# Exploring interface tension as a tool to control the morphology of binary organic semiconductor nanoparticles prepared by nanoprecipitation

Alexandre Holmes, Ingemar Persson, Marc Schmutz, Alejandro Salinas-Villasenor, Matthieu Gamarde, Valentina Maruzzo, Beatrice Galliano, Julie Cairney, Sylvain Chambon, Christine Lartigau-Dagron, et al.

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## ► To cite this version:

Alexandre Holmes, Ingemar Persson, Marc Schmutz, Alejandro Salinas-Villasenor, Matthieu Gamarde, et al. Exploring interface tension as a tool to control the morphology of binary organic semiconductor nanoparticles prepared by nanoprecipitation. Nanoscale, 2025, 17 (36), pp.21121-21131. 10.1039/d5nr01734c. hal-05287135

HAL Id: hal-05287135

https://univ-pau.hal.science/hal-05287135v1

Submitted on 5 Dec 2025

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# Exploring Interface Tension as a tool to control the Morphology of Binary
Organic Semiconductor Nanoparticles prepared by Nanoprecipitation

Alexandre Holmes¹, Ingemar Persson², Marc Schmutz³, Alejandro Salinas-Villasenor¹, Matthieu Gamarde¹,⁴,⁵, Valentina Maruzzo¹, Beatrice Galliano¹, Julie Cairney⁶,⁷, Sylvain Chambon⁸, Christine Lartigau-Dagron*¹, Natalie Holmes⁶,⁷,⁸, Antoine Bousquet*¹

1. Université de Pau et Pays de l'Adour, CNRS, IPREM, Pau, France

2. Lund University, Centre for Analysis and Synthesis, S-2210 00, Lund, Sweden

3. Université de Strasbourg, CNRS, Institut Charles Sadron, PLAMICS, F67000 Strasbourg, France

4. InnoVent, Villeneuve d'Ascq, France

5. Université de Bordeaux, CNRS, Bordeaux INP, IMS, UMR 5218, F-33400 Talence, France

6. Australian Centre for Microscopy and Microanalysis, University of Sydney, Sydney, NSW 2006, Australia.

7. School of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney, Sydney, NSW 2006, Australia.

8. The University of Sydney Nano Institute, Faculty of Science, University of Sydney, Sydney, NSW 2006, Australia.

## Abstract:

The ability to control the morphology of organic semiconductor nanoparticles is of paramount importance for applications in organic photovoltaics, photocatalysis, and photo-triggered biological applications. In this paper, we demonstrate that nanoprecipitation is a powerful technique to provide a variety of morphologies in binary blends of organic semiconductors. By investigating seven different donor:acceptor couples we demonstrate that the resulting morphology is primarily governed by the interfacial tension between the two photo-active components. The structure of the particles is also influenced by the interactions between the medium and the materials. Indeed, we show that the medium should not be considered solely as water; rather, the surfactant employed and the organic solvent in which the materials are dissolved also play crucial roles. By manipulating these parameters, P3HT:PC61BM nanoparticles were produced via nanoprecipitation, exhibiting either intermixed, Janus, or core-shell morphologies depending on the dispersive medium. Cryo-TEM and STEM-EDX were utilized to image the internal structure of the particles for couples involving PC61BM and non-fullerene acceptors (NFA) such as Y6 and a polymer P(DTS-DAP). The increased surface tension between the donors and PC61BM generally results in the formation of Janus nanoparticles. Conversely, the NFAs used in this study exhibit a higher compatibility for the donor, thereby promoting in some cases an intermixed structure.
TOC

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Morphological control of binary organic semi-conductor nanoparticles prepared by nanoprecipitation.

