Covalent Organic Framework as a Precursor Additive Toward Efficient and Stable Perovskite Solar Cells

In recent years, there has been significant progress in the use of organic–inorganic hybrid perovskites for photovoltaic applications. Engineering the compositions and/or morphology of perovskites has become the most effective method to address the challenges related to photovoltaic efficiency and stability. Herein, an amino‐functionalized covalent organic framework (NH2‐COF) as a precursor additive to modulate the crystallization of perovskites is incorporated. The NH2‐COF is found to decrease the defect concentration, reduce the nonradiative recombination within the perovskite layer, and further promote carrier transport. Correspondingly, the solar cells based on the NH2‐COF‐modified perovskites deliver a champion power conversion efficiency of 22.13% with a fill factor of 0.773 under AM 1.5G 100 mW cm−2 illumination. Furthermore, the device retains approximately 81% of its initial efficiency after 1000 h of aging under ambient conditions at a temperature of 30 °C and relative humidity ranging from 45% to 55%. It is believed that this work would provide a facile and efficient strategy to prepare high‐quality perovskite films for efficient and stable solar cells.

In recent years, there has been significant progress in the use of organicinorganic hybrid perovskites for photovoltaic applications.Engineering the compositions and/or morphology of perovskites has become the most effective method to address the challenges related to photovoltaic efficiency and stability.Herein, an amino-functionalized covalent organic framework (NH 2 -COF) as a precursor additive to modulate the crystallization of perovskites is incorporated.The NH 2 -COF is found to decrease the defect concentration, reduce the nonradiative recombination within the perovskite layer, and further promote carrier transport.Correspondingly, the solar cells based on the NH 2 -COF-modified perovskites deliver a champion power conversion efficiency of 22.13% with a fill factor of 0.773 under AM 1.5G 100 mW cm À2 illumination.Furthermore, the device retains approximately 81% of its initial efficiency after 1000 h of aging under ambient conditions at a temperature of 30 °C and relative humidity ranging from 45% to 55%.It is believed that this work would provide a facile and efficient strategy to prepare high-quality perovskite films for efficient and stable solar cells.
[19] The construction of COFs with various building blocks through predesigned strategies can endow them with high responsivity to visible light and tunable bandgaps, which also help prevent the decomposition of perovskite materials. [30,31]Wu et al. first incorporated a conjugated three-dimensional (3D) COF based on spirobifluorene cores linked via imine bonds (hereafter referred to as SP-3D-COFs) into PSCs, and the devices utilizing SP-3D-COF 2 exhibited a remarkable enhancement by 18.0% in average PCE with a lower leakage current and a higher rectification ratio than those of reference devices without doping. [17]Besides, Li et al. demonstrated that doping donor-acceptor type COFs (DA-COFs) in the perovskite layer significantly improved the device performance, achieving a best PCE of 23.19% with excellent air stability. [19]n addition, Mohamed et al. utilized the highly crystalline 2D COFs to facilitate the crystal growth of the perovskite film and enhanced the interfacial charge transfer, thus resulting in an increase in the PCE from 17.40% to 19.80%. [18]To date, the utilization of COFs as additives in PSCs has been limited due to the complexity of this technology.
In this work, we introduced an amino-functionalized COF (NH 2 -COF), featuring a high specific surface area and pore volume as well as abundant active sites, as an effective additive in perovskite precursors.NH 2 -COF containing multiple organic functional groups could establish additional contact sites with perovskite, thereby optimizing the crystal growth of the film.Furthermore, the addition of NH 2 -COF was found to decrease the defect concentration and nonradiative recombination within the perovskite layer.As a result, the PCE of PSCs with the NH 2 -COF additive improved from 20.44% to 22.13%.Moreover, the protective framework enhanced the stability of the PSCs under humidity and thermal stress, which maintained 81% of initial efficiency after 1000 h aging under ambient conditions at a temperature of 30 °C and relative humidity ranging from 45% to 55%, higher than that of 77% observed in the reference device.

