Direct C–H arylation-derived low crystallinity guest acceptor for high efficiency organic solar cells

Abstract

The majority of host/guest materials used in organic solar cells (OSCs) are currently synthesized via the Stille reaction, which suffers from poor atom/step economy, low cost-effectiveness, and environmental risks. Therefore, organic photovoltaic materials synthesized through low-cost and green methods are highly required. Here, an A–D–D–A type guest acceptor D-IDT was designed and synthesized using a tin-free direct C–H activation strategy and introduced into the classical D18:BTP-eC9 host system. Compared to the A–D–A type guest acceptor S-IDT, the D-IDT shows a greater π-conjugation but much weaker intermolecular interactions. Its low crystallinity results in good miscibility with the host acceptor BTP-eC9, which effectively promotes earlier assembly of BTP-eC9 and faster aggregation transition. This allows the formation of a smaller phase separation in the active layer, resulting in efficient exciton dissociation and charge transport. Moreover, the voltage loss of the OSC device reduces by 18 mV when D-IDT is incorporated into the binary system. As a result, the efficiency of the D-IDT-controlled device is increased to 19.92% compared to the device with S-IDT (17.66%). This work provides valuable guidelines for the exploration of guest materials via the C–H activation reaction, while controlling the crystallization kinetics to fine-tune the assembly behavior of the host acceptor.

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Broader context

Organic solar cells (OSCs), as a representative of the new generation of photovoltaic technologies, have attracted considerable attention due to their advantageous properties, including lightweight, transparency, flexibility and cost-effectiveness. However, most of the organic photovoltaic materials employed in OSCs necessitate the Stille reaction between bromine and organo-tin molecules, which is characterized by poor atom/step economy, cost-effectiveness and environmental pollution. In this study, a low-crystallinity acceptor, D-IDT, was designed and synthesized via a direct C–H activation strategy and introduced into the classical D18:BTP-eC9 host system. The introduction of D-IDT promoted faster aggregation of BTP-eC9 and inhibited its excessive self-assembly behavior, resulting in smaller domains in the D-IDT-controlled ternary film. Consequently, the device exhibited enhanced exciton dissociation and charge transport, along with reduced non-radiative voltage loss, resulting in an excellent power conversion efficiency (PCE) of 19.92%. This study elucidates the relationship between the crystalline properties of the guest material and the blend morphology of the active layer, providing a molecular design guideline for the development of efficient guest components.

1. Introduction

Organic solar cells (OSCs), as a representative of the new generation of photovoltaic technology, have attracted considerable attention due to their lightweight, transparent, flexible and cost-effective nature.^1,2 With the development of new materials and the optimization of device processes, the power conversion efficiency (PCE) of OSCs has been continuously increased.^3–5 Recently, the PCE of single-junction OSCs has reached 20%.^6–9 As far as we know, most of the organic photovoltaic materials employed in OSCs are required to undergo the Stille reaction between bromidic and organo-tin molecules, which has the disadvantages of poor atom/step economy, low cost-effectiveness, and environmental pollution.^10,11 To address these issues, the development of low-cost and green synthesis methods to construct photovoltaic materials is highly required. At present, the direct C–H activation has emerged as the most economical and environmental friendly way to building C–C bonds and expanding the π-conjugation, including two types of C–H/C–X (X: halogen) and C–H/C–H cross-coupling reactions.^12–16 In organic photovoltaic materials, much research has been focused on the C–H/C–X cross-coupling, while less attention has been paid to the C–H/C–H cross-coupling.^17–19 This challenge remains to be explored.