Introduction

Organic semiconductors have been widely investigated in the last decades for a wide range of applications, and more specifically for their ability to convert the energy provided by the sun.¹,² Their π-conjugated backbones often functionalized by alkyl side chains allow for solution-processability, significantly reducing the cost of implementation in electronic devices.³ However, most of usual organic solvents exhibit a high toxicity. Thus, solubilizing/dispersing organic semiconductors in water is relevant to efficiently use their attractive properties such as light absorption or emission, heat generation or conductivity in biological/aqueous media, or simply to increase the safety of their handling.⁴⁻⁷ For example, the active layer of organic photovoltaics (OPV) is usually casted from an organic solution in which the semiconductors are solubilized.³ Recently, aqueous inks of organic semiconducting nanoparticles were designed to achieve greener processing of OPV, with impressive efficiencies above 10%.⁸⁻¹¹ Another pertinent subject regarding aqueous dispersions pertains to their application in the photocatalytic (PC) production of hydrogen (H₂), wherein water serves as the preferred medium, supplying the protons.¹²⁻¹⁴

In these applications, the best performances were obtained when a blend of two organic semiconductors (electron donor and acceptor materials) were used.⁸,¹⁵ These two materials present slightly shifted energetic levels, allowing the dissociation of the photo-generated excitons into free charges, that will then be diffusing through the materials towards the interface with electrodes (in OPV) or water (in PC). It has been well-established for decades that the optimization of the morphology of the donor:acceptor (D:A) blend is essential in achieving optimal performance. Striking the right balance between maximizing the D:A interfaces to enhance exciton dissociation and maintaining the longest possible pristine domains for the diffusion of free charges (thereby preventing recombination) is
critical. Thanks to the achievement of optimised donor:acceptor blends, power conversion efficiencies up to 20.8% were achieved in organic solvent-based OPV.¹⁶

Recently, cryogenic transmission electron microscopy (cryo-TEM), synchrotron-based scanning transmission X-ray microscopy (STXM) and low energy/low dose rate scanning transmission electron microscopy (STEM) energy dispersive X-ray spectroscopy (EDX) mapping succeeded to reveal the internal composition structure of D:A nanoparticles.¹⁷,¹⁸ Depending on the methodology employed, whether miniemulsion or nanoprecipitation, as well as the parameters utilized during the synthesis of the nanoparticles, various internal morphologies may be obtained, such as core-shell (CS), Janus (J), or intermixed (I) structures (Figure 1a). The specific D:A organization will ultimately influence the performance of the application. For instance, Holmes *et al.* demonstrated that CS nanoparticles resulted in suboptimal organic photovoltaic (OPV) performance, due to charges becoming trapped within the core of the nanoparticles. It was only after appropriate annealing, which formed percolating channels through the particles, that the OPV performance was enhanced by a factor of five.¹⁹ Similarly, in the context of photocatalysis, Kosco *et al.* demonstrated that a D:A intermixed morphology proved more advantageous than a core-shell structure when using the same semiconductors, resulting in a ninefold improvement in hydrogen production.²⁰

Recent studies have emphasized that nanoprecipitation is a suitable method for nanoparticle preparation,²¹ owing to its straightforward principle, scalability, and performance in various applications.²²⁻²⁵ In the nanoprecipitation process, the organic solvent diffuses rapidly into water, providing only microseconds for the D:A pairs to organize through nucleation and growth,²⁶ a process that was initially thought to lead to intermixed nanoparticles.²⁷,²⁸ However, we previously reported the spontaneous formation of Janus nanoparticles (presenting a biphasic face-to-face separation) by nanoprecipitation in the case of P3HT:PCBM nanoparticles.²⁹ Consequently, it is crucial to understand the driving forces behind D:A nanoparticle formation when employing the nanoprecipitation technique. Beyond organic semiconductors, the formation of complex morphologies through nanoprecipitation has been documented,³⁰,³¹ with experimental observations supported by computational predictions of the final morphology.³² Yet, there are few reports that analyse the key parameters influencing structuration during nanoprecipitation.

In this study, the experimental nanoparticle morphologies obtained by nanoprecipitation for a range of donor:acceptor pairs were compared with the phase diagram developed by Li *et al.* Cryo-TEM and STEM-EDX were utilized to experimentally characterize the internal morphology of each system.³² The observed morphological alterations were subsequently correlated with the photophysical properties of the resulting dispersions (light absorption and emission). The definition of the dispersive medium
was first investigated. While it is generally considered as pure water, it is herein demonstrated that the
nature of the organic solvent and the surfactant are of primary importance and significantly influences
the P3HT:PC61BM morphology. Then the range of D:A nanoparticles was extended to fully polymeric
systems, fullerene and non-fullerene-based acceptors (NFAs) (Figure 1b) to investigate the versatility
of the phase diagram and the range of obtained morphologies.
<figure><img src="image_4.png" /><figcaption>**Figure 1.** (a) Schematic representation of core-shell (CS), Janus (J) and intermixed (I) nanoparticles, alongside the expected impact towards charge generation and extraction. (b) Chemical structure of all donor and acceptor materials used in this work. The blue box highlight donor materials while the red box highlights acceptor materials.</figcaption></figure>

