Over 14% Efficiency Single‐Junction Organic ... - Polymer

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Over 14% Efficiency Single-Junction Organic Solar Cells Enabled by Reasonable Conformation Modulating in Naphtho[2,3-b:6,7-b′]difuran Based Polymer

Bing Zheng, Feng Qi, Yu Zhang, Ming Zhang, Panfeng Gao, Feng Liu,* Tianchen Li, Donghui Wei, Meixiu Wan, Guangming Chen, Lijun Huo,* and Lei Jiang

DOI: 10.1002/aenm.202003954

1. Introduction

Bulk heterojunction (BHJ) organic solar cells (OSCs) have been in favorable devel-opment in recent years, on account of their adjustable structure, flexibility, low cost, and more.[1–6] The joint advantages result in power conversion efficiencies (PCEs) of the state-of-the-art photovoltaic devices over 17%.[7–12] The development of p-type polymer donors plays a crucial role in the improvement of the photovoltaic performance. One of the prototypical het-eroacenes, benzo[1,2-b:4,5-b′]dithiophene (BDT), as a star electron donor building block, has been widely used in the skel-eton of photovoltaic polymers.[13–17] By chemical modifications of the BDT unit, the main chain and side chain engi-neering can effectively modulate absorp-tion behavior and energy level and thus improve solar cell performance. So far, the

PCE of ≈16–17% was achieved with the BDT-based copolymer: Y6 in the binary system.[18–23]

With the development of donor materials, other benzodi-chalcogenophene (BDC) derivatives, benzo[1,2-b:4,5-b′]difuran (BDF) via replacing thiophene with furan rings have also received great attention.[24–31] Due to the smaller atom radius of oxygen in furan than that of the corresponding sulfur atom in thiophene unit, the former generates lesser steric hindrance and induces tighter packing, higher charge–carrier mobility.[25] In 2015, Huo et al. found that the BDF-based materials possessed remarkable planar molecular character to form more ordered molecular stacking and realized an impressive photovoltaic result.[32] In comparison with BDF, naphtho[2,3-b:6,7-b′]difuran (NDF) further extended π-conjugation system with fusing two furan units on the naphthalene ring, which can enhance the intermolecular packing and form a highly rigid coplanar back-bone. Meanwhile, the extended conjugated structure can pro-vide strong electron-delocalized capability to enhance charge separation, transport, and photovoltaic property.[33–35] The extending BDT unit to dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene (DTBDT) unit to construct efficient photovoltaic polymer is a successful case.[36–39] Intrigued by this effec-tive molecular strategy, the NDF-based materials should have more potential in enhancing photovoltaic properties than BDF.

As one kind of abundant product from renewable resources, furan and its fused-ring derivatives, have provoked great interest in the context of devel-oping efficient photovoltaic materials. However, the power conversion efficiency (PCE) of furan-based photovoltaic materials has lagged behind its thiophene counterparts. In this work, in consideration of the ordered π–π stacking via extending conjugation to further improve the charge mobility, a novel furan fused-ring derivative of naphtho[2,3-b:6,7-b′]difuran (NDF) based copolymer of NDF-3T is designed and synthesized. Because of its favorable linear molecular conformation, the NDF-3T possesses a high crystallinity, as well as ordered and dense π–π stacking. Subsequently, the NDF-3T-based device exhibits an efficient PCE of 14.21%, which is higher than that of the analogue naphthodithiophene (NDT) counterpart (10.86%). To the best of the authors’ knowledge, the PCE is also the best record in furan-based photo-voltaic materials. More importantly, the development of line NDF shows great potential in construing highly efficient photovoltaic materials and can be referenced to other furan fused-ring structures.

B. Zheng, F. Qi, Y. Zhang, P. Gao, T. Li, M. Wan, Prof. L. HuoSchool of ChemistryBeihang UniversityBeijing 100191, P. R. ChinaE-mail: [email protected]. Zhang, Prof. F. LiuSchool of Chemistry and Chemical EngineeringShanghai Jiao Tong UniversityShanghai 200240, P. R. ChinaE-mail: [email protected]. D. WeiCollege of Chemistry and Molecular EngineeringZhengzhou UniversityZhengzhou 450001, P. R. ChinaProf. G. ChenCollege of Materials Science and EngineeringShenzhen UniversityShenzhen 518061, P. R. ChinaProf. L. JiangKey Laboratory of Bio-inspired Materials and Interfacial ScienceTechnical Institute of Physics and ChemistryChinese Academy of SciencesBeijing 100190, P. R. China

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.202003954.

Adv. Energy Mater. 2021, 2003954

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Nevertheless, up to now, there is only limited progress on NDF-based polymers for OPV applications. All of the extended π-conjugated structures are angular-shaped “zigzag” and the related device performance is not satisfactory (PCE <7%). For example, Zhang group synthesized angular-shaped NDF-based copolymer, revealing the PCE of the device is only 1.83%.[40] Zou et al. reported NDF-based polycyclic arene; as a result, the device exhibited 6.9% efficiency with a 0.25% 1,8-diiodooc-tane (DIO) additive.[41] Considering the distinguished char-acters of linear and high planar molecular structure of NDF, further increasing the efficiencies of NDF-based copolymers is a significant effort.

