Side-Chain Length Modulation in Thieno[3,2‑c]isochromene Small-Molecule Donors for High-Performance Ternary Fullerene Organic Solar Cells

Abstract

Side-chain engineering is an effective strategy for precisely tuning the crystallinity and active-layer morphology of high-performance bulk-heterojunction (BHJ) organic solar cells (OSCs). To systematically elucidate the side-chain length–property relationship across a complete homologous series, we herein designed and employed a small-molecule donor TiC6, which, together with the previously reported TiC8 and TiC12 analogues, completes the thieno­[3,2-c]­isochromene-based donor family with alkyl chains varying from hexyl to dodecyl. Taking the PTB7-Th:PC₇₁BM system as a platform, we employed these three donors as the third component in ternary OSCs and systematically compared their photovoltaic performance and stability. Our comparative study reveals a non-monotonic chain-length dependence: the TiC8-based device delivers the highest PCE of 11.12%, superior to TiC6 (9.80%) and TiC12 (10.42%), which is attributed to optimized π–π stacking. Notably, the stability trend exhibits a reverse order; the TiC12-based device shows dramatically superior thermal stability, retaining 92% of its initial PCE after 48 h at 100 °C, far exceeding TiC8 (86%) and TiC6 (77%) under identical conditions. All ternary devices substantially outperform the binary PTB7-Th:PC₇₁BM control (9.16%) and exhibit markedly enhanced long-term storage stability (e.g., TiC6 retains 90% after 360 h at RT, whereas the control drops to 63% within 10 min at 100 °C). This work not only expands the donor library but also provides unprecedented insights into the side-chain length–property relationship, demonstrating that optimizing efficiency and thermal stability requires distinct chain-length choices.

Introduction

Bulk-heterojunction (BHJ) organic solar cells (OSCs) have emerged as a promising photovoltaic technology owing to their lightweight nature, cost-effectiveness, translucence, flexibility, and compatibility with roll-to-roll solution processing. − Driven by the development of novel donor–acceptor materials and device technologies, particularly the emergence of Y6-series acceptors, − the power conversion efficiencies (PCEs) of BHJ-OSCs have recently achieved tremendous improvements, with state-of-the-art devices now exceeding 21%. However, there remains a considerable gap before commercial production, necessitating further research, especially in terms of improving device performance and long-term stability. The limited absorption range of the donor and acceptor in the active-layer hinders the further enhancement of binary OSC performance. The ternary strategy, which introduces a third component, effectively resolves this issue by not only broadening the absorption spectrum to enhance the short-circuit current density (J _SC), but also further optimizing the active-layer morphology to improve the fill factor (FF). , However, only a few ternary OSC systems can achieve superior performance relative to their binary counterparts, owing to the complexity of the ternary blend films. − Moreover, the specific action mechanism of the third component remains largely ambiguous.

Generally, nanoscale phase separation in the active-layer is a critical factor governing the performance of OSCs. To achieve an ideal phase-separation morphology, various methods have been employed, such as solvent additives, solvent vapor annealing (SVA), and thermal annealing (TA), etc. , An in-depth understanding of the morphological evolution of the active-layer during processing is of great significance for boosting OSC performance. In addition, chemical structure modifications such as main-chain engineering, side-chain engineering, and heteroatom substitution of donor–acceptor materials are also important strategies to effectively regulate molecular aggregation and stacking behavior, − thereby influencing the morphology and photovoltaic performance of OSCs. This structure-morphology-performance relationship is particularly critical for the more complex active-layers of ternary OSCs, where the photovoltaic performance is extremely sensitive to the chemical structure of the donor–acceptor materials. − Therefore, meticulous design of the chemical structure of donor–acceptor materials is an effective pathway toward achieving superior photovoltaic performance in ternary OSCs, and elucidating the structure–morphology correlation provides valuable guidance for the development of new materials. Furthermore, the stability of the active layer is one of the most critical factors concerning device stability. Owing to the metastable nature of the OSC active-layer, which tends to evolve toward a thermodynamic equilibrium state and thereby form large-scale phase separation, achieving both high performance and high stability simultaneously remains a formidable challenge.