## Results & Discussion

The prediction of the binary nanoparticle morphology *via* the nanoprecipitation technique is primarily contingent upon a thorough understanding of the underlying mechanism. The formation of nanoparticles is governed by the miscibility between an organic solvent, in which the materials are initially dissolved, and a non-solvent, which in this case is water. Upon introducing a small volume of the organic solution into water, the solubilized materials reach a saturated concentration within the mixed medium. This triggers the first stage of nucleation, swiftly followed by a second stage in which the nuclei grow through the accumulation of the surrounding insoluble material.

Li *et al.* developed a model based on molecular dynamics in which the final nanoparticle morphology is driven by the interactions between both D and A materials, but also by their individual interactions towards the dispersive medium.³² These interactions are expressed by the donor:acceptor interfacial tension (γ^D:A) and organic semiconductor:medium interactions (γ^D:M and γ^A:M) (Figure 2a). If the
interfacial interaction between the acceptor material and the medium, the acceptor solubility, is lower than that of the donor material, it indicates a propension for the acceptor to precipitate first in the dispersive medium, forming the core of the particle. Then the precipitation of the donor material follows, creating the core-shell (CS) morphology with a donor shell. In cases where both materials exhibit similar solubility the dispersive medium, both are expected to precipitate simultaneously, resulting in the formation of Janus nuclei. The latter then grow with preferential accumulation, leading to the formation of Janus (J) nanoparticles. Based on this model, an additional morphology can be observed: the off-centered core-shell morphology, which represents a transitional state between the Janus and core-shell configurations.

While this model has been successfully applied to non-conjugated polymers, it remains essential to explore its versatility and limitations. For instance, although the model describes the formation of homopolymer nanoparticles, it is typically resulting from a very low compatibility between the two materials and characterized by high yD:A values, the presence of intermixed nanoparticles is not accounted for. However, one might anticipate that highly miscible materials, when yD:A values are low, would exhibit reduced phase segregation. Furthermore, this model does not address the potential role of complex solvent mixtures or surfactant molecules at the interface, which could influence the donor:acceptor (D:A) structuration. While water (W) serves as the non-solvent in our study, the organic solvent or surfactant used during the preparation of the aqueous dispersion may also be considered part of the medium during the nanoparticle's formation process. Consequently, the first parameter examined in this report is the dispersive medium used in the nanoprecipitation.
<figure><img src="image_5.png" /><figcaption>Figure 2. (a) Theoretical evolution of nanoparticle morphology as a function of their interfacial interactions of donor (D, blue color) and acceptor (A, brown color) materials with the medium (M). Adapted from ref. 32. (b) Plotted interfacial interactions of P3HT:PC61BM nanoparticles for various media, with identification of the expected morphology (c) Chemical structure of surfactants and organic solvents investigated during the preparation of P3HT:PC61BM nanoparticles.</figcaption></figure>

# 1. Determination of the dispersive medium nature with P3HT/PC61BM as a case study

P3HT:PC61BM nanoparticles were investigated due to their cost-effectiveness, distinct contrast in electron microscopy, and the valuable insights gained from previous studies.²¹,²⁹ Composite nanoparticles were produced using the nanoprecipitation method, and experimental conditions led to nanoparticles with a size ranging from 40 to 100 nm (Table 1, Table S1).