Results and Discussion
As illustrated in Figure 1a, the perovskite film with a composition of Cs 0.05 FA 0.85 MA 0.10 Pb(I 0.90 Br 0.10 ) 3 was fabricated using the antisolvent dripping method from a precursor solution incorporating a certain amount of NH 2 -COF.The fabrication details were described in the Experimental Section.NH 2 -COF was synthesized following a recent report, [32] and the concentration was optimized to 0.04 mg mL À1 (Table S1, Supporting Information).To understand the interaction between the NH 2 -COF molecule and perovskite, we performed X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) measurements.Compared to the pristine perovskite film, the two peaks of NH 2 -COF-doped film in Pb 4f core level XPS spectra show an apparent shift toward lower binding energy (by %0.33 eV), indicating the presence of chemical bonding between the NH 2 -COF molecule and perovskite (Figure 1b).In addition, a comparable shift of 0.25 eV was observed in the I 3d core level spectra upon NH 2 -COF doping, as depicted in Figure 1c.Similar results were also found in the N 1s core level spectra (Figure S1, Supporting Information).[35] Meanwhile, there is an interaction between ─NH 2 in NH 2 -COF and halide (I À ) in perovskite lattice through hydrogen bonding. [33]From the XRD measurements of the control and doped perovskite films (Figure 1d), both samples exhibit pronounced sharp peaks at (110), (112), ( 202), (220), and (211) planes.Compared to the control film, the NH 2 -COF-doped one shows stronger peaks at (110) plane, and weaker peaks at 2θ = 11.4 and 12.6°, which originate from δ-phase of perovskite and PbI 2 (001), respectively, indicating the improved crystallinity and stability after the introduction of NH 2 -COF. [36]The observable improvement may be due to the interaction of NH 2 -COF molecules with Pb.Based on the obtained results, a schematic representation depicting the molecular interaction between NH 2 -COF and perovskite is presented in Figure 1e.
We then conducted steady-state photoluminescence (PL) measurements on the perovskite films to investigate the effects of NH 2 -COF additive on film quality.As shown in Figure 2a, the PL intensity of the perovskite film significantly enhanced after NH 2 -COF doping, together with the obvious blue shift of emission peak centered from 789 nm for control film to 781 nm for the NH 2 -COF-doped film, which may be attributed to the reduction of defect density and suppression of nonradiative recombination in the doped perovskite film. [33,37]eanwhile, changes in the carrier lifetime can be quantified by the time-resolved PL (TRPL) spectroscopy (Figure 2b).The decay data of the two samples were fitted using a biexponential function (SI), and the detailed parameters are listed in Table S2, Supporting Information.Compared to the control sample (256.03ns), the perovskite film doped with NH 2 -COF possesses a much longer carrier lifetime (448.79 ns), which suggests that it has a lower density of defects. [38]In addition, we conducted UV-visible absorption spectroscopy characterization on perovskite films with or without addition of NH 2 -COF to investigate its effects on film optical properties.Figure S2, Supporting Information, shows that perovskite film with NH 2 -COF exhibited enhanced absorption in the range of 400-700 nm.In addition, the bandgap of NH 2 -COF-doped perovskite film obtained from the Tauc plots (Figure S3, Supporting Information) is determined to be %1.56 eV, which is comparable to that of control film (%1.57eV).
To gain a deeper understanding of the effect of NH 2 -COF additive on the perovskite film crystallization and morphology, additional measurements were conducted.Top-view scanning electron microscopy (SEM) images of pristine and NH 2 -COFdoped (with a concentration of 0.04 mg mL À1 ) perovskite films are displayed in Figure 2c,d.The pristine perovskite film shows a smooth morphology with apparent grain boundaries, while the perovskite film doped with NH 2 -COF exhibits a larger average crystal grain size.Additionally, the grains in NH 2 -COF-doped film exhibit tighter bonding to neighboring ones with narrower boundaries between each other, which provides more consecutive and compact channels for the motion of photons and charge carriers.The improved morphology of perovskite film with reduced defects and nonradiative recombination centers will contribute to the enhancement of photovoltaic performance. [39]urthermore, based on the results from PL intensity mapping presented in Figure 2e,f, we found a more uniform distribution of intensity in the perovskite film modified with NH 2 -COF compared to the pristine film, which can be attributed to the reduction of trap states and nonradiative recombination within the perovskite film. [40]o investigate the impact of NH 2 -COF doping on photovoltaic performance, PSCs were manufactured using a planar n-i-p structure of glass/FTO/SnO 2 /Cs 0.05 FA 0.85 MA 0.10 Pb(I 0.90 Br 0.10 ) 3 / spiro-OMeTAD/MoO 3 /Ag, as depicted in Figure 3a.Each constituent layer of the cell can be easily discerned from the crosssectional SEM image (Figure 3b).The optimization of the NH 2 -COF concentration is presented in Figure S4, Supporting Information, and the optimal condition for the device was achieved at a concentration of 0.04 mg mL À1 .Figure 3c depicts the current density versus voltage ( J-V ) curves of the optimized devices with and without adding NH 2 -COF, which were tested under air mass (AM) 1.5G illumination, with an intensity of 100 mW cm À2 , and the relevant device parameters are summarized in Table 1.The control device achieved a best PCE of 20.44% accompanied by a short-circuit current density ( J sc ) of 24.56 mA cm À2 , an open-circuit voltage (V oc ) of 1.105 V, and a fill factor (FF) of 0.754.The performance of the device doped with NH 2 -COF is significantly improved, delivering an enhanced PCE of 22.13% with a J sc of 24.94 mA cm À2 , an increased V oc of 1.148 V, and an FF of 0.773.Additionally, we have summarized previous reports on PSCs doped with COFs (Table S3, Supporting Information).Our work has achieved a relatively higher PCE, indicating that our approach can further enhance the performance of PSCs.As shown in Figure 3d, the NH 2 -COF-doped cell shows an improved stabilized power output (SPO) of 21.62%, compared to 19.25% of the control device, which is consistent with the results from the J-V test.Similarly, the integrated current density of the doped device obtained from the external quantum efficiency (EQE) test data is 23.65 mA cm À2 (Figure 3e), with an error of only 5% compared to the data collected in the J-V test, whereas the integrated J of the control device is 22.25 mA cm À2 , again demonstrating an effective improvement in the performance of the NH 2 -COF cell.
From the standpoint of commercialization, stability represents a major factor impeding the further development of PSCs.To comprehend the influence of NH 2 -COF on the stability of devices, the shelf storage stability was assessed in an ambient environment (30 °C, relative humidity of 45-55%) for a duration of 1000 h (Figure 3f ).The device doped with NH 2 -COF maintains 81% of its initial efficiency, which is higher than that of the control device (77% remaining).The improved stability observed in the NH 2 -COF-doped device may be attributed to the more effective elimination of defects.In addition, experiments were conducted to study the air stability of perovskite films under high humidity (relative humidity 45-55%, temperature %30 °C) in Figure 3g, and the films prepared under the two conditions were monitored on different days.It was found that the perovskite films modified by NH 2 -COF exhibited a slower transition to the yellow phase than the control one did, also verifying an improvement in the stability of the perovskite film modified by NH 2 -COF.To further verify the improved stability against moisture of the doped devices, the water contact angle on the perovskite film surface was measured (Figure S5, Supporting Information).The contact angle of control device (59.5°) is significantly smaller than that of NH2-COF-doped device (80.1°), which demonstrates the enhanced hydrophobicity of the film, thus proving the improvement of moisture stability.
To gain insights into the intrinsic dynamics of carriers, we characterized the control and target films or devices incorporated with NH 2 -COF to unearth the underlying reasons for the enhanced performance in PSCs.The space-charge-limited current (SCLC) characterization was utilized to investigate the density of defects in perovskite films after doping with NH 2 -COF.The J-V curves of the hole-only device based on a structure of glass/FTO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/perovskite/spiro-OMeTAD/MoO 3 /Ag are shown in Figure 4a.The linear relation indicates an ohmic response of the device in the region of low bias voltage.In addition, the current increases rapidly with increasing voltage and surpasses the kink point, indicating that the trap states are completely filled.The trap-state density (n trap ) can be calculated from the trap-filled limit voltage (V TFL ) using the equation, where ε r is the relative dielectric constant of perovskite, ε 0 is the vacuum permittivity, and L is the thickness of the perovskite layer.The hole-only device with NH 2 -COF dopants exhibits a V TFL of 0.210 V, which is lower than that of the control device (0.287 V).The corresponding trap density n trap is calculated to be 0.27 Â 10 16 cm À3 for the NH 2 -COF-doped device and 0.37 Â 10 16 cm À3 for the control device.The results demonstrated that the NH 2 -COF can decrease the density of trap states in the perovskite film.In addition, the trap density of state (tDOS) of the perovskite films was further analyzed by thermal admittance spectroscopy.In Figure 4b, the spectra are divided into three regions according to energy.In the left Band 1 and right Band 3 regions, it can be seen that the NH 2 -COF-doped device has a lower defect state density than the control device, which indicates that the shallowest-and deepest-level defect states were suppressed, respectively.Furthermore, to fully investigate the mechanism of the improvement in device V oc , electrochemical impedance spectroscopy was utilized to study the charge-transfer resistance (R tr ) and recombination resistance (R rec ) in these devices.As shown in Figure 4c and Table S4, Supporting Information, the NH 2 -COF-doped device demonstrates a lowered R tr of 31.13Ω and an elevated R rec of 243.8 Ω, which suggests enhanced charge transfer and suppressed carrier recombination, providing a smooth channel for carrier extraction and creating a barrier against recombination.Meanwhile, the trend of V oc was further measured under different light intensities.The relationship between V oc and light intensity (I) is linearly fitted by the equation  where n is the ideal factor to evaluate the dominant recombination mechanism, a larger n value indicates much nonradiative recombination, K B , T, and q are the Boltzmann constant, temperature, and elementary charge, respectively.Ideal factor n of the doped device is 1.54, while that of the control group is 2.18 (Figure 4d), which indicates that the defect-induced nonradiative recombination within the device is effectively suppressed after doping NH 2 -COF, which also explains the improved V oc in the relevant device.A similar result can be obtained from the J-V curves under dark (Figure 4e).Additionally, the devices doped with NH 2 -COF exhibited an increased built-in potential (V bi ) from 0.981 to 1.028 V, indicating an enlarged force driving charge carrier separation, as well as the depletion region width, contributing to the observed high V oc (Figure 4f ).

