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An Organic–Inorganic Hybrid Material Based on Benzo[ghi]perylenetri-imide and Cyclic Titanium-Oxo Cluster for Efficient Perovskite and Organic Solar Cells

2021; Chinese Chemical Society; Volume: 4; Issue: 3 Linguagem: Inglês

10.31635/ccschem.021.202100825

2096-5745

Zhou Zhang, Faming Han, Jie Fang, Chaowei Zhao, Shuai Li, Yonggang Wu, Yuefeng Zhang, Shengyong You, Binghui Wu, Weiwei Li,

Conducting polymers and applications

Open AccessCCS ChemistryCOMMUNICATION1 Mar 2022An Organic–Inorganic Hybrid Material Based on Benzo[ghi]perylenetri-imide and Cyclic Titanium-Oxo Cluster for Efficient Perovskite and Organic Solar Cells Zhou Zhang†, Faming Han†, Jie Fang†, Chaowei Zhao, Shuai Li, Yonggang Wu, Yuefeng Zhang, Shengyong You, Binghui Wu and Weiwei Li Zhou Zhang† Institute of Applied Chemistry, Jiangxi Academy of Sciences, Nanchang 330096 College of Chemistry and Environmental Science, Hebei University, Baoding 071002 , Faming Han† State Key Laboratory for Physical Chemistry of Solid Surfaces, Pen-Tung Sah Institute of Micro-Nano Science and Technology, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 Guangxi Key Laboratory of Low Carbon Energy Materials, College of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Guilin 541004 , Jie Fang† Institute of Applied Chemistry, Jiangxi Academy of Sciences, Nanchang 330096 , Chaowei Zhao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Institute of Applied Chemistry, Jiangxi Academy of Sciences, Nanchang 330096 , Shuai Li Institute of Applied Chemistry, Jiangxi Academy of Sciences, Nanchang 330096 , Yonggang Wu College of Chemistry and Environmental Science, Hebei University, Baoding 071002 , Yuefeng Zhang Institute of Applied Chemistry, Jiangxi Academy of Sciences, Nanchang 330096 , Shengyong You Institute of Applied Chemistry, Jiangxi Academy of Sciences, Nanchang 330096 , Binghui Wu State Key Laboratory for Physical Chemistry of Solid Surfaces, Pen-Tung Sah Institute of Micro-Nano Science and Technology, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 and Weiwei Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Institute of Applied Chemistry, Jiangxi Academy of Sciences, Nanchang 330096 Beijing Advanced Innovation Center for Soft Matter Science and Engineering and State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029 https://doi.org/10.31635/ccschem.021.202100825 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Perovskite and organic solar cells usually require electron-transport interlayers to efficiently transport electrons from the photoactive layer to the metal electrode. In general, pure organic or inorganic materials are applied into the interlayers, but organic–inorganic hybrid materials have been rarely reported for this application. In this work, we report using the first titanium-oxo cluster-based organic–inorganic hybrid as the interlayer material by introducing large π-conjugated benzo[ghi]perylenetriimides as an organic part via a simple ligand-exchange reaction. This new hybrid material showed excellent solubility, well-aligned energy levels, and excellent electron mobilities, enabling its great potential application as an interlayer in solar cells such as perovskite and organic solar cells, providing high power conversion efficiencies of <20% and 16%, respectively. Therefore, we claim that our present work introduces a new class of cluster-based organic–inorganic hybrid interlayer materials that exhibit promising application in organic electronics. Download figure Download PowerPoint Introduction Titanium oxide (TiO2) nanomaterials have been commonly adopted as electron-transport layers (ETLs) in different thin-film solar cells such as dye-sensitized solar cells (DSSCs), perovskite solar cells (PSCs), and organic solar cells (OSCs).1–10 The suitable energy level alignment between TiO2 and light-harvesting semiconductors endows the ability to extract electrons efficiently and block holes with reducing series recombination.11–14 Meanwhile, the application of TiO2 in photovoltaics is still inhibited by low electron mobility and rough surface with a large amount of defects,15–17 which is unfavorable for ohmic contact between TiO2 and organic/inorganic photoactive layers. Consequently, this causes severe electron accumulation and recombination at the interface, resulting in low power conversion efficiencies (PCEs) and evident