In ternary OSCs, three different host/guest components are mixed to form the active layer, opening up opportunities to maximize efficiency.^20,21 The introduction of a third component (guest component) has the great advantages of broadening the absorption range, optimizing the blend morphology and reducing voltage losses.^22–24 However, fine tuning the crystallization kinetics of the multicomponent blended film is very difficult. This is due to the fact that the blend morphology of the active layer is sensitive to changes in the molecular structure of the third component.²⁵ Recently, several key strategies have been reported, such as reduction of electrostatic potential, asymmetrization of terminal groups, oligomerization of small molecules, halogenated molecules, etc.^26–29 Currently, the focus of research in the field of ternary OSCs is the guest acceptor, i.e. ITIC- and Y-series.^30,31 Nevertheless, they have two significant limitations. First, their synthesis processes usually suffer from the Stille reaction, thus resulting in low cost-effectiveness and poor up-scalability for industrialization. Second, despite numerous efficient guest acceptors being reported, molecular design guidelines for the guest component have rarely been proposed.^9,32–35 In particular, elucidating the relationship between the molecular structure of the guest component and the blend morphology of the active layer is a great challenge.

Crystallinity is an important parameter for organic semiconductors, being closely related to the molecular structure. It is a common goal in OSCs to enhance the crystallinity of the molecules, as this contributes to improved charge mobility and therefore high efficiency.^36–39 Nevertheless, it remains uncertain whether molecules with high crystallinity are feasible for the third component. In this work, we introduced a large π-conjugated indene thiophene (IDT) unit based on an A–D–A type S-IDT (ITIC-series) and constructed an A–D–D–A type D-IDT to broaden the absorption range and shift up the lowest unoccupied molecular orbital (LUMO) energy level. Interestingly, we found that the π-conjugation system of D-IDT molecules was expanded, but the intermolecular interactions were significantly decreased, showing a low crystallinity. The in situ UV-vis spectra revealed that D-IDT could promote earlier aggregation of BTP-eC9 and faster aggregation transition than S-IDT, resulting in smaller phase separation in the D-IDT-controlled ternary film. This more desirable morphology of the active layer resulted in higher exciton dissociation and charge transport, and lower voltage losses. As a consequence, the D-IDT-controlled device exhibited an impressive PCE of 19.92%, while that of the device based on S-IDT was 17.66%. This is due to the higher open circuit voltage (V_OC), short-circuit current (J_SC) and fill factor (FF) of the former.

2. Results and discussion

The chemical structures of S-IDT and D-IDT, along with the corresponding synthetic routes, are illustrated in Fig. 1a and b and Scheme S1 in the ESI.† Typically, the compound 2IDT-CHO was synthesized via the Stille reaction between bromidic and organo-tin compounds (Fig. 1a). In pursuit of a more economical and environmentally sustainable approach, we developed an alternative synthetic route for 2IDT-CHO through direct C–H activation using a more efficient C–H/C–H cross-coupling strategy, as opposed to the conventional C–H/C–X cross-coupling reaction (Fig. 1a). This strategy eliminates the reliance on highly flammable reagents (such as n-butyllithium) and highly toxic compounds (such as organo-tin reagents), generating only hydrogen gas (H₂) as a benign byproduct, thereby significantly reducing the environmental impact of the synthesis. Moreover, the C–H/C–H cross-coupling strategy offers notable practical advantages, including fewer synthetic steps, higher overall yields, and the elimination of complex pre-functionalization processes (such as the preparation of organotin reagents required in the Stille reaction). This streamlined approach not only reduces reagent costs but also minimizes reaction time and operational complexity. To obtain satisfactory yields, we tried to optimize the catalyst and solvent for the C–H activation reaction (Table S1, ESI†). Ultimately, a high yield of 84% can be achieved using Pd(OAc)₂ and Cu(OAc)₂ as catalysts, N,N-dimethylformamide as solvent and K₂CO₃ as base. In the last step of the reactions (Scheme S1, ESI†), 90% yields were obtained by employing BF₃·OEt₂ as a catalyst in the Knoevenagel condensation developed by Zhang's group.^40,41 In addition, the objective molecules were carefully confirmed using ¹H NMR, ¹³C NMR, and HR-TOF-MS spectra (Fig. S1–S11, ESI†). Despite D-IDT possessing a lower solubility in chloroform (20 vs. 36 mg mL⁻¹) compared to S-IDT, both are enough for solution-processing.

