The Role of Solvent Additive Processing in High Performance Small

Oct 15, 2014 - ... Additive Processing in High Performance Small. Molecule Solar Cells. Louis A. Perez,. †,‡,§. James T. Rogers,. †,‡,§. Mic...
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The Role of Solvent Additive Processing in High Performance Small Molecule Solar Cells Louis A. Perez,†,‡,§ James T. Rogers,†,‡,§ Michael A. Brady,†,‡,§ Yanming Sun,† Gregory C. Welch,†,○ Kristin Schmidt,# Michael F. Toney,# Hiroshi Jinnai,∇ Alan J. Heeger,†,‡,§,∥ Michael L. Chabinyc,†,‡,§ Guillermo C. Bazan,*,†,‡,§,∥ and Edward J. Kramer*,‡,§,⊥ †

Center for Polymers and Organic Solids, ‡Materials Research Laboratory, §Departments of Materials, ∥Chemistry and Biochemistry, and ⊥Chemical Engineering, University of California, Santa Barbara, California 93106, United States # Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States ∇ Institute for Materials Chemistry and Engineering (IMCE), Kyushu University, CE80, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: The use of small volumes of a high boiling point liquid as a “solvent additive” is a deposition processing method that has been implemented in most high/record performing polymer:fullerene-based bulk heterojunction (BHJ) solar cell devices. Recently, solvent additive processing has been employed in a solution processable small molecule (SPSM) BHJ system, viz., 5,5′bis{(4-(7-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4c]pyridine}3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene:[6,6]-phenyl C71 butyric acid methyl ester (p-DTS(PTTh2)2:PC71BM), when a small amount, 0.25 v/v %, diiodooctane (DIO) was added to the casting solution, several key device metrics increased, leading to a high power conversion efficiency (PCE) of 6.7%. X-ray diffraction experiments show that the amount of additive added to the casting solution to make p-DTS(PTTh2)2:PC71BM thin films has several effects on the structure at multiple length scales: for example, the number and orientation of p-DTS(PTTh2)2 crystallites, different π−π stacking distances, and the nanoscale domain size. Additionally, we utilize energy filtered transmission electron microscopy (EFTEM), a technique that significantly enhances the contrast between p-DTS(PTTh2)2 and PC71BM in real space, to further verify the effect of increasing domain size as the additive concentration is increased. Tomographic reconstruction of the TEM micrographs provides a 3D representation of the BHJ structure. These studies show how domain size and tortuosity in all dimensions change due to solvent additive processing, and the overall finding is that the nanostructures of p-DTS(PTTh2)2 have enhanced connectivity when 0.25 v/v % DIO was used. Finally, we show evidence of solvent additive retention in p-DTS(PTTh2)2:PC71BM films when 1 v/v % DIO is used (but absent for 0.25%). This finding, in conjunction with the appearance of two populations of π−π stacking distances when 1 v/v % DIO is used, leads to the identification of one of the specific points of interaction between DIO and p-DTS(PTTh2)2.

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INTRODUCTION Organic photovoltaic (OPV) materials are under intense study because they offer the possibility of producing low-cost solar cells via solution methods that can enable high throughput fabrication on flexible substrates.1−4 The most implemented active layer architecture for OPV devices, a bulk heterojunction (BHJ), consists of a mixture of a photoactive narrow bandgap conjugated donor material and a soluble fullerene acceptor.5,6 It is postulated that optimal BHJ morphologies consist of robust bicontinuous nanoscale domains of each moiety, on the order of the exciton diffusion length, which extend vertically from each electrode, thereby increasing the surface area of the domains and forming continuous conducting pathways for efficient charge extraction and transfer. 7,8 An optimal morphology, however, is seldom achieved during film © XXXX American Chemical Society

formation; therefore, a number of processing techniques, such as thermal and solvent annealing, have been explored to control the blend morphology to attain the multiple structural requirements.9,10 Solvent additive processing is a common BHJ deposition technique during film formation with widespread utility in high performing/record breaking conjugated polymer:fullerene solar cells.11−16 This technique entails the addition of a small amount, ∼2−6 v/v %, of a high boiling point liquid to the casting solution and has been shown to impact several key solar cell device metrics, such as the short circuit current (Jsc), open Received: August 29, 2014 Revised: October 14, 2014

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circuit voltage (Voc), fill factor (FF), and ultimately the power conversion efficiency (PCE). Despite the advantages and widespread use of additives, exactly how the additive improves solar cell device performance is still under debate. Several studies on how solvent additives affect the morphology of a BHJ blend using various structural characterization techniques show that the additive can affect BHJ structural organization on multiple length scales.17−19 Additives have also been shown to modify the internal order within the phases, and can increase the crystallinity of the conjugated polymer donor.20,21 Akin to conjugated polymer:fullerene BHJ processing, solvent additive treatment has been applied to solution processable small molecule (SPSM) BHJ systems with similar desirable effects on the Jsc, FF, and PCE. A recent SPSM BHJ system comprised of 5,5′-bis{(4-(7-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine}-3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene (p-DTS(PTTh2)2 and [6,6]phenyl C71 butyric acid methyl ester (PC71BM) showed record improvements in PCE when small amounts of the solvent additive 1,8-diiodooctane (DIO) were added to the chlorobenzene (CB) casting solution (Figure 1).22,23 Solar cell

