Tailoring polymer acceptors by electron linkers for achieving efficient and stable all-polymer solar cells

Abstract The trade-off between efficiency and stability is a bit vague, and it can be tricky to precisely control the bulk morphology to simultaneously improve device efficiency and stability. Herein, three fused-ring conducted polymer acceptors containing furan, thiophene and selenophene as the electron linkers in their conjugated backbones, namely PY-O, PY-S and PY-Se, were designed and synthesized. The electron linker engineering affects the intermolecular interactions of relative polymer acceptors and their charge transport properties. Furthermore, excellent material compatibility was achieved when PY-Se was blended with polymer donor PBDB-T, resulting in nanoscale domains with favorable phase separation. The optimized PBDB-T : PY-Se blend not only exhibits maximum performance with a power conversion efficiency of 15.48%, which is much higher than those of PBDB-T : PY-O (9.80%) and PBDB-T : PY-S (14.16%) devices, but also shows better storage and operational stabilities, and mechanical robustness. This work demonstrates that precise modification of electron linkers can be a practical way to simultaneously actualize molecular crystallinity and phase miscibility for improving the performance of all-polymer solar cells, showing practical significance.


INTRODUCTION
Solution-processed bulk-heterojunction (BHJ) polymer solar cells (PSCs) composed of p-type conjugated polymer donors (P D s) blended with n-type small molecule non-fullerene acceptors (SM-NFAs) or conjugated polymer acceptors (P A s) have made significant efficiency improvements, with power conversion efficiencies (PCEs) rapidly improving from 14% to over 17% for NFA-based PSCs [1][2][3][4][5][6] and from 11% to over 15% for all-polymer solar cells (all-PSCs) [7][8][9][10][11] over the past two years, respectively. Extensive research into control of specific aspects of the materials such as electronic energy levels, optical bandgaps and intermolecular interactions, has resulted in the availability of a myriad of photovoltaic materials for PSC applications [2,[12][13][14]. In contrast, stability studies have been deprioritized because they often produce unsatisfactory results. This is because the resulting finely mixed, and phase-separated regions in the delicate BHJ structure are typically metastable [15,16], which is generally far away from the thermodynamic equilibrium, resulting in rapid performance attenuation of devices for many intrinsic factors (e.g. molecular structure [17][18][19], donor/acceptor (D/A) compatibility [20] and molecule migration [21], etc.) and external stresses (e.g. irradiation [22,23], heating [15,24] and mechanical stress [25,26], etc.). For instance, many organic compounds chemically degrade under light and oxygen conditions [27], and the blend morphology of active layers can evolve via molecular dimerization and migration [28], crystallization and/or phase segregation under extended exposure to heat, illumination and thermal cycling conditions [15]. Thus, it is important to emphasize that continued progress in SM-NFA-and all-polymer-based PSCs is still challenging, even though their efficiencies are approaching the required threshold considered for commercial viability [29,30].
Several specific approaches such as designing organic photovoltaic materials (e.g. suitably extending the conjugated planarity of molecules [31] and reducing the crystallinity of photovoltaic polymers [17]), modifying the degree of polymerization [19], selecting suitable D/A pairs [32], cross-linking between D/A components [33] and incorporating solid additives into the active layer [15] have been demonstrated to partially solve specific stability issues of BHJ active layers, especially in terms of storage stability [33], photostability [17,19] and thermal stability [31]. Among these strategies, designing active layer materials is perhaps the most impactful way to balance potential trade-offs between achieving desirable photovoltaic properties and introducing instability in BHJ micromorphology. In other words, the molecular geometry and intermolecular packing of photovoltaic materials are important considerations to keep high-performance PSCs under environmental operation conditions.
