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Highly Conjugated Three-Dimensional Covalent Organic Frameworks Based on Spirobifluorene for Perovskite Solar Cell Enhancement Chenyu Wu, Yamei Liu, Hui Liu, Chenghao Duan, Qingyan Pan, Jian Zhu, Fan Hu, Xiaoyu Ma, Tonggang Jiu, Zhibo Li, and Yingjie Zhao J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06291 • Publication Date (Web): 14 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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Journal of the American Chemical Society

Chenyu Wu,a+ Yamei Liu,a+ Hui Liu,a Chenghao Duan,b Qingyan Pan,a Jian Zhu,c Fan Hu,a Xiaoyu Ma,a Tonggang Jiu,b* Zhibo Lia* and Yingjie Zhaoa* Key Laboratory of Biobased Polymer Materials, Shandong Provincial Education Department; School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China b Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China c Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China a

ABSTRACT: Highly conjugated three-dimensional covalent organic frameworks (3D COFs) were constructed based on spirobifluorene cores linked via imine bonds (SP-3D-COFs) with novel interlacing conjugation systems. The crystalline structures were confirmed by powder X-ray diffraction and detailed structural simulation. A 6- or 7-fold interpenetration was formed depending on structure of linking units. The obtained SP-3D-COFs showed permanent porosity and high thermal stability. In application for solar cells, simple bulk doping of SP-3D-COFs to the perovskite solar cells (PSCs) substantially improved the average power conversion efficiency (PCE) by 15.9% for SP-3D-COF 1 and 18.0% for SP-3D-COF 2 as compared to the reference undoped PSC, while offering excellent leakage prevention in the meantime. By aid of both experimental and computational studies, a possible photo-responsive perovskite-SP-3D-COFs interaction mechanism was proposed to explain the improvement of PSC performance after SP-3D-COFs doping.

Covalent organic frameworks (COFs), firstly reported by Yaghi and co-workers, not only extended the concept of “molecules”, but also exhibited extensive promising applications.1,2 Over the past decade, COFs have become star materials in many fields including gas storage and separation,37 heterogeneous catalysis,8-14 semiconducting device,15-19 sensor,20-22 energy storage23-26 and drug delivery.27 While most of the reported COFs adopt two-dimensional (2D) layered structures, 3D COFs are still less reported. Typically, electroactive COFs were limited to conjugated and layered two-dimensional (2D) systems, built from a great diversity of macro-aromatic molecules.28 Rigid 3D COFs were generally excluded from electroactive COFs due to “unavoidable” presence of unconjugated tetrahedral building blocks, which prevented formation of conjugated 3D COFs.29-33 Hence, constructing conjugated 3D COFs linked by rigid covalent bond is still of great challenge. As rigid and highly stable conjugated 3D structures could provide more permanent redox-active sites and constant photoelectric properties,34 they would be promising candidates for photocatalytic and photoelectric applications. To tackle the synthetic challenge of making conjugated 3D COFs, we dedicated to designing a new strategy by using the bi-planar conjugated tetrahedral spirobifluorene as the core-linking unit. As a tetragonal-disphenoid shaped molecule,35,36 spirobifluorene can not only serve as tetrahedral nodes for 3D structures, but also have shown great potentials for optoelectronic devices due to its unique conjugated structure.37-41 In addition,

the orthogonal configuration of two planar intermolecular fluorene units bears strong rigidity and can increase the stability of resulting 3D COFs. Herein, we report the first successful construction of highly conjugated 3D COFs with spirobifluorene as the core building unit and imine bonds as conjugated linkages. Two examples of the 3D COFs with high crystallinity were obtained. Detailed structural simulation combined with powder X-ray diffraction (PXRD) measurements provided an indepth view of the structural information. Interestingly, by simple bulk doping of SP-3D-COFs into the perovskite layers of PSCs, we observed power conversion efficiency (PCE) remarkably improved by 15.9% for SP-3D-COF 1 and 18.0% for SP-3D-COF 2.