Hence, here we synthesized a novel linear NDF-based copolymer by introducing 3,3“-carboxylic acid-2-octyldodecyl ester-2,2′:5′,2”-terthiophene to the backbones of NDF-3T, in which the ester-substituted terthiophene acceptor block can provide a further tuning in energy level and reasonable solu-bility. As a comparison, the naphtho[2,3-b:6,7-b′]dithiophene (NDT)-based analogue, referred to as NDT-3T, was also synthe-sized (Scheme 1).[42] It is worth mentioning that economiza-tion of side chains on donor units can satisfy both in low-cost

and simple synthetic routes. Additionally, to match the energy level and absorption of polymers, the non-fullerene ITIC-4Cl was used as acceptor in the OSCs. Their photophysical, elec-trochemical, hole mobility, and device characterization were carefully investigated and discussed. As a result, the NDF-3T: ITIC-4Cl delivered a champion PCE of 14.21% compared with NDT-3T: ITIC-4Cl (10.86%), which is the best PCE for NDF-based polymers and the highest PCE record for furan based photovoltaic materials so far. Meanwhile, from the successful case of linear NDF building block, it is preliminarily proved that the developing linear conformation of furan fused-ring polymer is an efficient molecular strategy in constructing highly efficient furan-containing polymer donor.

2. Results and Discussion

2.1. Synthesis and Characterization

The detailed synthetic routes for monomers and polymers are provided in Scheme 1. The 3,7-dibromo-2,6-dihydroxynaphthalene

Scheme 1. Synthetic routes of the monomers and polymers of NDF-3T and NDT-3T.



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(2) is a key intermediate in the syntheses of compound (5), conveniently obtained from 2,6-dihydroxynaphthalene via bromination and selective debromination. Then the interme-diate compound was coupled with trimethylsilylacethylene by a typical Sonogashira reaction after acetylation on the hydroxyl moieties. Secondly, the cyclization steps were carried out in the mixed solution of N, N-dimethylacetamide and water of Cs2CO3 to give acceptable yield for the parent compound (5). Subsequently, the tin monomers of compound (6) (Figure S1, Supporting Information) and NDT were prepared according to the previous methods. For the accepter unit, first, thiophene-3-carboxylic acid was easily lithiated by lithium diisopropylamine and then reacted with tetrabromomethane to afford compound (7). Then 9-(bromomethyl)nonadecane was reacted with com-pound (7) to obtain compound (8) in the dichloromethane (DCM) solution of (dimethylamino)pyridine (DMAP) and N,N′-dicyclohexylcarbodiimide (DCC). Afterward, Stille coupling reac-tion between 2,5-bis(trimethylstannyl)thiophene and compound (8) was accomplished by utilizing a catalyst Pd(PPh3)4 to synthe-size the intermediated product (9). Subsequently, the polymeric monomer was easily available after a bromination reaction. The polymers were prepared under typical Stille-coupling polyconden-sation in toluene with Pd(PPh3)4 as the catalyst under refluxing reaction. Both obtained target polymers of NDT-3T and NDF-3T exhibited good solubility in chloroform, chlorobenzene (CB). The molecular weights of two polymers were determined by gel permeation chromatography (GPC) in a chloroform eluent with a polystyrene standard. Comparable number-average molecular weights of 126 and 113 kDa and the corresponding polymer dis-persity index (PDI) of 2.0 and 1.9 were obtained for NDT-3T and NDF-3T, respectively. The thermal properties of the two polymers were investigated by thermogravimetric analysis (TGA) and dif-ferential scanning calorimetry (DSC). The TGA curves of the two polymers showed that the decomposition temperatures (Td) at 5% weight loss of the copolymers of NDT-3T and NDF-3T were 358 °C and 373 °C, respectively (seen in Figure S2, Supporting Information), indicating the two polymers have adequate thermally stability for OSCs and other optoelectronic device applications. At the same time, their DSC curves showed ignorable difference in their glass transition process in the temperature range from 25 °C to 300 °C (Figure S3, Supporting Information).

