Interchain supramolecular interactions drive nearly 21% efficiency organic solar cells

Domain: biology
Published: 2026-03-30
Editorial outcome: major_revision

Abstract
A small-molecule acceptor, S-Cb, substituted with a cyclobutyl group that introduces
high ring strain, was designed and synthesized. Thanks to the rigid and planar structure of
cyclobutyl, S-Cb can form interchain supramolecular interactions through hydrogen bonding with
L8-BO at the external side chains. This clamping effect not only effectively suppresses the
electron-phonon coupling but also promotes the formation of high-quality acceptor alloy phases
in the ternary active layer, thereby optimizing carrier behaviors and reducing non-radiative energy
loss. The clamping effect reaches its maximum when S-Cb and L8-BO are in equal proportion,
where organic solar cells (OSCs) based on D18:S-Cb:L8-BO achieved an impressive efficiency of
20.93%, with a certified efficiency of 20.74%. In summary, the cyclobutyl-mediated interchain
supramolecular interactions suppress the electron-phonon coupling and optimize the acceptor alloy
phase for efficient ternary OSCs.

Introduction
Organic solar cells (OSCs) offer significant potential for applications in wearable electronics and
building-integrated photovoltaics, which stems from their lightweight design, mechanical
flexibility, semi-transparency, and compatibility with solution-based, large-area printing
techniques1-9. However, compared to the most advanced silicon-based or organic-inorganic hybrid
perovskite solar cells10-12, the power conversion efficiency (PCE) of OSCs still lags significantly
behind, which is mainly due to the strong electron-phonon coupling in organic semiconductor
materials13-20. The electron-phonon coupling refers to the interaction between charge carriers and
lattice vibrations. Charge carriers undergo scattering by phonons and exchange energy with the
lattice, resulting in a rapid relaxation process that leads to undesirable non-radiative
recombination16,17. Therefore, mitigating the electron-phonon coupling in active layer is
considered to be crucial for enhancing the photovoltaic performance of OSCs14,21-24.

The electron-phonon coupling in organic semiconductors primarily originates from two
distinct mechanisms: (i) the high-frequency intramolecular vibrations, such as C-C stretching
within the conjugated backbone, and (ii) the low-frequency intermolecular motions, including out-
of-plane bending and in-plane rotation of entire molecules. The coupling strength associated with
specific intramolecular modes (i) can be directly quantified by the corresponding Huang-Rhys
factor (S-factor). In contrast, the disordered and collective nature of the intermolecular motions (ii)
makes their direct microscopic quantification challenging. Therefore, macroscopic structural
parameters like the free volume ratio (FVR) of the active layer are often employed as effective
proxies to gauge the extent of such molecular-scale thermal fluctuations21. Generally, a smaller
FVR indicates more restricted molecular motion, which helps reduce electron-phonon coupling.

Current small-molecule acceptors (SMAs) consist of a conjugated backbone with soluble alkyl
chains around25-44. It is acknowledged that C-C bonds between sp²-hybridized C atoms are more
robust due to higher bond energy, and alternated C-C single and double bonds in conjugated
backbone form a delocalized π-structure, therefore, the rigidity of conjugated backbone is
significantly greater than that of alkyl chains linked by sp³-hybridized C atoms. As a result, C-C
stretching vibrations in large π-conjugated backbone of SMAs are much weaker than those in
flexible alkyl chains. Additionally, the conjugated backbones of SMAs readily form π-π stacking
in the solid state, which is able to effectively reduce thermal motion within molecular backbone
through π-π interactions. In view of this, we hypothesize that increasing the rigidity of alkyl chains
in SMAs can effectively suppress electron-phonon coupling in the active layers of OSCs.

Compared to straight or branched alkyl chains, cyclic ones exhibit greater rigidity due to their
ring strain45. As the ring size decreases, the angles between C-C single bonds become increasingly
compressed to enhance ring stain and force cyclic alkyl chains to adopt a more planar conformation,
which improves the rigidity of alkyl chain to a certain extent. Among common cyclic alkyl chains,
such as cyclohexyl, cyclopentyl, cyclobutyl and cyclopropyl, the angles of C-C bond are
approximately 109°, 108°, 90°, and 60°, respectively. Cyclohexyl and cyclopentyl groups can
largely maintain the ideal tetrahedral angle of 109.5° for sp³-hybridized carbon atoms, resulting in
low ring strain. These rings adopt characteristic conformations—the non-planar envelope-like
conformation for cyclopentane and the stable chair conformation for cyclohexane. In contrast,
cyclopropyl exhibits the highest ring strain and a rigid planar geometry. Although its pronounced
ring strain could, in principle, enhance structural rigidity, the cyclopropyl ring is prone to ring-
opening reactions under various chemical conditions, including those encountered during the
synthesis of Y6-type derivatives, which limits its practical utility in such materials. Cyclobutyl
with a C-C bond angle of about 90°that significantly deviates from the ideal tetrahedral angle
possesses considerable ring strain that imparts notable conformational rigidity. To partially relieve
this strain, the cyclobutane ring adopts a slightly folded butterfly-like conformation, with a
dihedral angle between the ring planes of only about 14°, indicating a relatively flat structure. This
combination of substantial rigidity, moderate steric bulk, and sufficient synthetic stability makes
the cyclobutyl group an attractive structural motif in the design of organic photovoltaic materials.
Moreover, the structural characteristics of cyclobutane place it between flexible alkyl chains and
rigid aromatic groups, ensuring molecular solubility while imparting moderate rigidity to the alkyl
chain region.

Taking these into consideration, introducing a cyclobutyl group into the central core of SMAs
is expected to reduce electron-phonon coupling in the active layer, primarily because, on one hand,
the relatively rigid cyclobutyl can partially suppress C-C stretching vibrations of alkyl chains. On
the other hand, cyclobutyl substitution may alter molecular packing modes of SMAs, potentially
reducing thermal motion of SMAs. Motivated by these insights, a new SMA namely S-Cb with
cyclobutyl groups substituted at thiophene β-position was designed and synthesized. Cyclobutyl
cyclization strategy can effectively increase the rigidity of S-Cb and reduce the Stokes shift in
contrast to that of L8-BO, revealing a suppressed relaxation process caused by electron-phonon
coupling. Simultaneously, the bandgap of S-Cb was narrowed along with molar extinction
coefficients and crystallinity enhanced. It is discovered that the rigid cyclobutyl group on S-Cb
can clamp with the bifurcated alkyl chains of L8-BO via hydrogen bonding. These interchain
supramolecular interactions facilitate the formation of a highly ordered acceptor alloy phase, which
significantly reduces the S-factor and FVR of ternary active layer and suppresses electron-phonon
coupling for lower non-radiative energy loss (E ). Binary device based on D18:S-Cb achieved a
loss
PCE of 19.63%, slightly lower than that of D18:L8-BO-based device (19.85%). Ternary device
based on D18:S-Cb:L8-BO gained an impressive PCE of 20.93% with a certified efficiency of
20.74% when the weight ratio of two acceptors was 1:1. This work demonstrates that the
introduction of rigid cyclobutyl side chains facilitates the formation of interchain supramolecular
interactions, thereby suppressing electron-phonon coupling and optimizing acceptor alloy phase
for enhanced photovoltaic performance of ternary devices.

Results
Molecular structure and rigidity
Molecular structures of polymer donor D18 and investigated SMAs S-Cb and L8-BO are presented
in Fig. 1a. The detailed synthetic routes for S-Cb is illustrated in Supplementary Fig. 1 and follows
a procedure similar to that used for Y6 and its derivatives41. The intermediates and final product
in the S-Cb synthesis were characterized using 1H NMR, 13C NMR, and mass spectrometry (MS),
confirming the correct chemical structures. Evaluation reveals that if L8-BO is synthesized via the
same route as S-Cb, the alkyl chain combinations in both materials would lead to comparable
large-scale production costs for S-Cb relative to L8-BO. It is worth noting that the
(bromomethyl)cyclobutene, the precursor of carboxylic acid containing one extra carbon atom,
required for S-Cb is commercially available at a lower price than the 2-butyloctyl bromide used
for L8-BO. Thermal stability of S-Cb and L8-BO were assessed using thermogravimetric analysis
(TGA) measurement, which revealed that 5% decomposition temperature (T ) of S-Cb is 324 °C,
d
approximately 10°C lower than that of L8-BO (335 °C, Supplementary Fig. 2). This decrease in
thermal stability is likely attributed to the inherent ring strain in cyclobutane unit, which facilitates
ring opening under elevated thermal stress.

In order to evaluate the change in rigidity of SMAs upon cyclobutyl substitution, temperature-
dependent UV-vis absorption spectra of S-Cb and L8-BO neat films were measured
(Supplementary Fig. 3). The redshifts in the spectra after thermal annealing as-cast films were
analyzed to assess thermal transition that can be quantified by the deviation metric (DM ), helping
determine glass transition temperature (T ) of SMAs46. T , defined as the lowest temperature at
g g
which a molecular segment starts to move, is positively correlated with molecular rigidity.
Generally, the higher the rigidity, the higher the T . As shown in Fig. 1b, the T values of L8-BO
g g
and S-Cb determined by the intersection of two linear fitting curves are found to be 87.2 oC and
92.8 oC, respectively. Although alkyl chains attached to the N-position of S-Cb’s central core is
longer than that of L8-BO, the T of S-Cb can still be enhanced through side chain cyclization in
g
thiophene β-position, indicating that cyclization strategy can effectively increase the rigidity of
SMAs.

