“Steric‐Locking” Polymer Acceptor Enabled 20.53% Efficiency With Suppressed Energetic Disorder and Enhanced Mechanical Robustness in Green‐Solvent Processed All‐Polymer Solar Cells

ABSTRACT

The realization of high‐performance organic photovoltaics via environmentally benign manufacturing is pivotal for sustainable energy. While all‐polymer solar cells (all‐PSCs) offer superior stability and mechanical resilience, achieving high efficiencies in nonhalogenated green solvents remains a formidable challenge. Here, we report a “steric‐locking” strategy for polymer acceptor design that enables a record‐breaking power conversion efficiency of 20.53% (certified 19.79%) in o‐xylene‐processed all‐PSCs. The introduction of a steric‐locking guest polymer acceptor (PY‐IDT) into the PM6:PYF‐T‐o host matrix profoundly regulates the crystallization kinetics and suppresses the excessive self‐aggregation of the host acceptor. This molecular‐level structural refinement significantly reduces energetic disorder and minimizes non‐radiative voltage loss. Consequently, the suppressed energetic disorder and refined nanostructured domains yielded a concurrent leap in open‐circuit voltage (0.942 V) and fill factor (82.11%). Furthermore, the steric‐locked morphology demonstrates exceptional mechanical robustness, maintaining 92.6% of its initial efficiency after 1000 bending cycles. This work establishes a new efficiency benchmark and provides a universal chemical framework for developing high‐performance, sustainable, and flexible optoelectronics.

By leveraging a “steric‐locking” effect, this work precisely modulates crystallization kinetics and minimizes energetic disorder in all‐polymer solar cells, achieving a record efficiency of 20.53% via green‐solvent processing. This morphological refinement not only minimizes non‐radiative voltage losses but also imparts exceptional mechanical robustness to the active layer, demonstrating the immense potential of high‐performance, environmentally benign organic photovoltaics.

1. Introduction

The rapid evolution of flexible and wearable electronics necessitates power sources that harmonize high conversion efficiency with environmental sustainability and mechanical durability [1, 2, 3]. Organic photovoltaics (OPVs) have emerged as pivotal components in this technological paradigm [4, 5, 6, 7, 8]. Among these, all‐polymer solar cells (all‐PSCs), utilizing both polymer donors and acceptors, stand out due to their superior morphological stability and intrinsic mechanical toughness compared to their counterparts based on small‐molecule acceptors [9, 10, 11, 12, 13]. Recent advancements in polymer acceptors (PAs) have rapidly propelled the power conversion efficiency (PCE) of all‐PSCs to surpass 19%, positioning them as high‐performance contenders alongside small‐molecule non‐fullerene acceptor (NFA) systems [14, 15, 16, 17, 18, 19, 20].

Despite the significant progress achieved, the commercialization of all‐PSCs still faces several critical challenges [21, 22, 23]. These primarily include the reliance on halogenated solvents during fabrication [24, 25], the severe energetic disorder stemming from polymer chain entanglements [26, 27], and the intrinsic trade‐off between efficient charge transport and mechanical flexibility, where over‐crystallization can lead to significant mechanical embrittlement [28, 29, 30]. Current approaches employ device engineering (solvent additives [31], solvent vapor annealing [32], and thermal annealing [33]) to optimize the nanoscale phase separation and molecular designs (steric hindrance groups [26], random copolymerization [34]) to mitigate polymer chain entanglement and enhance green‐solvent compatibility. While significant strides have been made in optimizing specific performance parameters of all‐PSCs, achieving a synergistic balance among high efficiency, long‐term stability, and minimized energetic disorder remains a sophisticated challenge [28, 35, 36, 37, 38, 39]. In particular, the inherent energetic disorder in polymer blends acts as a fundamental bottleneck, leading to severe recombination losses that hinder the simultaneous attainment of high voltage and robust fill factors (FF). This limitation is further exacerbated in all‐PSCs processed with eco‐friendly, nonhalogenated solvents, where the difficulty in regulating morphology typically leads to a substantial efficiency penalty, thereby obstructing the path toward sustainable large‐scale manufacturing.

To synergistically address these critical issues, we propose a “Steric‐Locking” molecular design strategy. Specifically, we designed and synthesized a new polymer acceptor, PY‐IDT, incorporating a bulky indole‐dithiophene (IDT) unit, and strategically introduced it into the PM6:PYF‐T‐o ternary system. The core of this strategy lies in leveraging the geometric steric hindrance of the bulky IDT unit to achieve a dual regulatory effect. Beyond acting as a crystallization kinetics modulator to temper the host acceptor's propensity for excessive self‐aggregation and phase separation, it concurrently facilitates a more favorable molecular orientation within the ternary blend, promoting compact and ordered π‐π stacking while significantly reducing conformational disorder. As a result, this structural refinement within the ternary blend dramatically suppresses the trap density and energetic disorder, evidenced by a reduced Urbach energy (E _u) of 23.66 meV, and curbs non‐radiative voltage losses (ΔV _non‐rad) by 0.012 V versus PYF‐T‐o‐based binary devices. These physical improvements translate into enhanced charge carrier mobility and extraction, thereby markedly increasing FF and short‐circuit current density (J _SC). Consequently, the corresponding ternary rigid devices (PM6:PYF‐T‐o:PY‐IDT) achieved a record‐high PCE of 20.53% (certified: 19.79%) for green‐solvent‐processed all‐PSCs, while flexible devices attained a notable efficiency of 19.08%. Additionally, the steric‐locked morphology imparted exceptional stability, including superior thermal stability and outstanding mechanical robustness (retaining 92.6% of initial PCE after 1000 bending cycles).

