Comparison of the Photovoltaic Characteristics and Nanostructure of

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Comparison of the Photovoltaic Characteristics and Nanostructure of Fullerenes blended with Conjugated Polymers with Siloxane-Terminated and Branched Aliphatic Sidechains Do Hwan Kim, Alexander L. Ayzner, Anthony Lucas Appleton, Kristin Schmidt, Jianguo Mei, Michael F Toney, and Zhenan Bao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm303572d • Publication Date (Web): 28 Dec 2012 Downloaded from http://pubs.acs.org on January 11, 2013

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Chemistry of Materials

Comparison of the Photovoltaic Characteristics and Nanostructure of Fullerenes blended with Conjugated Polymers with Siloxane-Terminated and Branched Aliphatic Sidechains

Do Hwan Kim,1,3,† Alexander L. Ayzner,1,2, † Anthony L. Appleton,1 Kristin Schmidt,2 Jianguo Mei,1 Michael F. Toney,2,* Zhenan Bao1,*

1

Department of Chemical Engineering, Stanford University, 381 North-south Mall, Stanford, California 94305-5025, USA 2

Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA

3

Department of Organic Materials and Fiber Engineering, Soongsil University, Seoul, 156-743, Korea

†

These authors equally contributed to this work.

*Corresponding authors: [email protected], [email protected]

ACS Paragon Plus Environment

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Abstract All-organic bulk heterojunction solar cells based on blends of conjugated polymers with fullerenes have recently surpassed the 8% efficiency mark and are well on their way to the industrially-relevant ~15% threshold. Using a low bandgap conjugated polymer, we have recently shown that polymer sidechain engineering can lead to dramatic improvements in the in-plane charge carrier mobility. In this manuscript, we investigate the effectiveness of siloxy sidechain derivatization in controlling the photovoltaic performance of polymer: [6,6]-phenyl-C[71]-butyric acid methyl ester (PC71BM) blends and hence its influence on charge transport in the out-of-plane direction relevant for organic solar cells. We find that in neat blends, the photocurrent of the polymer with siloxy sidechains (PII2T-Si) is four times greater than in blends using the polymer with branched aliphatic sidechains (PII2T-Ref). This difference is due to a larger out-of-plane hole mobility for PII2T-Si brought about by a largely face-on crystallite orientation as well as more optimal nanoscale polymer:PC71BM mixing. However, upon incorporating a common processing additive 1,8-diiodooctane (DIO) into the spin-casting blend solution and following optimization, PII2T-Ref:PC71BM OPV device performance undergoes a large improvement and becomes the better performing device, almost independent of DIO concentration (>1%). We find that the precise amount of DIO plays a larger role in determining the efficiency of PII2T-Si:PC71BM, and even at its maximum, the device performance lags behind optimized PII2TRef:PC71BM blends. Using a combination of atomic force microscopy and small- and wide-angle X-ray scattering, we are able to elucidate the morphological modifications associated with the DIO-induced changes in both the nanoscale morphology and the molecular packing in blend films.

Keywords: Organic photovoltaics, siloxy sidechain derivatization, DIO, molecular packing


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Introduction Organic photovoltaics (OPVs) based on solution-processed conjugated polymers have attracted considerable attention because of their potential flexibility, inexpensive processing, and light weight.1-3 In particular, the bulk heterojunction (BHJ) architecture has enjoyed wide popularity due to the efficient light harvesting that such a microstructure affords.4-6 However, power conversion efficiencies (PCEs) of BHJ OPVs are strongly affected by spontaneous phase segregation of the donor and acceptor components on the nanometer length scale in the blend film, which leads to a complex coexistence of pure donor and acceptor regions and intermixed regions within the film. The ideal microstructure must strike a fine balance between maximizing the rate of exciton dissociation at the heterojunction, which favors small domains, and minimizing the rate of bimolecular recombination while optimizing charge carrier mobilities, which favor large electron-hole separations (i.e. large domains). We have recently shown a dramatic improvement in the polymer field-effect mobility via side chain engineering using a conjugated backbone that yields good overlap with the solar emission spectrum.7 By changing the solubilizing sidechain from a common branched aliphatic moiety (PII2TRef) to a linear saturated carbon chain terminated by siloxane groups (PII2T-Si), (Scheme 1) we observed a significant decrease in the polymer π-stacking distance between backbones. Since the πstacking direction corresponds to one of the facile charge transport directions in the film, this synthetic strategy affords a concomitant large increase in the charge carrier mobility in the transistor configuration up to 2.48 cm2V-1s-1.7 However, in a BHJ OPV, a decrease in the spacing between the polymer π-stacking planes alone is not sufficient to guarantee an increase in the PCE since the device photocurrent is a complex and sensitive function of a large number of variables.8-10 In the search for optimum morphologies with large internal quantum efficiencies, a sizable region of parameter space needs to be explored. Specifically, variables such as polymer chemical structure,11, 12molecular weight,13, 14 processing variables and conditions (blend ratio,1 concentration,1 solvent additive,15-17 thermal18 and solvent annealing,19 etc.) have been investigated in an attempt at


