Insight into the efficiency‐stability‐cost balanced organic solar cell based on a polymerized nonfused‐ring electron acceptor

Organic solar cells (OSCs) have attracted extensive attention from both academia and industry in recent years due to their remarkable improvement in power conversion efficiency (PCE). However, the Golden Triangle (the balance of efficiency‐stability‐cost) required for large‐scale industrialization of OSCs still remains a great challenge. Here, a new nonfused‐ring electron acceptor (NFREA) BF and its polymerized counterpart PBF were designed and synthesized, and their photovoltaic performance, storage stability and material cost were systematically investigated. When blended with a widely‐used polymer donor PBDB‐T, the PBF‐based all‐polymer solar cell (all‐PSC) displayed a record high PCE of 12.61% for polymerized NFREAs (PNFREAs) with an excellent stability (95.2% of initial PCE after 800 h storage), superior to the BF counterpart. Impressively, PBF‐based all‐PSC possesses the highest industrial figure‐of‐merit (i‐FOM) value of 0.309 based on an efficiency‐stability‐cost evaluation, in comparison to several representative OSC systems (such as PM6:Y6 and PBDB‐T:PZ1). This work provides an insight into the balance of efficiency, stability, and cost, and also indicates that the PNFREAs are promising materials toward the commercial application of OSCs.


INTRODUCTION
10][11][12][13][14][15][16][17][18][19][20][21] While organic photovoltaic is an excellent technology in terms of its high efficiency, the goal of long-term stability and low cost remains a challenge for the whole community.Therefore, to demonstrate the potential of OSC materials and devices for industrial viability, it is vital to maintain the balance among efficiency, stability, and cost (so-called "Golden Triangle").
The challenge of balancing the "Golden Triangle" can be divided into two parts: (1) how to balance efficiency and stability, and (2) how to balance efficiency and cost.First, Li et al. proposed a pioneering strategy of polymerized smallmolecule acceptors (PSMAs) (generally referring to polymerized fused-ring electron acceptors, PFREAs) in 2017.With many efforts, the all-polymer solar cells (all-PSCs) based on PFREAs can not only achieve PCEs over 17%, [22][23][24][25][26][27][28][29][30][31][32] but also significantly improve the device storage and operation stability, showing a good way to balance efficiency and stability.Second, in view of the high product cost of fused-ring electron acceptors (FREAs), Chen et al. proposed the concept of nonfused-ring electron acceptors (NFREAs) in 2018. [33]43][44][45][46][47][48][49][50] Recently, we proposed a novel strategy of polymerized NFREA (PNFREA), [51] followed by various efforts from other groups, [52][53][54][55][56] exhibiting the potential of achieving low cost as well as excellent thermal/morphological stability.However, the PCEs of the PNFREAs usually lagged behind their NFREAs counterparts, which hinder their industrial viability.Herein, a novel PNFREA named PBF was constructed, where the NFREA (BF) and thiophene were employed as the key building block and the linker (Figure 1).Compared with the small-molecule acceptor BF, the polymer acceptor PBF exhibited enhanced thermal stability/light-absorbing ability and tighter π-π packing.Thus, the PBF-based device delivered a record efficiency (12.61%) for PNFREAs, much higher than that of BF-based one (11.10%),with an excellent storage stability (95.2% of initial PCE after 800 h storage).Moreover, the matrix analysis of the "Golden Triangle" shows that the industrial figure of merit (i-FOM) value of PBF-based OSC is 0.309, much higher than those of reputable systems such as PM6:Y6 (0.192) and PBDB-T:PZ1 (0.167).