2. Results

2.1. Materials, Optoelectronic, and Molecular Packing Properties

The molecular structures of polymers PM6, PYF‐T‐o, and PY‐IDT are presented in Figure 1a, with the detailed synthetic schemes provided in Scheme S1. High‐temperature gel permeation chromatography (HT‐GPC) was employed to determine the molecular weight and polydispersity index of PY‐IDT (Figure S1). As shown in Figure S2, the maximum absorption peaks of PYF‐T‐o and PY‐IDT in dilute chloroform solution are located at 810 and 748 nm, respectively. Furthermore, as displayed in Figure 1b, the introduction of PY‐IDT broadens the spectral response range of the acceptor phase, which is beneficial for enhancing the photocurrent. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the three polymers were characterized by cyclic voltammetry (CV) in Figure S3, with corresponding parameters in Table S1. As demonstrated in Figure 1c, the higher LUMO level of PY‐IDT (−3.72 eV) relative to PYF‐T‐o (−3.84 eV) contributes to an enhancement of open‐circuit voltage (V _OC) in photovoltaic devices. Concurrently, the uplifted HOMO/LUMO levels of PY‐IDT facilitate cascade energy‐level alignment, which enhances exciton dissociation and charge transport kinetics in all‐PSCs.

FIGURE 1.

Molecular structures, absorption, packing, and geometry simulation. (a) Chemical structures of PM6, PYF‐T‐o and PY‐IDT. (b) UV–vis absorptions of PM6, PYF‐T‐o, PY‐IDT and PYF‐T‐o:PY‐IDT films. (c) Energy levels of PM6, PYF‐T‐o and PY‐IDT. (d) 1D GIWAXS line‐cuts of PYF‐T‐o, PY‐IDT neat films and PYF‐T‐o:PY‐IDT blend films. The geometry and surface ESP of (e) PYF‐T‐o and (f) PY‐IDT optimized by DFT calculation using Multiwfn.

To investigate the influence of PY‐IDT on the aggregation structure of the acceptors, grazing‐incidence wide‐angle x‐ray scattering (GIWAXS) was employed to analyze the molecular packing behavior in the films (Figure 1d and Figure S4 and Table S2). The neat PYF‐T‐o and PY‐IDT films exhibit π–π stacking distances of 3.92 and 3.93 Å and crystal coherence lengths (CCL) of 16.20 and 15.49 Å, respectively, while the PYF‐T‐o:PY‐IDT blend film demonstrates a shortened π–π stacking distance of 3.89 Å and an extended CCL of 18.85 Å. These results indicate that the addition of PY‐IDT leads to tighter π–π stacking and higher structural order within the acceptor blend films, facilitating charge transport throughout the active layer. The quality of the active layer morphology also depends on the compatibility between the donor and acceptor components. First, the molecular geometry and electrostatic potential (ESP) distribution of the acceptors were analyzed by density functional theory (DFT) calculations (Figure 1e,f). DFT results reveal that PYF‐T‐o achieves good molecular planarity that facilitates tight π‐π packing with elevated average ESP (3.851 KJ mol^{− 1}), thus enhancing donor‐acceptor interaction. Conversely, the PY‐IDT molecule exhibits a relatively low ESP (2.529 KJ mol^{− 1}), attributable to the bulky IDT unit effectively suppressing rotation of single bonds between adjacent connecting units, thereby enhancing planarity. This steric bulk may impede intermolecular stacking, consequently weakening interactions with donor molecules. These differences in structural characteristics and electrostatic properties jointly influence the compatibility of the acceptors with the donors. Contact angle (CA) measurements were further employed to investigate the surface tension of the materials and assess the intermolecular interactions, with relevant data provided in Figure S5 and Table S3. Specifically, the Flory‐Huggins interaction parameters (χ) for PM6:PYF‐T‐o, PM6:PY‐IDT and PM6:PYF‐T‐o:PY‐IDT were calculated as 0.013, 0.040, and 0.009, respectively. The smaller χ value of PM6/PYF‐T‐o:PY‐IDT indicates that the incorporation of PY‐IDT enhances donor‐acceptor miscibility, which is beneficial to alleviating excessive phase separation and promotes the formation of an interpenetrating network.