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improving the PCE towards market viability; however much remains to be done in discerning the path forward to achieve 15% PCE.2

Among the many approaches available for tuning the film

microstructure, incorporation of small mole fractions of solvent additives in the blend solution used to cast the thin film stands out due to its simplicity and reproducible performance enhancement.20,

21

Herein, we explore the impact of varying volume fractions of the common solvent additive 1,8diiodooctane (DIO) on the final nanostructure and molecular orientation of BHJ blend films utilizing PII2T-Ref and PII2T-Si as the donor materials and [6,6]-phenyl-C[71]-butyric acid methyl ester (PC71BM) as the archetypal electron acceptor, and the associated influence on device performance. Prior to adding DIO, we find that PII2T-Si:PC71BM blends show a significantly higher short-circuit current than PII2T-Ref:PC71BM blends. After adding a small amount of DIO, we are able to induce an improvement in the device photocurrent for both polymers. However, despite a closer π-stacking distance, the highest efficiency of PII2T-Si:PC71BM devices is lower than that of PII2T-Ref:PC71BM cells at the optimum DIO volume fraction. In order to gain insight into the above differences, we have examined the blend microstructure and molecular packing in significant detail as a function of DIO content using a combination of atomic force microscopy (AFM), photoluminescence (PL) spectroscopy, and grazing-incidence wide-angle and small-angle X-ray scattering (GIWAXS and GISAXS). With the help of these techniques, we show that for blends with PII2T-Ref, DIO helps to produce a well-defined interpenetrating phase-segregated network, where the average size of the polymer domains is of the same order as the common conjugated polymer exciton diffusion length (10-20 nm).22-26 In contrast, the effect of DIO on PII2T-Si:PC71BM blends is milder and shows a different dependence on DIO solution concentration than PII2TRef:PC71BM blends. Moreover, the phase-segregated domains in these films are significantly larger (> 80nm) than the corresponding domains in PII2T-Ref:PC71BM blend films. We argue that part of the reason for these different trends comes about due to differences in solution-phase aggregation of the two polymers.


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Results and Discussion Solar Cell Device Fabrication and Characterization. Scheme 1 presents the chemical structure of PII2T-Ref, PII2T–Si, and PC71BM. The detailed synthetic procedures and optoelectronic properties for pristine PII2T-Ref and PII2T-Si polymers have been reported in our previous study.7 To determine the impact of solubilizing sidechains on solar cell performance,

photovoltaic

characteristics

were

investigated

with

the

architecture

of

ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al. We used a blend ratio of 1:1 by weight for PII2T-Ref and 1:2 for PII2T-Si, which was determined for each polymer individually as part of an optimization process encompassing a wide range of device manufacturing factors (e.g. solvent, solution concentration, blend ratio, and annealing conditions). (Figure S1 and S2) The BHJ films were deposited from hot chloroform solution prepared at 55°C and had thicknesses on the order of 170nm for PII2TRef:PC71BM and 150nm for PII2T-Si:PC71BM after optimization. We first examined the characteristics of OPVs that were prepared by spin-casting PII2TRef:PC71BM and PII2T-Si:PC71BM blend films from pure chloroform without DIO to directly observe the impact of solubilizing sidechains on device output (from here on referred to as neat blends). Current density-voltage (J-V) characteristics for the two polymer:PC71BM blends under AM1.5G illumination at 100 mWcm-2 are shown in Figure 1. Only modest PCEs of 0.5% for PII2T-Ref:PC71BM device and 1.6% for PII2T-Si:PC71BM device were obtained. However, it is important to point out that in neat films, the short-circuit current (Jsc) of PII2T-Si:PC71BM cells is approximately four times greater than in PII2T-Ref:PC71BM devices. Incorporating an additive to the blend solutions from which the BHJ layers are spin-cast is widely used for the fabrication of more efficient OPVs. Since performance of blends spin-cast from neat solvent solutions differed substantially between the two polymers, it is instructive to examine the manner in which a common solvent additive, i.e. DIO, affects the photovoltaic efficiency of the two polymer:PC71BM blends. Thus, we varied the fraction of DIO from 1 to 6% (v/v) in the blend solution


