Integration of Metal‐Organic Framework With Polymer Electrolyte Enables Efficient and Robust Organic Solar Cells

ABSTRACT

Electron transport layers (ETLs) are crucial for reducing the interfacial energy barrier in organic solar cells (OSCs). However, conventional polymer‐based ETLs often suffer from limited conductivity and severe interfacial defects, hindering the improvement in both efficiency and stability. Herein, we develop a hybrid ETL by incorporating a nickel‐based metal‐organic framework (Ni‐MOF) into a polymeric ETL (NDI‐Br). The incorporation of Ni‐MOF introduces abundant Ni centers that coordinate with the bromide anions in NDI‐Br, generating strong interactions that enhance interfacial charge transport and reduce interfacial recombination. OSCs based on the hybrid ETL, NDI‐Br:Ni‐MOF (10:1), exhibit a remarkable efficiency of 20.7%, largely outperforming that (19.1%) of devices based on pristine NDI‐Br. Moreover, the hybrid ETL demonstrates exceptional thickness tolerance (maintaining 17.3% efficiency at 80 nm) and thermal stability with a T₉₀ lifetime exceeding 1000 h. This work provides a versatile interfacial engineering strategy that can well address the efficiency‐stability trade‐off in OSCs, paving the way for further promoting the practical use of OSCs.

A Ni‐MOF‐assisted doping strategy is developed to regulate the electronic structure and molecular packing of polymeric ETLs in OSCs. The incorporation of Ni‐MOF induces intermolecular electronic interactions, enhances charge transport, suppresses interfacial recombination, and improves film stability. As a result, the optimized devices achieve a high PCE of 20.7% along with significantly enhanced thermal stability, demonstrating a promising pathway toward efficient and durable OSCs.

1. Introduction

As an important emerging photovoltaic technology, organic solar cells (OSCs) have aroused significant interest owing to their distinctive merits including ultra‐light weight, intrinsic stretchability, and optical transparency. These features make OSCs promising for novel applications in wearable electronics, building‐/vehicle‐integrated photovoltaics, and even spacecraft for low‐earth orbits [1, 2, 3, 4]. Over the past decade, innovation in active layer materials, notably polymer donors and non‐fullerene acceptors (NFAs), have driven remarkable progress in power conversion efficiency (PCEs) of p‐i‐n structured OSCs. State‐of‐the‐art PCEs now exceed 20%, narrowing the efficiency gap with conventional photovoltaics [5, 6, 7, 8]. Except for the active layer, interfacial layers (ILs) are also vital for improving the efficiency and stability of OSCs [9, 10, 11]. This stems from the multifaceted roles of ILs. For example, they improve energy‐level alignment between active layer and electrodes, passivate interfacial defects to reduce the charge accumulation and recombination, adjust surface profiles to regulate the active‐layer morphology, and reinforce tolerance of interfacial contact [12].

Given the importance of ILs, great effort has been devoted to exploring novel IL materials especially electron‐transporting layers (ETLs) that hold promise for balancing the efficiency and stability in OSCs. The early research in ETLs focused on inorganic materials including metals with low work function (WF) (e.g., Ca), alkali metal salts (e.g., LiF), and n‐type metal oxides (e.g., TiO₂) [13, 14, 15]. However, these ETL films are typically deposited using complex and energy‐intensive methods, such as thermal evaporation or high‐temperature sintering. In contrast, organic ETLs can be processed into films by facile solution method and show easily tunable structures and energy levels [16, 17, 18]. Therefore, they have emerged as a promising alternative to inorganic ETLs. The typical organic ETLs are alcohol‐soluble polyelectrolytes, for example, poly[(9,9‐bis(3′‐((N,N‐dimethyl)‐N‐ethylammonium)propyl)‐2,7‐fluorene)‐alt‐5,5′‐bis(2,2′‐thio‐phene)‐2,6‐naphthalene‐1,4,5,8‐tetracaboxylic‐N,N′‐di(2‐ethylhexyl)imide]dibromide (NDI‐Br) [19]. These polymeric ETLs perform well in OSCs owing to their excellent film‐forming ability and well‐matched energy levels. However, their device performances are largely dependent on their molecular weight and polydispersity, that is, suffering from severe batch‐to‐batch variations [20]. In addition, these polymers usually show limited electrical conductivity due to their less efficient π–π stacking, even though a self‐doping from electron‐donating side‐chains has already occurred [21]. To address this, integration of external species into polymer ETLs to induce an extra doping is highly desirable [22]. Meanwhile, these extra dopants are expected to have an additional function in alleviating the batch‐to‐batch variability of polymer host.