Synthesis and characterization
The detailed synthetic route for the target acceptors was displayed in Figure S1.The aldehyde intermediate (BO-CHO) was efficiently synthesized via C─H activation reaction, [46] followed by Knoevenagel condensation reaction to facilely afford the NFREA (BF) and dibromo-monomer (BF-Br).Finally, the PNFREA (PBF) was obtained via Stille cross-coupling polymerization between BF-Br and 2,5bis(trimethylstannyl)thiophene (T-Sn).The chemical structures of key intermediates and the NFREA BF were fully characterized with 1 H/ 13 C NMR, which are summarized in the Supporting Information.The number-average molecular weight (M n ) and dispersity (Đ) of the polymer acceptor PBF were measured to be 21.7 kDa and 1.23, respectively, by high-temperature gel permeation chromatography at 150 • C with 1,2,4-trichlorobenzene as eluent (Figure S2).Thermogravimetric analysis (TGA) (Figure S3) showed that PBF possessed a higher onset (at 5% weight loss) of thermal decomposition temperature (363 • C) than BF (324 • C).In addition, differential scanning calorimetry (DSC) indicated that BF had an endothermic peak at 224 • C, while no obvious thermal transitions was observed for the polymer acceptor PBF in the 50-250 • C range (Figure 2A).These results demonstrated an excellent thermal stability of PBF, thus showing a great potential for application in the highly-stable OSCs. [57]he UV-vis absorption spectra of BF and PBF were measured in both dilute solutions and thin films (Figure 2B and Figure S4).In comparison with BF, PBF exhibited red-shift absorption spectra of 23 and 27 nm in the solution and solid state (Table 1), respectively, which may benefit from the enhanced conjugated backbone and tighter π-π packing of PBF and will facilitate the solar harvesting.The electrochemical properties and energy levels of the two acceptors were measured by cyclic voltammetry (CV).As shown in Figure 2C, the lowest unoccupied molecular orbital (LUMO)/highest occupied molecular orbital (HOMO) energy levels were estimated to be −3.84/−5.46eV for BF and −3.76/−5.42eV for PBF, respectively.The slightly higher-lying energy levels of PBF in comparison with those of BF were consistent with theoretical calculated results (Figure S5).Two-dimensional grazing incidence wide-angle X-ray scattering (2D-GIWAXS) was employed to investigate molecular packing of the BF and PBF in neat films.As shown in Figure 2D,E, the neat films of both acceptors exhibited a preferential face-on orientation with respect to the substrate.The (010) peaks of neat BF and PBF films were located at 1.66 and 1.71 Å −1 , and the corresponding π-π stacking distances were calculated to be 3.78 and 3.67 Å, respectively.The tighter π-π packing in the PBF film may facilitate the electron transport in the blend films. [58]

Photovoltaic properties
The photovoltaic properties of the two acceptors were investigated by fabricating OSC devices with an inverted structure of ITO/ZnO/active layers/MoO 3 /Ag.The PBDB-T was selected as the donor since it possesses a complementary absorption spectrum with the acceptors.The current density-voltage (J-V) curves of the two optimized devices were shown in Figure 3A, and the corresponding photovoltaic parameters were summarized in Table 2 and Table S1.The PBDB-T:BF based device delivered a moderate PCE of 11.19%, with an open-circuit voltage (V oc ) of 0.881 V, a short-circuit current density (J sc ) of 20.32 mA cm −2 , and a fill factor (FF) of 62.48%.It is worth noting that a significantly improved PCE of 12.61% can be achieved in the PBDB-T:PBF-based all-PSC, accompanied by increased J sc of 21.54 mA cm −2 and FF of 69.35%, which is a record efficiency among the PNFREAs-based all-PSCs so far.As shown in Figure 3B, the PBF based device displayed a broader photo-electron response, and the integrated J sc values calculated from the EQE spectra were 19.58 and 20.96 mA cm −2 for BF and PBF based devices, respectively, which are in good agreement with the values determined from the J-V curves within 5% mismatch.In addition, the storage stability of the BFand PBF-based devices were studied in Figure 3C.A distinct efficiency decrease (63.4% of initial PCE after 800 h storage) was observed in BF-based device, while a much higher storage stability (95.2% of initial PCE after 800 h storage) was achieved in the PBF-based all-PSC device.These results suggested that the PBF-based all-PSC achieved simultaneous improvement in efficiency and stability, which is highly desirable for industrialization of organic photovoltaic technology.
The space charge limited current (SCLC) method was employed to investigate charge transport properties of blend films (Figure S6).The hole/electron mobilities were calculated to be 2.  for BFand PBF-based devices were calculated to be 60.6 and 84.2 μs, respectively.These results demonstrated a shorter τ ext and a longer τ rec in PBF-based device, which is well in agreement with the higher electron mobility and more suppressed charge recombination.The exciton dissociation properties were investigated by measuring the relationship of the photocurrent density (J ph ) and effective voltage (V eff ). [59]As shown in Figure 3D, the exciton dissociation probabilities (P diss = J ph /J sat , where J sat is the saturated J ph at a sufficiently high V eff ) of the devices based on PBDB-T:BF and PBDB-T:PBF were calculated to be 92.1% and 98.1%, respectively, indicating more efficient exciton dissociation in PBF-based device.In addition, the recombination behavior was explored by plotting the dependence of V oc and J sc against the incident light intensity (P light ) (Figure S8).The V oc −lnP light dependent of the curves showed that the PBDB-T:BF-and PBDB-T:PBF-based devices exhibited the slopes of 1.51 and 1.36 kT/q, respectively, (where k, T, and q are the Boltzmann constant, Kelvin temperature, and elementary charge, respectively), suggesting less additional trap-assisted recombination involved in the PBDB-T:PBF blend system. [60]oreover, the power-law dependence between J sc and P light can be expressed as J sc ∝ P light β .Theoretically, the β value closer to unity indicates a weaker bimolecular recombination during sweep-out. [61]The β values for the BFand PBFbased devices were 0.96 and 0.99, respectively, indicating that bimolecular recombination was significantly suppressed at short-circuit condition.[64] Compared with the BF-based film, the excitons were mostly generated in the middle and bottom region of the PBF blend film, which facilitates the transport of hole and electron to the corresponding electrode after exciton dissociation.According to exciton generation contours, the simulated exciton generation rate (G) was observed in the active layer (Figure S9).The bottom region of PBF-based film produced more excitons than BFbased film, which efficiently contributed to the photocurrent, resulting in an enhanced J sc .