Fig. 1 (a) Molecular design strategy for direct C–H activation. (b) The chemical structures of S-IDT and D-IDT. (c) The energy level diagram of D18, BTP-eC9, S-IDT and D-IDT. Absorption spectra of (d) S-IDT and (e) D-IDT solutions with toluene under different temperatures. (f) The normalized absorption spectra of D18, BTP-eC9, S-IDT and D-IDT neat films. 2D GIWAXS patterns of the (g) S-IDT and (h) D-IDT neat films. (i) The line-cut of profiles along the IP and OOP of the neat films corresponding to the 2D GIWAXS patterns.

To estimate the energy levels of the two acceptors, the electrochemical properties were measured using cyclic voltammetry. The LUMO/highest occupied molecular orbital (HOMO) energy levels of S-IDT and D-IDT were calculated to be −4.02/−5.88 eV and −3.75/−5.53 eV, respectively (Fig. S12, ESI†). These values are consistent with the DFT-calculated values presented in Fig. S13 (ESI†). The significantly up-shifted HOMO/LUMO in D-IDT compared to S-IDT was attributed to the insertion of an electron donating unit (IDT). In addition, the energy level diagrams of D18,⁴² D-IDT, and BTP-eC9⁴³ showed a cascade-like energy level alignment (Fig. 1c), which can enhance charge transfer and reduce voltage loss.^44,45

The UV-Vis spectra of the materials in the neat films and solutions were measured. Upon heating of the dilute solutions (from 25 to 95 °C), the absorbance of 0-0 peak decreased and the absorption spectrum was blue-shifted (Fig. 1d and e). Notably, the 0-0 peak of D-IDT was blue-shifted by 10 nm, whereas the S-IDT was only 5 nm. Comparing the absorption spectra in the solutions and films (Fig. S14 and Table S2, ESI†), the 0-0 peak of S-IDT was red-shifted from 689 to 754 nm (65 nm), while only 25 nm red-shift was observed for D-IDT. The results indicated that the intermolecular interactions of D-IDT were significantly weaker than S-IDT. This should be attributed to the fact that D-IDT possessed a significant torsion angle between the two IDT units (Fig. S15, ESI†). In addition, both of them exhibited very good complementary absorption compared to the absorption ranges of PM6 and BTP-eC9 (Fig. 1f), which facilitates the harvesting of more sunlight. To confirm the intermolecular interactions and crystallinity behavior of the guest acceptors, grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were conducted. The (010) π–π stacking peak at 1.73 Å⁻¹ (d = 3.63 Å) in the out-of-plane (OOP) direction and the (100) lamellar stacking peak at 0.40 Å⁻¹ (d = 15.7 Å) in the in-plane (IP) direction were observed in the S-IDT neat film (Fig. 1g, i and Table S3, ESI†), suggesting a clear face-on crystalline orientation. In contrast, the D-IDT neat film displayed an unclear crystalline orientation with a very weak crystallinity (Fig. 1h and i). The distances of the (010) and (100) peaks were increased to 4.47 Å and 16.8 Å, respectively (Table S3, ESI†).

