Crystallization of Sensitizers Controls Morphology and Performance in

Mar 17, 2017 - Organic solar cells based on multinary components are promising to further boost the device performance. The complex interplay of the m...
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Crystallization of Sensitizers Controls Morphology and Performance in Si-/C-PCPDTBT-Sensitized P3HT:ICBA Ternary Blends Xiaoyan Du,† Xuechen Jiao,∥ Stefanie Rechberger,‡ José Darío Perea,§ Markus Meyer,† Negar Kazerouni,§ Erdmann Spiecker,‡ Harald Ade,∥ Christoph J. Brabec,§,⊥ Rainer H. Fink,*,†,‡ and Tayebeh Ameri*,§ †

Department of Chemistry and Pharmacy, ‡Institute of Micro- and Nanostructure Research, & Center for Nanoanalysis and Electron Microscopy (CENEM), and §Institute of Materials for Electronics and Energy Technology (I-MEET), FAU Erlangen-Nürnberg, 91058 Erlangen, Germany ∥ Department of Physics, North Carolina State University, Raleigh, North Carolina 27695-7548, United States ⊥ Bavarian Center for Applied Energy Research (ZAE Bayern), Haberstr. 2a, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: Organic solar cells based on multinary components are promising to further boost the device performance. The complex interplay of the morphology and functionality needs further investigations. Here, we report on a systematic study on the morphology evolution of prototype ternary systems upon adding sensitizers featuring similar chemical structures but dramatically different crystallinity, namely poly(3-hexylthiophene) (P3HT) and indene-C60-bisadduct (ICBA) blends with poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)2,1,3-benzothiadi-azole)-5,5′-diyl] (Si-PCPDTBT) and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT), employing energy-filtered transmission electron microscopy (EFTEM) and resonant soft X-ray scattering (RSoXS). In addition, a combined density functional theory (DFT) and artificial neuronal network (ANN) computational approach has been utilized to calculate the solubility parameters and Flory−Huggins intermolecular parameters to evaluate the influence of miscibility on the final morphology. Our experiments reveal that the domain spacing and purity of ICBA-rich domains are retained in SiPCPDTBT-based systems but are strongly reduced in C-PCPDTBT-based ternary systems. The P3HT fiber structure are retained at low sensitizer content but dramatically reduced at high sensitizer content. The theoretical calculations reveal very similar miscibility/compatibility between the two sensitizers and ICBA as well as P3HT. Thus, we conclude that mainly the crystallization of Si-PCPDTBT drives the nanostructure evolution in the ternary systems, while this driving force is absent in CPCPDTBT-based ternary blends.

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INTRODUCTION

transport are much more complex in ternary solar cells than in binary systems. So far, several models have been proposed for the ternary solar cells, namely charge transfer,16−19 energy transfer,20,21 alloy formation,22 and two CT states or parallel like model,23 which are very system dependent. In all cases, the morphology change upon adding a third component in the host matrix must be well investigated in order to fully understand the working mechanism. In previous publications,16−18 the low band gap heteroanalogues poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadi-azole)5,5′-diyl] (Si-PCPDTBT)24 and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-

Organic solar cells (OSCs) have improved considerably in power conversion efficiency (PCE) over the past two decades owing to the effort of new material development, advanced device engineering, and further understanding of device working mechanism.1−6 The most intensively studied polymer:fullerene binary single junction devices based on newly developed polymers still face the problem of having a narrow absorption range over the whole solar spectrum. To increase the absorption range, tandem and/or ternary solar cells are considered to be the most promising strategies. Tandem solar cells have achieved great success by device engineering and materials development.7−12 Although ternary solar cells are much more promising in terms of device fabrication, there are still only a few successful cases.13−22 Understanding the mechanism of success/failure of ternary solar cells is critical for further optimization of such devices. Charge generation and © XXXX American Chemical Society