2.2. Photovoltaic Performances and Energy Disorder Analysis

Conventional device structures were fabricated using o‐xylene to investigate the influence of PY‐IDT on their photovoltaic performances. Detailed device architecture and fabrication procedures are provided in Supporting Information. As presented in Figure 2a and Table 1, PM6:PYF‐T‐o delivered a PCE of 17.89%, with a V _OC of 0.888 V, a J _SC of 26.50 mA cm^{− 2}, and an FF of 75.41%. In comparison, the PM6:PY‐IDT device obtained a PCE of 17.56% with a higher V _OC of 0.972 V but lower J _SC (24.17 mA cm^{− 2}) and FF (74.57%) due to its limited near‐infrared absorption spectrum. By incorporating PY‐IDT into the binary blend, the optimized ternary device achieved a V _OC of 0.942 V, a J _SC of 26.54 mA cm^{− 2}, and an FF of 82.11%, leading to a record‐high PCE of 20.53% (certified as 19.79%, Figure S6). This performance ranks among the highest reported for halogen‐free all‐PSCs (Figure 2b and Table S4). The external quantum efficiency (EQE) spectra of the all‐PSCs were measured to further explore the physical mechanism of the device's performance improvement (Figure 2c). The PM6:PYF‐T‐o‐based device exhibits a broad photo‐response from 430 to 880 nm, whereas the weak aggregation of the acceptors and the unfavorable molecular packing in the PM6:PY‐IDT result in a poor EQE response in the 640–770 nm range. The PM6:PYF‐T‐o:PY‐IDT device exhibited enhanced EQE response covering the 450–700 nm range, which collectively contributes to achieving a higher J _SC. Furthermore, the integrated J _SC from EQE curves for the PM6:PYF‐T‐o, PM6:PY‐IDT and PM6:PYF‐T‐o:PY‐IDT devices were 25.44, 23.20 and 25.61 mA cm⁻², respectively, which were consistent with the J _SC measured by the J–V curves. To probe the practical application and manufacturing prospects of the all‐PSC, we prepared the large‐area devices with an active area of 1 cm². As presented in Table S5, the ternary all‐PSC produces a PCE of 19.33%, with a V _OC of 0.940 V, a J _SC of 25.86 mA cm⁻², and an FF of 79.52%. The results reveal that the ternary blend in our work is suitable for large‐area coatings in an environmentally friendly way, which is crucially important for practical applications of organic photovoltaics. The mechanism of the increase of V _OC in the ternary devices will be further discussed from the viewpoint of energy loss analysis below.

FIGURE 2.

Photovoltaic performance and energy loss analysis of PYF‐T‐o‐ and PY‐IDT‐based binary and ternary devices. (a) J–V curves of PM6:PYF‐T‐o, PM6:PY‐IDT, and PM6:PYF‐T‐o:PY‐IDT devices. (b) The summary of reported representative all‐PSCs. (c) The EQE spectra, and (d) EQE_EL curves of PM6:PYF‐T‐o‐, PM6:PY‐IDT‐, and PM6:PYF‐T‐o:PY‐IDT‐based devices. (e) Histograms of energy loss items. (f) The normalized FTPS‐EQE spectra for three all‐PSCs. (g) J–V characteristics of hole‐only devices. (h) J _ph‐V _eff curves for three all‐PSCs. (i) Photo‐CELIV curves for three all‐PSCs.

TABLE 1.

The photovoltaic parameters for these all‐PSC devices.

Devices	V OC (V)	J SC (mA cm−2)	J SC, cal (mA cm−2)	FF (%)	PCE (%)
PM6:PYF‐T‐o	0.888	26.50	25.44	75.41	17.89
PM6:PY‐IDT	0.972	24.17	23.20	74.57	17.56
PM6:PYF‐T‐o:PY‐IDT	0.942	26.54	25.61	82.11	20.53