Table 1. Solar Cell Figures of Merit of Al/pDTS(PTTh2)2:PC71BM/MoOx/ITO/Glass Devices with Varying Solvent Additive DIO Concentrations Added to the p-DTS(PTTh2)2:PC71BM Casting Solutiona DIO v/v (%)

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

0 0.25 0.60 1.0

0.80 0.78 0.74 0.70

12.5 14.4 9.2 2.0

45.2 59.3 47.5 31.1

4.52 6.70 3.20 0.43

a

Small additions of DIO (0.25 v/v %) lead to optimal performance; however, as the DIO concentration increases, several characteristics decrease drastically such as the FF and Jsc, which ultimately leads to low PCEs (Voc = open circuit voltage, Jsc = short circuit current, FF = fill factor, Pout/in = power out/in, and PCE = power conversion efficiency = (Pout/Pin) = (VocJscFF)/Pin). The data are taken directly from refs 22 and 23.

increase device performance.28−30 These studies highlight the need to understand the impact of processing and specific material combination to obtain a clear picture of how chemical structure influences the performance of a molecular semiconductor blend.31 Herein, we report on the effects of solvent additive concentration on the morphology of p-DTS(PTTh2)2:PC71BM BHJ blends at multiple length scales. We utilized several advanced structural characterization probes to delineate a comprehensive morphological description of the effects of solvent additive processing. Previous studies of SPSM analogues to p-DTS(PTTh2)2 were limited to only probing optimized solvent additive casting conditions.23 We find here by X-ray scattering experiments that the amount of additive used to cast p-DTS(PTTh2)2:PC71BM thin films has several effects on the BHJ blend at multiple length scales: for example, the orientation of p-DTS(PTTh2)2 crystallites, the population of different π−π stacking distances, and the nanometer-scale domain size. Energy filtered transmission electron microscopy was used to verify the effect of additive addition on domain size correlations and show agreement with small-angle X-ray scattering experiments. Tomographic reconstruction of the electron microscopy images is performed and provides a 3D representation of the BHJ structure. The reconstructions show that domain size and tortuosity in all dimensions are affected by the amount of solvent additive. Finally, we show evidence of solvent additive retention, days after casting and vacuum treatment, in p-DTS(PTTh2)2:PC71BM films when 1 v/v % DIO is used.

Figure 1. (Top and bottom left) Chemical structures of the compounds used in this study: p-DTS(PTTh2)2, the SPSM electron donor, and PC71BM, the electron acceptor, which are blended in the BHJ solar active layer. CB is the solvent that p-DTS(PTTh2)2:PC71BM is cast from, and DIO is a solvent additive. (Bottom right) Sample architecture used in this study: p-DTS(PTTh2)2:PC71BM is the BHJ active layer (70:30 w/w % mixture), MoOx is the hole transport layer, ITO is the transparent cathode, and glass is the substrate.

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devices with p-DTS(PTTh2)2:PC71BM as the BHJ active layer had a PCE of 4.5% without solvent additive treatment, but this increased to 6.7% when 0.25 v/v % of DIO was added to the CB casting solution (Table 1). The increase in PCE due to processing with 0.25 v/v % DIO is attributed to an increase in both the Jsc and the FF. The other interesting aspect of solvent additive processing in p-DTS(PTTh2)2:PC71BM is that several solar cell metrics deteriorate drastically above the optimized conditions, where the addition of only 1.0 v/v % of DIO to the casting solution resulted in large drops in both the Jsc and the FF, leading to a negligible PCE of 0.43%; thus, the PCE decreases by over an order of magnitude from the as-cast case. A significant drop in PCE when a slight excess over the optimum amount of additive is used has also been observed in other SPSM systems.24−27 The addition of small amounts of polymer, such as poly(dimethylsiloxane) (PDMS), as additives to the casting solution of SPSM BHJs has also been shown to

RESULTS AND DISCUSSION Effect of the Solvent Additive on the Molecular Organization/Crystallinity of p-DTS(PTTh2)2 in p-DTS(PTTh2)2:PC71BM Blends. A prevalent method to probe structure-processing relationships on the atomic/molecular level in thin films is grazing incidence wide-angle X-ray scattering (GIWAXS).32 GIWAXS has been a valuable tool for investigating structure-processing relationships in thin film organic electronics.33,34 In particular, if molecular order is present, the following can be readily determined from GIWAXS: crystallite formation, lattice plane spacing, crystallite correlation lengths, orientation of crystallites, and polymorph formation.35,36 GIWAXS diffraction patterns of p-DTS(PTTh2)2:PC71BM with 0, 0.25, and 1.0 v/v % DIO added to the film casting solution are shown in Figure 2. The intense B

