Multifunctional all‐polymer photovoltaic blend with simultaneously improved efficiency (18.04%), stability and mechanical durability

One of the most appealing material systems for solar energy conversion is all‐polymer blend. Presently, the three key merits (power conversion efficiency, operation stability and mechanical robustness) exhibited a trade‐off in a particular all‐polymer blend system, which greatly limit its commercial application. Diverting the classic ternary tactic of organic solar cells based on polymer, nonfullerene small molecule and fullerene, herein we demonstrate that the three merits of a benchmark all‐polymer blend PM6:PY‐IT can be simultaneously maximized via the introduction of a polymerized fullerene derivative PPCBMB. Importantly, the addition of the guest component promoted the power conversion efficiency of PM6:PY‐IT blend from 16.59% to 18.04%. Meanwhile, the device stability and film ductility are also improved due to the addition of this polymerized fullerene material. Morphology and device physics analyses reveal that optimal ternary system contains well‐maintained molecular packing and crystallinity, being beneficial to keeping favorable charge transport and the reduced domain size contributed to charge generation and ductility improvement. Furthermore, the ternary photovoltaic blend was successfully used as photocatalysts, and an excellent heavy metal removal from water was demonstrated. This study showcases the multi‐functions of all‐polymer blends via the use of polymerized fullerenes.


INTRODUCTION
The future of world energy market depends on the final winner of the competition of various sustainable resource technologies, among which the organic solar cells (OSCs) with long history and attractive intrinsic advantages, are # Tao Liu and Kangkang Zhou contributed equally to this work.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2022 The Authors. Aggregate published by SCUT, AIEI, and John Wiley & Sons Australia, Ltd. endowed great expectation for decades. [1][2][3][4][5][6][7][8][9][10] To date, the best power conversion efficiency (PCE) and device lifetime of this type of photovoltaic (PV) are over 19% and 10 years, respectively, demonstrating the foreseeable large-scale commercialization. [11][12][13][14][15][16][17][18][19][20][21][22] However, the key competitiveness of OSCs is its high-level mechanical durability, which benefits from polymers, but the top PCEs are all realized by small molecular acceptors. These photoactive layers comprising small molecule acceptors are found to be brittle, which is not able to fill the demand of complex-scene application. Thereby, developing highly efficient, stable and mechanically durable all-polymer systems is an important take for researchers. [23][24][25][26][27][28][29][30] From the perspective of device engineering, ternary tactic has been successfully and widely used to achieve efficiency and stability improvements. [31][32][33][34][35][36][37] Among these reports, combining fullerene and non-fullerene acceptors to create a ternary matrix is a classic strategy, which is widely proven effective. [38][39][40][41][42][43] Analogy to polymerized Y-series acceptors, the polymerized fullerene materials could also be effective in realizing efficient and stable all-polymer blend, yet only the small molecule PCBM was tried to be the guest for host system. [44] Besides, the addition of PCBM can increase the film ductility due to favorable morphology evolution, thereby the polymerized PCBM with better stretchability should be a better candidate to enhance the mechanical performance. [44,45] In this work, we put forward the use of a seldomly explored polymerized fullerene material, named PPCBMB (see Figure 1A), with the famed blend system PM6:PY-IT [46,47] to build a multifunctional ternary all-polymer matrix, which exhibited top performance in both all-polymer solar cells and photocatalysts for heavy metal removal. The ternary system PM6:PY-IT:P-PCBM-B with weight ratio of 1:0.8:0.2 shows a record power conversion efficiency (PCE) of 18.04%, outperforming the binary control and other ternary blends with different ratios. A series of characterizations show the rise of PPCBMB in the blend results a monotonously weakened crystallinity, and a reduced and then increased phase separation in films. Thus, the optimal blend contains the most suitable thin film morphology for efficient charge generation and well-kept charge transport. Besides, the intrinsically high mobility of P-PCBM-B contributes to better electron transport. In addition, these morphology features and intrinsic properties lead to promoted mechanical performance of the photoactive layer. Moreover, the airfabricated devices and thick-film devices based on optimal ternary blend demonstrate decent values. The device stability of target system is also better than binary control. To extend the application of the established material combinations, the ternary blend is used as photocatalyst to remove heavy atoms for a water purification. As a result, the cationic ions Pb(II), Cu(II), Cd(II) and Cr(VI) are effectively removed.

