Molecular Engineering of Polymer Enabling Stability of Hybrid Perovskite Solar Cells

How to inhibit the formation of point defects in the preparation process of hybrid perovskite for improving the stability and photovoltaic characteristics has been a hot research issue in recent years. Herein, molecular engineering of polyvinylpyrrolidone (PVP) is focused as a Lewis base for an additive to improve the interface matching of hybrid perovskite solar cells. Based on the molecular binding strategy, the film crystallinity, bond binding, interfacial transport, and water resistance can be effectively regulated, resulting in preferred photovoltaic characteristics. Herein, the highest photoelectric conversion efficiency of perovskite solar cells reaches 19.7% as well as the improved stability of solar cells. Moreover, combined with first‐principles calculation, the mechanism of PVP macromolecules inducing perovskite interfacial binding through molecular engineering is revealed, which provides a feasible route for using organic macromolecules to regulate the crystallinity and photovoltaic characteristics of hybrid perovskite devices.


Introduction
In the past ten years, the research field of lead halide perovskite solar cells (PSCs) has made remarkable progress, which is mainly attributed to the excellent photoelectric properties of perovskite materials, such as low cost, solution processability, adjustable bandgap, low exciton binding energy, high light absorption coefficient, long carrier diffusion length, and lifetime.Due to the above excellent photoelectric performance, the laboratory certification efficiency of PSCs reached the current 25.8%, [1] among which the highest efficiency of MAPbI 3 PSCs reached 21.9%.However, the highest efficiency achieved by singlejunction PSCs at present is still far lower than the theoretically calculated Shockley-Queisser ultimate efficiency of 30.5%, [2][3][4] and there is still considerable room for improvement.In related research fields, it is a common strategy to use the basic principle of Lewis acid-base pairing to regulate the photovoltaic performance of perovskite devices.In the Lewis acid-base concept, alkali is defined as an electron donor and acid as an electron acceptor.[7] This adduct is connected by coordination bonds (coordination covalent bonds), in which two shared bonding electrons come from the same atom, constituting Lewis base coordination.
It's well known that it is easy to form uncoordinated Pb 2þ on the perovskite crystal surface due to the low formation energy of hybrid halide perovskite in the preparation process of PSCs. [8,9][16][17] At the same time, with the overflow of Pb 2þ in perovskite, the number of point defects in perovskite thin films will increase, and the lattice mismatch of perovskite will increase.Based on the principle of Lewis acidbase pairing, the overflow of Pb 2þ can be effectively suppressed by adding Lewis base reagent into perovskite precursor solution to improve the crystallinity of perovskite and the photovoltaic performance of PSC devices.[20] Up to now, PVP has been widely studied as a passivation layer between the transmission layer and the perovskite light absorption layer, which can effectively optimize the interface contact between the DOI: 10.1002/aesr.202300099How to inhibit the formation of point defects in the preparation process of hybrid perovskite for improving the stability and photovoltaic characteristics has been a hot research issue in recent years.Herein, molecular engineering of polyvinylpyrrolidone (PVP) is focused as a Lewis base for an additive to improve the interface matching of hybrid perovskite solar cells.Based on the molecular binding strategy, the film crystallinity, bond binding, interfacial transport, and water resistance can be effectively regulated, resulting in preferred photovoltaic characteristics.Herein, the highest photoelectric conversion efficiency of perovskite solar cells reaches 19.7% as well as the improved stability of solar cells.Moreover, combined with first-principles calculation, the mechanism of PVP macromolecules inducing perovskite interfacial binding through molecular engineering is revealed, which provides a feasible route for using organic macromolecules to regulate the crystallinity and photovoltaic characteristics of hybrid perovskite devices.perovskite light absorption layer and the transmission layer. [21,22]t the same time, it is of great research significance to use molecular engineering [23][24][25][26] to regulate the crystallinity and photovoltaic characteristics of perovskite through active groups.However, the underlying mechanism is still not unambiguous.
In this work, a molecular engineering method is introduced via a Lewis base organic macromolecule-PVP, added to the precursor of perovskite to prepare PSCs.During the preparation of perovskite thin films, the grains will nucleate around PVP.By adjusting the PVP concentration, perovskite grains' morphology was optimized.At the same time, introducing PVP can effectively reduce the defects of perovskite lattice mismatch and high point defect density, thus effectively inhibiting the nonradiation recombination of carriers as well as improving the stability.The Ab initio molecular dynamics (AIMD) simulation and electrostatic potential (ESP) calculation also support the molecular binding.

