# Halogen Bonding as a Tool to Control Morphology and Charge Transport in Organic Semiconductors

Tero-petri Ruoko, Marc-Antoine Stoeckel, Rakesh Puttreddy, Chi-yuan Yang, Silan Zhang, Ziang Wu, Nuno R Candeias, Paolo Samorì, Han Young Woo, Simone Fabiano, et al.

► To cite this version:

Tero-petri Ruoko, Marc-Antoine Stoeckel, Rakesh Puttreddy, Chi-yuan Yang, Silan Zhang, et al. Halogen Bonding as a Tool to Control Morphology and Charge Transport in Organic Semiconductors. Angewandte Chemie International Edition, 2025, 64 (21), 10.1002/anie.202424979. hal-05212710

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# Halogen Bonding as a Tool to Control Morphology and Charge Transport in Organic Semiconductors

Tero-Petri Ruoko,\* Marc-Antoine Stoeckel, Rakesh Puttreddy, Chi-Yuan Yang, Silan Zhang, Ziang Wu, Nuno R. Candeias, Paolo Samorì, Han Young Woo, Simone Fabiano,\* and Arri Priimagi\*

**Abstract:** The solid-state structure and morphology of organic semiconductors (OSCs) are critical in determining the performance of organic electronic devices, as they directly influence charge carrier mobility. Improved molecular packing and ordering are key to achieving better device performance. While halogen bonding (XB) has been extensively used in supramolecular crystal engineering, its potential for organic electronics remains largely untapped. Here, we show that strong and directional XB can significantly enhance the morphology and optoelectronic properties of a pyridine-substituted naphthalene diimide (NDI) derivative. By employing various XB donors with differing interaction strengths and comparing them to a nonbonding reference compound, we achieve a two-orders-of-magnitude improvement in field-effect mobility and $I_{ON}-I_{OFF}$ ratio. Furthermore, the geometric substitution pattern of the XB donors is found to strongly influence molecular packing, crystalline domain formation, and, ultimately, device performance.

## Introduction

Organic semiconductors (OSCs) are key active materials for a wide range of (opto-)electronic applications,

including light-emitting diodes,[1,2] photovoltaics,[3,4] field-effect transistors,[5,6] thermoelectrics,[7,8] and electrochemical devices.[9,10] The energetics and charge transport properties of OSCs[11–14] are strongly influenced by their molecular packing and film morphology,[15] which in turn directly affect the performance of OSC-based devices.[16]

The self-assembly of molecules into thin films is governed by the interplay of intramolecular, intermolecular, and interfacial interactions. Fine-tuning intermolecular interactions among the constituent $\pi$-conjugated (macro-)molecules is particularly effective for achieving optimal OSC film morphologies. Weak intermolecular interactions, such as van der Waals forces,[17] $\pi-\pi$ stacking,[18] noncovalent chalcogen interactions,[19,20] and hydrogen bonding (HB),[21–23] can be fine-tuned to achieve uniform molecular packing, thereby enhancing overall morphology. HB, commonly observed in naturally occurring $\pi$-conjugated pigments,[21] has been used in synthetic OSCs as conformational locks to noncovalently planarize the semiconductor core.[24–26] However, its reliance on specific functional groups and susceptibility to competing interactions in polar environments can limit its applicability in synthetic $\pi$-systems, particularly those processed from organic solvents.

Halogen bonding (XB), by contrast, offers a highly directional and tunable noncovalent interaction (R-X···B), where a halogen atom (X) covalently bound to an electron-withdrawing group (R) interacts with a nucleophile site (B), such as a lone electron pair, another halogen, or a $\pi$ system.[27,28] Although XB shares similarities with HB,[29] it provides several advantages for organic electronics. XB features superior directionality (nearing 180°), enabling precise molecular alignment and packing, and a tunable strength that increases in the order $F \ll Cl < Br < I$.[28,30] Unlike HB, which requires polar functional groups and can lead to solubility issues,[31] moisture sensitivity,[32] and electron traps