Given the significant increase in V _OC from 0.888 to 0.942 V observed in the ternary device, the energy loss mechanisms were investigated to understand the origin of this enhancement upon incorporating PY‐IDT. Fourier‐transform photocurrent spectroscopy (FTPS) EQE and electroluminescence quantum yield (EQE_EL) measurements were employed. The corresponding parameters and results are provided in Table S6 and Figure 2d–f and Figure S7. Figure 2d quantifies the EQE_EL values as 1.30 × 10⁻⁴ (PM6:PYF‐T‐o), 3.56 × 10⁻⁴ (PM6:PY‐IDT), and 2.10 × 10⁻⁴ (PM6:PYF‐T‐o:PY‐IDT), respectively corresponding to ΔE ₃ values of 0.230, 0.204, and 0.218 eV. The reduction of ΔE ₃ to 0.218 eV in the ternary device, which is 0.012 eV lower than that of the PM6:PYF‐T‐o binary device (0.230 eV), directly reflects a suppression of non‐radiative recombination. Notably, the ΔE ₂ value of the ternary device is reduced to 0.041 eV compared to those of the PM6:PYF‐T‐o (0.043 eV) and PM6:PY‐IDT (0.096 eV) devices. This reduction in ΔE ₂ originates from suppressed energetic disorder, as evidenced by diminished E _u [40]. As shown in Figure 2f, the E _u values determined from FTPS‐EQE spectra for the three devices are 25.74 meV (PM6:PYF‐T‐o), 26.01 meV (PM6:PY‐IDT), and 23.66 meV (PM6:PYF‐T‐o:PY‐IDT), respectively. This direct quantification of reduced energetic disorder provides a physical basis for the observed reduction in ΔE ₂. To further evaluate the role of “Steric‐Locking” strategy in suppressing energy disorder, Stokes shift (Δλ) between the absorption and photoluminescence peaks was measured for PYF‐T‐o, PY‐IDT, and PYF‐T‐o:PY‐IDT blend films (Figure S8). A reduced Δλ indicates weakened structural relaxation between the excited and ground states, reflecting decreased molecular conformational disorder [41, 42]. Compared with the pure PYF‐T‐o film (Δλ = 40 nm) and pure PY‐IDT film (Δλ = 55 nm), the Δλ of the PYF‐T‐o:PY‐IDT blend film is reduced to 37 nm. The significant reduction of Δλ for the PYF‐T‐o:PY‐IDT blended system confirms decreased conformation disorder, primarily attributed to the steric hindrance effect of PY‐IDT. Specifically, the steric hindrance of PY‐IDT effectively “locks” the molecular backbone conformation, reducing torsional conformations in high‐energy states and thereby lowering energy disorder. Overall, the incorporation of PY‐IDT simultaneously mitigates both radiative loss below the bandgap (ΔE ₂, by reducing energy disorder) and non‐radiative loss (ΔE ₃, by suppressing nonradiative recombination), thereby collectively contributing to the reduction in total energy loss and the significant enhancement of V _OC.

The intensification of energy disorder is usually closely related to the formation of trap states in the active layer [43]. To investigate the origin of nonradiative loss, the trap state densities in different devices were measured using the space‐charge‐limited current (SCLC) method. As shown in Figure 2g, the trap‐fill‐limited voltages (V _TFL) for the PM6:PYF‐T‐o, PM6:PY‐IDT, and PM6:PYF‐T‐o:PY‐IDT devices were determined to be 0.34, 0.41, and 0.29 V, respectively. The lower V _TFL observed in the ternary system corresponds to a reduced trap state density, which suppresses charge recombination and improves charge transport, thereby curbing energy loss. Charge carrier dynamics were employed to investigate the differences in exciton dissociation and charge collection behavior in different device systems. As displayed in Figure 2h, the photocurrent density (J _ph)‐effective voltage (V _eff) characteristic was measured [44, 45]. The PM6:PYF‐T‐o:PY‐IDT device exhibited an exciton dissociation efficiency (η _diss) of 99.46%, outperforming both from the PM6:PYF‐T‐o (98.27%) and PM6:PY‐IDT (97.54%) devices. The enhanced η _diss values indicate more efficient charge generation and transfer along with suppressed charge recombination, which well explains the significantly improved J _SC and FF in the ternary device. The charge recombination mechanisms in the devices were investigated by analyzing the light intensity dependence of J _SC. The relationship between J _SC and P _light is described by the power law J _SC ∝ P _light ^α, where the value of α is used to quantify bimolecular recombination [46]. As shown in Figure S9a, the α values of the PM6:PYF‐T‐o, PM6:PY‐IDT, and PM6:PYF‐T‐o:PY‐IDT devices are 0.961, 0.949, and 0.982, respectively. The α value close to 1 in the ternary device indicates suppressed bimolecular recombination. To probe the trap‐assisted recombination behavior in the devices, the V _OC was analyzed as a function of P _light according to the relation V _OC ∝ (nKT/q) ln (P _light), as depicted in Figure S9b. The n value of the PM6:PYF‐T‐o:PY‐IDT device decreased to 1.06 relative to 1.24 for PM6:PYF‐T‐o device and 1.31 for PM6:PY‐IDT device, indicating suppressed trap‐assisted recombination. Furthermore, charge carrier dynamics were investigated through transient photocurrent (TPC) measurements. As shown in Figure S9c, the charge extraction times for PM6:PYF‐T‐o, PM6:PY‐IDT, and PM6:PYF‐T‐o:PY‐IDT devices were determined to be 0.56, 0.62, and 0.47 µs, respectively. The shorter charge extraction time observed in the ternary blend suggests more efficient carrier extraction. Charge carrier lifetime was investigated by the transient photovoltage (TPV) measurements. As depicted in Figure S9d, the ternary device exhibited a longer charge carrier lifetime (7.70 µs) than those of PM6:PYF‐T‐o (5.44 µs) and PM6:PY‐IDT (4.47 µs). The prolonged carrier lifetime observed in the PM6:PYF‐T‐o:PY‐IDT device demonstrates that incorporating PY‐IDT into PM6:PYF‐T‐o effectively suppresses charge recombination. These enhancements are beneficial to increasing J _SC and FF. In addition, photo‐induced charge extraction by linearly increasing voltage (Photo‐CELIV) was measured to investigate charge transport properties. As shown in Figure 2i, the PM6:PYF‐T‐o:PY‐IDT device achieved a charge carrier mobility (µ) of 4.94 × 10^{− 4} cm² V^{− 1} s^{− 1}, superior to PM6:PYF‐T‐o (3.28 × 10^{− 4} cm² V^{− 1} s^{− 1}) and PM6:PY‐IDT (2.70 × 10^{− 4} cm² V^{− 1} s^{− 1}). This enhanced charge transport capability contributes to the higher J _SC observed in the ternary device. In summary, systematic analysis from multiple device physics perspectives reveals that the incorporation of PY‐IDT effectively reduces energy disorder and trap state density, promotes more efficient charge transport and collection.