To explore the photovoltaic performances of the two guest acceptors, devices with a conventional structure of ITO/BrBACz/D18:acceptors/PDINN/Ag were fabricated and characterized.⁴⁶ The current density–voltage (J–V) characteristics are displayed in Fig. 2a, and the corresponding photovoltaic parameters are summarized in Table 1. The reference device with D18:BTP-eC9 showed a high PCE of 18.51% with a V_OC of 0.851 V, a J_SC of 28.10 mA cm⁻², and an FF of 0.774. In contrast, the PCEs of the binary devices based on D18:S-IDT and D18:D-IDT were only 10.50% and 4.35%, respectively. This is due to their significantly lower external quantum efficiency (EQE) values (Fig. 2b). When doped with D-IDT (20 wt%), the V_OC, J_SC and FF of the ternary device were greatly improved to 0.875 V, 28.85 mA cm⁻² and 0.789, respectively, resulting in an outstanding PCE of 19.92% while for the S-IDT controlled ternary device, there are negative effects in terms of J_SC and FF. The PCE of the device was decreased to 17.66%. To explain the different J_SC values, the EQE spectra of the devices were characterized (Fig. 2b). The integrated J_SC values of the D18:BTP-eC9, D18:eC9:S-IDT- and D18:BTP-eC9:D-IDT-controlled devices are 27.77, 26.94 and 27.93 mA cm⁻², respectively, which are consistent with the J_SC values measured on the J–V curves. As shown in Fig. 2b, an improvement in the EQE values is observed in the 600–800 nm region when doped with D-IDT, leading to improved J_SC. In addition, in order to evaluate the broader applicability of D-IDT introduction with regard to the performance of OSCs, an investigation was conducted into two additional systems: PM6:BTP-eC9 and D18:L8-BO. The J–V characteristics of these OSCs are presented in Fig. S16 (ESI†). It was found that the introduction of D-IDT consistently enhanced device performance in both systems. This finding indicates that the tunability of device performance is not limited to our specific systems but can also be effectively achieved in other systems.

Fig. 2 (a) J–V characteristics of the corresponding devices. (b) EQE spectra of the corresponding devices. (c) Light-intensity dependent V_OC plots of the corresponding devices. (d) Hole and electron mobilities of the corresponding devices. (e) Voltage loss parameters of the corresponding devices. (f) EQE_EL curves of the corresponding devices. 2D transient absorption spectra of D18:BTP-eC9 (g), D18:BTP-eC9:S-IDT (h), and D18:BTP-eC9:D-IDT (i) blend films with an excitation of 800 nm.

Table 1 Photovoltaic parameters (AM 1.5G, 100 mW cm⁻²) of OSCs

Active layers	V OC [V]	J SC [mA cm^−2]	J ^EQESC [mA cm^−2]	FF	PCE (ave. ± dev.)^a [%]
a Average values of 10 individual cells are given in parentheses.
D18 : BTP-eC9 (1 : 1.3)	0.851	28.10	27.77	0.774	18.51 (18.32 ± 0.14)
D18 : S-IDT (1 : 1)	0.830	17.55	17.14	0.721	10.50 (10.23 ± 0.20)
D18 : D-IDT (1 : 1)	0.955	8.07	7.82	0.564	4.35 (4.09 ± 0.27)
D18 : BTP-eC9 : S-IDT (1 : 1.1 : 0.2)	0.848	27.36	26.94	0.761	17.66 (17.45 ± 0.19)
D18 : BTP-eC9 : D-IDT (1 : 1.1 : 0.2)	0.875	28.85	27.93	0.789	19.92 (19.83 ± 0.08)

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To evaluate the mechanism of charge recombination, the dependence of the V_OC and J_SC properties on the light intensity (P_light) of the devices were measured. As shown in Fig. 2c, the fitted slopes of S-IDT- and D-IDT-controlled devices were 1.12 and 1.10, respectively, which are lower than that of the reference device (n = 1.13). It indicates that trap-assisted recombination is effectively suppressed in the ternary devices. The α values of the ternary devices are higher than that of the binary device (0.998 vs. 1.021) and closer to 1 (Fig. S17, ESI†), suggesting that the bimolecular recombination losses of the device are inhibited by the introduction of D-IDT. To investigate the charge transport properties, the hole and electron mobilities of the devices were measured using the space-charge-limited current (SCLC) model. The hole and electron mobilities were calculated to be 2.39 × 10⁻⁴ and 2.03 × 10⁻⁴ cm² V⁻¹ s⁻¹, 2.15 × 10⁻⁴ and 1.77 × 10⁻⁴ cm² V⁻¹ s⁻¹, and 2.51 × 10⁻⁴ and 2.30 × 10⁻⁴ cm² V⁻¹ s⁻¹ for D18:BTP-eC9, D18:BTP-eC9:S-IDT, and D18:BTP-eC9:D-IDT systems, respectively (Fig. 2d and Fig. S18, ESI†). The enhanced mobility contributes to improved charge transport and reduced charge recombination, resulting in high J_SC and FF.^47,48