Synthesis and structural characterization The building block A (3,3',6,6'-tetraamine-9,9'-spirobifluorene) was synthesized according to the literature.36 The synthesis of SP-3D-COFs through the imine condensation reaction of A, D (1,4-phthalaldehyde) or A, G (4,4'-biphenyldicarbaldehyde) was performed in a mixture of 1,2dichlorobenzene/n-butanol/acetic acid at 130 °C for 3 days, giving micro-granular yellow powder SP-3D-COF 1 and SP3D-COF 2, respectively (Scheme 1). Model compounds C and F were also prepared by the same reaction (Figure S14) except that B (benzaldehyde) or E (4-phenylbenzaldehyde) was used. By comparison, SP-3D-COF 1 showed very similar characteristic peaks in 13C CP/MAS spectrum with C

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Scheme 1. (A) Synthesis of SP-3D-COF 1 and the corresponding model compound C; (B) synthesis of SP-3D-COF 2 and the corresponding model compound F.

(Figure S5, bottom). The carbon peak of C=N bond appeared at 158 ppm and proved the existence of imine bonds. Similar results were also observed for SP-3D-COF 2 and F (Figure S6, bottom). In addition, FT-IR spectra of the SP-3DCOFs confirmed the formation of the imine linkage with the characteristic C=N stretching band at 1625 cm-1 (Figure S7). Moreover, the SP-3D-COFs showed excellent thermal stability up to 450 °C according to the thermogravimetric analysis (TGA) (Figure S8). Scanning electron microscopy (SEM) revealed that both SP-3D-COFs exhibited uniform granularshaped aggregation morphology (Figure S9). In order to elucidate the crystalline structure and unit cell parameters of SP-3D-COFs, we performed PXRD analysis (Figure 1). Considering that SP-3D-COF 1 and SP-3D-COF 2 are constructed with the tetragonal-disphenoid shaped building block A (D2d dihedral symmetry) and the linear building block D or G, we assumed that such a combination should yield a lattice with diamond (dia) topology, similar to the reported COF-300.42 Predicted crystal structures were established by Materials Studio 7.0 Software with the calculated PXRD patterns obtained from the Reflex module. As dia nets are often interpenetrated,29,33,42,43 we considered possible structures of SP-3D-COFs with different degrees of interpenetration, adopting either normal (Figure S10A and S10C) or reversed (Figure S10B and S10D) C=N orientations. Structures with degrees of interpenetration higher than 6 for SP-3D-COF 1 or 7 for SP-3D-COF 2 were found to cause structural distortion and thus excluded (Table S1-2). After detailed comparison of PXRD profiles calculated from each possible structure with the experimental profiles (Figure S11-12), minor corrections to unit cell parameters were performed with best candidates for precise characterization. Eventually, a 6-fold-interpenetrated diamond (dia-c6) net with normal C=N bond orientations was identified for SP-3D-COF 1 (Figure 2A) while a dia-c7 net with normal C=N orientations was identified for SP-3D-COF 2 (Figure 2B). In view of the fact that the conjugated segments of SP-3D-COFs are proposed to align in the same

family of crystal planes shown as Figure 2 (i.e. {110} for the P42/NNM space group or {100} for the I41/AMD space group, depending on interpenetration), the {110} planes (P42/NNM) or {100} planes (I41/AMD) should exhibit predominant signals because they have much higher atom density than all other planes. Indeed, this feature was well reproduced in the experimental PXRD profiles (Figure 1), evidencing the presence of coplanar conjugated segments in SP-3D-COFs. As observed in the experimental PXRD profile (Figure 1A, black), SP-3D-COF 1 exhibited intense peaks at 5.4°and 10.8°corresponding to the {110} and {220} Bragg planes of the space group P42/NNM (Table S3), respectively. By zooming into the range after the primary {110} peak, we obtained more details of the PXRD spectra and referred the corresponding structural information along the c-axis (denoted in Figure 1A, inset). For SP-3D-COF 2, {200} and {400} planes of the space group I41/AMD (Table S4) were identified at 4.5°and 8.9°(Figure 1B, black). The calculated PXRD patterns (Figure 1, green) matched excellently well with the experimental results. Furthermore, Pawley refinement yielded PXRD profiles (Figure 1, red) with negligible difference compared with the experimental patterns. At the same time, a unit cell for SP-3D-COF 1 was obtained from Pawley refinement with parameters of a = b = 23.2 Å, c = 8.9 Å and α = β = γ = 90° (residuals Rp = 8.29%, Rwp = 12.06%), very close to the predicted crystalline structure (a = b = 22.6 Å, c = 8.7 Å and α = β = γ = 90°); whereas the unit cell obtained for SP-3D-COF 2 possesses parameters of a = 39.9 Å, b = 39.8 Å, c = 6.4 Å and α = β = γ = 90° (residuals Rp = 11.09%, Rwp = 16.15%), also very close to the predicted crystalline structure (a = b = 39.6 Å, c = 6.4 Å and α = β = γ = 90°). It should be noted that because dia-c6 bears P42/NNM symmetry whereas dia-c7 bears I41/AMD, the {100} planes of SP-3D-COF 2 in principle correspond to the (110) planes of SP-3D-COF 1. Hence, a=b of the SP-3D-COF 2 unit cell needs to be divided by 21/2 when comparing to that of the SP-3D-COF 1 unit cell (Figure S13).