Results and Discussion
Figure 1a-d shows the X-ray photoelectron spectroscopy (XPS) spectra of the MAPbI 3 perovskite (PVK) thin film.The binding energies of the I 3d 5/2 and I 3d 3/2 orbitals of the pure perovskite thin film are 629.72 and 618.17 eV (Figure 1a).After adding PVP, the binding energies of I 3d 5/2 and I 3d 3/2 orbitals decrease to 629.58 and 618.1 eV, respectively (Figure 1c).Similarly, Figure 1b,d shows XPS patterns of the lead element of the perovskite film, and the binding energies of the Pb 4f 7/2 and Pb 4f 5/2 orbital of the pure perovskite film are 142.61 and 137.78 eV, respectively.The binding energies of the Pb 4f 7/2 orbital and Pb 4f 5/2 orbital are reduced to 142.58 and 137.68 eV after adding PVP, respectively.The reduced binding energies after adding PVP indicate that the functional groups in PVP can form strong coordination with iodine atoms and lead atoms. [27]By implementing the carbonyl group, which is included in the PVP molecule as the Lewis base, the point defect, which is related to Lewis acid formation in the preparation process of the perovskite film, can be passivated by Lewis acid-base pairing.Therefore, the adverse effects of the point defect on the device performance can be depressed.The interaction between additive (PVP) and PbI 2 was further elucidated by liquid 1H NMR (1H NMR with dimethyl-d6 sulfoxide as solvent).After adding PbI 2 , the proton resonance signals of ─CH 2 ─N─ and CH 2 ─C═O in PVP changed from 3.13 to 3.16 ppm (%0.03 PPM), and 2.03 to 2.06 ppm (%0.03 PPM), respectively (Figure 1e).This forefield shift may be due to the interaction of carbonyl (C═O) and nitrogen (N) atoms in PVP molecules with perovskite to form Pb 2þ intermediate ligands, resulting in reduced electronic cloud density around the C═O and N atoms (Figure 1f ).
In device preparation, the upper film always grows on the underlying film as the virtual substrate.Therefore, the smooth surface morphology can often bring better device performance (Figure S1, Supporting Information).In PSCs, the roughness of the film surface will affect the contact between the perovskite layer and the carrier transport layer.The smoother the film surface, the better the charge transport and the smaller the V OC loss.Different perovskite films were characterized by an atomic force microscope (AFM) to explore the changes in surface morphology and roughness before and after adding PVP.As shown in Figure 2a,b, the perovskite grains in the pure perovskite thin film are small and uniformly dispersed, and the root mean square (RMS) roughness of the thin film is 31.4nm.After the addition of PVP, the RMS of the perovskite thin film is 20.7 nm, and the perovskite becomes a strip (Figure S2, Supporting Information).Based on the effective mobility model, the mobility is dominated by Coulomb scattering, phonon scattering, and surface rough- μ rough . [28]Therefore, the reduced RMS will benefit interfacial carrier mobility, further contributing to interface transport and extraction.Figure 2c shows the XRD pattern of the perovskite thin film.The image shows that the crystal phase of MAPbI 3 is cubic, and the peak intensities of (110) and ( 220) are relatively high, proving that the perovskite presents a cubic phase structure growing along the Z-axis direction.After adding PVP, the grain size of perovskite becomes smaller, which is consistent with the AFM morphology.Moreover, the high-contact-angle value of PVP-modified PVK demonstrates the surface's tendency to repel water, consistent with the optimized cell stability (Figure S3, Supporting Information). [29]sing the pure and PVP-added perovskite thin films to prepare devices, it can be observed that the PVP-added devices show higher spectral responsivity in the wavelength range of 300-800 nm, not only showing higher external quantum efficiency (EQE) but also increasing the integrated current to 22.35 mA cm À2 in comparison to the integrated current of pure perovskite thin film as 21.77 mA cm À2 , proving that the perovskite light absorption layer can better convert the absorbed light energy into electric energy after adding PVP (Figure 3d).
