# GIXRD Study of PM6: Y12 Films for Stretchable OPV Applications

Lydia Abbassi, Mouaad Yassine Aliouat, Souren Grigorian, Gilbert A

Chahine, Maxime Dupraz, Nils Blanc, David Duché, Jörg Ackermann,
Stéphanie Escoubas

► To cite this version:

Lydia Abbassi, Mouaad Yassine Aliouat, Souren Grigorian, Gilbert A Chahine, Maxime Dupraz, et al.. GIXRD Study of PM6: Y12 Films for Stretchable OPV Applications. Journal of Applied Polymer Science, 2025, 142 (33), pp.e57324. 10.1002/app.57324. hal-05240544

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RESEARCH ARTICLE

OPEN ACCESS

# GIXRD Study of PM6: Y12 Films for Stretchable OPV Applications

Lydia Abbassi¹,² ID | Mouaad Yassine Aliouat¹,³ ID | Souren Grigorian⁴ ID | Gilbert A. Chahine⁵ ID | Maxime Dupraz⁶ ID |
Nils Blanc⁶ ID | David Duché¹ ID | Jörg Ackermann² | Stéphanie Escoubas¹ ID

¹Aix-Marseille Université, Université de Toulon, CNRS, IM2NP, Marseille, France | ²Aix-Marseille Université, CNRS, CINAM, Marseille, France | ³Univ.

Grenoble Alpes, CEA, Leti, Grenoble, France | ⁴Department of Physics, University of Siegen, Siegen, Germany | ⁵SIMaP, Université Grenoble Alpes,

CNRS, Grenoble INP, Grenoble, France | ⁶Université Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, Grenoble, France

**Correspondence:** Stéphanie Escoubas (stephanie.escoubas@im2np.fr)

**Received:** 20 January 2025 | **Revised:** 18 April 2025 | **Accepted:** 4 May 2025

**Funding:** This work was supported by Aix-Marseille Université.

**Keywords:** mechanical properties | morphology | optical and photovoltaic applications | structure-property relationships | x-ray

## ABSTRACT

Recent advancements in organic photovoltaics (OPVs) have focused on improving both efficiency and mechanical robustness, yet critical insights into the structural and mechanical properties of the materials remain underexplored, particularly in the context of intrinsically stretchable OPVs. These devices hold great promise for applications in wearable and portable electronics, offering exciting possibilities for energy generation in innovative use cases. This study presents novel findings on the structural and mechanical behavior of PM6:Y12 blends, processed using a green solvent and two deposition techniques: spin-coating and blade-coating. We demonstrate that the blade-coating technique, performed in air, results in more ordered blend layers, with a significant increase in crystalline domain size from 67 Å to 136 Å. Additionally, we examine the effects of annealing and focus on the behavior of as-cast blend layers. In an original approach, we studied the mechanical response of these films by in situ GIXRD under tensile strain on stretchable TPU/PEDOT:PSS substrates, revealing that the as-cast PM6:Y12 blend can sustain up to 7% tensile strain. These findings provide valuable insights into the structural and mechanical properties of PM6:Y12 blends, providing a pathway for the development of stretchable OPVs.

## 1 | Introduction

Solar energy is gaining increasing importance as a field of research and industry in the context of climate change, as the need for renewable, green sources of energy is shown [1, 2]. Organic photovoltaic (OPV) cells have emerged as a highly promising technology in the renewable energy landscape, offering unique advantages such as flexibility, lightweight design, and potential for low-cost, large-area production with a diminished carbon print. In the last decade, the rapid growth of OPV technology has allowed for reaching astonishing efficiencies (up to 20% as of 2024), which has brought increasing attention to this branch in the scientific community [3–8]. Unlike traditional silicon-based

solar cells, OPVs can be fabricated on thin, flexible substrates, making them suitable for a wide range of applications, from portable devices and building-integrated photovoltaics to next-generation wearable technology. Nowadays OPV modules close to industrial conditions have been demonstrated with an efficiency of 14.5% on a glass substrate using a nonhalogenated solvent and layers processed by more relevant techniques such as blade-coating [9].