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signals in Figure 2a−c are from the π−π stacking (q ≈ 18 nm−1) and long unit cell axis (q ≈ 3.5 nm−1) lattice planes in pDTS(PTTh2)2 crystallites. The spotty arcs at ∼21.5 and ∼25 nm−1 are from the ITO layer, and the background scattering from ∼15−20 nm−1 is from amorphous regions of pDTS(PTTh2)2 and PC71BM, and the MoOx/ITO/glass substrate. The nonadditive treated and 0.25 v/v % DIO samples, Figure 2a and b, show that p-DTS(PTTh 2 ) 2 crystallites take on many different orientations within the film. The continuous intensity rings for both the long unit cell axis and the π−π stacking reflections demonstrate the nearly random distribution in crystallites of p-DTS(PTTh2)2 when 0 and 0.25 v/v % DIO is used. The GIWAXS diffraction pattern changes significantly when 1 v/v % DIO is added to the casting solution (Figure 2c), where a number of additional reflections appear on the diffraction pattern. The additional reflections indicate that the degree of crystallite perfection is greater when 1 v/v % DIO is used instead of 0 or 0.25 v/v % DIO. In addition, the π−π and long unit cell axis diffraction is pronouncedly anisotropic when 1 v/v % is added to the casting solution, which shows that p-DTS(PTTh2)2 crystallites become more textured at this additive concentration. The orientation dependence of p-DTS(PTTh2)2 crystallites due to solvent additive processing with DIO is more clearly visualized and easier to compare when line cuts are taken from diffraction patterns, as shown in Figure 3. Line profile cuts were

Figure 2. Grazing incidence wide-angle diffraction patterns of pDTS(PTTh2)2:PC71BM with (a) 0, (b) 0.25, and (c) 1.0 v/v % DIO added to the film casting solution.

rings, arcs, and spots in the diffraction patterns shown in Figure 2 correspond to Bragg reflections from ordered lattice planes in p-DTS(PTTh2)2 crystallites.37 There is no evidence of scattering from PC71BM, but this is likely due to its intrinsic amorphous nature of spin-cast PC71BM films and/or the low PC71BM content in blends of 70:30 SPSM:PC71BM; by contrast, in polymer systems, there is usually up to 2−4 times more PC71BM than polymer. The data in Figure 2 were taken using an area detector, which enables simultaneous capture of scattering at most scattering polar angles (χ) in a single exposure; thus, expeditious crystallite orientation determination is possible.38 Scattering directly along the qz axis (ordinate) is due to ordered lattice planes that are oriented orthogonally to the substrate normal, while scattered intensity on the abscissa, qxy, corresponds to lattice planes oriented parallel to the substrate normal. The two strongest scattering

Figure 3. Grazing incidence wide-angle X-ray scattering (GIWAXS) line cuts (a) out-of-plane (along the nominally qz direction) and (b) in-plane (along the qxy direction), taken from the diffraction patterns in Figure 2, of p-DTS(PTTh2)2:PC71BM with 0, 0.25, 1.0 v/v % DIO added to the film casting solution.

taken nearly out-of-plane (∼qz) (Figure 3a) and in-plane (∼qxy) (Figure 3b) at χ values 11° and 88°, respectively. An intensity correction, I(χ) sin(χ), was applied to the line cuts to account for the sample-experiment geometry.33,39,40 Azimuthally integrated intensity (q = ((qxy)2 + (qz)2)1/2) line cuts over all crystallite orientations of p-DTS(PTTh2)2:PC71BM with 0, 0.25, and 1.0 v/v % DIO added to the film casting solution are C

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up of this region that also shows this splitting is in the Supporting Information (Figure S5). The splitting of the π−π stacking peak indicates that two populations of π−π stacking distances are now present. The π−π stacking peak shifts to a lower q value, which means the average real space intermolecular distance increases in some p-DTS(PTTh2)2 crystallites from 0.349 to 0.353 nm. The intensity of the lower q (larger real space distance) π−π stacking peak increases when 1 v/v % is used, which suggests a larger population of this π−π stacking distance exists. There is no evidence of a structural change or peak splitting for the long unit cell axis or other reflections due to additive processing when an azimuthal integration is performed on the full diffraction pattern (Supporting Information Figure S1). Dynamic secondary ion mass spectrometry (DSIMS) was used to determine if any residual additive remains in the films after casting and drying under high vacuum.42,43 This technique has been used on a number of BHJ films to study the effects of different processing techniques on the composition as a function of vertical depth.44−47 The following secondary ion masses were monitored to distinguish between the particular species of interest in the film: 34S for p-DTS(PTTh2)2, 127I for DIO, and 130InO for the substrate (Figure 5). The 34S isotope was used instead of 32S to distinguish mass signals between 32S and 32O2. A ∼150 nm thick layer of polystyrene (PS) was laminated onto each sample prior to the DSIMS experiments to act as a capping layer that allows a steady-state damage profile to build up before the top interface of the film is encountered and to also serve as a calibration layer in determining the etching rate. It is noted that these samples were subject to high vacuum during the DSIMS experiments. The iodine signal in the nonadditive treated sample (Figure 5a) is at background level and shows no change in counts as the depth profile transitioned from the PS layer to the BHJ layer. The InO signal increase in the BHJ layer is attributed to holes or surface irregularities that exist in the samples. The 0.25 v/v % DIO treated sample (Figure 5b) shows possible evidence of retained DIO with a slight increase of the iodine signal when the BHJ layer is reached; however, the iodine signal is still very weak. The 1 v/v % DIO treated sample (Figure 5c) shows a noticeable jump in the iodine signal once the BHJ layer is reached (∼600 s) that is uniform until the bottom of the film is reached where another jump in the signals occurs (∼1200 s). The sulfur signal increase at the PS/BHJ interface when additive is used indicates that the donor material potentially surface segregates. The depth profile for the 1 v/v % DIO sample is not as homogeneous as 0 and 0.25 v/v %, which is due to a larger surface roughness for 1 v/v % DIO treated samples. Several BHJ solar cell device processes, such as exciton dissociation and charge transfer, are dependent on the interface area, domain size, and morphology of the two components.7,8,48 Therefore, to fully delineate the effects and relevance of solvent additive processing on the microstructure of p-DTS(PTTh2)2:PC71BM BHJ blends, nanometer length scale structures were also probed, as discussed below. Effect of the Solvent Additive on the Morphology of p-DTS(PTTh2)2:PC71BM Blends. Grazing incidence smallangle X-ray scattering (GISAXS) is a technique analogous to GIWAXS, but probes larger length scale correlations or structure (∼10−100 nm).49 GISAXS is a widely used technique that has been used to characterize the assembly of nanoparticles, quantum dot arrays, silica mesospheres, and