[*] T.-P. Ruoko, R. Puttreddy, A. Priimagi
Chemistry and Advanced Materials, Faculty of Engineering and Natural Sciences, Tampere University, Tampere FI-33720, Finland
E-mail: tero-petri.ruoko@tuni.fi
arri.priimagi@tuni.fi

M.-A. Stoeckel, S. Fabiano
Wallenberg Initiative Materials Science for Sustainability, Department of Science and Technology, Linköping University, Norrköping SE-60174, Sweden
E-mail: simone.fabiano@liu.se

M.-A. Stoeckel, C.-Y. Yang, S. Fabiano
n-ink AB, Norrköping SE-60221, Sweden

M.-A. Stoeckel, C.-Y. Yang, S. Zhang, S. Fabiano
Laboratory of Organic Electronics, Department of Science and Technology, Linköping University, Norrköping SE-60174, Sweden

Z. Wu, H. Y. Woo
Department of Chemistry, College of Science, Korea University, Seoul 136-713, Republic of Korea

N. R. Candeias
LAQV-REQUIMTE, Department of Chemistry, University of Aveiro, Aveiro 3810-193, Portugal

P. Samorì
University of Strasbourg, CNRS, ISIS UMR 7006, 8 Allée Gaspard Monge, Strasbourg 67000, France

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

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© 2025 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbH. 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.
that hinder charge transport,[33] XB is less polar and more
robust to environmental factors,[34] making it better suited for
OSC applications.

XB has demonstrated its versatility in fields like crys-
tal engineering,[35,36] soft materials,[37,38] and biomolecular
systems.[39,40] In optoelectronics, it has been used to enhance
the luminescence of organic dyes[41–43] and to improve inter-
facial engineering in perovskite solar cells.[44–46] However,
its potential for controlling OSC morphology and charge
transport remains largely untapped. Most existing studies
focus on crystal engineering[47–49] or halogen interactions
in halogenated OSCs,[50–53] with only a limited number of
halogen-bonded OSC devices reported.[54–56] Furthermore,
systematic investigations into the use of XB-donating addi-
tives to tune OSC morphology are lacking. While XB is widely
utilized in supramolecular crystal engineering to optimize
molecular assembly,[57–60] its tunable interaction strength has
yet to be fully harnessed in organic electronics.

Here, we employ XB to control the morphology of
an *n*-type naphthalene diimide (NDI)-based small molecule
functionalized with pyridine groups in the 2,6-positions,
comparing it to a reference NDI lacking XB-accepting groups.
Blending the semiconductors with strong XB donors, such
as 1,4-diiodotetrafluorobenzene, induces highly directional
XB interactions with the pyridine nitrogen. The result-
ing solution-processed XB semiconductors exhibit higher
crystallinity and larger crystalline domains, influenced by
the halogen substitution geometry of the donor molecules.
These morphological improvements lead to a two-orders-
of-magnitude enhancement in both field-effect electron
mobility and *I*<sub>ON</sub>-*I*<sub>OFF</sub> current ratio compared to the pristine
molecule. Comprehensive experimental and theoretical char-
acterization reveals the mechanisms by which XB-induced
morphology changes enhance semiconductor performance.
These findings highlight XB as a powerful tool for tailoring
the morphology and functionality of organic semiconductors,
paving the way for further advancements in the field.

Results and Discussion

The chemical structures of the XB-accepting NDI (NDI<sub>Pyr</sub>) and the nonbonding reference (NDI<sub>Ph</sub>) are shown in Figure 1a. NDI was chosen as the π-conjugated core due to its symmetrical substitution capability, remarkable charge transport properties, environmental stability, and solution processability.[61-64] To enhance solubility in common solvents, the NDI core was N,N'-substituted with linear *n*-dodecyl (C₁₂) chains. The XB-accepting NDI<sub>Pyr</sub> and the nonbonding reference NDI<sub>Ph</sub> were functionalized with 4-pyridyl and phenyl groups in the 2,6-positions, respectively. For the blends, we employed six different additives (see Figure 1a): the nonhalogen-bonding reference HFB, the hydrogen-bond donor TFB (which lacks XB ability), the brominated XB donor DBTFB, and three iodinated XB donors **1,2-DITFB**, TITFB, and **1,4-DITFB**. The iodinated compounds were selected to study the effect of iodine substitution geometry on morphology and device performance. The concept of XB-controlled packing is illustrated in Figure 1b,