Ab initio molecular dynamics (AIMD) simulation was performed to study the interaction between perovskite and PVP interface.The (001) PbI 2 -terminated perovskite surface with the most stable and lowest-energy configuration serves as the slab model.Four PVP polymer monomers loaded on perovskite (001) crystal surface represents the PVP polymers.With the evolution of time, lead (Pb) and oxygen (O) gradually approach under electrostatic interaction and reach a stable configuration state at about 260 fs in which the Pb─O bond length is 2.30 Å and then there is a slight fluctuation after stable configuration (Figure 3a-d).The simulation results indicate that the bonding ability between the O in the carbonyl group of PVP and the Pb in perovskite increases over time under electrostatic force.It is also found that lead ions in perovskite can interact with O atoms in unsaturated carbonyl groups in PVP, which can provide a basis for PVP to induce perovskite crystallization transformation in the process of thin-film preparation.Furthermore, the ESP calculation was performed to demonstrate the driving force source of Pb─O bonding (Figure 3e).It can be noted that the regions around the O atom of carbonyl functional group and the N atom are red and blue, respectively, indicating that O in the carbonyl functional group has a negative ESP value and N atom has a positive ESP value.The results indicate that O in PVP is more likely to anchor and interact with Pb in perovskite, which is beneficial for the passivation of unsaturated Pb atoms in perovskite and limits the migration of Pb 2þ .This interaction coincides with the shift of binding energies for the Pb orbital.In short, the AIMD simulation and ESP calculation as well as characterization results of XPS indicate a strong interaction between PVP and perovskite, which is beneficial for passivating unsaturated Pb atoms on the perovskite surface to reduce the nonradiative recombination of carriers, passivating possible inherent defects such as I vacancies and Pb interstitial defects.Moreover, perovskite is prone to encounter Pb 2þ in the crystallization process, which would gather at the carbonyl position by adding PVP.Then, the process induces the modified topological morphology, which undergoes spherical-to-tangible nuclei forming long strips with PVP macromolecules as templates (Figure 3f ).This agglomerated structure also contributes to good cell stability due to the water resistance.
To further explore the interfacial effect of PVP on the photovoltaic properties, the photoluminescence (PL) and time-resolved PL spectroscopy (TRPL) were analyzed, as shown in Figure 4a-c.The results showed that the intensity of the PL peak in the PVPmodified perovskite film was higher than that of the control specimen.the same time, with the concentration regulation of PVP, the peak position of PL spectra is given a certain blue shift.For every 1 mg PVP added, the PL peak changed to about 1 nm.Additionally, the carrier lifetime of pure perovskite is 45.9 ns according to TRPL spectra (Figure 4b).The perovskite film with PVP exhibits a larger carrier lifetime of 92.6 ns (Table S1, Supporting Information).While considering the electron transport