RESULTS AND DISCUSSIONS
First of all, we examined the basic properties of PPCBMB and the host system (PM6:PY-IT). The host systems are the same batches with those used in some prior studies. [47][48][49] Meanwhile, the polymerized fullerene derivative was synthesized according to the literature. [46] The estimated molecular weights, against polystyrene standards, are 7.9 KDa (M n ) and Then we investigated the ultraviolet-visible (UV-Vis) absorption of both neat and blend films, as given in Figure 1B-D. The PPCBMB's film has an absorption range complementary to PM6 and PY-IT, which is similar to what happened among PM6, Y6 and PCBM. Next, the absorption profiles of combined acceptor films show that increasing the content of the guest acceptor, will decrease but not eliminate the absorption peak of PY-IT, indicative of their moderate miscibility and tendency of separately crystallizing in films instead of co-crystallization. As for the blend films, the normalized absorbances tell that 20% PPCBMB enhance the 0-0 vibrational peak of PM6, but its binary film has the lowest intensity for this peak. This suggests suitable addition of PPCBMB can induce stronger aggregation of PM6, which could be beneficial to photon utilization and hole transport.
The devices were fabricated via a conventional structure composed of ITO/ HTL (PEDOT:PSS-PA)/ active layer/ ETL (PFN-Br-MA)/ Ag. [50,51] The current density versus voltage (J-V) characteristics of all systems and their external quantum efficiency (EQE) spectra for N 2 glovebox fabricated devices are plotted in Figure 2A,B, with relative parameters summarized in Table 1. The optimal efficiency as high as 18.04% for small area device is achieved by 20% incorporated PPCBMB, whose improvement is enabled by concurrently enhanced   Figure 2G, with details given in Table S1. Next, we compared the stability of two binary and optimal ternary systems, as shown in Figure 2C (shelf-like) and 2D (light soaking). The glass-encapsulated cells exhibit a best stability performance when PPCBMB's content comes to 100%, and ternary cells are better than PY-IT based binary devices. Considering the high initial PCE, the ternary devices should convert much more solar energy to electricity in a considerable period. To further explore the scalability of this high-performance ternary system, a series of solar cells were fabricated in ambient atmosphere, with large active area (1 cm 2 ), with different thicknesses. As listed in Table 1 (plotted in Figure 2E,F), the large-area devices made in air can still show a 16.94% efficiency, and maintain >14% PCE when film thickness comes to 535 nm. These results are significant as compared with other reports in Figure 2H,I, (Table S2 and S3).
Then the mechanical properties of photovoltaic films were carefully studied by using different measurements. The crack-onset-strain (COS) values, as an index for the film's breaking stretchability under tension was assessed by filmon-elastomer (FOE). [52] The crack formation images for each system are displayed in Figure S2. The various crack shapes indicates that the crack generation and propagation of the blend films become more difficult with the increase of PPCBMB's content. The COS values ( Figure 3A) for them are 9.6 ± 1.2%, 10.5 ± 1.3%, 10.9 ± 1.1%,11.6 ± 1.4%, 12.3 ± 1.1% and 13.1 ± 1.3%, respectively, with the increase of PPCBMB's content. Moreover, the stress-strain curves of films gained from film-on-water (FOW) tests are drawn in Figure 3B, pointing to a similar trend that fracture strain of film increase from 6.6%, to 6.8%, 6.9%, 7.1%, 7.2% and 7.4% following PPCBMB's increase. Both methods demonstrate that film's ductility is monotonously enhanced by adding the guest acceptor. Besides, we can also derive the elastic modulus of the films from these curves, which are shown in Figure 3C. The variation tendency well follows the Halpin-Tsai model that we found effective to describe the variation of PM6:N3:PC 71 BM ternary system. [45] This is further confirmed by the results from PFNQM measurements, given in Figure S3 and Figure 3D. Since the PM6:PY-IT binary system was donor-acceptor interpenetrating network when the blend ratio varies in a wide range (Davies model), [53] the addition of PPCBMB will not affect the co-continuity of donor and acceptor(s).
Apparently, the morphology is an important factor to correlate the change of device efficiency and film's mechanical property, thus we firstly pay attention to the molecular packing of each system with the help of grazing incidence wide-angle X-ray diffraction (GIWAXS) technique. [54][55][56] The 2D patterns, derived out of plane (OOP) & in plane (IP) directional intensity profiles, as well as calculated parameters including d-spacing, peak area, coherence length (CL), relative degree of crystallinity (rDoC) and the face-on fraction of the (010) peaks for all systems are presented in Figure 4. The rDoC and face-on fraction variation is not so significant when adding different ratio PPCBMB, compared to quickly F I G U R E 4 GIWAXS results. 2D pattern, out of plane and in plane direction intensity profiles and parameter summary of (010) peaks alongside out of plane orientation dropped CL and peak area. This feature indicates that PY-IT's general crystallization is not clearly higher than PPCBMB, but its crystallite size is larger than the latter one. As for optimal ternary film, its -stacking order is reduced, but still at a high level, which will not deteriorate the electron transport. Besides, the intrinsic high mobility of fullerene derivative can enhance its electron transport. [57] Furthermore, the hole transfer property tuning effect enabled by fullerene derivates can compensate the possible loss in a satisfying degree.
