Hydrogen-bonded polymer network-directed crystallization of FA-based perovskite for high-efficiency and stable solar cells - Nat. Commun.

• Research highlights:

  1. A chemical additive with multiple functions is designed to suppress the composite mesophase and promote the definite crystallization of the meteorite α phase.
  

  
  

  2. The addition of reagents generates in-situ polymerization during the formation of the diamond ore film, forming a hydrogen bond network to stabilize the α phase.

  
  

  3. The polymeric additives make the bare perovskite film have a strong hydrophobic effect on liquid water. When the unencapsulated device is soaked in continuous sunlight for 1000h and in an air environment with 50% humidity for 2000h, the efficiencies are 24.10% and 95%, respectively.

  
  

  1. Problems existing in perovskite solar cells

  The (PSCS) based (fapbi3)-based (PSCS) caused widespread concern in 2010, with 25.7% of the (PCE), unfortunately, below 150°C, the photoactive black phase (α-FAPbI3) undergoes a nasty phase transition to the non-meteorite yellow phase (δ-FAPbI3). One strategy to increase bI3α-FAPbI3 is to introduce additives into pure FAPbI3, such as volatile salts (methyl copper hydride (MACl), alkalized methyl aluminum (MAI) ions), formate (HCOO-), potassium acid salt (SCN-)) and cations (methylenedicopper (MDA2+), isopropylamine (iPAmH+)), while most of these PSCs still require high fabrication temperatures. to 150°C
  

  
  

  2. Introduction to achievements

  In view of this, Wang Fangfang and Qin Tianshi from Nanjing University of Technology reported a multifunctional fluorination additive FTPA, which can inhibit the complex mesophase of perovskite and significantly promote α- Directional growth of FAPbI3. Due to the improvement of charge transport balance, low defect density and gradient energy level alignment, the corresponding PSCs show excellent PCE of up to 24.10%. In addition, due to the formation of hydrogen-bonded polymer network in the perovskite film α- FAPbI3 makes PSCs have good light, moisture and thermal stability. It opens up a successful prospect for the rational screening of highly effective and stable FAPbI3-based PSCs molecular additives.
  

  
  

  3. Results and discussion

  Key point 1: hydrogen bond interaction between FTPA and perovskite
  

  In this paper, the mesophase, nucleation and crystallization of perovskite thin films during spin-coating and annealing were monitored and studied by in-situ X-ray diffraction (XRD), in-situ UV-vis absorption spectra and in-situ grazing incidence wide-angle X-ray scattering (GIWAXS). We found that the fundamental reason for the mixed halide perovskite dilemma is that due to the multiple crystal nucleation paths generated by various intermediate phases during the film forming process (Fig. 1a), it generally cannot grow into uniform and highly crystalline films. Therefore, the formation of perovskite thin films with orderly crystal orientation, low defect density and stable quality is crucial for low non-radiative energy loss and long-term stability of PSCs. In order to overcome these problems, we have successfully designed a multifunctional fluorinated molecule 3-fluoro-4-methoxy4 ', 4 "- bis (((4-vinylbenzyl ether) methyl) triphenylamine (FTPA) (see Supplementary Fig. 1 for the molecular synthesis route and 1H NMR, and Supplementary Note 1 for the synthesis steps of FTPA) is used as an additive of FA0.95MA0.05Pb (I0.95Br0.05) 3 perovskite (FAMA). The four important design criteria of the molecule are shown in Fig. 1a, (i) triphenylamine is used as the core of the molecule to carry out efficient hole transport and energy level adjustment on the body and surface of the perovskite film; (ii) The interaction of fluorine and oxygen atoms with perovskite inhibits the formation of multiple intermediate phases and promotes α- Directional crystallization of FAPbI3. (iii) As a solubilizing unit, the flexible ether group makes FTPA a viscous liquid (complementing the inert diagram in Figure 1), which is conducive to the continuous interaction between molecules and perovskite during spin-coating and annealing. (iv) Vinyl enables FTPA to be polymerized in situ and filled in the grain boundary of perovskite to form a hydrogen bond network, which can be effectively stabilized under the operating conditions of PSCs α- FAPbI3.
  

