Gaining Further Insight into the Solvent Additive-Driven Crystallization

Jun 22, 2016 - Gaining Further Insight into the Solvent Additive-Driven Crystallization of Bulk-Heterojunction Solar Cells by in Situ X-ray Scattering...
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Gaining Further Insight into the Solvent Additive-Driven Crystallization of Bulk-Heterojunction Solar Cells by in Situ X‑ray Scattering and Optical Reflectometry Felix Buss,† Benjamin Schmidt-Hansberg,† Monamie Sanyal,‡ Carmen Munuera,§ Philip Scharfer,† Wilhelm Schabel,† and Esther Barrena*,∥ †

Institute of Thermal Process Engineering, Thin Film Technology, Karlsruhe Institute of Technology (KIT), Kaiserstrasse 12, 76131 Karlsruhe, Germany ‡ Department of Metastable and Low-Dimensional Materials, Max Planck Institute for Intelligent Systems, Stuttgart, Germany § Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid 28049, Spain ∥ Instituto de Ciencia de Materiales de Barcelona (ICMAB-CSIC), Campus UAB, Bellaterra 08193, Spain S Supporting Information *

ABSTRACT: The use of solvent additives has become a successful strategy to control the structural evolution upon film formation in bulk-heterojunction (BHJ) solar cells. Nonetheless, a complete understanding of the additive’s role in the phase separation mechanisms and organization of donor and acceptor materials in BHJs is still lacking. In this work we gain further insight about the specific role that a widely used additive, 1,8-octanedithiol (ODT), has in the crystallization of both PCPDTBT and PC71BM, directly after wet film deposition using blade-coating. By in situ X-ray scattering and optical reflectometry, we correlate the additive-driven crystallization with the evolution of film composition from the earliest time of solvent evaporation. It is shown that ODT influences the evolution of both PCPDTBT and PC71BM. ODT leads to prolonged crystallization time during the ODT-drying dominated period corresponding to an overall solvent content (x) of 75 wt % > x > 15 wt % and delays the onset of PC71BM aggregation to x < 20 wt %, being pushed out of the crystalline polymer domains.

1. INTRODUCTION In the past decade, the power conversion efficiency (PCE) of bulk-heterojunction (BHJ) organic solar cells has been steadily increasing, reaching values above 10%.1−3 The BHJ concept, in which the photovoltaic layer is formed from a solution of donor and acceptor materials (either polymeric or molecular) in a common solvent or solvent mixture, allows solution processing and the implementation of wet coating and printing techniques. The highly intermixed three-dimensional structure that develops during the subsequent drying step has a profound impact on essentially all of the critical mechanisms of light-toelectricity conversion: light harvesting and exciton generation, exciton separation, and carrier transport to the electrodes. The optimization of the structural properties of BHJs (phase separation, spatial distribution, and crystallinity of the components) to achieve the best photovoltaic performance for a given combination of donor and acceptor materials still remains one of the key challenges in organic solar cells. Although a variety of postprocessing protocols have been applied to optimize the BHJ morphology, i.e., postdeposition thermal and solvent annealing, the use of solvent additives during casting is the most advantageous from a manufacturing perspective because it avoids the need of additional fabrication steps.4−8 The utility of additives was first reported in 2007 by Peet et al. for the low-band-gap polymer poly[2,6-(4,4-bis(2© XXXX American Chemical Society

ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) with [6,6]-Phenyl C61 butyric acid methyl ester (PC61BM). It was shown that the incorporation of a small amount of 1,8-octadecanedithiol (ODT) into the PCPDTBT:PCBM solution in chlorobenzene leads to a doubling of the PCE (from 2.8% to 5.5%) as compared to the films processed from pure chlorobenzene.9 The effect of adding an additive is particularly notable for this system for which the crystallization of PCPDTBT is significantly hindered by the fullerene component and for which attempts to improve the photovoltaic performance through thermal or solvent annealing methods were not successful. Subsequent studies showed that the addition of ODT promotes PCPDTBT ordering and larger scale phase separationboth factors contributing to the dramatic increase of PCE.9−12 Several studies have identified two important properties of ODT also shared by other additives like diiodooctane (DIO): (i) a selective (differential) solubility of the fullerene component and (ii) a higher boiling point than the host solvent.11 The efficacy of ODT or DIO has been Received: January 27, 2016 Revised: June 13, 2016

