Crystallinity and phase separation induced morphological modulation for efficient ternary all-polymer solar cells

All-polymer solar cells (all-PSCs) have gained broad research attention in recent years because of their desirable advantages, including easier chemical structure tunability, excellent morphological stability, and high film ductility.^1–4 During decades of endeavors in polymer acceptor innovation through molecular design and polymerized small-molecule acceptor (PSMA) strategy, power conversion efficiencies (PCEs) of binary all-PSCs have surged to over 17%.⁵ One pivotal issue impeding the advance of all-PSCs is the difficulty of morphological modulation in bulk-heterojunction (BHJ) devices, as highly entangled polymer chains and large molecular weight of polymers would cause excessive phase separation with decreased miscibility between polymer donors (P_Ds) and polymer acceptors (P_As), resulting in an undesirable bi-continuous interpenetrating network.^6–8 Various approaches have been employed to fine-tune the nanoscale morphology of all-PSCs involving utilization of additives, selection of different solvents, technology of thermal annealing, adoption of ternary strategy, and application of layer-by-layer structures.^9–19 Among the diverse methods mentioned earlier, ternary strategy stands out because an optimal addition of the third component can broaden the absorption spectrum of the original system, influence the crystalline features of host polymers, regulate the morphology of photoactive blends, and realize roll-to-roll production.^20–23 In the past few decades, numerous efforts have been dedicated to ternary organic solar cells (TOSCs) and a decent device performance with an obviously improved PCE was realized.^24,25

The ternary strategy is commonly applied to improve the photovoltaic performance of organic solar cells (OSCs). All-PSCs have obtained substantial developments due to the emergence of various n-type polymers realized via not only the polymerization of specific building blocks, such as perylene diimide (PDI), naphthalene diimide (NDI), bithiophene imide (BTI), and B–N unit, but also the PSMA strategy.^26,27 It is worth noting that with the synthesis of A-DA′D-A type SMAs, the PCE of all-PSCs has surged to over 15% because of the utilization of polymer acceptors, such as PYT, PJ1, L14, and PY-IT.^28–32 On this basis, a series of ternary works have been carried out to further improve the device performance through fine-tuned microstructures. By introducing a B ← N-type polymer acceptor BN-T to the high-performance binary system PM6:PY-IT, Liu et al. elevated the PCE to over 16% owing to the superior photoelectric properties brought by tuned crystalline and phase separations.³³ In addition, they applied the classic NDI based acceptor N2200 into PM6:PY-IT blends to modulate the morphology and gained an advanced efficiency.³⁴ Recently, Cui et al. have fabricated efficient ternary all-PSCs based on the PM6:PY-IT:PDI-2T system by virtue of manipulated vertical and horizontal morphologies through the synergy of sequential solution deposition approach and ternary strategy.³⁵ It implies that the traditional NDI, PDI, and B–N based polymer acceptors are promising as the third component for fabricating efficient all-PSCs.^36–39 Naphthalene diimide–indacenodithienothiophene (NDI-IDT) based copolymer BL-102 is intrinsically potential to be applied in ternary all-PSCs.⁴⁰ First, the IDT backbone with an extended fused-ring would cause a better π-electron delocalization, which is conductive to the intramolecular charge transfer for a broadened absorption spectrum.^41–43 Specifically, a large aromatic hydrocarbon side chain on the IDT skeleton can produce great steric hindrance effects to suppress the solution-phase aggregation and solid-state crystalline characteristics, endowing BL-102 with the ability of poor self-aggregation. Hence, the photoactive layer morphology can be modulated by introducing BL-102 to achieve a satisfactory interpenetrating network with a moderate phase separation for the fabrication of efficient all-PSCs.