  
  

  The strength of the interaction between additives and perovskite precursors determines the mesophase, so it plays a key role in the management of the crystallization process. FTPA can form hydrogen bond with charged formamidine (FA+), which is characterized by 1H NMR (Fig. 1b). In deuterated DMSO solution, the resonance signal of protonated ammonium in FAI is 8.787 ppm (Fig. 1b). With the addition of FTPA, at 8.965 ppm and 8.629 ppm, the resonance signal of ammonium is split into two, which indicates that the interaction between FTPA and FA+results in different chemical environments of the two kinds of ammonium in FA+. These results are consistent with the calculated results of DFT, that is, the fluorine atoms and oxygen atoms on the FTPA have strong hydrogen bonds with the ammonium atoms in FA+. In addition, it can be seen from the photos (Fig. 1b) that the transparent FAI solution turns yellow after adding FTPA, confirming the formation of FTPA-FA+complex in the solution. For the sample containing PbI2, the ammonium resonance signal intensity decreased, but it was still in a split state, indicating that although there was interaction between PbI2 and FTPA-FA+complex, the interaction between FA+and FTPA was stronger than that between FA+and PbI2 (Supplementary Fig. 2 and Supplementary Note 2). We further proved the interaction between FTPA and Pb ions by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). In the film characterization and devices discussed below, the content of FTPA in all film samples is constant, that is, the molar ratio of FTPA in perovskite precursor is 5% (43.13 mg mL − 1). In the FTIR spectrum (Fig. 1c and Supplementary Fig. 3), the C=N stretching peak of pure FAI moved from 1695 cm − 1 to 1698 cm − 1 after adding FTPA, and further moved to 1709 cm − 1 after adding PbI2. As shown in Fig. 1d, the XPS spectrum shows that after adding FTPA into perovskite, the main peaks of 143.15 eV (Pb 4f5/2) and 138.25 eV (Pb 4f7/2) move to the low binding energy region (142.70 eV and 137.80 eV respectively), while the F1 s peak of perovskite after adding FTPA is opposite (Supplementary Fig. 4). This may be due to the interaction between F of FTPA and uncoordinated Pb ions. In addition, compared with the control film, the small metal Pb peak near the main peak in the FTPA perovskite film almost disappeared. Supplementary figure 4 shows the I3d XPS spectra designated as Pb-I chemical species. Compared with the control group (630.45 eV and 618.95 eV), at 630.05 eV and 618.55 eV, I 3d3/2 and I 3d5/2 peaks of FTPA samples moved to the region with lower binding energy. This indicates that the Pb-I bond is weakened due to the interaction with the FTPA-FA+complex. We also noticed that after adding FTPA, the C-C=O peak (288.26 eV) related to oxygen/water in the control membrane C 1s was significantly inhibited, indicating that FTPA can slow down the degradation of the calcium titanate layer. The above results show that due to the strong interaction between the FTPA unit and perovskite, the mesophase (FTPA-FAI-PbI2) can be formed.

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• Fig. 1 Molecular design and interaction of FTPA and perovskite

  
  

  
  

  Key point 2: in-situ monitoring of the nucleation and crystallization of perovskite films