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DOI: 10.1021/acs.macromol.6b00193 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules attributed to selective solvation of the fullerene, enabling improved polymer order.11,13−17 Although currently additives are commonly used in the fabrication of BHJ organic solar cells, there is still incomplete understanding of their role in the mechanisms of phase separation and organization of donor and acceptor material. Consequently, the choice of a successful additive remains still a matter of trial and error. Real-time, in situ studies of the film formation during solidification and structure evolution have proven to be a powerful tool to elucidate the mechanisms by which additives influence the photovoltaic performance.18−23 Rogers et al. employed grazing incident wide-angle X-ray scattering (GIWAX S) to investigate the r ole o f ODT in PCPDTBT:PC71BM in chlorobenzene (CB) by monitoring the structural evolution after spin coating the films for 2 min.13 They saw the presence of the (100) peak already after the first 2 min of film formation, which suggests that the addition of ODT in the solution leads to a reduction of the nucleation barrier for polymer crystallization. The position of the (100), associated with the alkyl chain packing of PCPDTBT, varies slightly between different works. It was found at q = 0.59 Å−1 (spacing of 10.64 Å) without additive (ODT or DIO) and with shifted q with solvent additive (with q = 0.51 Å−1 and q = 0.54 Å−1).10,23 Rogers et al. reported in both cases, i.e., with and without solvent additive, a mixture of two polymorphs with (100) at q = 0.55 Å−1 along the out-of-plane direction and (100)′ at q = 0.50 Å−1 along the in-plane direction. Additionally a broad scattering ring at q ∼ 1.4 Å−1 was observed which was ascribed to scattering from amorphous PCPDTBT (possibly with contributions from the amorphous PC71BM and possible residual solvent or additive). Because this structural feature overlaps with the strongest reflection of the fullerene component, also a broad ring with q ∼ 1.3 Å−1 for PC71BM and q ∼ 1.4 Å−1 for PC61BM, it is difficult to separate the possible contributions from both. In this work we perform an in situ investigation of the specific role that ODT has in the crystallization of both PCPDTBT and PC71BM, directly after wet film deposition using blade-coated instead of spin-coated films. Blade coating (or knife coating) is a processing method for the fabrication of large area films on rigid or flexible substrates. In contrast to some of the previous works, where the structural evolution during the first minutes of spin coating was not accessible, a custom fabricated drying setup with an integrated blade coater allows us to characterize the development of crystalline order from the earliest instant of the film drying and correlate the structure with the composition of the wet film by combined GIWAXS/reflectometry (Figure 1).24,25 We show that ODT leads to prolonged crystallization time during the ODT-drying-dominated period corresponding to an overall solvent content of 75 wt % > xDCB+ODT > 15 wt % and elucidate its role in the ordering of PCPDTBT and the aggregation of PC71BM.