There is a common consensus that compared to SM-NFA-based PSCs, all-polymer systems are considered to present more potential for practical applications because of their low molecular diffusion coefficients, remarkable operational stability and thermal stability, and robust mechanical properties [11,[34][35][36][37][38][39]. For instance, some research groups have demonstrated that all-polymer systems based on poly{[N,N -bis(2-octyldodecyl) -naphthalene1,4,5,8-bis(dicarboximide)-2,6-diyl]alt-5,5 -(2,2 -bithiophene)} (N2200) or its derivatives as P A s exhibited better storage stability and thermal stability compared to the corresponding SM-NFAs-based blend morphologies [39][40][41]. In addition, the PM6 : PY-S (PY-S) all-polymer system, reported in our previous works [7,26], also showed better storage stability and mechanical stability than those of the PM6 : Y5-C20-based system. Despite this, it is worth noting that the PY-S-based all-PSCs also showed a remarkable reduction in PCE in a short period of hundreds of hours, with just ∼76% of their initial efficiencies retained. Some highly efficient all-polymer systems reported by us with PCEs >14% also showed unfavorable environmental stabilities [36,42]. Undeniably, bulky and low-planar P A materials compared to SM-NFAs are more desirable from a stability standpoint. However, the stability metrics for all-polymer systems, especially in terms of the BHJ morphological stability, will be determined on a case-by-case basis as these are dependent on the micromorphology of D/A materials originating from relevant intermolecular interactions of P D s or P A s. This is because, on the one hand, intermolecular interactions need to be sufficiently strong to form suitably phase-separated interpenetrating networks and to facilitate exciton dissociation and charge transport. On the other hand, intermolecular interactions should not be so strong as to invite molecular crystallite aggregates, molecular rearrangement and increased vertical disorder in active layers during operation. As such, there is an urgent need for further endeavors towards the intermolecular interactions of designed photovoltaic materials and to find promising approaches to effectively fine-tune the strength of intermolecular forces and also to find matching materials to carefully modify molecular compatibility of donor and acceptor materials to meet the stability requirements of high-performance PSC applications.
A plausible avenue is to tune the electron linker in conjugated backbones, which can effectively fine-tune intermolecular interactions. Based on this assumption, herein we demonstrate such a methodology using a highly efficient Y5-C20derivative P A s and changing the electron linkers (furan, thiophene and selenophene) to form a new series of fused-ring conducted P A s, namely PY-O, PY-S [7,26] and PY-Se. The three synthesized P A s exhibit comparable optical and electrochemical properties, but PY-Se possesses stronger crystallinity behavior in the solid-state compared to the PY-O and PY-S P A s. Furthermore, a PY-Se-based active layer is demonstrated to benefit both crystallinity in blends and miscibility with a medium band-gap P D PBDB-T (Poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b ] dithiophene))-alt-(5,5-(1 ,3 -di-2-thienyl-5 ,7 -bis (2-ethylhexyl)benzo[1 ,2 -c:4 ,5 -c ]dithiophene-4,8-dione)]). As a result, PBDB-T : PY-Se devices possessed a much higher PCE of 15.48% compared to the PY-O-(9.80%) and PY-S-based devices (14.16%). The PBDB-T : PY-Se blend also showed better storage and operational stabilities and higher tensile strength. This study illustrates that modification of electron linkers could be a promising strategy to alter π -π stacking interaction of the polymer acceptors and fine-tune their molecular miscibility with a specific P D .