  The interaction between additives and perovskite affects the nucleation and crystallization process of perovskite, which usually occurs rapidly in the spin-coating and initial annealing stages. Therefore, in order to understand the process of mesophase transformation to perovskite, we have carried out in-situ tracking of XRD (Fig. 2a) to study different processes. For the control FAMA perovskite, although only a small amount of MA+was used in the perovskite precursor, a complex mesophase was formed from DMSO/DMF solvent in the wet perovskite film without the use of anti-solvent, such as the solvent phase MA2Pb3I8 · 2DMSO (2 θ= 6.73 °, 7.38 °, 9.33 °) and δ- FAPbI3 (2 θ= 11.9°)。 The formation of MA2Pb3I8 · 2DMSO is due to the strong interaction between perovskite precursor and DMSO, which is confirmed by the XRD pattern of MA2Pb3I8 · 2DMSO mesophase (Supplementary Figure 5), which is consistent with previous reports. After the extraction of DMF/DMSO with anti-solvent, most of the intermediate solvent phase is transformed into α Phase perovskite. However, MA2Pb3I8 · 2DMSO and δ- FAPbI3。 In the process of anti-solvent assisted crystallization, Cl - in MACl in perovskite precursor enters the perovskite lattice, resulting in δ- The diffraction peak of FAPbI3 moved to a high angle region of 12.3 °. stay δ- The diffraction peak next to FAPbI3 (12.3 °) is the residual MA2Pb3I8 · 2DMSO diffraction peak at 11.87 °, which is different from that before the use of anti-solvent δ- FAPbI3 diffraction peaks overlap, see the enlarged XRD diagram in Supplementary Fig. 5. During the annealing process of 30 s~5 min, δ- FAPbI3 peak (from 12.3 ° to 11.9 °) and α- The position of FAPbI3 peak (from 14.15 ° to 13.9 °) is gradually transferred to the low angle region because Cl ion is replaced by I ion. It is noteworthy that, δ- FAPbI3 in α- FAPbI3 always exists in the crystallization process, which may be due to the two competing ways of crystal nucleation starting from the mesophase, as shown in Figure 1a.
  

  
  

  Therefore, due to the complexity of perovskite intermediate α- The nucleation and crystal growth of FAPbI3 are not feasible. After annealing at 100 ℃ for 1 hour, α The peak of phase perovskite is dominant, but due to the thermal decomposition of unstable non-perovskite yellow phase, PbI2 (2 θ= 12.8 °) peak. With the addition of FTPA (Figure 2a), MA2Pb3I8 · 2DMSO and δ- The intermediate phase of FAPbI3 was obviously inhibited. The wet perovskite film without antisolvent showed no obvious crystal phase and no obvious diffraction peak due to the hydrogen bond interaction between FTPA and perovskite precursor. With the increase of annealing time, α The phase peak gradually increases and the half peak width gradually narrows. After annealing for 1h, perovskite film only appears α- The peak of phase (Fig. 2a and Supplementary Fig. 8), which is attributed to the single crystal path during the formation of perovskite. In addition, the in-situ polymerization of FTPA in perovskite membrane is stable α- Another reason for FAPbI3 will be discussed later. In order to clearly see the mesophase of the perovskite sample, the optical microscope image of the wet perovskite film without anti-solvent was measured, as shown in Fig. 2b and Supplementary Fig. 9. In the control perovskite film, the needle structure of MA2Pb3I8 · 2DMSO and δ- The blocky structure of FAPbI3 verifies the above two competing crystal mesophase. After adding FTPA, due to the hydrogen bond interaction between FTPA and perovskite precursor, the wet perovskite film shows amorphous characteristics, without obvious crystallization. This also shows that liquid FTPA can continuously inhibit the formation of mesophase during the spinning coating process.

  
  