Figure 1. (a) Scheme of the drying channel designed for real-time grazing incidence X-ray scattering and laser reflectometry experiments. (b) Schematic representation of the X-ray scattering geometry. simultaneously monitored using a drying channel with a reflectometric setup as described elsewhere.24,26,27 For the analysis of the reflectometry measurements the refractive index n = 1.9737 of the blend was obtained from Klein et al. and determined from the same material batch.28 The real-time GIWAXS studies were performed at beamline ID10B in the European Synchrotron Radiation Facility (ESRF) (Grenoble, France) at 40 °C under a nitrogen flow rate of 0.15 m/s. Two-dimensional (2D) diffraction patterns were acquired by an X-ray area detector (MarCCD) in intervals of 18 s at beamline ID10B at ESRF (E = 13.35 keV). The incident angle was set below the critical angle of the substrate (0.135°) but above the critical angle of the blend (0.11°). It should be taken into account that the intensity collected with a flat detector in grazing incidence geometry is not a direct map of reciprocal space and misses a part of the reciprocal space along the vertical direction of the sample.29−31 We give values as a function of the magnitude of the scattering vector q determined from the total scattering angle (Figure SI.1). The point detector was mounted in a second arm of the diffractometer and used to align the sample and to collect X-ray reflectivity data before and after each drying experiment. The acquisition time for each frame was 5 s. After each frame, the sample was moved horizontally in the direction of coating by 0.6 mm (the horizontal beam size was controlled by the horizontal slit width which was 0.5 mm) to measure a fresh region of the film not previously exposed to direct radiation to avoid sample damage. The coherence correlation length is measured from the full width half-maximum (fwhm) of the peaks L ≈ 2π/fwhm(q). This is a convolution of coherence length of the lamellar stacking and crystalline quality. The scattered intensity over time is calculated in a selected region of the 2D diffraction maps (ROI) with the background subtracted. Radial line profiles are obtained from a sector plot radially averaged over an angular width of 20°.

3. RESULTS AND DISCUSSION We use the high boiling solvent o-dichlorobenzene (DCB) instead of the more commonly used chlorobenzene (CB) in order to have a longer time to monitor the initial stages of the structural evolution. Similarly as for CB, adding ODT to DCB led to enhanced donor/acceptor phase separation and improved photovoltaic performance of PCPDTBT:PC71BM solar cells.32 Figure 2 displays the reflectometer signal (a) and the calculated wet film thickness (b) for the drying of PCPDTBT:PC71BM, both in neat DCB and with 3 wt % ODT. The drying process without ODT is initially governed by evaporation of the solvent at a constant rate. The thickness of the film decreases linearly as manifested by the regular appearance of interference fringes. The drying rate strongly decreases after ≈100 s, as indicated by the transition from interference fringes to only small changes in the reflectometer data. During this falling rate period, at high solid fraction, only

2. EXPERIMENTAL SECTION PCPDTBT and PC71BM (>99%) were used as received. Two different solutions were prepared: PCPDTBT:PC71BM (1:3.4 by weight) in pure o-dichlorobenzene (DCB) and in a mixture of DCB and 1,8octanedithiol (ODT, 3 wt %). The solid contents were kept constant at 4.4 wt % for both solutions in order to achieve equal dry film thicknesses. For the drying experiments the solutions were coated on 35 mm × 60 mm silicon substrates by doctor blading at 400 μm slit width, 10 mm/s coating speed, and 60 μL of solution. Immediately after deposition the structural and thickness evolution was B