RESULTS AND DISCUSSION
To effectively fine-tune the intermolecular interactions, a series of P A s (PY-O, PY-S and PY-Se; Fig. 1A) were designed by polymerization of Y5-C20 building blocks and incorporation of different electron linkers (furan (O), thiophene (S) and selenophene (Se)). Compound Y5-C20-Br was synthesized via a published method [26]. The synthesis routes of the three P A s are outlined in Fig. 1A. Characterization information and detailed synthesis are also provided in the Supplementary data (Experimental section). The M w distribution plots of these three P A s are shown in Figs S1-S3. Weight-average molecular weights (M w s) of obtained P A s are between 18.2 and 20.7 kg mol -1 , and polydispersity indexes (PDIs) are between 1.9 and 2.0 (Table S1). These data were determined by high-performance gel chromatography with polystyrene standards. Notably, these comparable M w s and PDIs allow direct comparison of the material properties independent of the degree of P A polymerization [39]. The normalized absorption spectra of these three P A s in chloroform solution and solid film are exhibited in Fig. S4, and cyclic voltammograms are presented in Fig. S5. The corresponding optical and electrochemical parameters are summarized in Table S1. In the solutions, the absorption peak (λ max sol ) increases from 756 nm of PY-O to 776 nm of PY-S, and further increases to 783 nm of PY-Se. The corresponding P A s also show linearly increasing maximum film absorption wavelength (λ max film ) and onset wavelength (λ edge ) values according to the sequential chalcogen elements. Those indicate that heterocycles containing S and Se as electron linkers, as compared to the furan aromatic linker, can enhance the backbone interactions in the P A s. Of note is that the electron push-pull properties of the molecular backbones of these three P A s can also affect their absorption spectra in solutions. In addition, the most significant absorption feature of PY-Se is the maximum absorption coefficient of 1.03 × 10 5 cm −1 in the solid state, which is higher than Natl Sci Rev, 2022, Vol. 9, nwab151 those of PY-O (0.95 × 10 5 cm −1 ) and PY-S (1.01 × 10 5 cm −1 ) neat films. Besides, all of the P A s with different chalcogen heterocycles exhibit well-matched lowest unoccupied molecular orbital/highest occupied molecular orbital (LUMO/HOMO) energy levels with PBDB-T, with enough offsets for effective charge transfer, as depicted in Fig. S5B. Notably, the comparable LUMO/HOMO values of the designed P A s suggest that the corresponding chalcogen heterocycles as electron linkers have little influence in energy levels. In addition, molecular simulation was carried out using density functional theory (DFT) with B3LYP/6-31G (d, p) basis set. In particular, the HOMO and LUMO levels and related electron distributions were calculated (Figs S6-S8). The trend of variation for molecular orbital energy levels is consistent with the results obtained from the CV tests (Table S1).
To shed light on the effects of the electron linkers on the molecular-packing arrangements in the solid-state, we conducted two-dimensional grazing-incidence wide-angle X-ray scattering (2D-GIWAXS) measurements. Figure 1B-D presents 2D-GIWAXS patterns of the P A films (PY-O, PY-S and PY-Se), and the relevant crystallographic parameters of these pristine films are presented in Table S2. All of the P A s adopt a preferential face-on orientation exhibiting similar prominent (010) diffraction peaks located at q z = 1.63Å −1 in the out-of-plane (OOP) direction. As with the above-discussed optical properties, the crystalline correlation lengths (CCLs) of these three P A s also exhibit a linear increase (CCL PY-O = 16.71Å, CCL PY-S = 18.05Å, CCL PY-Se = 18.42Å) according to the sequential chalcogen elements. This result reflects the crystallinity behaviors of the neat P A films and the strength of intermolecular interactions in the solid-state and also implies their charge transport properties [43,44]. Thus, the electron mobilities of the neat P A s films were further measured using the space-charge-limited-current (SCLC) method (Fig. S9). The electron mobility of PY-Se is 2.76 ×10 -4 cm 2 V -1 s -1 , slightly higher than those of the pristine PY-O (1.48×10 -4 cm 2 V -1 s -1 ) and PY-S (1.90 × 10 -4 cm 2 V -1 s -1 ) films, as depicted in Fig. 1F. These results suggest that PY-Se with a selenophene as an electron linker shows enhanced aggregation strength and better molecular crystallinity in the thin film compared to the PY-O and PY-S P A s, indicating that the electron linker engineering can effectively modify the intermolecular interactions of P A s.