Although the fused-ring acceptors such as ITIC and Y6 series have achieved great success, their synthesis is complex and relatively costly. Therefore, the lower-cost classical fullerene system is still a good choice for OSC research. To systematically elucidate how side-chain length governs the photovoltaic performance and stability of ternary fullerene OSCs, we herein designed and employed a small-molecule donor TiC6, together with the previously reported TiC12 and TiC8 analogues, completes a homologous series of thieno­[3,2-c]­isochromene-based donors with alkyl chains varying from hexyl to dodecyl. Compared to their long-chain counterparts (TiC8 and TiC12), the short-chain TiC6 typically exhibits distinct advantages in terms of precursor cost, synthetic efficiency, and the ease of post-purification. Furthermore, the shorter C6 alkyl chain in TiC6 imposes minimal steric hindrance, facilitating a closer intermolecular π–π stacking distance. − This structural feature is highly conducive to the formation of highly ordered crystalline structures, thereby enhancing the charge carrier mobility, and incorporated them as third components into the classic PTB7-Th:PC₇₁BM system to fabricate ternary OSCs. The chemical structures of PTB7-Th, TiC12, TiC8, TiC6 and PC₇₁BM are shown in Figure a. The effects of TiC12, TiC8 and TiC6 on the optical, electrochemical, morphological, photovoltaic, and stability properties were systematically investigated and discussed in detail. Our comparative study across the C6/C8/C12 series reveals two unprecedented findings: (i) the photovoltaic performance follows a non-monotonic chain-length dependence, with the TiC8-based ternary device delivering the highest PCE of 11.12%, superior to TiC12 (10.42%) and TiC6 (9.80%); and (ii) the thermal stability exhibits a reverse trend, with TiC12 retaining 92% of its initial PCE after 48 h at 100 °C, far outperforming TiC8 (86%) and TiC6 (77%). Notably, all three ternary systems substantially outperform the binary PTB7-Th:PC₇₁BM control (9.16%) and exhibit dramatically enhanced storage and thermal stability, which is attributed to hydrogen-bonding interactions between the TiC series donors and PC₇₁BM that stabilize the blend morphology. This work not only demonstrates the effectiveness of the ternary strategy but also provides unprecedented insights into the side-chain length–property relationship, revealing a critical efficiency–stability trade-off that offers valuable guidance for the rational design of high-performance and durable OSCs.

1.

(a) Chemical structures of PTB7-Th, TiC6, TiC8, TiC12 and PC₇₁BM, (b) UV–vis absorption spectra of the PTB7-Th, TiC6, TiC8, TiC12 and PC₇₁BM neat films, (c) Energy level diagrams of PTB7-Th, TiC6, TiC8, TiC12 and PC₇₁BM.

Results and Discussion

The absorption characteristics of PTB7-Th, TiC12, TiC8, TiC6, and PC₇₁BM neat films were carefully investigated, with the corresponding spectra and parameters presented in Figure b and Table . The wide-bandgap small-molecule donors (TiC12, TiC8, and TiC6), the medium-bandgap polymer PTB7-Th, and the acceptor PC₇₁BM exhibit complementary absorption ranges spanning from 300 to 800 nm, which is beneficial for capturing more photons and thus improving the short-circuit current density (J _SC). Additionally, compared with TiC8 neat film (560 nm), the maximum absorption peak of TiC6 is slightly blue-shifted at 580 nm, which may arise from the subtle effect of alkyl chain length on the molecular configuration. The absorption onsets of PTB7-Th, TiC6 neat films are 780 and 700 nm, corresponding to the optical bandgaps (E_g ^opt) of 1.62 and 1.77 eV, respectively. While the polymer donor PTB7-Th dominates the absorption in the long-wavelength region (600–750 nm), the small-molecule donors (TiC12, TiC8, and TiC6) exhibit strong absorption in the visible range of 450–600 nm, effectively filling the absorption valley of the binary PTB7-Th:PC₇₁BM system. This broadened photon harvesting capability is expected to enhance the J _SC of the ternary devices.

1. Photophysical and Electrochemical Properties of TiC12, TiC8 and TiC6.

Donors	λmax [nm]	εmax [L mol–1 cm–1]	λmax [nm]	λonset [nm]	Eg opt [eV]	E HOMO [eV]	E LUMO [eV]	E g CV [eV]
TiC12	578	2.13 × 105	580	700	1.77	–5.45	–3.26	2.19
TiC8	578	1.69 × 105	560	685	1.81	–5.49	–3.27	2.18
TiC6	578	1.78 × 105	580	700	1.77	–5.50	–3.28	2.11

^aAbsorption in trichloromethane solution.