To gain insights into the effects of increased rigidity in SMAs on electron-phonon coupling,
photoluminescence (PL) spectroscopy of pure L8-BO and S-Cb films were further measured (Fig.
1c). The PL peaks of L8-BO and S-Cb are 902.5 nm and 903.2 nm, respectively. By combining
these values with absorption peaks (Supplementary Table 1), the Stokes shifts of L8-BO and S-Cb
neat films were calculated to be 1420 cm-1 and 1382 cm-1, respectively. The reduction in the Stokes
shift indicates a decrease in non-radiative E caused by relaxation process of excitons as they
loss
transition from excited state to the ground state, suggesting electron-phonon scattering and energy
exchange with the lattice are suppressed. Furthermore, the Full Width at Half Maximum (FWHM)
of PL spectrum of S-Cb is 101.6 nm, significantly lower than that of L8-BO (111.9 nm). The
narrowing of FWHM in S-Cb PL spectrum is attributed to a reduction in vibrational modes, which
results from increased atomic ordering within S-Cb molecule, leading to fewer vibrational energy
levels in the molecule14. PL experiments reveal that increasing the rigidity of alkyl chain in SMAs
can effectively suppress electron-phonon coupling.

Parallel measurements of the photoluminescence quantum yield (PLQY) for L8-BO and S-
Cb neat films were conducted, with the results from six independent measurements shown in Fig.
1d. The average PLQY was 9.9% for L8-BO and 10.7% for S-Cb. The slightly higher PLQY of S-
Cb suggests that the cyclobutyl substituent helps to suppress electron-phonon coupling to reduce
the non-radiative transition rate (k ) to some extent, according to the relationship PLQY =
nr
𝑘𝑟
,
𝑘𝑟+𝑘𝑛𝑟
where k is the radiative transition rate.
r

To decouple whether this suppression originates from the inherent molecular rigidity or the
molecular packing effects, we further examined their properties in dilute solution (Supplementary
Fig. 4). At the same concentration (3.0 × 10-⁶ mmol mL⁻¹), the PL intensity of S-Cb is 2.4 times
that of L8-BO. Comparison of normalized absorption and PL spectra reveals that S-Cb has a
smaller Stokes shift (838 cm⁻¹ for S-Cb vs. 862 cm⁻¹ for L8-BO) and a narrower emission FWHM
(55.9 nm for S-Cb vs. 61.0 nm for L8-BO). Most notably, the PLQY of S-Cb in dilute solution
reaches 29.2%, nearly double that of L8-BO (16.6%). Considering the negligible molecular
aggregation in dilute solution, these results strongly suggest that the inherent rigidity of the
cyclobutyl group can effectively suppress intramolecular electron-phonon coupling, thereby
reducing exciton-molecular vibration interaction and mitigating k caused by vibrational
nr
relaxation.

We posit that this intramolecular vibrational suppression effect of the cyclobutyl group
persists in the solid state. However, the transition from solution to film leads to a much more
pronounced decrease in PLQY for S-Cb than for L8-BO, revealing that S-Cb undergoes a more
significant aggregation-caused quenching (ACQ) effect in the solid state, likely attributable to the
cyclobutyl substitution promoting a stronger propensity for H-aggregation as corroborated by the
more distinct 0-1 vibrational shoulder in its absorption spectrum (Fig. 1c), which will exacerbate
electron-phonon coupling and facilitate non-radiative energy dissipation47. The weaker overall
electron-phonon coupling observed in S-Cb compared to L8-BO is likely primarily due to the
effective reduction of molecular vibrations by the rigid cyclobutyl group. Simultaneously, its
shorter alkyl chain might also enhance molecular packing (particularly H-aggregation), which
exacerbates ACQ and partially counteracts the benefits gained from rigidity.

Photophysical and crystallization properties
Subsequently, impacts of cyclobutyl cyclization on molecular absorption, crystallinity, and energy
levels were investigated. The normalized UV-Vis absorption spectra of S-Cb and L8-BO in dilute
solution and neat films are shown in Supplementary Fig. 5 and Fig. 1c, respectively. Compared to
L8-BO, S-Cb exhibits a slight red shift of approximately 2 nm in solution. To quantify their
extinction coefficients, five different concentrations of dilute S-Cb and L8-BO solutions were
prepared, and their absorption spectra were measured (Supplementary Fig. 6). The plots of
maximum absorbance versus concentration yielded molar extinction coefficients of 2.15 × 10⁵ and
2.04 × 10⁵ M⁻¹ cm⁻¹ for S-Cb and L8-BO, respectively. In the solid state, S-Cb shows a more
pronounced 0-1 vibronic shoulder and a narrower optical bandgap (1.36 eV for S-Cb vs. 1.38 eV
for L8-BO), indicating that the cyclobutyl substitution enhances the molecular packing behavior
of SMA. Additionally, the absorption spectra of S-Cb and L8-BO films with varying thicknesses
were measured (Supplementary Fig. 7). The fitted absorption coefficients for S-Cb and L8-BO
films were 4.49 × 10⁴ and 3.97 × 10⁴ cm-¹, respectively (Supplementary Table 1). Overall,
cyclobutane-substitution improves absorption properties of S-Cb.

Differential scanning calorimetry (DSC) measurements were performed for S-Cb and L8-BO
to investigate the crystallization behavior of two acceptor molecules. As shown in Fig. 1e, both
SMAs exhibit distinct melting peaks near their decomposition temperatures, with melting
temperatures (T ) of 318.4 °C for S-Cb and 318.1 °C for L8-BO. However, the enthalpy change
m
(△H ) associated with the melting peak of S-Cb (94.1 J/g, determined by integrating the area of
m
the melting peak only up to the red line, and the portion after the red line was excluded because
the thermal effect in that interval includes additional heat absorption due to the thermal
decomposition of S-Cb) is substantially higher than that of L8-BO (35.6 J/g), indicating that
cyclobutyl substitution markedly enhances the intermolecular interactions and thus crystallization
tendency of S-Cb.

Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were conducted on
neat films of S-Cb and L8-BO to gain further insights into how the cyclobutyl substitution affect
molecular packing behavior of S-Cb. The corresponding 2D GIWAXS patterns and 1D line-cut
profiles are shown in Supplementary Fig. 8 and Fig. 1f, respectively. Both S-Cb and L8-BO films
exhibit well-defined face-on orientations. Notably, the (010) and (100) diffraction peak intensities
of S-Cb are stronger than those of L8-BO, suggesting enhanced crystallinity in S-Cb film. Analysis
of the 1D profiles reveals that π-π stacking distance of S-Cb (3.62 Å) is slightly larger than that of
L8-BO (3.57 Å), likely due to the larger steric hindrance introduced by the 2-butyloctyl group
attached to the N atom of S-Cb. However, S-Cb shows a longer π-π coherence length (CL of
12.05 Å for S-Cb and 10.92 Å for L8-BO), indicating a more ordered π-π stacking arrangement.
Furthermore, the lamellar packing distance and CLs for S-Cb are 13.78 Å and 32.40 Å,
respectively-both significantly improved compared to those of L8-BO (14.61 Å and 22.58 Å).
These results suggest that cyclobutyl substitution primarily enhances crystallinity by promoting
lamellar packing, instead of intermolecular π-π stacking. Electron mobilities of neat S-Cb and L8-
BO films were measured by using space-charge limited current (SCLC) method. By fitting the
current density-voltage (J-V) curves (Supplementary Fig. 9), the electron mobilities were
determined to be 2.44 × 10⁻⁴ and 2.88 × 10⁻⁴ cm² V⁻¹ s⁻¹ for S-Cb and L8-BO, respectively. The
slightly lower electron mobility of S-Cb is consistent with the GIWAXS results, as the steric
hindrance from the 2-butyloctyl group in S-Cb increases the π-π stacking distance, thereby
impeding charge transport.