To further investigate the molecular compatibility of donor and acceptor materials, we used water ( Fig. 2A) and ethylene glycol (EG, Fig. 2B) to con-duct surface energy measurements of these three P A s and PBDB-T introduced as P D in this work (see Fig.  S10). As shown in Fig. 2C, the corresponding surface energy values are 33.37 mN m -1 for PY-O, 36.80 mN m -1 for PY-S, 39.52 mN m -1 for PY-S and 45.14 mN m -1 for PBDB-T. Thus, the Flory-Huggins interaction parameter (χ ) values between donor and acceptors, are calculated from experimentally measured contact angles (Fig. S10), and summarized in Table S3. As a direct consequence (see Fig. 2C To further investigate the crystallization at nanoscale, 2D-GIWAXS measurements of pristine PBDB-T film (Fig. S11) and its blend films with different P A s ( Fig. 2G-I) were carried out. The relevant crystallographic parameters of these films are summarized in Table S4. For the pristine PBDB-T film, the polymer backbone preferred an obvious face-on orientation relative to the substrate, supported by the prominent (010) reflection peak at 1.69Å −1 (crystal coherence length, CCL = 28.56Å) in the OOP direction and lamellar (100) peak at 0.294Å −1 (d 100 = 21.37) in the IP direction. After blending with PBDB-T, all the blends show obvious face-on orientation relative to the substrate. In the IP direction (Fig. 2J), these all-polymer blend films exhibit obvious (100) Table 1. The higher PCEs for the PBDB-T : PY-Se system are attributed to improvements in all photovoltaic parameters. Although these three systems show comparable energy levels, their V OC values are slightly different. The above-mentioned blend morphologies of these systems may lead to associated energetic loss mechanisms (Fig. S16) and thus cause slight V OC variation (Table 1). External quantum efficiency (EQE) measurements of these systems, as plotted in Fig. 3B, were carried out to explain the difference in the measured J SC values from J − V plots.
To clarify the larger difference of J SC and FF values, we studied the charge photogeneration of the three all-PSCs. The photocurrent density (J ph ) versus the internal voltage (V in ) curves of the devices are shown in Fig. 3C. This result indicates that the high J SC and FF values obtained for PBDB-T : PY-S and PBDB-T : PY-Se systems result from the charge collection being efficient enough at the internal electric field. In contrast, the PBDB-T : PY-O device did not exhibit an apparent saturation regime for J ph even at high V in (> 1 V), which is mainly attributed to a decrease in limited charge extraction and recombination [46]. We further investigated the J ph at high V in regimes (V in = 4 V), 19. Based on this point, photoluminescence (PL) spectra were further used to study the effects of the electron linkers in the P A s on exciton dissociation and charge transport properties in these blends. As shown in Fig. 3D, the PL emission of acceptors is quenched 79.4% in the PY-O-based blend, 83.9% in the PY-S-based blend and 86.4% in the PY-Se-based blend, respectively. This result illustrates that the exciton dissociation of the PBDB-T : PY-O blend is a vital limiting factor for the lower J SC as compared to the other two systems. Additionally, the hole and electron mobilities of these three systems were investigated by analyzing the J − V characteristics of single-carrier devices ( Fig. S18 for hole-only mobilities and Fig. S19 for electron-only mobilities, respectively). As depicted in Fig. 3E, the PBDB-T : PY-Se blends show morebalanced hole-and electron-mobilities of 3.16 × 10 -4 cm 2 V -1 s -1 and 3.28 × 10 -4 cm 2 V -1 s -1 in devices compared to the PBDB-T : PY-S system (a μ h of 2.74×10 -4 cm 2 V -1 s -1 and a μ e of 2.55 × 10 -4 cm 2 V -1 s -1 ) and PBDB-T : PY-O system (a μ h of 2.04 × 10 -4 cm 2 V -1 s -1 and a μ e of 1.56 × 10 -4 cm 2 V -1 s -1 ). Notably, the low and unbalanced electron and hole mobilities of the optimized PBDB-T : PY-O system indicate that its blend is transport-limited, also supported by the above-mentioned J ph analysis (Fig. 3C).