To explain the enhanced V_OC of the ternary OSCs, a voltage loss (V_loss) analysis was conducted, the corresponding data are listed in Fig. 2e and Table S4 (ESI†).⁴⁹ According to reported work, the total energy loss can be classified into three categories (ΔE_loss = ΔE₁ + ΔE₂ + ΔE₃ = E_g − V_OC).⁵⁰E_g can be obtained by taking the crossing point between the normalized absorption and emission spectra of the active layer (Fig. S19, ESI†).⁵¹ Compared to the reference device, the ΔE₂ values of the ternary devices with D18:BTP-eC9:S-IDT and D18:BTP-eC9:D-IDT were decreased from 0.081 eV to 0.070 and 0.066 eV, respectively. This suggested that the introduction of the guest acceptor can effectively reduce the radiative recombination from the absorption below the bandgap. In addition, the ΔE₃ value is determined by the electroluminescent quantum efficiency (EQE_EL) of the device. After the introduction of D-IDT in the binary system, the ΔE₃ value of the device was reduced by only 0.003 eV (Fig. 2f), indicating that non-radiative recombination voltage loss has not been significantly suppressed. As for the S-IDT-treated device, it displayed a higher ΔE₃ value (0.196 vs. 0.205 eV). As a consequence, the total voltage loss of the D-IDT-controlled device was 0.534 eV, which is significantly lower than those of the devices with D18:BTP-eC9 (0.552 eV) and D18:BTP-eC9:S-IDT (0.563 eV). Therefore, the V_OC of the D-IDT-controlled device was improved from 0.851 to 0.875 V.

To understand the charge transfer process of the blend films, femtosecond transient absorption (fs-TAS) spectroscopy was performed (Fig. 2g–i and Fig. S20, ESI†). The ground-state bleaching (GSB) signals of BTP-eC9 appear at 800–860 nm, while the excited-state absorption (ESA) signals of the photo-excited local excitation (LE) state occurred at 880–940 nm. After ∼0.50 ps, the GSB signals (520–600 nm) of the D18 could be observed, suggesting the ultrafast hole transfer kinetics from BTP-eC9 to D18. In addition, the D18:BTP-eC9:D-IDT blend film showed stronger GSB signals, indicating more efficient hole transfer. The kinetic traces of the GSB of D18 at 586 nm were monitored for the blend films (Fig. S21, ESI†). The values of τ₁ and τ₂ correspond to exciton dissociation and diffusion, respectively, and are obtained through the application of a double exponential decay function.^52,53 The τ₁/τ₂ values were calculated to be 1.37/27.6 and 0.88/16.8 ps for the D18:BTP-eC9:S-IDT and D18:BTP-eC9:D-IDT blend films, respectively, while those of D18:BTP-eC9 blend film were 1.11/20.5 ps. The shorter τ₁ and τ₂ values indicated that the addition of D-IDT could improve exciton dissociation and diffusion, resulting in a higher J_SC and FF. In addition, exciton generation contour plots were generated by film-depth-dependent analysis.^42,54 The D18:BTP-ec9:D-IDT-ternary film exhibited higher exciton concentrations in the wavelength range of 650–950 nm than the other two blend films (Fig. S22, ESI†). Moreover, the maximum exciton generation rate (G_max) value of the D18:BTP-eC9:D-IDT blend film increased from 1.78 × 10²⁸ to 1.86 × 10²⁸ nm⁻³ s⁻¹. In contrast, the G_max value of the D18:BTP-eC9:S-IDT blend film decreased to 1.67 × 10²⁸ nm⁻³ s⁻¹. The result is consistent with the τ₁ value.