Received: December 15, 2016 Revised: February 8, 2017

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Macromolecules benzothiadiazole)] (C-PCPDTBT)25 were reported as nearinfrared (NIR) sensitizers in poly(3-hexylthiophene) (P3HT) and indene-C60-bis-adduct (ICBA) blends. It has been demonstrated that there is ultrafast hole transfer from Si-/CPCPDTBT to P3HT after photoexcitation.17 Although the optical properties of Si-PCPDTBT and C-PCPDTBT are quite similar, the former leads to an increased PCE in ternary cells while the latter leads to a severe PCE decrease, which mainly comes from low short-circuit current densities (JSC) and fill factor (FF) (see Table S1). Based on the similarities in energy levels and optical properties of the two sensitizers, the different functionality mainly arises from their influence on the morphology of the host matrix, which in turn has a large influence on charge generation and transport properties. Considering the surface energy derived from contact angle measurements, photoluminescence (PL) spectra, and charge carrier mobility measured by space charge limited current (SCLC) method, it is proposed that the strong recombination by adding C-PCPDTBT are due to the fact that the CPCPDTBT tends to stay in the amorphous mixed P3HT:ICBA and ICBA-rich domains, and thus both the electron transport and hole transport are disturbed, while Si-PCPDTBT interacts more with P3HT-rich domains rather than ICBA-rich domains, which benefits both hole and electron transport.26 However, these predictions based on surface energy might not fully reflect the bulk nanomorphology. At this point, it is essential to characterize the nanomorphology and structural evolution upon addition of sensitizers, especially the changes in fullerene-rich domains and nanostructures of both host and sensitizer polymers. The driving force for the final morphology formation needs to be further clarified beyond the surface energy calculation prediction. Chemically sensitive characterization methods are still needed to obtain deeper insight into the nanomorphology of these ternary blends in order to fully understand the structure−property relationship. For chemical sensitive characterization, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy of light elements like C, N, and O has been proven as a sensitive method to differentiate polymers and fullerene derivatives27 and thus allows for superior chemically sensitive imaging or scattering. The two complementary techniques based on NEXAFS spectroscopy are scanning transmission X-ray microspectroscopy (STXM) in real space and resonant soft X-ray scattering (RSoXS) in reciprocal space.28,29 STXM has been used to measure the absolute composition distribution in polymer:fullerene blends and the miscibility of fullerenes in different polymers.30−33 However, state-of-the-art STXM still faces the limitation of spatial resolution which is around 30 nm in routine operation and thus limits its application to systems with large-scale phase separation. To overcome this limitation but still take advantage of the high chemical sensitivity of NEXAFS spectroscopy, RSoXS was developed, with a q-range corresponding to structure size down to 10 nm and all the way up to micrometers.34,35 The high absorption and scattering contrast at resonant energies of fullerene and polymers has been widely used to determine the domain spacing and domain purities in many kinds of binary systems.36−40 For real-space imaging, conventional bright-field transmission electron microscopy (BFTEM) lacks chemical sensitivity in carbon-rich blend materials. Therefore, energy-filtered TEM (EFTEM), based on electron energy loss spectroscopy (EELS), is used to visualize the material contrast.41−45

In the semicrystalline P3HT and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) binary systems, the crystallization of P3HT and partial miscibility are the main driving force for the solid state nanostructure.43 In ternary systems, the polymer crystallization and the compatibility between polymers and polymer:fullerenes are much more complex. The resulting final morphology could be determined by both crystallization kinetics and equilibrium miscibility. Thus, besides crystallinity, the compatibility/miscibility between the third component and both host polymer and fullerenes must be considered in order to understand the solid-state nanostructure of the ternary blends. Many methods have been reported to determine the miscibility. For example, traditional different calorimetric methods are commonly used to determine the glass transition temperature (Tg), which is a criterion to distinguish the miscibility.46 Also, the difference in surface energy between two components26 and the quenching effect of photoluminescence depending on the amount of quencher can be used to deduce the miscibility.47 In addition, dynamic secondary ion mass spectrometry (SIMS) was explored to study the interdiffusion of fullerenes into polymers upon annealing.48−51 NEXAFS spectroscopy was used to directly measure the composition of polymer:fullerene blends to achieve equilibrium concentrations upon annealing, thus creating a miscibility phase diagram for amorphous portions of the systems investigated.48 However, the later two methods are more suitable to highly phaseseparated systems and are especially not practical to determine the compatibility between polymers. We have previously reported on a combined computational approach based on density functional theory (DFT) and artificial neural networks (ANN) for predicting the solubility parameters of fullerenes.52 In addition, the solubility parameters of the polymers could be determined by the experimental binary solvent gradient method.53,54 A numerical approach can be used to determine relative solubility and then the Flory−Huggins intermolecular parameter χ1,2, which is a fundamental metric of molecular interaction and miscibility, for a polymer:polymer:fullerene ternary systems. Several studies on correlations between the device performances with the molecular interaction parameters have been reported.55−57 While the importance of the interaction parameters for the formation as well as the stability of the complex nanostructures is recognized, a correlation of the device performance and the parameters is not straightforward yet and needs to be studied more extensively.55−57 In this article, we utilize EFTEM and RSoXS together with STXM as complementary chemically and/or material-sensitive characterization techniques to follow the nanostructure evolution of the P3HT:ICBA matrix upon addition of PCPDTBT-based NIR sensitizers. High contrast images for polymer:fullerene nanostructures were obtained by EFTEM measurements. The changes in ICBA-rich domains were revealed by RSoXS measurements. The compatibility/miscibility between polymer and fullerene as well as polymers and polymers were determined to distinguish the driving force for the morphology formation employing thermodynamic model based on ab initio DFT calculations combined with an ANN.52 A detailed morphology model is proposed based on the combined morphology studies and correlated with device performance. The results for the prototype ternary systems presented here pave the way to understand ternary systems in general and further optimize the nanostructure to get functional devices. B