The highest occupied molecular orbital (HOMO) energy levels of D18, L8-BO, and S-Cb
neat films were determined by ultraviolet photoelectron spectroscopy (UPS), as shown in
Supplementary Fig. 10. The measured HOMO levels (HOMOUPS) are -5.09 eV, -5.20 eV, and -5.14
eV for D18, L8-BO, and S-Cb, respectively. To accurately calculate the corresponding lowest
unoccupied molecular orbital (LUMO) levels, cyclic voltammetry (CV) measurements were
further performed on L8-BO and S-Cb neat films (Supplementary Fig. 11). The HOMO/LUMO
levels derived from CV curves (HOMOCV/ LUMOCV) are -5.63/-3.88 eV for L8-BO and -5.57/-
3.93 eV for S-Cb. The fundamental gap (E fund = E CV – E CV) is thus determined to be
g LUMO HOMO
1.75 eV for L8-BO and 1.64 eV for S-Cb, while that of D18 (2.24 eV) is taken from our previous
report40. Consequently, the LUMO levels were calculated using E = E UPS + E fund,
LUMO HOMO g
yielding values of -2.85 eV for D18, -3.45 eV for L8-BO, and -3.50 eV for S-Cb. It is noteworthy
that the E fund physically represents the minimum energy required to completely remove an
g
electron from the HOMO level, corresponding to the difference between the ionization energy (IE)
and the electron affinity (EA). Therefore, using E fund instead of the optical gap (E opt) provides a
g g
more rigorous basis for estimating the LUMO level. The trend in HOMO and LUMO levels
revealed by UPS and CV is consistent, providing mutual verification. Moreover, the energy levels
of L8-BO and S-Cb align well with those of D18 (Fig. 1g), which is expected to create an effective
driving force for charge separation at the donor/acceptor interface. However, the slightly lower
LUMO level of S-Cb compared to that of L8-BO may adversely affect the enhancement of the V .
OC

Photovoltaic performance
The photovoltaic performance of S-Cb was evaluated by fabricating OSCs devices with an
architecture of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS)/active layer/poly(9,9-bis(3-(N,N-dimethyl)-N-ethylammonium-propyl)-2,7-
fluorene-alt-2,7-(9,9-dioctylfluorene)) dibromide:melamine (PFN-Br:MA)48/Ag. Detailed
fabrication procedures are provided in the Methods section, and J-V curves of optimized device
are shown in Fig. 2a with corresponding photovoltaic parameters are summarized in Table 1. The
D18:S-Cb-based OSCs achieved a V of 0.888 V, a J of 27.57 mA cm⁻², and an FF of 80.2%,
OC SC
yielding a PCE of 19.63%. For comparison, the D18:L8-BO-based device showed a slightly higher
PCE of 19.85%, with a V of 0.925 V, a J of 26.83 mA cm⁻², and an FF of 80.0%. In view of
OC SC
complementary photovoltaic parameters of D18:S-Cb and D18:L8-BO devices, a ternary blending
strategy was considered with L8-BO introduced as a third component into D18:S-Cb blend to form
ternary devices. As a result, a significant performance enhancement was observed. Upon adding
25% weight ratio of L8-BO, the D18:S-Cb:L8-BO device was able to achieve a PCE of 20.44%,
a
great improvement in photovoltaic performance compared to that of D18:S-Cb-based devices,
which is attributed to simultaneous improvements in V , J , and FF. When L8-BO content was
OC SC
increased to 50%, the PCE of D18:S-Cb:L8-BO-based devices can be further improved, resulting
in an impressive PCE of 20.93%, along with a third-party certified efficiency as high as 20.74%
(a V of 0.913 V, a J of 27.90 mA cm⁻², and an FF of 81.4%, Fig. 2b and Supplementary Fig.
OC SC
12). When L8-BO ratio was increased to 75%, although V continued to rise, both J and FF
OC SC
declined, leading to a reduced PCE of 20.40%. 10 to 15 data points were collected for these OSC
devices, as shown in Supplementary Fig. 13 (Based on sample size, mean, and standard deviation,
F- and t-test statistical analysis of the photovoltaic parameters across different L8-BO content
indicated no statistically significant differences between the groups). The average PCEs and its
standard deviation are presented in Table 1, demonstrating good device reproducibility.

To further verify the general applicability of S-Cb as a ternary component, we replaced L8-
BO with another high-performance acceptor, BTP-eC9, to fabricate ternary devices based on
D18:S-Cb:BTP-eC9 with a BTP-eC9 content of 50 wt%. The J-V curves and EQE spectra of the
champion device are presented in Supplementary Fig. 14, with the corresponding photovoltaic
parameters summarized in Supplementary Table 2. The binary device based on D18:BTP-eC9
yielded a PCE of 19.21%, slightly lower than that of D18:S-Cb-based OSCs. In contrast, the
ternary device based on D18:S-Cb:BTP-eC9 (1:1) achieved an enhanced PCE of 20.23%, with an
V of 0.876 V, a J of 28.54 mA cm⁻², and an FF of 80.9%. The efficiency enhancement,
OC SC
primarily attributed to the notable improvement in FF, represents an absolute increase of
approximately 1%. The magnitude of this gain is comparable to that observed when introducing
S-Cb into the D18:L8-BO binary system. These results demonstrate that cyclobutyl-substituted S-
Cb serves as a versatile ternary component, exhibiting excellent compatibility with acceptor
materials bearing either linear or branched alkyl side chains and effectively boosting device
performance.

The EQE spectra of optimal OSCs based on D18:S-Cb:L8-BO with different L8-BO ratios
are shown in Fig. 2c. It can be observed that device based on D18:S-Cb exhibits a higher EQE
response than the device based on D18:L8-BO in the ranges of 300-540 nm and 620-950 nm.
Additionally, as the proportion of L8-BO increases from 25% to 75%, the EQE response intensity
of D18:S-Cb:L8-BO first increases and then decreases. The optimal EQE spectrum response
occurs when 50% of L8-BO is added, with improvements observed in 380-950 nm wavelength
range. When the proportion of L8-BO increases to 75%, the EQE response is between that of
D18:S-Cb and D18:L8-BO. Finally, the EQE-integrated J for D18:S-Cb:L8-BO-based OSCs
SC
with 0%, 25%, 50%, 75%, and 100% L8-BO contents are 26.56, 26.85, 27.09, 26.29, and 25.78
mA cm-², respectively, which agrees well with measured J
SC
with a reasonable error of
approximately 4%.

Carrier behavior and energy loss
Femtosecond-resolved transient absorption spectra (fs-TAS) were measured for D18:S-
Cb:L8-BO active layer with varying L8-BO ratios to probe photoinduced carrier dynamics in OSC
devices. Spectral cuts at representative pump-probe delay times and corresponding 2D TAS
contour plots are shown in Supplementary Fig. 15 and Supplementary Fig. 16, respectively. Donor
polaron kinetics were fitted using the sum of exponentials, as shown in Fig. 2d. The kinetics of
donor polarons photobleaching (PB) were used as primary basis for analyzing free charge
generation and sub-nanosecond recombination. It is clearly observed that D18:L8-BO active layer
facilitates very rapid singlet exciton dissociation, whereas this kinetic process slows down
significantly in D18:S-Cb blend. Conversely, the rapid dissociation process in D18:L8-BO may
lead to an excessive concentration of electron-hole pairs that results in charge accumulation and
promotes free charge recombination. In contrast, the charge recombination process in D18:S-Cb
slows down, as indicated by its more gradual decay trend (green curve, 0% L8-BO), while the
D18:L8-BO exhibits a faster decay after 100 ps (red curve, 100% L8-BO), which is beneficial for
achieving higher EQE and FF in devices. When coming into ternary blend, the singlet exciton
dissociation dynamics are further slowed, particularly when the proportion of L8-BO increases to
50% and 75%. With a 25% L8-BO ratio, free charge recombination in D18:S-Cb:L8-BO ternary
blend will be slightly suppressed compared with no addition. Recombination process can be mostly
inhibited when L8-BO proportion is increased to 50%, which correlates well with enhanced EQE
and FF in ternary OSCs based on D18:S-Cb:L8-BO with 50% L8-BO content. However, when L8-
BO ratio increases to 75%, free charge recombination in D18:S-Cb:L8-BO blend will raise, leading
to a decrease in both J and FF for the corresponding device.
SC

Carrier mobilities of optimal D18:S-Cb:L8-BO active layer with varying L8-BO contents was
measured with resulting J-V curves displayed in Supplementary Fig. 17. Corresponding hole and
electron mobilities (μₕ and μₑ) fitted using SCLC method are summarized in Fig. 2e. D18:S-Cb
active layer exhibits hole and electron mobilities of 6.45×10⁻⁴ and 5.96×10⁻⁴ cm² V⁻¹ s⁻¹,
respectively, higher than those of D18:L8-BO (3.49×10⁻⁴ and 2.19×10⁻⁴ cm² V⁻¹ s⁻¹, respectively).
It should be noted that the high and balanced carrier mobility of D18:S-Cb helps reduce charge
recombination, resulting in a lower recombination rate compared to D18:L8-BO. When L8-BO is
introduced, both the hole and electron mobilities of D18:S-Cb:L8-BO decrease to varying extents
relative to those of D18:S-Cb. D18:S-Cb:L8-BO with 50% L8-BO content exhibits the highest
hole and electron mobilities with a lowest μₕ/μₑ ratio in ternary blend, which helps further reduce
charge recombination (consistent with the fs-TAS test results) to promote improvements in J and
SC
FF.