## Influence of the surfactant.

Surfactants are key in the preparation and processing of donor:acceptor nanoparticles for energy-related applications, providing both stability and tuning interfacial properties.³³,³⁴ Previous reports highlighted the impact of sodium 4-(2-(thiophen-3-yl)ethoxy)butane-1-sulfonate (TEBS) in miniemulsion, resulting in intermixed morphologies rather than core-shell structures when sodium dodecyl sulphate (SDS) was employed.²⁰,³³,³⁵⁻³⁸ However, the literature lacks of morphological investigation in the case of the nanoprecipitation when introducing aromatic surfactants. The
conjugated materials were first solubilized in THF and then precipitated in aqueous solutions of surfactants (entries 1 and 2 in Table 1). Our previous study evidenced that a Janus morphology was achieved for P3HT:PC<sub>61</sub>BM synthesised in the absence of surfactant or with CTAB or pluronic F127.<sup>21,29</sup> Herein, two anionic surfactants are compared, presenting a similar hydrophilic head but different hydrophobic tails: sodium dodecyl sulphate (SDS) and the aromatic surfactant sodium 4-(2-(thiophen-3-yl)ethoxy)butane-1-sulfonate (TEBS) (Figure 2c). Previous studies, focused on miniemulsion, report the influence of TEBS as increasing intermixed morphologies rather than core-shell structures when SDS was employed, resulting in enhanced photocatalytic efficiency.<sup>20,33,35,36</sup> Herein, interfacial interactions were calculated from P3HT and PC<sub>61</sub>BM surface energies, each medium surface tensions (considering water, THF and surfactant), and resulting contact angles (see equations and values in Supporting Information, Tables S2-S10, Figures S1 and S2). All surface tension ratios were calculated and reported in the Figure 2b diagram. While a purely aqueous medium would lead to a core-shell (CS) morphology (with the acceptor forming the shell), the introduction of the organic solvent (THF) or the surfactant (SDS), or both into the medium causes a shift towards a Janus morphology.

<table><caption>Table 1: Experimental conditions and resulting size of D:A nanoparticles.</caption><thead><tr><th>Entry</th><th>Donor:Acceptor</th><th>Solvent</th><th>[D/A]<br>(mg/mL)</th><th>Solvent/W<br>Vol. ratio</th><th>Surfactant</th><th>[surfactant]<br>(mg/mL)</th><th>Size<br>(nm)</th><th>PdI</th></tr></thead><tbody><tr><td>1</td><td>P3HT:PC<sub>61</sub>BM</td><td>THF</td><td>1.0</td><td>1/8</td><td>SDS</td><td>3.0</td><td>60</td><td>0.13</td></tr><tr><td>2</td><td>P3HT: PC<sub>61</sub>BM</td><td>THF</td><td>0.5</td><td>1/8</td><td><b>TEBS<sup>a</sup></b></td><td>0.5</td><td>90</td><td>0.08</td></tr><tr><td>3</td><td>P3HT: PC<sub>61</sub>BM</td><td><b>Pyridine</b></td><td>1.0</td><td>1/8</td><td>SDS</td><td>3.0</td><td>50</td><td>0.21</td></tr><tr><td>4</td><td>P3HT: PC<sub>61</sub>BM</td><td><b>Picoline</b></td><td>1.0</td><td>1/8</td><td>SDS</td><td>3.0</td><td>40</td><td>0.19</td></tr><tr><td>5</td><td>PTQ10:PC<sub>61</sub>BM</td><td>THF</td><td>1.5</td><td>1/8</td><td>SDS</td><td>3.0</td><td>80</td><td>0.19</td></tr><tr><td>6</td><td>P(CPDT-DPP):PC<sub>61</sub>BM</td><td>THF</td><td>2.0</td><td>1/4</td><td>SDS</td><td>3.0</td><td>80</td><td>0.28</td></tr><tr><td>7</td><td>P3HT: P(DAP-DTS)</td><td>THF</td><td>2.0</td><td>1/4</td><td>SDS</td><td>3.0</td><td>70</td><td>0.15</td></tr><tr><td>8</td><td>P3HT:Y6</td><td>THF</td><td>2.0</td><td>1/4</td><td>SDS</td><td>3.0</td><td>90</td><td>0.17</td></tr><tr><td>9</td><td>P(CPDT-DPP):Y6</td><td>THF</td><td>2.0</td><td>1/4</td><td>SDS</td><td>3.0</td><td>90</td><td>0.29</td></tr><tr><td>10</td><td>PTQ10:Y6</td><td>THF</td><td>0.5</td><td>1/8</td><td>SDS</td><td>3.0</td><td>90</td><td>0.18</td></tr></tbody></table>