In parallel to the enhancement of efficiency and industrial relevance, there is a growing interest in new applications of OPV such as stretchable OPV cells. Indeed, OPV is one of the very few technologies that allow by its organic nature to produce not

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). Journal of Applied Polymer Science published by Wiley Periodicals LLC.
only flexible but also stretchable cells. The development and optimization of materials for substrates, active layers, charge transport layers, and electrodes has allowed stretchable OPV's to reach encouraging efficiencies while maintaining good performances under high stages of mechanical strain [10–13], with nowadays record efficiency of 14.2% and maintaining up to 80% of initial PCE under 52% tensile strain [14]. This showcases the importance of adequate material selection in this process. For instance, a system that has proven to be particularly effective is the combination of the polymer donor PBDB-T-F, also known as PM6 (Poly[[4,8-bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl]-2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c']dithiophene-1,3-diyl]-2,5-thiophenediyl]), with the small molecule acceptor (SMA) BTP-4F (2,2'-(2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2',3":4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)dimalononitrile), also known as Y6 [15, 16]. So far, all presented work on stretchable OPV was done with materials that are only soluble in halogenated solvent as Y6 for example. Interestingly, SMA Y12, also known as BTP-4F-12 (2,2'-(2Z,2'Z)-((12,13-bis(2-butyloctyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2',3":4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)dimalononitrile) is a modified molecule from Y6 structure, with similar characteristics but with an enhanced solubility in o-xylene [17]. PM6 polymer and Y12 SMA chemical structures are displayed in Figure S1 of the Supporting Information document. PM6:Y12 blend possesses ideal complementary optical absorption properties and electrical properties that support high power conversion efficiencies. Moreover, the chemical stability and solution-processability in o-xylene allow large-scale fabrication using a greener approach, lowering health and environmental risks of OPV processing.

Achieving consistent high performance in stretchable OPVs requires not only a good selection of materials but also a precise understanding of how these materials are crystallized, as nanoscale morphology strongly influences photovoltaic efficiency. Grazing Incidence X-ray Diffraction (GIXRD) has become an indispensable technique for studying the crystalline properties of thin layers used as active layers in OPV structures, enabling researchers to probe molecular packing, polymer chain and molecule orientation, but also various structural parameters within the active layers. By revealing these properties, GIXRD studies yield insights into how these factors impact charge transport and mechanical stability. Furthermore, multiple studies have shown the relationship between crystallinity, efficiency, and mechanical behavior for stretchable active layers [10, 11].

The first part of our study compares two deposition techniques for the active layer: spin-coating and blade-coating. Blade-coating in air using nonhalogenated inks can be considered as a first step toward industrialization and is particularly interesting as it minimizes material waste and aligns more with scalable production methods. For this, it is valuable to compare layer morphology for samples produced using both techniques. Moreover, it is also of interest to industrial users to use the lowest possible

annealing temperatures. The second part focuses on the effects of thermal annealing, which has been shown in numerous studies to enhance crystalline ordering [18, 19] as well as phase separation [20–23]. The present study addresses the effect of thermal annealing on the crystallinity of PM6:Y12 blend, as we intend to go toward lower annealing temperatures in our process since stretchable thermoplastic substrates present poor temperature resistance.

In this study, we investigate the structural properties of PM6:Y12 films processed via spin-coating and blade-coating, with a particular focus on potential anisotropic effects and differences in molecular organization through an analysis of the crystalline parameters. Furthermore, we examine how thermal annealing influences phase separation. Finally, the mechanical behavior of the as-cast blend on a stretchable thermoplastic polyurethane (TPU) substrate has been investigated by in situ GIXRD measurements under tensile strain, allowing us to assess the mechanical performance of TPU/PEDOT:PSS/PM6:Y12 stacks under tensile strain. To the best of our knowledge, no prior studies have explored the morphology of the PM6:Y12 blend under the processing conditions mentioned and for stretchable OPV applications.

## 2 | Comparative Study Between Spin-Coating and Blade-Coating Techniques

This first part aims to compare the crystalline properties of thin films based on PM6 donor and PM6:Y12 blend on a rigid (glass) substrate. For this purpose, samples were processed either by spin-coating in inert atmosphere or doctor-blading in air conditions using identical ink formulation. Samples are then measured in two different directions in x,y plane, noted as parallel and perpendicular and shown in Figure 1 below (for samples deposited by blade-coating, the parallel measurement refers to the X-ray beam being parallel to the deposition direction, while for spin-coating, this direction is arbitrary). For each material, the 2-dimensional GIXRD images measured in the reciprocal space are integrated in order to plot intensity along both in-plane (IP) and out-of-plane (OOP) directions. For this, a radial integration of the image is performed along a chosen integration for every q value, as explained in the experimental section. From the GIXRD IP and OOP plots, the peaks corresponding to the lamellar distance usually labeled 100 and the $\pi-\pi$ stacking labeled 010, are fitted using a Gaussian function from which the crystalline parameters can be extracted. These parameters are the stacking distances in both reciprocal and real space (q and d, respectively, with $q = \frac{2\pi}{d}$) and coherent domain length $L_{CD}$, calculated from peak FWHM (Full-width at half maximum) using Scherrer's equation:

$$L_{CD} = \frac{K \cdot \lambda}{FWHM \cdot \cos\theta} \qquad (1)$$

With K being the Scherrer's constant (0.9 in our case), $\lambda$ being the X-ray wavelength and $\theta$ the Bragg angle.