It is worth noting that the high and balanced charge transport properties in devices generally lead to reduced carrier recombination losses. Using the transient photovoltage (TPV) and CE techniques, which can depict the charge carrier lifetime τ (Fig. S20) as a function of charge carrier density n (Fig. S21) under open-circuit conditions, τ (n), we shed light on the differences of carrier recombination mechanisms in these all-polymer systems. As exhibited in Fig. 3F, a lower recombination order value R (R = 2.02), which was calculated via the equation of τ = τ 0 (n 0 /n) λ (where τ 0 and n 0 are constants and λ is the so-called recombination exponent) [47], was found for the PBDB-T : PY-Se device as compared to PBDB-T : PY-O (R = 2.29) and PBDB-T : PY-S (R = 2.07). The non-radiative voltage losses of these systems calculated from the electroluminescence EQE (Fig. S22)  photovoltaic parameters in the PBDB-T : PY-Se devices.
The relationships between molecular structure and morphological stability were emphatically studied to perfect the potential assessment of the investigated P A s based on different electron linkers. We firstly explored the long-time stored stability of the corresponding devices tested in a nitrogen glove box at room temperature. As shown in Fig. 4A, the PY-O-and PY-S-based devices exhibited inferior storage stability to that of the PY-Se-based device. Their performances decreased to 51.5% and 72.5%, respectively, of their initial efficiencies after 600 h storage, while the PBDB-T : PY-Se device decreased to only 84.3% PCE loss within the same time frame. This degradation trend of stored devices is identical to the attenuation trend of the devices exposed to light stress, as presented in Fig. 4B. After continuous light-soaking, PY-O-and PY-S-based films showed significant light-induced losses within 216 hours (75.29% and 81.75%), while PY-Se-based film was 85.03% of quenching efficiency over the same period. Light-induced degradation affects all the photovoltaic parameters (see Fig. S23). This trend is further confirmed by the change in PL intensity (Figs S24 and S25). The initial PL quenching rates of these blends were decreased after light-soaking for 216 hours, with PY-O-and PY-S-based blends gaining PL intensity more accelerated than the PY-Se-based system, underlining that PY-Se is thermally more stable.
We further demonstrated that the stability of photoactive layers could be quickly and reliably analyzed by measuring the space-charge-limited current of hole-only or electron-only devices under illumination (Figs S26 and S27). This indicates that the performance degradation probably originates from decreased and increasingly unbalanced electron and hole mobilities (Table S9). To further gain insight into the charge recombination behaviors after lightsoaking, charge carrier lifetime (Fig. S28) as a function of charge carrier density (Fig. S29) for the allpolymer solar cells was investigated (Fig. S30). For the PBDB-T : PY-Se devices, the recombination order (R) slightly increased from 2.02 for the fresh devices (0 h) to 2.08 for the corresponding device under one sun illumination for 216 h. In contrast, the R value of PBDB-T : PY-O devices significantly increased from 2.29 for the fresh devices (0 h) to 2.53 for the corresponding device under one sun illumination for 216 h (Table S9). Increased recombination order in OSCs can be linked to trap-mediated recombination and/or reduced mobility. All these physical characterizations as mentioned above suggest that the PBDB-T : PY-Se system shows much more stable blend microstructure.
