. Depletion of PCBM at the Cathode Interface in P3HT/ PCBM Thin Films as Quantified via Neutron Reflectivity Measurements. Adv. Mater. 2010, 22, 2444−2447. (13) Lee, K. H.; Zhang, Y.; Burn, P. L.; Gentle, I. R.; James, M.; Nelson, A.; Meredith, P. Correlation of Diffusion and Performance in Sequentially Processed P3HT/PCBM Heterojunction Films by TimeResolved Neutron Reflectometry. J. Mater. Chem. C 2013, 1, 2593− 2598. (14) Guralnick, B. W.; Kirby, B. J.; Majkrzak, C. F.; Mackay, M. E. Morphological Characterization of Plastic Solar Cells Using Polarized Neutron Reflectivity. Appl. Phys. Lett. 2013, 102, No. 083305. (15) Kiel, J. W.; Kirby, B. J.; Majkrzak, C. F.; Maranville, B. B.; Mackay, M. E. Nanoparticle Concentration Profile in Polymer-Based Solar Cells. Soft Matter 2010, 6, 641−646. (16) Kiel, J. W.; Mackay, M. E.; Kirby, B. J.; Maranville, B. B.; Majkrzak, C. F. Phase-Sensitive Neutron Reflectometry Measurements Applied in the Study of Photovoltaic Films. J. Chem. Phys. 2010, 133, No. 074902. (17) Kiel, J. W.; Eberle, A. P. R.; Mackay, M. E. Nanoparticle Agglomeration in Polymer-Based Solar Cells. Phys. Rev. Lett. 2010, 105, No. 168701. (18) Kohn, P.; Rong, Z.; Scherer, K. H.; Sepe, A.; Sommer, M.; Müller-Buschbaum, P.; Friend, R. H.; Steiner, U.; Hüttner, S. Crystallization-Induced 10-nm Structure Formation in P3HT/PCBM Blends. Macromolecules 2013, 46, 4002−4013. (19) Wu, W.-R.; Jeng, U.-S.; Su, C.-J.; Wei, K.-H.; Su, M.-S.; Chiu, M.-Y.; Chen, C.-Y.; Su, W.-B.; Su, C.-H.; Su, A.-C. Competition Between Fullerene Aggregation and Poly(3-hexylthiophene) Crystallization Upon Annealing of Bulk Heterojunction Solar Cells. ACS Nano 2011, 5, 6233−6243. (20) Kozub, D. R.; Vakhshouri, K.; Orme, L. M.; Wang, C.; Hexemer, A.; Gomez, E. D. Polymer Crystallization of Partially Miscible

4. CONCLUSIONS Numerous studies have investigated the use of novel additives of different chemistry and size to modify OPV performance. In this study, we probe the effect of P3HT additives that only differ from the donor polymer in regard to their size, and improvements in PCE up to 36% are observed. GI-SAXS studies reveal that there is a critical loading limit that is inversely proportional to the MW of the additive. Exceeding this loading level coarsens BHJ film morphology and induces swelling of domain sizes in the active layer that appear to decrease PCE. No discernible differences in P3HT crystallinity appear in the GI-WAXS measurements for any of the additivemodified films. In total, these findings demonstrate that the physical size and loading levels of an additive, which affect the molar mass distribution of the conjugated polymer used to make the device, are just as important as chemical design. Although these results are specific to BHJ films of P3HT and PCBM, this work presents powerful insight into the potential for low-MW additives to affect BHJ film morphology and device performance that also should extend to higher performing donor-type polymers as they are developed and tested.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13078. GPC and NMR characterizations of low-MW P3HTs, tables summarizing types of oligomeric additives used in P3HT devices and their effect on performance, and table of calculated molecular weight of P3HT mixtures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Enrique D. Gomez: 0000-0001-8942-4480 S. Michael Kilbey II: 0000-0002-9431-1138 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.D.S. acknowledges support from TN-SCORE, a multidisciplinary research and training program sponsored by NSF (NSF EPSCoR EPS 1004083) and from the Bredesen Center at UT-Knoxville. SMKII acknowledges support from NSFCBET (Award no. 1512221). T.P.L., Y.L., and E.D.G. acknowledge support from NSF under Grant no. DMR1609417. This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract DEAC02-05CH11231. Manolis Doxastakis is thanked for helpful discussions.

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REFERENCES

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Impact of Low Molecular Weight Poly(3-hexylthiophene)s as

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