where the halogen-bonded complex **NDI<sub>Pyr</sub>·1,4-DITFB** is
expected to form somewhat linear chains along the pyridine
direction, whereas the NDI-cores are expected to π-stack
strongly in the perpendicular direction.[65]

Density functional theory (DFT) calculations at the PBE1PBE/LanL2DZ (I), 6-31G(d,p) levels were performed to evaluate the complexation energies between one **NDI<sub>Pyr</sub>** and two XB donor molecules (Figure 2a). The results show that **NDI<sub>Pyr</sub>·DBTFB** has a complexation energy of −6.3 kcal mol<sup>−1</sup>, whereas the iodinated XB donors form stronger complexes with **NDI<sub>Pyr</sub>**, with similar energies of −10.6 kcal mol<sup>−1</sup> (**1,2-DITFB**), −10.7 kcal mol<sup>−1</sup> (**TITFB**), and −11.0 kcal mol<sup>−1</sup> (**1,4-DITFB**). These comparable complexation energies among the iodinated donors provide a solid foundation for examining how the iodine substitution geometry affects the packing and crystallization of the NDI complexes. Similar calculations for the complexation between **NDI<sub>Ph</sub>** and two **1,4-DITFB** molecules show a weaker complexation energy of −5.58 kcal mol<sup>−1</sup> (Figure S1, Supporting Information). The complexation does not occur through an interaction with the phenyl moieties but with the carbonyl groups in **NDI<sub>Ph</sub>** that act as better halogen bond acceptors.

Time-dependent density functional theory (TD-DFT) and
orbital analysis of **NDI<sub>Pyr</sub>** and **NDI<sub>Pyr</sub>·1,4-DITFB** reveal dis-
tinct differences in the nature of their first vertical excitation
(Figure 2b,c). For **NDI<sub>Pyr</sub>**, the first excitation predominantly
involves a transition between the frontier orbitals, with
additional contributions from buried occupied orbitals. Upon
coordination with two **1,4-DITFB** molecules, the first excita-
tion becomes dominated by transitions between the frontier
orbitals, with the HOMO and LUMO discretely localized in
the XB donor and acceptor, respectively. Similar behavior was
observed for **NDI<sub>Pyr</sub>** complexes with **DBTFB**, **1,2-DITFB**,
and **TITFB** (Figure S2), although the first vertical transition
in **NDI<sub>Pyr</sub>·DBTFB** includes significant contributions from
several other occupied orbitals (Table S1). Furthermore, the
LUMO level of the complexes decreases by up to 0.28 eV
depending on the strength of the XB interaction, as shown
in the frontier molecular orbitals in Figure S2.

We confirm the occurrence of XB in the solid-state
using Raman spectroscopy. Typically, the Raman peaks of
the uncomplexed C–X bonds appear in the 100–400 cm⁻¹
range and redshift upon XB formation due to a weakening
of the C–X vibration.[67,68] The Raman spectra of drop-
cast films of **1,4-DITFB**, **NDI<sub>Pyr</sub>**, and **NDI<sub>Pyr</sub>·1,4-DITFB**
(Figure 3a) demonstrate this effect. Specifically, the C–I
Raman peak of **1,4-DITFB**, initially at 158 cm⁻¹, redshifts to
153 cm⁻¹ upon XB formation. Similarly, the Raman peaks of
**NDI<sub>Pyr</sub>** at 199 and 237 cm⁻¹ blueshift to 204 and 240 cm⁻¹,
respectively, due to the I···N interaction.[69] These shifts are
consistently observed in all iodinated XB blends, with the
largest changes for **NDI<sub>Pyr</sub>·1,4-DITFB** (Figure S3), indicating
that its molecular packing strongly favors XB interaction.
In contrast, the Raman spectra of **NDI<sub>Pyr</sub>·DBTFB** and the
reference blends show no observable changes, suggesting that
the interaction strengths are insufficient to induce detectable
spectral shifts.