^bAbsorption in the film state.

The electrochemical properties of TiC12, TiC8, and TiC6 were determined by cyclic voltammetry (CV). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels were calculated from the reduction potentials (φ_red) and oxidation potentials (φ_ox) using the equation: E _LUMO/HOMO = −e­(φ_red/ox + 4.8) (eV). the corresponding LUMO/HOMO values of the three donor materials are listed in Table , and the energy level diagrams are shown in Figure c. The small molecules TiC12, TiC8 and TiC6 possess significantly deeper HOMO energy levels (−5.45/–5.49/–5.50 eV, respectively) than PTB7-Th (−5.27 eV). It is noteworthy that the HOMO and LUMO energy levels of PTB7-Th, TiC12, TiC8, TiC6 and PC₇₁BM exhibit a cascade-like alignment, which may facilitate electron and hole transport in the ternary OSCs. Furthermore, the substantial LUMO energy level offset between the donors and PC₇₁BM provides a sufficient driving force for exciton dissociation.

To investigate the influence of the alkyl-chain length of the third component on the photovoltaic performance, OSCs with a conventional structure of ITO/PEDOT:PSS/Active-layer/Ca/Al were fabricated, where the ITO is indium tin oxide, and PEDOT:PSS is poly­(3,4-ethylenedioxythiophene):polystyrene sulfonate. The active-layers were composed of binary or ternary blend films of PTB7-Th, TiC12, TiC8, TiC6 and PC₇₁BM. The optimal D:A weight ratios were 1:1.5 for the binary OSCs (PTB7-Th:PC₇₁BM) and 0.8:0.2:1.5 for the ternary OSCs (PTB7-Th:TiC12:PC₇₁BM, PTB7-Th:TiC8:PC₇₁BM, and PTB7-Th:TiC6:PC₇₁BM), with the blend solution concentration fixed at 25 mg/mL. The 1,8-Diiodooctane (DIO) was used as solvent additive with a ratio of 3% by volume. The details of device fabrication were shown in the Supporting Information (SI). The corresponding current density-voltage (J–V) curves and the external quantum efficiency (EQE) spectra are shown in Figure , and the performance parameters of the optimal binary and ternary cells are summarized in Table .

2.

(a) J–V curves and (b) EQE curves of the optimal binary and ternary OSCs.

2. Photovoltaic Parameters of Optimized Binary and Ternary OSCs.

Active layer components	D:A ratio	V OC [V]	J sc [mA cm–2]	FF [%]	PCE [%]
PTB7-Th:PC71BM	1:1.5	0.789	16.75	69.4	9.16 (9.02 ± 0.13)
PTB7-Th:TiC12:PC71BM	0.8:0.2:1.5	0.790	17.92	73.6	10.42 (10.25 ± 0.21)
PTB7-Th:TiC8:PC71BM	0.8:0.2:1.5	0.789	19.42	72.1	11.12 (10.86 ± 0.25)
PTB7-Th:TiC6:PC71BM	0.8:0.2:1.5	0.791	17.00	72.8	9.80 (9.66 ± 0.14)

^aThe average values and standard deviations were calculated from 20 devices.

It can be observed that all ternary OSCs of PTB7-Th:TiC12:PC₇₁BM, PTB7-Th:TiC8:PC₇₁BM and PTB7-Th:TiC6:PC₇₁BM exhibit similar V _OC values to the binary OSCs of PTB7-Th:PC₇₁BM, but show significantly higher J _SC and FF, which could attribute to the morphological improvement induced by the third components (TiC12, TiC8, and TiC6). As a result, the TiC8-based ternary device exhibited a champion PCE of 11.12%. In addition, the TiC12 and TiC6 based ternary devices also achieved satisfactory PCE values of 10.42% and 9.80%, respectively, with the FF of the TiC12-based ternary device reaching 73.6%. Interestingly, the TiC6-based ternary device exhibits enhanced V _OC, J _SC, and FF values of 0.791 V, 17.00 mA cm^–2, and 72.8%, respectively. This overall improvement in parameters compared to the PTB7-Th:PC₇₁BM binary counterpart leads to superior device performance. The corresponding statistical diagram is shown in Figure a. The above results indicate the effectiveness of this type of small molecules in enhancing the performance of OSCs based on PTB7-Th:PC₇₁BM. However, the photovoltaic performance difference among the three ternary OSCs indicate that the alkyl-chain length of the third component has a significant impact on the device performance. It is also noteworthy that the V _OC values of the ternary OSCs remain almost unchanged with the addition ratio of the small molecules (TiC12, TiC8 and TiC6) ranging from 15.0 wt % to 25.0 wt %, suggesting that all three ternary OSCs follow a cascade charge-transfer model.