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profiles of the X-ray scattering intensity obtained from the 2D GIWAXS images (Figure 3b) allow a better observation of the characteristic PC71BM scattering features at q = 1.34 Å−1 (strongest signal), q = 1.90 Å−1, and q = 0.66 A−1 (shoulder), in agreement with GIWAXS data obtained for neat PC71BM films.10,33,34 There is no evidence of structural order of PCPDTBT confirming that the polymer crystallization is impeded by interaction of PCPDTBT and PC71BM due to the miscibility of the two components as previously reported.10,11,13,23,32 The evolution of the solvent mass fraction and the integrated intensity of the ring with drying time are shown in the Figures 3c and 3d, respectively. It can be seen that the aggregation of PC71BM occurs after ∼100 s, during the last stage of drying corresponding to a solvent mass fraction of xDCB < 13 wt %. The onset of PC71BM aggregation in Figure 3d is expected to take place between the last GIWAXS image without (90 s) and the first image with PC71BM structural features (108 s). During this time interval the solvent mass fraction decreases over a wide range, leading to an imprecise determination of the solubility limit. In comparison with the published PC71BM solubility limit (corresponding to xDCB ∼ 95 wt % at room temperature35), the aggregation of PC71BM occurs at a higher solid mass fraction than expected. This is another evidence of the strong fullerene−polymer interaction that keeps PC71BM and PCPDTBT both intermixed. The estimated coherence length L of the PC71BM-rich phase of the blend, calculated using the expression L = 2 π/Δ, where Δ corresponds to the full width at half-maximum (fwhm) of the Bragg scattering peak, gives a coherence length of L = 28 Å for the strongest PC71BM scattering feature. Such small coherence length points to the short-range order of PC71BM forming nanoaggregates as commonly observed in blends with other polymers.34,36 The solidification process is remarkably different when the solution contains ODT as shown in Figure 4a. The first observable X-ray scattering signature from the blend is now an isotropic scattering ring at q = 1.37 Å−1 that appears after 108 s and coincides with the onset of the ODT-dominated drying period. The peak position, with a larger q value than that reported for PC71BM, and absence of the accompanying structural features of PC71BM allow us to assign this ring to an amorphous phase of the polymer, plausibly mixed with PC71BM and/or ODT as reported by Rogers et al.13,20 This is shortly followed by the crystalline formation of PCPDTBT with the presence of the (100) and (100)′ reflectionsand followed only after the evaporation of the majority of ODT by the clear apparition of the structural features due to PC71BM as we will show in detail in the following. Figures 4b and 4c display the radial profiles of the (100) peak and diffraction ring, respectively. The time-dependent change of the ring and (100) intensities is compared and correlated with the time evolution of the solvent mass fraction in Figure 4d. As aforementioned, the scattering diffraction from the mixed amorphous phase of PCPDTBT at q = 1.37 Å−1 appears when there is still a relatively low concentration of solid in the film (xDCB+ODT ≈ 75 wt %). The intensity of the amorphous phase reaches rapidly a steady stage, showing only a slight decrease during the ODT-dominated drying (Figures 4b and 4d). There is a small change in the ring’s broadness which corresponds to a change of crystalline coherence length from 19 to ∼27 Å, which in all the cases indicates very short-range order. At 576 s, the diffraction features characteristic for PC71BM clearly emerge; a shoulder at q ≈ 0.65 Å−1 and peak at

Figure 2. Drying kinetics of PCPDTBT:PC71BM in pure DCB and a mixture of DCB/ODT. Drying experiments were conducted at 40 °C and a nitrogen flow of 0.15 m/s. (a) Reflectometer signal during the drying process. (b) Calculated thickness evolution. Upon the addition of 3 wt % ODT there are two drying stages: a first period where mainly DCB evaporates until 116 s and a prolonged second constant rate period which is governed by ODT evaporation (116−651 s).

residual solvent evaporates from the film leading to small changes of film thickness that are unresolved by the reflectometry setup. When the film is processed with ODT, we observe initially a similar drying period characterized by the constant evaporation rate of DCB. After ≈110 s a drying period dominated by slow evaporation of ODT molecules follows due to the extremely low vapor pressure of ODT (boiling point: 269−270 °C). This is observed as an oscillatory change in the reflectometry signal due to changes in the optical thickness up to ≈650 s. Afterward, the reflectometer signal exhibits only slight changes (falling rate period) which likely originates from the slow removal of residual molecules (DCB and ODT) from the film. Thus, processing with ODT leads to a prolonged drying time for the structure formation (6 times longer compared with additivefree processing) when only ODT remains as a solvent in the film (ODT-dominated drying). In the following figures, the thickness evolution is converted into solvent mass fractions in order to relate the appearance of GIWAXS signals to the solubility limit of the fullerene and to interpret the aggregation mechanisms of PCPDTBT and PC71BM (see Experimental Section). Note that in the case of DCB the solvent mass fraction is referred to as xDCB since DCB is the only solvent in the drying film. For DCB with ODT the overall solvent content, i.e. xDCB+ODT, is given. The drying characteristicscomposition and GIWAXSof the pure DCB solutions processed without ODT are depicted in Figure 3. Selected 2D GIWAXS patterns of the film evolution are shown in Figure 3a. Over time we observe the disappearance of scattering intensity from DCB (broad rings at q ∼ 1.1 Å−1 and q ∼ 1.8 Å−1) and the evolution of a broad diffraction ring at q = 1.34 Å−1 due to PC71BM. Radial line C