  The effect of FTPA on the formation of spin-coated perovskite films was studied by in situ UV-Vis absorption spectroscopy and in situ GIWAXS. In Fig. 2c and Supplementary Fig. 10, it can be found that with the increase of spin coating time, the absorption of FTPA by perovskite precursor is significantly enhanced, while the control film is significantly weakened, which may be due to the stronger internal interaction of FTPA-perovskite complex than the solvent intermediate. The in-situ GIWAXS model and intensity distribution are shown in Fig. 2d and Supplementary Fig. 11 respectively. In the initial spin-coating stage, the nucleation signal of wet perovskite film cannot be recognized. After dropping anti-solvent on the control perovskite film for 30s, MA2Pb3I8 · 2DMSO (q=0.47 ⊙ − 1,0.52 〞 − 1,0.66 〞 − 1) δ- FAPbI3 (q=0.85 ⊙ − 1), PbI2 (q=0.88 〞 − 1) and α- Diffraction signal of FAPbI3 (q=1.01 ⊙ − 1). This confirms that the phase evolution of FA mixed anionic perovskite crystal is complex, because there are many kinds of possible crystals and the difference of formation energy is very small. After adding FTPA, the mesophase of MA2Pb3I8 · 2DMSO disappears. FTPA except FAPbI3· δ Phase sum α In addition to the phase (weak) signal, a new phase appeared in the perovskite film at q=0.9523 Ω− 1, which may be the signal of the FTPA · FAI · PbI2 complex. In addition, the formation and crystallization of perovskite films during annealing were monitored by in-situ UV vis absorption spectroscopy and GIWAXS. As shown in Fig. 2e and Supplementary Fig. 12, the absorption intensity of perovskite films increases from about 15 s after annealing, indicating the formation of perovskite crystals. The absorption wavelength range increased from 400~600 nm to 400~800 nm, and the mesophase changed to α- FAPbI3。 FTPA added α The deposition time of phase perovskite in the film is 10 s, which is longer than that of the control film (6 s). As expected, FTPA effectively inhibits the complex mesophase and delays the crystallization kinetics of perovskite, thus achieving the directional growth of perovskite films.

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• Fig. 2 During spin-coating and annealing, the mesophase, nucleation and crystallization process of control and FTPA-based perovskite films were monitored in situ.

  
  

  Key point 3: morphology, carrier extraction and energy level behavior of perovskite film

  We apply FTPA to the bulk of perovskite film (B-FTPA) and further apply it to the surface of B-FTPA film (BS-FTPA) to passivate defects, minimize bandgap penalty, and improve charge extraction and transmission. During the annealing process, FTPA containing vinyl can be in-situ polymerized on the body or surface of perovskite film by atom transfer radical polymerization (ATRP). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to study the morphology of control, B-FTPA and BS-FTPA perovskite films. As shown in Fig. 3a, white PbI2 phase can be clearly observed in the control film. The grain size of B-FTPA perovskite film becomes larger and PbI2 phase disappears. The surface of the perovskite film is completely covered by the polymerized FTPA, and its thickness is enough to distinguish the grain boundary of the bottom layer. The grain size distribution of these films is shown in Figure 13. The average grain size of the control film is~550 nm, while the average grain size of B-FTPA and BS-FTPA films is~820 nm and~870 nm, respectively. High resolution transmission electron microscopy (HR-TEM) images confirmed the in-situ polymerization of FTPA in perovskite film bulk. Fig. 3b shows that perovskite particles show obvious lattice stripes, while polymerized FTPA with amorphous morphology exists between or at the edge of crystalline perovskite particles. The lattice spacing of perovskite crystal is 3.18 ⊙, which corresponds to the (200) plane of the cubic phase of FAPbI3 crystal. Therefore, during the annealing process, with the growth of grains, FTPA polymerizes in the intergranular region, and the hydrogen bond interaction delays the crystallization of perovskite, thus inducing the directional growth of perovskite films and forming good morphology.
  

  
  