For the neat PM6 layer, the intense peak in the $q_z$ direction (lamellar distance) as well as the peak in the $q_x$ direction ($\pi-\pi$ stacking) indicate a preferential edge-on orientation (see
<figure><img src="image_3.png" /><figcaption>**FIGURE 1** | Schematic illustration of the GIXRD setup in both parallel and perpendicular configurations (The rectangular-shaped samples shown in this schematic are only for illustration purposes, as the real samples are square-shaped). [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

Figures 2 and 3) for both deposition methods. For the out-of-plane 100 peak, $L_{CD}$ is larger in the case of blade-coating (Tables 1 and 2), indicating better crystallinity. For both deposition techniques, comparing the 100 peaks, along in-plane and out-of-plane directions, reveals similar lamellar distances with differences smaller than 1Å between parallel and perpendicular measurements (around 19Å for IP distance and around 22Å for OOP distance) (see Tables 1 and 2), suggesting no in-plane anisotropic effect of deposition techniques on neat PM6 layers. Our findings thus demonstrate that blade-coating does not induce significant in-plane structural anisotropy compared to spin-coating. Furthermore, blade-coating yields a slight enhancement in the crystallinity of the PM6 donor material. We attribute this increase in domain size to differences in the solvent evaporation rate, which are influenced by both the mechanical impact of the blade-coating technique and the in situ heating applied during the coating process, inducing a rearrangement in the chains. In contrast, spin-coated samples were processed without heating during film deposition.

For the PM6:Y12 blend layer deposited on glass, we measured similar 100 intensities in both $q_x$ and $q_z$ directions (see Figures 2 and 3), indicating no preferential edge-on nor face-on orientation within the blend but rather an isotropic orientation. The $\pi-\pi$ peak appears, this time, on the $q_z$ direction, indicating the existence of a small face-on orientation within the sample. This time again, no in-plane anisotropic effect is noted for both deposition techniques, as the lamellar and $\pi-\pi$ stacking distances are nearly identical between parallel and perpendicular measurements for both methods. It is possible to draw this conclusion since the measurements are carried on square samples (dimensions specified in the Supporting Information), meaning that the footprint is the same for parallel and perpendicular configurations. Additionally, lamellar stacking also shows larger domain

sizes with blade-coating for the PM6: Y12, indicating a more ordered material and with higher coherence length than pristine PM6. Since our blend material does not present an important crystallinity (less than 3 orders of diffraction), we cannot calculate information regarding microstrains and the g factor.

Results obtained on PM6:Y12 blend confirm that blade-coating does not induce significant in-plane structural anisotropy, but yields a significant enhancement in the blend crystallinity.

### 3 | Effect of Thermal Annealing on PM6:Y12 Structure

In the second part of this study, we investigated the effect of thermal annealing on PM6:Y12 blend layers deposited by blade-coating onto glass/PEDOT:PSS substrates. Prior to this investigation, we verified that the underlying PEDOT:PSS layer has no significant impact on crystallinity. This conclusion is based on the absence of notable changes in molecular orientation, interlayer distances, and domain sizes associated with lamellar and $\pi-\pi$ stacking (refer to Figures S2, S3 and Table S1 in the Supporting Information document for GIXRD images, q plots and crystalline parameters). Since we have already demonstrated that the blade-coating technique shows isotropic in-plane deposition, the following measurements were only carried out in one single arbitrary in-plane direction.