As shown in Figure b, PTB7-Th:PC₇₁BM binary and the three ternary OSCs exhibit almost the same EQE response range (300 nm to 800 nm). Compared with the PTB7-Th:PC₇₁BM binary device, the TiC6-based ternary device exhibits significantly enhanced absorption intensity in the 450–650 nm range, which is consistent with the results shown in Figure b. The quantum efficiencies of all three ternary OSCs are significantly higher than that of the binary counterpart. Therefore, the TiC12, TiC8 and TiC6 third components effectively enhance the photo-response intensity of the ternary OSCs.

The dark J–V characteristics of the optimal PTB7-Th:PC₇₁BM, PTB7-Th:TiC12:PC₇₁BM, PTB7-Th:TiC8:PC₇₁BM, and PTB7-Th:TiC6:PC₇₁BM based OSCs are shown in Figure a. The optimal OSCs are characterized by low reverse-bias leakage currents, avoiding the detrimental charge injection from electrodes below the turn-on voltage that usually penalizes FF and J _SC. Most importantly, the best ternary devices show pronounced rectification ratios and heavily suppressed reverse dark currents. Such electrical behaviors indicate that the introduction of small quantities of TiC12, TiC8, or TiC6 leads to fewer defects and diminished shunt resistance effects in the photoactive layers, which directly contributes to the improved FF and J _SC values.

3.

(a) Current density-voltage characteristics of the optimal binary and ternary OSCs in the dark. (b) The J _ph versus V _eff curves of the cells based on PTB7-Th:PC₇₁BM, PTB7-Th:TiC12:PC₇₁BM, PTB7-Th:TiC8:PC₇₁BM and PTB7-Th:TiC6:PC₇₁BM (0.8:0.2:1.5). (c) The hole mobility of devices based on PTB7-Th:PC₇₁BM system and (d) the corresponding electron mobility.

To comprehensively elucidate the exciton dissociation kinetics in both binary and ternary devices, we investigated the evolution of photocurrent density (J _ph) as a function of effective applied voltage (V _eff), with the relevant data presented in Figure b. Herein, Jph is defined as the difference between the current densities under illumination and in the dark (J _ph = J _L – J _D), while the effective voltage is determined by the difference between the voltage at zero photocurrent (V ₀) and the applied bias (V _a), expressed as V _eff = V ₀ – V _a. Under the saturation current density (J _sat) state, since the current is solely limited by the absorbed photon flux, it is reasonable to deduce that all photogenerated excitons undergo complete dissociation and are efficiently collected by the respective electrodes. This phenomenon corroborates the superior exciton dissociation capability of the ternary systems. By calculating the J _ph/J _sat ratio under short-circuit conditions, we evaluated the charge dissociation efficiency (η _diss) of the optimal devices. The results indicate that the binary PTB7-Th:PC₇₁BM device exhibits an η _diss of 95.43%, whereas the ternary devices incorporating TiC12, TiC8, and TiC6 show remarkable increases to 97.60 %, 97.20 %, and 96.48 %, respectively. Although the introduction of TiC small molecules with varying alkyl side-chain lengths enhances exciton dissociation efficiency, the role of TiC6 remains significant. Specifically, the incorporation of TiC6 boosts the η _diss from 95.43% in the binary system to 96.48%, demonstrating its ability to effectively facilitate the conversion of photogenerated excitons into free charges and reduce the probability of exciton recombination. Regarding saturation characteristics, the TiC6-based ternary device achieves current saturation at a relatively lower effective voltage, indicating a stronger internal driving force for exciton dissociation and optimized charge transfer capabilities at the donor/acceptor (D/A) interfaces. Despite its slightly lower enhancement margin compared to the longer side-chain TiC12 and TiC8, TiC6 still effectively modulates the active-layer morphology to provide sufficient active sites for exciton dissociation. This fully validates the positive role of the third component with a thieno­[3,2-c]­isochromene backbone in optimizing the charge generation process.