shown in the Supporting Information (Figure S1). The position and breadth of the long unit cell axis (smallest detectable reciprocal lattice vector of the unit cell) and π−π stacking peaks for 0 and 0.25 v/v % DIO at both orientations are similar. The intensity of the π−π stacking peak, however, is more intense in the in-plane direction (Figure 3b), which indicates there is a slight preponderance of p-DTS(PTTh2)2 with an edge-on orientation (π−π stacking is perpendicular to the substrate normal). The π−π stacking peak in-plane (Figure 3b) for 1 v/v % DIO is much stronger than 0 or 0.25 v/v %, and its intensity out-of-plane (Figure 3a) is barely detectable, indicating that when 1 v/v % DIO is used, the molecular orientation of the π−π stacking planes in p-DTS(PTTh2)2 crystallites mainly adopt an edge-on orientation. Despite the difference in the crystallinity and orientation of p-DTS(PTTh2)2 crystallites when using 1 v/v % additive, it is noted that the crystallite correlation lengths (CCL) of π−π stacking lattice planes do not vary significantly with varying DIO amounts. The CCL is inversely proportional to the full-width-at-half-maximum (fwhm) of a Bragg reflection peak and was calculated to be ∼10 nm for the π−π stacking direction.41 The CCL of the π−π stacking peak does show a slight dependence when 0.25 and 1 v/v % DIO are used as evidenced by a slight narrowing (Figure 3b) of the peak width relative to the case of 0 v/v % DIO. The nearly constant CCL, however, suggests that the additive does not increase coherence length (due to paracrystalline disorder) but instead enhances the crystallinity. A closer look of the π−π stacking peak shows another interesting difference between nonadditive and additive treated samples. Line profiles of the intensity azimuthally integrated over all orientations in the π−π stacking regime of pDTS(PTTh2)2:PC71BM with 0, 0.25, and 1.0 v/v % DIO added to the film casting solution are shown in Figure 4. The prominent π−π stacking peak position of p-DTS(PTTh2)2 gradually shifts from 18 nm−1 (light gray dotted line) to 17.8 nm−1 (dark gray dotted line), with splitting of the peak when the DIO amount is 1 v/v %. A GIWAXS image plot of a close-

Figure 4. Azimuthally integrated intensity (q = ((qxy)2 + (qz)2)1/2) over all orientations in the π−π stacking regime, taken from the diffraction patterns in Figure 2, of p-DTS(PTTh2)2:PC71BM with 0, 0.25, and 1 v/v % DIO added to the film casting solution. The vertical lines at 17.8 and 18 nm−1 serve as a guide to the prominent peak positions. D

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Figure 6. Grazing incidence small-angle X-ray scattering (GISAXS) patterns of p-DTS(PTTh2)2:PC71BM with (a) 0, (b) 0.25, and (c) 1.0 v/v % DIO added to the film casting solution. The rod-shaped object in the middle is the beam stop.

indicates that correlated domains of p-DTS(PTTh2)2 and/or PC71BM are present in the film. The scattered intensity shifts to lower scattering vector q, and the maxima are located closer to qy = 0 for 0.25 v/v % DIO; therefore, the domain size increases when 0.25 v/v % DIO is used. In addition to scattering from pDTS(PTTh2)2 or PC71BM domains in the GISAXS patterns of 0 and 0.25 v/v %, there are strong intensity oscillations along the qz direction that originate from scattering from standing waves that form in the film due to the coupling of incident and reflected X-rays from the substrate.56−58 The standing waves are also present in X-ray reflectivity (XRR) measurements below the critical angle of total reflection of the ITO substrate for 0 and 0.25 v/v % p-DTS(PTTh 2)2 :PC71BM films (Supporting Information Figure S3). A requisite for standing waves to form is that diffuse scattering is limited by maintaining low roughness in all layers of the sample; therefore, the presence of standing waves when 0 and 0.25 v/v % DIO is used indicates that the BHJ film has low roughness. The GISAXS intensity diminishes when 1.0 v/v % DIO is used (Figure 6c), which indicates that the domain correlations are at q values below the detection limit (qmin ≈ 0.06 nm−1, dmax ≈ 105 nm). In addition, the standing wave patterns are absent when 1.0 v/v % DIO is used, which indicates that the BHJ layer becomes more rough and supports DSIMS analysis.