Metal‐organic frameworks (MOFs) could be the possible candidates for serving as the extra dopants to fulfill the above dual goals. MOFs are a class of porous and highly ordered materials, which are formed by metal clusters coordinated with organic ligands [23]. It has been established the three‐dimensional (3D) porous structure of MOFs can form a physical barrier to impede the direct contact between active layer and electrodes in perovskite photovoltaics, thereby suppressing charge recombination caused by defects on electrode surface [24]. Moreover, MOFs can induce non‐covalent interactions with other interfacial components to enhance the electrical conductivity, rendering improved electron extraction at the interfaces [25]. Therefore, it is promising to develop hybrid ETLs by inclusion of MOFs into the host of polymer ETLs to address the bottleneck of both efficiency and stability of OSCs.

Herein, we developed a hybrid ETL, NDI‐Br:Ni‐MOF (10:1), by incorporating a nickel‐coordinated MOF, that is, Ni‐MOF, into the classic polymeric ETL, NDI‐Br (Figure 1a). The selection of Ni‐MOF is based on two rules: 1) it can provide Ni centers with sufficient empty orbitals that can interact with electron‐rich species; 2) it has sufficient oxygen‐ and sulfur‐based moieties that enable good solubility or dispersibility in protonic solvents such as methanol to ensure solution processing. The inclusion of Ni‐MOF effectively facilitates the electrical conductivity through inducing an efficient doping between the Ni cations in Ni‐MOF and the bromide anions at the side‐chain terminal of NDI‐Br. This suppresses the interfacial defects and relevant charge recombination at the interfaces, promoting the PCE to 20.7% in OSCs. More importantly, the intimate interactions between Ni‐MOF and NDI‐Br could result in the enrichment of polymer chains at the surface of Ni‐MOF frameworks, thus stabilizing the distributions of both components in the hybrid ETL. This may account for the improved thermal stability of resultant OSCs, retaining ∼90% of initial PCE after ∼1000 h continuous annealing at 65°C.

FIGURE 1.

(a) Chemical structure of NDI‐Br and single‐crystal structures of Ni‐MOF. Br 3d peaks of XPS for (b) neat NDI‐Br and (c) NDI‐Br:Ni‐MOF (10:1). (d) ESR spectra of NDI‐Br, Ni‐MOF and their blend as solid powder, where g is the g‐factor.

2. Results and Discussions

2.1. Interactions Between Ni‐MOF and NDI‐Br

Figure 1a illustrates the chemical structure of NDI‐Br and the single‐crystal structures of Ni‐MOF in a unit cell and in molecular‐packing mode (front view) [24]. The Ni‐MOF crystal consists of metal nodes interconnected by oxygen‐ and sulfur‐containing ligands, forming an ordered porous architecture with abundant coordination sites (see the side‐view of 3D structure shown in Figure S1). The lone pair electrons on the bromide ions of NDI‐Br would be available for the coordination with the nickel centers in Ni‐MOF. We therefore performed x‐ray photoelectron spectroscopy (XPS) to elucidate if the non‐covalent interactions can be formed between Ni‐MOF and NDI‐Br. By comparing the global spectra of XPS for Ni‐MOF and its blend with NDI‐Br, we observe the Ni 2p characteristic peak in neat Ni‐MOF film is maintained in the NDI‐Br:Ni‐MOF (10:1, weight ratio) blend film (Figure S2). However, the peak is too weak to give more in‐depth and reliable information by general peak fitting.