UV-vis absorption spectra of spin-coated films of **NDI<sub>Pyr</sub>** and **NDI<sub>Pyr</sub>·1,4-DITFB** (Figure 3b) further support the role
<figure><img src="image_4.png" /><figcaption>**Figure 1.** a) Chemical structures of the compounds used in this study: nonbonding *N*,*N*'-bis(dodecyl)-*(2,6)*-diphenyl-1,8:4,5-naphthalenetetracarboxdiimide (NDI<sub>Ph</sub>) and XB acceptor *N*,*N*'-bis(dodecyl)-*(2,6)*-dipyridyl-1,8:4,5-naphthalenetetracarboxdiimide (NDI<sub>Pyr</sub>), nonbonding additive hexafluorobenzene (HFB), HB donor 1,2,4,5-tetrafluorobenzene (TFB), brominated XB donor 1,4-dibromotetrafluorobenzene (DBTFB), and the iodinated XB donors 1,2-diiodotetrafluorobenzene (1,2-DITFB), 1,3,5-triiodotrifluorobenzene (TITFB), and 1,4-diiodotetrafluorobenzene (1,4-DITFB). b) A hypothetical illustration of the XB-driven organization of the NDI<sub>Pyr</sub>-1,4-DITFB complex. XB between NDI<sub>Pyr</sub> and 1,4-DITFB is indicated with pink ellipses.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** a) DFT-calculated complexation energies between one NDI<sub>Pyr</sub> and two XB donor molecules. b,c) Representation of the natural transition orbitals<sup>[66]</sup> and orbital composition for the first vertical excitations of b) NDI<sub>Pyr</sub> and c) NDI<sub>Pyr</sub>-1,4-DITFB. Representation of pure frontier molecular orbitals can be found in Figure S2.</figcaption></figure>

of XB. XB results in a blueshift of the absorption onset from 459 to 408 nm, while the first absorption peak, corresponding to the π-π* transition of the NDI core, shifts from 394 to 382 nm. Additionally, the absorption onset of NDI<sub>Pyr</sub>-1,4-DITFB becomes steeper, indicating more uniform molecular packing, reduced energetic disorder in the film, and lower

scattering, as evidenced by the reduced sub-bandgap slope. The UV-vis spectral shifts are not merely due to packing changes but are associated with an energetic shift in the semiconductor caused by XB formation as seen by the shifts in the absorption onset and maximum positions as well as the varying vibrational contributions in the spectra.
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Spectroscopic proof of halogen bonding in the **NDI<sub>Pyr</sub>·1,4-DITFB** complex. a) Raman spectra, b) UV-vis spectra, and c) PYS emission yields of **NDI<sub>Pyr</sub>** and **NDI<sub>Pyr</sub>·1,4-DITFB** film samples.</figcaption></figure>