Conclusion
In conclusion, the utilization of NH 2 -COF as an additive to perovskite solutions was demonstrated as an effective strategy to prepare highly efficient PSCs.We added a small amount of NH 2 -COF molecules into the perovskite precursor solution to modify the crystallization of Cs 0.05 FA 0.85 MA 0.10 Pb(I 0.90 Br 0.10 ) 3 , where the interaction between -NH 2 group in the COF molecule and Pb in perovskite lattice leads to the effective passivation of the trap states within the film.Accordingly, the highest PCE of the PSC incorporating NH 2 -COF with a concentration of 0.04 mg mL À1 reached 22.13% under simulated one sun illumination.The efficiency maintained 81% of its initial value after 1000 h aging under humid ambient conditions (30 °C, relative humidity: 45-55%).Our work has shown that the COF materials with -NH 2 functional groups exhibited great potential to enhance the efficiency and stability of PSCs, which provide an alternative route to fabricating more efficient and stable photovoltaic devices.
Fabrication of PSCs: First, FTO glass substrates underwent sequential ultrasonic cleaning using detergent water, acetone, and isopropanol solvents for a duration of 1 h and were subsequently dried using N 2 .Then, the FTO substrates were treated under UV-ozone for 15 min.For the deposition of the electron transport layer, a 100 μL mixture of SnO 2 and ammonium hydroxide was spin-coated onto clean FTO substrates at 3000 rpm for 25 s.Before depositing the perovskite film, the substrates were annealed at 165 °C for 30 min and transferred into a N 2 box (O 2 and H 2 O both below 2.0 ppm).To deposit perovskite thin films, 30 μL of 1.20 M Cs 0.05 FA 0.85 MA 0.10 Pb(I 0.90 Br 0.10 ) 3 precursor solution, where a concentration of 0.04 mg mL À1 of NH 2 -COF was added, was spin-coated at 1000 rpm for 10 s (acceleration rate at 200 rpm s À1 ) and 4000 rpm for 20 s (acceleration rate at 2000 rpm s À1 ).Next, the as-deposited perovskite films were annealed at 100 °C for 60 min.After the substrates cooled down to room temperature, 30 μL of the hole transport solution (containing 90 mg spiro-OMeTAD, 35.5 μL of a 520 mg mL À1 lithium bis(trifluoromethylsulfonyl) imide in acetonitrile, 20.6 μL of 4-tert-butyl pyridine and 1 mL of chlorobenzene) was spin-coated onto the perovskite layers at 4000 rpm for 20 s to form approximately 200 nm of the hole transport layer.Finally, 8 nm MoO 3 and 120 nm Ag were deposited by thermal evaporation under 10 À6 mbar to complete the whole device fabrication.