Figure 4 shows GIXRD images of PM6:Y12 layers deposited by blade-coating on glass/PEDOT:PSS substrates for a layer annealed at 100°C and for an as-cast layer. For both samples, annealed at 100°C and as-cast, two lamellar peaks can be observed in the IP direction. The first peak (at $q = 0.22 \AA^{-1}$ for the annealed sample and $q = 0.23 \AA^{-1}$ for the as-cast one) is linked
<figure><img src="image_4.png" /><figcaption>**FIGURE 2** | GIXRD patterns for PM6 and PM6:Y12 layers deposited on glass substrates by spin-coating and blade-coating, and measured in the parallel direction. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**FIGURE 3** | GIXRD integrated plots in q for PM6 and PM6:Y12 layers deposited on glass by spin-coating and blade-coating. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

to the Y12 acceptor lamellar stacking (GIXRD images, q plots and crystalline parameters available on Figures S4, S5 and Table S2 on the supporting information document), whereas the second peak at $q=0.31 Å^{-1}$ bears lamellar stacking

contributions from both materials. The first peak appears to be prominent in the case of the as-cast PM6:Y12, although it is also present but less intense for the sample annealed at 100°C for 2 min, whereas it is indiscernible for the sample annealed
<table><caption>**TABLE 1** | Crystalline parameters for PM6 layers deposited on glass by spin-coating and blade-coating and measured in the parallel and perpendicular directions.</caption><thead><tr><th rowspan="3"></th><th rowspan="3"></th><th colspan="6">100 (lamellar)</th><th colspan="6">010 (π-π stacking)</th></tr><tr><th colspan="3">IP</th><th colspan="3">OOP</th><th colspan="3">OOP</th></tr><tr><th>q (Å<sup>-1</sup>)</th><th>d (Å)</th><th>L<sub>CD</sub> (Å)</th><th>q (Å<sup>-1</sup>)</th><th>d (Å)</th><th>L<sub>CD</sub> (Å)</th><th>q (Å<sup>-1</sup>)</th><th>d (Å)</th><th>L<sub>CD</sub> (Å)</th></tr></thead><tbody><tr><td rowspan="2">Spin-coating</td><td>||</td><td>0,29</td><td>22,0</td><td>55,4</td><td>0,33</td><td>18,8</td><td>62,3</td><td>1,68</td><td>3,74</td><td>17,8</td></tr><tr><td>⊥</td><td>0,28</td><td>22,2</td><td>55,4</td><td>0,33</td><td>19,1</td><td>62,4</td><td>1,70</td><td>3,70</td><td>18,0</td></tr><tr><td rowspan="2">Blade-coating</td><td>||</td><td>0,29</td><td>21,9</td><td>58,3</td><td>0,33</td><td>19,2</td><td>83,6</td><td>1,69</td><td>3,72</td><td>18,3</td></tr><tr><td>⊥</td><td>0,29</td><td>21,7</td><td>57,6</td><td>0,34</td><td>18,5</td><td>79,2</td><td>1,73</td><td>3,63</td><td>20,3</td></tr></tbody></table>

<table><caption>**TABLE 2** | Crystalline parameters for PM6:Y12 layers deposited on glass by spin-coating and blade-coating, measured in the parallel and perpendicular directions.</caption><thead><tr><th rowspan="3"></th><th rowspan="3"></th><th colspan="6">100 (lamellar)</th><th colspan="6">010 (π-π stacking)</th></tr><tr><th colspan="3">IP</th><th colspan="3">OOP</th><th colspan="3">OOP</th></tr><tr><th>q(Å<sup>-1</sup>)</th><th>d(Å)</th><th>L<sub>CD</sub>(Å)</th><th>q(Å<sup>-1</sup>)</th><th>d(Å)</th><th>L<sub>CD</sub>(Å)</th><th>q(Å<sup>-1</sup>)</th><th>d(Å)</th><th>L<sub>CD</sub>(Å)</th></tr></thead><tbody><tr><td rowspan="2">Spin-coating</td><td>||</td><td>0,30</td><td>21,1</td><td>67,2</td><td>0,32</td><td>19,9</td><td>93,3</td><td>1,75</td><td>3,60</td><td>19,4</td></tr><tr><td>⊥</td><td>0,30</td><td>21,1</td><td>69,2</td><td>0,32</td><td>19,8</td><td>85,3</td><td>1,75</td><td>3,60</td><td>18,5</td></tr><tr><td rowspan="2">Blade-coating</td><td>||</td><td>0,30</td><td>21,2</td><td>137</td><td>0,32</td><td>19,8</td><td>108</td><td>1,72</td><td>3,65</td><td>18,8</td></tr><tr><td>⊥</td><td>0,30</td><td>21,2</td><td>143</td><td>0,32</td><td>19,8</td><td>114</td><td>1,75</td><td>3,60</td><td>17,3</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>**FIGURE 4** | GIXRD images of PM6:Y12 layers deposited by blade-coating on glass/PEDOT:PSS substrates, annealed (100°C) and as-cast. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