Morphology characterization
The 2D-GIWAXS measurement was used to further understand the molecular stacking behavior in blend films.The corresponding 2D-GIWAXS patterns and the line-cut profiles along the OOP and IP were shown in Figure 4A,B.Similar to neat films, the PBDB-T:BF and PBDB-T:PBF As shown in Figure 4C, the root-mean-square surface roughness (R q ) of PBDB-T:BF and PBDB-T:PBF blends were observed to be 2.64 and 2.11 nm, respectively.AFM phase images showed that PBDB-T:PBF blend featured more ordered fibrous phase separation morphology in comparison with the PBDB-T:BF blend.In addition, the contact angles of neat films exhibited the Flory-Huggins interaction parameters were 0.39 for PBDB-T and BF, and 0.24 for PBDB-T and PBF, respectively, suggesting that PBF showed better miscibility with PBDB-T (Figure S10).Thus, we concluded that the enhanced crystallinity and more favorable phase separation in the PBDB-T:PBF blend film led to the efficient exciton dissociation and improved charge transport.

Matrix analysis of the "Golden Triangle"
The industrial figure of merit (i-FOM = PCE × Stability/SC) is commonly used to evaluate the industrial interest and application potential of OSCs, where SC refers to the synthetic complexity index. [65]The "Stability" is measured as the ratio of the device efficiency illuminated under one sun for 200 h in nitrogen-filled glove box to the initial PCE.The SC index is used to describe the cost of active layer materials, which depends on five parameters, including the number of synthetic steps (NSS), the reciprocal of total yields (RY), the number of column chromatographic purifications (NCC), the number of isolation/purification (NUO), and the number of hazardous chemicals used for their preparation (NHC). [66]enerally, the SC value can be calculated according to the following formula: Considering the diversity of active layer composition (single-component, binary, ternary, and even more complex systems), [67] the cost analysis can be thus obtained by the following extended formula (w AL , the weight fraction of each material in the active layer): Here, i-FOM values for various types of acceptors (including the FREA, PFREA, NFREA, and PNFREA) based blend systems were calculated.Note that widely-used PBDB-T (D 1 ) and PM6 (D 2 ) were selected as donors for the above calculation.The synthetic schemes of the involved donor/acceptor materials were shown in Figures S11-S17, and the relevant parameters were summarized in Tables S2  and S3.The NSS and RY values of these FREAs and their polymerized derivatives were calculated to be 10-17 As shown in Figure 5B, the industrialization potential of representative OSCs was further evaluated by a matrix analysis of the "Golden Triangle".Among the blend systems, PM6:Y6-based OSC showed the highest efficiency of 16.88% with poor stability (68.1%) and high SC AL values (59.7), and the IT-4F counterpart also suffered from the same problem (65.3% of initial PCE after 800 h storage, SC AL = 67.8).Note that despite their excellent device stability (≈90%), PFREAbased all-PSCs (such as PBDB-T:PZ1 and PBDB-T:PN-Se) still suffered from the complex synthesis (SC AL = 51.6-74.2).Impressively, the blend systems based on NFREA and PNFREAs possessed smaller SC AL values (36.6-45.7)due to their concise synthetic routes, especially PNFREA based all-PSCs significantly improved the stability (>90%) compared with NFREA-based OSCs.Thus, the PBF-based all-PSC in this work has the advantages of low cost (SC AL = 38.9),excellent stability (95.2%), and high efficiency (12.61%), resulting in an extraordinary i-FOM value of 0.309, much higher than those of representative systems such as PM6:Y6 (0.192) and PBDB-T:PZ1 (0.167) (Figure 5C).