Thin film morphology of the active layer is the critical factor for achieving high performance OSCs.^55,56In situ UV-Vis measurements were employed to elucidate the crystallization kinetics of the film formation process, thereby providing guidelines for finely tuning the phase separation of blend films.⁵⁷ The film formation process can be divided into three stages. Stage I (dissolved state): the positions of the absorption peaks for the donor/acceptor remain unchanged. Stage II (nucleation and growth): the positions of the absorption peaks for the donor/acceptor start to red-shift as the solvent evaporates. Stage III (film state): all absorption signals are stabilized after removing residual solvents. As shown in Fig. 3, the three stages were completed in less than 300 ms for chloroform solvents. Here, we focused on the acceptor BTP-eC9, since the donor D18 showed almost no change in the position of the absorption peaks. For D18:BTP-eC9 binary system, the aggregation state transition of BTP-eC9 finished at 125–250 ms. For the D18:BTP-eC9:S-IDT ternary sample, the time was delayed to 150–275 ms. However, the aggregation transition of BTP-eC9 was faster in the D18:BTP-eC9:D-IDT ternary system (75–175 ms). The results indicated that introduction of D-IDT promoted rapid nucleation and assembly of BTP-eC9, which suppresses excessive self-aggregation of BTP-eC9. However, the addition of IDT presented the opposite effect. Thus, a smaller phase separation can be observed in the D18:BTP-eC9:D-IDT blend film, as characterized below. Although the molecular structure differences between the two guest acceptors are small, their modulation of the film morphology is significantly different. Therefore, it is highly desirable to propose more guidelines for molecular design. To explain the differences in crystallization kinetics, we conducted contact angle measurements. The contact angles of water and diiodomethane liquid drops were determined (Fig. S23 and Table S5, ESI†). The surface energies (γ) of D18, BPT-eC9, S-IDT, and D-IDT were calculated to be 29.23, 37.62, 42.40, and 34.15 mN m⁻¹, respectively. Consequently, the Flory–Huggins interaction parameters (χ) were 0.014 and 0.084 K for BPT-eC9:S-IDT and BPT-eC9:D-IDT, respectively. This suggested that D-IDT possessed higher miscibility and compatibility with the host acceptor BTP-eC9 than S-IDT, contributing to the inhibition of self-aggregation of BTP-eC9.

Fig. 3 (a) In situ 2D UV-vis absorption contour maps of three blend systems during the spin-coating process. (b) In situ UV-vis absorption line profiles of the corresponding systems. (c) Time evolution of the acceptors peak location extracted from the corresponding absorption contour map.

In order to investigate the morphological properties of the active layers, a series of measurements were conducted using atomic force microscopy (AFM), GIWAXS and grazing-incidence small-angle X-ray scattering (GISAXS). Compared to the binary film, the root-mean-square roughness (R_q) values of the ternary films were similar (Fig. 4a). All the three films had fibril networks, which facilitates exciton dissociation and charge transport.²³ However, their phase-separated nanostructures were significantly different (Fig. 4b). The estimated diameters of the nanofibers of the D18:BTP-eC9:S-IDT and D18:BTP-eC9:D-IDT ternary films were 15.0 ± 4.5 and 8.9 ± 1.4 nm, respectively (Fig. 4c and Fig. S24, ESI†), while that of the binary film of D18:BTP-eC9 was 11.0 ± 2.4 nm. The smaller nanofibers, the more p–n junctions.

Fig. 4 (a) AFM height image, (b) phase images and (c) fibril diameter distributions of blended films based on D18:BTP-eC9, D18:BTP-eC9:S-IDT and D18:BTP-eC9:D-IDT.