Figure 5. Dynamic secondary ion mass spectrometry (DSIMS) of pDTS(PTTh2)2:PC71BM with 0, 0.25, and 1.0 v/v % DIO added to the film casting solution. A polystyrene (PS) capping layer was laminated on each film.

copolymers in thin films.50 GISAXS has also been used in a number of studies to characterize BHJ blends.33,51−53 Similar to the GIWAXS measurements, small-angle scattering patterns were collected on a 2D area detector; however, due to the small incident angle, qx is negligible, and therefore the abscissa is now in terms of qy (also commonly referred to as qpar) and not qxy as described for GIWAXS.54,55 GISAXS patterns of p-DTS(PTTh2)2:PC71BM with 0, 0.25, and 1.0 v/v % DIO added to the film casting solution are shown in Figure 6. The intensity on the scattering patterns corresponds to scattering from large length scale correlations of p-DTS(PTTh2)2 or PC71BM domains (∼10−100 nm). A significant amount of scattering is observed for 0 and 0.25 v/v % DIO (Figure 6a and b), which E

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important aspect of BHJ solar cell operation is the ability to efficiently extract and transport charge vertically to each electrode. This aspect would ideally require nanoscale phase separated bicontinuous interpenetrating domains of both the donor and the acceptor components throughout the vertical direction of the film.64 The GISAXS data give evidence of domain formation in p-DTS(PTTh2)2:PC71BM blends; however, it is difficult to interpret how the solvent additive affects the ability for continuous vertical domains to form. Despite the importance of the existence of bicontinuous interpenetrating networks in BHJ solar cells, characterization and quantification of BHJ structures is challenging. The common methods that have been used to probe BHJ structures have been electron microscopy methods such as cross-sectional scanning electron microscopy (SEM) and TEM.65,66 However, a common issue with the aforementioned techniques is that the components of organic BHJ solar cells are typically similar in chemical composition and density. These similarities render it difficult to distinguish specific moieties accurately unless there is a large degree of phase separation; therefore, a technique capable of generating contrast between polymer or SPSM and fullerene components that can be resolved in three dimensions is necessary to fully characterize the morphology of organic BHJ blends. A technique that has recently been employed to achieve chemical contrast in organic BHJ blends with several nanometer resolution is energy filtered transmission electron microscopy (EF-TEM).67−72 The ability to selectively form images with electrons that have lost characteristic amounts of energy allows EF-TEM to achieve unprecedented contrast. In addition, with successful implementation of EF-TEM, energy filtered transmission electron microscopy tomography (EFTEMT) enables three-dimensional (3D) imaging of BHJ structures.73−76 Electron energy loss spectra (EELS) were recorded for pure components of p-DTS(PTTh2)2 and PC71BM and used to determine electron energy loss values at which there is maximum contrast between them (Supporting Information Figure S2). The maximum contrast between p-DTS(PTTh2)2 and PC71BM is achieved when electrons with energy losses between 18 and 22 eV are selected for imaging. This energy loss difference range was calculated by finding the energy range corresponding to the maximum value of the difference between the EELS spectra of p-DTS(PTTh2)2 and PC71BM. EF-TEM images, formed from electrons that have an energy loss between 18 and 22 eV, of p-DTS(PTTh2)2:PC71BM with 0, 0.25, and 1.0 v/v % DIO added to the film casting solution are shown in Figure 8. The light regions in each image represent pDTS(PTTh2)2-rich domains. There is a subtle difference in the morphology between 0 and 0.25 v/v % DIO with both showing small uniform domains (∼20−40 nm); however, with 1.0 v/v % DIO, a disperse distribution of very large domains (>100 nm) of p-DTS(PTTh2)2 is present. The EF-TEM, therefore, supports what was deduced from the GISAXS experiments: 0 and 0.25 v/v % DIO have small domains of each component, and large domains form when 1 v/v % DIO is used. While EF-TEM is a powerful method for identifying the presence of distinct SPSM-rich and fullerene-rich phases, conventional 2D TEM images like those in Figure 8 provide only a 2D projection of 3D structures of the components comprised in the film. Tomographic reconstruction was performed by recording EF-TEM images over a range of tilt angles to produce 3D representations on the effect of additive

In-plane (qy) line profile cuts taken from the scattering patterns in Figure 6 of p-DTS(PTTh2)2:PC71BM with 0, 0.25, and 1.0 v/v % DIO added to the film casting solution are shown in Figure 7; Iq2 corrected intensity plots are in the

Figure 7. Grazing incidence small-angle X-ray scattering (GISAXS) inplane (qy) line cuts, taken from the scattering patterns in Figure 6, of p-DTS(PTTh2)2:PC71BM with 0, 0.25, and 1.0 v/v % DIO added to the film casting solution.

Supporting Information. Line cuts of 0 and 0.25 v/v % DIO have a broad peak centered at ∼0.31 and 0.16 nm−1, respectively. This peak originates from nanometer length scale correlations between the p-DTS(PTTh2)2and PC71BM domains or quasi-periodicity. The change in the peak position shows that the introduction of additive causes the real space domain spacing to increase. The real space domain correlation spacing corresponds to an average distance of ∼20 and 39 nm for 0 and 0.25 v/v %, respectively. The broadness of the peak appears to increase when 0.25 v/v % was used as well, which could indicate that the domain size correlation distances are more disperse than when no additive is used; however, there are a number of things that could affect the broadness such as a distribution in domain sizes and shapes and the interconnectivity of the domains. When 1 v/v % is used, there is no peak or visible scattering features present. The absence of an obvious scattering feature indicates that the length scales of the correlations are outside the instrumental detection limits (i.e., d > 100 nm) or do not exist. The GISAXS measurements show that not only does DIO have a significant effect on the molecular ordering and orientation of p-DTS(PTTh2)2 crystallites, but it also strongly affects the BHJ nanometer-scale structure. The multilength scale effect of additives on the structure of a BHJ blend has been shown in some polymer:fullerene BHJ systems; however, it is rare to see any significant evidence of spatial correlation between domains (periodicity) from hard X-ray small-angle scattering because there are often only subtle or no detectable scattering features.15,20,59,60 GISAXS domain size analysis of BHJ systems has been mainly limited to Guinier analysis for estimations of the radius of gyration due to the lack of scattering features.61−63 The appearance of scattering in the small-angle regime demonstrates that there are regions of electron density contrast, which would indicate that appreciably pure domains of each component are present. Real Space Images and 3-D Morphology of Solvent Additive Treated p-DTS(PTTh2)2:PC71BM Blends. Another F