We therefore investigated the Br 3d peaks as an alternative, since the Br 3d signal is intense in the global spectra of NDI‐Br:Ni‐MOF (10:1) (Figure S2b). Through fitting of Br 3d peaks for NDI‐Br and NDI‐Br:Ni‐MOF (10:1), we find neat NDI‐Br displays two groups of Br 3d peaks with binding energy (BE) centered at 68.6/67.5 eV (3d_3/2/3d_5/2) and 71.4/70.3 eV (3d_3/2/3d_5/2), respectively (Figure 1b). The former (main peaks) should stem from the bromide anions (Br⁻) in NDI‐Br, while the latter could be resulted from the electron transfer from Br⁻ to the electron‐deficient NDI backbone [20, 26]. The NDI‐Br:Ni‐MOF (10:1) composite preserves both of these peaks, with obviously reduced BEs at 68.2/67.2 eV (3d_3/2/3d_5/2) and 71.1/70.1 eV (3d_3/2/3d_5/2) (Figure 1c). Meanwhile, the composite exhibits an evident change in the shoulder peak shape, splitting off an extra peak at BEs of 69.0/68.2 eV (Figure 1c). These combined results could be attributed to the coordination of Br⁻ (in side‐chain terminal of NDI‐Br) with Ni³⁺/Ni²⁺ centers (in Ni‐MOF) that have unoccupied 3d orbital [27].

To directly probe the Ni‐related electronic changes, additional XPS measurements were performed using samples with largely increased Ni‐MOF content (NDI‐Br:Ni‐MOF = 1:1). As shown in Figure S3b, the Ni 2p signal becomes sufficiently intense for reliable analysis. Notably, compared with neat Ni‐MOF, both the Ni 2p_3/2 and Ni 2p_1/2 peaks exhibit discernible shifts to the higher BE region in the mixed system, indicating a change in the local electronic environment of Ni ions (both Ni³⁺ and Ni²⁺) upon blending with NDI‐Br. In addition to the peak shifts, a noticeable evolution of Ni³⁺ peak shape is also observed after blending. Taken together, the consistent changes observed from the XPS spectra of Br 3d and Ni 2p provide complementary evidence for the presence of interfacial electronic interaction between Ni ions and Br⁻ species. Such interactions in hybrid ETL could facilitate the charge transfer at the interface to improve the electrical conductivity. Meanwhile, it might help form a denser film with reduced surface defects and improved interfacial robustness, thus contributing to both efficiency and stability of resultant OSCs.

Electron spin resonance (ESR) measurements were performed to further confirm the occurrence of electron transfer (doping) between NDI‐Br and Ni‐MOF in the hybrid ETL. Considering the requirement of sufficient spin density for reliable ESR detection, solid‐state samples are employed for all ETL samples. For the NDI‐Br:Ni‐MOF hybrid, we directly mixed the powders of these two components with a weight ratio of 1:0.5. The varied Ni‐MOF fraction compared to that used for XPS measurement is intentionally designed for enhanced ESR intensity with improved signal‐to‐noise ratio. As shown in Figure 1d, neat Ni‐MOF exhibits almost no ESR response, whereas neat NDI‐Br shows a discernible signal attributed to intrinsic self‐doping arising from interactions between electron‐rich bromide anions and the electron‐deficient NDI backbone. Upon blending with Ni‐MOF, the ESR signal is significantly enhanced, with a characteristic g‐factor of ∼2.003, indicating an increased population of unpaired electrons. This pronounced enhancement suggests that intermolecular electronic interactions are substantially strengthened in the hybrid system. In conjunction with XPS results, which reveal intermolecular interactions between Br⁻ containing species and Ni centers, the enhanced ESR signal can be ascribed to charge redistribution at the materials interfaces. Such interactions facilitate partial electron transfer and promote the electron density, which may be contributive to enhanced electrical conductivity of the hybrid ETL film.