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Figure 1. (a) Chemical structures of the various components in the active layer and (b) corresponding normalized NEXAFS spectra recorded from thin films of the pure materials.

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EXPERIMENTAL METHODS

RESULTS AND DISCUSSION Figure 1 shows the chemical structure of the component materials in the active layer and the corresponding C K-edge NEXAFS spectra of the single-component films as spectroscopic references. The spectra are normalized to pre-edge and post-edge. The ICBA spectrum shows the highest C 1s to π*resonance at the lowest energy (284.5 eV) due to its high conjugation and strong delocalization of the orbitals. The second resonance at 285.0 eV, which is absent for PCBM, is due to the lower symmetry of ICBA compared to PC61BM.27 The P3HT homopolymer shows a prominent resonance at 285.2 eV, while for Si-PCPDTBT and C-PCPDTBT push−pull D−A copolymers, the energy levels are more split due to the various chemical surroundings of C atoms. Si-PCPDTBT and C-PCPDTBT show similar core level excitations, with three main π*-resonance at 284.3, 284.9, and 286.0 eV. The strong absorption contrast between ICBA and polymer compounds at the C K-edge makes STXM measurement feasible. In RSoXS measurements, there are two main contributions to the scattering intensity: one is vacuum contrast originating from surface roughness/bulk voids; the other is material contrast originating from polymer-rich and fullerene-rich interdomain scattering. The choice of scattering energy is based on the material contrast function

Sample Preparation. The photoactive solutions were prepared from different mixing ratios of P3HT (purchased from BASF, Sepiolid P200, regioregularity > 98%, Mw < 50 000 g/mol) and Si-PCPDTBT (provided by Solarmer, Mw ∼ 40 000 g/mol, PDI ∼ 1.8) or CPCPDTBT (purchased from 1-Material, Mw ∼ 38 000 g/mol, PDI ∼ 2.3) with the overall polymer concentration of 1 wt % in odichlorobenzene, blended with ICBA (purchased from Nano-C) with a total polymer:ICBA weight ratio of 1:1. The active layers were doctor bladed from prepared solutions on top of an ultrathin PEDOT:PSS film and annealed at 150 °C for 10 min. Then the films were floated in water and transferred to Cu grids for EFTEM and STXM analysis. Samples for RSoXS were floated onto a Si3N4 window (∼100 nm thick). The films for EFTEM were approximately 50 nm thick, while samples for RSoXS and STXM were approximately 100 nm thick. Near-Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy and Scanning Transmission X-ray Microspectroscopy (STXM). NEXAFS spectroscopy and STXM were performed at the PolLux end station of the Swiss Light Source (Paul Scherrer Institut, Villigen, Switzerland).29 A 25 nm zone-plate focusing is used to achieve around 30 nm lateral resolution. The transmitted X-ray intensity was detected by means of photon multiplier tube. The energy-dependent optical density is calculated through the Lambert− Beer law. The absorption spectra were taken near the carbon K-edge from 278 to 320 eV with 0.1 eV step size in the 278−292 eV range. STXM images were obtained by raster scanning the sample through the focal point and measuring the transmitted photons. Resonant Soft X-ray Scattering (RSoXS). RSoXS experiments were performed in transmission mode at Beamline 11.0.1.2 of the Advanced Light Source (Lawrence Berkeley National Laboratory, Berkeley, CA).34 All measurements were conducted under vacuum (1 × 10−7 Torr) to reduce the air absorption of soft X-ray beam. The size of incident X-ray beam was set as 200 μm × 300 μm by collimating slits. The scattering patterns were collected by a PI-MTE CCD detector cooled down to −45 °C with 2048 × 2048 pixels. The lowand high-q patterns were collected at the sample-to-detector distance of ∼170 and ∼50 mm, respectively. The final sample-to-detector distance was refined by fitting the well-defined PS300 scattering pattern. The 1D reduction of the scattering pattern was performed by using the customized Nika analysis package.58 Transmission Electron Microscopy. The TEM investigations were performed using an FEI Titan Themis3 300 TEM with a high brightness field emission gun (X-FEG) operated at 200 kV equipped with a high-resolution Gatan imaging filter (GIF Quantum) for electron energy loss spectroscopy (EELS) and energy-filtered TEM (EFTEM).