hysteresis in planar PSCs.18 To smooth the interface with the photoactive layer and reduce the defect in the TiO2 layer, some organic materials, including perylene diimide (PDI) derivative PDI2,19 well-known fused-ring electron acceptor 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC),20 and a conjugated polymer electrolyte,21 were introduced between TiO2 and the active layers, providing improved efficiencies in PSCs and OSCs. However, these organic materials have difficulty forming tight contact and perfect alignment of the energy levels with inorganic TiO2. In this work, we developed a solution-processed organic–inorganic hybrid material that could be compatible with both TiO2 and photoactive layers to act as a universal interfacial layer for PSCs and OSCs. We used Ti-oxo clusters (TOCs) as the inorganic component to construct this hybrid material. As molecular analogue of TiO2, TOCs are of atomically precise structures and have attracted increasing attention due to their potential in constructing organic–inorganic hybrid assemblies and providing models for studying the correlation between structure and function.22–25 To date, TOCs have been mainly applied in photocatalysis, host–guest ions selectivity, supporting single-atom site/metallic nanomaterials, and porous materials.26–38 Although organic chromophore appending TOCs have exhibited efficient electron communication and considerable photocurrent in model DSSCs,39–41 there are only a few reports about their application in high-performing photovoltaics.42,43 One challenge is how to attach organic dyes onto TOCs since the trial-and-error one-pot approach is relatively inefficient for bulky dye ligands protecting TOCs.44 A feasible way is to functionalize as-synthesized clusters such as ligand exchange at surface labile sites.45–47 Although this protocol has been proven to be rather ripe for simple organic ligands,48–50 the large π-conjugated materials with photoelectronic properties attached to TOCs have been rarely reported. In our previous work, we synthesized a series of stable cyclic TOCs (CTOCs) ligated with –OH and –COOH units, in which they exhibited highly exchangeable properties with other alcohols.37 This finding intrigued us to introduce conjugated materials into TOCs via rational molecular design. Herein, we reported the construction of an organic–inorganic hybrid material based on a benzo[ghi]perylenetriimide dye ( BPTI-OH, Supporting Information Scheme S1) and a TOC ( CTOC-3) (Scheme 1) and applied it as an interlayer in planar PSCs and OSCs. BPTI unit as a π-extended PDI exhibits deep frontier energy levels and strong crystallinity, which could be potentially used as electron-transporting material.51 The targeted hybrid material CTOC-3-BPTI (Scheme 1) was simply synthesized via ligand exchange between BPTI-OH and ethylene glycolates in CTOC-3. CTOC-3-BPTI was then applied as an interlayer into PSCs and OSCs, exhibiting enhanced PCEs over 20% and 16%. We also used some advanced techniques to study the physical properties of this hybrid material, revealing its outstanding performance as an interlayer to tune the work function and electron-transporting properties in solar cells. Scheme 1 | Synthetic route of the organic–inorganic hybrid material CTOC-3-BPTI. Download figure Download PowerPoint Results and Discussion The chemical structures of BPTI-OH, CTOC-3-BPTI, and its synthetic route are depicted in Scheme 1. The hierarchical structure of CTOC-3 is simplified into eight Ti4 cycles for clarity. The overall structure of CTOC-3 involving the labile alcohol site is shown in Supporting Information Figure S1. The detailed synthetic procedures are presented in the Supporting Information. In general, the involatile monocoordinated ethylene glycol ligands in CTOC-3 were initially replaced by volatile isopropanol via recrystallization process, yielding CTOC-3- i OPr. Powder X-ray diffraction (XRD) indicated that the crystal structure of CTOC-3 was maintained after ligand exchange with isopropanol ( Supporting Information Figure S2). This product was then mixed with BPTI-OH in CH2Cl2 solution and stirred at room temperature. The reaction process of BPTI-OH and CTOC-3 was monitored by UV–vis absorption spectra. As shown in Figure 1a, when 1 equiv CTOC-3- i OPr was treated with 17 equiv BPTI-OH, the intensity of absorption peaks of BPTI-OH were significantly reduced in 1 h. After 24 h, the onset of