The Fourier transform photonic spectroscopy EQE (Supplementary Fig. 18) and
electroluminescence (EL) spectra (Fig. 2f) of D18:S-Cb:L8-BO system were measured to assess
E s of OSCs based on Shockley-Queisser (S-Q) theoretical framework. The calculated radiative
loss
and non-radiative E 49 data are summarized in Supplementary Table 3. For D18:S-Cb:L8-BO
loss
blend with 0%, 25%, 50%, 75%, and 100% L8-BO content, the measured EQE values were
EL
determined to be 1.93 × 10⁻⁴, 2.35 × 10⁻⁴, 3.12 × 10⁻⁴, 1.51 × 10⁻⁴, and 1.45 × 10⁻⁴, respectively,
corresponding to non-radiative E (ΔE ) of 0.222, 0.217, 0.210, 0.229, and 0.230 eV, respectively,
loss 3
according to ΔE = -kTln(EQE ), where k is the Boltzmann constant, and T is Kelvin temperature.
loss 3 EL
The slightly higher EQE and reduced ΔE of D18:S-Cb-based OSCs are likely attributed to
EL 3
suppressed electron-phonon coupling. It can be observed that L8-BO addition further enhances
EQE of ternary active layer, especially when L8-BO content reaches half of the total acceptor
EL
amount, where the EQE significantly increases, and ΔE is further suppressed. However, when
EL 3
L8-BO content is increased to 75%, the EQE of D18:S-Cb:L8-BO decreases, indicating an
EL
increase in ΔE (the reason will be discussed in the theoretical calculation section). E s of D18:S-
loss 3
Cb:L8-BO-based OSCs with 0%, 25%, 50%, 75%, and 100% L8-BO content were calculated to
be 0.523, 0.517, 0.515, 0.512, and 0.520 eV, respectively. It should be noted that D18:S-Cb-based
OSCs obtain a slightly larger E than that of D18:L8-BO-based OSCs due to a larger additional
loss
radiative E (ΔE ).
loss 2

Working mechanism of ternary device
As L8-BO content gradually increases, the V of D18:S-Cb:L8-BO-based ternary devices
OC
exhibits a linear improvement (Supplementary Fig. 19), suggesting that the potential mechanism
for improved performance of ternary devices may be the formation of acceptor alloy phase.
To thermodynamically probe the ease of forming an acceptor alloy phase between S-Cb and
L8-BO, we evaluated their compatibility by measuring the contact angles of their neat films and
deriving the corresponding surface tensions. By using ethylene glycol (EG) and water as probe
liquids (contact angles of 65.40°/94.19° for L8-BO and 61.66°/89.77° for S-Cb, respectively,
Supplementary Fig. 20), the surface tensions (λ) of L8-BO (λ ) and S-Cb (λ ) were fitted
L8-BO S-Cb
via Wu’s model to be 39.68 mN/m and 40.33 mN/m, respectively. The Flory-Huggins interaction
parameter χ, estimated from Tχ ∝(√λ − √λ )2 was found to be exceptionally small (χ
S-Cb L8-BO
∝ 0.0026). This minimal χ value indicates excellent thermodynamic miscibility between S-Cb
and L8-BO, providing a key thermodynamic rationale for their facile formation of a homogeneous
acceptor alloy phase in the blend.

To further validate the working mechanism of ternary devices, we investigated the trend of
energy levels in S-Cb:L8-BO blend films as L8-BO content varied by testing their CV curves
(Supplementary Fig. 21). It was found that as L8-BO ratio increased, the LUMO energy levels of
S-Cb:L8-BO blend exhibited a linear increase, while the HOMO energy levels showed the opposite
trend. We also tested the DSC curves of S-Cb:L8-BO mixture (Supplementary Fig. 22), where only
one single crystallization peak can be observed as L8-BO content gradually increased, indicating
that S-Cb can form a good alloy phase with any ratio of L8-BO. However, as L8-BO content
increased from 25% to 75%, the crystallization temperature gradually decreased, and the enthalpy
change of corresponding crystallization peak significantly lowered, suggesting that an increase in
L8-BO content will weaken the crystallization performance of acceptor alloy phase. These two
experiments results revealed that the operating mechanism of ternary device involves the
formation of a S-Cb:L8-BO acceptor alloy phase.

Single crystal analysis and theoretical simulations
To deeply understand formation mechanism of S-Cb:L8-BO alloy phase at a molecular scale,
single crystal of S-Cb were analyzed, and theoretical simulations were employed to visualize
intermolecular interactions between S-Cb and L8-BO. The single crystal of S-Cb were successfully
grown using a three-solvent diffusion method with chlorobenzene as the good solvent, n-hexane
as the poor solvent, and toluene as the buffer solvent. The crystallographic parameters of S-Cb are
summarized in Supplementary Table 4. The results of single crystal are shown in Figs. 3a-d. We
found a Y-type dimer packing with overlapping molecular backbones in S-Cb (Fig. 3e). The π-π
interaction distance between dimer molecules is about 3.31Å. Similarly, we analyzed three types
of dimer stacking from L8-BO single crystal, namely S-type, S*-type and M-type dimer packing
with overlapping molecular backbone (Figs. 3f-h). The difference between S-type and S*-type is
that S* only overlaps at the end groups. The π-π stacking distances of these types are 3.21Å, 3.32Å,
and 3.27Å, respectively, which are common stacking types and π-π distances in single crystals of
Y6-series SMAs.

Based on quantum chemical calculation, the weak interaction analysis of dimers found that
the overlapping visual in green color had an obvious π-π interaction, as shown in Figs. 3i-l. Overall,
regardless of the type of molecular stacking found in the single crystal growth of S-Cb or L8-BO,
we use δG (See Supplementary Information for its definition) to quantitatively describe the
interaction strength of the four types of stacking (Y, S, S* and M), which are 1.59, 1.43, 1.32 and
1.46 respectively, that is, these types of molecular packing have π-π interactions with similar
strengths. Further, the excited state theoretical calculation analysis of these dimer types shows that
the distance of electron-hole separation d in the first excited state is 18.05 Å, 17.34 Å, 19.88 Å
e-h
and 17.86 Å, respectively, as shown in Figs. 3m-p. The difference in electron-hole separation, on
the one hand, depends on the type of molecular packing, and on the other hand, it is determined
by Coulomb attraction energy (E )50. For weak-bound carriers dominated by π-π weak interactions,
c
E is determined by the following Equation (1):
c
E = ∬
ρhole(r1)ρele(r2)
d d (1)
c r1 r2
|r1−r2|
Where ρhole and ρele are the density of holes and electrons, respectively, and |r −r | is the
1 2
electrons-holes separation distance. That is, the Coulomb attraction energy and the separation
distance between electrons and holes d are inversely proportional. Our theoretical calculation
e-h
results show that the Coulomb attraction energies of the four stacking types are very close, namely
1.69, 1.87, 1.53 and 1.70 eV, respectively. In summary, the four stacking types obtained by S-Cb
and L8-BO single crystal have very close π-π stacking distances, π-π interaction strengths and
Coulomb attraction energies, as well as the resulting charge separation in the excited state.

Interestingly, we used molecular dynamics simulations to change different ratios of L8-BO
in D18:S-Cb:L8-BO to simulate the formation of real blend films (Supplementary Fig. 23), and
we found a unique dimer formed between S-Cb and L8-BO. As shown in Fig. 4a, we controlled
the ratio of S-Cb:L8-BO to be 0.75:0.25 (dark blue), 0.50:0.50 (dark red), and 0.25:0.75 (light
blue), and statistically analyzed the pair distribution g with different dimer packing
S-Cb-L8-BO
distances. According to theoretical calculation results, when the two ratios of acceptors are very
different, that is, the ratio is 0.75:0.25 (dark blue) and 0.25:0.75 (light blue), the dimer packing
distances of the two are concentrated in the range of 2.75- 4.31 Å. This stacking distance usually
means that the two acceptors exist in a way that the molecular skeleton overlaps up and down,
which is like the four types in single crystals mentioned above. More importantly, when the ratio
of the two acceptors is 0.50:0.50 (dark red) , a new dimer pair distribution peak is obtained in the
stacking distance range of 1.56 to 2.73 Å, which means that S-Cb and L8-BO interact together
more compactly.

To further explore this new stacking mode, we extracted all dimers of S-Cb and L8-BO with
stacking distances ranging from 1.56 to 2.73 Å from the simulation results of molecular dynamics.
As shown in Fig. 4d, the nearly planar cyclobutyl side chains of S-Cb are clamped in the dendritic
side chains of L8-BO. This incredible intermolecular stacking mode is achieved through hydrogen
bonds on the molecular side chains. The dark blue part in Fig. 4d shows this attraction between
hydrogens. Compared with the π-π interaction, the hydrogen bond makes the two molecules more
firmly clamped combined. Compared with the four stacking modes in the single crystal, the δG of
the clamp type is only 0.65, and the clamp distance between them is only in the range of 2.23 to
2.96 Å. By counting the S-Cb and L8-BO stacking types at different ratios, as shown in Fig. 4b,
we found that when the ratio of the two acceptors is 0.50:0.50 (center), the clamp effect (refers to
a supramolecular interaction wherein the nearly planar and rigid cyclobutyl side chain of S-Cb is
spatially locked into the branched alkyl chain pocket of L8-BO via intermolecular hydrogen bonds,
e.g., C-H···H-C, effectively clamping the side chains of the two acceptors together) is most
obvious, and the clamp type ratio reaches 27%; when the ratios of the two acceptors are different,
0.75:0.25 (left) and 0.25:0.75 (right), the clamp effect still exists but is weakened to varying
degrees, depending on the content of S-Cb, 16% and 8%, respectively. More interestingly, we also
counted the impact of the clamp effect on the packing of acceptors and donors. The results show
that when the ratio of the two acceptors is the same (0.50:0.50), the stacking of donors and
acceptors is the largest, at 28%; and when the ratios of the two acceptors are different, the stacking
of donors and acceptors will also be affected, at 20% and 17% respectively, and the influence may
also be related to the content of S-Cb. We believe that the reason for this phenomenon may be that
the clamp effect caused by mixing two different acceptors originates from the difference in the
side chains of the two molecules. The nearly planar alkyl chain of S-Cb is more likely to be
captured by the dendritic side chains than the linear alkyl chain, and the probability of being
clamped increases to a certain extent with the increase of S-Cb, reaching the maximum when the
ratio is 0.50:0.50; the clamp dimer is combined by hydrogen bonds, and does not affect the further
stacking of the molecular backbone with other molecules, that is, the molecules form a fiber-like
chain, and the clamp molecular chain still can be arranged with other molecules (including donors)
through backbone stacking. This is also the reason why the stacking probability of the donor and
the acceptor increases when the ratio of the two different acceptors is 0.50:0.50.