2.2. Performance of Hybrid ETL in OSCs

Conventional devices with structure of indium tin oxide (ITO)/JJ36:HPWO/active layer/ETL/Ag (Figure S4) were fabricated to reveal the performance of NDI‐Br:Ni‐MOF hybrid ETL in OSCs, where JJ36 is a p‐type self‐assembled monolayer (SAM) and HPWO is tungsten‐based polyoxometalate (POM) [28]. The active layer comprises a donor:acceptor blend of D18:L8‐BO with a small portion (10% weight ratio versus that of donor) of acceptor guest, DYO‐IT, included to fine‐tune the intermolecular interactions and bulk morphology [29]. The energy levels of diverse components in different layers of OSCs are depicted in Figure S6, with relevant cyclic voltammograms (CV) results of ETL materials shown in Figure S5 and Table S1. It is observed the incorporation of Ni‐MOF into NDI‐Br maintains a cascade energy level alignment between different functional layers, which is favorable for ensuring an efficient electron extraction in resulting OSCs.

The reference devices based on the pristine NDI‐Br exhibit a maximum PCE of 19.1% in D18:L8‐BO:DYO‐IT‐based OSCs, with an open‐circuit voltage (V _OC) of 0.906 V, a short‐circuit current density (J _SC) of 26.5 mA cm⁻², and a fill factor (FF) of 79.4% (Figure 2a and Table 1). The addition of varied contents of Ni‐MOF into NDI‐Br induces different degree of increasement in the PCEs (Figure S7 and Table S2). The optimal mixing ratio of NDI‐Br:Ni‐MOF was screened to be 10:1, giving a champion PCE of 20.7% (V _OC = 0.917 V, J _SC = 27.6 mA cm⁻², FF = 81.5%). Note that the three parameters are simultaneously increased compared to those of reference devices based on neat NDI‐Br (PCE = 19.1%, Table 1). These improvements could be attributed to Ni─Br coordination as revealed by the XPS results, which not only promotes the electron extraction but potentially reduces the charge recombination at the interface by decreasing the interfacial defects.

FIGURE 2.

(a) J–V curves of the D18:L8‐BO:DYO‐IT‐based devices with different ETLs. (b) Relevant EQE curves and the integral current densities. (c) Dark J–V characteristics of NDI‐Br and NDI‐Br:Ni‐MOF devices. (d) Thermal stability of devices based on different ETLs under 65°C aging.

TABLE 1.

Photovoltaic parameters of D18:L8‐BO:DYO‐IT‐based devices comprising different ETLs measured under one sun, AM1.5 G illumination

	V OC	J SC	J cal b	FF	PCE c
ETL	(V)	(mA cm−2)	(mA cm−2)	(%)	(%)
NDI‐Br	0.906	26.5	25.5	79.4	19.1
	(0.900 ± 0.004)	(26.3 ± 0.5)		(78.6 ± 0.9)	(18.6 ± 0.4)
NDI‐Br:Ni‐MOF a	0.917	27.6	26.4	81.5	20.7
	(0.915 ± 0.004)	(27.6 ± 0.2)		(80.9 ± 0.5)	(20.5 ± 0.2)

^a The weight ratio is 10:1.

^b The current density calculated from EQE.

^c The average values in bracket are obtained from 10 devices.

Figure 2b displays the external quantum efficiency (EQE) curves of devices based on NDI‐Br and NDI‐Br:Ni‐MOF (10:1). The integrated current densities were determined as 25.5 and 26.4 mA cm⁻², respectively (Table 1), within 4% mismatch with respect to those obtained from J−V measurement. In the wavelength range of 450–850 nm, the hybrid‐ETL device exhibits obviously superior EQE response than that of reference device comprising neat NDI‐Br as ETL. This might be attributed to the enhanced electron extraction in the hybrid‐ETL device (to be discussed later), considering the almost identical absorption spectra of NDI‐Br and NDI‐Br:Ni‐MOF (Figure S8). In addition, the incorporation of Ni‐MOF leads to a pronounced reduction in the dark current density (Figure 2c), suggesting a suppressed charge carrier leakage in the hybrid‐ETL device. The shunt resistance (R_sh) was calculated to be 0.7 × 10³ and 2.1 × 10³ Ω cm², respectively, for NDI‐Br and NDI‐Br:Ni‐MOF‐based devices. These results imply that the interaction between NDI‐Br and Ni‐MOF might help reduce the interfacial defects, contributing to mitigated J _SC loss.