As illustrated in Fig. 5a and Fig. S25 (ESI†), the 2D GIWAXS patterns showed that the three blended films had clear face-on orientations with (010) peaks in the OOP direction, which is favorable for efficient charge transport. Notably, there is no significant change in the π–π distance, lamellar distance, and crystal coherence length (CCL) value for any of these three blend films (Table S6, ESI†). In order to quantify the nanoscale phase separation and determine the domain sizes of each phase, the blend films were measured using GISAXS (Fig. 5b). The GISAXS fittings showed that the domain sizes for the intermixed domains were 36 nm, 53 nm, and 27 nm for the D18:BTP-eC9, D18:BTP-eC9:S-IDT, and D18:BTP-eC9:D-IDT blend films, respectively (Fig. S26, ESI†). Compared to the D18:BTP-eC9 film, the D18:BTP-eC9:S-IDT film shows larger intermixed amorphous phases, indicating a relatively poor phase separated morphology, which is consistent with the results observed in the AFM phase images. However, for the D18:BTP-eC9:D-IDT film, D-IDT imparts a more favorable phase separation to the D18:BTP-eC9 system and its smaller mixed phase contributes to improved charge collection and reduced charge recombination, resulting in improved device performance.

Fig. 5 (a) 2D GIWAXS and (b) 2D GISAXS patterns of the three blend films based on D18:BTP-eC9, D18:BTP-eC9:S-IDT and D18:BTP-eC9:D-IDT.

In addition to the improvement of the PCE, the stability of OSCs is critical for commercial applications. Here, inverted devices (ITO/SnO₂/active layers/MoO₃/Ag) were fabricated to evaluate their long-term stability. As shown in Fig. S27 (ESI†), after 200 h of illumination, the PCEs of the D-IDT-treated ternary devices decreased by only 8%, while that of the binary device dropped by 22%. Meanwhile, after 200 h of thermal treatment at 65 °C, the ternary devices based on D-IDT retained more than 76% of its initial PCE, compared with 63% for the control device (Fig. S27, ESI†). The results indicate that the introduction of D-IDT in the binary devices can effectively improve the stability of the devices.

3. Conclusions

In summary, a low crystallinity acceptor, D-IDT, was designed and synthesized by using a direct C–H activation strategy via a C–H/C–H cross-coupling reaction. Subsequently, A–D–D–A typed D-IDT and A–D–A typed S-IDT, which have completely different crystallinities, were introduced as a third component in the D18:BTP-eC9 binary system for photovoltaic applications, respectively. The absorption spectra and GIWAXS indicated that the intermolecular interactions of D-IDT were very weak, despite having a larger π-conjugated system than S-IDT. The in situ absorption spectra showed that the introduction of D-IDT promoted faster aggregation of BTP-eC9 and inhibited its excessive self-assembly behavior compared to S-IDT, resulting in smaller domains in the D-IDT-controlled ternary film. Consequently, the D-IDT-controlled device displayed a higher exciton dissociation and charge transport, a lower non-radiative voltage loss. As a result, an excellent PCE of 19.92% was achieved in the D18:BTP-eC9:D-IDT-based device, which was significantly higher than that of the S-IDT-based device. This result reveals the relationship between the crystalline properties of the guest material and the blend morphology of the active layer, which provides a molecular design guideline for the development of highly efficient guest components. Moreover, the economical and green synthesis routes contribute to the commercialization of OSC technology.

Author contributions

P. D., X. R., Y. H. and D. Y. conceived and designed the research. P. D. and X. R. synthesized the organic photovoltaic materials. X. Y. and H. W. helped with device fabrication. J. Z. and F. C. performed the energy loss measurements. Z. S., X. L., X. C., J. W., L. X. and G. C. helped with the basic characterization. P. D. and D. Y. drafted the manuscript. D. Y. and Z. G. revised the paper. Z. G. supervised this project.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Science Fund for Distinguished Young Scholars (No. 21925506), the National Natural Science Foundation of China (No. U21A20331, 22439004, and 22409202), and the National Postdoctoral Program for Innovative Talents (No. BX20230386).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ee00542f

‡ P. D. and X. R. contributed equally to this work.

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