Since the O atom radius of furan is smaller than S atom radius of thiophene, the conformation of two polymer back-bones caused some different influence. To better understand

the structure-property correlations, the best optimized confor-mations of NDT-3T and NDF-3T are obtained via employing density functional theory (DFT) at the B3LYP/6-31G* (d, p) level by the Gaussian 09 software. As shown in Figure 1a, three repeating units are calculated, in which the alkyl chains are simplified into the carbomethoxy groups for convenient calculation. The conformation of NDT-3T shows obviously zigzag shape, however, NDF-3T possessed linear conforma-tional structure. The optimized geometries of the NDT-3T trimers show that the dihedral angles between the adjacent 3-carbomethoxy-thiophene, thiophene, NDT motif in the cen-tral repeating unit are 35.2°, 44.2°, and 10.9°. After the NDT group was replaced with NDF, the aforementioned dihedral angles were changed to 35.5°, 43.8°, and 2.1° for NDF-3T, respectively. Obviously, compared to the polymeric conforma-tion of NDT-3T, that of NDF-3T exhibits better linear degree since the smaller radius of oxygen atom in NDF-based conju-gated polymer could decrease steric hindrance and offer denser crystal packing. Additionally, due to the migration of charge carriers, the Figure S4, Supporting Information, shows that two polymers have consecutive positive molecular electrostatic potentials (ESP).

The UV–vis absorption spectra of the two polymers in dilute chloroform solution (10–5 m) and in solid films are illustrated in Figure 1b; and Figure S5, Supporting Information, and the detailed absorption parameters are summarized in Table 1. In the diluted solution, the absorption spectra of the poly-mers are exhibited through two spectral features. Although, the absorption spectra of short wavelength region are not obvious from 350 to 450 nm, which is assigned to localized π–π* transitions. Whereas the other one at longer wavelength region from 450 to 700 nm is attributed to intramolecular charge transfer. Compared with the solution samples, both

Figure 1. a) Molecular twist angles between side chain and backbone of polymers based on three repeat units determined using DFT at the B3LYP/6–31g (d, p) level. b) the normalized UV–vis absorption spectra of donor polymers and acceptor in neat films.

Table 1. Molecular weights, thermal, and physicochemical properties of the polymers.

Polymer λabsmax [nm] λonset

a) [nm] Egb) [eV] HOMO [eV] LUMO [eV] Eg

c) [eV]

solution film

PNDT-3T 500 527 629 1.97 −5.39 −3.39 2.00

PNDF-3T 519 532 632 1.96 −5.46 −3.37 2.09

a)Absorption edge of the polymer films; b)Calculated from the absorption edge of the polymer films: Eg

opt = 1240/λedge; c)Egec = ELUMO–EHOMO.



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solid films display bathochromic shift absorption resulting from the strengthened aggregation behavior in solid states. In addition, the maximum absorption peak of NDF-3T exhibits a little redshift by 5 nm in comparison with NDT-3T counter-part, coupling with a more pronounced shoulder peak located at ≈574 nm, which is ascribed to the stronger aggregation of NDF-3T in solid state. From the absorption onsets in films (λonset) of NDT-3T and NDF-3T of 629, 632 nm, correspond-ingly their optical bandgaps are 1.97 and 1.96 V, respectively. Considering the wide bandgap property of NDT-3T and NDF-3T, the low bandgap acceptor of ITIC-4Cl was adopted to pro-vide a complementary absorption in favor of improving the photovoltaic performance.

The electrochemical properties of NDT-3T and NDF-3T were evaluated using electrochemical cyclic voltammetry (CV) measurement in 0.1 m tetrabutylammonium hexafluorophos-phate (Bu4NPF6) acetonitrile solution. The CV curves of both polymers were obtained and shown in Figure S6, Supporting Information. Both NDT-3T and NDF-3T exhibit p-type char-acteristics owing to the irreversible oxidation peaks with neg-ligible reduction peaks. Subsequently, their lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels are calculated according to their onset oxidation and reduction potentials, respectively. The measured LUMO levels of two polymers are 3.39 and 3.37 eV, respectively. In contrast to the comparable LUMO levels, the HOMO level of NDF-3T (−5.46 eV) is deeper compared with NDT-3T (−5.39 eV), which coincides with larger electronegativity of O atom than S atom and a higher Voc could be expected in NDF-3T based device. Meanwhile, the electrochemical bandgap (Eg

ec) of them are 2.00 and 2.09 eV, respectively. In Figure 2a, the energy level diagrams of the polymer donors and ITIC-4Cl acceptor exhibit matchable energy levels between donor and acceptor, signifying their interfaces can occur efficient charge separation.

The grazing incidence wide angle X-ray scattering (GIWAXS) measurement has been widely used to investigate the mole-cular packing and crystalline features within the thin films of these two polymers. The (2D GIWAXS profiles and their cor-responding line-cuts of in-plane (IP) and out-of-plane (OOP) direction are provided in Figure 3a,b. For NDT-3T and NDF-3T neat films, it was seen that both of them exhibited a prefer-able face-on molecular orientation and the distinct (010) π–π stacking diffraction peaks were observed at 1.75 and 1.80 Å–1 in the out-of-plane (OOP) direction, respectively, corresponding to the π–π stacking distance of 3.59 and 3.49 Å. Meanwhile, the lamellar scattering was also observed in the NDT-3T (21.14 Å) and NDF-3T (21.22 Å) neat films in the in-plane (IP) direction. It means that NDF-3T owns a smaller π–π stacking distance and enhanced intermolecular stacking degree than NDT-3T. The results in GIWAXS measurements suggest that, in compar-ison with the zigzagged conformation of NDT-3T, the straight linear conformation of NDF-3T could be efficiently realized by replacing S atom by O in BDC building block, which is benefi-cial to form denser π–π stacking and stronger crystallinity.