Experimental observations of nanoparticles prepared with SDS revealed a Janus morphology (Figure 3a, Figure S3), as observed in Cryo-TEM images where the PCBM domains appear darker compared to P3HT due to diffraction contrast. Interestingly, when the surfactant in the predictive model was altered, a stronger interface tension between Water:THF:TEBS and P3HT suggested a transition from the Janus to a CS morphology with acceptor shell (Figure 2b). Surprisingly this shift was not observed experimentally, and, on the contrary, the presence of a particle blend comprising Janus NPs alongside

<footnote><sup>a</sup> in bold the parameter changing compared to entry 1</footnote>
core-shell structures featuring a well-defined donor shell (P3HT) around an acceptor core (PCBM) was identified (Figure 3b, Figure S4). This result underscores the importance of considering the surfactant component in predictive models to accurately determine internal morphology. Notably, TEBS, which includes a hydrophobic tail containing a thiophene like in the P3HT repetitive units, may lead to preferential aggregation of P3HT at the outer shell of the nanoparticle, and could result in an higher threshold of water solubility for P3HT due to strong intermolecular interactions.³⁹ The CS nanoparticles displayed distinct crystalline P3HT domains with clear lamellar stacking observed throughout the nanoparticle shell (Figure S5). TEBS is a surfactant which differs from classical one, such as SDS or CTAB. It only slightly decreases the water surface tension from 73 to 69 mN/m, and does not form micelles, consistently leading to larger nanoparticles.³³In miniemulsion, TEBS was found to promote an intermixed morphology between P3HT and PCBM (using chloroform as a solvent), whereas SDS under the same conditions resulted in a PCBM core with a P3HT shell.²⁰,³³,³⁵⁻³⁸ This was attributed to TEBS’s higher miscibility with Chloroform/P3HT solution, encouraging the creation of smaller dispersed donor domains rather than a single large shell. However, in the present case, this greater miscibility is not observed, as indicated by the high contact angle of a water:THF:TEBS drop on a P3HT surface (90°), which is notably higher than that of water:THF:SDS (59°) and even higher than water:THF without any surfactant (72°). This suggests that the interaction with THF may be influencing the γ<sup>D:M</sup> interface tension, requiring further exploration.
<figure><img src="image_6.png" /><figcaption>Figure 3. Cryo-TEM images of P3HT:PC<sub>61</sub>BM nanoparticles prepared through nanoprecipitation in presence of different media: (a) THF precipitated in a SDS-containing water medium, (b) THF precipitated in a TEBS-containing water medium, (c) pyridine precipitated in a SDS-containing water medium and (d) picoline precipitated in a SDS-containing water medium.</figcaption></figure>

**Influence of the organic solvent.** After we established that the surfactant influences the morphology of binary nanoparticles produced through nanoprecipitation, the organic solvent was the next parameter to be studied. THF, pyridine, and picoline were selected as organic solvents due to their ability to solubilize P3HT and PC<sub>61</sub>BM, as well as their miscibility with water. The selection of these solvents from an extended list was based on Hansen solubility parameters (Table S9) and experimental considerations, leading to the exclusion of some of them. Some solvents, like acetone, were ruled out due to their low solubility for P3HT, while others, like methyl-THF, were excluded due to their low water-miscibility, both of which would result in aggregation. The size of the nanoparticles was observed to decrease slightly as the solvent was changed from THF to pyridine, and then to picoline (Figure 3c, 3d and Figures S6 and S7). Previous reports on nanoprecipitation have described such influence of the
solvent on nanoparticles size.⁴⁰,⁴¹ More efficient mixing was linked to a higher solvent/water miscibility, resulting in the formation of smaller polymer nanoparticles. The Hansen solubility parameters (HSP) of water, THF, pyridine and picoline were compared, and the distance Ra between molecules were calculated (formulas in SI and Table S11, the HSP being 3D coordinates). Indeed, the smaller the distance Ra the more miscible the molecules are. For water/solvent molecules, Ra evolves in the order picoline > pyridine > THF, therefore the smallest nanoparticles should have been obtained with THF. Therefore, we hypothesized that the pKa of the aromatic amines could also play a role. However, pyridine and picoline have a pKa of 5.2 and 6 respectively, so at pH 7.5 (the pH of our water/SDS solution), these organic solvents are mainly in their neutral form (1/30 and 1/200 of the picoline and pyridine molecules are protonated, respectively). The NPs size difference could also be explained by the solubility of active materials in solvent in the initial organic mixture. Since it was not possible to measure the contact angle of good solvents on P3HT or PCBM films because of solubility, HSP were also calculated. Ra, reported in Table S11, showed that both P3HT and PCBM solubility is higher in picoline than in pyridine and then in THF, helping to obtain smaller nanoparticles.