the spectra was red-shifted from 486 to 509 nm. Besides, the ratio of absorption intensity between the second peak (∼434 nm) and the third peak (∼465 nm) was also significantly enhanced. This is consistent with the change of absorption spectra of BPTI-OH from solution to film ( Supporting Information Figure S3), indicating the aggregation of BPTI during the reaction process. The absorption intensity was continuously reduced after 48 h, and there was no apparent change after 72 h, indicating that the reaction was finished in 72 h. Column chromatography was used to purify the product as a yellow solid. It exhibited excellent solubility in nonpolar and polar solvents such as n-hexane and CH2Cl2 (Figure 1b), facile for solution-processed photovoltaic devices. Figure 1 | (a) Reaction between CTOC-3 and BPTI-OH monitored by UV–vis spectra. (b) The molar absorption coefficient of BPTI-OH in CH2Cl2, CTOC-3-BPTI in CH2Cl2, and n-hexane. (c) The 1H NMR spectra of BPTI-OH and CTOC-3-BPTI in chloroform-d1. Download figure Download PowerPoint The proton nuclear magnetic resonance (1H NMR) spectra of BPTI-OH and CTOC-3-BPTI are shown in Figure 1c, in which the chemical shifts of 9.1–10.3 ppm were assigned to the aromatic Hs in BPTI-OH upshifted to 8.5–9.6 ppm in CTOC-3-BPTI. The integral ratio between aromatic H atoms of BPTI and those from C–CH2CH2-O of CTOC is 16:1, consistent with the molar ratio of BPTI:CTOC in CTOC-3-BPTI ( Supporting Information Figure S4). This was confirmed further by the thermogravimetric analyses (TGA, Supporting Information Figure S5). We tried to obtain a single-crystal XRD and perform mass spectroscopy data of CTOC-3-BPTI to deduce convincing structural information, but both attempts failed. However, the coexistence of BPTI and CTOC in CTOC-3-BPTI was confirmed by diffusion-ordered spectroscopy (DOSY), where signals of BPTI and CTOC were unified with a diffusion coefficient (D) of 1.51 × 10−10 m2/s (log D = −9.82) ( Supporting Information Figure S6). When added a large volume of methanol-d4 (50 μL) to unload all BPTI, the unique signals of CTOC-3-BPTI dissociated into two individual species with D of 1.23 × 10−10 m2/s (log D = −9.91) and 2.45 × 10−10 m2/s (log D = −9.61), corresponding to CTOC-3 and BPTI-OH, respectively ( Supporting Information Figure S7). These measurements illustrated that an equilibrium reaction existed between BPTI-OH and CTOC-3. When utilized enough BPTI-OH and reaction time, CTOC-3-BPTI was obtained, which also decomposed when added large volumes of alcohol. We then demonstrated the application of CTOC-3-BPTI as an interlayer into PSCs with a device configuration of fluorine-doped tin oxide (FTO)/TiO2/ CTOC-3-BPTI/perovskite/Spiro/Au (Figure 2a). The fabrication procedures are described in the Supporting Information. We used mixed cation Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3, cesium formamidinium methylammonium lead mixed halide [(CsFAMA)PbX3] as the perovskite active layer. We also prepared the devices without an interlayer and with CTOC-3 and BPTI-OH as interlayers for comparison. The current density–voltage (J–V) curves of the best performing PSCs with different interlayers are presented in Figure 2b and Supporting Information Figure S8, with the photovoltaic parameters summarized in Table 1. The PSC based on the perovskite layer directly deposited onto TiO2 without interlayer exhibited a PCE of 16.02%. When CTOC-3 or BPTI-OH was used as the interlayer, open-circuit voltages (Vocs) and short-circuit current densities (Jscs) were slightly enhanced, and fill factors (FFs) were almost unchanged so that the PCEs were slightly increased to 16.82% and 17.21%. Surprisingly, when CTOC-3-BPTI was used as an interlayer, all the photovoltaic parameters were simultaneously improved, boosting the PCE to 20.14%. The enhanced Jscs could also be reflected by the incident photon-to-current conversion efficiencies (IPCEs, Figure 2c), where PSC with the CTOC-3-BPTI interlayer showed a high photoresponse in the range of 300–800 nm. Moreover, hysteresis phenomena were significantly reduced for the CTOC-contained interlayers ( CTOC-3 and CTOC-3-BPTI, Supporting Information Figure S8), unlike the devices without or with BPTI-OH as interlayer. This indicated that the CTOC core is responsible for hysteresis suppression. The histograms of over 30 devices in Figure 2d revealed the high reproducibility of the fabricated PSCs in this