Combining the results of the GIWAXS and DFT dihedral angle simulated results, the different packing modes of NDT-3T and NDF-3T can be interpreted as following. As demonstrated in Scheme 2, due to the more twisted backbone conformation of NDT-3T, ordered inter-chain packing can be achieved only under H-aggregation mode. Under the J-aggregation mode, the inter-chain packing will be non-ordered, because the inter-chain distance between the adjacent main chains is unequal. In contrast, the straight linear conformation of NDF-3T has almost constant d-spacing between the adjacent backbones under whether J-aggregation or H-aggregation modes, and thus the film of NDF-3T exhibits a clear (010) peak in the OOP pro-file of the GIWAXS analysis.[37] On the other hand, since the NDT-3T has more difficulty in forming ordered inter-chain packing than NDF-3T, the face-to-face π–π stacking will be

Figure 2. a) Energy level diagram of the related materials used in PSCs devices. b) Device structure of the PSCs used in this work.

Figure 3. a) 2D GIWAXS images of NDT-3T and NDF-3T neat films, respectively. b) the corresponding IP and OOP line cuts.



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definitely weakened in the film of NDT-3T. The packing modes in Scheme 2 provide a reasonable interpretation for the dif-ferent behaviors of NDT-3T and NDF-3T in GIWAXS analysis.

2.2. Photovoltaic Device

To investigate the photovoltaic properties of the these two polymers, bulk heterojunction OSCs were prepared based on an inverted device structure of ITO/ZnO/polymers:ITIC-4Cl/MoO3/Ag (Figure 2b). The photovoltaic characteristics were measured under Air Mass 1.5G illumination (100 mW cm–2). The results of systematic optimization of OSCs fabricated with various D:A ratios, additive volume content, and annealing temperatures were shown in Figure S7, Supporting Informa-tion, and the device parameters were summarized in Table S1, Supporting Information. The raw devices showed distinctly differential PCEs of 9.58% and 12.89% with Jsc values of 15.68 and 18.96 mA cm–2 for NDT-3T:ITIC-4Cl and NDF-3T: ITIC-4Cl cells, respectively. The pronounced increased Jsc from the former to the latter probably is contributed to the more efficient dissociation of the excitons and free charge trans-mission. The traditional 1,8-diiodooctane (DIO) additive was incorporated into these two blends, respectively, leading to a higher enhancement both in Jsc and FF for NDF-3T:ITIC-4Cl (Jsc = 19.61 mA cm–2, FF = 73.6%) than NDT-3T:ITIC-4Cl (Jsc = 15.97 mA cm–2, FF = 70.6%). Upon thermal annealing processing at 90 °C in nitrogen atmosphere for 15 min, more enhancement of the photovoltaic parameters was observed in NDF-3T blend than in NDT-3T blend. The champion PCE of NDF-3T blend exhibited an impressive PCE of 14.21% (Voc = 0.91 V, Jsc = 20.80 mA cm–2, FF = 75.1%) but that of NDT-3T blend is relatively lower value of 10.86% (Voc = 0.90 V, Jsc = 16.19 mA cm–2, FF = 74.5%) (Figure 4a). It is worth noting that the PCE achieved by NDF-3T based device is the highest record for furan-based photovoltaic materials to the best of our knowl-edge. And the utilization of the low-lying HOMO of NDF-3T as

donor could effectively realize the coordination performance of high Voc (>0.90 V) and high PCE (>14%). Although the HOMO level of NDF-3T is distinctly deeper than that of NDT-3T, the difference in Voc values between NDF-3T and NDT-3T are slight, which may be ascribed to higher energy loss occurred in the NDF-3T:ITIC-4Cl blends.[43] The related device parameters are summarized in Table 2.

In comparison with NDT-3T based device, the NDF-3T based device exhibits a pronounced improvement of PCE, which is mainly attributed to the increased Jsc and FF. To explain the phenomenon, the incident photon to converted electron (IPCE) were measured under AM 1.5G spectrum. As shown in Figure 4b, both NDT-3T and NDF-3T have IPCE values above 40% in the range of ≈370–770 nm, however, the NDF-3T based device exhibits a higher IPCE response than NDT-3T especially in the range from ≈400 to 800 nm, even achieving a maximal IPCE value over 80% at 560 nm. Hence, we can conclude that the contribution of IPCE response to the photocurrent is higher in NDF-3T based device, indicating a more efficient photon-harvesting.