C = |Δδ 2 + Δβ 2|E 4 = Δn2E 4

(1)

where n = 1 − δ + iβ is a material’s complex index of refraction; δ is dispersion, β is absorption, and E is the incident photon energy. The calculated material contrast function for different polymers and ICBA is shown in Figures S1 and S2 (see the Supporting Information). The energy-dependent scattering profiles for the P3HT:ICBA blend is shown in Figure S3, which clearly shows that the resonant energy of ICBA gives the highest scattering intensity in the binary blend. Based on the calculated material contrast function, 284.2 eV was chosen for RSoXS measurements to eliminate fluorescence background as it is below absorption edge and provides high material contrast at the same time. In the following, the nanomorphology of binary as well as ternary blends will be discussed in detail. Nanomorphology in Binary Blends. In the first instance, the morphology of three polymer:fullerene binary blends (weight ratio 1:1) is compared. EFTEM investigations have been performed to directly reveal the nanomorphology in real space. Figure 2 shows elemental maps based on EFTEM imaging of sulfur (S) using the S L-edge and of carbon (C) C

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emphasized (see Figure S1 for contrast function calculation). It is noted that the center-to-center spacing of ICBA-rich domains is probed considering the high material contrast at this energy. The dominant domain spacing was represented by the long period d = 2π/q, where q is the peak location in the spatial frequency distribution.59 The scattering profiles at nonresonant energy (270 eV) are also shown in Figure S4 to give mass− thickness variation related information. As displayed in Figure 3, the three binary blends exhibit distinct scattering profiles with largely different scattering maximum. The dominant domain spacing for P3HT:ICBA is around 27 nm and for Si-PCPDTBT:ICBA around 58 nm. For C-PCPDTBT:ICBA, the broad peak with much lower scattering intensity represents a dominate domain spacing around 38 nm. The domain spacing in P3HT:ICBA blends derived from RSoXS correlates well with the distance between P3HT fibers as observed in EFTEM. The scattering profile of Si-PCPDTBT:ICBA is much broader than P3HT:ICBA, implying more dispersed structures. It is noted that the dramatically different scattering intensity for PCPDTBT-based binary blends indicates that the Si-PCPDTBT:ICBA blend has much larger phase separation than in C-PCPDTBT:ICBA blend, in agreement with the EFTEM results. Nanomorphology in Ternary Blends. The nanomorphology evolution of P3HT:ICBA host matrix upon addition of PCPDTBT-based sensitizers were studied and discussed in this section. The EFTEM investigation and RSoXS profiles for the ternary blends with different concentrations of Si-/CPCPDTBT are shown in Figures 4 and 5, respectively. For P3HT (80%):Si-PCPDTBT (20%):ICBA ternary blends, EFTEM investigations (see Figures 4a and 4e) reveal that the P3HT fibers are well preserved, with several structures quite similar to those in the Si-PCPDTBT:ICBA binary blend (see Figure 2b). From the RSoXS profile (Figure 5a), the q value of the dominant domain spacing remains at the same position as for the binary P3HT:ICBA host matrix. Addition of small amounts of Si-PCPDTBT (20%, weight ratio with respect to P3HT) does not change the P3HT:ICBA phase separation in the films. The reduced scattering intensity is most probably due to the reduced effective scattering volume which is influenced by the location of semicrystalline Si-PCPDTBT in the ternary blends. For P3HT (50%):Si-PCPDTBT (50%):ICBA ternary blends, EFTEM reveals more structures similar to those in the Si-PCPDTBT:ICBA binary blend (Figure 2b). In addition, the P3HT fibrous network is not as obvious as in the case of small Si-PCPDTBT contents. These EFTEM results further confirm the previous speculation that the ordering of Si-PCPDTBT in the ternary films remains or is marginally affected.26 As seen in RSoXS profiles in Figure 5a, the dominant domain spacing increases slightly; a second dominant peak appears around q = 0.05 nm−1, and its intensity increases when more Si-PCPDTBT crystals are present in the blends as shown in EFTEM micrographs. This larger domain spacing is most probably due to the hierarchical structure related to the crystallization of SiPCPDTBT. For P3HT (80%):C-PCPDTBT (20%):ICBA ternary blends, the elemental maps (Figures 4c and 4g) reveal that the P3HT fibers are still present, and no new polymer structures appear as in the Si-PCPDTBT case. The P3HT fiber structure disappears completely for P3HT (50%):C-PCPDTBT (50%):ICBA ternary blends (Figures 4d and 4h). The ICBA-rich domain spacing derived from RSoXS profiles (Figure 5b) increases from ∼30 to ∼50 nm when 20% C-PCPDTBT is added and