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Figure 1. Indexed experimental (black), Pawley refined (red) PXRD patterns with their difference (blue) and the calculated pattern (green) from (A) dia-c6 net of SP-3D-COF 1 and (B) dia-c7 net of SP-3D-COF 2 with normal C=N bond orientations. Inset: zoomed view of detailed PXRD profile without the primary peak.

Figure 2. Structural representation of conjugated 3D COFs (A) SP-3D-COF 1 (dia-c6) and (B) SP-3D-COF 2 (dia-c7). Top left: ball-and-stick images; top right: representation of interpenetration; bottom: space-filling model views perpendicular (left) and parallel to 1D channels (right).



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Table 1. Summary of photovoltaic performance of the CH3NH3PbI3 based PSCs with and without doping SP-3DCOF. The average PCE values are each averaged by 16 devices. Treatment

Figure 3. (A-B) N2 adsorption-desorption isotherms (77 K) of (A) SP-3D-COF 1 and (B) SP-3D-COF 2; (C-D) pore size distribution from the quenched solid density functional theory of (C) SP-3D-COF 1 and (D) SP-3D-COF 2.

The permanent porosities of SP-3D-COF 1 and SP-3DCOF 2 were investigated by nitrogen adsorption measurements at 77 K. As shown in Figure 3A-B, both SP-3D-COFs displayed type I isotherm with sharp uptake observed within the low-pressure region (P/P0 < 0.05), which is a typical characteristic of micro-porosity. The Brunauer-Emmett-Teller (BET) surface area of SP-3D-COF 1 was obtained as SBET = 641 m2 g−1 and the total pore volume was calculated at P/P0 = 0.99 to be Vp = 0.45 cm3 g-1. By the application of the non-local density functional theory (NLDFT) model, pore size distribution (PSD) of SP-3D-COF 1 (Figure 3C) was obtained, showing a pore limiting diameter of 1.2 nm, identical to that calculated from the simulated crystal model (1.2 nm, Figure 2A). In contrast, SP-3D-COF 2 with larger pores exhibited a higher BET surface area and total

Figure 4. (A) Device architectures; (B) corresponding dark currents density-voltage (J-V) curves; (C) J-V characteristic curves under AM 1.5G 100 mW cm-2 simulated solar light; (D) EQE spectra of the undoped reference and SP-3D-COFs doped PSC devices. *Avg. PCE: average power conversion efficiency. ETM: electron transporting materials; HTM: hole transporting materials; PC61BM: [6,6]-phenyl-C61 butyric acid methyl ester; ITO: indium-tin-oxide; P3CT-K: poly[3-(4carboxylbutyl)thiophene-K.

Voc

Jsc

FF

PCE max. (avg.)

(V)

(mA/cm2)

(%)

(%)

w/o doping

1.028

22.50

72.1

16.70 (15.83)

w/doping_1

1.026

23.20

79.8

18.95 (18.34)

w/doping_2

1.031

23.60

78.3

19.07 (18.68)