In addition, it is well known that FF is always greatly affected by hole and/or electron mobility. The carrier mobilities of these two polymers were thereby measured by the space–charge–limited current (SCLC) method. The corresponding results in pure and blend films are summarized in Table S1, Sup-porting Information. The NDF-3T pure film shows higher hole motility (μh) value of 9.61 × 10–4 cm2 V–1 s–1 than NDT-3T (μh = 3.38 × 10–4 cm2 V–1 s–1). The higher mobility of NDF-3T could be attributed to the more strengthened stacking caused by planar topological architecture of furan derivatives.[44] In com-parison with the pure films, in blend films, the hole mobility ratios of NDF-3T: ITIC-4Cl relative to NDT-3T:ITIC-4Cl are further raised from 2.8 (9.61 × 10–4/3.38 × 10–4 cm2 V–1 s–1) to 4.8 (8.13 × 10–4/1.70 × 10–4 cm2 V–1 s–1) fold. That means the hole mobility of NDF-3T-based blend film is well improved, leading to the potential enhancement in FF and Jsc values. Further-more, the more balanced electron and hole mobility ratios of

Scheme 2. The demonstration of the top views of the packing modes in NDT-3T and NDF-3T.



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NDF-3T: ITIC-4Cl (0.91) than that of NDT-3T: ITIC-4Cl (0.24) also accounted for the reason of the higher FF in NDF-3T based devices. In a word, the high and balanced mobility is greatly beneficial to charge transport and suppression of carrier recom-bination. Meanwhile, the results show that the charge mobility enhancement by modulating polymeric mainchain conforma-tions is an effective strategy in increasing FF.

To study their charge-recombination mechanism, the rela-tionship between photocurrent (Jsc) and light intensity (Plight) was investigated according to the generalized power law: Jsc∝Plight

S. The value of S is theoretically expected to be close to 1, which means less charge-recombination.[45,46] Figure 4c illustrates that the S values for the optimized devices of NDT-3T:ITIC-4Cl and NDF-3T:ITIC-4Cl were 96.7% and 99.0%, respectively. That illustrates that NDF-3T based devices have less bimolecular recombination and more efficient charge car-riers than NDT-3T based devices. In addition, to gain a better understanding of the charge dissociation and collection prop-erties of different blends in solar cells, the photocurrent den-sity (Jph) versus the effective voltage (Veff) of the devices were measured. Jph equals to JL−JD, where JL is the photogenerated current density under illumination and JD is the dark current

density under dark. Veff is the value of bias voltage when Jph = 0. The probability of charge dissociation P (E, T) could be described from the ratio of Jph/Jsat, where Jsat is the value of Jph saturates when Veff ≥ 2 V.[47,48] As shown in Figure 4d, the calculated P (E, T) for the optimized NDT-3T:ITIC-4Cl and NDF-3T:ITIC-4Cl solar cells are 93.4% and 94.6%, respectively. The result suggested that NDF-3T based devices had more suf-ficient exciton dissociation and charge extraction compared with NDT-3T based devices, resulting in a higher Jsc and FF. As a result, 14.21% is a new record for NDF-based non- fullerene polymer solar cells (NF-PSCs) (Figure 4e).

Considering the effects of the morphology on photovoltaic performance, atomic force microscope (AFM) and transmis-sion electron microscope (TEM) were used to characterize the morphological features of the OSCs (Figure 5). As deter-mined by AFM, the NDT-3T: ITIC-4Cl and NDF-3T: ITIC-4Cl blend surfaces have root-mean-square (rms) roughness values of 2.23 and 2.91 nm, respectively. With adding DIO as addi-tive, both RMS values of blend films were reduced (RMS = 1.05 nm for NDT-3T blend and RMS = 1.21 nm for NDF-3T blend). After the annealing process, both of them strengthened to a coarse degree from their RMS values. In the meantime,

Table 2. Summary of device parameters of polymers: ITIC-4Cl-based devices based on optimized conditions under the illumination of AM 1.5 G, 100 mW cm−2.

Polymer: ITIC-4Cl Treatment Voc [v] Jsc [mA cm–2] FF [%] PCEmax [PCEavga)] Jsc

b) [mA cm−2]

as cast 0.94 15.68 65.0 9.58 (9.28) 15.38

NDT-3T 0.5% 0.91 15.97 70.6 10.26 (9.79) 15.76

90 °C 0.90 16.19 74.5 10.86 (10.56) 15.95

as cast 0.94 18.96 72.3 12.89 (12.45) 18.49

NDF-3T 0.5% 0.92 19.61 73.6 13.28 (12.77) 19.06

90 °C 0.91 20.80 75.1 14.21 (13.98) 20.38

a)All average values were calculated from ten devices; b)Calculated Jsc values.