CONCLUSION
In summary, a new PNFREA PBF was constructed by the corresponding NFREA (BF) and a thiophene unit.Through the polymerization strategy, PBF possessed enhanced thermal stability, broader absorption window, and tighter π-π packing.Consequently, the PBDB-T:PBF-based all-PSC showed a higher PCE of 12.61% and better storage stability than BF-based device due to the enhanced/balanced charge transport, favorable phase separation, and stable film morphology.Importantly, the i-FOM value of PBF-based blend was much higher than those of representative highperformance systems (such as Y6-and PZ1-based blends), which is mainly due to the facile synthesis and low cost enabled by the nonfused-ring structure.This work revealed PNFREAs can be considered as a promising approach to achieve the efficiency-stability-cost balanced OSCs.

F I G U R E 1
The proposed pathway to achieved efficiency-stability-cost balanced OSCs in this work.

F
I G U R E 2 (A) DSC traces of BF and PBF.(B) UV-vis absorption spectra of PBDB-T and the two acceptors in thin films.(C) Cyclic voltammogram plots of BF and PBF measured in 0.1 M n-Bu 4 NPF 6 acetonitrile solution at a scan rate of 100 mV s −1 .(D) 2D-GIWAXS patterns of BF and PBF neat films.(E) The corresponding 1D line-cuts in the in-plane (IP, dotted line) and out-of-plane (OOP, solid line) directions.TA B L E 1 Optical, electrochemical, and molecular packing properties of BF and PBF.from the reduction/oxidation onset of the CV curves; b) Calculated according to 1D line-cuts of 2D-GIWAXS.
10/1.74 × 10 −4 and 4.01/3.77× 10 −4 cm 2 V −1 s −1 BFand PBF-based blend films, respectively.The increased and more balanced hole/electron mobilities (μ h /μ e = 1.06) in the PBF-based blend may be one of the reasons for the higher J sc and FF values.For more information about the charge carrier dynamics, transient photocurrent (TPC) and transient photovoltage (TPV) measurements were investigated as shown in Figure S7.The charge extraction time (τ ext ) obtained from the decay traces of TPC were estimated to be 0.47 and 0.45 μs for BFand PBF-based devices, respectively.Furthermore, the carrier lifetime (τ rec ) F I G U R E 3 (A) J−V curves and (B) EQE curves of the BF-based OSC and PBF-based all-PSC.(C) Normalized PCE for the long-term stability of the device in a nitrogen-filled glove box.(D) Photocurrent-effective voltage curves.(E,F) Simulated exciton generation contours within active layer (unit, 10 25 m −3 s −1 nm −1 at its position and wavelength) from FLAS spectra.TA B L E 2 Detailed device parameters of PBDB-T:BF-based OSC and PBDB-T:PBF-based all-PSC.

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I G U R E 4 (A) 2D-GIWAXS patterns, (B) line-cut profiles along the OOP and IP directions, and (C) AFM images for the blends based on BF and PBF.blend films possessed a dominant face-on molecular orientation, providing efficient transport channels for free carries between the electrodes.In details, the (010) π-π stacking diffraction peak of BF-based blend in OOP direction was located at 1.66 Å −1 , and the counterpart peak for PBFbased blend was located at 1.68 Å. Correspondingly, the π-π stacking distance of PBF-based blend (3.74 Å) was obviously shorter than that of PBDB-T:BF blend (3.78 Å).Meanwhile, the crystal coherence lengths (CCLs, calculated via Scherrer equation) of the corresponding peaks were 21.2 and 21.7 Å for BF-and PBF-based blend films, respectively, suggesting the enhanced intermolecular stacking in the PBDB-T:PBF film.Further insights into the morphology of the blend films were investigated by atomic force microscopy (AFM).

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I G U R E 5 (A) The matrix analysis of parameters related to synthetic complexity (SC) value and (B) efficiency-stability-cost balance in the "Golden Triangle".(C) The industrial figure of merit (i-FOM) against synthetic complexity of active layer (SC AL ) plots of the representative systems.and 24.6-158.7,respectively, while nonfused-ring structured acceptors (NFREA and PNFREAs) possessed smaller NSS (6-11) and RY (2.2-9.1)values.The corresponding values of NCC, NUO, and NHC showed a similar trend (Figure 5A), which is attributed to the facile synthesis and high yield due to the simpler structure of NFREA and PNFREAs.

A
U T H O R C O N T R I B U T I O N S X.G. and Y.W. contributed equally to this work.The synthetic works were carried out by X.G.Y.W. carried out the device fabrication and measurements.Z.H. and Z.D. provided DFT calculation and AFM images.G.L. and G.L. performed FLAS measurements.J.Z. and Z.W. performed GIWAXS measurements.H.H., X.Z., and Y.C. supervised and directed this project.X.G., Z.X., and H.H. prepared the manuscript.All authors discussed the results and commented on the manuscript.