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for PII2T-Ref:PC71BM and from 0.25 to 3% (v/v) for PII2T-Si:PC71BM. We note that the parameter space spanned by the variables which affect device performance is indeed vast; therefore, out of practical device optimization considerations, our work is limited to a subset of this space, captured by such parameters as the blend ratio, film thickness and DIO content. Figure 2 shows J-V characteristics of BHJ OPV devices made from PII2T-Ref:PC71BM and PII2T-Si:PC71BM blends as a function of DIO content. The relevant photovoltaic parameters obtained from Figure 2 have been summarized in Figure 3a for PII2T-Ref:PC71BM devices and Figure 3b for PII2T-Si:PC71BM devices. First, as shown in Figure 3a, we observed that devices based on PII2T-Ref:PC71BM blends showed an optimal efficiency at 4% DIO with a similar trend with fill factor (FF). In contrast, Jsc drastically increases after incorporation of 1% DIO and then plateaus around 2% DIO. Devices with 4% DIO achieved average PCEs of 5.2%, with the champion device reaching 5.6%. The fact that the photocurrent does not change past 2% DIO but the PCE is maximal at 4% is explained by the maximum in the fill factor, as shown in Figure 3a. External quantum efficiency measurements, as shown in Figure S3, display increased photon harvesting efficiency across all wavelengths after adding DIO, consistent with the broad band illumination experiments in Figure 2. As all blend films show similar light absorption properties (Figure S5), it is likely that this nontrivial change of FF over a range of DIO concentration is affected by subtle changes in the polymer:PC71BM blend morphology and molecular packing. We will address this point in the next section. In stark contrast to the blends described above, PII2T-Si:PC71BM solar cells display a different trend in OPV performance as a function of DIO concentration in Figure 3b. First, the photocurrent roughly doubles upon adding just 0.25% DIO. Both the current and FF quickly reach a maximum between 0.25% and 0.5%, after which both Jsc and FF drop with additional DIO. By 3%, the device characteristics are decreased to or below the neat blend level. The peak efficiency attained for 0.25% DIO was on average 3.5% over 10 devices. However, this value is still lower than that of the best PII2T-Ref:PC71BM devices, which gave a mean PCE of 5.2%. In order to better understand the large


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difference in photocurrent for the neat PII2T-Si:PC71BM blends relative to PII2T-Ref:PC71BM as well as the drastic reversal upon adding DIO, we next go on to examine the blend microstructure in some detail via a combination of atomic force microscopy (AFM) and X-ray scattering.

Blend microstructure. First, we used AFM to investigate the nanoscale topography of the polymer:PC71BM blend films spun from solutions with and without DIO. Figure 4a shows that PII2T-Ref films blended with PC71BM processed from chloroform without DIO exhibit large aggregates with a domain size of 200~400 nm. Increasing the fullerene mole fraction further to a 1:2 polymer:fullerene weight ratio results in concomitant growth of surface features up to a few micrometers. Therefore, we believe that these large features are likely due to fullerene clustering. However, addition of 1% DIO results in a thin film surface with fine nanowire domains with widths on the order of 20 nm (Fig. 4(b)). These morphological features are present for all blend films where DIO was used, irrespective of DIO amount and blending ratio with PC71BM. (Fig. S7 and S8) In contrast, in Figure 4c, the AFM image of a neat PII2T-Si:PC71BM film shows smaller aggregates than the neat PII2T-Ref:PC71BM blend. Upon adding 0.5% DIO, as shown in Figure 4d, PII2T-Si:PC71BM blend shows a network with elongated nanowire bundles on the order of 80 nm in width; this is significantly larger than the features in PII2TRef:PC71BM films after adding DIO. Furthermore, when 3% DIO was incorporated into the PII2TSi:PC71BM blend solution, the film topography became macroscopically much rougher, which made it difficult to obtain good AFM images. Since (surface sensitive) AFM studies do not provide definitive information on the bulk 3D phase separation in BHJ films, we performed grazing incidence X-ray scattering experiments to more clearly elucidate the relationship between photovoltaic performance and blend microstructure. For these experiments, the X-ray incidence angle was chosen to be above the critical angle for total external reflection of the organic film but below that of the Si substrate. This ensured that the beam probed the


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entirety of the organic film with minimal penetration into the inorganic substrate. We start by examining how DIO affects the formation of the interpenetrating morphological network of percolated electronand hole-transporting pathways. A means to probe the blend microstructure in semiconducting polymer:PC71BM films is provided by GISAXS. The scattered X-ray intensity at small angles carries information about electron density inhomogeneities on the mesoscale (of order 10 nm). On this length scale, molecular details of conjugated polymer:PC71BM blends are not visible, making this technique complementary to the crystal packing information provided by GIWAXS.27, 28 Figure 5a shows the reduced 1D GISAXS in-plane cuts near the image horizon for the neat PII2T-Ref:PC71BM and PII2T-Si:PC71BM blend films plotted on a log-log scale. We have subtracted the background due to scattering by the underlying PEDOT:PSS film in the presented data.29 There are no obvious structural features evident in the scattering profiles in either blend, making interpretation challenging. Nevertheless, the intensity of the PII2T-Ref:PC71BM blend is seen to cross that of PII2TSi:PC71BM at low Q (< 1 nm-1). It is expected that at Q values below our measured range, the scattering curves will turn over and reach the so-called Guinier plateau, which roughly characterizes the size of the scattering aggregate structure. In the range of momentum transfer available to us, we were unable to see the Guinier plateau for neat blend samples, indicating scattering aggregate sizes greater than ~ 80 nm. To resolve larger structural features, methods like USAXS or RSoXS have to be performed.6,30 The curve crossing mentioned above may suggest that the Guinier region occurs at lower Q (larger R) for the PII2T-Ref:PC71BM blend than for the PII2T-Si:PC71BM blend. Although these considerations are consistent with the AFM images, which showed large structural features, it must be born in mind that the surface structure need not be representative of the bulk morphology, and the GISAXS intensity in our experiment is an average over the film thickness. In Figure 5b we display the small-angle scattering profiles for thin films incorporating DIO that correspond to the maximal PCE for both polymers. In the PII2T-Ref:PC71BM blend with 4% DIO, the Guinier plateau is visible at low Q, which allows us to estimate an effective radius of gyration of the