Figure 4. a) J–V characteristics of NDT-3T:ITIC-4Cl, and NDF-3T:ITIC-4Cl based solar cells with optimized conditions under the illumination of AM1.5G, 100 mW cm−2. b) The corresponding IPCE spectra of NDT-3T:ITIC-4Cl and NDF-3T:ITIC-4Cl based devices. c) Photocurrent density (Jph) versus light intensity characteristics of PSCs based on NDT-3T:ITIC-4Cl and NDF-3T: ITIC-4Cl based devices. d) Photocurrent density (Jph) as a function of effec-tive voltage (Veff) characteristics of these two optimized devices. e) PCE values based on NDF-containing polymer donors reported during the years 2012–2020.



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NDF-3T blend showed stronger crystalline property than that of NDT-3T blend, which means, NDF-3T blend would facili-tate effective charge transfer and alleviative exciton recombina-tion.[49] Meanwhile, TEM micrographs of the polymer:ITIC-4Cl blends are shown in Figure 5d,h. The NDF-3T blend showed the fiber-like morphology, which could promote charge separa-tion and transport, beneficial to BHJ OSCs. On the contrary, owing to the relatively large degree of aggregation disorder for NDT-3T, the polymer crystalline domains were easily destroyed and penetrated by the acceptor. As a result, excessive miscibility between donor and acceptor in NDT-3T blend increased exciton recombination, resulting in lower photovoltaic performance (Figure 6).

GIWAXS and resonant soft X-ray scattering (RSoXS) were used to assess the differences in photovoltaic performance of these two polymer-based blend films, and Figure 7a,b shows the images of GIWAXS measurements and the corresponding line cuts of these blend films. ITIC-4Cl as a fullerene-free

acceptor, was reported to have a (100) lamellar peak located at q = 0.37 Å–1 and an obvious π–π stacking diffraction peak at q ≈ 1.79 Å–1 with relative strong crystallinity in previous research. For the blends, no (100) peak of ITIC-4Cl was observed in the NDT-3T and NDF-3T: ITIC-4Cl blend films, demonstrating better miscibility between donor and acceptor. Although the two copolymers display similar face-on molecular packing ori-entation, NDF-3T blends exhibit more tight molecular packing features in the OOP direction in comparison with the NDT-3T-based blend film (3.63 Å), supported by the smaller π–π stacking distance of 3.53 Å. As a result, the NDF-3T-based blend film has better charge transport capability in the vertical direction of substrate than NDT-3T blend films, consistent with the better photovoltaic results. RSoXS results were shown in Figure 7d, that NDT-3T blends show a distinct peak at ≈0.00745 Å–1, corre-sponding to a center-to-center distance of 84.3 nm. The NDF-3T blends present relatively different characteristics, that a broad shoulder could be observed at similar position, but with much

Figure 5. AFM height images (2 × 2 µm) of a) NDT-3T:ITIC-4Cl, b) with 0.5% DIO, c) with 0.5% DIO and annealing at 90 °C, e) NDF-3T:ITIC-4Cl, f) with 0.5% DIO, g) with 0.5% DIO and annealing at 90 °C. TEM images d) NDT-3T: ITIC-4Cl blend, and h) NDF-3T: ITIC-4Cl blend with 0.5% DIO and annealing at 90 °C.

Figure 6. Schematic diagram of the polymers and corresponding active layer microstructures.



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weaker intensity, indicating better miscibility between NDF-3T and ITIC-4Cl.

To evaulate the photo-degradation properties of NDT-3T and NDF-3T based photovoltaic devices upon light exposure, the stability measurements were carried out under continuous 1 sun illumination in nitrogen atmosphere and presented in Figure S8, Supporting Information. It was found that both NDT-3T and NDF-3T based devices showed comparable stable Voc in Figure S8, Supporting Information. Moreover, NDF-3T based device exhibited slightly higher FF than NDT-3T based device. Notablely, the Jsc of NDT-3T device showed more decayed character than that of NDF-3T device. In other words, in comparison with the device of NDT-3T blend, that of NDF-3T presented better photostability under continuous illumination. That implies that the chalcogen replacement not only affects the crystalline and its photovoltaic device, but also leads to dif-ferent long-term photostability.

3. Conclusion

In summary, to fulfill an effective enhancement in PCE for furan derivatives, a novel NDF-based wide-band-gap polymer, NDF-3T, was designed and applied in OSCs. As a comparison, the NDT-based analogue, NDT-3T, was also prepared. By replacing S atom by O atom, the backbone conformation of NDF-3T shows more straight linear char-acter and more ordered inter-chain packing than that of NDT-3T from theory simulations to experimental results.

On the other hand, the ordered inter-chain packing for NDF-3T can prevent extreme permeation of acceptor, con-tributing to decreased exciton recombination and increased charge transfer. Therefore, the photovoltaic device based on NDF-3T obviously outperformed NDT-3T based counterpart. The champion device based on NDF-3T blend shows a PCE of 14.21% with high FF of 75.1%, which is the highest PCE reported for NDF-based single-junction OSCs and it is also the best PCE record for furan-containing polymers to the best of our knowledge. It is worth mentioning that econo-mization of side chains on donor units can satisfy both in low-cost and high-performance. The comprehensive high photovoltaic performance of NDF-3T suggests that NDF is a promising building block in organic electronic materials, and the method of linear conformational modulation is proved to have great potential in enhancing photovoltaic performance for furan-based materials.