Photoemission yield spectroscopy (PYS) was used to measure the ionization potentials of the two NDI semiconductors, both with and without additives. PYS is a nondestructive technique that analyzes the electronic structure of thin films in their native environment without requiring ultra-high vacuum conditions. As shown in Figure 3c, the ionization potential of **NDI<sub>Pyr</sub>** decreases significantly in the **NDI<sub>Pyr</sub>·1,4-DITFB** complex. No energetic changes are observed for blends of **NDI<sub>Ph</sub>** and **1,4-DITFB** in either UV-vis or PYS measurements (Figure S4). For **NDI<sub>Pyr</sub>**, nonbonding additives such as **HFB** and **TFB** slightly reduce the ionization potential from 5.39 ± 0.04 eV (pristine **NDI<sub>Pyr</sub>**) to 5.28 ± 0.07 eV (**NDI<sub>Pyr</sub>·HFB**) and 5.25 ± 0.03 eV (**NDI<sub>Pyr</sub>·TFB**), respectively. However, XB additives induce a much greater reduction (Figure S5), with the largest shift of -0.54 eV observed for **NDI<sub>Pyr</sub>·1,4-DITFB** complex, lowering the ionization potential to 4.85 ± 0.04 eV (Figure 3c). This substantial decrease is attributed to significant changes in molecular packing and orientation within the film compared to pristine **NDI<sub>Pyr</sub>** (see below).[70,71] Interestingly, while **1,2-DITFB** and **TITFB** reduce the ionization potential to 5.15 ± 0.02 and 5.13 ± 0.13 eV, respectively, the weaker halogen bond donor **DBTFB** induces a larger reduction to 5.02 ± 0.15 eV. Despite **1,2-DITFB** and **TITFB** being stronger XB donors than **DBTFB**, the difference is likely due to distinct supramolecular packing enabled by linear halogen bonding with para-substituted XB donors. Additionally, XB formation exerts a pulling effect on the electron density of the NDI core, although this effect appears weaker than the packing-related changes, as evidenced by the larger ionization potential shift observed for **DBTFB** than for **1,2-DITFB** and **TITFB**.

Atomic force microscopy (AFM) of solution-processed thin films of **NDI<sub>Pyr</sub>** and its blends was conducted to study the influence of molecular packing on morphology. The AFM topography images (Figure 4a–d) reveal significant differences in grain sizes and roughnesses between **NDI<sub>Pyr</sub>** and the three different iodine-containing XB blends. **NDI<sub>Pyr</sub>·1,2-DITFB** shows a dramatic decrease in grain size, while **NDI<sub>Pyr</sub>·TITFB** exhibits a slight increase. Notably, **NDI<sub>Pyr</sub>·1,4-DITFB** shows a substantial increase in grain size and a more uniform distribution, indicating a significant reduction in grain boundaries. In contrast, no visible differences were observed in the AFM topography between **NDI<sub>Pyr</sub>** and the nonbonding blends or **NDI<sub>Pyr</sub>·DBTFB** (Figure S6).

To quantify these morphological changes, power spectral density (PSD) analysis was performed, which enables detailed characterization of surface morphologies and grain sizes (correlation length represented by the B parameter in the ABC model) (Figure S7).[72] The analysis shows that crystal sizes decrease in blends with non-XB additives, as these additives disrupt the molecular packing of **NDI<sub>Pyr</sub>**, acting as impurities that inhibit the formation of long-range ordered phases. In contrast, **NDI<sub>Pyr</sub>·DBTFB**, **NDI<sub>Pyr</sub>·TITFB**, and **NDI<sub>Pyr</sub>·1,4-DITFB** exhibit larger crystal sizes compared to pristine **NDI<sub>Pyr</sub>**, with the differences between **TITFB** and **1,4-DITFB** arising from the greater uniformity of the crystal grains in the **1,4-DITFB** blend (Figure 4). Interestingly, PSD analysis reveals that grain sizes for **NDI<sub>Pyr</sub>·1,2-DITFB** are comparable to those for blends with nonbonding additives. This is attributed to a geometric limitation, as the molecular structure of **1,2-DITFB** does not support efficient packing in the end-to-end direction, reducing its ability to enhance long-range order in the film.