Figure 2. EFTEM investigation showing elemental maps of sulfur (a− c) and carbon (d−f) of polymer:ICBA binary blends with weight ratio of 1:1. (a, d) P3HT:ICBA; (b, e) Si-PCPDTBT:ICBA; (c, f) CPCPDTBT:ICBA. The white arrows in Si-PCPDTBT:ICBA blends highlight the polymer structures.

using the C K-edge of the three binary polymer:ICBA blends. Since only the polymers comprise sulfur, the sulfur signal is used to distinguish the polymers from ICBA. Because of the difference of the carbon content of the polymers (CP3HT = 40.0 at. %, CSi‑PCPDTBT = 40.5 at. %, CC‑PCPDTBT = 41.9 at. %) and ICBA (83.0 at. %), the carbon signal can be used to represent ICBA. The three binary blends clearly show different structural features of both polymers and ICBA-rich domains. In the P3HT:ICBA blend (Figures 2a and 2d), P3HT forms random fibrous networks, with fiber diameters of about 10 nm, in a matrix of P3HT:ICBA mixed area. In the Si-PCPDTBT:ICBA blend (Figures 2b and 2e), the bright structures in the S map correspond to the dark features in the C map and thus are attributed to Si-PCPDTBT domains. In the case of CPCPDTBT:ICBA (Figures 2c and 2f), no clear structure is visible, which indicates a good intermixing of the two materials. These results revealed distinct domains of both P3HT and SiPCPDTBT in the polymer:ICBA blends, while the CPCPDTBT:ICBA blend has no obvious structures at the detected length scale. In order to get further information on the ICBA-rich domains, RSoXS was performed on these binary blends. Figure 3 shows the corresponding Lorentz corrected and thickness normalized RSoXS scattering profiles at 284.2 eV, where the compositional contrast between polymer and fullerene is

Figure 3. Lorentz-corrected RSoXS scattering profiles of binary blends at 284.2 eV. The dominant domain spacing is labeled for clarity. The intensity of C-PCPDTBT was scaled by 10 times. D

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Figure 4. Elemental maps of sulfur (a−d) and carbon (e−h) of P3HT:Si-PCPDTBT:ICBA (a, b, e, f) and P3HT:C-PCPDTBT:ICBA (c, d, g, h) ternary blends with weight ratio of 0.8:0.2:1 (a, c, e, g) and 0.5:0.5:1 (b, d, f, h). The white arrows highlight the polymer structures.

Figure 5. Lorentz corrected scattering profiles of (a) P3HT:Si-PCPDTBT:ICBA blends and (b) P3HT:C-PCPDTBT:ICBA ternary blends recorded at 284.2 eV. The dominant domain spacing is labeled for clarity.