The space charge-limited current (SCLC) method was employed to measure the hole (μh) and electron (μe) mobilities of the optimal active layers, utilizing hole-only devices with a structure of ITO/PEDOT:PSS/active layer/MoO₃/Al and electron-only devices with a structure of ITO/ZnO/active layer/Ca/Al, respectively. As shown in Figure c,d, both the μh and μe values of the ternary active layers are higher than those of the PTB7-Th:PC₇₁BM binary active layer. Specifically, the μh and μe values for the PTB7-Th:TiC6:PC₇₁BM ternary active layer are calculated to be 2.13 × 10^–4 cm² V^–1 s^–1 and 2.60 × 10^–4 cm² V^–1 s^–1, respectively.The hole (μh) and electron (μe) mobilities of the optimal binary active layer based on PTB7-Th:PC₇₁BM are 1.68 × 10^–4 cm² V^–1 s^–1 and 1.60 × 10^–4 cm² V^–1 s^–1, respectively. Specifically, the μh and μe values of the PTB7-Th:TiC6:PC₇₁BM ternary active layer are increased by approximately 26.8% and 62.5%, respectively, compared to those of the optimal binary counterpart. Furthermore, the μh/μe ratio remains well-balanced at ∼0.82, which is highly beneficial for suppressing bimolecular recombination and facilitating efficient charge extraction, thereby contributing to the enhanced J _SC and FF observed in the ternary devices.

To gain insight into the origin of the superior performance of the ternary cells, the surface morphologies of the optimal binary and ternary blend films were characterized by atomic force microscope (AFM), respectively. The AFM height images reveal significant morphological evolution upon the incorporation of the third component, TiC12, TiC8, TiC6, into the PTB7-Th:PC₇₁BM binary system (Figure a–e). The binary blend film (Figure f) exhibits a relatively smooth surface with a root-mean-square (RMS) roughness of 1.54 nm. While this indicates good miscibility, however, pronounced phase aggregation is evident, which may limit charge transport efficiency. In contrast, the introduction of TiC6 induces a moderate increase in surface roughness, rising from 1.72 nm (0.85:0.15:1.5) to 2.01 nm (0.75:0.25:1.5) as the TiC6 content increases. This trend suggests that TiC6 acts as a crystallization driver within the blend matrix. As shown in Figure b, at a TiC8 ratio of 20 wt %, the ternary blend film exhibits the smallest RMS roughness of 1.38 nm and better morphology. Upon the addition of 20 wt % TiC12, the ternary blend film exhibits a moderate RMS roughness of 1.69 nm (Figure a). The TiC6-containing blend shows excessively fine phase separation with small domain sizes, while the TiC12-containing blend displays overly large phase domains due to the enhanced aggregation tendency of the longer C12 side chains, which reduce the donor–acceptor interfacial area, limiting exciton dissociation efficiency. The TiC8-containing blend, with its optimal domain size and surface roughness, provides the most balanced morphology for efficient exciton dissociation and charge transport.

4.

AFM height images (a) for the ternary blend film of PTB7-Th:TiC12:PC₇₁BM, (b) PTB7-Th:TiC8:PC₇₁BM, (c–e) PTB7-Th:TiC6:PC₇₁BM and AFM height images (f) for the binary blend films of PTB7-Th:PC₇₁BM. Panel (a) is reproduced with permission from ref , copyright 2026 MDPI. Panel (b) is reproduced with permission from ref , copyright 2021 The Royal Society of Chemistry; permission conveyed through Copyright Clearance Center, Inc.

Beyond the as-cast morphology, the thermal stability of the blend films is another critical factor influencing device longevity. To assess this, the films were subjected to thermal annealing for 48 h, and the resulting morphological changes were examined by AFM. As shown in the AFM images in Figure S3, the TiC6-containing ternary blend film maintains a more stable nanoscale phase separation morphology after 48 h of thermal annealing, with less pronounced changes in domain size and surface roughness compared to the binary blend, whereas the binary blend film shows significant phase aggregation.