Compared with the four stacking types discussed previously, we found through theoretical
calculations that the clamp dimer has a smaller Coulomb attraction of only 1.15 eV, which greatly
enhances the electron-hole separation distance to 42.36 Å, which means that electrons and holes
can be more effectively separated at the interface between the acceptor pure domain and blend
interface (Fig. 4e). At the same time, we compared the effects of the stacking types in single
crystals and the clamp type on the Huang Rhys factor, and Equation (2) is:
S = λ (2)
ℏω
Where λ is the reorganisation energy and ω is the molecular vibration frequency. The maximum
values of the Huang Rhys factor (S ) of the four dimer types are 1.02, 1.03, 1.03 and 1.00, and
max
the average values (S ) are 0.02, 0.04, 0.01 and 0.02, respectively. Correspondingly, the S of
avg max
the clamp-type dimer is 0.81, and the S is 0.005 (Fig. 4c and Supplementary Fig. 24). This may
avg
be because the clamp stacking relies on hydrogen bonds between side chains to suppress the high-
frequency vibration51 and weaken the reorganization energy of the low-frequency vibration
mode52,53. In addition, a lower free volume was found in the ratio of 0.50:0.50 (Fig. 4f). The
clamping effect helps to reduce both the Huang-Rhys factor and the free volume of ternary active
layer, thereby effectively suppressing electron-phonon coupling to improve EQE of D18:S-
Cb:L8-BO ternary active blend and suppress ΔE 21.
loss 3

To quantitatively evaluate the electron-phonon coupling strength, temperature-dependent PL
measurements were performed on L8-BO, S-Cb neat films, and their blended film with a 1:1
weight ratio. The temperature was varied from 300 K down to 80 K in steps of 20 K. The PL
spectra and corresponding fitting results are provided in Supplementary Fig. 25. The broadening
of the PL emission linewidth with increasing temperature can generally be described by the
following model19,54,55:
Γ(T) = Γ + aexp(− Ea )+bT (3)
i kbT
where, Γ represents the inhomogeneous linewidth of the film, a denotes the density of non-
i
radiative recombination centers, E is regarded as the energy barrier for back charge recombination
a
from electronic to excitonic states in Y-series non-fullerene acceptors, and b is defined as the
electron-phonon coupling coefficient. In the low-temperature region (80-200 K), the contribution
of the second (nonlinear) term to the linewidth is minor; therefore, an approximate linear fitting
was applied to data in this range to extract the electron-phonon coupling constant b. The fitting
yields b values of approximately 0.305, 0.284, and 0.163 meV·K⁻¹ for neat L8-BO, S-Cb, and the
S-Cb:L8-BO blend, respectively.
These results indicate that the electron-phonon coupling in neat
S-Cb is slightly weaker than that in neat L8-BO. Notably, blending the two materials at a 1:1
weight ratio leads to a more pronounced suppression of the coupling, a trend consistent with our
theoretical calculations. We attribute this enhanced suppression to the formation of larger dimer-
like structures via hydrogen-bonding interactions between the side chains of S-Cb and L8-BO,
which likely further restrains molecular vibrations.

H-H interaction of clamping effect
To verify the inter-H interaction between cyclobutyl side chain of S-Cb and 2-butyloctyl side chain
of L8-BO, which is observed in theoretical calculations, we performed 1H-1H NOESY NMR
spectroscopy on S-Cb, L8-BO, and S-Cb:L8-BO (1:1) mixture (Supplementary Fig. 26). The
hydrogen atoms labeling Ha-Hh and their positions on S-Cb and L8-BO are marked in
Supplementary Fig. 26a. The 1H-1H NOESY NMR spectrum of S-Cb (Supplementary Fig. 26b)
shows long-range H-H interactions between Ha and Hb, Hd and Hc, and Hd and Hb on cyclobutyl
side chain. In contrast, no long-range H-H correlation is observed between Hf and Hg in L8-BO
(Supplementary Fig. 26c), likely due to the flexibility of branched alkyl chains, which causes Hf
and Hg to be spatially distant. It is worth noting that a strong H-H spatial interaction exists between
Hf and Hh, whereas such an interaction does not occur between Ha and He in S-Cb due to greater
spatial distance between Ha and He in S-Cb than that between Hf and Hh in L8-BO as revealed by
single crystal analysis (Supplementary Fig. 27a). This phenomenon reveals that cyclization
strategy can reduce the steric hindrance acting on the terminal double bonds to promote the
planarity of S-Cb, which is consistent with the single crystal results (Supplementary Fig. 27b). As
can be observed in 1H-1H NOTESY NMR spectrum of S-Cb:L8-BO blend solution with equal ratio
(Supplementary Fig. 26d), in addition to the H-H correlations within the individual molecules of
S-Cb and L8-BO, new intermolecular long-range H-H correlations appeared between Hb and Hg,
and Ha and Hf, which indicates that Hb and Hg, and Ha and Hf, are spatially very close.
Furthermore, the long-range H-H correlation intensity between Ha and Hf is weaker than that
between Hb and Hg, suggesting that the rigid cyclobutyl side chain of S-Cb can effectively clamp
the branched alkyl chain at the branching point of L8-BO, which is in agreement with the
molecular dynamics results.

To verify the generality of the clamping effect induced by the cyclobutyl group, specifically
probing whether effective long-range interactions exist with the linear alkyl chains on acceptor
molecule, we performed ¹H-¹H NOESY NMR spectroscopy on a 1:1 blend of S-Cb and BTP-eC9.
The relevant hydrogen atoms on BTP-eC9 are labeled as Hi-Hk, as illustrated in Supplementary
Fig. 26a. The NOESY spectrum reveals distinct spatial proximity correlations between the
hydrogen atoms of the cyclobutyl group on S-Cb (Ha) and those on the linear alkyl chains of BTP-
eC9 (Hj and Hi) (Supplementary Fig. 26e). The intensity of this interaction appears stronger than
that previously observed in the S-Cb:L8-BO system, which can be attributed to the lower steric
hindrance of linear alkyl chains compared to branched ones that allows the cyclobutyl group to
approach the main body of the linear chain more closely. This result provides evidence for a
favorable spatial proximity and orientation between the cyclobutyl group of S-Cb and the linear
alkyl chain-based acceptor, forming a crucial structural foundation for the clamping effect. This
finding further reinforces that the cyclobutyl-induced clamping effect of S-Cb may a general
phenomenon applicable to acceptor molecules functionalized with either branched or linear alkyl
side chains.

To further prove the existence of C-H…H-C hydrogen bond, we have performed the Fourier-
transform infrared (FTIR) spectroscopy on neat films of S‑Cb and L8‑BO, as well as their blend
film with a weight ratio of 1:1, corresponding spectra are provided in Supplementary Fig. 28. By
comparing the FTIR transmission spectra of the neat S‑Cb and L8‑BO films, we assigned the
characteristic C‑H stretching vibration of the cyclobutyl group at approximately 2954 cm⁻¹.
Notably, in the S‑Cb:L8‑BO blend film, this C‑H vibration peak exhibits a small but discernible
shift to a higher wavenumber (blue‑shift of about 2 cm⁻¹) compared to the neat S‑Cb film,
accompanied by an increase in peak intensity and broadening of the line shape. These spectral
changes align with the trend expected when the cyclobutyl C‑H bond acts as a potential
hydrogen‑bond acceptor. It is worth noting that the considerable strain and the resulting bent bonds
enhance the polarity of the C-H bonds on cyclobutyl, endowing the hydrogen atoms with a partial
positive charge-though still weaker than that of hydrogens bonded directly to oxygen or nitrogen.
The compact steric profile and the weakly electrophilic hydrogen of the cyclobutyl group allow it
to act as a hydrogen-bond acceptor, forming weak but non-negligible hydrogen bonds. We consider
that the relatively small magnitude of the shift may be attributed to the moderate polarity of the
cyclobutyl C‑H bond, which could limit the strength of its hydrogen‑bond accepting interaction,
thereby resulting in only subtle yet detectable alterations in the FTIR spectrum.

Clamping effect on ternary morphology
Intermolecular stacking properties of D18:S-Cb:L8-BO active layer with varying L8-BO contents
were explored through GIWAXS experiment, and the resulting 2D GIWAXS patterns and
corresponding 1D cutting line profiles are shown in Supplementary Fig. 29. It reveals that all
D18:S-Cb:L8-BO blend films exhibit a well-defined face-on orientation, which facilitates the
vertical transport of charge carriers. The π-π stacking distances/CLs for D18:S-Cb and D18:L8-
BO blend films are 3.742/15.09 Å and 3.683/13.80 Å, respectively. Although the π-π stacking
distance of D18:S-Cb is slightly larger than that of D18:L8-BO, the cyclobutyl substitution is able
to enhance CL of D18:S-Cb blend film, which is consistent with pure film measurements of S-Cb
and L8-BO and helps explain a higher carrier mobility of D18:S-Cb than that of D18:L8-BO. As
L8-BO content increases to 25%, 50%, and 75%, the π-π stacking distances/CLs of D18:S-Cb:L8-
BO active layer are found to be 3.642/15.42 Å, 3.611/16.97 Å, and 3.630/14.90 Å, respectively.
This trend supports the variation in carrier mobility of D18:S-Cb:L8-BO blend films. It can be
concluded that the clamping effect between S-Cb and L8-BO plays a crucial role in regulating the
morphology of ternary active layer. When S-Cb and L8-BO are in equal proportions, the clamping
effect between S-Cb and L8-BO reaches its maximum, significantly reducing the π-π stacking
distance and enhancing CLs of active layer. However, when the ratio of S-Cb to L8-BO is unequal,
especially when L8-BO is in excess, the clamping effect is lowered, which weakens the π-π
stacking. GIWAXS results indicate that the clamping effect between S-Cb and L8-BO is beneficial
for improving π-π stacking properties in ternary active layer.