To further investigate the effect of ETL thickness on device performance, OSC devices with varied ETL thicknesses were fabricated. The NDI‐Br:Ni‐MOF devices exhibit a stronger tolerance to increased ETL thickness, maintaining 84% (from 20.7% to 17.3%) of its initial PCE after the ETL thickness increased from 10 to 80 nm (Figure S9 and Table S3). These findings may be attributed to the enhanced conductivity via the doping between NDI‐Br and Ni‐MOF (to be discussed later). Moreover, a good universality is demonstrated for the NDI‐Br:Ni‐MOF hybrid ETL, delivering obviously improved PCEs in various active‐layer systems (Figure S10 and Table S4). To evaluate the generality of the MOF‐incorporation strategy, we further explored the use of commercial‐available Zn‐based MOF, ZIF‐8 (structure shown in Figure S11) in the composite ETL of OSCs. The Zn cation in ZIF‐8 can also provide empty orbitals while affording sufficient dispersibility in methanol. When blending ZIF‐8 with NDI‐Br, the corresponding device shows a noticeable improvement in PCE compared to that based on pristine NDI‐Br (Figure S12 and Table S5), indicating that the incorporation of MOFs as interfacial modifiers represents a general and effective strategy for improving ETL properties.

To elucidate if improved electrical conductivity of hybrid ETL can mitigate the influence of batch‐to‐batch variations on the OSCs performance, we performed a systematic comparison using different batches of NDI‐Br with rationally varied number‐average molecular weights (M_n) and polydispersity index (PDI). Gel permeation chromatography (GPC) analysis confirms that the selected batches exhibit noticeable differences in M_n and PDI (Figure S13 and Table S6). For devices based on neat NDI‐Br (Figure S14a and Table S7), varied batches result in clear changes in J _SC (varying from 25.5 to 26.5 mA cm^{− 2}, ∼3.9% variation) and PCE (ranging from 18.3% to 19.2%, ∼4.9% variation). These results demonstrate that device performance based on polymeric ETLs is indeed sensitive to batch‐to‐batch variations even though the molecular weights and PDIs do not vary significantly. In contrast, upon incorporation of Ni‐MOF, the batch‐to‐batch variation is significantly suppressed (Figure S14b). The corresponding devices exhibit highly consistent photovoltaic parameters, with J _SC (27.7–27.9 mA cm^{− 2}), V _OC (0.905–0.909 V), and FF (81.1%–81.6%) showing minimal variation. Notably, the PCE values are tightly distributed within 20.4%–20.6%, corresponding to only ∼1.0% variation, substantially lower than that of the neat‐NDI‐Br devices. This improved reproducibility can be attributed to the enhanced electrical conductivity of the hybrid ETL, which reduces the sensitivity of charge transport and extraction processes in polymer batches. These results clearly demonstrate that improving ETL conductivity is an effective strategy to mitigate batch‐to‐batch variation in polymeric ETL‐based devices.

The long‐term operational stability of NDI‐Br:Ni‐MOF‐based OSCs was evaluated under both thermal aging and maximum‐power‐point (MPP) tracking. The unencapsulated devices were initially subjected to continuous thermal annealing at 65°C. The NDI‐Br:Ni‐MOF‐device exhibits outstanding thermal stability, retaining ∼90% of its initial PCE after ∼1000 h, whereas the PCE of control NDI‐Br‐device decays to 80% of original value within only ∼500 h (Figure 2d). Moreover, the MPP tracking under continuous one‐sun illumination also confirms the better stability of hybrid ETL, showing a much higher efficiency retention compared to that of pristine NDI‐Br in OSCs (Figure S15). These findings indicate the incorporation of Ni‐MOF into conventional ETLs can efficiently alleviate the efficiency‐stability trade‐off in OSCs, underscoring the great potential of the MOF‐doping strategy in OSCs.