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Figure 9. Image of a tomographic reconstruction of p-DTS(PTTh2)2:PC71BM with 0.25 v/v % DIO taken from the energy filtered tomography images at varying angles. The white/gray and black regions represent p-DTS(PTTh2)2- and PC71BM-rich regions, respectively.

intensity indicates that there are disconnected/tortuous domains throughout the films thickness. Visual inspection reveals that the intensity of the spots in the video of 0.25 v/v % DIO treated films appears more persistent than that in the nonadditive treated case. The consistent intensity shows evidence of continuous domains throughout the thickness of the film with numerous connective paths that should permit efficient charge transport throughout the film and limit recombination of carriers. This could explain why the FF and ultimately the PCE for 0.25 v/v % DIO is greater than when no additive is used. The video for 1.0 v/v % DIO treated films shows that the large irregular domains shown in Figure 8c of pDTS(PTTh2)2 form throughout the film. Correlation of Structural Characterization Data to Device Performance. The results from this study show that the amount of additive added to the casting solution of a small molecule BHJ has several effects on the structure and morphology of spun-cast films. X-ray scattering experiments show that there is little observable change of the molecular order of p-DTS(PTTh2)2 when 0.25 v/v % DIO is used as compared to the nonadditive treated film. When 1 v/v % DIO was used, however, the molecular order, a structural aspect typically associated with high performing donors in BHJ solar cells, is much greater. Specifically, the population of pDTS(PTTh2)2 crystallites and relative degree of crystallinity increase above 0.25 v/v %, while the crystallite coherence length and therefore level of crystalline disorder remain the same with additive concentration. Additionally through the use of GIWAXS, we found that the orientation of p-DTS(PTTh2)2 crystallites is strongly edge-on when 1 v/v % DIO is used, whereas the orientation of crystallites when 0 or 0.25 v/v % DIO is used is nearly random and therefore has a much higher population of face-on orientation; the face-on geometry is postulated to be the preferred orientation in BHJs because the average charge transport direction is commensurate with the device geometry.39,78 Finally, GIWAXS results for 1 v/v % show an appreciable splitting of the π−π stacking peak, where there is a high population of the larger π−π stacking distance. These structural attributes of crystallite length scales partially explain the evolution of p-DTS(PTTh2)2:PC71BM OPV device performance with DIO additive concentration. Although the degree of crystallinity and molecular order improves at 1 v/v %, device testing revealed that Jsc, FF, and PCE all drop

Figure 8. Energy filtered transmission electron microscopy (EF-TEM) images, formed from electrons that have an energy loss between 18 and 22 eV, of p-DTS(PTTh2)2:PC71BM with (a) 0, (b) 0.25, and (c) 1.0 v/v % DIO added to the film casting solution. The light regions in each image represent p-DTS(PTTh2)2-rich domains.

concentration on domain shape and tortuosity throughout the films.77 Videos of the tomographic reconstructions demonstrate a top-to-bottom view of the morphological changes in the films and can be found in the Supporting Information.77 An image example of a top and side view of a full 3D tomographic reconstruction of p-DTS(PTTh2)2:PC71BM with 0.25 v/v % DIO is shown in Figure 9. The white/gray and black regions in Figure 9 represent p-DTS(PTTh2)2- and PC71BM-rich regions, respectively. The tomographic video of p-DTS(PTTh2) 2:PC71BM without DIO shows that there are numerous domains of each component; however, the appearance or intensity of domains tends to oscillate during the video if one focuses on individual regions. This oscillating G

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performing small molecule:fullerene BHJ blend at several DIO additive concentrations and uses these attributes to provide a nonempirical, physical basis for the optimization of OPV performance with a widely used and record-performing additive. To effectively combine structural analysis and performance with device design to create high performing OPVs, we must be able to observe these changes associated with solvent additives, correlate the structural changes with device performance, and, last, discover the mechanism by which this widely used solvent additive interacts with the blended semiconductors. As described previously, DSIMS was performed to evaluate the solvent retention of this blend as a function of additive concentration. The solvent additive, DIO, was only strongly retained following device drying and testing at a concentration of 1 v/v %. Consequently, this concentration also showed significant changes in molecular ordering, relative degree of crystallinity, and spacing along the π−π stacking and intermolecular transport direction, as well as the formation of large, faceted SPSM domains. These results together, from DSIMS, GIWAXS, and EFTEM, are an initial clue in the literature as to the mechanism of solvent additive optimization in BHJ OPVs and suggest that DIO may function as a nucleation agent or perhaps modify casting solution solubility, forcing these molecules to crystallize before deposition. While more work is needed and laboratory studies are under way to solve this critical piece of the puzzle in solvent additive processing of SPSM OPVs, this report suggests that DIO as an additive is retained for long periods following casting and drying and has a strong interaction with this high performing SPSM.