Concerning the morphology, the Janus behavior obtained with THF seems to be gradually lost when the solvent is switched to pyridine and even more with picoline (Figure 3c and 3d), already proving the effect of the solvent nature. These cryo-TEM observations agree with the dispersions photophysical behavior. Figure 4a highlights that the absorption spectra of P3HT:PC<sub>61</sub>BM dispersions from THF present clear shoulders at 550 and 600 nm, characteristic of P3HT crystallization. On the contrary, the dispersions made from pyridine and picoline does not show these shoulders and additionally present a maximum of absorption slightly blue-shifted below 500 nm, also suggesting a lower trend to crystallize. Figure 4b reports the photoluminescence spectra of the P3HT:PC<sub>61</sub>BM dispersions. As expected, the mixing of PCBM with P3HT lead to the donor emission quenching by D:A electron transfer, which is lower in the case of the CS morphology obtained with the TEBS surfactant. For the three other dispersions, the quenching is almost quantitative indicating a sufficient degree of intermixing even in the case of the Janus morphology. Calculating y<sup>D:M</sup> and y<sup>A:M</sup> with pyridine and picoline leads to lower values than with THF and thus the modelled morphology would lead to pristine particles made of one materials (bottom left of the phase diagram in Figure 2b)). This is obviously not the case, the cryo-TEM images and the photophysical properties suggesting more an intermixed morphology. These results demonstrate that the materials:(good)solvent interactions should be taken into consideration, not only at the dispersion stage but also during the solubilization step, to define the morphology of binary nanoparticles prepared by nanoprecipitation.
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>Figure 4. (a) Absorption and (b) emission spectra of P3HT:PC61BM nanoparticles prepared with different surfactants and organic solvents. Photoluminescence data were collected on diluted dispersions presenting an absorption of 0.1 at the excitation wavelength, 500 nm.</figcaption></figure>

## 2. Extension to different Donor:Acceptor systems

From a selection of several donor and acceptor materials, interfacial tensions were calculated (Figure 5a, Table S8 and S9). Out of 9 donor:acceptor couples, 7 were selected in order to compare different miscibility: three PC61BM based blends showing high D:A interfacial tensions γD:A (above 10 mN m⁻¹), three Y6 based blends with lower γD:A ranging from 1 to 8 mN m⁻¹ and one polymer:polymer blend (1.7 mN m⁻¹). Contact angles were performed for all systems using four different media: water, water:THF, water:SDS and water:THF:SDS, and the interfacial interactions with the medium were calculated for each donor:acceptor blends (Tables S7 and S8). The predicted morphologies were significantly affected by the selected medium, with a shift from a predominant core-shell (with a donor shell) when water alone was used as the medium (Figure S2). However, and based on our previous investigation, both THF and SDS must be considered in the dispersive medium. This led to a shift of morphologies to mostly
Janus ones (Figure 5b), except for P(CPDT-DPP):Y6, which should display a CS morphology with a Y6 core (Figure 5c).

All dispersions were prepared from a THF solution quickly added to an aqueous solution containing SDS. Experimental conditions were optimized to achieve nanoparticle sizes under 100 nm, stable and reproducible dispersions (entries 5 to 11 in Table 1, Table S1, Figure S8).

<figure><img src="image_9.png" /><figcaption>Figure 5. (a) Mapping of interfacial tension between donor (X axis) and acceptor (Y axis) materials, scaling from 1 to 16 mN m⁻¹. (b) Plotted interfacial interactions of donor:acceptor nanoparticles selected in this work in a water:THF:SDS medium. (c) Schematic representation of all morphologies predicted by the model, classified in two categories: low miscibility (red box) and high miscibility (green box).</figcaption></figure>