study. Figure 2 | (a) The device configuration of planar PSCs. Comparison of (b) best reverse scan J–V curves and (c) IPCE of devices with different interlayers. (d) Histograms of efficiencies in both reverse and forward scans among 30 cells with CTOC-3-BPTI as interlayer. Download figure Download PowerPoint Table 1 | Photovoltaic Parameters of Best PSCs without or with CTOC-3, BPTI-OH, and CTOC-3-BPTI as Interlayers Interlayers Voc (V) Jsc (mA/cm2) FF (%) PCE (%) Without 1.022 21.60 72.57 16.02 CTOC-3 1.070 21.78 72.13 16.82 BPTI-OH 1.035 22.80 72.96 17.21 CTOC-3-BPTI 1.119 23.81 75.58 20.14 Next, we studied the origination of improved Jsc, Voc, and FF in the PSC based on the CTOC-3-BPTI interlayer. First, scanning electron microscopy (SEM, Supporting Information Figures S9 and S10) and X-ray diffraction (XRD, Supporting Information Figure S11) measurement found that the perovskite layers on different interlayers exhibited similar grain size and crystallinity, indicating that the varied performance was due to the interface. We then used Kelvin probe force microscopy (KPFM) to obtain the contact potential difference (CPD), which inherently correlated with the variation of work function (Figures 3a–3c, Supporting Information Figures S12–S15).52–54 Measured by the same tip, the CPD significantly increased from −2.3 mV of bare TiO2 to 86.2 mV of the CTOC-3-BPTI-modified TiO2, corresponding to an elevation of work function by ∼90 mV. For comparison, CPD was enhanced to 30.8 mV with the CTOC-3 interlayer and 9.3 mV with the BPTI-OH interlayer (Figure 3c). These results indicate that both CTOC-3 and BPTI-OH altered the surface work function, partially explaining the enhanced work function by CTOC-3-BPTI. Besides, other factors such as the possible intramolecular charge transfer between CTOC and BPTI and the amorphous nature of CTOC-3-BPTI generated a dense interlayer ( Supporting Information Figure S2), also responsible for its significantly enhanced work function. This directly led to the energy level of the CTOC-3-BPTI interlayer to localize in the middle of those of TiO2 underlayer (−4.10 eV) and (CsFAMA)PbX3 layer (−3.93 eV), promoting efficient electron transport between the perovskite and TiO2 ( Supporting Information Figure S16). Besides, the clear hysteresis suppression by the CTOC-containing interlayers might correlate with the trap passivation, indicating an optimized interface between TiO2 and (CsFAMA)PbX3 layer. Figure 3 | (a and b) Two-dimensional topography spatial maps of bare TiO2 and CTOC-3-BPTI/TiO2. (c) CPD of TiO2 without and with different interlayers. (d) TRPL of perovskite films on different substrates. Download figure Download PowerPoint The steady-state and time-resolved photoluminescence (TRPL) were utilized to evaluate the charge-transfer kinetics across the TiO2/interlayer/perovskite interfaces. The (CsFAMA)PbX3 film showed a primary PL peak at 760 nm. The intensity was significantly reduced in the presence of ETL, especially with the CTOC-3-BPTI interlayer, as shown in Supporting Information Figure S17. This could be associated with effective electron transfer from the perovskite to ETL. From TRPL measurement, the average lifetime of perovskite cast on FTO was 59.9 ns, while this value was reduced to 37.2 ns on FTO/TiO2 and further reduced to 16.7 ns on FTO/TiO2/ CTOC-3-BPTI (Figure 3d). These results supported a significant enabling of charge transfer by the CTOC-3-BPTI interlayer. Additionally, the charge-carrier transport and recombination properties of PSCs were characterized using electrochemical impedance spectroscopy (EIS). The Nyquist plots of devices without and with CTOC-3-BPTI interlayer subjected to one sun illumination and no bias voltages applied during testing are shown in Supporting Information Figure S18. The introduction of the CTOC-3-BPTI interlayer resulted in a significant increase of the observed arcs in the intermediate frequency region between 1 and 1000 kHz. Since the perovskite/hole-selective layer interfaces were identical in both cases, the higher frequency of device with CTOC-3-BPTI interlayer belongs to more efficient charge transport through an interface, as the lower frequency was related to the charge recombination. All these factors synergistically contributed to the enhanced PCEs in PSCs. We further applied CTOC-3-BPTI as an interlayer into OSCs with the device configuration of