To further understand impacts of clamping effect between S-Cb and L8-BO on the
morphology of ternary active layer, photo-induced force microscopy (PiFM) was employed to
image the characteristic Fourier-transform infrared (FTIR) absorption peaks of the polymer donor
(1578 cm⁻¹ for D18) and two acceptors (1426 cm⁻¹ for S-Cb and 1532 cm⁻¹ for L8-BO,
Supplementary Fig. 30). PiFM images of D18:S-Cb:L8-BO with 0%, 25%, 50%, 75%, and 100%
L8-BO ratios are shown in Fig. 5. By comparing the phase diagrams of D18:S-Cb and D18:L8-
BO blend films, as presented in Fig. 5a and 5t, significant morphological differences between
D18:S-Cb and D18:L8-BO blend films can be clearly observed. The D18:S-Cb blend exhibits a
hilly surface feature, while the D18:L8-BO blend shows a smooth fish-scale morphology. Fig. 5c
displays the distribution morphology of S-Cb (green) acceptor component in D18:S-Cb, which
reveals a peak-like morphology compared to that of L8-BO acceptor component (Fig. 5v) in
D18:L8-BO, which can be attributed to the rigidity of cyclobutyl that effectively improves
molecular crystallization properties of S-Cb. Fig. 5b shows the distribution morphology of
polymer donor D18 (red) in the D18:S-Cb component, which corresponds to the portion remaining
after S-Cb is removed from D18:S-Cb blend. D18 component in D18:S-Cb showed a more obvious
pit structure, which in turn indicated that S-Cb could form a better pure acceptor phase. The phase
length scales of D18 and S-Cb components in D18:S-Cb blend are comparable to those in D18:L8-
BO and are approximately 15 nm and 35 nm, respectively (Supplementary Fig. 31), which favors
excitons transfer to donor/acceptor interfaces before quenching. Additionally, the PiFM combined
images of D18:S-Cb blend (Fig. 5d) show a more significant donor-acceptor phase separation
compared to D18:L8-BO. The stronger crystallinity of S-Cb helps S-Cb acceptor phase to
precipitate faster, facilitating the formation of a more continuous and high-quality donor-acceptor
nanofiber network structure and carrier transport channel, which enhances electron and hole
mobilities. In contrast, the PiFM combined images of D18:L8-BO show more light yellow areas,
indicating a higher degree of donor-acceptor blending phase. Overall, the cyclobutyl cyclization
strategy increases the donor-acceptor phase separation by enhancing molecular crystallinity.
When L8-BO is introduced with a ratio of 25% and 75%, the phase diagrams of D18:S-Cb:L8-
BO active layers show a leopard-like surface morphology (Fig. 5e and 5o), which is likely due that
the clamping effect between S-Cb and L8-BO is not strong enough when the mass ratios of S-Cb
to L8-BO are 3:1 and 1:3, preventing forming a high-quality acceptor alloy phase between S-Cb
and L8-BO. From PiFM images of S-Cb and L8-BO components in D18:S-Cb:L8-BO blend (Fig.
5g, 5h, 5q, and 5r), clear crystalline phases and nanofiber network structures of S-Cb and L8-BO
can be observed, indicating more pure phases of S-Cb and L8-BO components in D18:S-Cb:L8-
BO ternary blend with L8-BO content of 25% and 75%, particularly evident when the L8-BO
content reaches 75%. However, when the ratio of S-Cb to L8-BO is the same, the clamping effect
between the two acceptors is maximized, and S-Cb and L8-BO are able to form a high-quality
acceptor alloy. As shown in Fig. 5i and 5m, the crystalline phases and nanofiber network structures
of S-Cb and L8-BO components become less prominent, which is significantly different from the
morphology of S-Cb and L8-BO components observed in D18:S-Cb:L8-BO blend with 25% or
75% L8-BO content. The uniform and smooth microstructure of S-Cb and L8-BO components in
D18:S-Cb:L8-BO indicating that the two are more inclined to completely intermixed. Interestingly,
the addition of L8-BO significantly increases the phase length scale of D18 from 15 nm to ~25 nm,
while reducing the phase length scale of acceptor phases from 35 nm to ~25 nm (Supplementary
Fig. 31). This appropriately matched phase length scale between donor and acceptor is considered
to be beneficial for improving carrier transport and reducing charge recombination, consistent with
test results above. Moreover, PiFM combined images of D18:S-Cb:L8-BO blend with 50% L8-
BO content presents a good donor-acceptor nano-interpenetrating network structure along with a
more uniform donor-acceptor distribution compared to D18:S-Cb:L8-BO blend with 25% and 75%
L8-BO content.

To further elucidate the role of clamping effect between S-Cb and L8-BO on morphology
stability, PiFM measurements of degraded blend films under 100°C for one day was conducted
with results shown in Supplementary Fig. 32. It can be clearly observed from PiFM images
(Supplementary Figs. 32a-d and 32j-m) that the degraded blend films of D18:S-Cb and D18:L8-
BO underwent a great morphological change compared to previously fresh ones. The appearance
of pore structures in both donor and acceptor components indicates severe phase aggregation. The
phase length scales of D18, S-Cb, and L8-BO components in D18:S-Cb or D18:L8-BO blends all
increase to ~ 60 nm. In contrast, the morphology of D18:S-Cb:L8-BO with 50% L8-BO content
can be well maintained (Supplementary Figs. 32e-i), and the phase length scales of D18, S-Cb,
and L8-BO are slightly increased to ~ 28 nm for donor and ~30 nm for acceptors, which benefitted
from the strong clamping effect between S-Cb and L8-BO and significantly slow down the
aggregation process of acceptor phases. These results informed us that the clamping effect between
S-Cb and L8-BO plays a crucial role in helping to form high-quality acceptor alloy phases.

Thermal- and photo-stability
To further investigate the impact of the clamping effect on thermal- and photo-stability of OSCs,
we fabricated inverted devices with a structure of ITO/ZnO/active layer/MoO /Ag. The stability
x
test results for devices based on D18:S-Cb:L8-BO with 0%, 50%, and 100% L8-BO content are
presented in Supplementary Fig. 33. Under continuous annealing at 100 °C, D18:S-Cb device
exhibited significantly better thermal stability than the D18:L8-BO device, which can be attributed
to the higher T of S-Cb resulting from its short and rigid cyclobutyl substituent. Furthermore,
g
when 50% L8-BO was incorporated into the D18:S-Cb active layer, the device stability showed
improvement within the first 160 h of heating. However, beyond 160 h, the thermal stability of the
ternary device declined more rapidly compared to the D18:S-Cb binary device. We speculate that
during the early stage of heating, the clamp effect mediated by hydrogen bonding helps to stabilize
the morphology. With prolonged thermal stress, this hydrogen-bond network may gradually
degrade, causing the ternary blend to evolve toward a morphology resembling that of D18:L8-BO,
thus accelerating the decay in device performance.

It is worth noting that, under continuous heating at 100 °C, these weak interactions provide
limited stabilization, which explains why the stability improvement is modest and eventually
deteriorates over extended periods, leading to the observed trend of initial stability followed by
rapid decay in the ternary system. In contrast, under continuous illumination, such hydrogen-
bonding interactions appear less susceptible to degradation. Consequently, the device based on
D18:S-Cb:L8-BO with 50% L8-BO content demonstrated markedly better photo-stability than the
D18:S-Cb binary device. Although the overall thermal- and photo- stability of the ternary devices
remains inferior to state-of-the-art values reported in the literature56—particularly in terms of
thermal stability-these results provide indirect evidence supporting the role of the hydrogen-bond-
guided clamping effect in modulating blend morphology and device stability.