w/o doping: without bulk doping; w/ doping_1: with SP3D-COF 1 bulk doping; w/ doping_2: with SP-3D-COF 2 bulk doping. pore volume, with SBET = 1582 m2 g−1 and Vp = 0.97 cm3 g-1 (at P/P0 = 0.99), respectively. PSD calculated for SP-3D-COF 2 (Figure 3D) showed a major peak at 1.5 nm, again in good agreement with the value calculated from the simulated model (1.6 nm, Figure 2B). Studies on PSC performance with SP-3D-COF bulk doping The reported photo-responsive applications of spirobifluorene and the extended conjugation in SP-3D-COFs have driven us to explore their application for solar energy harvesting. The past decade has seen a surge in the development of PSCs, with certified power conversion efficiency (PCE) rocketing from single digit to 22.1%.44,45 Here, the obtained 3D COFs have uniformly ordered porous frameworks through the orthogonal configuration of bi-planar spirobifluorene units as tetrahedral nodes. More importantly, rigid and long-range conjugated systems (1D conjugated segments) interlaced across the dia nets combined with high degrees of interpenetration could possibly provide numerous electron-transporting channels in the frameworks with highly ordered alignment. Therefore, we considered that SP-3D-COFs could exhibit advanced photoelectric properties and can be employed in solar cell devices. In this regard, we employed the as-prepared COFs as doping materials in photovoltaic devices to explore their potential photoelectric applications. To this end, we chose the inverted pi-n PSC as our model and bulk doped SP-3D-COF 1 or SP3D-COF 2 into the CH3NH3PbI3 layer. Figure 4A illustrates the device structure used in this study: ITO/P3CTK/CH3NH3PbI3 (SP-3D-COF)/PC61BM/ZnO/Al. At first, the dark current density characteristics were fitted for devices with and without doping SP-3D-COF 1 and 2 in Figure 4B. We found that the device doped with COF exhibited excellent diode characteristics with a lower leakage current and a higher rectification ratio than those reference devices without doping. In particular, after doping SP-3D-COF into the CH3NH3PbI3 film, the dark current was drastically reduced, which indicated contribution of doped SP-3D-COF to leakage prevention of the PSC device. To investigate the impact of doping SP-3D-COF on the performance of the PSCs, we obtained the current density-voltage (J-V) characteristics under AM 1.5G irradiation (100 mW cm -2) as depicted in Figure 4C and the detailed parameters as summarized in


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Table 1. The reference device based on pure CH3NH3PbI3 showed a Voc of 1.028 V, a short circuit current (Jsc) of 22.50 mA cm-2, a fill factor (FF) of 72.1% and a resulted maximum PCE of 16.70% (average PCE of 15.83%). After doping SP3D-COFs into the CH3NH3PbI3 perovskite layer, the devices exhibited a maximum PCE of 18.95% and an average PCE of 18.34% for SP-3D-COF 1, and a maximum PCE of 19.07% and an average PCE of 18.68% for SP-3D-COF 2, respectively. Very encouragingly, the average PCE over reference devices was greatly improved by 15.9% after SP-3D-COF 1 doping and by 18.0% after SP-3D-COF 2 doping, respectively. In line with this major enhancement, the device parameters were also significantly improved after doping SP3D-COFs, especially for the Jsc and FF of the devices (Table 1), which suggested that bulk doping of SP-3D-COFs into active layer can improve the film quality of the perovskite layer in the meantime. EQE spectra presented the higher efficiency of photon-electron conversion, which is in good agreement with the data measured by the J-V curves (Figure 4D). PCE distributions of the devices shown in Figure 5 demonstrated much enhanced performance for those with doping in the CH3NH3PbI3 layer, compared with reference devices without SP-3D-COFs. It is clear to observe a significant up-shift trend for the values of Jsc, FF and PCE from undoped device to doping SP-3D-COFs based device. At the same time, the data distributions also became narrow when SP-3D-COFs were doped into perovskite film. This means that the devices could not only present better performance but also afford enhanced stability and reproducibility by using hybrid SP-3D-COFs/perovskite as active layers. Mechanism studies of perovskite-SP-3D-COF interactions: from an experimental perspective Encouraged by the dramatic effect of SP-3D-COFs on PSCs as bulk doping materials, we conducted a systematic investigation of the material properties to reveal the mechanism therein. Solid-state UV-Vis spectroscopy measurements on the SP-3D-COFs powder as well as the model compounds (C and F) were performed. Strong absorption in the range

Figure 5. The device parameter distribution maps (A) Voc, (B) Jsc, (C) FF and (D) PCE of the undoped reference devices (black), the SP-3D-COF 1 doped devices (blue) and the SP3D-COF 2 doped devices (red).