Structurally, TiC6 possesses a rigid conjugated backbone featuring fluorine (F) atoms and alkyl side chains (as seen in the molecular schematic Figure a). The fluorine atoms facilitate strong non-covalent FcdotsS and FcdotsH intramolecular interactions, promoting a planar molecular conformation and enhancing π–π stacking capabilities. When incorporated into the host matrix, TiC6 likely undergoes co-crystallization or forms ordered aggregates with the donor PTB7-Th due to their structural similarities. This “alloy-like” behavior optimizes the molecular packing and prevents the large-scale phase separation often observed in binary systems dominated by fullerene aggregation. Moreover, the improved morphology of the photoactive layer also contributes to higher device stability.

In addition to the high efficiency requirements for OSCs, stability is also a critical demand for its practical application. Therefore, We investigated the room temperature storage stability and thermal stabilities of PTB7-Th:PC₇₁BM (1:1.5), PTB7-Th:TiC12:PC₇₁BM (0.8:0.2:1.5), PTB7-Th:TiC8:PC₇₁BM (0.8:0.2:1.5) and PTB7-Th:TiC6:PC₇₁BM (0.8:0.2:1.5) based devices in the nitrogen glovebox. As shown in Figure , compared with the PTB7-Th:PC₇₁BM based OSCs, all the ternary OSCs exhibit an enormously improved stability. During room-temperature storage in the nitrogen glovebox, the PCE of the control device decreased sharply to 65% of its initial value after 68 h. In contrast, all ternary devices retained over 90% of their original PCE values after more than 360 h. In addition, PTB7-Th:PC₇₁BM based OSCs exhibited extremely poor thermal stability, with the PCE value dropping sharply to 50% of its initial value after about 0.3 h at 100 °C in the nitrogen glovebox. The poor stability of the PTB7-Th: PC₇₁BM based OSCs is mainly attributed to the self-aggregation tendency of fullerenes. However, all ternary devices demonstrated significantly improved time stability and thermal stability. Among them, PTB7-Th:TiC6:PC₇₁BM-based OSCs maintained about 77% of their original PCE after 48 h at 100 °C. The above results demonstrate that incorporating TiC series small molecules into PTB7-Th:PC₇₁BM-based OSCs is an effective strategy to enhance device stability, which may be attributed to the immobilization of the donor–acceptor phase in the active-layer by these small molecules. − Interestingly, the stability of the ternary devices improves with increasing alkyl-chain length of the small-molecule third component. This may be due to the fact that longer side chains enhance molecular interactions, thereby preventing relative molecular sliding and stabilizing the active-layer morphology.

5.

(a) Room temperature stability and (b) thermal stability at 100 °C of devices based on PTB7-Th:PC₇₁BM, PTB7-Th:TiC12:PC₇₁BM, PTB7-Th:TiC8:PC₇₁BM and PTB7-Th:TiC6:PC₇₁BM.

As the alkyl side-chain length increases from C6 to C12, the steric bulk of the side chains increases progressively. The longer C12 chains introduce greater steric hindrance, which disrupts the π–π stacking order and increases the π–π stacking distance. This leads to a less favorable molecular packing arrangement for charge transport, resulting in reduced charge carrier mobility − and, consequently, lower photovoltaic efficiency compared to TiC8. In contrast, the C8 chain provides an optimal balance, sufficient to promote favorable phase separation while not excessively disrupting the π–π stacking, thereby achieving the highest PCE of 11.12%.

From the perspective of polymer physics, chain length has a well-established influence on the glass-transition temperature (T _g). According to the Fox–Flory relationship, T _g increases with increasing chain length (or molecular weight) and approaches an asymptotic value at high molecular weights. This is because longer chains have fewer chain ends per unit mass, and chain ends act as regions of enhanced free volume and faster segmental relaxation, which lower T _g. In our system, TiC12 with the longest C12 side chains possesses the highest effective T _g among the three derivatives, meaning that its active-layer morphology is more resistant to thermal perturbation. At the annealing temperature of 100 °C, the C12-based blend remains further below its T _g, thereby exhibiting superior morphology resilience and retaining 92% of its initial PCE after 48 h. In contrast, the shorter C6 chains resultin a lower T _g, making the morphology more susceptible to thermal degradation, which is consistent with the observed retention of only 77%.