To gain further insight into the crystallinity of the thin film samples, grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were performed (Figure 4e–h). The 2D-GIWAXS pattern of **NDI<sub>Pyr</sub>** reveals its polycrystalline nature, with sharp, smeared diffraction spots forming distinct rings. Major diffraction peaks are observed at 0.329 Å⁻¹ for (100) lamellar alkyl stacking (19.1 Å) with a higher-order diffraction at 0.671 Å⁻¹, 0.496 Å⁻¹ for (001) end-to-end stacking (12.7 Å), and 1.37 Å⁻¹ for (010) π-stacking (4.59 Å).[73] The scattering pattern suggests either a monoclinic or triclinic unit cell, with previous studies reporting a triclinic cell for similar NDIs lacking pyridine substituents at the 2,6-positions.[74] The pyridine substitution increases the end-to-end packing distance, as indicated by the (001) diffraction, which occurs at roughly half the momentum transfer of unsubstituted C₁₂-NDI.[74] Alkyl chains of **NDI<sub>Pyr</sub>** predominantly pack perpendicular to the surface,[75] but the smeared rings indicate partial random orientation. When **NDI<sub>Pyr</sub>** is blended with the nonbonding additive **HFB**, the (001) and (010) peaks weaken, and the rings become more pronounced, suggesting increased random orientation (Figure S8). For **NDI<sub>Pyr</sub>·DBTFB**, distinct scattering spots largely disappear, and the (100) spot becomes an intense ring, accompanied by strengthened (010) and (001) rings, indicating highly random orientation despite the AFM results showing larger crystal grains. A similar but less
<figure><img src="image_7.png" /><figcaption>**Figure 4.** a–d) AFM topography images and e–h) 2D-GIWAXS scattering patterns of NDI<sub>Pyr</sub>; a,e), NDI<sub>Pyr</sub>·1,2-DITFB; b,f), NDI<sub>Pyr</sub>·TITFB; c,g), and NDI<sub>Pyr</sub>·1,4-DITFB; d,h) film samples.</figcaption></figure>

pronounced effect is observed for **NDI<sub>Pyr</sub>·1,2-DITFB**, where the weaker (001) diffraction supports the conclusion that this XB geometry disrupts end-to-end packing.

Conversely, **NDI<sub>Pyr</sub>·TITFB** and **NDI<sub>Pyr</sub>·1,4-DITFB** exhibit improved packing orientation. **NDI<sub>Pyr</sub>·1,4-DITFB** shows the least orientational smearing and a larger number of distinct higher-order diffraction spots. The 1D-GIWAXS profiles (Figure S9) further confirm this trend, showing that **NDI<sub>Pyr</sub>·1,4-DITFB** features improved packing orientation in both out-of-plane and in-plane directions. In the out-of-plane direction, only the (h00) lamellar diffractions are visible for **NDI<sub>Pyr</sub>·1,4-DITFB**, while **NDI<sub>Pyr</sub>·TITFB** also shows (010) π-stacking. For other samples, strong (001) end-to-end diffractions are observed in the out-of-plane direction, but for **NDI<sub>Pyr</sub>·1,4-DITFB**, this diffraction is confined to the in-plane direction.

Additionally, azimuthal peak width analysis of the lamellar (100) and (200) diffractions (Figure S10) highlights that most peak broadening is due to disorder rather than crystal size effects.[73,75] **NDI<sub>Pyr</sub>·1,4-DITFB** exhibits narrower peak widths compared to all other blends, indicating that the strong directionality of halogen bonding enhances crystal growth uniformity and thin film morphology. This finding underscores the ability of halogen bonding to improve molecular alignment and packing regularity in OSC thin films.

To evaluate the impact of XB on charge transport properties, solution-processed organic field-effect transistors (OFETs) were fabricated using **NDI<sub>Pyr</sub>** and its blends. The transfer curves for **NDI<sub>Pyr</sub>** and **NDI<sub>Pyr</sub>·1,4-DITFB**, along with the field-effect mobilities for all blends, are shown in Figure 5. Additional transfer curves for all blends, including *I*<sub>ON</sub>–*I*<sub>OFF</sub> current ratios and threshold voltages, are presented in Figure S11, while the output curves are provided in Figure S12. The electron mobility of pristine **NDI<sub>Pyr</sub>** is relatively low (1.2 × 10<sup>−7</sup> ± 0.3 × 10<sup>−7</sup> cm<sup>2</sup> V<sup>−1</sup> s<sup>−1</sup>), primarily due to its large π-stacking distance (0.459 nm), caused