In this contribution, we addressed the morphology control problem in the state-of-the-art PM6:PY-IT all-polymer system through the steric-hindrance effect induced photoactive layer morphology modulation and achieved a desirable device performance. Atoms in the molecule occupy a certain space, and repulsion would occur when they are approaching in space. Therefore, this behavior is called the steric-hindrance effect, which would influence molecular stacking, morphological and charge transport properties. The large tetrahexylphenyl substituents on the backbone of the copolymer acceptor BL-102 endow it with large steric-hindrance, delivering the property of suppressed self-aggregation. As a result, the blending films showed suppressed aggregation but more ordered molecular stacking (3 wt. %) and, thus, realized PCE optimization. In the ternary case, complementary absorbance enhanced utilization of sunlight for obviously improved short-circuit current density (J_SC) and the addition of BL-102 generated optimal morphology with a balanced phase separation and intermixing metrics, which is favorable for effective dissociation of excitons. In addition, a highly ordered molecular packing orientation perpendicular to the substrate can be observed in ternary blends with a larger π–π stacking coherence length, improving the charge transport ability to a great extent. Therefore, a faster hole transfer (HT) and electron transfer (ET) process was realized combined with balanced charge carrier mobilities, which is favorable for charge transfer and transport. The optimized ternary device exhibited an improved PCE with obviously elevated J_SC and fill factor (FF) values. This work further illustrates the vital importance of morphology control in the ternary-blending process for achieving a desirable photovoltaic performance and confirms the feasibility of the steric-hindrance effect for regulating the nanoscale morphology to obtain efficient all-PSCs.

Figure 1(a) shows the chemical structures of PM6, PY-IT, and BL-102. As shown in Fig. 1(b), the device structure used in this work was ITO/PEDOT:PSS/BHJ/PDINN/Ag. Figure 1(c) exhibits the energy level diagram of the selected materials. The absorbance of neat and donor:acceptor (D:A) blending films is displayed in Figs. 2(a) and 2(b), respectively. The two optical absorption peaks of BL-102 at 400 and 740 nm provide complementary absorbance, assisting in the elevated photon harvesting and an improved J_SC value. PM6 manifested a characteristic emission peak at ∼695 nm, while BL-102 showed an emission peak at 825 nm and PY-IT possessed an emission peak at 855 nm. Photoluminescence (PL) spectra of the blend films shown in Fig. 2(d) demonstrate the obvious quenching effects of the PM6 emission peak that results from the electron transferred from donors to acceptors.

The current density–voltage (J–V) characteristics of the optimal binary and three different ternary all-PSCs taken under AM 1.5G (100 mW cm⁻²) conditions are depicted in Fig. 2(e), and Table I gives the detailed photovoltaic performance parameters. The optimal PM6:PY-IT based binary all-PSCs exhibited a high PCE value of 14.96% with an open-circuit voltage (V_OC) value of 0.949 V, a J_SC value of 23.41 mA cm⁻², and a FF of 67.31%. Adding the third component BL-102 into the binary system led to an elevated J_SC, improved V_OC, and enhanced FF in the PM6:PY-IT:BL-102 (3 wt. %) device and, herein, achieved a PCE value as high as 15.43%. Further increasing the doping concentration decreased the PCE values as the ternary system PM6:PY-IT:BL-102 with various blending ratios showed lower J_SC, V_OC, and FF values, compared to the controlled device. The external quantum efficiency (EQE) spectra of the different all-PSC systems were measured and are plotted in Fig. 2(f), which are in pair with the J_SC values gained from J–V measurements.

The nanoscale morphology of all-PSCs is pivotal in determining the overall photovoltaic performance since a proper phase separated bi-continuous interpenetrating network together with a predominant face-on orientation is conductive to the effective exciton dissociation at the D/A interface and, meanwhile, the efficient charge transport through the photoactive layer.^44–47 Herein, atomic force microscopy (AFM) was adopted to probe the morphology evolution of the blend films, while the grazing-incidence wide-angle X-ray scattering (GIWAXS) measurement was employed to investigate the molecular stacking and crystalline characteristics. The AFM phase and height images (Fig. 3) of the four different blends exhibited a well-dispersed nanofibrous network synchronously. The addition of BL-102 leads to a fine-tuned bi-continuous network. As sketched in Fig. 3, the binary system showed rougher surface morphology delivering larger root-mean-square (RMS) roughness values of 1.10 nm, while ternary systems exhibited a quite smooth film surface. The abrasive surface morphology in the binary film originates from the strong molecular aggregation induced larger phase separation. In contrast, the phase separation was suppressed as ternary systems showed decreased RMS values, which is consistent with the assumption that the inferior self-aggregation property of BL-102 has caused the restrained aggregation of the original polymers but a well-defined intermixing behavior in the photoactive layer.^48–50 Nonetheless, the optimized ternary system showed a moderate RMS value of 1.02 nm, indicating a balanced phase separation and intermixing behaviors.^18,51–53 The AFM results were in accordance with the charge transport characteristics, where the hole mobilities (μ_h) and electron mobilities (μ_e) of the four different blend films were measured through the space-charge-limited current (SCLC) method. The measured μ_h and μ_e values for the PM6:PY-IT blend were 1.37 × 10⁻⁵ and 2.57 × 10⁻⁵ cm² V⁻¹ s⁻¹, respectively, with a μ_h/μ_e ratio of 0.53. In contrast, the ternary blending film showed slightly changed μ_h and μ_e values (μ_h = 1.76 × 10⁻⁵ and μ_e = 2.44 × 10⁻⁵ cm² V⁻¹ s⁻¹) but a more balanced μ_h/μ_e ratio of 0.72 (Table S1). The results mentioned earlier imply that a proper amount of BL-102 (3 wt. %) addition would form a desirable nanoscale interpenetrating network and, meanwhile, maintain an excellent phase separation behavior. Therefore, the effective exciton dissociation and efficient charge transport process could be realized, contributing to elevated J_SC and FF values.