layer's architecture, the PVP-modified perovskite shows a lower carrier lifetime.It indicates that after adding PVP, a better interface contact can be obtained between the perovskite and electron transport layer, resulting in carriers effectively transporting to both ends of the device.This coincides with the common sense that the nonradiative recombination of carriers in the perovskite film is significantly inhibited after adding PVP, and the interface contact  between the perovskite film and the transport layer becomes better.Figure 4d-f shows the UV-vis spectra.It can be found from the image that after adding PVP, the perovskite film shows higher spectral responsivity than the perovskite film without PVP in the wavelength range of 300-800 nm.Moreover, the absorption peak position of the UV-vis spectrum also changed.According to the results, the bandgap of perovskite decreased to some extent after adding PVP, which indicates that the addition of PVP can regulate the crystal integrality or ordering of perovskite film.
To evaluate the performance of the device (Figure S4, Supporting Information), the photoelectric conversion efficiency (PCE) of the device is tested.The test data are shown in Table 1. Figure 5a,b shows the J-V curves of PSCs prepared using pure perovskite film and PVP-modified one.The results show that the device's performance is the best when the concentration is 4 mg mL À1 .The PCE values increased from 18.19% to 19.71%, the J SC values increased from 21.89 to 22.50 mA cm À2 , the V OC values increased from 1.067 to 1.094 V, and the FF values rose from 77.84% to 80.01% compared to the devices fabricated using pure perovskite films.After the addition of PVP, the device performance has been significantly improved.In contrast, the hysteresis behavior has been significantly improved, which indicates that the addition of PVP effectively reduces defect density.The hysteresis index dropped from 6.9% to 4.9% (Table S2, Supporting Information).On the other hand, the decrease of hysteresis index can also explain the reduction of defect states in the device, which shows that introducing PVP can effectively passivate the point defects produced in the preparation of perovskite films.
In order to further investigate the effect of PVP on the longterm stability of the devices, a reference device and a batch of devices added to PVP were prepared and placed in the same environment at the same time.The effects of temperature and humidity on the device performance were explored by adjusting the temperature and humidity of the storage environment.Figure 5c,d shows the performance degradation profiles of the resulting devices under different conditions.Other conditions being controlled, 931 h of continuous monitoring at 30% RH was performed.And the results showed that the PVP-added perovskite device exhibited better humidity stability, maintaining 89% after 931 h of storage, higher than 80% of the reference device.When the samples were stored at 50 °C and humidity less than 10% RH, other conditions were controlled to be the same.After continuous monitoring for 50 h, the efficiency of the PVPadded device remained at 85%, which was higher than 76% of the reference device.In other words, the perovskite exhibited higher-temperature stability after the PVP was added.In  summary, the humidity and temperature stability of the perovskite can be effectively improved after the addition of PVP, which not only comes from the hydrophobicity of organic materials, but also dramatically affects the crystallinity of the perovskite as well as the denser and more uniform surface morphology effectively resisting the erosion of moisture.