at 120°C for 10 min shown in the first part of the study. This suggests that higher annealing temperatures and more prolonged durations reduce phase separation between the donor and the acceptor materials. Even though this effect has not yet been reported on PM6:Y12 films, a relevant study on OPV active layers using the P3HT donor polymer and a fullerene derivative acceptor [24] indicates that thermal annealing can lead to the migration of the acceptor from a pure phase into a mixed phase, while concurrently causing the donor to separate from the mixed phase into a pure donor phase. This observation is consistent with our findings, where the pure Y12 phase becomes less pronounced, as shown by the reduced intensity of the first 100 peak in as-cast PM6:Y12 samples. The mixed phase becomes more prominent, indicated by the relative increase in the second 100 peak. The as-cast sample also shows higher coherent domain lengths along the lamellar stacking in the IP direction, especially for the Y12-related peak. Azimuthal integration along the 100 ring linked to both

PM6 and Y12 shows less isotropic orientation for the samples annealed at 100°C and the as-cast samples, as well as a higher edge-on orientation proportion compared to those annealed at 120°C (see Table 4). Thus, the annealing at higher temperatures reduces blend edge-on orientation toward a more isotropic one (Figure 5 and Table 3).

There is thus a clear distinction in the morphology of as-cast and annealed blends. In the case of PM6:Y12-based systems, annealing has been beneficial for achieving some of the highest device efficiencies reported [25]. Annealed active layers often exhibit improved exciton dissociation and charge collection, leading to enhanced overall device performance. This effect has been showcased in a previous report on PM6:IT-4F [26] where annealed samples exhibited enhanced hole and electron mobilities in donor and acceptor phases and weaker carrier recombination, leading to higher photocurrent and power conversion efficiency.
## 4 | Deposition on a Stretchable TPU Substrate

Following our examination of the blend's structural properties, we now shift our focus to stretchable TPU substrates sequentially coated with PEDOT:PSS and PM6:Y12. TPU was our substrate of choice, as it has shown superior wettability properties as well as a high elasticity and optical transparency compared to other elastomers and has been used extensively in stretchable OPV fabrication in the literature [10, 11]. Before investigating the structural change within the material during tensile testing, it is important to make sure that the substrate itself is not causing any significant change nor degradation in the active layer morphology. In this part, we analyze the same blend material deposited on a TPU substrate covered by a PEDOT:PSS layer. Figure 6 shows a GIXRD image and q plots for as-cast TPU/PEDOT:PSS/PM6:Y12. The active layers keep a predominant face-on orientation with the presence of two lamellar peaks, similar to the as-cast active layer on glass. The diffuse halo observed at $q=1.4Å^{-1}$ is linked to the amorphous TPU substrate. For lamellar and $π-π$ stacking distances (see Table 5), no major change is observed in comparison to the glass/PEDOT:PSS/PM6:Y12 sample, as the dominant orientation remains face-on

with a slight increase in the face-on oriented population and a decrease in the isotropic orientation (72% face-on, 20% Edge-on and 8% isotropic orientation for TPU sample). Characteristic distances are kept unchanged (lamellar distances are around 28Å and 20Å for Y12 and mixed peaks respectively, and $π-π$ stacking distance 3,4Å for both samples). We can, however, highlight a slight increase in the lamellar domain size along the IP direction for both materials (shifting from 323Å to 393Å for the Y12 peak and from 174Å to 197Å for the mixed peak) and also a slight increase in the OOP direction.

Since no major change in chain/molecule orientation, distances, and domain size is observed in this part of the study, TPU substrate was considered to be suitable for depositing materials stacks for OPV. In the last part, we will show the first in situ GIXRD results obtained under uniaxial stretching.

<figure><img src="image_7.png" /><figcaption>**FIGURE 5** | GIXRD integrated plots in q for PM6:Y12 layers deposited on glass/PEDOT:PSS substrates, as-cast and annealed (100°C). [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

## 5 | Distance Evolution Under Mechanical Strain

We exploit in situ GIXRD measurements under uniaxial tensile stress to study the behavior of polymer chains under stretching. This involves tracking the evolution of a characteristic distance (lamellar or $π-π$) to identify structural changes within the material. For that purpose, the samples were fixed on a uniaxial tensile stage compatible with the diffractometer (see schematic in Figure 7).

The two samples consist of a set of TPU/PEDOT:PSS/PM6 and TPU/PEDOT:PSS/PM6:Y12 stacks. We have chosen to monitor the evolution, under tensile strain, of the distance in the OOP direction; this corresponds to the lamellar distance for the pure PM6 sample due to its edge-on orientation, and the $π-π$ stacking distance for the PM6:Y12 blend, which is related to the predominantly face-on proportion. The two graphs in Figure 8 display strain evolution as a function of stretching.