200-500 nm was observed for both SP-3D-COFs (Figure 6A). As expected, both SP-3D-COFs displayed 66 nm redshift in the maximum absorption wavelength (λmax) as compared to model compound C (for SP-3D-COF 1, blue line, Figure 6B) and model compound F (for SP-3D-COF 2, red line, Figure 6B), which clearly verified the formation of extended conjugation systems.46 The electron mobility of both SP-3D-COF 1 and SP-3DCOF 2 doped active perovskite layer were then measured by the method of space charge limited current (SCLC) (Figure 6C). Both doped layers exhibited enhanced electron mobility (3.42×10-3 for SP-3D-COF 1 and 4.44×10-3 cm2 V-1 s-1 for SP-3D-COF 2) compared with the reference undoped perovskite layer (2.64×10-3 cm2 V-1 s-1). As the fine SP-3DCOFs powders were evenly dispersed in the perovskite layer in our method, it is proposed that SP-3D-COFs can contribute to crystal structure formation of CH3NH3PbI3. This is not surprising when we took the 3D crystalline structure of SP-3D-COFs into consideration, because they may serve as crystallization template for CH3NH3PbI3 as good heterogenous nucleation sites, leading to better perovskite crystallization and hence increasing electron mobility. In this regard, this template effect will facilitate close contact between SP-3D-COFs and perovskite, where the electrons could easily transport through their boundaries. This is further proved by subsequent UV absorption measurements for the SP-3D-COFs doped perovskite active layers. From the absorption spectra in the range 500-800 nm (Figure 6D), in which range the absorption was solely contributed from CH3NH3PbI3, we clearly observed an obvious increase in absorption between 740-780 nm after SP-3D-COF doping, especially after introduction of SP-3D-COF 2. As reported in literature,47 the promoted band-edge absorption of the perovskite film in the wavelength range of 740-780 nm can be attributed to the enhanced crystallinity of perovskite films.

Figure 6. (A) Solid-state UV-Vis spectra of the SP-3D-COFs; (B) UV-Vis spectra of C and F in solution; (C) the electron mobility of the pure CH3NH3PbI3 layer and SP-3D-COFs doped active perovskite layers; (D) solid-state UV-Vis spectra of the SP-3D-COFs doped perovskite active layers.



Hence, this observation strongly supported the above assumption that the doping SP-3D-COFs can result in close contact with perovskite, and also help crystallization of the latter. Due to the presence of larger biphenyl end-groups protruding from the surface, SP-3D-COF 2 could form even closer contact with perovskite and contribute to better perovskite crystallization, which was supported by the higher electron mobility and stronger absorption in the range of 740-780 nm regime. Indeed, the close physical contacts are prerequisite of possible photo-electric interactions between SP-3D-COFs and CH3NH3PbI3 in the perovskite layer. Furthermore, the EQE spectra exhibited significant increase within 300-400 nm (Figure 4D), which is exactly where SP-3D-COFs strongly absorb light (Figure 6A) and apparently indicates the contribution of SP-3D-COFs in light-harvesting. Considering this significant contribution of doping SP-3D-COFs on EQE within their absorption range 300-400 nm and their close contact with CH3NH3PbI3 in the perovskite layer, it is plausible to propose that photo-responsive interactions between SP-3D-COFs and

CH3NH3PbI3 can have synergistic contribution to PSC enhancement. However, since the model compound C and F bear similar function groups to SP-3D-COFs and also exhibit strong absorption, they should afford photo-responsive interaction with CH3NH3PbI3 in a similar way. Therefore, we subsequently doped the model compound C or F to the perovskite layer of the reference PSC and studied the device parameters before and after doping. Indeed, the introduction of model compound C or F as doping materials led to improved PCE of PSCs relatively by 2-4%, which clearly demonstrated the contribution of photo-responsive interactions between the small molecules and CH3NH3PbI3 on PSC performance, thus validating our hypothesis. However, we posited that the small molecule model compounds C and F do not bear long range structures to aid spatial charge separation. Indeed, the SP-3D-COFs doped PSCs exhibited relative improvement of PCE by 16-18%, much higher than those of the model-compound-doped. Therefore, the extended conjugation feature of SP-3D-COFs must dominantly contribute to the dramatic PCE improvement (vide infra).