2.3. Impact of Hybrid ETL in Charge Dynamics

Electrochemical impedance spectroscopy (EIS) measurements were deployed to elucidate the interfacial charge transport in OSCs. The equivalent circuit for the devices is illustrated in Figure S16, where R _s denotes the series resistance of the whole device, R ₁ and C ₁ represent the resistance and capacitance of the active layer, and R ₂ and C ₂ are the resistance and capacitance at the interfaces [30]. As shown in Figure 3a and Table S8. the R _s and R ₂ are determined as 0.9 and 21.6 Ω for the control devices, while those for the target devices are 0.7 and 18.3 Ω, respectively. The reduction in both R _s and R ₂ indicates fewer interfacial defects and better charge transport in the hybrid ETL. We further used the space‐charge limiting current (SCLC) method to study the charge transport properties, for example, electron mobility (µ _e), in different devices. The µ _e was calculated by fitting the dark current to a single‐electron SCLC model, J = 9ε₀ε_r µV²/8d ³, Where µ is the zero‐field mobility, ε₀ is the vacuum permittivity, ε_r is the relative permittivity, d is the thickness of the active layers, and V is the effective voltage [31]. The NDI‐Br:Ni‐MOF‐based electron‐only devices deliver a µ _e of 2.80 × 10⁻⁴ cm² V⁻¹ s⁻¹, largely surpassing that of 1.09 × 10⁻⁴ cm² V⁻¹ s⁻¹ for neat NDI‐Br devices (Figures 3b and S17). In addition, the electrical conductivity (σ) of different ETL layers was measured using sandwich structure of ITO/ETL/Ag [32]. As shown in Figure 3c, NDI‐Br:Ni‐MOF exhibits a higher σ of 4.80 × 10⁻⁴ S m⁻¹ compared to that of 2.47 × 10⁻⁴ S m⁻¹ for NDI‐Br, which echoes the improved electron mobility in the doping film.

FIGURE 3.

(a) EIS plots for different devices. (b) Electron mobility of devices comprising different ETLs. (c) I−V curves for sandwiched devices (ITO/ETL/Ag) based on NDI‐Br and NDI‐Br:Ni‐MOF. (d) TPC curves for devices based on different ETLs. (e) Relationship of bimolecular recombination coefficient versus carrier density. (f) C–V curves measured at 100 kHz in the dark.

Transient photocurrent (TPC) and transient photovoltage (TPV) measurements were conducted to reveal the charge recombination and extraction behaviors in OSCs with different ETLs [33]. From the TPC results illustrated in Figure 3d, we obtain a much shorter charge extraction (CE) time (0.42 µs) for NDI‐Br:Ni‐MOF device with respect to that of 0.54 µs for NDI‐Br device (Table S9). This validates that doping in ILs is an effective way for enhancing CE in OSCs. For TPV, usually a slower decay rate of photovoltage indicates a longer lifetime of photo‐generated charge carriers. The TPV results yield an obviously longer charge carrier lifetime of 47.7 µs for NDI‐Br:Ni‐MOF device relative to that of 29.1 µs for NDI‐Br device (Figure S18 and Table S10). This confirms that the incorporation of Ni‐MOF significantly inhibits the charge recombination in devices. To further clarify the charge recombination in OSCs, we performed CE measurement to reveal the dependence of carrier density (n) and carrier lifetime (t) on light intensity (P _light) [34]. It is evident that the NDI‐Br:Ni‐MOF device has a higher carrier density and longer carrier lifetime compared to those of NDI‐Br device (Figure S19a,b). The increased carrier density is consistent with the improved conductivity, accounting for the enhanced J _SC in hybrid‐ETL devices. The n‐t relationship is established based on the n‐P _light and t‐P _light correlations (Figure S19c), from which the bimolecular recombination coefficient (k _rec) was obtained using the equation: K _rec = 1/[(λ + 1)nt] [35], where λ is the recombination order. The K _rec‐n relationship was depicted in Figure 3e, where we observe that NDI‐Br:Ni‐MOF device delivers a significantly smaller k _rec compared to that of NDI‐Br device. This clearly demonstrates the suppressed bimolecular recombination in OSCs via doping of Ni‐MOF, consistent with the improved J _SC and FF in the hybrid‐ETL devices.