2.3. MD Simulation Analysis

To clarify the aggregation behavior under different acceptor conditions and the influence of the “Steric‐Locking” design of PY‐IDT, molecular dynamics (MD) simulations were performed to construct the spatial arrangement of donor and acceptor molecules. The radial distribution functions (RDF) g(r) used in this work were calculated using the molecule‐surface‐based‐surf mol method in GROMACS, detailed clarification is provided in the supporting information. Figure 3a–c shows the configurations at thermal equilibrium, where blue, red, and green represent the donor PM6, the acceptor PYF‐T‐o, and PY‐IDT, respectively. Molecular coordinates were extracted from the MD trajectories to calculate the g(r) between components in the blend systems. For PM6:acceptor blends, a higher intensity of the first peak in the RDF profile indicates tighter molecular packing within the donor or acceptor domains [47]. As shown in Figure 3d, the donor–donor (D–D) peak intensities are similar across the three blend films, suggesting little influence on donor molecular packing. In contrast, the acceptor–acceptor (A–A) RDF (Figure 3e) displays a higher peak for the ternary system PM6:PYF‐T‐o:PY‐IDT compared to the PM6:PYF‐T‐o and PM6:PY‐IDT, indicating denser acceptor‐molecule packing. Moreover, as shown in Figure 3f, the donor–acceptor (D–A) RDF reveals closer intermolecular packing between donor and acceptor in the ternary blend, which favors the formation of a uniform and continuous donor–acceptor interface, providing a structural basis for charge separation and transport.

FIGURE 3.

Molecular dynamics simulations for different all‐PSCs. (a–c) Stabilized distributions of PM6:PYF‐T‐o, PM6:PY‐IDT, and PM6:PYF‐T‐o:PY‐IDT blends from MD simulations. Color code: PM6 (blue), PYF‐T‐o (red), PY‐IDT (green). (d–f) Statistical simulation results of stacking distances between donor–donor (D–D), acceptor–acceptor (A–A), and donor‐acceptor (D–A) pairs across different systems. (g,h) The molecular centroid distances between acceptor‐acceptor (A–A), and donor–acceptor (D–A) pairs in the MD‐simulated PM6:PYF‐T‐o, PM6:PY‐IDT, and PM6:PYF‐T‐o:PY‐IDT blends. (i) Distribution of simulated solvent‐accessible surface area (SASA) data among different components.

The molecular centroid distances for A–A and D–A pairs were further evaluated. As illustrated in Figure 3g, the PM6:PYF‐T‐o and the ternary blends exhibit almost identical A–A spacings (5.31 nm), whereas PM6:PY‐IDT displays a notably elongated A–A distance of 5.58 nm. This elongation likely originates from the larger steric hindrance that impedes intermoiety packing in the PM6:PY‐IDT system. Furthermore, as depicted in Figure 3h, the PM6:PYF‐T‐o and the ternary blends also exhibit similar D–A spacing (2.04 nm), while the D–A distance of PM6:PY‐IDT increased (5.95 nm). This is consistent with the trend observed in their A–A molecular centroid distance (Figure 3g). This suggests that while PY‐IDT itself possesses large steric hindrance, its strategic incorporation into the ternary matrix does not disrupt, but rather reinforces the molecular network. Consistent with this, solvent accessible surface area (SASA) measurements (Figure 3i) demonstrate a pronounced interfacial contraction in the acceptor domain of the ternary system. This indicates that the PY‐IDT guest induces a more compact phase arrangement with optimized domain size, thereby improving charge transport and phase separation. In summary, MD simulations reveal that the “steric‐locking” effect of PY‐IDT modulates the molecular packing behavior, leading to tighter acceptor domains and improved donor–acceptor interfacial contact, which collectively establish a morphological foundation for superior device performance.