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underlying structure giving rise to the scattering. In fact, as shown in the SI, the Guinier regime is evident even at 1% DIO and we obtain a radius of gyration of ~ 22 nm; this value changes slowly but monotonically to reach ~ 28 nm by 6% DIO (see SI). From AFM images it appears that the surface features that resemble polymer nanowires approximately correspond to the effective radius of gyration that we extract from GISAXS data. However, AFM only probes surface features that need not be representative of the bulk morphology, and we emphasize that is unclear from the scattering data alone whether this effective aggregate size corresponds to polymer or fullerene aggregates. The fact that the low-Q plateau is visible for all the PII2T-Ref:PC71BM blend films where DIO was added indicates that the effective domain size is drastically reduced when the additive is used relative to the neat blend film. We note that the extent of PL quenching (shown in the SI) increased in the film spun from a solution with the additive relative to the neat film and varied very little beyond 1 % DIO; this is due to an increase in the quenching interfacial area; this result is entirely consistent with the GISAXS data. In contrast to the maximal PCE blend with PII2T-Ref, the scattering curves for the highest PCE PII2T-Si:PC71BM blend films still do not have a Guinier plateau in our measured Q range. In fact, this was the case regardless of the amount of added DIO (see SI). This indicates that the radius of gyration of the scattering inhomogeneities is considerably larger than in the case of PII2T-Ref:PC71BM blends after adding DIO. Apart from a lack of a Guinier regime, there is another quantitative difference between the two blend films. This is the functional form of the scattered intensity in the Q range from ~0.1 to ~0.7 nm-1for both polymer:PC71BM blends, approximately corresponding to length scales R (~ Q-1) of order 2-10 nm. Concentrating on films that correspond to the maximal PCE devices, on a log-log plot the PII2T-Ref:PC71BM film with 4% yields a slope of -3.7, indicating that the GISAXS intensity scales as Q − p with p = 3.7. For the PII2T-Si:PC71BM with 0.5% DIO , p = 3.1. The fits (in the high Q region) are shown as dashed curves in Figure 5b and are explained below. We expect the electron densities of the crystalline and amorphous polymer regions to be similar in analogy with previous work.31 With this assumption, the blend morphology on the 10-100 nm scale


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can be viewed as an interpenetrating network of nearly pure PC71BM aggregates within a conjugated polymer matrix (both pure, mostly crystalline polymer and amorphous polymer containing some fullerene molecules). Since the interface between these two components is expected to be irregular, it is reasonable to analyze the power law scaling of the scattered intensity within the concept of fractal geometry. 32-34 That is, we assume that in the thin film the interface between the PC71BM and polymer rich components is self-similar when examined over a constrained length scale range (2-10 nm here). For morphologies where the interface exhibits such fractal scaling, the SAXS intensity I ∝ Q − ( 2 D − d s ) , where d s is the fractal dimension of the interfacial surface. 35-36 In analogy with previous work, 37 we interpret the power law decay of the intensity as due to fractal scaling of the fullerene-polymer interface, which yields the fractal dimension of the surface d s ~ 2.3 and d s ~ 2.9 for PII2T-Ref and PII2T-Si blends with DIO, respectively. We explain the meaning of these results in the discussion.