Additionally, to develop a quantitative understanding of the mechanical stabilities or properties of all-polymer systems depending on acceptor types, we further employed a pseudo-free-standing tensile test on a water surface that can directly yield stress-strain (S-S) curves of mechanical properties (Fig. 4C). The detailed mechanical values, including elastic modulus, crack-onset strain (COS), toughness and tensile modules for the blend films are summarized in Table S10. Compared to the small molecule Y5-C20-based blend with a COS of 4.76%±0.39%, all the all-polymer systems show higher elongation values of 8.70%±0.42%-9.57%±0.35%. The excellent mechanical stability of the all-polymer blends was also confirmed by calculation of toughness. As presented in Fig. 4D was found, resulting from increased acceptor chain length by polymerization. The tensile behaviors of the Y5-C20-and PY-Se-based blends were compared using optical microscopy during the tensile tests, as depicted in Fig. 4E, indicating that there is a dramatic difference in their fracture response under tensile strain. Notably, as compared to the PY-Sand PY-Se-based blends, PY-O-based film shows a relatively low fracture toughness. It is mainly attributed to the low miscibility of PBDB-T and PY-O (Fig. 2C), resulting in limited chain entanglement and large domain sizes in the blend (Fig. 2D) [48]. In contrast, the improved mechanical properties of the PY-S-and PY-Se blends can be attributed primarily to the ductility of the polymer films imposed by the entangled polymer chains and their bi-continuous interpenetrating networks with nm-scale domains. Of particular note is the increased stress at the same strain in the PY-Se-based blend compared with PY-S, as a result of improved hardness of the crystalline domains resulting from its stronger intermolecular interactions. Previously, it was found that electron linkers can regulate intermolecular arrangement and crystallinity, thus affecting blend morphology and device efficiency [49,50]. The relationship between intermolecular interactions and phase separation in blends is generally considered in most cases [34,[49][50][51], but the effects of D/A compatibility as well as their intermolecular interactions on relevant stability issues are often neglected. In this work, we systematically elucidated the detailed influence of the electron linkers on efficiency and stability of the investigated all-polymer systems and summarized the corresponding results using visualized radar charts (Fig. 5)-a straightforward approach but the start of thinking about how we provide a comprehensive evaluation of design strategy.
As provided in Fig. 5, we can conclude that the higher degree of molecular crystallinity found for PY-Se because of its strong intermolecular interactions is reflected in enhanced stabilities compared to the other two systems. This finding for the morphological degradation rate of the active layer as a function of intermolecular interaction energy is consistent with all our experimental results as mentioned above. It already allows for quite deep insight into the fundamental mechanisms behind electron linker engineering. In addition, P D − P A compatibility (or phase miscibility) not only determines the blend morphological characteristics (Fig. 3), thus affecting the device efficiency and stability [52,53] ( Fig. 2A; Fig. 4A and B), but also influences the mechanical robustness of relevant active layers, strongly supported by tensile test results (Fig. 4D). It seems plausible that the molecular crystallinity and phase compatibility are not directly related, supported by our analysis results, which is inconsistent with some previous findings [35,54,55], especially in N2200based systems [56][57][58]. Nonetheless, we can conclude with care that both intermolecular interactions and D-A compatibility simultaneously determined the blend morphological characteristics, which result in the performance differences of all-polymer systems based on various electron linkers (Fig. 5).

CONCLUSION
In summary, a series of narrow band-gap polymer acceptors PY-X (O, S, Se) containing furan, thiophene and selenophene as the electron linkers in their conjugated backbones were designed and synthesized for application in all-PSCs. The electron linker engineering significantly affects the physical and chemical properties and intermolecular interactions of relative P A s and their charge transport properties. A PBDB-T : PY-Se system with remarkable D/A compatibility showed maximum performance with a PCE of 15.48%, much higher than those of PBDB-T : PY-O (9.80%) and PBDB-T : PY-S (14.16%) devices, supported by the optimized bulk microstructure with respect to its physical mechanisms in parallel. Note that the achieved PCE value (15.48%) is also one of the highest values in the all-PSCs reported. Additionally, the PY-Se-based blend displayed much higher storage stability and light-soaking stability than those of the other two systems. Better toughness values have also been realized in the PBDB-T : PY-Se blend, mainly resulting from suitable D/A compatibility for achieving favorable domains with nanoscale phase separation and meanwhile maintaining relatively stable morphology with suitable intermolecular interactions. Of particular note is the in-depth analysis of the effect of electron linkers on intermolecular interactions and molecular miscibility and its influence on BHJ morphology and device performance. The strategy of precise modification of electron linkers could be a practical way to simultaneously actualize molecular crystallinity and phase miscibility for improving the performance of all-polymer solar cells, showing practical significance.

SUPPLEMENTARY DATA
Supplementary data are available at NSR online.

FUNDING
This work was supported by the National Natural Science Foundation of China (51773157 and 52061135206). We also thank the opening project of Key Laboratory of Materials Processing and Mold and Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology) for support.