Capacitance–voltage (C–V) measurement was performed to quantify the built‐in potential (V _bi) of devices according to the formula, 1/C ⁻² = 2(V _bi‐V)/(A ² qNε_rε₀), where C represents the device capacitance, V is the applied voltage, A is the device area, q is the elementary charge, N is the carrier concentration, ε_r is the relative dielectric constant of the active layer, and ε₀ is the vacuum permittivity [36]. From the extracted Mott‐Schottky curves (Figure 3f), the parameters derived from the linear fitting were summarized in Table S11. The NDI‐Br:Ni‐MOF device shows a slightly higher carrier concentration, which should be due to the electron transfer from Br⁻ to Ni²⁺ centers. Moreover, the V _bi for the NDI‐Br:Ni‐MOF device is 0.791 V, much higher than that of NDI‐Br device (V _bi = 0.745 V), according well with the higher V _OC in solar cells.

2.4. Microstructure and Morphology of Hybrid ETL

Fourier transform infrared spectroscopy (FT‐IR) was carried out for the films of NDI‐Br, Ni‐MOF, and their blend, NDI‐Br:Ni‐MOF. The characteristic peak at 1012 cm⁻¹ for Ni‐MOF is absent in NDI‐Br (Figure S20), affording the chance to distinguish these two components in their composite film. Since the 1012 cm⁻¹ peak remains visible in the hybrid film, we performed IR‐triggered atomic force microscopy (IR‐AFM) for the NDI‐Br:Ni‐MOF (10:1) blend film using this peak as probe. It is observed the small fraction of Ni‐MOF can be uniformly dispersed across most regions of NDI‐Br matrix (Figure 4a). However, we can still locate at some rare, nanoscale agglomeration regions (Figure 4b), suggesting the MOF‐rich clusters do exist within the composite ETL film. It is noteworthy these cluster regions only account for a very small proportion and do not dominant the overall morphology of the entire film.

FIGURE 4.

(a) IR‐AFM image of NDI‐Br:Ni‐MOF at location 1, showing a relatively uniform compositional distribution. (b) IR‐AFM image of NDI‐Br:Ni‐MOF at location 2, where some rare nanoscale MOF‐cluster‐rich regions can be observed. (c) Cross‐sectional TEM image of the entire device. SEM images of (d) neat NDI‐Br, (e) neat Ni‐MOF, and (f) NDI‐Br:Ni‐MOF.

Scanning electron microscopy (SEM) based on the secondary‐electron mode reveals distinct surface morphologies for the ETL films. The NDI‐Br film exhibits a fine granular microstructure (Figure 4d), whereas the Ni‐MOF film shows flake‐like aggregates distributed across the surface (Figure 4e). In the NDI‐Br:Ni‐MOF composite film, most regions display a relatively uniform matrix, indicating good compatibility between Ni‐MOF and NDI‐Br (Figure 4f). Meanwhile, a small number of clusters composed of fine grains are observed, which can be attributed to partial agglomeration of Ni‐MOF. The blurred boundaries of these domains might suggest the adsorption of NDI‐Br chains on the Ni‐MOF surface, consistent with the presence of intermolecular interactions. Note that the large features observed in SEM images cannot be interpreted as surface roughness, since the SEM contrasts are influenced by multiple factors including compositional variation, crystallographic orientation, and local surface potential. Given the ultrathin nature (∼10 nm) of the spin‐coated ETL film and the observed uniform ETL layer without noticeable discontinuities revealed by cross‐sectional transmission electron microscopy (TEM, Figure 4c), we further believe the large‐size “cluster‐like” contrast in SEM primarily reflects the lateral surface morphology of the composite ETL film rather than the vertical roughness. Overall, these results provide a consistent picture that the composite ETL is predominantly uniform with very limited local aggregation, which would not influence the device performance as confirmed by the photovoltaic results discussed previously.