The above analysis reveals that maximizing efficiency and maximizing stability impose opposing requirements on the side-chain length: shorter chains favor tighter π–π stacking and better charge transport (higher efficiency), while longer chains favor higher Tg and greater morphology resilience (better stability). TiC8 represents the optimal compromise for efficiency, whereas TiC12 represents the optimal choice for stability. This trade-off, to the best of our knowledge, has not been previously disclosed in the PTB7-Th-based donor system, and we believe it provides valuable guidance for the rational design of ternary OSC materials that target either high efficiency or long-term operational stability.

Materials and Methods

Unless otherwise stated, all reagents and solvents were obtained from commercial sources. Chlorobenzene (CB) and 1, 8-diiodooctane (DIO) were purchased from Aldrich and used without further purification. PTB7-Th and PC₇₁BM were purchased from J&K Scientific Co., Ltd. Poly­(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) (Clevios PVP Al 4083) was obtained from H.C. Starck, Germany. Molybdenum oxide (MoO₃), Ca and aluminum (Al) were purchased from Alfa Aesar Co., Ltd. All materials were used without further purification. TiC12, TiC8 and TiC6 was synthesized by our research group. ,, More detailed experiments methods and synthetic protocols are provided in SI.

Conclusions

In summary, a series of small molecules (TiC12, TiC8, TiC6) with different side-chain lengths were introduced as third components into the classical fullerene system (PTB7-Th:PC₇₁BM) to fabricate ternary OSCs enabling a systematic investigation into the side-chain length-property relationship. All ternary devices exhibit obviously improved performance compared with the binary control (9.16%), which is attributed to morphology optimization induced by the small-molecule additives. Our comparative study across the C6/C8/C12 series reveals two key insights: (1) the photovoltaic performance follows a non-monotonic chain-length dependence, with TiC8 delivering the highest PCE of 11.12%, superior to TiC12 (10.42%) and TiC6 (9.80%); and (2) the thermal stability shows a reverse trend, with TiC12 retaining 92% of its initial PCE after 48 h at 100 °C, far outperforming TiC8 (86%) and TiC6 (77%). These results demonstrate that maximizing efficiency and maximizing stability require distinct side-chain length choices, a trade-off that has not been previously disclosed in this donor system. This work demonstrates that side-chain engineering serves as an effective and generalizable strategy for modulating the molecular packing, π–π stacking behavior, and phase separation morphology of the active layer in ternary blend systems. Through systematic variation of the side-chain architecture, we have established a clear structure-property relationship that links side-chain design to the crystallization behavior, molecular orientation, and nanoscale morphology of the blend films, ultimately leading to improved device performance. Importantly, the side-chain engineering strategy presented here is not limited to fullerene-based systems; rather, it offers a broadly applicable approach for rational molecular design in organic photovoltaics. By revealing the intrinsic relationship between side-chain structure, molecular packing, and device performance, this work provides universal guidance for the future design of long-lifetime and low-cost photovoltaic materials.

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c09370.

• General materials; synthesis of TiC6, TiC8 and TiC12; General Measurements; OSC Device Fabrication and Measurement; J–V curves of PTB7-Th:TiC6:PC₇₁BM devices with different donor ratios (Figure S1); (a–c) 2D GIWAXS images and corresponding (d) in-plane and (e) out-of-plane cuts of the films (Figure S2); (a) AFM height images of PTB7-Th:TiC6:PC₇₁BM blend films under thermal annealing at 100 °C; (b) AFM height images of PTB7-Th:PC₇₁BM blend films under thermal annealing at 100 °C (Figure S3); performance parameters of PTB7-Th:TiC6:PC₇₁BM devices of different proportions; refs , are cited in the Materials (Table S1) (PDF)

Conceptualization, X.L. and W.T.; methodology, X.L. and W.T.; software, W.T.; validation, R.L. and J.L.; formal analysis, X.L. and W.T.; investigation, W.T.; resources, X.L.; data curation, R.L. and J.L.; writingoriginal draft preparation, W.T.; writingreview and editing, X.L.; visualization, X.L.; supervision, X.L.; project administration, X.L.; and funding acquisition, X.L. All authors have read and agreed to the published version of the manuscript.

This work was supported by the Outstanding Youth Project of Hunan Education Department (No. 25B0765) and the Hunan Provincial Natural Science Foundation (No. 2026JJ80591).

The authors declare no competing financial interest.