by the pyridine substituent being oriented perpendicular to the NDI core (Figure S13), which reduces the transfer integral between adjacent molecules and limits charge transport. The addition of non-XB additives **HFB** and **TFB** enhances mobility by ~200% (3.4 × 10<sup>−7</sup> ± 0.2 × 10<sup>−7</sup> and 3.7 × 10<sup>−7</sup> ± 0.2 × 10<sup>−7</sup> cm<sup>2</sup> V<sup>−1</sup> s<sup>−1</sup>) likely due to a minor charge transfer effect from the aromatic additives that enhances transistor performance.[76,77] However, these additives disturb the molecular packing of **NDI<sub>Pyr</sub>**, as confirmed by morphological analysis. Surprisingly, **NDI<sub>Pyr</sub>·1,2-DITFB** exhibits mobility comparable to the non-XB blends (3.6 × 10<sup>−7</sup> ± 0.5 × 10<sup>−7</sup> cm<sup>2</sup> V<sup>−1</sup> s<sup>−1</sup>). This suggests that the smaller grain size and disturbed packing counteract any potential energetic benefits provided by XB, as smaller grains increase grain boundary density and act as charge traps.

In contrast, the other XB donors significantly enhance mobility, with an increase of 400% for **NDI<sub>Pyr</sub>·DBTFB** (5.9 × 10<sup>−7</sup> ± 0.7 × 10<sup>−7</sup> cm<sup>2</sup> V<sup>−1</sup> s<sup>−1</sup>), 1400% for **NDI<sub>Pyr</sub>·TITFB** (1.7 × 10<sup>−6</sup> ± 0.4 × 10<sup>−6</sup> cm<sup>2</sup> V<sup>−1</sup> s<sup>−1</sup>), and 5900% for **NDI<sub>Pyr</sub>·1,4-DITFB** (6.9 × 10<sup>−6</sup> ± 0.4 × 10<sup>−6</sup> cm<sup>2</sup> V<sup>−1</sup> s<sup>−1</sup>) compared to pristine **NDI<sub>Pyr</sub>**. The *I*<sub>ON</sub>–*I*<sub>OFF</sub> current ratio also improves by nearly two orders of magnitude, reaching 10<sup>4</sup> for **NDI<sub>Pyr</sub>·1,4-DITFB**. Additionally, **NDI<sub>Pyr</sub>·1,4-DITFB** exhibits a significant threshold voltage shift, attributed to a LUMO level reduction (Figure S2) that enhances electron injection, alongside improved molecular packing reducing the trap density. Interestingly, despite DFT modeling indicating a similar pyridine rotation angle to pristine **NDI<sub>Pyr</sub>**, **NDI<sub>Pyr</sub>·1,4-DITFB** shows markedly better performance. In comparison, the mobility of **NDI<sub>Ph</sub>** decreases by 85% after blending with **1,4-DITFB** (8.4 × 10<sup>−7</sup> ± 0.5 × 10<sup>−7</sup> and 1.4 × 10<sup>−7</sup> ± 0.3 × 10<sup>−7</sup> cm<sup>2</sup> V<sup>−1</sup> s<sup>−1</sup> for pristine **NDI<sub>Ph</sub>** and **NDI<sub>Ph</sub>+1,4-DITFB**, respectively). This is a direct result of XB occurring with the carbonyl moieties in **NDI<sub>Ph</sub>**, resulting in worse molecular packing for the blend. The dramatic improvements in transistor performance for blends of **NDI<sub>Pyr</sub>**
<figure><img src="image_8.png" /><figcaption>**Figure 5.** a) Schematic structure of an OFET device, b) mobility enhancement with $NDI_{Pyr}$ blends relative to pristine $NDI_{Pyr}$ (linear regime, $V_d$ = 40 V, $L$ = 2.5 µm), c) transfer curves of $NDI_{Pyr}$ and $NDI_{Pyr·1,4-DITFB}$ ($V_d$ = 40 V, $L$ = 2.5 µm), and d) output curves of $NDI_{Pyr}$ and $NDI_{Pyr·1,4-DITFB}$ ($V_g$ = 40 V, $L$ = 2.5 µm).</figcaption></figure>