Methods
Device fabrications and characterizations
OSCs devices were fabricated by employing a conventional structure of
ITO/PEDOT:PSS/D18/PFN-Br:MA/Ag. The patterned ITO-coated glass substrates were scrubbed
with detergent, followed by ultrasonic treatment in deionized water, acetone, and isopropanol
sequentially. The substrates were then dried overnight in an oven. Prior to use, the glass substrates
were treated with UV-ozone for 30 minutes to enhance their work function and cleanliness.
PEDOT:PSS (Al4083 from Heraeus) was spin-coated onto the ITO substrate at 7500 rpm for 30
seconds and subsequently dried at 160°C for 15 minutes under a nitrogen atmosphere. A blend
solution of D18:S-Cb:L8-BO (D18 concentration: 4 mg/mL, with a weight ratio of D18 to acceptor
at 1:1.4, and the ratio of L8-BO to acceptors varying from 0, 0.25, 0.50, 0.75 to 1.00) in chloroform,
with 5 mg/mL DCBB as an additive, was pre-dissolved at 100°C for 20 minutes. Once the solution
cooled to approximately 60°C, it was spin-coated onto the PEDOT:PSS layer at 2000 rpm for 30
seconds and then annealed at 100°C for 1 minute. A thin layer of PFN-Br (0.5 mg/mL in methanol
with 0.25% melamine by weight) was then spin-coated on the active layer at 3000 rpm.
Subsequently, Ag was deposited by thermal evaporation at a pressure of 3×10⁻⁴ Pa through a
shadow mask. The optimal thickness of the active layer, measured with a Bruker Dektak XT stylus
profilometer, was approximately 110 nm. The current density-voltage (J-V) characteristics of the
devices were measured using a Keysight B2901NA Source Meter inside a glove box under AM 1.5G
(100 mW cm⁻²) illumination from an Enlitech solar simulator. The device contact area was 0.042
cm², and the illuminated area during testing was 0.040 cm², determined by a mask. External
quantum efficiency (EQE) spectra were measured using a Solar Cell Spectral Response
Measurement System AQE-R3011 (Enlitech Co., Ltd.), with light intensity at each wavelength
calibrated using a standard monocrystalline silicon photovoltaic cell.

Theoretical simulations
Gaussian 16 (Revision C.02) code was used to perform density functional theory (DFT)
calculations at the non-empirically tuned B3LYP-D3(BJ)/TZVP level of theory. The excited state
single point energy was calculated under TD CAM-B3LYP/Def2TZVP level for high precision
calculations. The wavefunction software Multiwfn and VMD were used for post-possess.
Molecular dynamics simulations were conducted with the GROMACS 2023 software package. A
spherical cut-off of 1.2 nm for the summation of van der Waals (VDW) interactions and short-
range Coulomb interactions and the particle-mesh Ewald solver for long-range Coulomb
interactions were used throughout. The simulations were carried out with three-dimensional
periodic boundary conditions using the leapfrog integrator with a time step of 1.0 fs. The
electrostatic potentials were fitted using RESP charges. The velocity rescaling thermostat and
Berendsen barostat under the NPT ensemble were applied to control the temperature and pressure,
respectively.