further increases to ∼60 nm when 50% C-PCPDTBT is added. In addition, the scattering intensity decreases drastically compared to host matrix. It is noted that volume fraction normalization and contrast normalization have been performed for accurate intensity comparison (Figure S5), and thus this finding indicates that the ternary blends are very well mixed. It is known that the morphology of the P3HT:PCBM system is not thermally stable and PCBM microcrystals are often observed after long time annealing.60 Thus, thermal instability leads to morphology study more sensitive to film aging problems. The thermal stability of the morphology of the system is studied by STXM. Films prepared at device conditions, namely, annealed at 150 °C for 10 min as well as for long time (15 h) annealing stress, were investigated as shown in Figure S6. The STXM micrographs of the P3HT:ICBA blend at different energies are shown in Figure S7. The micrograph taken at resonant energy of ICBA (284.5 eV) features the highest material contrast. It is noted that the nanostructure could not be fully resolved in STXM; however, inhomogeneity of ICBA-rich domains is still probed besides thickness contrast. It is obvious that adding Si-PCPDTBT does not severely affect ICBA aggregations on the probed length scale. After annealing for 15 h, there are no extra micrometersized crystals observed. The coarser microstructure under annealing stress for all three films might be due to the further aggregation of ICBA-rich domains. The STXM images for CPCPDTBT-based ternary blends (not shown here) did not

show clear structure due to the intermixing of C-PCPDTBT with ICBA, which is already observed in EFTEM images (see above). In summary, the nanostructure of the two sensitizers in ternary blends is dramatically different compared to each other: Si-PCPDTBT tends to form crystalline domains, while CPCPDTBT is completely amorphous. The structural features are retained in ternary blends, indicating minor perturbation on their structure formation by either P3HT or ICBA. The two sensitizers feature dramatically different impact on the ICBA aggregations: while Si-PCPDTBT has a minor influence on ICBA-rich domain spacing at low concertation but induces further phase separation at high concentration, C-PCPDTBT mixes strongly with ICBA in the ternary blends. The influence of the two sensitizers on the P3HT nanostructure is quite similar. At low sensitizer content, the nanostructure of P3HT is not severely affected, while at high sensitizer content, the fiber structures of P3HT is severely reduced in both ternary systems. It is noted that from differential scanning calorimetry (DSC) results on P3HT:PCBM binary blends, the melting peak of P3HT disappears when its weight ratio is less than 70%.61,62 Our previous DSC results on P3HT:C-PCPDTBT binary blends indicate no influence on the P3HT crystallinity by adding C-PCPDTBT.61,62 Considering the lower tendency for ICBA to aggregate or crystallize compared to PCBM, it is expected that this drastically reduced amount of P3HT fibers in E

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of these two sensitizers with ICBA in the amorphous region. The interaction strength between Si-PCPDTBT and P3HT is also very close to that between C-PCPDTBT and P3HT. In such a case, nanostructure formation is not determined by the miscibility between polymers in the amorphous region either. In summary, the theoretical calculations of interaction strength based on the determination of the Hildebrand solubility parameters through our previous thermodynamic model suggest that the interactions between the two sensitizers with P3HT and ICBA are quite similar. The mixing enthalpies in the amorphous mixed domains are thus not the driving force for the nanostructure formation. The main driving force for Si-/ C-PCPDTBT-based binary and ternary blends is thus the crystallization enthalpies of the sensitizers. At this point, it is important to note that the Si-PCPDTBT is known to have much stronger intermolecular π−π interaction than CPCPDTBT.24 In consequence, Si-PCPDTBT tends to form pure crystalline phases and forms favorable phase separation with both ICBA and P3HT. While C-PCPDTBT is dominantly amorphous, there is no driving force for such phase separation but rather well intermixing with ICBA-rich domains and the mixed P3HT:ICBA region. Nanostructure Model and Correlation with Device Performance. Based on the present EFTEM, RSoXS, STXM, and theoretical calculation results, a morphology model of the P3HT:ICBA blend with different sensitizers is proposed as shown in Scheme 1. Here, the morphology changes are

ternary blends at high sensitizers content (50%) is kinetically determined during the crystallization process of diluted P3HT. Comparison of Relative Miscibility through Theoretical Calculation. It is known that the nanostructures of P3HT:PCBM system is mainly driven by P3HT crystallization and partial miscibility,43 which should also be the case for P3HT:ICBA binary blends. To understand the driving force for the nanostructure formation in ternary blends, both the crystallinity of the third component and its compatibility/ miscibility with ICBA and the host polymer have to be considered. For this purpose, we explored a numerical approach to determine the relative miscibility for the material systems discussed here. We determined a dimensionless number according to eq 2, which indicates the strength of interactions:63 ΔHmix = χ1,2 ϕ(1 − ϕ) (2) RT Here, ΔHmix is the enthalpy of mixing, R is the gas constant, T is the temperature, χ1,2 is the Flory−Huggins intermolecular parameter, and ϕ is the volume fraction. The Flory−Huggins intermolecular parameter χ1,2 is determined according to eq 3, ν χ1,2 = 0 (δT1 − δT 2)2 (3) RT