Figure 7 (A) Schematic energy level diagram of SP-3D-COFs doped PSCs with energy levels of SP-3D-COFs calculated from plane-wave DFT; (B-C) geometry optimized framework from the plane-wave DFT calculations of (B) SP-3D-COF 1 and (C) SP3D-COF 2, colored by the calculated valence charge distribution from subsequent valence charge analysis, with valence charge 7-0 corresponding to RGB(255,0,0)-RGB(255,255,0); (D) proposed mechanism of photoinduced charge separation.

z

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Mechanism studies of perovskite-SP-3D-COF interactions: from a computational perspective The close boundary contact between SP-3D-COFs and CH3NH3PbI3 drived us to gain an insight into the possible

photo-responsive interactions from an energy level perspective. The frontier orbital energy levels of both SP-3DCOF 1 and SP-3D-COF 2 were obtained from the geometryoptimized framework by employing the plane-wave density

Figure 8 Structure and frontier orbitals of (A) model compound C, (B) abstracted planar 1D model and (C) abstracted module including one intact spirobifluorene for conjugated segment of SP-3D-COF 1; (D) model compound F, (E) abstracted planar 1D model and (F) abstracted module including one intact spirobifluorene for conjugated segment of SP-3D-COF 2.



function theory (DFT, Figure 7B-C). Both SP-3D-COFs were revealed to share a same highest occupied molecular orbital (HOMO) level of -4.7 eV and a lowest unoccupied molecular orbital (LUMO) level of -2.9 eV. Combining reported energy levels of all the other components48,49 used in the PSC of this work, we could propose that the excited electron of SP-3DCOFs can easily transport through CH3NH3PbI3 to Al. whereas the excited electron of CH3NH3PbI3 can make up the electron vacancy in HOMO of SP-3D-COFs left by the previous process and restore SP-3D-COFs (Figure 7A), fulfilling one cycle of photo-responsive perovskite-SP-3DCOFs interaction (Figure 7D). This process requires two criteria: (a) HOMOs and LUMOs of SP-3D-COFs should be fully exposed to the perovskite-SP-3D-COFs interface; (b) the fully exposed HOMO and LUMO of SP-3D-COFs should be spatially well separated to prevent charge recombination. Firstly, We computationally studied model compound C (Figure 8A) and F (Figure 9D) and found photoinduced donor-acceptor (D-A) charge transfer (CT) characters very similar to the photo-responsive D-A type molecules with strong CT characters reported by Miyake et al.50-54 Indeed, spirobifluorene is considered an electron-donor37,55 whereas phenyl/biphenyl groups are electron accepters.53,56 Additionally, we observed a low-lying symmetrybreaking CT state in model compound C (Figure 8A). While the symmetry-breaking CT character is very common in AD-A scaffolds,53 what interested us is that this character indicates the excited electrons could transfer freely through the the central sp3 of the spirobifluorene moiety. Indeed, it has been found a couple of decades ago that photo-excited electrons can be delocalized through 1-3 alkyl carbons.57 The difference between C and F could be explained by the lower-lying π* orbitals of the biphenyl group53 of F, which provides higher electron affinity for the excited electron to be freely delocalized to both planar conjugated halves, in contrast to C(see Figure S14). We then considered these phenomena in the extended structure of SP-3D-COFs. As SP-3D-COFs were synthesized by the Schiff-Base reactions (Scheme 1), the surfaces of SP3D-COFs typically consist of highly electron-donating amino-spirobifluorene moieties and highly electron-withdrawing aldehyde-phenyl/biphenyl moieties as the end groups. Hence, it’s highly likely that HOMOs of SP-3D-COFs are all distributed on the surface amino-spirobifluorene moieties whereas all LUMOs on the surface aldehyde-phenyl/biphenyl moieties. If this is true, HOMOs and LUMOs of SP-3D-COFs should be fully exposed and scattered on surfaces of for perovskite-SP-3D-COFs can be explained, but also that the ability of SP-3D-COFs to prevent charge recombination via spatially separated, excellently meeting the aforementioned two criteria. Finally, to prove the existence of this “end-group effect”, we performed DFT calculations on abstracted model conjugated segments of SP-3D-COF 1 (Figure 8B-C) and SP-3DCOF 2 (Figure 8E-F), with the D-A scaffold rationally simplified to make allowance for modern computing capacity. As expected, the planar model of an 1D conjugated segment terminated with different end groups exhibited a HOMO localized on the D end (amino-fluorene moiety) and a LUMO localized on the A end (aldehyde-phenyl/biphenyl moiety)