with strong directional XB are attributed to a combination of improved molecular packing with larger, more uniform grain sizes and a modulation of electronic levels facilitated by XB. While the measured mobility values remain low due to the intrinsically low mobility of $NDI_{Pyr}$, this work serves as a proof of principle, highlighting the potential of XB for optimizing the morphology and electrical performance of OSCs. Future studies will focus on exploring higher-mobility systems to further enhance performance.

## Conclusion

Our results demonstrate that external halogen-bond (XB) donors can effectively enhance the morphology of organic semiconductors with suitable XB-accepting moieties. Crystal sizes determined with AFM were affected by the directionality of the XB donors, with the largest crystal size observed for the two 1,4-substituted donors, **DBTFB** and **1,4-DITFB**. GWAXS measurements revealed that the directionality of XB improved the crystal growth uniformity and thin film morphology, consistent with the enhanced uniformity observed in UV-vis absorption spectra. Additionally, XB significantly reduced the ionization potential of the NDI semiconductor compared to its pristine form, indicating its potential for tuning energy levels. The effects on OSC morphology scaled with XB strength, highlighting the potential for precise control over intermolecular packing through halogen substitution. Interestingly, while all iodinated XB donors exhibited similar complexation energies with $NDI_{Pyr}$, the resulting morphologies and performances varied significantly. The

supramolecular assembly of the 1,4-substituted bifunctional XB donors facilitated the alignment of $NDI_{Pyr}$, suggesting that optimizing thin film deposition methods to enhance alignment could further boost performance.

This work establishes halogen bonding with external donors as a powerful tool for controlling the morphology and performance of organic semiconductors. Further efforts will focus on optimizing molecular design, including planarized XB semiconductor molecules, such as core-extended perylene diimides with suitable functionalization in the bay positions. Further work is underway to also functionalize the semiconductors to form halogen bonds without the need for external donors. These advancements hold significant potential for enhancing device performance and expanding the applicability of XB in organic electronics.

## Supporting Information

The authors have cited additional references within the Supporting Information.[78–100]

## Acknowledgements

This work is financially supported by the Academy of Finland (SUPREL project, no. 326416). The work is conducted as part of the Finnish Center of Excellence program on Life Inspired Hybrid Materials Research (LIBER, no. 346107) and the Finnish Flagship program on Photonics Research and Innovation (PREIN, no. 320165). T.-P.R. acknowledges
the Research Council of Finland Postdoctoral (no. 340103) and Academy Researcher Fellowships (no. 363033) and EU H2020 Marie Sklodowska-Curie actions (no. 101022777). N.R.C. acknowledges FCT/MCTES, Fundação para a Ciência e Tecnologia and Ministério da Ciência, Tecnologia e Ensino Superior FCT for financial support under the Scientific Employment Stimulus (CEE-CINST/2018) and projects PTDC/QUI-QOR/1131/2020 and UIDB/50006/2020. CSC-IT Center for Science Ltd., Finland, is acknowledged for the allocation of computational resources. S.F. acknowledges financial support from the Knut and Alice Wallenberg Foundation (2021.0058, WWSC, and the Wallenberg Initiative Materials Science for Sustainability WISE) and the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU 2009-00971). This work made use of Tampere Microscopy Center facilities at Tampere University.

Open access publishing facilitated by Tampereen yliopisto ja Tampereen ammattikorkeakoulu, as part of the Wiley - FinELib agreement.

[Correction added on 12 July 2025, after first online publication: FinELib 2023 funding statement has been added.]

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

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

**Keywords:** Charge transport · Halogen bonding · Morphology · OFET · Organic semiconductor

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