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Figure 3. Drying process of PCPDTBT:PC71BM cast from DCB. (a) Selected 2D GIWAXS images. The first image shows the signal of the SiO2 substrate. The subsequent images show the 2D GIWAXS data collected during the drying process. The film composition (solvent mass fraction) is given below each image. (b) Radial profile scans (at an angle of 45°) over time. The arrows indicate the evolution direction of the characteristic q values for dichlorobenzene (DCB) and PC71BM as drying proceeds. (c) Solvent content calculated from laser interferometry. (d) Intensity of the PC71BM ring (q ∼ 1.34 Å−1) over time.

q = 1.90 Å−1. This is accompanied by a progressive shift of the ring to q = 1.34 Å−1 and increase of intensity in the last stage of drying, additionally reflecting the relative increase in the population of nanoscale PC71BM aggregates. The time evolution of the radial profiles over extended drying time is shown in Figure SI.3 for the films processed with and without ODT. These observations show that during the ODTdominated period PC71BM is partly solubilized, allowing the ordering of PCPDTBT. This interpretation is supported by in situ reflectometry and absorption studies by Van Franeker et al., who found that in solutions containing >2 vol % additives polymer aggregation precedes liquid−liquid phase separation that would otherwise result in the formation of large fullerene domains.8 The development of PCPDTBT crystallinity during the ODT-dominated drying period is evidenced by the increase of intensity of the (100) peak at q = 0.53 Å−1 (Figure 4c,d). The (100)′ peak becomes also visible along the in-plane direction at q = 0.541 Å−1. The total scattered intensity from the (100) crystallites was not accompanied by an increase of correlation length (L100 ∼ 135 Å), suggesting that the crystalline fraction of the film increases as a result of additional nucleation events rather than by further growth of the initially formed crystallites. The fact that the amorphous phase is observed before the emergence of the (100) reflection might imply the early formation of polymer aggregates as nuclei for PCPDTBT crystallization. On the other hand, the well-defined spot shape of the (100) reflection along the out-of-plane direction indicates an interface-induced ordering. This interface could be either the top liquid/air interface or the bottom liquid/solid interface. We find plausible that the nucleation

starts at the film−air interface promoted by a higher concentration of ODT at that interface due to the lower molecular density of ODT compared to DCB (0.95 vs 1.28 g/ cm3) and lower surface energy (30.2 vs 36.4 mN/m) at 40 °C processing temperature.37 Between 558 and 666 s, i.e., upon evaporation of most ODT, a rigid shift of the (100) reflection from q = 0.530 to 0.575 Å−1 is clearly visible, which is accompanied by a decrease of intensity. The decrease in spacing from 11.9 to 10.9 Å indicates a densification of the alkyl packing. The fact that this shift occurs rather abruptly at the time at which the aggregation of PC71BM is visible suggests that the more compact packing of PCPDTBT occurs as a result of demixing of PC71BM and PCPDTBT. This process also leads to less ordered crystalline orientation as manifested in a slight increase of the orientation distribution of PCPDTBT (Figure SI.3). These observations suggest that although PC71BM remains solubilized by ODT for 75 wt % > xDCB+ODT > 20 wt %, a fraction is also partially intermixed with PCPDTBT. In accordance with the fact that only a fraction of PC71BM is expected to be completely dissolved in ODT, we expect the onset of PC71BM aggregation at a similar overall solvent content as in the case without ODT, which is the case as can be seen when comparing the evolution of the scattering intensity of the ring at different solvent mass fraction values for film drying in DCB and in DCB/ODT which are shown in Figure 5.38,39 There, a detailed analysis of the peak positions and their assignment to contributions from DCB, PC71BM, and PCPDTBT is given. Residual amounts of solvent might be retained within the dried films as indicated by the continual D