dramatically from 0.25 to 1 v/v %. While this improved molecular order is usually correlated with electronic transport enhancement, the drop in Jsc and FF that we observed is partially associated with crystallite texture differences between 0.25 and 1 v/v %. Even with improved molecular ordering at 1 v/v %, edge-on texture is dominant and causes a reduction in device performance. The more random texture at 0.25 v/v % allows for more three-dimensional transport and more effective charge collection at electrodes. Adding to this current and collection efficiency drop at 1 v/v % is the dominant population of crystallites with increased π−π spacings, where the increased distance between cofacial stacks hinders hole transport in a typically dominant transport direction. These structural features of crystallite length scales, however, do not completely explain the device performance variation with DIO additive concentration and do not account for performance differences at low additive concentrations. In considering the structural hierarchy in these BHJ blends, the other critical spatial correlation is that among ∼10−100 nm domains. GISAXS and EFTEM analysis has depicted the change in this domain structure as the concentration of DIO additive in the starting casting solution is varied: the domain size increases with DIO concentration, first from ∼20 to ∼40 nm from 0 to 0.25 v/v %, and then to a much larger size of ∼100 nm and larger for 1 v/v %. In addition, the correlation of these domains decreases dramatically above 0.25 v/v %; below this concentration, domains are uniformly spaced and sized throughout the film in a bicontinuous manner, while above this concentration, DIO domains are arranged in a more dispersed fashion, with a distribution of large sizes. Perhaps more important than this domain correlation, however, is the domain tortuosity/connectivity, observable with EFTEM. The connectivity of domains is most enhanced at 0.25 v/v %, with poorer connectivity (more tortuous, nodular domains) with no additive treatment, and nearly lacking connectivity at 1 v/v %. Finally, DSIMS, XRR, and GISAXS were utilized to probe yet another correlation on the mesoscale, the surface roughness. The increase in surface roughness above 0.25 v/v % corroborates the GISAXS and EFTEM domain analysis, providing support that the morphology is coarsened at high DIO additive concentrations. This coarsening completes the story of changes in OPV performance with processing additive concentration, in showing that the optimized 0.25 v/v % DIO device has an ideal mesoscale, domain structure. While the average domain sizes for both 0 and 0.25 v/v % DIO are of reasonable scale for efficient exciton dissociation, the improved domain connectivity (reduced tortuosity) and slightly larger distribution of sizes at 0.25 v/v % allow for superior charge transport and collection and thus PCE. The strong drop in both Jsc and FF from 0.25 to 1 v/v % DIO can be largely explained by the large p-DTS(PTTh2 ) 2 domains and very low connectivity at 1 v/v %. The size of these domains (∼100− 150 nm) prevents efficient exciton dissociation between donor and acceptor and thus free charge carrier generation, while the low connectivity (high tortuosity) among them prevents the effective transport of charges through the device and their collection at the electrodes.33 This thorough characterization, using state of the art X-ray scattering, electron microscopy, and spectroscopic techniques, has detailed numerous microstructural changes associated with the variation of solvent additive concentration used in the initial OPV casting solution. This study describes the structural attributes, at both the nano- and the mesoscale, of a high

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CONCLUSIONS In summary, we have now provided details on how casting solution of p-DTS(PTTh2)2:PC71BM BHJ blends influences structure, morphology, and ultimately the solar cell device performance. Structural changes occur across multiple length scales. At the molecular level it was found by GIWAXS experiments that the p-DTS(PTTh2)2 crystallite population and degree of crystallinity increase, and crystallite orientation becomes more textured. In addition, it was found that a double population of π−π stacking distances forms, and there is evidence of DIO retention in the film after casting when 1 v/v % DIO is used. Upon inspection of the nanometer-scale structure by GISAXS and EF-TEM, it became apparent why using 1 v/v % DIO instead of the optimized amount, 0.25 v/v %, led to a significant decrease in the short circuit current and device performance: the domain size of the p-DTS(PTTh2)2 phase grows as the amount of DIO increases, and the correlation between domains weakens (e.g., less tendency for periodicity). There is also a slight difference in the domain size of 0 and 0.25 v/v % DIO treated films; however, the degree of domain connectivity and uniformity throughout the thickness of the film is more consistent in the 0.25 v/v % treated films. The results from this study have direct implications for future optimization and design of solvent additive processed SPSM BHJ blends and will be key in leading future studies to elucidate the driving force and exact point of interaction of the additive.