  In general, the insulating polymer or cross-linked molecule in the perovskite film can passivate the defect, but it will hinder the carrier migration. The charge mobility and well state density of pure electronic devices (Fig. 3c) and pure hole devices (Fig. 3d) were characterized by space charge limiting current (SCLC) measurements. Compared with the control device, the lower hole (or electron) trap density (Nt) obtained in B-FTPA and BS-FTPA can be attributed to the passivation of perovskite film body and surface defects. The calculated electron mobility of control, B-FTPA and BS-FTPA devices is comparable, 1.98 respectively × 10−3cm2V−1 s−1、1.51 × 10 − 3cm2V − 1 s − 1 and 1.14 × 1010−3cm2V−1 s−1。 Corresponding hole mobility of B-FTPA perovskite (2.74 × 10 − 4 cm2V − 1 s − 1) vs control (1.01 × 10 − 5 cm2V − 1 s − 1) increased by one order of magnitude due to the pure FTPA (2.97) with triphenylamine as the molecular core × 10 − 2 cm2V − 1 s − 1) has excellent hole mobility (Supplementary Fig. 17 and Supplementary Note 5). The well-designed FTPA, as the connection bridge of particles, can minimize the electrolytic coupling or insulation between perovskite crystals, which can inhibit the non-radiative recombination in perovskite films.

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• Fig. 3 Comparison, morphology, carrier extraction and energy level characterization of B-FTPA and BS-FTPA perovskite films

  
  

  In order to evaluate the dynamics of charge extraction, time-resolved photoluminescence (PL) was measured (Fig. 3e), and the data fitted with the double exponential equation were summarized in the supplementary table 1. And control membrane（ τ 2=282.3 ns), the PL life of B-FTPA perovskite is longer（ τ 2=325.1 ns), which shows that the non-radiative recombination is strongly inhibited because the FTPA improves the Schottky order of bulk perovskite. In addition, the average PL life of BS-FTPA perovskite film is due to the hole extraction of the FTPA cover（ τ Avg) is 50.1 ns. SpiroOMeTAD is used as hole transport layer on S-FTPA film, PL τ Avg further decreased to 26.9 ns. The change trend of steady-state PL spectrum (supplementary figure 18) is consistent with the time-resolved PL measurement results. Ultraviolet photoelectron spectroscopy (UPS) (Fig. 3f, g) shows that Fermi energy level (EF) shifts downward from − 4.81eV (control) to − 4.93eV (B-FTPA), indicating that the p-doping level of B-FTPA perovskite is higher than that of the control. The valence electron and maximum valence electron (EV) of B-FTPA are − 5.39eV and − 5.15eV, respectively, and the high-occupied orbit (HOMO) of Spiro-OMeTAD is − 4.97eV, respectively. Therefore, adding FTPA to the body and surface of perovskite film can lead to gradient energy level arrangement, thus promoting hole transport/extraction.

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• Table 1 Device parameters of champion PSCs based on different device structures and perovskite modified by FTPA

  
  