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Figure 4. Drying process of PCPDTBT:PC71BM cast from a mixture of DCB and ODT. (a) Selected 2D GIWAXS images. ODT induces strong ordering of PCPDTBT indicated by the apparition of the (100) peak. The radial cuts used for the radial profiles in parts b and d are marked with dashed lines. (b, c) Radial profile scans at an angle of 45° and 0°, respectively, to show the evolution of the intensity in the ring and (100) peak over time. (d) Solvent content (DCB + ODT) over time compared to integrated intensities of parts b and c. ODT-drying dominated period corresponds to 75 wt % > xDCB+ODT > 15 wt %. During this period the intensity of the ring is mainly dominated by amorphous PCPDTBT (blue) and after 576 s by aggregation of PC71BM (red). At a similar time, the PCDPTBT (100) peak shifts from q = 0.53 Å−1 to q = 0.575 Å−1.

presence of a peak at q = 1.09 Å−1. The higher overall ring intensity in the dried films, in comparison with processing without ODT, is attributed to an increased amount of fullerene nanoclusters in ODT-processed films; see Figures 5b (576 s) and 5c (882 s). Although it has been demonstrated that ODT leads to enhanced separation between polymer and fullerene domains,11 the coherence length estimated for PC71BM in the dried films is similar regardless of the use of additive, indicating that ODT does not significantly affect the crystalline size of the PC71BM aggregates in the film. Figure 6 summarizes schematically the structural evolution of the PCPDTBT:PC71BM blend with/without ODT as processing additive. During film drying the crystallization of PCPDTBT is suppressed in the additive-free films presumably due to strong intermixing with PC71BM. Without ODT, the aggregation of PC71BM is delayed until most of the solvent has evaporated. A primary role of the ODT is to provide extended time for crystallization after the removal of the majority of the solvent (ODT-dominated drying) while PC71BM is partially dissolved in ODT. This enables the aggregation of amorphous polymer shortly followed by the formation of crystalline nuclei. After the slow removal of ODT, PCBM is expelled from PCPDTBT domains, and the aggregation of PC71BM occurs allowing the ordering and densification of the PCPDTBT packing. Although the comparison of the drying without and with ODT reveals that the aggregation of PC71BM occurs at a

similar solid mass fraction, the “dry” state is delayed about 6 times for ODT-processed films. The general aspects of the structural evolution during drying coincide with previous studies using DB as solvent,10,13,23,40 indicating that the intermolecular PCPDTBT−PC71BM interaction, causing the formation of a mixed amorphous state, is the cause of the suppression of crystallization in the additive-free films. As consequence of this, as shown by Morana et al., there is a charge transfer complex involved in a multistep charge recombination process in additive-free PCPDTBT−PCBM blends, which is significantly quenched upon addition of ODT.32 Schaffer et al. have additionally demonstrated by combining GIWAXS and grazing incidence small-angle X-ray scattering (GISAXS) that ODT enhances laterally the microphase separation of PCPDTBT and PC71BM in producing an enlargement of slightly larger domains of more pure materials and vertical phase separation.40 Overall, the use of ODT leads to a morphology with enhanced crystallinity, phase purity, and nanoscale domain size, which presumably leads to lower recombination and more efficient carrier generation. The role of ODT during film formation seems to be general for other polymers: provide larger time for polymer crystallization and at the same time delaying the onset of fullerene aggregation as recently reported for P3HT:PCBM blends.19 The role of solvent additives have also been demonstrated to influence the phase separation and degree of E

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Figure 5. (a) Comparison of radial profile scans for PCPDTBT:PC71BM cast from DCB and DCB/ODT over time showing the evolution of the ring. The corresponding mass fractions are given in wt %. (b, c) Peak analysis for selected scans of part a. Using DCB as the only solvent, a strong PC71BM signal at q = 1.34 Å−1 evolves over time, beginning at 108 s (xDCB < 13 wt %).

Figure 6. Schematic drying process of PCPDTBT:PC71BM cast from pure DCB and a mixture of DCB/ODT. In the initial state all components are dissolved. If no ODT is added, PC71BM impedes polymer crystallization due to strong interactions between polymer and fullerene. At solvent contents