Grazing‐incidence wide‐angle x‐ray scattering (GIWAXS) measurements were performed to elucidate the influence of Ni‐MOF incorporation on the molecular packing of NDI‐Br. The 2D GIWAXS patterns of Ni‐MOF, NDI‐Br, and NDI‐Br:Ni‐MOF are shown in Figure 5a–c, with the corresponding line‐cuts presented in Figure 5e. The pristine NDI‐Br film exhibits characteristic diffraction peaks at 1.479 Å^{− 1} (010) in the out‐of‐plane (OOP) direction and 0.366 Å^{− 1} (100) in the in‐plane (IP) direction, which are assigned to π–π stacking and lamellar stacking, respectively (Table S12). The corresponding d‐spacing/crystalline coherence length (CCL) values are 4.2 Å/7.9 Å for the (010) peak and 17.2 Å/25.5 Å for the (100) peak. Upon incorporation of Ni‐MOF, the hybrid film maintains similar peak positions while exhibiting reduced full width at half maximum (FWHM) for both the OOP (010) and IP (100) reflections, corresponding to increased CCLs of 8.1 and 31.6 Å, respectively (Figure 5d). Notably, the CCL enhancement is more pronounced in the lamellar stacking direction (from 25.5 to 31.6 Å), whereas the π–π stacking remains less changed (from 7.9 to 8.1 Å). This anisotropic structural evolution suggests that Ni‐MOF primarily promotes lamellar ordering without significantly perturbing the π–π stacking framework. Such behavior can be attributed to the bridging effect of Ni‐MOF, that is, the Ni centers provide multiple interaction sites that connect adjacent NDI‐Br chains, thereby reducing structural disorder and enhancing long‐range coherence along the lamellar direction. As a result, the improved molecular ordering facilitates charge transport and suppresses interfacial recombination in the NDI‐Br:Ni‐MOF‐based OSCs.

FIGURE 5.

2D GIWAXS patterns of (a) Ni‐MOF, (b) NDI‐Br, and (c) NDI‐Br:Ni‐MOF. (d) Diagrams of CCL and d‐spacing in different directions. (e) GIWAXS line‐cuts of different ETLs.

3. Conclusions

In summary, we herein develop a hybrid ETL by incorporating a nickel‐based metal‐organic framework (Ni‐MOF), into the classic polymeric ETL, NDI‐Br. The inclusion of Ni‐MOF effectively facilitates the electrical conductivity through inducing an efficient doping between the Ni centers in Ni‐MOF and the bromide anions at the side‐chain terminal of NDI‐Br. This enhances the interfacial charge transport, suppresses the interfacial defects and relevant charge recombination at the interfaces. OSCs based on the hybrid ETL, NDI‐Br:Ni‐MOF, exhibit a remarkable efficiency of 20.7%, largely outperforming that (19.1%) of devices based on pristine NDI‐Br. Moreover, the hybrid ETL demonstrates exceptional thickness tolerance, that is, maintaining 17.3% efficiency under a thick ETL film of 80 nm. More importantly, the intimate interactions between Ni‐MOF and NDI‐Br stabilize the distributions of both components in the hybrid ETL. This accounts for the improved thermal stability of derived OSCs, retaining ∼90% of initial efficiency after ∼1000 h continuous annealing at 65°C. Overall, this work provides a versatile interfacial engineering strategy that can well address the efficiency‐stability trade‐off in OSCs.

Author Contributions

Ting Wang: conceptualization, methodology, data curation, investigation, validation, formal analysis, writing – review and editing, writing – original draft. Yizhe Liu: investigation, methodology. Jifa Wu: investigation, formal analysis, data curation, validation. Jiashuai Wang: data curation, formal analysis. Zhenyan He: data curation, formal analysis. Yibo Zhou: data curation. Xiaoke Zhang: formal analysis. Wei Wang: data curation, formal analysis. Yuang Fu: data curation, formal analysis. Xinhui Lu: investigation, supervision. Xiaobing Peng: supervision. Baobing Fan: conceptualization, methodology, supervision, funding acquisition, project administration, writing – review and editing. Fei Huang: conceptualization, methodology, supervision, funding acquisition, project administration, writing – review and editing. Yong Cao: funding acquisition, writing – review and editing, project administration, supervision, conceptualization.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: anie72769‐sup‐0001‐SuppMat.docx.

Supporting File 2: anie72769‐sup‐0002‐data.zip.

Data Availability Statement

All data supporting the findings of this study are provided in the manuscript and Supporting Information as well as Research Exchange (ReX) system.