Molecular Packing in Blends Having examined the manner in which the film morphology evolves with DIO for the two polymer:PC71BM blends, we now turn to the impact of DIO on molecular packing of the crystallites in the semicrystalline polymers. Figure 6a shows 2D GIWAXS images for PII2T-Ref:PC71BM blend. Without any DIO, the diffraction pattern of the polymer is very similar to the pure film, as reported previously.7 The film is uniaxially textured, and polymer crystallites form a 2-D powder in the substrate plane. Polymer chains pack in an edge-on manner, such that π-stacking between adjacent chains occur in-plane, corresponding to (0k0) Bragg reflections near the horizon. The sidechains stack out-of-plane, giving rise to the (h00) Bragg rod centered about Qxy = 0. 38,39 A broad ring around 1.3 Å-1 due to scattering by the PC71BM is also visible, indicating isotropically oriented fullerene clusters. Upon adding 2% DIO, we observe a dramatic change in the polymer crystallite orientation distribution. Specifically, the (h00) peak intensity becomes much less concentrated around the Qz axis, and intensity is seen to spread around a Q = (Qxy2+Qz2)1/2 = constant ring. This is consistent with ACS Paragon Plus Environment

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previous reports with similar additives.21 As expected, the (010) reflection, corresponding to the πstacking direction, now shows considerable intensity near the Qz axis. This implies that adding DIO builds in a population of crystallites, where the π-stacking direction is now largely normal to the substrate plane – a situation that is conducive to efficient out-of-plane charge transport in the direction relevant for the photovoltaic device architecture. To determine whether this crystallite re-orientation affects the efficiency of vertical charge carrier transport, we measured the bias dependence of injected current with carrier-selective electrodes. Using the space charge limited current (SCLC) formalism, 40,41 we estimated hole and electron mobility values in the vertical direction, where we have assumed a commonly used functional form for the electric field dependence of the mobility. 42 As shown in Figure 7a, we find that the SCLC hole mobility is correlated with face-on crystalline population density; the hole mobility was found to be around 4.0x10-4 cm2V-1s-1 for PII2TRef:PC71BM device without DIO and 1.3x10-3 cm2V-1s-1 for PII2T-Ref:PC71BM device with 4% DIO, respectively. (Table 1) This suggests that the partial face-on orientation induced by DIO allows charges to traverse the device thickness more efficiently relative to the edge-on texture, which is largely expected. However, whereas the hole mobility increases by three fold, the electron mobility did not change significantly (Table 1). On upon adding 0.5% DIO to the PII2T-Si:PC71BM blend, we also found a increase in the hole mobility relative to the neat blend, though the improvement was smaller than in PII2T-Ref:PC71BM blend films. In addition, the electron mobility increased by a significant factor after adding 0.5% DIO, such that the ratio of the hole to electron mobility decreased from ~ 6 to ~ 2 relative to the neat blend. We observe a further slight increase in the intensity of the out-of-plane (010) peak when we increase the DIO volume fraction from 2 to 4%, which is consistent with the performance increase of the 4% device; adding even more DIO brings about relatively little additional change in both device performance and X-ray scattering patterns relative to blends with lower additive amounts. The latter point is illustrated further in Figure 6b, which shows the (normalized) intensity of the (200) peak as a


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function of polar angle with varying amounts of DIO. The (200) reflection was chosen instead of the (100) peak because the latter is superimposed on a reflectivity background. One can see the spreading out of intensity along the ring as soon as 1% DIO is added. More DIO further spreads this intensity, though all additional changes up to 6% are smaller. We note that the (200) pole figures in Fig. 6b need to be corrected by sin(polar angle) to accurately represent the crystallite volume fraction.43 Figure 8 shows GIWAXS images of PII2T-Si:PC71BM blends with no DIO and an image with 0.5% and 1.5% DIO. Images of films incorporating larger DIO amounts showed very little difference. As was the case with PII2T-Ref:PC71BM, the scattering pattern of PII2T-Si:PC71BM blends without DIO is similar to diffraction of the pure PII2T-Si film.4 Unlike in the neat PII2T-Ref:PC71BM blend, even before DIO is added it is clear that the (h00) Bragg peaks show both in-plane and out-of-plane orientation, and there is a distinct out-of-plane π-stacking peak. It is then not surprising that changes in the scattering pattern that take place upon adding DIO are more subtle than was the case with PII2T-Ref:PC71BM blends, since the initial and final states in this case are more similar. Although neat PII2T-Si:PC71BM blends show both out-of-plane and in-plane crystallite orientations, adding DIO further spreads intensity of the (h00) peaks along rings of constant radius in Figure 8, suggesting that similar changes in the orientation distribution are taking place as in PII2T-Ref:PC71BM blends. We note that this trend held true for all DIO volume fractions that we explored.