<table><caption>**TABLE 4** | Orientation proportions for PM6:Y12 layers deposited by blade-coating on glass/PEDOT:PSS with different annealing conditions.</caption><thead><tr><th></th><th>Isotropic<br>(%)</th><th>Face-on<br>(%)</th><th>Edge-on<br>(%)</th></tr></thead><tbody><tr><td>120°C, 10 min</td><td>85</td><td>11</td><td>3,4</td></tr><tr><td>100°C, 2 min</td><td>79</td><td>6,3</td><td>14</td></tr><tr><td>As-cast</td><td>24</td><td>54</td><td>20</td></tr></tbody></table>

<table><caption>**TABLE 3** | Crystalline parameters for PM6:Y12 layers deposited on glass/PEDOT:PSS substrates by blade-coating, annealed (100°C), and as-cast.</caption><thead><tr><th rowspan="3"></th><th colspan="6">100 (lamellar)</th><th colspan="6">010 (π-π stacking)</th></tr><tr><th colspan="3">IP</th><th colspan="3">OOP</th><th colspan="3">OOP</th></tr><tr><th>q (Å<sup>-1</sup>)</th><th>d (Å)</th><th>L<sub>CD</sub> (Å)</th><th>q (Å<sup>-1</sup>)</th><th>d (Å)</th><th>L<sub>CD</sub> (Å)</th><th>q (Å<sup>-1</sup>)</th><th>d (Å)</th><th>L<sub>CD</sub> (Å)</th></tr></thead><tbody><tr><td rowspan="2">Annealed (100°C)</td><td>0,23</td><td>27,1</td><td>174</td><td>0,33</td><td>19,1</td><td>112</td><td>1,77</td><td>3,5</td><td>16,1</td></tr><tr><td>0,31</td><td>20,5</td><td>144</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">As-cast</td><td>0,22</td><td>28,3</td><td>323</td><td>0,33</td><td>19,0</td><td>65</td><td>1,83</td><td>3,4</td><td>32,3</td></tr><tr><td>0,31</td><td>20,4</td><td>174</td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><caption>TABLE 5 | Crystalline parameters for as-cast PM6:Y12 layer deposited on a TPU/PEDOT:PSS substrate.</caption><thead><tr><th colspan="3">100 (lamellar)</th><th colspan="3">010 (π-π)</th></tr><tr><th colspan="3">IP</th><th colspan="3">OOP</th></tr><tr><th>q (Å<sup>-1</sup>)</th><th>d (Å)</th><th>L<sub>CD</sub> (Å)</th><th>q (Å<sup>-1</sup>)</th><th>d (Å)</th><th>L<sub>CD</sub> (Å)</th></tr></thead><tbody><tr><td>0,22</td><td>27,9</td><td>393</td><td>0,33</td><td>19,0</td><td>71,5</td></tr><tr><td>0,31</td><td>20,4</td><td>197</td><td></td><td></td><td></td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>FIGURE 6 | GIXRD image and integrated plot in q for as-cast PM6:Y12 layer deposited on a TPU/PEDOT:PSS substrate. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>FIGURE 7 | Illustration of the in situ GIXRD setup with the tensile stage. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

Stretching represents how much the substrate is elongated relative to its initial length $L_0$ (between the two jaws of the tensile stage). Strain $\epsilon$, on the other hand, describes the relative change in stacking distances $d$ (either lamellar or $\pi-\pi$ stacking distance) at the nanoscale. Stretching and deformation are given by the Equations (2) and (3):

$$ \text{Stretching (\%)} = \frac{L - L_0}{L_0} \times 100 \qquad (2) $$

with $L_0$ representing the initial sample length and $L$ the sample length at the given stage of stretching, and:

$$ \epsilon(\%) = \frac{d - d_0}{d_0} \times 100 \qquad (3) $$

With $d_0$ representing the initial stacking distance and $d$ the stacking distance at the given stage of stretching.