4. Experimental SectionSynthesis and Characterization: The detailed synthesis, procedures,

and characterization of all compounds are given in the Supporting Information.

General Procedure for the Stanyllation of Compounds (6) and (10): (6) 2,7-Bis(trimethylstannyl)naphtho[2,3-b:6,7-b′]difuran: Naphtho[2,3-b:6,7-b′]difuran (416.42 mg, 2.00 mmol) was dissolved in dry THF (100 mL)at −78 °C, and n-butyllithium (2.4 m solution in hexane, 2.92 mL, 7.00 mmol) was added to the sample dropwise. The mixture was stirred under this temperature for 30 min, then restored up to

Figure 7. a) 2D GIWAXS images of NDT-3T: ITIC-4Cl blend and b) NDF-3T: ITIC-4Cl blend. c) the corresponding IP and OOP line cuts and d) I–q RSoXS profiles of the blend films.



© 2021 Wiley-VCH GmbH2003954 (9 of 10)

room temperature for 1 h. Me3SnCl (1 m solution in hexane, 8 mL, 8 mmol) was added dropwise after cooling down to −78 °C and continously stirred for a 30min. Subsequently, the mixture was stirred overnight by warm up to room temperature. Water (100 mL) was added into the mixture to quench reaction, and extracted twice with dichloromethane. The crude product was purified by recrystallization with ethanol.

1H NMR (400 MHz, CDCl3) δ 7.97 (s, 2 H), 7.88 (s, 2 H), 6.92 (t, J = 4.0 Hz, 2 H), 0.29 (m, 18 H). 13C NMR (CDCl3, 25 °C): δ 168.4, 156.1, 128.9, 128.5, 117.2, 117.0, 105.8. HRMS: 533.9864.

(10) 5,5′-dibromo-3,3“-Carboxylic acid-2-octyldodecyl ester-2,2′:5′,2”-terthiophene: The solution of 3,3“-carboxylic acid-2-octyldodecyl ester-2,2′:5′,2”-terthiophene (3.590 g, 4.0 mmol) and N-bromosuccinimide (1.6 g, 8.9 mmol) was added in chloroform (25 mL) and stirred at 0 °C, then allowed to undergo reaction at room temperature for 28 h. The NaOH aqueous solution and 100 mL of water were added for diacidification after the reactant was cooled at 0 °C. The mixture was extracted with chloroform, then organic layer was subsequently dried (MgSO4) and concentrated in vacuo. The crude product was purified by recrystalization to separate out yellow solid.

1H NMR (400 MHz, CDCl3) δ 7.42 (s, 2 H), 7.34 (s, 2 H), 4.13 (d, J = 7.6 Hz, 4 H), 1.67 (s, 2H), 1.24 (s, 64 H) 0.87 (t, J = 8.0 Hz, 12 H). 13C NMR (CDCl3, 25 °C): δ 162.0, 143.5, 135.3, 132.8, 129.5, 128.6, 111.1, 68.0, 37.3, 32.0, 31.3, 29.7, 29.6, 29.5, 29.4, 29.3, 26.7, 22.7, 14.1.

General Procedure for the Polymerization: Compound (6) (0.15 mmol) and (10) (0.15 mmol) were dissolved into 9 mL of toluene in a flask under nitrogen, respectively. The mixture was flushed with nitrogen for 10 min, and 15 mg of Pd(PPh3)4 was added into the flask. The solution was flushed with nitrogen for an additional 25 min. Then, the reaction mixture was stirred for 12 h at 110 °C. The polymer was obtained when the cooled solution was precipitated in 100 mL of methanol. Subsequently the polymer was collected by filtration and purified by washing, extracted on a Soxhlet extractor with methanol and hexane in succession. The final product was obtained by precipitating the chloroform solution in methanol.

Instrumentation: 1H and 13C NMR spectra were recorded on a Bruker AVANCE 300 MHz spectrometer using CDCl3 as the solvent. The molecular weight of polymers was determined by gel permeation chromatography (GPC) relative to polystyrene standards with chloroform as the eluent. Thermal gravimetric analysis (TGA) was performed on a Perkin-Elmer Pyris 1 thermogravimetric analyzer. UV–vis absorption measurements were carried out on a Hitachi (model U-3010) UV–vis spectrophotometer. Cyclic voltammetric (CV) measurements were carried out in a conventional three-electrode cell using a platinum plate as the working electrode, a platinum wire as the counter electrode, and an Ag/Ag+ electrode as the reference electrode on a Zahner IM6e Electrochemical workstation in a tetrabutylammonium hexafluorophosphate (Bu4NPF6) (0.1 m) acetonitrile solution at a scan rate of 20 mV s−1.