Conclusion
In this research work, an organic macromolecule PVP as an additive was implemented as the crystallization template of perovskite through molecular engineering, which optimizes the interface contact between the perovskite light absorption layer and transport layer and then promotes the carrier transport at the interface.The introduction of PVP has successfully improved the photovoltaic characteristics of PSCs, achieving the highest PCE value of 19.71%.At the same time, the PSCs show higher humidity and temperature stability.Based on the first-principles analysis, the experiment expounds the mechanism of PVP as an additive to regulate the crystallinity and photovoltaic characteristics of perovskite materials via molecular binding.This research route provides a feasible scheme for organic macromolecules to regulate the performance of PSCs through matched molecular engineering.

Experimental Section
Film Preparation and Device Fabrication: The etched fluorine-doped SnO 2 substrates (FTO, 15 Ω per square) were ultrasonically cleaned in an ultrasonic bath for 20 min with detergent, deionized water, acetone, and ethanol in sequence.After drying, a compact TiO 2 layer was deposited with a precursor solution of 0.6 mL titanium diisopropoxide bis (acetylacetonate) and 0.4 mL acetylacetonate in 9 mL anhydrous ethanol by spray pyrolysis along with using N 2 as the carrying gas at 450 °C.After drying, a mesoporous TiO 2 layer was coated on the substrate by spin coating for 20 s at 5000 rpm with a ramp rate of 2000 rpm s À1 , using a solution obtained by diluting the commercially available titanium dioxide slurry with anhydrous ethanol in a weight ratio of 1: 6.Then, it was heated at 100 °C for 10 min and annealed at 450°C for 30 min to remove organic components.MAPbI 3 perovskite precursor solution was prepared by dissolving a mixture of PbI 2 (1.2 mmol, 553.21 mg) and MAI (1.2 mmol, 190.75 mg) into the 1 mL DMF/DMSO (v/v, 1/4), mixed solution.A consecutive spin-coating process deposited the perovskite film at 1200 rpm for 10 s (Ramp rate 600 rpm s À1 ) and 4200 rpm for 30 s (Ramp rate 1400 rpm s À1 ) on the surface of TiO 2 layer.At 10 s before the program ended, 110 μL of chlorobenzene dripped on the spinning substrate, and the film was then annealed at 100 °C for 15 min.
Film and Device Characterizations: The ultraviolet-visible absorption spectrum was recorded by a spectrophotometer (UV-2600, Shimadzu).Standardized differential pulse voltammetry (DPV) was measured by an electrochemical workstation (Zennium X, Zahner).The current densityvoltage characteristics were measured at 100 mW cm À2 (AM 1.5G illumination) via Newport solar simulator (model 91160) and Keithley 2400 source/meter.The EQE spectrum was recorded using a computercontrolled device consisting of a xenon light source (Spectral Products As-Xe-175), monochromator (Spectra Products CM110), and potentiostat (LabJack U6 DAQ board), which was calibrated by a certified reference solar cell (Fraunhofer ISE).Steady-state PL was measured by a spectrometer (FLS1000, Edinburgh Instrument).A picosecond pulsed diode laser (EPL-470) measured the time-resolved PL (TRPL) decay transient.The surface morphology of the films was characterized by scanning electron microscope (SEM) and AFM.The X-ray diffraction (XRD) pattern of the thin film was measured by a TTR-18KW diffractometer, in which Cu Kα radiation (λ = 0.15405 nm) was operated at 10 000 W power (40 kV, 250 mA).XPS was performed by a scanning XPS microprobe (K-Alpha).
Density Functional Theory (DFT) Calculations: Molecular dynamics simulation: The ab initio molecular dynamics of the interface between perovskite and PVP was performed through CP2K package under a constant temperature and constant volume (NVT) ensemble for efficient and accurate electronic structure calculations. [30,31]The Nosé-Hoover hot bath was used to control the system temperature (300 K).The total simulation time was 5 ps, with a time step of 1 fs.The cut off energy was set to 450 Ry.The valence electrons and core electrons were described by double-zeta basis sets (DZVP-MOLOPT, Gaussian basis sets for accurate calculations on molecular systems in gas and condensed phases) and Goedecker-Teter-Hutter pseudopotentials, respectively.A PBE-D3 functional was used to correct the interaction between perovskite I-Pb terminal surface and PVP.The trajectory was obtained every 20 fs until 5 ps after system equilibrium.The (001) Pb-I surface of MAPbI 3 was constructed through a 3 Â 3 supercell with 15 Å vacuum layers in the z-direction.In order to simplify the structural model of PVP molecules, the basic monomer of PVP was implemented to replace PVP molecule.
Calculation of ESP: The molecular ESP was calculated using the Gaussian 09 package with B3LYP functional and 6-311G basis set. [32]

Figure 2 .
Figure 2. a,b) Topological morphology and c) preferred orientation of perovskite films; d) incident-photon-to-current conversion efficiency (IPCE) curves under monochromatic irradiation at room temperature.The integrated photocurrent J SC was calculated from the overlap integral of the IPCE with standard AM 1.5G spectrum.

Figure 3 .
Figure 3. a-d) Molecular dynamics simulation for PVP-modified perovskite; e) ESP of PVP molecules (oxygen atom and nitrogen atom are red and blue, respectively).f ) Crystallization model for a strip perovskite.

Figure 5 .
Figure 5. a,b) J-V characteristics of PSCs.c,d) The PCE decays of PSCs.

Table 1 .
Photovoltaic parameters of devices with different concentrations of PVP.