Discussion We showed that the small angle scattering intensity obeyed a power law in the Q range ~ 0.1 to 0.7 nm-1, which we interpreted as scattering from a blend with a fractal polymer/fullerene interfacial morphology. For films that yielded the highest PCE, we found that the fractal dimension of the interfacial surface was larger for PII2T-Si:PC71BM blends relative to PII2T-ref:PC71BM (2.9 vs. 2.3). In so far as an larger fractal dimension implies a larger degree of branching (i.e., more jagged), the larger fractal dimension for PII2T-Si:PC71BM blends suggests that the interface between electron


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density heterogeneities (i.e., the pure fullerene aggregates and the polymer-rich domains) are more branched and jagged than for PII2T-Ref:PC71BM. That is, the clustering is pictured as more dendritic. Although this dendritic-like morphology may result in enhanced exciton harvesting relative to PII2T-Ref:PC71BM blends due to a larger donor/acceptor interfacial area, a nanoscale morphology with a larger surface fractal dimension (more jagged) might increase the chances that a charge carrier will reach a dead end as it traverses the path between electrodes that is roughly in line with the direction of the electric field vector. This renders the carrier essentially trapped or significantly slowed down by the local topology. In the absence of additional time-resolved measurements, we speculate that these considerations would lead to a larger bimolecular electron-hole recombination rate for the best performing PII2T-Si:PC71BM blend relative to the champion PII2T-Ref:PC71BM device, resulting in a reduced short-circuit current for the former blend device – consistent with our current-voltage measurements. However, we note that the larger radius of gyration (aggregate size) in PII2TSi:PC71BM blends may also act to decrease the probability of a non-geminate electron-hole encounter. It is intriguing to ask why the performance of PII2T-Ref:PC71BM and PII2T-Si:PC71BM blends progress in a different manner as a function of additive content. We believe the answer partly lies with the propensity of the polymer with the siloxane sidechains to aggregate in solution, driven by the strong interactions between rigid, planar π-electron systems. Indeed, unlike PII2T-Ref, we find that PII2T-Si solutions at device-relevant concentrations are thermodynamically unstable, and partial precipitation can be observed in solutions left undisturbed for one day after initial dissolution. Further evidence for this is shown in Figure S9 in the SI, where the normalized absorption spectrum of a dilute pure PII2T-Si solution in chloroform is shown after all the powder had dissolved – forming an optically clear solution – and allowed to sit undisturbed for one day (blue curve). The red curve shows the same solution after heating to ~ 55 oC and cooling back down to room temperature. It is clear that in the initial solution (albeit after sitting for one day), the optical bandgap is smaller than after the solution had been heated,


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which is indicative of aggregation. Furthermore, one can clearly see a scattering tail on the red side of the first singlet exciton transition, which is due to large suspended aggregates and crystallites. After heating, the bandgap slightly opens, which is due to partial dissolution of aggregates. The enhanced solution-phase aggregation in PII2T-Si is due to the fact that the siloxy-terminated sidechains have a linear segment, which allows polymer backbones to come closer together as evidenced from the closer π−π stacking distance in solid state.7 PII2T-Ref is unable to achieve similar backbone proximity due to the fact that the branched sidechains tend to prevent the polymer backbones from making closer contacts. We note that it is difficult to assess that state of solution phase aggregation using the wellestablished dynamic (visible) light scattering technique. This is due to the fact that at blend solution concentrations relevant for device fabrication, multiple scattering effects become significant, making calculations of particle size distributions very difficult. Although solution-phase aggregation is likely present to some extent in PII2T-Ref, we speculate that the range and strength of the interaction potential between the polymer backbone as well as the sidechain and a PCBM molecule is different than for PII2T-Si due to dipole and/or solid state polarization effects. It should also be borne in mind that both the fullerene cage and the solubilizing ester group of PC71BM can interact with the siloxane sidechain. Indeed, in neat films incorporating no DIO additive, blends with PII2T-Si did not show very large clusters on the surface by AFM, whereas PII2T-Ref did. One possible explanation for this is that the interaction between PII2T-Si and PC71BM is stronger than that in PII2T-Ref, thereby giving significantly more coarse phase segregation in the latter case, whereas the stronger interaction in the former case prevents such large-scale separation. A difference in the polymer-PC71BM interaction strength would likely have an impact on the manner in which varying volume fractions of DIO affect the end-state thin film morphology and exciton quenching. Unfortunately, owing to the very small fluorescence quantum yield of PII2T-Si, we were unable to investigate this further using PL quenching measurements.


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In light of these observations, we can begin to understand why DIO affects the two polymer:PC71BM blends in such a different manner. Since PII2T-Ref with branched sidechains is highly soluble in the spin-coating solvent, there is presumably relatively little solution-phase polymer aggregation in the neat solution. The interaction with the substrate then likely dictates the polymer crystallite orientation, giving edge-on packing. In contrast, DIO likely induces some aggregation in solution, which results in a very different polymer crystallite texture in the dried film. However, PII2TSi already tends to aggregate in solution even in the absence of DIO. Therefore, the polymer texture in the film is not very different from the corresponding neat blend. When DIO is added to this blend solution, relatively small additive volume fractions likely already cause further polymer aggregation in order to minimize the solution free energy, since DIO is a poor solvent for the polymer. As a result, it takes a very small amount of DIO to produce the optimal device. Even with this small optimal DIO volume fraction, domain sizes in the solid state are larger than in PII2T-Ref:PC71BM blends. It is conceivable that because PII2T-Ref:PC71BM with DIO shows aggregates on the order of 20 nm, and since the polymer-fullerene interface fractal dimension was lower than PII2T-Si:PC71BM blends (less jagged), the network of opposite charge carrier pathways percolating through the bulk of the PII2T-Ref:PC71BM blend film contains more direct and less tortuous paths bridging the top and bottom electrodes. At the same time, the larger particles with a larger surface fractal dimension in PII2TSi:PC71BM blends could form more isolated clusters as well as moredead-ends, which may act as charge traps and lead to a larger carrier recombination rate. 44 Therefore, even though the hole mobility through a π-stacked assembly is larger for PII2T-Si, a partially unfavorable blend morphology does not allow us to take full advantage of this fact in bulk heterojunction films. Current work is underway to decrease the tendency of PII2T-Si to aggregate in solution while preserving the larger hole mobility by virtue of extending the linear aliphatic piece of the siloxy-terminated sidechain.