The GIWAXS measurements were carried out to investigate the crystalline information. Figure 4 shows the two-dimensional (2D) GIWAXS patterns of and the 1D linecut profiles of the neat and blending films. PM6 and PY-IT prefer a face-on molecular orientation since obvious π–π stacking peaks (010) in the out-of-plane (OOP) direction were identified, for which PM6 and PY-IT exhibit a (010) peak at 17.34 and 16.36 nm⁻¹ in the OOP direction, respectively. Nevertheless, BL-102 seemed to be amorphous as we can only observe a poor lamellar peak at 3.45 nm⁻¹ along the in-plane (IP) direction as well as a ring-like peak located at 10.10 nm⁻¹ in the OPP direction without any π–π stacking information, and they indicated the faint crystalline packing capacity and weaker self-aggregation tendency of BL-102, resulting from the suppressed molecular aggregation caused by the steric-hindrance effect between the fused-ring conjugated backbone and large aromatic hydrocarbon side chain of the IDT units.^54,55Figures 4(e) and 4(f) show that all blending films displayed face-on molecular orientation and the π–π stacking peaks in the OOP direction for 1:1:0, 1:1:0.03, and 1:1:0.1 films were observed at 16.81, 16.74, and 16.99 nm⁻¹, corresponding to the π–π stacking distance of 0.374, 0.375, and 0.370 nm, respectively (Table S3, supplementary material). The almost consistent π–π stacking distance values in the OOP direction for the above three blend films indicated quite the same degree of difficulty for electron hopping between different polymer chains through π–π conjugated orbitals.⁵⁶ Nevertheless, the crystalline coherence length (CCL) values for π–π peaks were obviously different: 2.15, 2.75, and 2.52 nm for 1:1:0, 1:1:0.03, and 1:1:0.1 systems, respectively. The CCL results implied that a trace amount of BL-102 addition (3 wt. %) can induce highly ordered molecular face-on packing, but excessive doping (10 wt. %) can destroy the originally well-organized molecular stacking with a reduced CCL, which may cause an ineffective charge-carrier transport in BHJ films.⁵⁷ In comparison with the control system, 2D and 1D GIWAXS measurements have confirmed that the 1:1:0.03 ternary film possesses a comparative π–π stacking distance yet more ordered and enhanced molecular packing, which is favorable for the charge transport in the vertical direction and promoted charge collection, generating a satisfied photovoltaic performance with higher J_SC and FF values.

Transient absorption (TA) spectroscopy was employed to monitor the HT dynamics. The ground state bleaching (GSB) signals of neat PM6 films are located at 525–680 nm, whereas pure PY-IT films exhibit the GSB band at 625–850 nm [Figs. S2(b) and S2(d), supplementary material]. Moreover, the positive signal centered at around 900 nm in the neat PY-IT film was the contribution of excited state absorption (ESA), while PM6 monomer exhibited broad photoinduced absorption bands in the 800–1200 nm region [Figs. S2(a) and S2(c), supplementary material].⁵⁸ To investigate the HT process, the acceptor PY-IT was selectively excited by an 800 nm pump pulse and the temporal evolution of the signal is depicted in Figs. 5(a) and 5(c). The kinetics of 625 nm was extracted as PM6 GSB dynamics because the signal of the photoexcited PY-IT is absent there [Figs. S3(a) and S3(b), supplementary material]. Therefore, the rising dynamics of PM6 GSB in PM6:PY-IT blends presents the hole transfer process, which was fitted by a signal exponential function.⁵⁹ The ternary film with 3 wt. % BL-102 doping concentration displayed the fastest HT rate with the value of 4.07 × 10¹² s⁻¹, significantly higher than the control device (2.97 × 10¹² s⁻¹). However, further doping (5 and 10 wt. %) led to a decreased HT rate [Figs. S3(c) and S3(d), supplementary material]. TA results agree well with the proposal from AFM and GIWAXS measurements that the moderate phase separation combined with the more ordered molecular packing in the ternary system (3 wt. %) led to an enlarged D/A interface for an improved HT efficiency and fine-tuned morphology for faster exciton diffusion.