The general trend of the two curves can be described as a primary negative evolution of strain followed by a gradual return to the initial value. The first part reflects a compressive stress faced by the polymer chains in OOP direction, which is related to a Poisson effect (compression in width and thickness in response to stretching along the length). The second part of the curve indicates the return of the chains to their initial dimension following the relaxation of stress. This can be interpreted as degradation of the layer in that area, isolating the measured chains from the rest. This tensile trend was previously observed and described by M. Aliouat et al. in 2020 for the donor polymer PCE11 [27] and PCE11:PC<sub>71</sub>BM blends [28]. An interesting data point is the minimum on the curves: this represents the maximum elongation before layer degradation which could be related to the COS% (Crack Onset) value of the material and, by extension, to the maximum elongation that the cell can withstand during operation. This value is 5% for pure PM6 and 7% for PM6:Y12, which is notably close to the PM6:Y12 blend COS%
<figure><img src="image_10.png" /><figcaption>**FIGURE 8** | (Left) Evolution of lamellar strain for TPU/PEDOT:PSS/PM6 under stretching; (Right) Evolution of π-π stacking strain for TPU/PEDOT:PSS/PM6:Y12 under stretching. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

value reported in literature (5.83%) [29]. This reduced COS% value, compared to those reported earlier by M. Aliouat et al. reaching values beyond 15% [27, 28] can be mainly attributed to the structure of PM6 polymer and Y12 molecule which both contain an important number of fused rings (see chemical structures in Figure S1 of Supporting Information) leading to an increased rigidity and brittleness of the blend material. It is also worth mentioning that the underlying PEDOT:PSS layer may restrict active layer deformation where further investigation is needed to understand their mutual interactions. Although these values can provide insights into the behavior of both the polymer and the blend, it remains difficult to generalize them since we do not have sufficient statistics on the same type of sample.

## 6 | Conclusion

In this study, we conducted a structural investigation of thin organic layers using GIXRD. We analyzed the active layer materials: PM6 donor and PM6:Y12 blend, processed with a green solvent, comparing two deposition techniques: spin-coating and blade-coating. The results revealed no significant differences between the methods, except for slightly larger domains in the blade-coated films, indicating a better crystallinity with this technique. Furthermore, we examined the effect of annealing on the materials, noting the presence of a peak related to Y12 corresponding to the lamellar distance along the IP direction in samples either unannealed or annealed at lower temperatures. This peak disappeared at higher annealing temperatures of 120°C, suggesting a decrease in phase separation with increased annealing. We succeeded in depositing these layers on TPU substrate while maintaining the same distances and crystallinity. This allowed us to study the evolution of these distances induced by uniaxial stress. The behavior of PM6 and PM6:Y12 blend layers on TPU/PEDOT:PSS during in situ stretching shows that the chains underwent compression until reaching the COS% of the

layer, estimated at 5% for neat PM6 and 7% for blend PM6:Y12, after which it slowly reverted to its original length postdegradation. These results show the limitations of using this system for stretchable OPV applications, as every component of the cell must sustain at least 10% strain without cracking or delaminating. This suggests that further strategies are needed to enhance the mechanical compliance, such as using all-polymer active layers, high molecular weight polymers, or additives, in order to increase stretchability.

## 7 | Experimental Section

### 7.1 | Materials

PM6 polymer and Y12 molecule were purchased from ONE-Material, TPU kit (Clearflex 95) was supplied by Smooth-On, and TPU pellets (Clear, F98A) from Lubrizol. High conductivity PEDOT:PSS solution (PH1000, 1:2.5 w/w ratio, 1.3% concentration) was purchased from Clevios. DMF (N, N-dimethylformamide, >99%), DMSO (Dimethyl sulfoxide), GOPS ((3-glycidyloxypropyl)trimethoxysilane) and o-xylene (anhydrous, 97%) were supplied by Sigma Aldrich, and tetralin (4-(2,5-Dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphtha lene-1,2-dicarboxylic Anhydride) by TCI. Capstone FS-30 was supplied by Apollo Scientific.

#### 7.1.1 | Glass Substrate Preparation

Glass substrates were precut at 2.5×2.5 cm² then were ultrasonicated sequentially in soap distilled water, distilled water, acetone, then isopropanol, and were dried with argon after each bath. For PEDOT:PSS-free samples, glass substrates were subjected to a UV-Ozone treatment at 80°C for 15 min for wettability improvement.
### 7.1.2 | TPU Substrate Preparation

For static measurements samples: TPU pellets were dissolved in DMF at the concentration of 200mg/mL⁻¹ and stirred for 48 h. TPU solution was spin-coated twice at 1500 rpm for 1 min on clean glass substrates; each layer was annealed at 100°C for 10 min. A final annealing was carried out at 80°C for 30 min. For measurements under tensile strain samples: Clearflex 95 kit was used by mixing part A and Part B at a 1:1.5 volume ratio. The mixture was deposited by blade coating on clean glass microscope slides (2.5×7.5 cm) with a gap height of 150 µm a volume of 1 mL, and a speed of 3⁻¹mm/s; the resulting layers were left to cure at room temperature for 48 h.