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Summary We have shown that the polymer with siloxy sidechains leads to significantly greater photocurrents when blended with PC71BM. We believe this is primarily due to a significant fraction of polymer crystallites assuming a face-on orientation, which is associated with good charge transport. In contrast, neat blends utilizing PII2T-Ref display edge-on texture, which is not conducive to efficient out-of-plane hole transport. In addition, polymer chains and fullerene molecules are blended on a finer length scale in neat blends for PII2T-Si, which leads to a larger exciton harvesting rate. After incorporating small amounts of a common solvent additive (DIO), the performance of both polymer:PC71BM blends increases substantially. However, this enhancement is larger for PII2TRef:PC71BM with branched sidechains, which is largely due to dramatically reduced domain size, which improves exciton harvesting, and a re-orientation of polymer crystallites to a largely face-on arrangement. Furthermore, we find very different trends in device-relevant parameters as a function of DIO for the two polymers. We believe that the reason PII2T-Si:PC71BM blends are unable to achieve higher power conversion efficiencies can be partially traced to the solution-phase aggregation of polymer chains without DIO. The siloxy-terminated sidechains brings the polymer backbones closer and thereby increases their self-interaction; this lowers the solution free energy and leads to eventual largescale aggregation.

Experimental Section Materials. PII2T-Ref/PII2T-Si and PC71BM (Nano-C inc.) were used in this study. The detailed synthetic procedure and optoelectronic properties for pristine PII2T-Ref and PII2T-Si polymers have been reported in our previous study.7 Device fabrication and characterization. PII2T-Ref:PC71BM and PII2T-Si:PC71BM devices were fabricated

on

glass

substrates

with

the

architecture

of


indium

tin

oxide

(ITO)/(3,4-

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ethylenedioxythiophene (PEDOT:PSS)/polymer:PC71BM/LiF/Al. The ITO coated glass substrate was ultrasonically cleaned in detergent, acetone, and isopropyl alcohol and dried under nitrogen. The substrates were placed in a UV ozone chamber for 30 min prior to the deposition of PEDOT:PSS (Clevios P VP AI 4083) by spin-casting from solution to form a 30 nm thick ﬁlm. The substrate was annealed for 20 min at 150 °C onto hotplate in air, and then transferred into a glove box to deposit the active layer and counter electrode of LiF/Al. The active layers with a mixture of polymer:PC71BM were deposited onto PEDOT:PSS from hot chloroform solution prepared at 55 °C and had thicknesses on the order of 170nm for PII2T-Ref:PC71BM and 150nm for PII2T-Si:PC71BM after optimization. 1,8diiodooctane (DIO) was used as a processing additive. We used a blend ratio of 1:1 by weight for PII2T-Ref (10 mg/ml) and 1:2 for PII2T-Si (6 mg/ml), which was determined for each polymer individually as part of an optimization process. Subsequently, the devices were annealed at 100 °C for 30 min, and then a LiF/Al (1 nm/100 nm) was deposited by thermal evaporation in a vacuum of about 5×10-6 mbar, where the active area of the cells was 0.09 cm2. Hole-only/electron-only devices were fabricated with ITO/MoO3/active layer/MoO3/Au and ITO/Al/active layer/LiF/Al electrodes to measure the hole and electron mobilities, respectively. The electrical characteristics were measured using Keithley 2400 source meter in the dark and under AM1.5 solar illumination (91160 Oriel 300W solar simulator equipped with a 6258 ozone-free Xe lamp). The light intensity was calibrated using an NREL calibrated silicon photodiode with a KG5 filter which had a spectral mismatch factor error of less than 2% for the devices analyzed. Spectroscopies. UV-vis absorption spectra were recorded with a UV-vis spectrophotometer (Cary 6000i) at room temperature. Thin films for UV-vis in solid state were prepared by spin-casting on glass from polymer solutions in chloroform. The photoluminescence excitation/emission measurements were performed by using the Nanolog spectrofluorometer for nanomaterials (Horiba Jobin Yvon). A 450-W broadband cw Xenon lamp and a monochrometer supplied the excitation light in the range of 550– 845nm with 5nm steps. External quantum efficiency (EQE) measurements were taken at short circuit