**D:A materials with high interface tension.** The low compatibility between the polymers (P3HT, PTQ10, P(CPDT-DPP)) and PC₆₁BM was expressed by high interfacial tensions. According to the calculated interaction parameters, Janus morphologies are predicted for the three polymer: PC₆₁BM:Water/THF/SDS mixture in all three systems (Figure 5b). Experimentally, the structure of PTQ10:PC₆₁BM and P(CPDT-DPP):PC₆₁BM nanoparticles was identified as Janus through Cryo-TEM imaging (Figure 6a, Figures S9-S10). Interestingly, in the P(CPDT-DPP):PC₆₁BM samples, off-centered core-shell morphology was observed, with larger polymer domains at the surface of the particles. This is consistent with the lower γD:M for P(CPDT-DPP) compared to PTQ10, indicating a greater tendency for the donor to cover the PC₆₁BM at the medium interface. Additionally, larger P(CPDT-DPP):PC₆₁BM
nanoparticles were observed, displaying multifaceted morphologies (Figure S10). Such morphologies
could result from the nanoparticle growth mechanism, where multiple Janus nuclei aggregate to form
multi-patched nanoparticles.<sup>30</sup>

<figure><img src="image_10.png" /><figcaption>Figure 6. a) Cryo-TEM images for donor: PC<sub>61</sub>BM systems (polymers in light grey, PC<sub>61</sub>BM in dark grey). b) STEM-EDX C-K, O-K, N-K, F-K, and S-K maps of PTQ10: PC<sub>61</sub>BM particles</figcaption></figure>

As we evidenced in previous works²⁹,⁴² PC₆₁BM allows for an easy identification of the internal morphology by TEM. However, when using NFAs or polymers acceptors with limited short-range order, cryo-TEM present limitations with a very low contrast between materials. To overcome this issue, non-destructive STEM-EDX is a relevant technique which relies on the unique elemental identifiers of materials with nm resolution.¹⁷ In the case of donor and acceptor blends showing different elemental composition, a X-ray mapping can be used to localise specific elements. To attest the validity of the morphology extracted with STEM-EDX, the PTQ10:PC₆₁BM Janus system was first investigated, and evidenced once again a Janus morphology. Figure 6b shows the STEM-EDX C-K, O-K, N-K, F-K, and S-K maps of the particles (see EDX spectra and additional maps in Figure S11). C is homogeneously distributed in the particles while S and N, atoms only present in the PTQ10 structure, are arranged anisotropically within the particles, in agreement with a Janus morphology.
**D:A materials with low interface tension.** The use of NFAs significantly enhanced the miscibility of polymer donor and acceptor blends, as confirmed by lower interfacial tensions < 8 mN m⁻¹ (Figure 5a). While cryo-TEM was not able to reveal these D:A morphologies (see supplement Figures S12-S15), the high-resolution elemental mapping offered by STEM-EDX allowed us to investigate in detail the nanoparticle composition of donor-acceptor blends without short-range.

Focusing on P3HT-based systems, we can see that the shift from PC₆₁BM, to P(DAP-DTS) and finally Y6 significantly reduced the interfacial tension between both donor and acceptor (Figure 5a). From the clear depiction of Janus nanoparticles for P3HT:PC₆₁BM, the morphology of P3HT:P(DAP-DTS) is more challenging to unravel. Both CS and Janus morphology are observed in the STEM-EDX maps and reported in the left and right side of Figure 7a, respectively. Concerning the CS morphology, P3HT was observed in the core with an increased abundance of the sulphur composition. On the contrary, N-Si N-K and N-O maps showed a higher concentration in the shell of the particles (see additional image in Figure S16). The presence of the oxygen was attributed to the hydrolysis of the thioether side chains of P(DAP-DTS) in water. The dual CS and Janus morphology is indeed explained, by the phase diagram, in which P3HT:P(DAP-DTS) blends lies on the predicted boundary between Janus and CS.