### 7.1.3 | PEDOT: PSS Solution Preparation and Layer Deposition

High-conductivity PEDOT:PSS solution was prepared by mixing 94.5%wt of PH1000 solution, 5%wt DMSO, and 0.5%wt Capstone FS-30 fluorosurfactant to enhance wettability in addition to 0.5%wt GOPS to increase adhesion between the substrates and the PEDOT:PSS films. The mixture was stirred overnight at room temperature. PEDOT:PSS layers were deposited by the blade-coating technique with a gap height of 80 µm, a solution volume of 35 µL and at a speed of 1.5⁻¹mm/s. For annealed samples, PEDOT:PSS layers were annealed at 120°C for 15 min; for as-cast samples, they were left to dry at room temperature for 90 min before active layer deposition.

### 7.1.4 | Neat and Blend Photoactive Materials Solution Preparation and Deposition

PM6 and Y12 inks were prepared at the respective concentrations of 30 mg/mL and 40 mg/mL, and PM6:Y12 at a total concentration of 30 mg/mL with a ratio of 1:1.2, all in a solvent mix of 96.5% o-xylene and 3.5% tetralin. All solutions were stirred overnight at room temperature. Layers are deposited by blade-coating with a gap height of 60 µm, a solution volume of 35 µL and at a speed of 4⁻¹mm/s. The sample was placed on a heating table set at 50°C during the blade-coating process. For annealed samples, layers were annealed either at 120°C for 10 min or at 100°C for 2 min. For glass samples, active layer thickness measured using a mechanical profilometer (Bruker DEKTAK XT) is around 200 nm, and PEDOT:PSS layers are 90 nm thick.

## 8 | GIXRD Experimental Setup

GIXRD measurements are carried out at ESRF (Grenoble), at the beamline BM02-D2AM [30]. The working energy is set to 15 keV. The sample-to-detector distance chosen is 20 cm to allow the detection of all signals relevant to our study. Calibration is performed at the beginning of the experiment using LaB₆ powder. The incidence angle range is set between 0.05° and 0.12° and is optimized for each sample. Measurements are carried out under atmospheric conditions, with each scan lasting 10s to collect sufficient signal intensity without damaging the material. Some samples are measured consecutively in both the parallel and perpendicular directions, with the sample holder rotated by 90°

for this purpose. A 2D IMXPAD D5 photon counting detector was used to collect the diffracted signal in two directions simultaneously ($q_z$ and $q_x/y$). The detector consists of 8 modules with a total area of 12×7.5 cm² and generates images of 560×960 pixels, with pixel dimensions of 130×130 µm². A lead beamstop is used in order to protect the detector from the direct beam. Data analysis was performed using Python-based scripts developed at the beamline as well as open-source software developed at the ESRF, including PyFAI [31]. This script was used for 2D image extraction, radial integration for generating 1D q plots, and azimuthal integration for generating pole figures. For radial integration, the azimuthal range was over 20° for both IP and OOP directions. For orientation proportions, we calculated the relative ratios for areas under the pole figures (obtained by azimuthal integration along a q interval corresponding to the width of the 100 signal) for each orientation range. To determine peak position and FWHM for relevant peaks, Open-source software Fityk was used to fit the plots with a Gaussian model.

## Author Contributions

**Lydia Abbassi:** data curation (lead), writing – original draft (lead). **Mouaad Yassine Aliouat:** investigation (equal), validation (equal), writing – review and editing (supporting). **Souren Grigorian:** investigation (equal), resources (supporting), writing – review and editing (supporting). **Gilbert A. Chahine:** investigation (supporting), software (supporting). **Maxime Dupraz:** investigation (supporting), software (supporting). **Nils Blanc:** investigation (supporting), software (supporting). **David Duché:** funding acquisition (supporting), supervision (equal), writing – review and editing (equal). **Jörg Ackermann:** supervision (equal), writing – review and editing (equal). **Stéphanie Escoubas:** funding acquisition (lead), investigation (equal), supervision (lead), writing – review and editing (lead).

## Acknowledgments

We express our heartfelt thanks to the beamline personnel at the synchrotron ESRF—beamline BM02, especially Stephan Arnaud and Ravi Raj Purohit Purushottam Raj Purohit for their exceptional support in setting up the equipment and providing the data treatment script.

## Conflicts of Interest

The authors declare no conflicts of interest.

## Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author.

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## Supporting Information

Additional supporting information can be found online in the Supporting Information section.