Figure 1 .
Figure 1.a) Schematic illustration of the preparation of perovskite films doped with NH 2 -COF in the spin-coating process, b) Pb 4f, c) I 3d core level XPS spectra of pristine and doped perovskite films, d) XRD patterns of the control and NH 2 -COF-doped perovskite films, and e) schematic illustration of the molecular interaction between NH 2 -COF and perovskite.

Figure 2 .
Figure 2. a) Steady-state PL spectra and b) TRPL spectra of the perovskite films, top-view SEM images of c) pristine and d) doped perovskite films, normalized PL mapping of the e) control and f ) NH 2 -COF-modified perovskite films.

Figure 3 .
Figure 3. a) The device architecture of the manufactured PSCs, b) their corresponding cross-sectional SEM image with a scale bar of 400 nm, c) J-V curves of the champion devices, d) the SPO of the champion control and NH 2 -COF cell, e) EQE curves and integrated current density ( J) of the control and NH 2 -COF-doped device, f ) long-term stability of unencapsulated devices under 30 °C and 45-55% relative humidity, and g) the moisture stability of control and NH 2 -COF-doped films.

Figure 4 .
Figure 4. a) SCLC measurements of the hole-only devices, b) trap density of states (t-DOS) characterization, c) Nyquist plots, d) light-intensity dependent V oc measurements, e) the dark-current plots of the control and NH 2 -COF-doped perovskite films, and f ) the Mott-Schottky plots of the control and COF-doped devices.

Table 1 .
Photovoltaic parameters of the devices based on control and NH 2 -COF-doped perovskite films.