Time-resolved photoluminescence (TRPL) spectroscopy was utilized to figure out the ET dynamics.⁶⁰ 1D TRPL spectrum and 2D time-resolved fluorescence images of the four different blend films are detected at 680 nm and are shown in Figs. 5(d) and 6, respectively. Table S4 (supplementary material) summarizes the fitting parameters of Fig. 5(d). The 2D TRPL characteristics were analogous with the 1D TRPL results, where all the ternary devices showed a shorter average lifetime (τ_avg) at 680 nm in contrast to the binary controlled system, indicating a faster ET process in ternary systems.⁶¹ In particular, the ternary device PM6:PY-IT:BL-102 (1:1:0.03) possessed the shortest τ_avg value (53.19 ps), representing the most efficient ET in this case, which may account for the improved PCE value in this circumstance with promoted FF and J_SC values.

In summary, we utilized the steric-hindrance effect to modulate the photoactive layer morphology and achieved a desirable photovoltaic performance in ternary blending devices. The addition of BL-102 induced a more favorable molecular stacking and aggregation behavior, causing an improved π–π CCL in the OOP direction combined with a suitable trade-off between phase separation and miscibility domains. Thus, a well-defined bi-continuous interpenetrating network with an ordered molecular face-on orientation can be achieved in the optimal ternary device. The improved photoactive morphology led to a faster ET and HT process as well as balanced charge carrier mobilities, which were beneficial for the effective dissociation of excitons and charge transporting process. This work highlights the possibility of photoactive layer morphology regulation via the steric-hindrance effect and paves way for the fabrication of efficient ternary all-PSCs.

See the supplementary material for the solar cell device fabrication; the details of device characterization, photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements, grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements, and transient absorption (TA) spectrum measurements; the hole and electron transport performance of the hole-only/electron-only films in the SCLC measurement (Fig. S1 and Table S1); the 2D and 1D TA characteristics of the neat PM6 and PY-IT films (Fig. S2); the dynamic curve of PM6 GSB signal probed at 625 nm, the kinetic curve of PY-IT probed at 625 nm, the PM6 GSB dynamics curves in ternary (1:1:0.05) films probed at 625 nm, and the PM6 GSB dynamics curves in binary and ternary (1:1:0.1) films probed at 625 nm (Fig. S3); the corresponding information for Figs. 4(a)–4(f) (Tables S2 and S3); the J–V curves of PM6:BL-102 (1:1) blends under AM 1.5G conditions (Fig. S4); the TRPL decay kinetics of the neat PM6 film detected at 680 nm (Fig. S5); and the detailed fitting parameters for TRPL decay curves detected at 680 nm for the neat PM6 film and blending films (PM6:PYIT:BL-102) (Table S4).

This work was supported by the Shandong Provincial Natural Science Foundation (Grant No. ZR2021QF016), Major Program of Natural Science Foundation of Shandong Province (Grant No. ZR2019ZD43), Natural Science Foundation of China (Grant No. 52073162), and Key Lab of Fluorine and Silicon for Energy Materials and Chemistry of Ministry of Education, Jiangxi Normal University (Grant No. KFSEMC-202203). The authors also acknowledge the National Natural Science Foundation of China (Grant No. 52073207) for financial support.

The authors have no conflicts to disclose.

Mengzhen Sha: Writing – original draft (lead). Bili Zhu: Investigation (equal). Qian Wang: Investigation (equal). Ping Deng: Writing – review & editing (equal). Xunfan Liao: Investigation (equal). Hang Yin: Conceptualization (lead); Writing – review & editing (lead). Xiaotao Hao: Writing – review & editing (equal).

The data that support the findings of this study are available within the article and its supplementary material.