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EXPERIMENTAL SECTION

Materials. 5,5′-Bis{(4-(7-hexylthiophen-2-yl)thiophen-2-yl)[1,2,5]thiadiazolo[3,4-c]pyridine}-3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene, p-DTS(PTTh2)2, was synthesized according to procedures H

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close-up of the π−π stacking peak splitting seen in 1 v/v % DIO treated p-DTS(PTTh2)2:PC71BM thin films; HR-XRR; Iq2 GISAXS plots; and further details on EF-TEM and EF-TEMT measurements and data workup. The EF-TEMT videos are named as follows: 0 v/v% DIO (cm5031987_si_002), 0.25 v/v % DIO (cm5031987_si_003), 1 v/v% DIO (cm5031987_si_004). This material is available free of charge via the Internet at http://pubs.acs.org.

published elsewhere.22 [6,6]-Phenyl C71 butyric acid methyl ester (PC71BM) was purchased from Solenne BV and stored in a N2 filled glovebox. 99.9% anhydrous chlorobenzene (CB) and 1,8-diiodooctane (DIO) were purchased from Sigma-Aldrich and stored in a N2 atmosphere glovebox. Sample Preparation. All samples were fabricated by the same device scientist, Dr. Yanming Sun, who produced the results in refs 22 and 23. Multiples of the samples were made and tested to ensure that they performed at the same level as published. Precut 1.5 × 1.5 cm glass substrates with a ∼150 nm indium tin oxide (ITO) top layer were used as the substrates. The substrates were first cleaned with a cotton swap and detergent followed by ultra sonication in water, acetone, and isopropyl alcohol. The substrates were then dried overnight at 100 °C. Molybdenum oxide (MoOx) films were thermally evaporated onto ITO substrates at a rate of 0.1 Å s−1 under a vacuum of about 1 × 10−6 Torr, resulting in ∼9 nm thick films. A solution containing a mixture of p-DTS(PTTh2)2:PC71BM (70:30 w/w) in CB/(0, 0.25, and 1 v/v %) DIO at a total solids concentration of 40 mg mL−1 was spin-cast on top of the MoOx film at 1600 rpm for 40 s. Samples were annealed for 10 min after casting at 70 °C. The final sample architecture used in these studies is as follows: glass/ITO/ MoOx/p-DTS(PTTh2)2:PC71BM. Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS). GIWAXS was performed on beamline 11-3 at the Stanford Synchrotron Radiation Lightsource (SSRL) with an X-ray energy of 12.735 keV. Scattered X-rays were collected on a MAR345 image plate detector. The sample-to-detector distance was 400 mm and was calibrated with a LaB6 standard. Samples were exposed for 15 s with an incident angle of 0.12° while under a He environment to minimize beam damage and reduce background from air scattering. The data were worked up with the WxDiff program provided at the beamline.38 High-Resolution X-ray Reflectivity (HR-XRR). HR-XRR measurements were conducted at beamline 8-ID-E at the Advanced Photon Source (APS) of Argonne National Laboratory, with an X-ray energy of 7.35 keV. Reflected X-rays were collected by an avalanche photodiode (APD), attached to a linear high-precision motorized vacuum feed-through located 370 mm downstream of the sample position.79 Grazing Incidence Small-Angle X-ray Scattering (GISAXS). GISAXS experiments were conducted at beamline 8-ID-E at the Advanced Photon Source (APS) of Argonne National Laboratory, with an X-ray energy of 7.35 keV. Scattered X-rays were collected on a twodimensional Pilatus 1MF Pixel Array Detector with 981 × 1043 pixel resolution. The sample-to-detector distance was 1.975 m and was calibrated with a silver behenate standard. Samples were exposed for 30 s with varying incidence angles between 0.18° and 0.24°, while under vacuum to minimize beam damage and reduce air scattering. The data were worked up with a MatLab plug-in script written and provided at the beamline. Dynamic Secondary Mass Ion Spectrometry (DSIMS). DSIMS measurements were performed with a Physical Electronics 6650 dynamic SIMS using a 2 keV, 40−50 nA beam of O2+ ions at 45° off normal incidence, which was rastered over a 0.09 mm2 region. Charge neutralization was accomplished using a static, defocused, 0.7−1 keV electron beam. Negative ions of C, H, S, InO, and I were monitored as a function of time from an electrically gated area that was 15% of the rastered area. Energy Filtered Transmission Electron Microscopy (EF-TEM) and Tomography (EF-TEMT). Electron tomography was performed using a JEOL JEM-2200FS microscope that combines a 200 kV field emission gun and an in-column energy filter (Omega Filter). Images were taken at 1° increments over a range of ±68° with an image acquisition time of 2 s at each step (exposure times were limited to 2 s to prevent beam damage).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address ○

Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Center for Energy Efficient Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under award no. DC0001009. L.A.P. acknowledges support from the ConvEne IGERT Program (NSF-DGE 0801627) and a Graduate Research Fellowship from the National Science Foundation (GRFP). M.A.B. wishes to thank Graduate Research Fellowship support from the National Science Foundation and California Nanosystems Institute. H.J. gratefully acknowledges the financial support received through a Grant-in-Aid (no. 24310092) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. We thank Tom Mates (UCSB) for help with the DSIMS experiments and Zhang Jiang, Joseph Strzalka, and Jin Wang at the Advanced Photon Source for valuable discussions about GISAXS. We also thank Chance C. Holland of UCSB for help in editing the figures in this manuscript. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University, and the Advanced Photon Source (APS), an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, supported by the U.S. DOE under contract DE-AC02-06CH11357. The MRL Shared Experimental Facilities are supported by the MRSEC Program of the NSF under award no. DMR 1121053, a member of the NSF-funded Materials Research Facilities Network.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Azimuthally integrated intensity GIWAXS of p-DTS(PTTh2)2:PC71BM; EELS spectra of p-DTS(PTTh2)2; a I

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dx.doi.org/10.1021/cm5031987 | Chem. Mater. XXXX, XXX, XXX−XXX