2.4. Blend Film Morphology and Crystallization Kinetics Analysis

To validate these molecular packing properties and investigate the crystallization kinetics process, multi‐scale characterization (in situ UV–vis, atomic force microscopy (AFM), GIWAXS) was conducted. As shown in Figure 4a–f, in situ UV–vis absorption spectroscopy was employed to monitor the deposition kinetics and crystallization process of the film in real time. Based on the temporal evolution of the characteristic absorption peaks of the donors and acceptors, the film formation can be distinctly divided into three stages: the initial dissolution stage (green region) with unchanged absorption; the aggregation stage (blue region) showing a distinct redshift upon continued solvent evaporation; and a final stage (yellow region) yielding a stable dry film [48, 49]. Specifically, in the PM6:PYF‐T‐o binary system, the aggregation onset times for the donor and acceptor were 976 and 957 ms, respectively, with corresponding aggregation durations of 83 and 129 ms. In contrast, the PM6:PY‐IDT binary system exhibited markedly shortened first‐stage assembly (206 ms for the donor and 215 ms for the acceptor) with accelerated second‐stage aggregation rates (69 ms for the donor and 55 ms for the acceptor). The in situ UV–vis kinetics reveal an intriguing phenomenon: despite PY‐IDT exhibiting a weaker intrinsic aggregation tendency than PYF‐T‐o, the aggregation onset in the PM6:PY‐IDT binary blend occurs substantially earlier (Figure 4b). This behavior is elucidated by considering the interplay between thermodynamic driving forces and kinetic constraints. Thermodynamically, the higher Flory‐Huggins interaction parameter (χ = 0.040) confirms a significantly poorer miscibility between PM6 and PY‐IDT compared to the PM6:PYF‐T‐o system (χ = 0.013). During the early stages of solvent evaporation, this immiscibility accelerates liquid‐liquid phase separation, leading to localized enrichment of PM6 that prematurely triggers the aggregation of the donor phase (with the onset time shortened from 976 to 206 ms). Kinetically, while phase separation is initiated early, the bulky steric hindrance of PY‐IDT severely restricts its own excessive self‐stacking and drives the phase‐separated domains into a rapidly kinetically solidified state. The combination of this thermodynamically premature phase separation and kinetic steric constraint successfully explains how the blend traps into a well‐dispersed, nanoscale morphology at a very early stage without oversized aggregation. This phenomenon primarily originates from the inherently weaker aggregation tendency of PY‐IDT molecules and the introduced steric hindrance effects, which collectively accelerate the overall aggregation kinetics and also modulate the aggregation behavior of the donor. In the ternary blend, PY‐IDT further shortened these stages and induced an acceptor peak blueshift, indicating suppressed excessive aggregation. This promotes optimized nanoscale phase separation and an interpenetrating network, leading to enhanced crystallinity and charge transport.

FIGURE 4.

The film‐forming kinetics and crystallization behavior for different blend systems. In situ absorption spectrum of (a) PM6:PYF‐T‐o, (b) PM6:PY‐IDT, and (c) PM6:PYF‐T‐o:PY‐IDT. (d–f) The related maximum absorption peak location for different blend films. (g–i) 2D GIWAXS patterns and (j) 1D line‐cut profiles of PM6:PYF‐T‐o, PM6:PY‐IDT, PM6:PYF‐T‐o:PY‐IDT blend films. (k) The histogram of CCL and π–π stacking distance in OOP direction for different blend films.

To correlate the solution behavior with the solid‐state morphology, the surface morphological differences of the blend films were further investigated by AFM. As depicted in Figure S10, the ternary film obtained a lower root‐mean‐square (RMS) roughness (0.64 nm) vs. binary counterparts PM6:PYF‐T‐o (0.76 nm) and PM6:PY‐IDT (0.96 nm), with a smoother, fibrous surface‐consistent with UV–vis‐derived aggregation regulation. To probe the molecular packing characteristics of the blend films, GIWAXS measurements were performed, as depicted in Figure 4g–j. All films exhibit a predominant “face‐on” orientation, as evidenced by a distinct (010) π–π stacking peak in the out‐of‐plane (OOP) direction, which favors vertical charge transport. Compared to the PM6:PYF‐T‐o film (d _π–π = 3.79 Å), the PM6:PY‐IDT blend shows a larger π–π distance of 3.83 Å (q _z = 1.64 Å^{− 1}), attributable to the greater steric hindrance of PY‐IDT. In contrast, the ternary blend exhibits the smallest d _π–π (3.70 Å) and the strongest (010) peak intensity, indicating tighter and more ordered molecular packing. Consistent with this, the ternary film also possesses the largest coherence length (CCL = 17.67 Å) vs the binary films (16.63 and 16.16 Å), confirming its enhanced crystallinity (Figure 4k and Table S7). In summary, the incorporation of PY‐IDT effectively modulates the film‐forming kinetics, suppressing excessive acceptor aggregation and optimizing the blend morphology. This regulation yields a smoother, fibrillar surface and more compact, ordered molecular packing, synergistically enhancing crystallinity and charge transport while reducing energetic disorder.