Device Fabrication and Characterization: An inverted architecture was fabricated with ITO/ZnO/active layer/MoO3/Ag. The ITO-coated glass substrates were sequentially ultrasonicated in soap water, deionized water, acetone, and isopropyl alcohol for at least 15 min, and ultimately dried in an oven overnight. The ITO-coated glass substrates were treated by UV-ozone for 10 min. The ZnO precursor (zinc acetate dihydrate) was spin-coated on top of the pre-cleaned ITO (4000 rpm, 20 s). After coating, ZnO films were annealed at 200 °C for 30 min. The active layers were deposited in a glove box by spin coating. The solution of active layer was chloroform solution containing 8mg mL−1 of the donor, 8 mg mL−1 of the acceptor and 0.5% DIO as additive. The mixed solution was spin cast at 2000 rpm for 40s atop ZnO layer to form the active layer. The optimized thickness of active layer was ≈110 nm. Then, the active layers were treated by thermal annealing at 90 °C for 5min under nitrogen conditions. Then the thin films were transferred into a vacuum evaporator connected to the glove box. MoO3 (5nm) and Ag (100 nm) were deposited sequentially by thermal evaporation under 10–5 Pa. The active area of the devices was 4.50mm2. Current density-voltage (J–V) characteristics were measured by a Keithley 2400 Source Measure Unit,

in N2 atmosphere under an AM 1.5G solar simulator with an irradiation light intensity of 100mw·cm–2. The external quantum efficiency (EQE) of the devices was measured by using a QEX10 solar cell EQE measurement system (PV measurements.Inc.). The light intensity at each wavelength was calibrated with a standard single-crystal Si photovoltaic cell.

Atomic Force Microscopy (AFM) Characterization: AFM images were investigated on a Dimension Icon AFM (Bruker) in a tapping mode.

Transmission Electron Microscopy (TEM) Characterization: TEM images were performed on a JEOL JEM-1400 transmission electron microscope. TEM samples were prepared as follows: First, the active layer was spin cast on the top of ITO/PEDOT:PSS substrates; Then, the active layer film was peeled off and floated onto the surface of deionized water; Finally, the floated films were picked up on a carbon film 200 mesh copper grid for TEM measurements.

Grazing Incidence Wide-Angle X-Ray Scattering (GIWAXS) Characterization: GIWAXS measurements were performed at beamline 7.3.3 at the Advanced Light Source. Samples were prepared on Si substrates using identical blend solutions as those used in devices. The 10 keV X-ray beam was incident at a grazing angle of 0.12°–0.16°, selected to maximize the scattering intensity from the samples. The scattered X-rays were detected using a Dectris Pilatus 2 m photon counting detector.

Resonant Soft X-Ray Scattering (RSoXS) Characterization: RSoXS transmission measurements were performed at beamline 11.0.1.2 at the Advanced Light Source (ALS). Samples for R-SoXS measurements were prepared on a PEDOT:PSS modified Si substrate under the same conditions as those used for device fabrication, and then transferred by floating in water to a 1.5 mm × 1.5 mm, 100 nm thick Si3N4 membrane supported by a 5 mm × 5 mm, 200 µm thick Si frame (Norcada Inc.). 2D scattering patterns were collected on an in-vacuum CCD camera (Princeton Instrument PI-MTE). The sample detector distance was calibrated from diffraction peaks of a triblock copolymer poly(isoprene-b-styrene-b-2-vinyl pyridine), which has a known spacing of 391 Å. The beam size at the sample was approximately 100 µm by 200 µm.

Space-Charge-Limited Current (SCLC): The current density–voltage (J–V) characteristics of the hole or electron only devices were fitted by the Mott–Gurney law:

J V Lrε ε µ( )( )= 9/8 /02 3 (1)

where J is the current density, εr is the dielectric permittivity of the active layer, ε0 is the vacuum permittivity, L is the thickness of the active layer, μ is the mobility. V = Vapp−Vbi, where Vapp is the applied voltage, Vbi is the offset voltage (Vbi is 0 V here). The mobility can be calculated from the slope of the J0.5 ≈ V curves.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis work was financially supported by the National Natural Science Foundation of China (NSFC) (No. 21774003), Key Laboratory of Bio-inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing Municipal Science & Technology Commission (No. Z181100004418012, 2212032) and funded by Beihang University Youth Talent Support Program (YWF-18-BJ-J-218).

Conflict of InterestThe authors declare no conflict of interest.


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© 2021 Wiley-VCH GmbH2003954 (10 of 10)

Data Availability StatementThe data that supports the findings of this study are available in the supplementary material of this article.

Keywordscrystallinity, molecular conformation, naphthodifuran, non-fullerene, polymer solar cell

Received: December 25, 2020Revised: February 6, 2021

Published online:

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