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for both SP-3D-COF 1 (Figure 8B) and SP-3D-COF 2 (Figure 8E), reinforcing our hypothesis. Taking the through-spirobifluorene CT into consideration, encouragingly, we observed the HOMO still localized on the D end despite the presence bridging sp3 carbon while the LUMO localized on the far-side A end (Figure 8C for SP-3D-COF 1 and Figure 8F for SP-3D-COF 2). In short, by studying the model segments, we were able to confirm that (a) HOMOs will be localized on the aminospirobifluorene end group on the surface of crystalline SP3D-COFs; (b) the excited electron can transfer through the bridging sp3 carbon to locate a lowest-lying π* of an A end in another conjugated segment; (c) if multiple A ends are available, the LUMO will be localized on the A end farthest from D ends (the further an A end is from D ends, lowerlying π* it would gain, as shown in Figure S15-16). These observed features of SP-3D-COFs exactly match the aforementioned criteria we set for SP-3D-COFs and comprehensively validated our interpretation on the enhanced PSC performance. Furthermore, because of the larger area of biphenyl-group A ends on the surface, SP-3D-COF 2 could form closer and larger contact with CH3NH3PbI3, which would serve as better heterogeneous nucleation template (explained better perovskite film quality after SP-3D-COF 2 doping as compared to SP-3D-COF 1) and also provide more exposed LUMOs for efficient perovskite-SP-3D-COFs interaction (explained slightly better PSC enhancement by doping SP-3D-COF 2 than SP-3D-COF 1). Overall, from a dynamic perspective, when SP-3D-COFs were excited by light, the intrinsic long-range CT characters of Schiff-Base synthesized SP-3D-COFs will generate electrons on the LUMO-localized A ends and pass that electron to LUMO of CH3NH3PbI3 whereas generate holes on the HOMO-localized D ends and accept an excited electron from nearby CH3NH3PbI3; as HOMOs and LUMOs tend to spatially stay far from each other due to the aforementioned “far-side π* selection rule” of LUMO, charge recombination can be avoided and the generated electrons and holes can diffuse to the perovskite regions through the close-contact perovskite-SP-3D-COF interface by following energy level relationships (Figure 7A). By this process, the enhanced power conversion efficiency (PCE) of SP-3D-COFs doped PSCs can be well explained.

In this work, two highly conjugated 3D COFs were prepared using spirobifluorene as the core building unit. This is a successful attempt to explore new building blocks for constructing crystalline structures of 3D COFs. Detailed structural analysis and simulation revealed the interpenetrated 3D nets of the obtained COFs. Both of the 3D COFs show permanent porosity determined from N2 adsorption experiments. Owing to the interlacing long-range conjugated systems bearing spirobifluorene units in the frameworks, SP3D-COFs were demonstrated to be the first 3D COFs applied in perovskite solar cells and exhibited comprehensive device enhancement. With the help of DFT calculation, a possible perovskite-SP-3D-COFs interaction mechanism was proposed and substantially corroborated by experimental supports containing electron transport mobility, absorption,


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morphology and so on. These 3D COFs with novel conjugated structures exhibit vast potential in further improving performance of PSCs as long-term and light-weight bulk doping materials. The Supporting Information is available free of charge on the ACS Publications website. Synthetic procedures, structural simulation, device fabrication, atomic coordinates used in DFT calculations, computational details, SEM, FTIR, NMR, TGA including Figure S1-S16, Table S1-S31 (PDF).

[email protected], [email protected], [email protected]. +These

authors contributed equally.

The authors declare no competing financial interests.

We thank Prof. Xin Zhao from SIOC for the suggestions and guidance. We thank Rongran Liang from SIOC for the help of simulation. The work was supported by the National Natural Science Foundation of China (21604046), the National Young Thousand Talents Program, Shandong Provincial Natural Science Foundation, China (ZR2016XJ004).

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Highly Conjugated Three-Dimensional Covalent ... - ACS Publications

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