2.5. Charge Carrier Dynamics

To investigate the carrier dynamics in blend films in depth, transient absorption spectroscopy (TAS) measurements were employed. Polymer acceptors were excited at a wavelength of 800 nm, and their two‐dimensional (2D) spectra are presented in Figure S11. Ground‐state bleaching (GSB) signals were observed near 800 nm for PYF‐T‐o and PY‐IDT, while the GSB signals in the 500–660 nm range were assigned to PM6 (Figure S12). As shown in Figure 5a–f, the GSB decay of the acceptors is consistent with the enhanced bleaching signal of PM6 in the 500–600 nm range, which indicates efficient hole transfer from acceptors to the donor. Extraction of the acceptor GSB signals from different blend films (Figure 5g) reveals that the GSB signal of the acceptor in PM6:PY‐IDT decays the slowest, suggesting the slowest hole transfer process. In contrast, the acceptor GSB decays faster in PM6:PYF‐T‐o and the ternary blend films, reflecting a more rapid hole transfer process. This result is consistent with the faster generation process observed in the donor's GSB signal (Figure 5h) [50]. Furthermore, a double‐exponential function was used to fit the hole transfer process, yielding two time constants (τ₁ and τ₂). These constants correspond to the ultrafast dissociation of excitons at the donor‐acceptor interface and the diffusion of excitons toward the interface within the donor/acceptor domains, respectively [51, 52]. As shown in Figure 5i, both τ₁ (0.338 ps) and τ₂ (1.6 ps) in the ternary device are the shortest, whereas τ₁ (0.425 ps) and τ₂ (2.4 ps) in PM6:PY‐IDT are the longest. This phenomenon is mainly attributed to the large steric hindrance of PY‐IDT molecules, which results in larger domain sizes that are unfavorable for exciton diffusion and dissociation. However, when PY‐IDT is incorporated into the PM6:PYF‐T‐o system, it regulates film formation kinetics to suppress excessive molecular aggregation and form appropriate domain sizes, thereby facilitating exciton diffusion and dissociation.

FIGURE 5.

Charge carrier dynamics for different blend systems. (a–c) The 2D color spectra and (d–f) 1D transient absorption spectra of PM6:PYF‐T‐o, PM6:PY‐IDT, and PM6:PYF‐T‐o:PY‐IDT blend films. (g) Acceptor GSB and (h) donor GSB kinetic curves of all‐PSCs blend films treated under different conditions. (i) The statistical histograms for τ₁ and τ₂.

2.6. Device Stability and Mechanical Flexibility

Owing to the inherent high degree of polymerization and chain entanglement of polymer materials, stress can be dissipated more effectively during bending, thereby imparting excellent mechanical robustness [53, 54]. Leveraging these mechanical advantages of the polymer system, corresponding flexible devices were fabricated, with a schematic of the device structure shown in Figure 6a. Photovoltaic performance measurements revealed that the ternary system achieved a PCE of 19.08% (Figure 6b and Table S8). This high efficiency is attributed to the optimized molecular packing order and suppressed energy disorder resulting from the incorporation of PY‐IDT. To further evaluate the mechanical stability of the flexible devices, a systematic study was conducted on the performance of OSCs based on different devices under different bending radii (Figure 6c). After 300 bending cycles at a curvature radius of 1 mm, the ternary devices retained approximately 95% of its initial PCE, demonstrating exceptional mechanical stability. Under the same conditions, the PM6:PYF‐T‐o and PM6:PY‐IDT binary devices retained 90% and 87% of their initial PCE, respectively.

FIGURE 6.

Photovoltaic performance and mechanical stability of flexible devices. (a) Flexible device architecture images for all‐PSCs. (b) J–V curves for flexible all‐PSCs. (c) Normalized mechanical stability with different bending radii (R) after continuous bending for 300 cycles. (d) Normalized long‐term bending tests with 1000 cycles at R = 3 mm. (e) Corresponding summarizes of the degradation of photovoltaic parameters after 1000 bending cycles at R = 3 mm. (f) Parameter comparison of PM6:PYF‐T‐o, PM6:PY‐IDT, and PM6:PYF‐T‐o:PY‐IDT.