In the case of P3HT:Y6 blends, which exhibit the lowest interfacial tension among all the systems studied, the model predicts a Janus morphology; however, this prediction places it far on the upper side of the phase diagram. Indeed, it has been documented that P3HT and Y6 have a strong compatibility, which discourages phase segregation.43,44 This behavior is further supported by the UV-visible spectrum of P3HT:Y6 nanoparticles, which shows a blue-shift in the P3HT contribution (Figure S18, 480 nm) compared to other P3HT-acceptor nanoparticles. Similarly, a hypsochromic shift in the Y6 contribution (Figure S18, 768 nm) is evidenced in P3HT:Y6 nanoparticles, in contrast to pristine Y6 nanoparticles (820 nm). In addition, all dispersion displayed complete Photoluminescence quenching, indicating a well optimized morphology to promote exciton dissociation (Figure S19). Consequently, the morphology of P3HT:Y6 nanoparticles is likely to be intermixed rather than Janus-like. Such a low interfacial tension likely promotes high miscibility between the two components, resulting in very low photocatalytic efficiencies because of low charges extraction.45
<figure><img src="image_11.png" /><figcaption>Figure 7. STEM-EDX maps of (a) P3HT:P(DAP-DTS) particles showing C-K, O-K, N-K, Si-K, and S-K maps, and (b) P(CDPT-DPP):Y6 particles showing C-K, O-K, N-K, F-K, and S-K maps.</figcaption></figure>

In the case of P(CPDT-DPP):Y6, a CS morphology is expected from the model. However, the only different element between both systems is F (Y6). Notably, it was shown that PTQ10:PC61BM mapping of F-K alone may not be enough to identify internal morphology (Figure 6b). The relatively weak F terminations may be broken during scanning resulting in loss of F over time.¹⁷ One can however quantify the elemental ratio of common elements to differentiate elemental concentrations in the blends (Table S12). The O/C ratio is higher for P(CPDT-DPP) while the N/C is for Y6 which gives contrast in the O-K (abundant in the shell), and N-K maps (abundant in the core, Figure 7b). This CS morphology agrees with the model, Y6 forming the core due to a higher interface tension with the medium than the polymer.⁸

Finally, PTQ10:Y6 nanoparticles were synthesized and analyzed by cryo-TEM (Figure S15). As expected, no information on the internal morphology could be obtained, but images showed that the size distribution (ranging from 50 to 100 nm) and the shape dispersity of the sample are high. Indirect characterization, such as photoluminescence, allowed to ensure a proper exciton quenching, but this does not attest for a specific morphological identification (Figure S20). In that case, since the donor and
acceptor were composed of the same elements in a very similar ratio, STEM-EDX were not investigated.
This highlights the necessity for the development of additional advanced characterization techniques
to better elucidate the structure of some binary organic nanoparticles.

## Conclusion

In this paper we have shown that nanoprecipitation can lead to different morphologies in binary blends of organic semiconductors. The morphology is driven by the interface tension between donor and acceptor that will dictate the extent of D:A segregation even though the microsecond is the time-scale of particle formation. The particles structuration is also affected by the interaction between the medium and the photo-active materials. Herein, we proved that the medium should not only be considered as water alone, but the surfactant used and the organic solvent in which the materials are solubilised should be included in calculations, at both the solubilization and the dispersion steps. By playing with these parameters, P3HT:PC61BM nanoparticles were produced by nanoprecipitation with an intermixed, a Janus or a core-shell morphology. Cryo-TEM and STEM-EDX were used to image the inner particles structuration for couples made with PC61BM and NFA. A higher surface tension between donors and PC61BM usually leads to Janus nanoparticles in the water:THF:SDS medium. On the contrary, the NFAs used in this study have a higher compatibility with the donor and favoured an intermixed constitution. Controlling the morphology of these nanoparticles is of primary importance for OPV, photocatalysis or photo-triggered biological application.

## Conflict of interest

There are no conflicts of interest to declare

## Data availability statement

The data supporting this article have been included as part of the Supplementary Information and
others can be shared on demand to the corresponding author.
## Acknowledgments

The authors thank E2S UPPA for funding the research. This project has received additional financial support from the institut Carnot ISIFOR through the 2022 ISIFOR call of proposals and by ANR agreement (ANR - 22 CARN 0019 01). This project has also received financial support from the CNRS through the MITI interdisciplinary programs through its exploratory research program. S.C. acknowledges the support provided by the ANR through the WATER-PV project N°ANR-20-CE05-0002-01. The company InnoVent is acknowledged for funding through the CIFRE process. The authors acknowledge the technical and scientific assistance of Sydney Microscopy & Microanalysis, the University of Sydney node of Microscopy Australia. We further thank the support of the University of Sydney Core Research Facilities.

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