Afterwards, we used atomic force microscope (AFM) and transmission electronic microscope (TEM) measurements to evaluate the change of phase separation caused by adding different ratio third component. Figure 5 displays the AFM height and phase images and TEM images. The film surface goes through an insignificant change, which provides insufficient information of phase segregation's variation but tells all films are smooth. The TEM images, in contrast, provides more information about what we would like to know. Compared with others, the optimal ternary film shows a more significant fibril network, which could be beneficial to charge transport. Then, the power spectral density (PSD) calculation results show that the phase separation length scales are about 33 nm, 13 nm, 11 nm, 14 nm, 21 and 26 nm, respectively, with the rise of the PPCBMB content. The reduced phase separation of film in ternary systems appear to be similar to what we observed before, which is supposed to be beneficial to charge generation and avoiding large number of crack initiation sites.
Having fully characterized the morphology, we focus on the charge transport, recombination, dissociation and collection, inferred before based on morphology investigation. The mobility of photovoltaic blend with weight ratios of 1:1:0, 1:0.8:0.2, 1:0.6:0.4 and 1:0:1 for PM6:PY-IT:PPCBMB was evaluated by the space charge limited current (SCLC) method based on hole-only and electron-only devices ( Figure 6A,B). [58][59][60] The results (µ h , µ e and µ h /µ e ) are shown in Figure 6C, from which the optimal ternary film can be found with highest µ h and µ e , and lowest µ h /µ e , indicative of most intensive and balanced charge transport. These results are also consistent with morphology study. Then we investigated the recombination change in different systems, by comparing the V OC /J SC vs light intensity relationships ( Figure 6D,E). The fitted ideal factors are 1.18, 1.17, 1.28 and 1.40, respectively, for PPCBMB weight ratios of 0, 20%, 40% and 100%. Meanwhile, the calculated bimolecular recombination parameters are 0.97, 0.98, 0.96 and 0.95. These results indicate that trap-assisted and bimolecular recombination of the best ratio system is well suppressed compared with others, consistent with the charge transport variation tendency. Moreover, the charge extraction of them were also studied, as provided in Figure 6F, the J ph vs V eff relationship. The saturated current density (J sat ) values are 25.38, 25.76, 24.86 and 12.42 mA/cm 2 , respectively, which corresponds to dissociation and collection efficiencies of (92.2%, 93.8%, 91.5%, 90.7%) and (83.5%, 86.3%, 80.9%, 73.4%) (Figure 7). Furthermore, in Figure 7, the multi-functions of the novel ternary photovoltaic blend constructed in this work is demonstrated by a set of experiments on heavy atom removal, where the redox, chelation and adhesion are supposed to concurrently drive the water purification. PM6:PY-IT:PPCBMB with coconut shell carbon substrate can reduce the content of cationic heavy metal ions Cd(II), Cu(II), Pb(II), Zn(II) and Al(III) to an undetectably low level within 7 minutes under standard illumination (100 mW/cm 2 ), and anionic heavy metal ion Cr(VI), completely removed to undetectable within 9 min [61][62][63] , which is better than all reported amphoteric adsorption materials. As for the stability of the all-polymer based membrane, it can still be highly effective after 10-round recycle ( Figure S4). This is a new breakthrough and attempt to apply all-polymer ternary blends as photocatalytic materials to treat multiple heavy metal ions in water, and uplift the cost-effectiveness of water purification by reducing the processing steps for various kinds of elements.

CONCLUSION
In summary, we reported a previously unexplored ternary all-polymer blend with record high performance in both solar cells and heavy metal removal. Owing to the introduction of a polymerized fullerene derivative PPCBMB, the PCE of non-halogenated solvent processed all-polymer solar cells based on PM6:PY-IT binary system was improved from 16.59% to 18.04%, and the shelf-like and light-soaking lifetime is prolonged significantly. Moreover, the film ductility also benefits from the addition of PPCBMB, and the elastic modulus well follow our previously confirmed Halpin-Tsai model. The morphology characterization reveals well-kept crystallinity, crystallite size and face-on orientation in the ternary film is beneficial to maintain the favorable charge transport property, which is also assured by SCLC testing results. The reduced domain size for optimal ternary blend is also consistent with its better photon utilization, and promoted ductility since it can reduce the number of crack initiation sites. In addition, the application of this ternary blend extends from photovoltaic devices to heavy metal removal related water purification, and a very successful case is demonstrated. Thereby, our work offers a successful case for ternary design strategically, realizes high performance all-polymer solar cells engineeringly, and offers understanding of efficiency-ductility-morphology's inner correlation, as well as its multi-functions.