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using monochromated white light from a tungsten lamp which was modulated by an optical chopper. The current from the devices were measured as a function of wavelength and compared to the current obtained from a photodiode with a NIST traceable calibration photocurrent action spectrum. Morphological and Structural Characterization. Thickness was measured by a Dektak 150 profilometer (Veeco Metrology Group). AFM images were taken using tapping mode (light tapping regime) using a Multimode AFM (Veeco). 2D-GIXD images were collected in reflection mode with a planar area detector in a He atmosphere at beamline 11-3 of the Stanford Synchrotron Radiation Lightsource. The sample-detector distance was nominally set to 400 mm, and the incidence angle was 0.12 degrees; the X-ray wavelength was 0.9758 Angstroms. Slits were set to 150 µm and 50 µm in the horizontal and vertical directions, respectively. GIXD images were calibrated and analyzed using WxDiff software provided by Stefan Mannsfeld.45 For details about the sample chamber, please see Ref 39. The GIXD samples were prepared by spin-casting the same polymer blend solutions used for fabricating solar cell devices onto silicon wafers at 1000 rpm for 60 s.

Acknowledgements. This work was partially supported by the Center for Advanced Molecular Photovoltaics, award no. KUS-C1-015-21, made by King Abdullah University of Science and Technology. We also acknowledge support from the Global Climate and Energy Program at Stanford and the Camille and Henry Dreyfus Postdoctoral Program in Environmental Chemistry. GIXD measurements were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences.

Supporting Information Available: OPV, EQE, PL, UV-vis, and AFM characteristics of PII2TRef:PC71BM blends and GISAXS profiles of PII2T-Ref or PII2T-Si:PC71BM blends. This material is available free of charge via the Internet at http://pubs.acs.org


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Table 1. Charge carrier mobility measured by SCLC model as a function of DIO amount. Sample no DIO PII2T-Ref 1% DIO 4% DIO 6% DIO no DIO PII2T-Si 0.5%DIO 3%DIO

µh(cm2/Vs) 3.99×10-4 7.01×10-4 1.31×10-3 8.68×10-4 3.53×10-4 5.56×10-4 1.90×10-8

µe(cm2/Vs) 4.09×10-4 4.79×10-4 6.38×10-4 5.42×10-4 5.69×10-5 2.86×10-4 -

µh/µe 0.7 1.7 2.0 1.6 6.2 1.9 -

PCE (%) 0.5 2.7 5.2 4.1 1.6 3.4 1.1

Figure Captions Scheme 1. Schematic representation of molecular structure of PII2T-Ref, PII2T-Si, and PC71BM Figure 1. Current-voltage characteristic of optimized PII2T-Ref:PC71BM and PII2T-Si:PC71BM solar cells without DIO. Figure 2. Current-voltage characteristics of (a) PII2T-Ref:PC71BM and (b) PII2T-Si:PC71BM solar cells as a function of DIO content. Figure 3. The DIO dependency of photovoltaic properties (power conversion efficiency (PCE, black), fill factor (FF, red), and current density (Jsc), blue) of (a) PII2T-Ref:PC71BM and (b) PII2T-


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Si:PC71BM devices. Figure 4. AFM topographs of the PII2T-Ref:PC71BM (a) without DIO and (b) with 1% DIO v/v, and the PII2T-Si:PC71BM (c) without DIO and (d) with 0.5% DIO v/v. Figure 5. In-plane GISAXS profiles of PII2T-Ref:PC71BM and PII2T-Si:PC71BM blend films (a) without DIO and (b) with DIO. Red lines are fits to power-law for high Q in (b) Figure 6. (a) 2D GIXD images and (b) polar angle dependency of (200) Bragg reflection of PII2TRef:PC71BM blend films as a function of DIO content. Figure 7. Experimental dark-current densities of PII2T-Ref:PC71BM blend films as a function of DIO content for (a) a hole-only device and (b) an electron-only device. Figure 8. 2D GIXD images of PII2T-Si:PC71BM blend films as a function of DIO content.

Scheme 1.


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2

2

J (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

PII2T-Si:PC71BM

0

PII2T-Ref:PC71BM

-1 -2 -3 -4 -5 -6 -7 -0.6 -0.4 -0.2 0.0

0.2

0.4

0.6

0.8

1.0

V (V)

Figure 1.


23


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Figure 2.


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Figure 3.


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Figure 4.


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Figure 5.


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Figure 6.


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Figure 7.


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Figure 8.


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Table of Contents Graphic:


31

Comparison of the Photovoltaic Characteristics and Nanostructure of

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