where ν0 is the lattice molar volume and δT1 and δT2 are the Hildebrand solubility parameters for two components under discussion. For the determination of the Hildebrand solubility parameters, our previous thermodynamic model based on ab initio DFT quantum chemical calculations combined with an ANN was employed and listed in Table S2.52 The calculated Hildebrand solubility parameters of the polymers are in good agreement with the values determined by the experimental binary gradient method.53 More detailed calculations can be found in the Supporting Information. The Flory−Huggins intermolecular parameters and the interaction strength are shown in Table 1. It is important to note that the values were

Scheme 1. Proposed Morphology Models of P3HT:ICBA Blends upon Addition of Low Content of Different Sensitizersa

Table 1. Intermolecular Parameter χ1,2 and Interaction Strength (ΔHmix/RT) for Polymer:ICBA and Polymer:Polymer Binary Blends with Weight Ratio of 1:1 binary blends

intermolecular parameter (χ1,2)

interaction strength (ΔHmix/RT)

P3HT:ICBA C-PCPDTBT:ICBA Si-PCPDTBT:ICBA P3HT:C-PCPDTBT P3HT:Si-PCPDTBT

0.241 02 0.000 90 0.002 94 0.237 55 0.256 60

0.058 07 0.000 22 0.000 72 0.059 25 0.063 97

a

The semicrystalline Si-PCPDTBT crystallizes in the ternary blend and has a minor influence on ICBA aggregations, while the amorphous C-PCPDTBT tends to mix with ICBA. In addition, the P3HT fibers remain in the two ternary blends.

discussed in three aspects: nanostructures of P3HT, ICBA-rich domains, and the sensitizers. The host P3HT:ICBA matrix contains P3HT fibers, ICBA-rich domains, and amorphous mixed P3HT:ICBA. In ternary blends, semicrystalline SiPCPDTBT forms crystals and preferentially locates in amorphous P3HT:ICBA mixed region, which is verified by the unchanged ICBA-rich domain spacing and the wellpreserved P3HT fibers. Considering the high miscibility of the Si-PCPDTBT and ICBA in amorphous state as the theoretical calculations suggested, the minority noncrystalline Si-PCPDTBT is incorporated in the mixed regions. In contrast, there is no driving force for phase separation between the amorphous C-PCPDTBT and ICBA, leading to intermixed

estimated under the same conditions, and we concentrate our discussion on the relative trend given by the interaction parameters rather than on the absolute values. It is worth mentioning that the calculations are describing the transition from the melt into the solid state for truly amorphous systems and hence are more related to the mixing behavior of amorphous region. For the polymer:fullerene binary blends, the interaction strength between P3HT and ICBA is extremely higher than that between Si-/C-PCPDTBT and ICBA, indicating much higher miscibility in Si-/C-PCPDTBT:ICBA blends than that in P3HT:ICBA blends. The interaction parameters of Si-PCPDTBT and ICBA are quite close to that between C-PCPDTBT and ICBA, indicating similar miscibility F

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but absent in C-PCPDTBT-based ternary blends. We observe a significant relationship of the determined nanomorphology with the FF and Jsc. It is noted that the characterization of the complex nanomorphology and determination of the driving forces for the morphology formation are of great importance in this field, especially for studies trying to relate the solubility/miscibility parameters with the device performance. The present morphology studies reveal fundamental guidelines toward designing potential sensitizers for functional organic ternary solar cells to further expand the absorption range and enhance the device performance.