Building upon the evaluation of device changes under long‐term stress, this study systematically investigated the mechanical stability and thermal stability. As displayed in Figure 6d, the long‐term stability of the devices under milder strain was investigated. After 1000 bending cycles at a radius of 3 mm, the ternary device maintained 92.6% PCE, significantly outperforming the PM6:PY‐IDT device (91.1% retention). As shown in Figure 6e, the performance degradation is primarily attributed to the decrease in FF and V _OC, while J _SC remains relatively stable. The superior mechanical stability of the ternary system stems from the introduction of PY‐IDT, which effectively suppresses excessively large phase domains and enhances donor‐acceptor miscibility. Furthermore, the performance changes of rigid devices under long‐term thermal stress were measured. As shown in Figure S13, after 1400 h of thermal aging at 80°C, the PCEs of the PM6:PYF‐T‐o and PM6:PY‐IDT binary devices decreased significantly to 71.2% and 64.1% of their initial values, respectively. In contrast, the PM6:PYF‐T‐o:PY‐IDT ternary device exhibited markedly enhanced thermal stability under identical conditions, retaining 79.8% of its initial PCE. This improvement is attributed to the steric hindrance of PY‐IDT, which suppresses polymer chain mobility within the blend, while its dense molecular packing effectively inhibits thermal‐induced molecular motion. These effects collectively prolong the thermodynamic process of PM6:PYF‐T‐o:PY‐IDT films, thereby enhancing thermal stability. As illustrated in Figure 6f, a comprehensive analysis across five key metrics (rigid/flexible efficiency, thermal/mechanical stability, FF) reveals that the PY‐IDT ternary device achieves synergistic enhancements. This improvement is attributed to the sterically bulky unit in PY‐IDT, which optimizes film‐formation kinetics in non‐halogenated solvents. The optimized kinetics suppress excessive aggregation and oversized phase domains, while reducing energy disorder, collectively boosting performance and stability.

Generally, enhancing polymer crystallinity to achieve a high FF often increases film brittleness, resulting in an inherent trade‐off between electrical performance and mechanical flexibility. However, the ternary system in this work effectively mitigates this conflict. This synergistic effect originates from the multi‐scale hierarchical morphology induced by the unique “steric‐locking” effect of the guest PY‐IDT. MD simulations reveal that the bulky IDT units of PY‐IDT precisely modulate the microstructure of the blend. Although the ternary blend exhibits a more compact overall molecular packing (reduced SASA), its localized steric hindrance moderately disrupts the long‐range crystalline order. This develops a unique coexisting structure where relatively ordered domains (sustaining efficient charge transport pathways) interpenetrate with localized disordered regions (serving as sites for mechanical stress dissipation). Under this hierarchical configuration, the film can flexibly accommodate bending strain through the conformational relaxation of polymer segments within the disordered regions without disrupting the global charge carrier percolation network. This kinetically locked, robust morphology successfully harmonizes high crystallinity with enhanced mechanical toughness, offering a viable pathway for fabricating all‐polymer solar cells with simultaneously outstanding efficiency and mechanical durability.

3. Discussion

In conclusion, we have developed a “steric‐locking” strategy that effectively coordinates the relationship between molecular‐level architectural design and the macroscopic optoelectronic properties of polymer heterojunctions. This “steric‐locking” mechanism functions as a pivotal conformational regulator: by introducing localized steric hindrance, it effectively modulates the crystallization kinetics and suppresses the deleterious self‐aggregation of the host acceptor. The resulting conformational stabilization reinforces an exceptionally ordered and compact molecular arrangement within the ternary bulk‐heterojunction, as evidenced by a remarkably low Urbach energy of 23.66 meV and a reduced trap state density. Consequently, this structural refinement yields a record‐breaking PCE of 20.53% (certified: 19.79%) for green‐solvent‐processed all‐PSCs, characterized by simultaneously improved V _OC (0.942 V) and FF (82.11%). Beyond electronic optimization, the steric‐locked interpenetrating network provides a structural scaffold that ensures exceptional thermal stability (T₈₀ = 1400 h at 80°C) and mechanical resilience (92.6% PCE retention after 1000 bending cycles). By harmonizing high‐efficiency energy conversion with environmentally benign processing and exceptional durability, this conceptual framework provides a universal blueprint for the next generation of scalable, flexible, and sustainable organic semiconductor technologies.

Author Contributions

K.G. and Y.S. conceived the ideas and visualization. Y.S. and L.S. conducted the device fabrication, device measurements, and data collection. H.G. synthesized the PY‐IDT. Y.K. and X.M. conducted the E _loss and TEM and analyzed the results. L.W. performed the data collection and analysis for molecular dynamics simulation. T.G., C.H., and W.Z. performed the data collection and analysis for TA. J.Z. and G.Z. collected Eloss data of these devices. X.W. and R.Y. conducted the GIWAXS and analyzed the results. X.M., L.W., Y.K., Y.F., and W.Z. contributed to the research preparation and data analysis. The manuscript was mainly written by Y.S., H.G., Y.K., L.S., and K.G. All authors contributed to discussions, manuscript writing, and revisions.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: adma74003‐sup‐0001‐SuppMat.docx.

Data Availability Statement

All the data generated in this study are provided in the Article file, Supplementary Information file, and Source Data file. Source data are provided in this paper.