PiFM measurements
PiFM results were acquired using a VistaScope microscope from Molecular Vista inc. at Songshan
Lake Materials Laboratory. All PiFM experiments were excited by a pulsed quantum cascade laser
(Block Engineering) with a gap-free narrowband tunable wavenumber of 760-1950 cm−1. The
spectral linewidth is ~2 cm-1 with a wavenumber resolution of 0.5 cm−1. The PiFM experiment
here was operated at the sideband excitation with the laser-frequency modulated at f = f - f ,
m 1 0
where f is the first mechanical eigenmode resonances of the cantilever that is used for PiF signal
0
detection, while f denotes the second ones recorded for the AFM topography of the sample. The
1
probe is an Pt-coated tip with a resonant frequency of ~350 kHz (PPP-NCHPt-MB, Nanosensors).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Wei Gao and colleagues report organic solar cells achieving 20.93% efficiency (20.74% certified) through the incorporation of cyclobutyl-substituted acceptor S-Cb in ternary blends with D18 and L8-BO. They propose that these rigid cyclobutyl groups enable “clamping” interactions with L8-BO via hydrogen bonding, suppressing electron-phonon coupling and improving device performance. This manuscript reports a meaningful efficiency improvement in organic solar cells and employs comprehensive characterization techniques. However, the mechanistic claims, particularly regarding electron-phonon coupling suppression via the “clamping effect” are not definitively proven. The work represents a solid incremental advance through molecular engineering rather than a conceptual breakthrough. Despite this, I recommend publication subject to the following revisions.\n\nMaior Comments\n1) The proposed mechanism linking cyclobutyl substitution to efficiency improvements involves multiple intermediate effects that are inadeauately deconvoluted. The authors show changes in the molecular rigidity (T increase of 5.6 °C), crystallinity (4 x higher enthalpy), morphology (altered phase separation), and intermolecular interactions. However, the relative contributions remain unclear and improvements are automatically attributed to the clamping effect of cyclobutyl substitution. The authors need to provide a good statement on these intermediate effects and how much it contributes. For instance in line 198 the authors state 'Both S-Cb and L8-BO films exhibit well-defined face-on orientations, indicating that cyclobutyl substitution does not disrupt the preferential molecular orientation. The emphasis on electron-phonon coupling suppression appears somewhat arbitrary given that morphological improvements alone could explain the performance gains.\n2) The hydrogen bonding-mediated clamping between S-Cb and L8-BO feels overstated. MD simulations how the effect only at 50:50 ratio, raising questions about why this specific stoichiometry is special. Should we not see this in higher S-CB concentrations as well? Additionally, the effect of the “unique dimer” has not been adequately explained.\n3) The 1H-1H NOESY NMR provides only indirect evidence of proximity, not definitive proof of the proposed binding mode\n4) I find that the correlation between computed Huang-Rhys factors and device performance is tenuous\n5) While 10-15 devices were tested per condition, no statistical analyses determine whether performance differences are significant given the reported standard deviations (0.16-0.27%). The optimal 50:50 ratio could be within experimental error of other compositions.\n6) The study examines only one system. Claims about the broader applicability of cyclobutyl-mediated interactions require demonstration in other donor-acceptor combinations. Should we expect the same behaviour with other binary and ternary blends as well? Or is the clamping effect only applicable to this particular combination.\n\nMinor Comments\n1. The single crystal analysis, while thorough, may have limited relevance to solution-processed films with substantial disorder.\n2. PiFM characterization is sophisticated but interpretation of \"acceptor alloy phase\" from IR absorption alone is problematic.\n3. Stability testing (100°C, 1 day) is insufficient for practical relevance. Light stability and longer-term studies are needed for a proper conclusion.\n4. The Stokes shift reduction (2.3 nm) is marginal and within measurement uncertainty.\n5. Fig. 4f showing free volume is difficult to interpret. Quantitative analysis would be preferable to color maps. I can’t really see where the free volume is.\n6. The manuscript would benefit from discussing synthetic complexity and scalability of cyclobutyl-modified acceptors.\n\nBased on the above, I recommend a major revision to address these points. My recommendation is for the authors to:\n1. Tone down claims about the clamping effect mechanism\n2. Address how that morphological improvements are or are not the primary performance driver\n3. Add statistical analysis of device performance differences\n4. Provide more comprehensive stability data or acknowledge this limitation\nThe efficiency achievement is noteworthy and the work is technically sound, making it suitable for publication after these revisions address the overclaimed mechanistic insights.", "reviewer_2": "In this manuscript, the authors reported a small-molecule acceptor substituted with a cyclobutyl group, that can form interchain supramolecular interactions through hydrogen bonding with L8-BO at the external side chains. The authors tried to demonstrate the clamping effect can suppress the electron-phonon coupling and promote the formation of high-quality acceptor alloy phases in ternary active layer with various characterization methods. And optimized carrier behaviors and reduced non-radiative loss were achieved, delivering an efficiency of 20.93%. The results are impressive, however, due to the following issues, e.g., the negligence of the longer butyloctyl groups, the manuscript should be revised significantly before it can be considered for publication.\n\n1. The unit of Stokes shift should be wavenumber (cm-1) instead of nm. So, in page 7, if the authors compare the wavenumbers, I am not sure whether the corresponding deduction can be valid.\n2. Besides FWHM, the fluorescence quantum yield in thin film should be another key parameter that is informative for the investigation on electron-phonon coupling.\n3. As is well known, the fluorescence quantum yield in solution is normally unaffected by the flexible side chains, why the use of cyclobutyl group can itself reduce the electron-phonon coupling. I mean the cyclobutyl group may only play an important role in suppress the high-frequency vibration of the conjugated backbone by the tight packing induced by the cyclobutyl groups..\n4. In Page 12, the slightly larger π–π stacking distance is attributed to the steric hindrance introduced by the rigid cylcobutyl group, which is wrong. Compared with hexyldecyl group on L8-BO, the cylcobutyl group is less sterically hindered. So the possible reason should be the larger butylhexyl group on the nitrogen atoms.\n5. The energy level of S-Cb and L8-BO should also be examined by UPS, and be compared with D18.\n6. Why did the authors use PFN-Br:MA as the electron transport layer?\n7. Since the authors reported a very high PCE, I suggest the value to be examined by the NIM, China.\n8. In Page 14, the authors expressed the significant slowdown of charge recombination. what is the basis?\n9. Why does the larger LUMO-LUMO energy offset induce the lager ΔE2? The authors should reveal under which conditions the energy losses were evaluated, CT or S-Q theory.\n10. The authors should define the δG, what is physical meaning of the term?\n11. The clamping effect In Figure 4d and 4e is not indicated clearly, and I suggest the authors to redraw the picture to clearly show the clamping dimer geometry. Especially, the hydrogen bonding should be clearly indicated and confirmed with solid evidence.\n12. For the PiFM, why 1426 cm-1 is chosed for S-Cb and 1532 cm-1 for L8-BO. Are they selected for the specific functional groups?\n13. The authors demonstrate that ternary system shows better morphological stability under high temperature. I think it is necessary to examine and discuss the stability of OPV devices and make a comparison.\n14. Traditionally, the alloy mechanism was characterized by the linear correlation betweeen Voc and the ratio of the third component.", "reviewer_3": "This study reports a novel cyclobutyl-substituted small-molecule acceptor (S-Cb) that forms interchain supramolecular “clamp” interactions with L8-BO, effectively suppressing electron–phonon coupling and enabling ternary organic solar cells (OSCs) with an impressive efficiency of 20.93% (certified 20.74%). This work is highly innovative, systematic, and thorough. The molecular design is rational, the experimental support is comprehensive, and the combination of morphology characterization, device physics, and theoretical simulations provides a convincing mechanism for the performance enhancement. The reported efficiency is among the highest in the field of OSCs. This manuscript presents a timely and impactful contribution to the field of high-efficiency OSCs. The clamp strategy offers a new pathway to suppress electron–phonon coupling and optimize active layer morphology. The work meets the high standards of Nature Communications and is recommended for acceptance after minor revisions.\n\n1. The introduction of a rigid cyclobutyl side chain to enhance molecular rigidity and suppress electron–phonon coupling is a clever and underexplored strategy. Compare S-Cb with cyclopentyl/cyclohexyl analogues to highlight ring-strain superiority. \"Clamping effect\" requires clearer definition (e.g., schematic illustrating H-bond positioning).\n2. The “clamping effect” between S-Cb and L8-BO is convincingly demonstrated through GIWAXS, PiFM, NMR, and molecular dynamics simulations. Although NMR shows spatial proximity between S-Cb and L8-BO, FTIR or temperature-dependent NMR could further confirm the presence and strength of hydrogen bonding.\n3. Quantification of Electron–Phonon Coupling is required. While Stokes shift, FWHM, and Huang–Rhys factors are provided, temperature-dependent PL or charge transport measurements could help quantify the electron–phonon coupling constant more directly.\n4. The morphological stability under thermal stress (100°C) is shown in Supplementary Fig. 22, but light-soaking stability should be included to evaluate operational stability.\n5. A PCE of 20.93% (certified 20.74%) with low non-radiative loss and good reproducibility represents a state-of-the-art result. The non-radiative loss (ΔE₃) increases again at 75% L8-BO content. Please discuss whether this is related to morphological deterioration or reduced clamp efficiency.\n6. Alloy-phase evidence should be consolidated. Specify if \"acceptor alloy\" implies mixed crystalline/amorphous phases. CV shows linear LUMO shift; DSC shows single crystallisation peak. However, GIWAXS (100) peak drift is ≤0.04 Å, much smaller than high-quality alloys (≥0.1 Å). Statistical significance (error bars, χ-parameter) is absent. MD g(r) alone is insufficient; orientational order parameter P₂ and Flory–Huggins χ should be included to demonstrate thermodynamic miscibility.\n7. Synthetic Scalability: Cyclobutyl synthesis complexity may hinder industrial adoption. A brief cost/feasibility assessment is needed. Industrial viability (e.g., ink formulation, large-area printing compatibility) should be discussed."}, "author_response": "Reply: Thank you for this critical and insightful comment. The increase in T upon cyclobutyl substitution primarily indicates the enhanced molecular rigidity of S-Cb compared to L8-BO. This greater rigidity can suppress vibrations in the alkyl chain region, contributing to the observed smaller Stokes shift and higher PLQY for S-Cb. Correspondingly, energy loss analysis reveals a lower non-radiative energy loss (△E ) 3 in the D18:S-Cb-based device. DSC curves show a higher melting enthalpy for S-Cb than for L8-BO, suggesting a stronger crystallization tendency. Subsequent GIWAXS studies confirm that S-Cb exhibits improved packing order in both neat and D18- blended films. This optimized microstructure leads to higher and more balanced charge carrier mobility in S-Cb neat films and D18:S-Cb blends, which effectively suppresses charge recombination (as verified by TAS). Consequently, the binary devices based on D18:S-Cb achieve a higher J (corresponding to a higher EQE response) and FF than SC their D18:L8-BO counterparts. Through systematic comparison with L8-BO, our comprehensive characterization elucidates the impact of cyclobutyl substitution on S- Cb across multiple aspects: optical absorption, energy levels, suppression of electron- phonon coupling, PLQY, non-radiative energy loss, crystallinity, film morphology, photoelectric conversion processes (including dissociation, transport, and recombination), and ultimately photovoltaic performance.\n\nThe hydrogen-bond interaction between the cyclobutyl group in S-Cb and the 2- butyloctyl chain in L8-BO was an unexpected result revealed by molecular dynamics (MD) simulations. In the latter part of the manuscript, we focus on this clamping effect, demonstrating its existence albeit weak strength. We illustrate its role in suppressing electron-phonon coupling and, more importantly, in modulating the blend morphology (including donor-acceptor phase separation and acceptor alloy phase). These effects collectively contribute to the performance enhancement of ternary ternary devices. We apologize for any inconvenience caused by the ordering and logical flow of the discussion in our manuscript.\n\nIn the revised manuscript, we have further elaborated the formation mechanism of this hydrogen-bond interaction: the considerable ring strain and resulting bond angle bending in the cyclobutyl group enhance the polarity of the C–H bonds, endowing the hydrogen atoms with a partial positive charge (though still weaker than that of hydrogens directly bonded to oxygen or nitrogen). The compact steric profile and weakly electrophilic hydrogen of the cyclobutyl group allow it to function as a hydrogen-bond acceptor. Furthermore, we have experimentally confirmed the existence of this interaction using 2D NMR and FTIR. After the “unique dimer” structure was revealed by theoretical calculations, our subsequent work focused on exploring its effects. Considering that this hydrogen bond may facilitate an alternating ordered arrangement of S-Cb…L8-BO… molecules, thereby promoting intermolecular π–π stacking, we first investigated its role in modulating blend morphology. We understand your concern that effective morphology can also be achieved in ternary blends without this clamping effect—which is indeed commonly thought in L8-BO-based ternary systems. However, we note that in typical ternary systems with L8-BO as the main acceptor, the efficiency usually peaks when the third component is around 20 wt%, and further increasing its ratio often leads to performance degradation. In contrast, in D18:S-Cb:L8-BO ternary system, device efficiency improves significantly when the S- Cb ratio reaches 25 wt%, and a further increase to 50 wt% still yields an approximately 0.5% enhancement in efficiency—a phenomenon not observed in other L8-BO-based ternary systems. We tentatively attribute this to the further morphology optimization induced by the clamping effect, which is also supported by GIWAXS and PiFM measurements.\n\nIn the revised manuscript, we also quantitatively analyzed the electron-phonon coupling effect by temperature-dependent photoluminescence (PL) spectroscopy of neat S-Cb, neat L8-BO, and their 1:1 blend films. As shown in Supplementary Fig. 25, the PL emission peak of the S-Cb:L8-BO (1:1) blend is markedly narrower than those of the neat films, and its electron-phonon coupling constant decreases to 0.163 meV K⁻¹. This significant suppression of electron-phonon coupling stems from the restriction of alkyl chain vibrations in both S-Cb and L8-BO imposed by the clamping effect, while the dimer structure also constrains the overall motion of acceptor molecules. Moreover, thermal stability tests revealed that the performance of D18:S- Cb:L8-BO (1:1) devices first increased and then decreased upon heating. We speculate that this may be due to the gradual disruption of the clamping effect in the initial stage, followed by a faster decay in stability as the ternary morphology evolves toward that of the D18:L8-BO binary system. This observation indirectly supports the existence and role of the clamping effect. Finally, the clamping effect contributes to significant improvements in PCE of the ternary devices by optimizing blend morphology and effectively suppressing electron-phonon coupling.\n\n[... Author response continues for all points ...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have conducted meaning additional experiments to verify my comments and I can now recommend this manuscript for publication.", "reviewer_2": "The authors provided extensive revisions to the manuscript and a comprehensive response to my previous comments. The efforts to clarify the scientific content and to elaborate on the material design are evident and have improved the manuscript. However, there remains some concerns need to be addressed.\n> There is some conceptual confusion regarding the definitions of radiative energy loss (ΔE2) and non-radiative energy loss (ΔE3). Both ΔE2 and ΔE3 are linked to the energy offset between the donor and the acceptor. However, in the authors’ definitions, ΔE2 is solely ascribed to this energy offset, whereas ΔE3 is attributed to other factors such as defects and vibronic coupling. Furthermore, it should be noted that in the current donor-acceptor system, the key factor influencing radiative/non-radiative recombination should be the HOMO offset, rather than LUMO offset. Additionally, Fig. R4 shows that the CT energy level equals the LUMO energy level of the acceptor, and the LE energy level equals the LUMO energy level of the donor. What Is the basis for this assumption?\n> In DSC measurements, the scanning temperature should typically be maintained below the onset temperature of material degradation. However, in this work, the DSC tests were conducted at temperatures approaching Td (where the material had already undergone 5% degradation). Notably, for S-Cb, the DSC scanning temperature significantly exceeded Td.", "reviewer_3": "The authors have addressed the reviewers' concerns. The revised manuscript is substantially improved and it may be acceptable for publication in its present form."}, "author_response": "Reply: We sincerely appreciate your recognition of our work and the time you have invested in reviewing our manuscript. Thank you for your valuable and constructive comments. We have revised the manuscript carefully and thoroughly based on your suggestions and the feedback from the other reviewers and enhanced overall quality."}]

Decision letter