domains and dramatically reduced purity of ICBA-rich domains. Here, the present findings on nanomorphology of the ternary blends are compared with the device functionalities. The device parameters are shown in Table S1. For devices with addition of Si-PCPDTBT, the Jsc of all ternary cells is higher than that of binary devices (8.0 mA cm−2) and reaches the maximum (10.0 mA cm−2) at 20% concentration. The FF increases at 20% concentration (from 60% to 65%) and decreased at higher concentration due to decreased charge carrier mobility and also increases recombination as shown by previous SCLC and transient photovoltage (TPV) measurements.16 It is worth noting that even with 50% Si-PCPDTBT, the FF is not severely affected (87% of the binary device), and the Jsc is higher compared to the binary devices, which shows completely different behavior compared to C-PCPDTBT-based ternary solar cells. For devices with addition of C-PCPDTBT, even with low content of sensitizer (20%), the Jsc decreases from 8.0 to 6.4 mA cm−2, and the FF decreases from 60% to 54%. The dramatically different changes in the device performance are related to the morphology changes in terms of P3HT and the sensitizers nanostructures and the ICBA-rich domain spacing and purity. For Si-PCPDTBT-based ternary devices, the dominant ICBA-rich domain spacing around 30 nm is retained with sensitizer concentration up to 50%, which guarantees the efficient electron transport, while the intermixing of CPCPDTBT with ICBA as reflected in both EFTEM and RSoXS results is detrimental to charge transport. As for the hole transfer from sensitizers to P3HT (as suggested from previous findings), the crystalline sensitizer in the matrix of mixed regions is definitely better than the sensitizer well-mixed with ICBA, since this intermixing leads to high charge recombination, as indicated from the reduced FF in CPCPDTBT-based ternary blends. At higher sensitizer contents, the reduced amount of P3HT fibers which reflecting the decreased P3HT ordering is another crucial factor for low Jsc and FF due to higher recombination. We should point out that severe morphology destruction and device performance drop were not observed for P3HT:PCBM at low concentration of CPCPDTBT, most probably due to higher aggregation tendency of PCBM compared to ICBA.26,58,64

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02699. Device parameters, chemical contrast function, additional RSoXS profiles, STXM micrographs, detailed theoretical calculation information (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: rainer.fi[email protected] (R.H.F.). *E-mail: [email protected] (T.A.). ORCID

Rainer H. Fink: 0000-0002-6896-4266 Author Contributions

X.D. and X.J. contributed equally to this work. S.R. and M.M. conducted the TEM experiments. J.D.P. performed the theoretical calculation. N.K. helped in preparing samples. R.H.F., E.S., C.B., H.A., and T.A. contributed to discussions and manuscript preparation. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

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The authors declare no competing financial interest.

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CONCLUSIONS In conclusion, the difference in morphology modification of P3HT:ICBA upon the addition of Si-/C-PCPDTBT as sensitizers was thoroughly analyzed by combining EFTEM, RSoXS, and STXM as complementary chemical and/or material sensitive characterization techniques. The relative compatibility/miscibility of the polymers and ICBA were calculated to evaluate the driving force for the ternary morphology. The domain spacing and purity of ICBA-rich domains retain in Si-PCPDTBT systems but is strongly reduced in C-PCPDTBT-based ternary systems. The two sensitizers have similar and minor influence on P3HT fiber structures at low sensitizer’s content. The reduced P3HT fiber structures at high sensitizer content are most probably kinetically determined during the crystallization process of diluted P3HT in ternary blend solutions. The miscibility calculations reveal that the miscibility/compatibility between the two different sensitizers and ICBA as well as P3HT is quite close. We thus conclude that the driving force for morphology changes in the ternary blends is the crystallization of sensitizers itself, which is present in Si-PCPDTBT-based ternary solar cells

ACKNOWLEDGMENTS We acknowledge experimental support at the PolLux beamline by Drs. B. Watts, J. Raabe, and A. Späth. S. Langner and N. Li are acknowledged for inspiring discussions on the miscibility calculations. The STXM study was funded by the Deutsche Forschungsgemeinschaft within the Research Training Group GRK 1896. X.Y.D. and T.A. obtained financial support through the Bavarian initiative “Solar technologies go hybrid”. X.Y.D. gratefully acknowledges the China Scholarship Council (CSC) for her research grant. J.D.P. is funded by a doctoral fellowship grant of the Colombian Agency COLCIENCIAS. T.A., S.R., and C.J.B. acknowledge the projects of Organic Semiconductors for NIR Optoelectronics (OSNIRO, grant 607585), DFG grant BR 4031/2-2, and Synthetic Carbon Allotropes (SFB953). Work by X.C.J. and H.A. was supported by ONR grant N000141410531. Soft X-ray scattering data were acquired at Beamline 11.0.1.2 at the Advanced Light Source (ALS), which is supported by the Director Office of Science and Office of Basic Energy Sciences of the U.S. Department of Energy under Contract DE-AC02-05CH11231. G

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DOI: 10.1021/acs.macromol.6b02699 Macromolecules XXXX, XXX, XXX−XXX