ITO/poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT-PSS)/poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione)] (PM6):2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6)/ CTOC-3-BPTI/Ag. A well-known organic electrolyte 2,9-bis[3-(dimethyloxidoamino)propyl]anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10(2H,9H)-tetrone ( PDINO) was also used as an interlayer for comparison.55 The chemical structures of these materials and the OSC configuration are presented in Figures 4a and 4b. We obtained a PCE of 15.43% when using CTOC-3-BPTI as interlayer, which was slightly lower than that by using PDINO as interlayer (15.84%), as shown in Figure 4c and Table 2. This was mainly due to the reduced FFs, although the Jsc was slightly enhanced, as confirmed by the external quantum efficiencies (EQE, Figure 4d). Surprisingly, when CTOC-3-BPTI/ PDINO double interlayer was applied, the PCE was enhanced to 16.71% with a high Jsc of 27.22 mA/cm2, enhanced Voc of 0.845 V, and FF of 72.66% (Table 2). Figure 4 | (a) Chemical structures of PM6, Y6, and PDINO. (b) Device architecture of the OSC. (c) J–V curves and (d) EQE of best OSCs with single PDINO, CTOC-3-BPTI, and double CTOC-3-BPTI/PDINO ETL. EH, 2-ethylhexyl. Download figure Download PowerPoint Table 2 | Photovoltaic Parameters of OSCs with CTOC-3-BPTI and PDINO Single ETL, and CTOC-3-BPTI/PDINO Double ETL, Respectively Interlayers Voc (V) Jsc (mA/cm2) Calculated Jsc (mA/cm2) FF (%) PCE (%) μe (cm2/V S) PDINO 0.823 (0.831) 25.75 (25.57) 24.49 74.76 (73.57) 15.84 (15.64) 0.00317 CTOC-3-BPTI 0.820 (0.821) 26.20 (25.98) 25.20 71.79 (71.68) 15.43 (15.30) 0.0212 CTOC-3-BPTI/ PDINO 0.845 (0.837) 27.22 (26.64) 25.93 72.66 (73.75) 16.71 (16.47) 0.0894 Values in the brackets are the average of six independent devices. The electron transport across different interlayers was investigated by fabricating electron-only devices with an architecture of ITO/ZnO/Y6/ETL/Ag. As shown in Supporting Information Figure S19 and Table 2, the electron mobility of the device with PDINO single interlayer was 0.00317 cm2/V S, which was increased to 0.0212 and 0.0894 cm2/V S for devices with CTOC-3-BPTI and CTOC-3-BPTI/ PDINO interlayers, respectively. This remarkable enhancement of electron mobilities indicated that the new interlayer was able to improve the electron transport from the photoactive layers to the electrode.56 Conclusion For the first time, we have developed an organic–inorganic hybrid material CTOC-3-BPTI as an interlayer for application in PSCs and OSCs, in which high PCEs of over 20% and 16% could be obtained. The enhanced performance was attributed to work function tuning of the interface and the improved electron mobilities, resulting in efficient electron injection from the photoactive layer into the electrode. Also, we intend to emphasize that combining organic and inorganic materials in a single molecule could simultaneously enhance the adhesion ability between an inorganic electrode and an organic layer, possibly responsible for the improved PCEs. Therefore, we envision that the organic–inorganic hybrid materials based on oxo-clusters would open a new avenue for a class of materials for application in organic electronics. Supporting Information Supporting Information is available and includes general procedure and characterization of materials and solar cells. Conflict of Interest There is no conflict of interest to report. Funding Information This study is jointly supported by MOST (nos. 2018YFA0208504 and 2017YFA0204702) and NSFC (51773207, 52073016, 5197030531, and 21801213) of China. The Fundamental Research Funds for the Central Universities further supported this work (no. XK1802-2), Open Project of State Key Laboratory of Supramolecular Structure and Materials (no. sklssm202043), and Jiangxi Provincial Department of Science and Technology (nos. 20192ACB20009, 20192BBEL50026, 20202ACBL213004, and 20203BBE53062). 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Google Scholar Previous articleNext article FiguresReferencesRelatedDetails Issue AssignmentVolume 4Issue 3Page: 880-888Supporting Information Copyright & Permissions© 2021 Chinese Chemical SocietyKeywordsperovskite solar cellsorganic–inorganic hybrid materialperylene diimideorganic solar cellsTi-oxo clustersAcknowledgmentsThe authors thank Prof. Nanfeng Zheng from Xiamen University for constructive discussion and support. Downloaded 1,739 times PDF DownloadLoading ...