  Point 4: Photovoltaic performance and stability of PSCs

  The cross section scanning electron microscope images of the control and BS-FTPA PSCs are shown in Fig. 4a. After the addition of FTPA, the irregular crystal of the control perovskite is transformed into the integral crystal which is difficult to distinguish between the grain boundaries. We believe that FTPA can reduce the grain boundary by inhibiting the complex mesophase, thus regulating the nucleation and crystal growth process of perovskite films, and thus promoting the carrier migration. The spatial distribution of FTPA in perovskite films was further studied by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The section image (Fig. 4a), three-dimensional image (Supplementary Fig. 19) and depth profile (Supplementary Fig. 20) of ToF-SIMS confirm the uniform distribution of FTPA on the body and surface of perovskite film. Figure 4b shows the current density - voltage (J-V) curve of the BS-FTPA PSCs compared with the B-FTPA, and the specific photovoltaic parameters are shown in Table 1. The maximum power conversion efficiency (PCE) of the control battery is 22.48%, the JSC is 24.46 mA cm − 2, the VOC is 1.143 V, and the filling factor is 80.38%. The champion BS-FTPA PSC showed excellent maximum PCE of 24.10%, JSC of 24.43 mA cm − 2, VOC of 1.182 V, and filling factor of 83.45%. The measurement results of incident photoelectron conversion efficiency (IPCE) (Fig. 4c) show that the comprehensive JSC of the control and BS-FTPA PSCs are 23.22 mA cm − 2 and 23.32 mA cm − 2, respectively, which are in good agreement with the measured values of JSC under the solar simulator. It is noteworthy that BS-FTPA PSCs have a high VOC of 1.182 V, which is 93% of the Shockley-Queisser limit VOC (1.27 V) at the 1.55 eV absorption threshold (Supplementary Figure 21). Calculation of non-radiation restructuring losses（ Δ VOC loss) is only 0.10 ev (supplementary figure 22). In addition, the Ulbach energy (Eu) of the BS-FTPA device (Fig. 4d) is 14.5 meV, which is one of the lowest values among the reported high-performance PSCs, indicating that the defect density in the BS-FTPA perovskite film is very low. Statistical analysis of photovoltaic parameters based on 30 devices shows that BS-FTPA devices have good repeatability, with an average PCE of 23.75%, which is higher than the control device (21.76%) (Figure 4e). The device was aged in a thermostat (23 ± 2 ° C, supplementary figure 25), and simulated 1 solar illumination in nitrogen atmosphere (ISOS-LC-1) (figure 4f). The PCE of the control device decreased to 59% of the initial PCE after aging for 1000 hours, while the PCE of the BS-FTPA device only decreased to 5% of the initial PCE. We also monitored the PCE change of the unpacked device in the ambient air of 25 ± 5 ° C and 50 ± 10% relative humidity for 2000 hours (ISOS-D-1) (Figure 4g). The PCE of the control device decreased to 95% of the initial value after aging for 150 h, while the BSFTPA device still maintained 95% of the initial PCE after aging for 2000 h. Generally, in n-i-p type PSCs, Li-doped Spiro-OMeTAD always brings degradation problems to devices, mainly due to the hygroscopic characteristics of Li-TFSI. Water molecules can easily penetrate the perovskite structure, which further reduces the performance of PSCs. The excellent humidity stability of BS-FTPA PSCs can be attributed to the hydrophobic effect of fluorine-containing polymerized FTPA on the surface of GBs and perovskite. The contact angle of water increased significantly from 47.4 ° to 100.8 ° (Fig. 4h). Surprisingly, the BS-FTPA membrane can still maintain the black phase of FAPbI3 after soaking in water for more than 5 minutes, while the control membrane without FTPA immediately turns yellow due to decomposition into PbI2. The results show that FTPA can inhibit the permeation of water molecules in all directions. In addition, the long-term stability of the unpacked PSCs under nitrogen atmosphere at 65 ° C was also investigated (ISOS-T-1, Figure 4i). After aging for 500 h, the initial efficiency of B-FTPA and BS-FTPA devices remained stable at more than 80% and 85%, respectively, while the control device remained above 58% only after 500 h. The polymer network formed in the perovskite film inhibits the ion migration, thus improving the thermal stability of FTPA-based PSCs.
  

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• Figure 4 Photovoltaic characteristics and stability of PSCs based on contrast, B-FTPA and BS-FTPA

  
  

  4. Summary
  

  Here, we have monitored and analyzed the mesophase, nucleation and crystallization process of perovskite films during spin-coating and annealing. It is found that the complexity of the mesophase is the main reason for the disordered crystallization of mixed halide perovskite, which affects the corresponding photovoltage performance and stability of PSCs. Based on this understanding, we developed a multifunctional fluorination additive FTPA, which can inhibit the complex mesophase of perovskite and significantly promote α- Directional growth of FAPbI3. Due to the improvement of charge transport balance, low defect density and gradient energy level alignment, the corresponding PSCs show excellent PCE of up to 24.10%. In addition, due to the formation of hydrogen-bonded polymer network in the perovskite film α- FAPbI3 makes PSCs have good light, moisture and thermal stability. It opens up a successful prospect for the rational screening of highly effective and stable FAPbI3-based PSCs molecular additives.
  

  
  

  5. References

  Orientated crystallization of FA-based perovskite via hydrogen-bonded polymer network for efficient and stable solar cells

  https://www.nature.com/articles/s41467-023-36224-6